Zeeman study of H- local modes of Kramers rare-earth ions in CaF2 and SrF2

Splittings of the infrared absorption lines of doubly degenerate (X, Y) loc al-mode vibrations of H- ions are observed for hydrogenic C4 upsilonH- cent ers in CaF2 and SrF2 crystals containing Kramers rare-earth ions. These spl ittings are interpreted as electron-local mode phonon interaction effects i n which the twofold degenerate (X, Y) local-mode vibrations, coupled to the Kramers-degenerate doublet electronic ground states of the rare-earth ions , are split into distinct doublets. The dominant contribution to these ener gy splittings comes from second order perturbation terms of the electron-ph onon interaction. Nonlinear Zeeman shifts and splittings of the local mode lines can be quantitatively accounted for without introducing any additiona l parameters.