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

Abstract

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(³P_J) + C₂H₂(X¹Σ⁺_g) → l/c-C₃H + H(²S_1/2) reaction in experiments conducted with pulsed supersonic beams and variable beam crossing angle configuration at two relative translational energies E_T = 0.80 and 3.5 kJ mol⁻¹. H(²S_1/2) atoms were detected by time-of-flight mass spectrometry with sequential excitation to the ²P^°_J state using a laser beam tuned at the Lyman-α 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-α 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 ΔH^°₀ = −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(³P_J) + C₂H₂(X¹Σ⁺_g) → l/c-C₃H + H(²S_1/2) and C(³P_J) + C₂H₂(X¹Σ⁺_g) → C₃(X¹Σ⁺_g) + H₂(X¹Σ⁺_g) reaction channels in experiments conducted with supersonic continuous beams under 45° crossing angle configuration using “soft” electron-ionisation mass spectrometry time-of-flight detection at E_T = 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 E_T. Formation of C₃(X¹Σ⁺_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 E_T = 3.5 kJ mol⁻¹ obtained with the two different crossed molecular beam techniques for the C(³P_J) + C₂H₂(X¹Σ⁺_g) → l/c-C₃H + H(²S_1/2) channels. An estimate of the exoergicity of the C(³P_J) + C₂H₂(X¹Σ⁺_g) → c-C₃H + H(²S_1/2) pathway from the extent of the translational energy release corroborates the value of ΔH^°₀ = −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.