Scientific Publications 2006
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2006. "Comparison of Frozen-Density Embedding and Discrete Reaction FieldSolvent Models for Molecular Properties." Physical Chemistry Chemical Physics. PCCP 8(20):2349-2359. doi:10.1039/b601997h Abstract We investigate the performance of two discrete solvent models in connection with density functional theory (DFT) for the calculation of molecular properties. In our comparison we include the discrete reaction field (DRF) model, a combined quantum mechanics and molecular mechanics (QM/MM) model using a polarizable force field, and the frozen-density embedding (FDE) scheme. We employ these solvent models for ground state properties (dipole and quadrupole moments) and response properties (electronic excitation energies and frequencydependent polarizabilities) of a water molecule in the liquid phase. It is found that both solvent models agree for ground state properties, while there are significant differences in the description of response properties. The origin of these differences is analyzed in detail and it is found that they are mainly caused by a different description of the ground state molecular orbitals of the solute. In addition, for the calculation of the polarizabilities, the inclusion of the response of the solvent to the polarization of the solute becomes important. This effect is included in the DRF model, but is missing in the FDE scheme. A way of including it in FDE calculations of the polarizabilities using finite field calculations is demonstrated.
2006. "Resonance Raman Scattering of Rhodamine 6G as Calculated Using Time-DependentDensity Functional Theory." Journal of Physical Chemistry A 110:5973-5977. doi:10.1021/jp0610867 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. In this work, we present the first calculation of the resonance Raman scattering (RRS) spectrum of rhodamine 6G (R6G) which is a prototype molecule in surface-enhanced Raman scattering (SERS). The calculation is done using a recently developed time-dependent density functional theory (TDDFT) method, which uses a short-time approximation to evaluate the Raman scattering cross section. The normal Raman spectrum calculated with this method is in good agreement with experimental results. The calculated RRS spectrum shows qualitative agreement with SERS results at a wavelength that corresponds to excitation of the S1 state, but there are significant differences with the measured RRS spectrum at wavelengths that correspond to excitation of the vibronic sideband of S1. Although the agreement with the experiments is not perfect, the results provide insight into the RRS spectrum of R6G at wavelengths close to the absorption maximum where experiments are hindered due to strong fluorescence. The calculated resonance enhancements are found to be on the order of 105. This indicates that a surface enhancement factor of about 1010 would be required in SERS in order to achieve single-molecule detection of R6G.
2006. "Characterization and Oxidation of Chromium(III) by Sodium Hypochlorite in Alkaline Solutions." Inorganica Chimica Acta 359(10):3237-3242. Abstract Chromium exists in nuclear waste sludges and is a problematic element in the vitrification process of high-level nuclear wastes. It is therefore necessary to treat the waste sludges to remove chromium prior to vitrification, by caustic leaching or oxidation of Cr(III) to Cr(VI). The objective of this study is to investigate the effect of oligomerization of Cr(III) on its oxidation by hypochlorite in alkaline solutions. Monomeric, dimeric and trimeric Cr(III) species in solution were separated by ion exchange. The kinetics of the oxidation of the separated species by hypochlorite in alkaline solutions was studied by UV/Vis absorption spectroscopy, and compared with the oxidation by hydrogen peroxide previously studied. Results indicate that hypochlorite can oxidize Cr(III) to Cr(VI) in alkaline solutions, but the rate of oxidation by hypochlorite is slower than that by hydrogen peroxide at the same alkalinity and concentrations of oxidants. The rate of oxidation of Cr(III) by both oxidants decreases as the concentration of sodium hydroxide is increased, but the oxidation by hypochlorite seems less affected by the degree of oligomerization of Cr(III) than that by peroxide. Compared with the oxidation by hydrogen peroxide where the major reaction pathway has an inverse order with respect to CNaOH, the oxidation by hypochlorite has a significant reaction pathway independent of [OH−].
2006. "Effect of irradiation temperature on dynamic recovery in gallium nitride." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 242(1-2):431-433. Abstract A single crystal gallium nitride film on sapphire was successively irradiated to a fluence of 4.5 Au3+/nm2 in different areas at varied temperatures ranging from 150 to 800 K. The temperature dependence of disorder on both the Ga and N sublattices has been investigated using a 3.736 MeV He+ backscattering analysis along the <0001>- and <10-11>-axial channeling directions. Significant dynamic recovery of disorder occurs over the applied temperature range. There is a higher degree of disorder on the N sublattice observed along the <10-11> axis. Some of the defects produced during the irradiation in GaN are effectively shielded by the <0001> axis.
2006. "Hydrogen behavior in Mg+-implanted graphite." Journal of Materials Research 21(4):811-815. doi:10.1557/JMR.2006.0121 Abstract A graphite wafer has been implanted with Mg+ to produce a uniform Mg concentration. Subsequent H+ implantation covered both the Mg+-implanted and unimplanted regions. Ion-beam analysis shows a higher H retention in graphite embedded with Mg than in regions without Mg. A small amount of H diffuses out of the H+ implanted graphite during thermal annealing at temperatures up to 300°C. However, significant H release from the region implanted with both Mg+ and H+ ions occurs at 150°C; further release is also observed at 300°C. The results suggest that there are efficient H trapping centers and fast pathways for H diffusion in the Mg+ implanted graphite, which may prove highly desirable for reversible H storage.implanted graphite, which may prove highly desirable for reversible H storage.
2006. "Irradiation-induced nanostructures in cadmium niobate pyrochlores." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 250(1-2):188-191. Abstract This paper reports the formation processes of crystalline Cd nanostructures (nanoparticles and nanowires) on ion-cut surfaces of cadmium niobate pyrochlores (Cd2Nb2O7). Irradiation with 3 MeV He+ ions has been performed at low temperatures (≤ 295 K) to induce material decomposition and aggregation of host atoms. The irradiation also leads to surface exfoliation due to rupture of gas (He and O2) filled blisters. Nanoparticles and nanowires are observed on the ion-cut surfaces at low and higher doses, respectively. The nanostructures are examined and characterized using a suite of experimental tools. Both the nanoparticles and nanowires are found to be single crystals that primarily consist of metallic Cd.
2006. "Temperature response of 13C atoms in amorphized 6H-SiC." Applied Physics Letters 89(26):art. no.:261902, (3 pages). doi:10.1063/1.2422892 Abstract Implantation of 13C2+ ions was employed to produce a concentration profile in 6H-SiC at 140 K. In-situ study of 13C implanted species was performed using the resonant reaction of 13C(p,gamma)14N at Ep=1.748 MeV. Significant 13C diffusion in the amorphized SiC does not occur up to 1130 K. The presence of Au implants (1.9 at.%) does not affect the 13C behavior. High-energy H+ irradiation also does not promote the 13C diffusion. The results suggest that C atoms are readily trapped locally in the SiC structure during disordering, which is important to understanding the amorphization processes in SiC.
2006. "Direct evidence of N aggregation and diffusion in Au+ irradiated GaN." Applied Physics Letters 89(2):021903-1-3. Abstract A surface amorphized layer and a buried disordered structure were created in gallium nitride (GaN) irradiated using 1.0 MeV Au+ ions to fluences of 25 and 70 Au+/nm2 at room temperature. Bubbles of N2 gas within both the amorphized and disordered GaN are formed. A gradient profile with a lower N concentration in the amorphized region is observed, which provides direct evidence of N loss by diffusion in the Au+ irradiated GaN. These results are important to understanding the amorphization processes in GaN and may have significant implications for the design and fabrication of GaN-based devices.
2006. "Aerosol Composition and Source Apportionment in the Mexico City Metropolitan Area with PIXE/PESA/STIM and Multivariate Analysis." Atmospheric Chemistry and Physics 6(12):4591-4600. Abstract Aerosols play an important role in the atmosphere but are poorly characterized, particularly in urban areas like the Mexico City Metropolitan Area (MCMA). The chemical composition of urban particles must be known to assess their effects on the environment, and specific particulate emissions sources should be identified to establish ef- 5 fective pollution control standards. For these reasons, samples of particulate matter _2.5 µm (PM2.5) were collected during the MCMA-2003 Field Campaign for elemental and multivariate analyses. Proton-Induced X-ray Emission (PIXE), Proton-Elastic Scattering Analysis (PESA) and Scanning Transmission Ion Microscopy (STIM) techniques were done to determine concentrations of 19 elements from Na to Pb, hydrogen, and 10 total mass, respectively. The most abundant elements from PIXE analysis were S, Si, K, Fe, Ca, and Al, while the major emissions sources associated with these elements were industry, wind-blown soil, and biomass burning. Wind trajectories suggest that metals associated with industrial emissions came from northern areas of the city whereas soil aerosols came from the southwest and increased in concentration during 15 dry conditions. Elemental markers for fuel oil combustion V and Ni correlated with a large SO2 plume to suggest an anthropogenic, rather than volcanic, emissions source. By subtracting major components of soil and sulfates determined by PIXE analysis from STIM total mass measurements, we estimate that approximately 50% of PM2.5 consisted of carbonaceous material.
2006. "A Disseminator for Rapid, Selectable and Quantitative Delivery of Low-and Semi-Volatile Liquid Species to the Vapor Phase." Review of Scientific Instruments 77(09):094103. doi:10.1063/1.2349298 Abstract Nelson and co-workers introduced a quantitative method for disseminating liquid samples to the vapor phase using a lead screw to depress the plunger of a syringe whose tip was mounted into the flow of a carrier gas. In order to measure quantitative vapor-phase infrared spectra, we have modified a commercial device to improve the accuracy and precision for quantitative vapor delivery. Design changes have focused on disseminating reactive or low-volatility liquids by heating only the syringe tip and dispensed liquid. Performance features include quantitative vapor-phase generation with >3 orders of magnitude concentration range, including for low volatility species, with most equilibration times <40 s. The method has been vetted by comparing the gas-phase IR data versus IR spectra taken using both gravimetric (NIST) and vapor (PNNL) generation techniques. Quantitative vapor spectra of low volatility samples are reported.
2006. "The Quantitative Infrared and NIR Spectrum of CH2I2 Vapor: Vibrational Assignments and Potential for Atmospheric Monitoring." Atmospheric Chemistry and Physics 6(9):2581-2591. Abstract Diiodomethane (CH2I2) photolysis in the presence of ozone is a suggested precursor to new particle aerosol formation, particularly in coastal areas. As part of the PNNL database of gas-phase infrared spectra, the quantitative absorption spectrum of CH2I2 has been acquired at 0.1 cm-1 resolution. Two strong b2 symmetry A-type bands at 584 and 1114 cm-1 are observed, but are not resolved at 760 Torr and appear as B-type. In contrast, the b1 symmetry C-type bands near 5953, 4426 and 3073 cm-1 are resolved with rotational structure, including Q-branches with widths ≤ 1 cm-1. The quantitative infrared and near-infrared vapor-phase spectra (600 - 10,000 cm-1) are reported for the first time and discussed in terms of ambient monitoring. FT-Raman spectra and ab initio calculations are used to complete vibrational assignments.
2006. "Carrier Dynamics in a-Fe2O3 (0001) Thin Films and Single Crystals Probed by Femtosecond Transient Absorption and Reflectivity." Journal of Applied Physics 99(5):Article: 053521 (6 pages). Abstract Femtosecond transient reflectivity and absorption is used to measure the carrier lifetimes in α-Fe2O3 thin films and hematite single crystals. The results from the thin films show that initially excited hot electrons relax to the bandedge within 300 femtoseconds and then recombine with holes or trap within 5 pioseconds. The trapped electrons have a lifetime of hundreds of picoseconds. The trapped electrons have a lifetime of hundreds of picoseconds. Transient reflectivity measurements from hematite (α-Fe2O3) single crystals show similar but slightly faster dynamics. In hematite, the transient reflectivity displays oscillations due to the formation of longitudinal acoustic phonons generated following absorption of the ultrashort excitation pulse.
2006. "Probing Electron Transfer Dynamics at MgO Surfaces by Mg-Atom Desorption." Journal of Physical Chemistry B 110(37):18093-18096. doi:10.1021/jp064092b Abstract Desorption of a weakly bound adsorbate from a porous solid was studied for the case of N2 on amorphous solid water (ASW). Porous ASW films of different thickness were grown on Pt(111) by ballistic deposition. N2 adsorption and desorption kinetics were monitored mass-spectrometrically. Temperature programmed desorption spectra show that with the increasing film thickness, the N2 desorption peak systematically shifts to higher temperatures. The results are explained and quantitatively reproduced by a simple model, which assumes that the N2 transport within the film is faster than the depletion rate to vacuum. The local coverage at the pore mouth determines the desorption rate. For thick ASW films (>1 μm), the assumption of the fast equilibration within the film is shown to be no longer valid due to diffusion limitations. The mechanisms of the adsorbate transport are discussed.
