Relaxation of H2 vibrational motion by wall collisions

The method of computer molecular dynamics was applied to an important technological problem where experimental data may be extremely difficult to obtain: collisional de-excitation at surfaces of vibrationally highly excited hydrogen molecules. The calculations have been carried out primarily for vibrational states about the v = 6 level. The technique is illustrated by means of a specific example and the detail that can be obtained from such calculations concerning the dynamics of the interaction is shown. Molecular states excited to the v = 4, v = 6, and v = 11 vibrational levels and the J = 1 rotational level have been examined. The translational temperature of the molecules is chosen to be 5000K, and the wall is given thermal motion characteristic of a temperature of 5000K. Different angles of approach have been studied, and different impact points on the surface have been selected. After the first collision, the impact point becomes random because of the stochastic nature of subsequent molecular trajectories. For any given collision with the wall, the molecule may pick up or lose vibrational, rotational, and translational energy. Careful sampling and averaging of the results of many trajectory runs is needed in order to arrive at meaningful conclusions about the probability of de-excitation as a function of such parameters as the number of wall collisions, the residence time, the wall temperature, and the microscopic nature of the wall itself