Polarized Raman study ofNO2−in KCl, KBr, KI, and NaBr

The ${\mathrm{NO}}_2^{\mathrm{-}}$-molecular impurity is shown to exhibit different equilibrium orientations depending on the alkali-halide host lattice. The polarized Raman intensities of the three internal modes reflect a twofold axis for the ${\mathrm{NO}}_2^{\mathrm{-}}$ ion in KI and NaBr and a threefold axis in KCl and KBr. For KCl and KBr the quasifree rotation of the molecule around the O-O axis is responsible for the axial behavior of the derived polarizability tensor. The Raman-active gap mode in KI is identified as an ${\mathit{A}}_2$ mode of the ${\mathit{C}}_{2\mathit{v}}$ point group. This mode involves as such a purely rotational motion of the impurity around its dipole axis and is therefore different from the three ir gap modes. The 60-${\mathrm{cm}}^{\mathrm{-}1}$ mode in KI is shown to possess ${\mathit{A}}_1$ character and this necessitates a reconsideration of the fine structure of the ${\nu }_1$(${\mathit{A}}_1$) mode (1308 ${\mathrm{cm}}^{\mathrm{-}1}$), previously explained as a Fermi resonance between the combination mode of this 60-${\mathrm{cm}}^{\mathrm{-}1}$ mode with the asymmetric-stretching mode and the totally-symmetric-stretching mode. In relatively highly doped crystals (>1 mol %), different and new polarized Raman properties are detected, revealing symmetry lowering and preferential orientation.