Scientific Publications 2007
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2007. "An In Situ Study of the Martensitic Transformation in Shape Memory Alloys Using Photoemission Electron Microscopy." Journal of Nuclear Materials 361(2-3):306-312. doi:10.1016/j.jnucmat.2006.12.008 Abstract Thermally-induced martensitic phase transformations in polycrystalline CuZnAl and thin-film NiTiCu shape memory alloys were probed using photoemission electron microscopy (PEEM). Ultra-violet photoelectron spectroscopy shows a reversible change in the apparent work function during transformation, presumably due to the contrasting surface electronic structures of the martensite and austenite phases. In situ PEEM images provide information on the spatial distribution of these phases and the evolution of the surface microstructure during transformation. PEEM offers considerable potential for improving our understanding of martensitic transformations in shape memory alloys in real time.
2007. "Study of Martensitic Phase transformation in a NiTiCu Thin Film Shape Memory Alloy Using Photoelectron Emission Microscopy." Advanced Functional Materials 17(1):161-167. doi:10.1002/adfm.200600611 Abstract The thermally-induced martensitic phase transformation in a polycrystalline NiTiCu thin film shape memory alloy was probed by photoelectron emission microscopy (PEEM). In situ PEEM images reveal distinct changes in microstructure and photoemission intensity at the phase transition temperatures. In particular, images of the low temperature, martensite phase are brighter than that of the high temperature, austenite phase, due to the relatively lower work function of the martensite. Ultra-violet photoelectron spectroscopy shows that the effective work function changes by about 0.16 eV during thermal cycling. In situ PEEM images also show that the network of trenches observed on the room temperature film disappear suddenly during heating and reappear suddenly during subsequent cooling. These trenches are also characterized by atomic force microscopy at selected temperatures. We describe implications of these observations with respect to the spatial distribution of phases during thermal cycling in this thin film shape memory alloy.
2007. "Evaluation of the Influence of Amino Acid Composition on the Propensity for Collision-Induced Dissociation of Model Peptides Using Molecular Dynamics Simulations." Journal of the American Society for Mass Spectrometry 18(9):1625-1637. doi:10.1016/j.jasms.2007.06.005 Abstract The dynamical behavior of model peptides was evaluated with respect to their ability to form internal proton donor-acceptor pairs using molecular dynamics simulations. The proton donor-acceptor pairs are postulated to be prerequisites for peptide bond cleavage resulting in formation of b and y ions during low energy collision-induced dissociation in tandem mass spectrometry (MS/MS). The simulations for the polyalanine pentamer Ala5H+ were compared to experimental data from collision energy-resolved surface induced dissociation (SID) studies. The results of the simulation are insightful into the events that likely lead up to the fragmentation of peptides. 9-mer polyalanine-based model peptides were used to examine the dynamical effect of each of the 20 common amino acids on the probability to form donor-acceptor pairs at labile peptide bonds. A continuous range of probabilities was observed as a function of the substituted amino acid. However, the location of the peptide bond involved in the donor-acceptor pair plays a critical role in the dynamical behavior. This influence of position on the probability of forming a donor-acceptor pair would be hard to predict from statistical analyses on experimental spectra of aggregate, diverse peptides. In addition, the inclusion of basic side chains in the model peptides alters the probability of forming donor-acceptor pairs across the entire backbone. In this case there are still more ionizing protons than basic residues, but the side chains of the basic amino acids form stable hydrogen bond networks with the peptide carbonyl oxygens and thus act to prevent free access of “mobile protons” to labile peptide bonds. It is clear from the work that the identification of peptides from low-energy CID using automated computational methods should consider the location of the fragmenting bond as well as the amino acid composition.
2007. "Catalyst Screening and Kinetic Studies Using Microchannel Reactors." Catalysis Today 125(1-2):29-33. Abstract A multi-parallel microchannel reactor system is described, as related to catalyst screening and discovery for heat-intensive heterogeneous catalytic reactions. Example systems are detailed, in which the rapid heat transfer of the screening device is utilized to maintain isothermal operation in multiple channels for catalyst screening as well as kinetic investigations. The advantages of the system and pertinent results are discussed, specifically for Fischer-Tropsch synthesis, methanol oxidation to formaldehyde, and methanol steam reforming.
2007. "A Red Cy3-Based Biarsenical Fluorescent Probe Targeted to a Complementary Binding Peptide." Journal of the American Chemical Society 129:8672-73. doi:10.1021/ja070003c Abstract We have synthesized a red biarsenical fluorescent probe, AsCy3, with good photostability, low pH sensitivity, high absorbance and good quantum yield. It is directed specifically to a small tetracysteine peptide binding motif Cy3TAG (CysCysLysAlaGluAlaAlaCysCys) in the presence of other tetracysteine tags. This new probe provides a FRET partner to biarsenical dye FlAsH, making this discovery an important step toward a whole toolkit of colored probes directed to different small peptide motifs.
2007. "Three-Dimensional Mapping of Ozone-Induced Injury in the Nasal Airways of Monkeys Using Magnetic Resonance Imaging and Morphometric Techniques." Toxicologic Pathology 35(1):27-40. Abstract ABSTRACT Age-related changes in gross and microscopic structure of the nasal cavity can alter local tissue susceptibility as well as the dose of inhaled toxicant delivered to susceptible sites. This article describes a novel method for the use of magnetic resonance imaging, 3-dimensional airway modeling, and morphometric techniques to characterize the distribution and magnitude of ozone-induced nasal injury in infant monkeys. Using this method, we are able to generate age-specific, 3-dimensional, epithelial maps of the nasal airways of infant Rhesus macaques. The principal nasal lesions observed in this primate model of ozone-induced nasal toxicology were neutrophilic rhinitis, along with necrosis and exfoliation of the epithelium lining the anterior maxilloturbinate. These lesions, induced by acute or cyclic (episodic) exposures, were examined by light microscopy, quantified by morphometric techniques, and mapped on 3-dimensional models of the nasal airways. Here, we describe the histopathologic, imaging, and computational biology methods developed to efficiently characterize, localize, quantify, and map these nasal lesions. By combining these techniques, the location and severity of the nasal epithelial injury were correlated with epithelial type, nasal airway geometry, and local biochemical and molecular changes on an individual animal basis. These correlations are critical for accurate predictive modeling of exposure-dose-response relationships in the nasal airways, and subsequent extrapolation of nasal findings in animals to humans for developing risk assessment.
2007. "Structural Analysis of Conserved Oligomeric Golgi Complex Subunit 2." Journal of Biological Chemistry 282(32):23418-23426. doi:10.1074/jbc.M703716200 Abstract Add this sentance to the front of abstracts that you cut and paste into ERICA "The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. The conserved oligomeric Golgi (COG) complex is strongly implicated in retrograde vesicular trafficking within the Golgi apparatus. Although its mechanism of action is poorly understood, it has been proposed to function by mediating the initial physical contact between transport vesicles and their membrane targets. An analogous role in tethering vesicles has been suggested for at least six additional large multisubunit complexes, including the exocyst, a complex essential for trafficking to the plasma membrane. Here we report the solution structure of a large portion of yeast Cog2p, one of eight subunits composing the COG complex. The structure reveals a six-helix bundle with few conserved surface features but a general resemblance to recently determined crystal structures of four different exocyst subunits. This finding provides the first structural evidence that COG, like the exocyst and potentially other tethering complexes, is constructed from helical bundles. These structures may represent platforms for interaction with other trafficking proteins including SNAREs (soluble N-ethylmaleimide factor attachment protein receptors) and Rabs.
2007. "Advances in the Surface Science of TiO2 – A Global Perspective." Hyomen Kagaku. Journal of the Surface Science Society of Japan 28(10):561-572. Abstract TiO2 rutile single-crystal surfaces have served has useful prototypical, well-defined specimens for fundamental investigations of oxide surface science for many years. As a result of both experimental and theoretical efforts, we have gained considerable insight into the structural, electronic, thermochemical and photochemical properties of pristine as well as defective surfaces. In this brief review, I summarize some of the recent advances that have been made in the laboratories of participants of the International Workshop of Oxide Surfaces (IWOX) series, principally on TiO2(110).
2007. "Growth, Electronic and Magnetic Properties of Doped ZnO Epitaxial and Nanocrystalline Films." Applied Physics A, Materials Science and Processing 88(1):1-5. doi:10.1007/s00339-007-3948-2 Abstract We have used oxygen plasma assisted metal organic chemical vapor deposition along with wet chemical synthesis and spin coating to prepare CoxZn1-xO and MnxZn1-xO epitaxial and nanoparticle films. Co(II) and Mn(II) substitute for Zn(II) in the wurtzite lattice in materials synthesized by both methods. Room temperature ferromagnetism in epitaxial Co:ZnO films can be reversibly activated by diffusing in Zn, which occupies interstitial sites and makes the material n-type. O-capped Co:ZnO nanoparticles, which are paramagnetic as grown, become ferromagnetic upon being spin coated in air at elevated temperature. Likewise, spin-coated N-capped Mn:ZnO nanoparticle films also exhibit room temperature ferromagnetism. However, the inverse systems, N-capped Co:ZnO and O-capped Mn:ZnO, are entirely paramagnetic when spin coated into films in the same way. Analysis of optical absorption spectra reveal that the resonances Co(I) ↔ Co(II) + e-CB and Mn(III) ↔ Mn(II) + h+VB are energetically favorable, consistent with strong hybridization of Co (Mn) with the conduction (valence) band of ZnO. In contrast, the resonances Mn(I) ↔ Mn(II) + e-CB and Co(III) ↔ Co(II) + h+CB are not energetically favorable. These results strongly suggest that the observed ferromagnetism in Co:ZnO (Mn:ZnO) is mediated by electrons (holes).
2007. "Properties of Structurally Excellent N-doped TiO2 Rutile." Chemical Physics 339(1-3):27-35. doi:10.1016/j.chemphys.2007.04.024 Abstract We have used plasma-assisted molecular beam epitaxy to synthesize structurally near-perfect crystalline films of TiO2-xNx rutile for the first time. These materials allow the properties of TiO2-xNx to be elucidated without the interfering effects of oxygen vacancy defects. In the absence of such defects, the extent of N incorporation in the lattice is limited to 2 ± 1 at. % of the anions. Substitutional N (NO) exhibits a -3 formal charge due to charge transfer from shallow-donor interstitial Ti(III), which forms during epitaxial growth. Hybridization between NO and adjacent lattice Ti ions occurs, resulting in new states off the top of the rutile valence band and an apparent band gap reduction of ~ 0.5 eV. It is not yet known if these new states result in mobile electron-hole pair creation upon irradiation, but experiments are planned to answer this important question.
2007. "Quantitative analysis of human immunodeficiency virus type 1-infected CD4(+) cell proteome: Dysregulated cell cycle progression and nuclear transport coincide with robust virus production." Journal of Virology 81(14):7571-7583. Abstract Relatively little is known at the functional genomic level about the global host response to HIV-1 infection. Microarray analyses by several laboratories, including our own, have revealed that human immunodeficiency virus type 1 infection causes significant changes in host mRNA abundance and regulation of several cellular biological pathways. However, it remains unclear what consequences these changes bring about at the protein level. Here we report the expression levels of ~3,200 proteins assessed in the CD4+ CEMx174 cell line after infection with HIV-1 LAI, using liquid chromatography-mass spectrometry coupled with stable isotope labeling and the accurate mass and time (AMT) tag approach. Further, we found that 687 (21%) proteins changed in abundance at the peak of virus production at 36h post-infection. Pathway analysis revealed that the differential expression of proteins were concentrated in select biological pathways, exemplified by ubiquitin conjugating enzymes in the ubiquitination, carrier proteins in nucleo-cytoplasmic transport, cyclin-dependent kinase in cell cycle progression, and pyruvate dehydrogenase of the citrate cycle. Moreover, we observed changes in the abundance of proteins with known interactions with HIV-1 viral proteins. Our proteomic analysis captured changes in the host protein milieu at the time of robust virus production, accompanied by a moderate accumulation of G1/G0-phase cells. We will discuss the contributions of these changes to virus production in the infected cells.
2007. "In-situ ion scattering and x-ray photoelectron spectroscopy studies of stability and nanoscale oxidation of single crystal (100) InAs." Applied Physics Letters 90(20):, doi:10.1063/1.2740200 Abstract Preparation of clean and smooth surfaces of InAs(100) by hydrogen molecular cleaning (HMC) along with in-situ studies of the nanoscale oxidation of pristine surfaces is studied. Removal of native oxides has been verified in-depth by in-situ nuclear reaction analysis(NRA) using the 16O(d,p)17O reaction and XPS. Further, ion channeling studies have been performed to verify atomically smooth surfaces after post-cleaning. Stability and kinetic boundaries of the cleaned InAs(100) surfaces against oxidation have also been experimentally derived and studied by NRA. These results are important not only to prepare clean surfaces of InAs, but also to understand fundamentals of oxide/III-V semiconductor interfaces.
2007. "On the stability and oxidation of single crystal (100) InAs surfaces." ECS Transactions 11(4):409-419. doi:10.1149/1.2779577 Abstract We report on a method to prepare clean and smooth surfaces of InAs (100) along with in-situ high-resolution studies of the nanoscale oxidation of the pristine surface. A hydrogen molecular cleaning (HMC) technique has been developed that results in complete removal of native oxide. This has been verified in-depth by in-situ nuclear reaction analysis (NRA) using the 16O(d,p)17O reaction and X-ray photoelectron spectroscopy. Further, ion channeling studies have been performed to verify atomically smooth surfaces after post cleaning. We derive kinetic boundaries for oxide formation on cleaned InAs surfaces using NRA measurements.
2007. "Functional Scaffolds of Bicontinuous, Thermoresponsive L3-phase Silica/Hydroxyapatite Nanocomposites." Journal of Materials Chemistry 17(3):238-242. doi:10.1039/b609409k Abstract The silicified L3-poly(N-sopropylacrylamide)/hydroxyapatite nanocomposites by the integration of thermosensitive polymer and heterogenious continuity in 3-D interconnected porous structure have been prepared for the highly controlled drug release without the burst release at the initial time during step-wised temperature changes, and demonstrated the in-vitro cytotoxicity test for biocompatibility.
2007. "Synthesis and characterization of lithium-doped tin dioxide nanocrystalline powders." Materials Chemistry and Physics 102(2-3):176-180. doi:10.1016/j.matchemphys.2006.11.022 Abstract In order to develop a better understanding of p-doping in SnO2 nanoparticles, a gel-sol method of synthesis is employed to systematically examine the influence of reaction and growth conditions such as pH, dopant concentration, and calcinations temperature. The extent of Li doping in nanoparticles is characterized using nuclear reaction analysis (NRA) and the nanostructure with high-resolution electron transmission microscopy (HR-TEM) and X-ray diffraction (XRD) techniques. The NRA reports that the lithium incorporation only takes place at alkaline pH (> 8). The XRD and TEM results indicate that the crystallite size of exhibits a maximum at a pH of 8 increasing (decreasing) with calcinations temperature (dopant concentration), while preserving the casserite crystal structure (tetragonal rutile) under these reaction conditions. A preliminary analysis suggests a nanoscopic phase separation of lithium-poor crystalline core and lithium rich amorphous shell may occur with increased dopant concentration.
2007. "Effects of Dimerization of Serratia marcescens Endonucleaseon Water Dynamics." Biopolymers 85(3):241-252. doi:10.1002/bip.20641 Abstract The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. The dynamics and structure of Serratia marcescens endonuclease and its neighboring solvent are investigated by molecular dynamics (MD). Comparisons are made with structural and biochemical experiments. The dimer form is physiologic and functions more processively than the monomer. We previously found a channel formed by connected clusters of waters from the active site to the dimer interface. Here, we show that dimerization clearly changes correlations in the water structure and dynamics in the active site not seen in the monomer. Our results indicate that water at the active sites of the dimer is less affected compared with bulk solvent than in the monomer where it has much slower characteristic relaxation times. Given that water is a required participant in the reaction, this gives a clear advantage to dimerization in the absence of an apparent ability to use both active sites simultaneously.
2007. "A Sol-Gel-Modified Poly(methyl methacrylate) Electrophoresis Microchip with a Hydrophilic Channel Wall." Chemistry - a European Journal 13(22):6461-6467. doi:10.1002/chem.200700242 Abstract A sol-gel method was employed to fabricate a poly(methyl methacrylate) (PMMA) electrophoresis microchip that contains a hydrophilic channel wall. To fabricate such a device, tetraethoxysilane (TEOS) was injected into the PMMA channel and was allowed to diffuse into the surface layer for 24 h. After removing the excess TEOS, the channel was filled with an acidic solution for 3 h. Subsequently, the channel was flushed with water and was pretreated in an oven to obtain a sol-gel-modified PMMA microchip. The water contact angle for the sol-gel-modified PMMA was 27.4° compared with 66.3° for the pure PMMA. In addition, the electro-osmotic flow increased from 2.13×10-4 cm2 V-1 s-1 for the native-PMMA channel to 4.86×10-4 cm2 V-1 s-1 for the modified one. The analytical performance of the sol-gel-modified PMMA microchip was demonstrated for the electrophoretic separation of several purines, coupled with amperometric detection. The separation efficiency of uric acid increased to 74 882.3 m-1 compared with 14 730.5 m-1 for native-PMMA microchips. The result of this simple modification is a significant improvement in the performance of PMMA for microchip electrophoresis and microfluidic applications.
2007. "Fabrication of Poly(methyl Methacrylate) microfluidic chips by redox-initiated polymerization." Electrophoresis 28(16):2897-2903. Abstract In this report, a method based on the redox-initiated polymerization of methyl methacrylate (MMA) has been developed for the rapid fabrication of PMMA microfluidic chips.The new fabrication approach obviates the need for special equipment and significantly simplifies the process of fabricating microdevices. The attractive performance of the novel PMMA microchips has been demonstrated in connection with contactless conductivity detection for the separation and detection of ionic species.
2007. "Effect of γ-Al₂O₃ substrate on NO₂ interaction with supportedBaO clusters." Surface Science 601(12):L65-L68. doi:10.1016/j.susc.2007.04.112 Abstract The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. BaO oxide is the main storage component of the NO sub x storage and reduction catalysts. Herein, the interactions between the NO₂ molecule and the unsupported as well as γ-Al₂O₃ supported BaO clusters have been studied using the first principle density functional theory calculation. Our results indicated that there is a strong synergetic effect involving both the BaO clusters and the surface of the γ-Al₂O₃ substrate toward NO₂ adsorption. The interfacial region between the monodispersed BaO cluster and the substrate surface that allows NO₂ to bond with the cluster and the surface simultaneously was shown to be optimal for NO₂ adsorption.
2007. "N incorporation and electronic structure in N-doped TiO2(110) rutile." Surface Science 601(7):1754-1762. doi:10.1016/j.susc.2007.01.051 Abstract Epitaxial TiO2-xNx film growth under anion-rich conditions is characterized by nearly balanced incorporation rates for substitutional N (NO) and interstitial Ti (Tii). Tii donors fully compensate and stabilize N3-, but preclude the formation of p-type material. Hybridization occurs between Tii(IV) and NO3-, but the value of x is limited to ~0.02 under these conditions. Tii(IV)-NO3- states occur above the valence band maximum of pure TiO2, riving rise to enhanced optical absorption in the visible up to ~2.5 eV. Much higher NO and Tii concentrations result from using cation-rich conditions.
2007. "Oxidation of Ethanol to Acetaldehyde over Na-promoted vanadium oxide catalysts." Applied Catalysis. A, General 332(2):263-272. doi:10.1016/j.apcata.2007.08.024 Abstract Sodium-promoted vanadium oxide catalysts supported on MCM-41 and TiO2 (anatase) were investigated for the partial oxidation of ethanol to acetaldehyde. The catalysts were prepared by incipient wetness impregnation with a vanadium oxide content of 6 wt. %. The experimental characterization was performed by X-ray diffraction (XRD), N2 adsorption, temperature programmed reduction (TPR), and diffuse reflectance UV-Vis. Temperature programmed oxidation (TPO) was also used to identify carbon deposits on the spent catalysts. The presence of sodium plays a strong role in the dispersion and reducibility of the vanadium species as detected by TPR analysis and optical absorption spectroscopy. While sodium addition increases the dispersion of the VOx species, its presence also decreases their reducibility. Additionally, TPO of the spent catalysts revealed that an increase in the Na loading decreases the carbon deposition during reaction. In the case of the catalysts supported on MCM-41, these modifications were mirrored by a change in the activity and selectivity to acetaldehyde. Additionally, on the VOx/TiO2 catalysts the catalytic activity decreased with increasing sodium content in the catalyst. A model in which sodium affects dispersion, reducibility and also acidity of the supported-vanadia species is proposed to explain all these observations.
2007. "The Dewetting Transition and The Hydrophobic Effect." Journal of the American Chemical Society 129(15):4847-4852. doi:10.1021/ja069242a Abstract The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. A molecular-level description of the behavior of water in hydrophobic spaces is presented in terms of the coupled effects of solute size and atomic solute-solvent interactions. For model solutes with surface areas near those of protein contacts, we identify three different regions of solute-water interaction to be associated with three distinctly different structural characteristics of water in the intersolute region: dry, oscillating, and wet. A first orderlike phase transition is confirmed from the wet to dry state bridged by a narrow region with liquid-vapor oscillations in the intersolute region as the strength of the solute-water attractive dispersion interaction decreases. We demonstrate that the recent idea that cavitation in the intersolute region of nanoscopic solutes is preceded by the formation of a vapor layer around an individual solute is not the general case. The appearance of density waves pulled up around and outside of a nanoscopic plate occurs at lower interaction strengths than are required to obtain a wet state between such plates. We further show that chemically reasonable estimates of the interaction strength lead to a microscopically wet state and a hydrophobic interaction characterized by traps and barriers to association and not by vacuum induced collapse.
2007. "Molecular Dynamics Modeling of the Thermal Conductivity of Irradiated SiC as a Function of Cascade Overlap." Journal of Applied Physics 101(2):023527. doi:10.1063/1.2431397 Abstract SiC thermal conductivity is known to decrease under irradiation. To understand this effect, we study the variation of the thermal conductivity of cubic SiC with defect accumulation induced by displacement cascades. We use an empirical potential of the Tersoff type in the framework of non-equilibrium molecular dynamics. The conductivity of SiC is found to decrease with dose, in very good quantitative agreement with low temperature irradiation experiments. The results are analyzed in view of the amorphization states that are created by the cascade accumulation simulations. The calculated conductivity values at lower doses are close to the smallest measured values after high temperature irradiation, indicating that the decrease of the conductivity observed at lower doses is related to the creation of point defects. A subsequent decrease takes place upon further cascade accumulation. It is characteristic of the amorphization of the material and is experimentally observed for low temperature irradiation only.
2007. "Evolution of the Electronic Properties of Sn-n Clusters (n=4-45) and the Semiconductor-to-Metal Transition." Journal of Chemical Physics 126(6):064505. doi:10.1063/1.2435347 Abstract The electronic structure of Snn − clusters _n=4–45_ was examined using photoelectron spectroscopy at photon energies of 6.424 eV _193 nm_ and 4.661 eV _266 nm_ to probe the semiconductor-to-metal transition. Well resolved photoelectron spectra were obtained for small Snn −clusters _n_25_, whereas more congested spectra were observed with increasing cluster size. A distinct energy gap was observed in the photoelectron spectra of Snn − clusters with n_41, suggesting the semiconductor nature of small neutral tin clusters. For Snn − clusters with n_42, the photoelectron spectra became continuous and no well-defined energy gap was observed, indicating the onset of metallic behavior for the large Snn clusters. The photoelectron spectra thus revealed a distinct semiconductor-to-metal transition for Snn clusters at n=42.
2007. "Endohedral Stannaspherenes M@Sn12-: A Rich Class of Stable Molecular Cage Clusters." Angewandte Chemie International Edition 46(5):742-745. doi:10.1002/anie.200603226 Abstract Experimental and theoretical evidence is reported that the icosahedral stannaspherene Sn12 2- cage can trap an atom from any of the transition-metal series or the f-elements, giving rise to a myriad of stable endohedral clusters. A selected set of M@Sn12 - cluster has been created using laser vaporization and characterized by photoelectron spectroscopy. It is shown that these clusters maintain perfect or pseudo-icosahedral symmetry with the central metal atom inducing very little distortion to the stannaspherene cage. The doped atom in M@Sn12 - keeps its quasiatomic nature with large magnetic moments. The endohedral stannaspherenes might thus be viewed as “superatoms”, yielding potentially a rich class of new building blocks for clusterassembled materials with tunable magnetic, electronic, and chemical properties.
2007. "A Photoelectron Spectroscopic and Computational Study of Sodium Auride Clusters, NanAun-(n=1-3)." Journal of Physical Chemistry A 111(31):7555-7561. Abstract Negatively charged sodium auride clusters, NanAun- (n ) 1-3), have been investigated experimentally using photoelectron spectroscopy and ab initio calculations. Well-resolved electronic transitions were observed in the photoelectron spectra of NanAun - (n ) 1-3) at several photon energies. Very large band gaps were observed in the photoelectron spectra of the anion clusters, indicating that the corresponding neutral clusters are stable closed-shell species. Calculations show that the global minimum of Na2Au2 - is a quasi-linear species with Cs symmetry. A planar isomer of D2h symmetry is found to be 0.137 eV higher in energy. The two lowest energy isomers of Na3Au3- consist of three-dimensional structures of Cs symmetry. The global minimum of Na3Au3 - has a bent-flake structure lying 0.077 eV below a more compact structure. The global minima of the sodium auride clusters are confirmed by the good agreement between the calculated electron detachment energies of the anions and the measured photoelectron spectra. The global minima of neutral Na2Au2 and Na3Au3 are found to possess higher symmetries with a planar four-membered ring (D2h) and a six-membered ring (D3h) structure, respectively. The chemical bonding in the sodium auride clusters is found to be highly ionic with Au acting as the electron acceptor.
2007. "Research Activities at U.S. Government Agencies in Subsurface Reactive Transport Modeling." Vadose Zone Journal 6(4):805-822. doi:10.2136/vzj2006.0091 Abstract The fate of contaminants in the environment is controlled by both chemical reactions and transport phenomena in the subsurface. Our ability to understand the significance of these processes over time requires an accurate conceptual model that incorporates the various mechanisms of coupled chemical and physical processes. Adsorption, desorption, ion exchange, precipitation, dissolution, growth, solid solution, redox, microbial activity, and other processes are often incorporated into reactive transport models for the prediction of contaminant fate and transport. U.S. federal agencies use such models to evaluate contaminant transport and provide guidance to decision makers and regulators for treatment issues. We provide summaries of selected research projects and programs to demonstrate the level of activity in various applications and to present examples of recent advances in subsurface reactive transport modeling.
