Lattice Dynamics and Electron-Phonon-Coupling Field and Vibronic Spectra

Details of the dynamical properties of crystalline lattices are tested against vibronic spectra accompanying electronic transitions of ${\mathrm{Sm}}^{2+}$ probe ions. Dispersion relations and shell-model parameters are presented for Ca${\mathrm{F}}_2$, Sr${\mathrm{Cl}}_2$, and Eu${\mathrm{F}}_2$. Considerable information is also obtained on the symmetry and nature of the electron-phonon-coupling field. It is shown that long-range Coulombic and highly local, non-Coulombic fields are involved. Furthermore, it is indicated that previously reported deficiencies in shell models concerning the prediction of the dielectric constant and the dynamical polarization have a probable origin in the neglect of extended, partially overlapping, electronic distributions.