Trajectory surface hopping study of the C + CH reaction

The influence of electronically nonadiabatic transitions in the C(³P_g) + CH(X ²Π) → C₂(X ¹Σ⁺_g, a ³Π_u) + H(²S_g) reaction is investigated by using Tully's fewest-switches version of the trajectory surface hopping method. A diabatic model of the first two ²A' potential energy surfaces coupled by a conical intersection is used. The diatomic CH has the internal state (ν = 0, j = 0) and batches of 20 000 trajectories are computed for four collision energies, E = 0.1, 0.3, 0.5 and 0.7 eV. We find that the reaction dynamics does not exhibit a statistical character, despite the existence of deep wells along the reaction path. Only the distribution of scattering angle shows a good agreement between trajectories and phase space theory results. A strong excess of vibrational energy is disposed on both electronic products C₂(X ¹Σ⁺_g) and C₂(a ³Π_u), correlated with a lack of recoil energy. With all trajectories starting on a single potential surface, we obtain an electronic branching ratio X : a close to 2 : 3, only slightly dependent on the collision energy.