Weak magnetic field enhancement of the luminescence from F centre pairs in alkali halides

Whenever a member of a F centre pair is excited optically at low temperature, it can return to the ground state either radiatively or by formation of a F′ centre, the excited electron being transferred by a fast tunnel effect to the neighbouring centre. This process is dependent on the spin symmetry of the pair, which is in turn determined by the competitive influence of the different local hyperfine fields and of an applied magnetic field. The latter causes a decrease of the tunnelling probability, i.e. an increase of the luminescent yield in two steps; the first critical value is given by the hyperfine field and the second one by the temperature according to the Boltzmann factor. The quantum yield is found to have an exponential dependence of the concentration, of the volume in which tunnelling is possible, and of the average tunnelling frequency. Spin‐lattice relaxation and EPR, by mixing between the spin state populations, reduce the effect. Application of the theory to experimental values obtained for the luminescence intensity as a function of the applied field in KI, KCI, and KBr yields the average tunnelling frequency (about ten times the radiative probability), the effective range (85 Å), and the absolute value of the luminescent quantum yield.