AVS2001 Session SS2-TuM: Water at Surfaces

Tuesday, October 30, 2001 8:20 AM in Room 122

Tuesday Morning

Start	Invited?	Item
8:20 AM		SS2-TuM-1 A Molecular Beam Study of Water Adsorption, Desorption, and Clustering on Pt(111) J.L. Daschbach, B.M. Peden, G. Teeter, R.S. Smith, B.D. Kay (Pacific Northwest National Laboratory) Adsorption, desorption, and clustering are investigated by molecular beam techniques. Specular He atom scattering is used to probe the sub-monolayer H2O surface coverage on Pt(111) over the temperature range 22K to 185K. Structural rearrangements of H2O adsorbed at 22K are studied as a function of initial coverage and temperature in the coverage range from 0.01 ML to 1.0 ML by measuring the specular He intensity using linear temperature ramps in the range 0.1 K/s to 10 K/s. Either or both of two transitions are observed depending on coverage, with the first corresponding to the onset of surface diffusion and trapping of mobile H2O by step edges or defects, and the second corresponding to Oswald ripening of 2-D condensed phase islands. Adsorption and desorption kinectics are interrogated isothermally by measuring the H2O coverage in time as a function of H2O beam flux and temperature. At temperatures between 150 K and 165 K the adsorption and desorption spectra are linear in time and therefore independent of coverage. In this region the desorption kinetics are strictly zero-order and can be measured with high precision. The zero order kinetics are a consequence of the existence of a 2-D two phase H2O system present on the Pt surface. At temperatures above 172 K, depending on flux, a transition to non-zero order kinetics is observed with the kinetics consistent with first order. This transition occurs when the system has moved from a two-phase coexistence region to a single phase 2-D gas.
8:40 AM		SS2-TuM-2 Water/MgO Interactions after High Temperature Annealing C.J. Hirschmugl, M. Harland (University of Wisconsin-Milwaukee); R. Plass (Sandia National Laboratories); K. Rewolinski, M. Gadjardziska-Josifovska (University of Wisconsin-Milwaukee) Infrared transmission measurements of MgO(100) and MgO(111) samples as a function of annealing (between 900-1500K) and exposure to air, reveal strikingly different absorption signatures. The measurements are taken 30 degrees from normal incidence to couple to vibrations of adsobates perpendicular and parallel to the surface. Previously, it has previously been surmised that MgO(111) samples annealed to 1500 K undergo a reconstruction at the surface, while MgO(100) does not undergo any similar reconstruction. Notably, the infrared absorption observed for both terminations of MgO(100) and (111) were similar. Samples annealed to 900 K show broad water absorption bands between 3400-3600 cm^-1, corresponding to a layer of hydrogen bonded water at the surface. However, this prominent feature disappeared for samples annealed to 1500 K, and a series of weak, sharper bands appeared over the same frequency range. An examination with D₂O will be used to identify the nature of the bands due to interaction between MgO and water.
9:00 AM		SS2-TuM-3 Morphology of Vapor-Deposited Ice at Low Temperatures by Atomic Force Microscopy S.C. Fain (University of Washington); J.M.K. Donev (University of Washington and Pacific Northwest National Laboratory); S.A. Joyce (Pacific Northwest National Laboratory) Atomic force microscopy (AFM) is being used to study the morphology of multilayer films of vapor-deposited water ice on various substrates as a function of deposition and annealing temperatures below 150K. These experimental studies are coordinated with theoretical studies by other investigators to provide improved understanding of ice growth and properties of ice for use in other fields. For example, nucleation and growth of crystalline ice particles in the upper atmosphere is of central importance for cloud formation, global energy balance, and dynamics of ozone depletion. For these experiments the films are deposited in-situ in ultra-high vacuum from an effusive doser at an angle of 67 degrees from the surface normal. The first measurements were made for ice films deposited on Au(111) on mica. These films were profiled by a probe tip attached to a quartz crystal (Omicron needle sensor) which provides nanometer resolution of surface features. Films deposited below 100K appear relatively flat, adopting the surface texture of the Au(111) substrate. Three-dimensional clusters typically 30 nm high form after annealing these films up to 130K. The lateral dimensions of the clusters depend on the initial coverage. These changes are produced by surface diffusion producing a non-wetting film. The rearrangement happened even if the annealing was done without imaging. Previous thermal desorption measurements by Kay and coworkers [J. Chem. Phys. 91, 5120-5121 (1989) and Surface Science 367, L13-L18 (1996)] have inferred amorphous ice clusters surrounded by bare substrate for films deposited on Au(111) at low-temperatures. Further measurements are in progress using non-contact AFM (Omicron beam-deflection) with the same deposition geometry and different substrates. Supported by Department of Energy's Office of Biological and Environmental Research, a U. W. Nanotechnology Fellowship Award to J.M.K.D., and NSF KDI 99-80125.
9:20 AM		SS2-TuM-4 Initial Growth of Water on Ru(001) and Cage Formation of CD₃Cl Y. Lilach, V. Buch, M. Asscher (The Hebrew University, Israel) The adsorption of H₂O on Ru(001) and the coadsorption system H₂O+CD₃Cl/Ru(001) were studied using Temperature Programmed Desorption (TPD) and work function change (ΔΦ) measurements. We developed a kinetic model that fits the measured ΔΦ upon water adsorption at 80K. The model indicates that at very low coverages water monomers dominate, while as coverage increases di- tri- and tetramers are formed. Water tetramers were observed recently by IR measurements to be the dominant species in similar adsorption conditions. The effective ΔΦ contribution of these species suggests an adsorbed cyclic tetramer, with inclined water dipoles. Molecular Dynamics (MD) simulations using the TIP4P potential energy surface for the water-water interaction, were performed as a means for gaining deeper insight into the experimental results. The effective dipoles of the small clusters obtained from the MD simulations were in good agreement with the dipoles predicted by the kinetic model. Adsorbed CD₃Cl molecules were found to be compressed and then caged under H₂O layers, as indicated by complex ΔΦ curves monitored upon adsorption. These are explained by the following sequence (a) H₂O molecules initially compress CD₃Cl into separate islands. Desorption from this stage suggests that the structure of methyl chloride resembles that of multilayer CD₃Cl, namely chlorine down in the first layer, while in the second layer it flips up. (b) Hydrophobic displacement of CD₃Cl from surface sites to become trapped within the water layer. Further increase of the water coverage produces a tight cage of the CD₃Cl molecules inside the ice structure, as indicated by a sharp, explosive desorption at 165 K.
9:40 AM		SS2-TuM-5 Using Nanoscale Amorphous Films to Study Mixing, Transport and Phase Separation in Deeply Supercooled, Metastable Binary Solutions of Methanol, Ethanol, and Water P. Ayotte, Z. Dohnalek, G.A. Kimmel, R.S. Smith, B.D. Kay (Pacific Northwest National Laboratory) The relatively large diffusivities exhibited by amorphous deposits of water, methanol, and ethanol at cryogenic temperature (<160K) opens up the possibility to investigate liquid-phase kinetics in the deeply-supercooled, metastable thin film regime. Compositonally tailored, multilayer films grown by molecular beam dosing techniques are particularly well suited for the characterization of diffusion and desorption kinetics as shown previously for thin film water. While it is well-known that liquid mixtures of alcohol and water exhibit non-ideal solution behavior due to hydrophobic solvation, the complexity of their liquid-solid phase diagrams has been interpreted in terms of formation and decomposition of different crystalline hydrates (stochiometric and/or clathrates). We use molecular beam scattering and programmed desorption (both TPD and isothermal) to study the desorption and mixing kinetics as well as the crystallization of mixed multilayer ices of water, ethanol, and methanol. The desorption spectra exhibit complex features that depend strongly on both the film's composition and thickness. Analysis of the desorption spectra using a kinetic model that describes liquid solution evaporation reveals both the extent of mixing and the details of the solvation kinetics in the metastable liquids. We compare these results with existing liquid-solid phase diagrams for these binary mixtures. While thin binary films of methanol and ethanol exhibit ideal behavior, binary mixtures of water and either alcohol display strongly non-ideal behavior presumably caused by precipitation of solid crystalline hydrates from the deeply-supercooled, metastable liquid solutions. P. Ayotte is an NSERC Postdoctoral Fellow. Pacific Northwest Laboratory is a multiprogram National Laboratory operated for the U. S. Department of Energy by Battelle Memorial Institute under contract DE-AC06-76RLO 1830.
10:00 AM		SS2-TuM-6 Crystallization Kinetics of Amorphous Solid Water on Hydrophobic and Hydrophilic Substrates G. Teeter, Z. Dohnalek, R.S. Smith, G.A. Kimmel, B.D. Kay (Pacific Northwest National Laboratory) The crystallization kinetics of thin film (5-200 BL) Amorphous Solid Water (ASW) has been studied using Temperature Programmed Desorption (TPD) and Fourier Transform Infrared Spectroscopy (FTIR). ASW thin films were deposited by molecular beam on clean, w ell-ordered Pt(111) and decane (C₁₀H₂₂) thin film substrates in order to probe the dependence of H₂O crystallization kinetics on substrate-H₂O interactions. H₂O wets the Pt(111) surface and is non-wetting on the decane substr ate. In both cases crystallization of the ASW thin film proceeds via nucleation and 3D growth of the crystalline phase. As-deposited ASW kinetically wets the decane substrate at low temperature, but TPD and FTIR results indicate that crystallization is accompanied by de-wetting the H₂O film from the decane substrate. Details of the experimental techniques, results, and implications will be presented.t. Pacific Northwest National Laboratory is a multiprogram National Laboratory operated for the U.S. Department of Energy by Battelle Memorial Institute under contract DE-AC06-76RLO 1830. a.
10:20 AM		SS2-TuM-7 A Two Step Dissociative Ionization and Desorption Mechanism for Water Ion Cluster Emission from a Pt Field Emitter Tip C.J. Rothfuss, V.K. Medvedev, E.M. Stuve (University of Washington) Our recent work on the behavior of water molecules adsorbed on a Pt emitter tip in high fields (~1 V/ Å) has provided an understanding of the behavior of water dissociation and ion cluster [H⁺(H₂O)_n] emission. Water ion emission from an emitter tip has traditionally been treated as a single step event. While differing n cluster types have been observed, typically ranging to 10 or above, no detailed explanation of the mechanism for cluster formation has been suggested. Mass resolved ramped field desorption experiments from field adsorbed water layers (T>170K) have given field dependencies for specific ion cluster masses. As the field was ramped, each cluster type was observed, in turn, beginning with high n clusters and transitioning to lower n clusters. The emission of high n clusters was energetically favored, while low n clusters were favored kinetically. However, at low temperatures (T<150K), all observed ions appeared simultaneously at a critical onset field as the tip potential was ramped, demonstrating a common limiting reaction step. This ion burst contained only those clusters observed in the field adsorbed work at the onset field or below. Constant field, temperature cycled ion emission experiments have given solvation energies for protons resident in the surface adsorbed layer. Spatially resolved field ion microscopy of the ion cluster emission has shown localization of the ion emission event. These results have lead us to suggest a 2 step mechanism for ion emission, where dissociation is a distinctly separate event from desorption of the ion cluster. At low temperatures the dissociative ionization event is rate limiting, while at higher temperatures dissociation occurs freely, making the ion cluster emission the rate determining step.
10:40 AM		SS2-TuM-8 Bonding of Water to Metal Surfaces Studied with Core-level Spectroscopies H. Ogasawara, D. Nordlund (Uppsala University, Sweden); B. Brena (University of Stockholm, Sweden); L.-Å. Näslund, M. Nagasono (Uppsala University, Sweden); L.G.M. Petterson (University of Stockholm, Sweden); A. Nilsson (Uppsala University, Sweden and Stanford University) The chemical interaction between water and a metal surface is important in many practical fields, including corrosion, electrochemistry, molecular environmental science and heterogeneous catalysis. On smooth metal surfaces, (e.g. fcc(111) or hcp(0001)), water molecules adsorb intact with strong intermolecular forces between the water molecules. This leads to the formation of a hexagonal two-dimensional ice lattice proposed as a "bilayer structure".¹ The unit cell contains two water molecules, which are different in the interaction with respect to the metal surface. We have studied the water "bilayer structure" on Pt(111) with X-ray absorption and X-ray emission spectroscopies. These two techniques provide atom specific information about electronic structure. We have identified two different water-metal chemical bonds in the "bilayer structure". ¹ ¹ P.A. Thiel and T.E. Madey, Surface Science Reports 7, 211 (1987).
11:00 AM		SS2-TuM-9 The Interaction of Carbon Monoxide with Hydrogen-bonding Molecular Ice Surfaces J.W. Dever, M.P. Collings, M.R.S. McCoustra (University of Nottingham, UK) Carbon monoxide (CO) is the second most abundant molecule in the Universe after hydrogen. It's interaction with solid grain surfaces is key therefore to understanding gas-grain interactions in the interstellar medium, whether those interactions relate to the freezing out of molecules on grains in cold, dark molecular clouds or their eventual reappearance in the gas phase and their role in radiative cooling of warm, collapsing proto-stellar objects. Using a combination of temperature programmed desorption (TPD) and reflection-absorption infrared spectroscopy (RAIRS), we have conducted state-of-the-art surface physicochemical studies of the interaction of CO with hydrogen-bonded molecular ice (water, ammonia and methanol) surfaces that represent a model of the surfaces of some types of interstellar grains. We will present the results of these first detailed ultrahigh vacuum studies of realistic gas-grain interactions and their interpretation in terms of simple models of the CO-ice interaction.
11:20 AM		SS2-TuM-10 Water Reactivity with MgO(100)Thin Film Surfaces Studied by Electron Stimulated Desorption D. Cáceres, I. Colera, I. Vergara, R. González (Universidad Carlos III de Madrid, Spain); E.L. Román, J.L. de Segovia (CSIC, Spain) The adsorption of two isotopic forms of water, D₂ ¹⁶O and H₂ ¹⁸O, with well characterised MgO(100) thin film surfaces at 300 K was studied by Electron Stimulated Desorption, ESD. Oriented MgO(100) thin films were grown on Si(100) at 940 K by rf-sputtering with a Mg target and at a total pressure of 7 x 10^-3 mbar (10% O₂ and 90% Ar). These films were characterized by X-Ray diffraction, Auger Electron Spectroscopy (AES), X-Ray Photoelectron Spectroscopy (XPS), and Electron Energy Loss Spectroscopy (EELS). Previous ESD results^1,2 of O⁺ desorbed species from clean surfaces of bulk MgO(100) showed a bimodal energy distribution structure with peaks at 6 and 9 eV. Their origin was assigned to O⁺ desorption from surface defects and to desorption from the oxide, respectively. The aim of the present ESD study of MgO(100) thin films is twofold: (i) to compare the previous results on bulk defective surfaces (stepped or with oxygen vacancies) with a thin film surface with no steps, and (ii) to study the reactivity of water with these thin films, and the influence of surface defects on the hydrolisis processes. D and ¹⁸O isotopes were used to distinguish them from the H contamitation and from the ¹⁶O of the oxide, respectively. Ion yield experiments using H₂ ¹⁸O show ¹⁸O⁺ and ¹⁸OH⁺ desorbed ions in addition to the usual H⁺ and ¹⁶O⁺ ions. The adsorption of ¹⁸O is related to the ¹⁶O defective sites on the MgO(100) thin films. Preliminary results indicate that the Feibelman-Knotek mechanism is responsible for the desorption processes. Ion kinetic distribution curves at different electron energies are also discussed. ¹ I. Colera, E. Soria, E. Román and R. González, Vacuum 48 nº 7-9 pag 647 to 649 (1997) ¹ ² I. Colera, R. González, E. Soria, J.L. de Segovia, E.L. Román and Y. Chen, J. Vac. Sci. Technol. A 15(3) pag 1698-1703 (1997).
11:40 AM		SS2-TuM-11 The Influence of Calcium Carbonate Coatings on Contamination Reactivity T. Droubay, S.A. Chambers (Pacific Northwest National Laboratory) High energy-resolution x-ray photoemission spectroscopy (XPS) is used in conjunction with scanning probe microscopy to investigate the influence of calcium carbonate coatings on the adsorption of aqueous CrO₄^2- on epitaxial Fe₃O₄/MgO(001) surfaces prepared by oxygen plasma assisted molecular beam epitaxy (OPA-MBE). Deposition of calcium carbonate films was accomplished with the use of a polymer-induced liquid-precursor (PILP) process, which utilizes polyaspartate to suppress growth of crystal aggregates and results in the formation of flat films. Of the three polymorphs of CaCO₃, the films primarily consist of a combination of vaterite and calcite with undetectable amounts of aragonite. X-ray photoemission analysis and atomic force microscopy reveal a thin polymer interfacial layer between the underlying Fe₃O₄ and carbonate overlayer which acts as a template for vaterite growth. An increase of the intensity in the low binding energy shoulder of the Fe 2p^3/2 core-level and a decrease in the Fe(III) non-charge transfer satellite is evidence of an increase in Fe(II) at the magnetite surface as a result of the aqueous growth environment. This result is consistent with surface reduction found during the interaction of Fe₃O₄/MgO(001) with liquid water. The effects of carbonate coating on substrate reactivity, emphasizing electron transfer reactions responsible for the reductive immobilization of Cr(VI)O₄^2- will be discussed. These results have application in chromate remediation efforts centered on the reduction of chromate contamination by Fe(II).