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

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 same 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 values 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 independent of the isotopologue in spectroscopic databases). On the theoretical side, a refinement of the simple model described in [Lamouroux et al. JQSRT 111;2010:2321] is proposed which enables the determination of the air-broadening coefficients for the ¹³CO₂ isotopologue from the ¹²CO₂ values. 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 half-width 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 air-broadening coefficients while the simple model leads to slightly smaller isotopic effects. In contrast, for the air-pressure shift coefficients, a quite large disagreement is observed between the calculations and the experimental data.