Decomposing the First Absorption Band of OCS Using Photofragment Excitation Spectroscopy

Photofragment excitation spectra of carbonyl sulfide (OCS) have been recorded from 212-260 nm by state-selectively probing either electronically excited S((1)D) or ground state S((3)P) photolysis products via 2 + 1 resonance-enhanced multiphoton ionization. Probing the major S((1)D) product results in a broad, unstructured action spectrum that reproduces the overall shape of the first absorption band. In contrast, spectra obtained probing S((3)P) products display prominent resonances superimposed on a broad continuum; the resonances correspond to the diffuse vibrational structure observed in the conventional absorption spectrum. The vibrational structure is assigned to four progressions, each dominated by the C-S stretch, ν1, following direct excitation to quasi-bound singlet and triplet states. The S((3)PJ) products are formed with a near-statistical population distribution over the J = 2, 1, and 0 spin-orbit levels across the wavelength range investigated. Although a minor contributor to the S atom yield near the peak of the absorption cross section, the relative yield of S((3)P) increases significantly at longer wavelengths. The experimental measurements validate recent theoretical work characterizing the electronic states responsible for the first absorption band by Schmidt and co-workers.