Abstract
Optical and EPR measurements of tetragonal symmetry erbium-fluoride, erbium-hydride, erbium-deuterium and erbium-tritium ion pairs in calcium fluoride are reported. A crystal-field analysis accounts for the observed 4f11 multiplet splittings and for the g-values obtained by EPR and Zeeman spectroscopy. The isotope shifts in the electronic and vibronic lines and in the EPR g-values are interpreted in terms of a point-charge model of the electron-phonon interaction between the erbium ion electronic states and the hydride ion local modes. Reasonable agreement with the observations is obtained.