Low-Temperature Recombination Luminescence in Alkali Halide Crystals

Luminescence observed upon low-temperature x-ray excitation of alkali-halide crystals has been identified as due to radiative electron-hole recombination in the perfect lattice. In KCl the emission spectrum is a single band, while in KBr, KI, and NaCl there are two bands. In crystals containing self-trapped holes, optical excitation of electron-excess defects such as $F$ centers causes transfer of electrons to holes with emission of recombination luminescence identical to that excited directly by x-ray irradiation. If the holes have been preferentially oriented, the optically excited luminescence from KCl and KBr exhibits partial polarization characteristic of the transitions involved. The origin of the recombination luminescence is revealed through these polarization properties, together with results on optical excitation and bleaching spectra, temperature dependence, excitation efficiency, and impurity independence. The relevance of this work to the process of electron tunneling from excited $F$ centers is discussed from an elementary point of view. The bleaching of $F$ and $F^{\prime }$ centers through the absorption of recombination luminescence is shown to be a potentially important factor in determining the growth rates of the various centers during high-energy irradiation.