SELECTED-ION FLOW DRIFT-TUBE STUDIES OF THE REACTION OF SI+(P-2) WITH C2H4 - OBSERVATION OF THE TERNARY REACTION WITH 2 CHANNELS - COLLISIONAL STABILIZATION AND COLLISIONAL DISSOCIATION

A selected ion flow drift tube study on the reaction of ground state Si+2P With C2H4 has been carried out in the pressure range from 0.14 to 0.52 Torr for average relative centre-of-mass kinetic energies, KE(CM), from near thermal to approximate to 2 eV, Two product ions have been observed, SiC2H3+ and SiC2H4+. The apparent binary (''effective'') rate coefficient, k(eff), and the product distribution have been determined as functions of KE(CM). The reaction rate coefficients of the binary (k(BIN)) and ternary (k(3)) channels have been determined by two procedures, from the pressure dependence of k(eff) (indicated by index ''PRESSURE'') and by using the observed product distribution (indicated by index ''PROD''). For KE(CM) < 0.1 eV k(3) obtained by both methods, k(3-PRESSURE) and k(3-PROD), are equal and their dependence on KE(CM) can be expressed in the form: k(3-PROD) = k(3-PRESSURE) alpha (KE(CM))(-0.5). It was observed that dependence of the reaction rate coefficient of the binary channel can be expressed also in the form k(BIN-PRESSURE) alpha (KE(CM))(-0.5). This similarity in the dependence on KE(CM) may indicate that both binary and ternary channels are proceeding via the same rate-determining process prior to separation into two channels. The situation is different for KE(CM) > 0.1 eV. Here k(3-PROD) decreased more rapidly with KE(CM) than k(3-PRESSURE), indicating that in the collisions of He atoms with the excited intermediate collision complex the dissociation of (SiC2H4)(+)* to SiC2H3+ takes place. Here we report the first observation of a ternary reaction having two product channels-collisional stabilization (product (SiC2H4+) and ''collision induced dissociation'' (product SiC2H3+) with probabilities alpha(1) and alpha(2) (where alpha(2) = 1 - alpha(1)), respectively al and al have been determined from k(3-PROD) and k(3-PRESSURE) For KE(CM) < 0.1 eV the stabilization channel is dominant (alpha(1) much greater than alpha(2)), for KE(CM) approximate to 0.2 eV both channels have the same probability (alpha(1) = alpha(2)), and for KE(CM) > 0.2 eV the ''collision induced dissociation'' of the intermediate complex, producing SiC2H3+, is dominant (alpha(2) > alpha(1)).