Hyperfine Pressure Shift of Hydrogen in Helium

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Abstract
The hyperfine pressure shift (HPS) of hydrogen atoms in a helium atmosphere is calculated over a wide temperature range (50–700°K). Results are not very sensitive to the exact form of the H–He potential, but depend strongly on the values of the spin density at the hydrogen nucleus at different H–He interatomic separations. Spin densities are evaluated by the spin-optimized self-consistent-field (SOSCF) method and are corrected for the long-range Van der Waals interaction. It is argued that long-range spin density shifts are indeed negative as suggested by several previous calculations, in contrast with a recent five-configuration treatment. Our results are 20%–25% higher than experimental values. Most of the error is attributed to the uncertainties concerning the method of including long-range effects.