Electron Paramagnetic Resonance Optical and Absorption Studies of the $V_1\left({\mathrm{Li}}^{+}\right)$ Center in KCl:${\mathrm{Li}}^{+}$

An interstitial chlorine atom created by x or $\gamma$ irradiation at low temperatures can be stabilized by a ${\mathrm{Li}}^{+}$ ion in KCl and form a $V_1\left({\mathrm{Li}}^{+}\right)$ center, which is a bent (∼ 8°) $\mathrm{Cl}_2^{}{}_{}{}^{-}$ molecule ion occupying a single negative-ion vacancy next to a substitutional ${\mathrm{Li}}^{+}$. The two chlorine nuclei are inequivalent, and the internuclear axis is tipped 26° away from $〈001〉$ in a {110} plane. The angular variation of the electron paramagnetic resonance (EPR) linewidth indicates a further weak hyperfine (hf) interaction with two other chlorine nuclei. One deduces that the nuclear configuration of the $V_1\left({\mathrm{Li}}^{+}\right)$ center has the shape of a nonplanar $Y$, this in contrast to the $H$ and $V_1\left({\mathrm{Na}}^{+}\right)$ centers in KCl, whose nuclear configurations are either linear or almost linear. The symmetry of the $V_1\left({\mathrm{Li}}^{+}\right)$ center is consistent with the well-known $〈111〉$ off-center position of the ${\mathrm{Li}}^{+}$ ion. The optical absorption bands of $V_1\left({\mathrm{Li}}^{+}\right)$ are situated at 293, 354, and 618 nm and they are $\sigma$ polarized. Optical anisotropy can be produced in these bands with $〈110〉$-polarized light, and a disorientation temperature $T_D=23.\mathrm{}5$ °K is found.