Raman-scattering study of theLi+andFA(Li+) centers in KBr

It was recently shown for the $F_A$(${\mathrm{Li}}^{+}$) center in KCl:${\mathrm{Li}}^{+}$ that polarized resonant Raman scattering spectra analyzed with the Zhou-Goovaerts-Schoemaker behavior-type (BT) method [Phys. Rev. B 29, 5509 (1984)] allow one to establish the off-center position of the ${\mathrm{Li}}^{+}$ impurity ion in this center. The present study reports the resonant Raman spectra of $F_A$ ${(}^7$ ${\mathrm{Li}}^{+}$) and $F_A$ ${(}^6$ ${\mathrm{Li}}^{+}$) in KBr. The BT analysis of the polarized Raman data establishes a $C_{4v}$ defect symmetry indicating the on-center position of the ${\mathrm{Li}}^{+}$ ion in this defect, in agreement with the on-site location of the isolated ${\mathrm{Li}}^{+}$ ion in KBr. Among the many observed modes, $F_A$(${\mathrm{Li}}^{+}$) in KBr possesses a vibrational mode at 227 ${\mathrm{cm}}^{\mathrm{-}1}$, which reflects the motion of ${\mathrm{Li}}^{+}$ along the fourfold defect axis, as indicated by its regular isotope effect. Its vibrational energy strongly exceeds that of the 17-${\mathrm{cm}}^{\mathrm{-}1}$ infrared mode of the isolated ${\mathrm{Li}}^{+}$ ion, demonstrating that the adjacent electron in the $F_A$(${\mathrm{Li}}^{+}$) center strongly disturbs the potential experienced by the ${\mathrm{Li}}^{+}$ ion. A mode at 28 ${\mathrm{cm}}^{\mathrm{-}1}$ also associated with the ${\mathrm{Li}}^{+}$ impurity exclusively appears under $F_{A1}$ excitation and shows by the absence of an isotope effect very small participation of ${\mathrm{Li}}^{+}$ motion. The other dynamical modes of KBr:$F_A$(${\mathrm{Li}}^{+}$) are defect-induced lattice modes with peak positions close to those of the unperturbed F center. However, they possess different relative peak heights and polarized Raman intensities. A comparison with the vibrations of the $F_A$(${\mathrm{Li}}^{+}$) center in KCl is presented and the low-frequency Raman- and infrared-active modes of the isolated ${\mathrm{Li}}^{+}$ 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.