Translational Energy Dependence of the Reaction Cross Section for K+CH3I→ KI+CH3 from 0.1 to 1 eV (c.m.)

Abstract

A molecular beam study of the reaction K+CH3I→ KI+CH3 over an extended range of collision energy is described. A thermal K beam is crossed with a seeded supersonic nozzle beam of CH3I. The excitation function (energy dependence of the reaction cross section) for this reaction was measured from 0.1 to 1 eV (c.m.). The total flux of the KI reaction product was determined by integration over the laboratory spherical polar coordinates (Θ, Φ) of the in-plane and out-of-plane angular distributions of KI measured by a two-filament differential surface ionization detector mounted on a goniometer. By seeding the CH3I with a variety of lighter gases, its velocity could be changed from δ ≈ 530–2330 m/sec. The average center of mass collision energy, Ē, was thereby varied from 0.1 to 1 eV. The densities of both beams were monitored during the scattering experiments, so that relative cross sections vs Ē were obtained with a precision of ≈ ± 10%. The supersonic CH3I beam speed distributions were quite narrow (FWHM ≈ 20%). The K speed distribution was essentially Maxwellian. The measured total reaction cross section rises to a maximum at Ē=0.18(± 0.03) eV, beyond which it decreases continuously. No evidence was found for the occurrence of additional (high energy) processes, such as chemiionization or formation of alternative products.