High‐resolution Raman investigation of the vibrational relaxation of two‐phonon bound states in solid CO2 at different temperatures

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Abstract
The Raman spectra of the bound states Ω+ and Ω− of the Fermi diad ω1:2ω2 in crystalline CO2 have been measured at several temperatures under very high resolution. Band profiles have been determined for each of the two factor group components, one Ag and one Fg, of the two states.The evolution with temperature of the bandwidths has been interpreted in terms of four‐phonon decay processes. A single down‐process, involving a lattice phonon at 60 cm−1, is shown to be sufficient to reproduce the experimental data for the Ω+ state. In the same way, a single up‐process, involving the same lattice phonon, is sufficient to fit the experimental data for the Ω− state. The variation of the factor group splitting of the Ω− component as a function of temperature has also been determined.