Quantum Stereodynamics Of H-2 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 H-2 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 H-2 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 Delta m(J) not equal 0 type rotational inelastic scattering depends on the initial alignment (m(J)) of the impinging molecule. The origin of this steric effect was uncovered by quasi-classical trajectory calculations, which show that the Delta m(J) not equal 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 H-2 interatomic distances. This correlation is further confirmed by reduced-dimensional quantum calculations of the same system but with a fixed H-2 bond, which exhibit a significant reduction of Delta m(J) not equal 0 type transitions, due apparently to the inability of H-2 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.