ESR AND OPTICAL-ABSORPTION STUDY OF V1 CENTER IN KCL.NACL

The results of optical experiments show that the V1 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 a110a. The optical measurements show further that the V1 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 Na+. These optical measurements have been correlated with the anisotropic ESR spectrum of the V1 center, which is best observed at 35 K. It is found that the V1-center ESR spectrum is similar to that of an H center; however, the four nuclei of the Cl43- molecule ion are all inequivalent, and the internuclear axis of the two central nuclei lies in a {100} plane and is tipped (5.7a°±0.3a°) away from the a110a direction. Furthermore, the central molecular bond is bent by about 3.5a°. The ESR spectra reveal that the V1 center is an H center trapped by a substitutional Na+ impurity. The Na+ lies in the {100} plane containing the Cl43- molecule ion and is in a nearest-neighbor position to the central nuclei of the molecule ion. The reorientation of V1 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 V1 ESR spectrum averages into an isotropic line at g=2.020, with a linewidth of 100 G. © 1969 The American Physical Society.