Vibrational relaxation and hot luminescence in impurity centres with a strong vibronic coupling

The results of theoretical and experimental study of vibrational relaxation processes in impurity centres with a strong electron-phonon interaction are presented. General expressions are obtained which describe the temporal evolution of the configurational-coordinate distribution function and the envelope of the emission spectrum consisting of ordinary and hot luminescence. The problem is solved exactly in the harmonic approximation with allowance made for both linear and quadratic vibronic coupling providing a small number of actual configurational coordinates. The hot luminescence bands in KI-Tl positioned at 3.1 and 2.25 eV are detected. The computations show that in Tl + centres the main portion of the excess vibrational energy relaxes in a time shorter than the mean period of vibrations ( approximately=0.5 ps) and, therefore, the observed spectral features are caused by the first classical turning point in the configurational-coordinate relaxation law. Thus, the centres under study serve as an example of ultrafast relaxing systems. Other weak spectral features are related with the complicated structure of adiabatic potential surfaces in the excited state and are interpreted as a barrier and post-tunnel hot luminescence.