V Optical Absorption Strengths of Defects in Insulators : The f-Sum Rule, Smakula's Equation, Effective Fields, and Application to Color Centers in Alkali Halides

Abstract

This chapter discusses the optical strengths of defects in insulators. It reviews theory for the strength of optical absorption by defects in insulating solids. Experimental results for color centers in alkali halides are considered as an example of these ideas and possible experimental tests of the theory is discussed in the chapter. On general grounds, it is not possible completely to decouple the absorption of a particular defect from that of the crystal as a whole. Smakula's classical relation between the integrated absorption cross section and defect oscillator strength may be generalized, provided that the effective field ratio, oscillator strength, and mass are given with their proper quantum mechanical interpretation. The idea of a local effective field are considered in the chapter from both classical and quantum mechanical viewpoints. In the limit of small overlap between defect and host, the classical approximation is sufficient. The distributions of oscillator strength as a function of energy for hydrogenic defects and the F-center are also discusses in the chapter