Luminescence quenching of F centers due to nonradiative de-excitation during lattice relaxation

F centers have no luminescence in some crystals. This occurs only in cases intermediate between the adiabatic and the nonadiabatic limits, when we adopt a reasonable damping rate of the configuration coordinate whose oscillation is triggered by photon excitation at an energy E. When $\text{(}\text{EE}{}_1\text{)}\text{E}{}_1\text{> 1 ? 4Λ}$, with E₁  absorption-peak energy, the luminescence quantum efficiency decreases steeply. Here Λ  S/E₁ represents Bartram and Stoneham's parameter with S  lattice-relaxation energy. Otherwise, the nonradiative de-excitation requires a thermal activation, but the activation energy is much smaller than that required at thermal equilibrium.