S. Holloway

The dissociation of diatomic molecules at surfaces

We present an exposition of the various theoretical models currently in use for describing the dynamics of molecular dissociation at surfaces. We begin by outlining the representations of the nuclear and electronic dynamics and how these define the potential energy surfaces for the interactions. Strategies for solving the nuclear motion follow with particular emphasis being paid to a quantum description on the electronic ground state which is in line with experiments employing hyperthermal molecular beams. These can be performed in either a time-dependent or time-independent fashion and both approaches are considered. Following this, the methods that have been developed for treating the dissipative motion as the molecule nears the surface are presented. This is divided into energy loss to the electronic subsystem and to the substrate atomic vibrations. The final part of the review shows how the results of theoretical simulations have been usefully applied to rationalize data obtained from molecular beam scattering experiments.

Electrostatic adsorbate-adsorbate interactions: The poisoning and promotion of the molecular adsorption reaction

The electrostatic interaction between two adsorbates, and, in particular, between an adsorbed atom and an adsorbed or adsorbing molecule is studied. Based on self-consistent calculations of the electrostatic potential around a series of atoms outside a jellium surface, it is shown that a simple electrostatic interaction can explain a large number of experimental observations concerning the influence of pre-adsorbed atoms on the adsorption rate, stability and adsorption configuration of simple molecules on metal surfaces. The role of pre-adsorbed alkalis as promoters and of electronegative atoms like P, S, Cl and O as poisons for the adsorption of electron acceptor molecules like H2, O2, N2 and CO is discussed, as well as the relative magnitude of the influence of the alkalis and the electronegative atoms. The peculiar effects that pre-adsorbed atoms have on molecules like H2O and NH3 are ascribed to the large intra-molecular electron transfer in these molecules.

Microscopic model for the poisoning and promotion of adsorption rates by electronegative and electropositive atoms

Abstract A theoretical framework for comparing binding energies and activation energies for adsorption on surfaces based on the effective medium theory is presented. It is applied to a microscopic description of the mechanisms underlying the promotion and poisoning of the molecular adsorption process on metal surfaces by co-adsorbed species. Based on self-consistent calculations of the electronic structure of adsorbed electropositive and electronegative atoms, estimates of the range and strength of this so-called electronic factor in heterogeneous catalysis are made which are in good agreement with recent experimental studies.

The influence of potential energy surface topologies on the dissociation of H2

In this work we present a theoretical study of the dissociative adsorption of hydrogen molecules from a series of model potential energy surfaces. The aim is to discover those particular topological features in the potential surface which are responsible for determining the vibrational state‐to‐state cross sections in both the dissociated and the scattered flux. The potential energy surface is two‐dimensional, and is chosen to be deliberately simple; a combination of Morse potentials and a Gaussian barrier. A quantum wave packet is chosen to represent the molecule and the dynamics are solved by a spectral grid method. Results show that the location of the barrier influences the scattering cross sections markedly. Early barriers result in vibrationally excited adsorbed species while late barriers produce translationally hot atoms. The individual state distributions resulting from the two model potentials are quite different. In addition, results are given for a potential where the activation barrier is dee...

ROTATIONAL MOTION AND THE DISSOCIATION OF H2 ON CU(111)

The influence of rotational state on the dissociation probability of H2 on Cu(111) has been investigated with 3‐ and 4‐dimensional close‐coupling wave packet calculations. Recent experimental results have shown that the energetic threshold for dissociative adsorption increases and then decreases as the J state is continuously increased. This trend can be faithfully reproduced by modeling the H2 as a planar (cartwheel) rotor scattering from a flat surface. The agreement disappears when the model is extended to a 3‐dimensional rotor. Further, the degenerate mJ states have a spread of dissociation probabilities which results in a broad smearing of the dissociation threshold. This effect, which is absent from experiment, increases with Ji. These shortcomings can be partially corrected by corrugating the potential in the azimuthal coordinate in accord with recent ab initio results. The dynamical calculations also exhibit strong rotational inelasticity for the scattered fraction, during dissociation. Since this...

A theoretical study of the dissociation of H2/Cu

We present calculations for the dissociative adsorption of hydrogen molecules on a Cu surface as a function of initial translational energy and vibrational quantum state. Classical, semiclassical, and fully quantum calculations are performed and the results compared. The potential energy surface was based upon a total energy calculation for H2 on a small Cu cluster and has been previously employed in dynamical simulations. Our results show that for low primary beam energies, dissociation occurs primarily via tunneling through the activation barrier in the vibrational coordinate. Populating the initial vibrational states is shown to enhance reactivity, but not simply by a total energy shift. By changing the hydrogen isotope it is shown that tunneling effects can persist up to quite high molecular masses. This occurs because the activation barrier lies in the vibrational coordinate, where the reduced mass of the molecule determines the dynamics.

Charge transfer, vibrational excitation, and dissociative adsorption in molecule–surface collisions: Classical trajectory theory

The consequences of charge transfer processes occurring when a molecular beam of diatomic molecules is directed upon a solid surface are here considered. In analogy with resonance electron scattering from molecules or harpooning processes in atom–diatom collisions, the incident beam could either be scattered into a highly vibrationally excited molecular state, dissociatively scattered, or dissociatively adsorbed due to formation of temporary negative molecular ions which enable redistribution of the incident translation energy of the beam into intramolecular degrees of freedom. In this work, the exact classical trajectories for the diatomic molecule, including internal vibrational motion, are calculated for motion over model diabatic potential surfaces in which surface hopping due to charge transfer/harpooning is accounted for. Connections between classes of trajectories and topological features of the potential energy surfaces (PES) are illustrated. The model is used to study the average translational to...

Steering effects in non-activated adsorption

Abstract Classical and quantum calculations of the dissociation dynamics of H 2 on W(100) have been performed on an ab initio PES. The results show conclusively that at low translational energies the dissociation is dominated by strong steering in an essentially direct process. Starting from a value near unity, the dissociation falls with increasing energy because the steering has less time to operate and is therefore less effective. No precursor state is involved in the process. By examining the quantum flux we see that molecules oriented with their axes perpendicular to the surface reflect at high translational energy, but not at low translational energy. Some molecular trapping occurs as a result of rotational excitation which gives rise to sharp peaks in the quantum dissociation probability.

The dissociative adsorption of hydrogen : two-, three-, and four-dimensional quantum simulations

A quantum wave packet calculation for the activated dissociative adsorption of H2 is presented. Restricting the motion of the molecule to lie within a plane normal to the surface we have treated all four molecular degrees of freedom exactly. We compare results obtained using two‐, three‐, and four‐dimensional simulations on the same potential and show that by restricting the molecular orientation, important dynamical effects are lost. The potential employed in the calculations has been obtained using the effective medium approximation. In the simulations it has been possible to treat dissociation, rotations and diffraction on an equal footing. By including a rotational degree of freedom, it is seen that strong orientational effects occur near to the transition state and result in an anisotropic selectivity in the dissociation. By examining the state‐to‐state scattering probabilities, it is possible to use the nonreacting (scattered) fraction to provide information on the reactive potential energy surface.

On the dynamics of the associative desorption of H2

The dynamics of activated associative desorption is discussed with particular reference to the system H2–Cu and to the partitioning of the energy released among the various product degrees of freedom. It is argued that a simple theory based on transition‐state concepts should hold for this system because the potential energy surface (PES) divides naturally into reactant and product regions, separated by a ‘‘seam’’ or ‘‘ridge’’ at which it is reasonable to assume a thermal distribution of desorbing trajectories. Using a PES constructed in accordance with available electronic structure calculations we consider the angular distributions and translational, vibrational, and rotational energy distributions of the desorbing molecules. It is shown that, whereas the rotational energy reflects the surface temperature, the vibrational energy is markedly enhanced because the energetically low‐lying regions of the ridge in the PES correspond to an H–H bond distance that is distended as compared with the gas‐phase equi...