Atomic processes induced by electronic excitation in insulating crystals.

We have studied two examples of atomic processes induced by electronic excitation in insulating crystals. One is the bistable defect system in $\rm CdF\sb2{:}M\sp{3+},$ the other is the athermal halogen atom desorption. The bistable (shallow-deep) defect systems associated with trivalent impurities (In, Ga, Y and Sc) in CdF$\sb2$ are examined. The equilibrium lattice relaxation around the defect and the wavefunction of the electron bound to the impurities are determined by minimizing the energy of the defects. The impurity-fluorine interatomic potentials determined using the electron-gas model of Gordon and Kim are used, and the defect electron is treated by the extended-ion method. In order to compare the deep and shallow states using the same discrete lattice model, a very large cluster of atoms is treated. Two groups of trivalent impurity centers are found. With In and Ga, there is a low, but clearly identified, potential barrier which separates the deep level from the shallow one. In Sc and Y, only a simple shallow level state is obtained. The analysis of the results shows that the difference is to be attributed to the short-range potential of the trivalent impurity centers. On the basis of present work, we predict that Tl$\sp{3+}$ would exhibit similar bistable behaviour. Energetic halogen atom desorption observed from certain alkali halide crystal under electron or photon stimulation is being investigated. The relaxed structure of the localized one-center (Frenkel-type free exciton) and two-center (self-trapped exciton: scSTE) electronic excitations are studied. It is shown that the triplet scSTE state undergoes an instability on and near the surface similar to that in the bulk. For the first and second layers' scSTE below the (100) surface, the excited electron localizes with preference closer to the surface. Further below, there is no preference. Only when the off-center relaxation of the scSTE propels $\rm Br\sbsp{2}{-}$ toward the surface can there be a desorption. The third layer scSTE in KBr leads to desorption with a possible kinetic energy of about 1 eV, the trajectory undergoes deviation as the ejected Br$\sp0$ clears the surface. In NaBr, the scAPES of third layer's scSTE encounters a barrier as in the bulk, no energetic desorption is expected in NaBr as the experiment shows. The presence of a In$\sp+$ adjacent to a scSTE in the third layer, on the side closer to the surface, results in the excited electron localizing on the Br site nearer to the surface and on In$\sp+,$ pushing the V$\sb{k}$ center deeper into the bulk. We believe that this causes the observed inhibition of Br$\sp0$ desorption in In-doped KBr. In CaF$\sb2,$ though the excited electron localizing on the surface for the first layer's scSTE below the (111) surface, the F atom desorption is still expected due to the different lattice structure with the ejecta's trajectory undergoing correction to approach the normal of the surface. In all the examples studied here, the large atomic displacement is induced by the excited electron