Scattering of High-Velocity Neutral Particles. XVI. Ar–Ar, Ar–He, and Ar–H2

Measurements are reported for the scattering of argon beams with energies between about 200–1500 eV by room-temperature argon, helium, and hydrogen. Two separate investigations in essentially the same apparatus are involved, and interaction potentials are derived for Ar–Ar and Ar–He at internuclear distances where scattering measurements have not previously been made. For Ar–H2 there are no previous direct scattering results. The Ar–Ar system was studied in both investigations, and the combined results are represented by V(r) = 3.25 × 103 / r9.97eV, 1.96 Å r 2.41 Å. There is good agreement between this result and potentials from other scattering measurements and shock compression of liquid argon. One series of new measurements was made for Ar–He. The derived potential is V(r) = 22.2 / r5.42eV, 1.40 Å r 1.81 Å, which agrees well with other scattering results and with measurements of mutual diffusion coefficients at elevated temperatures. A combination of He–He potentials obtained from scattering with the present Ar–Ar results leads to a derived He–Ar potential which is in good agreement with that obtained from the direct measurements. One series of measurements was made for Ar–H2, and the resulting potential is V(r) = 159 / r6.28eV, 1.81 Å r 2.36 Å, which is about a factor of 2 higher than that derived from previous measurements on He–He, He–H2, and Ar–Ar. It is also higher by about the same amount than the Ar–H2 potential derived from other scattering measurements on Ar–Ar and H2–H2. Mutual diffusion coefficients for Ar–H2 at elevated temperatures also suggest that this potential is high. The present potential for Ar–Ar agrees well with theoretical calculations based on a delta-function model and a statistical (Thomas–Fermi–Dirac) model. The Ar–He potential also agrees with calculated potentials based on these models and with that obtained from ab initio Hartree–Fock calculations.