MODIFIED QUASI-CHEMICAL APPROXIMATION IN THE LATTICE-GAS THEORY ALLOWING FOR THE INTERNAL PARTITION-FUNCTIONS OF ADATOM GROUPS IN AN ADSORBED LAYER

The quasi-chemical approximation of the lattice-gas theory is widely u sed for description of adsorption-desorption processes. In its present form, however, it is incapable of describing some experimental result s: wide ranges of variation of the pre-exponential factor in the desor ption-rate constant with changing coverage, the compensation effect, t he shift of thermal desorption spectra toward lower temperatures with increasing initial coverage for some systems with lateral attraction b etween adatoms, etc. In the present paper a modification of the quasi- chemical approximation is proposed, which, in the authors' opinion, si gnificantly increases its capabilities of describing experimental resu lts. This is achieved by including an entropy factor alpha(0), which r eflects a change of the internal partition functions of adsorbed parti cles by their interactions with nearest neighbors. This parameter is i nterpreted as the ratio of the internal partition function of a dimer to that of two isolated monomers on the surface. It always differs fro m unity. The introduction of the parameter alpha(0) in the lattice-gas theory leads to replacing the quantity eta = exp(-u/kT) by the quanti ty eta(1) = alpha(0) exp(-u/kT) in all formulae of the theory, their a nalytical form being unchanged. However, this involves cardinal change s of the coverage dependence of the Arrhenius pre-exponential factor i n the desorption rate constant, of the thermal desorption spectra beha vior with varying the initial coverage, as well as of the critical tem perature of the first-order phase transition.