ESR and Optical-Absorption Study of theV1Center in KCl: NaCl

Abstract

The results of optical experiments show that the $V_1$ center in KCl has a strong transition at 357 nm and a weak transition at 560 nm. Both transitions have the same polarization and the transition moments are oriented approximately along $〈110〉$. The optical measurements show further that the $V_1$ center disorients in the neighborhood of 17 K, and suggest that it consists of an $H$ center trapped in the immediate vicinity of a substitutional ${\mathrm{Na}}^{+}$. These optical measurements have been correlated with the anisotropic ESR spectrum of the $V_1$ center, which is best observed at 35 K. It is found that the $V_1$-center ESR spectrum is similar to that of an $H$ center; however, the four nuclei of the ${\mathrm{Cl}}_4^{3-}$ molecule ion are all inequivalent, and the internuclear axis of the two central nuclei lies in a {100} plane and is tipped (5.7°±0.3°) away from the $〈110〉$ direction. Furthermore, the central molecular bond is bent by about 3.5°. The ESR spectra reveal that the $V_1$ center is an $H$ center trapped by a substitutional ${\mathrm{Na}}^{+}$ impurity. The ${\mathrm{Na}}^{+}$ lies in the {100} plane containing the ${\mathrm{Cl}}_4^{3-}$ molecule ion and is in a nearest-neighbor position to the central nuclei of the molecule ion. The reorientation of $V_1$ centers is sufficiently rapid to result in lifetime broadening of the ESR lines above 35 K, and at 50 K the ESR spectrum shows a collapsing of certain four-line groups into single lines, which indicates that a restricted interstitial jump is occurring. At 80 K, the $V_1$ ESR spectrum averages into an isotropic line at $g=2.020\mathrm{}$, with a linewidth of 100 G.