Separation of O, C, and S isotopes by two-step, laser photodissociation of OCS

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Abstract
Isotopes of O, C, and S have been separated by two-step, laser photodissociation of OCS. The technique utilizes isotopically selective vibrational excitation of OCS in the ν2 (bending) vibrational mode with a line-tuned CO2 laser (λ∼9.4 μm), followed by photodissociation with a KrF excimer laser (λ=249 nm) and chemical scavenging of the sulfur atoms. Enrichment factors ranging from 1.5 to 3.5 were obtained for different isotopes. A sensitive IR absorption apparatus was used to measure absorption of the high energy CO2 laser pulse by OCS as a function of pressure and laser fluence. Average absorptions ranging up to 1.5 photons/molecule were attributed to rapid rotational relaxation and in part to sequential absorption up the ladder of ν2 vibrational levels. The 249 nm photodissociation cross section of OCS was measured as a function of absorbed IR energy. An excitation of one CO2 laser photon per molecule, or two quanta of ν2 vibration per molecule, increased the photodissociation cross section by a factor of 9 over the thermal (295 K) cross section. A rate constant of (4.8±1.0)×105 s−1 Torr−1 was inferred for the exchange of ν2 vibrational energy between isotopic varieties of OCS.