Dichroism of theFandMBands in KCl

The dichroism of the $F$ and $M$ absorption bands that can be produced by optical bleaching of the $F$ band with polarized light at 87°K is critically dependent on the concentrations of $F$ and $M$ centers. The changes in half-widths and peak positions of the $F$ and $M$ bands that accompany the development of dichroism suggest that the $F$-band dichroism is due to a spatial association of $F$ and $M$ centers. It is probable that an excited $F$ center can transfer energy to a neighboring $M$ center. Dichroism of the $F$ and $M$ bands can be produced at room temperature by optical bleaching of either band with polarized radiation.