Studies of Atomic Hydrogen Spin-Exchange Collisions at 1 K and Below

Spin-exchange interactions during collisions between hydrogen (H) atoms cause the DeltaF = 1, Deltam_{ rm F} = 0 hyperfine transition of the H atom to be broadened and shifted from its unperturbed frequency. A recent theory suggests that hyperfine interactions during these collisions lead to frequency shifts in an oscillating H maser which depend in a non-linear way upon the atomic density. These non-adiabatic contributions are of considerable technological concern as they may limit the ultimate frequency stability of H masers operating at cryogenic temperatures. In the first of two experiments described here, we examine the influence of H-H spin-exchange collisions on the oscillation frequency of a H maser operating at 0.5 K. The results of this study are consistent with the theoretical predictions. The second study involves several experiments with mixtures of H and deuterium (D) atoms at cryogenic temperatures. It makes use of the observation that for comparable H and D densities, the spin-exchange broadening of the DeltaF = 1, Delta m_{rm F} = 0 hyperfine transition of the H atom is dominated by H-D collisions. We have used hyperfine magnetic resonance techniques on this transition to study interactions between H and D atoms confined by liquid ^4He (l-^4He) walls at 1 K and zero magnetic field. The resonance signal intensity gives a measure of the H density in the mixture while the broadening of the transition gives a simultaneous measure of the D density. Measurements are made of several spin-exchange and recombination cross sections. An improved measurement of the energy required to force a D atom into l-^4He is made. These results also set a lower bound for the effective mass of a D quasiparticle in l -^4He.