STUDIES OF ASTROPHYSICALLY IMPORTANT MOLECULAR IONS WITH ULTRASENSITIVE INFRARED LASER TECHNIQUES Richard J. Saykally Department of Chemistry UC Berkeley Berkeley, CA 94720 Over the last decade, modeling of the chemistry occurring in inter- stellar gas clouds has evolved drfmatically. The early qualitative versions of Solomon and Klemperer and Herbst and Klemperer,2 which first predicted the preeminence of ion-molecular reactions in these cold, diffuse environments, are now supplanted by the modern sophisti- cated models of Mitchell, Ginzberg and Kuntz,3 Prasad and Huntress,4 Watson,5 and others. Quantitative predictions of molecular abundances are now given for an impressive number of species, including some two dozen of the most important molecular ions. In Table 1, the molecular ions expected to be most abundant in cold (50 0 K), dense cm-3) clouds are listed, along with abundances (relative to Hi) predicted by Ginzberg, and Kuntz,3 appropriate for a cloud density of 10 cm- 3 . Generally, the stable, closed-shell "proton adducts" are the most abundant ions in cold, dense clouds, because open-shell species general- ly react rapidly with H2 until these thermodynamically favored products are finally obtained. Actually, the synthesis of these ions is most likely to proceed from the direct proton transfer reaction of the principal reactant ion, H3+' with abundant stable molecules (e.g., CO, H20, CO 2 , NH 3 , etc.) to yield the respective proton adducts. Until the present, only three such protonated species have been definitively identified in interstellar clouds--HOC+, HNN+ and HCS+. In addition, CH+ (the unstable isomer of HCO+)6 and HC0 2 + 7 have been tentatively identified. This situation is simply the result of a general lack of high-resolution laboratory spectroscopic information on polyatomic ions. While approximately 45 diatomic ions have now been studied at high resolution in the laboratory,8,9 only 15 polyatomic species lO have thus far been detected by spectroscopic techniques capable of resolving their rotational energy level structures. Definitive identification of molecular transitions observed by microwave, millimeter; or infrared astronomical techniques clearly requires comparison with such high- precision laboratory data for the general cases of nonlinear molecules, even though the linear molecules HCO+, HNN+, and HCS+ were, in fact, originally identified quite reliably from millimeter observations alone. 8 ,9 403 G. H. F. Diercksen et al. (eds.), Molecular Astrophysics, 403-419. © 1985 by D. Reidel Publishing Company.