Electron Spin Resonance of Neutral Hydrogen Atoms in SrF2and BaF2

The electron-spin-resonance spectra of Sr${\mathrm{F}}_2$ and Ba${\mathrm{F}}_2$, each containing neutral hydrogen atoms in interstitial sites, is investigated. In all cases, the electron-proton contact hyperfine interaction is resolved. Also resolved are the smaller superhyperfine interactions with the nuclei of the eight neighboring ${\mathrm{F}}^{-}$ ions. It is found that the magnitude of these interactions is very sensitive to the host lattice. In particular, the superhyperfine interaction decreases by about a factor of two as one proceeds from smallest to largest lattice parameter. It is also found that the difference between the electron-proton interaction parameter $B$ and the corresponding quantity for the free atom is positive, and decreases from 39.9±0.1 Mc/sec for Ca${\mathrm{F}}_2$ to about 4.1±0.1 Mc/sec for Ba${\mathrm{F}}_2$, at room temperature. We have also observed forbidden transitions corresponding to simultaneous flips of the electron and an associated fluorine nucleus, and we discuss the intensity of these in terms of the effective magnetic fields acting on the ${\mathrm{F}}^{-}$ sites.