Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session V34: Condensed Phase Dynamics, Structure and Thermodynamics |
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Sponsoring Units: DCP Chair: Bruce Garrett, PNNL Room: LACC 511A |
Thursday, March 24, 2005 11:15AM - 11:27AM |
V34.00001: Time-resolved electron diffraction measurement of laser-induced solid-solid transition of \textit{La}$_{1-x}$\textit{Sr}$_{x}$\textit{MnO}$_{3}$ Hyuk Park, Shouhua Nie, Xuan Wang, Rick Clinite, Jim Cao For a limited range of compositions (x), \textit{La}$_{1-x}$\textit{Sr}$_{x}$\textit{MnO}$_{3}$ is known to undergo a reversible solid-solid structural phase transition from an orthorhombic to a rhombohedral with increasing temperature or external magnetic field. The direct and real time monitoring of this structural change provides a unique mean to uncover the transition mechanism and pathways. By initiating this structural change with fs optical excitations, the timescale and structural kinetics were directly monitored with femtosecond electron diffraction. The transition time was found to be several hundred ps, depending on the doping level. Based on our observations on the temporal evolutions of the diffraction pattern, including the Bragg peak position, width and intensity, the transition is most likely thermal driven. [Preview Abstract] |
Thursday, March 24, 2005 11:27AM - 11:39AM |
V34.00002: Three dimensional phase behaviour of core-softened potentials David Quigley, Matt Probert Interest in core-softened pair potentials originates in the work of Stell and Hemmer which proposed that such models may produce a liquid-liquid phase transition in a single component system. In this work we concentrate of the `shoulder' form of core-softened potential, constructed as a superposition of the Lennard-Jones potential and an outer Gaussian minimum. This potential has been subject to much study in two dimensions, in which various liquid anomalies have been reported. The behaviour of these models in three dimensions is unclear, with previous studies limited to small or meta-stable regions of the phase diagram. We have mapped the full stable phase diagram in three dimensions for a family of core softened potentials as a function of the outer well depth, using a variety of computational techniques. In the solid, we note a replacement of fcc structure by simple hexagonal with increasing outer well depth, with further phase transitions at high pressure. Trends in melting temperature and the location of the critical point are also explored. We then search for anomalies over the full thermodynamically stable liquid, paying particular attention to the region close to the melting temperature. [Preview Abstract] |
Thursday, March 24, 2005 11:39AM - 11:51AM |
V34.00003: Energy Transfer Dynamics in Novel Macrocyclic Polymers Jerainne Johnson, Rong Chen, Thieo Hogen-Esch, Stephen Bradforth We have characterized the spectroscopy and dynamics of a novel molecular architecture that mimics natural light harvesting. Macrocyclic poly (9,9- dimethyl-2-vinylfluorene) PDMVF has chromophores attached to the polymer backbone that are arranged in a similar topology to the biological design in light harvesting antenna LH2. The polymer repeat can be tuned from 12 -- 142 identical chromophore units. Ultrafast depolarization and exciton annihilation (EA) techniques have been used in analysis and provide complementary evidence of the occurrence of energy transfer (ET). Our results predict Forster type hopping on $\sim $1 ps timescale. Direct evidence of ET is manifested in the depolarization, where the decay time constants are substantially faster than those obtained for reorientation of the free chromophore in solution. The time constants obtained are 700 - 900 fs and 7 - 8 ps. We speculate that the fast component results from ET dynamics with distinct bi-exponentiality due to conformational disorder. EA is a more direct way of measuring ET, since knowledge of the polymer conformation is not required. Isotropic transient absorption signals for PDMVF show strong dependence on excitation intensity. The population decay is due to S$_{1}$-S$_{1}$ EA lifetimes in the polymers, the amplitude of which depend on ring size, while an additional component at higher intensities corresponds to higher order annihilation dynamics. [Preview Abstract] |
Thursday, March 24, 2005 11:51AM - 12:03PM |
V34.00004: Chemical Kinetics at the Critical Point of Solution James Baird, Yeong Kim We have measured the rate of decomposition of acetone dicarboxylic acid in a mixture of isobutyric acid + water near its consolute point. At temperatures close to the critical solution temperature, the rate constant oscillated in magnitude by about 10 percent as it passed through two complete cycles of slowing down followed by speeding up. This is to be compared with our observations of the rates the SN1 hydrolysis reactions of 2-chloro-2-methylbutane in isobutyric acid + water and 2-bromo-2-methylpropane in triethylamine + water near their respective consolute points. In both mixtures, we observed a single cycle of slowing down above the critical temperature followed by speeding up above it [J. Phys. Chem. A 107, 8435 (2003)]. Whereas we can find no ready explanation for the speeding up, we suggest that because none of these mixtures contains any inert components, the observed slowing down should belong to the Griffiths-Wheeler class of strong critical effects [Phys. Rev. A 2, 1047 (1970)]. To check this hypothesis, we measured the rate of decomposition of ethyl diazoacetate in isobutyric acid + water near the critical solution temperature. In this mixture, isobutyric acid is inert, and no critical effect was observed. [Preview Abstract] |
Thursday, March 24, 2005 12:03PM - 12:15PM |
V34.00005: Light Scattering Study of a fluorinated Alkyl Methacrylate Polymer in Carbon Dioxide Ji Guo, Joseph M. Desimone, Michael Rubinstein The solution properties of fluorinated homopolymers in liquid and supercritical carbon dioxide were studied by light scattering. Poly(fluoroalkyl methacrylate) samples were fractionated by carbon dioxide (CO$_{2})$ to achieve narrow polydispersity and the refractive index of each sample in CO$_{2}$ was measured with a high pressure optical setup. Molecular weight, size, and interaction parameters of the polymer dissolved in CO$_{2}$ were studied as a function of temperature and CO$_{2}$ density using both static and dynamic light scattering. The solvent quality of CO$_{2}$ was shown to quantitatively improve with temperature and CO$_{2}$ density. We observed both $\theta $-temperature and $\theta $-density for poly(fluoroalkyl methacrylate) in CO$_{2}$. The hydrodynamic radius was found to increase with the temperature and density of CO$_{2}$. We demonstrate that the second virial coefficient of the polymer in CO$_{2}$ can be expressed in terms of the universal interaction parameter in the good solvent regime. This confirms that polymers in CO$_{2}$ have the same universal behavior as in organic solvents. [Preview Abstract] |
Thursday, March 24, 2005 12:15PM - 12:27PM |
V34.00006: Vibrational energy relaxation of water in Aerosol OT reverse micelle Yoonsoo Pang, Zhaohui Wang, John Deak, Dana Dlott An IR-Raman technique with mid-IR pump and anti-Stokes Raman probe is used to investigate reverse micelle mixture of Aerosol OT, water, and carbon tetrachloride, where polar water phase and nonpolar oil phase is separated by a monolayer of surfactant molecules. Anti-Stokes Raman scattering is only dependent on the population of vibrationally excited states, thus time-dependent population changes of parent/daughter vibrations can be monitored with this technique. Vibrational energy from nanodroplet of water is transferred to the surfactant head group in 1.8 ps and then out to solvent in 10 ps. Vibrational energy directly pumped into the surfactant tail group results in a slower 20-40 ps energy transfer to solvent. This energy transfer cannot be explained by ordinary heat transfer, but the specific vibrational energy relaxation pathway such as sulfonate stretch of surfactant molecules should be used. We can change the water-to-solvent energy transfer rate by adopting different size of reverse micelles or changing pump frequency over the broad OH stretch mode of water due to hydrogen bond network. Water molecules confined in nanometer scale reverse micelles have very different properties from bulk water and we have found many differences between the vibrational dynamics of water in these reverse micelles and those of bulk water. [Preview Abstract] |
Thursday, March 24, 2005 12:27PM - 12:39PM |
V34.00007: Femtosecond x-ray measurements of inertial atomic-scale motion Aaron Lindenberg, Kelly Gaffney, Jerry Hastings, Jorgen Larsson, Ola Synnergren, Klaus Sokolowski-Tinten, Jonathan Sheppard, Christian Blome, Carl Caleman Using a new source of femtosecond x-rays at the Stanford Linear Accelerator Center, we present new measurements of the first step in the ultrafast transition from solid to liquid. We show that the transition is predominantly inertial in character and point to analogies with intrinsic dynamics in the liquid state. [Preview Abstract] |
Thursday, March 24, 2005 12:39PM - 12:51PM |
V34.00008: The Effect of Substrate Hydrophobicity in Ink Transport During Dip-Pen Nanolithography Jennifer R. Hampton, Arrelaine A. Dameron, Rachel K. Smith, Paul S. Weiss We have investigated the transport mechanism of the inks typically used in dip-pen nanolithography by patterning both hydrophilic and hydrophobic thiol inks on the same Au\{111\} substrate. The use of two inks with opposite contrast in lateral force microscopy images allows visualization of the later-patterned ink with respect to a previously-patterned structure. When hydrophobic ink is written on top of a pre-existing hydrophilic structure, the second ink is observed at the outsides of the hydrophilic structure. However, in the reverse case, the hydrophilic ink displaces the previously patterned hydrophobic ink. This striking difference highlights the important role substrate hydrophobicity plays in determining the transport properties of thiol inks in dip-pen nanolithography. [Preview Abstract] |
Thursday, March 24, 2005 12:51PM - 1:03PM |
V34.00009: A density functional approach for ab initio calculations in the presence of a solvent Jean-Fran\c cois Bri\`ere, Sahak Petrosyan, David Roundy, Tom\'as Arias The study of a system in contact with liquid water typically involves a large number of atoms and averaging over a large number of configurations, making ab initio study of such systems prohibitive. This talk presents a formally exact way to circumvent this problem by separating the free energy of the system into three parts: a Kohn-Sham theory for the solute electron density, a classical density functional theory for the water molecular density, and a free energy of interaction between these two systems. We shall present various approximations for the interaction functional within this new joint density functional theory (JDFT) framework. [Preview Abstract] |
Thursday, March 24, 2005 1:03PM - 1:15PM |
V34.00010: Terahertz Spectroscopy of Hydrogen-Bonded Molecular Complexes in Solution Timothy Korter, Anna Joseph Hydrogen bonding is ubiquitous in nature and governs a wide array of chemical and biological processes. Although the hydrogen bond is well studied, its low-frequency vibrations -- the large-amplitude motions involving stretching and bending along the actual hydrogen-bond coordinates -- have been rarely investigated. These vibrations largely fall in the terahertz (THz) or far-infrared region (0.1-6 THz). Here we present pulsed THz spectroscopic investigations of intermolecular hydrogen bonding in several molecular clusters in non-aqueous solutions. In order to access spectral data above 3 THz, a pulsed THz spectrometer utilizing gallium phosphide (GaP) crystals was constructed to provide $>$6 THz of continuous spectral bandwidth. Molecular systems to be discussed include the mixed dimeric systems of phenol:pyridine and 4-fluorophenol:pyridine, as well as the phenol trimer. Experiments will also be presented on the 7- azaindole dimer which serves a model system for the phenomenon of excited- state double-proton transfer. In all cases, density functional theory calculations were used to assign the observed vibrational features to specific vibrational motions of the complexes. Finally, preliminary optical-pump-THz-probe experiments concerning the excited-state dynamics of these molecular complexes will be discussed. [Preview Abstract] |
Thursday, March 24, 2005 1:15PM - 1:27PM |
V34.00011: Atomistic simulations of surface segregation in bimetallic Pt-M catalyst nanoparticles (M=Ni, Re, Mo) Guofeng Wang, M.A. Van Hove, P.N. Ross, M.I. Baskes To design Pt bimetallic catalyst nanoparticles for their applications in fuel cells, we have developed interatomic potentials within the modified embedded atom method for Pt-Ni, Pt-Re, and Pt-Mo alloy systems. These potentials were used in an investigation of the segregation of Pt atoms to the surfaces of nanoparticles with sizes from 2.5 nm to 5 nm. We draw the following conclusions from our Monte Carlo simulations at T= 600 K. Due to surface segregation, (1) Pt-Ni nanoparticles form a surface-sandwich structure in which the Pt atoms are strongly enriched in the outermost and third layers while the Ni atoms are enriched in the second layer; (2) the equilibrium Pt-Re nanoparticles adopt a core-shell structure, with a highly Pt-enriched shell surrounding a Pt-Re core; and (3) Pt slightly segregates to the surface of Pt-Mo nanoparticles and preferentially occupies the facet sites in the surfaces. This work was supported by the U.S. Department of Energy under Contract No. DE-AC03-76SF00098 at LBNL and W-7405-ENG-36 at LANL. [Preview Abstract] |
Thursday, March 24, 2005 1:27PM - 1:39PM |
V34.00012: Platinum and platinum hydroxide nanomaterials by laser ablation in liquids William Nichols, Takeshi Sasaki, Naoto Koshizaki Recently, laser ablation has been extended to solid targets immersed in liquids to enhance the variety of reactions possible as well as to use the rapid quenching in the solution to stabilize metastable phases. To control the ablation and reaction conditions it is imperative to have an understanding of how the laser parameters effect the nanomaterial formation. Here, we report the effect of wavelength and fluence on the ablation mechanism and chemical reactivity during laser ablation of a platinum target immersed in water. Results suggest the ablation mechanism can be changed among thermal, explosive and reactive sputtering by choice of fluence. Further, control of the reactivity of ablated species in the water is also possible through proper selection of laser parameters. We discuss the formation of platinum hydrogels through reactive quenching of the laser-ablated species by the water followed by crystallization into platinum oxide based nanomaterials. [Preview Abstract] |
Thursday, March 24, 2005 1:39PM - 1:51PM |
V34.00013: H-bonding network and the far Infrared spectrum of liquid water Manu Sharma, Raffaele Resta, Roberto Car We have performed Car-Parrinello molecular dynamics simulation based on maximally localized Wannier functions on a computational sample of liquid water containing 64 molecules in a cubic supercell. Using the Wannier function centers along the molecular dynamics trajectory, we have calculated the Infrared spectrum of liquid water and analyzed in detail the far Infrared region of the spectrum. We have devised several correlation functions that allow us to ``cleanly'' isolate the features of the spectrum in the far Infrared region. We observe that the $\sim $200 cm$^{-1}$ peak in the Infrared spectrum arises due to dynamical charge transfer between a water molecule and its nearest neighbors. Furthermore, we provide evidence that this peak is strongly anisotropic in nature and is Infrared active only when the local H-bonding environment in liquid water changes asymmetrically. [Preview Abstract] |
Thursday, March 24, 2005 1:51PM - 2:03PM |
V34.00014: Probe and control of coherent lattice motions using femtosecond electron diffraction Shouhua Nie, Hyuk Park, Xuan Wang, Rick Clinite, Jim Cao Coherent phonon control provides a mean to directly control the lattice dynamics and drive the lattice into novel and non-equilibrium states, which are not accessible by the conventional thermal excitations. Previously, this technique has been mostly used with fs optical spectroscopy as an indirect probe of lattice motions. Recent advances in time-resolved X-ray diffraction have provided a capability of more direct measurement by monitoring the corresponding diffraction intensity change, albeit with the limitation of probing only single Bragg peak. Here we report a direct and real-time probe and control of coherent lattice motions with fs electron diffraction. By recording a complete diffraction pattern, the lattice motions can be directly measured with sub milli-{\aa}ngstr\"{o}m spatial resolution on the fs time scale, which make it possible to determine the lattice vibrational modes without any ambiguity. In addition, by using a sequence of excitation pulses, we were able to control both the amplitude and phase of coherent lattice motions. [Preview Abstract] |
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V34.00015: Nonlinear Orientational Response of Permanent Dipoles in a Mean Field Potential to Alternating Fields William Coffey, Yuri Kalmykov, Derrick Crothers, Pierre-Michel D\'ejardin It is shown how the existing theory of the dynamic Kerr effect and nonlinear dielectric relaxation based on the noninertial Brownian rotation of noninteracting rigid dipolar particles may be generalized taking into account interparticle interactions using a mean field potential. The results (available in simple closed form) suggest that the frequency dependent nonlinear response provides a new method of measuring the Kramers escape rate (or in the analogous problem of magnetic relaxation of fine single domain ferromagnetic particles, the superparamagnetic relaxation time). [Preview Abstract] |
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