Showing papers in "The Journal of Physical Chemistry in 2004"

Full Valence Band Photoemission from Liquid Water Using EUV Synchrotron Radiation

Abstract: The valence band photoelectron spectra of liquid water (H2O and D2O) are studied in the photon energy range from hν = 60 to 120 eV. The experiments use a 6 μm diameter liquid-jet free vacuum surface at the MBI undulator beamline of the synchrotron radiation facility BESSY. Photoelectron emission from all four valence molecular orbitals (MOs) is observed. In comparison to those of the gas phase, the peaks are significantly broadened and shifted to lower binding energies by about 1.5 eV. This is attributed primarily to the electronic polarization of the solvent molecules around an ionized water molecule. Energy shifts, peak broadening, and relative peak intensities for the four MOs differ because of their specific participation in the hydrogen bonding in liquid water. Relative photoionization cross sections for MOs were measured for hν = 60, 80, and 100 eV. The main difference for liquid water, as compared to the gas phase, is the relative intensity decrease of the 1b2 and 3a1 orbitals, reflecting changes o...

Ionic Conduction and Ion Diffusion in Binary Room-Temperature Ionic Liquids Composed of [emim][BF4] and LiBF4

Abstract: Binary room-temperature ionic liquid (RTIL) samples including a lithium salt were prepared by mixing 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) with LiBF4. The ionic conductivity, viscosity, thermal properties, and ion self-diffusion coefficients in [emim][BF4] and the binary [Li][emim][BF4] at six concentrations of LiBF4 ranging from 0.25 to 1.50 M were measured at various temperatures. The self-diffusion coefficients of the individual components, [emim], BF4, and Li, were measured by using 1H, 19F, and 7Li pulsed gradient spin−echo NMR, respectively. Since the Walden product holds similar to typical solution electrolytes, the ion conduction mechanism is interpreted using a flux basis electrolyte theory. The ions form associated structures and diffuse under the influence of the counterions in the binary IL systems. An attempt to correlate the ion diffusion with the ionic conduction was made in the framework of the Nernst−Einstein relationship. The Li net transference number and the appar...

Nucleation and Growth of Single-Walled Carbon Nanotubes: A Molecular Dynamics Study

Abstract: Molecular dynamics simulations based on an empirical potential energy surface were used to study iron catalyzed nucleation and growth of single-walled carbon nanotubes (SWNTs). The simulations show...

Photoemission from Aqueous Alkali-Metal−Iodide Salt Solutions Using EUV Synchrotron Radiation

Abstract: The valence band photoemission of aqueous alkali-metal halide solutions is studied for photon energies from 90 to 110 eV. A 6 μm diameter liquid microjet provides a free vacuum surface, allowing water molecules to evaporate without collisions, and hence enables the direct detection of photoelectrons originating from the liquid. The experiments were performed at the MBI undulator beamline of the synchrotron radiation facility BESSY. Here, we focus on the determination of electron binding energies of solvated anions and cations. The effect of different countercations (Li+, Na+, K+, and Cs+), and salt concentrations is systematically investigated. Electron binding energies of the solvated ions are found to differ considerably from those in the gas phase; contrary to intuition, the energies do not depend on the salt concentration. Measured binding energies can be surprisingly well explained within a simple dielectric cavity model. For a NaI aqueous solution, negative surface excess is inferred from the evolut...

CH··F Hydrogen Bonds. Dimers of Fluoromethanes

Abstract: The nature of the CH···F H-bond is studied by examining the potential energy surfaces for dimers of the fluoromethanes, including all homo- and heterodimers involving CH4, CFH3, CF2H2, CF3H, and CF4. Several of the surfaces encompass two separate minima. Any pair capable of forming a cyclic structure does so, this geometry being more stable than any other minima that might occur on the surface. Such cyclic dimers are bound by 2−3 kcal/mol. Noncyclic geometries are only very weakly bound. In all cases, the C−H bonds that participate directly in the H-bonds undergo a contraction and associated blue shift in their stretching frequencies, whereas the C−F bonds manifest an elongation. The most strongly bound of all dimers studied pairs CH3F with CHF3, for which the C−H bond contracts by 0.0023 A, and its stretching frequency increases by 39 cm-1. The most stable cyclic dimer of fluoroform, containing one single and one bifurcated hydrogen bond, is reported for the first time and thus resolves the longstanding ...

Electrochemical Micromachining of p-Type Silicon

Abstract: Electrochemical micromachining (ECM) of p-type Si substrates is accomplished in HF-based solutions by applying nanosecond potential pulses between the substrate and a tungsten tool electrode. With sufficiently high potential pulses, the silicon potential locally reaches the electropolishing regime and microstructures may be machined. ECM precision is investigated as a function of pulse height, pulse duration, solution composition, and silicon doping level. Results show that micrometer precision may be obtained with highly doped substrates and that experimental data can be explained within a simple model, taking the charging time of the interface capacitance into account. In highly doped p-Si, well-defined microstructures can be realized without application of a mask on the surface. In addition, the isotropy of the process allows fabrication of structures not constrained by the crystal direction. In the case of low-doped material, ECM is only possible for very short pulses (<3 ns).

Substitution Patterns in Mono BN-Fullerenes: Cn (n = 20, 24, 28, 32, 36 and 40)

Abstract: Semiempirical MNDO and Density Functional Theory (DFT) are applied to investigate the structure and properties of C n - 2 BN fullerenes, where n = 20, 24, 28, 32, 36, and 40. Low-mass fullerenes are of particular interest because of their high curvature and increased strain energy owing to adjacent pentagonal rings. The most important factor for stability is the connectedness of the heteroatoms. The BN group prefers to replace a short CC bond. N atoms tend toward smaller angles, leading them toward participation in pentagons over hexagons. The BN pair prefers hexagon-pentagon over pentagon-pentagon junctions. No systematic trends are observed in the effects of doping upon the HOMO-LUMO gap, ionization potential, and electron affinity. Whereas MNDO is unable to reliably predict the most stable isomers, single-point DFT calculations at MNDO-optimized geometries correctly reproduce the full DFT relative energy trends.

Temperature effects on a hydroxyapatite precursor solution

Abstract: Multinuclear NMR spectroscopy has been used to monitor synthesis of hydroxyapatite (HAp) from diethyl hydrogen phosphonate and calcium diethoxide in solution at two different temperatures. Acetyl 2-hydroxyethyl phosphonate, bis(2-hydroxyethyl) phosphonate, and acetyl ethyl phosphonate have been identified for the first time in this reaction solution as intermediates. The formation of these compounds is shown to be crucial in controlling the phase purity of the final hydroxyapatite product. A possible mechanism for the formation of acetyl 2-hydroxyethyl phosphonate is discussed.

Strain and microcrystallite size in synthetic lamellar apatite

Abstract: Synthetic lamellar mixed A−B type carbonate containing apatite with two different Ca/P ratios (1.67 and 1.82) has been produced from a lamellar calcium acetate phosphonate precursor to investigate ...

Lysozyme adsorption to charged surfaces : A Monte Carlo Study

Abstract: Lysozyme adsorption to charged surfaces was studied by Monte Carlo simulations at different protein concentrations, protein net charges, ionic strengths, and surface charge densities. The lysozyme was represented by a hard sphere with embedded positive and negative surface charges parametrically dependent on the solution pH. A short-range attractive protein−protein potential was included to represent attractive non-Coulomb forces. The charged surface was described by a hard wall with embedded charges representing a mica surface. The protein adsorption was favored by high protein concentration, high protein net charge, low ionic strength, and high surface charge density. Nevertheless, adsorption appeared also for a weakly negatively charged protein to the negatively charged surface as a result of an electrostatically favorable protein orientation at the surface. While a multipole expansion including monopole and dipole moments only was insufficient to explain preferential orientation, an expansion includin...

Photoinduced electron and energy transfer processes in a bichromophoric pyrene-perylene bisimide system

Abstract: The synthesis and photophysical properties of a system consisting of a bay-functionalized perylene bisimide, containing four appended pyrene and two coordinating pyridine units, and its reference system are described. A complete study of their photophysical properties was obtained using UV-vis absorption, steady state and time-resolved emission, and femtosecond transient absorption. Analysis of the data, obtained from time-resolved emission and femtosecond transient absorption spectroscopy, showed the presence of both photoinduced electron and energy transfer processes. A high yield (>90%) and fast photoinduced energy transfer (k(en) approximate to 6.2 x 10(9) s(-1)) is followed by efficient electron transfer (70%, k(et) approximate to 6.6 x 10(9) s(-1)) from the pyrene units to the perylene bisimide moiety. The energy donor-acceptor distance, R = 8.6 Angstrom, is calculated from the experimental energy transfer rate using Forster theory. Temperature-dependent time-resolved emission spectroscopy showed an increase of the acceptor emission lifetime with decreasing temperature. It also indicates the presence of different conformations because two different electron transfer barriers (0.08 and 0.42 eV) were found. These barrier values were corroborated by a theoretical analysis of the energetics of the process using Marcus theory, indicating average donor-acceptor distances of 4.5 Angstrom (room temperature) to 11 Angstrom (at low 7).

Optimal choice of dividing surface for the computation of quantum reaction rates

Abstract: We consider the calculation of quantum mechanical rate constants for chemical reactions via algorithms that utilize short-time values of the symmetrized flux-flux correlation function. We argue that the dividing surface that makes optimal use of the short-time quantum information is the surface that minimizes the value at the origin of the symmetrized flux-flux correlation function. We also demonstrate that, in the classical limit, this quantum variational criterion produces the same dividing surface as Wigner's variational principle. Finally, we argue that the quantum variational criterion behaves in a nearly optimal fashion with respect to the minimization of the extent of re-crossing flux.