A consistent theory was presented of a mechanism for unusual photostructural changes observed in glassy semiconductors. The theory takes into account that in these materials negative-U centers were the basic charge carriers of which the electron ground state was 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 was related to a large atomic displacement in the soft mode, it was shown that metastable so-called defects could indeed be created in the original structure. These were identified as the photostructural changes. The theory provides consistent answers to open basic questions. First calculations of the transition probabilities related to this phenomenon were presented.

Photostructural Changes in Glassy Semiconductors - Franck-Condon and Relaxation Transitions in Negative-U Centers. M.I.Klinger, V.Halpern, F.Bass: Physica Status Solidi B, 2002, 230[1], 39-45