Role of dipole image forces in molecular adsorption

Abstract

Electrostatic image force energy W was calculated for a finite-size (extended) dipole located in vacuum near a plane surface of a condensed-matter substrate using the HF molecule over graphite as an example. The spatial dispersion of substrate static dielectric permittivity was taken into account, with the contributions of both free and bound charge carriers being considered in various realistic models. The dependences of W on the distance z between the dipole and the substrate were shown to be finite at all z’s, contrary to the classical point-dipole case. An existence of a crossover between the preferable normal and planar orientations of extended dipole with respect to the surface was found. The applicability of point-dipole approximation for the calculation of W(z) was discussed. Numerical quantum chemical calculations were carried out for the HF molecule near two graphite layers. The results obtained confirm the validity of non-local electrostatic approach beyond the region of Pauli repulsion (in the closest vicinity to the interface). On the other hand, at large distances z, the quantum chemical consideration becomes less reliable owing to the computational restrictions, whereas the electrostatics preserves its capabilities and demonstrates, in particular, the subtle orientation crossover.