Study of theF′Center in Several Alkali Halides

The $F^{\prime }$ 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-$F^{\prime }$ conversion. The observed spectra exhibited two distinctive features. In each salt the $F^{\prime }$ 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 $F^{\prime }$ band. This absorption is attributed to $F^{\prime }$ absorption such that the remaining $F$ center is left in an excited $2p$ or $2s$ state. A semicontinuum-model variational calculation using a correlated Hylleraas wave function yields, for reasonable values of the parameters, $F^{\prime }$ 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.