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The interaction between two ligated nanoparticles depends on whether they are isolated or immersed in a liquid solvent. However, very little is known about the influence of solvent vapor on the interaction between two ligated nanoparticles. Recent experiments yield the surprising result that the cyclic exposure of solvent free suspended monolayers of dodecane thiol ligated gold nanoparticles (AuNPs) to water vapor and dry nitrogen generates reversible cyclic decreases and increases in Young's modulus of the monolayer, implying corresponding cyclic changes in the AuNP–AuNP interaction. We examine how water vapor interacts with an isolated dodecane thiol dressed AuNP and how water vapor affects the interaction between a pair of nanoparticles, using all-atom molecular-dynamics simulations. We find that there is condensation of water molecules onto the ligand shell of an AuNP in the form of clusters of 100–2000 molecules that partially cover the shell, with most of the water in a few large clusters. A water cluster bridges the AuNPs, with a sensibly constant number of water molecules for AuNP–AuNP separations from the edge-to-edge contact up to center-to-center separations of 100 Å. The wet AuNP–AuNP interaction has a slightly deeper and wider asymmetric well than does the dry interaction, a change that is qualitatively consistent with that implied by the observed water vapor induced change in Young's modulus of a monolayer of these AuNPs. We find that macroscopic analyses of water drop–deformable surface interactions and dynamics provide both guidance to understanding and qualitatively correct predictions of the phenomena observed in our simulations. [ABSTRACT FROM AUTHOR]

We report studies of (i) the isothermal density dependent sequences of phases in two-dimensional systems of particles with repulsive pair potentials devised by Truskett [J. Chem. Phys. 145, 054901 (2016)] and Torquato [Phys. Rev. E 88, 042309 (2013)] to support a high-density Kagome lattice phase and (ii) transient structured fluctuations close to a transition to a Kagome lattice. The commonalities in the sequences of phases in these systems and other 2D systems suggest the existence of a universal mechanism driving all to favor similar packing arrangements as the density is increased, but the simulations also show that the only such general rule proposed, namely, the Süto theorem, is not a necessary condition for the support of multiple distinct lattice structures by a particular pair potential. The transient fluctuations in the liquid close to the liquid-to-Kagome phase transition have Kagome symmetry, whereas deeper in the liquid phase, the fluctuations have hexagonal symmetry. When the transition is string-to-Kagome phase, the transient structured fluctuations in the string phase have both six-fold and other than six-fold symmetries. The path of the string-to-Kagome transition in the Truskett system involves intermediate honeycomb configurations that subsequently buckle to form a Kagome lattice. The path of the string-to-Kagome transition in the Torquato system suggests that the Kagome phase is formed by coiled strings merging together; increasing density generates a Kagome phase with imperfections such as 8-particle rings. [ABSTRACT FROM AUTHOR]

We report the results of molecular dynamics simulations of the properties of a pseudo-atom (united atom) model of dodecane thiol ligated 5-nm diameter gold nanoparticles (AuNPs) in a vacuum as a function of ligand coverage and particle separation in three states of aggregation, namely, the isolated AuNPs, the isolated pair of AuNPs, and a square lattice of four AuNPs. Our calculations show that the ligand density along a radius emanating from the core of an isolated AuNP has the same gross features for all values of the coverage; it oscillates around a constant value up to a distance along the chain corresponding to the position of the fourth pseudo-atom and then smoothly decays to zero, reflecting both the restricted conformations of the chain near the core surface and the larger numbers of conformations available further from the core. Interaction between two AuNPs generates changes in the ligand distributions of each. We examine the structure and general shape of the ligand envelope as a function of the coverage and demonstrate that the equilibrium structure of the envelope and the deformation of that envelope generated by interaction between the NPs are coverage-dependent so that the shape, depth, and position of the minimum of the potential of mean force display a systematic dependence on the ligand coverage. We propose an accurate analytical description of the calculated potential of mean force as a function of a set of parameters that scale linearly with the ligand coverage. Noting that the conformational freedom of the ligands implies that multiparticle induced deviations from additivity of the pair potential of mean force are likely important; we define and calculate a "bond stretching" effective pair potential of mean force for a square lattice of particles that contains, implicitly, both the three- and four-NP contributions. We find that the bond stretching effective pair potential of mean force in this cluster has a different minimum and a different well depth from the isolated pair potential of mean force. Previous work has found that the three-particle contribution to deviation from pair additivity is monotonically repulsive, whereas we find that the combined three- and four-particle contributions have an attractive well, implying that the three- and four-particle contributions are of comparable magnitude but opposite sign, thereby suggesting that even higher order correction terms likely play a significant role in the behavior of dense assemblies of many nanoparticles. [ABSTRACT FROM AUTHOR]

We report the structure of transient fluctuations in the liquid phase of a two-dimensional system that exhibits several ordered phases with different symmetries. The density-temperature phase diagram of the system studied, composed of particles with a repulsive shouldered soft-core pair interaction, has regions with stable liquid and hexatic phases, a square solid phase, two separate hexagonal solid phases, and a quasi-crystalline phase with 12-fold symmetry. We have examined the character of the structured fluctuations by computing the same-time aperture cross correlation function of particle configurations in several fluid regions near to and far from phase transition lines. The two primary goals of our study are (1) determination if the spectrum of structures of the fluctuations in the liquid is broader than or limited to the motifs exhibited by the ordered phases supported by the system and (2) determination of the density domains in the liquid that support particular transient structured fluctuations. In the system studied, along a low-temperature isotherm in the temperature-density plane that intersects all the ordered phases we find that the liquid phase exhibits structured fluctuations with hexagonal symmetry near both liquid-hexatic transition lines. Along the same isotherm and in the stable liquid between the lower density hexatic-to-liquid and the higher density liquid-to-square solid transitions, we find that transient hexagonal ordered fluctuations dominate the liquid region near the hexatic-to-liquid transition and square ordered fluctuations dominate the liquid region near the liquid-to square solid transition, but both of these structured fluctuations occur at all densities between these transition lines. At a higher temperature, at phase points in the liquid above, but close to the density maximum of an underlying transition, there are ordered fluctuations that can be correlated with the structure of the lower temperature phase. Although it is expected that very close to a liquid-ordered phase boundary a structured fluctuation in the liquid will have the same symmetry as the ordered phase, it is not obvious that structured fluctuations in thermodynamic states deep in the liquid phase will be similarly restricted. The most striking result of our calculations is that no evidence is found in the liquid phase for structured fluctuations with other symmetries than those of the ordered phases of the system. [ABSTRACT FROM AUTHOR]

Experimental studies of the variation of the mean square displacement (MSD) of a particle in a confined colloid suspension that exhibits density variations on the scale length of the particle diameter are not in agreement with the prediction that the spatial variation in MSD should mimic the spatial variation in density. The predicted behavior is derived from the expectation that the MSD of a particle depends on the system density and the assumption that the force acting on a particle is a point function of position. The experimental data are obtained from studies of the MSDs of particles in narrow ribbon channels and between narrowly spaced parallel plates and from new data, reported herein, of the radial and azimuthal MSDs of a colloid particle in a dense colloid suspension confined to a small circular cavity. In each of these geometries, a dense colloid suspension exhibits pronounced density oscillations with spacing of a particle diameter. We remove the discrepancy between prediction and experiment using the Fisher-Methfessel interpretation of how local equilibrium in an inhomogeneous system is maintained to argue that the force acting on a particle is delocalized over a volume with radius equal to a particle diameter. Our interpretation has relevance to the relationship between the scale of inhomogeneity and the utility of translation of the particle MSD into a position dependent diffusion coefficient and to the use of a spatially dependent diffusion coefficient to describe mass transport in a heterogeneous system. [ABSTRACT FROM AUTHOR]

Previous studies have demonstrated that when experimental conditions generate non-adiabatic dynamics that prevents highly efficient population transfer between states of an isolated system by stimulated Raman adiabatic passage (STIRAP), the addition of an auxiliary counter-diabatic field (CDF) can restore most or all of that efficiency. This paper examines whether that strategy is also successful in a non-isolated system in which the energies of the states fluctuate, e.g., when a solute is subject to collisions with solvent. We study population transfer in two model systems: (i) the three-state system used by Demirplak and Rice [J. Chem. Phys. 116, 8028 (2002)] and (ii) a four-state system, derived from the simulation studies of Demirplak and Rice [J. Chem. Phys. 125, 194517 (2006)], that mimics HCl in liquid Ar. Simulation studies of the vibrational manifold of HCl in dense fluid Ar show that the collision induced vibrational energy level fluctuations have asymmetric distributions. Representations of these asymmetric energy level fluctuation distributions are used in both models (i) and (ii). We identify three sources of degradation of the efficiency of STIRAP generated selective population transfer in model (ii): too small pulse areas of the laser fields, unwanted interference arising from use of strong fields, and the vibrational detuning. For both models (i) and (ii), our examination of the efficiency of STIRAP + CDF population transfer under the influence of the asymmetric distribution of the vibrational energy fluctuations shows that there is a range of field strengths and pulse durations under which STIRAP + CDF control of population transfer has greater efficiency than does STIRAP generated population transfer. [ABSTRACT FROM AUTHOR]

Optical matter (OM) systems consist of (nano-)particle constituents in solution that can self-organize into ordered arrays that are bound by electrodynamic interactions. They also manifest non-conservative forces, and the motions of the nano-particles are overdamped; i.e., they exhibit diffusive trajectories. We propose a data-driven approach based on principal components analysis (PCA) to determine the collective modes of non-conservative overdamped systems, such as OM structures, and harmonic linear discriminant analysis (HLDA) of time trajectories to estimate the reaction coordinate for structural transitions. We demonstrate the approach via electrodynamics-Langevin dynamics simulations of six electrodynamically-bound nanoparticles in an incident laser beam. The reaction coordinate we discover is in excellent accord with a rigorous committor analysis, and the identified mechanism for structural isomerization is in very good agreement with the experimental observations. The PCA-HLDA approach to data-driven discovery of reaction coordinates can aid in understanding and eventually controlling non-conservative and overdamped systems including optical and active matter systems. Optical matter consisting of nanoparticle constituents in solution is of key interest due to the exhibited self-assembling mechanisms. The authors propose a principal components analysis based data-driven approach to determine the collective modes of colloidal clusters mimicking optical binding used in colloidal self-assembly. [ABSTRACT FROM AUTHOR]

We offer a comparison of two different characterizations of favored local structure in an inhomogeneous liquid, using as vehicles simulations of the interfaces between a Lennard-Jones liquid and: (i) a flat Lennard-Jones wall, (ii) the 100 face of a face-centred cubic Lennard-Jones crystal, and (iii) a Lennard-Jones vapor. The interfacial density distributions in (i) and (ii) have oscillations extending several particle diameters into the bulk liquid, while in (iii) it monotonically decays from the bulk liquid to the vapor. We search for transient ordered fluctuations using the Aperture Cross Correlation Function (ACCF) of the system. Particles in the in the two layers closest to the surface of the FCC-liquid interface exhibit a four-fold mirroring of the surface structure; no structure is found elsewhere in this or the other interfaces. The absence of transient ordered structure in the liquid–vapor interface yields a different picture of that interface than derived from the cluster analysis reported by Godonoga et al. (Mol. Phys., 109 (7–10), 1393–1402 (2011)). We argue the advantages of using the ACCF to characterize transient ordered fluctuations as being both experimentally accessible in favourable cases and not reliant on a pre-assumed list of possible structures. [ABSTRACT FROM AUTHOR]

The effect of hydrodynamic interactions on the spreading of clusters of colloid particles in a quasi-one-dimensional channel is analyzed both experimentally and theoretically. An n-particle cluster spreading diffusion coefficient is defined, in terms of the displacement Δx(t) in time t, by Dn≡<[∑i=1nΔxi(t)]2>/2nt, where the average is taken over all groups of n adjacent particles. Our study focuses on the n-dependence of Dn with some attention to the dependence of Dn on colloid packing fraction. We find that the ratio of Dn to the infinite dilution self-diffusion coefficient DS0 increases as n increases, eventually saturating for large n. The observed dependence of Dn on n is in satisfactory agreement with the predictions obtained from both Stokesian dynamics simulations and hydrodynamic calculations using the method of reflections. [ABSTRACT FROM AUTHOR]

The control of population transfer can be affected by the adiabatic evolution of a system under the influence of an applied field. If the field is too rapidly varying or too weak, the conditions for adiabatic transfer are not satisfactorily met. We report the results of an analysis of properties of counterdiabatic fields (CDFs) that restore the adiabatic dynamics of a system by suppressing diabatic effects as they are generated. We observe that a CDF is not unique and find the one that has minimum intensity, and we provide natural upper and lower bounds to the integrated intensity of a CDF in terms of integrals of the eigenvalues of the system Hamiltonian. For Hamiltonians that are separable with respect to their parameters, we prove that the time integral of an associated CDF is path independent. Finally we explain why and when, in the neighborhood of an avoided crossing, a CDF can be approximated by Lorentzian pulses. [ABSTRACT FROM AUTHOR]

We report the results of a theoretical study of locally ordered fluctuations in a quasi-two-dimensional (quasi-2D) binary hard sphere mixture as monitored by the aperture cross-correlation function. Systems with thickness less than two large hard sphere diameters were studied, over a range of large hard sphere density and relative concentration of large and small hard spheres in the liquid state, for two ratios of sphere diameters, 0.2 and 0.3. In several major respects the occurrence and character of the structured fluctuations in a quasi-2D binary hard sphere mixture have similarities with those in a quasi-2D one-component hard sphere fluid, as reported by Sheu and Rice [J. Chem. Phys. 128, 244517 (2008)]. Specifically, our studies establish that structured fluctuations with both hexagonal and square symmetries can be found in the liquid phase well below the freezing density and that the occurrence of particular structured fluctuations is correlated with the symmetry of the solid to which the liquid freezes. However, there is also a major difference, specifically, the presence of the smaller spheres in the binary mixture can suppress the occurrence of all structured fluctuations. We attribute the affect of a small volume fraction of small spheres in a dense binary mixture of small and large spheres on the occurrence of structured fluctuations to the unfavorable entropy change associated with demixing of the small and large spheres. [ABSTRACT FROM AUTHOR]

We report the results of a theoretical study of locally ordered fluctuations in a quasi-two-dimensional colloid fluid. The fluctuations in the equilibrium state are monitored by the aperture cross-correlation function of radiation scattered by the fluid, as calculated from molecular dynamics simulations of near hard spheres with diameter σ confined between smooth hard walls. These locally ordered fluctuations are transient; their decay can be monitored as a function of the time between the cross-correlated scattered radiation signals, but only the single-time cross-correlated signals are discussed in this paper. Systems with thicknesses less than two hard sphere diameters were studied. For wall separation H in the range 1σ≤=H≤=1.57σ, the liquid freezes into a single hexagonal layer and the locally ordered fluctuations only exhibit hexagonal symmetry that extends well into the liquid region. For H>=1.57σ, hexagonal fluctuations persist in the dense liquid up to H=1.75σ, and fluctuations with square ordered symmetry, that of the solid to which the liquid freezes, only emerge at densities ∼2% below freezing. For H=1.8σ and 1.85σ, hexagonal ordered flucuations are no longer found, and the square ordered fluctuations dominate the dense liquid region as the system freezes into a two layer square solid. For H=1.9σ and 1.95σ, where the liquid freezes into a two layer hexagonal solid, both square and hexagonal ordered fluctuations are observed. At lower densities, the ordered fluctuations only exhibit square symmetry. Hexagonal ordered fluctuations appear at densities ∼7% below freezing and become more dominant as the density is increased, but the square ordered fluctuations persist until the system is converted into the solid. [ABSTRACT FROM AUTHOR]

We report the results of classical mechanics simulations of infrared multiphoton induced control of isomerization of FCN, ClCN, and BrCN in liquid Ar, using ab initio potential energy and dipole moment surfaces for the XCN molecules. The field induced isomerization and fragmentation dynamics of these molecules are found to be different from that of HCN in liquid Ar. In particular, the scheme that provides complete controlled conversion of HCN to CNH in liquid Ar fails to generate complete conversion of XCN to CNX in liquid Ar for X=F,Cl,Br. It is suggested that the sources of the differences in behavior arise from differences in the spectra of vibrational nonlinear resonances in HCN and XCN and to the occurrence of monodromy in the dynamics of the XCN molecules. [ABSTRACT FROM AUTHOR]

The authors report the results of measurements of the center of mass and relative pair diffusion coefficients in quasi-one-dimensional (q1D) and quasi-two-dimensional (q2D) binary colloid suspensions. The new results extend the findings of similar studies of one-component quasi-one-dimensional and quasi-two-dimensional colloid suspensions. Our principal new finding is that the presence of the smaller diameter component can destroy the oscillatory structure of the separation dependence of the q2D relative pair diffusion coefficient of the large particles even though the oscillatory character of the large particle equilibrium pair correlation function remains prominent, and that no such effect occurs with the q1D suspension. An interpretation of these results is proposed. [ABSTRACT FROM AUTHOR]

We report the results of simulation studies of the statistics of vibrational dephasing of a YCl (Y=H, D, T, and I) diatom in dense fluid Ar at two temperatures, including the effect of strong field driving on the energy level modulation statistics. The distribution of energy level modulations is found to be non-Gaussian with a high energy tail. Aspects of stimulated Raman adiabatic passage (STIRAP) between the vibrational levels of HCl in dense fluid Ar have been investigated. For HCl with nearly degenerate v=0→v=1 and v=1→v=2 transitions, the combined effect of modulation and power broadening reduces the STIRAP efficiency for population transfer from v=0 to v=2 of the order of 30%. However, if the transitions used have very different frequencies, as in the original model studied by Demirplak and Rice [J. Chem. Phys. 116, 8028 (2002)], the STIRAP efficiency for population transfer remains high, of the order of 80%, even with non-Gaussian modulation of energy levels. [ABSTRACT FROM AUTHOR]

We report the results of experimental determinations of the triplet correlation functions of quasi-two-dimensional one-component and binary colloid suspensions in which the colloid-colloid interaction is short ranged. The suspensions studied range in density from modestly dilute to solid. The triplet correlation function of the one-component colloid system reveals extensive ordering deep in the liquid phase. At the same density the ordering of the larger diameter component in a binary colloid system is greatly diminished by a very small amount of the smaller diameter component. The possible utilization of information contained in the triplet correlation function in the theory of melting of a quasi-two-dimensional system is briefly discussed. [ABSTRACT FROM AUTHOR]

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 Ga2 molecules for several layers into the bulk liquid, with the axes of the Ga2 molecules in successive layers oriented perpendicular to the diamond surface. No driving force for the proposed formation of Ga2 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 π complex with the Ga2 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 Ga2(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. [ABSTRACT FROM AUTHOR]

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 xBi=0.0032, 0.0023, 0.000 37, and 0.000 037. The monolayer coverage of the alloys with xBi=0.0023, and xBi=0.000 37 is about 0.85 and only very slightly temperature dependent. The monolayer coverage in the lowest-concentration alloy, with xBi=0.000 037, ranged from 0.82 at 29 °C to 0.58 at 110 °C. In none of these alloys, down to the lowest temperature used, 29 °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 xBi=0.000 037 suggest that near 80 °C there is a disordered phase-to-disordered phase transition. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 1013 W/cm2, 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. [ABSTRACT FROM AUTHOR]

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)]. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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 σL/σS=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 ρL*=NπσL2/4A≤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 ρL*>=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. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Two-pulse selective photochemistry that exploits population transfer via adiabatic passage is considered for the case that there are degenerate product states with different lifetimes. As an example, a four-level model system with a complex symmetric Hamiltonian is constructed. Analytical and numerical studies of this model system demonstrate that extensive control over the product branching ratio can be achieved by detuning either the pump pulse or the Stokes pulse while maintaining negligible population in the intermediate state. This control approach represents a significant simplification of both the Kobrak–Rice extended stimulated Raman adiabatic passage scheme and the Chen–Shapiro–Brumer strong-field control scheme. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The feasibility of efficient population transfer between an initial state and a decaying target state of the same parity without populating an intermediate state, in the presence of large-amplitude stochastic energy level fluctuations that mimic the dephasing in a solute molecule due to the influence of a solvent, is demonstrated theoretically. In particular, it is shown that a decaying target state, whose decay rate constant is large compared with the band width of picosecond laser pulses but small compared with the associated peak Rabi frequencies, can dramatically suppress the dephasing-induced nonadiabaticity associated with the dynamics of population transfer, resulting in, irrespective of the correlation time of stochastic energy level fluctuations, negligible population in the intermediate state and complete population transfer to the decaying target state. These results should further motivate experimental studies of optical control of molecular dynamics in a liquid. An interesting connection between our results and the quantum Zeno and anti-Zeno effects is also discussed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report the results of a study of crystallization in quasi-two-dimensional binary mixtures of large and small colloids. The experiments sample the parameter spaces of colloid particle diameter ratio, large particle density, and small particle packing fraction. The depletion potential between the large particles, induced by the presence of the small particles in the system, affects the density at which the large particles undergo a liquid-to-solid freezing transition. For systems with a large to small particle diameter ratio of 4.6, the addition of small particles increases the large particle liquidus transition density, a seemingly counterintuitive result given that the depletion potential is purely attractive when the small particle packing fraction is low. When the large to small particle diameter ratio is 8.8, the same trend in the large particle liquidus transition density is seen, but to a lesser extent. The other system properties for the system with diameter ratio 8.8 show the same trends as for the system with diameter ratio 4.6. Liquid-liquid phase separation is observed for binary mixtures with diameter ratios of 20 and 40. Although the particles used in our experiment can be well modeled as hard spheres, our results cannot be readily explained by extant descriptions of the depletion interaction developed for three-dimensional binary hard sphere mixtures. Inversion of the pair correlation functions obtained from our measurements yields a depletion interaction that is much stronger than predicted for the same densities and diameter ratio in a three-dimensional hard sphere mixture. Our results imply that the depletion interaction is strongly dependent on the degree of confinement of the system. [ABSTRACT FROM AUTHOR]

We report the results of a study of the influence of solvent fluctuations on the efficiency of selective population transfer from an initial state to a designated target state of a solute molecule. Our model of the influence of liquid fluctuations on the states of the solute assumes that dephasing is the dominant relaxation process, and utilizes an analog of the Kubo stochastic theory of line shape. The solvent fluctuations are represented as a Gaussian random process that independently modulates each of the energy levels of the solute molecule. For typical liquid densities the maximum amplitude of these fluctuations is taken to be of the order of 150 cm[sup -1], and the correlation time of the fluctuations is taken to be of the order of a few hundred femtoseconds, but we have also explored the effects of varying the fluctuation frequency and correlation time. It is shown that STIRAP (stimulated Raman adiabatic passage) generated population transfer to a designated target state of the solute remains efficient when the frequency of the solvent fluctuations is large or small relative to the inverse of the widths of the pump and Stokes pulses. It is further shown that extended STIRAP generated selective transfer to one of a pair of degenerate states of the solute remains efficient under the same conditions. These results suggest, subject to the accuracy of the representation of the influence of the solvent on the solute, that it should be possible, using coherent superpositions of states generated with picosecond excitation, to control population transfer, hence reactivity, for a class of reactions carried out in the liquid phase. [ABSTRACT FROM AUTHOR]

We report the results of studies, using digital video microscopy, of the spatial distribution of particles in a quasi-one-dimensional colloidal liquid. The system studied consists of a water suspension of silica particles with diameter σ=1.58 μm confined in a 3 μm×3 μm×2 mm silicone elastomer channel. The measured spatial distributions have been characterized by the nearest-neighbor distribution function and the pair correlation function, and the pair correlation function has been inverted, using the hypernetted chain approximation, to yield the effective colloid–colloid interaction. The effective colloid–colloid interaction is found to have an attractive potential well with depth of ∼0.3k[sub B]T at a colloid-colloid separation Rapprox. 1.18σ. Molecular-dynamics simulations of the nearest neighbor and pair correlation functions carried out using the empirically determined colloid–colloid interaction are in very good agreement with the experimental data. The experimental data obtained rule out any important charge–charge contribution to the measured colloid–colloid interaction. It is suggested that because the host liquid does not wet the channel wall, and the colloid particles are tightly confined in the long narrow channel, capillary forces determine the effective colloid–colloid interaction. This suggestion is consistent with the determination (also reported) that the effective colloid–colloid interaction in a quasi-two-dimensional suspension of the same particles is different from that in the quasi-one dimensional suspension. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Discusses the results of self-consistent quantum Monte Carlo simulations of the structure of the liquid-vapor interface of the alloy Sn[sub 0.09]Ga[sub 0.81]. Agreement of the calculations with the experimental results; Confirmation of the experimentally inferred existence of a partial second layer of tin.

This work examines, both experimentally and theoretically, nonradiative decay processes in a series of substituted naphthalenes. We report single vibronic level fluorescence lifetimes and fluorescence excitation spectra of jet-cooled 2-chloronaphthalene, 1- and 2-fluoronaphthalene and 1- and 2-methylnaphthalene over an energy range of about 0–4000 cm-1 in S1. While the 00 nonradiative rates of these molecules vary by a factor of 30, the energy dependences of the nonradiative rates are quite similar. At low vibrational energies the nonradiative rates depend sensitively on the level excited, but in general they increase with energy. As energy increases, the nonradiative rates become less sensitive to the level excited and eventually become almost independent of vibrational energy. We can qualitatively predict this behavior using a thermodynamic formalism which treats the density of states as an intramolecular entropy and avoids the calculation of vibrational coupling terms. In addition we report the fluorescence lifetime of the S1 vibrationless level of 1-chloronaphthalene, and single vibronic level fluorescence spectra of levels up to 1396 cm-1 in 2-chloronaphthalene. Most of the fluorescence excitation and fluorescence spectra show substantial enhancement of the origin inensity and vibrational mode mixing in S1 compared to naphthalene. We briefly discuss these substituent effects and make some tentative assignments of S1 vibrational levels. We also discuss substituent effects on the 00 fluorescence lifetimes. In particular, the order of magnitude increase in the decay rate of 1-chloronaphthalene relative to 2-chloronaphthalene cannot be explained using CNDO/S calculations. [ABSTRACT FROM AUTHOR]

We report the results of calculations, at five temperatures ranging from 243 to 313 K, of the distributions of non-short-circuited rings of hydrogen-bonded molecules in TIPS-2 water. These data are used to test two hypotheses concerning the use of ring transformations as collective excitations in water. The suggestion by Speedy that it is the concentration of five-membered rings that determines the anomalous properties of water, and the suggestion by Rice, Belch, and Sceats that it is the ‘‘reactions’’ 2(6 ring)⇄(5 ring)+(7 ring) and 8(6 ring)⇄2(4 ring)+4(8 ring) that determines these properties, are both shown to be incomplete. Further verification of the random network model assumption of separation of time scales for network relaxation and local molecular motion is presented. [ABSTRACT FROM AUTHOR]

We report the results of experiments which probe the vibrational energy redistribution which accompanies fragmentation of deuterobenzene–helium van der Waals molecules. Branching ratios for vibrational relaxation to various final states were obtained for initially prepared levels 61-He1,2, 61162-He1, 111171-He1, and 172-He1 in several deuterobenzene compounds. The probabilities of transitions to final states in the relaxation from 61-He1 show general agreement with an energy gap law, and relaxation from 61162-He1, 111171-He1, and 172-He1 favors small vibrational quantum number changes. The experiments were undertaken to illuminate how molecular symmetry influences the choice of pathways for vibrational energy redistribution associated with van der Waals molecule predissociation. The data for the various C6H6-nDn·He1,2 systems, however, show very different relaxation patterns. These patterns cannot readily be explained by the usual intramolecular vibrational coupling mechanisms. [ABSTRACT FROM AUTHOR]

The x-ray reflectivity and fluorescence, as functions of angle of incidence and angle of detection, are calculated for several models of the longitudinal density distribution in the liquid–vapor interface. The models studied range from one descriptive of the monotone density distribution in a dielectric liquid–vapor interface to one descriptive of the stratified density distribution predicted for the metal liquid–vapor interface. In addition, we report calculations for the liquid–vapor interfaces of several simple binary alloys, each characterized by varying degrees of surface segregation of the components. Our results suggest that x-ray reflectivity and fluorescence intensity variations with angle of incidence and angle of detection can be sensitive probes of density and composition variations along the normal to the interface for pure metals and alloys. [ABSTRACT FROM AUTHOR]

This work examines, both experimentally and theoretically, the lifetimes of nominally degenerate benzene 1B2u levels split by differential coupling of the vibrational angular momentum components. We report fluorescence lifetimes for the 61162, 61102, and 61172 levels of the S1 electronic state in cold C6D6 in a supersonic jet. The higher energy vibrational angular momentum component has the longer lifetime. Calculations of the nonradiative decay rates for these levels are presented. Calculations based on conventional radiationless transition theory, assuming the prepared state is a single vibronic level, are not able to explain the experimental results, and we conclude that another mechanism must be responsible for the observed lifetime differences. We present a qualitative explanation of the data that takes into account perturbations of zeroth-order harmonic oscillator states by cubic and quartic vibrational anharmonicities and by Coriolis coupling. These perturbations affect the split vibrational angular momentum components differently and are capable of producing differences in lifetimes. [ABSTRACT FROM AUTHOR]

We examine the dynamics of isomerization reactions and point out the existence of important phase space bottlenecks. A three-state statistical theory based on the behavior of trajectories and the properties of the phase space bottlenecks is proposed; this theory is better than standard two-state RRKM theory under some circumstances and offers a new way of conceptualizing isomerization. We test our ideas on the simple system, consisting of an equivalent double well coupled to a Morse oscillator, which was studied by DeLeon and Berne. Approximate methods of obtaining the required elementary rate constants in our theory are also discussed. [ABSTRACT FROM AUTHOR]

We report measurements of the in-plane (transverse) structure function, S||(k), of the liquid–vapor interface of Hg at 298 K, obtained using the method of grazing incidence x-ray diffraction. Our results show that: (i) The transverse structure function of the liquid–vapor interface of Hg is very similar to the bulk liquid structure function, but there are subtle differences between the two functions. (ii) As k→0, S||(k) appears to diverge as k-2, in agreement with theoretical predictions and computer simulation studies. (iii) The observable S||(k) for different longitudinal (parallel to the normal) density distributions in the liquid–vapor interface are very nearly the same, the largest differences being for momentum transfers less than that at the first peak of S||(k). For this reason the measurement of S||(k) is not a good monitor of the longitudinal density distribution. (iv) The interpretation of the differences in S||(k) in the small k region (but outside the region where the k-2 divergence overtakes other effects) requires consideration of the effect of the longer ranged part of the effective pair potential and assessment of the relative contributions of the ionic and electronic components of the liquid metal to the compressibility. A comparison of the shape of S||(k) in the threshold region just before the first peak and that of the same region in a bulk liquid metal leads to the weak conclusion that our data are consistent with the Monte Carlo simulations which predict that the liquid–vapor interface of Hg is stratified for about three atomic layers. [ABSTRACT FROM AUTHOR]

We present a novel approach to the control of selectivity of reaction products. The central idea is that in a two-photon or multiphoton process that is resonant with an excited electronic state, the resonant excited state potential energy surface can be used to assist chemistry on the ground state potential energy surface. By controlling the delay between a pair of ultrashort (femtosecond) laser pulses, it is possible to control the propagation time on the excited state potential energy surface. Different propagation times, in turn, can be used to generate different chemical products. There are many cases for which selectivity of product formation should be possible using this scheme. We illustrate the methodology with numerical application to a variety of model two degree of freedom systems with two inequivalent exit channels. Branching ratios obtained using a swarm of classical trajectories are in good qualitative agreement with full quantum mechanical calculations. [ABSTRACT FROM AUTHOR]

The classical dynamics pertinent to van der Waals molecule vibrational predissociation of a T-shaped model for HeI2(B) is examined. An interesting phase space structure involving nonlinear resonances and stochastic motion is found. For low initial vibrational excitations of the I2 partner of the vdW complex the relevant part of phase space is dominated by quasiperiodic motion indicating a purely quantal mode of decay (‘‘dynamical tunneling’’), but for higher initial vibrational excitations van der Waals molecule predissociation is a classically allowed process. Classically determined rates of decay agree to within a factor of 3 with the rates calculated from quantum mechanics. [ABSTRACT FROM AUTHOR]

A plot of final vibrational action vs initial vibrational phase for a two degree of freedom model for He + I2 collisions is examined and shown to exhibit a chattering region. The chattering region is shown to exhibit a very intricate structure, including an infinite number of regions of regularity which we term icicles. Structure is evident on all scales of examination and a fractal dimension close to 2 is obtained for some parts of the chattering region. The survival probability associated with the complexes shows an initial fast decay, due to the icicles, but can be roughly characterized over a longer and wider time range by a more slowly decaying exponential. [ABSTRACT FROM AUTHOR]

We report an analysis of the vibrational spectrum of the water dimer, including Fermi resonance and the adiabatic coupling between intra- and intermolecular modes. Three recently proposed pair potentials for water, each of which includes a representation of the intra- and intermolecular force fields, are tested by comparing the predicted and observed infrared spectra. Our results show that dynamical effects in the vibrational spectrum are underestimated by these potentials. Despite the considerable uncertainty in the experimental data it is clear that currently available water–water potentials are too inaccurate to be trusted except for predictions of the qualitative properties of the vibrational spectrum. [ABSTRACT FROM AUTHOR]

We describe a representation of coherent anti-Stokes Raman spectroscopy (CARS) suitable to the description of time-dependent measurements. This representation includes the standard energy frame formulation as a special case when cw fields are involved. The traditional CARS cw field frequency matching condition ω0=ω1-ω2+ω3 must be generalized for non-cw fields; in that case it refers to the Fourier component at ω0 of the convolution of the radiation field with the wave packet recurrences. The influence of resonance, both in the ground and electronically excited states, on the decay of time delayed CARS signals is discussed. As expected, intramolecular vibrational redistribution on the ground state potential surface of a molecule causes the CARS signal to decay in time. Model calculations show that quantum beats in the CARS signal may be observed, reflecting either a small number of coupled states (strong and regular recurrences) or sequential coupling of states (weak and early recurrences). [ABSTRACT FROM AUTHOR]

A time-dependent formulation of two-photon spectroscopy is employed to show that selectivity of reactivity can be achieved via coherent two-photon processes. The problem of finding the optimum waveform (i.e., coherent pulse sequence) that will maximize the formation of a desired chemical species is formulated as a problem in the calculus of variations, and solved for two different cases. [ABSTRACT FROM AUTHOR]

We report a study of very low energy collision induced vibrational relaxation using an approximate resonant state formalism which relates the inelastic cross section to the properties of metastable states. A study of models loosely based on the He+I2 (B 3Π0+u) system reveals that the combined effect of a low collision energy resonance and high initial diatomic vibrational excitation can lead to a large enhancement of the vibrational relaxation cross section. However, even in the absence of resonances the cross section, for very low energies, increases with decreasing energy. It is found that this effect increasingly enhances the contribution of a resonance to the cross section the closer the resonance is to zero collision energy. Because the density of resonances near zero collision energy is small the collision dynamics near zero collision energy is very sensitive to the nature of the potential energy surface. [ABSTRACT FROM AUTHOR]

We report the results of two studies of aspects of the consistency of truncated nonlinear integral equation based theories of freezing: (i) We show that the self-consistent solutions to these nonlinear equations are unfortunately sensitive to the level of truncation. For the hard sphere system, if the Wertheim–Thiele representation of the pair direct correlation function is used, the inclusion of part but not all of the triplet direct correlation function contribution, as has been common, worsens the predictions considerably. We also show that the convergence of the solutions found, with respect to number of reciprocal lattice vectors kept in the Fourier expansion of the crystal singlet density, is slow. These conclusions imply great sensitivity to the quality of the pair direct correlation function employed in the theory. (ii) We show the direct correlation function based and the pair correlation function based theories of freezing can be cast into a form which requires solution of isomorphous nonlinear integral equations. However, in the pair correlation function theory the usual neglect of the influence of inhomogeneity of the density distribution on the pair correlation function is shown to be inconsistent to the lowest order in the change of density on freezing, and to lead to erroneous predictions. [ABSTRACT FROM AUTHOR]

It is conjectured that the hard sphere system has several distinct solid phases, all but one of which are metastable. The bifurcation theory analysis of freezing is extended to the description of the transition between a supercooled liquid and a disordered solid by defining a restricted phase space for the disordered solid. This approach leads to the prediction of a first order transition between a supercooled hard sphere fluid and a disordered metastable hard sphere solid. The results of the calculation are in qualitative agreement with the results of Woodcock’s molecular dynamics computer simulations. [ABSTRACT FROM AUTHOR]

We report new measurements of the rotational state dependence of the initial fluorescence decay of the vibrationless 1B3u state of pyrazine. Through examination of excited state dynamics from a Green’s function point of view the apparently inconsistent data from several sources are reconciled. [ABSTRACT FROM AUTHOR]

Recent measurements of the rotational state dependence of the initial fluorescence decay of the vibrationless 1B3u state of pyrazine are analyzed from several points of view. The relationship of these analyses to recent ideas dealing with quantum chaos is discussed. [ABSTRACT FROM AUTHOR]