Crossed-beam studies on the dynamics of the C + C2H2 interstellar reaction leading to linear and cyclic C3H + H and C3 + H2

The dynamics of the C + C2H2 reaction has been investigated using two crossed molecular beam apparatus of different concepts. Differential cross sections have been obtained for the C(3PJ) + C2H2(X1S+g ) - l/c-C3H + H(2S1/2) reaction in experiments conducted with pulsed supersonic beams and variable beam crossing angle configuration at two relative translational energies ET = 0.80 and 3.5 kJ mol_1. H(2S1/2) atoms were detected by time-of-flight mass spectrometry with sequential excitation to the 2P1J state using a laser beam tuned at the Lyman-a transition around 121.567 nm and ionisation by a second laser beam at 364.7 nm. Doppler–Fizeau spectra of the H atoms were recorded with the Lyman-a laser beam parallel to the relative velocity vector of the reagents. These spectra could be fitted using a forward convolution process including two contributions. The recoil energy distribution functions of both contributions were taken as statistical, with total energies corresponding to a reaction exoergicity DH10 = _11 kJ mol_1 for the major one, assigned to the c-C3H + H path, and _1.5 kJ mol_1 for the minor one, assigned to the l-C3H + H path. The angular distribution was taken as also statistical (uniform) for the minor contribution but somewhat backward peaked for the major one. Differential cross sections have been obtained for the three energetically allowed and competitive C(3PJ) + C2H2(X1S+g ) - l/c-C3H + H(2S1/2) and C(3PJ) + C2H2(X1S+g ) - C3(X1S+g ) + H2(X1S+g ) reaction channels in experiments conducted with supersonic continuous beams under 451 crossing angle configuration using ‘‘soft’’ electron-ionisation mass spectrometry time-of-flight detection at ET = 3.5 and 18.5 kJ mol_1. From measurements of angular and time-of-flight distributions at the mass-to-charge ratios m/z = 37 and 36, product angular and translational energy distributions have been determined in the centre-of-mass system for both linear- and cyclic-C3H isomer formation as well as for C3 production. The variations of the dynamics and product branching ratios with collision energy have been characterized. The ratios c-C3H/l-C3H and C3/C3H from the C(3P) reactions have been both found to decrease with increasing ET. Formation of C3(X1S+g ) from the C(3P) reaction has been rationalized in terms of intersystem crossing between triplet and singlet C3H2 potential energy surfaces. There is good agreement between the results at ET = 3.5 kJ mol_1 obtained with the two different crossed molecular beam techniques for the C(3PJ) + C2H2(X1S+g ) - l/c-C3H + H(2S1/2) channels. An estimate of the exoergicity of the C(3PJ) + C2H2(X1S+g ) - c-C3H + H(2S1/2) pathway from the extent of the translational energy release corroborates the value of DH10 = _11 kJ mol_1 obtained from the Doppler–Fizeau measurements. The overall results have been discussed in the light of the available theoretical information on the relevant triplet and singlet C3H2 potential energy surfaces, and compared with the results of previous related kinetic and dynamic work as well as of theoretical calculations of the reaction dynamics.