2009. "Water as a Catalyst: Imaging Reactions of O-2 with Partially and Fully Hydroxylated TiO2(110) Surfaces." Journal of Physical Chemistry C 113(5):1908-1916. Abstract The reactions of molecular oxygen with bridging hydroxyl groups, OHb, formed by H2O dissociation on bridging oxygen vacancies of TiO2 (110) are studied at low and high OHb coverages as a function of the O2 exposure, using scanning tunneling microscopy (STM), temperature programmed desorption (TPD), and electron simulated desorption (ESD) techniques. On partially hydroxylated surfaces, the sudden simultaneous disappearance of oxygen vacancies and oxygen adatoms formed by O2 dissociation is observed at high O2 exposures. On fully hydroxylated TiO2 surfaces, which enable us to compare results of STM, TPD and ESD studies, most of OHb’s are removed via reacting with O2. Hence, fully hydroxylated TiO2 surfaces can be converted to nearly stoichiometric surfaces, albeit with some amount of adsorbed molecular water. Formation of mobile H2O molecules and water-assisted diffusion of the reactants plays an important role in the kinetics of the processes on both partially and fully hydroxylated surfaces.
2009. "Nonthermal Water Splitting on Rutile TiO2: Electron-Stimulated Production of H-2 and O-2 in Amorphous Solid Water Films on TiO2(110)." Journal of Physical Chemistry C 113(11):4451-4460. Abstract Electron-stimulated desorption (ESD) of H2, O2 and H2O from 0 - 60 ML films of amorphous solid water (ASW) adsorbed on TiO2(110) are investigated as function of film thickness and isotopic composition. For 100 eV incident electrons, both the H2 and O2 ESD yields have maxima when the ASW coverage is ~ 20 monolayer (ML), while the H2O ESD yield increases monotonically with water coverage. All the products reach a coverage-independent yield above 40 - 50 ML. Experiments using isotopically layered films of H2O and D2O demonstrate that the molecular hydrogen is produced in reactions that occur preferentially at or near both the ASW/TiO2 interface and the ASW/vacuum interface. However, electronic excitations or ionic defects created within the interior of the ASW films by the energetic electrons can subsequently migrate to the interfaces where they initiate reactions. Electron irradiation of ASW films results in the formation of bridge-bonded hydroxyls on the TiO2(110). These hydroxyls do not contribute to the H2 produced near the ASW/TiO2 interface. Instead, the results suggest that this H2 is produced from a stable precursor, trapped near the substrate. The proposed mechanism for the H2 production near the ASW/TiO2(110) interface is supported by a kinetic model that semi-quantitatively reproduces the main features of the non-thermal reactions.
2009. "Chemical Reactivity of Reduced TiO2(110): The dominant role of surface defects in oxygen chemisorption." Journal of Physical Chemistry C 113(28):12407-12411. doi:10.1021/jp901989x Abstract O2 chemisorption on reduced, rutile TiO2(110) with various concentrations of oxygen vacancies (Ov) and bridging hydroxyls (OHb) is investigated with scanning tunneling microscopy, temperature programmed desorption and electron-stimulated desorption. On the annealed surface, 2 oxygen molecules can be chemisorbed per Ov. The same amount of O2 chemisorbs on surfaces where each Ov is converted to two OHb’s by exposure to water (i.e. 1 O2 per OHb). Surfaces with few or no Ov’s or OHb’s can be created by exposing the hydroxylated surface to O2 at room temperature, and the amount of O2 that chemisorbs on these surfaces at low temperatures is only ~20% of the amount on the annealed (reduced) surface. In contrast, the amount of chemisorbed O2 increases by more than a factor of two when the OHb concentration is enhanced – without changing the concentration of sub-surface Ti interstitials. The results indicate that the reactivity of TiO2(110) is primarily controlled by the amount of electron-donating surface species such as Ov’s and/or OHb’s, and not Ti3+ interstitials.
2009. "No Confinement Needed: Observation of a Metastable Hydrophobic Wetting Two-Layer Ice on Graphene." Journal of the American Chemical Society 131(35):12838-12844. Abstract The structure of water at interfaces is crucial for processes ranging from photocatalysis to protein folding. Here, we investigate the structure and lattice dynamics of two-layer crystalline ice films grown on a hydrophobic substrate - graphene on Pt(111) - with low energy electron diffraction, reflection-absorption infrared spectroscopy, rare-gas adsorption/desorption, and ab-initio molecular dynamics. Unlike hexagonal ice, which consists of stacks of puckered hexagonal "bilayers", this new ice polymorph consists of two flat hexagonal sheets of water molecules in which the hexagons in each sheet are stacked directly on top of each other. Such two-layer ices have been predicted for water confined between hydrophobic slits, but not previously observed. Our results show that the two-layer ice forms even at zero pressure at a single hydrophobic interface by maximizing the number of hydrogen bonds at the expense of adopting a non-tetrahedral geometry with weakened bonds.
2009. "Electron-Stimulated Reactions and O-2 Production in Methanol-Covered Amorphous Solid Water Films." Journal of Chemical Physics 130(10):Art. No. 104710. Abstract The low-energy, electron-stimulated desorption (ESD) of molecular products from amorphous solid water (ASW) films capped with methanol is investigated versus methanol coverage (0 - 4 x 1015 cm-2) at 50 K using 100 eV incident electrons. The major ESD products from a monolayer of methanol on ASW are quite similar to the ESD products from bulk methanol film: H2, CH4, H2O, C2H6, CO, CH2O, and CH3OH. For 40 ML ASW films, the molecular oxygen, hydrogen, and water ESD yields from the ASW are suppressed with increasing methanol coverage, while the CH3OH ESD yield increases proportionally to the methanol coverage. The suppression of the water ESD products by methanol is consistent with the non-thermal reactions occurring preferentially at or near the ASW/vacuum interface and not in the interior of the film. The water and molecular hydrogen ESD yields from the water layer decrease exponentially with the methanol cap coverage with 1/e constants of ~ 0.6 x 1015 cm-2 and 1.6 x 1015 cm-2, respectively. In contrast, the O2 ESD from the water layer is very efficiently quenched by small amounts of methanol (1/e ~ 6.5 x 1013 cm-2). The rapid suppression of O2 production by small amounts of methanol is due to reactions between CH3OH and the precursors for the O2 - mainly OH radicals. A kinetic model for the O2 ESD which semi-quantitatively accounts for the observations is presented.
2008. "Tetraoxygen on Reduced Ti02(110): Oxygen Adsorption and Reactions with Oxygen Vacancies." Physical Review Letters 100(19):Art. No. 196102. doi:10.1103/PhysRevLett.100.196102 Abstract Oxygen adsorption on reduced TiO2(110) is investigated using temperature programmed desorption and electron-stimulated desorption. At low temperatures, two O2 molecules can be chemisorbed in each oxygen vacancy, Ov. These molecules do not desorb upon annealing to 700 K. Instead for 200 K < T < 400 K, the two O2 convert to another species which has four oxygen atoms, i.e. tetraoxygen, that decomposes at higher temperatures. In contrast when only 1 O2 is adsorbed in an oxygen vacancy, the molecule dissociates upon annealing above ~150 K to heal the vacancy in agreement with previous results.
2008. "Photoionization of Sodium Salt Solutions in a Liquid Jet." Journal of Physical Chemistry C 112(22):8359-8364. doi:10.1021/jp7102534 Abstract A liquid microjet was employed to examine the gas/liquid interface of aqueous sodium halide (Na+X-, X=Cl, Br, I) salt solutions. Laser excitation at 193 nm produced and removed cations of the form H+(H2O)n and Na+(H2O)m from liquid jet surfaces containing either NaCl, NaBr or NaI. The protonated water cluster yield varied inversely with increasing salt concentration, while the solvated sodium ion cluster yield varied by anion type. The distribution of H+(H2O)n at low salt concentration is identical to that observed from low-energy electron irradiated amorphous ice and the production of these clusters can be accounted for using a localized ionization/Coulomb expulsion model. Production of Na+(H2O)m is not accounted for by this model but requires ionization of solvation shell waters and a contact ion/Coulomb expulsion mechanism. The reduced yields of Na+(H2O)m from high concentration (10-2 and 10-1 M) NaBr and NaI solutions indicate a propensity for Br- and I- at the solution surfaces and interfaces. This is supported by the observation of multiphoton induced production and desorption of Br+ and I+ from the 10-2 and 10-1 M solution surfaces.
2007. "Hydrogen Bonding, H-D Exchange, and Molecular Mobility in Thin Water Films on TiO2(110)." Physical Review Letters 99(19):Art.No.196103. doi:10.1103/PhysRevLett.99.196103 Abstract Hydrogen bonding, H/D exchange and diffusion in water films ( 2 monolayers) on TiO2(110) have been studied using water electron-stimulated desorption. For T < 70 K, films with one water isotope adsorbed on the Ti4+ rows and another isotope on the bridging oxygen rows can be prepared. For T > 70 K, H/D exchange and molecular diffusion between these “layers” occur with a distribution of activation energies. The results demonstrate that all the water molecules that are directly bound to TiO2(110) – i.e. water in the first and second layers – are also hydrogen bonded to each other, thus influencing the chemistry of the water/TiO2(110) interface.
2007. "Site-Dependent Electron-Stimulated Reactions in Water Films on TiO2(110)." Journal of Chemical Physics 127(22):Art. No. 224706. Abstract Electron-stimulated reactions in thin (<3 monolayer, ML) water films adsorbed on TiO2(110) are investigated. Irradiation with 100 eV electrons results in electron-stimulated dissociation and electron-stimulated desorption (ESD) of adsorbed water molecules. The ESD yield for water molecules adsorbed on the bridging oxygen rows, H2OBBO, is 4-5 times greater than the ESD yield for water adsorbed on Ti4+ sites, H2OTi. In contrast, the probability for electron-stimulated dissociation of adsorbed water is comparable for both H2OTi and H2OBBO. The total electron-stimulated sputtering rate is larger for coverages greater than 1 ML due to the increased water ESD for those coverages. The water ESD yields versus electron energy (for 5 – 50 eV) are qualitatively similar for 1, 2 and 40 ML water films. In each case, the observed ESD threshold is at ~10 eV and the yield increases monotonically with increasing electron energy. The results indicate that excitations in the adsorbed water layer are primarily responsible for the ESD in thin water films on TiO2(110). Experiments on “isotopically layered” films with D2OTi and H2OBBO demonstrate that increasing the water coverage above 1 ML rapidly suppresses the electron-stimulated desorption of D2OTi and D atoms, despite the fact that the total water ESD and atomic hydrogen ESD yields increase with increasing coverage. The coverage dependence of the electron-stimulated reactions is probably related to the different bonding geometries for H2OTi and H2OBBO and its influence on the desorption probability of the reaction products.
2007. "Electron-Stimulated Oxidation of Thin Water Films Adsorbed on TiO2(110)." Journal of Physical Chemistry C 111(44):16319-16329. doi:10.1021/jp072479o Abstract Electron-stimulated reactions in thin (< 3 monolayer, ML) water films adsorbed on TiO2(110) are investigated. For electron fluences less than ~1×1016 e-/cm2, irradiation with 100 eV electrons results in electron-stimulated desorption (ESD) of atomic and molecular hydrogen, but no measurable O2. The ESD leaves adsorbed hydroxyls which oxidize the TiO2(110) surface and change the post-irradiation TPD spectra of the remaining water in characteristic ways. The species remaining on the TiO2(110) after irradiation of adsorbed water films are apparently similar to those produced without irradiation by co-dosing water and O2. Annealing above ~600 K reduces the oxidized surfaces, and water TPD spectra characteristic of ion sputtered and annealed TiO2(110) are recovered. The rate of electron-stimulated “oxidation” of the water films is proportional to the coverage of water in the first layer for coverages less than 1 ML. However, higher coverages suppress this reaction. When thin water films are irradiated, the rate of electron-stimulated oxidation is independent of the initial oxygen vacancy concentration, as is the final oxidized state achieved at high electron fluences. To explain the results, we propose that electron excitation of water molecules adsorbed on Ti4+ sites leads to desorption of hydrogen atoms and leaves an OH adsorbed at the site. If hydroxyls are present in the bridging oxygen rows, these react with the OH’s on the Ti4+ sites to reform water and heal the oxygen vacancy associated with the bridging OH. Once the bridge bonded hydroxyls have been eliminated, further irradiation increases the concentration of OH’s in the Ti4+ rows leading to the creation of species which block sites in the Ti4+ rows, perhaps H2O2 and/or HO2.
2007. "Crystalline Ice Growth on Pt(111) and Pd(111): Nonwetting Growth on a Hydrophobic Water Monolayer." Journal of Chemical Physics 126(11):Art. No. 114702. Abstract The growth of crystalline water films on Pt(111) and Pd(111) is investigated using temperature programmed desorption of the water films and of rare gases adsorbed on the water films. The water monolayer wets both Pt(111) and Pd(111) at all temperatures investigated (e.g. 20-155 K, for Pt(111)). However, crystalline ice films grown at higher temperatures (e.g. T>135 K) do not wet the monolayer. Similar results are obtained for crystalline ice films of D2O and H2O. Amorphous water films, which initially wet the surface, crystallize and dewet exposing the water monlayer when they are annealed at higher temperatures. Thinner films crystallize and dewet at lower temperatures than thicker films. For samples sputtered with energetic Xe atoms to prepare ice crystallites surrounded by bare Pt(111), subsequent annealing of the films causes water molecules to diffuse off the ice crystallites to reform the water monolayer. A simple model suggests that, for crystalline films grown at high temperatures, the ice crystallites are initially widely separated with typical distances between crystallites of ~14 nm or more. The experimental results are consistent with recent theory and experiments suggesting that the molecules in the water monolayer form a surface with no dangling OH bonds or lone pair electrons, giving rise to a hydrophobic water monolayer on both Pt(111) and Pd(111).
2006. "Electron-Stimulated Production of Molecular Oxygen in Amorphous Solid Water on Pt(111): Precursor Transport Through the Hydrogen Bonding Network." Journal of Chemical Physics 125(12):124702 (11 p.). Abstract The low-energy, electron-stimulated production of molecular oxygen from thin amorphous solid water (ASW) films adsorbed on Pt(111) is investigated. For ASW coverages less than ~60 monolayers (ML), the O2 ESD yield depends on coverage in a manner that is very similar to the H2 ESD yield. In particular, both the O2 and H2 ESD yields have a pronounced maximum at ~20 ML due to reactions at the Pt/water interface. The O2 yield is dose-dependent and several precursors (OH, H2O2 and HO2) are involved in the O2 production. Layered films of H216O and H218O are used to profile the spatial distribution of the electron-stimulated reactions leading to oxygen within the water films. Independent of the ASW film thickness, the final reactions leading to O2 occur at or near the ASW/vacuum interface. However for ASW coverages less than ~40 ML, the results indicate that dissociation of water molecules at the ASW/Pt interface contributes to the O2 production at the ASW/vacuum interface presumably via the generation of OH radicals near the Pt substrate. The OH (or possibly OH-) segregates to the vacuum interface where it contributes to the reactions at that interface. The electron-stimulated migration of precursors to the vacuum interface occurs via transport through the hydrogen bond network of the ASW without motion of the oxygen atoms. A simple kinetic model of the non-thermal reactions leading to O2, which was previously used to account for reactions in thick ASW films, is modified to account for the electron-stimulated migration of precursors.
2006. "Electron-Stimulated Production of Molecular Oxygen in Amorphous Solid Water." Journal of Physical Chemistry B 110(6):2723-2731. Abstract The low-energy, electron-stimulated production of molecular oxygen from pure amorphous solid water (ASW) films and ASW films co-dosed with H2O2 is investigated. Layered films of H216O and H218O are used to determine the spatial profile of the reactions in the films leading to O2. The O2 yield is dose-dependent, indicating that precursors are involved in the O2 production. For temperatures below ~80 K, the O2 yield at steady state is relatively low and nearly independent of temperature. At higher temperatures, the yield increases rapidly. The O2 yield is enhanced from H2O2-dosed water films, but the experiments show that H2O2 is not the final precursor in the reactions leading to O2. Instead, a stable precursor for O2 is produced through a multi-step reaction sequence probably involving the reactions of OH radicals to produce H2O2 and then HO2. The O2 is produced in a non-thermal reaction from the HO2. For relatively thick films, the reactions leading to O2 occur at or near the ASW/vacuum interface. However, the electronic excitations which initiate the reactions occur over a larger range in the film. A kinetic model which qualitatively accounts for all of the observations is presented.
2006. "Layer-by-Layer Growth of Thin Amorphous Solid Water Films on Pt(111) and Pd(111)." Journal of Chemical Physics 125(4):044713-1 - 044713-12. doi:10.1063/1.2218844 Abstract The growth of amorphous solid water (ASW) films on Pt(111) is investigated using rare gas (e.g. Kr) physisorption. Temperature programmed desorption of Kr is sensitive to the structure of thin water films and can be used to assess the growth modes of these films. At all temperatures that are experimentally accessible (20 – 155 K), the first layer of water wets Pt(111). Over a wide temperature range (20 – 120 K), ASW films wet the substrate and grow approximately layer-by-layer for at least the first 3 layers. In contrast to the ASW films, crystalline ice films do not wet the water monolayer on Pt(111). Virtually identical results were obtained on epitaxial Pd(111) films grown on Pt(111). The desorption rates of thin ASW and crystalline ice films suggest that the relative free energies of the films are responsible for the different growth modes. However at low temperatures, surface relaxation or “transient mobility” is primarily responsible for the relative smoothness of the films.
2005. "Low-LET Microbeam Investigation of the Track-End Dependence of Electron-Induced Damage in Normal Human Diploid Fibroblasts." Radiation Research 164(5):677-679. Abstract Using the electron microbeam located at Pacific Northwest National Laboratory, we have investigated the incident electron energy dependence on micronuclei formation in human diploid fibroblasts, AG01522, following non-targeted irradiations at 25 and 80 keV. Examining the dose response, we find that 25 keV electrons are more effective than 80 keV electrons at producing biological damage for a given dose. Our results demonstrating the induction of micronuclei as a function of incident electron energy offer direct support for the hypothesis that the track end is responsible for the biological damage occurring in the cell.
2005. "A Variable-Energy Electron Microbeam: A Unique Modality for Targeted Low-LET Radiation." Radiation Research 164(5):695-700. Abstract We have designed and constructed a low-cost, variable energy low-LET electron microbeam that utilizes energetic electrons to mimic radiation damage produced from y- and X -rays. The microbeam can access lower regions of the LET spectra, analogous to that produced by a 60Co source. The device has two operating modes, as a conventional microbeam targeting single cells or sub-populations of cells or as a pseudo broad beam source allowing for direct comparison with conventional sources. By varying the incident electron energy, the target cells can be selectively exposed to different parts of the energetic electron tracks, including the track ends.
2005. "Low-Energy Electron-Stimulated Luminescence of Thin H20 and D20 Layers on Pt(111)." Journal of Physical Chemistry B 109(33):15835-15841. Abstract The electron-stimulated luminescence (ESL) from amorphous solid water and crystalline ice films deposited on Pt(111) at 100 K is investigated as a function of the film thickness, incident electron energy (5 – 1000 eV), isotopic composition, and film structure. The ESL emission spectrum has a characteristic double-peaked shape that has been attributed to a transition between a superexcited state ( ) and the dissociative, first excited state ( ) in water: . Comparing the electron-stimulated luminescence and O2 electron-stimulated desorption (ESD) yields versus incident electron energy, we find the ESL threshold blue-shifted from the O2 ESD threshold by ~3 eV, which is close to the center of the emission spectrum near 400 nm and supports the assignment for the ESL. For thin films, radiative and non-radiative interactions with the substrate tend to quench the luminescence. The luminescence yield increases with coverage since the interactions with the substrate become less important. The ESL yield from D2O is ~ 4 times higher than from H2O. Using layered films of H2O and D2O, this sizable isotopic effect on the ESL is exploited to spatially profile the luminescence emission within the ASW films. These experiments show that most of the luminescence is emitted from within the penetration depth of the incident electron. However, the results depend on the order of the isotopes in the film, and this asymmetry can be modeled by assuming some migration of the excited states within the film. The ESL is very sensitive to defects and structural changes in solid water, and the emission yield is significantly higher from amorphous films than from crystalline ice.
2005. "Electron-Stimulated Sputtering of Thin Amorphous Solid Water Films on Pt(111)." Journal of Chemical Physics 123(5):054702 (1-7). Abstract The electron-stimulated sputtering of thin amorphous solid water films deposited on Pt(111) is investigated. The sputtering appears to be dominated by two processes: 1) electron-stimulated desorption of intact water molecules and 2) electron-stimulated reactions leading to the production of molecular hydrogen and molecular oxygen. The electron-stimulated desorption of water increases monotonically with increasing film thickness. In contrast, the total sputtering – which includes all electron-stimulated reaction channels – is maximized for films of intermediate thickness. The sputtering yield versus thickness suggests that erosion of the film occurs due to reactions at both the water/vacuum interface and the Pt/water interface. Experiments with layered films of D2O and H2O demonstrate significant loss of hydrogen due to reactions at the Pt/water interface. The electron-stimulated sputtering is independent of temperature below ~80 K and increases rapidly at higher temperatures. A simple one-dimensional random walk model qualitatively accounts for the experimental observations.
2005. "Monte Carlo Simulation of the Spatial Distribution of Energy Deposition for an Electron Microbeam." Radiation Research 163(4):468-472. Abstract Dosimetry calcutions characterizing the spatial variation of the energy deposited by the slowing and stopping of energetic are reported and compared with experimental measurements from an electron microbeam facility. The computations involve event-by-event, detailed-histories Monte Carlo simulations of low-energy electrons interacting in water vapor. Simulations of electron tracks with starting energies from 30 to 80 keV are used to determine energy deposition distributions in thin cylindrical rings as a function of penetration and radial distance from a beam source. Experimental measurements of the spatial distribution of an electron microbeam in air show general agreement with the density-scaled simulation results for water vapor at these energies, yielding increased confidence in the predictions of Monte Carlo track-structure simulations for applications of the microbeam as a single-cell irradiator.
2005. "Crystalline Ice Growth on Pt(111): Observation of a Hydrophobic Water Monolayer." Physical Review Letters 95(16):166102. Abstract The growth of crystalline water films on Pt(111) is investigated using rare gas physisorption. The water monolayer wets Pt(111) at all temperatures investigated (20-155 K). At low temperatures (T ≤ 120 K) where the water mobility is limited, additional water layers kinetically wet the monolayer surface. However, crystalline ice films grown at higher temperatures (T > 135 K) do not wet the water monolayer. These results are consistent with recent theory and experiments suggesting that the molecules in the water monolayer form a surface with no dangling OH bonds or lone pair electrons, giving rise to a hydrophobic water monolayer on Pt(111).
2005. "Role of Water in Electron-Initiated Processes and Radical Chemistry: Issues and Scientific Advances." Chemical Reviews 105(1):355-389. doi:10.1021/cr030453x Abstract An understanding of electron-initiated processes in aqueous systems and the subsequent radical chemistry these processes induce is significant in such diverse fields as waste remediation and environmental cleanup, radiation processing, nuclear reactors, and medical diagnosis and therapy. We review the state of the art in the physical chemistry and chemical physics of electron-initiated processes in aqueous systems and raise critical research issues and fundamental questions that remain unanswered.
2004. "Electron-Stimulated Reactions in Thin D2O Films on Pt(111) Mediated by Electron Trapping." Journal of Chemical Physics 121(8):3727-3735. Abstract We have measured the electron-stimulated desorption (ESD) of D2, O2 and D2O, the electron-stimulated dissociation of D2O at the D2O/Pt interface, and the total electron-stimulated sputtering in thin D2O films adsorbed on Pt(111) as a function of the D2O coverage (i.e. film thickness). Qualitatively different behavior is observed above and below a threshold coverage of ~2 monolayers (ML). For coverages less than ~2 ML electron irradiation results in D2O ESD and some D2 ESD, but no detectible reactions at the water/Pt interface and no O2 ESD. For larger coverages, electron-stimulated reactions at the water/Pt interface occur, O2 is produced and the total electron-stimulated sputtering of the film increases. An important step in the electron-stimulated reactions is the reaction between water ions (generated by the incident electrons) and electrons trapped in the water films to form dissociative neutral molecules. However, the electron trapping depends sensitively on the water coverage: For coverages less than ~ 2 ML, the electron trapping probability is low and the electrons trap preferentially at the water/vacuum interface. For larger coverages, the electron trapping increases and the electrons are trapped in the bulk of the film. We propose that the coverage dependence of the trapped electrons is responsible for the observed coverage dependence of the electron-stimulated reactions.
2004. "Electron-Stimulated Production of Molecular Hydrogen at the Interfaces of Amorphous Solid Water Films on Pt(111)." Journal of Chemical Physics 121(8):3736-3744. Abstract The electron-stimulated production of molecular hydrogen (D2, HD and H2) from amorphous solid water (ASW) deposited on Pt(111) is investigated. Experiments with isotopically layered films of H2O and D2O are used to profile the spatial distribution of the electron-stimulated reactions leading to hydrogen within the water films. The molecular hydrogen yield has two components that have distinct reaction kinetics due to reactions that occur at the ASW/Pt interface and the ASW/vacuum interface, but not in the bulk. However, the molecular hydrogen yield as a function of the ASW film thickness in both pure and isotopically layered films indicates that the energy for the reactions is absorbed in the bulk of the films and electronic excitations migrate to the interfaces where they drive the reactions
2003. "Molecular Beam Studies of Nanoscale Films of Amorphous Solid Water." Chapter 14 in Water in Confining Geometries, ed. V. Buch and J.P. Devlin, pp. 337-357. Springer-Verlag, New York, NY. Abstract In this article, we discuss the use of nanoscale thin films to explore the structural and physical properties of amorphous solid water.
2003. ""Electron-Simulated Reactions at the Interfaces of Amorphous Solid Water Films Driven by Long-Range Energy Transfer from the Bulk"." Physical Review Letters 90(16):166102. Abstract The electron-stimulated production of D2 from amorphous solid D2O deposited on Pt(111) is investigated as a function of film thickness. The D2 yield has two components with distinct reaction kinetics. Using isotopically layered films of H2O and D2O demonstrates that the D2 is produced in reactions that occur at both the Pt/amorphous solid water (ASW) interface and the ASW/vacuum interface, but not in the bulk. The energy for the reactions, however, is absorbed in the bulk of the films and electronic excitations migrate to the interfaces where they drive the reactions.
2003. "Temperature Independent Physisorption Kinetics and Adsorbate Layer Compression for Ar Adsorbed on Pt (111)." Journal of Chemical Physics 119(13):6776-6783. Abstract The influence of adlayer compression on the physisorption of Ar on Pt(111) is investigated using temperature programmed desorption and modulated molecular beams. We find that the difference in coverage between the compressed and uncompressed first laysers is ~10-15%. For coverages near one monolayer, this compression causes nearly temperature independent desorption kinetics over a wide temperature range (32K-42K). We present a theory that includes the effects of the compression on the desorption kinetics and explains the observed kinetics in terms of a competition between adsorbate-substrate and adsorbate-adsorbate interactions resulting in a continuous increase in the chemical potential near the completion of each successive layer.
2003. "The Deposition Angle-Dependent Density of Amorphous Solid Water Films." Journal of Chemical Physics 118(1):364-372. Abstract The index of refraction and thickness of amorphous solid water (ASW) films are determined using laser optical interferometry. From the film thickness, the density of ASW can be calculated directly since the molecular beam flux and the H2O condensation coefficient are both known. From the index of refraction the ASW density can also be determined using the Lorentz-Lorenz relationship. The densities determined via both methods agree within experimental uncertainty. For films deposited at 22 K using a collimated molecular beam, the index of refraction and density decrease monotonically as the deposition angle is varied from normal to oblique incidence. At normal incidence the films have an index of refraction of 1.285 and are fully dense (0.94 g/cm3). At glancing incidence (86?) the film has a refractive index of 1.05 and a density of 0.16 g/cm3, indicating a porosity exceeding 80%. The angle dependent film density is in good quantitative agreement with the results of ballistic deposition simulations of ASW film growth.
2002. "Spatially Resolved Single-Cell Irradiator to Study Bystander Responses to Low-LET Radiation." Radiation Research 158(3):375-376. Abstract Currently, the standards for human exposure to ionizing radiation are determined by extrapolating data obtained at high doses to the low-doese region using a linear, no-threshold model.
2002. "Structural and chemical characterization of aligned crystalline nanoporous MgO films grown via reactive ballistic deposition ." Journal of Physical Chemistry B 106(14):3526-3529 . doi: 10.1021/jp013801c Abstract Highly-porous (~ 90%), high-surface area (~ 1000 m2/g), thermally stable (1200 K) crystalline films of MgO are synthesized using a novel reactive ballistic deposition technique. The film consists of a tilted array of porous nanoscale crystalline filaments. Surprisingly, the individual filaments exhibit a high degree of crystallographic order with respect to each other. These films have chemical binding sites analogous to those on MgO(100). However the fraction of chemically active, high energy binding sites is greatly enhanced on the nanoporous film. This unique collection of properties makes these materials attractive candidates for chemical applications such as sensors and heterogeneous catalysts.
2002. "Structural and Chemical Characterization of Aligned Crystalline Nanoporous MgO Films Grown via Reactive Ballistic Deposition." Journal of Physical Chemistry B 106(14):3526-3529. Abstract Highly-porous (~90%), high-surface area (~1000 m2/g), thermally stable (1200K) crystalline films of MgO are synthesized using a novel reactive ballistic deposition techniques. The film consists of a tilted array of porous nanoscale crystalline filaments. Suprisingly, the individual filaments exhibit a high degree of crystallographic order with respect to each other. These films have chemical binding sites analogous to those on MgO (100). However, the fraction of chemically active, high energy binding sites is greatly enhanced on the nanoporous film. This unique collection of properties makes these materials attractive candidates for chemical applications such as sensors and heterogeneous catalysts.
2002. "A Beaker without Walls: Formation of Deeply Supercooled Binary Liquid Solutions of Alcohols from Nanoscale Amorphous Solid Films." Physical Review Letters 88(24):art. no. 245505. Abstract Layered nanoscale amorphous solid films of methoanol and ethanol undergo complete intermixing prior to the onset of measurable desorption at 120 K. This intermixing precedes and inhibits crystallization. Subsequent desorption of the film is described quantitatively by a kinetic model describing evaporation from a continuously remixing ideal binary liquid solution. This occurs at temperatures below the melting point of the binary mixture indicating ideal behavior for the supercooled liquid solution. This approach provides a new method for preparing and examining deeply-supercooled solutions.
2001. "Control of Amorphous Solid Water Morphology using Molecular Beams - II: Ballistic Deposition Simulations ." Journal of Chemical Physics 114(12):5295-5303. Abstract Ballistic deposition simulations of thin film growth were performed. The results of the simulations are compared to experiments of N2 adsorption by porous amorphous solid water thin films. The simulations are in qualitative agreement with the experimental observations: The porosity of the thin films is controlled by using a collimated beam to vapor deposit the films. Films with normal or near normal growth angles (q ~ 0?) are relatively dense and smooth. Films with larger growth angles are highly porous and the average pore size increases as the growth angle increases. The simulations indicate that for growth angles greater than ~70?, adsorption into the largest pores is not possible leading to the experimentally observed maximum in N2 adsorption by porous amorphous solid water at q = 70?.
2001. "Control of Amorphous Solid Water Morphology using Molecular Beams - I: Experimental Results ." Journal of Chemical Physics 114(12):5284-5294. Abstract The adsorption of N2 was used to investigate the porosity/morphology of thin films of amorphous solid water. Molecular beams were used to vapor deposit amorphous solid water films on a Pt(111) crystal at a variety of incident growth angles. The amount of N2 adsorbed by the amorphous solid water depends very sensitively on the growth angle and thermal history of the film. For normal and nearly normal incidence growth, the water films are relatively dense and smooth and adsorb only a small amount of N2. For larger growth angles, the films are porous and adsorb large quantities of N2 with apparent surface areas as high as ~2700 m2/g. The physical and chemical properties of amorphous solid water are of interest because of its presence in astrophysical environments. The observations have important implications for laboratory studies which use vapor deposited amorphous solid water films as analogs for astrophysical icy bodies such as comets.
2001. "Physisorption of CO on The MgO(100) Surface." Journal of Physical Chemistry B 105(18):3747-3751. Abstract The ability to grow thin MgO(100) films of quality approaching that of vacuum cleaved MgO(100) is demonstrated using low energy electron diffraction and temperature programmed desorption of CO. The highly ordered MgO(100) surfaces are used to study adsorption and desorption of CO. A linearly increasing sticking coefficient from 0.47 ? 0.03 to 0.90 is observed for relative CO coverages less than 0.8 ML. This is a consequence of constant but different sticking coefficient on bare (0.47 ? 0.03) and CO covered (0.90) MgO(100) surface. In TPD, the desorption of CO is dominated at very low coverages by desorption from the defects. At intermediate coverages (0.25 - 0.8 ML) the CO desorbs via first order desorption with the desorption energy of 17 ? 2 kJ/mol and preexponential factor of 1x10(superscript)15?2 s(superscript)-1 at 0.8 ML when the peak saturates. The desorption energy increases linearly as coverage decreases due to repulsive interactions between adsorbed CO molecules. Above Theta = 0.8 ML the adsorption occurs on fully CO covered MgO(100) surfaces and further increase in Theta is achieved by compression of the CO layer. This results in a sharp decrease in desorption energy which upon saturation of the first CO layer (Theta = 1 ML) and formation of c(4x2) ordered phase has value of ~9 kJ/mol.
2001. "Effect of Porosity on the Adsorption, Desorption, Trapping and Release of Volatile gases by Amorphous Solid Water." Journal of Geophysical Research. Planets 106(E12):33,387-33,392. Abstract We compare the adsorption, desorption, trapping, and release of Ar, N2, O2, CO, and CH4, by dense (non-porous) and highly porous amorphous solid water (ASW) films. Molecular beam deposition techniques were used to control the porosity of the vapor deposited ASW thin films. Experiments where the gas species was deposited on top and underneath of dense and porous ASW were conducted. The porous films were found to adsorb between 20 and 50 times more gas than the dense films. The desorption temperature of the adsorbed gas was also dependent on the porosity of the ASW film. The differences between desorption from porous and dense ASW films are attributed to differences in their ability to trap weakly physisorbed gases. The results were largely independent of the gas studied confirming that the adsorption and trapping of gases is dominated by the ASW porosity. These findings show that laboratory studies must account for the growth conditions and their effects on ASW morphology in order accurately predict the properties of astrophysical ices.
2000. "The Self-diffusivity of Amorphous Solid Water Near 150 K." Chemical Physics 258(3):291-305. Abstract Molecular beam techniques are used to create nanoscale thin films composed of different isotopes of amorphous solid water (ASW). The metastable ASW composites are then heated above the glass transition temperature, Tg, and the extent of isotopic intermixing is determined using temperature programmed desorption (TPD). The observed self-diffusion in the 150-160 K range is roughly a million-fold greater than that expected for crystalline ice. The magnitude and temperature dependence of the self-diffusivity are consistent with an amorphous solid that melts into a deeply supercooled liquid prior to crystallization. The overall temperature dependence for the diffusivity of liquid water, supercooled liquid water (238-273 K), and of ASW (150-160 K) is well described by the Vogel-Fulcher-Tamman equation. These results suggest that ASW above its Tg is a deeply supercooled metastable extension of normal liquid water prior to crystallizing near 160 K
2000. "The Effect of Underlying Substrate on the Crystallization Kinetics of Dense Amorphous Solid Water Films." Journal of Chemical Physics 112(13):5932-5941. Abstract The crystallization kinetics of thin, dense amorphous solid water films deposited on crystalline ice and Pt(111) substrates are investigated. A dramatic acceleration of the crystallization rate is observed for amorphous films on crystalline ice substrates. The acceleration originates from the absence of the nucleation process on the crystalline ice substrate which serves as a 2-dimensional nucleus for the growth of the crystalline phase. This contrasts with the crystallization on a Pt(111) substrate which proceeds via bulk nucleation and 3-dimensional growth. Activation energies for growth and nucleation are determined to be 56 and 140 kJ/mol, respectively using the apparent activation energies measured on crystalline ice and Pt(111) substrates. For amorphous films deposited on crystalline ice substrates, the crystallization rate decreases rapidly with increasing distance from the crystalline ice substrate most probably due to crystallization induced cracking of the films.
1999. "Controlling the Morphology of Amorphous Solid Water." Science 283:1505-1507. Abstract The morphology of amorphous solid water grown by vapor deposition depends strongly ont he angular distribution of the water molecules incident from the gas phase...
1999. "Quantum-Resolved Electron Stimulated Interface Reactions: D2 Formation from D2O Films." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 157:183-190.
1999. "Substrate Induced Crystallization of Amorphous Solid Water at Low Temperatures." Journal of Chemical Physics 110(12):5489-5492. Abstract We show that N2 monolayer desorption from ice surfaces is a quantitive, highly sensitive method for following the surface crystallization kinetics at low temperatures...