Quantum Stereodynamics of H₂ Scattering from Co(0001): Influence of Reaction Channels

Diffraction of light molecules from crystalline surfaces is known to provide useful insights into surface topology and molecule/surface interaction. It has been even suggested that molecular diffraction could be used to obtain relevant information about dissociative chemisorption. However, such a direct connection between the diffracted molecules and reactive channels has not been clearly established to date. Because of its low barrier, dissociative chemisorption H₂ on Co(0001) provides an ideal testing ground for examining the influence of reactive channels on diffraction and rotational inelastic scattering of molecules from the surface. Here, we report quantum state-to-state scattering dynamics of aligned H₂ from Co(0001) using time-dependent quantum dynamical methods on a full-dimensional potential energy surface determined from first-principles calculations. Our results show that the Δm_J ≠ 0 type rotational inelastic scattering depends on the initial alignment (m_Ji) of the impinging molecule. The origin of this steric effect was uncovered by quasi-classical trajectory calculations, which show that the Δm_J ≠ 0 events are substantially enhanced by “quasi-reactive” trajectories that access the dissociative channel, characterized by classical turning points that are close to the surface with elongated H₂ interatomic distances. This correlation is further confirmed by reduced-dimensional quantum calculations of the same system but with a fixed H₂ bond, which exhibit a significant reduction of Δm_J ≠ 0 type transitions, due apparently to the inability of H₂ to elongate and dissociate. This theoretical investigation suggests that the impact of reactive channels can be probed by scattering of aligned molecules from reactive metal surfaces.