Modified Shepard interpolation method applied to trapping mediated adsorption dynamics

The modified Shepard (MS) interpolation method is applied to H(2)/Pd(111) to investigate its performance for a system for which dissociative adsorption takes place through a direct as well as an indirect (i.e. dynamic trapping) mechanism. The input data were obtained from an available accurate potential energy surface (PES) interpolated by using the corrugation reducing procedure (CRP). Dissociation probabilities obtained from classical trajectory calculations with the MS-PES are in very good agreement with the results for the CRP-PES. Thus, this study confirms the MS method as a promising tool to tackle low energy adsorption dynamics of polyatomic molecules, usually dominated by trapping.

Seven-dimensional microcanonical treatment of hydrogen dissociation dynamics on Cu(111): Clarifying the essential role of surface phonons

A simple picture of the hydrogen dissociation/associative desorption dynamics on Cu(111) emerges from a two-parameter, full dimensionality microcanonical unimolecular rate theory (MURT) model of the gas-surface reactivity. Vibrational frequencies for the reactive transition state were taken from density functional theory calculations of a six-dimensional potential energy surface [Hammer et al., Phys. Rev. Lett. 73, 1400 (1994)]. The two remaining parameters required by the MURT were fixed by simulation of experiments. These parameters are the dissociation threshold energy, E(0)=79 kJmol, and the number of surface oscillators involved in the localized H(2)Cu(111) collision complex, s=1. The two-parameter MURT quantitatively predicts much of the varied behavior observed for the H(2) and D(2)Cu(111) reactive systems, including the temperature-dependent associative desorption angular distributions, mean translational energies of the associatively desorbing hydrogen as a function of rovibrational eigenstate, etc. The divergence of the statistical theory's predictions from experimental results at low rotational quantum numbers, J < or approximately 5, suggests that either (i) rotational steering is important to the dissociation dynamics at low J, an effect that washes out at high J, or (ii) molecular rotation is approximately a spectator degree of freedom to the dissociation dynamics for these low J states, the states that dominate the thermal reactivity. Surface vibrations are predicted to provide approximately 30% of the energy required to surmount the activation barrier to H(2) dissociation under thermal equilibrium conditions. The MURT with s=1 is used to analytically confirm the experimental finding that partial differential "E(a)(T(s))" partial differential E(t)= -1 for eigenstate-resolved dissociative sticking at translational energies E(t)<E(0)-E(v)-E(r). Explicit treatment of the surface motion (i.e., surface not frozen at T(s)=0 K) is a relatively novel aspect of the MURT theoretical approach.

Dynamical pruning of static localized basis sets in time-dependent quantum dynamics

We investigate the viability of dynamical pruning of localized basis sets in time-dependent quantum wave packet methods. Basis functions that have a very small population at any given time are removed from the active set. The basis functions themselves are time independent, but the set of active functions changes in time. Two different types of localized basis functions are tested: discrete variable representation (DVR) functions, which are localized in position space, and phase-space localized (PSL) functions, which are localized in both position and momentum. The number of functions active at each point in time can be as much as an order of magnitude less for dynamical pruning than for static pruning, in reactive scattering calculations of H2 on the Pt(211) stepped surface. Scaling of the dynamically pruned PSL (DP-PSL) bases with dimension is considerably more favorable than for either the primitive (direct product) or DVR bases, and the DP-PSL basis set is predicted to be three orders of magnitude smaller than the primitive basis set in the current state-of-the-art six-dimensional reactive scattering calculations.

The role of molecular rotation in activated dissociative adsorption on metal surfaces

The role of molecular rotation in dissociative adsorption of H2 on the activated NiAl(110) metal surface is systematically investigated by means of classical dynamics calculations performed on ab initio six-dimensional potential energy surfaces. The calculations show that molecules rotate abruptly when they are close to the surface and that this rotation allows the molecules to adopt the orientation that is more convenient for dissociation (i.e., nearly parallel to the surface). Also, in reactive sectors of the NiAl(110) unit cell, there is an "angular threshold" below which molecules cannot dissociate. This angular threshold goes down as the incidence energy increases, which explains the rise of the dissociation probability and the fact that it reaches a value close to 1 at incidence energies of the order of 2 eV. The fact that switching on molecular rotation favors dissociation establishes a competition between dissociation and rotational excitation of reflected molecules above the dissociation threshold. Measurements on rotational excitation might thus bring indirect evidence on the dissociation dynamics. Sample calculations for nonactivated Pd(111) and activated Cu(110) metal surfaces suggest that some of these conclusions may be of general validity.