The random walk of individual F centers, which causes F center diffusion in alkali halides after electron irradiation, was simulated in three dimensions with a Monte Carlo program. The F center random walk was considered to be thermally driven along the halogen sublattice. The resulting diffusion process supplies F centers to the surface. For cold (about room temperature) surfaces this causes alkali metal surface enrichment. The surface enrichment time dependence was predicted by the three-dimensional Monte Carlo simulation and compared with experimental data. Three different initial distributions for the F centers were considered. These distributions were chosen to coincide with the depth profile for: (i) the energy deposited by the irradiating electrons; (ii) the density of hot holes after their migration; and (iii) a Gaussian distribution located well below the surface. Although all three simulations with these initial distributions compare reasonably well with the experimental data, the third provided the best agreement with the data.

F Center Random Walk Diffusion after Electron Irradiation of Alkali Halides. C.A.Ordonez, D.P.Russell, M.Portillo: Surface Science, 1993, 290[1-2], 113-8