Translational Energy Dependence of the Reaction Cross Section forKplus;CH3Irarr;KIplus;CH3from 0.1 to 1 eV (c.m.)

摘要

A molecular beam study of the reactionKplus;CH3Irarr;KIplus;CH3over 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 coordinateslpar;THgr;,thinsp; PHgr;rpar;of the inhyphen;plane and outhyphen;ofhyphen;plane angular distributions of KI measured by a twohyphen;filament differential surface ionization detector mounted on a goniometer. By seeding the CH3I with a variety of lighter gases, its velocity could be changed fromdgr; ap; 530ndash;2330thinsp;msol;sec. The average center of mass collision energy,Emacr;, 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 vsEmacr;were obtained with a precision ofap; plusmn; 10percnt;. The supersonic CH3I beam speed distributions were quite narrowlpar;FWHMap; 20percnt;rpar;. The K speed distribution was essentially Maxwellian. The measured total reaction cross section rises to a maximum atEmacr;equals;0.18lpar;plusmn; 0.03rpar;thinsp;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.