AB-INITIO QUANTUM AND MOLECULAR-DYNAMICS OF THE DISSOCIATIVE ADSORPTION OF HYDROGEN ON PD(100)

The dissociative adsorption of hydrogen on Pd(100) has been studied by ab initio quantum dynamics and ab initio molecular-dynamics calculati ons. Treating all hydrogen degrees of freedom as dynamical coordinates implies a high dimensionality and requires statistical averages over thousands of trajectories. An efficient and accurate treatment of such extensive statistics is achieved in a three-step approach: In a first step we evaluate the ab initio potential-energy surface (PES) fore nu mber of appropriate points in configuration space. Then (as step 2) we determine an analytical representation that serves as an interpolatio n between the actually calculated points. In an independent third step dynamical calculations are performed on the analytical representation of the PES. Thus the dissociation dynamics is investigated without an y crucial assumption except for the Born-Oppenheimer approximation whi ch is anyhow employed when density-functional-theory calculations are performed. The ab initio molecular dynamics is compared to detailed qu antum-dynamical calculations on exactly the same ab initio PES. The oc curence of quantum oscillations in the sticking probability as a funct ion of kinetic energy is addressed. They turn out to be very sensitive to the symmetry of the initial conditions. At low kinetic energies st icking is dominated by the steering effect, which is illustrated using classical trajectories. The steering effect depends on the kinetic en ergy, but not on the mass of the molecules, as long as no energy trans fer to the substrate atoms is considered. The comparison between quant um and classical calculations of the sticking probability shows the im portance of zero-point effects in the hydrogen dynamics. The dependenc e of the sticking probability on the angle of incidence is analyzed; i t is found to be in good agreement with experimental data. The results show that the determination of the potential-energy surface combined with high-dimensional dynamical calculations, in which all relevant de grees of freedom are taken into account, leads to a detailed understan ding of the dissociation dynamics of hydro en at a transition metal su rface.