F–M Conversion in Additively Colored KCl Crystals

The photochemical reaction of F→M conversion in an additively colored KCl crystal was investigated as a function of the concentration of F centers, under the condition such as differently colored crystals absorb the equal number of incident photons. The process of the F band bleaching is separated into two steps of the fast and slow bleaching. During earlier stages of F light irradiation the F band displayed the fast bleaching and its amount in each crystals was equal, and independent on the F center concentration, but the growth rate of the M band was proportional to the square root of the concentration of F center. An absorption band, which may be Petroff's A band, was induced on the red side of the F band, but the appearance of this band was dominant in the crystal lightly colored. These experimental results show that the mechanism of the optical F→M conversion depends on the F center concentration, and that the M center is converted from the F center through certain intermediate states. In the case of the F centers converted from U centers, the growth of the M band was small as compared with that of additively colored crystals containing only F centers, and the growth of the A band was also negligibly small.