Cesium ion desorption from oxygen and carbon adlayers on platinum surfaces with nanosecond time resolution: Variation of desorption parameters with time available for surface diffusion and degree of surface heterogeneity

Abstract

Measurements of ionic cesium desorption from surfaces at temperatures above 1100 K have been made with the so-called field reversal method. Time constants down to 30 ns have been measured with very low surface densities of Cs. The prexponential factors for desorption from clean platinum and graphite covered platinum surfaces are close to 10 12 s −1, in agreement with simple desorption theory. However, for slightly oxygen covered surfaces and for carbon layers formed at low temperatures the preexponential and the desorption energy become significantly larger, while a strict Arrhenius form is retained. This is proposed to be due to the influence of surface inhomogeneity and surface diffusion, in line with the theoretical description of simultaneous diffusion and desorption recently given by Nordholm. In the oxygen case, the adsorbed oxygen is considered to be the origin of the heterogeneity, while in the carbon layer case the surface, which has been formed at low temperature, is disordered and may contain many strongly binding sites. The influence of bulk diffusion on the desorption is also described.