Molecular Beam Study of the K+CH3I Reaction: Energy Dependence of the Detailed Differential Reactive Cross Section

A molecular beam study has been carried out on the reaction of a velocity‐selected K beam with a crossed beam of CH₃I (from a glass capillary‐array source). Velocity analysis of the angular distribution of the reactively scattered KI has yielded detailed differential cross sections at three incident relative kinetic energies $\text{Ē= 1.77,}$ 2.80, and 3.71 kcal/mole. The center‐of‐mass (c.m.) differential cross sections were obtained from the lab data by a least‐squares computer ``inversion'' program providing for deconvolution of beam velocity distributions. The c.m. cross section is ``backward'' peaked (KI recoiling opposite to the incident K, in accord with the literature) with a very sharp (30%–40% FWHM) recoil velocity distribution which maximizes at 80% of the energetically allowed KI recoil velocity. This distribution shifts systematically to higher velocities with increasing incident collision energy. Averaging over all angles yields an overall product c.m. translational energy distribution which shifts upwards as the incident translation energy is increased. The average energy released into relative product translation varies from 15 to 17.5 kcal/mole over the above range of Ē; the average fraction of the total available energy released is $\text{≈ 0.58.}$ thus incident translational energy is efficiently converted to relative translational energy of product recoil. The detailed differential cross sections at each energy were scaled relative to one another and suitably integrated to yield relative values of the total reactive cross section. The present results confirm previous observations of an increase in reaction cross section with increasing Ē up to $\text{≈ 4\hspace{0.17em}}\mathrm{kcal/mole}.$