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

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Abstract
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

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