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  • 11
    Electronic Resource
    Electronic Resource
    Adsorption of polydisperse polymer chains (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2255-2258 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The adsorption of polydisperse ideal polymer chains is shown to be sensitive to the large N tail of the distribution of chains. If and only if the number of chains decays more slowly than exponentially is there an adsorption transition like that of monodisperse infinite chains. If the number decays exponentially the adsorption density diverges continuously at a temperature which is a function of the mean chain length. At low coverages, chains with repulsive monomer–monomer interactions show the same qualitative behavior. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479496
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  • 12
    Electronic Resource
    Electronic Resource
    Raman spectra and calculated vibrational frequencies of size-selected C16, C18, and C20 clusters (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 9652-9655 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The surface plasmon polariton-enhanced Raman spectra of size-selected C16, C18, and C20 clusters isolated in nitrogen matrices are presented along with the calculated vibrational frequencies for the ring and linear chain isomers. The Raman spectra, recorded at a range of excitation wavelengths from 457.9 to 670 nm, show strong resonance enhancement for the three clusters. The calculated vibrational frequencies for ring and linear chain isomers and the cage and bowl structures for C20 are compared to the experimental frequencies. Systematic shifts in the series of peaks in the 200 cm−1 region for C16, C18, and C20 suggest that the observed isomers have the same geometry, thereby ruling out the bowl and cage isomers for C20. The measured spectra appear to be most consistent with the linear chain isomer. This high-energy isomer may be produced during neutralization of the deposited cluster ions. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477632
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  • 13
    Electronic Resource
    Electronic Resource
    The coil–globule transition of polymers of long rigid monomers connected by flexible spacers (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 7477-7482 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A simple model of a polymer with long rigid segments which interact via excluded volume repulsions and short ranged attractions is proposed. The coil–globule transition of this model polymer is strongly first order, the globule is crystalline and the coil which coexists with the globule is swollen. A virial expansion truncated at low order is shown to provide a very poor approximation to the free energy and so a cell theory is used to calculate the free energy of the globule. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474987
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  • 14
    Electronic Resource
    Electronic Resource
    A surface-enhanced hyper-Raman and surface-enhanced Raman scattering study of trans-1,2-bis(4-pyridyl)ethylene adsorbed onto silver film over nanosphere electrodes. Vibrational assignments: Experiment and theory (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 4313-4323 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present theoretical and experimental studies of the infrared (IR) spectroscopy, normal Raman spectroscopy (NRS), surface-enhanced Raman spectroscopy (SERS), and surface-enhanced hyper-Raman (SEHRS) spectroscopy of trans-1,2-bis(4-pyridyl)ethylene (BPE). This centrosymmetric molecule is expected to have no common Raman and hyper-Raman lines provided that it is not strongly perturbed by adsorption on the surface. The measured SERS spectrum, obtained under electrochemical conditions on Ag film over nanosphere (AgFON) electrodes, shows a well defined C=C stretch band that is not seen in the SEHRS spectrum, but many of the other bands overlap closely. We use ab initio calculations for isolated BPE to assign the spectra, and find excellent agreement between the calculated and measured IR and SERS spectra, and good agreement between the calculated and measured SEHRS spectrum. We find that the apparently overlapping IR, SERS, and SEHRS bands are in fact due to modes that have similar vibrational characteristics but different symmetry. Our results indicate that SEHRS spectra are consistent with the expected (three photon) selection rules and intensities. This rules out an alternative mechanism in which the observed spectrum arises from surface second harmonic generation (SHG) followed by SERS excited at the second harmonic frequency. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471241
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  • 15
    Electronic Resource
    Electronic Resource
    Stability of the nematic phase of a mixture of aligned cylinders with respect to the smectic and columnar phases (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 2622-2627 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The theory for the stability of the nematic phase of parallel cylinders due to Mulder [Phys. Rev. A 35, 3095 (1987)] is generalized to binary mixtures. It is shown that adding cylinders of a different length to a one-component nematic phase of cylinders postpones the nematic–smectic A transition, enhancing the range of the nematic. This is a result of the two different length scales each favoring a density modulation of a different wavelength. When the ratio of the lengths of the cylinders is large a cusp appears in the limit of stability of the nematic phase. This cusp corresponds to a large jump in the wavelength of the density modulation from a wavelength characteristic of ordering of the short rods to that characteristic of the longer rods. The cusp is, however, masked by a transition to a columnar phase unless the diameters of the two species of rod are significantly different. The limit of stability with respect to the smectic phase bears a striking resemblance to the phase behavior of a mixture of two mesogens studied some time ago by Sigaud et al. [J. Phys. (Paris) Colloq. 40, C3 (1979)]. Their results have hitherto only been described by Landau theory and via lattice "analogs'' which have no obvious connection with the underlying behavior at the molecular level. In contrast, theories of specific model molecules such as the cylinders studied here offer insight into behavior at the molecular level. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468693
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  • 16
    Electronic Resource
    Electronic Resource
    The gas, liquid, and solid phases of dimerizing hard spheres and hard-sphere dumbbells (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 939-946 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The complete phase diagram of a model associating molecule is determined, including the gas, liquid, and solid phases, the regions of coexistence between these three phases, and the location of the critical and triple points. The model molecule is a hard sphere with two very different attractive interactions, one a short ranged and directional attraction and the other a mean field. The first interaction only forms dimers as a molecule can only interact in this way with one other molecule. This saturable attraction mimics hydrogen and chemical bonding. The second interaction is an approximation for the dispersion forces between molecules. Thermodynamic functions for the liquid and gas phases of this model molecule are obtained from an existing theory for associating fluids but a new theory is developed for the solid phase. This is believed to be the first microscopic theory of a model associating molecule in the solid phase. In the low temperature limit no monomers are present; the system is then a fluid or solid of hard-sphere dumbbells. Simulation data are available in this limit and it is shown that in both the fluid and solid phases the theoretical predictions are close to those of simulation. The pressure equation of state for dumbbells is the most accurate theory available for the solid phase. An approximation for the free energy of a solid mixture of spheres and dumbbells is also presented. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469161
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  • 17
    Electronic Resource
    Electronic Resource
    Thermodynamic perturbation theory for association with bond cooperativity (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 1113-1120 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A simple model of a hydrogen-bonding molecule is considered, each model molecule can bond to two other molecules. This bonding is not, in contrast to previous work, taken to be pairwise additive. The association energy contains a three-body term which either increases or decreases the strength of the bonds in chains of three or more molecules, i.e., it makes the bonding cooperative. Motivation for this form of the intermolecular potential is provided by the behavior of hydrogen fluoride: the bond strength of a hydrogen fluoride dimer is considerably less than the bond strength in chains of three or more molecules. An analytical theory for the free energy of this model molecule is derived via a new approach which is sufficiently general to accommodate the non-pairwise additive interaction. Calculations are performed which show the effect of the cooperative bonding on vapor–liquid coexistence. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471955
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  • 18
    Electronic Resource
    Electronic Resource
    Phase separation in mixtures of a rodlike colloid and two or more rodlike polymers (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 10632-10636 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A suspension of rodlike colloidal particles and rodlike liquid crystalline polymers is modelled as a mixture of thick (colloidal) and thin (polymeric) hard rods. Extensive immiscibility in the fluid phase is observed in the mixtures. For two species of polymers, one species much longer than the other, we observe two demixing critical points. For three polymer species we find three critical points. Polymer molecules of length l induce an attractive interaction of range l between colloidal rods. Two different polymers induce effective attractions of two different ranges. The range of the attraction determines the density at which the demixing occurs. The attractions of different ranges create demixing at different densities. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472949
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  • 19
    Electronic Resource
    Electronic Resource
    Discrete dipole approximation for calculating extinction and Raman intensities for small particles with arbitrary shapes (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 869-875 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a discrete dipole approximation (DDA) method to determine extinction and Raman intensities for small metal particles of arbitrary shape. The Raman intensity calculation involves evaluation of surface electromagnetic fields, and thus is relevant to surface enhanced Raman scattering (SERS) intensities. We demonstrate convergence of the method by considering light absorption and scattering from an isolated spheroid, from an isolated tetrahedron, from two coupled spheroids, and from a spheroid on a flat surface. We also examine comparisons with traditional T-matrix methods. Extensions and simplifications of the method in studies of clusters and arrays of particles are presented. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469787
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  • 20
    Electronic Resource
    Electronic Resource
    Experimental determination of the Compton profile of C60 through binary encounter electron spectroscopy (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 10413-10416 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The method of 0° electron spectroscopy was used to study binary encounter electrons resulting from hard collisions between 1.5 MeV/u C6+ ions and the electrons in a C60 vapor target. The Compton profile of C60 was then extracted from the electron spectra using an impulse approximation treatment. The experimental results are in excellent agreement with theoretical Compton profiles of C60. The C60 Compton profile is compared with that of atomic carbon, as well as those for graphite and diamond. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469889
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