VANDERWAALS INTERACTION BETWEEN A MOLECULE AND A SOLID-SURFACE

A general expression for the Van der Waals interaction energy between a molecule and a solid surface is derived, which depends on the orientation of the molecule with respect to the surface, and which is valid for all molecule-surface separation z, provided that the retardation effect can be neglected. Two special cases are discussed: (1) molecules with axial symmetry, and (2) molecules with a permanent electric dipole moment. In all cases the leading term of the asymptotic expansion of the energy is the z-3 term, same as the atom-surface interaction case. The Van der Waals energy favors the head on position of molecular adsorption on a solid surface with either the axis of symmetry or the direction of the permanent dipole moment perpendicular to the surface. The water molecule H2O is taken as an example to calculate the permanent dipole moment contribution to the Van der Waals energy, which shows that it has about the same order of magnitude of the contribution from the induced dipole moment for z to be around a few Angstroms.