Vibronic and resonance Raman spectra of NO2impurity ions in the body-centered alkali halide crystals

The low-temperature resonance secondary radiation spectrum as well as the absorption and luminescence excitation spectra of NO₂⁻ impurity ions in cesium halides have been studied. The energy relaxation processes and NO₂⁻ equilibrium orientation and reorientation problems have also been discussed. It was shown that the systems under study were characterized by average Stokes' losses and strong lattice distortions, exemplified by the Generation of a number of low-frequency local and pseudolocal vibrations. The inhomogeneous broadening in CsCl-NO₂⁻ and CsI-NO₂⁻ spectra was extremely large for the simple molecular impurity systems, leading to the interesting peculiarities of energy relaxation processes. Unlike some alkali halide crystals with NaCl structure the impurity NO₂⁻ does not rotate in the lattices with CsCl structure. The NO₂⁻ equilibrium orientation in cesium halides was fixed: both the molecular axis and the axis perpendicular to the molecular plane were directed in (100) directions of the crystal.