Snow in the environment acts as a host to rich chemistry and provides a matrix for physical exchange of contaminants within the ecosystem. The goal of this review is to summarise the current state of ...knowledge of physical processes and chemical reactivity in surface snow with relevance to polar regions. It focuses on a description of impurities in distinct compartments present in surface snow, such as snow crystals, grain boundaries, crystal surfaces, and liquid parts. It emphasises the microscopic description of the ice surface and its link with the environment. Distinct differences between the disordered air-ice interface, often termed quasi-liquid layer, and a liquid phase are highlighted. The reactivity in these different compartments of surface snow is discussed using many experimental studies, simulations, and selected snow models from the molecular to the macro-scale. Although new experimental techniques have extended our knowledge of the surface properties of ice and their impact on some single reactions and processes, others occurring on, at or within snow grains remain unquantified. The presence of liquid or liquid-like compartments either due to the formation of brine or disorder at surfaces of snow crystals below the freezing point may strongly modify reaction rates. Therefore, future experiments should include a detailed characterisation of the surface properties of the ice matrices. A further point that remains largely unresolved is the distribution of impurities between the different domains of the condensed phase inside the snowpack, i.e. in the bulk solid, in liquid at the surface or trapped in confined pockets within or between grains, or at the surface. While surface-sensitive laboratory techniques may in the future help to resolve this point for equilibrium conditions, additional uncertainty for the environmental snowpack may be caused by the highly dynamic nature of the snowpack due to the fast metamorphism occurring under certain environmental conditions. Due to these gaps in knowledge the first snow chemistry models have attempted to reproduce certain processes like the long-term incorporation of volatile compounds in snow and firn or the release of reactive species from the snowpack. Although so far none of the models offers a coupled approach of physical and chemical processes or a detailed representation of the different compartments, they have successfully been used to reproduce some field experiments. A fully coupled snow chemistry and physics model remains to be developed.
In this Letter, we study a localized stellar overdensity in the constellation of Ursa Major, first identified in Sloan Digital Sky Survey (SDSS) data and subsequently followed up with Subaru imaging. ...Its color-magnitude diagram (CMD) shows a well-defined subgiant branch, main sequence, and turnoff, from which we estimate a distance of 630 kpc and a projected size of 6250 x 125 pc super(2). The CMD suggests a composite population with some range in metallicity and/or age. Based on its extent and stellar population, we argue that this is a previously unknown satellite galaxy of the Milky Way, hereby named Ursa Major II (UMa II) after its constellation. Using SDSS data, we find an absolute magnitude of Mv6-3.8, which would make it the faintest known satellite galaxy. UMall's isophotes are irregular and distorted with evidence for multiple concentrations; this suggests that the satellite is in the process of disruption.
Silver(I) oxide (Ag2O) micro- and nanoparticles were electrochemically synthesized by anodizing a sacrificial silver wire in a basic aqueous sulfate solution. Ag2O particles were released from the ...silver electrode surface during synthesis producing a visible sol “stream”. The composition of these particles was established using selected area electron diffraction, X-ray diffraction, and X-ray photoelectron spectroscopy. The shape of Ag2O crystallites could be adjusted using the potential of the silver wire generator electrode. The generation of a dispersed Ag2O sol and the observed shape selectivity are both explained by a two-step mechanism involving the anodic dissolution of silver metal, Ag0 → Ag+(aq) + 1e-, followed by the precipitation of Ag2O particles, 2Ag+ + 2OH- → Ag2O(s) + H2O. Within 100 mV of the voltage threshold for particle growth, cubic particles with a depression in each face (“hopper crystals”) were produced. The application of more positive voltages resulted in the generation of 8-fold symmetric “flower”-shaped particles formed as a consequence of fast growth in the 〈111〉 crystallographic direction. The diameter of flower particles was adjustable from 250 nm to 1.8 μm using the growth duration at constant potential.
We have investigated the reaction of ultrathin Al2O3/NiAl(110) films with water vapor from ultrahigh vacuum to relative humidities (RH) up to 10%, over the temperature range from −5 to 65 °C, using ...ambient pressure X-ray photoelectron spectroscopy (APXPS) and scanning tunneling microscopy (STM). The APXPS experiments show a sharp onset of oxide/hydroxide film growth at ∼0.01% RH, coupled with an increase of the oxygen-to-aluminum ratio, most likely because of the adsorption of OH and H2O species at the oxide surface and their reaction with Al from the NiAl substrate, which leads to a thickening of the oxide film at RH > 0.01%. The STM measurements support the results from APXPS and reveal a change in surface morphology when the Al2O3/NiAl(110) sample was exposed to relative humidities greater than 0.01%.
We present five new satellites of the Milky Way discovered in Sloan Digital Sky Survey (SDSS) imaging data, four of which were followed up with either the Subaru or the Isaac Newton Telescopes. They ...include four probable new dwarf galaxies--one each in the constellations of Coma Berenices, Canes Venatici, Leo, and Hercules--together with one unusually extended globular cluster, Segue 1. We provide distances, absolute magnitudes, half-light radii, and color-magnitude diagrams for all five satellites. The morphological features of the color-magnitude diagrams are generally well described by the ridge line of the old, metal-poor globular cluster M92. In the past two years, a total of 10 new Milky Way satellites with effective surface brightness k sub(u) 28 mag arcsec super(-2) have been discovered in SDSS data. They are less luminous, more irregular, and apparently more metal-poor than the previously known nine Milky Way dwarf spheroidals. The relationship between these objects and other populations is discussed. We note that there is a paucity of objects with half-light radii between 640 and 6100 pc. We conjecture that this may represent the division between star clusters and dwarf galaxies.
The interaction of water with the Fe3O4(001) surface was investigated in a combined ambient pressure X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) study. The uptake of ...molecular water and hydroxyl species on the (001) surface of a natural magnetite sample at near-ambient conditions was quantified using O 1s spectra taken in the p(H2O) range from 10–9 to 2 Torr. At low p(H2O) (≤10–4–10–5 Torr) and room temperature, we found that water does not adsorb dissociatively on the surface, except on defect sites. In contrast, progressive dissociation into surface hydroxyl species was observed between 10–4 and 10–2 Torr p(H2O). The onset of hydroxylation coincides with the increasing presence of molecular water species on the surface, which demonstrates the key role played by cooperative interactions between adsorbed water molecules, leading to dissociation and surface hydroxylation. The measured O 1s chemical shifts of hydroxyl and molecular water species from both isotherm and isobar data are on average ∼1.2 eV and ∼3.3 eV, respectively, relative to lattice oxygen. The chemical shift of the hydroxyl species on magnetite(001) agrees with previously reported values for hydroxyl species on iron oxyhydroxides such as goethite (α-FeOOH). DFT calculations including an on-site Coulomb repulsion parameter (generalized gradient approximation (GGA) + U approach) predict O 1s surface core-level shifts (SCLS) at the clean (21/2×21/2)R45° reconstructed Fe3O4(001) surface of up to ∼−1 eV depending on the specific bonding configuration of the surface O atoms. Hydroxyl groups formed by the dissociation of isolated water molecules at O vacancies have an SCLS value of ∼1.2 eV. With increasing coverage there is a transition toward partial dissociation on the (001) surface. The calculated SCLS for hydroxyl and adsorbed water are 1.2–1.9 and 2.6–3.0 eV, respectively, and compare very well with our experimental results. Final-state effects obtained within the Slater–Janak approach thus have the dominant contribution. In addition, the modest reduction of the work function (∼0.5 eV) predicted by DFT calculations for the mixed adsorption of dissociated and intact water molecules agrees well with work function changes measured experimentally. Finally, the similarity between isotherm and isobar data and the DFT calculations for the C-free Fe3O4(001) surface indicate that surface hydroxylation is indeed substrate induced and not catalyzed by the presence of adventitious carbonaceous species. Both theory and experiment show the importance of cooperative effects between adjacent water molecules in the dissociation reaction.
Molybdenum disulfide nano- and microribbons were synthesized by a two-step, electrochemical/chemical synthetic method in which MoO2 “precursor” nanowires were first electrodeposited size-selectively ...on a highly oriented pyrolytic graphite (HOPG) surface. These precursor wires were then converted to MoS2 by exposure to H2S at 800−900 °C. The MoS2 ribbons prepared using this method had the 2H crystal structure of bulk MoS2, were organized into parallel arrays of hundreds of ribbons, and were up to a millimeter in length. The electronic properties of these nanoribbon arrays were probed after transferring them onto an insulator surface.
We have studied the reaction of water vapor with a polycrystalline vanadium surface using ambient pressure X-ray photoelectron spectroscopy (AP-XPS) which allows the investigation of the chemical ...composition of the vanadium/water vapor interface at p(H2O) in the Torr range. Water dissociation on the vanadium surface was studied under isobaric conditions at p(H2O) ranging from 0.01 to 0.50Torr and temperatures from 625K to 260K, i.e. up to a relative humidity (RH) of ~15%. Water vapor exposure leads to oxidation and hydroxylation of the vanadium foil already at a pressure of 1×10−6Torr at 300K (RH~4×10−6%). The vanadium oxide layer on the surface has a stoichiometry of V2O3. Initial adsorption of molecular water on the surface is observed at RH>0.001%. Above a RH of 0.5% the amount of adsorbed water increases markedly. Experiments at increasing temperatures show that the water adsorption process is reversible. Depth profile measurements show a thickness for the vanadium oxide layer of 3–5 mono layers (ML) and for vanadium hydroxide of 1–1.5 ML over the whole RH range in the isobar experiments. The thickness of the adsorbed water layer was found to be in the sub-ML range for the investigated RH's.
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•We studied the interaction of water vapor with a polycrystalline vanadium foil.•The V-foil is easily hydroxylated and oxidized by dissociatively adsorbed water.•For the investigated relative humidities water coverage in the sub ML range was found.•Desorption experiments show that the molecular water adsorption process is reversible.
Using a combination of X-ray photoemission and near-edge X-ray absorption spectroscopy (NEXAFS) as well as density-functional theory (DFT), we have investigated the adsorption of acetone on ice in ...the temperature range from 218 to 245 K. The adsorption enthalpy determined from experiment (45 kJ mol(-1)) agrees well with the adsorption energy predicted by theory (41 to 44 kJ mol(-1)). Oxygen K-edge NEXAFS spectra indicate that the presence of acetone at the ice surface does not induce the formation of a pre-melted layer at temperatures up to 243 K. DFT calculations show that the energetically most favored adsorption geometry for acetone on ice is with the molecular plane almost parallel to the surface.
X-ray photoelectron spectroscopy (XPS) is used to monitor the heterogeneous reaction of hydroxyl radicals (OH) and ozone with thin films (∼5 Å) of coronene. Detailed elemental and functional group ...analysis of the XPS spectra reveals that there is a competition between the addition of oxygenated functional groups (functionalization) and the loss of material (volatilization) to the gas phase. Measurements of the film thickness and elemental composition indicate that carbon loss is as important as the formation of new oxygenated functional groups in controlling how the oxygen-to-carbon ratio (O/C) of the coronene film evolves during the surface reaction. When the O/C ratio of the film is small (∼0.1) the addition of functional groups dominates changes in film thickness, while for more oxygenated films (O/C > 0.3) carbon loss is an increasingly important reaction pathway. Decomposition of the film occurs via the loss of both carbon and oxygen atoms when the O/C ratio of the film exceeds 0.5. These results imply that chemically reduced hydrocarbons, such as primary organic aerosol, age in the atmosphere by forming new oxygenated functional groups, in contrast to oxygenated secondary organic aerosol, which decompose by a heterogeneous loss of carbon and/or oxygen.
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