Selected ion flow drift tube studies of the reaction of Si + ( 2P) with C 2H 4. Observation of the ternary reaction with two channels: collisional stabilization and collisional dissociation

Abstract

A selected ion flow drift tube study on the reaction of ground state Si +( 2P) with C 2H 4 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 ≈ 2 eV. Two product ions have been observed, SiC 2H 3 + and SiC 2H 4 +. 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 ∝ ( 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 ∝ ( 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 (SiC 2H 4) +∗ to SiC 2H 3 + takes place. Here we report the first observation of a ternary reaction having two product channels—collisional stabilization (product (SiC 2H 4 +) and “collision induced dissociation” (product SiC 2H 3) with probabilities α 1 and α 2 (where α 2 = 1 - α 1), respectively. α 1 and α 2 have been determined from k 3-PROD and k 3-PRESSURE. For KE CM < 0.1 eV the stabilization channel is dominant ( α 1 ⪢ α 2), for KE CM ≈ 0.2 eV both channels have the same probability ( α 1 = α 2), and for KE CM > 0.2 eV the “collision induced dissociation” of the intermediate complex, producing SiC 2H 3 +, is dominant ( α 2 > α 1).