Small molecular ions as impurity centres in crystals
ABSTRACT The present status and results of the spectroscopic study of molecular ions, inserted into alkali halide crystals, are reviewed. The vibronic spectra of a number of molecular centres (O2-, S2-, NO2-, PO2-) contain clear-cut series of zero-phonon lines. For these centres the data about the parameters of intramolecular vibrations, anharmonic constants and adiabatic potentials are discussed. The librational–rotational motion of impurity molecules is revealed in the spectra of infrared absorption, vibronic absorption, luminescence and Raman scattering. Molecular centres are also perspective objects for the spectroscopic study of the local lattice dynamics and anharmonic effects. In the case of the NO2- centres the data on the hot luminescence spectrum and characteristic relaxation times are considered.