Johan Strömquist

The dynamics of H absorption in and adsorption on Cu(111)

Abstract We have studied theoretically the dynamics of H adsorption on and absorption in Cu(111) using a classical molecular dynamics approach. A key ingredient of this study is plane-wave and pseudopotential calculations of potential energy curves for the major high symmetry sites. These calculations are based on density functional theory and the generalized gradient approximation. The extracted chemisorption bond parameters from the energy curves are in good agreement with available experimental data. We find that the calculated energy barriers for absorption of an adsorbed atom are lowered dramatically by relaxations of the Cu atoms; these barriers are so low that, even in the rigid surface lattice situation, the absorption of an incident H atom is non-activated for impacts close to the so-called fcc and hcp hollow sites. The model interaction potential that we have used in the dynamics calculations is determined from the calculated potential energy curves and its form is taken from a semi-empirical effective medium theory for binary compounds. The main results of the dynamics calculations are: the relaxations and thermal fluctuations of the Cu atom do not affect the absorption of H in the surface; the energy transfer to the phonons is rather inefficient so the H atom has to make a large number of collisions with the surface atoms before it sticks either in the surface adsorption well or in the subsurface absorption well; a simple estimation shows that the energy transfer to electron-hole pairs can be as efficient as the energy transfer to phonons; our results are consistent with experiments, which indicate that subsurface sites can be populated by an incident atomic H beam and show that the scattering probability is small.

A first-principles potential energy surface for Eley–Rideal reaction dynamics of H atoms on Cu(111)

We have performed first-principles total-energy calculations of low-dimensional sections of the electronically adiabatic potential energy surface (PES) that are relevant for the Eley–Rideal (ER) reaction of H atoms on a rigid Cu(111) surface. These calculations were performed within density-functional theory using a plane-wave and pseudopotential method and the generalized gradient approximation for the exchange-correlation energy. The calculated energy points for various configurations of one and two atoms on the Cu(111) surface were used to construct a model PES that can be used in ER reaction dynamics calculations.

Motion of ‘‘hot’’ oxygen adatoms on corrugated metal surfaces

We have investigated the likelihood of a high transient mobility for hyperthermal adatoms resulting from dissociative adsorption of a diatomic molecule, in particular O2 on Al(111), using dynamical simulations on model potentials fitted to available first principle data. We find no evidence for a large transient mobility, compatible with the conclusions by Brune et al. [Phys. Rev. Lett. 68, 624 (1992)], for hyperthermal O atoms moving on the chemisorption potential energy surface for the O/Al(111) system. Our findings are more compatible with the STM results for O2 on Pt(111). We have also examined the possibility that one of the O atoms moves further away from the surface as a neutral species. That could result in that some O atoms leave the surface as neutral species (abstraction) or extended trajectories along the surface if a weakly bound neutral state for the O atom exists with sufficiently long lifetime.

Ballistic motion and abstraction in dimer dissociation at surfaces: Cl2 on K

Non-traditional dissociation dynamics at surfaces, with charge transfer and diabatic motion, is studied for the model system of Cl-2 molecules impinging on the metal K. Three-dimensional molecular dynamics on simple but physically reasonable potential-energy surfaces is used to investigate the final fates of the dissociation fragments (Cl, Cl-) of the molecular ion, Cl-2(-), that is formed by electron transfer (harpooning) to Cl-2 from the K surface. The model study demonstrates that both abstraction (emission of neutrals) and ballistic motion can result from dissociative chemisorption. Our qualitative reasoning has strong implications on other chemisorption systems, like O-2 on alkali and other metals and halogens on Si, e.g., a proposed explanation for ballistically moving O atoms on Al.

QUANTUM-MECHANICAL WAVE PACKET CALCULATION OF PHOTOINDUCED SURFACE REACTION: O2/PT(111)

We present a two-dimensional quantum-mechanical wave packet study of photoinduced reaction of O2 on Pt(111) within a mechanism of hot electron/hole excitation of a molecular resonance. Based on three constructed potential energy surfaces including the molecule-surface and intra-molecular coordinates, photoexcitation is simulated by nonadiabatic electronic transitions between the ground state (the chemisorbed O2−) and a negative ion resonance state (the O22− shape resonance) or a neutral O2 state. The wave packet dynamics exhibits a fast energy exchange between the two bonds, giving a comparable yield for desorption and dissociation. The calculated branching ratio, BR=0.5–1.0, between desorption and dissociation and the mean kinetic energy of the desorbed molecules 〈Ekin〉/2kB= 990 K are comparable with the reported experimental data measured in desorption by nanosecond laser pulses, while the vibrational temperature is first predicted by this calculation. In addition our results indicate the importance of ...

Density functionals and van der Waals interactions at surfaces

Abstract Recent progress in the construction of a density functional for van der Waals interactions is described, towards the background of successes of local and semi-local exchange-correlation density functionals for dense matter. The functional should apply for atoms, molecules, surfaces, and other objects. The only input needed consists of the electron densities of the interacting fragments or surfaces, and their static polarizabilities or static image planes, respectively, which can be easily evaluated in a ground-state density-functional calculation. Results for well separated atoms, molecules, and atoms/molecules outside surfaces are in agreement with those of other, more elaborate, calculations. A description of the asymptotic van der Waals interaction between two parallel surfaces is also given.