TEMPERATURE-PROGRAMMED DESORPTION OF POSITIVE-IONS AND NEUTRAL MOLECULES FROM ALKALI-HALIDE LAYERS DEPOSITED ON A METAL-SURFACE

To elucidate the process of thermal ionic and neutral desorption from an ionic crystal surface, a thin film (theta approximate to 10(2)-10(3 ) molecular layers) of alkali halide (e.g. NaCl) deposited on a platin um plate was heated to a high temperature (T) at a constant rate (beta approximate to 2-65 K s(-1)), and the desorption rates of both positi ve ions and neutral molecules were simultaneously measured as function s of T and a using a dual-ion source system developed in our laborator y. Analysis of the relation between beta and the peak appearance tempe rature (T-p approximate to 700-1300 K) of each desorbing particle show s that the activation energies for desorbing Na+ and NaCl are 3.0 +/- 0.2 and 2.0 +/- 0.1 eV, respectively, while the respective frequency f actors are similar to 4 x 10(11) and similar to 5 x 10(10) s(-1). Deso rption parameters evaluated at T-p suggest that NaCl and Na+ are desor bed only from upper layers (more than theta/2) and mainly from high wo rk function sites on Pt at theta < 1 layer, respectively. The data obt ained similarly with other alkali halides also indicate that the desor ption process of each particle follows the first-order kinetics with r egard to each surface concentration.