Measurement of atomic-hydrogen spin-exchange parameters at 0.5 K using a cryogenic hydrogen maser

Using a cryogenic hydrogen maser, suitably modified to have electronic control of both the resonance frequency and the quality fatter of the external cavity, we have measured a number of spin-exchange parameters for an atomic-hydrogen (H) gas at a temperature of 0.5 K. These results are relevant to the ultimate achievable frequency stability for cryogenic H masers and, when coupled with accurate calculations of the spin-exchange parameters, serve as a sensitive test of the H-H interatomic potentials. We find evidence for a frequency shift not predicted by semiclassical theories of spin exchange. In the context of a fully quantum mechanical hydrogen-atom spin-exchange theory [B. J. Verhaar et al., Phys. Rev. A 35, 3825 (1987) and J. M. V. A. Koelman ed al., Phys. Rev. A 38, 3535 (1988)], this frequency shift is attributed to the influence of hyperfine interactions during spin-exchange collisions. Our findings are generally in agreement; with these predictions; however, the sign of the hyperfine-induced frequency shift appears to differ from theory.