Photostructural changes in glassy semiconductors: Franck-condon and relaxation transitions in negative-U centers

A consistent theory is presented of a mechanism for unusual photostructural changes observed in glassy semiconductors. The theory takes into account that in these materials negative-U centers are the basic charge carriers of which the electron ground state is formed by strong self-trapping of a singlet electron pair in a soft atomic mode in a glass. As a gap-light generated excited state of a negative-U center is related to a large atomic displacement in the soft mode, it is shown that metastable "defects" can indeed be created in the original structure. These are identified as the photostructural changes. The theory provides consistent answers to open basic questions. First calculations of the transition probabilities related to this phenomenon are presented.