Measurement of a hyperfine-induced spin-exchange frequency shift in atomic hydrogen

We have measured a hyperfine-induced spin-exchange frequency shift in the atomic-hydrogen ground-state hyperfine transition. A recent quantum-mechanical treatment of low-energy hydrogen-hydrogen scattering by Koelman {ital et} {ital al}. (Phys. Rev. A 38, 3535 (1988)) predicts such frequency shifts to become large at low temperature, and to affect the performance of atomic clocks such as the cryogenic hydrogen maser. The experiment reported here was performed with a hydrogen maser operating near room temperature, where the reported hyperfine effects are predicted to be small, but measurable. Using an adiabatic fast passage (AFP) technique to vary the incoming atomic population in the masing states from approximately 100% (AFP on) to 50% (AFP off), we determined the change in the dimensionless hyperfine-induced frequency-shift parameter {Omega} to be {Omega}{sub on}{minus}{Omega}{sub off}=5.38 (1.06){times}10{sup {minus}4}. The theoretical prediction at this temperature is {Omega}{sub on}{minus}{Omega}{sub off}={minus}0.76{times}10{sup {minus}4} to {minus}1.12{times}10{sup {minus}4}, for the range of masing-state populations used in the present experiment. We review the relevant theory, report our experimental method and results, and discuss possible reasons for the discrepancy between experiment and theory.