Papers

AVS1997 Session SS-TuM: Gas-Surface Dynamics

Tuesday, October 21, 1997 8:20 AM in Room A1/2-A

Tuesday Morning

Start	Invited?	Item
8:20 AM		SS-TuM-1 Molecular Dissociation at Surfaces: Is there Life beyond the Reaction Coordinate? S. Holloway (University of Liverpool) In the simplest treatments of molecular dissociation, it is standard practice to comment how motion procedes along the reaction coordinate from a reactant configuration (an intact molecules and a clean surface) to a product state describing the dissociated fragments adsorbed on the surface. Over the past decade, there have been many theoretical studies that have had as their focus, the extension of these simple models to higher dimensions which take as their starting point a potential energy surface that is deemed 'reliable,' by those in the club who calculate such things. It is tempting, when looking at these fearfully complicated energy surfaces, to make an initial interpreation of the dynamical processes that may occur. Again, this is done in such a way as to look for major topographical features such as low energy pathways or saddle points and then 'one-dimensionalise' the anticipated motion. Is this in any way useful or meaningful? In this talk, I shall give some examples for the dissociation of diatomics at activated and non-activated surfaces with the emphasis on explaining how motion evolves, taking the reactants through the possible dynamical bottlenecks to the product region. I will compare classical and quantum techniques and results for the dynamics that arise and discuss similariries and differences. I will illustrate that to some extent it is possible to perform a Born-Oppenheimer like separation of the interenal and external molecular degrees of freedom in order to arrive at a deeper understanding of the quantum dynamics.
9:00 AM		SS-TuM-3 Vibrational Relaxation of H₂ Scattered from Pd(111) M. Gostein, E.K. Watts, G.O. Sitz (University of Texas, Austin) Molecular vibration has long been believed to play an important role in the activated dissociation of H₂ on certain metals. Experimental measurements of state-to-state scattering dynamics help to elucidate this role and the nature of the H₂/metal interaction. We have investigated the dynamics of vibrationally excited H₂ scattering from Pd(111), using molecular beam and laser spectroscopy techniques. We have observed the relaxation of H₂ in the rovibrational state (v=1, J=1) into (v=0, J=5 and J=7) upon scattering from the Pd sample. The relaxation probability is 0.04 ± 0.01 into J=5 and 0.03 ± 0.01 into J=7. Relaxation does not occur when the surface is saturated with H atoms at low temperature. Furthermore, relaxation occurs with significant loss of vibrational energy, 50-120 meV, to the substrate. This suggests that the vibrational relaxation involves energy dissipation to substrate electrons, so that the scattering is electronically nonadiabatic. On an H-saturated surface, the survival probability of the incident H₂ (v=1, J=1) is nearly unity, 0.97-1.00. However, for scattering from a clean surface the (v=1, J=1) survival probability is only 0.05 ± 0.01, which is low compared the value of ~0.4 for the (v=0, J=1) state, suggesting vibrationally enhanced dissociation.
9:20 AM		SS-TuM-4 Steric Preferences in Surface Chemistry: Energy and Internal State Dependence of Alignment in D₂(v,J) Associative Desorption From Cu(111) H. Hou (IBM Almaden Research Center); S.J. Gulding (University of California, Santa Barbara); C.T. Rettner (IBM Almaden Research Center); A.M. Wodtke (University of California, Santa Barbara); D.J. Auerbach (IBM Almaden Research Center) We have measured, for the first time, the kinetic energy and internal state dependence of the steric preference for a surface reaction, the associative desorption of D₂ from Cu(111). We did so by observing the state specific rotational polarization of desorbing D₂(v,J) using REMPI/Field-Free Time-of-Flight methods. At low translational energy and high rotational state,J, the desorbing D₂ is strongly aligned with its bond axis parallel to the surface. The degree of alignment decreases sharply with increasing kinetic energy, approaching zero at 0.8 eV. The degree of alignment also decreases with decreasing J, but is relatively insensitive to the vibrational state. Through detailed balance, these results indicate that the dissociative adsorption of D₂ on Cu(111) shows a strong steric preference for broadside collisions when the molecules strike the surface at low translational energy. Furthermore, this steric preference exhibits a dramatic dependence on the collision energy, all but disappearing as the energy is raised above the activation barrier for the reaction. These measurements bare directly on the geometry of the transition state and the dynamics of the reaction in the critical energy regime near the barrier to dissociation.
9:40 AM		SS-TuM-5 H Atom Abstraction of D Adsorbed on Si(100): Dynamical Evidence for an Eley-Rideal Mechanism S.A. Buntin (National Institute of Standards & Technology) The reaction dynamics of the H atom abstraction of D from a monodeuteride-terminated Si(100) surface are investigated. H atoms in the range of nominally 1 to 3 eV are generated by laser photolysis of HI, and the kinetic energy distribution of the scattered HD products is determined using a quadrupole mass spectrometer with time-of-flight (TOF) analysis. For an incident H atom kinetic energy of nominally 1 eV, the HD products have a mean kinetic energy of 1.2-1.3 eV and the distribution extends up to the available-energy limit, providing dynamical evidence for a direct Eley-Rideal mechanism for this abstraction reaction. A cursory angular dependence, obtained from TOF spectra recorded for a limited set of scattering geometries, indicates no product energy/angle coupling. For higher incident H atom kinetic energies, the width of the HD product kinetic energy distribution increases while the mean value remains unchanged, suggesting that energy loss to the substrate becomes more significant and the reaction becomes less Eley-Rideal-like.
10:00 AM		SS-TuM-6 Trajectory of H and D Desorption from a Passivated Si(100) Surface Induced by Electronic Excitation P. von Allmen (Motorola Phoenix Corporate Research Laboratories); K. Hess (University of Illinois, Urbana-Champaign) We use first principles methods to study the desorption mechanism of hydrogen and deuterium from a Si(001) surface induced by the electronic current of a scanning tunneling microscope (STM). We focus on the high voltage regime where the desorption is mediated by the excitation of an electron from a bonding to an anti-bonding state. The surface is modeled by an eight-layers slab of silicon atoms which are described by a non-local pseudo-potential. We find that the desorption barrier of a single hydrogen atom is 3.8 eV when the system is in the electronic ground state. We show that it is necessary to construct a localized excited state to have desorption induced by a single electronic excitation and we determine the desorption path for the hydrogen atom and the gain in kinetic energy along it. The hydrogen atom has stored enough kinetic energy to overcome the barrier after about 3 fs while the deuterium atom remains trapped after the same time. The hydrogen desorption starts with the pairing at the surface of two hydrogen atoms from neighboring dimers and follows with the desorption of the H2 molecule.
10:20 AM		SS-TuM-7 Dissociative Sticking of O₂ on Al(111) L. Österlund, H. Ternow, I. Zori@aa c@, B. Kasemo (Chalmers University of Technology, Sweden) The dissociative sticking probability of O₂ on Al(111) has been measured as a function of incident translational and vibrational energy at different oxygen coverages. The initial sticking probability rises from 10^-2 for E_trans=30 meV to near unity in the range 0.6-2 eV with an apparent activation barrier of about 0.2 eV for the dissociation event. The vibrational energy is much more efficient, in comparison to translational energy, in promoting sticking. The initial sticking probability is insensitive to surface temperature in the interval 80s<650K indicating a direct sticking mechanism, with negligible contribution from phonons towards overcoming the adsorption barrier(s). An unusual angular dependence of the sticking coefficient is observed. The experimental results are discussed in terms of the calculated adiabatic potential energy surface, and an alternative scenario based on a charge transfer model. The latter is used to discuss the recent observation [1] of widely separated O atoms after O₂ dissociation on Al(111), as well as the observed chemiluminscence from polycrystalline Al films. The oxygen sticking dynamics at finite O-atom coverage is also presented. The saturation coverage of atomic oxygen on Al(111) shows an almost twofold increase as E_trans is increased, again with an apparent activation energy of 0.2 eV. The possible role of an abstraction reaction mechanism (O-atom emission) in explaining the observed finite coverage data is discussed. ¹1. H. Brune et al., Phys. Rev. Lett. 68, 624 (1992)
10:40 AM		SS-TuM-8 Double-Precursor Mechanism in the Adsorption of Oxygen on Pd(111) P. Sjovall, P. Uvdal (Lund University, Sweden) The adsorption of O₂ on Pd(111) was studied using supersonic molecular beam techniques. The dependencies of the initial sticking coefficient on translational energy (0.08 - 0.7 eV) and substrate temperature (100 - 650 K) indicate an adsorption mechanism including two different molecular precursor states. At low translational energies (0.08 eV), the O₂ molecules are trapped into a physisorbed state, before moving into a molecularly chemisorbed state, from which dissociation occurs. At the higher translational energies, the O₂ molecules get direct access to the chemisorbed state, while trapping into the physisorbed state becomes less efficient. The substrate temperature dependence of the sticking coefficient suggests that direct dissociative sticking is not a major adsorption pathway, even at the highest translational energies studied. At the lowest translational energies, ≤0.1 eV, the sticking coefficient falls off with increasing translational energy and the incident angle dependence is relatively weak, consistent with trapping into a physisorbed state. At the higher translational energies, at which direct adsorption into the chemisorbed precursor state is expected to dominate, the translational energy dependence shows signs of weakly activated adsorption. Furthermore, for these translational energies a strong incident angle dependence is observed, in which the sticking coefficient is reduced as the incident angle becomes more glancing. This dependence, which cannot be explained be simple normal energy scaling, is discussed in terms of shielding effects on a strongly corrugated potential.
11:00 AM		SS-TuM-9 Direct Collisionally Activated and Trapping-Mediated Chemisorption of Neopentane on Clean Pt(111): The Activity of Surface Defect Sites J.F. Weaver, M.A. Krzyzowski, R.J. Madix (Stanford University) Molecular beam results indicate that the dissociative chemisorption of neopentane on Pt(111) occurs by both direct collisionally activated and trapping-mediated mechanisms. The contribution of each reaction channel was assessed by directly measuring the initial dissociation probability, S_o, for a range of incident translational energies (E_i), incident angles (θ_i) and surface temperatures. The results indicate that direct dissociation dominates at translational energies greater than about 110 kJ/mol, exhibits normal energy scaling and is nearly independent of the surface temperature. At translational energies less than about 100 kJ/mol the trapping-mediated pathway is the controlling dissociative adsorption mechanism. Contrary to direct dissociation, the trapping-mediated dissociation probability decreases with increasing incident energy and is only weakly dependent on the incident angle. In fact, at 500 K the dissociation probabilty scales with E_icos^0.6θ_i, in excellent agreement with the scaling observed for molecular adsorption of neopentane at 105 K. Additionally, the trapping-mediated dissociation probability decreases with increasing surface temperature, indicating that the barrier to dissociation from the molecular adsorption state lies below the zero energy level. Experiments performed on Pt(111) surfaces with differing defect-site densities further indicate that trapping-mediated dissociation is facilitated by surface defects. The measured surface temperature dependence of the trapping- mediated dissociation probability was accurately reproduced by a model which assumes that the rate of migration to defects is rapid compared to the dissociation and desorption rates and that dissociation occurs exclusively at defect sites. The observations that trapping-mediated dissociation occurs preferentially at surface defects and by a lower energy pathway than direct dissociation indicate that the dissociation barrier near defect sites is lower than on the terraces, and that only trapped molecules which have a long residence time can frequently sample favorable configurations near the defects for dissociation.
11:20 AM		SS-TuM-10 State Resolved Dynamics of Methyl Desorption from GaAs Q.-S. Xin (Southern Illinois University); H. Wang, X.-Y. Zhu (University of Minnesota) We have used state-resolved characterization as a tool to establish the molecular basis for the macroscopic kinetic behavior in a model metal-organic chemical vapor deposition (MOCVD) reaction involving the breaking of a single carbon-metal bond: the desorption of methyl radical from GaAs(100). In the experiment, the vibrational state distribution of CD₃ desorbed from GaAs(100) was determined by resonance-enhanced multiphoton ionization spectroscopy (REMPI). Since CD₃ is pyramidal on the surface and planar in the gas phase, the extent of vibrational excitation in the umbrella mode is related to the location of the transition state. Indeed, vibrational population ratios in the umbrella mode of CD₃ thermally desorbed from GaAs(100) is superthermal: N_v=2/N_v=0 = 0.6, N_v=3/N_v=1 = 0.5, which are more than four times those expected from a thermal equilibrium at the desorption temperature. In contrast to vibration, the mean translation energy of the methyl radical is approximately thermal neutral. CD₃ must desorb promptly from the surface with an "early" transition state. The present study provides a quantitative basis for further theoretical investigations.