Optical transitions in the centres with soft dynamics in the final state

Abstract

Anomalous optical spectra of the centres with strongly-reduced local elastic springs in the final electronic states are studied. In this case the transitions take place to the vicinity of a flat potential curve in the configurational coordinate space which corresponds to a very strong mode mixing at the electronic transition. To take this mixing into account we apply the operator transform method. We have found that the mixing results in a strong enhancement of the effect of low-frequency vibrations, causing an intensive low-energy phonon sideband. In this case the spectrum (or its envelope) has the shape of an asymmetrical lambda-letter. Suchtype spectra have been observed experimentally. The spectra may also reveal the Airy-type oscillations if the potential of the final state has a finite slope. This-type oscillations have been observed in the hot luminescence; the oscillating structure of zero-point lines in the absorption spectra of super-fluid 4He droplets probably has an analogous origin. © 2005 IOP Publishing Ltd.