Dipole—Dipole Scattering in Molecular Beams. Variation of Total Cross Section with Velocity and Rotational Overlap

Abstract

Total cross sections were measured for scattering of velocity-selected beams of alkali halide molecules KCl, CsCl, and CsBr by HBr and CH3I and, for comparison, for Kr and Xe. The pressure in the scattering chamber was monitored by a second crossed beam of alkali atoms, which allowed direct measurement of the ratios of cross sections for different molecules interacting with the same scattering gas. As expected from the first-order selection rules, the results indicate that the contribution of rotationally inelastic transitions produced by the angle-dependent dipole—dipole force depends strongly on the extent to which the rotational spectra of the two molecules overlap. Inelastic scattering appears to be dominant for the CH3I systems, where the overlap is very good. The cross sections are ∼5000 Å2 (compared with ∼1400 Å2 for elastic van der Waals scattering; both corrected for angular resolution) and the velocity dependence is much weaker than for elastic scattering. In the Appendix simple approximate formulas are derived from which the first-order inelastic dipole—dipole cross section can be calculated without numerical integrations over impact parameters and rotational distributions.