Jahn-Teller Effect in an Orbital Triplet Coupled to Both $E_{\mathrm{g}}$ and $T_{2\mathrm{g}}$ Modes of Vibration: Experimental Evidence for the Coexistence of Tetragonal and Trigonal Minima

We have observed magnetic resonance in the ${}_{}{}^3{}_{}{}^{}T_{1\mathrm{u}}^{}{}_{}{}^{}$ excited levels of ${\mathrm{Ga}}^{+}$ in KBr using an optical detection technique. The $A_T$ and $A_X$ emissions take place from excited states which display tetragonal and trigonal symmetries respectively. A model assuming nearly equal linear coupling to $E_{\mathrm{g}}$ and $T_{2\mathrm{g}}$ modes and an additional bilinear coupling to totally symmetric modes, $A_{1\mathrm{g}}$, accounts for the available data: magnetic resonance spectra, and temperature and pressure dependence of the optical spectra.