Influence of the isotopic constitution of the O2 −-center in alkali halides on its reorientation time

The theory of simple phonon induced reorientation of an O₂ ^?-center in alkali halides is refined to include the effect of the symmetry of its wave function. It is shown that the diagonal terms of the strain matrix and the off-diagonal ones lead to different coupling mechanisms between phonons and the impurity. For a rotation byπ/2 of a homonuclear molecule, either one or the other of the coupling mechanisms becomes ineffective, depending upon the isotopic constitution of the molecule and its nuclear spin state. Expressions for the reorientation time are obtained.