Electron transfer by the tunnel effect and its influence on the F center luminescence in alkali halides

Concentration quenching plays an important role in luminescence phenomena, and is generally accounted for by pair interactions. In alkali halides containing F centers, whenever a member of an F center pair is optically excited at low temperature, it can return to its ground state either radiatively or by the formation of an intermediate F' center. In this case, the excited electron is transferred by fast tunnelling to a neighboring center. This process depends on the spin symmetry of the pair, which is determined by the inhomogeneity of the hyperfine field. Effects of applied magnetic field, spin lattice relaxation and spin-spin interaction on the tunnelling probability are investigated. Experimental confirmation of the theory is presented for the case of KCl