AVS2001 Session SS2-FrM: Gas-Solid Dynamics: Theory and Experiment

Friday, November 2, 2001 8:40 AM in Room 121

Friday Morning

Start	Invited?	Item
8:40 AM		SS2-FrM-2 Chemisorption of O₂ on Al(111): Non-Adiabatic Pathway to Abstraction, and Simulation of STM Data J.Z. Sexton, A.J. Komrowski, A.C. Kummel (University of California, San Diego); M. Binetti, O. Weisse, E. Hasselbrink (Universitat Essen, Germany) There exists controversy in the literature surrounding the initial oxidation of the Al(111) surface. (1) DFT calculations show the activated reaction of O₂ with Al(111) cannot be modeled using a single potential energy surface and consistency with the observed reaction barrier requires a non-adiabatic charge transfer process.¹ The calculations show that the initial charge transfer is most favorable for end-on collisions with Al(111) which is consistent with an abstraction mechanism.¹ (2) Abstractive chemisorption in the oxidation of aluminum is a process in which an oxygen molecule reacts with the surface producing one atom bound to the surface and one atom ejected into the gas phase. We have experimentally verified this mechanism using supersonic molecular beams to dose the clean Al(111) surface with variable incident energy O₂. The evidence is as follows: (a) STM images show a transition between exclusively single O-adatom reaction products to more pairs of O-adatom reaction products as the O₂ translational energy is raised from 0.025 eV (thermal) to 0.8 eV. (b) The ejected O-atoms have been detected in the gas phase with REMPI laser spectroscopy. This is consistent with an end-on, non-adiabatic reaction. (3) Interpretation of O-Al(111) reacted sites in STM images can be interpreted in terms of long range dissociation, abstraction, or diffusion.² The main issue of the controversy is the assignment of single O-Al(111) reacted sites on the room temperature surface. We are performing cluster and slab calculations to simulate the appearance of O - Al(111) features in STM images. The existence of nearly degenerate adsorption sites for oxygen adatoms and local relaxation of the Al surface complicates the interpretation of the STM images. ¹Y. Yourdshahyan, et al, Sol. St. Comm. 117, 531 (2001). ²M. Schmid and P. Varga, in AVS Symposium Abstracts, SS-TuP4, Seattle, WA, (1999) p. 109.
9:00 AM		SS2-FrM-3 Surface Temperature Effects in Molecular Beam Scattering: Inelastic State-to-state Scattering and the Dissociation of Hydrogen and Deuterium Z. Wang (University of Texax, Austin); G.R. Darling, S. Holloway (University of Liverpool, UK) We have carried out quantum mechanical wavepacket calculations on a parameterized LDA potential energy surface for the surface temperature dependence of the inelastic scattering of hydrogen and deuterium molecules from Cu(111). We find, in agreement with experimental data, that the surface temperature dependence of each cross-section invariably has an Arrhenius form with an 'activation energy' that depends upon the translational energy of the molecules. Additional calculations for the dissociation probability reveal very similar results. On the basis of the wavepacket calculations, it has been possible to develop a simple model to explain the results. The model is quite general, robust and results will be presented for vibrationally inelastic scattering and dissociative adsorption. We shall discuss exactly the origins of the Arrhenius form for a system where thermal equilibrium is never established, and the meaning of the derived "activation energy" in terms of the parameters entering into the dynamical model.
9:20 AM		SS2-FrM-4 Monte Carlo Simulation of the Adsorbate Assisted Adsorption in the Case of CO/ZnO U. Burghaus (Ruhr-Universitaet Bochum, Germany) Very recently and for the first time the adsorption dynamics of a prototype molecule has been studied in detail on a metal oxide surface [Becker, Kunat, Boas, Burghaus, Woell, JCP 113 (2000) 6334], namely for the systems CO/O-ZnO and CO/Zn-ZnO, which both show a distinct increase in the adsorption probability with increasing CO coverage. This phenomenon (referred as adsorbate assisted adsorption) is in contrast to traditional precursor models. The presented Monte Carlo algorithm [submitted to Surf. Rev. and Lett.], which predicts the enhancement of adsorption by pre-adsorbates, assumes different adsorption probabilities for molecules scattered on bare and already occupied sites. Additionally, the effect of the mass-mismatch of the adsorbate and the surface atoms is considered. Thus, presented is an MC version of the analytical so-called modified Kisliuk model [e.g., R.J. Madix, et al., Surf. Sci. 470 (2001) 226]. In contrast to most of the analytic models, the MCS scheme includes lateral interaction energies, the influence of defects, and the effect of cooperative precursor dynamics. Although adsorbate assisted adsorption has also been observed on metal surfaces, the polar surfaces of ZnO are especially well suited to test the algorithm and the influence of the mass-mismatch on the energy transfer processes involved, since O-ZnO and Zn-ZnO differ solely by the mass of the atoms in the first surface layer. MC simulations will be presented which explain the main effects observed experimentally for both polar surfaces of ZnO.
9:40 AM		SS2-FrM-5 Ultrafast Energy Flow Studied by Femtosecond Vibrational Spectroscopy S. Roke, A.W. Kleyn, M. Bonn (Leiden University, The Netherlands) We present a simple model describing femtosecond surface vibrational spectroscopy as a tool to study ultrafast surface chemical dynamics. We compare our calculations to recent experiments in which the interaction between CO and the Ru(0001) surface is studied using the femtosecond surface vibrational spectroscopy: sum frequency generation (fs-SFG). After short-pulsed excitation of the metal leading to desorption of CO, a transient red-shift and broadening in the infrared spectrum of the C-O stretch vibration on a picosecond time-scale are observed. The data are successfully modeled by considering the response of adsorbed CO to heating of the system by a femtosecond laser pulse. The calculations match the experimental data very well and demonstrate that fs-SFG is a very powerful tool to study the dynamics of molecules at surfaces. Detailed analysis shows that this method will enable us to detect transition states and reaction intermediates in chemical reactions at surfaces and determine the transition state lifetime. Results of first experiments on energy flow in a reacting system will be shown at the meeting.
10:20 AM	Invited	SS2-FrM-7 Vibrational Energy Transfer and Energy "Pooling" in Adsorbate Layers: CO on NaCl S.A. Corcelli, J.C. Tully (Yale University) Adsorbate vibrational excitations on insulator surfaces can be very long-lived if the adsorbate frequency is much greater than the Debye frequency of the solid. This allows time for vibrational excitations to resonantly hop among neighbors a great many times before de-excitation. Sometimes this will result in two adjacent adsorbate molecules both excited to v=1. As a result of anharmonicity, the state with one molecule in v=2 and one in v=0 has a slightly lower energy than both molecules in v=1. Similarly, 3 quanta of vibration shared by two neighboring adsorbate molecules produce a lower total energy if all 3 quanta reside on one of the neighbors. This produces a driving force for "pooling" of energy into highly excited molecules. This effect has been demonstrated dramatically by George Ewing and coworkers¹ who observed population of vibrational levels as high as v=15 for CO on NaCl. We have developed a perturbation theory approach to calculate all of the operative rate processes; vibrational relaxation, resonant hopping, pooling, and radiation. We have used these rates in a kinetic Monte Carlo simulation of the energy pooling process. Our results are in qualitative agreement with experiment, and reveal interesting phenomena such as self-trapping and Ostwald ripening. In addition, we predict an enormous C-12/C-13 isotope effect. ¹ H.-C. Chang and G. E. Ewing, Phys. Rev. Lett. 65, 2125 (1990).
11:00 AM		SS2-FrM-9 HD Scattering on Cu (001): Rotationally Mediated Bound State Resonances and Evidence for Interaction of the Bound States with the Surface Phonons L.V. Goncharova, A.V. Ermakov, B.J. Hinch (Rutgers University) Seeded HD molecular beams have been used to study the laterally averaged HD - Cu (001) physisorption potential by measurements of rotationally mediated bound state resonances (BSR). The physisorption well depth of 30.5 meV was confirmed using a Le Roy plot by combining our HD data with existing H₂ and D₂ data. The positions of all rotationally mediated bound state resonance energies are determined well by the results of calculations using different potentials. BSR energies from isotropic potential calculations are considerably higher than the experimentally determined energies, as well as those of the values calculated with an anisotropic potential. So the anisotropic part of the potential cannot be neglected. Calculations predict lower energy for m_J = 0, indicating that the molecule prefers to have its molecular axis perpendicular to the surface (cartwheel rotation). A considerable shift of the bound state levels in the presence of an adsorbate (chemisorbed hydrogen) is also observed. Resonance energies do not change with the surface temperature. The Debye-Waller analysis of the intensities and the widths of the resonance features show a coupling between the resonant states and surface phonons. The strongest coupling is for the states lower in the potential well.