Deviations from two-dimensionality in classical adsorbed films

The statistical mechanical theory of adsorption is presented as an expansion about the two-dimensional approximation. Two corrections to the latter are additive in the perturbation limit. One contribution, arising from the lateral variation of the substrate potential, has been previously evaluated. The second correction term is due to the particles' mutual interaction when their separation vector is not parallel to the surface. The latter is expressed in terms of the radial distribution function. Its contribution to the deviation from two-dimensionality is shown to be comparable to the first term for the case of a Lennard-Jones model.