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Stuart A Rice

Publications and source records attributed to Stuart A Rice.

At least 19 recordsLinked to original sources
PubMed2005

Melting of quasi-two-dimensional crystalline Pb supported on liquid Ga.

Experimental studies have shown that the Pb monolayer that segregates in the PbGa alloy liquid-vapor interface forms a two-dimensional hexagonal crystal that melts at 341 K, and it has been speculated that the disordered phase formed is hexatic. This paper reports the results of simulation studies of the in-plane structure of the outermost stratum of the liquid-vapor interface of a dilute Pb in Ga alloy. These simulations are based on four major improvements to a previous study. First, the simulation studies involve considerably more atoms and considerably longer equilibration runs than considered in the previous work of Chekmarev, Oxtoby, and Rice. Second, a more accurate nonlocal pseudopotential representation of the interactions in the system is used. Third, the amplitude of the out-of-plane motion of the Pb atoms is constructed to have the observed value. Fourth, an approximation to the role of the liquid Ga substrate is provided by adding a layer of Ga atoms to the layer of Pb atoms. The results of our simulation studies show that the Ga layer adjacent to the Pb layer has a profound influence on that layer's properties. In particular, it is shown that in the two-layer PbGa system the Pb layer forms, at low temperature, a stable two-dimensional crystal on top of liquid Ga. This two-dimensional crystal melts at a temperature close to that found experimentally. It is found that the crystalline Pb layer is transformed to the liquid state via two intermediate hexatic phases that differ in the magnitude of the bond orientation order. Each of the phase transitions along this melting pathway is first order. The temperature range over which each hexatic phase is stable is small. The profound influence of out-of-plane motion is demonstrated by a comparison of the results of simulations of a quasi-two-dimensional (Q2D) and of a strictly two-dimensional monolayer of Pb. The melting transition in the Q2D one-layer system is first order, directly to the liquid, with no intervention of a hexatic phase. The melting transition in the strictly 2D system involves two stages: a first-order transition to an intermediate hexatic phase followed by melting of the hexatic to a liquid phase. The latter transition is continuous over a small temperature range. An examination of the role of defects in the melting process reveals a picture rather different from that postulated in the Kosterlitz-Thouless-Halperin-Nelson-Young theory of 2D melting.

Journal Article↗
PubMed2005

Influence of hydrodynamic coupling on pair diffusion in a quasi-one-dimensional colloid system.

The effect of hydrodynamic interaction on the separation dependence of the center of mass and relative pair diffusion coefficients of colloid particles in a quasi-one-dimensional system, including the influence of proximate walls, is calculated using the method of reflections. There is excellent agreement between the theoretical predictions and the experimental data. We show that the separation dependence of the relative pair diffusion coefficient has an oscillatory structure on the scale length of the correlation length in the system, and we directly relate that oscillatory structure to the pair correlation function of the system.

Biopolymers↗
PubMed2005

A theoretical study of the structure of the liquid Ga-diamond (111) interface.

We present the results of a computer simulation study of the structure of the interface between liquid Ga and the (111) face of diamond, with which we reinterpret the findings from an x-ray reflectivity study of that interface [W. J. Huisman, J. F. Peters, M. J. Zwanenburg, S. A. de Vries, T. E. Derry, D. Abernathy, and J. F. van der Veen, Nature (London) 390, 379 (1997); Surf. Sci. 402-404, 866 (1998)]. That experimental study has been interpreted to show that the contact of Ga with the (111) face of diamond induces the formation of Ga(2) molecules for several layers into the bulk liquid, with the axes of the Ga(2) molecules in successive layers oriented perpendicular to the diamond surface. No driving force for the proposed formation of Ga(2) molecules is identified. The simulations reported in this paper are based on a model that permits chemical binding of Ga, as a dimer, to the C=C double bonds in the reconstructed (111) face of diamond, thereby identifying the driving force for dimerization. We show that an isolated pi complex with the Ga(2) axis perpendicular to the C=C double bond is stable. We then modify the pseudopotential-based self-consistent Monte Carlo simulation scheme for describing inhomogeneous liquid metals, using the calculated potential-energy surface of Ga(2)(C=C) in the region close to the diamond surface. In this model only the Ga adjacent to the diamond is composed of dimers. The interfacial density distribution obtained from the simulations predicts an x-ray reflectivity that is in good agreement with that observed.

Journal Article↗
PubMed2005

Anomalous behavior of the depletion potential in quasi-two-dimensional binary mixtures.

We report an experimental determination of the depletion interaction between a pair of large colloid particles present in a binary colloid mixture that has a high density of large particles and is tightly confined between two parallel plates, as a function of the small colloid particle density. The bare interaction between the large particles in the one component large colloid suspension, and the effective potential between the large particles in the binary colloid suspension represented as a pseudo-one-component fluid, were obtained by inverting the Ornstein-Zernike equation with the hypernetted chain closure. The depletion interaction is defined by subtracting the bare potential from the effective potential at fixed large colloid density. We find that the depletion potential in the quasi-two-dimensional (Q2D) system is purely attractive and short ranged as described by Asakura-Oosawa model. However, the depth of the depletion potential is found to be almost an order of magnitude larger than the counterpart depletion potential predicted for the same density and diameter ratio in a three-dimensional system. Although it is expected that the confining walls in the Q2D geometry enhance the excluded volume effects that generate entropic attraction, the observed enhancement is much larger than predicted for a Q2D binary mixture of hard spheres. We speculate that this anomalously strong confinement-induced depletion potential is a signature of characteristics of the real confined binary colloid mixture that are not included in any extant theory of the depletion interaction, specifically the omission of the role of the solvent in those theories. One such characteristic could be differential wall or particle wetting that generates a wall induced one-particle effective potential that confines the centers of the small particles to lie closer to the midplane between the walls than expected from the wall separation and the direct particle-wall interaction, thereby enhancing the depletion interaction.

Journal Article↗
PubMed2005

Depletion interaction in a quasi-two-dimensional binary colloid mixture: Monte Carlo simulations.

We report the results of extensive calculations of the depletion interaction between the large hard spheres in a quasi-two-dimensional (q2D) binary mixture of large and small hard spheres. Two definitions of the depletion interaction have been examined, and the dependencies of both on the large and small sphere densities, and the confining wall separation have been explored. The results of the simulations show that the depletion interaction is enhanced relative to its magnitude in a three-dimensional binary mixture with the same density, composition and sphere diameter ratio and that it has a complex dependence on the large sphere-large sphere separation. There are qualitative differences between the properties of q2D and mathematical 2D systems that are relevant to the interpretation of experimental data.

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PubMed2005

Phase transitions in the liquid-vapor interface of dilute alloys of Bi in Ga: new experimental studies.

We report the results of measurements of x-ray reflectivity and grazing incidence x-ray diffraction from the liquid-vapor interfaces of four dilute alloys of Bi in Ga with mole fractions x(Bi)=0.0032, 0.0023, 0.00037, and 0.000037. The monolayer coverage of the alloys with x(Bi)=0.0023, and x(Bi)=0.00037 is about 0.85 and only very slightly temperature dependent. The monolayer coverage in the lowest-concentration alloy, with x(Bi)=0.000037, ranged from 0.82 at 29 degrees C to 0.58 at 110 degrees C. In none of these alloys, down to the lowest temperature used, 29 degrees C, can we find any evidence for crystallization of the Bi monolayer that segregates as the outermost stratum of the liquid-vapor interface. Drawing on theoretical arguments we propose that the transitions inferred from the second-harmonic generation and plasma generation studies of dilute Bi in Ga alloys are from the liquid state to the hexatic state of the Bi monolayer. The data for the alloy with x(Bi)=0.000037 suggest that near 80 degrees C there is a disordered phase-to-disordered phase transition.

Journal Article↗
PubMed2005

From random walk to single-file diffusion.

We report an experimental study of diffusion in a quasi-one-dimensional (q1D) colloid suspension which behaves like a Tonks gas. The mean squared displacement as a function of time is described well with an ansatz encompassing a time regime that is both shorter and longer than the mean time between collisions. The ansatz asserts that the inverse mean squared displacement is the sum of the inverse mean squared displacement for short time normal diffusion (random walk) and the inverse mean squared displacement for asymptotic single-file diffusion (SFD). The dependence of the 1D mobility in the SFD on the concentration of the colloids agrees quantitatively with that derived for a hard rod model, which confirms for the first time the validity of the hard rod SFD theory. We also show that a recent SFD theory by Kollmann leads to the hard rod SFD theory for a Tonks gas.

Journal Article↗
PubMed2005

Controlled subnanosecond isomerization of HCN to CNH in solution.

We report a study of control of the HCN-->CNH isomerization in a liquid Ar solution. We show, using molecular dynamics simulations, nearly complete conversion from HCN to CNH can be achieved in solution on the subnanosecond time scale without requiring laser pulse shaping or molecular alignment. The mechanism of the isomerization reaction involves multiphoton rovibrational excitation on the ground electronic state potential energy surface coupled with rapid rovibrational relaxation in solution. The results demonstrate the important role of rotation-vibration coupling in multiphoton excitation of small molecules and constitute the first realistic computational demonstration of fast, robust, and high-yield laser field manipulation of solution-phase molecular processes.

Journal Article↗
PubMed2005

Influence of polydispersity on the effective interaction in a quasi-two-dimensional pseudo-one-component colloid fluid.

We report the results of simulations of one-component and binary quasi-two-dimensional polydisperse hard sphere colloid fluids. When the polydisperse one-component system is regarded as an effective monodisperse one-component system, our results show that polydispersity gives rise to effective pair attraction and soft repulsion. These effective interactions depend on both the degree of polydispersity and the system density. At low density only the effective soft repulsion survives and the effective attraction becomes negligible. For the binary system, the depletion interaction develops extra features due to polydispersity, specifically a repulsive interaction with range twice the average particle diameter. At low density this extra feature vanishes. To carry out our study we have devised a method for continuous sampling of the polydisperse mixture according to a prescribed particle diameter distribution.

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PubMed2005

Depletion interaction in a quasi-two-dimensional colloid assembly.

We address several aspects of the character of the depletion interaction in a quasi-two-dimensional (Q2D) colloid system. First, we consider how, given information concerning the pair and triplet correlation functions, the depletion interaction can be efficiently and accurately determined. For this purpose we introduce a method based on the Born-Green equation using the assumption that for the Q2D binary mixtures of interest to us the depletion interaction is accurately represented as a sum of pair potentials. We then verify, by direct calculation, that three-particle contributions to the depletion interaction are negligibly small in the region of thermodynamic state space of interest to us. Second, we develop a representation of the dependence of the depletion interaction in a Q2D colloid system on the thickness of the confining parallel plates. Third, we report the results of extensive simulations of Q2D binary hard-sphere mixtures for a range of cell thickness, large and small particle number densities, and a ratio of sphere diameters q=0.3 .

Journal Article↗
PubMed2005

Isomerization and dissociation dynamics of HCN in a picosecond infrared laser field: a full-dimensional classical study.

We report a full-dimensional study of the classical dynamics of HCN-->HNC isomerization and of HCN rovibrational dissociation driven by a strong but nonionizing picosecond infrared laser field. The dynamics of the isolated molecule and of the molecule in liquid Ar have both been studied. Our theoretical and numerical results show that when all degrees of freedom are accounted for the field induced molecular dynamics can be totally different from what was found in previous studies, where the HCN molecule is restricted to a plane containing the external field. It is shown that as HCN is driven by an infrared laser field, the rotation of the H atom around the C-N bond provides an important and highly efficient energy absorption mechanism. In the presence of a monochromatic picosecond infrared laser field with an intensity of 10(13) W/cm(2), this energy absorption mechanism generates considerable HCN-->HNC isomerization yield or high rovibrational dissociation yield without molecular preorientation or prealignment. Our study of the field induced isomerization and dissociation dynamics of the same system in liquid Ar shows that the picosecond isomerization dynamics is insignificantly affected by the surrounding atomic liquid whereas the dissociation yield may be greatly suppressed in a high density liquid. The implications of this study for full-dimensional quantum dynamics of multiphoton rovibrational excitation and dissociation of triatomics are briefly discussed.

Journal Article↗
PubMed2005

Influence of hydrodynamic coupling on the density dependence of quasi-one-dimensional diffusion.

The effect on the short time one particle diffusion coefficient of hydrodynamic interaction between pairs of colloid particles, and between colloid particles and the walls of a quasi-one-dimensional cylindrical channel, are calculated using the method of reflections. The nonzero size of the colloid particle is accounted for in the analysis, and the theoretical predictions are compared with the experimental data of Lin, Cui, Lee, and Yu for the short time one particle diffusion coefficient of colloids in a square open channel [Europhys. Lett. 57, 724 (2002)].

Journal Article↗
PubMed2004

Influence of a depletion interaction on dynamical heterogeneity in a dense quasi-two-dimensional colloid liquid.

We report the results of digital video microscopy studies of the large particle displacements in a quasi-two-dimensional binary mixture of large (L) and small (S) colloid particles with diameter ratio sigma(L)/sigma(S)=4.65, as a function of the large and small colloid particle densities. As in the case of the one-component quasi-two-dimensional colloid system, the binary mixtures exhibit structural and dynamical heterogeneity. The distribution of large particle displacements over the time scale examined provides evidence for (at least) two different mechanisms of motion, one associated with particles in locally ordered regions and the other associated with particles in locally disordered regions. When rhoL*=Npisigma(L) (2)/4A< or =0.35, the addition of small colloid particles leads to a monotonic decrease in the large particle diffusion coefficient with increasing small particle volume fraction. When rhoL* > or =0.35 the addition of small colloid particles to a dense system of large colloid particles at first leads to an increase in the large particle diffusion coefficient, which is then followed by the expected decrease of the large particle diffusion coefficient with increasing small colloid particle volume fraction. The mode coupling theory of the ideal glass transition in three-dimensional systems makes a qualitative prediction that agrees with the initial increase in the large particle diffusion coefficient with increasing small particle density. Nevertheless, because the structural and dynamical heterogeneities of the quasi-two-dimensional colloid liquid occur within the field of equilibrium states, and the fluctuations generate locally ordered domains rather than just disordered regions of higher and lower density, it is suggested that mode coupling theory does not account for all classes of relevant fluctuations in a quasi-two-dimensional liquid.

Journal Article↗
PubMed2004

Complete quantum control of the population transfer branching ratio between two degenerate target states.

A five-level four-pulse phase-sensitive extended stimulated Raman adiabatic passage scheme is proposed to realize complete control of the population transfer branching ratio between two degenerate target states. The control is achieved via a three-node null eigenstate that can be correlated with an arbitrary superposition of the target states. Our results suggest that complete suppression of the yield of one of two degenerate product states, and therefore absolute selectivity in photochemistry, is achievable and predictable, even without studying the properties of the unwanted product state beforehand.

Journal Article↗
PubMed2004

Anomalous hydrodynamic interaction in a quasi-two-dimensional suspension.

We study the correlated Brownian motion of micron-sized particles suspended in water and confined between two plates. The hydrodynamic interaction between the particles exhibits three anomalies. (i) The transverse coupling is negative; i.e., particles exert "antidrag" on one another when moving perpendicular to their connecting line. (ii) The interaction decays with interparticle distance r as 1/r(2), faster than in unconfined suspensions but slower than near a single wall. (iii) At large distances, the pair interaction is independent of concentration within the experimental accuracy. The confined suspension thus provides an unusual example of long-range, yet essentially pairwise, correlations even at high concentration. These effects are shown to arise from the two-dimensional dipolar form of the flow induced by single-particle motion.

Journal Article↗
PubMed2004

Measurement-assisted coherent control.

Two advantageous roles of the influence of measurement on a system subject to coherent control are exposed using a five-level model system. In particular, a continuous measurement of the population in a branch state in the Kobrak-Rice extended stimulated Raman adiabatic passage scheme is shown to provide a powerful means for controlling the population transfer branching ratio between two degenerate target states. It is demonstrated that a measurement with a large strength may be used to completely shut off the yield of one target state and that the same measurement with a weak strength can dramatically enhance the robustness of the controlled branching ratio against dephasing.

Journal Article↗
PubMed2004

Wavelength dependence of liquid-vapor interfacial tension of Ga.

The wave-vector dependence of the liquid-vapor interfacial tension of Ga, gamma(q), has been determined from diffuse x-ray scattering measurements. The ratio gamma(q)/gamma(0)=1 for q<0.05 A(-1) decreases to 0.5 near q=0.22 A(-1), and increases strongly for larger q. The observed form for gamma(q)/gamma(0) is consistent with the prediction from the Mecke-Dietrich theory when the known stratified liquid-vapor interfacial density profile of Ga and a pseudopotential based pair interaction with appropriate asymptotic (r--> infinity ) behavior are used. The detailed behavior of gamma(q)/gamma(0) depends on the particular forms of both the interfacial density profile and the asymptotic falloff of the atomic pair interaction.

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