DESORPTION FROM MULTILAYERS

Unimolecular desorption of a multilayer adsorbate on a solid surface is studied by the use of a multilayer lattice gas model with restricted atomic stacking. Mutual interactions between adatoms are limited to first nearest neighbors and treated in the quasi-chemical approximation with a local atomic configuration expansion. The desorption rate is derived by summing up desorption probabilities of individual atoms, which are assumed to be of a classical Arrhenius form and which depend on nearest-neighbor atomic configurations and atomic densities in the upper layers. When the lattice gas system is in the two-phase coexistence regime, the desorption is found to follow zero- or quasi-zero-order kinetics. It is also found that the system is well described by the bilayer model proposed by Asada and Masuda [Surf. Sci. 207 (1989) 517) at relatively low temperatures compared to the critical temperatures.