Radiative and nonradiative processes in the optical cycle of the F-3(+) center in LiF

Lithium fluoride stands apart from the other alkali halide crystals because of its good physical properties and the excellent thermal stability of the color centers. Among them the F-3(+) after absorbing at similar to 450 nm emits at similar to 535 nm, with an unusual dependence on time, excitation power, and temperature. Accurate optical measurements as a function of temperature have confirmed the existence of a triplet state, which acts as a trap for a sizable fraction of the excited F-3(+) centers. In addition, the singlet and triplet states are connected by nonradiative transitions involving multiphonon processes, which have been crucial in assessing the energy position of the ground triplet state similar to 1 eV below the relaxed excited singlet state.