Librational motion of theHA(Li+)center in KCl

It is shown from the analysis of some subtle features of the ESR spectra that the $\mathrm{Cl}_2^{}{}_{}{}^{-}$ axis of the $H_A\left({\mathrm{Li}}^{+}\right)$ center in KCl:${\mathrm{Li}}^{+}$ which makes a 26° angle with a $〈100〉$ direction, does not lie exactly in a {110} plane at 4.2 K, but lies outside it by about 1.5°. Such a geometry implies the existence of a librational motion over a 3° angle. The behavior of the ESR lines both as a function of temperature and under $〈110〉$ uniaxial stress indicates that such a motion exists, and that it is very likely a tunneling motion at 4.2 K with a rate of about 1 × ${10}^7$ Hz.