Surface Lifetimes of Alkali Metals on Molybdenum

Abstract

The surface lifetimes (τ¯) of all the alkali metals on a polycrystalline molybdenum surface in the temperature region 900–1350°K were measured. The temperature dependence followed an Arrhenius expression of the form τ¯° exp(l¯/kT). Desorption energies (l¯) were found to be 3.14, 2.60, 2.53, 2.31, and 2.10 eV for Li, Na, K, Rb, and Cs. The corresponding pre-exponential factors (τ¯°) were 3× 10−16, 3×10−15, 2× 10−14, 1×10−13 and 3×10−13sec. From these data, ion and neutral desorption energies were calculated using a model employing a partial surface charge for the adsorbed particle and the Schottky relation. The validity of the Schottky relation for an adsorbed alkali was supported by analysis of recently available theory of partial charges. An explanation of the variation of the pre-exponential factors is presented on the basis of relative mobility of the adsorbed species. A pre-exponential factor was calculated using a partition function for an adsorbate with mixied localized and nonlocalized character. The extremes of completely localized and completely nonlocalized adsorption led to pre-exponentials that spanned the experimental values.