M. Alducin

Competition between electron and phonon excitations in the scattering of nitrogen atoms and molecules off tungsten and silver metal surfaces

We investigate the role played by electron-hole pair and phonon excitations in the interaction of reactive gas molecules and atoms with metal surfaces. We present a theoretical framework that allows us to evaluate within a full-dimensional dynamics the combined contribution of both excitation mechanisms while the gas particle-surface interaction is described by an ab initio potential energy surface. The model is applied to study energy dissipation in the scattering of N(2) on W(110) and N on Ag(111). Our results show that phonon excitation is the dominant energy loss channel, whereas electron-hole pair excitations represent a minor contribution. We substantiate that, even when the energy dissipated is quantitatively significant, important aspects of the scattering dynamics are well captured by the adiabatic approximation.

Low sticking probability in the nonactivated dissociation of N2 molecules on W(110)

The six-dimensional potential energy surface for the dissociation of N2 molecules on the W(110) surface has been determined by density functional calculations and interpolated using the corrugation reducing procedure. Examination of the resulting six-dimensional potential energy surface shows that nonactivated paths are available for dissociation. In spite of this, the dissociation probability goes to a very small value when the impact energy goes to zero and increases with increasing energy, a behavior usually associated with activated systems. Statistics on the dynamics indicate that this unconventional result is a consequence of the characteristics of the potential energy surface at long distances. Furthermore, two distinct channels are identified in the dissociation process, namely, a direct one and an indirect one. The former is responsible for dissociation at high energies. The latter, which includes long-lasting dynamic trapping in the vicinity of a potential well above the W top position, is the leading mechanism at low and intermediate energies.

The role of exchange-correlation functionals in the potential energy surface and dynamics of N2 dissociation on W surfaces

We study the dissociative adsorption of N(2) on W(100) and W(110) by means of density functional theory and classical dynamics. Working with a full six-dimensional adiabatic potential energy surface (PES), we find that the theoretical results of the dynamical problem strongly depend on the choice of approximate exchange-correlation functional for the determination of the PES. We consider the Perdew-Wang-91 [Perdew et al., Phys. Rev. B 46, 6671 (1992)] and Perdew-Burke-Ernzerhof (RPBE) [Hammer et al., Phys. Rev. B 59, 7413 (1999)] functionals and carry out a systematic comparison between the dynamics determined by the respective PESs. Even though it has been shown in earlier works that the RPBE may provide better values for the chemisorption energies, our study brings evidence that it gives rise to a PES with excessive repulsion far from the surface.

Electron–Hole Pair Effects in Polyatomic Dissociative Chemisorption: Water on Ni(111)

The influence of electron-hole pairs in dissociative chemisorption of a polyatomic molecule (water) on metal surfaces is assessed for the first time using a friction approach. The atomic local density dependent friction coefficients computed based on a free electron gas embedding model are employed in classical molecular dynamics simulations of the water dissociation dynamics on rigid Ni(111) using a recently developed nine dimensional interaction potential energy surface for the system. The results indicate that nonadiabatic effects are relatively small and they do not qualitatively alter the mode specificity in the dissociation.

Vibrational deexcitation and rotational excitation of H2 and D2 scattered from Cu(111): Adiabatic versus non-adiabatic dynamics

We have studied survival and rotational excitation probabilities of H(2)(v(i) = 1, J(i) = 1) and D(2)(v(i) = 1, J(i) = 2) upon scattering from Cu(111) using six-dimensional (6D) adiabatic (quantum and quasi-classical) and non-adiabatic (quasi-classical) dynamics. Non-adiabatic dynamics, based on a friction model, has been used to analyze the role of electron-hole pair excitations. Comparison between adiabatic and non-adiabatic calculations reveals a smaller influence of non-adiabatic effects on the energy dependence of the vibrational deexcitation mechanism than previously suggested by low-dimensional dynamics calculations. Specifically, we show that 6D adiabatic dynamics can account for the increase of vibrational deexcitation as a function of the incidence energy, as well as for the isotope effect observed experimentally in the energy dependence for H(2)(D(2))/Cu(100). Furthermore, a detailed analysis, based on classical trajectories, reveals that in trajectories leading to vibrational deexcitation, the minimum classical turning point is close to the top site, reflecting the multidimensionally of this mechanism. On this site, the reaction path curvature favors vibrational inelastic scattering. Finally, we show that the probability for a molecule to get close to the top site is higher for H(2) than for D(2), which explains the isotope effect found experimentally.

Dissipative effects in the dynamics of N2 on tungsten surfaces

The role of electron-hole pair excitations in the dynamics of N(2) on W(100) and W(110) is evaluated using a theoretical model that accounts for the six-dimensionality of the problem in the whole calculation. The six-dimensional potential energy surface is determined in each case from an extensive grid of energies calculated with density functional theory. Dissipative effects due to electron-hole pair excitations are introduced in the classical dynamics equations through a friction force. Corresponding electron friction coefficients are calculated for each atom in the molecule with density functional theory in a local density approximation. Our results show that electronic friction plays a very minor role in the dissociative dynamics of N(2) in both tungsten faces. A similar conclusion is reached when we calculate the energy lost by the reflecting molecules.

Non-adiabatic effects during the dissociative adsorption of O2 at Ag(111)? A first-principles divide and conquer study

We study the gas-surface dynamics of O2 at Ag(111) with the particular objective to unravel whether electronic non-adiabatic effects are contributing to the experimentally established inertness of the surface with respect to oxygen uptake. We employ a first-principles divide and conquer approach based on an extensive density-functional theory mapping of the adiabatic potential energy surface (PES) along the six O2 molecular degrees of freedom. Neural networks are subsequently used to interpolate these grid data to a continuous representation. The low computational cost with which forces are available from this PES representation allows then for a sufficiently large number of molecular dynamics trajectories to quantitatively determine the very low initial dissociative sticking coefficient at this surface. Already these adiabatic calculations yield dissociation probabilities close to the scattered experimental data. Our analysis shows that this low reactivity is governed by large energy barriers in excess of 1.1eV very close to the surface. Unfortunately, these

Dissociative dynamics of spin-triplet and spin-singlet O2 on Ag(100)

We study the dissociative dynamics of O(2) molecules on the Ag(100) surface. Initially, the impinging molecules are either in the spin-triplet ground state or in the spin-singlet excited state. The molecule-surface interaction is obtained in each case by constructing the six-dimensional potential energy surface (PES) from the interpolation of the energies calculated with spin-polarized and non-spin-polarized density functional theories, respectively. Classical trajectory calculations performed in both PESs show that O(2) molecules initially in the spin-triplet ground state only dissociate for incidence energies above 1.05 eV. This result is consistent with molecular beam experiments performed in this system. Interestingly, our results also suggest that for the spin-singlet O(2) dissociation occurs even for incidence energies as low as 50 meV. We propose the use of spin-singlet excited O(2) molecules to improve the otherwise low dissociative reactivity of O(2) at clean Ag(100).

Electron-hole pair effects in methane dissociative chemisorption on Ni(111).

The dissociative chemisorption of methane on metal surfaces has attracted much attention in recent years as a prototype of gas-surface reactions in understanding the mode specific and bond selective chemistry. In this work, we systematically investigate the influence of electron-hole pair excitations on the dissociative chemisorption of CH4/CH3D/CHD3 on Ni(111). The energy dissipation induced by surface electron-hole pair excitations is modeled as a friction force introduced in the generalized Langevin equation, in which the independent atomic friction coefficients are determined within the local-density friction approximation. Quasi-classical trajectory calculations for CH4/CH3D/CHD3 have been carried out on a recently developed twelve-dimensional potential energy surface. Comparing the dissociation probabilities obtained with and without friction, our results clearly indicate that the electron-hole pair effects are generally small, both on absolute reactivity of each vibrational state and on the mode specificity and bond selectivity. Given similar observations in both water and methane dissociation processes, we conclude that electron-hole pair excitations would not play an important role as long as the reaction is direct and the interaction time between the molecule and metal electrons is relatively short.

Role of physisorption states in molecular scattering: A semilocal density-functional theory study on O2/Ag (111)

We simulate the scattering of O2 from Ag(111) with classical dynamics simulations performed on a six-dimensional potential energy surface calculated within semilocal density-functional theory. The enigmatic experimental trends that originally required the conjecture of two types of repulsive walls, arising from a physisorption and chemisorption part of the interaction potential, are fully reproduced. Given the inadequate description of the physisorption properties in semilocal density-functional theory, our work casts severe doubts on the prevalent notion to use molecular scattering data as indirect evidence for the existence of such states.