Thermal Production ofF-Aggregate Centers in KCl

The optical absorption spectrum of a KCl crystal x rayed at liquid nitrogen temperature has been followed during warming in the dark to about 320°K. This measurement not only provides a means for the study of $M$-center formation by the aggregation of separated $F$ centers, but also the formation of other $F$-aggregate centers. The principal features of this measurement are as follows. A small initial $M$ band appears superimposed on the $F^{\prime }$ band at liquid nitrogen temperature upon completion of x raying. Warming to about 207°K results in the disappearance of the $F^{\prime }$ band with the $M$ band remaining essentially unchanged. At 207°K the onset of $F$-center migration is indicated by the initiation of growth of the $M$ band and several longer wavelength bands. The $M$ band shows continued growth with increasing temperature, increasing by over an order of magnitude upon reaching room temperature. Owing to a particularly large $M$-center concentration (in the presence of few $R$ centers), Okamoto's $M_4$ excited $M$-center band is directly observed. Absorption structure appearing to the long-wavelength side of the $N$ bands at about 1350 mμ and other structure between the $M$ and the $N$ bands likewise show steady growth with increasing temperature above 207°K, but became unstable at higher temperature. The 1350-mμ structure, which shows a steady increase over the temperature range 207-248°K, declines above 248°K and disappears completely by 269°K. A possible fundamental role of luminescence in the production of $F$-aggregate centers by varied methods is mentioned, and the primary influence of local vibrations of the lattice in affecting the optical transition of the $M$ center is discussed.