Shape of physical adsorption potentials

The possibility is considered that all physical adsorption potentials have the same shape. Evidence is collected from experiments (atomic scattering, low-energy electron diffraction, thermodynamics) and theory. New calculations are performed for He and Ar interacting with an Ar solid. The assembled information indicates that the shapes are, indeed, very similar. The length scale is found to be l=(C3D)13, where D is the well depth and C3 is the coefficient of the dispersion force. It appears that alkali halide surfaces provide a softer reduced repulsion than other systems; this is tentatively interpreted in terms of an induced dipole attraction. Comparison is made with other universal potential functions and with a simple model based on overlap with substrate charge density.