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  • 1
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report results of our growth and characterization of GaN films using low-pressure chemical vapor epitaxy with a new nitrogen source, hydrazoic acid (HN3). This growth technique allows for low-temperature deposition, low III/V ratios, and increased deposition rates (up to ∼2–3 μm/h). The deposited films show Ga:N atomic ratios of 1±0.25 based on our x-ray photoelectron spectroscopy analyses, and the He(II) UPS (ultraviolet photoelectron spectroscopy) spectra compare favorably with the semi-ab initio calculations for the GaN valence bands and with the reported UPS data for single crystal GaN films. X-ray and Raman spectra show deposited films crystallized in the expected wurtzite structure. We find these epitaxial films to be efficient light emitters in the blue or yellow region of the spectrum, depending upon growth conditions. Our photoluminescence time-decay kinetics confirm the excitonic nature of the blue emission. Lastly, far infrared time-domain spectroscopy shows the low carrier concentration of this material. © 1995 American Institute of Physics.
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  • 2
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Multilayer soft x-ray mirrors with an absorber consisting of the mixture Mo0.5Si0.5 have been fabricated by electron-beam evaporation in UHV. This has been done to get soft x-ray normal incidence mirrors for 80–100 eV photon energy with enhanced thermal stability and still high reflectivity. The thermal stability is studied by baking them at temperatures between 600 and 950 °C. The results were compared with multilayers of pure Mo and Si, which were also fabricated by electron-beam evaporation. After each baking step the x-ray mirrors are characterized by small angle CuKα x-ray diffraction. The reflectivity of the first-order Bragg peak is nearly constant up to 20 min baking at 900 °C. Further we present the normal incidence soft x-ray reflectivity for wavelengths between 12 and 18 nm of a Mo0.5Si0.5/Si mirror with 12 double layers (N=12) and of a Mo0.5Si0.5/Si mirror as deposited with 33 double layers (N=33). With the latter a reflectivity of 46% is achieved.
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  • 3
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 62 (1993), S. 910-912 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 100 fs pulses are generated in passively mode-locked erbium fiber lasers with small negative group-velocity dispersion. The pulses are obtained at a pump power significantly higher than the mode-locking threshold. An optimization of fiber lengths leads to the elimination of pedestal formation and the generation of stable pulse trains with high contrast ratios.
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  • 4
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The unphysical flow of zero-point energy (ZPE) in classical trajectory calculations is a consequence of the fact that the classical phase-space distribution may enter regions of phase space that correspond to a violation of the uncertainty principle. To restrict the classically accessible phase space, we employ a reduced ZPE γεZP, whereby the quantum correction γ accounts for the fraction of ZPE included. This ansatz is based on the theoretical framework given in Paper I [G. Stock and U. Müller, J. Chem. Phys. 111, 65 (1999), preceding paper], which provides a general connection between the level density of a system and its relaxation behavior. In particular, the theory establishes various criteria which allows us to explicitly calculate the quantum correction γ. By construction, this strategy assures that the classical calculation attains the correct long-time values and, as a special case thereof, that the ZPE is treated properly. As a stringent test of this concept, a recently introduced classical description of nonadiabatic quantum dynamics is adopted [G. Stock and M. Thoss, Phys. Rev. Lett. 78, 578 (1997)], which facilitates a classical treatment of discrete quantum degrees of freedom through a mapping of discrete onto continuous variables. Resulting in negative population probabilities, the quasiclassical implementation of this theory significantly suffers from spurious flow of ZPE. Employing various molecular model systems including multimode models with conically intersecting potential-energy surfaces as well as several spin-boson-type models with an Ohmic bath, detailed numerical studies are presented. In particular, it is shown, that the ZPE problem indeed vanishes, if the quantum correction γ is chosen according to the criteria established in Paper I. Moreover, the complete time evolution of the classical simulations is found to be in good agreement with exact quantum-mechanical calculations. Based on these studies, the general applicability of the method, the performance of the classical description of nonadiabatic quantum dynamics, as well as various issues concerning classical and quantum ergodicity are discussed. © 1999 American Institute of Physics.
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  • 5
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An accurate and efficient numerical scheme is presented for calculating electrical conditions inside wire-duct electrostatic precipitators. A Galerkin finite-element method with quadratic interpolation is employed in solving Poisson's equation to yield the electric potential solution. A backward difference method is utilized to compute the space-charge density from the continuity equation. The two methods are iteratively applied until convergence criteria for electric potential and current density are met. Computed potential and electric field values show good agreement with analytic solutions and experimental measurements. Comparisons between the present scheme and a finite-difference scheme show that the finite-element method offers distinct advantages in predicting the electrical characteristics of precipitators.
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  • 6
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A W/Si multilayer was used to determine the degree of circular polarization of the soft x-ray radiation of the European Synchrotron Radiation Facility helical undulator HELIOS I. The multilayer, manufactured by vapor deposition serves as a wideband tunable polarization analyzer in the photon energy range from 520 to 930 eV. The characterization of the multilayer's analyzing power, varying from 0.82 to 0.25 for these energies, indicates that it operates close to its calculated specifications. The lack of phase-shifters applicable in this energy range was overcome by a detailed analysis of the unpolarized background identified as radiation from the magnetic lattice. In this way, the degree of circular polarization of HELIOS I was determined to exceed 0.85 for hν〉685 eV. © 1997 American Institute of Physics.
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  • 7
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: In understanding the macroscopic response of polycrystalline structural materials to loading, it is frequently essential to know both the three-dimensional distribution of strain and of microtexture. The methods must be nondestructive, however, if the evolution of quantities, such as strain at a fatigue crack tip, are to be studied. This paper describes approaches for high resolution synchrotron x-ray diffraction tomography of polycrystalline materials. Preliminary experiments are reported on partially cracked compact tension samples of Al-Li 2090 and on model samples of randomly packed, millimeter-sized single crystals. Polychromatic beams collimated to diameters as small as 30 μm have been used, and collecting the spatial distribution of diffracted intensity on image storage plates as a function of sample-to-detector separation allowed inference of the depth of the volume elements contributing to diffraction. The precision to which one can determine the depths of volume elements will be discussed as well approaches for three-dimensional, nondestructive strain mapping. © 1996 American Institute of Physics.
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  • 8
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The wide matrix rocking curves of the in situ eutectic composite TaSi2-Si make it attractive as a wide-bandpass monochromator for synchrotron radiation. Wafers with Si[111], Si[110], or Si[100] orientation were studied to determine the origin of the wide rocking curves. The high degree of preferred orientation of the TaSi2 rods relative to the Si matrix was examined using synchrotron Laue patterns and the TaSi2 [100], TaSi2 [003], and TaSi2 [102] reflections. Double and triple axis diffractometry were used to show that the large widths were due to strain and mosaic and not long-range bending; copper radiation (for some double axis results) and 120 and 160 keV synchrotron radiation were used. At 8 keV, rocking curve widths were about twenty times broader than those from perfect Si, and peak reflectivities approached 20%. Rocking curves from Si[333] and Si[444] (120 and 160 keV, respectively) had identical profiles and reflectivities of about 25%. The triple axis results show compressive strains in the Si matrix along Si[111] (i.e., parallel to the rods) and dilational strains orthogonal to the rods. These results confirm the promise of TaSi2-Si as a wide-bandpass optical element for synchrotron radiation. © 1996 American Institute of Physics.
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 5298-5309 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Potential-energy functions of the three lowest singlet states of pyrazine have been computed on the self-consistent field, second-order Møller–Plesset (SCF/MP2) level as a function of ab initio determined ground-state normal coordinates. The electronic-structure calculations confirm the existence of a conical intersection of the S1(nπ*) and S2(ππ*) surfaces near the minimum of the latter surface. A vibronic-coupling model involving four spectroscopically active vibrational modes is constructed on the basis of the ab initio data. Absorption, resonance-Raman, fluorescence, and femtosecond time-resolved pump–probe spectra are computed for this model using previously developed methods. The results are compared with experimental data where available.
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 7184-7194 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a computer simulation of the real-time detection of ultrafast electronic decay dynamics in polyatomic molecules with femtosecond laser pulses. The intramolecular non-Born-Oppenheimer quantum dynamics is treated numerically exactly for a two-state three-mode vibronic coupling model representing the conically intersecting S1 and S2 excited states of pyrazine. The pump–probe signal is evaluated in lowest order perturbation theory with respect to the radiation–matter interaction by numerical integration over the pump and probe pulses. We discuss in some detail the dependence of the pump–probe signal on the properties of the laser pulses (frequencies and pulse durations). The calculations predict a dramatic (∼12 000 cm−1) and ultrafast (∼20 fs) red shift of the stimulated-emission signal as well as distinctive quantum beats in the pump–probe signal as a function of the delay time. Both effects are very pronounced and should therefore be relatively easily detectable experimentally. They are expected to be generic features of ultrafast internal-conversion processes in polyatomic molecules.
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