Abstract

Consider a system of several spatially separated potential wells (trapping sites) in impure crystal or in a large molecule. Let few (but not all!) of these traps to be filled with electrons, which we call "outer" or "extra" electrons. As a practical realization of this situation one can think of a crystalline or a molecular complex with impurity centers in various charge states [1-3]. Consider, e.g., a 2-site impurity subsystem AB with an extra electron initially residing on site B. Due to the random fields of other crystalline defects the (filled) energy level of an electron on site B may be slightly raised relatively to the corresponding (vacant) level of the site A. As a result, the outer electron can make a spontaneous tunnel transition (TT) from the site B to the site A. An excess of energy can be emitted as either phonon(s) or a low energy photon (infrared or microwave). Therefore, the initial configuration of the impurity pair is, in fact, a configurationally excited state which can decay to a lower lying (ground) state configuration through the exponentially slow spontaneous TT B → A of an extra electron [2, 3].

© 1984 Optical Society of America