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The desorption of Na+ under electron irradiation at low energies (

A Monte Carlo random walk model was used to study the creation and dynamics of F- and H-centers induced by electron irradiation in alkali-halide single crystals. The purpose of the study was to observe the effects of depth dependent electron energy deposition near the surface region using beam energies of 500 eV or less and relatively low currents (< 100 μA/cm2). The calculations were carried out using the parameters appropriate for an FCC LiF single crystal at room temperature. Depth dependent dynamics of F- and H-centers are also presented as well as F—H center recombination rates, and thermal and non-thermal halogen production rates at the surface. The model demonstrates that F—H center recombination dramatically affects the F-center distribution. F-center distributions are found to be peaked at the surface and do not reflect the initial energy deposition profile.

In initial observations during Auger studies of NaCl, we have observed stable, clearly resolved Auger peaks. With time, however, some of the Auger spectra reveal the occurrence of strong negative charging. This charging appears to be a result of irradiation-induced accumulation of sodium metal on the sample surface. We report the dependence of the strong negatively charged state on temperature. The results are consistent with the above model; the strongly negatively charged surface cannot be maintained at temperatures sufficiently high that sodium metal evaporation occurs.

We study temperature programmed desorption of sodium and chlorine from an electron irradiated sodium chloride (100) surface. In this experiment, the sodium chloride crystal is irradiated with 500 eV electrons. After the electron beam is turned off, the crystal is heated and desorption of the alkali atoms is observed. This delayed desorption of sodium is analyzed using fractional order desorption equations. The results clearly show that the desorption is not a first order process. The data are interpreted in terms of a thick layer of sodium forming on the surface.