Recombination Luminescence in Alkali Halides

A new type of luminescence in nominally pure alkali halide crystals was studied. At room temperature the excitation was most efficient in the wavelength region that has been associated with band-to-band transitions. It is suggested that the emission results from the capture of a hole by an $F$ center. In agreement with this model the emission (430 mμ in KCl) is the same as that which is observed when a crystal containing $\alpha$ centers is irradiated in the $\alpha$ band at low temperature. From the thresholds of the excitation spectra the following band gaps are derived: NaCl, 8.0 eV; KCl, 8.1 eV; KBr, 7.3 eV; and KI, 5.8 eV. A comparison with external photoelectric thresholds as obtained by other investigators yields the electron affinities: NaCl, 0.8 eV; KCl, 0.6 eV; KBr, 0.9 eV; and KI, 1.6 eV.