Luminescent Excitation Spectra ofFandMCenters in KCl

Luminescent excitation spectra were measured for the $F$ and the $M$ centers in KCl; in particular, for the $F$ center and for two transitions of the $M$ center: the $M_1$ and $M_2$ transitions. In all three cases, the excitation spectra were nearly double-Gaussian in shape, and the efficiency for luminescence was nearly independent of the wavelength of the exciting light. A comparison of the dichroic absorption spectrum with the excitation spectrum of the $F$-band region of crystals with $M$ centers present and oriented provided further evidence for the existence of the $M_2$ transition proposed by van Doorn and Haven and by Okamoto. The efficiency for luminescence for $M$ centers upon $M$-band excitation was equal to the efficiency for luminescence for $F$ centers in pulse-annealed crystals of low $F$-center concentrations. New absorption and luminescent excitation bands in NaF are also reported.