Merging Beams, A Different Approach to Collision Cross Section Measurements

TLDR

The technique is applicable to ion‑neutral, ion‑ion, and neutral‑neutral reactions. Two co‑propagating molecular beams are merged along a common axis, and the instrument’s principles and design are described. The merging‑beam method measures two‑body collision cross sections from thermal to several hundred eV, enabling high‑resolution studies of labile species; it was applied to resonant charge transfer of Ar, yielding results that agree with prior data and validate the technique.

Abstract

A different approach has been developed to measure two-body collision cross sections in the energy range from thermal to several hundred electron volts. Two molecular beams are merged and move in the same direction along a common axis. Ion-neutral reactions are discussed but the techniques could also be used for ion-ion and neutral-neutral collisions. Advantages of the merging beams technique over conventional beam-gas and crossed beam methods include accessibility (with good energy resolution) to the energy region from a few tenths of one to a few electron volts; the possibility of measuring cross sections for two general, labile species; and relative ease in collecting and detecting products for total reaction cross section measurements. The principles of the method and details of the instrument are discussed. A cross section for the resonant charge transfer of Ar was measured. Reasonable agreement with some previous measurements, and other tests, indicate that the basic principles of a merging beams technique are valid and that the apparatus performs satisfactorily.