State-selected C2H2+C2H4 reaction: Controlled by dynamics or statistics?
Résumé
The reaction between acetylene ion and ethylene is revisited to investigate possible dynamical effects in one of the reactions, which are considered as benchmark systems for statistical models. Reactant ions are produced by photoionisation with synchrotron radiation and are selected in internal energy by a coincidence method between threshold photoelectrons and photoions. Measured absolute reaction cross-sections decrease with collision energy (0.1-1 eV CM), but increase with acetylene ion vibrational energy for the three exothermic channels giving C4H5+, C3H3+ and C2H4+ ionic products. Even though RRKM calculations are shown to fit rather well experimental results for both the bimolecular (C2H2+ + C2H4) and unimolecular (C4H6+) systems, some experimental results clearly indicate dynamical effects in this reaction for the C2H4+ production channel, in particular the strongly backward peaked C2H4+ angular distribution. We propose an alternative model combining reaction control both by dynamics and by statistics: the first step in the reactant approach is the capture between reactants, followed by a charge transfer process controlled by dynamics. In a second step, either the two reactants separate leading to C2H4+ ionic products, or there is a rearrangement into 1,3-butadiene (C4H6+) ion which dissociates statistically into C4H5+ + H and C3H3+ + CH3 products. Charge transfer is shown to be possible at intermediate intermolecular distances, slightly shorter than the capture distance, provided that one takes into account the charge-quadrupole interaction in addition to the ion-induced dipole interaction. This work clearly shows that a good fit of experimental data by RRKM calculations does not prove that the reaction elementary mechanism is controlled by statistics.