Study of the F′ Center in Several Alkali Halides
The F′ band has been produced and measured in the following alkali halides: NaCl, KCl, KBr, KI, RbCl, RbI, CsBr, and CsI. Additively colored samples were optically bleached in the F band at an optimum temperature to produce a maximum F-to-F′ conversion. The observed spectra exhibited two distinctive features. In each salt the F′ band had a shoulder on the high-energy side of the principal peak, except in NaCl where the band was bell-shaped. CsBr and CsI exhibited a more complex structure. The shoulder in the potassium and rubidium salts is attributed to low-lying d-like states in their conduction-band structure. In all salts there was a small second absorption shoulder on the high-energy tail of the F′ band. This absorption is attributed to F′ absorption such that the remaining F center is left in an excited 2por 2s state. A semicontinuum-model variational calculation using a correlated Hylleraas wave function yields, for reasonable values of the parameters, F′ binding energies in fair agreement with the observed optical absorption edges. An absorption-cross-section calculation yields absorption curves in good agreement with experiment if one excludes the aforementioned structure.