Radiationless transitions and kinetics of the F-3(+) centre luminescence in LiF

Among alkali halide crystals, lithium fluoride is of particular interest be cause of its good physical properties and the thermal stability of several kinds of colour centres, which are laser active in the visible and in the n ear infrared at room temperature. In particular F-3(+) and F-2 centres, whi ch emit with high efficiency in the green and red spectral regions, possess almost the same absorption band. This coincidence, added to unusual optica l properties of the F-3(+) centre, prompted careful studies of the optical cycle of these centres. Measurements at liquid-helium temperature (LHeT) ha ve further clarified the role of a triplet state which acts as a trap for a sizeable fraction of excited F-3(+) centres. The nonradiative transitions connecting singlet and triplet states are now much better known and a compr ehensive theoretical explanation is within reach.