Robert Mulliken

Kuhn:

I would say, sir, if it is agreeable to you, that we start off by talking shout the general question of your background with the sciences and your family background in general. I realize that your father himself was a scientist of some real distinction but still I’d be interested in knowing how you worked into the sciences, bow you acquired your interests, what sort of a career it looked as though one could have in the sciences. The whole situation is one that has changed a good deal over the last fifty years.

Mulliken:

Yes. Well, I mentioned my grandfather; and my father and Arthur Noyes worked together. They were friends and they worked in the family woodshed or what do you call it — the wood house chamber they call it. They made a laboratory there and did experiments and for a textbook they used Mrs. Somebody’s Conversations on Chemistry. Do you know that book?

Kuhn:

Yes! Mrs. Marcet.

Mulliken:

Published about 1800 or 1810.

Kuhn:

Yes, they’re not the only ones by any means who used this.

Mulliken:

I have one or two copies which come from there and meant to bring one over, but you know about it.

Kuhn:

Yes indeed I do.

Mulliken:

So they did their experiments and they had a fire or two and so on, and they went to MIT I guess. At any rate, then Arthur Noyes went to Leipzig and my father went to Leipzig a year later, I think, and got his PhD there. I have the PhD thesis — it happens to be up there [on Mulliken’s bookshelf] but that’s going farther back. Then I read proofs from my father’s books so the terminology of organic chemistry was not unfamiliar.

Kuhn:

Starting at what age did you begin to read proofs for him?

Mulliken:

There I’m not sure. It was not at a very early age. Unfortunately I don’t remember things all carefully dated; they’re very much jumbled up.

Kuhn:

When you read proof did you read it with him or with someone else so that you were comparing the proof and the manuscript?

Mulliken:

Oh, he was there and I was there. Perhaps I read it and he listened. I’m not sure. If really don’t remember but there was a great deal of it so that perhaps opened s sort of interest in organic chemistry which my father was doing. But in high school — this high school had a special relation with MIT in that they had a rather large scholarship fund for students in the high school to go to MIT or possibly other places, but the enp1iasis was on MIT.

Kuhn:

What high school is this?

Mulliken:

Newburyport High School. So they had a science option that was a regular part of the curriculum specializing in science, and that was stimulated I suppose by the existence of this (Wheelwright) fund.

Kuhn:

Was it clear to you from a relatively early age that you were going on into science?

Mulliken:

Well, I was interested in many things so there was somewhat of a conflict because I was interested in almost anything, but I was very much interested in science. There was a book by Robert Kennedy Duncan which I read which impressed me very much. Do you know that book? It was sort of a ‘frontiers of science,’ [The New Knowledge; A Popular Account of the New Physics and the New Chemistry in Their Relation to the New Theory of Matter. 1905] telling all the interesting things. That probably was in high school. Oh, and I meant to mention that later on, with regard to the Bohr theory end so on, there is Foote and Mohler. Do you know that? That was a very important landmark; I mention it now just so as not to forget it. That I found very interesting, but that was later. In high school I didn’t have the highest rating in the class but maybe the second or third. I don’t remember. At any rate, I had to give an essay and the essay was on the electron; it was entitled, “The Electron: What it is and what it does.” That and perhaps the Kennedy Duncan shows trends of interest. At that time people bad just recently been getting acquainted with electrons. It wasn’t so long after J. J. Thomson and so on. In high school I was also the weather prophet! I looked at the barometer and made a prediction for the local newspaper about what the weather would be. First I looked at what the government had said in the morning paper, then I looked at the barometer and at the sky, and then I made a prediction for the evening paper.

Kuhn:

How was your batting average?

Mulliken:

Oh, pretty good I guess.

Kuhn:

It’s not a part of the country where one can hope for immense reliability.

Mulliken:

No, the reliability wasn’t very good for the government’s predictions. Mine were a little bit closer to the time perhaps. I had an advantage.

Kuhn:

When you speak of having been interested in some things other than science and there having been some real conflicts, what other things particularly were in conflict with science for your main interest?

Mulliken:

An interest in what now would come under the heading of social sciences or humanities — all those things. There was one time — I don’t remember whether it was in high school or in college — when I thought the best thing would be to be a philosopher. Then I got hold of a book on philosophy and I found that all the philosophers, every one, had entirely different ideas, no two could agree. I concluded they were all wrong and science was obviously much preferable, much better from that point of view. Well, I thought perhaps when I reached the age of 60 I might [be a philosopher]. Nobody below the age of 60 should try to be a philosopher because they couldn’t know enough. Well, now I would say that nobody can ever know enough to be properly prepared to be a philosopher. That was one thought. I was interested in languages; I was very good in languages. I had a course in French and German in high school and when I got to MIT I found that the course in German which was required at MIT was quite unnecessary for me. I knew much more; the high school had done it. Also I took a year of Latin as an extra in high school and have since found that very helpful in knowing something about the origin of words. In high school I used to go out in the country on a bicycle. There wasn’t much competition from automobiles and I studied the plants, got acquainted with all the wild plants, made a lot of observations and found there was one species of goldenrod which forms hybrids with about six other species. I don’t know that that has ever been recorded yet in the literature.

Kuhn:

Did you do the cross-pollination experiments yourself?

Mulliken:

No, I didn’t do any experiments. My behavior, I believe, was more like that of a theoretical person. No, I was not very much inclined to do experiments in the laboratory. In fact, I didn’t learn much about that until later and I still wasn’t inclined; I wasn’t mechanically minded and that sort of thing. Then I went to MIT. I thought it might be nice to go to Harvard but we had this (Wheelwright) fund and we didn’t have any great amount of extra funds in the family.

Kuhn:

Was the desire to go to Harvard the feeling that you still might not want to go into the sciences?

Mulliken:

Well, possibly, and that the background would be broader, but I guess I didn’t have enough independence or initiative or something to try very hard to see if there might be an exception. No, I guess I was very impressed with authority at that time and had not learned how to go out and do things that were different, other than getting up early and going out bicycling.

Kuhn:

In this period before you got to MIT had there been teachers or books besides Duncan, which you mentioned, which you remember as particularly influential in pointing you toward science?

Mulliken:

No, no teachers particularly. There were a physics teacher and a biology teacher in the high school and that was all very nice, but they didn’t do anything especially that I remember to stir up interest. They may have called my attention to some books, but — well, that’s about it I guess.

Kuhn:

So that really, as of the time you went to MIT, you were presumably going into the sciences but without any great sense of vocation about it. You might well not have gone. Is that a fair —?

Mulliken:

Yes, it might have been otherwise, but actually I guess I had a very strong feeling that the scientific way was the right way and that experience with philosophy is an indication. I always felt that science is the way to keep your feet on the ground but then I like to explore in other directions. I’ve always been interested, since high school or perhaps before, in the fact that the scientific method ought to be much more widely used in every direction. People have gradually come to that view more and more but still it isn’t very much put into practice. So on that subject perhaps it isn’t irrelevant to bring this in, which is a talk I gave. The University of Chicago trustees give a dinner every year for the faculty and one representative of the trustees, one of the faculty, and the president give a speech, so I gave this.

Kuhn:

Oh, I’m very glad to learn of this.

Mulliken:

This was reproduced there later; it was published in Science 1936, I think, and a few words or footnotes have been omitted here perhaps but it has not been altered very much.

Kuhn:

Yes, I hadn’t realized that this was that long ago.

Mulliken:

I’ve noticed that some people recently have used almost the same words but perhaps that is quite natural. Looking at this, it in some ways seems pretty naive; it’s also a little bit in the Hutchins’ style. [Robert M.] Hutchins was the president. It’s dated in that respect and there are a few uses of words which are not quite accurate but I think on the whole it is not very different from what I would say now or what anyone ought to say in my opinion. … There are other matters of interest here and evidently they were more impressed when they got this out by what [Norman] Hilberry had to say. He was director of the Argonne Lab at that time. But he was a friend of mine all the way from the graduate school. We come to that a little later. There’s a great deal of connection there. This is very very different from what I have here, but this I think is quite nice too.

Kuhn:

Good, I shall hold on to this if I may and certainly I’ll have a look.

Mulliken:

Of course the original article in Science you could find; I’m not sure if it’s referred to here or not.

Kuhn:

There’s, I think, no exact reference but it refers to its being a considerably older talk.

Mulliken:

Anyway, 1936 is about the date. I might say that I think at the age of eight — this might possibly have some interest — I concluded I wasn’t going to kill anybody because if I did the government would punish me. I thought that was a very good reason and from that time on it has seemed to me that capital punishment was a good idea; it keeps a lot of people from killing other people. That was the way I thought about it. I know that nowadays it’s popular to say it has no effect. Well, it has no effect if it isn’t carried into effect, of course. That was a sample perhaps of thinking at the age of eight; I don’t know if that’s scientific or not.

Kuhn:

That’s not irrelevant.

Mulliken:

But in some ways I think that by the age of 16 I had reached practically all the conclusions which I’ve had since, including this general point of view that’s expressed in there, other than some refinements. I guess that’s rather a usual experience.

Kuhn:

You clearly, to this extent then, had by the age of 16 quite a refined idea of what the scientific method was.

Mulliken:

Oh, I hadn’t come in contact with the experimental side. There was something lacking which I learned a little later and that’s mentioned here incidentally; let me see if I can find it. I was really very deficient in keeping my feet on the ground.

Kuhn:

You certainly do say there that it the quantitative analysis course that brought you from the notion of —

Mulliken:

It was very uninteresting. I didn’t like it but I began to learn; and there were still some further stages required.

Kuhn:

Would it disturb you if I smoked?

Mulliken:

No, certainly not. I’ve been trying the experiment since December 2nd in which I don’t. [Discussion of stopping smoking here omitted]

Kuhn:

You said you had had both physics and biology; had you had chemistry also in high school?

Mulliken:

Oh, perhaps not. I don’t seem to remember any.

Kuhn:

How about mathematics? How far had that gone?

Mulliken:

I think in those days they did better than they usually do now. At any rate it was a good sound training; it was a good high school, a good course, and a good preparation for science.

Kuhn:

It included trigonometry and solid geometry?

Mulliken:

I suppose; I don’t remember. I might even have a book from then but probably not. However, the solid geometry I didn’t like very well. In that respect I’m not very well suited to be a physicist.

Kuhn:

Particularly one concerned with molecules! You’ve done notably with that disability.

Mulliken:

Yes; I don’t like them too complicated — crystal structure, that I wouldn’t do well with.

Kuhn:

Was your decision to go to MIT equivalent really to the decision to go into science?

Mulliken:

Oh, more or less. Then I took the chemistry course. There weren’t many students in the chemistry course because MIT was for engineering. That perhaps was the point at which I wasn’t entirely pleased; the thought that this was perhaps not the ideal place.

Kuhn:

Because of the engineering emphasis.

Mulliken:

Because of the major emphasis on engineering, also even in physics. Now this is something of interest: they may have had one or two people working for a PhD in physics and in chemistry two or three — I don’t know but I suppose you could find the statistics if you wanted to — but at any rate there were very few. Certainly physics was highly unpopular; there were very few people going into it and unfortunately also the professor, Professor [C. R.] Cross, was not very inspiring. He was professor of physics, so my physics didn’t go very well and the chemistry was much more interesting. The chemistry was very interesting surely, aside from having to do this quantitative analysis. Then I got to physical chemistry — Professor [M. S.] Sherrill. Professor Sherrill worked extraordinarily hard but somehow I couldn’t understand what he was saying. That was rather unfortunate. Here were the things that I was really going to go into but the chemical thermodynamics and so on just didn’t take very well.

Kuhn:

Did he have a book that be used along with this? Or was there a book that you read in physical chemistry?

Mulliken:

There was a book — Noyes and Sherrill — and then Professor Noyes gave me a research problem which had to do with spectra, as a. matter of fact — to understand or to explain somehow the colors of different cobalt salts, complex ions. Well, that’s something that later I was quite interested in, but I disappointed him tremendously. He and my father were very disillusioned and thought I’d never be a scientist, because instead of starting to do anything in the laboratory I just looked up the literature. I got a very complete collection of the literature and they were very unhappy.

Kuhn:

What sort of an explanation of the color of cobalt salts would they have anticipated your giving in this period?

Mulliken:

I don’t know. I don’t remember what it was. Perhaps you might say I didn’t hit it off with Arthur Noyes. He was always a bachelor and perhaps dust didn’t have the right touch. He was an old pal of my father and he tried to get my father to go to Cal Tech when he went there but my father felt the family, his mother and sister and (everybody), so he didn’t go. It would have been interesting. Well, later on, I don’t remember the sequence, I did some research with James Norris. It was just a little thing in organic chemistry and it was very simple but I got quite interested. I was quite interested in the odors of all these different compounds and how these various compounds which were not obviously similar sometimes had similar odors or compounds which were related had somewhat related odors, and so on. This is something that has never been explained as far as I know, but is quite interesting. So that was the basis for this article. [“Reactions between alcohols and aqueous solutions of hydrochloric and hydrobromic acids”, J. Amer. Chem. Soc. 2:42, 1920], which was the second publication under my name. So Professor Norris wrote it up sometime afterward. The work was done in, say, the spring of ‘17 or maybe the fall of ’16 — something like that.

Kuhn:

Now by this time you were quite clear that you were going to be a research chemist.

Mulliken:

Well, I seemed to be headed in that direction. It seemed to be the natural thing to do, and although I guess I had some questions about various other directions I might go in, I seemed to be going in that direction. However also, being in the engineering school, I thought perhaps I should look into engineering. I couldn’t make up my mind I guess. So I did some courses in chemical engineering and attended the chemical engineering practice school in the summer; that was very enjoyable. I had no prejudice against engineering; I mean, I didn’t feel that was to be despised and that pure science was the only thing. I thought that all these things were worthy and this experience was helpful. We visited various chemical plants or places where chemical processes were going on — a pulp mill up in Maine, a caustic soda and chlorine producing place somewhere, and the chemical company in Everett, Massachusetts, where I discovered that hydrochloric acid, which is supposed to be sour, tastes sweet if you breathe enough of it. In this chemical plant the atmosphere was so thick with hydrochloric acid it tasted sweet.

I also learned that if you breathe enough of sulfur dioxide you go through various periods of suffocation and come out finding you can tolerate a pretty heavy concentration — a sulfuric acid plant. You may see an interest in various aspects of odor and perhaps physiology; however, I never did want to go into biology because it seemed to me that biology is quite trivial. It’s just an accident. There’s a little scum on the surface of the earth which is called life and that’s an extraordinarily minor thing in the universe. That was the point of view basically, you see, from which I went into physical science and not into biological science in spite of a certain interest in various things biological and human and so on. I think I have a good sense of smell; I classify myself somewhat with the dogs since I had always this great interest in various kinds of odors. Oh yes, — one tine Bertrand Russell was visiting Chicago and I talked with him about how I felt at the age of 16. At that time I was almost completely inhuman in some ways; my feeling was that everything — life — everything was mechanical or to be explained mechanically. Well, I was ‘scientific’ of course. Well, Russell said he felt that way at the age of eight! But that was a very strong element. Although science was interesting and some aspects of life were interesting, still I didn’t see really much point in it. It was sometime later that I concluded it was more interesting to live than not and gradually I got more normal and more sociable. In fact sometime later I felt it was a great accomplishment that I had become fairly normal but the abnormality came to quite a large extent just from rational thinking about things and applications of a scientific sort of thought to all sorts of things.

Kuhn:

Do you speak of yourself as having been also interested in the humanities? Did you read novels? How about music and the arts?

Mulliken:

I read a reasonable number, not a terrific number, of books, novels and so on, and built up some education. I had an aunt who was a teacher of music; she sang beautifully and played beautifully. She tried to teach me how to play the piano but unfortunately the primitive exercises which were supposed to be the first step were so boring that I just wouldn’t do it, so I never got very far in music in any way. I’m in the state of a primitive savage more or less, but I do have an appreciation. One deficiency might be indicated — a symptom of it might be that I find it very hard to keep time in dancing; there was something lacking there.

Kuhn:

Do you listen to music? Do you enjoy listening to it?

Mulliken:

I enjoy very much listening to good music but I’m very distressed when I go to the airport and bear what they hand out. There are some pieces which — there’s one which might be entitled “Little Boobies at Play on the Beach” and it’s all so light and “twiddle twiddle” I’d rather hear the Marseillaise or something of that sort or some jazz or almost anything but what they do. If I had a little time to spare I’d try to organize an association to combat this. I take it that you —

Kuhn:

I would be glad to join!

Mulliken:

I have a reprint somewhere but I’ve got so many piles of stuff that I don’t know where it is just now. Somebody wrote an article on this subject which is pretty good. If I find it I might send you a copy; perhaps you would start this society.

Kuhn:

I have the same disability of time for starting the society.

Mulliken:

Perhaps you could find somebody else to.

Kuhn:

I will certainly keep eyes open for the right man.

Mulliken:

I once wrote to the presidents of an airline which was putting on this stuff, explaining to him why I went on the other plane which didn’t play such stuff.

Kuhn:

Good. Well look, when you finished at MIT you went on for graduate work to Chicago rather than going on at MIT.

Mulliken:

There seem to be a few places where the information that you have doesn’t seem to fit. It says, ‘why did I break off my studies,’ [Outline, p. 1]. Well, in 1917 there was a war on. That was the reason; any able-bodied citizen was supposed to get in the army. I wasn’t very disposed to get into the army but I could do scientific research so I went into the Chemical Warfare Service. I think that’s mentioned in the [biographical sketch].

Kuhn:

Did you go straight to the Chemical Warfare Service?

Mulliken:

Yes. Well, it amounted to the same thing. It was a civilian position of junior chemical engineer at the old American University in Washington. They had this laboratory — it was Conant, James B. Conant. We had a small room not much bigger than this, somewhat longer and I guess no wider. In the middle of the room there was a row of hoods taking up the fumes, some benches at one end and at the other end, and the door over there. We had about all the poison gases known to man in that little room and with my usual interest in odors and with some indiscretion — one time there was a test tube of liquid hydrogen cyanide so I took a breath of this; then I breathed it out and nothing happened. I did get into trouble with mustard gas, though, and the less said about that the better. That was poor technique. Actually I had some good ideas then about how to make the different varieties of mustard gas better but unfortunately I was still in an inhibited state and so I didn’t say anything about these. Those ideas, I think, would have been better than the experimental work I was doing.

Kuhn:

What were you actually doing?

Mulliken:

We had tear gas, sneeze gas, and all sorts of things. Oh, just some of what you might call cookery with mustard gas. It’s not a very nice thing to play with. This then was as a civilian. Then after awhile they decided all these boys should be put in the army so I got put in the army for about a couple of months. This was 1918, and then the war ended and also a flu epidemic came along and so I spent most of the time in the hospital. I did a little drilling on the field; I have a photograph of that — it’s with an oversized uniform and very comical looking, taken along with Roscoe Gerke. Well, after awhile I got transferred. I asked to be transferred and was transferred to this section under Tolman, but that was only these few months before the war was over. However, we did some things with smoke particles; I don’t recall just what was the reason but it had to do with behavior of smokes on the battlefield. Roscoe Gerke, who has been in rubber research ever since, was my pal, so to speak, at that time. Well, that’s the first publication really with my name on it [“Relat. betw. Intensity of Tyndall Beam & Size of Particles”, J. Am. Chem. Soc. 41, 1919]. Lots of other names on it; it’s not really very significant. Most of these I have copies of if you are — [interested]. Well, that explains that. Then I guess all this time I had the feeling the proper thing to do was to go on and do some graduate work and get a PhD. I got that idea somewhere.

Kuhn:

Did this wartime experience change, in any way that you’re conscious of, your attitude toward science, your desire to do it?

Mulliken:

No, after all the first part of it was organic chemistry along the same lines as what I did for this little bachelor’s, B.S., thesis. Then this very brief interlude where we blew smoke, or Roscoe Gerke blew smoke and we looked at the diffusion of the particles and decided they were particles. Well, then I learned something about Einstein’s equations for diffusion and, it seemed to me, Smoluchowski came in there and that was moderately interesting. Incidentally, here is Harry Smyth [A co-author of the paper “… Intensity of Tyndall beam…” See above]. Then I got a job with the New Jersey Zinc Company. I think the main reason for doing that was to save some money to get a start on graduate work because in those days people didn’t expect to have their expenses paid and nobody else was going to pay expenses. I went there for awhile and that was quite interesting, many varied experiences. First I was put into en analytical laboratory and I didn’t like that so after awhile I said — I mean at first I was just assigned to the analytical laboratory.

After a month or so I did get up initiative enough to say that I wanted to do something more like research, so I got into some research on mixing up rubber with carbon black and this and that. A little experience then in rubber — well, that was industrial research. As I mentioned before, I had nothing against it, I had perfectly good respect and everything for industrial research, for engineering and for anything else. But still, I thought the best thing, the thing I wanted to go into, was basic science research. I had been interested in the electron, you see, but by this time it seemed to me that the nucleus was really more of a problem. That was where the most fundamental things would lie and Harkins was about the only person I’d heard of, in this country at least, who was showing any interest in the nucleus. That is why I came here. I have no memory at all about how I heard about this, that and the other, but somehow, somewhere, in general reading or talking with people, I picked these things up. It must have been readily accessible in the environment of reading or otherwise, but on the other hand it’s certainly true that hardly anybody was thinking about nuclei or interested in that direction.

Kuhn:

In this country.

Mulliken:

Yes.

Kuhn:

Do you have any notion why it is?

Mulliken:

Well, there wasn’t very much anywhere in the world. I don’t remember the chronology, but Aston and Rutherford were at work already, and just a few people.

Kuhn:

Yes. Rutherford about this time was going to Cambridge from Manchester. There was a significant group in Vienna and another in Paris and again Berlin; those would be the places where this sort of thing was going on.

Mulliken:

I have this rather strong residual impression which perhaps is valid —. It’s very hard to know whether anything is really so or not from memory. Perhaps the reason I made the statements I did about hardly anybody’s being interested was simply my impression from looking around in this country or looking at the American journals of the time or something of that sort. Perhaps my feeling was not valid, or not as valid, for Europe; but it is certainly true that until some years later, relatively few scientists were working on the nuclei anywhere — just a very few. Well, then I went to Chicago and I got a scholarship.

Kuhn:

Immediately? You got one in your first year?

Mulliken:

I think so.

Kuhn:

In spite of feeling that you had to make some money before you could go to graduate school.

Mulliken:

Well, perhaps. I don’t know how much I may have saved before and probably the scholarship was not very ample. Somehow I made out. I felt that I was not really doing like other people; I didn’t have any outside job while I was at the university. I felt I was a sort of spoiled academic type, theoretical, and not in contact with the facts of life properly. Oh yes, earlier I never carried newspapers but one time I thought I’d like to earn s money and did get a job at a pottery but I lasted only about one day because I just walked out. I was irresponsible. I hadn’t gotten the idea of what work was like. It was only very gradually that I got that idea. Well, with Harkins — I guess I didn’t tell Harkins at first that the reason I came there was because I was interested in nuclei — so perhaps he said, “What would you like to do?” And he was also very much interested in surface tension so I started doing some surface tension work. I didn’t really think that was the right thing to do but somehow I was in the habit of not doing what I really thought was the right thing to do.

Kuhn:

This was experimental work?

Mulliken:

This was experimental work; little drops coming off a tip and dropping down. Really it was not too uninteresting; I have a feeling that it was rather interesting but it just wasn’t the most important thing. So after doing that for six months or so, I told Harkins I’d like to do something having more to do with nuclei. Well, of course he was trying to do something about separating isotopes which is not getting very close to the nuclei but at least it was along that general line, so I started working on that. What I did for a PhD thesis essentially was just repeating what Bronsted and Hevesy had done with a slight variation I think. I think that Bronsted and Hevesy were rather annoyed that Harkins butted into this; Harkins was always having difficulties with other people. He was always claiming that he wasn’t being recognized. He really was very good, very enterprising — just the mere fact of his recognizing the importance of this work. He built the first cyclotron here and all sorts of things. If he hadn’t had this unfortunate characteristic of getting into these controversies about who did it first and so on, he would have had much better a reputation. Anyway some of the students who worked with him are still around.

Kuhn:

To what extent were you also taking course work as you went on?

Mulliken:

I don’t remember very much, but there were some courses, a moderate number of courses. As I recall, the research started right away. Now in our system people have to pass examinations and that usually takes a year or so before they get to that point. I personally don’t think that’s a very good system. I think that people should be doing research all along, but that is in many cases now possible in practice as research assistant on a government contract and so on. That’s the apprenticeship system which I think is a good system. It’s working gradually toward independence. I did that work which was more or less a repetition of the Bronsted-Hevesy, but then I went into some theoretical discussions. When I got the PhD they were much disappointed at my very sketchy — well, poor—knowledge of physical chemistry. I still hadn’t made up what I hadn’t learned back at MIT.

Kuhn:

How much of the mathematics that physical chemistry was increasingly coming to need had you had at this stage of the game?

Mulliken:

Well, if you can see Noyes and Sherrill or perhaps Lewis — when did Lewis’ book on thermodynamics come out? — whatever mathematics is there in the thermodynamics.

Kuhn:

You had this also but in mathematics courses? Or had you picked it up just in conjunction with learning the thermodynamics?

Mulliken:

Yes, only I hadn’t learned it very well unfortunately.

Kuhn:

But you hadn’t had much mathematics in mathematics course work.

Mulliken:

Oh, did I have s mathematics courses here? Well, probably, but I don’t remember. I must have done quite a number of courses here in physics and chemistry, quite a lot of it was in physics, but about mathematics I’m not so sure. There wouldn’t have been much time to do physics, chemistry, mathematics and research.

Kuhn:

How far had the mathematics gone at MIT as part of the chemistry curriculum?

Mulliken:

Oh, there was calculus. Yes, there was calculus at MIT and I was assigned to a certain section and began to learn calculus very nicely. Then I was assigned to another section where the instructor was just unintelligible so I’m afraid I didn’t learn as well as I should have there. What other than calculus I don’t recall. Of course you have differential and integral calculus. Maybe I’ll find the books that are still here that would give an indication. The physics text I couldn’t learn from; I had difficulty there because about three times on every page there would be a statement that could be interpreted one way or another. There were too many of these bifurcations where I didn’t know which was the meaning — two to the nth power of ambiguities is just too much. Afterwards I concluded that other students probably adapted the more probable meaning and went on all right, but I couldn’t do that, that was my trouble; which of course also has its silver lining in the dark cloud looking at things more carefully and critically in research.

Kuhn:

With whom else did you work besides Harkins?

Mulliken:

We came to this. We can follow this outline: atomic structure [Outline, p. 2]. I don’t remember very much about MIT but I’m sure I must have spent some time in the library reading this and that, maybe the journals. At any rate, at Chicago I took at least two courses with Millikan, probably two, in which he talked about the quantum theory and that is where I first heard about the quantum theory. It seemed to me an awful mess, but he was full of enthusiasm so I got some impressions in spite of my feeling that this just didn’t fit together and so on.

Kuhn:

Do you suppose you still have notes on those lectures of Millikan’s?

Mulliken:

Probably not. I’m afraid not. I’ll take a look afterwards. But certainly he was just full of enthusiasm about all these strange things by Planck and Einstein and Bohr. At the same time I did reading in the library. You asked about Lewis-Langmuir [p. 2, Outline]; I read those papers of Langmuir’s and then back to Lewis with great care and interest. I was tremendously interested. Professor [H. J.] Schlesinger, the professor of inorganic chemistry, did a great deal later in developing boron chemistry. He gave some lectures that were very interesting to me about oxides of nitrogen. He gave a whole course on oxides of nitrogen and all their relations, how they related to making sulfuric acid and this and that; how some were green and some were brown and so on. I was always interested in color. My mother went to art school and was quite competent as a painter so I had tendencies that way. I might mention one thing that I did in high school possibly; I went through a French dictionary collecting all the irregular verbs and made a collection of about a hundred of them with all their forms. A very fascinating thing. But some years later I decided that this was really unworthy and I threw it away. That was something like collecting these hybrid goldenrods. I did collect some of these botanical specimens and mount them up but not very neatly; I was a poor experimentalist. However, working with Harkins I did get started on doing experimental work. I began to get the idea. Then I got the postdoctoral National Research Fellowship and said at first, “I want to continue; I just got started separating isotopes and I want to really develop this.” I’ve got these papers. I was thinking of every possible way of separating isotopes and I wrote these little — it ought to come under the heading of ‘theory’ or at least ‘paper work’.

Kuhn:

These are really analyses of the feasibility and effectiveness of various methods.

Mulliken:

Yes, and some equations. I did one or two crude experiments in which I tried to do things that sounded plausible. A lot of photochemical methods and all sorts of things. But what I actually did is describe in this thing [showing a diagram]: I built up this nice equipment which I was quite proud of. I call this the ‘first isotope factory’. There were six units set up on the table in the basement, in the chemical laboratory which is the next building over that way. I would feed into one unit — this was a sort of fractionation process with successive enrichments — the product one would go into the next and so on. This was just the same kind of system that has been used since in principle. About here there was a boiler to evaporate mercury, and here was a membrane, a cylinder of filter paper which I sealed together with sodium silicate. I don’t know how I happened to come on that but probably somebody suggested it. Then there was water cooling. The vapor from here went up, but since the evaporation was at low pressure there was a partial separation and the lighter isotope went up preferentially. When it got through to this filter paper the lighter isotope went through preferentially, so out here was collected something enriched in lighter isotope and out here something enriched in heavy isotope. Then they go through a succession of stages. I guess that was the most enterprising experimental operation that I did, accompanied by various equations and so on.

Well, just a little more about this. [A. J.] Dempster was here and was doing mass spectroscopy and of course he was a leader in that. One time I went over to visit Dempster because I wanted to learn about all sorts of things. He was separating isotopes in different ways in his mass spectrograph; well, he thought I was a Harkins spy! The feeling was very strong. He thought that Harkins was going to try to learn his secrets and set up a competing enterprise. So I had a lot to do with the physics department, their courses and interests. You inquire [p. 3, Outline] about the ‘spirit’ of physics. At MIT it wasn’t very good, just sort of a sideline. The chemical engineering I mentioned. About Harkins you had a question: “Harkins, unacknowledged anticipations of important developments.” [p. 2, Outline]. He thought that he was discoverer of the neutron in a way, but that’s the quantum theory. He saw the neutron in the nuclei because he looked at the abundance of the elements and noticed the odd and even alternation. I’m sorry, I haven’t looked this up. I also would have to see whether Rutherford had got some actual neutrons before this, but either at about the same time or earlier; at least before Rutherford came out, I think that Harkins saw the neutron in the nuclei.

Kuhn:

You mean as a matter of the theoretical demand for such a particle?

Mulliken:

Yea, the systematics of the structure, the systematics of the building up of the elements of the nuclei just shoved that.

Kuhn:

I’ve also heard a story about Harkins which is secondhand and may or may not be right to the effect that Harkins — this would be before you knew him — sent a paper to the Physical Review on, I think, the Quantization of the Rotator. This would be a little earlier than Bjerrum and still before Bohr.

Mulliken:

I see. I never heard about that.

Kuhn:

— And that it was refused, as it might very well have been.

Mulliken:

I never heard about that. He was very free to talk about all such things. He was a great friend of A. C. Lunn and he often said Lunn had sent a paper to the Physical Review which was turned down and which anticipated the quantum mechanics. But Lunn was of such a temperament that, well, he was just sour about it and never did anything more.

Kuhn:

Anticipated quantum mechanics — this is which formulation?

Mulliken:

Maybe wave mechanics. I don’t know. I never saw the paper and I don’t really know to what extent this is true, whether it was somewhat similar or whether it went quite a long way, but Harkins was very emphatic that Lunn had been very badly treated and that his article had been turned down.

Kuhn:

It may be a story of this sort via Harkins that is responsible for the report I had had.

Mulliken:

I think so. It sounds a little bit like it. It certainly would be interesting to check on that manuscript but I’m afraid —. Lunn has died, his son has died; his son went into science but I don’t know what happened. I think he committed suicide. An unhappy family, I guess, and I should say an unnecessarily unhappy family; what’s the use of being unhappy? Then I’m not sure what happened to Mrs. Lunn but very likely she’s died. There might conceivably be a possibility of finding this manuscript and checking on it; quite possibly it would turn out not to be of great interest, but on the other hand I just don’t know.

Kuhn:

Well, it may well be that there are papers still in existence that the Holton-King project [on the History of Recent Physics in the U.S.] has already located and I will be in touch with them to see what they’ve been able to find out.

Mulliken:

Yes, I would think very likely they would have heard about this. Here’s where [S. K.] Allison also might have beard about it because Allison worked with Harkins. Who else — there were others. Well, Robert Moon worked with Harkins on the cyclotron but that is somewhat later and probably not of so much interest to you.

Kuhn:

Harkins was appreciably more — or was he — mathematically inclined than most people doing this sort of work around here at the time.

Mulliken:

I don’t recall anything particularly mathematical. He was more physical, say, and he certainly went into these experimental projects, building things. [break for lunch]

Kuhn:

Well, we pick up more or less where we left off. We had talked about your period in Chicago; before we take you to Harvard I’d like to ask you about the period ‘22-‘23, this very special period in the development of physics with respect to quantum mechanics in particular. How much sense of that had you picked up? How aware were you of quantum mechanics as a whole and of the sorts of problems it was running into?

Mulliken:

I was not so concerned or aware of the most fundamental problems. I was interested in the molecules and the atoms and the spectra; the theory was something to help understand them.

Kuhn:

Now you hadn’t done much even with spectra at this point, had you?

Mulliken:

Oh, at this point I hadn’t done anything at all with spectra. There was just this little item that I was very fond of colors, which was one attraction in that direction, my mother having been an artist and so on — entirely irrelevant to the science.

Kuhn:

How much do you suppose you knew about the Bohr atom?

Mulliken:

As soon as Bohr’s paper came out — I’ve got a note here — I was wildly enthusiastic.

Kuhn:

Which paper?

Mulliken:

The Bohr theory of the building up of the elements. When did the hydrogen atom come out?

Kuhn:

The hydrogen atom is 1913.

Mulliken:

1913. Well, then I heard about that from Millikan in the lectures here and of course then I thought, “Well that’s nice, but then there are a lot of these things that don’t seem to make sense: why should the electrons jump; why should they go around and then jump?” It seemed very arbitrary.

Kuhn:

Did you worry about why they didn’t radiate while they were going around?

Mulliken:

Well, I said, “I don’t understand it and there must be something wrong,” I suppose. I had a feeling this couldn’t be the real answer but I didn’t try particularly to do anything about it except that I felt rather uncomfortable. It was only, let’s say largely, Millikan’s enthusiasm that made me willing to contemplate these awful things. But of course the hydrogen atom was pretty good, but when the Aufbau came along that was really exciting and that connected with chemistry too. Then, considering the molecules, all these primitive ideas about molecules were simply trying to develop the ‘Aufbauprinzip’ on empirical grounds for the molecules. At first there wasn’t any very good theoretical basis, so I tried to do what could be done in a very empirical way. Then the quantum mechanics came along and Hund showed how to use it for the diatomic molecules. A series of papers here show that when I first saw Hund’s papers then I was of course immediately converted to doing it the right way; before that I’d been using the sort of makeshifts based on the Bohr theory or whatever you want to call it. Here I can perhaps look at this a little bit.

Kuhn:

Well, I think we should look at it. I only point out that jumping already to the new quantum theory we’re getting too far ahead because still, in terms of this progression, I really would like to see how you come from the interest in isotopes. I’ll admit that the two tie together.

Mulliken:

I was very much interested in isotopes so I had this National Research Fellowship. Now the fact is, the National Research Fellowship board — I seem to recall that A. A. Noyes was on it — said, “Now it’s not good for you to keep staying at Chicago where you’ve gotten your PhD and keep on doing this.” I thought this was really very exciting and wanted to keep on doing it, but they said that wasn’t a good idea and I should propose something else. “Please propose something else.”

Kuhn:

Now you’d already held a fellowship here for two years.

Mulliken:

I don’t know — it might be one and a half or so. At any rate, at the time of the discussion it probably wasn’t much more than a year. I’m sorry, I don’t know. Well, so they said, “Please propose something else.” I looked around and I talked this over with Norman Hilberry — you remember he has an article there and later he was Compton’s right-hand man in the Manhattan project and then later he was director of the Argonne Laboratory. I used to talk to him a great deal and we were very interested. He never did very much in a scientific way but he was just full of ideas; that is, in research he never did much but he was full of ideas. Well, I guess I went to the library and looked around and I sent in a proposal to go and work with Rutherford on beta-ray spectra.

I thought beta-ray spectra was a subject which had a lot of promise. After all, that has to do with nuclear structure too, doesn’t it? But the fellowship board said, “Well, you don’t know enough physics. You haven’t had enough experience in experimental physics and what not.” And they said, “Try again.” I thought that was rather bad, but still I wanted to get some more support for doing some more research. I think Hilberry had a lot to do with suggesting that band spectra would be a good subject. Well, I don’t mind taking suggestions and it seemed like a good suggestion so I looked around. I really don’t know, maybe he would recollect but perhaps he wouldn’t, but certainly I talked to him about this whole business. [F. W.] Loomis, I think, had published something about an isotope effect in MCI in the infra-red, a chlorine isotope effect. Then the thought was, why shouldn’t there be an isotope effect in an electronic spectrum? I don’t know just how this happened but I came upon this thing of Jevons about what he called the boron nitrate bands. It was very easy to say what to expect so I probably said in the proposal to the National Research Council what sort of thing would be expected. At any rate I looked at Jevons’ picture and there should be 20% of the B¹⁰ isotope, I guess it was, and it should be quite clearly visible, and sure enough, there it was, all over the place. So I proposed to work on isotope effect in band spectra and then they said all right.

Kuhn:

And you simultaneously proposed to go to Harvard to do this?

Mulliken:

Probably. Well, the question would naturally come up where to go. Of course Birge was interested at that time. Perhaps I hadn’t heard much about him at that time, I don’t know, but I know Saunders was at Harvard and was doing atomic spectra and they had good facilities for spectroscopy, so that was arranged. Oh, here is a question [p. 2, outline] whether the new field required some studying up. I had to learn about spectroscopy and spectroscopic instruments. I really needed to learn a great deal of physics which I was not very well up on, but I didn’t go about that too much. I just went ahead and did some research. It was just very interesting. Professor Saunders had a number of spectra; he brought me some spectra and as soon as I got started he presented me these puzzles in addition to various things that I wanted to do anyway with cases where isotopes appeared. Of course Kemble was there to advise and encourage on the theoretical side, so there was experimental and theoretical advice and the equipment. There was a good grating spectrograph there and everybody was very nice so I just got to work on all sorts of spectra. It so happened that this boron nitride Jevons had gotten using active nitrogen which is a very interesting thing or substance or whatever you call it.

He had gotten it by putting boron chloride into active nitrogen but there were many other spectra coming out of active nitrogen which, among others, if there’s a little air in the nitrogen or a little oxygen, there are these very nice nitric oxide bands. At that time I don’t know whether anybody was sure they were nitric oxide but there were all sorts of spectra which just turned up very readily and so the job was to photograph them, measure them, analyze them, and explain them. Well, the explanation of the spectra involved the matters of rotation and vibration, but also it raised questions about the electronic states and so gradually my emphasis shifted onto understanding the electronic states. But the structure of the spectra showed various things about the properties of the electronic states, the angular momenta, the electronic angular momenta and so on. However, at that time the theory was not available and so I tried to get some explanation which was better than what there was before. But at the same time then Hund came along with the Hund quantum mechanical vector model, shall we say, with the different cases of band structure. Now why did he do that? I had got a lot of things sorted out on the basis of the old quantum theory, so that provided a lot of material which perhaps made it easier for him to translate it into more modern terms because of what I had done before, in 1924 and ‘25 and maybe into ‘26. Well, the first word that I knew of about the Heisenberg theory came in 1926 in some lecture at MIT.

Kuhn:

Born lectured at MIT in the fall of ‘25 and spring of ‘26.

Mulliken:

I seem to remember early in 1926 being at least the first time that I heard of it.

Kuhn:

Were you still in Cambridge in 1926?

Mulliken:

Yes, I stayed there another year. Kemble got me some kind of a stipend and I stayed there some more. Then I thought, “Well now I should learn some more physics,” so I spent some time trying to learn more physics and at the same time doing more research on the spectra.

Kuhn:

So you were three years at Cambridge.

Mulliken:

I guess so. It must have been the fall of ‘26 that I went to New York University. But in the period ‘24 to ‘25 or ‘26 apparently I just took over this subject and organized everything that was available, got things much more greatly clarified as compared with what they had been before, but at the same time in ‘25 I visited all these people who had done anything on the subject. I visited — the comet tail bands [F.] Baldet at the observatory at Meudon in Paris and I visited Kayser and Konen. Kayser of course worked on atomic spectra so I wanted to see him, and Konen had written a very interesting book called Das Leuchten der Gase. That was a very interesting source book, very stimulating to see. Well, I visited him in I guess ‘25.

Kuhn:

That book had still been useful to you, had it? It actually came out just before the Bohr atom, so that it was over a decade old.

Mulliken:

Oh, yes. Well, there was empirical material, information about spectra, interesting spectra of molecules all of which called out for explanation.

Kuhn:

Yes. I’d forgotten that it had molecular as well as atomic spectra.

Mulliken:

Oh certainly, oh yes. I had lunch, or maybe it was dinner, with Konen one time when the French were still occupying the Rhineland and while we were having dinner a Moroccan walked through the room, a Moroccan orderly of a French officer. Professor Konen apologized for having this interruption but he said he couldn’t help it: the French officer was quartered in his house! And I visited Paschen in Berlin. One or two of these visits may have been 1927 but I think nearly all of them —. I visited Paschen and I saw the principal people in atomic spectroscopy, Fowler in London at Imperial College and R. C. Johnson I guess was one of his students who started work on band spectra. I visited [Th. R.] Merton in Oxford. Do you know Merton?

Kuhn:

No, I guess I don’t know Merton.

Mulliken:

Well, he was a well known spectroscopist, especially for atomic spectra perhaps. He was also a country gentlemen; I saw him many years later in the Atheneum, sitting there enjoying himself. He was a combination spectroscopy professor and country gentleman. I visited Hulthen in Lund and Hulthen gave me a grand welcome with all kinds of drinks. They didn’t have as high taxes then I guess as they do now. He had the band play the ‘Star Spangled Banner’ and it was a grand visit. But that was right across the channel from Copenhagen and of course I visited Copenhagen. I went up to see Bohr. I must have written these people ahead of time but I don’t seem to recall that. I don’t know how I would have arranged all these visits without having written them, so I probably wrote. It would be interesting to see those letters. I found that Bohr wouldn’t be in Copenhagen but he would be in the country, so he said to come up and see him in the country and he came to the station to meet me on his bicycle. We went out to his house and I met his wife and five sons on the sandy seashore and I talked to him about Rydberg series, I think. This was a subject which Bohr had been considerably concerned with and I was interested in although I was interested in the molecules, still the atoms and molecules were related. I’m just now trying to get a paper on Rydberg series of molecules finished. But there was one [shows paper to Kuhn].

Kuhn:

“Electronic states of the helium molecule,” Proc. Nat. Acad. Sci. U.S.A. 12, 1926. March 1926.

Mulliken:

And it was sent in February. I would say that I didn’t quite believe this, I guess, but being easily persuaded I adopted the symbols which Birge proposed and which then Hund later just translated into Greek because there was a strong analogy, though somewhat superficial, of the S, P states of the atoms to the Σ, π states of the molecules. But this was before they knew that the doublets were really triplets, I think.

Kuhn:

I’ll confess great ignorance. Under what experimental circumstances can you get helium bands?

Mulliken:

Oh, they’re quite easy. Some condensers in the circuit gives out a periodic discharge through a spark. That excites the atoms and the excited atoms get together with the normal atoms and form molecules; they’re quite important. There is a great deal of study now of excitation processes and so on. Now let’s see, we should translate these letters into Greek letters — [reading] — “certain restrictions in their combinations” — well, this is following more or less an analogy to what was known of atomic spectra — “ j=0 for the S state.” “I adopted Mecke’s valuable suggestion …” Oh, yes, I visited Mecke in Germany. “Lines are missing. This gets rid of the quarter integer quantum numbers used by (Krutch) and Curtis leaving half integers,” which somehow is more respectable, and so forth. Well, so this represents something right in that transition period. People didn’t know or were just beginning to hear. This might have been written just before I heard that there were some new developments but there were all these things that could be said without —

Kuhn:

In the courses of the analyses that you were doing, clearly this is a period, as the books on your shelves indicate, when you were reading Sommerfeld carefully, wrestling with Born’s Atommechanik. How much was the old theoretical quantum mechanics that you were learning in that way playing a role in the analyses here of band spectra and of the interaction with electronic states?

Mulliken:

Well, in so far as it was relevant it was being used; but the molecule is not an atom and we did not know exactly what to do with it, what kind of states it would have. But then the idea was to follow through as well as possible to make it analogous. But then at the same time here was a series which had been in the Proc. of the National Academy. I would think that, although now we wouldn’t reproduce these or do anything much with them because they don’t have much of any value other than historical, they might be useful in showing what was going on. Here — [reading] — “according to the correspondence principle —.” You asked something about the correspondence principle. “Changes in the state of rotation or precession according to the theory developed by Lena, Heurlinger and Kratzer.” … Oh, yes. Fourier component parallel to the vector j. Anyway, it’s correspondence or atom mechanics. Kratzer and Kramers and Pauli obtained a formula which was like this [showing Kuhn] and I tried to use this more or less. “And sigma is the component of electronic angular momentum.” Oh yes. And here’s an epsilon. That wasn’t the right formula, but at any rate these quantities — what I did after that was sort of develop some systematics of interpreting these and finding out what quantities of that sort might be, what their values might be and what they might mean. Here were various examples of bands. Oh, here’s the Langmuir — his suggestion of the idea. Let’s see — “adaptability to band spectra of the alternation and displacement laws of line spectra of which use has been made was first pointed out by Mecke, also by Birge in a private communication.” “Mecke also pointed out certain analogies,” and so on. Then there is Birge and Sponer; Birge and Sponer came along and it’s hard to say just what the relations were. Then Hulthen had a lot of work on the zinc-cadmium-mercury hydride bands which, however, got some reinterpretation, some modified interpretation here. Nitrogen, carbon monoxide and NO — I guess the BO and CN and so on being analogous to sodium maybe came first and then this made NO analogous to aluminum and so on as suggested by Birge and Sponer. Then — here — postulates. Here is a set of sort of semi-empirical postulates to try to understand these things: that the j has integral values for odd molecules and half integral values for even molecules. It seems like it’s just the other way around. [Searching through papers]. This is getting a little further into more general thoughts, which may or may not be sensible, on the relation of molecular and atomic states. “Model for hydrogen originally proposed by Lenz, more recently discussed by Nordheim” — various models coming out of the Bohr theory.

Kuhn:

You suggested that you think you first heard of the revision of the theory by Heisenberg from Born’s lecture.

Mulliken:

Well, if I had to, without any special prompting, guess, I would say April 1926, but that may be wrong.

Kuhn:

I think it was a little earlier.

Mulliken:

It could have been earlier or it might have been that I went to the third lecture in the series, or having developed various thoughts in terms of the other theory; I was rather preoccupied about getting it all written out and not having to stop and learn all about something new, at least not in a great rush. I guess perhaps that was the psychology.

Kuhn:

Did the matrix mechanics papers create much excitement around you?

Mulliken:

Oh, of course. Lots of people were saying, “Well, here’s the real thing at last.”

Kuhn:

Do you remember who did say that?

Mulliken:

No, I don’t. Well, the people who ought to know that’s my general impression. Among the people who ought to know were Kemble and maybe Van Vleck — I just don’t remember the chronology.

Kuhn:

Van Vleck I think by that time had left Harvard. Yes, he did catch on fairly quickly to this. Kemble, although I think he was interested, makes no use of this sort or of the Schrodinger equation for a while.

Mulliken:

Is that so? Well, I certainly heard and got a very strong impression that certainly “here is the thing”. I felt I still can almost feel it rather unhappy that I couldn’t take this up right away the way some other people were eagerly sopping it up, but they were better prepared, partly the physics and partly the mathematics.

Kuhn:

How did your sense of what came when you heard of matrix mechanics compare with the somewhat but not so very much later developments with the Schrodinger equation?

Mulliken:

Well, I’m sure it seemed much more strange so that I guess the Schrodinger equation was somewhat of a relief that it wasn’t quite so bad. The summer of 1925 was before these things.

Kuhn:

Yes, the summer of 1925 was just before they broke. That was when you were in Europe.

Mulliken:

Yes. That was when I made that extensive visit and saw almost everybody who had any possible connection with band spectra and, to a large extent, with atomic spectra.

Kuhn:

What did you feel you had gotten from that trip? Did you get any sense of large scale differences between physics in America and physics in Europe? Did get a lot of data? Did you get new experimental ideas?

Mulliken:

Well, it was after all my first visit to Europe and so the main thing was getting acquainted with all these various people whose backgrounds other than their science were quite varied. I should have mentioned [W.] Jevons in that connection. I wrote some papers saying that these bands that Jevons said were boron nitride were not, they were boron oxide and my proof was quite clear as to why it happened. It was queer; when you put boron chloride into nitrogen you might expect to get boron nitride but it just wasn’t. And furthermore, there was a little bit of oxygen in there. We bad nitric oxide bands that you never got rid of and later we analyzed some of them Harry Barton, Jenkins and I you might have seen that. Two papers on nitric oxide bands. Well, so I wrote a letter to Nature about it and then Jevons wrote a letter to Nature saying it wasn’t so, that of course it was boron nitride. Then I was going to write another one saying, well it really was, but I was in very frequent communication with Birge, many long letters, so Birge said, “Why don’t you write to Jevons instead of writing a letter to Nature?” So I wrote a letter to Jevons saying “I’m coming over next summer and how about coming to see you?” He said all right and I went to see him. Then he said it wasn’t his idea to write the letter to Nature, it was his boss, the professor at the college where he was, namely E. N. da C. Andrade who has written various books but many people have found him hard to get along with.

Kuhn:

Was Jevons in fact convinced that you were right?

Mulliken:

Well, yes — at least as soon as I talked to him.

Kuhn:

Did you try to persuade Andrade also?

Mulliken:

Oh, no. I didn’t see Andrade at that time. I wasn’t especially interested in persuading Andrade.

Kuhn:

Was there a third letter to Nature admitting that it was boron oxide?

Mulliken:

Not that I know of, no, I don’t think so. This must be it [shows paper].

Kuhn:

Yes, that’s the first paper I think.

Mulliken:

Oh, Professor Lyman of course was there and although I didn’t see so much of him that was part of the atmosphere. It was a big spectroscopic atmosphere. Then Baxter in chemistry, so I saw the people in chemistry too. But now you surprise me by saying [p.3, outline] that I said that the SiN does not show the half quantum numbers. I’d forgotten all about anything of that sort. That is in here, is it, somewhere?

Kuhn:

I don’t think it’s in this one. I think it’s in a second paper.

Mulliken:

I meant to look that up before you came but I haven’t.

Kuhn:

That’s a paper in the Physical Review.

Mulliken:

Yes, well I have that here, but does the abstract say anything of that sort? Oh, yes. Well, it just says n and n is presumably an integer.

Kuhn:

“The agreement is also definite but not yet conclusive evidence against the existence of half quantum numbers for SiN”. [Science Abstracts 1926, No. 92] “The exact opposite to that for BO.”

Mulliken:

Yes, well, it’s saying it was not conclusive and — ‘well, that’s just the way the thing looked.’

Kuhn:

Did you worry about the fact that one would suppose if there were a theoretical reason for getting half quantum numbers in one case one ought presumably to get them in the other?

Mulliken:

Well, it would seem to be that one should, but perhaps one reason for not worrying would be that there were just 80 many different things happening, interesting things, that I didn’t dwell on it. Is there a discussion here?

Kuhn:

I’m not perfectly sure.

Mulliken:

There are many other interesting features — oh, yes — “agreement of the observed is calculated” —.

Kuhn:

It really doesn’t extend much, with respect to this problem, over what’s said in the abstract.

Mulliken:

Well, say it’s about a 70 percent statement then. It’s not a 99% or a 98%... Well, that’s about as far as it goes. As for the general theory, there were so many things shifting around there the rotational structure, the sometimes half quantum numbers and sometimes whole quantum numbers — well, perhaps a more rigorously theoretical point of view might have worried more about it. Here’s a band which Saunders — “The band discussed in this paper was called to the writer’s attention by Professor Saunders who had noticed it.” He gave me this picture and said, “Can you explain this?” I looked at it and said, “Well goodness, this is very interesting. What happens? Here these go up and then they suddenly stop and the same happens on both ends.” Then there was another picture. Professor Henry Crew lent me a picture, a plate, so in this case I perhaps didn’t make one myself. The explanation here is not the final accurate explanation. Well, that’s an example showing there were so many varieties of interesting things, but they did rather naturally lead to the —.

Kuhn:

That one was the Physical Review of April 1925.

Mulliken:

Yes, here was this [looking at papers]. And then there was this from Nature — you know about that — and then Number 4. Here is this series having to do with the electronic structures so that I was trying to explain them while making these postulates. That was where these postulates started that were given in here. [Four papers in Phys. Rev., 1926-27].

Kuhn:

This series is really a fuller version of the series in the Proc. of the National Academy.

Mulliken:

That’s right.

Kuhn:

Yes, and there is the place where the Hund theory suddenly comes in.

Mulliken:

Number 4 and the Hund theory, yes. So here I made a lot of progress.

Kuhn:

Do I gather then from what you say, that really the Hund paper or the beginning of the Hund series of papers really was something of an eye opener to you at this point?

Mulliken:

All right, yes. Let’s see, when was this sent in? This is Number 3, [of the sequence “Electronic States and Band Spectrum Structure in Diatomic Molecules” in Phys. Rev.] November 13, 1926, from Washington Square College ‘appreciation to Kemble’ — so it was probably written sometime in 1926. It seems to me that I had one long paper and then Kemble said to me that I ought to divide it up into several, so I divided it up into several. It was probably mostly written early in 1926. If there were then some feelings or misgivings that perhaps this ought to be done differently and that we now knew how to do it, there was also the feeling “well, here it is.” Of course there was some kind of an awareness that the new quantum mechanics might improve things but we had the feeling, “well, here it is; this is probably worthwhile and good as far as it goes.” It certainly seemed to be progress. I think I have reprints of those which are available, but Number 4 I don’t think I have. I have one but not an extra one. But let’s look at it if you would like; while we’re on the subject it would be bearable. [Reading]: “Interpretation of certain types of bands” — and here — “an interpretation of certain other types of bands” — and here — “intensity relations in the bands.” Here: “As observed and calculated there were” — there’s some theoretical basis — well, somehow Honl and London had some formulas. Now how did they get them?

Kuhn:

I’m not sure how those were done but there were plenty of bases for doing intensity calculations in old quantum theory.

Mulliken:

1925; well, that’s probably the older. So there were all sorts of data to be digested in terms of what was available. The CH [No. 6 in the series]; this is really electronic levels. It must be later that there’s a detailed analysis of CH bands which is quite interesting. Oh yes — that was using the — I guess maybe Van Vleck came in there. But here [No.] 4 introducing Hund’s theory but also introducing some further kinds of bands which needed discussion. Now the beginning of this should be instructive. The language could be — … Looking at this it isn’t very obvious that it’s quantum mechanics, is it?

Kuhn:

No. I think in practice that in Hund’s paper he is now using wave functions though it’s been long enough since I’ve looked at the paper. There is really no place here, except perhaps for the background effects in Hund’s work, where it becomes very clear that one has made a transition to new as against old quantum theory.

Mulliken:

So the general thinking is in terms of the vector model, and it’s, as you say, not so different. “Phenomenon of alternating intensities” [in No. 4] Now, what about that? “Hund’s theory gives no explanation for the phenomenon of alternating intensities in band lines; also it is not easy to account for the precise nature of the selection rules in lambda-type doubling.” Oh, yes. Well, that required some further application. Kronig and Van Vleck came in. After all, not everything could be done at once and there were a number of features, but as for these alternating intensities, here is reference to Heisenberg, Spring of 1927, in terms of the wave mechanics, you see. Wave mechanics — it doesn’t say “matrix mechanics” — I don’t know what that proves. Heisenberg was pretty strong about matrix mechanics.

Kuhn:

Yes, and he still was in the spring of ‘27 too.

Mulliken:

Oh yes. “In terms of the Hund series it is evident that this notation doesn’t mean the same as in the atomic,” and so on. Then I would say that this transition is rather special, but I don’t seem to have enough —. Now what do we have? Oh, the CH bands — here’s a detailed interpretation of the structure of certain CH bands [No. 6 in the Phys Rev. series]. “— using the correspondence principle —” Here are some intensities — where did that come from? I don’t know unless it was Honl and London in 1925.

Kuhn:

Well, it may have been because they certainly were concerned with intensity formulas and they got a number of formulas that stood up.

Mulliken:

Here’s ‘Empirical Relations’ [No. 8 in the series] — oh, this is 1929, isn’t it? A Kronig reference and also Van Vleck — “all these conclusions in agreement with Van Vleck” — well then there’s another article as things gradually came in. Now where do we go? Now of course Van Vleck, Kemble and Slater were there and Oppenheimer was there at various times. Slater went away to Copenhagen some time during that time, but there was a time when he and I were rooming in the same place, not in the same room but next door, outside where we could see the trolley wires. Perhaps you’ve heard of that. There was a copper arc when the trolley left the wire. Green flash.

Kuhn:

Did you and he talk much at that point about the problems?

Mulliken:

Oh, yes, we talked about them in so far as we were capable of communicating with each other. We must have talked a great deal about the scientific problems. Of course he had done a thesis on the solid state. Well, and he knew his physics better than I did I suppose; he must have, and maybe I knew more chemistry. But he had done his thesis on something about solid state, I’ve forgotten what. Then he went into the theoretical side. Somewhere along in there Oppenheimer appeared above the horizon as a graduate student and got his PhD I guess and went abroad. Then we saw the Born and Oppenheimer paper and various other things.

Kuhn:

Was the Born-Oppenheimer paper itself an important paper for you?

Mulliken:

Yes. Well, it gave some justification but let us say it was clear empirically that things were a certain way, so it was nice to have some backing, but it wasn’t so essential for us.

Kuhn:

It didn’t change anything in the way one thought about these things. Among these various developments that come right in ‘25 and ‘26, one of the ones that clearly would relate most directly to the problems you were thinking about is electron spin.

Mulliken:

Oh, yes. Well, that must have emerged somewhere. This helium molecule, the singlets and doublets. We didn’t have electron spin — well now here we have electron spin, don’t we, in 1927 so it must have come in somewhere.

Kuhn:

The suggestion was made in 1925, almost at the same time as the matrix mechanics. But you know, there are people to whom that seems an absurd idea. In Europe, at least, there was much back and forth discussion about it. I wonder if you remember discussions of it in this country.

Mulliken:

Wentzel can tell you about that early history and how Kronig really had the idea first but Pauli sat on him and so it never —

Kuhn:

You and Wentzel were at very different places at the time this notion came out and I’m interested also in its reception at different places.

Mulliken:

I can only imagine what it must have been. I can imagine that it must have been quite comforting to find a simple explanation of these very funny things which — who was it used to talk about ‘Rumpf’ and so on? It wasn’t Sommerfeld, was it?

Kuhn:

Well, a lot of people. Sommerfeld uses it, and Heisenberg perhaps.

Mulliken:

Somebody talked about these strange quantum numbers that didn’t seem to make any sense. I certainly remember that feeling that here were these things that didn’t make much sense and then here something came along and it was solved. You might not understand why there should be a spin but at least now you could put these things together in a vector model and everything fit. But the impact apparently hadn’t arrived in time to — but that would have to be not only the spin perhaps but the use of the spin in explaining the helium molecule. The helium molecule’s levels were very analogous to those of the atom, so we had to wait for Heisenberg with the exchange and so on, symmetry properties, and putting the spin into that. Perhaps it was a little bit later before the helium problem was clarified. It seems to me that de Broglie’s thesis was, as far as I know, not known for scene time. Awareness of that was somewhat delayed, I would say, but again I’m possibly imagining things in trying to reconstruct a very faint impression.

Kuhn:

It certainly was true in a number of places that it was known only later. Most people discovered it only after they had discovered the Schrödinger equation.

Mulliken:

Yes, ‘27 or ‘28.

Kuhn:

Well, ‘26. You see, Schrodinger refers to de Broglie.

Mulliken:

Oh, so he does, yes! Oh, yes, yes, yes. No, I don’t think that anybody was aware of de Broglie before Schrodinger’s work came out, no. [scanning outline] Duane-Compton [p.3, outline] — well, as I say, if you see Allison he can tell you about that. The “Zweideutigkeit” of the electron [p.3 outline] — well, that has to do with this electron spin problem. Oh, I probably talked to Slater about various things. He’s probably the one I talked most with although I can’t seem to visualize any particular conversation or particular time. If Van Vleck was around I talked with him. I’m sure I didn’t talk much with Oppenheimer and only perhaps saw him a few times. Foote and Mohler have mentioned. How I happened to choose [the band spectra ascribed to BN] [p. 3, outline] — well, we’ve already gone into that. And about the half integral quantum numbers — that, let’s say, would be an approximation to what fitted the empirical date, but I think Kemble had idea that that might be significant and encouraged it so really he is somewhat responsible for my (getting) that out. Another person who was there at Harvard for some part of the time was Turner, [L.A.] Turner, and [H.A.] Barton I’ve already mentioned. Oh, yes, after I went there as a National Research Fellow, then F.A. Jenkins came and Allison came, I’ve forgotten which one came first but they had both been working with [W.D.] Harkins, hadn’t they? I think so.

Kuhn:

I’m not sure of that.

Mulliken:

Yes. So they followed my example and then Jenkins worked with me quite a bit on some of these spectra. The Lewis-Langmuir theory [p. 4, outline]; I showed you something, although I don’t remember where it was, to indicate that that certainly had a lot to do with the picture of the nitrogen molecule and so on. Well, here you have some questions. I don’t know if you really want to make anything serious out of them.

Kuhn:

Well, I would say that this is intended at least to point you into an area that I do very much hope you will take off and ride with, and this is the question of the development of molecular spectroscopy in terms of molecular orbitals as against its development in terms of interacting atoms.

Mulliken:

Oh, yes. I have made various remarks but maybe it would be good to draw them together, to summarize them. Then, first of all, Bohr’s ‘Aufbauprinzip’ for atoms made a very great impression and so I thought something similar for molecules would be nice. If you translate orbits into orbitals for atoms, then for molecules it’s molecular orbitals; it’s something that goes around all the atoms or however many atoms there are and the ‘Aufbauprinzip’ transferred to molecules simply means molecular orbitals.

Kuhn:

Now I Bohr would deny that flatly.

Mulliken:

Really? Well that’s what it meant to me anyway and Hund evidently agreed or felt the same way. Hund took this up.

Kuhn:

I don’t mean to say that I don’t think one can try Bohr theory in terms of molecular orbitals, but if one’s approach is via ‘Aufbau’, then Bohr himself talks about, in the early papers, building molecules out of atoms. You build up atoms out of a nucleus and adding electrons, but you build up molecules by bringing together two atoms or two ions and in that sense one would almost say that the approach via an ‘Aufbau’ in which you put together atoms to make molecules is more likely to lead to the Heitler-London approach.

Mulliken:

I see. Well then it could be. In any event as a matter of fact, it was a few years, two or three years, before maybe. Was it 1923 that Bohr’s ‘Aufbau’ came out?

Kuhn:

‘22 and there was a note in Nature late in ‘21.

Mulliken:

I have a sort of a feeling that it was after getting to Harvard that I was really looking at that intensively although it may not be so. I’m pretty sure that as soon as I heard about it I thought it was very exciting, but then perhaps there was an interval and I got the ‘Aufbau’ idea. Then when it came to this it seemed very natural to try it out, though I think there were some reasons probably why it appeared appropriate. As a matter of fact, this spectroscopy — molecular orbitals are spectroscopic orbitals in contrast to chemical orbitals which would be more in the ordinary way of thinking in terms of atoms and building molecules out of atoms. You would use localized orbitals which would very likely be atomic orbitals. But in spectroscopy you’re almost automatically forced to use orbitals of the whole system, which in special cases may be largely localized so I think there’s probably a good reason why it was clearly reasonable. One example would be the electronic states of the helium molecule. Here [showing a paper] there were these Rydberg series, highly analogous to the atomic series, so at least for the higher excited states here was something going around the whole molecule but it was almost practically the same as atomic in that case. Then it’s perhaps rather natural to go down to the smaller ones and have them go around the whole molecule, but as a matter of fact there is some paper in which somebody — I’m not sure it wasn’t Bohr tried to have an orbit that went around both nuclei. It was some well known person and there were all kinds of troubles with it. You couldn’t make it behave right in Bohr’s theory. There was certainly that effort. You don’t recollect what that paper might have been?

Kuhn:

I wonder whether you’re thinking of the Pauli paper on the hydrogen molecule ion.

Mulliken:

It may be Pauli.

Kuhn:

That was an early one. Bohr in fact also had a hydrogen molecule theory.

Mulliken:

‘23 or so. Well all right, I’m thinking of such papers, yes. There they were trying to have an orbit going around the whole, but of course it was only two nuclei, so that was the molecular orbital idea. But here we have these electronic states so analogous to atomic states, and if you explain the atomic states in terms of orbitals of the atoms, well then it’s just quite natural. I think you could say that the approach via spectroscopy tends to lead quite naturally in that direction.

Kuhn:

You remember that when you pointed to the spot at which you had referred to the Lewis-Langmuir model, this is a sentence in which you say that the idea of using the octet as the arrangement —

Mulliken:

Oh, well that’s another case that Langmuir talked about, the nitrogen molecule and the carbon monoxide molecule as having a group of eight electrons jointly around the two nuclei and this again pointing rather in the same direction, having two inner shells but only one outer shell of eight plus two extra electrons. Those two extra electrons are now in a rather interesting situation. We have a Professor (Husinaga) here; I suggested he should do something and he showed that the molecular orbitals of the nitrogen molecule do indeed resemble the 2s and 2p atomic orbitals of a single atom except for this extra pair and they look like a 3d but shrunken down as the imprisoned pair, Langmuir’s imprisoned pair.

Kuhn:

That is very interesting.

Mulliken:

The orbital that corresponds to the 2s of the atom, however — the molecular s — is very different from a 1s and very different from a 2s. It’s recognizable as an s but you couldn’t say it was a 1s or a 2s; it’s a molecular s, but still it’s atom-like and spherically symmetrical, that is, nearly. I hadn’t thought of that before but I guess there are probably some good reasons for believing in an ‘Aufbauprinzip’ for molecules in terms of orbitals. Well, first orbits and then orbitals. Everybody had tried to make some kind of orbits and it didn’t fit, but after all it must be that something like that was the idea. As soon as the quantum mechanics was available then those who were fluent with it could use it to fit that. So Hund had first got these papers on the mostly rotational structure of band spectra and then he got at the electronic states. The summer of 1927 I spent in Gottingen talking very much with Hund and seeing Born and Franck, but Franck more than Born; seeing other people who were there; riding out on a bicycle and getting wet in the showers and so forth, and finally going at the end of the summer on this trip in the Black Forest with Hund. I was talking to various people who were in Gottingen; there were some Americans visiting there. Well I didn’t go to Europe for a year, you see — just whenever an opportunity presented itself. Hund and I were talking very closely together about that and I was presenting all these things organized from a rather empirical viewpoint and then he translated properly or explained them in the right way. That summer I must have talked with him about the electronic states because then in 1928, just after that, I got out a paper on the, electronic states really rather in terms of what he had been doing. Then he got out some papers that were more or less similar.

Kuhn:

Did you correspond with him a good deal when you were back in this country?

Mulliken:

Well, moderately. Not so much.

Kuhn:

By the time you saw him in the summer of ‘27 — and you were both, I think, already then quite clearly established as utilizing an orbital approach — the Heitler-London paper was then out. This must have been one of the things you discussed.

Mulliken:

Well, it must have been in 1927. I visited a number of places that summer. I visited Zurich and called on Schrodinger and had a chat with him about things. His chair collapsed while I was talking with him but he said — he didn’t say it in these words but the idea translated into some kind of English, would be, “I’ve got two chaps here who have got something you might find very interesting,” and that was Heitler and London. So he took me in to introduce me to Heitler and London and they told about what they had been doing. Well, I didn’t feel too enthusiastic about that way of doing it. I didn’t take to it very much and didn’t absorb it very well at that time.

Kuhn:

Had you worried at all about the valence problem in the course of your own spectroscopic work?

Mulliken:

Oh, yes. There’s somewhere in these [papers], there’s something about bonding and antibonding electrons. Herzberg brought in that terminology or the German equivalent, bonding and anti-bonding electrons. But here somewhere is a paper on the bonding power of electrons and valence. Oh certainly, there were these papers — let’s see, that doesn’t occur at this point, does it? It comes in a little later, about 1930 perhaps. Well I think Lennard-Jones came along in 1929 and emphasized the additive and subtractive. Hund had emphasized the united-atom relations and I went along with that but noted that some were bonding and some were anti-bonding. That certainly comes empirically out of the band spectrum works. Given these molecular orbitals — and they were obviously there and you were shifting them around — then a purely empirical derivation shows that there must be molecular orbitals having certain properties without and entirely aside from any theoretical derivation that there should be such orbitals. There must be certain ones which perhaps I called ‘s’ and ‘p’ and Hund renamed sigma and pi. That was the ‘28 paper. Could that have been before Hund introduced the sigma and pi? Oh, yes it must have been. Perhaps it was between ‘27 and ‘28 that he developed this and so I was proceeding with the molecular orbital idea but quite empirically and further developing these ideas of the analogy. This 1928 paper I like very much from the historical point of view as a piece of detective work. I’ve got a copy of that translated into modern terminology, or extracts from it, which I could give you [perhaps “Correlation of Molecular and Atomic Electron States,” Phys. Rev. 32, 1928].

Kuhn:

Actually I’d almost sooner work from it in its untranslated form.

Mulliken:

The translation is just translating s and p into sigma and pi.

Kuhn:

Good. I’d be glad to have a copy of it in this form also and I can get at in the journal for more careful work. You speak about this initial reaction of being somewhat unhappy about the Heitler-London approach.

Mulliken:

Well, I thought, “Yes, well we’ve got a nice explanation in terms of” — the name wasn’t invented yet — “molecular orbitals.” Anybody who has some favorite theory is naturally inclined to be somewhat annoyed if there is some other theory which is quite different. Either one must show that his is right and the other is wrong or if not that, at least one’s got to spend a lot of time understanding it, and that prevents him from having time to go ahead and do some other things he’d like to do. I’m afraid that may sound —

Kuhn:

No, this is a quite typical and quite important phenomenon. How did that situation change? My impression is that there was really a good deal of arguing in certain places between adherents of the two approaches.

Mulliken:

Well there was a good deal of arguing, sure. Let us say, in the crudest form of each theory they looked very different and it looked as if you had to say that either this was right or that was right, but it was, I think, Slater, especially, who pointed out that if you improve each, they come together, they came toward each other. Each one needs to be corrected with a little bit of the other or, in the case of molecular orbitals, the first idea was, “Here is something, an entity, which is characteristic of the molecule.” But then it was Lennar-Jones who emphasized the construction of the molecular orbitals [as] the linear combination of atomic orbitals. By doing so he was able to give a good explanation of the bonding and anti-bonding properties of molecular orbitals which I could see empirically but which I tried to explain in terms of promoted or unpromoted orbitals. [My explanation] was really not sufficient; it was partly right but really not sufficient. The unpromoted orbitals tended to be for bonding, the promoted orbitals tended to be anti-bonding, but they weren’t always; sometimes they were bonding. That was explained in terms of the linear combination of atomic orbitals, ‘LCAO’, which Lennard-Jones emphasized. In fact Lennard-Jones wanted to say that was the theory of molecular orbitals but I had to argue with him somewhat that that is merely a rough approximation to molecular orbitals and is not representing the true, essential molecular orbital. In a way, by stressing the ‘LCAO’ form, the apparent shortcomings of the molecular orbital system were over emphasized. After that there were various steps. One could shrink these atomic orbitals in the Heitler-London theory; one could shrink them also in the linear combination of atomic orbitals and in the molecular orbital theory; then one could polarize them, and so on. All of that made each method better, but it wasn’t too clear that they were brought together. Well, there were various steps.

Kuhn:

Would you say that as of 1928 — I pick this date rather arbitrarily and you can change it — it was clear that in this whole area of the investigation of molecular spectra and the related problems, that there were a series of schools — a Heitler-London school, a molecular orbital school — with some geographical localization? There were certain places in which one approach was being used and other places in which another approach was being used so that people were somewhat talking past each other?

Mulliken:

I don’t know. The way I was thinking was not in such terms as to notice things quite in that framework. I would say there were some people who were stronger for one thing than for another, but whether they were more abundant in one particular place I don’t know.

Kuhn:

Who would you point to as the particularly strong proponents of one or another of these views?

Mulliken:

Of course Pauling and Slater developed the Heitler-London theory, but for a long time not much was said about Slater. Pauling was a much better showman. Lately I was astonished to hear Pauling credit Slater with a lot of it. At any rate he developed — of course this would go beyond the Heftier-London and now it’s the ‘Heitler-London-Slater-Pauling,’ or the ‘valence bond theory’ or whatever you call it. Pauling made a special point of making everything sound as simple as possible and in that way making it very popular with the chemists but delaying their understanding of the true —. He left them with a pretty crude idea and made them feel that that was satisfactory, whereas something better could have been done. So anywhere Pauling went that became popular. At first it wasn’t clear, but it turned out later — gradually — that the molecular orbital approach is really better for calculations. The valence bond method required great flocks of resonance structures when the molecule got complicated, and to make any calculations with those is still just about hopeless. The molecular orbital method has proved more adaptable to real computations. Now I must say that nobody knew about that at first and at first it looked as if the Heitler-London was more suitable for the simple approach and perhaps a better description in some ways of the chemical bonding. Now I have a favorite argument that Lewis’ electron pair bond is considerably better described by a pair of electrons in a molecular orbital than by the Heitler-London bond. If the chemical bond has any polarity, it’s necessary to add an ionic term, to mix an ionic and a covalent term, that is, a Heitler-London plus an ionic, to represent the bond. That is a rather messy description, whereas the molecular orbital — this is not the general spectroscopic molecular orbital but the chemical molecular orbital, the localized molecular orbital — fits very nicely to concept. I’ll give you some of these reprints but here is this one which says something about this.

Kuhn:

This is May 1960, The Vortex.

Mulliken:

Which is not a very well known magazine but is put out in Berkeley. The chemists, of course, know that very well. So here you have a few historical answers… And that is rather the theme of one of my perhaps chief interests.

Kuhn:

“What are the electrons really doing in molecules?” I can see that there’s a good deal of rather useful scientific and biographical information here. [Mulliken continues to ply Kuhn with reprints]…

Mulliken:

(Roton) came here in 1950 or maybe 1946. He had had a very good training with Kronig in the Delft Institute of Technology, so he came here and got his PhD with me although he is really — well, the German word is “selbststandig”. I don’t know that I contributed too much to his education. He was already very well trained in mathematics. But anyway, the subjects of interest were, under this heading, “What are the electrons really doing in molecules.” Let’s say I got him interested in the subject but he is really responsible for all the development of the machine programs by which we are finding out what the electrons are really doing. Here is a picture of the situation in that field right afterwards, for the next period of development. [reading] “… to understand the early days of quantum mechanics. For the first few years many of the world’s best theoretical physicists engaged in calculations on molecules.” Well, there are Heitler and London among others. That was it. And here is an enumeration of the difficulties and then what has been done about it in Japan, and by Slater and here; but we’ve just now got beyond those stages and that’s been done right here. So this is sort of a (cycle). There’s a great deal more progress since that time, but this does give you some of the historical perspective I think. There is one other reference which gives a survey which I’d like to not forget, but I don’t have a reprint. [finds article] I didn’t write it in French; somebody else translated it for me. It’s just the very first part of it which gives some survey.

Kuhn:

“Quelques aspects de la theorie des orbitales moleculaires,” Journal de Chimie Physique, volume 46, 1949, nos. 9 and 10, page 497 f. Then numbers 11 and 12 pick up on page 675.

Mulliken:

I’m not sure that most of this matters too much. It’s mostly the introduction. That was l948 when there was a big symposium in Paris and I held forth and Pauling held forth; I think we each took about three hours, largely because they had to translate everything into French after we spoke … [Mulliken examines one of the numbers] Yes. I call the Heitler-London and the valence bond the atomic orbital method and the other the molecular orbital method. This then gives my impression, briefly, of the history, so that may be something of importance for your purpose. Of course you may not believe what it says!

Kuhn:

Mistake and doubt comes later; first we find out what it says!

Mulliken:

Well, I tried to make it right!

Kuhn:

That sort of thing is extremely useful, particularly at the early stages.

Mulliken:

There are so many different things that you might not find them readily, so I think digging them out is useful. I had one other item. I seem to have only one copy here so I might show you what there is.

Kuhn:

It’s a paper in the Physical Review, volume 32, pages 186-222, August 1928 perhaps “Assignment of quantum numbers for electrons in molecules”!. Here is Langmuir and Lewis again and N₂ you see that theme, and also the octet. I think that is something that is especially relevant perhaps. That does contain the survey. There’s lots more.

Kuhn:

I see that this is the translated and excerpted version of the (1928) paper.

Mulliken:

Yes, and I don’t remember what all is left out, but perhaps this is the most important. Here was the CO plus N₂ etc.; then Birge came in. This is of course rather primitive and this seems to be pre-Hund although it’s 1928 and I had been talking with Wind in ‘27. Perhaps he hadn’t gotten around to that; but I’d talked with him about all these things and so then he thought about it and he came out with whatever it was. Here’s promotion of electrons, binding energies — [reading] — “A necessary condition for the formation of the molecule is that the total binding energy must first increase faster than the nuclear repulsion,” and so on and so on. “The total energy as it may be considered as the sum of its parts”; “individual electrons may then be considered as contributing to the bonding”. Oh, yes, bonding electrons and bonding power. There are some other discussions of that but perhaps this is a good example: “Here are two kinds of bonding power judged by whether removing an electron tends to increase or decrease the dissociation energy or to increase or decrease the distance between the nuclei or the vibration frequency.” “Hund has suggested —.”

Kuhn:

Tell me, when did you put this excerpted version together?

Mulliken:

Oh, a year or two ago. The purpose of that is that Donald Ramsay of Ottawa persuaded me that I should get some of these papers out and reprinted in a book form and he got the Pergamon Press to agree. I don’t know whether or not that was a wise choice, but at any rate they said that was fine and they’d be very glad to do it. I must say that I find it very hard to bring myself to look at all these old papers.

Kuhn:

Are you going ahead with it though?

Mulliken:

Yes, at least in principle, if we don’t both die before we get it done. We made one mistake: there were two quite interesting tables in the 1930 papers and Herzberg in his book on diatomic molecules has in part followed along a similar plan and got much more elaborate tables, but in some respects he has not done everything that was done in those early tables. Then Ramsay and I thought we would bring those tables up to date. Well, just that little item has produced a lot of delay but prior to that — I have a lot of annotations — I thought that if this was to be put out there should be a number of annotations to point out later developments. There should also be a correction of any absolute errors or misprints, so some attention was devoted to that. I think that if we once get over this hurdle of the revised tables, which will really be quite nice once finished, then the best thing will just be to put it out without any change. Well, I never quite saw — they were so long ago — how that was worthwhile, but various people said it would be, especially certain ones, so I agreed to do it. In the meantime there are such competing interests of newer developments that it’s just very hard to find the time. This subject [apparently showing Kuhn another article] has been extremely interesting in developing — lots of people have gotten going on that since 1950 when I did something, but at the same time this subject is in a most extremely exciting stage so it’s rather hard to do anything with these old papers.

Kuhn:

I can see, yes indeed I can.

Mulliken:

Do you have one of those? That was a picture of which I happen to have a good supply. It’s not up to date but it’s of a certain time and contains something (of interest). I’ve already mentioned European visits which you didn’t seem to know about… I spent a half a year in 1930 and another half year in 1932, making a total of about a year in Europe.

Kuhn:

There is in this period, as you know, a number of people getting out of graduate school and going more or less straightaway to Europe for one or two years. You didn’t do that. Did you consider doing that at all? In fact, I know you did because you have talked about your project to go to Rutherford for beta spectra.

Mulliken:

Yes. If somebody had provided me with some funds I would have gone. Let’s see [examining outline, p.4] — New York, New York University, Washington Square — I was there two years. There were two divisions, the Washington Square and the Heights and they were very far apart and had a hard time getting together. If you are interested in what went on there possibly F. W. Loomis could tell you more; he was the chairman of the department. Also Hilberry was there and Hilberry got me the job there. We had been sort of pals at the University of Chicago when I was a graduate student; he was in physics and I was in chemistry and we belonged to the Gamma Alpha fraternity — not the rah-rah boys but a graduate fraternity. He got me to New York and we were roommates there in New York on Washington Square during the prohibition days which was a very interesting situation. Perhaps I remember more about that than just what the scientific atmosphere was: it wasn’t a very wonderful faculty on the whole.

Kuhn:

Did you see much of the group at Columbia while you were there?

Mulliken:

Well, I must have visited there occasionally but it seemed a little bit far to get up there. I don’t remember specifically, but Rabi was there, and Kronig.

Kuhn:

Kronig I think was there; Rabi I guess was back at least before you left. He’d of course gone to Europe and I’m not quite certain whether he was already back at the time you arrived in New York or whether he’d have arrived back during the time you were there.

Mulliken:

That was ‘26-‘28. Well, I knew those various people of course and probably saw them more often than I recollect. There were some meetings of the Physical Society sometimes. There were those three lectures I gave at Columbia in ‘27 to ‘28, so that was during the period I was at Washington Square. They asked me to give some lectures there so I did. Then later in the spring Compton tried to get me to come to Chicago and R.W. Wood wanted me to come to Baltimore. Then the people at New York said, “Well, we’ll make you the head of the department” and Loomis said he would resign if I would stay there. I really didn’t think I was up to it, however; somehow or other I had some sort of loyalty to Chicago and decided to come here. Oh, yes. R. W. Wood said he would get me a higher salary than anybody in the department and higher than his, in fact, I guess. So that was my peak of popularity. That was after I bad been publishing lots and lots of papers.

Kuhn:

You had already of course published a good deal before you went to NYU and one of the things I wanted to ask you was whether there were other options of jobs available to you in ‘26 when you went there.

Mulliken:

In ‘26 I didn’t do anything about looking around or stirring anything up. Hilberry came along and so on; this was in the physics department, so that is part of the transfer that I had to —. But meantime in ‘26, this year I stayed [at Harvard] after the National Research Fellowship, it was Kemble who got me a small stipend and I stayed. The idea was largely to learn more physics but I’m afraid I didn’t do as good a job at that as I should have. Anyway, I went to NYU and taught certain physics courses and wrote some motes on electromagnetic theory which I took at now and wonder if I knew all that. It’s just standard stuff.

Kuhn:

What sort of problems did the transition from establishing yourself in one field and working into another present to you?

Mulliken:

Well, the problem was that there was a great deal to learn which I didn’t entirely accomplish satisfactorily, partly because I was so anxious to do something else, to get some research done.

Kuhn:

Have there been physicists who have wanted to hold you at arm’s length, who have said, “Well, after all, you’re a chemist; you can’t do this!”?

Mulliken:

Nobody ever said that; I’m sure there are a lot of them who consider that I don’t really belong entirely, especially if it’s theoretical, and others who, mistakenly perhaps, think that I know a great deal more than I do about some things. Actually my interests could be said to be somewhat more on the chemical side and the intermediate position is a position of unstable equilibrium which is always a good place to be, really. It’s a fruitful place to work in but it entails a certain amount of painful ignorance and unless one is something very super-extraordinary he’s likely to be ignorant of a lot of things. Also some of these areas which started in physics have been gradually taken over by chemistry so that is a reason that it becomes rather natural to associate. Then this second interest, so much interest in the (???) (???), was pretty much chemistry. On the other hand, I have the spectroscopy of the small molecules and atoms as tending to come back somewhat into physics again. Getting the mechanism of electrical discharges, plasmas, and various astrophysical problems which involve excitation of atoms and of simple and small molecules and so on, interaction of atoms and molecules; a good deal of that is fairly much at home in physics.

Kuhn:

Having really moved largely into chemistry for awhile and are now moving back?

Mulliken:

Well, no. I’m dividing my attention but this is rather distressing because all these different fields are growing rapidly and there are new fields which are related and there’s a problem of what to do. It would be better to drop some of these things. Then I have all sorts of people writing to me about many things which I’m interested in. It’s very hard to drop some subjects completely. There was the hyper-conjugation which I really ought to write some surveys about because Professor (Dewar) who was here and now has gone away, went out on a campaign essentially to say that there any such thing as hyper-conjugation or at least hardly anything, and he put that in a most exaggerated form. Well, that’s rather annoying but it would interfere with other interests to go and carefully work that out and counter each point. I’m afraid I’m going off on various tangents right now.

Kuhn:

Let’s take one more look back here [to the theory of molecular orbitals].

Mulliken:

You’ve got most of everything. That was an interesting point about the ‘Aufbauprinzip’, how you think that Bohr wouldn’t have — [see earlier in interview].

Kuhn:

No, I think that Bohr would have indicated that one went at molecules not by adding electrons but by adding atoms.

Mulliken:

Yes, well I really don’t know. When I first talked with him it was I think about Rydberg states and quantum defects and I think it was because I was interested in these helium molecules. Well, not only, certainly not, but I was interested in the molecules and this was closely related. — aspects of the picture. I’ve referred to 1925 and 1927 and visits around and talks with people and getting acquainted with them. Now in 1930 I had a Guggenheim Fellowship, but that was six months or half of one [i.e. ‘half of a fellowship’], so this was also a honeymoon trip. A little bit of conflict there but compromise. We stayed at Leipzig and Hund and Heisenberg were there. [E.] (Huckel) was just getting his PhD, I think, working more or less with Hund; Teller was Heisenberg’s assistant. Debye was there in the nearby Institute of Physical Chemistry. This was the Institute of Physics. Here there are one or two items which are not strictly of scientific interest, but Teller, as Heisenberg’s assistant couldn’t, of course, be better than Heisenberg at anything. Heisenberg was the best at ping pong or Tisch Tennis and Teller was next.

Heisenberg was still pretty young at that time and one day it became known that he had been elected to the Saxon Academy of Sciences, so they put on a celebration and he came dressed up in a long grey beard. There was a party with probably coffee and cakes or something, I’ve forgotten what, but there were some goings on. There were some Americans there, I don’t remember just who. But skipping from that — my wife was there of course — we almost starved in a pension in Leipzig. At that time Hitler was ranting and raving and we could have gone to a rally but I didn’t go. I only heard him on the radio. I couldn’t understand why there was all this furor about Hitler, why anybody would be interested, but Hund explained that. I think it was 1932, later, when we were sitting in a hotel lobby with Hund and Hund was explaining to us how things were developing, how they were going and what was going to happen. That night at about 2 a.m. somebody knocked on the door and said it was the police. They said “your passport, bitte!” so I got out of bed and showed them the passport and they went away. But things were casting their shadows. That was ‘32. We spent a certain amount of time in Leipzig. Hund was a good friend and he had a new house. Then we went up to Gottingen and spent some time with Franck. I gave some talks in various places, in Gottingen and in Copenhagen I guess. I remember one time I gave a talk in Cambridge with Dirac sitting in the front row; I didn’t think I had anything to say that would really be important to him but I was quite flattered that he was there. I think it may have been ‘32; I was talking about application of group theory to the classification of molecular electronic states. I talked about that at Gottingen and elsewhere. In the winter of ‘32 to ‘33 we were in a pension on Unter den Linden [in Berlin] which was all demolished, I guess, in the war, but there were communist storm troopers and Nazi storm troopers marching up and down the street below our window every morning, first one and then the other. Then a little later there was the burning of the Reichstag and at that time we moved to Vienna.

Kuhn:

You were in Berlin, though, for the Reichstag fire.

Mulliken:

No, I think it was somewhat after we left. Yes, I think we were in Vienna. We went to Switzerland. Oh, my wife had her appendix removed in Berlin. First she was in Gottingen and had some trouble but the doctor there didn’t believe in operations although his name was Stich.

Kuhn:

Maybe that’s why!

Mulliken:

As a matter of fact, I heard it said that Professor Franck had had trouble and hadn’t been properly treated. I shouldn’t repeat rumor, but he had a good deal of ill health which stemmed from something in that period. Of course he left — it couldn’t have been very long after that. We asked people there who was the best surgeon in Germany and who was the second best. When we got to Berlin I decided the second best surgeon would be good enough; I didn’t want to take any chances but I thought that would be adequate. That was Herr Professor Doktor (Nordmann) and he operated and removed my wife’s appendix, leaving a scar about one half inch long. He was an expert on that. He worked on Marlene Dietrich and such people.

Kuhn:

Who was the first best surgeon?

Mulliken:

Oh, the first best surgeon became Hitler’s physician. Sauerbruch was his name. I don’t know what he would have done.

Kuhn:

No scar at all!

Mulliken:

None at all, obviously! But it was all right. We wouldn’t have wanted a Nazi, would we? But it just turned out that it happened that way. Well, we went to Switzerland and first we took a place that was cheap but it rained for a couple of days and we decided that would never do, so we went to St. Moritz which is really elegant and spent a week there. Then we went on to Austria where my wife has some cousins, and from Austria — this was either in ‘30 or in ‘32 — we made a trip to Budapest and visited the Teller family. We had lunch with Teller’s mother and father and sister but we were a half hour late or something which was very wicked. I saw some of the Hungarian physicists and at that time I saw that famous (wave making) machine which, as I said, was apparently the origin of the one in India. Well, all that is pretty much gossip I guess. About Teller — there had been a Communist regime in Hungary for a year or more and they had some very strong impressions. This I think helps to account for Teller’s very strong views in more recent years. In 1930 we became acquainted with all sorts of people; I probably visited a larger fraction of the universities in Germany than in this country, then and later.

There was a meeting of the Bunsen Gesellschaft in Heidelberg in 1930 which was quite a famous meeting, and all sorts of people were there. They had a meeting in the Heidelberg castle with beer and wine served in the cellars next to the famous Heidelberger [Tonne] — you know that immense cask — and there was dancing. Everybody had so many beer and wine tickets they were giving them away; the Herr Direktor of the I.G. Farben Industrie was there — or some of them were there — and they were being slapped on the back by some of the young fellows. This was quite an occasion and at that time I did give a paper at the meeting. This was a paper in which I didn’t say very much because I did give it in German and that reduced the speed by a factor of one half. [Shows paper] Here we see united-atom descriptions of various molecules and some other things, and — oh, I guess these are footnotes. Well, that’s that. This is a meeting [of the Faraday Soc.] I didn’t go to, in 1929, but this [showing the proceedings] might, or does, I think, reflect some of the historical situation; it was about band spectra and atomic nuclei, [Reading]: “Revealed by our present knowledge, the phenomena of alternating intensities and alternate missing lines … “Let us see what is here. The hydrogen molecule — ortho and parahydrogen; all right, well that is all right. [Next] “Molecules of complex nuclei”, “Nitrogen”, “Nuclear particles; protons and electrons” [reading] “14 protons, 7 electrons.” Now: “If each particle has a spin of one-half, how does it happen that the spin of the nitrogen nucleus is one? — or the alternating intensities?” That problem was not yet clarified.

Kuhn:

That paper was written when?

Mulliken:

1929.

Kuhn:

Is that Transactions of the Faraday Society, November 1929? [This is perhaps the article by Mulliken, “Band Spectra & Atomic Nuclei”].

Mulliken:

Yes. Here it discusses the nuclear spins as obtained from band spectra and certain ones are not surprising. It says, “The relatively large values for sodium, chlorine 35, iodine bismuth, suggest that the normal state of a nucleus doesn’t necessarily correspond to a maximum pair-wise cancellation of spins.” Also, “The value Jn-1 which is definitely indicated for nitrogen by the measured alternating ratio is very puzzling, since it cannot be accounted for by protons plus electrons.” So here everybody is being puzzled about that. “We’re left with the possibility suggested by Kronig that ordinary nitrogen happens to be a mixture of two or more isobaric isotopes of different nuclear spin in just such a proportion as to account for the 2:1 ratio. Such isobaric isotopes we presume to be somewhat analogous to isomers of a complex molecule such as hexane,” and so on.

Kuhn:

Did you find that a reasonable possibility at that time?

Mulliken:

No, I doubt it. Then there’s still another difficulty in nitrogen independently; one needs the spin and statistics both together; independently and separately they are wrong, contrary to conclusion. Maybe this page would be a point of interest to you. “It would seem here that there is an error in Rasetti’s measurements” … and so on. I think possibly I almost had an idea to explain it but here’s where the neutron comes in — or didn’t come in yet — and here’s where Harkins had some ideas. I don’t remember the historical situation, but that seems to fit in.

Kuhn:

Yes, very definitely.

Mulliken:

Another matter — this is probably less of interest, but at any rate — “Notation for spectra of diatomic molecules.” Here’s the situation.

Kuhn:

Physical Review, 1930.

Mulliken:

The atomic people had gotten together and gotten out a notation; that was Russell, Shenstone and I’ve forgotten just who. So I just took it on myself to write to everybody and make some proposals, asking for suggestions and agreement or disagreement. I made some compromises, introduced [subscripts] g and u for ‘gerade’ and ‘ungerade’ — that was sort of a political move to put in something for the Germans. I got about 96% agreement among all the people who were important in the general field, Dieke being the most refractory. These Dutchmen are particularly stubborn; it’s my observation that they’re extraordinarily stubborn, almost more than anybody else. But all these other people agreed and then this was a standard notation; this was accepted. This was no international commission appointed by the international union, but it was the right way to get agreement.

Kuhn:

Yes. Did it do the trick quite quickly?

Mulliken:

Yes. Just by getting in touch with practically everybody in the field, getting them to agree. Of course thereafter some might have diverged or new people could have come up and not agreed, but a while after that Herzberg came along with his book and in his book he said, “This is the official international nomenclature.” It seemed to me that was a much more effective way of getting some notation agreed on than these international commissions which seem to forever endlessly quarrel. In thermodynamics I guess you know how much trouble they’ve had getting agreement on what letters to use in thermodynamics. Later, in 1955, Herzberg got me to work on this thing, [showing Kuhn] for polyatomic molecules, but I forgot to put my name on it so it came out with no name. That was probably a mistake because, although it says that it was prepared at the request of this commission, still it might have been more effective if it had said who prepared it.

Kuhn:

It has not had so much impact as the first one, you feel.

Mulliken:

No, I think not, but one reason is that I started using certain letters or choice of axes, something like that, where Herzberg used a different choice and so here were these two different things. It had to do particularly with what is a (B1) state and what is a (B2)state. What is a (B1) state in my papers is a (B2) state in Herzberg’s and vice versa, and that sort of thing — no end of confusion. So I proposed a compromise on this but Herzberg insisted on my putting in this to follow my choices throughout, but I don’t know how well this is being followed. The London Chemical Society decided this was official but the American Chemical Society hasn’t. Well, that is not particularly to do with your subject but it’s another thing. And there were conferences [showing Kuhn a conference proceedings] — well, that is 1942 which is beyond your limit. It might show you that certain people were at certain places or still alive or something that you might have some interest in.

Kuhn:

That’s Science, October 1942, reporting the spectroscopy conference at Chicago and this is really the same conference reported in the Review of Modern Physics.

Mulliken:

Yes, one is a report on it, or survey, and the other is sort of an apology or justification for having the conference in 1942 when the war was going on. Here’s a letter from somebody. I’m sorry, I don’t know just who it was. Well then, that does cover the odds and ends I had in mind.

Kuhn:

I’m very, very grateful to you.