Some Low-Temperature Properties of theR,M, andNCenters in KCl and NaCl

Abstract

Temporary changes in the absorption spectrum of a colored NaCl crystal can be produced at 77 and 4°K by irradiating the crystal with a strong auxiliary light. The changes in the absorption spectrum indicate that the $R$, $M$, and $N$ centers are not randomly distributed but are located near each other. It is concluded that previous arguments based upon less complete observations are not valid and that one cannot decide from this type of experiment whether the $R_1$ and $R_2$ bands arise from transitions in two distinct centers or from two transitions in the same center. Measurements of the degree of polarization of the emission excited with $R_1$- or $R_2$-band light indicate that the concentration of other centers probably affects this number. It is concluded that this measurement may not determine the symmetry of these centers. Measurements of the temperature dependence of the half-widths of the absorption and emission bands of the $M$ center indicates that the optical transition is primarily influenced by local interactions of the center with neighboring ions and not by interactions with the long-range phonons of the lattice.