Scattering of thermal He beams by crossed atomic and molecular beams . II

Differential cross sections for He-Ar scattering at room temperature have been measured. The experimental consistency of these measurements with others performed in different laboratories is demonstrated. Despite this consistency, the present van der Waals well depth of 1.78 meV, accurate to 10%, is smaller by 20% to 50% than the experimental values obtained previously. These discrepancies are caused by differences between the assumed mathematical forms or between the assumed dispersion coefficients of the potentials used in the present paper and those of previous studies. Independent investigations have shown that the previous assumptions are inappropriate for providing accurate potentials from fits to experimental differential cross section data for He-Ar. We use two forms free of this inadequacy in the present analysis: a modified version of the Simons-Parr-Finlan-Dunham (SPFD) potential, and a double Morse-van der Waals (MSV) type of parameterization. The resulting He-Ar potentials are shown to be equal to within experimental error, throughout the range of interatomic distances to which the scattering data are sensitive. The SPFD or MSV potentials are combined with a repulsive potential previously determined exclusively from fits to gas phase bulk properties. The resulting potentials, valid over the extended range of interatomic distances r $ 2.4 A., are able to reproduce all these bulk properties quite well, without adversely affecting the quality of the fits to the DCS.