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  • 1995-1999  (146)
  • 1990-1994  (167)
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  • 1
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Journal of the American Chemical Society 113 (1991), S. 2020-2033 
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Accounts of chemical research 26 (1993), S. 496-501 
    ISSN: 1520-4898
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Il nuovo cimento della Società Italiana di Fisica 17 (1995), S. 983-991 
    ISSN: 0392-6737
    Keywords: General, theoretical, and mathematical biophysics (including logic of biosystems, quantum biology, and relevant aspects of thermodynamics, information theory, cybernetics, and bionics) ; Molecular dynamics, conformational changes, and pattern recognition in biomolecules ; Physical chemistry of solutions of biological macromolecules
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We introduce a general scheme to assign statistical weights to alla priori pathways in conformation space for an aggregate of interacting particles that undergoes a series of unimolecular events in a limited time frame. Illustrations of such systems are biopolymers that fold intramolecularly or macromolecules of biological relevance that exert their function onto themselves through a series of structural rearrangements under the severe time constraints imposed by enzymatic timing. We endow the space of kinetically controlled pathways with a regular measure induced by a stochastic process built upon a complex potential energy landscape. This process simulates the progressive and opportunistic exploration of basins of attraction of critical points in the molecular potential energy. The derivation is general and holds for any diffusion-like process subject to time constraints and governed by thermal fluctuations upon a complex energy landscape. Within this framework, the ensemble of physically relevant kinetically arrested states becomes a cross-section of the ensemble of pathways at a fixed instant.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4673-4686 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The paper presents and extends an earlier model for the relaxation dynamics within solids and desorption of adsorbates on surfaces. The model retains the discrete nature of the adsorbates while adopting a continuum representation for the solid. Extensive one-dimensional model calculations are carried out along with sample calculations in the two-dimensional case. Specifically, in the one-dimensional case, the formulation is for a diatom with harmonic intramolecular bond and a Morse potential between the diatom and the remaining solid as well as an atomic inclusion in a two-dimensional bulk solid. The excitation mechanisms are initial disturbances that take the system away from its equilibrium configuration and acoustic pulses incident on the molecule. An extensive parametric study is carried out by varying the width, amplitude, and frequency content of the pulse along with the strength of the Morse bond. These calculations permit an understanding of the roles of the various parameters in determining the relaxation rate associated with energy transfer and desorption processes for the case of the diatom bound to the solid surface as well as an atom as a guest in the solid. The observed physical phenomena span the linear to the highly nonlinear regime.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 2927-2937 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Analytic solutions of the optimal fields giving selective local excitation in harmonic molecules are presented, and calculated by a numerically stable algorithm. It is shown that the optimal fields can be decomposed into a finite number of monochromatic laser fields, with the following properties: (a) the number of component frequencies is equal to the number of normal modes of the molecule; (b) the component frequencies go to the molecular vibrational normal mode frequencies in the limit of low amplitude; (c) there is an inverse relationship between the amplitude of the fields (as controlled by the weighting factor for the field fluence term) and the time T at which the objective is reached; (d) there exists a limiting form for the optimal fields as the controlling time T→∞ (which had been observed empirically in previous work and now has a regorous mathematical basis). The intensity of the component laser fields, and thus the optimal field can be reduced to any desired level with a corresponding lengthening of the pulses. Thus by varying the design criteria one may determine a corresponding entire family of optimal fields with each member equivalently leading to the desired final local excitations. The optimization procedure clearly illustrates the trade off between optimal pulse length and amplitude. In order to try and achieve the control objectives with consideration of laboratory constraints on the laser fields a formalism is presented for designing the optimal fields with constrained functional forms. An illustration based on linear chain molecules shows that the objectives may be quite well achieved by using realistically constrained optimized fields.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 7125-7135 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper examines the role of the potential surface of the rigid rotor He–H2 system upon a variety of transport and relaxation cross sections, including some involving field effects. The technique of functional sensitivity analysis is used to explore these issues. Three different levels of cross sections were studied: microscopic, thermally averaged, and effective cross sections. The cross sections studied were found to be sensitive to differing components of the potential energy surface, with some cross sections being more sensitive to either the slope or the magnitude of the potential components. The degree of information loss in the progression from microscopic to bulk observables is highly dependent on the individual phenomenon.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 7114-7124 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper examines the effect of infinitesimal functional variations in a three-dimensional vibration–rotation He–H2 potential surface on several different levels of physical observables: inelastic cross sections, rate constants, and energy level populations. Earlier equations for a rigid-rotor system are extended and a comparison of the current results with earlier rigid-rotor results is made. A significant difference in the sensitivity of observables to the potential components has been observed between those observables which are purely rotationally inelastic and those which are vibrationally inelastic. The region of highest sensitivity is dependent upon the energy or temperature as well as the states related by the individual observable. Significant information loss has been observed in the transition from the microscopic observables to the macroscopic ones for those observables which are vibrationally inelastic.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 364-376 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A quantum mechanical formalism for the optimal control of physical observables of microsystems is developed. The computational procedure for numerical implementation is presented. Three illustrative examples with a model Morse oscillator show that optimal pumping fields can be found to reach the physical objectives: selective excitations, steering a system to a specified state, and breaking a bond.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1158-1166 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The possibility of controlling unimolecular–dissociation processes with multiple laser fields in the collisional regime is examined. Employing the Bloch equations to describe optical excitation and decay processes, optimal control theory is used to design amplitude modulated fields which produce the desired excited-state products. The selectivity of the product distribution of a simple four-state photodissociation system is shown to have a square-root dependence on the relative value of the mean dephasing time T2 to the pulse length τ, i.e, (T2/τ)1/2. The equivalence between T2 decay and phase disruptions occurring in a random-walk fashion is also examined. In the Appendix it is shown that the essential effect of the system temperature is to introduce a Boltzmann population factor on the product selectivity without affecting the nature of the optimal field.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 2305-2314 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A general iterative inversion procedure based on functional sensitivity analysis is presented for determining the gas–surface interaction potential from low energy elastic scattering data. Formally, Tikhonov regularization, singular function analysis, and a recently developed exact transformation technique are implemented to render the inversion stable and efficient. Specifically, the simulation of helium scattering from a rigid periodic xenon monolayer on the graphite (0001) face is considered. It is found that the functional sensitivity densities of the diffraction intensities with respect to the He–Xe/C(0001) potential contain profound information, thus are invaluable in guiding the inversion of scattering data to yield the potential. Although, unequivocal determination of the full three-dimensional potential from the inevitably incomplete experimental data may be difficult, we demonstrate that simulated input data consisting of a finite number of polar scan specular intensities can be used to accurately recover the underlying He–Xe/C(0001) potential. The recovered potential has been obtained without imposing any explicit functional form on the potential per se. The resulting procedure is quite promising for treating real laboratory data.
    Type of Medium: Electronic Resource
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