Scattering of Thermal Rare Gas Beams by Argon. Influence of the Long-Range Dispersion Forces

Abstract

Total cross sections have been measured for scattering atomic beams of He, Ne, Ar, Kr, and Xe by argon. A modulated beam technique was employed. The apparatus had an angular resolution of 1.5 minutes of arc. The data were, correlated using the Massey-Mohr theory, assuming an attractive interatomic potential $V\left(r\right)=-\frac{C}{r^6}$; in which case the cross section $Q$ is given by $Q=b{\left(\frac{C}{v_r}\right)}^{\frac{2}{5}}$ where $b$ is a constant and $v_r$ is the relative velocity. Values of $C$ may be estimated from theoretical expressions and from experimental data on transport properties in gases. By using these sources for $C$, we may calculate the corresponding $Q\text{'}\mathrm{s}$. The cross sections obtained from our experiment were higher than those calculated. For the argon-argon interaction, for example, a theoretical expression for $C$ (that of Kirkwood and Müller) indicates that $Q=410\mathrm{}$ ${\mathrm{A}}^2$. From viscosity data we infer that $Q$ should be 383 ${\mathrm{A}}^2$. Our experimental value is 468 ${\mathrm{A}}^2$. The discrepancy is beyond experimental error.