Abstract
OBSERVATIONS of photo-electric ‘activation’ (lowering of the photo-electric threshold energy) through the interaction of small quantities of oxygen with various metals are recorded in the literature1. Such activation, for which no satisfactory theory has been advanced, is obtained without heat treatment and most of the metals concerned possess a low work function (below about 3 eV.). Large admissions of oxygen always cause the reverse effect: the threshold is shifted to shorter wave-lengths. The phenomenon of ‘thermal regeneration’, that is the removal of oxygen from the surface of certain metals and semiconductors by heating in vacuo, is also well established. The oxygen which disappears from the surface in these cases is not desorbed; it is either incorporated by regions below the surface2,3 or it is effectively aggregated into islands on the surface as the result of recrystallization processes to leave bare areas4. We have observed photoelectric activation in nickel films (work function about 5.1 eV.) after interaction with oxygen followed by thermal regeneration of the surface. There appears, therefore, to be a close parallel between thermal regeneration and photo-electric activation. Further, we have found that regeneration of a nickel surface to which oxygen has been admitted at 20° C. occurs spontaneously on standing in vacuo at that temperature.
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