Dynamics of the C+C2H2 reaction from differential and integral cross-section measurements in crossed-beam experiments

Abstract

The reaction between atomic carbon and acetylene has been investigated using complementary crossed molecular beam techniques. Differential cross sections have been obtained for the reactions of both ground and excited carbon atoms, C(3PJ, 1D2)+C2H2(X 1Σg+), in experiments conducted with continuous supersonic beams, mass spectrometric detection, and time-of-flight analysis at a relative translational energy of 29.3 kJ mol−1. The reaction C(3PJ)+C2H2(X 1Σg+) has been found to lead to C3H+H and C3+H2 products in comparable amounts. Both H and H2 elimination pathways are found to proceed through the formation of a C3H2 long-lived intermediate complex whose lifetime may be comparable to its rotational period. The spin-forbidden H2 elimination channel is attributed to the occurrence of intersystem-crossing between the triplet and singlet manifolds of the C3H2 potential-energy surfaces. The reaction C(1D2)+C2H2(X 1Σg+) has been found to lead to formation of C3H+H, with a C3H center-of-mass angular distribution strongly forward peaked, indicating a short-lived intermediate complex. Integral cross sections have been obtained for the C(3PJ)+C2H2(X 1Σg+)→C3H+H(2S1/2) reaction in experiments conducted with pulsed, supersonic molecular beams in the range of relative translational energies ET=0.38−25.5 kJ mol−1, the H(2S1/2) product being detected by laser-induced fluorescence. The reaction has been found to be without a barrier, relative integral cross sections being proportional to (ET)−0.80±0.03 below ET=10 kJ mol−1. These findings provide direct evidence that the C(3PJ)+C2H2 reaction can occur under the physical conditions prevailing in dense interstellar clouds and, in particular, that it may be the source of both C3H and C3 species in these extreme environments.