Rotational energy relaxation time of vibrationally excited molecules

Abstract

The influence of the molecular vibrational state on the rotational energy relaxation time is studied in the state-to-state approach. The rotational levels of the molecules are described by the nonrigid rotator model; their interaction is described by the variable soft sphere model. According to this model, collision cross sections for N2-N, O2 -O, NO-O interactions are calculated for different vibrational and rotational levels of molecules. On the basisof the kinetic theory of nonequilibrium processes, the rotational relaxation time is introduced for each vibrational level. Relaxation times are calculated numerically in the wide temperature range, and a comparison with the relaxation time obtained by the well-known Parker formula is carried out. The influence of various multiquantum rotational transitions on the accuracy of the calculation of the rotational relaxation time is analyzed; the convergence of the solution is shown with increasing the maximum possible number of quanta transferred during the transition. It is shown that the vibrational state of the molecule significantly affects the rotational energy relaxation time in the state-to-state approach; when calculating the state-specific transport coefficients, the use of the Parker formula can lead to a noticeable error. In the one-temperature approximation for moderate temperatures, the Parker formula provides a satisfactory agreement with the results obtained by averaging the state-resolved relaxation times with the Boltzmann distribution over vibrational energy.