Optical detection of nuclear and electronic magnetic resonances of close F-center pairs in potassium chloride

Abstract

The luminescence of close F center pairs (produced by short F band irradiation near room temperature) was measured at LHeT in a static magnetic field (kG) under the influence of a microwave and/or a radio-frequency (rf) field of suitable frequency. EPR, revealed by an increase of the luminescence, is modulated by the rf, giving ENDOR spectra similar to those obtained for single centers by conventional methods. The rf alone induces also changes of the luminescence (up to 1%) giving NMR spectra with an intensity depending on the optical excitation and the rf power. The relaxation rate, extrapolated to zero light intensity, is equal to the inverse electronic spin-lattice relaxation time. These observations can be explained by a pair model with isotropic spin exchange, calculated in the high field limit (Zeeman energy » hyperfine energy ≥ exchange energy). The states, characterized by electronic and nuclear quantum numbers, have different luminescent probabilities and the optical cycle transfers the pairs into the nonradiative states. Any magnetically induced transition tends to restore the population equilibrium and increases the luminescence. The calculated intensity ratio EPR:NMR:ENDOR agrees with the observations.