Optical-absorption and photo emission edges in insulating solids

In this paper, the author considers in general terms the theory predicting the position of optical-absorption edges and also photoemission edges in insulating solids. It is seen that these two experimental techniques may probe fundamentally different properties of solids and that by proper analysis of these results it is possible to deduce directly the effects of "excitonic" processes in soft-x-ray spectra. By constructing appropriate limits of the general theory, it becomes possible to perform a series of model calculations for several systems to illustrate the salient features. In general, a very favorable comparison with experiment is obtained. Based upon these models, one is also able to give a quantitative size to such various effects as relaxation, correlation, and electron-hole interaction strength. Finally, we are able to resolve the controversy among Brown et al. , Menzel et al. , berg and Dehmer, and Kunz et al. in favor of the excitonic model for explaining the optical absorption spectrum of LiF.