Adsorption of Barium onto Oriented Tungsten Surfaces

Abstract

The Kelvin and the retarding-field contact potential difference techniques have been used to follow the work-function changes during the adsorption of barium onto both polycrystalline and single-crystal tungsten surfaces. A double-evaporation technique was devised in order to obtain conditions under which the barium could be evaporated without the pressure exceeding, at best, 3 × 10−10 torr. Contrary to the results of a previous similar investigation which used a thermionic method, the work functions of barium covered (110) and (100) tungsten surfaces differed considerably. For films, estimated to be greater than five monolayers in thickness, the final work functions on the (110) and (100) surfaces, respectively, were 2.955 ± 0.005 eV and 2.560 ± 0.005 eV. A model is suggested in which the barium on the (100) surface forms a polycrystalline deposit, while that on the (110) surface compares closely with a single-crystal (110) plane in barium metal but with a high density of defects. The shape of the work function vs coverage curves have been compared with those predicted by a recent semiempirical theory. Unfortunately, for a worthwhile comparison an accurate knowledge is required of the polarizability of the barium atom, the adsorbate density, and the monolayer position, none of which were determined in the present work. By allocating reasonable values to the polarizability and the monolayer position a fit between the experimentally and theoretically determined curves to within ±0.04 eV could be obtained on the (110) surface using the adsorbate density predicted by the proposed model. With no restriction on the adsorbate density for the (100) plane the fit could be improved to ±0.02 eV. Removal of this restriction for the (110) plane also improved the agreement between theory and experiment to within ±0.02 eV.