Atomic hydrogen and deuterium mixtures at 1 K

Abstract It has been predicted that the spin exchange broadening cross section for collisions between atomic hydrogen and deuterium (H and D) is over 300 times as large as the corresponding value for H-H collisions at 1 Kelvin. By monitoring the broadening of the ΔF =1, Δm F =0 hyperfine transition of the H atom, we were able to infer the density of D atoms present in a mixture of H and D confined by superfluid liquid helium ( l - 4 He) walls at temperatures between 1.0 and 1.2 K. Measurements have been made of the energy required to dissolve a D atom into l - 4 He as well as the rate constants for the H-D- 4 He and D-D- 4 He gas phase recombination processes. The theoretical spin exchange broadening cross section has been indirectly confirmed.