Short-Range Effects on the Pressure Shift for a Nitrogen Atom in a Rare-Gas Atmosphere

The short-range contribution to the pressure shift of the hyperfine constant has been analyzed for a nitrogen atom in interaction with a rare-gas atom, and specific calculations are performed for helium as buffer gas. Two mechanisms are found to contribute to the short-range shift; the first, due to the finite spin density at the nitrogen nucleus from the distorted $2p$ valence electrons, contributes only 1% of the experimental shift. The second mechanism arises from the exchange polarization of $1s$ and $2s$ cores by the perturbed valence electrons, and produces a shift of 0.76 cps/mm Hg, as compared to the experimental value of 0.27 cps/mm Hg. The results are in disagreement with earlier assumptions of a negligible short-range effect. Possible sources that could influence the theoretical value to improve agreement with experiment are discussed.