A study has been made of the luminescent emission of $F$, $M$, and $R$ centers in NaCl, KCl, and LiF. When such centers are excited by polarized light, the resulting emission can be analyzed for polarization. This yields information on the symmetry properties of the emitting center. In the case of $M$ centers and of $F$ centers made at 77°K the results obtained are consistent with the models widely used for these centers.

In the case of the $R_1$ and $R_2$ centers the situation is more complex and it appears that as far as optical properties are concerned some relationship exists between these centers. Excitation in either band yields the same emission band with very similar polarization properties. This has been interpreted in terms of an energy transfer process between $R_1$ and $R_2$ centers.

When crystals containing $F$, $M$, and $R$ centers are investigated, the behavior of the $F$-band emission is markedly changed from the case where only $F$ centers are present. The characteristic $F$-center emission disappears and instead $F$-band excitation gives the emissions characteristic of either the $R_1$ or $M$ centers. Under these conditions the emission excited by polarized $F$-band light is polarized. Experiments were conducted to test whether these effects might be ascribable to the presence of "higher" excited states of the $M$ and $R$ centers in the region of the $F$ band. This does not appear to be the case. It is therefore concluded that energy transfer processes can occur between the $F$ center and the $M$ and $R$ centers. The observed dichrosim of the $F$ band is ascribed to an interaction of $F$ centers with nearby $M$ and $R_1$ centers.

• Received 24 January 1957

DOI:https://doi.org/10.1103/PhysRev.106.684