2009. "The Oil-Water Interface: Mapping the Solvation Potential." Journal of the American Chemical Society 131(3):1037-1042. doi:10.1021/ja805962x Abstract Ions moving across the oil water interface are strongly impacted by the continuous changes in solvation. The solvation potential for Cs+ is directly measured as they approach the oil-water interface (“oil” = 3-methylpentane), from 0.4 to 4 nm away. The oil-water interfaces are created at 40K using molecular beam epitaxy and a softlanding ion beam, with pre-placed ions. The solvation potential slope was determined at each distance by balancing it against an increasing electrostatic potential made by increasing the number of imbedded ions at that distance, and monitoring the resulting ion motion. The potential approaches the Born model for greater than z>0.4nm, and shows the predicted reduction of the polarizability at z<0.4nm.
2008. "Pyroelectricity of Water Ice." Journal of Physical Chemistry B 112(20):6379-6389. doi:10.1021/jp073870c Abstract Water ice usually is though to have zero pyroelectricity by symmetry. But biasing it with ions breaks the symmetry because of the induced partially dipole-aligned. This unmasks a large pyroelectricity. Ions were soft-landed upon 1 micron films of water ice at ≥ 160K. When cooled below 140 to 150 K this locks-in the dipole-alignment. Workfunction measurements of these films then show high and reversible pyroelectric activity from 30 to 150K. For an initial ~10V induced by the deposited ions, the at 160K, the observed bias below 150K varies approximately as 10V*(T/150K)2 This implies water has pyroelectric coefficients as large as that of many commercial pyroelectrics such as lead zirconate titanate (“PZT”). The pyroelectricity of water ice, not previously reported, is in reasonable agreement with that predicted via a monte carlo simulation of TIP4P ice. This is observed in crystalline and compact amorphous ice, deuterated or not. This implies that for water ice between 0 and 150K (such as astrophysical ices), temperature changes can induce strong electric fields (~10,000,000 V/m) that can influence their chemistry and trajectories or binding.
2008. "Proton segregation on a Growing Ice Interface." Surface Science 602(17):2886-2893. doi:10.1016/j.susc.2008.07.008 Abstract Hydronium segregates to the surface of H2O (D2O) ice films grown on Pt(111) above 151K (158K). This is observed as from the voltage that develops across the films, utilizing work function measurements. The dependence of this voltage on the film’s thickness is explained by a simple equilibrium model: as the film grows, most of the surface ions migrate so as to remain at the ice-vacuum interface, while a fixed percentage (~0.05%) take the thermodynamically-unfavored route, to become incorporated into the growing bulk ice. This model implies a delta-G of about +0.1eV for the movement of ions from the ice surface into the bulk ice.
2007. "Reply to Comment on Dissociation of Water Buried Under Ice on Pt(111)." Physical Review Letters 99(10):109602. Abstract [1] on our Letter ‘‘Dissociation of Water Buried under Ice on Pt(111)’’ [2] raises some interesting points. In the original Letter, we made two sets of observations: (1) that thick water films ( > 150 monolayers) absorbed at temperatures higher than usually used in ultrahigh vacuum studies ( > 153 K for H2O) on Pt(111) showed a positive charging that behaved much as would be expected for positive ions trapped on the ice surface during the film growth, and (2) that the thick ice films, when desorbed carefully in a two-step process, showed a consistent pattern of high temperature programmed desorption (TPD) features spanning 170 to 200 K. Based on this, we concluded that water was dissociating at the Pt(111) surface under the thick ice film and in amounts greater than a few tenths of a monolayer. Zimbitas et al. [1] argue that the high temperature peaks can be explained by a nonwetting phenomena that they see for adsorption above 152 K (for H2O), that leads to large water clusters sitting upon a single monolayer of water. Thus, they conclude that our evidence for dissociation of water is weak.
2007. "Dissociation of water buried under ice on Pt(111) ." Physical Review Letters 98(1):Art. No. 016105 . Abstract Water on Pt(111) is generally thought to be non-dissociative. However, by adsorbing a thick ice film (>150 monolayers (ML)), substantial (~0.16 to 1 ML) dissociation of the "buried water" occurs for T > 151K . New temperature-programmed desorption peaks signal the dissociation (after careful isothermal pre-desorption of the overlying ice films). The buried water likely dissociates via the elevated temperatures and/or solvation changes experienced under the ice. Dissociation also creates large work function changes (up to +9V!) due the trapping of ~0.007ML H3O+ at the vacuum-ice interface of the growing ice.
2007. "Design and Performance of a Novel Instrument for Soft-Landing of Biomolecular Ions on Surfaces." Analytical Chemistry 79(17):6566-6574. doi:10.1021/ac070600h Abstract A new ion deposition apparatus was designed and constructed in our laboratory. Our research objectives were to investigate interactions of biomolecules with hydrophilic and hydrophobic surfaces and to carry out exploratory experiments aimed at highly-selective deposition of spatially defined and uniquely selected biological molecules on surfaces. The apparatus includes a high-transmission electrospray ion source, quadrupole mass filter, bending quadrupole that deflects the ion beam and prevents neutral molecules originating in the ion source from impacting the surface, an ultrahigh vacuum (UHV) chamber for ion deposition by soft landing, and a vacuum-lock system for introducing surfaces into the UHV chamber without breaking vacuum. Ex situ analysis of surfaces following soft-landing of mass-selected peptide ions was performed using 15 keV Ga+ time-of-flight secondary ion mass spectrometry (TOF-SIMS) and grazing incidence infrared reflection-absorption spectroscopy (IRRAS). It will be shown that these two techniques are highly complementary methods for characterization of surfaces prepared with a range of doses of mass-selected biomolecular ions.
2007. "Hydronium Ion Motion in Nanometer 3-Methyl-pentane Films." Journal of Chemical Physics 127(2):Paper # 024704. doi:10.1063/1.2748756 Abstract An ion soft-landing approach was applied to study the motion of hydronium (D3O+) and cesium (Cs+) ions from 84 to 104 K in glassy 3-methylpentane (3MP) films vapor-deposited on Pt(111). Both ions were found to have very similar mobilities in 3MP. The span of ion mobilities probed is from ~10-18 to ~10-13 m2 V-1 s-1. Ion transport in these films was studied as a function of film thickness and electric field strength. The drift velocity was found to be linear with applied field below about 2 x10(8) V/m, and deviates from linearity above this. To a large extent, D3O+ and Cs+ motion in 3MP was well predicted by a simple continuum-based ion mobility model in films from 25 to 20,000 monolayers thick, (including pronounced perturbations 7 monolayers from both the vacuum and Pt interfaces). The mobility varies with temperature more slowly than predicted by Stokes’ law, which may be due to extended inhomogeneous structures in the 3MP near the 77 K glass transition.
2006. "Analysis of Individual Environmental Particles Using Modern Methods of Electron Microscopy and X-Ray Microanalysis." Journal of Electron Spectroscopy and Related Phenomena 150(2-3):260-274. Abstract Understanding the composition of particles in the atmosphere is critical because of their health effects and their direct and indirect effects on radiative forcing, and hence on climate. In this manuscript, we demonstrate the utility of single particle off-line analysis to investigate the chemistry of individual atmospheric particles using modern, state-of-the-art electron microscopy and time-of-flight secondary ionization mass spectrometry techniques. We show that these methods provide specific, detailed data on particle composition, chemistry, morphology, phase and internal structure. This information is crucial for evaluating hygroscopic properties of aerosols, understanding aerosol aging and reactivity, and correlating the characteristics of aerosols with their optical properties. The manuscript presents a number of analytical advances in methods of electron probe particle analysis along with a brief review of a number of the research projects carried out in the authors’ laboratory on the chemical characterization of environmental particles. The obtained data offers a rich set of qualitative and quantitative information on the particle chemistry, composition and the mechanisms of gas-particle interactions which are of high importance to atmospheric processes involving particulate matter and air pollution.
2006. "A New Approach to Determining Gas-Particle Reaction Probabilities and Application to the Heterogeneous Reaction of Deliquesced Sodium Chloride Particles with Gas-Phase Hydroxyl Radicals." Journal of Physical Chemistry A 110(36):10619-10627. doi:10.1021/jp063263+ Abstract The reaction kinetics for gaseous hydroxyl radicals (OH) with deliquesced sodium chloride particles (NaClaq) were investigated using a novel experimental approach. The technique utilizes the exposure of substrate-deposited aerosol particles to reactive gases followed by chemical analysis of the particles using computer-controlled scanning electron microscopy with energy-dispersive analysis of X-rays (CCSEM/EDX) capability. Experiments were performed at room temperature and atmospheric pressure with deliquesced NaCl particles in the micron size range at 70-80% RH and with OH concentrations in the range of 1 to 7×109 cm-3. The apparent, pseudo first-order rate constant for the reaction was determined from measurements of changes in the chloride concentration of individual particles upon reaction with OH as a function of the particle loading on the substrate. Quantitative treatment of the data using a model that incorporates both diffusion and reaction kinetics yields a lower-limit to the net reaction probability of γnet > 0.1, with an overall uncertainty of a factor of two.
2005. "Sticky Ice Grains Aid Planet Formation: Unusual Properties of Cryogenic Water Ice." The Astrophysical Journal 620(2):1027-1032, Pt. 1. Abstract For planets to form in the nebula around a new-born star, they must hurry: Ina few million years or less the star's stellar winds will disperse much of the remaining dust (Briceno, et. al 2001). How do the dust grains stick together fast enough to form gravitationally bound planetsimals? The distinct properties of cryogenic (5-100 K) amorphous water ice, which composes or coats the grains, could be the key. Measurements are presented that show this ice readily acquires persistant macroscopic electric dipoles, strongly enhancing grain-grain adhesion. It is also highly mechanically inelastic (about 10% rebound). Together these explain this efficient sticking.
2005. "Erratum To "Heterogeneous Chemistry of Individual Mineral Dust Particles From Different Dust Source Regions: The Importance of Particle Mineralogy" [Atmos.Environ.38 (36)(2004)6253 –6261]." Atmospheric Environment 39(2005):395. Abstract The publisher regrets that there was an error in the information in Table 1 incorrectly identifies the mineral dust and their corresponding composition. The corrected table is given below. In addition, the second full sentence below the table should read “Smaller amounts ≤1% of other elements such as V and Ti were also observed but are not listed in Table 1 and is the reason why the percentages do not total to 100 in Table 1.” In the original article it states that K was not included in Table 1 but clearly it is.
2005. "Processing of Soot in an Urban Environment: Case Study from the Mexico City Metropolitan Area." Atmospheric Chemistry and Physics 5:3033-3043. Abstract Chemical composition, size, and mixing state of atmospheric particles are critical in determining their e ffects on the environment. There is growing evidence that soot aerosols play a particularly important role in both climate and human health, but still relatively little is known of their physical and chemical nature. In addition, the atmo- 5 spheric residence times and removal mechanisms for soot are neither well understood nor adequately represented in regional and global climate models. To investigate the effect of locality and residence time on properties of soot and mixing state in a polluted urban environment, particles of diameter 0.2–2.0 µm were collected in the Mexico City Metropolitan Area (MCMA) during the MCMA-2003 field campaign from various sites 10 within the city. Individual particle analysis by di fferent electron microscopy methods coupled with energy dispersed X-ray spectroscopy, and secondary ionization mass spectrometry show that freshly-emitted soot particles become rapidly processed in the MCMA. Whereas fresh particulate emissions from mixed-tra ffic are almost entirely carbonaceous, consisting of soot aggregates with liquid coatings suggestive of unburned 15 lubricating oil and water, ambient soot particles which have been processed for less than a few hours are heavily internally mixed, primarily with ammonium sulfate. Single particle analysis suggests that this mixing occurs through several mechanisms that require further investigation. In light of previously published results, the internally-mixed nature of processed soot particles is expected to a ffect heterogeneous chemistry on 20 the soot surface, including interaction with water during wet-removal.
2005. "Optical, Physical and Chemical Properties of Tar Balls Observed During the Yosemite Aerosol Characterization Study." Journal of Geophysical Research. D. (Atmospheres) 110:Art. No. D21210. doi:10.1029/2004JD005728 Abstract The Yosemite Aerosol Characterization Study of summer 2002 (YACS) occurred during an active fire season in the western U. S., and provided an opportunity to investigate many unresolved issues related to the radiative effects of biomass burning aerosols. Single particle analysis was performed on field collected aerosol samples using an array of electron microscopy techniques. Amorphous carbon spheres, or “tar balls”, were present in samples collected during episodes of high particle light scattering coefficients that occurred during the peak of a smoke/haze event. The highest concentrations of light-absorbing carbon from a dual-wavelength aethalometer (λ = 370 and 880 nm) occurred during periods when the particles were predominantly tar balls, indicating they do absorb light in the UV and near-IR range of the solar spectrum. Closure experiments of mass concentrations and light scattering coefficients during periods dominated by tar balls did not require any distinct assumptions of organic carbon molecular weight correction factors, density, or refractive index compared to periods dominated by other types of organic carbon aerosols. Measurements of the hygroscopic behavior of tar balls using an environmental SEM indicate that tar balls do not exhibit deliquescence, but do uptake some water at high (~83 %) relative humidity. The ability of tar balls to efficiently scatter and absorb light, and to absorb water has important implications for their role in regional haze and climate fence.
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. "Response to Comments on "Reactions at Interfaces as a Source of Sulfate Formation in Sea-Salt Particles"." Science 303(5658):1. Abstract Our paper (1) demonstrated that the oxidation of chloride at the interface will be a new source of alkalinity that can modulate the acidification of particles during the day when gaseous OH is present. We proposed that this modulation of the acidity of sea-salt particles could potentially affect the rate of uptake and oxidation of SO2 [S(IV)], which is very sensitive to pH.
2004. "Heterogeneous Chemistry of Individual Mineral Dust Particles from Different Dust Source Regions: The Importance of Particle Mineralogy." Atmospheric Environment 38(36):6253-6261. Abstract The heterogeneous chemistry of individual dust particles from four different dust source regions is investigated on a particle-by-particle basis using state-of-the-art scanning electron microscopy techniques including computer-controlled scanning electron microscopy/computer-controlled X-ray analysis (CCSEM/EDX). Morphology and compositional changes of individual particles as they react with nitric acid are observed. Clear differences in the reactivity of mineral dusts from these four different dust regions with nitric acid could be observed. Mineral dust from source regions containing high levels of calcium, such as those found in parts of China and Saudi Arabia, are found to react to the greatest extent. Calcium containing minerals, such as calcite (CaCO3) and dolomite (CaMg(CO3)2), react to form nitrate salt whereas other calcium containing minerals such as gypsum (CaSO4·2H2O) do not react. The importance of particle chemical composition and mineralogy in the heterogeneous chemistry of mineral dust aerosols is definitively borne out in this study of individual dust particles.
2003. "Time-Resolved Aerosol Collector for CCSEM/EDX Single Particle Analysis." Aerosol Science and Technology 37(3):246-260. Abstract An automated Time Resolved Aerosol Collector (TRAC) has been developed for sequential sampling of field-collected aerosols for laboratory-based Computer Controlled Scanning Electron Microscopy/Energy Dispersed X-ray (CCSEM/EDX) single particle analysis. The collector is optimized for use of grid-supported 20 nm carbon films as deposition substrates. The carbon films have low enough X-ray background to permit EDX analysis down to 0.1-0.2 ?m particles, including detection of low-Z elements: C, N, & O. The TRAC provides unattended sampling onto a set of 151 individual grids, at sequential time intervals as short as 1 min. After collection, the samples are sealed and refrigerated pending analysis. The utility of the TRAC-CCSEM/EDX approach is exemplified using the aerosol samples collected during the Texas 2000 Air Quality Studies (Aug. 15 ? Sept. 15, 2000). We are able to quantitatively follow the time evolution in the relative contribution of non-volatile particles such as ammonium sulfate, mineral dust, sea salt, carbonaceous, etc. in the aerosol make up. The results show, among other things, the diurnal cycles in appearance of fine carbonaceous & ammonium sulfate particles & substantial mixing/coating of mineral particles with ammonium sulfates.
2003. "Reactions at Interfaces As a Source of Sulfate Formation In Sea-Salt Particles." Science 301(5631):340-344. Abstract Understanding the formation of sulfate particles in the troposphere is critical because of their health effects and their direct and indirect effects on radiative forcing, and hence on climate.
2003. "The Transformation of Solid Atmospheric Particles into Liquid Droplets Through Heterogeneous Chemistry: Laboratory Insights into the Processing of Calcium Containing Mineral Dust Aerosol in the Troposphere." Geophysical Research Letters 30(3):1148. doi:10.1029/2002GL016563 Abstract [1] Individual calcium carbonate particles reacted with gas- phase nitric acid at 293 K have been followed using Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray (EDX) analysis as a function of time and relative humidity (RH). The rate of calcium carbonate to calcium nitrate conversion is significantly enhanced in the presence of water vapor. The SEM images clearly show that solid CaCO3 particles are converted to spherical droplets as the reaction proceeds. The process occurs through a two-step mechanism involving the conversion of calcium carbonate into calcium nitrate followed by the deliquescence of the calcium nitrate product. The change in phase of the particles and the significant reactivity of nitric acid and CaCO3 at low RH are a direct result of the deliquescence of the product at low RH. This is the first laboratory study to show the phase transformation of solid particles into liquid droplets through heterogeneous chemistry.
2003. "Probing Heterogeneous Chemistry of Individual Atmospheric Particles Using Scanning Electron Microscopy and Energy-Dispersive X-ray Analysis." Analytical Chemistry 75(19):5170-5179. Abstract In this paper, we demonstrate the utility of single-particle analysis to investigate the chemistry of isolated, individual particles of atmospheric relevance such as NaCl, sea salt, CaCO3, and SiO2. A variety of state-of-th-art scanning electron microscopy techniques, including environmental scanning electon microscopy and computer-controlled scanning electron microscopy/energy-dispersive X-ray analysis, were utilized for monitoring and quantifying phase transitions of individual particles, morphology, and compositional changes of individual particles as they react with nitric acid.
2003. "Structure of Ultrathin MgO Films on Mo(001)." Thin Solid Films 445(1):90-95. Abstract We have studied the structure of ultrathin MgO films grown on a single crystal Mo(001) surface. Scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) were used to investigate the effect of substrate temperature and oxygen partial pressure on the growth and morphology of these films.
2003. "Nanometer-Resolved Interfacial Fluidity." Journal of the American Chemical Society 125(17):5176-5185. Abstract Confined liquids can have properties that are poorly predicted from bulk parameters. We resolve with 0.5 nm resolution the nanoscale perturbations that interfaces cause on fluidity, in thin 3-methylpentane (3MP) films. The films of glassy 3MP are much less viscous at the vacuum-liquid interface and much more viscous at the 3MP-metal interface, compared to the bulk of the film. We find that the viscosity at the interfaces continuously returns to the bulk value over about a 3 nm distance. The amorphous 3MP films are constructed using molecular beam epitaxy on a Pt(111) substrate at low temperatures (<30 K). Ions are gently inserted at specific distances from the substrate with a 1 eV hydronium (D3O+) or Cs+ ion beam. The voltage across the film, which is directly proportional to the position of the ions within the film, is monitored electrostatically as the film is heated at a rate of 0.2 K/s. Above the bulk glass transition temperature (T-g) of 3MP (77 K), the ions are expected to begin to move down through the film. However, ion movement is observed at temperatures as low as 50 K near the vacuum interface, well below the bulk T-g. The fitted kinetics predict that at 85 K, the glass is about 6 orders of magnitude less viscous near the free interface compared to that of the bulk.
2002. "Quantitative Time-Resolved Monitoring of Nitrate Formation in Sea Salt Particles Using a CCSEM/EDX Single Particle Analysis"." Environmental Science and Technology 36(23):4948-4955. Abstract Progress of the chlorine depletion and nitrate enrichment in individual sea salt particles was detected as a function of time using aerosol samples collected during the TexAQS 2000 experiment. We demonstrate that the time-resolved collection approach coupled with the automated EDX single particle analysis made it possible to follow in great detail the time evolution of sea salt particles within a diverse aerosol mixture. Using a custom built Time Resolved Aerosol Collector (TRAC), particulate samples were taken sequentially on grid-supported 20 nm carbon films with a time resolution of 10 minutes between two consecutive samples. The samples were analyzed in the laboratory using Computer Controlled Scanning Electron Microscopy with Energy-Dispersed analysis of X-rays (CCSEM/EDX). Between midnight of 08/16/00 and the early morning of 08/17/00, a steady, particularly sea salt rich aerosol was observed at the measurement site, which later showed the effects of atmospheric processing. During the night of 08/17/00 the sea salt particles were almost unprocessed, having elemental composition close to that of seawater. By 12 noon, the evolving atmosphere was able to completely convert them, predominantly to sodium nitrate particles. During the next night, this process had nearly stopped and fairly virgin sea salt particles appeared again.
2002. "On Deposition Efficiency of Point-To-Plate Electrostatic Precipitator." Journal of Aerosol Science 33(3):405-409. Abstract The deposition efficiency of a point-to-plate electrostatic precipitator onto a target substrate was studied in this work. The efficiency when measured in the traditional way, by the loss of particles from the exiting air, was like in many others such devices, typically high (80-90%). However the actual useful deposition onto the target substrate was never more than 50% and sometime as low as a few percent only. Therefore, special caution should be taken in design and testing of electrostatic precipitators for quantitative sampling of aerosols.
2002. "NMR Investigation of the Quasi-Brine Layer in Ice/Brine Mixtures." Journal of Physical Chemistry B 106(43):11226-11232. Abstract We report the study of a liquid-like phase that is found in dilute NaCl aqueous solutions frozen at temperatures below the liquid-to-solid phase transition temperatures of H2O and NaC1-2H2O. The fractions of water and NaC1 in this sub-eutectic quasi-liquid phase were measured by NMR spectroscopy, and the experimental results compared to predictions derived from an equilibrium thermodynamic analysis. A numerical model derived from the NaC1 water phase diagram and freezing point depression curve was found to provide a quantitative description of the quasi-liquid fraction for data measured above the eutectic temperature. The relevance and application of these findings to the study of chemistry in polar regions are discussed.
2001. "Sculpting the Oil-Water Interface to Probe Ion Solvation." Journal of Physical Chemistry B 105(13):2483-2498. Abstract Solvation of ions at oil-water interfaces is so important in cell wall and enzyme function, colloidal chemistry, fuel cells and other areas, that substantial effort has been made to understand the process via molecular-scale simulations of well-specified model systems. Here we report a series of experiments probing ion transport and solvation in composite films that have a geometrical specificity that rivals what theory is routinely able to employ. Proton/hydronium transport across the water-organic interface has been studied using a novel "soft-landed" ion technique that allows precision tailoring of the interfaces (including initial ion position) and sensitive monitoring of the ion motion via the electric potential generated by the ions. There are two main findings. First, the ion solvation by water at the organic/aqueous interface is probed continuously for various monolayers of water present in simple and complex sandwiched structures. The potential trap created by the solvation can be systematically overwhelmed by the collective electric field of the ions, giving us some unique information about both the trap depth and shape. A simple Born model for the trap reproduces some, but not all the details experimentally observed. Second, ion transport in several organic glasses is well predictable by continuum viscosity models at electric fields up to approximately 108 V/m.
2001. "Automated Single-Particle SEM/EDX Analysis of Submicron Particles down to 0.1 mu m." Analytical Chemistry 73(5):1023-1029. Abstract An automated single particle SEM/EDX analysis of aerosols deposited onto grid supported carbon film of 15-25 nm thickness is demonstrated. Use of the carbon film gives exceptionally low background in the SEM/EDAX and allows satisfied automated analysis of particles down to 0.1 mm size, including analysis of low-Z elements (C, O and N). In this work, six lab-generated 0.1-2 mm aerosols were tested for their elemental composition. The EDAX yields reasonably accurate quantitative results featuring all the elements presented in the tested compounds, namely C, O, N, Na, S, Al, Si and Cl. Furthermore, the carbon film has very low backscattered electron (BSE) yield compared to that from the particle, so in the BSE mode the particle image is seen with very high contrast. This greatly improves quality and speed of the automated mapping of particles by SEM prior to EDAX. The presented approach is believed to be a significant improvement over the automated single particle analysis in the electron microprobe based techniques, which usually provide limited information on low-Z elements and usually could not go below ~0.5 mm.
2000. "Ion Transport in Micelle-Like Films: Soft-Landed Ion Studies." Langmuir 16(9):4259-4265. Abstract N/A
2000. "Ozone Loss on Atmospheric Soot Aerosols." Journal of Geophysical Research. D. (Atmospheres) 105(D8):9767-9771. Abstract The fractal-like structure of atmospheric soot (e.g., elemental carbon) provides a large surface area available for heterogeneous chemistry in the upper troposphere and lower stratosphere. One potentially important reaction is ozone decomposition on soot. Although extensively studied in the laboratory, a wide range of reaction probabilities have been observed (g~10-3 to g~10-7) which have been attributed to differences in reactivity between fresh (i.e., nonoxidized) versus aged (i.e., oxidized) soot. The importance in understanding soot-ozone chemistry is particularly important in light of recent nighttime field measurements made over Portland, Oregon. The data revealed episodes of an anticorrelation between ozone mixing ratio and aerosol surface area density. During these episodes a single scattering albedo in the range 0.8-0.9 was measured, indicating an increased absorptive component of the aerosol, perhaps due to elemental carbon. In addition, an increase in the concentration of aerosols contained in the small size range of the fine mode (<0.1-0.15 mm) was observed, suggestive of new aerosol formation. In this article we attempt to explain these field observations. One explanation of the field observations is ozone loss occurring on atmospheric soot aerosol. Here we present laboratory results obtained using a static aerosol reactor that indicate that direct ozone loss on soot aerosol is unlikely under ambient conditions in the troposphere. An alternative and more likely explanation of the field data is based on ozone-mediated organic aerosol production. This could occur by either nighttime nitrate radical oxidation or direct ozone oxidation of hydrocarbons as suggested previously.
2000. "A Chamber Investigation of Nitric Acid-Soot Aerosol Chemistry at 298 K." Journal of Atmospheric Chemistry 37(2):113-123. Abstract Long-pathlength infrared absorption spectroscopy was used to investigate nitric acid-soot aerosol chemistry at 298 K and 0.5 % relative humidity. Experiments were performed by introducing nitric acid vapor (PHNO3~3 Pa, Ptotal~40 kPa) into a teflon-coated chamber and initiating acquisition of infrared spectra at 3 minute time intervals. After 36 minutes of data collection, soot powder was rapidly expanded into nitric acid contained in the chamber to generate a soot-HNO3 aerosol. Infrared spectra collected before, and after, soot introduction to the chamber were used to characterize chamber wall reaction processes and soot aerosol chemistry, respectively. Three soot types were investigated (Degussa FW2, Cabot Monarch 1000, and crystalline graphite), each yielding similar chemistry. Upon soot introduction to the chamber both HNO3 uptake and NO2 production occurred, with the molar ratio of HNO3 uptake to NO2 production varying from 1.2 to 2.9 for the three soot types studied. Unreacted HNO3 was present at the conclusion of each of the aerosol experiments, indicating incomplete conversion of HNO3 into NO2. This observation suggested that "active" sites at the soot surface responsible for the reduction of HNO3 are not regenerated (i.e., formed) in the reaction process. In essence, a titration occurred between these active sites and HNO3. The NO2 concentrations produced, the soot mass concentrations used, and the BET measured specific surface area of the powders allowed computation of the surface density of active sites of ~4.0x10-18 m2/active site (describing all three powders studied). This is the first reported measurement of surface density of active sites for nitric acid chemistry on soot. Since atmospheric heterogeneous reactions that exhibit surface deactivation may, in principle, affect trace gas concentration, we perform an assessment in this regard.
1999. "Probing Aqueous-Organic Interfaces with Soft-Landed Ions." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 157(1-4):259-269.
1999. "Ion Penetration of the Water-Oil Interface." Science 286(5449):2482-2485. Abstract A novel ion soft-landing approach, in tandem with a molecular beam, was used to create a pure-ion-containing water/oil interface and to observe how the solvation of hydronium (D3O+) ions slowed down their motion in an organic liquid, 3-methylpentane. The delay of the ion motion was mainly due to an energy barrier between the interface and bulk 3-methylpentane that varied with the thickness of the added water. A Born-type model, combined with simple viscosity and kinetics models, accurately predicted the ion motion delay and gave an energy barrier of 0-38 kJ/mol for 0-35 bilayers of water.
1999. "Soft-Landed Ion Diffusion Studies of Vapor-Deposited Hydrocarbon Films." Journal of Chemical Physics 111:2222-2234. Abstract Cesium and hydronium ions were deposited with a "soft-landing" ion beam (1 eV) on n-hexane and 3-methyl-pentane vapor-deposited thin films on a Pt (111) surface at 27 K...
1999. "Reply to "comment on "Ferroelectricity in Water Ice'" by R.W. Whitworth." Journal of Physical Chemistry B 103(38):8194.
1999. "Immobility of Protons in Ice from 30 to 190 K." Nature 398(04/01/1999):405-407. Abstract Transport properties in water ice of hydronium and H-bonding defects are notoriously hard to study, and remain controversial. We present a new class of data using soft-landed hydronium ions (D3O+) deposited on crystalline expitaxially-grown, 600 to 5060 monolayer water (D2O) films. A work function probe is used to measure the ion motion and H-bond defect migration. In contrast to many previous notions we found: 1) The hydronium did not diffuse in crystalline water films up to at least 190 K. 2) D-defects diffuse at about 124 K at zero field. This temperature shifts linearly with field strength, extrapolated to zero K at 1.6 x 108 V/m. 3) Ice grown at 160 K and subsequently dosed with hydronium at 160 K shows an active dielectric constant (-1/T2) down to 30 K. This suggests that L defects have zero activation barrier (or tunnel) for their migration.
1998. "erroelectricity in Water Ice." Journal of Physical Chemistry B 103(38):8194. Abstract partially proton-oprdered ice I (cubic) was grown from the vapor phase, from 40 to nearly 150 K. It is believed to be metastable, and oriented by the asymmetry of the solid-vacuum interface during growth.