Beam Studies of the Energy Dependence of Hot-Hydrogen-Atom Reactions with Cyclohexane

Beam studies of the energy dependence of hydrogen-atom-molecule reactions having appreciable thresholds are reported. Such investigations spanning the entire chemical range promise to be of importance in understanding the dynamics of reactive collisions. This work examines the interactions with solid cyclohexane of tritium ion beams, both T+ and T2+, having controlled energy between 1–200 eV. These ions undergo efficient charge transfer at the surface of the target to form hot tritium atoms of similar velocity. Labeled products are formed by hot-atom reactions, and their yields are measured by radio–gas chromatography. At high energies (≥ 30 eV), absolute yields for T+ beams are close to those found by the nuclear recoil technique. With T2+, high-energy yields are about 20% lower; this apparently corresponding to the fraction of ions forming unreactive T2 upon charge exchange. At lower energies, yields decline. The threshold for the T for H replacement is at 1.5 ± 0.5 eV. Hexenes and hexanes appear to result from processes in which a C–C as well as a C–H bond is ruptured. The threshold for such double displacement is at 4.5 ± 0.5 eV. Provided certain rather drastic assumptions are made, the energy dependence of the yields can be used to derive approximate excitation functions.