Photodissociation of vibrationally excited OCS

A two-step photodissociation method has been used to measure the photodissociation cross section of vibrationally excited OCS(ν1) at λ=249 nm. The method employs an NH3 laser to pump OCS molecules to the ν1=1 vibrational state, followed by photodissociation by a KrF excimer laser and mass spectrometric analysis of the chemically scavenged photodissociation products. The experiments determine the photodissociation cross section ratio σ d(ν1)/σ d295 K =4.5±0.9, where σ d295 K is the 249 nm photodissociation cross section of thermal (295 K) OCS. An analysis of our previously reported data for the ν2=2 vibrational state of OCS gives the photodissociation cross section ratio σ d(2ν2)/σ d295 K =8.4±1.1. The measured cross section ratios are used in an analysis of temperature dependent photoabsorption spectra to obtain the photoabsorption cross section ratios σ a(0)/σ a295 K =0.41±0.04 and σ a(ν2)/σ a295 K =2.5±0.6 for the vibrationless and ν2=1 states of OCS, respectively. Isotopically selective, two-step photodissociation through the OCS(ν1) vibrational state was used to separate 33S or 34S isotopes with enrichment factors of β=2.2.