Optical properties of the MeNO₂ ferroelectrics and MeNO ₃-NO₂^- compounds

The lowest singlet-singlet and singlet-triplet electronic transitions of the nitrite molecular ion, responsible for the ferroelectricity of the nitrite crystal salts, have been investigated in a temperature range of 1.6-300 K. Polarized well-resolved spectra enabled dealing with separate spin substates of the excited triplet level, and for the nitrite-ion impurity incorporated in the nitrate crystals, to elucidate the effect of the symmetry correlations on the imbedding features of the impurity center. Parameters of the spin-orbit and electron-phonon interactions have been derived, and the microscopic characteristics of the crystal, mostly affecting the electron-phonon coupling, are deduced.