Resonant Raman scattering and dynamics of theFA(Li+) modes in KCl

By means of the behavior-type method, the polarized resonant Raman intensities of the three $F_A$(${\mathrm{Li}}^{+}$) modes in KCl are analyzed, taking into account the optically induced reorientation of the defect. The $F_A$ ${(\mathrm{}}^7$ ${\mathrm{Li}}^{+}$) center is known to possess two high-frequency Raman-active vibrations, at 216 and at 266 ${\mathrm{cm}}^{\mathrm{-}1}$, which exhibit a regular ${\mathrm{Li}}^{+}$ isotope shift and a low-frequency resonance at 47 ${\mathrm{cm}}^{\mathrm{-}1}$, which shows a small and anomalous isotope effect. The investigation of their polarized Raman data necessitated an extension of the behavior-type method to the case of resonant scattering. In spite of reorientation effects the Raman scattering excited at the wavelengths of the $F_{A1}$ or $F_{A2}$ band is found to be dominated by the resonant contribution of the defects which can still absorb the incident light. From the analysis of the polarized Raman intensities of the 216-${\mathrm{cm}}^{\mathrm{-}1}$ mode the defect is shown to have only a mirror-plane symmetry, which independently demonstrates the off-axis position of the ${\mathrm{Li}}^{+}$ ion in $F_A$(${\mathrm{Li}}^{+}$). This deviation from $C_{4v}$[001] symmetry is not observed in the data for the 47- and the 266-${\mathrm{cm}}^{\mathrm{-}1}$ modes, but within the $C_{1h}$ symmetry group all the modes belong to the representation A’, i.e., they are even under reflection. The polarized Raman study, together with the isotope shifts of the Raman modes and considerations of the normal modes of the defect, permits a more detailed description of the dynamics of the $F_A$(${\mathrm{Li}}^{+}$) center: The 266- and the 216-${\mathrm{cm}}^{\mathrm{-}1}$ vibrations strongly involve the ${\mathrm{Li}}^{+}$-ion motion within the mirror plane, essentially parallel and perpendicular to the defect axis, respectively. The low-frequency mode at 47 ${\mathrm{cm}}^{\mathrm{-}1}$ is an activated band mode in which the ${\mathrm{Li}}^{+}$ ion hardly participates, similar to the 43-${\mathrm{cm}}^{\mathrm{-}1}$ mode of the naked ${\mathrm{Li}}^{+}$ impurity.