Isotopologue dependence of γ and δ for the CO2-air Collision System: 13CO2 vs 12CO2

Abstract

Fourteen transitions of the 20012-00001 band of 13CO2 in air have been recorded at room temperature using a cavity ring-down spectrometer linked to an optical frequency comb referenced to a GPS-disciplined Rb oscillator. The comb-coherence transfer (CCT) technique was applied to obtain a RF tunable narrow-line laser comb-disciplined source using an external cavity diode laser and a simple electro-optic modulator. For each transition a multi-spectrum fit was used with a speed dependent Nelkin-Ghatak line profile to analyze the high S/N (several thousands) spectra recorded at five pressures from 50 to 750 Torr. The air-broadened half-width and air-induced line shift were retrieved for each transition studied. Measurements of 12CO2 in air at total pressures from 75 to 760 Torr were made on the FT spectrometer at DLR, Germany, for transitions in the 30012-00001 and 20012-00001 bands, allowing comparison with the 13CO2 data. A refinement to the simple model proposed in the Appendix of Lamouroux et al.1 was made by assuming that γ is proportional to v rel(T), the thermally averaged relative speed which is proportional to T1/2 and (1/µ) 1/2 where µ is the reduced mass. From the power law temperature dependence n, one can show that γ is proportional to v rel(T)2−2n and thus to (1/µ) 1−n. This approximate but very simple expression allows the determination of γ(13CO2) from γ(12CO2). Complex Robert-Bonamy-Ma calculations were made for 12CO2 and 13CO2 in collision with N2 and O2 for J ” from 0 to 85 for the 20012-00001 band. The intermolecular potential was optimized to the data of Devi et al.2 for the 30012-00001 band of 12CO2. From these data the 13CO2-air and 12CO2-air half-widths and line shifts can be computed and the isotopologue effect studied. The agreement between the measurements, model, and calculations will be presented. 1 J. Lamouroux et al., JQSRT, textbf111, 2321 (2010). https://doi.org/10.1016/j.jqsrt.2010.03.006 2 V. M. Devi, et al., JQSRT 111, 2355 (2010). doi:10.1016/j.jqsrt.2010.06.003