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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 9114-9121 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photoinduced dissociative electron attachment of CH3Br on Pt(111) has been examined by measuring CH3 photofragment translational energy distributions from CH3Br adsorbed on Xe multilayers deposited upon CH3Br covered Pt(111). Subvacuum level, photoexcited substrate electrons produced by 308 nm laser irradiation were found to propagate through the Xe layer where attachment to CH3Br led to fragmentation. A simple model for surface dissociative electron attachment, DEA, was found to quantitatively predict the observed fragmentation dynamics for the CH3Br/Xe/CH3Br/Pt(111) system. The role of the local work function in surface DEA processes was explored by examining the dependence of the photofragment yield upon the thickness of the intermediate Xe layer and the fractional coverage of the Pt(111) bound first layer of CH3Br. Three dimensional variation of the local work function above the substrate was required to account for the observed DEA dynamics.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 9102-9113 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using the formalism of extended irreversible thermodynamics we derive the generalized constitutive equations for the polarization vector and the symmetric and antisymmetric parts of the total stress tensors (the sum of the Maxwell and viscous stress tensors) of a viscoelastic polar fluid. The analysis of these equations shows several interesting features for a permanent dipolar system. The diffusive transport of polarization charge and the diffusion of the transverse polarization component are two elementary physical processes in terms of which complex cases of dielectric relaxation can be described, taking into account both translational and rotational motions of the dipolar particles in a viscoelastic fluid. The calculations for several cases of the complex dielectric susceptibility are presented. In these results, the coupling between polarization and hydrodynamics predicts several modes of dielectric energy dissipation. These various channels are due to the intimate coupling between different degrees of freedom, taken into account by the constitutive equations. The calculation of the complex shear and rotational viscosities complete the presentation.
    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 96 (1992), S. 9138-9143 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Simulations of the molecular dynamics of n-CNH2N+2 in the n-alkane/urea clathrate have been performed for N=13,23,33, for a period of 1 ns. Parallel simulations were performed starting from conformations in which all CH2–CH2 bonds were in the trans conformation, and starting from a conformation in which one of the terminal CH2–CH2 bonds was in a gauche conformation. These simulations suggest less than 3% of the terminal CH2–CH2 bonds would be in a gauche state at equilibrium. The gauche content is much smaller at the CH2–CH2 bonds further removed from the ends of the chain. No significant differences are seen in the simulations performed with n-alkanes with N=13,23,33. The mobility of the C–H bond vectors about the axis defined by the channel is greater at the ends than in the middle of the chain. The angular distribution is nearly randomized in 1 ns for the C–H vector in a methyl group, but a longer time scale would be required for randomization of this distribution for the C–H vector bonded to the central methylene group in C33H68.
    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 96 (1992), S. 9144-9146 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The effects of the finite size of impurities upon the heterogeneous nucleation rate is examined. Simple arguments based upon probability theory are used to find the relative nucleation rate, p( j), on particles containing j nuclei. The expression for p( j) is used in turn to compute the overall nucleation rate and average number of nuclei on an impurity as a function of time.
    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 96 (1992), S. 9133-9137 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A deviation from mean-field behavior is observed in the static susceptibility and correlation length measured with small angle neutron scattering as a function of temperature near the phase boundary of a relatively low molecular weight critical polymer mixture. The possibility of a fluctuation influenced crossover from mean-field to nonmean-field behavior is considered.
    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 96 (1992), S. 9157-9160 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The surface of the polymer mixture is simulated by Monte-Carlo method via condensing the polymer mixture against the solid wall and examining the free surface parallel with the wall. Depending on interactions present in the mixture and on the composition we find three representative situations of surface segregation effect in polymer mixtures described as segregation in, segregation out, and surface double layer. The segregation occurs across a realistic diffusive surface profile in contrast to theoretical approaches to this phenomenon working with a steplike density profile across the surface. Among the advantages of the present simulation, like the presence of compressibility of the studied system, there are also certain limitations, such as the lattice approximation.
    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 96 (1992), S. 9147-9156 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The compressible random phase approximation (RPA) is combined with the lattice cluster theory to study how compressibility and monomer molecular structure affect small angle neutron coherent scattering intensities and quantities like the apparent radius of gyration and correlation length in binary polymer blends. Blend compressibility implies that the partial structure factors Sαβ(k) depend on three macroscopic interaction parameters χαβ, which, in turn, are functions of (at least) three microscopic van der Waals energies, temperature, pressure, polymerization indices, and monomer molecular structures. The {χαβ} are determined from the lattice cluster theory for the extended lattice model of compressible, structured monomer binary blends. The traditional incompressible RPA analysis is applied to our computed Sαβ(k) and uncovers deficiencies in extracting the apparent radius of gyration and correlation length. The calculated structure factor, apparent radius of gyration, and correlation length for polystyrene/polyvinylmethylether blends are compared with the experimental data of Han et al. without using new adjustable parameters.
    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 96 (1992), S. 3240-3250 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structures of the (100) surfaces of silicon and germanium generally have been interpreted in a static manner in the past. We present molecular dynamics (MD) simulations that show these surfaces to consist of a mixture of rapidly interconverting buckled and unbuckled dimers. Over a time average, the surface is found to have long p(2×1) rows of symmetric, unbuckled dimers, as seen in recent scanning tunneling microscopy images of silicon. However, higher order unit cells are observed in He scattering and low energy electron diffraction experiments at low temperatures. We present a dynamical interpretation of the structure to explain both sets of observations. The simulations have been performed on different size slabs at both constant energy and constant temperature utilizing a new method for effective removal of heat from an exothermic system while retaining the correct dynamics. Several different interaction potentials were analyzed in an attempt to find the most realistic one for simulations of these surfaces. The effect of surface defects and annealing were also investigated. The surface phonon densities of states were calculated and for Si(100) are in good agreement with experiments and other theoretical treatments. Such simulations and structural analyses are reported for the first time for Ge(100).
    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 96 (1992), S. 3262-3268 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Metastable and stable phases of water adsorbed at low coverages on Cu(111) were studied using vibrational spectroscopy (both time-resolved and high-resolution electron energy-loss spectroscopies) and temperature programmed desorption. The two water species have distinct spectroscopic signatures and different desorption kinetics. The more stable species appears to be three-dimensional clusters of bulklike, polycrystalline ice while the metastable species was observed to have a higher desorption rate despite a larger activation enthalpy for desorption. A simple model is proposed to explain this intriguing result. The conversion from the metastable to the stable species, as well as the affects of preadsorbed carbon monoxide on the adsorption/desorption kinetics, are also discussed.
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