IR SPECTROSCOPY OF GAS-PHASE IONS BY ONE-PHOTON IR PHOTODISSOCIATION IN A FAST ION BEAM

Abstract

Photodissociation of vibrationally excited polyatomic ions, ranging in size from four to thirteen atoms, has been observed following absorption of single $CO_{2}$-laser photons. In a number of cases the infrared wavelength dependence of the process shows well defined peaking, which is interpreted as reflecting normal-mode frequencies of the ion. Kinetic energy release and fragmentation rate information was obtained for both photon-induced and metastable decomposition of a number of the ions. RRKM theory modeling indicates that these data are compatible with a quasiequilibrium-theory description of the unimolecular decomposition for all of the larger ions considered, but the energy release in the $CF_{3}X^+ (X=1, Br, Cl)$ cases is unaccountably low for such a picture. Considered as equivalent to the last photon of a multiphoton dissociation process, these results are relevant to understanding infrared spectroscopy and multiphoton photochemistry of polyatomic molecules.