FIELD IONIZATION FROM HELIUM FILMS

Field ionization from He films has been investigated from 4.2 to 1.6°K on clean and oxygen covered W emitters. At low fields (1.8-2.2 V/Å) electron tunneling from He controls current generation; reasonable values of the He ionization potential are obtained by fitting the i-F data to WKB tunneling or simple perturbation-theory models. At higher fields current becomes supply limited, and it is concluded that the latter occurs by diffusion of He from the shank over distances whose lower limits can be estimated to be ∼10 -3 cm. The variation of supply-limited current with pressure is interpreted in terms of such a diffusion model. It is concluded that adsorption of He is stronger on oxygen covered than on clean W.