Recent Papers in the field

  • Adsorption, diffusion, and dissociation of NO, N and O on flat and stepped Ru(0001)
    B. Hammer * hammer@ifa.au.dk, Surface Science, Vol. 459 (3) (2000) pp. 323-348
    The chemisorption, diffusion and dissociation of nitric oxide, NO, on flat and stepped Ru(0001) surfaces are investigated using density functional theory. In the Perdew-Wang-91 GGA approximation (PW91) for the exchange-correlation energy, the NO chemisorption potential energy (calculated relative to the gas-phase NO) is 2.73 and 3.10eV on flat and stepped Ru(0001), respectively. The NO(a) experiences a diffusion energy barrier of 0.33eV on the flat Ru(0001) terrace, while the barrier for diffusion across the steps is 0.9-1.1eV. The barriers for attachment to and detachment from the steps are in the range of 0.4-0.8eV. A number of strongly inclined, but metastable, NO configurations are found. The NO dissociation is calculated to be highly activated (Ea1.3eV) on the flat Ru(0001) but is found to be only slightly activated (Ea0.1-0.5eV) when monatomic Ru steps are present at the surface. The product N and O atoms prefer chemisorption in two- and threefold configurations right behind, or at, the step edges. The reason for this is the energetically higher Ru4d-band positions at the step edges. The N and O atoms are subject to considerable energy barriers for diffusion over the Ru(0001) terraces (ENd=0.79eV and EOd=0.54eV), and they experience even larger barriers for interlayer
    diffusion. Combining the NO dissociation and the N plus O chemisorption results, two mechanisms are found to cause the higher reactivity of the step edges in terms of NO-bond activation. The main mechanism is the ability of the reaction site to provide rebonding of the non-interacting reaction products. The other mechanism is the minimization of the degree of repulsive interaction of the N and O atoms in the transition state. Chemisorption energies, and diffusion and dissociation energy barriers are further calculated in the PBE and RPBE exchange-correlation descriptions. The PBE results are, as expected, very close to the PW91 results. The RPBE results, however, show largely reduced bonding energies and energy barriers that vary by ±0.2eV, depending on the reaction geometry.
    • Precursor-mediated adsorption of oxygen on the (111) surfaces of platinum-group metals
      A. Eichler, F. Mittendorfer, and J. Hafner; Physical Review B Volume 62,  4744-4755 (2000).
      The dissociative adsorption of oxygen on the (111) surfaces of platinum, palladium, and nickel has been investigated using ab initio
      local-spin-density calculations. For all three surfaces, adsorption is shown to be precursor-mediated and the structural, energetic,
      vibrational and electronic properties of the precursors are in very good agreement with the available experimental information. The
      investigation of the transition states shows that on Pt and Pd the barriers for dissociation are comparable to (or at sufficiently high
      coverage even higher than) the desorption barriers. In combination with large energies for atomic adsorption, this also leads to a high
      barrier for associative desorption—in agreement with observation. In contrast, the dissociation barrier for O2 on Ni(111) is low and
      occurs already for a less stretched molecule. The trends in molecular and atomic adsorption and in the dissociation barriers are
      discussed in relation to the geometric and electronic properties of the substrate and to the sticking probabilities observed in
      molecular-beam experiments. 
    • Quantum Monte Carlo simulation of atomic motion
      Wahnstrom G, Mattsson TR; COMPUTER PHYSICS COMMUNICATIONS 122: 477-479, Sp. Iss. SI SEP-OCT 1999
      The path-integral formulation of quantum mechanics is a suitable starting point for quantum simulation studies. We have considered H diffusion on the Ni (100)-surface at room temperature and below using the quantum Monte Carlo technique. The transition rate is determined using the path-centroid formulation and the simulations are based on a potential derived from recent first-principles calculations of the total energy. We find a marked change of the temperature dependence for the diffusion constant around 60 K, indicating that quantum tunneling between the ground states starts to dominate the diffusion process. The simulation results are compared with recent experimental findings. (C) 1999 Published by Elsevier Science B.V. All rights reserved.
    • Diffusion of Xe on Ni(111)
      Nabighian E, Zhu XD; CHEMICAL PHYSICS LETTERS 316: (3-4) 177-180 JAN 14 2000

      We have measured the diffusion rate D(T) for Xe on Ni(111) from 30 to 60 K at coverages of theta = 0.04 and theta = 0.16. By fitting D(T) to Arrhenius forms D-0 exp(-E-d/RT), we found the diffusion to be characterized by small activation energy and unusually small diffusivity: E-d = 0.33 +/- 0.02 kcal/mol and D-0 = 2 x 10(-9 +/-0.2) cm(2)/s at theta = 0.04, and E-d = 0.40 +/- 0.04 kcal/mol and D-0 = 3 x 10(-8 +/- 0.3) cm(2)/s at theta = 0.16. We discuss the significance of our result in the light of similar observation of small diffusion barriers and unusually small diffusivities for Xe on W(110) and for Al diffusion on Al(111) and on Au(111) reported recently. 

    • Analysis of the Arrhenius shape of the diffusion coefficient on an fcc (111) surface
      Masin M, Chvoj Z; SURFACE REVIEW AND LETTERS  7: (3) 219-225 JUN 2000
      We study the temperature and coverage dependence of an effective diffusion barrier assuming an Arrhenius shape of the chemical diffusion coefficient for a system of interacting particles. The previously published model of the diffusion of an fee (111) surface with bivariate trap is used. The presence of two nonequivalent occupation sites and interaction result in a non-Arrhenius shape of the diffusion coefficient and a coverage- and temperature-dependent effective diffusion barrier. The temperature dependence of effective energy E-a shows a minimum at low temperatures (at approximately 150 K) for strong interactions. The coverage dependence of E-a has a deep minimum in the vicinity of Theta = 1/2, even in a system without interaction.
    • Tracer surface diffusion at high pressures: Molecular-dynamics study
      Zeiri Y; J CHEM PHYS 113: (9) 3868-3873 SEP 1 2000 
      Molecular-dynamics simulations were employed to investigate the influence of high pressure on tracer surface diffusion. A model potential was used to describe the interaction among the various species in the system. The different binding energy values and masses used in this model simulation correspond to surface diffusion of N-2 on Ru(001) surface under pressure of Ar. A pronounced enhancement in the magnitude of the diffusion coefficients was observed when pressure increased from P = 0 to P = 200 atm. The relationship between diffusion coefficient and three parameters that characterize the system was explored. It was found that the gas temperature and the nature of gas-adsorbate interaction (i.e., attractive or repulsive) have only a negligible influence on the diffusion coefficient. However, a marked variation in the diffusion coefficient was observed when the magnitude of gas-substrate binding energy was altered. The temperature dependence of the surface diffusion coefficient exhibits an Arrhenius behavior for all cases investigated. The relationship between the pressure and both pre-exponential factor and activation energy for surface diffusion was discussed based on a detailed analysis of the diffusion mechanism. The diffusion mechanism was deduced by careful examination of large number of
      individual trajectories.

    • Surface self-diffusion of hydrogen on Cu(100): A quantum kinetic equation approach
      Pouthier V, Light JC; J CHEM PHYS 113: (3) 1204-1216 JUL 15 2000 
      The self-diffusion of hydrogen on the (100) copper surface is investigated using a quantum kinetic equation approach. The dynamics of the adatom is described with a multiple-band model and the surface phonons represent the thermal bath responsible for the diffusion mechanism. Using the Wigner distribution formalism, the diffusive motion of the adatom is characterized in terms of the correlation functions of the adatom-phonon interaction. The diffusion coefficient exhibits two terms related to phonon mediated tunneling (incoherent part) and to dephasing limited coherent motion (coherent part). The competition between these two contributions induced a transition from a thermally activated regime to an almost temperature independent regime at a crossover temperature T*. A numerical analysis is performed using a well-established semiempirical potential to describe the adatom-surface interaction and a slab calculation to characterize the surface phonons. These calculations show that two-phonon processes represent the relevant contribution involved in the adatom-phonon coupling. The temperature dependence of the diffusion constant is thus presented and the relative contribution of the incoherent versus the coherent part is analyzed. Both contributions exhibit a change of behavior around 100 K from an
      exponential to a power law temperature dependence as the temperature decreases. This change is due to the confinement of the motion of the adatom in the ground energy band at low temperature. The incoherent part is shown to be the dominant contribution at high temperature and is characterized by an activation energy and a prefactor equal to Delta E=0.49 +/- 0.01 eV and D(0)approximate to 2.44x10(-3) cm(2)/s, respectively. At low temperature, the power law dependence of the two contributions is different since the coherent part increases slowly as the temperature decreases whereas the incoherent part decreases. The crossover temperature is estimated to be equal to T*=125 K. Below T*, the coherent part becomes the main contribution and the diffusion constant exhibits an almost temperature independent behavior. 

    •  Energetic and entropic contributions to surface diffusion and epitaxial growth
      Steltenpohl A, Memmel N; PHYSICAL REVIEW LETTERS  84: (8) 1728-1731 FEB 21 2000
      For Pd/Pd(111) an exceptionally high barrier (350 meV) for surface self-diffusion and a negative additional energy Delta E = -53 meV for step-down diffusion are measured. Both findings agree with the proposed role of free-electron-like surface states. Despite the negative value of Delta E layer-by-layer growth is not observed. This is related to the low preexponential factor for step-down diffusion. Preadsorption of oxygen increases Delta E but flattens the films. Again this is due to the prefactors for diffusion. The present results demonstrate the importance of entropic effects for diffusion and growth.

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