The F-center formation efficiencies around the Br K-shell absorption edge (KBr), the Rb K-shell (RbCl), and the I K-shell (KI) in single crystals were studied by using monochromatic X-rays at liquid He temperatures. It was noted that the relaxation of a hole in the K-shell of Rb and I ions did not exhibit the enhancement of formation efficiency that was seen in the case of Br K-shell excitation in KBr. It was concluded that 3 parameters had to be taken into consideration with regard to F center formation. These were the depth of the core, the efficiency of the Auger cascade, and the radius of the halogen atom. A hole in a deep core was expected to decay into several valence holes via Auger cascade or to dissipate most of its energy via fluorescence. The hole in the deeper core generated more valence holes via the Auger cascade. Thus, in alkali halides with a heavy halogen ion, multiple ionization due to Auger cascades seemed to lead to more efficient defect formation. However, the efficiency of X-ray fluorescence increased, while that of the Auger cascade decreased rapidly, with increasing atomic number of the ion. Then the most of the absorbed X-ray energy is dissipated by the X-ray fluorescence.

Y.Kondo, Y.Suzuki, K.Shimoyama, M.Yanagihara: Materials Science Forum, 1997, 239-241, 587-90