Crossed-Molecular-Beam Measurements of the Total Cross Sections of Ar–N2, Ar–Ne, Ar–He, and Ar–H2 at Thermal Energies

Total cross sections have been measured for the scattering of Ar beams from perpendicular intersecting beams of N2, Ne, He, and H2. The beams were at 300°K and contained Maxwellian velocity distributions. The slit system approximated ideal slits, and the scattering-center density was calculated from rate-of-flow measurements and a modification of the usual beam-intensity equations. In addition, beams of He and H2 were scattered from an intersecting beam of Ar. Beam reversal reduced the angular-resolution correction to less than 1%. The background scattering of the primary beam could be estimated without recourse to pressure-gauge readings. Comparison of the experimental results was made with the Schiff—Landau—Lifshitz formula and calculations with the partial-wave WKB approximation with an assumed Lennard-Jones (12, 6) potential. The L-J total cross sections do not agree with the experimental results for Ar–He and Ar–H2 systems. The L-J angular-resolution corrections do not bring the beam-reversal measurements in line with the measurements with Ar as the scattering gas.