ANISOTROPIC TRANSLATIONAL ENERGY-DISTRIBUTION DUE TO GAS-PHASE COLLISIONS IN RAPID DESORPTION OF MOLECULES FROM SURFACES
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The velocity distribution of NO molecules rapidly desorbing from an LiF(100) surface, in the presence of gas-phase collisions, is calculated using a direct Monte Carlo simulation procedure. The gas-phase collisions are found to transform the initial Maxwell-Boltzmann like distribution into an ellipsoidal Boltzmann distribution. In this respect the rapid desorption process is found to be similar to the supersonic expansion process in which case the gas-phase collisions also convert the random thermal motion of the gas molecules and the rotational energy into directed motion along the beam axis. Even though the total velocity distributions show non-equilibrium behavior, the velocity distribution of the molecules desorbing near the normal direction in the few gas-phase collisions limit is found to be reasonably well represented by a Maxwell-Boltzmann distribution if the temperature is allowed to vary as a function of desorption angle. However, the ellipsoidal Boltzmann distribution function characterized by two temperatures is found to describe the velocity distributions of the molecules desorbing into specific desorption angles as well as the total velocity distribution of the molecules desorbing in all the directions very well. The translational energy corresponding to the peak of the time-of-flight distribution calculated using an ellipsoidal distribution is found to be greater than the value predicted by the Knudsen layer model. The probable reasons for this discrepancy are analyzed. We find that under the conditions at which postdesorption collisions are important, the mean translational energy of the desorbed molecules need not necessarily be directly related to the surface temperature. The implications of these results for the existing mechanisms for rapid desorption are discussed. 1988.
