Experimental and theoretical studies of CO2 spectra for planetary atmosphere modelling: region 600-9650 cm-1 and pressures up to 60 atm
Résumé
Extensive experimental studies of room-temperature carbon dioxide absorption coefficients are reported for a wide range of wavenumbers and pressures requested in atmospheric spectra modelling. The quality of measurements is optimised by the use of two complementary setups with long- and short-path optical cells for low and high gas densities, respectively. The recorded spectra provide a representative picture of band-shape evolutions from gaseous to nearly liquid phases of CO2 and enable a theoretical analysis of line-mixing effects. Various kinds of vibrational bands (Σ ← Σ, Π ← Σ as well as Π ← Π transitions) are modelled using a specific, non-Markovian in the general case, approach of Energy-Corrected Sudden type which is based on the symmetric relaxation matrix and, in contrast to the standard ECS model used for infrared absorption calculations, ensures automatically the fundamental relations of detailed balance and double-sided sum rules. Moreover, this method properly accounts for the vibrational angular momenta of the initial and final molecular states and allows including Coriolis resonances via the usual Herman-Wallis factors in the dipole transition moments. With a set of ECS parameters previously obtained for isotropic and anisotropic Raman spectra modelling, completely neglected imaginary part of the relaxation operator and a simple change in the tensorial rank to get the dipole absorption case in the working formulae, the computed spectra reproduce quite correctly the vibrotational band shapes up to 20 amagat without any additional parameter. An empirical correction factor tentatively introduced to account globally for the Coriolis effects on the relaxation matrix leads to better matches with high-density band shapes but its role merits further studies with an accurately modelled imaginary part of the relaxation matrix.