Spin exchange and recombination in a gas of atomic hydrogen at 1.2 K.

Abstract

We report an accurate measurement of the longitudinal spin-exchange broadening cross section for the {Delta}{ital F}=1, {Delta}{ital m}{sub {ital F}}=0 hyperfine transition of the hydrogen atom at 1.2 K. The result, {sigma}=0.51(2) A{sup 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 {ital ab} {ital initio} H atom pair potentials. A by-product of our work is an improved measurement of the two body recombination rate constant {ital k}=1.9(1){times}10{sup {minus}33} cm{sup 6}/s for the gas phase reaction H+H+{sup 4}He{r_arrow}H{sub 2}+{sup 4}He. {copyright} {ital 1996 The American Physical Society.}