J.B.A. Mitchell

TEMPERATURE DEPENDENCE OF THE RATE CONSTANT FOR THE CL-+CH3BR REACTION DOWN TO 23 K

Rate constants measured for the Cl−+CH3Br→CH3Cl+Br− reaction increase by over two orders of magnitude when the temperature is reduced from 300 to 23 K. Calculations done with the rotating bond approximation provide a simple explanation of this unusually strong negative temperature dependence of the rate constant.

Nanoparticle plasma ejected directly from solid copper by localized microwaves

A plasma column ejected directly from solid copper by localized microwaves is studied. The effect stems from an induced hotspot that melts and emits ionized copper vapors as a confined fire column. Nanoparticles of ∼20–120 nm size were revealed in the ejected column by in situ small-angle x-ray scattering. Optical spectroscopy confirmed the dominance of copper particles in the plasma column originating directly from the copper substrate. Nano- and macroparticles of copper were verified also by ex situ scanning electron microscopy. The direct conversion of solid metals to nanoparticles is demonstrated and various applications are proposed.

X-ray synchrotron radiation probing of an ethylene diffusion flame

Experimental results are presented from an experiment in which ionization created by a high energy X-ray synchrotron radiation beam intersecting an ethylene diffusion flame is measured. The ionization signal is believed to arise from X-ray absorption by aggregated soot particles. Comparisons are made between these ionization profiles and those that are a result of natural flame ionization. Visible light absorption measurements were also performed to determine the soot volume fraction in the beam and the results compared with the X-ray induced signals.

X-ray Photoelectron Spectroscopy of Isolated Nanoparticles.

X-ray photoelectron spectroscopy (XPS) is a very efficient and still progressing surface analysis technique. However, when applied to nano-objects, this technique faces drawbacks due to interactions with the substrate and sample charging effects. We present a new experimental approach to XPS based on coupling soft X-ray synchrotron radiation with an in-vacuum beam of free nanoparticles, focused by an aerodynamic lens system. The structure of the Si/SiO2 interface was probed without any substrate interaction or charging effects for silicon nanocrystals previously oxidized in ambient air. Complete characterization of the surface was obtained. The Si 2p core level spectrum reveals a nonabrupt interface.

ON THE VOLATILE ENRICHMENTS AND HEAVY ELEMENT CONTENT IN HD189733b

Favored theories of giant planet formation center around two main paradigms, namely the core accretion model and the gravitational instability model. These two formation scenarios support the hypothesis that the giant planet metallicities should be higher or equal to that of the parent star. Meanwhile, spectra of the transiting hot Jupiter HD189733b suggest that carbon and oxygen abundances range from depleted to enriched with respect to the star. Here, using a model describing the formation sequence and composition of planetesimals in the protoplanetary disk, we determine the range of volatile abundances in the envelope of HD189733b that is consistent with the 20-80 M ? of heavy elements estimated to be present in the planets envelope. We then compare the inferred carbon and oxygen abundances to those retrieved from spectroscopy, and we find a range of supersolar values that directly fit both spectra and internal structure models. In some cases, we find that the apparent contradiction between the subsolar elemental abundances and the mass of heavy elements predicted in HD189733b by internal structure models can be explained by the presence of large amounts of carbon molecules in the form of polycyclic aromatic hydrocarbons and soots in the upper layers of the envelope, as suggested by recent photochemical models. A diagnostic test that would confirm the presence of these compounds in the envelope is the detection of acetylene. Several alternative hypotheses that could also explain the subsolar metallicity of HD189733b are formulated: the possibility of differential settling in its envelope, the presence of a larger core that did not erode with time, a mass of heavy elements lower than the one predicted by interior models, a heavy element budget resulting from the accretion of volatile-poor planetesimals in specific circumstances, or the combination of all these mechanisms.

The dissociative recombination of hydrocarbon ions. III. Methyl-substituted benzene ring compounds

The recombination of electrons with cyclic ions produced via ion – molecule reactions between atomic precursor ions and methyl-substituted benzene ring compounds (toluene , ortho-, and para-xylene and mesitylene) has been studied at 300 K using a flowing afterglow Langmuir probe-mass spectrometer apparatus. Differing amounts of energy can be deposited into the daughter ions depending upon which atomic precursor is used. It has been found that same-mass daughter ions formed from different precursors displayed different recombination rate coefficients indicating that different isomeric forms were reacting. In particular , the benzene ring of the toluene cation expands to a seven-membered ring following isomerization to the cycloheptatriene form. H atom abstraction allows two different isomeric daughter ions to be formed that do not interconvert and that display different recombination rates. A similar behavior was observed for the xylenes and for mesitylene. All recombination rates lie in the range from 10-7 to 10 -6 cm3 s-1 and display no apparent relation with size nor with the aromaticity of the ions .

Observations of Ball-Lightning-Like Plasmoids Ejected from Silicon by Localized Microwaves

This paper presents experimental characterization of plasmoids (fireballs) obtained by directing localized microwave power (<1 kW at 2.45 GHz) onto a silicon-based substrate in a microwave cavity. The plasmoid emerges up from the hotspot created in the solid substrate into the air within the microwave cavity. The experimental diagnostics employed for the fireball characterization in this study include measurements of microwave scattering, optical spectroscopy, small-angle X-ray scattering (SAXS), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Various characteristics of these plasmoids as dusty plasma are drawn by a theoretical analysis of the experimental observations. Aggregations of dust particles within the plasmoid are detected at nanometer and micrometer scales by both in-situ SAXS and ex-situ SEM measurements. The resemblance of these plasmoids to the natural ball-lightning (BL) phenomenon is discussed with regard to silicon nano-particle clustering and formation of slowly-oxidized silicon micro-spheres within the BL. Potential applications and practical derivatives of this study (e.g., direct conversion of solids to powders, material identification by breakdown spectroscopy (MIBS), thermite ignition, and combustion) are discussed.

First in-flight synchrotron X-ray absorption and photoemission study of carbon soot nanoparticles

Many studies have been conducted on the environmental impacts of combustion generated aerosols. Due to their complex composition and morphology, their chemical reactivity is not well understood and new developments of analysis methods are needed. We report the first demonstration of in-flight X-ray based characterizations of freshly emitted soot particles, which is of paramount importance for understanding the role of one of the main anthropogenic particulate contributors to global climate change. Soot particles, produced by a burner for several air-to-fuel ratios, were injected through an aerodynamic lens, focusing them to a region where they interacted with synchrotron radiation. X-ray photoelectron spectroscopy and carbon K-edge near-edge X-ray absorption spectroscopy were performed and compared to those obtained for supported samples. A good agreement is found between these samples, although slight oxidation is observed for supported samples. Our experiments demonstrate that NEXAFS characterization of supported samples provides relevant information on soot composition, with limited effects of contamination or ageing under ambient storage conditions. The highly surface sensitive XPS experiments of airborne soot indicate that the oxidation is different at the surface as compared to the bulk probed by NEXAFS. We also report changes in soot’s work function obtained at different combustion conditions.

A new apparatus for the measurement of X-ray absorption by flame generated particles

An apparatus is described that allows a flame, burning under vacuum, to be irradiated by an X-ray beam from a synchrotron radiation source. Initial results concerning the ionization of flame generated soot particles are described.

Characteristics of Arcs Between Porous Carbon Electrodes

Arcs between carbon electrodes present some specific differences compared with metallic arcs. The arc voltage is higher, but does not attain a stable value displaying large fluctuations. Indeed, the arcs are produced by the direct sublimation of the electrodes, without passing through a molten phase. The arc production is also facilitated by both circuit breaking and electric field breakdown. In this paper, arcing has been examined under various conditions (voltage, current, and opening or fixed interelectrode gap) and particular attention has been given to the ability of the system to sustain the arc due to reignition. Small-angle X-ray scattering has been used to examine the formation of particles within carbon arcs and again the results are different from what is found with metallic electrodes, the particles being larger with rougher surfaces. The ultimate aim of this paper was to gain new knowledge concerning these arcs and their consequences for electrical safety (arc faults in wires).