It was recalled that the shallow electrons which were associated with certain impurities or interstitial Ag were believed to play key roles in some photo-induced processes. Theoretical calculations were here performed using a method which had been developed for treating excited electron centers in insulators. In the present crystals, the Frenkel-pair energies were found to vary monotonically as a function of the vacancy-interstitial separation; which reflected the energetic tendency for interstitial diffusion towards the vacancy. In order to deduce the very small activation energy for interstitial Ag diffusion, a simple model was used for quadrupolar deformation of the Ag ion. It was found that the barrier to diffusion from the second cell to the vacancy was substantial (0.4eV) when compared with the much smaller value far from the vacancy (0.07eV). The calculations showed that the electron could be bound to the Ag vacancy in a diffuse orbital when an interstitial Ag ion was present nearby. The diffuse electron was found to reduce further the activation energy at a moderate distance from the vacancy. Moreover, the large barrier in the second cell from the vacancy was significantly reduced by the diffuse electron.

Role of the Excited Electron in the Diffusion of Interstitials in AgCl and AgBr. C.Fu, K.S.Song: Physical Review B, 1999, 59[4], 2529-36