Total Collision Cross Sections for the Interaction of Molecular Beams of Cesium Chloride with Gases. Influence of the Dipole-Dipole Force upon the Scattering

Total cross sections (Q) for the interaction of beams of CsCl with a number of molecules were measured using an apparatus of ca 4′ angular resolution in which the temperature of the scattering gas could be varied from 200°—735°K. The temperature dependence of Q was studied for Ar, CH4, CH2F2, CHF3, CF4, NO, H2S, NH3, and for cis- and trans-CHCl=CHCl. Relative values of Q at 300°K were measured for eight additional gases. The data were correlated using the Massey-Mohr theory, assuming an intermolecular potential V(r) = —C/r6, so that Q=b(C/vr)2/5, where vr is the relative velocity and b a known constant; the potential constant C was estimated from formulas for the dispersion, dipole-induced dipole, and dipole-dipole forces. For the nonpolar gases the observed small temperature dependence of Q agrees within experimental error (±3%) with that expected from the temperature dependence of vr. The theoretical values of Q differ by a nearly constant factor from the experimental results; thus relative cross sections (Q*) are predicted with fair accuracy. For the polar gases the Q's are large, decreasing significantly with increasing temperature. Because of the large dipole-dipole interaction, the approximate theoretical treatment (based on the limiting temperature-dependent dipole-dipole contribution to C) accounts only semiquantitatively for the observations.