De-excitation and equipartition in H2-surface collisions

Abstract

We have used computer molecular dynamics to study de-excitation of vibrationally-excited H/sub 2/ molecules undergoing repeated collisions with a wall maintained at some temperature T(W). We have calculated the average loss or gain of vibrational, rotational, translational, and total molecular energy as a function of the number of collisions for a statistically significant number of molecules having some initial vibrational state v'' ranging from v'' = 2 to v'' = 12. For initial translational and rotational temperatures around 500/sup 0/K, we have obtained a consistent picture of rapid vibrational de-excitation during the first collision, with a corresponding increase in translational kinetic energy and with rotational excitation from an initial J = 1 state up through values greater than J = 14. Wall collisions are found to provide an effective and rapid kinematic mechanism for V-T, V-R, and R-T energy transfer. The rate of loss of total molecular energy to the wall (accommodation) is discussed and compared with the rate of energy redistribution (equipartition) among the vibrational, rotational, and translational degrees of freedom of the molecules.