Fundamentals

The weakest form of adsorption to a solid surface is called physical adsorption, or physisorption. It is characterized by the lack of a true chemical bond between adsorbate and substrate. If this is true, some other attractive force must exist that binds a gas phase species to a solid. One possibility that suggests itself is the ubiquitous van der Waals interaction. To see its origin, consider a closed shell atom that sits a distance, z, above a solid surface. We restrict our attention to distances z ≪ c/ωp (~ 1000 Å) so that the finite propagation velocity of light can be ignored. Even at these distances, a mutual attraction between the atom and the surface exists that arises from the interaction of the polarizable solid with dipolar quantum mechanical fluctuations of the atomic charge distribution. Put another way, the atomic electrons are attracted to their images in the solid.

A one-dimensional harmonic oscillator model of the hydrogen atom is sufficient to capture the essential physics of the van der Waals, or dispersion, force between an atom and a solid. Let the oscillator coordinate, r, represent the projection of the electron's orbital motion along the normal to the surface. Consider first the image system appropriate to a perfectly conducting substrate (Fig. 8.1).