Rolf Gross

Recollections of Research on the HF cw Laser I came to Aerospace in 1965 and for the first two years, I worked on high-altitude atmospheric reactions associated with atmospheric re-entry, with substances like 0 and N. This work came to an end; the Air Force was no longer greatly interested, and we were fishing around for some other work that would fit the background of the Kinetics’ Department (we also had rocket propulsion experience) and give us a new lease on life.

R. Hartunian, the laboratory director, decided that someone should look into gas dynamic lasers. He came to me because I was new, and because he thought that I would push it. He asked me to design a cw laser. Mirels had nothing to do with this work at this point in time. Ted Jacobs was the man who was driving it. It was about 1966 or 1967 when we started collecting the literature, and caught up with what Pimentel, Kompa, and the people in Berkeley were doing. Jacobs, who had done his thesis under Penner at Cal Tech on radiation kinetics, had good connections with Pimentel and others in that scientific specialty.

At this time, we decided we needed to get our feet wet and we began a simple laser experiment. Jacobs decided we would work on HF because of the promise that a gas dynamic HF laser was possible. The laboratory director, R. Hartunian, was unhappy with this decision because he wanted a blue, not an infrared laser. He went as far as to sabotage our work behind our backs.

In the winter of 1966/67, I was sent to Berkeley to discuss our work with Pimentel. Pimentel told me that in his lab nobody worked on HF lasers any longer; Kompa had gone back to Germany, Kasper had left for a teaching job at UCLA, and Corneille who had done the HCL work had gone to Gambia with the Peace Corps and was teaching physics there. Pimentel had no idea as to how to make a cw HF laser. Jacobs and I decided that only some clever gas dynamics could ever control the highly exothermic chemical reactions. How it should be done concretely first became clear in Germany. At the Shock Tube Symposium in Freiburg in 1967 I met Theodore Just from the DFVRL in Porz, an old G3ttingen classmate of mine, who was experimenting with H₂-O₂ and H₂-F₂ ramjet engines. They were doing spectroscopic measurements and reaction kinetics in supersonic jets using fairly novel methods. We wanted to use HF because it was a molecule that we knew would lase. It also was a simple molecule. We thought we understood its kinetics and its spectroscopy. From the German work, there appeared to be two alternate ways to control the very fast reaction kinetics to obtain cw operation; (1) a diffusion flame and (2) a shock-initiated flame. I opted for the second approach because it promised a well-defined, locally stable reaction zone behind the shock wave, hence a defined inversion.

I gave a seminar at Aerospace in the late summer of 1967, sketching out what I thought a cw HF laser should look like using the standing shock concept. Thereafter I had many conversations with Mirels on this, but he was less than enthusiastic. We did some initial experiments in a shock tube to test the idea. We observed HF lasing behind a moving shock wave, still today the only such experiment. The funding was in-house. In those days, we had sizeable discretionary funds, and the work wasn’t very expensive. However to expand our work despite the still negative position of Aerospace management, we decided to find money on the outside.

Jacobs knew Major Oglukian at AFWL, and he, Bod Giedt, and I went there and spent 2 days huddling with Oglukian and the director of AFWL. They finally gave us $300,000 from their discretionary funds for the HF work at Aerospace. The money was obtained by a letter of agreement only because we were part of the Air Force and did not have to submit formal proposals. By that time, Don Spencer in Mirels group was working on a shock tube driven gas dynamic CO₂ laser experiment. One day Jacobs had the idea that our HF—laser experiment should be done with an arc and Mirels’ group owned the arc. Jacobs without telling me sold our idea to Mirels.

The two men were very different. Jacobs was outgoing and always trying to make connections and give away his ideas; Mirels has since mellowed, but at that time he was secretive, defensive, and insecure. Mirels turned my experiment over to Spencer, and I was only allowed to peripherally watch what Spencer was doing. I had continued with my shock wave idea, and Spencer pursued the diffusion flame concept. The eventual success of Spencer’s work and my inability to make a shock stabilized laser finally led to the decision to abandon the shocked flame concept. I had used F₂O in my early photolysis experiments and in the shock tube, since it’s more reactive than SF. The arc group tried to use it instead of SF₆. They fed a fairly large amount of F₂O into the arc and had a bad accident with a large F₂ flame and even larger amount of toxic F₂O released. After this accident Jacobs suggested to consult the people at Edwards Air Force Rocket Propulsion Lab. Giedt and I went out to talk to them.

We eventually persuaded them to let us use their equipment for a large scale F₂-H₂ laser. Jacobs hired a professional engineer, Bill Caskill, who headed the extensive field project at RPL for several years. I tried to stay away from that project. Laser Isotope Separation: Three US groups conceived of US in concerto at the same time. One was Steve Harris, another was the AVCO group, and the third was S.L. Meyer, an old and idiosyncratic chemist at Aerospace. Meyer came into my office one day and said, “I want to separate uranium with your overtone with either an HF or CO₂ laser. All three groups submitted proposals to the AEC, and the AEC turned them all down except Meyer’s, who was promised $70,000. AVCO founded the US project in Seattle. Harris, who has 10 ideas a day, anyway, simply shelved his proposal. We did separate hydrogen and deuterium with the cw HF laser. We chose CH300 because it matched our HF-laser lines. The results of this experiment were never confirmed by others and may have been due to a “dirt-effect” in the absorption cell.

Since work at Aerospace was becoming increasingly hectic and job stability less and less stable, I went to Germany in the fall of 1971, to work with Kompa. When I got back in 1972, I found that the director of the Aerospace laboratory division, Gil King, had got together with Batelle in Columbus and submitted a joint proposal to the AEC for a 2.5 million dollar project to separate uranium. Batelle would supply the experience with uranium compounds and we the laser. The AEC just laughed at this proposal, because in the meantime Manson Benedict had chaired a “blue ribbon” committee for the AEC which determined that US was not practical. King had also taken the work out of Meyer’s and our laboratory’s hands and given it to a new man in another part of the laboratories who knew nothing about it and who was not getting anywhere with it. Meyer and I were never allowed to pursue our work any further.

Contacts with foreign scientists: Of the various foreign groups working on chemical lasers, only the Soviet work exerted a real influence upon our HF-work. They taught me how to handle F₂-H₂ reactions in simple ways. In exchange they led us down at least one dead alley. The Soviets did very interesting work. I had no access to their large scale technical HF laser work, if any existed, but I did repeatedly have access to their scientific work. N. O. Basov and his chief chemical laser scientist, A. N. Oraevskij, invited me to a conference in 1969, which I was able to attend after much haggling with the Air Force. This established very useful contacts between me and Basov’s laboratory. Later they also translated my book on chemical lasers.