12 research outputs found
Emergence of Bulk CsCl Structure in (CsCl)nCs+ Cluster Ions
The emergence of CsCl bulk structure in (CsCl)nCs+ cluster ions is
investigated using a mixed quantum-mechanical/semiempirical theoretical
approach. We find that rhombic dodecahedral fragments (with bulk CsCl symmetry)
are more stable than rock-salt fragments after the completion of the fifth
rhombic dodecahedral atomic shell. From this size (n=184) on, a new set of
magic numbers should appear in the experimental mass spectra. We also propose
another experimental test for this transition, which explicitely involves the
electronic structure of the cluster. Finally, we perform more detailed
calculations in the size range n=31--33, where recent experimental
investigations have found indications of the presence of rhombic dodecahedral
(CsCl)32Cs+ isomers in the cluster beams.Comment: LaTeX file. 6 pages and 4 pictures. Accepted for publication in Phys.
Rev.
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Effects of gasâwall partitioning in Teflon tubing and instrumentation on time-resolved measurements of gas-phase organic compounds
Recent studies have demonstrated that organic compounds can partition from the gas phase to the walls in Teflon environmental chambers and that the process can be modeled as absorptive partitioning. Here these studies were extended to investigate gasâwall partitioning of organic compounds in Teflon tubing and inside a proton-transfer-reaction mass spectrometer (PTR-MS) used to monitor compound concentrations. Rapid partitioning of C8âC14 2-ketones and C11âC16 1-alkenes was observed for compounds with saturation concentrations (câ) in the range of 3âŻĂâŻ104 to 1âŻĂâŻ107âŻÂ”gâŻmâ3, causing delays in instrument response to step-function changes in the concentration of compounds being measured. These delays vary proportionally with tubing length and diameter and inversely with flow rate and câ. The gasâwall partitioning process that occurs in tubing is similar to what occurs in a gas chromatography column, and the measured delay times (analogous to retention times) were accurately described using a linear chromatography model where the walls were treated as an equivalent absorbing mass that is consistent with values determined for Teflon environmental chambers. The effect of PTR-MS surfaces on delay times was also quantified and incorporated into the model. The model predicts delays of an hour or more for semivolatile compounds measured under commonly employed conditions. These results and the model can enable better quantitative design of sampling systems, in particular when fast response is needed, such as for rapid transients, aircraft, or eddy covariance measurements. They may also allow estimation of câ values for unidentified organic compounds detected by mass spectrometry and could be employed to introduce differences in time series of compounds for use with factor analysis methods. Best practices are suggested for sampling organic compounds through Teflon tubing
Crossover between ionic/covalent and pure ionic bonding in magnesium oxyde clusters
An empirical potential with fluctuating charges is proposed for modelling
(MgO)_n clusters in both the molecular (small n) and bulk (n->infty) regimes.
Vectorial polarization forces are explicitely taken into account in the
self-consistent determination of the charges. Our model predicts cuboid cluster
structures, in agreement with previous experimental and theoretical results.
The effective charge transferred between magnesium and oxygen smoothly
increases from 1 to 2, with an estimated crossover size above 300 MgO
molecules
Reducing the environmental impact of surgery on a global scale: systematic review and co-prioritization with healthcare workers in 132 countries
Background
Healthcare cannot achieve net-zero carbon without addressing operating theatres. The aim of this study was to prioritize feasible interventions to reduce the environmental impact of operating theatres.
Methods
This study adopted a four-phase Delphi consensus co-prioritization methodology. In phase 1, a systematic review of published interventions and global consultation of perioperative healthcare professionals were used to longlist interventions. In phase 2, iterative thematic analysis consolidated comparable interventions into a shortlist. In phase 3, the shortlist was co-prioritized based on patient and clinician views on acceptability, feasibility, and safety. In phase 4, ranked lists of interventions were presented by their relevance to high-income countries and lowâmiddle-income countries.
Results
In phase 1, 43 interventions were identified, which had low uptake in practice according to 3042 professionals globally. In phase 2, a shortlist of 15 intervention domains was generated. In phase 3, interventions were deemed acceptable for more than 90 per cent of patients except for reducing general anaesthesia (84 per cent) and re-sterilization of âsingle-useâ consumables (86 per cent). In phase 4, the top three shortlisted interventions for high-income countries were: introducing recycling; reducing use of anaesthetic gases; and appropriate clinical waste processing. In phase 4, the top three shortlisted interventions for lowâmiddle-income countries were: introducing reusable surgical devices; reducing use of consumables; and reducing the use of general anaesthesia.
Conclusion
This is a step toward environmentally sustainable operating environments with actionable interventions applicable to both highâ and lowâmiddleâincome countries
Atmospheric phosphorus deposition at a montane site: Size distribution, effects of wildfire, and ecological implications
International audienceThe dry deposition of atmospheric particulate matter can be a significant source of phosphorus (P) to oligotrophic aquatic ecosystems, including high-elevation lakes. In this study, measurements of the mass concentration and size distribution of aerosol particles and associated particulate P are reported for the southern Sierra Nevada, California, for the period July-October, 2008. Coarse and fine particle samples were collected with Stacked Filter Units and analyzed for Total P (TP) and inorganic P (IP) using a digestion-extraction procedure, with organic P (OP) calculated by difference. Particle size-resolved mass and TP distributions were determined concurrently using a MOUDI cascade impactor. Aerosol mass concentrations were significantly elevated at the study site, primarily due to transport from offsite and emissions from local and regional wildfires. Atmospheric TP concentrations ranged from 11 to 75 ng mâ3 (mean = 37 ± 16 ng mâ3), and were typically dominated by IP. Phosphorus was concentrated in the coarse (>1 ÎŒm diameter) particle fraction and was particularly enriched in the 1.0-3.2 ÎŒm size range, which accounted for 30-60% of the atmospheric TP load. Wildfire emissions varied widely in P content, and may be related to fire intensity. The estimated dry depositional flux of TP for each daily sampling period ranged between 7 and 118 ÎŒg mâ2 dâ1, with a mean value of 40 ± 27 ÎŒg mâ2 dâ1. Relative rates of dry deposition of N and P in the Sierra Nevada are consistent with increasing incidence of N limitation of phytoplankton growth and previously observed long-term eutrophication of lakes