Mean adsorption lifetimes of cesium chloride on nickel surfaces

Abstract

A pulsed-molecular-beam method was applied to the study of the mean adsorption lifetimes of adsorbed CsCl on single crystal and polycrystalline nickel surfaces. The experiments were conducted in an ultra-high vacuum system free of hydrocarbon contaminates and with pressures less than 2 x 10/sup -8/ Torr. A molecular beam of CsCl was formed from species effusing from a double-oven effusion cell equipped with a conical orifice. A rotating chopper served to pulse the molecular beam; the CsCl molecular beam was on for 10.7 percent of the period of the cycle. The molecular beam struck a nickel target inclined at a 45/sup 0/ angle. Four targets were used in these experiments: Ni(111) surface, Ni(100) surface, clean polycrystalline Ni surface, and a contaminated polycrystalline Ni surface. The mean adsorption lifetimes were determined by measuring the rate of evaporation of CsCl from the surface by means of a surface ionization detector. The desorption has been found to follow first-order kinetics in cases of single crystal targets for which equilibrium surface coverage of the adsorbate is much less than 0.001 monolayer. The activation enthalpy for the desorption process, ..delta..H*, and the preexponential, tau/sub 0/, are dependent on surface structure and surface cleanliness, but theymore » are not dependent on surface coverage. In an attempt to interpret the present values of tau/sub 0/, a mobile adsorption model must be used.« less