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  • 1
    Electronic Resource
    Electronic Resource
    Fracture of polymer chains in extensional flow: Experiments with DNA, and a molecular-dynamics simulation (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 2650-2662 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The behaviors of polymer chains in simple shear flow and in flows with a large component of extension are now considered to be qualitatively different [R. B. Bird, C. F. Curtiss, R. C. Armstrong, and O. Hassager, Dynamics of Polymeric Liquids, 2nd ed. (Wiley, New York, 1987), Vol. 2, Chap. 13]. To examine the extensional case, we have fractured DNA molecules in solution in steady sink (primarily extensional) flow. DNA was chosen for this study because it can be obtained as monodisperse, unique-sequence material, and because the size distribution of the fracture fragments can be obtained by gel electrophoresis. Dilute monodisperse T7 DNA (Mw=26×106) solutions were recirculated through a 0.13 mm orifice in a flat plate. The flow field upstream of the orifice closely approximated an ideal sink flow, being free of vortices under the conditions used, DNA concentrations less than 12.5 μg/ml and flow rates less than 0.045 cm3/s. The kinetics of the fracture at low flow rates showed an initial lag period followed by a period of first-order rate; the lag period disappeared at higher flow rates while the first-order period persisted. The fracture rate increased exponentially with the flow rate. Contrary to the classic theory by Frenkel, our experimental results showed fracture products broadly distributed in size; this anomaly was explained by a bead-spring molecular-dynamics computer simulation. The simulation showed that just prior to chain fracture the chain was aligned parallel to the flow but contained many folds, so that points of maximum stress were not usually at the molecular center. These results suggest that the residence time in the converging flow was too short for the chain to reach complete extension.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457960
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  • 2
    Electronic Resource
    Electronic Resource
    A stationary phase study of mj rainbows in molecule-corrugated surface scattering (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 6515-6522 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using the stationary phase limit of the quantum mechanical full sudden S matrix for diatom-corrugated rigid surface scattering of Proctor, Kouri, and Gerber [J. Chem. Phys. 80, 3845 (1984)] we obtain expressions for the rotational actions j and mj of an inelastically scattered diatom. By integrating over all points on the lattice, we reduce these to functions of the polar angles θ and φ only. It is found that j is a strong function of θ, but almost independent of φ, whereas the reverse is true for mj. Both j(θ) and mj(φ) are continuous plots which exhibit extrema known to produce rainbow behavior in inelastic gas-phase scattering theory. We propose that this implies the existence of rainbows in the mj distribution, and show the dependence of these rainbows on various potential parameters, including corrugation, potential repulsion parameter, and lattice constant. The results explain earlier trajectory studies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455371
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  • 3
    Electronic Resource
    Electronic Resource
    The effect of reagent rotation on chemical reactivity: F+H2 revisited (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6906-6918 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Classical trajectory calculations on the gas phase reaction F+H2 ( j)→HF+H have been carried out. Different reactivity trends were seen depending on whether there was a chemically significant and anisotropic well in the entrance channel of the potential surface. For those in which there is no such well, rotation may decrease reactivity at low values of j, but increases it thereafter. The reaction cross section SR ( j) decreases slowly from j=0, reaching a minimum near j=6 then increases again. This behavior has been reported for several systems, including H+H2, and seems to be the "canonical'' behavior for SR ( j) for most direct chemical reactions. For F+D2 the minimum does not occur until j=8. However, this does correspond to the same amount of rotational energy as the minimum for F+H2 . For potentials in which there is a deep anisotropic well, it is found that the j=0 results are dominated by the presence of the well, and that the SR ( j=0) is anomalously high. On such surfaces there is normally a sudden drop in cross section from j=0 to j=1, followed by an increase. The experimental findings of Lee's group [J. Chem. Phys. 82, 3045 (1985)] that the cross section increases on going from j=0 to j=1 probably precludes the possibility of a chemically significant well in the entrance valley. The rotational product state distribution for both types of potential is dominated by kinematics away from threshold, and does not show the same trends as the reaction cross section. The mean product vibrational quantum number 〈v'〉 can decrease at low j, then increase at higher j. This occurs only at collision energies close to threshold, and on potentials which have a tight bend force constant at the transition state. The more general case for this reaction is that 〈v'〉 increases with j. For the case in which the potential has no well the differential cross section shows precisely the same trends as does SR( j). All these trends can be explained using a simple model we have recently proposed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457359
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  • 4
    Electronic Resource
    Electronic Resource
    Cluster catalyzed chemisorption of H2 on Si(111)(1×1): Effects of collision speed and cluster geometry (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 6667-6676 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have carried out classical dynamical simulations of collisions of Ar12H2 clusters with a rigid Si(111)(1×1) solid surface for a variety of collision speeds and two different cluster geometries. At low cluster temperatures, the most stable cluster geometry is that with the H2 on the outside of the cluster. It is found that dissociative chemisorption of the hydrogen can occur with this cluster geometry at H2 collision energies as low as 0.05 eV; this is markedly lower than the 0.4 eV threshold for "bare'' hydrogen on this surface. The reactivity increases with collision energy until a steric limit of roughly 33% reaction is attained. This corresponds to that fraction of collisions in which the H2 can be caged by the Ar cluster on the surface. The other isomer, with the H2 inside the Ar cluster, is less reactive at low energies, since the H2 does not have free access to the surface. However, the reactivity begins to rise steeply away from threshold as the H2 becomes sufficiently energetic to rearrange the Ar "solvent'' shell. Studies of the details of the dynamics reveal that the kinetic energy of the Ar cluster can be "pooled'' to allow access to relatively high energy channels, such as chemisorption, dissociative trapping, and even collision-induced dissociation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465857
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  • 5
    Electronic Resource
    Electronic Resource
    Evaluation of thermal rates for reactions with intermediate wells: Removal of bound state contributions to quantum flux correlation functions (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 3516-3525 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We consider the calculation of rates of chemical reactions that have bound intermediate states (i.e., wells along the reaction path) using flux correlation function methods. When time-dependent wave packets are used to evaluate the propagator matrix elements, and the dividing surface is located at a point where bound states have nonzero probability density, the standard expression for the flux correlation function shows infinitely long lived oscillations due to these bound states, making the evaluation of rate constants numerically ill-behaved. However, if the bound part of the initial wave packet is projected out, the resulting continuum-only propagators produce rapidly decaying correlation functions, and numerically well-behaved rate constants. We illustrate this projection operator approach by considering a one-dimensional reaction path model in which the potential is taken to be an Eckart well, and the dividing surface is located at the minimum. In another application, we consider a two degree of freedom model of H2 dissociative chemisorption on a rigid metal surface. This application is sufficiently complex that it is impractical to calculate the chemisorption rate using conventional flux correlation function methods, but with the projected wavepacket approach, the problem is made relatively easy. We also consider a second approach to the treatment of bound states in which the flux correlation function is altered to remove implicitly the bound state contributions to the propagator at long times. This second expression can be used with the full propagator, eliminating the need to construct and project explicitly the bound states. This should be advantageous when many bound states are present.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466149
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  • 6
    Electronic Resource
    Electronic Resource
    Island size effects in nanoparticle-enhanced photodetectors (1998)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 73 (1998), S. 3815-3817 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report the effect of metal-island size variation in nanoparticle-enhanced photodetectors. Nanoparticle size was controlled by varying the deposition and annealing conditions used to produce the metal-island films. Increasing the size of silver-island particles fabricated onto 165 nm thick silicon-on-insulator (SOI) photodetectors resulted in a dramatic increase in the observed photocurrent. A nearly factor-of-20 photocurrent enhancement was observed for light of wavelength 800 nm, a significant improvement over previously reported results. The improvement is linked to two physical effects: the increased scattering efficiency of the larger nanoparticles and a qualitative change in the resonance characteristics of the metal-island film due to radiative coupling to the SOI waveguide modes. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.122903
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  • 7
    Electronic Resource
    Electronic Resource
    Electrical response of superconducting YBa2Cu3O7−δ to light (1989)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 54 (1989), S. 1175-1177 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have studied the effect of visible and near-infrared light on the electrical properties of superconducting thin films of YBa2Cu3O7−δ. The responses we observe can all be explained in terms of light heating the film to a temperature above the superconducting transition. No evidence that the light directly affects superconductivity was found.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.101483
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  • 8
    Electronic Resource
    Electronic Resource
    Propagation of terahertz bandwidth electrical pulses on YBa2Cu3O7−δ transmission lines on lanthanum aluminate (1989)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 54 (1989), S. 2265-2267 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We study the propagation of terahertz bandwidth electrical pulses on high critical current density c-axis oriented YBa2Cu3O7−δ (YBCO) coplanar transmission lines deposited epitaxially on the low dielectric loss substrate lanthanum aluminate (LaAlO3). The losses on the YBCO transmission line are lower than on an equivalent gold line on the same substrate at temperatures below 50 K. At higher temperatures, absorption and dispersion of the ultrashort electrical pulses are observed in reasonable agreement with Mattis–Bardeen theory.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.101519
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  • 9
    Electronic Resource
    Electronic Resource
    Ion beam induced damage and superlattice formation in epitaxial YBa2Cu3O7−δ thin films (1989)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 54 (1989), S. 1178-1180 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have studied the effect of ion beam irradiation on the microstructure of epitaxial YBa2Cu3O7−δ thin films. The ion beam induced defects are found to cluster in small (〈100 A(ring)) disordered areas. The size and density of the disordered areas are found to increase with the ion fluence. The presence of these small disordered areas can lead to the reduction of phase coherence of the electron pair wave function. Ion beam irradiation is also found to reduce the orthorhombicity of the lattice structure. A new incommensurate superlattice phase due to ion beam induced defect ordering has also been observed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.101484
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  • 10
    Electronic Resource
    Electronic Resource
    Persistent currents in ceramic and evaporated thin-film toroids of Ba2YCu3O7 (1988)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 52 (1988), S. 328-330 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have demonstrated the existence of circumferential persistent currents in Ba2YCu3O7 ceramic toroids at 77 and 4.2 K and in thin films at 77 K. The field dependence of the critical current densities obtained from these persistent current measurements at 77 K is in fair agreement with that obtained from transport measurements. The persistent critical current densities for fields less than 10 G are 150 A/cm2 for the ceramic at 77 K and 105 A/cm2 for the thin film. A long term experiment showed that the resistivity is less than 3×10−18 Ω cm at 77 K.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.99454
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