It was recently shown for the F~(Li+) center in KC1:Li+ that polarized resonant Raman scattering spectra analyzed with the Zhou-Goovaerts-Schoemaker behavior-type (BT) method [Phys. Rev. 8 29, 5509 (1984)] allow one to establish the off-center position of the Li+ impurity ion in this center. The present study reports the resonant Raman spectra of F"(Li ) and F& ( Li+) in KBr.The BT analysis of the polarized Raman data establishes a C4, defect symmetry indicating the oncenter position of the Li+ ion in this defect, in agreement with the on-site location of the isolated Li+ ion in KBr. Among the many observed modes, F& (Li+) in KBr possesses a vibrational mode at 227 cm ', which reflects the motion of Li+ along the fourfold defect axis, as indicated by its regular isotope e6'ect. Its vibrational energy strongly exceeds that of the 17-cm infrared mode of the isolated Li+ ion, demonstrating that the adjacent electron in the F&(Li+) center strongly disturbs the potential experienced by the Li+ ion. A mode at 28 cm ' also associated with the Li impurity exclusively appears under F» excitation and shows by the absence of an isotope e6'ect very small participation of Li+ motion. The other dynamical modes of KBr:F&(Li+) are defect-induced lattice modes with peak positions close to those of the unperturbed F center. However, they possess di6'erent relative peak heights and polarized Raman intensities. A comparison with the vibrations of the F&(Li ) center in KC1 is presented and the low-frequency Ramanand infrared-active modes of the isolated Li+ impurity in KBr are also reported. It is suggested that both static and dynamic anharmonicities are important for the description of these low-frequency modes.
