Emission Efficiency and Energy Transfer in Color Centers at High Concentrations

Color Centers in alkali halides are a well known class of point defects, whose optical properties have been studied carefully since long times, in particular their efficient emissions after optical excitation in the absorption bands. However, there are a certain number of intrinsic and extrinsic factors which can greatly influence the emissions up to a complete absence of them. In general these phenomena, which are often referred to as quenching effects, occur during the optical cycle when energy exchanges between different types of color centers, and color centers and lattice vibrations of crystals, take place. Among them, the decrease of the quantum efficiency on the density number of color centers, i.e. concentration quenching, occupies a special place both for basic and applied research. In this lecture, the historical background of the emission and emission quenching of the F center will be reviewed with emphasis on the concentration quenching. Afterwards, the luminescent properties of color centers, like F_A, {F}₂ {F}_3^ + ... , will be described in details, and it will result that, while the concentration quenching effect of the F center is enough well understood, this is not the case of the previous more complex centers. Indeed, the energy transfer phenomena which decrease the luminescence intensity are not well known yet, an aspect still awaiting to be unraveled both from the experimental and theoretical point of view.