Effects of the off-center position of theLi+ion on the optical properties of theFAcenter in KCl

The off-axis symmetry of the $F_A$ center in KCl:${\mathrm{Li}}^{+}$ has been investigated by optical experiments. We have studied static and dynamic reorientational properties of the $F_A$ center by measurements of absorption and emission. The off-axis model of the center has been confirmed. A theoretical treatment, which takes into account the small deviation of the center with respect to the crystal axes, is in good agreement with the experimental results. Best fittings of the theory to the experimental data yield a value of the off-axis angle theta=8°.