My 45 years of astrochemistry: memoirs of Takeshi Oka.

I decided to devote my research career to studies of interstellar molecules on December 17, 1968. On that day I read in the Sussex Library of the National Research Council of Canada (NRC) in Ottawa the December 16 issue of Physical Review Letters, in which the discovery of interstellar NH3 through radio emission between inversion levels of the J = K metastable NH3 was reported by the group of Charlie Townes. Totally unaware that such work had been underway, I was thunderstruck. The discovery revealed rich chemistry in interstellar space and initiated the concept of “Molecular Clouds” with unexpected high density, which was to affect astronomy in a profound way. I also had my personal reason to get excited. My paper on the Δk = 3n selection rule on collisions of NH3 with He had just appeared in the October 1 issue of The Journal of Chemical Physics (JCP). Applying the powerful technique of microwave double resonance that I inherited from my thesis supervisor, Koichi Shimoda, at the University of Tokyo, I was studying selection rules of spectroscopy and collisions. In spectroscopy, I worked as an anarchist, demonstrating that the selection rules in the textbooks are by no means sacred and many “forbidden” transitions are allowed. In studying collisions, I worked as a lawmaker, revealing experimentally that the changes of rotational levels, which appear random, follow approximate but simple selection rules. Such subtleties of selection rules in spectroscopy and collisions, however, are washed out in the laboratory due to high pressure and result in thermal distributions according to the maximum entropy principle. In the low densities of interstellar space, however, the two processes may be competitive, and the intricacy of the selection rules is manifest in their raw forms! On that day I was to give my progress report at Herzberg’s group meeting, but I scrapped my prepared report and talked about the discovery of interstellar NH3. I sent my JCP paper to Townes. I sent it without an accompanying letter since I was sure he would understand it right away. Sure enough, in the May 1, 1969 issue of The Astrophysical Journal (ApJ), the authors (including Townes) used the collisional Δk = 3n rule for the analysis of interstellar NH3. 3 In this paper they stated that the Δk = ± 3 radiative transition (spontaneous emission) occurs via the electric octopole moment of NH3 and takes longer than the age of the Universe. I immediately thought that this could not be right. The spontaneous breakdown of symmetry must produce a small dipole moment to cause the forbidden transitions. Jim Watson completed the theory. We found that the forbidden (J, K) = (2, ± 2)→ (1, ∓1) spontaneous emission has a lifetime of 300 years, 8 orders of magnitude less than the age of the Universe. Thirty years later, when H3 + was discovered in the Galactic center, this spontaneous emission, which was somewhat academic for NH3, became essential since its lifetime is only 27.6 days and competitive with collisions in diffuse clouds. Jim Watson is a supreme theorist, and his work greatly inspired me and influenced my experimental research. The theory of vibration−rotation interactions was originated by Edward Teller, founded by Bright Wilson, and completed by Jim. His 1968 paper on the simplification of the Wilson− Howard Hamiltonian is the most beautiful paper on theoretical spectroscopy by any of my contemporaries. Following NH3, H2O and H2CO were discovered in interstellar clouds. They each appeared in a most dramatic way in the centimeter region − NH3 in emission from metastable levels, H2O as an intense maser with a spectacular velocity profile, and H2CO in strong absorption of the 2.73 K cosmic background radiation. All of those manifestations of extraordinary molecular distributions added fuel to my fascination with interstellar molecules since each of them is the result of the subtle interplay of collisional and radiative selection rules. My first paper in ApJ 1970 was on the cooling of H2CO due to radiative selection rules analogous to adiabatic demagnetization. Although Townes and Cheung’s analysis using collisional selection rules became the orthodoxy, I still think my explanation is alive. Spectroscopists fall in love with molecules; my first love was H2CO. I obtained my Ph.D. at the University of Tokyo in 1960 by performing microwave spectroscopy of H2CO in Shimoda’s lab. Koichi Shimoda is a genius experimentalist and did first-class experiments with very limited budgets in impoverished post-World War II Japan. It was tremendous luck for me that Charlie Townes stayed in Shimoda’s lab for half a year while on sabbatical leave in 1956 when I was a freshman graduate student in Shimoda’s lab. I learned greatly from listening to discussions between the two physicists. Following Shimoda’s design, Eizi Hirota and I constructed a microwave spectrometer in Yonezo Morino’s lab in Chemistry; that was the beginning of Hirota’s great work on free radicals, which was to blossom from 1975 to 1990 at the Institute of Molecular Sciences in Okazaki. The subtle interplay between the radiative and collisional effects has become my bread and butter and was later applied to H3 + in the Galactic Center (Section 7) and the anomalous diffuse interstellar bands toward Herschel 36.