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

A selected ion flow drift tube study on the reaction of ground state S i+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 bee n observed, SiC2H3+ and SiC2H4+. The apparent binary (''effective'') r ate coefficient, k(eff), and the product distribution have been determ ined as functions of KE(CM). The reaction rate coefficients of the bin ary (k(BIN)) and ternary (k(3)) channels have been determined by two p rocedures, from the pressure dependence of k(eff) (indicated by index ''PRESSURE'') and by using the observed product distribution (indicate d by index ''PROD''). For KE(CM) < 0.1 eV k(3) obtained by both method s, k(3-PRESSURE) and k(3-PROD), are equal and their dependence on KE(C M) can be expressed in the form: k(3-PROD) = k(3-PRESSURE) alpha (KE(C M))(-0.5). It was observed that dependence of the reaction rate coeffi cient of the binary channel can be expressed also in the form k(BIN-PR ESSURE) 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 cha nnels. 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 prod uct channels-collisional stabilization (product (SiC2H4+) and ''collis ion induced dissociation'' (product SiC2H3+) with probabilities alpha( 1) and alpha(2) (where alpha(2) = 1 - alpha(1)), respectively al and a l 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 tha n alpha(2)), for KE(CM) approximate to 0.2 eV both channels have the s ame probability (alpha(1) = alpha(2)), and for KE(CM) > 0.2 eV the ''c ollision induced dissociation'' of the intermediate complex, producing SiC2H3+, is dominant (alpha(2) > alpha(1)).