Duration of hydrogen-atom spin-exchange collisions

Interruption of the hyperfine interaction during electron spin-exchange collisions shifts the hydrogen-atom ground-state ΔmF=0 hyperfine transition frequency in proportion to the spin-exchange collision rate and to the average time TD during which the exchange interaction interrupts the hyperfine interaction. Measurements of the thermal average TD for hydrogen-hydrogen collisions at 308 ^∘{}K in an atomic hydrogen maser confirm the predictions of a semiclassical theory and a numerical estimate using straight-line collision trajectories. Measurements of much longer TD for hydrogen-atom collisions with O₂, NO, and NO₂ molecules are consistent with the formation of long-lived intermediate complexes during some collisions.