Rotronic Jahn-Teller effect for diatomic molecular impurities at cubic sites in ionic crystals

The theory of hindered rotation of diatomic molecular impurities at cubic sites in ionic crystals is generalized to account for the orbital degeneracy of the molecule, with use of a crystal-field approach. This theory explains the observed orientations of the molecular axis, the orientations of the $\pi$ orbitals along either or axes, and the dominance of 90° tunneling in some cases, even using only the lowest-order term in the octahedral crystal field ($l=4\mathrm{}$). It is shown that the coupling of the degenerate orbital to the rotation in the crystal field generates an instability if the molecular axis is parallel to a direction, leading to a Jahn-Teller effect resulting from rotational-electronic (rotronic) coupling. Analytic expressions for the lowest branch of the adiabatic potential energy as a function of the orientation of the molecular axis are calculated for $\Sigma$, $\Pi$, and $\Delta$ states, and the orbital wave functions are determined for $\Pi$-state molecules. The case of a diatomic molecular impurity at a site of tetrahedral symmetry is discussed.