H-Ar potential from high-resolution differential cross-section measurements at thermal energy

We report high-resolution differential cross-section measurements for the systems H-Ar and D-Ar at a relative energy of 67 meV. The primary beam is velocity selected (FWHM 10%) and the secondary beam is produced by a low- temperature source. The experimental resolution allows us to resolve the ''fast- oscillation'' structure of the cross section. Our experimental results are compared with the effective cross sections obtained from potentials recently proposed for these systems. The present data cannot accomodate the potential forms deduced from two separate measurements of the energy dependence of the total cross section, but are not in disagreement with the recent theoretical results of Wagner, Das, and Wahl. The sensitivity of the measured properties to the attractive and to the weakly repulsive parts of the potential is discussed, and we show that a compensation effect between the two does not allow a unique determination of the potential. (AIP)