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  • 1
    Electronic Resource
    Electronic Resource
    Computer simulation of interlayer water in 2:1 clays (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 7434-7445 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Monte Carlo computer simulation has been used to study water confined between the layers of 2:1 clay minerals. The model systems are based on natural Mg and Na smectites. The simulation cells contain one clay layer, 64 water molecules and four magnesium or eight sodium interlayer cations. These atoms and molecules interact with each other through a new set of effective pair potentials, which we discuss. The calculations are conducted in constant (N,p,T) ensembles, at T=300 K and with a uniaxial pressure, p, of 1 M Pa applied normal to the clay sheets. All the molecules, including the clay sheets, are therefore allowed to move during the simulations. The calculated equilibrium layer spacing is 14.7±0.1 A(ring) with interlayer Mg2+ and 14.2±0.1 A(ring) with interlayer Na+. These spacings compare with experimental values of 15.1 A(ring) and 14.5 A(ring), measured for Mg and Na saturated Chambers montmorillonite, at 79% relative humidity. The corresponding densities and average potential energies of the interlayer water molecules are 1.38±0.04 g cm−3 and −17.63±0.02 kcal mol−1, respectively, for Mg smectite and 1.14±0.04 g cm−3 and −11.77±0.02 kcal mol−1, respectively, for Na smectite. We analyze and compare the interlayer structures in the two systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460175
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  • 2
    Electronic Resource
    Electronic Resource
    Nonperturbative approximate solution for Lane–Emden equation (1993)
    College Park, Md. : American Institute of Physics (AIP)
    Journal of Mathematical Physics 34 (1993), S. 4364-4369 
    Details
    ISSN: 1089-7658
    Source: AIP Digital Archive
    Topics: Mathematics , Physics
    Notes: A nonperturbative approximate analytical solution is derived for the Lane–Emden equation using the Adomian decomposition method. The solution is in the form of a power series with easily computable coefficients. The Padé approximants method is used to accelerate the convergence of the power series. Comparison with some known exact and numerical solutions shows that the present solution is highly accurate.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.530005
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  • 3
    Electronic Resource
    Electronic Resource
    Atomistic computer simulation of the clay–fluid interface in colloidal laponite (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 3727-3733 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Monte Carlo and molecular dynamics computer simulations have been used to study the structure and dynamics of the interlayer aqueous solution in a colloidal sodium laponite clay at 277 K. The system studied has a clay–clay spacing of 34.06 Å, and contains 1200 interlayer water molecules and 24 sodium counterions. The density profiles for interlayer species show two distinct layers of surface water as one moves away from the clay particles. The innermost of these layers is strongly oriented to form hydrogen bonds to the surface oxygen atoms. Radially averaged pair distributions have been calculated as a function of distance from the clay surfaces, and show that throughout our system the water structure is significantly perturbed from the bulk. In particular, we observe an increase in the second nearest-neighbor oxygen–oxygen distance, similar to that reported for low-density water at 268 K [A. K. Soper and M. A. Ricci, Phys. Rev. Lett. 84, 2881 (2000)]. The majority of the sodium counterions are fully hydrated by six water molecules. These hydrated ions have a strong tendency to remain close to the solid surfaces, as so-called "outer-sphere" complexes. However, we also observe cations further from the clay sheets, in the diffuse layer. Diffusion of water and cations in the plane of the clay sheets is comparable to that in the bulk, but is significantly reduced normal to the clay sheets. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1343839
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  • 4
    Electronic Resource
    Electronic Resource
    Semiclassical maps: A study of classically forbidden transitions, sub-h structure, and dynamical localization (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 531-544 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Representing the dynamics of a continuous time molecular system by a symplectic discrete time map can much reduce the computational time. The question then arises of whether semiclassical methods can be effectively applied to this reduced description: as in the classical case, the map should prove to be a much more computationally efficient description of the dynamics. Here we study the semiclassical propagation of the standard map, or kicked rotor, based on a Herman–Kluk propagator. This is a very interesting playground to test the feasibility of a semiclassical mapping approach, since it demonstrates a wealth of quantum and classical dynamical behavior: As the kick strength increases, the system goes from being very nearly integrable, through mixed phase space, to chaotic. The map displays phenomena that occur in generic molecular systems, so this study is also a test of how well semiclassics can describe such phenomena. In particular, we discuss (i) classically forbidden transport: the significance of branches of the semiclassical integrand in the complex phase plane must be understood in order for the semiclassics to be meaningful; (ii) sub-h structure: in the nearly integrable regime, the semiclassics can be poor due to the presence of islets of area less than Planck's constant in phase space; (iii) dynamical localization: in the chaotic regime, the classical momentum diffuses, whereas the quantum localizes. Our results show that semiclassics also localizes, and we can confirm directly the theory that dynamical localization is due largely to phase interference. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480545
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  • 5
    Electronic Resource
    Electronic Resource
    X-ray diffraction studies of solutions of lithium in ammonia: The structure of the metal–nonmetal transition (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 2991-2996 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structure of solutions of lithium in ammonia has been studied at 0, 2, 8, and 22 mol % metal (MPM) and 200 K by wide-angle x-ray diffraction. The principal diffraction peak shifts from 2.14(2) Å−1 at 0 MPM to 1.93(3) Å−1 at 22 MPM, reflecting the 30% decrease in overall density as the solution expands to accommodate the excess electrons. We find that the solvent is significantly perturbed over both the short- and intermediate-length scales. The nearest neighbor (N–N) coordination number decreases from 11.8(10) at 0 MPM to 7.6(10) at 22 MPM. In addition, electrostriction around the fourfold coordinated lithium ions causes N–N correlations to become progressively shorter as concentration is increased. At 22 MPM a strong diffraction prepeak is located at 1.05(3) Å−1. Upon dilution to 2 MPM, our experiments find that this feature shifts to 1.29(5) Å−1. We conclude that the prepeak observed in our experiments is a signature of polaronic solvent cavities of approximate radius 2.6 Å. The first solvation shell of an excess electron then contains about 7 ammonia molecules, the second shell about 30 ammonia molecules. This picture is in excellent agreement with interpretation of magnetic resonance data. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1436120
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  • 6
    Electronic Resource
    Electronic Resource
    High-resolution infrared spectroscopic determination of the nν5±n vibrational energies of HCCN and DCCN (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 9331-9339 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The high-resolution infrared kinetic spectra of HCCN and DCCN have been recorded, the former between 3395 and 3481 cm−1 and the latter in the 2517–2593 cm−1 region. A total of four dominant Q branches were observed and have been subsequently assigned to the ν1+2ν5±2←ν5±1, ν1+3ν5±3←2ν5±2 bands of the two isotopomers. The band origins have been determined to be 3420.666(1), 2544.743(2), 3460.912(2), and 2573.480(1) cm−1 respectively. These new values have been used in conjunction with the previously obtained ν1+2ν5±2←2ν5±2 band origins of HCCN and DCCN to obtain 2ν5±2←ν5±1 vibrational energies of 212.822(1) and 133.106(1) cm−1 for HCCN and DCCN, respectively. Semirigid bender calculations of the energy levels of these molecules have been carried out. By adjusting the height of the barrier to linearity, excellent agreement with experimental data has been attained. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1413981
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  • 7
    Electronic Resource
    Electronic Resource
    Far-infrared spectra and two-dimensional potential energy surface for the ring-bending and ring-twisting vibrations of 5,6-dihydro-4H-thiopyran (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 9506-9511 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: 5,6-Dihydro-4H-thiopyran has been synthesized and its far-infrared spectrum has been recorded. Eleven ring-bending bands originating at 120.7 cm−1 and four ring-twisting bands originating at 274.5 cm−1 were observed. Twelve sum and difference bands in the 383–397 and 148–166 cm−1 regions were also observed and these facilitated the construction of a detailed energy map including numerous excited vibrational states of the two coupled vibrations. The two-dimensional potential energy surface, which satisfactorily fits the observed data, was determined to be V=9.48 ×104x4−4.13×104x2+1.37×104τ4−1.82×104τ2+1.10 ×105x2τ2, where x and τ are the bending and twisting coordinates, respectively. The minima on the potential energy surface correspond to twisting angles of ±48° (half-chair conformation). The lowest energy bent (boat) conformation corresponds to a saddle point 1500 cm−1 above the twisted conformation on the potential energy surfaces, and the barrier to planarity was estimated to be 6000 cm−1. Both of these values have large uncertainties since the vibrational data only extend to 800 cm−1 above the potential surface minimum. The relatively low bending energy and high barrier to planarity can both be explained by the low force constant for the C–S–C angle bending. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468820
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  • 8
    Electronic Resource
    Electronic Resource
    Time-resolved doubly bent crystal x-ray spectrometer (1988)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 59 (1988), S. 1822-1824 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: X-ray spectroscopy is an essential tool in high-temperature plasma research. We describe a time-resolved x-ray spectrometer suitable for measuring spectra in harsh environments common to many very high-energy density laboratory plasma sources. The spectrometer consisted of a doubly curved Si(111) crystal diffraction element, a WL-1201 (ZnO:Ga) phosphor, a coherent fiber-optic array, and two visible streak cameras. The spectrometer design described here has a minimum time resolution of 1.3 ns with 2.8-eV spectral resolution over a 200-eV-wide bandpass in the 6–7-keV region of the spectrum. Complete system spectral throughput calibrations were done at the Cornell High Energy Synchrotron (CHESS). Details of the design and calibration results are presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1140071
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  • 9
    Electronic Resource
    Electronic Resource
    The structure of interlayer water in vermiculite (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 5751-5760 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Neutron diffraction, in conjuction with substitution of deuterium (D) for hydrogen (H), has been used to determine the structure of interlayer water in sodium- and nickel-substituted vermiculites. We have measured the intensities of the (00l) Bragg reflections as a function of relative humidity and H/D content, up to l=30. Difference analysis has then been used to obtain separate density profiles, ρ(z), for both the hydrogen atoms and the oxygen atoms plus the clay sheets. Ni–vermiculite was studied at 84% relative humidity, while Na–vermiculite was studied at both 88% and 30% relative humidity. At these values the layer spacings are 14.40, 14.96, and 11.78 A(ring), respectively. We find that each interlayer nickel ion is coordinated octahedrally to 6.0 water molecules. All of these water molecules are oriented to form a strong hydrogen bond to the adjacent clay surface. We also find that extra water is located close to the clay layers. This additional water is situated within the hexagonal rings of SiO4 and AlO−4 tetrahedra, which comprise the clay surfaces. In the 14.96 A(ring) phase of Na–vermiculite there are an average of 4.9 interlayer water molecules per cation. About half of these water molecules are oriented to form a hydrogen bond to one of the clay surfaces. Additional water is found close to the clay surface, occupying the same hexagonal ring sites as in 14.40 A(ring) Ni–vermiculite. In the 11.78 A(ring) phase of Na–vermiculite there are an average of 2.1 water molecules per interlayer cation. The oxygen atoms of these interlayer water molecules are found close to halfway between the layers, while the hydrogen atoms are directed towards one of the adjacent clay sheets.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460457
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  • 10
    Electronic Resource
    Electronic Resource
    Measurement of mass diffusion coefficients using nonexponential forced Rayleigh scattering signals (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 267-274 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Recent reports on mass-diffusion forced Rayleigh scattering (FRS) experiments have emphasized that the detected signal arises as a difference between two exponentially decaying fields diffracted from "complementary" ground-state and photoproduct population gratings. A mass-diffusion coefficient has nevertheless often been extracted by forcing a single-exponential fit to the data, especially in cases where the decay appears to be monotonic. In this paper, we use simulations and experiments to evaluate the accuracy of single-exponential fits for FRS profiles, and we propose a useful alternative method for obtaining a meaningful rate constant in cases in which the error in the single-exponential analysis is large. We begin by noting from the complementary grating model that (1) severe deviations from single-exponential decay can occur for an arbitrarily small (but nonzero) difference in the ground-state/photoproduct rate constants, and (2) the first cumulant of a FRS decay—in contrast to that of a dynamic light scattering profile—does not (in general) represent a physically useful decay rate. These statements apply to both monotonic and nonmonotonic decays. We then show that a combination of the first two FRS cumulants provides a physically useful mean rate constant. Finally, to address these issues experimentally, we have reexamined the diffusion of methyl red (MR) through 2-propanol at room temperature, a system previously analyzed using single-exponential fits. The new experiments, carried out at higher sensitivity than the previous studies, show that the MR/2-propanol signal is nonmonotonic. The geometric-mean diffusion coefficient obtained from the curvature of the local maximum is compared to the diffusion coefficient inferred from single-exponential fits, and it is found that the latter is larger by nearly a factor of 2. The results reported here should prove important in improving the accuracy of the FRS technique. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476560
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