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  • 1
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    Molecular dynamics study of the mechanical loss in amorphous pure and doped silica (2014)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2014-08-02
    Description: Gravitational wave detectors and other precision measurement devices are limited by the thermal noise in the oxide coatings on the mirrors of such devices. We have investigated the mechanical loss in amorphous oxides by calculating the internal friction using classical, atomistic molecular dynamics simulations. We have implemented the trajectory bisection method and the non-local ridge method in the DL-POLY molecular dynamics simulation software to carry out those calculations. These methods have been used to locate the local potential energy minima that a system visits during a molecular dynamics trajectory and the transition state between any two consecutive minima. Using the numerically calculated barrier height distributions, barrier asymmetry distributions, relaxation times, and deformation potentials, we have calculated the internal friction of pure amorphous silica and silica mixed with other oxides. The results for silica compare well with experiment. Finally, we use the numerical calculations to comment on the validity of previously used theoretical assumptions.
    Print ISSN: 0021-9606
    Electronic ISSN: 1089-7690
    Topics: Chemistry and Pharmacology , Physics
    Published by American Institute of Physics (AIP)
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  • 2
    Electronic Resource
    Electronic Resource
    Transition states of charge-transfer reactions: Femtosecond dynamics and the concept of harpooning in the bimolecular reaction of benzene with iodine (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 5153-5156 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We describe an approach for real-time studies of the transition-state dynamics of charge-transfer reactions. An application to the bimolecular reaction of benzene with iodine is reported. The measured 750±50 fs transient growth of the free I atom product elucidates the nature of the transition state and the mechanism for the dissociative charge-transfer reaction. The mechanism is formulated in relation to the impact geometry and the dative bonding, which are crucial to condense-phase and surface reactions, and is supported by molecular dynamics. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470603
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  • 3
    Electronic Resource
    Electronic Resource
    On the assignment of Jahn–Teller effects in the ultraviolet absorption spectrum of Ag3 (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 6312-6317 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A Jahn–Teller linear-plus-quadratic Hamiltonian is shown to account for most of the observed band positions and intensities in the absorption and emission spectra of Ag3. Coupling parameters obtained for a simultaneous fit to absorption and emission results are k=1.93, g=0.25 for the E' ground state and k=0.19, g=0.02 for the E‘ excited state. At higher vibrational energies, simple Jahn–Teller calculations predict fewer bands than observed. Calculations including spin–orbit coupling with larger linear coupling partially quenched by the Ham effect offer a reasonable explanation for this higher energy structure. Splittings and intensity sharing at lower vibrational energies, however, demand higher-order Jahn–Teller coupling, indicating the need for more extensive calculation simultaneously incorporating quadratic Jahn–Teller effects, spin–orbit coupling, and perhaps anharmonicity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467093
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  • 4
    Electronic Resource
    Electronic Resource
    Femtosecond real-time probing of reactions. XXI. Direct observation of transition-state dynamics and structure in charge-transfer reactions (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 6216-6248 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper in the series gives our full account of the preliminary results reported in a communication [Cheng, Zhong, and Zewail, J. Chem. Phys. 103, 5153 (1995)] on real-time femtosecond (fs) studies of the transition state of charge-transfer (CT) reactions, generally described as harpooning reactions. Here, in a series of experimental studies in a molecular beam, and with the help of molecular dynamics, we elucidate the microscopic elementary dynamics and the structure of the transition states for the isolated, bimolecular reaction of benzenes (electron donor) with iodine (electron acceptor). The transition state is directly reached by fs excitation into the CT state of the complex Bz⋅I2, and the dynamics is followed by monitoring the product build up or the initial transition-state decay. We further employed the fs resolution in combination with the kinetic-energy resolved time-of-flight and recoil anisotropy techniques to separate different reaction pathways and to determine the impact geometry. Specifically, we have studied: (1) the temporal evolution of the transition state (τ
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472478
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  • 5
    Electronic Resource
    Electronic Resource
    Bimolecular reactions observed by femtosecond detachment to aligned transition states: Inelastic and reactive dynamics (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 7864-7867 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: With fs radical detachment and kinetic energy-resolved time-of-flight (KETOF) mass spectrometry, we are able to study the transition state dynamics of the bimolecular reaction CH3I+I, inelastic and reactive channels; the collision complex is coherently formed (1.4 ps) and is long lived (1.7 ps). We also report studies of the dynamics of I2 formation. Direct clocking of the CH3I dissociation, hitherto unobserved, gives 150 fs for the C–I bond breakage time and 0.8 A(ring) for the repulsion length scale. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472606
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  • 6
    Electronic Resource
    Electronic Resource
    Isomeric structures of the electronically excited acetylene⋅Ar complex: Spectroscopy and potential calculations (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 2850-2862 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Acetylene⋅Ar complex in the S1 state has been characterized through laser fluorescence excitation spectra in the acetylene A˜←X˜, 3n0 (n=0–4) bands region. Two isomeric structures have been determined for the acetylene(A˜)⋅Ar complex from rotational band analysis, even though only one structure was known to exist for the ground state acetylene(X˜)⋅Ar. The in-plane isomer has the Ar atom situated in the molecular plane of the trans-bent acetylene, 3.77 A(ring) from the acetylene center of mass and tilted from the H atoms. The out-of-plane isomeric structure, directly inverted from the rotational constants, has argon 3.76 A(ring) away from the acetylene center of mass and 18° tilted from the C2 rotational axis. This structure is most likely due to large amplitude bending motions away from the equilibrium position which is along the C2 axis. Axis switching effect in the rotational band analysis for both isomers has been examined and found to be negligible. (Formulas for calculating the three-dimensional axis switching angles are detailed in the Appendix.) Three van der Waals vibrational mode frequencies have been determined from the vibrational progressions in the spectra; the stretching fundamental of the out-of-plane isomer is 28 cm−1; the in-plane bending fundamental, and the out-of-plane bending first overtone of the in-plane isomer are 11 and 17 cm−1, respectively.The isomeric structures have been compared with the results from a pairwise-atom potential calculation with parameters transferred from the ones previously derived for C2H4⋅Ar potential calculations. It was found that when the set of parameters that most closely reflects the electronic density distribution in C2H2(A˜) orbitals was used, two potential minima mimicking the two isomeric structures were generated. This potential calculation can even qualitatively reproduce the complex spectral shift induced by the ν3 mode excitation in acetylene. Combining the observed spectral shifts and previous experimental and theoretical studies of acetylene(X˜)⋅Ar, we have estimated the binding energy of the out-of-plane C2H2(A˜)⋅Ar isomer to be 179 cm−1, and that of the in-plane isomer to be 170 cm−1. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470723
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  • 7
    Electronic Resource
    Electronic Resource
    Electronic spectroscopy of silver dimer rare gas complexes (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 6249-6256 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrationally resolved electronic spectra are reported for the metal dimer-rare gas complexes Ag2–Ar and Ag2–Kr. These spectra are obtained using resonant two-photon photoionization in the energy region near the Ag2 B←X electronic transition (280–285 nm). Both complexes exhibit extensive activity in three vibrational modes, making it possible to determine vibrational constants, anharmonicities, and cross-mode couplings. An unusual cancellation of factors results in the Kr complex (ω'e =72.6 cm−1) having nearly the same metal-rare gas stretching frequency as the Ar complex (ωe=73.9 cm−1). Progressions extending over a significant range of the excited state potential surfaces make it possible to derive the excited state dissociation energies (D'0=755 and 1205 cm−1 for Ar and Kr, respectively). Combination with the red-shifted electronic state origins yields the corresponding ground state dissociation energies (D(large-closed-square)0=275 and 394 cm−1 for Ar and Kr, respectively). Potential energy surfaces are investigated for excited and ground states of both complexes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461545
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  • 8
    Electronic Resource
    Electronic Resource
    A photoemitted electron-impact ionization method for time-of-flight mass spectrometers (1993)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 64 (1993), S. 2211-2214 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A simple and efficient electron-impact ionization method for time-of-flight mass spectrometers is described. This method utilizes the photoelectrons emitted from the accelerating electrode surfaces upon UV laser irradiation as the electron source. Since no modification of the spectrometer is required, it provides a convenient way to perform electron-impact ionization in a time-of-flight mass spectrometer originally designed for laser ionization. The detection sensitivity (∼109/cm3) and mass resolution (∼150) achieved by this method in our apparatus are sufficient for diagnostic purposes for cluster beam experiments. The observed ion intensities suggest that the photoelectron current density produced is comparable to those generated by conventional thermionic emission electron sources. This technique is generally applicable as an ionization method in time-of-flight mass spectrometers and is particularly useful when the available lasers are not suitable for photoionization detection due to wavelength or intensity limitations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143962
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  • 9
    Electronic Resource
    Electronic Resource
    Reflectron time-of-flight mass spectrometer for laser photodissociation (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 2177-2186 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We describe a new reflectron time-of-flight mass spectrometer configuration for laser photodissociation of mass-selected ions and the initial performance characteristics observed for this instrument. Ions are produced by laser photoionization within the acceleration region of the instrument or by laser vaporization in an external pulsed-nozzle cluster ion source. Mass selection is accomplished with pulsed deflection plates at the end of an initial drift section. Laser photodissociation of selected ions takes place at the turning point in the ion trajectory in the reflectron. The transit time through a second drift section defines the fragment ion masses. Optimized operating conditions and the role of mass discrimination in this instrument are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143135
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  • 10
    Electronic Resource
    Electronic Resource
    Characterization and minimization of flicker in silicon light valves (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 831-837 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have performed systematic characterizations of flicker in silicon light valves. It was found that there were four conduction mechanisms accounting for the flicker. These four mechanisms were residual dc charge on the silicon surface, voltage holding capability of the liquid crystal cell, voltage holding capability of the silicon panel, and parasitic capacitor coupling of the pixel. Major causes of these four mechanisms were identified. Solutions of flicker minimization were obtained for each mechanism. Among these solutions, offset of common voltage was found very useful to compensate for residual dc charge and parasitic capacitor coupling. Frame rate multiplication was found very useful for the minimization of flicker due to low voltage holding capabilities of the liquid crystal cell and silicon panel. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1331073
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