Effects of Helium Buffer Gas Atoms on the Atomic Hydrogen Hyperfine Frequency

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Abstract
Calculations are made for the pressure, mass, and temperature dependence of the atomic hydrogen hyperfine frequency shift arising from the perturbing influence of helium buffer gas atoms. The calculated fractional pressure shift is found to be +1.73×10—9 mm Hg—1 compared with the experimental value of +3.7×10—9 mm Hg—1. From the standpoint of the calculation this discrepency may be related to a failure of the simple wave function employed to adequately characterize the unpaired electron spin density at the hydrogen nucleus. The results of the calculations for the temperature variation and mass dependence indicate that these are very small effects. The form of the mass dependent quantum statistical correction, used to elucidate the mass effect, suggests that the relatively large variations experimentally observed with other buffer gases, due to hydrogen isotope variation, are in error. Finally, a calculation is made for the pressure shift from a kinetic theory point of view and the result obtained (+1.79×10—9 mm Hg—1) is in fair agreement with the above mentioned quantum statistical calculation.