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  • American Institute of Physics (AIP)  (43)
  • American Meteorological Society (AMS)
  • 2000-2004  (43)
  • 1
    Electronic Resource
    Electronic Resource
    The effects of a dynamic lattice on methane self-diffusivity calculations in AlPO4-5 (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 3345-3350 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Canonical ensemble molecular dynamics simulations were conducted for methane diffusion in AlPO4-5 in order to assess the role of the lattice motion on adsorbate diffusivity in straight pore zeolites. Both a static lattice model and a full dynamic lattice model were used at a loading of 1.5 methane/unit cell at 295 K. Although recent simulation work has asserted that there should be a difference, we show that there is little difference in the observed methane diffusivity (1.26×10−7 m2/s) and passing frequency (0.305) when a static lattice approximation is used over a full dynamic lattice (1.33×10−7 m2/s and 0.328). Furthermore, we introduce a methodology for handling lattice motion in molecular simulations by utilizing the normal vibrational modes in a harmonic crystal approximation. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480917
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  • 2
    Electronic Resource
    Electronic Resource
    Laser spectroscopy of jet-cooled ethyl radical: Infrared studies in the CH2 stretch manifold (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 1823-1834 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A glow discharge, slit supersonic expansion in conjunction with direct infrared laser absorption methods has been utilized to record high resolution vibration–rotation spectra of the CH3–CH2 ethyl radical. The slit supersonic expansion results in efficient rotational cooling from discharge temperatures down to Trot(approximate)14 K, permitting unambiguous rotational assignment and spectral analysis for the first time. Furthermore, a discharge on/discharge off data collection scheme permits clean discrimination between spectral contributions from radical vs precursor absorption. Spectra for both symmetric and asymmetric CH2 stretch manifolds are observed. Least-squares fits of transition frequencies out of the K=0 ground state manifold to a near prolate top model Hamiltonian reproduce the data to within the 7 MHz experimental uncertainty and provides rotational constants for both ground and vibrationally excited symmetric/asymmetric CH2 stretch states. The band origins for the CH2 stretch vibrations [3037.018 96(12) cm−1 and 3128.693 69(13) cm−1] are in reasonable agreement with ab initio theory; though predictions for relative intensities of the two bands are off by nearly an order of magnitude and indicate that the transition moment vector is tilted 33° away from each C–H bond toward the C–C bond axis. Structural analysis based on the measured B and C rotational constants imply a C–C bond distance of 1.49 Å. This is consistent with partial ((approximate)15%) double bond character for the ethyl radical carbon frame and in excellent agreement with theoretical predictions. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480746
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  • 3
    Electronic Resource
    Electronic Resource
    Femtosecond stimulated emission pumping: Characterization of the I2− ground state (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 8847-8854 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Femtosecond stimulated emission pumping in combination with femtosecond photoelectron spectroscopy is used to characterize the potential energy function of the I2−(X˜ 2Σu+) ground state up to vibrational energies within 2% of the dissociation limit. The frequency and anharmonicity of this state are measured at a series of vibrational energies up to 0.993 eV by coherently populating a superposition of ground state vibrational levels using femtosecond stimulated emission pumping, and monitoring the resulting wave packet oscillations with femtosecond photoelectron spectroscopy. The dissociative I2−(A˜′ 2Πg,1/2) state is used for intermediate population transfer, allowing efficient population transfer to all ground state levels. Using the measured frequencies and anharmonicities, the X˜ 2Σu+ state has been fit to a modified Morse potential with the β-parameter expanded in a Taylor series, and the bond length, well depth, and υ=0–1 fundamental frequency set equal to our previously determined Morse potential [J. Chem. Phys. 107, 7613 (1997)]. At high vibrational energies, the modified potential deviates significantly from the previously determined potential. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481499
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  • 4
    Electronic Resource
    Electronic Resource
    Crossed beams studies of Mo(a 7S3) and Mo*(a 5S2) collisions with CH4 and C2H6 (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 4634-4643 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The interactions of Mo(a 7S3) and Mo*(a 5S2) with methane, CH4, and ethane, C2H6, were studied under single collision conditions using the crossed molecular beams technique. Ground state Mo(a 7S3) atoms were found to be unreactive at all collision energies studied up to 〈Ecoll〉=35.4 kcal/mol. Nonreactive scattering of Mo(a 7S3) with methane and ethane was studied and compared to collisions with Ne and Ar. A forward peaking center-of-mass angular distribution, T(aitch-theta), was necessary to simulate the elastic collisions with inert gases as well as inelastic collisions with the alkanes. At a collision energy of 14.4 kcal/mol with CH4 and 21.0 kcal/mol with C2H6, inelastic collisions were found to transfer ∼10% and ∼19% of the initial kinetic energy into alkane internal energy, respectively. For collisions of Mo*(a 5S2)+CH4, the dehydrogenation product, MoCH2, was observed at all collision energies studied down to 2.1 kcal/mol. The reaction Mo*(a 5S2)+C2H6→MoC2H4+H2 was observed down to 〈Ecoll〉=4.5 kcal/mol. For a given total energy (electronic+translational), it was found that electronic energy is highly effective in promoting this reaction whereas translational energy is ineffective. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481018
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  • 5
    Electronic Resource
    Electronic Resource
    Inherent temperature effects in magnetic tunnel junctions (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 7567-7569 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Theoretical studies of the temperature dependence of the tunneling magnetoresistance ratio (TMR) are presented. A successful elastic tunneling model has been extended to handle temperature dependence. It treats Fermi smearing and applies Stoner-like behavior to the exchange split band structure in the electrodes to calculate TMR(T). As expected, the effects of Fermi smearing are small, but small changes in the magnetic band structure produce large changes in TMR. For a Co/I/Co junction produced by LeClair et al. [Phys. Rev. Lett. 84, 2933 (2000)], calculations using bulk magnetization predicted 33% of the experimental loss of TMR from 0 to 300 K with only a 1.5% change in magnetization. A mere 3.2% change in magnetization produced 100% of the observed drop in TMR. These results imply larger than imagined intrinsic temperature dependence for TMR. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1357126
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  • 6
    Electronic Resource
    Electronic Resource
    Spin dependent tunneling at finite bias (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 5224-5226 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Results of theoretical studies of spin dependent tunneling in magnetic tunnel junctions at finite bias are presented. A simple model which extends Slonczewski's ideas is developed. For each spin it assumes tunneling from a single free electron band through a simple barrier. The model predicts a decrease in conductance ratio with bias in good agreement with experimental observations. We find that the decrease of the magnetoconductance ratio, universally seen in experiment, has an intrinsic component resulting from the underlying electronic structure. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.373302
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  • 7
    Electronic Resource
    Electronic Resource
    Using two-photon standing waves and patterned photobleaching to measure diffusion from nanometers to microns in biological systems (2002)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 73 (2002), S. 2128-2135 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A method of measuring molecular diffusion rates in microscopic sample volumes is described. This method utilizes the standing wave interference created by colliding two counterpropagating laser beams at the focus of two opposing microscope objectives, creating a periodic light distribution in a volume on the order of 1 fl. By using a Pockels cell to vary the laser intensity with a time resolution of milliseconds, we show how this experimental geometry can be used to perform ultrahigh resolution fluorescence recovery after patterned photobleaching (FRAPP) experiments. A mathematical treatment of the experiment shows that the laser excitation profile has two characteristic length scales, the width of the focal spot and the period of the standing wave, which permits the simultaneous measurement of dynamics on two separate length scales. This feature may be used to determine whether the measured diffusion is anomalous. We present experimental results using a femtosecond Ti:sapphire laser to create a two-photon excitation profile with a fringe visibility on the order of 100. This standing wave is used to demonstrate FRAPP in both model dye/polymer systems and in more complex systems like living cells stained with a fluorescent dye. By combining the advantages of standing wave microscopy and two-photon fluorescence recovery after photobleaching, this technique permits the measurement of very short length motions in localized sample volumes, which should be useful in both biology and the study of diffusion in microscopically heterogeneous systems. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1464656
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  • 8
    Electronic Resource
    Electronic Resource
    An octopole electrodynamic balance for three-dimensional microparticle control (2001)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 72 (2001), S. 3380-3385 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A new octopole electrode design improves the particle trapping stability of an electrodynamic balance (EDB) and allows one to make three-dimensional force measurements on a trapped particle. A conventional double-ring EDB was modified by splitting each ring into four equal segments that are electrically independent. Three dc sources were combined such that eight potentials were applied to the eight segments of the electrodes. An additional ac voltage was superimposed on each ring segment as in a conventional double-ring EDB. The resulting electric field has dc components in the x, y, and z directions, which can be controlled independently by the three dc supplies. The z component is used to balance and measure vertical forces such as gravity, radiation pressure, or phoretic forces. The x and y fields can be used to suppress lateral oscillations of the trapped particles, and lateral forces on the particle can be measured in terms of the x and y dc voltages. The apparatus and the electric fields are described herein, and the operation of the device is demonstrated for spheres and aggregated particles. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1384453
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  • 9
    Electronic Resource
    Electronic Resource
    Pd growth and subsequent Schottky barrier formation on chemical vapor cleaned p-type GaN surfaces (2002)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 732-738 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Characterization of chemical vapor cleaned, Mg-doped, p-type GaN(0001) surfaces and Pd contacts sequentially deposited on these surfaces has been conducted using x-ray and ultraviolet photoelectron spectroscopies and low-energy electron diffraction. The band bending and the electron affinity at the cleaned p-GaN surface were 1.4±0.1 eV and 3.1±0.1 eV, respectively. A previously unidentified band of surface states was observed at ∼1.0 eV below the Fermi level on this surface. The Pd grew epitaxially on the cleaned surface in a layer-by-layer mode and formed an abrupt, unreacted metal–semiconductor interface. The induced Fermi level movement with Pd deposition has been attributed to a complex interaction between extrinsic and intrinsic surface states as well as metal induced gap states. The final Schottky barrier height at the Pd/p-GaN interface was 1.3±0.1 eV; the interface dipole contribution was 0.4±0.1 eV. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1424060
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  • 10
    Electronic Resource
    Electronic Resource
    Electrical, structural and microstructural characteristics of as-deposited and annealed Pt and Au contacts on chemical-vapor-cleaned GaN thin films (2002)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 2133-2137 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Schottky contacts of Pt(111) and Au(111) were deposited on chemical-vapor-cleaned, n-type GaN(0001) thin films. The growth mode of the deposition, as determined by x-ray photoelectron spectroscopy analysis, followed the two-dimensional Frank–van der Merwe growth model. The resulting as-deposited metal films were monocrystalline and epitaxial with a (111)//(0002) relationship with the GaN. Selected samples were annealed for three minutes at 400 °C, 600 °C or 800 °C. The rectifying behavior of both contacts degraded at 400 °C; they became ohmic after annealing at 600 °C (Au) or 800 °C (Pt). High-resolution transmission electron micrographs revealed reactions at the metal/GaN interfaces for the higher temperature samples. X-ray diffraction results revealed an unidentified phase in the Pt sample annealed at 800 °C. A decrease in the room temperature in-plane (111) lattice constant for both metals, ranging from −0.1% to −0.5%, was observed as the annealing temperature was increased from 400 to 800 °C. This plastic deformation was caused by tensile stresses along the [111] direction that exceeded the yield strength as a result of the large differences in the coefficients of thermal expansion between the metal contacts and the GaN film. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1432127
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