Atomic hydrogen spin-exchange frequency shift cross section at 0.5 Kelvin

We present an accurate measurement of the dominant hydrogen atom spin-exchange frequency shift cross section at 0.5 K, made using a recirculating cryogenic hydrogen maser. This parameter is sensitive to the detailed form of the H-H interaction potentials, particularly at large interatomic separations, and thus provides a means for testing the accuracy of ab-initio potentials. We find /spl lambda/~/sub 0/=-21.7/spl plusmn/2.8 /spl Aring//sup 2/, whereas the most sophisticated theoretical calculations to date give /spl lambda/~/sub 0/=-11.8 /spl Aring//sup 2/. >