A mechanism is suggested for the nonradiative quenching of excited states of simple impurity centers, namely a crossover to the ground electronic state of the center immediately after excitation and before thermal equilibrium has had time to come about. It is proposed that this mechanism is responsible for the absence of strong luminescence in some impurity and color centers, e.g., the $F$-center in alkali-halide crystals. The quenching process proposed here is applicable only for certain geometries of the configuration coordinate diagram, which are discussed with a view to a criterion for strong luminescence. The quenching mechanism is consistent with known configuration coordinate curves, and leads to a prediction which is experimentally verified, to the effect that the wavelength of emitted radiation is not more than twice that of the light absorbed by the center. The same mechanism shows the possibility of photoconductivity arising from absorption in discrete lines even at low temperatures.

• Received 5 July 1955

DOI:https://doi.org/10.1103/PhysRev.100.603