276 research outputs found
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Inner-shell electron spectroscopy and chemical properties of atoms and small molecules
The program has been concerned with gas-phase carbon 1s photoelectron spectroscopy of a number of molecules of potential chemical interest. The primary goals have been to determine carbon 1s ionization energies with a view of relating these to other chemical properties such as electronegativity, acidity, basicity, and reactivity, in order to provide a better understanding of these fundamental properties. The role of electron-donating (methyl) and electron-withdrawing (fluoro) substituents on the carbon 1s ionization energies of substituted benzenes has been studied., and these results have been related to measurements of the reactivities of the same molecule as well as to their affinities for protons (basicity). Opportunities for investigation in unplanned areas have arisen, and the program has been modified to take advantage of these. One has been the realization that, under certain circumstances, inner-shell ionization energies may depend on the molecular conformation. Several examples of this phenomenon have been investigated and it has been shown that this technique provides a tool for the measurement of the energy differences between different conformers of the same substance. The other has been the demonstration that photoelectron recoil can lead to the excitation of vibrational modes that are forbidden in the normal view of photoemission and to rotational heating of the molecule that increases with the energy of the exciting radiation
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Small Molecule Photoelectron Spectroscopy : Recoil Effects, Stoichiometric Surprises, and Double-Core-Hole Ionization
This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Elsevier and can be found at: https://doi.org/10.1016/j.elspec.2012.12.004Three features of small-molecule photoelectron spectroscopy are considered (1) The atom from which a photoelectron
is emitted must have a recoil momentum equal to that of the emitted electron. This is shared among the various modes
of motion of the ion, leading to rotational and vibrational excitation. Furthermore, any initial velocity of the atom
(due to either translational, rotational, or vibrational motion) will lead to Doppler broadening. These effects are
observable and can, in general, be accounted for by simple models. In some cases, however, the simple models fail
and a deeper insight is necessary. (2) Inner-shell photoionization is essentially an atomic process, and it is expected
that the intensity for emission of a photoelectron from the core of an atom in a molecule will be independent of its
chemical environment. Recent measurements on the carbon 1s photoelectron spectra of three chloroethanes show that
this is not the case. At energies not far above the ionization threshold there are strong oscillations of the intensity
ratio (C[subscript Cl]/C[subscript H]) with increasing photon energy. These are similar to those seen in EXAFS and can be accounted for
by considering backscattering of the photoelectrons from the chlorine atoms. Moreover, even at high energies the
cross section for ionization has been found to depend on the chemical environment of the atom. These results have
important consequences for the use of inner-shell electron spectroscopy for quantitative analysis. (3) Single-core-hole
ionization energies have long been used as a tool for investigating chemical phenomena. Double-core-hole ionization
energies provide additional chemical information. By combining the single-hole and double-hole ionization energies
it is possible to determine the effects of the initial-state charge distribution and final-state charge rearrangement on
the chemical shifts and on other chemical properties. Until recently double-core-hole ionization energies have not
been experimentally accessible for first-row elements. New experimental techniques have, however, made it possible
to measure these not only for single sites in a molecule, but also for two different sites in the same molecule. The
chemical information that can be obtained from such measurements is discussed
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Angular Distribution of Fragments from Fission Induced by Heavy Ions in Gold and Bismuth
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Coriolis interaction and the division of energy between vibrational and rotational excitation induced by photoelectron recoil
The effect of the Coriolis interaction upon the sharing of energy between rotational and vibrational excitation
during an electronic transition is considered with particular emphasis on recoil-induced excitation during
photoionization. If there is a large change in equilibrium bond length upon ionization, then Coriolis coupling
leads to a significant transfer of energy between rotational and vibrational excitation. Experimental results for
valence ionization of Nâ and CO and for carbon 1s ionization of CO show evidence of this effect
Calibration of oxygen 1s ionization energies. Accurate energies for CO2, H2O, CO, and O2
Access to accurate reference data is a prerequisite in order to translate chemical shifts to an absolute scale for inner-shell ionization energies. Calibration standards for oxygen 1s (O 1s) ionization energies are less well established than, for instance, for carbon 1s. To improve upon this situation, adiabatic and vertical O 1s ionization energies for gaseous carbon dioxide (CO2) are critically reviewed and used to establish the most accurate values currently available: 541.085(17) and 541.253(17) eV, respectively. Combining these with new precise measurements of shifts in O 1s ionization energies for H2O, CO, and O2 allows us to establish equally accurate absolute ionization energies for these molecules as for CO2. The resulting adiabatic and vertical energies are 539.728(17) and 539.827(17) eV for H2O, 542.439(17) and 542.495(17) eV for CO, 543.285(17) and 543.294(17) eV for O2 (4ÎŁ final state), and 544.338(17) and 544.423(17) eV for O2 (2ÎŁ final state). It is proposed that O 1s in CO2 be adopted as a standard of higher precedence, and that H2O, CO, and O2 be used also. The O 1s ionization energies in these molecules occur in the range 540â543âŻeV. These proposed standards should provide optimal internal calibration for a wide range of oxygen-containing compounds.publishedVersio
Tubulin Polymerization Promoting Protein Affects the Circadian Timing System in C57Bl/6 Mice
The circadian timing system (CTS) is a complex set of cyclic cellular mechanisms which serve to synchronize discrete cell groups across multiple organ systems to adapt the bodyâs physiology to a (roughly) 24-hour clock. Many genes and hormones have been shown to be strongly associated with the CTS, some of which include the genes 'Bmal1, Period1, Period2, Cryptochrome1', and 'Cryptochrome2', and the hormone melatonin. Previous data suggest that microtubule dynamics play an important role in melatonin function as it relates to the CTS in vitro, though this relationship has never been explored in vivo. The purpose of this study was to determine whether disruption of microtubule regulation in C57Bl/6 mice results in measurable changes to the CTS. To study the potential effects of microtubule dynamics on the CTS in vivo, we utilized a mouse model of microtubule instability, knocked out for the tubulin polymerization promoting protein gene ('Tppp' -/-), comparing them to their wild type (WT) littermates in three categories: locomotor activity (in light/dark and dark/dark photoperiods), serial clock gene expression, and serial serum melatonin concentration. These comparisons showed differences in all three categories, including significant differences in locomotor characteristics under dark/dark conditions. Our findings support and extend previous reports that microtubule dynamics are a modulator of circadian rhythm regulation likely through a mechanism involving melatonin induced phase shifting
Noble Gases Identify the Mechanisms of Fugitive Gas Contamination in Drinking-Water Wells Overlying the Marcellus and Barnett Shales
Horizontal drilling and hydraulic fracturing have enhanced energy production but raised concerns about drinking-water contamination and other environmental impacts. Identifying the sources and mechanisms of contamination can help improve the environmental and economic sustainability of shale-gas extraction. We analyzed 113 and 20 samples from drinking-water wells overlying the Marcellus and Barnett Shales, respectively, examining hydrocarbon abundance and isotopic compositions (e.g., C2H6/CH4, δ13C-CH4) and providing, to our knowledge, the first comprehensive analyses of noble gases and their isotopes (e.g., 4He, 20Ne, 36Ar) in groundwater near shale-gas wells. We addressed two questions. (i) Are elevated levels of hydrocarbon gases in drinking-water aquifers near gas wells natural or anthropogenic? (ii) If fugitive gas contamination exists, what mechanisms cause it? Against a backdrop of naturally occurring salt- and gas-rich groundwater, we identified eight discrete clusters of fugitive gas contamination, seven in Pennsylvania and one in Texas that showed increased contamination through time. Where fugitive gas contamination occurred, the relative proportions of thermogenic hydrocarbon gas (e.g., CH4, 4He) were significantly higher (P \u3c 0.01) and the proportions of atmospheric gases (air-saturated water; e.g., N2, 36Ar) were significantly lower (P \u3c 0.01) relative to background groundwater. Noble gas isotope and hydrocarbon data link four contamination clusters to gas leakage from intermediate-depth strata through failures of annulus cement, three to target production gases that seem to implicate faulty production casings, and one to an underground gas well failure. Noble gas data appear to rule out gas contamination by upward migration from depth through overlying geological strata triggered by horizontal drilling or hydraulic fracturing
Surface oxides, carbides, and impurities on RF superconducting Nb and Nb3Sn: A comprehensive analysis
Surface structures on radio-frequency (RF) superconductors are crucially
important in determining their interaction with the RF field. Here we
investigate the surface compositions, structural profiles, and valence
distributions of oxides, carbides, and impurities on niobium (Nb) and
niobium-tin (Nb3Sn) in situ under different processing conditions. We establish
the underlying mechanisms of vacuum baking and nitrogen processing in Nb and
demonstrate that carbide formation induced during high-temperature baking,
regardless of gas environment, determines subsequent oxide formation upon air
exposure or low-temperature baking, leading to modifications of the electron
population profile. Our findings support the combined contribution of surface
oxides and second-phase formation to the outcome of ultra-high vacuum baking
(oxygen processing) and nitrogen processing. Also, we observe that
vapor-diffused Nb3Sn contains thick metastable oxides, while electrochemically
synthesized Nb3Sn only has a thin oxide layer. Our findings reveal fundamental
mechanisms of baking and processing Nb and Nb3Sn surface structures for
high-performance superconducting RF and quantum application
The Grizzly, November 12, 2015
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Electronic Properties of Chlorine, Methyl, and Chloromethyl as Substituents to the Ethylene Group-Viewed from the Core of Carbon
âSubstituent effectsâ is an important and useful concept in organic chemistry. Although there are many approaches to parametrizing the electronic and steric effects of substituents, the physical basis for the parameters is often unclear. The purpose of the present work is to explore the properties of chemical shifts in carbon 1s energies as a well-defined basis for characterizing substituents to an ethylene CâC moiety. To this end, high-resolution carbon 1s photoelectron spectra of six chloro-substituted ethenes and seven chloro-substituted propenes have been measured in the gas phase. Site-specific adiabatic ionization energies have been determined from the spectra using theoretical ab initio calculations to predict the vibrational structures. For two molecules, 3-chloropropene and 2,3-dichloropropene, the spectral analyses give quantitative results for the conformer populations. The observed shifts have been analyzed in terms of initial-state (potential) and relaxation effects, and charge relaxation has also been analyzed by means of natural resonance theory. On the basis of core-level spectroscopy and models, chlorine, methyl, and chloromethyl have been characterized in terms of their effect on the carbon to which they are attached (Îą site) as well as the neighboring sp² carbon (β site). The derived spectroscopic substituent parameters are characterized by both inductive (electronegativity) effects and the ability of each substituent to engage in electron delocalization via the Ď system. Moreover, the adopted approach is extended to include substituentâsubstituent interaction parameters
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