Spin Exchange and Recombination in a Gas of Atomic Hydrogen at 1.2 K

We report an accurate measurement of the longitudinal spin-exchange broadening cross section for the ΔF0ex0ex=0ex0ex1, ΔmF0ex0ex=0ex0ex0 hyperfine transition of the hydrogen atom at 1.2 K. The result, σ0ex0ex=0ex0ex0.51(2) ², is the first of its kind which is sufficiently accurate to clearly demonstrate disagreement with theory. Resolution of this discrepancy is relevant to the understanding of cold atom collisions and may involve a revision of the long range part of the ab initio H atom pair potentials. A by-product of our work is an improved measurement of the two body recombination rate constant k0ex0ex=0ex0ex1.9(1)×10^-33 cm⁶/s for the gas phase reaction H+H+⁴He→H₂+⁴He.