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  • 1
    Electronic Resource
    Electronic Resource
    Ultrafast extended x-ray absorption fine structure (EXAFS)—theoretical considerations (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 6238-6246 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Inspired by the recent experimental demonstration of ultrafast x-ray absorption spectroscopy, we present a framework for the calculation of extended x-ray absorption fine structure (EXAFS) spectra on the ultrafast (femtosecond to picosecond) time scale. Model calculations for gas phase I2, evolving under the influence of laser pumping, demonstrate that ultrafast EXAFS has the potential to serve as a direct probe of nuclear dynamics, including time-dependent interatomic separations and relative orientations. The feasibility of ultrafast EXAFS as a viable and useful experimental technique is discussed. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479928
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  • 2
    Electronic Resource
    Electronic Resource
    A simple physical picture for quantum control of wave packet localization (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 1441-1450 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Based on weak field quantum control theory, a semiclassical approximation relates the characteristic parameters of the tailored light field to corresponding classical dynamical quantities and thus reveals the underlying physical basis of wave packet focusing. A coordinate-dependent two-level-system approximation is employed to further analyze the molecular dynamics induced by short laser pulses, thus leading to a simple interpretation of the observed correlation between the pulse chirp and vibrational focusing and defocusing. Though our study is presented in the context of quantum control, the conclusions are general, providing an intuitive picture of the quantum coherence of light–matter interaction and a guideline for the design of tailored laser fields. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475151
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  • 3
    Electronic Resource
    Electronic Resource
    Quantum control of I2 in the gas phase and in condensed phase solid Kr matrix (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 8486-8503 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present experimental results and theoretical simulations for an example of quantum control in both gas and condensed phase environments. Specifically, we show that the natural spreading of vibrational wavepackets in anharmonic potentials can be counteracted when the wavepackets are prepared with properly tailored ultrafast light pulses, both for gas phase I2 and for I2 embedded in a cold Kr matrix. We use laser induced fluorescence to probe the evolution of the shaped wavepacket. In the gas phase, at 313 K, we show that molecular rotations play an important role in determining the localization of the prepared superposition. In the simulations, the role of rotations is taken into account using both exact quantum dynamics and nearly classical theory. For the condensed phase, since the dimensionality of the system precludes exact quantum simulations, nearly classical theory is used to model the process and to interpret the data. Both numerical simulations and experimental results indicate that a properly tailored ultrafast light field can create a localized vibrational wavepacket which persists significantly longer than that from a general non-optimal ultrafast light field. The results show that, under suitable conditions, quantum control of vibrational motion is indeed possible in condensed media. Such control of vibrational localization may then provide the basis for controlling the outcome of chemical reactions. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473906
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  • 4
    Electronic Resource
    Electronic Resource
    Chirped pulse enhancement of multiphoton absorption in molecular iodine (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 2309-2313 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Chirp effects on three photon absorption yields in I2 vapor are investigated using femtosecond pulses with center wavelengths in the region 550–600 nm. Enhancements of as much as a factor of 3 are observed for chirped pulses with respect to transform-limited, zero chirp pulses. Theoretical considerations and model calculations suggest that wave packet dynamics play an important role. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475615
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  • 5
    Electronic Resource
    Electronic Resource
    Detecting wave packet motion in pump–probe experiments: Theoretical analysis (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 5062-5072 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Zewail–Bersohn model [Ber. Bunsenges. Phys. Chem. 92, 373 (1988)] of pump–probe experiments is generalized to nonstationary wave packets and more realistic forms of probe pulses. The analysis illustrates the important role of probe linear chirp rate, as pointed out by Sterling, Zadoyan, and Apkarian [J. Chem. Phys. 104, 6497 (1996)], in detecting the motion of wave packets and the physical reason for the existence of optimal probe pulses to yield the best probe signal. Since the pump–probe process can be viewed as delayed two-photon resonant absorption, the probe signal can be readily optimized within the framework of quantum control theory, as discussed by Yan [J. Chem. Phys. 100, 1094 (1994)]. Numerical calculations based on quantum control theory are used to confirm our theoretical predictions. We point out that the same analysis can be extended to other impulsive nonlinear optical processes, such as multiphoton pump–probe absorption and stimulated Raman scattering. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473552
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  • 6
    Electronic Resource
    Electronic Resource
    Quantum control of dissipative systems: Exact solutions (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 5239-5248 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Optimal quantum control theory, which predicts the tailored light fields that best drive a system to a desired target, is applied to the quantum dissipative dynamics of systems linearly coupled to a Gaussian bath. To calculate the material response function required for optimizing the light field, the analytical solution is derived for the two-level Brownian harmonic oscillator model and the recently developed method for directly simulating the Gaussian force is implemented for anharmonic Brownian oscillators. This study confirms the feasibility of quantum control in favorable condensed phase environments and explores new quantum control features in the presence of dissipation, including memory effects and temperature dependence. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473522
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  • 7
    Electronic Resource
    Electronic Resource
    Ultrafast x-ray absorption probing of a chemical reaction (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 6066-6069 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ultrafast x-ray techniques can, in principle, allow us to more directly watch the time evolution of matter, with atomic spatial resolution and with time resolution on the scale of atomic motions such as the making and breaking of chemical bonds, in order to more directly observe the fundamental molecular dynamics underlying the concept of "mechanism'' in inorganic, organic, and biochemical reactions. As a step toward this goal, we have observed a chemical reaction process, photoinduced dissociation of gas phase SF6 molecules, detected by ultrafast near-edge x-ray absorption spectroscopy with time resolutions of 1.5–3 ps, near the sulfur K edge at a photon energy of 2.48 keV (4.98 A). © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471305
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  • 8
    Electronic Resource
    Electronic Resource
    Energy flow in an atom exchange chemical reaction in solution (1990)
    s.l. : American Chemical Society
    Journal of the American Chemical Society 112 (1990), S. 524-530 
    Details
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ja00158a008
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  • 9
    Electronic Resource
    Electronic Resource
    Controlling the Future of Matter (1995)
    s.l. : American Chemical Society
    Accounts of chemical research 28 (1995), S. 133-140 
    Details
    ISSN: 1520-4898
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ar00051a006
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  • 10
    Electronic Resource
    Electronic Resource
    Molecular dynamics of a model SN2 reaction in water (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 1356-1376 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics are computed for a model SN2 reaction Cl−+CH3Cl→ClCH3+Cl− in water and are found to be strongly dependent on the instantaneous local configuration of the solvent at the transition state barrier. There are significant deviations from the simple picture of passage over a free energy barrier in the reaction coordinate, and thus, a marked departure from transition state theory occurs in the form of barrier recrossings. Factors controlling the dynamics are discussed, and, in particular, the rate of change of atomic charge distribution along the reaction coordinate is found to have a major effect on the dynamics. A simple frozen solvent theory involving nonadiabatic solvation is presented which can predict the outcome of a particular reaction trajectory by considering only the interaction with the solvent of the reaction system at the gas-phase transition barrier. The frozen solvent theory also gives the transmission coefficient κ needed to make the transition state theory rate agree with the outcome of the molecular dynamics trajectories. This theoretical κ value, which is the implementation for the SN2 reaction of the van der Zwan–Hynes nonadiabatic solvation transmission coefficient, is in good agreement with the trajectory results. In contrast, a Kramers theory description fails dramatically.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452224
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