Electron-Phonon Interaction Effects in the Spectra of Hydrogenated, Deuterated, and Tritiated Crystals of Calcium and Strontium Fluorides Containing Cerium

Spectroscopic measurements of cerium-hydride, cerium-deuteride, and cerium-tritide ion pairs in calcium and strontium fluoride crystals are reported. The local-mode spectrum of the hydride ion shows a splitting of the doubly degenerate vibration which is attributed to electron-phonon interaction effects between the cerium $4f$ electronic states and the hydride-ion local-mode phonons. A dipole moment of $1.1e{a}_0$ for the hydride ion, where $a_0$ is the Bohr radius, is postulated to explain the observed local-mode spectrum. The electronic lines of the tetragonal cerium-hydride ion pairs display large isotope shifts of 23. 8 and 35. 1 ${\mathrm{cm}}^{-1\mathrm{}}$ for hydrogen to deuterium and hydrogen to tritium, respectively. Only the nondegenerate vibration of the hydride ion appears in the vibronic spectrum of the $4f\to 5d$ transition of cerium and has a higher frequency when coupled to the cerium-ion $5d$ electronic state than to the $4f$ electronic states. The intensity of these vibronics relative to their parent electronic lines is quantitatively explained on a configuration-coordinate model. Both the electronic line isotope shifts and the vibronic frequency shifts are attributed to electron—local-mode-phonon interaction effects in the cerium-ion $5d$ electronic state. A quantitative model for the electron—local-mode-phonon interaction with point-charge and dipolar interactions between the cerium ion and the hydride ion is set up and includes anharmonic effects. This model successfully explains the vibronic splitting in the local-mode spectrum, the occurrence of only one local-mode frequency in the vibronic spectrum of the cerium $4f\to 5d$ transition, and the relative intensity of these lines, but is unsuccessful in explaining the magnitude or sign of the observed isotopic effects. The estimated electronic line isotope shifts are ¼ the magnitude of the observed shifts and of opposite sign.