Abstract
Interruption of the hyperfine interaction during electron spin-exchange collisions shifts the hydrogen-atom ground-state $delta$m/subF/=0 hyperfine transition frequency in proportion to the spin-exchange collision rate and to the average time T/subD/ for hydrogen-hydrogen collisions during which the exchange interaction interrupts the hyperfine interaction. Measurements of the thermal average T/subD/ at 308 degreeK 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 T/subD/ for hydrogen- atom collisions with O$sub 2$, NO, and NO$sub 2$ molecules are consistent with the formation of long-lived intermediate complexes during some collisions.
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