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  • 1
    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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  • 2
    Electronic Resource
    Electronic Resource
    Photoinduced large amplitude motion as mechanism for pure electronic dephasing and its manifestation in continuous-wave and time-resolved spectroscopy (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 246-254 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It is demonstrated that photoinduced large amplitude (LA) dynamics on a picosecond time scale may result in electronic pure dephasing on a time scale of a few tens of femtoseconds. It is shown that LA photodynamics affects continuous-wave (cw) spectra (e.g., absorption and resonance-Raman) and transient spectra (e.g., photon-echo and pump–probe) in a rather different way. Calculations are presented for a two-dimensional model problem, consisting of a fast vibrational mode and a slow LA mode, which is considered as a simple model for isomerization. The spectroscopic signals for this model are compared to the results for a complementary model, where the fast vibrational mode interacts with a bath (e.g., the environment). It is shown that standard cw techniques such as absorption and resonance-Raman spectroscopy fail to clearly distinguish the two (physically rather different) model problems, as the ultrafast optical dephasing results in strong line broadening of these spectra. Time-resolved pump–probe spectroscopy, on the other hand, is not limited by electronic pure dephasing and thus allows for a clear discrimination of the two photophysical processes. Simulations of photon-echo experiments furthermore elucidate that slow intramolecular LA motion results in inhomogeneous broadening of optical spectra. Finally a novel time-resolved technique is proposed that is capable to reveal "sub-linewidth'' information on electronic transitions which are strongly broadened by homogeneous and inhomogeneous pure dephasing processes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468176
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  • 3
    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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  • 4
    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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  • 5
    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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  • 6
    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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  • 7
    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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  • 8
    Electronic Resource
    Electronic Resource
    Chaotic dynamics of heavy particle dispersion: Fractal dimension versus dispersion coefficients (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 1305-1308 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The chaotic dynamics of Lagrangian motion of particles in a steady Arnold–Beltrami–Childress (ABC) flow and a pseudoturbulence are investigated and the Lyapunov exponents and fractal dimensions of particle trajectories for different particle inertia and particle drift velocity are computed. The dispersion process of particles could be characterized by the fractal dimension and dispersion coefficients. The interesting behavior of fractal dimension of particle motion in ABC flow suggested the similarity of particle motion in ABC flow and in a mixing layer. The relationship between particle dispersion coefficient and fractal dimension was nearly linear for the pseudoturbulence.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.857579
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  • 9
    Electronic Resource
    Electronic Resource
    Mo0.5Si0.5/Si multilayer soft x-ray mirrors, high thermal stability, and normal incidence reflectivity (1993)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 63 (1993), S. 2207-2209 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.110529
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  • 10
    Electronic Resource
    Electronic Resource
    Passive mode locking in erbium fiber lasers with negative group delay (1993)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 62 (1993), S. 910-912 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.108516
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