7D Quantum Dynamics of H2 Scattering from Cu(111): The Accuracy of the Phonon Sudden Approximation

The correct prediction of elementary processes occurring when H-2 scatters from a metal surface is one of the main challenges of surface science. In the field, the scattering of H-2 from Cu(111) has been considered a prototype system for activated dissociative chemisorption. Experimental and theoretical work suggested that a proper description of some scattering experiments on this system might require going beyond the static surface approximation, to consider how the motion of the Cu atoms affects the scattering event. Previous work suggested that important effects of phonons on the dynamics can be incorporated in the Potential Energy Surface (PES) by including four degrees of freedom, that have approximately additive couplings with the hydrogen molecule: the 3 dimensional motion of the nearest 1st layer copper atom and the displacement of the nearest 2nd layer copper atom along the direction perpendicular to the surface [3]. In the present work, we extend the 6D dynamical model by including the perpendicular motion of the 2nd layer surface atom and we study this novel dynamical model with two techniques: an approximate method based on the Phonon Sudden Approximation (PSA) and an exact description using 7D wavepacket quantum dynamics. We consider how the inclusion and the excitation of the lattice degree of freedom affect some relevant processes: dissociative chemisorption, vibrational excitation of H-2 and state-to-state scattering probabilities fully resolved with respect to the vibrational states of the surface. We show that the PSA works in an excellent way for the system, thereby suggesting that this might be a viable way to study higher dimensional quantum models, incorporating four surface degrees of freedom that appear to be most relevant for H-2 scattering.