Dynamical Jahn-Teller and reorientation effects in the EPR spectrum of CaF2:O−

The ${\mathrm{O}}^{-}$ ion, substitutional for ${\mathrm{F}}^{-}$ in Ca${\mathrm{F}}_2$, shows a Jahn-Teller effect associated with the electronic degeneracy of the unfilled $p$ shell. The different Jahn-Teller distortions are coupled to each other via a partially quenched spin-orbit energy. The magnitude of this coupling, ∼0.15 ${\mathrm{cm}}^{-1\mathrm{}}$, may be estimated from an interpretation of the effects of applied stress on the low-temperature EPR spectrum. This coupling also results in reorientation of the center via a phonon-assisted tunneling process, whose magnitude can be estimated experimentally from the thermal broadening of the EPR in the range (5-20) °K, and theoretically, without adjustable parameters, from the results of the low-temperature experiments and the elastic constants of the host crystal. The results are in reasonable accord, giving support both to the proposed model for the center and to the usual theory for the phonon-assisted tunneling rates.