ELECTRONIC PHOTOCHROMISM - CASE-STUDY OF BISMUTH-GERMANATE

In this paper we examine the various mechanisms which can lead to photochromism, based on electronic rather than chemical effects. We make a distinction between passive and active photochromic materials. We show that in the case of solids, the simple models, adequate for organic molecules, no longer apply. Photochromism encountered in bismuth germanate (Bi4Ge3O12) requires the postulation of inhomogeneously broadened absorption due to trap states. It is suggested that the results indicate the emergence of a new technique of excited-state site selection spectroscopy.