Abstract
Two centers have been investigated which, like the FA 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 FB center, absorbs at around 636 nm and luminesces at 1240 nm, while the other, the FC center, absorbs at approximately 620 nm and luminesces at around 1310 nm. Like the F and FA centers, the FB and FC 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 FB and FC center on sodium concentration further indicate that the FB center contains two Na+ ions and the FC center three. From the details of their anisotropic behavior, it is likely that the F center lies between the two ions in the FB center and lies in the same (100) plane with the three ions in the FC center.

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