Electron-Stimulated Desorption of Neutral Ground-State Lithium Atoms from LiF and the Influence of Stored Defects

Abstract

We have investigated the ESD of neutral ground-state Li atoms from LiF single crystal <100> surfaces caused by 100eV electron irradiation at varied crystal temperatures between 300K and 800K. During irradiation desorption increases exponentially with temperature, limited only by the evaporation of the alkali metal layer on the surface. The obtained activation energy varies between 0.8eV and 1.6eV with current density and reflects the size of the metal clusters on the surface. Above 300°C a plateau is reached where the electron flux becomes the rate-limiting quantity. Above 450°C the evaporation of the crystal adds significantly to the desorption yield.

We have also studied the time-dependence of the desorption which continues after the end of irradiation. This delayed emission can be divided into two components by their different time constants: A fast decay of signal in less than 20ms, which is followed by a slow decay with a time constant of several seconds. The fast decay has been attributed to F-center diffusion. We propose to explain the slow decay in terms of aggregation of F-centers and subsequent disintegration of the aggregates.