Atomic-Beam Measurements of van der Waals Forces

It is expected that the strength of the van der Waals potential (—C/r6 term) may be calculated from measurements of the total scattering cross section Q if the collision is of sufficiently low energy. Unfortunately, experimental values obtained for C have been about a factor of 2 higher than those predicted by theory. This discrepancy would be resolved if experimental Q's were about 30% lower. This paper discusses a systematic experimental error which may account for the high values for Q. This error is the Ishii—Nakayama pumping effect, which occurs in the normal use of McLeod gauges. We have eliminated this error in measurements of Q for scattering metastable helium and neon in rare gases. The resulting C's, and corrected C's for previously published alkali scattering, no longer include the factor of 2 discrepancy. Comparisons are made with other experiments.