Raman scattering induced by V centers and their aggregates in alkali halides

The Raman spectroscopy technique allows one to determine the nature of stable non-paramagnetic V-centers created in KI and KBr X-irradiated in the range 100 to 300 K. In the early stage of X-irradiation, trihalide ions X3- (X = I, Br) are created. They are characterized by the 111 cm-1 (KI) and the 175 cm-1 (KBr) stretching vibrational mode of the linear ion. The X3- ions are responsible for the V2 (about 350 nm) optical absorption band in KI and of the V4 (275 nm) band in KBr. By increasing the X-ray dose, polyhalide ions characterized by the I2 vibrational mode in I-sub(n) (n=5,...) ions are formed. In the case of KI, it is shown that the polyiodides are precursors of iodine aggregates which have the iodine crystal structure. Studies of KI (Ca2+) and KBr (Sr2+) near RT proves that the X3-ions are stabilized by the divalent cations. (author)