Electron Spin Resonance of Hydrogen Atoms in CaF2

We report the results of a detailed investigation of the electron spin resonance of a hydrogen atom in an interstitial position in Ca${\mathrm{F}}_2$. The following aspects of the problem are treated in detail: (1) apparatus and sample preparation; (2) a spin Hamiltonian with four parameters describing the $g$ value and hfs of the hydrogen atom with the eight equivalent fluorine nuclei surrounding it; (3) the electron-nuclear double resonance (ENDOR) spectrum and the resonance linewidth; and (4) an attempt to calculate the parameters of the spin Hamiltonian starting from atomic and ionic wave functions. Our sample preparation technique has allowed us to deuterate the specimens, and we have obtained spin Hamiltonian parameters for the deuterium center as well. Most of the data were obtained at room temperature, but some data are available on the hydrogen center at 77°K. We have essayed an explanation of the small proton-deuteron and temperature-dependent differences of the spin Hamiltonian parameters.