Paper 5.2: N. Bloembergen, “Comments on the dissociation of polyatomic molecules by 10.6mm radiation,” Optics Comm. 15, 416–418, 1975.

My interest was aroused when I heard a presentation by Dr. C.P. Robinson of the Los Alamos National Laboratories on this subject at the Third Conference of the Laser, sponsored by the New York Academy of Sciences in April 1975. The interest of the Los Alamos group had in turn been fueled by Ambartsumian et al.'s pioneering paper announcing that this process could be isotope selective. (R.V. Ambarsumian, V.S. Letokhov, E.A. Ryabov and N.B. Chekalin, J.E.TP. Lett. 20, 273, 1974.) Clearly, this could not be a simple heating effect, but it was equally clear that it was not a nonlinear process involving thirty or more infrared photons simultaneously, as the yield of dissociation products was not proportional to a very high power of the intensity.

This early note distinguishes two stages, which turned out to be basic in further developments. An anharmonic vibrational mode is excited by a multiphoton transition, involving a relatively small number of photons. This step is isotope selective. Subsequently, the molecule is “heated” in the quasi-continuous vibrational phase space by a cascade of one-photon absorption and emission process. This leads to a unimolecular dissociation reaction which preserves isotopic selectivity.

I had been scheduled to give an invited talk, presenting a rather routine review of nonlinear spectroscopy, at a conference on laser spectroscopy held in Les Houches, France, in June 1975. The text for that talk appeared in the proceedings, Laser Spectroscopy, edited by S. Haroche, J.C. Pebay-Peyroula, T.W. Hansch and S. E. Harris, Springer Verlag, Berlin, 1975, pp. 31–38. That talk was, however, never presented. I volunteered to speak instead about an exciting new development: infrared multiphoton dissociation. The session chair and the audience readily approved of this change. My presentation, which is described in this note, infuriated V.S. Letokhov, the young leader of the group who had discovered the isotope selective process. He came to me after my lecture and told me that he had these same ideas already years ago. When I asked him where he had published them, he said that they were “too simple, obvious, and unimportant,” but he would show me his laboratory notes. I told him I could not read Russian and he should not get so excited if the problem was really so unimportant. Rem Khoklov, the leader of the Soviet delegation to the conference, had witnessed this rather heated exchange. He told Letokhov to make amends and the latter offered a formal apology after lunch. For several years, our groups remained fiercely competitive, but from about 1980 on, Letokhov and I have been on good collegial speaking terms. Letokhov's group has probably done the largest amount of work on the subject. He and his associates have written several books about it, including V.S. Letokhov's Nonlinear Laser Chemistry, Springer Verlag, Heidelberg, 1983, and Laser Spectroscopy of Highly Vibrationally Excited Molecules, edited by V.S. Letokhov, Adam Hilger, London, 1989.

In 1975, David Larson of the MIT Lincoln laboratory, was encouraged by our mutual colleague, Paul Kelley, to work on the quantum theory of the anharmonic oscillator transition. We published a note which is not reproduced here, “Excitation of polyatomic molecules by radiation,” D.M. Larsen and N. Bloembergen, Optics Comm. 17, 254–258, 1976.

I had close contact with the important experimental group at Los Alamos headed by C.P. Robinson. I spent several summers there as a consultant. I got aquainted with Cy Cantrell, who was a theoretician in the group. Our next theoretical communication was a presentation by David Larson, Cy Cantrell, and myself at a memorable conference held in Loen, Norway, in June 1976, preceding the larger International Quantum Electronics Conference, held in Amsterdam, Netherlands. At both conferences, many research groups presented papers on multiphoton dissociation processes as it had become a “hot topic”. The interested reader is referred to the conference proceedings Tunable Lasers and Applications, edited by A. Mooradian, T. Jaeger and P. Stokseth, Springer-Verlag, Berlin, 1976.