Isotopologue dependence of the CO2-air broadening and shifting coefficients: Experimental evidence and comparison with theory for 13CO2 and 12CO2

Abstract

Fifteen transitions of the 20012-00001 band of 13CO2 in air have been recorded at 296 K using a cavity ring-down spectrometer linked to an optical frequency comb referenced to a GPS-disciplined Rb oscillator. In parallel, measurements of 12CO2 in air were made with a Fourier transform spectrometer for transitions belonging to the 20012-00001 band, enabling comparison of the air-induced half-width and line shift coefficients of both isotopologues. For that a multi-spectrum fit procedure is adopted with a Hartmann-Tran (HT) line profile. A decrease of about 0.4% is demonstrated for the 13CO2 air-broadening coefficients compared to the corresponding value of the main isotopologue. To the best of our knowledge, this observation is the first experimental evidence of an isotopic effect on the air-broadening coefficients in CO2 (which are generally assumed to be identical in spectroscopic databases). On the theoretical side, a refinement to the simple model described in [Lamouroux et al. JQSRT 111;2010:2321] is proposed which enables the determination of γ(13CO2) from γ(12CO2). In addition, complex Robert-Bonamy-Ma (CRBM) calculations have been performed for 12CO2 and 13CO2 in collision with N2 or O2 for the 20012-00001 band with J” values from 0 to 85. From these CRBM data, the 13CO2-air and 12CO2-air halfwidth and line shift coefficients were computed and the obtained isotopologue effect on these parameters was compared to the experimental data. A good average agreement is achieved for the CRBM calculations for the airbroadening coefficients while the simple model leads to slightly smaller isotopic effect. In contrast, for the airpressure shift coefficients, a quite large disagreement is observed between the calculations and the experimental data.