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  • 1
    Electronic Resource
    Electronic Resource
    Probing nanoscale photo-oxidation in organic films using spatial hole burning near-field scanning optical microscopy (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 7864-7872 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Spatial hole burning near-field scanning optical microscopy (SHB–NSOM) is used to locally photopattern three species of organic thin films, poly(2-methoxy, 5-(2′-ethyl hexyloxy)–p-phenylene vinylene) (MEH–PPV), tris-8-hydroxyquinoline aluminum (Alq3) and dye-functionalized polyelectrolyte self-assembled layers, on a 100 nm length scale. In SHB–NSOM the film is illuminated with light from a stationary NSOM tip to induce photo-oxidation. The reduction in the fluorescence yield resulting from this exposure is then mapped using fluorescence NSOM (FL–NSOM). We have examined the localized photo-oxidation as a function of time, position, and environment free from the limits of far-field spatial averaging. In all of the thin film materials studied we find that the long-time diameter of the dark spot is much larger than the tip diameter and is a signature of energy migration. Characteristic lengths of the energy migration are extracted from this data by a simple diffusion model and are found to be of the order of a few hundred nanometers for each of the films studied. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481391
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  • 2
    Electronic Resource
    Electronic Resource
    Structural and energetic properties of the Br−–C2H2 anion complex from rotationally resolved mid-infrared spectra and ab initio calculations (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 1075-1080 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An infrared vibrational predissociation spectrum of the 79Br−–C2H2 anion complex has been recorded over the 2800–3400 cm−1 range. Bands are observed that correspond to excitation of bound and free C–H stretches of an acetylene molecule engaged in a linear hydrogen bond with Br−. The band associated with the bound C–H stretch displays rotationally resolved substructure. Lower J transitions are absent from the predissociation spectrum, indicating that the upper levels lie below the dissociation threshold. Analysis leads to constants for lower and upper states: v0=2981.28, B″=0.048 84, ΔB=9.3×10−4 cm−1, and a minimum J′=28 for dissociation. The rotational constants correspond to vibrationally averaged separation between Br− and the C2H2 center of mass of 4.11 Å in the ground state and 4.07 Å in the v3 state. A dissociation energy for Br−–C2H2 of 3020±3 cm−1 is estimated from the energy of the lowest dissociating level. The spectroscopically derived data are corroborated by ab initio calculations conducted at the MP2/aug-cc-pVTZ level. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481919
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  • 3
    Electronic Resource
    Electronic Resource
    Calculation of the vibrational linewidth and line shape of Raman spectra using the relaxation function. I. Method and application to nitrogen (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 1395-1403 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The spectral line shape of the fundamental vibration of nitrogen is calculated from molecular dynamics simulations by determining the Fourier transform of the relaxation function. It has been applied to the fluid phase at various pressures and temperatures, and to solid δ-N2. The validity of the assumption that the spectrum at relatively high temperatures and pressures can be calculated by assuming that these systems are in the fast modulation regime (Δτc(very-much-less-than)1), has been verified. A deviation of the vibrational line shape from the motional narrowing limit has been found for fluid nitrogen at low pressure, with a Kubo parameter, Δτc, equal to 0.23, and for the vibrational line of the molecules on the a sites in δ-N2, with Δτc equal to 0.075. It is concluded that the value of the Kubo parameter is not an unambiguous criterion for the fast modulation regime. Moreover, a detailed comparison reveals a difference in the dynamical behavior of the molecules on the a and c sites. It is shown that this procedure can also be used if one does not know whether or not closely spaced lines are to be expected. The present procedure is suited to calculate line shapes in the intermediate Kubo regime, e.g., in concentrated mixtures, where no simple relations are available. Finally it is shown that in nitrogen at low density and 126 K the ratio of the correlation time of the frequency autocorrelation function and the dephasing time is smaller than in CH3I, where the n dependence of the vibrational overtone is subquadratic. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480693
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  • 4
    Electronic Resource
    Electronic Resource
    Calculation of the vibrational linewidth and line shape of Raman spectra using the relaxation function. II. Application to the mixture neon–nitrogen with inhomogeneous broadening due to concentration fluctuations (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 1404-1412 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Raman spectra of nitrogen in the mixture neon–nitrogen have been measured for neon mole fractions, xM, of 0.10, 0.65, and 0.95 at 296 K and 408 K up to the solidification pressure. It was found that the frequency increases as a function of xM. The linewidth is strongly composition dependent, with a very large value for xM=0.65. Further, it is shown that the width decreases as a function of temperature. Molecular dynamics simulations have been performed in order to calculate the linewidth and shape, using the relaxation function. The simulations are in good agreement with experiment. It is shown that the increase in linewidth in the intermediate concentration range is due to an increase of the correlation time, rather than an increase of the amplitude of modulation. In this range inhomogeneous broadening due to concentration fluctuations occur: the vibrational line shape starts to deviate from a Lorentzian curve, and gets a Gaussian component. The simulations also provide insight into the relatively large decrease of the width as a function of temperature in the intermediate concentration range. When the temperature is increased from 296 to 408 K, the amplitude of modulation increases, but the correlation time decreases more. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480694
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  • 5
    Electronic Resource
    Electronic Resource
    An exchange functional for accurate virtual orbital energies (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 7002-7007 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We show that a simple Gaussian model for exchange yields Kohn–Sham virtual orbital energies that are in significantly better agreement with Hartree–Fock theory than those arising from functionals based on the uniform electron gas. Also we show that normalization of the Gaussian model significantly improves the accuracy of total exchange energies, and that reparametrizing a Becke-type asymptotic correction leads to total exchange energies which are only slightly less accurate than B88. Errors in HOMO–LUMO gaps for this new functional are typically less than a third of the corresponding B88 errors. Many-body perturbation theory using B88 exchange in the zeroth-order Kohn–Sham problem is shown to be divergent or very slowly convergent for some typically well-behaved closed shell systems; using the functional presented here, though, convergence is in each case at a rate comparable with normal Møller–Plesset perturbation theory. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481298
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  • 6
    Electronic Resource
    Electronic Resource
    Simulated current response in avalanche photodiodes (2002)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 2107-2111 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The time dependent current response to an impulse of injected carriers is calculated for an avalanche photodiode using Monte Carlo simulation. For low electric fields and long avalanche regions the results agree with the conventional model, which assumes that carriers travel always with their saturated drift velocities. However, while diffusion remains unimportant, for high fields and short avalanche regions, the conventional model underestimates the device speed. Monte Carlo simulations show that the mean downstream average velocity of ionizing carriers is significantly enhanced at high electric fields and agreement is restored if we allow for this effect in the conventional model. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1432122
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  • 7
    Electronic Resource
    Electronic Resource
    Midinfrared photoluminescence from IV–VI semiconductors grown on silicon (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 3514-3516 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Above room temperature continuous wave midinfrared photoluminescence has been observed from PbSe/Pb1−xSrxSe multiple quantum well structures grown by molecular beam epitaxy on (111) silicon. Emission energy from a sample with 10-nm-thick quantum wells varied from 336.1 to 343.7 meV as sample temperature was increased from 15 to 35 °C. At a heat sink temperature of 25 °C the emission energy varied from 336.8 to 339.9 meV as the current in the near-infrared diode pump laser was increased from 300 to 800 mA indicating an additional 8.2 °C of epilayer heating due to increased photon flux from the pump laser. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1347950
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  • 8
    Electronic Resource
    Electronic Resource
    Charge states of divacancies in self-implanted doped Si (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 5991-5996 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The charge states of divacancies induced by 5 MeV self-implantation of doped silicon were investigated by positron annihilation methods. For low doping concentrations, results were found to be in agreement with the predictions of Fermi statistics. For the case of heavily boron-doped silicon (nB=1×1019 cm−3) an anomalous single-negative divacancy charge state was detected. We attribute this to the introduction of new levels in the band gap, due to the capture of boron by divacancies, resulting in a boron-divacancy complex. Detailed analysis of positron annihilation spectra suggests that the boron does not reside on a nearest-neighbor site to the divacancy. Isothermal annealing experiments yield activation energy of 0.9±0.1 eV for migration of this defect. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1363682
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  • 9
    Electronic Resource
    Electronic Resource
    Thermal stability of Ta-pinned spin valves (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 6825-6827 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: It has recently been found that large uniaxial anisotropy fields in excess of 120 kA/m (1500 Oe) can be created in thin (3–5 nm) films of Co by obliquely sputtered Ta underlayers. This anisotropy can be used to pin the bottom film of a spin valve while having only a modest effect on the top "free" film, separated by a 2.5 nm Cu spacer layer. This article describes measurements of thermal stability in these Ta-pinned spin valves. Using room temperature giant magnetoresistance (GMR) as a measure, we find that the structure is stable under cumulative 20 min anneals at 25 °C intervals up to 300 °C; GMR decreases to zero upon further anneals up to 450 °C. Measurements taken at elevated temperatures reveal that GMR decreases linearly with temperature, extrapolating to zero at approximately 425 °C, while the anisotropy field is much less temperature dependent, remaining nearly constant up to 150 °C and gradually decreasing to 50% of its room temperature value at 325 °C. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1357147
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  • 10
    Electronic Resource
    Electronic Resource
    Study of quantum well intermixing caused by grown-in defects (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 88 (2000), S. 3090-3092 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We show that the degree of thermally induced quantum well intermixing is dependent on the growth quality of epitaxial layer structures. Two different undoped quantum well/barrier structures are studied: an InGaAs/InGaAsP 1.5 μm laser-like structure, and an InGaAs/InP structure. In both cases, the temperature of growth of one or more layers is altered and results compared with a control wafer. Wafers with material grown at the lower temperature display blueshifts of up to 80 meV in the QW emission energy during the early stages of rapid thermal annealing. Wafers grown at temperatures that are more standard exhibit excellent stability. We examine shifts of both the heavy hole and light hole transitions as a function of intermixing via transmission measurements, and conclude that interdiffusion occurs primarily on the group V sublattice. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1287406
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