Molecular collision cross sections and the effect of hydrogen on vibrational relaxation in carbon dioxide

Abstract

A numerical method previously employed for the calculation of vibrational excitation cross sections and vibrational relaxation in carbon dioxide has been applied to the case of vibrational excitation of carbon dioxide by collision with molecular hydrogen. Cross sections have been calculated for collision energies up to 2.5 eV and coupling between five molecular states, distributed between the three vibrational modes, has been retained. For the inter-molecular potential the Lennard-Jones parameters tabulated by Hirschfelder et al. have been used. The dependence of vibrational relaxation in carbon dioxide on hydrogen concentration has been determined in terms of these cross sections for temperatures from 300 to 600 °K. The results are in qualitative agreement with the sound absorption measurements of Winter and reproduce the experimental observation that hydrogen appears to be less efficient at 400 °K at inducing vibrational transitions in carbon dioxide than at 300 °K. It is concluded that such anomalous behaviour can be accounted for when proper allowance is made for the coupling between the molecular vibrational states.