Abstract

In a first step the temperature dependent lineshapes of F-center optical features are obtained analytically from the usual small-polaron theory and Condon approximation. Though the calculation may be considered already standard, it is improved in putting emphasis in retaining the full spectrum of lattice modes and in keeping control of the accuracy of the approximations. Although the latter is shown to be better than 0.1% for F-centers the theory fails in giving the temperature dependent shifts of the spectral features and the difference between the emission and absorption linewidths. A parameter-free test of the so obtained results with experiment shows good agreement, but significant deviations are also observed. Refining the theory it is shown that, in order to fully account for the optical properties of F-centers, one has to go beyond the current treatments, drop the Condon approximation, and consider the splitting of the F-electron energy levels due to phonons induced hybridization of the electronic states. The temperature dependence of the band shifts is calculated in this way, obtained excellent agreement with experiment. It is concluded that F-centers have appreciable intrinsic structure which may be of interest for further experimental investigation.