The mechanism for the formation of $M$ centers from $F$ centers upon irradiation in the $F$ band is discussed. The evidence favoring ${F_2}^{+}$ (or $M^{+}$) centers as intermediate states is shown to be less convincing than formerly believed. Measurements of the activation energy of the reaction rate have been made for $F$-to-$M$ photoconversion in additively colored KCl (0.35 eV), KBr (0.39 eV), and CsBr (∼0.1 eV). The "activation volumes" for $F$-to-$M$ conversion in KCl and CsBr have been measured. They are much smaller than those estimated for free anion diffusion. The same mechanism for $F$-to-$M$ conversion seems to obtain in these three salts.

• Received 18 March 1968

DOI:https://doi.org/10.1103/PhysRev.174.958