Strong spin-lattice coupling of Kramers doublets

Vibronic interactions can induce strong admixture by the magnetic field of other states into Kramers doublets leading to strong spin-lattice coupling of the Kramers doublets. The spin-lattice coefficients of the ground ${\Gamma }_7$ level of $d^9$ ions in tetrahedral symmetry are calculated in the cases of strong and moderate Jahn-Teller coupling of the ${}_{}{}^2{}_{}{}^{}T_2^{}{}_{}{}^{}$ state to $E$ modes of vibration. The electron paramagnetic resonance (EPR) of ${\mathrm{Cu}}^{2+}$ and ${\mathrm{Ag}}^{2+}$ in ZnS, ZnSe, and ZnTe is unobserved because of the strong spin-lattice coupling: the internal strains of the crystals are the cause of broadening of EPR lines thus too weak to be detected.