George Pimentel - Session I

Bromberg:

I’m in the office of Professor George Pimentel at the University of California, where we’re going to talk a little bit about his work on chemical lasers.

Pimentel:

Perhaps it would be reasonable for me to begin talking about my recollections of thinking about lasers; also, in partial answer to some of the questions that you formulated in advance about the extent to which I felt I could recollect that my work was affected by the observations that were coming out in physics, and the developments in lasers. I go back to one of my graduate logs, to the work of one of my former students, Rick Spratley, because Rick actually was the first one to have tried to operate a laser in the lab; the date on this is July 13, 1962. We decided to put together a ruby laser, and that represents a dating of when I was actively beginning to try to get to work with lasers. At that time, if I’m reconstructing this all together, I had the notion that lasers as light sources were bound to be important to me, and I couldn’t see exactly how, but I didn’t want to be the last one to discover that they were important. And I felt that if we got to work using lasers in the lab, that would presumably mean that it would be on our minds, we’d be getting familiar with how they operate and what they can do, and as I began to proceed using this, that would put me in a good position to exploit them. Now, I can tell you that my thinking, that lasers were going to be interesting and important, definitely was keyed by reading an article by (Arthur L.) Schawlow, as I recall, in SCIENTIFIC AMERICAN. I can’t date that precisely, but I think you will be able to date it.

Bromberg:

It’s one in which he mentions lasers and chemistry, isn’t it?

Pimentel:

I must say I don’t recall, but it’s —

Bromberg:

— looked at it.

Pimentel:

It’s got to be before ‘62. Art Shawlow has said a lot about lasers and chemistry much later than that.

Bromberg:

But there is an article, before he leaves Bell that has a sentence or two, maybe a paragraph.

Pimentel:

That might, however, I don’t recall at all being influenced by that. It was rather that I saw them as important kinds of light sources. I might just interject, what lasers and chemistry means is something that’s really worth a mention or two. One of the ways in which lasers might be used in chemistry is, in any spectroscopic application in which the very special qualities of the laser as a light source are of use, but you’re doing more or less conventional spectroscopy. In fact, this decision to put together a ruby laser was associated with the possibility that there might be some fairly conventional kind of use of it, that would be worth getting experienced with, so that we had lasers and the one I mention here is Raman spectroscopy. That is spectroscopic application in chemical problems. Another possible meaning of lasers in chemistry is a much more recent meaning, in which the very selective possibilities of excitation are used to influence chemical processes. That is one synonym for mode selectivity. Mode selectivity means that you use the tunability of the light source, plus the many photons you can get to excite a substantial number of molecules in a very selective way, so that you can have the chemistry be that of selectively excited molecules, rather than one that’s normally excited through thermal excitation.

Bromberg:

I just assumed that the Polanyi article of 1960 might have made an impression on you. Do you remember?

Pimentel:

Oh yes. But let me stick with and finish off this other. The third possible meaning of lasers in chemistry would be chemistry to drive lasers, and what you want to do, going back to the Schawlow article which I don’t have and I don’t recall this particular reference you make, I’m not saying it’s not there, is whether you can discern which kind of-lasers and chemistry be meant. Whether he meant chemical reactions might pump lasers, or something else. At this meeting in La Jolla, lots of people were thinking about the possibilities of chemical reactions pumping lasers, at least by 1965. Now, we actually refer to Polanyi’s paper in our first publication, January 19, 1965, the first sentence is, “Polanyi clearly predicted this possibility.”

Bromberg:

But at the time he predicted it, which was I guess about 1960, did it make a mark? Did it come to your attention then?

Pimentel:

Yes. I certainly saw that article. I remember, for instance, and I think it’s the 1960 article, I think he coined what was I think intended to be a sort of a humorous take-off on the physicists’ love of acronyms. He said, “This one will be called Eraser.” I don’t recall seeing that.

Bromberg:

Yes, he said both Eraser and Vaser. Something like that.

Pimentel:

That’s the article, ‘61.

Bromberg:

Well, I don’t have the reference with me, but —

Pimentel:

I think that’s the article we were talking about. That’s why I looked it up, because I didn’t recall it being in ‘60.

Bromberg:

I think the submission date is what I was looking at. It’s what I usually look at because I’m interested in when the person had the thought.

Pimentel:

The other dimension would be, when the rest of the world has a chance to hear about it.

Bromberg:

Yes, that’s right.

Pimentel:

I heard about it when I was published, and I did see it in time, but, anyway, so we’re not — in disagreement about that.

Bromberg:

So, you know, it sounds to me as if the chemical ideas like Polanyi’s come through the chemical journals but the ideas of the physicists like Schawlow are coming through the more general scientific — you wouldn’t read the PHYSICAL REVIEW articles of Schawlow and Townes or that kind of thing, would you? Or might you have?

Pimentel:

Probably not. I was interested in lasers and was following what was going on, but I’m not a regular reader of PHYSICAL REVIEW.

Bromberg:

You know about Maiman and so on.

Pimentel:

Oh yes, sure. That’s why I think I paid attention to this article in SCIENTIFIC AMERICAN. In any event, what really caused me to say, to myself, perhaps we can investigate this issue of chemical pumping of population inversion was because of the contemplation of what we were liable to be seeing with our rapid scan spectrometer. We were looking for the first time at chemical reactions, using infra-red radiation , looking at chemical reactions that were taking place in micro-seconds, and we recognized that the molecules that would be coming out might be vibrationally excited. In that case, we would see them in emission rather than in absorption. And of course seeing them in emission has the ingredients of chemical pumping, and so, just the possibility that we would be seeing them in emission made it possible that we might be able to demonstrate a chemical laser. Our attack was to go after an optical set-up that was suitable to see weak infra-red emission — that is to say, a multiple reflection cell. And that’s why I was so strongly urging Jerry Kasper, my graduate student, to put together a white cell, that would have a long effective optical path length so that we could look at weak sources. We expected that we would see infra-red emission, but that we would have to work very hard to go from that point to the point where we would actually be able to convert this into laser emission. The big surprise we got was that we got laser emission in this, what I would say is unconventional optical situation from the point of view of a laser spectroscopist, but conventional optics from the point of view of someone used to doing spectroscopy with very weak sources.

Bromberg:

So then you really had Kenneth Herr and Jerry Kasper in parallel on these two projects?

Pimentel:

That’s right. Ken was a year or two ahead of Jerry, and his apparatus was coming into operation, and we published our first articles on that — successful use of the micro-second spectrometer — at almost the same time, ‘65, right at the end of ‘64 and beginning of ’65. We had it actually operating a year or so before that but hadn’t seen any transient species. And the idea of looking at emission as opposed to absorption for transient behavior was an obvious possibility associated with the rapid scan. While Ken was working on absorption techniques, I decided that Jerry ought to be looking into the possibility that we could see things in emission, and there was the possibility of chemical lasers, and that was a nice pot at the end of the rainbow.

Bromberg:

In these papers, I notice that it’s a contract with the Air Force Office of Scientific Research. One thing I wondered, was that an ongoing contract for other work, or was that negotiated just for this? I looked through papers up to about 1970 and they all seem to be the OSR, Office of Scientific Research and I was just —

Pimentel:

They were OSR, Air Force, as opposed to ONR.

Bromberg:

That’s right.

Pimentel:

The Air Force had been supporting for more or less an ongoing period of several years, I can’t document exactly what that is without going into my records, for a number of years they had been supporting research on free radicals, because of the importance of these transient species in flames and consequently in combustion processes. I remember very well one visit I made back to Washington, DC, to give a talk to the Air Force people there about the progress of my research, and I was picked up at the airport, as I recall, by my project officer, and on the way to the meeting where I was to speak, I told him, “Now, there are two things I can talk about. One is what we are doing with the rapid scan spectrometer, which is more or less along the line of what the grant request was all about. But I’ve been moonlighting trying to do this other business of looking at emission, and we think we have a laser. Now, if it would embarrass you for me to talk about something that’s outside the project description, I won’t mention it. But on the other hand, it’s really quite exciting.” Of course, he was elated to hear that this had come and the fact that I was working outside the project description didn’t bother him a bit and as it turned out, everybody was very excited and enthusiastic about it.

Bromberg:

That was a tremendously high gain laser, the ACO, wasn’t it?

Pimentel:

No, it was the iodine laser that had the enormous gain.

Bromberg:

Oh, that was about the 100 decibels per meter or something enormous.

Pimentel:

Yes, it had this enormous gain, and I related to you earlier the anecdote of Jerry and me talking in one of our typical once a week or two times a week discussions of his progress, what and how we could measure the gain, and he told me that he thought it had to be such and such and expressed it in decibels. I’m not used to thinking in decibels and I interrupted the discussion to convert that into what the implications were of gain in terms that I understood, and I finally concluded that if this gain were correct that he had called out, that we would have an enormous gain per meter. There it is in our first publication, quoting 160 decibels per meter, and we were dealing with something about a meter long, and I immediately said I just couldn’t believe that because the implication would be that we wouldn’t even have to have mirrors on the cavity, and as I mentioned to you before, he contended that he was right, that it did have that high a gain, and he left and came back the next day and said, “I put an index card in place of one of the mirrors and it lives.” Then I began to believe him, that he had actually measured that high a gain.

Bromberg:

This is what you told these Air Force people? Or you don’t?

Pimentel:

I don’t remember whether we actually knew the gain then in those terms. I think I was much more excited about the hydrogen chlorine explosion laser than the iodine laser at the time, because I was preoccupied with a real chemical reaction that in principle could proceed without any light initiation, and the iodine laser is what we call a photo dissociation laser, in which the chemical bond is ruptured, but you have to put in light to cause that, so it’s somewhat less a chemical laser than the hydrogen chlorine explosion. Now, let me conclude this little anecdote about the Air Force. I thought that was an excellent example of what I consider to be the very enlightened attitude that they had established in AF OSR originally coming out of the Office of Naval Research, that they shouldn’t be telling people what to do. They should be finding people who are doing interesting creative things and let them do what they thought was going to be important. That kind of attitude is disappearing from the scene, unfortunately. Increasingly it’s the short range point of view that you have to judge whether there’s likely to be a mouse trap at the end, and predict it, and then keep your nose on the track, which mouse trap you’ve predicted.

Bromberg:

See, that’s also on the question I asked, whether as you got the ACL laser and the HF laser, whether they were indicating one direction or another or suggesting?

Pimentel:

No. They were so elated that this grand new direction had come out of this that my continued support was more or less assured, not with any enhancement of it, I don’t mean to say, but it more or less assured me that they would have continued interest and I could go on developing this. Of course, I was still interested in the rapid scan spectrometer, but now I had in parallel this activity on the chemical laser. That was what brought Karl Kompa to me.

Bromberg:

Yes, that was one of my questions, whether he came because he was interested specifically in lasers?

Pimentel:

I honestly can’t say I remember, whether he came because of the lasers. We were doing a number of things, the matrix isolation technique, this cryogenic technique that we developed in my lab, was more or less centered in my lab. Did I tell you about this technique at all?

Bromberg:

Not really.

Pimentel:

Well, I won’t spend a lot of time on it, but it was also directed at finding ways to do spectroscopic study of transient, normally transient species, such as occur in flames and explosions, and the difference between it and the rapid scan was, the rapid scan looked at real time, and this froze out the transient species, unless you do a leisurely spectroscopic study. So we were developing that technique, and that made people interested in coming into the lab, the rapid scan spectrometer, that’s something people were hearing about, and then the chemical lasers.

Bromberg:

I see, so the whole thing is coming out of a research program devoted to looking in various ways at these transient species.

Pimentel:

There was a coherence to it. I was interested in the possibility of understanding chemical reactions in more intimate detail by reason of being able to study directly the transient intermediates that had been out of reach before.

Bromberg:

Well, you know, it might not be such a bad idea, since this is the first tape anyway, to speak a little bit about how you came to that research program, and just get that a little bit clarified, because I think it might be important.

Pimentel:

That’s fine. It won’t take very long. It goes back near to the beginning of my research career. I was interested in the use of infra-red spectroscopy as a means of understanding molecular structure and its relationship to activity. Lots of people were recognizing that as an important tool in chemistry, prior to World War II. Only the physicists were exploring it, using it, showing how to use it. As the technique became available in chemistry, it was plain that chemists were going to be able to use it in a very powerful way to study chemical reactions. I was having some success by reason of my thesis work. I got onto the study of diborane and borane chemicals, these being very interesting from the point of view of chemical structure, but also extremely reactive and very difficult to work with from the point of view of reactivity and hazard to life. It may be that that work caused me to get interested in the general problem of the transient species that are associated with these intermediate sets of reactions. In any event, somewhere along in the early fifties I got into the desire to study transient species in flames. That was really where my interest in infra-red study of transient species began, as an attempt to detect transient species in flames.

Bromberg:

Was this the point that you got in touch with the Air Force for funding?

Pimentel:

I’d have to look back on that. I think at the time I was sponsored by the Office of Naval Research.

Bromberg:

I see, so it was a transfer.

Pimentel:

Yes. Somewhere along the line, I began to get Air Force support.

Bromberg:

What we’re always trying to do is get the intellectual story situated in the institutional context of funding and all that stuff.

Pimentel:

In any event, we began to look at emission from flames I used to say that this period of research was characterized by some of the most magnificent spectra of hot water that anybody had ever reported because we were looking for NH₂ and NH in flames and we saw all this amazing and beautifully detailed emission from hot water. The problem was to find these little pieces of grass in this enormous forest. Emission by hot water. And we did ultimately have a tiny little bit of success, but it was because of the difficulty of seeing a little bit of a low concentration but interesting species in the presence of the emission of everything else that was there at a high temperature that caused me, with one of my early post-docs, to invent the cryogenic technique that we call matrix isolation technique. We went off on this matrix isolation technique, beginning about 1951 to ‘56, or ‘57, somewhere in there, and at some point along the line, probably around 1960, I got back to the idea that maybe the way to go after the transient species in real time was to go after a rapid scan technique. The time was right for doing that because, again, the physicists were developing some instrumental capabilities that I saw as potentially being useful.

Bromberg:

Those were the new detectors at Perkin-Elmer and so on.

Pimentel:

It was the new detectors. It turned out that these germanium detectors were displaying very rapid response time. That made it possible to think in terms of micro-seconds instead of milliseconds for infra-red recording. My problem, of course, was to be able to afford to get access to the detector. That led to this — again, I think I told you that the Perkin-Elmer Corporation had Lt decided to try to market such a detector — the problem with it was that the cooling of the detector to a temperature near 20 degrees Kelvin or lower made the whole devise very expensive. And there just wasn’t any money to even contemplate buying such a detector. I wrote to the Perkin-Elmer Corporation, they being generally supportive of people like me trying to do research in that technique, and asked them whether there was any chance at all that they would provide me just with the germanium chips, and I would try to mount them and build the cold cell around the (computer? detector?) They sent me, to my great surprise and elation, something like a dozen of these, and said, “Pick out the best three and pay us,” I don’t remember, some nominal amount like $100 each for the three “and send the rest back.” So we did exactly that, and built ourselves a liquid hydrogen cooler in which we could cool the detectors, tested them, found three, and then it became evident that we now could move into this rapid scan technique.

Bromberg:

By the way, were you in very much touch with any of the laser firms around here? For example, Spectro-Physics?

Pimentel:

They didn’t exist then, to my knowledge.

Bromberg:

Well, they start in ‘61.

Pimentel:

Well, the one contact that is perhaps relevant, I think you may recall that when Jerry presented our first laser at this La Jolla meeting, the physicists there generally expressed doubt that we had seen laser emission because it was a most unconventional optical situation. And so when he got back to Berkeley, he said, “They want to see this operate in a conventional laser optical system,” and he went down to the Peninsula to one of these outfits, and I don’t know whether we’ve given them acknowledgement enough, but someone loaned us a —

Bromberg:

— I think you did, I can’t remember at this time what it was.

Pimentel:

One of the conventional, I think, neon laser tubes, with which we could then test — we don’t seem to… to give an — oh yes, here it is, this is a helium neon gas laser tube produced by PEK Labs Inc. Sunnyvale, California. So there’s an outfit that was in business in 1964, and I can’t honestly say that I remember that we had any contact earlier.

Bromberg:

And you didn’t think of buying a ruby laser, you just built one.

Pimentel:

Well, we did buy the ruby crystal. Then we built the rest of it.

Bromberg:

Which in a sense was interesting because there were companies like Trion and Hughes I think and TRG that were marketing ruby lasers at that point. There were, they were about 5000, that might —

Pimentel:

— it was entirely a money business. We were just not funded on the scale that physicists are used to being funded, and so, we were often faced with the question, not; can we get a hold of one (?) but, can we build one? For instance, this rapid scan spectrometer, it was such a gerryrigged affair that people would come in just to see it, you know, visitors from the East, and say “Where is it?” and I’d say, “You’re looking at it. It’s right in front of you. There it is.” Because it was so unconventional and gerryrigged and so on.

Bromberg:

Were there any other reactions, besides the physicists wanting a better cavity out of you, to that iodine, do you recall? How did the chemists react to that? How did your colleagues react?

Pimentel:

Well, people like Polanyi were quite excited and interested, I’m sure. Incidentally, in my manuscript folded here I have a letter that I wrote to John at the time. It has a little bit of historical significance.

Bromberg:

It looks as if he’s going to have a lot of good stuff in your files, when you finally —

Pimentel:

“Congratulations on your early prediction.” … (off tape)

Bromberg:

The word is that they were excited about it here at the lab, and he went to numerous international conventions to talk about it. You know, one thing, while you’re looking for that — what was the relation between the kind of thing you were doing and the kind of thing Kamar Patel was doing, with the vibrational rotational CO₂ levels? Was there an intersection of any importance there, or those were just two separate —

Pimentel:

I was aware of what he was doing, and paid attention to it. My inclination was to consider that anything you do in which you put a chemical system in a plasma causes you immediately to lose control of it, so that if you get light out, and that’s your aim, that’s great, but if you want to learn about chemistry, it’s bad. So I always took the attitude that the last thing you want to do is carry out your chemistry in a plasma, even though it might turn out that that’s the easy way to get light out. To give an illustration of this, we have done quite a bit of work on rotational lasers, HF rotational lasers, in which you see laser emission between two pure rotational states, with no vibrational change at all. The first discovery of those was by a fellow named Deutsch, very early in the game, something like ‘67 or so, maybe even ‘66, I don’t know, but he got it by putting things into a discharge. I considered that might be a chemical laser, but whoever is going to know? Because you have everything in a discharge there, the energy’s there, and so far beyond any chemical energy that —

Bromberg:

But I’m wondering, as I just glanced at the literature, you are the only group I could identify which was really interested in doing chemistry with lasers, and there were a lot of people at Aerospace and Deutsch at Raytheon and so on who were interested in making chemical lasers, — is that a correct appreciation or was there a whole universe out there of people interested in the same kind of approach you were?

Pimentel:

Well —

Bromberg:

— for example, in the May 1969 —

Pimentel:

— this gives you a sort of a glossary of all of the various people who were interested in this possibility, and it corroborates your remark. There were a lot of people who were interested in lasers. I was interested in the chemical reactions, and so who was looking at it from that point of view. John Polanyi, but he had an entirely different technique. This was chemiluminescence, you see. John felt that his technique gave him much better control and understanding of what was going on than my technique did, for him, so we’ve gone along more or less in parallel ever since, oftentimes studying the same chemical reactions, oftentimes after the same kinds of information, he by measuring chemiluminescence and me by trying to measure optical gains. He was one of the people who, on occasion, would see whether he could get laser emission out of a system, but because of his preoccupation with the chemistry he tended to say, “I’m learning about chemical reactions my way, and I know what I’m doing, I’m not sure those people do know what they’re doing.” Now, a group of people did show up beginning to look at it in the same way as we did. This fellow named Airey, who spent some of his time —

Bromberg:

— oh yes, he was at AVCO, wasn’t he?

Pimentel:

Yes, and then he went to NRL, as I recall, and he did some very nice work. The CO laser came along somewhere in here, and I’ve forgotten, Curt Wittig was one of the people who was a person — Wittig, he’s not at USC, one of the early people in the carbon monoxide laser, chemically pumped.

Bromberg:

So Airey was also interested in elucidating chemical reactions?

Pimentel:

I think that’s a fair statement. He went at it with the same kinds of interest that we did. That grew, more slowly. I think the immediate glamour aspect of it was what cause most of those people to be interested, rather than that those people were really interested in reactions and this is one of the ways you can learn about reactions.

Bromberg:

The immediate glamour was the very high gain, or was it just the chemical pumping?

Pimentel:

No — laser is a magic word.

Bromberg:

OK.

Pimentel:

See, one of the manifestations of our interest in lasers as a means to learn about reactions was our development of what we called the tandem technique.

Bromberg:

That’s something I wanted to find out about, how that was developed.

Pimentel:

See, the tandem technique is a very early variant of what is now called in-cavity chemistry. The in-cavity meaning that you have a laser that’s operating and you place some system inside the cavity to see the effect of the laser light under those conditions when you have this very high intensity. In our case, we called it the tandem technique because we would put two chemical reactors in the same optical cavity, one of which was known to give emission at the particular wavelength and transition, and then put a second chemical system where it was in doubt as to whether the two energy levels involved therewou1d have population or not. If what we called the slave system produced, in the course of the chemical reaction, a species that was emitting, with occupancy of the two states that we were studying, it would affect the gain of the cavity, the gain of the system, either if the population had a population inversion, or vice versa. We considered that very liberating, to be able to use a chemical laser to study the populations, whether it would act as a laser or not, because if you’re interested in a reaction, you’re interested in who gets the population, and it’s more incidental whether it happens to be enough to operate a laser independently. That gave us an additional step forward toward trying to get the quantitative kind of information that Polanyi was getting from chemiluminescence.