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  • 11
    Electronic Resource
    Electronic Resource
    High density polarized hydrogen gas target for storage rings (1995)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 66 (1995), S. 28-31 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A target of gaseous polarized hydrogen was formed by injecting polarized hydrogen atoms (produced by Stern–Gerlach spin separation) into a storage cell consisting of a cylindrical tube open at both ends. The target was placed in a storage ring to study the target characteristics (nuclear polarization, target thickness, radiation resistance). A weak transverse guide field (5 G) was applied to define the polarization direction. When atoms in a single hyperfine state were selected, the nuclear polarization of the target was measured to be 0.80±0.02. The areal density of the target under these conditions was (5.5±0.2)×1013 H/cm2, while for two spin states (applicable to experiments in high energy rings where a strong magnetic field can be applied to the target) the target thickness was found to be (8.2±0.3)×1013 H/cm2. The target polarization was unaffected by prolonged exposure of the target to beams up to 1 mA. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1146388
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  • 12
    Electronic Resource
    Electronic Resource
    TaSi2-Si composites as wide-bandpass optical elements for synchrotron radiation : The 9th National conference on synchrotron radiation instrumentation (1996)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 67 (1996), S. 3355-3355 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1147373
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  • 13
    Electronic Resource
    Electronic Resource
    Characterization of Si-TaSi2 in situ composites by synchrotron white beam topography and by double axis diffractometry (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 1737-1742 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In situ Si-TaSi2 composites are studied by synchrotron white beam topography and by double axis diffractometry. These results show that the single crystal Si matrix is of excellent quality: Rocking curve widths are between 40 and 60 s of arc, and the topographs do not exhibit asterism. Diffuse radial streaks in the Laue patterns originate from diffraction by the TaSi2 rods in the matrix: The K-absorption edge of filters placed in the incident x-ray beam produces a sharp change in contrast in the streaks, and this is used to determine the d spacings present in the streaks and to show that considerable preferred orientation exists between the TaSi2 rods and the Si matrix.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.354081
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  • 14
    Electronic Resource
    Electronic Resource
    Classical phase-space analysis of vibronically coupled systems (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 69-78 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Based on a recently introduced mapping formulation [G. Stock and M. Thoss, Phys. Rev. Lett. 78, 578 (1997)], a classical phase-space description of vibronically coupled molecular systems is developed. In this formulation the problem of a classical treatment of discrete quantum degrees of freedom such as electronic states is bypassed by transforming the discrete quantum variables to continuous variables. Here the mapping formalism is applied to a spin-boson-type system with a single vibrational mode, e.g., representing the situation of a photo-induced electron transfer promoted by a high-frequency vibrational mode. Studying various Poincaré surfaces-of-section, a detailed phase-space analysis of the mapped two-state problem is given, showing that the model exhibits mixed classical dynamics. Furthermore, a number of periodic orbits (PO's) of the nonadiabatic system are identified. In direct extension of the usual picture of trajectories propagating on a single Born-Oppenheimer surface, these vibronic PO's describe nuclear motion on several coupled potential-energy surfaces. A quasiclassical approximation is derived that expresses time-dependent quantities of a vibronically coupled system in terms of the PO's of the system. As an example, it is demonstrated that vibronic PO's may be used to calculate the time-dependent population probability of the initially excited electronic state. For the system under consideration, already two PO's are sufficient to qualitatively describe the short-time evolution of the nonadiabatic process. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1421067
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  • 15
    Electronic Resource
    Electronic Resource
    Ultrafast cis-trans photoswitching: A model study (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 1085-1091 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A quantum-mechanical model description of a molecular photoswitch is developed. It takes into account (i) the electronic curve crossing arising from the cis-trans twisting of a double bond, resulting in an ultrafast internal-conversion process of the system and (ii) the coupling of the initially excited chromophore (the "system") to the remaining degrees of freedom (the "bath"), affecting a vibrational cooling of the hot photoproducts. The latter mechanism is responsible for the localization of the molecule in the cis and trans configuration, respectively, thus determining the quantum yield of the photoreaction. Following a discussion of the validity and the numerical implementation of the Redfield formulation employed, detailed numerical studies of the time-dependent dissipative photoisomerization dynamics are presented. While the short-time dynamics ((approximately-less-than)1 ps) is dominated by the coherent wave-packet motion of the system, the time evolution at larger times mainly reflects the interaction between system and bath. The quantum yield of the cis-trans forward reaction (Yc→t) and the trans-cis backward reaction (Yt→c) is shown to depend on the energy storage of the photoreaction and, in particular, on the form of the system–bath coupling. On the other hand, it is found that Yt→c=1−Yc→t, that is the population probabilities of the cis and trans configuration at long times do not depend on the initial preparation of the system. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1428344
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  • 16
    Electronic Resource
    Electronic Resource
    Quantum-classical Liouville description of multidimensional nonadiabatic molecular dynamics (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 2001-2012 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The quantum-classical Liouville formulation gives a quantum-mechanical density-matrix description of the "quantum" particles of a problem (e.g., the electrons) and a classical phase-space-density description of the "classical" particles (e.g., the nuclei). In order to employ this formulation to describe multidimensional nonadiabatic processes in complex molecular systems, this work is concerned with an efficient Monte Carlo implementation of the quantum-classical Liouville equation. Although an exact stochastic realization of this equation is in principle available, in practice one has to cope with two major complications: (i) The representation of nonlocal phase-space operators in terms of local classical trajectories and (ii) the convergence of the Monte Carlo sampling which is cumbersome due to complex-valued trajectories with rapidly oscillating phases. Several strategies to cope with these problems are discussed, including various approximations to determine the momentum shift associated with a nonadiabatic transition, the on-the-fly generation of new trajectories at curve-crossings, and the localization of trajectories after irreversible electronic transitions. Employing several multidimensional model systems describing ultrafast photoinduced electron transfer and internal conversion, detailed numerical studies are performed which are compared to exact quantum calculations as well as to the "fewest-switches" surface-hopping method. In all cases under consideration, the Liouville calculations are in good agreement with the quantum reference. In particular, the approach is shown to provide a correct quantum-classical description of the electronic coherence. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1336576
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  • 17
    Electronic Resource
    Electronic Resource
    Characterization of the helical undulator HELIOS I in the 520 to 930 eV range using a multilayer polarimeter (1997)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 68 (1997), S. 1939-1944 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1148081
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  • 18
    Electronic Resource
    Electronic Resource
    Toward the development of high resolution synchrotron x-ray diffraction tomography of polycrystalline materials : The 9th National conference on synchrotron radiation instrumentation (1996)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 67 (1996), S. 3358-3358 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1147381
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  • 19
    Electronic Resource
    Electronic Resource
    Classical formulation of the spectroscopy of nonadiabatic excited-state dynamics (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 1545-1555 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A general classical formulation of the spectroscopy of nonadiabatically coupled electronic states is developed within the theoretical framework of the classical electron analog model due to Meyer and Miller. Adopting the Heisenberg representation for the calculation of the electric polarization, several approaches are presented to establish a quantum-classical correspondence. It is shown that a consistent classical formulation is obtained by replacing the quantum-mechanical commutators, which appear in quantum-mechanical perturbation theory, by the corresponding classical Poisson brackets which come from classical perturbation theory. The more usual practice of replacing the Heisenberg dipole operators directly by the corresponding classical dipole functions is shown to be an approximation to classical perturbation theory. For a coupled electronic three-state system explicit expressions are derived for the nonlinear polarization. Furthermore the practical application to femtosecond pump-probe spectroscopy is discussed in some detail.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465323
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  • 20
    Electronic Resource
    Electronic Resource
    Theory of resonance Raman scattering and fluorescence from strongly vibronically coupled excited states of polyatomic molecules (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5496-5509 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theoretical description of secondary emission from complex absorption bands of isolated polyatomic molecules is developed. The strong non-Born–Oppenheimer coupling associated with conical intersections of the multidimensional excited-state potential-energy surfaces is included in a fully microscopic manner by solving the time-dependent Schrödinger equation for appropriate model systems incorporating the most relevant electronic states and vibrational modes. The effect of the large number of remaining vibrational modes and of the weaker coupling with additional electronic states is modeled by phenomenological relaxation terms (lifetime broadening and pure dephasing) in the framework of the density-matrix formalism. Explicit eigenstate-free expressions for absorption, resonance Raman, and fluorescence spectra are derived via density-matrix perturbation theory. The computational feasibility of the resulting mixed microscopic/phenomenological theory is demonstrated for a simple three-mode model of the vibronic coupling of the S1(nπ*) and S2(ππ*) states of pyrazine. The effect of excited-state vibronic coupling and ultrafast S2→S1 internal conversion on resonance Raman and fluorescence spectra is analyzed on the basis of these model calculations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459619
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