Vibronic and impurity-induced Raman scattering from a Jahn-Teller—distorted impurity

Abstract

Raman scattering transitions between the hindered-rotational levels of the vibronically coupled $^{2}E_{g}$ ground state of ${\mathrm{Cu}}^{2+}$ in CaO have been studied as a function of temperature and applied uniaxial strain. The results are compared with a cluster model for the vibronic interaction, and the energies of the strain-split states fit the model well. The intensities of the transitions are calculated from the cluster-model vibronic wave functions. Only two parameters are available to fit the intensities of all observed levels as a function of applied strain. These intensity measurements are therefore a rigorous test of the accuracy of the vibronic wave functions. Good agreement is obtained for transitions observed in one of the two pertinent polarization geometries, but poor agreement is observed for the second geometry. These results are not fully understood, but they may be due simply to the inaccuracy of the cluster-model eigenfunctions. The impurity-induced Raman scattering from the vibrations of the host lattice is compared with calculations based on the shell-model lattice-dynamics eigenvectors for CaO. It is found that substantial coupling to the second-neighbor ${\mathrm{Ca}}^{2+}$ ions and the oxygen polarization are required in addition to the nearest-neighbor (oxygen) core coupling to explain the observed spectra. This apparent complexity of the electron-phonon interaction is the most likely explanation for the observation that the measured strain coupling coefficient for the ${\mathrm{Cu}}^{2+}$ ground state is a factor of 2.5 too small to be consistent with the other cluster-model parameters deduced from the fits to the vibronic levels.