Equilibrium orientations of diatomic-molecular impurities in cubic crystals determined by a polarized Raman study of the stretching mode

Polarized Raman measurements were performed on the $A_1$ stretching mode of static or reorienting diatomic molecular impurities in cubic crystals. The equilibrium orientation(s) of the molecular axes, most commonly lying along either the 〈110〉, 〈111〉, or 〈100〉 directions, can be determined by a behavior-type (BT) analysis of the Raman spectra. This approach allows one to establish the point group of the impurity ion without (i) applying any secondary fields or (ii) studying the experimentally less accessible tunneling sidebands. The correspondence between the polarized Raman intensities and the actual defect symmetry can be obscured when the stretching vibration is strongly coupled to reorientational degrees of freedom, or, when the anisotropy of its derived polarizability is relatively small. The BT analysis was tested on the stretching modes of ${\mathrm{CN}}^{\mathrm{-}}$ in KCl and KBr, ${\mathrm{OD}}^{\mathrm{-}}$ in KCl, and ${\mathrm{O}}_2^{\mathrm{-}}$ in KCl and KBr, establishing a $C_{3v}$, $C_{4v}$, and $D_{2h}$ point-group symmetry, respectively. The $C_{4v}$ symmetry for ${\mathrm{OH}}^{\mathrm{-}}$ in KCl and KBr is confirmed and a lower symmetry for ${\mathrm{OH}}^{\mathrm{-}}$ in NaCl is established. The ${\mathrm{SH}}^{\mathrm{-}}$ defect in KCl is shown to possess $C_{3v}$ symmetry. A comparison is made between the present Raman results and previous experimental work.