Valence bond study of fluorine hyperfine interactions near trapped hydrogen atoms in the alkaline earth fluorides

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Abstract
Hyperfine interactions of fluorine nuclei near trapped hydrogen atoms in CaF2, SrF2, and BaF2 are computed using a valence-bond model that includes electronic charge transfer from the nearest-neighbour fluorine ions to the trapped hydrogen atoms at both interstitial and substitutional sites, termed Hi0 and H20 centres respectively. The best agreement between theory and experiment is obtained by fitting the computed and experimental anisotropic hyperfine constants (HFCs) of the nearest-neighbour fluorine ions in order to assess the amount of charge transfer. The isotropic HFCs are then computed with the resulting wavefunction. In all cases, the amount of charge transfer is very small but nevertheless essential for explaining the anisotropic HFCs and the g-shift. In the case of the Hi0 centre the theoretical isotropic HFCs are smaller than the corresponding experimental values by 20% for SrF2 and BaF2, and by 17% for CaF2. For the H20 centre, on the other hand, quantitative agreement between theory and experiment can be achieved by including a small inward displacement of the nearest-neighbour F- ions, which reduces the undistended F--H distance by about 10%.

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