Sodium-perturbedRcenters (F3centers) in KCl crystals

Abstract

In KCl crystals containing $R$ (${F}_{3}$) centers and sodium-ion impurities, the absorption spectrum measured at 2\ifmmode^\circ\else\textdegree\fi{}K reveals two small zero-phonon lines on the low-energy side of the well-known ${R}_{2}$ zero-phonon line. We investigated the structure of the centers responsible for these lines by applying uniaxial stress along the high-symmetry directions [100], [110], and [111]. For the more intense line occurring at 13216 ${\mathrm{cm}}^{\ensuremath{-}1}$, the stress shifts, and the stress-induced dichroism, proved that the center must have a doubly degenerate ground state and trigonal symmetry, the same as the ${R}_{2}$ transition of the $R$ center, so the model proposed is an $R$ center perturbed by a single sodium ion equidistant from all three of the constituent $F$ centers. The weaker line arises from a different perturbed center, and the stress results are consistent with a model with the same symmetry as the $R$ center. Tentative models for the second perturbed center are proposed.