Temperature-programmed desorption of positive ions and neutral molecules from alkali halide layers deposited on a metal surface

Abstract

To elucidate the process of thermal ionic and neutral desorption from an ionic crystal surface, a thin film (¯θ≈ 10 2–10 3molecular layers) of alkali halide (e.g. NaCl) deposited on a platinum plate was heated to a high temperature ( T) at a constant rate (β ≈ 2–65K s −1), and the desorption rates of both positive ions and neutral molecules were simultaneously measured as functions of T and ¯θusing a dual-ion source system developed in our laboratory. Analysis of the relation between β and the peak appearance temperature ( T p ≈ 700–1300K) of each desorbing particle shows that the activation energies for desorbing Na + and NaCl are3.0 ± 0.2and2.0 ± 0.1eV, respectively, while the respective frequency factors are∼ 4 × 10 11and∼ 5 × 10 10s −1. Desorption parameters evaluated at T psuggest that NaCl and Na + are desorbed only from upper layers (more than¯/θ2) and mainly from high work function sites on Pt at¯θ< 1layer, respectively. The data obtained similarly with other alkali halides also indicate that the desorption process of each particle follows the first-order kinetics with regard to each surface concentration.