SCATTERING OF HIGH-VELOCITY NEUTRAL PARTICLES .16. 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-H 2 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/r 5.42eV, 1.40 Å<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/V6.28 eV, 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.