Vibrational Resonance Mode and the Temperature Dependence of Hyperfine Coupling of Mn²⁺ in BaO

The temperature dependence of the hyperfine coupling constant A of Mn 2+ in BaO is measured in the temperature range 4 to 300 ° K. It is shown that the theoretical temperature dependence of A given by Šimánek and Orbach [5] does not give a good agreement with the measured values. The temperature dependence of A is calculated by assuming that the interaction of the electronic states of the impurity ion with the resonance mode of vibration prevails over the interactions with the continuous phonon spectrum. To explain the discrepancy between the theoretical curve and the experimental data, the effect of anharmonic terms on the temperature dependence of hyperfine coupling is taken into account.