On the theory of helium field adsorption

Density functional theory has been used to model the field adsorption of helium on tungsten, This paper reports the main results, setting them in the context of a high-level review of the theory of rare-gas field adsorption. In a uniform external field the calculated covalent binding energies are not high enough to explain experimental desorption activation energies. However, local field enhancement above kink sites means both that the covalent contribution is higher, and that there is a polarisation contribution. Together these effects can account for observed activation energies. An approximate formula is established for the total short-range field-adsorption binding energy, as a function of external field and the local field-enhancement factor. It is confirmed that field adsorption is field-induced chemisorption, and shown that the mechanism of local field adsorption is a field-induced chemical bonding in which both covalent and polarisation effects play a significant part. This work represents a successful integration of past ideas, resolves the controversies of recent years, and advances the theory to a new phase.