Calculation of vibrational relaxation times in carbon dioxide using forced harmonic oscillator model

The study is devoted to the evaluation of vibrational relaxation times in carbon dioxide using the kinetic theory methods. There are several relaxation channels in CO2, such as vibrational-translational energy transitions within modes and vibrational-vibrational exchanges between different modes. The theoretical expressions for the relaxation times of such processes are derived; they depend on the energy variation in a specific collision and cross sections of corresponding transitions. The first-order perturbation theory as well as two modifications of the forced harmonic oscillator (FHO) model are implemented for the transition probabilities. Relaxation times are evaluated in the temperature range 500-10000 K; it is shown that they are strongly affected by the cross section model. It is found that the FHO model provides good agreement with experimental data at low and moderate temperatures; at high temperatures, both models give non-monotonic trends for the relaxation times. The reasons of such behaviour are analysed.