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  • Springer  (101)
  • American Institute of Physics (AIP)  (14)
  • American Association for the Advancement of Science (AAAS)  (8)
  • American Meteorological Society (AMS)
  • 2000-2004  (23)
  • 1990-1994  (86)
  • 1935-1939  (14)
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  • 1
    Electronic Resource
    Electronic Resource
    Approximate calculation of femtosecond pump–probe spectra monitoring nonadiabatic excited-state dynamics (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 4910-4922 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An approximate theory of femtosecond spectroscopy of nonadiabatically coupled electronic states is developed. Neglecting the commutators of vibrational Hamiltonians pertaining to different diabatic electronic states, the formulation represents a generalization of the semiclassical Franck–Condon approximation to the case of nonadiabatic dynamics. Explicit expressions for various time- and frequency-resolved spectra are derived which allow for a simple interpretation of femtosecond spectroscopy of vibronically coupled molecular systems. Employing multidimensional model problems describing (i) the nonadiabatic cis–trans isomerization of an electronic two-state system, and (ii) the S2→S1 internal conversion of pyrazine, exact reference data are compared to approximate calculations of transient absorbance and emission as well as time-resolved photoelectron spectra. In all cases considered, the approximation is shown to be appropriate for probe–pulse durations that are shorter than the period of the fastest relevant vibrational mode of the molecular system. Reducing the numerical costs of pump–probe simulations to the costs of a standard time-dependent wave-packet propagation, the approximate theory leads to substantial computational savings. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481045
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  • 2
    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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  • 3
    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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  • 4
    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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  • 5
    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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  • 6
    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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  • 7
    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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  • 8
    Electronic Resource
    Electronic Resource
    Ab initio characterization of the S1–S2 conical intersection in pyrazine and calculation of spectra (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 5298-5309 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462715
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  • 9
    Electronic Resource
    Electronic Resource
    Chaotic dynamics of particle dispersion in fluids (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 1789-1804 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An analysis of the Lagrangian motion for small particles denser than surrounding fluid in a two-dimensional steady cellular flow is presented. The Stokes drag, fluid acceleration, and added mass effect are included in the particle equation of motion. Although the fluid motion is regular, the particle motion can be either chaotic or regular depending on the Stokes number and density ratio. The implications of chaotic motion to particle mixing and dispersion are discussed. Chaotic orbits lead to the dispersion of particle clouds which has many of the features of turbulent dispersion. The mixing process of particles is greatly enhanced since the chaotic advection has the property of ergodicity. However, a high dispersion rate was found to be correlated with low fractal dimension and low mixing efficiency. A similar correlation between dispersion and mixing was found for particles convected by a plane shear mixing layer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858401
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  • 10
    Electronic Resource
    Electronic Resource
    Quantification of chaotic dynamics for heavy particle dispersion in ABC flow (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1073-1080 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A six-dimensional nonlinear dynamic system describing the Lagrangian motion of a heavy particle in the Arnold–Beltrami–Childress (ABC) flow was numerically studied. Lyapunov exponents and fractal dimension were used to quantify the chaotic motion. A single set of ABC flow parameters and a limited set of initial conditions were used. Given these restrictions, the following were found. (1) Attractor fractal dimension varies significantly with Stokes number, and, depending on inertia, periodic, quasiperiodic, and chaotic attractors may exist. (2) Particle drift reduces the fractal dimension when the drift is small. It can also cause irregular jumps when the drift parameter is close to one. (3) Quasiperiodic orbits on smooth two-dimensional manifolds were shown to be the most common ultimate solutions of the system when either the inertia or the drift is relatively large. (4) Different initial conditions can lead to different attracting sets; however, most of them have the same dimension. (5) A direct measure of dispersion based on mean square displacement was defined, but no relation between this dispersion measure and fractal dimension was found.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858088
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