FCenters in Additively Colored KCl:NaCl Crystals

Abstract

Two centers have been investigated which, like the $F_A$ center, are produced by optical conversion from $F$ centers in sodium-doped KCl crystals. They each have one absorption band on the long-wavelength side of the $F$ band, and they each contain one $F$ center. One of these, the $F_B$ center, absorbs at around 636 nm and luminesces at 1240 nm, while the other, the $F_C$ center, absorbs at approximately 620 nm and luminesces at around 1310 nm. Like the $F$ and $F_A$ centers, the $F_B$ and $F_C$ centers are each able to trap a second electron, forming a qualitatively similar primed center, and they each have a single ionized form which lacks visible and near-infrared transitions. The dependences of the $F_B$ and $F_C$ center on sodium concentration further indicate that the $F_B$ center contains two ${\mathrm{Na}}^{+}$ ions and the $F_C$ center three. From the details of their anisotropic behavior, it is likely that the $F$ center lies between the two ions in the $F_B$ center and lies in the same (100) plane with the three ions in the $F_C$ center.