Both the optical and infrared spectra of cerium and praseodymium tri-positive ions in the alkaline earth fluorines have been studied. Various charge compensation mechanisms have been employed including negative hydride, deuteride and tritide ions.

For the hydride centres the degenerate local mode lines are resolved into more than one component. These splittings are attributed to electron-phonon interaction effects between the low lying rare earth 4f electronic states and the hydride ion local mode phonons.

The 4f-5d electronic transitions of the cerium hydride type centres show large isotope shifts up to 50 cm⁻¹. Only the non-degenerate hydride ion vibration appears in the 4f-5d optical spectra and the vibrational interval is increased from absorption to fluorescence by as much as 15%. Both the isotope and vibronic shifts for the tetragonal cerium sites are attributed to electron-phonon interaction effects.

Simple models involving point charges and point dipoles account in a semi-quantitative way for several features of the spectra but fail to account for either the sign or magnitude of the isotope and vibronic shifts.