Collision‐induced scattering in CO2gas

Experimental Polarizability Transition Moments of CO2 for Excited Vibrational States

The Raman spectrum of CO2 from room temperature to 1800 K has been measured in a series of experiments. The differential Raman scattering cross-sections for the fundamental bands at 1285.41 cm-1 and 1388.18 cm-1 have been obtained from reference bands of H2 and N2 as intensity standards. The Raman cross-sections of CO2 hot bands, involving vibrational energy levels up to 5000 cm-1, were derived from those of the fundamental bands. The Raman cross-sections obtained this way were reduced to transition moments of the mean molecular polarizability, which make it possible to simulate the Raman spectrum of CO2 up to 2000 K. This paves the way for local or remote diagnostics of CO2 in hot environments using Raman based techniques.

Molecular dynamics simulations for CO2 spectra. III. Permanent and collision-induced tensors contributions to light absorption and scattering

Classical molecular dynamics simulations have been performed for gaseous CO(2) starting from an accurate anisotropic intermolecular potential. Through calculations of the evolutions of the positions and orientations of a large number of molecules, the time evolutions of the permanent and collision-induced electric dipole vector and polarizability tensor are obtained. These are computed from knowledge of static molecular parameters taking only the leading induction terms into account. The Laplace transforms of the auto-correlation functions of these tensors then directly yield the light absorption and scattering spectra. These predictions are, to our knowledge, the first in which the contributions of permanent and collision-induced tensors are simultaneously taken into account for gaseous CO(2), without any adjusted parameter. Comparisons between computations and measurements are made for absorption in the region of the ν(3) infrared band and for depolarized Rayleigh scattering in the roto-translational band. They demonstrate the quality of the model over spectral ranges from the band center to the far wings where the spectrum varies by several orders of magnitude. The contributions of the permanent and interaction-induced (dipole and polarizability) tensors are analyzed for the first time, through the purely permanent (allowed), purely induced, and cross permanent∕induced components of the spectra. It is shown that, while the purely induced contribution is negligible when compared to the collision-broadened allowed component, the cross term due to interferences between permanent and induced tensors significantly participates to the wings of the bands. This successfully clarifies the long lasting, confusing situation for the mechanisms governing the wings of the CO(2) spectra considered in this work.