Abstract

If desorbed particles have a sufficiently low number density they disperse collisionlessly. This means that if, for example, the mechanism is thermally activated, a time-of-flight spectrum yields the surface temperature through the relation kT = Ê/2, Ê being the most probable kinetic energy, and the signal is proportional to cosθ. If, on the other hand, the number density is high enough for a few (estimates range from 3 to 20) collisions to occur, a so-called Knudsen layer is formed. We first show how this problem can be solved analytically and then explore its consequences to thermally activated desorption. In particular, when there is a Knudsen layer, (1) 18-24% of the desorbed particles are found to recondense, (2) the temperature falls to 70-90% of the value at the surface, but this is more than compensated by a center-of-mass velocity, (3) the relation between kT and Ê changes significantly, and (4) the angular distribution becomes strongly forward peaked (~cos³θ). We finally consider the effect of a Knudsen layer on an electronic desorption process, especially on the energy spectrum and on the angular distribution.

© 1987 Optical Society of America