Elektronenspinresonanz-Untersuchungen zur Aggregation von F-Zentren in KCl

The ESR absorption line of F-centers in additively colored KCl-crystals is narrowed continuously from 46 to 35 Gauss (width between points of maximum slope) by light absorption in the F-band at room temperature. It is shown that this narrowing is due to a continuous changing of the F-centers during the illumination. There is no ESR signal observed which can be attributed to any one of the known F-aggregate centers M, R and N. The changing of the ESR signal is explained as an exchange narrowing which is caused by a loose aggregation of those F-centers which are not transformed into new centers. These aggregates are either pairs or larger clusters of F-centers. According to a calculation of the exchange frequency as a function of the distance between F-centers the observed exchange narrowing corresponds to an average distance of 4 to 5 lattice distances. ENDOR measurements are consistent with this analysis. In addition several observations on the influence of annealing and bleaching at higher temperature on the ESR-spectrum of F-centers in KCl are reported.