Relaxation of optically excited F centers

Abstract

The relaxation of optically excited F centers was studied by resonant secondary radiation (RSR), which occurs successively as Raman scattering, hot luminescence and ordinary luminescence. The RSR and its linear polarization were measured as a function of excitation photon energy mainly at 77 K, for F centers in seven alkali halides. Dynamic processes, including these optical processes were analyzed in terms of a semi-classical model based on the Franck-Condon principle in an A 1g anharmonic adiabatic potential derived from the Born-Mayer potential. It is proposed that, abruptly after the optical excitation, the F electron expands its orbital radius and changes its nature including decoupling from the T 2g Jahn-Teller mode. This model works particularly for KCl, RbCl and NaCl. The relaxation of F centers in KBr and KI is different from that in KCl, RbCl and NaCl. The difference is related to the characteristic differences in lattice dynamics, namely predominant coupling with localized modes and the T 2g frequency near that of the LO phonons. The relaxation of type II F A centers is attributed to two steps, one F-like and the other Li +-dominated.