Mechanisms of defect formation

To displace atoms from their lattice sites by irradiation with low-energy electrons or photons requires an indirect process in which momentum is obtained from the lattice. Many models have been proposed and this review describes the general principles of the excitation stages and the subsequent separation of the vacancies and interstitials. In the case of the alkali halides the details of an exciton absorption and replacement collision sequence of halogens is experimentally and theoretically well understood. The displacement of silver from the silver halide lattice is also mentioned in terms of exciton absorption and a diffusion stage. The reasons for the lack of definitive models for other materials are discussed. Quantitative estimates of defects formed by more energetic electron or ion bombardment may underestimate the defect yield and the additional displacements are attributed to the existence of low-energy processs. (author)