Duration of hydrogen-atom spin-exchange collisions

Interruption of the hyperfine interaction during electron spin-exchange collisions shifts the hydrogen-atom ground-state $\Delta m_F=0\mathrm{}$ hyperfine transition frequency in proportion to the spin-exchange collision rate and to the average time $T_D$ during which the exchange interaction interrupts the hyperfine interaction. Measurements of the thermal average $T_D$ 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 $T_D$ for hydrogen-atom collisions with ${\mathrm{O}}_2$, NO, and N${\mathrm{O}}_2$ molecules are consistent with the formation of long-lived intermediate complexes during some collisions.