Photoelectron-Photofragment Coincidence Studies on the Dissociation Dynamics of the OH-CH₄ Complex

Photoelectron-photofragment coincidence (PPC) spectroscopy was used to characterize the energetics and dynamics of the OH + CH₄ → H₂O + CH₃ reaction initiated by photodetachment of the OH^-(CH₄) anion complex. PPC measurements at a photon energy of 3.20 eV yielded stable (OH-CH₄ + e^-) and dissociative (OH + CH₄ (ν₁ or ν₃, v = 0, 1) + e^-) channels. The main channel is dissociation to OH + CH₄ + e^- with a low kinetic energy release (KER), peaking at 0.04 eV. Interpretation of the experimental results was supported by quantum chemistry and quasiclassical trajectory calculations. The anion potential energy surface was constructed at the correlated coupled cluster singles, doubles, and perturbative triples level with augmented correlation consistent polarized valence triple-ζ basis set, and previously calculated neutral potential energy surfaces were used. Quasiclassical simulation of the dynamics of the OH-CH₄ complex was carried out by selecting the momenta and coordinates from the Wigner distribution for the anion, providing the starting point for 4000 trajectories on the neutral potential energy surface. In agreement with the experimental results, most of the trajectories yield slowly recoiling OH + CH₄ reactants while some are trapped in the entrance channel van der Waals well.