Papers

AVS1997 Session SS1-MoA: Atmospheric and Environmental Surface Science

Monday, October 20, 1997 2:00 PM in Room B1/2

Monday Afternoon

Start	Invited?	Item
2:00 PM		SS1-MoA-1 Nanoscopic Droplets Studied using Scanning Polarization Force Microscopy F. Rieutord, M. Salmeron (Lawrence Berkeley National Laboratory) We shall present a study of spreading and wetting phenomena using scanning polarization force microscopy (SPFM). Like AFM, SPFM allows nanoscale studies of materials ¹ . It has the additional advantadge of inducing very little perturbation to the system under study due to its non-contact mode of operation,thus being well suited for soft materials such as liquid droplets. Both vertical and horizontal resolutions are far better than other available techniques and prove to be well adapted to the relevant length scales of most spreading and wetting phenomena. We measured the shape of nanoscopic droplets of sulfuric acid, both at equilibrium and during evaporation. Interpretation of the results constitutes a stringent test of wetting theories ² since the scale probed here matches the range of the relevant interactions governing wetting. Qualitative and quantitative interpretation of the contact angle dependance on the drop radius was obtained using recent models ³. ¹Hu J., Xiao X.-D., Salmeron M., Appl. Phys. Lett 67 (1995) 476 ²Brochard-Wyart F.,di Meglio J.M., Quere D., de Gennes P.G., Langmuir 7 (1991) 335 ³Elbaum M., Lipson S.G., Europhys. Lett. 29 (1995) 457
2:20 PM		SS1-MoA-2 Soft-Landed Ions to Recreate Aqueous Interfaces J.P. Cowin (Pacific Northwest National Laboratory); A.A. Tsekouras (University of Athens, Greece); M.J. Iedema (Pacific Northwest National Laboratory) We are recreating aqueous interfaces in UHV by combining ice-epitaxy (water and solvent ices) along with a novel mass-selected ion beam that produces aqueous-type ions (H₃O⁺, H⁺, NH₄⁺, Cs⁺..) at very low impact energies of 0.5 to 2 eV. We probe ion solvation, diffusion, extraction between solvent phases, and charge transfer. We measured the work function (WF) changes after dosing the D₃O⁺ or Cs⁺ ions on top of multilayers of frozen hexane and/or water, which reflects the position of the ion in the layer and the solvent's dielectric response. For Cs⁺ deposited onto 6 to 300 monolayer hexane films, we create up to 15 volts of work function change. Amorphous hexane ices (grown at 27 K) show ion diffusion (mobility) from 60 to 95 K., proportional to the field strength. This is competitively inhibited by the crystallization of the hexane. Crystallized hexane only barely allows ion transport, near the desorption temperature of 150K. Water ice grown at 27 K is amorphous. Ion deposition on it shows that amorphous ice defects, when they anneal (30 -120 K), have a strong transient dielectric response . Amorphous water ice pre-annealed to 140 K shows no ion diffusion, nor strong dielectric changes up to the glass temperature near 120 K. Crystalline water ice (grown at 140 K) shows very limited ion diffusion (for Cs⁺, hydronium work in progress) up to about 180 K. These observations allow formulating strategies for a wide range of surface double-layer charge transfer experiments.
2:40 PM		SS1-MoA-3 Effects of Water on the Chemistry and Morphology of NaCl Surfaces: Implications for Atmospheric Chemistry of Sea Salt Particles. J.C. Hemminger (University of California, Irvine) Near oceans, substantial concentrations of sea salt particles can be found in the atmosphere. In polluted urban areas it has been discovered that many of these particles are depleted of chlorine by reactions with oxides of nitrogen, which produce gas phase chlorinated products. Our experiments are aimed at learning the fundamental surface reactions which are important in this process and determining the reaction probabilities. We have used x-ray photoelectron spectroscopy to study the reaction of nitric acid with NaCl(100) surfaces. Our experiments measure the build-up of a sodium nitriate product layer as HCl is evolved. Reaction of dry nitric acid with NaCl results in the formation of a passivating ultrathin nitrate surface layer. We utilize transmission electron microscopy to show that in the presence of very low vapor pressures of water the ionic mobility in the nitrate layer is increased resulting in three dimensional crystal growth and phase separation of the nitrate and the NaCl exposing clean NaCl surfaces for further reaction. The adsorption of water on freshly cleaved NaCl(100) is molecular and completely reversible. In constrast, after corrosion by nitric acid, the nitrate covered NaCl surface is active for the dissociative adsorption of water to form hydroxide.
3:20 PM		SS1-MoA-5 Chemistry at the Surfaces of Model Tropospheric and Stratospheric Cloud Particles J.T. Roberts, J.L. Duncan, E. Guldan, L.R. Schindler (University of Minnesota) Chemical reactions that occur in the surface and near surface regions of ice and sulfuric acid particles are important in the stratosphere and troposphere. We seek to understand the mechanisms of these surface-mediated transformations. Our experimental method involves the study of ultrathin (<200 Å thick) films of ice and sulfuric acid that are deposited on single-crystal metal substrates. The films serve as convenient samples on which to study adsorption and desorption kinetics, and adsorbate structure. The large surface area to volume ratios of the films make it possible to distinguish between surface- and bulk-mediated processes. The following aspects of our recent work will be discussed: (1) the extent to which ice surface structure dictates reactivity, (2) the different surface chemical properties of solid and liquid sulfuric acid, and (3) the mechanistic consequences of the fact that the surface of an atmospheric ice particle is highly dynamic, i.e. the evaporation and condensation rates of water on ice are very large.
3:40 PM		SS1-MoA-6 H₂O Adsorption and Desorption on a Well-Defined α-Al₂O₃(0001) Surface J.W. Elam, C.E. Nelson, M.A. Tolbert, S.M. George (University of Colorado, Boulder) H₂O adsorption and desorption kinetics on a well-defined α-Al₂O₃(0001) surface characterized by LEED was studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD). This α-Al₂O₃(0001) surface models the exhaust from solid rocket motors that may impact atmospheric chemistry. The initial H₂O sticking coefficient was 0.02 and the hydroxyl coverage after saturation H₂O exposures at 298 K was 0.50 monolayers. H₂O adsorption at lower temperature resulted in higher hydroxyl coverages approaching one monolayer at 190 K. Dissociative adsorption was differentiated from molecular adsorption using H₂¹⁸O isotopic labeling experiments. H₂O desorption from the hydroxylated surface occurred over a broad temperature range from 300 K to 600 K. Modeling this TPD data was consistent with a range of H₂O adsorption energies between 23 and 42 kcal/mol. TPD experiments were conducted at different hydroxyl coverages established either by varying the H₂O exposure or annealing following a large H₂O exposure. These H₂O desorption studies revealed that the initial dissociative adsorption of H₂O is independent of site adsorption energy and that hydroxyl surface diffusion is negligible. These results predict that α-Al2O3 particles in rocket exhaust will be partially hydroxylated at stratospheric temperatures and pressures.
4:00 PM		SS1-MoA-7 An Investigation of Liquid Sulfuric Acid Films in Ultrahigh Vacuum. J.L. Duncan, L.R. Schindler, J.T. Roberts (University of Minnesota) Chemical reactions that occur in the surface and near surface regions of liquid sulfuric acid particles are of relevance to atmospheric chemistry. A method has been developed to synthesize ultrathin films of supercooled liquid sulfuric acid in ultrahigh vacuum (UHV). The liquid films, which were characterized with infrared spectroscopy and mass spectrometry, are water + sulfuric acid azeotropes that consist of approximately 98 mole % sulfuric acid. Exposure of supercooled H₂SO₄ films to near stratospheric pressures of D₂O or H₂O produces dilute H₂SO₄ + D₂O or dilute H₂SO₄ + H₂O films as seen by infrared spectroscopy. Future work will involve investigating interactions of these dilute liquid films with DCl, acetone and nitric acid.
4:20 PM		SS1-MoA-8 Reaction of H₂S on Naturally Occurring Pyrite: Effects of Short Range Order J.M. Guevremont, D.R. Strongin (Temple University); M.A.A. Schoonen (State University of New York, Stony Brook) Pyrite, FeS₂, the most abundant metal sulfide in nature plays an important role in environmentally relevant reactions, ranging from acid mine waste to the sorption of heavy metals in the environment. Electron spectroscopic studies in vacuum coupled with aqueous studies have investigated the reaction of H₂S with the striated (100) plane of FeS₂. Special emphasis has been placed on understanding the effect of the short-range order of pyrite on the thermal surface chemistry of H₂S, an environmentally relevant reactant on pyrite. Results obtained from photoemission of adsorbed xenon (PAX) studies suggest that H₂S binds strongly to defect sites that on the basis of experimental observations are proposed to be sulfur anion vacancies. Based on PAX and isotope labeling experiments, decomposition fragments resulting from H₂S dissociation at these defect sites migrate onto surface that is suspected to be stoichiometric surface of pyrite. Based partly on this experimental observation it is proposed that the defect sites are unique binding sites of reactivity. These sites do not undergo stoichiometric reaction with adsorbate, but instead can dissociate adsorbate and release the fragments to undergo subsequent chemistry on the less reactive surface characteristic of stoichiometric pyrite. Implications for these experimental results on pyrite reactions in nature are discussed.
4:40 PM		SS1-MoA-9 ToF-SIMS Studies of Organic Reactivity on Single Particles T.F. Fister, G. Strossman (Charles Evans & Associates); S.S. Summers (University of North Carolina, Chapel Hill); R.W. Odom (Charles Evans & Associates); R.W. Linton (University of North Carolina, Chapel Hill) Coal burning power plants are a significant source of atmospheric particulates. The flyash matrix consists of a large variety of toxic metals and hazardous organics such as polycyclic aromatic hydrocarbons (PAHs). The adsorbed organics can undergo a variety of transformations via reactions such as photooxidation and nitration, significantly changing their toxicity. These organic transformation rates are dependent, among other factors, on the particle composition. We have used time-of-flight secondary ion mass spectrometry (ToF-SIMS) to monitor PAH reactions on both silica gel and flyash. Benz[a]anthracene (BaA) and benzo[a]pyrene (BaP) were coated onto silica gel and flyash in submonolayer coverages. Coated particles were placed onto a graphite planchet and analyzed by ToF-SIMS. The sample was then removed and photolyzed for specific times and reanalyzed. By repeating this procedure, the photolysis on individual particles were monitored. The average BaA half-life on individual silica gel particles was 5 min with a relative standard deviation (RSD) of 25%. In addition to the BaA decay, the formation and decay of benzanthracene-7,10-dione (BaAD) was observed. BaP half-lives were much longer averaging 44 min with an RSD of 104%. This paper will also report on the photolysis of PAHs coated on flyash particles. BaP-coated flyash samples have been exposed to nitrogen dioxide at different levels of RH (relative humidity). The experimental apparatus makes it possible to setup a dynamic system for exposing the coated flyash to controlled nitrogen dioxide concentrations and %RH. ToF-SIMS analyses of exposed samples have confirmed the selective formation of nitro-BaP on individual particles, while adjacent particles in the same sample exhibit essentially unreacted BaP. Early results show a correlation between major flyash matrix elements and the degree of BaP reaction. These experiments are being expanded to include flyash from several regional sources.