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  • American Institute of Physics (AIP)  (28)
  • American Association for the Advancement of Science (AAAS)
  • 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
    The effect of reagent rotation in the reaction OH( j)+H2( j')→H2O+H (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 7424-7433 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Classical trajectory calculations on the reaction OH( j)+H2( j')→H2O+H have been carried out with j≤40 and j'≤15 on both the Schatz–Elgersma [Chem. Phys. Lett. 73, 21 (1980)] and the Rashed–Brown [J. Chem. Phys. 82, 5506 (1985)] potential energy surfaces. When there is no rotation in the OH, then a plot of reaction cross section, SR( j') resembles that for an atom–diatom system: Just above threshold, rotation decreases reactivity for small j', but increases it for high j'; at higher translational energies this trend is less obvious, but still present. When j'=0, then SR( j) is a complicated function, decreasing for low j, then climbing to a maximum, finally decreasing once more at very large values of j. We have also carried out calculations with isotopically substituted H in OH, and show that these effects scale as the mass of the hydrogen isotope. We show that this behavior is due to artifacts in both the potential surfaces. Using a simple model we are able to rationalize this behavior. Using this same model, together with a potential in which the artifacts are absent, we predict that rotation in OH will decrease reactivity rather less than was reported in previous trajectory studies [Schatz, J. Chem. Phys. 74, 1133 (1981); Rashed and Brown, J. Chem. Phys. 82, 5506 (1985)].
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454306
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  • 5
    Electronic Resource
    Electronic Resource
    The effect of OH rotation on the reaction OH(v=0, j)+H2→H2O+H (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 3698-3699 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reaction cross section for OH(v=0, j≤31)+H2→H2O+H has been calculated using quasiclassical trajectories at E=0.15, 0.20, and 0.30 eV. There is an initial increase in reactivity with j, probably due to long-range R→T transfer. At very high j, the cross section decreases, probably because of disruption of the transition state by the OH rotation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452968
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  • 6
    Electronic Resource
    Electronic Resource
    Optimal annealing schedules for two-, three-, and four-level systems using a genetic algorithm approach (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 7964-7978 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An annealing schedule, T(t), is the temperature as function of time whose goal is to bring a system from some initial low-order state to a final high-order state. We use the probability in the lowest energy level as the order parameter, so that an ideally annealed system would have all its population in its ground-state. We consider a model system comprised of discrete energy levels separated by activation barriers. We have carried out annealing calculations on this system for a range of system parameters. In particular, we considered the schedule as a function of the energy level spacing, of the height of the activation barriers, and, in some cases, as a function of degeneracies of the levels. For a given set of physical parameters, and maximum available time, tm, we were able to obtain the optimal schedule by using a genetic algorithm (GA) approach. For the two-level system, analytic solutions are available, and were compared with the GA-optimized results. The agreement was essentially exact. We were able to identify systematic behaviors of the schedules and trends in final probabilities as a function of parameters. We have also carried out Metropolis Monte Carlo (MMC) calculations on simple potential energy functions using the optimal schedules available from the model calculations. Agreement between the model and MMC calculations was excellent. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481397
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  • 7
    Electronic Resource
    Electronic Resource
    Ultralow noise high gain transimpedance amplifier for characterizing the low frequency noise of infrared detectors (1999)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 70 (1999), S. 1860-1867 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: An ultralow noise transimpedance amplifier with a gain of 1010, a bandwidth greater than 10 kHz, and an input equivalent noise power spectral density of 1.7×10−30 A2/Hz (single sided) for frequencies less than 10 Hz and a level of 5×10−30 A2/Hz at 1 kHz is described. This level of performance is achieved with standard electronic devices, rather than batteries, powering the amplifier. Very good agreement between theoretical and experimental noise measurements is achieved due to careful measurement and modeling of the significant noise sources. It is shown that the noise level of the 1010 Ω feedback resistor increases with frequency and contributes a significant level of noise for frequencies in the kHz frequency range. The usefulness of the amplifier is demonstrated through measurement of the noise of a HgCdTe planar photovoltaic 3–5 μm mid-wavelength infrared detector cooled to 80 and 120 K. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1149681
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  • 8
    Electronic Resource
    Electronic Resource
    Molecular beam scattering measurements of differential cross sections for D+H2(v=0)→HD+H at Ec.m.=1.5 eV (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 6396-6419 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reaction D+H2→HD+H has been studied in a crossed molecular beam scattering experiment at a most probable collision energy of Ec.m.=1.5 eV. Angular and time-of-flight distributions of the HD product have been measured over a wide range of angles. The experimental data are compared with quasiclassical trajectory (QCT) calculations on the LSTH potential hypersurface. The QCT calculations fit the experimental data quite satisfactorily. The sensitivity of the experiment to the different properties of the cross section has been investigated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451470
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  • 9
    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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  • 10
    Electronic Resource
    Electronic Resource
    Optical spectroscopy: An in situ diagnostic for pulsed laser deposition of high Tc superconducting thin films (1989)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 54 (1989), S. 179-181 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: High oxygen partial pressure has been found to be an important parameter for the pulsed laser deposition of as-deposited superconducting thin films with high Tc and Jc. The optical emissions from both elemental and oxide species ejected from the target of YBa2Cu3O7−x during the pulsed laser deposition process increase with the oxygen pressure, with the oxide emission showing a stronger pressure dependence than the elemental emission. The dynamics of the interaction between the oxygen atoms and the species in the laser-produced plume were studied by a wavelength and time-resolved measurement. The results are qualitatively explained using a simple model based on optical emission arising from inelastic and recombination collision between the elemental species and electronically excited oxygen atoms. The formation of oxides in the plume is shown to be essential for the production of higher quality superconducting films, indicating the value of optical spectroscopy as a diagnostic tool.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.101441
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