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The self-assembly of binary colloidal mixtures provides a bottom-up approach to create novel functional materials. To elucidate the effect of composition, temperature and pressure on the self-assembly behavior of size-asymmetric mixtures, we performed extensive dynamics simulations of a simple model of polymer-grafted colloids. We have used a core-softened interaction potential and extended it to represent attractive interactions between unlike colloids and repulsion between like colloids. Our study focused on size-asymmetric mixtures, where the ratio between the sizes of the colloidal cores was fixed at $\frac{\sigma_{B}}{\sigma_{A}}=0.5$. We have performed extensive simulations in the NPT and NVT ensembles to elucidate the phase behavior and dynamics of mixtures with different stoichiometric ratios. Our simulation results uncovered a rich phase behavior, including the formation of hierarchical structures with many potential applications. For compositions where the small colloids are the majority, sublattice melting occurs for a wide range of densities. As the temperature is decreased, the small colloids localize, akin to a metal-insulator transition, with the small colloids playing a role similar to electrons. Under these conditions the large colloids form a well-defined lattice, whereas small colloids can diffuse through the system. Our results are summarized in terms of phase diagrams.

Nanofluids-dispersions of nanometer-sized particles in a liquid medium-have been proposed for a wide variety of thermal management applications. It is known that a solid-like nanolayer of liquid of typical thicknesses of 0.5-1 nm surrounding the colloidal nanoparticles can act as a thermal bridge between the nanoparticle and the bulk liquid. Yet, its effect on the nanofluid viscosity has not been elucidated so far. In this article, we compute the local viscosity of the nanolayer using equilibrium molecular dynamics based on the Green-Kubo formula. We first assess the validity of the method to predict the viscosity locally. We apply this methodology to the calculation of the local viscosity in the immediate vicinity of a metallic nanoparticle for a wide range of solid-liquid interaction strength, where a nanolayer of thickness 1 nm is observed as a result of the interaction with the nanoparticle. The viscosity of the nanolayer, which is found to be higher than its corresponding bulk value, is directly dependent on the solid-liquid interaction strength. We discuss the origin of this viscosity enhancement and show that the liquid density increment alone cannot explain the values of the viscosity observed. Rather, we suggest that the solid-like structure of the distribution of the liquid atoms in the vicinity of the nanoparticle contributes to the nanolayer viscosity enhancement. Finally, we observe a failure of the Stokes-Einstein relation between viscosity and diffusion close to the wall, depending on the liquid-solid interaction strength, which we rationalize in terms of the hydrodynamic slip.

We report the laser intensity dependence of multiply charged atomic ions (MCAIs) Arn+ with 2 ≤ n ≤ 8 from argon clusters in focused nanosecond laser fields at 532 nm. The laser field, in the range of 1011–1012 W/cm2, is insufficient for optical field ionization but is adequate for multiphoton ionization. The MCAI sections of the mass spectra for clusters containing 3700 and 26 000 atoms are dominated by Arn+ with 7 ≤ n ≤ 9, extending to Ar14+. While the distributions of the MCAIs remain largely constant throughout the intensity range of the laser, the abundance of Ar+ relative to the abundances of the MCAIs increases dramatically with increasing laser intensity. Consequently, exponential fittings of the yields result in a larger exponent for Ar+ than for MCAIs, and the exponents of MCAIs with 2 ≤ n ≤ 8 are similar, with only slight variations for different charge states. The width of the arrival time and, hence, the corresponding kinetic energy of Ar+ also increases with increasing laser intensities, while the width of the arrival time of MCAIs remains constant throughout the range of measurements. These results call for more detailed theoretical investigations in this regime of laser–matter interactions.

Depletion interactions between colloids of discoidal shape can induce their self-assembly into columnar aggregates. This is an effect of entropic origin with important implications in a range of colloidal systems, particularly in the clustering of erythrocytes that determine the rheological properties of blood. Here, we investigate the equilibrium state reached by discoidal colloids in a solution of smaller depletant particles. We develop a thermodynamic model of depletion-induced aggregation based on self-assembly theory and solve it analytically. We test the validity of the model by using Langevin simulations of a system of discs and depletant particles in which the depletion interaction emerges naturally. In addition, we consider the effect of an attractive interaction between depletant and discoidal particles, which we show induces a re-entrant dependence of aggregation with temperature.

Ratcheted multi-step hopping electron transfer systems can plausibly produce directional charge transport over very large distances without requiring a source–drain voltage bias. We examine molecular strategies to realize ratcheted charge transport based on multi-step charge hopping, and we illustrate two ratcheting mechanisms with examples based on DNA structures. The charge transport times and currents that may be generated in these assemblies are also estimated using kinetic simulations. The first ratcheting mechanism described for nanoscale systems requires local electric fields on the 109 V/m scale to realize nearly 100% population transport. The second ratcheting mechanism for even larger systems, based on electrochemical gating, is estimated to generate currents as large as 0.1 pA for DNA structures that are a few μm in length with a gate voltage of about 5 V, a magnitude comparable to currents measured in DNA wires at the nanoscale when a source–drain voltage bias of similar magnitude is applied, suggesting an approach to considerably extend the distance range over which DNA charge transport devices may operate.

The interfacial stiffness for nematic–smectic B (nm–smB) interface in a liquid crystalline (LC) material is calculated using Capillary Wave Theory (CWT) and molecular dynamics simulations. The Gay–Berne (GB) pair potential with parameters κ, κ′, μ, and ν equal to 3, 5, 2, and 1 is used to model the LC material. Using a smart three-step recipe, we have obtained an nm–smB phase coexistence in our simulations where the nm and smB directors are nearly parallel to each other and perpendicular to the interface normal. The density profiles are used to compute the nm–smB coexisting density range, the interfacial width, and its position. The smectic phase is differentiated from the nematic phase by using the local bond order parameter (q6q6), which has helped us to demonstrate that the interface is indeed rough. Finally, the interfacial stiffness of the nm–smB interface is computed by following the CWT analysis and is found to be γnm−smB=0.39861kBT/σee2=0.04429/σss2, where σee and σss are the length and diameter of the GB LC particles.

An analysis of the structures, some energy related properties, and key aspects of the bonding nature of the microsolvated electron with four water molecules is presented. The study is based on an exhaustive potential energy surface scan of the ground state of (H2O)4− at the UCCSD(T)/6-311(3+,4+)G(d,p)//UMP2/6-311(3+,4+)G(d,p) level. A total of 18 structures, most of them not reported before, spanning in an energy range of 8.8 kcal mol−1 were found. The energetic stability of the clusters is dictated by the effect of the excess electron on their structures, on their partial fragmentation, and on the hydrogen bonds’ framework. Vertical detachment energies depend on the number of water molecules holding the excess electron in “direct contact” to their two protons at the same time and, to a lesser extent, also depend on the hydrogen bond sequence in the rest of the structure. In general, hydrogen bonds in (H2O)4− are of closed shell character, and there are other less common interactions assisted by the excess electron.

Propionitrile (CH3CH2CN, PN) is a molecule relevant for interstellar chemistry. There is credible evidence that anions, molecules, and radicals that may originate from PN could also be involved in the formation of more complex organic compounds. In the present investigation, dissociative electron attachment to CH3CH2CN has been studied in a crossed electron–molecular beam experiment in the electron energy range of about 0–15 eV. In the experiment, seven anionic species were detected: C3H4N−, C3H3N−, C3H2N−, C2H2N−, C2HN−, C2N−, and CN−. The anion formation is most efficient for CN− and anions originating from the dehydrogenation of the parent molecule. A discussion of possible reaction channels for all measured negative ions is provided. The experimental results are compared with calculations of thermochemical thresholds of the detected anions.

Glass formers are characterized by their ability to avoid crystallization. As monodisperse systems tend to rapidly crystallize, the most common glass formers in simulations are systems composed of mixtures of particles with different sizes. Here, we make use of the ability of patchy particles to change their local structure to propose them as monodisperse glass formers. We explore monodisperse systems with two patch geometries: a 12-patch geometry that enhances the formation of icosahedral clusters and an 8-patch geometry that does not appear to strongly favor any particular local structure. We show that both geometries avoid crystallization and present glassy features at low temperatures. However, the 8-patch geometry better preserves the structure of a simple liquid at a wide range of temperatures and packing fractions, making it a good candidate for a monodisperse glass former.

Dynamical or spatial properties of charged species can be obtained using electrostatic lenses by velocity map imaging (VMI) or spatial map imaging (SMI), respectively. Here, we report an approach for extracting dynamical and spatial information from patterns in SMI images that map the initial coordinates, velocity vectors, and angular distributions of charged particles onto the detector, using the same apparatus as in VMI. Deciphering these patterns required analysis and modeling, involving both their predictions from convolved spatial and velocity distributions and fitting observed images to kinetic energies (KEs) and anisotropy parameters (βs). As the first demonstration of this capability of SMI, the ensuing photoelectrons resulting from (2 + 1) resonant ionization of water in a selected rotational state were chosen to provide a rigorous basis for comparison to VMI. Operation with low acceleration voltages led to a measured SMI pattern with a unique vertical intensity profile that could be least-squares fitted to yield KE and β, in good agreement with VMI measurement. Due to the potential for improved resolution and the extended KE range achievable by this new technique, we expect that it might augment VMI in applications that require the analysis of charged particles and particularly in processes with high KE release.

Shape resonances appear when the system is trapped in an internuclear potential well after tunneling through a barrier. They manifest as peaks in the collision energy dependence of the cross section (excitation function), and in many cases, their presence can be observed experimentally. High-resolution crossed-beam experiments on the S(1D) + H2(j = 0) reaction in the 0.81–8.5 meV collision energy range reaction revealed non-monotonic behavior and the presence of oscillations in the reaction cross section as a function of the collision energy, as predicted by quantum mechanical (QM) calculations. In this work, we have analyzed the effect of shape resonances on the differential cross sections for this insertion reaction by performing additional QM calculations. We have found that, in some cases, the resonance gives rise to a large enhancement of extreme backward scattering for specific final states. Our results also show that, in order to yield a significant change in the state-resolved differential cross section, the resonance has to be associated with constructive interference between groups of partial waves, which requires not getting blurred by the participation of many product helicity states.

We report full quantum dynamical calculations for lifetimes of scattering resonances, among which are true metastable states, of the intermediate heavy ozone complex 50O3* of the 18O + 16O16O reaction, for any value of the total angular momentum quantum number J. We show that computations for nonzero values of J are mandatory in order to properly analyze resonances and time delays, with a view to establish a somewhat comprehensive eigenlife spectrum of the complex O3*. Calculations have been performed in a given low to moderate energy range, including the interval between zero-point energies (ZPEs) of reagents and product species. Quasi-bound states tend to be more numerous, and eigenlifetimes themselves are seen to increase with J, reaching unusually large values for J = 30. A very dense forest of O3* species is pictured already for J greater than 20, especially at the highest energies considered, leading to a quasi-continuum of metastable states. On the contrary, they appear as rather sparse and isolated at J = 0 and lower energies, including the domain between 18O16O and 16O16O ZPEs, embedded among many overlapping resonances that turn out to be not long-lived enough to be associated with genuine metastable states.

A realistic double many-body expansion potential energy surface (PES) is developed for the 2A″ state of the carbon–nitrogen–oxygen (CNO) system based on MRCI-F12/cc-pVQZ-F12 ab initio energies. The new PES reproduces the fitted points with chemical accuracy (root mean square deviation up to 0.043 eV) and explicitly incorporates long range energy terms that can accurately describe the electrostatic and dispersion interactions. Thermal rate coefficients were computed for the C(3P) + NO(2Π) reaction for temperatures ranging from 15 K to 10 000 K, and the values are compared to previously reported results. The differences are rationalized, and the major importance of long range forces in predicting the rate coefficients for barrierless reactions is emphasized.

Field Cycling (FC) 2H nuclear magnetic resonance (NMR) relaxometry was applied to study dynamics in Nafion NR 212 in the temperature range from 300 K to 190 K and water content of λ = 8.2. The sensitive time window of FC was extended up to eight decades using the temperature–frequency superposition principle and master curve. The rotational correlation times obtained from 2H FC NMR coincide with translational correlation times gained from static field 2H NMR diffusometry in the temperature range applied. This fact means that a long-range mass transport in Nafion is coupled to molecular rotations. It is assumed that confined water in Nafion has more ordered oxygen sublattices as compared with bulk water, on a short range is similar to ice. We discuss the possible role of D and L defects, typical for the ordered ice structure and using this concept to describe the processes of self-diffusion of confined water in Nafion, as well as the similarity of temperature and humidity dependence of self-diffusion and proton conductivity.

Vibrational predissociation (VP) has been observed in 16 bands of the C3Ar van der Waals complex near the 0 v2 0 - 000 (v2 = 2-, 4-, 2+) and 0 2- 2 - 100 bands of the A1Π-X1Σ+g system of C3. New higher resolution wavelength-resolved emission (WRE) spectra covering a wider spectral range have been recorded for many of these C3Ar bands, which show that most of the features observed in fluorescence must be reassigned as emission from the C3 fragment. Two types of VP processes have been recognized. The first type gives rise to vibrationally hot C3 fragments, mostly following |Δv| = 1, |ΔP| = 1 propensity rules, where P is the vibronic angular momentum of C3. The second type gives vibrationally cooled fragments. The VP processes can change abruptly from one type to the other with comparatively small differences in vibrational energy. Although the initial states are associated with both orbital components of the C3, A1Πu state, most of the VP fragments belong to the lower orbital component. A dipole-induced dipole model has been used to interpret the observed ΔP- propensities. Ab initio calculations of the binding energies of the ground and excited electronic states of C3Ar have been carried out; the calculated values are consistent with estimates of ≤144 cm-1 and 164 cm-1, respectively, given by the WRE spectra.

Recent studies of structural relaxation in Cu-Zr metallic glass materials having a range of compositions and over a wide range of temperatures and in crystalline UO2 under superionic conditions have indicated that the localization model (LM) can predict the structural relaxation time τα of these materials from the intermediate scattering function without any free parameters from the particle mean square displacement ⟨r2⟩ at a caging time on the order of ps, i.e., the "Debye-Waller factor" (DWF). In the present work, we test whether this remarkable relation between the "fast" picosecond dynamics and the rate of structural relaxation τα in these model amorphous and crystalline materials can be extended to the prediction of the local interfacial dynamics of model amorphous and crystalline films. Specifically, we simulate the free-standing amorphous Cu64Zr36 and crystalline Cu films and find that the LM provides an excellent parameter-free prediction for τα of the interfacial region. We also show that the Tammann temperature, defining the initial formation of a mobile interfacial layer, can be estimated precisely for both crystalline and glass-forming solid materials from the condition that the DWFs of the interfacial region and the material interior coincide.

Resonance electron attachment to short-tail analogs of coenzyme Q10 is investigated in the electron energy range 0 eV-14 eV under gas-phase conditions by means of dissociative electron attachment spectroscopy. Formation of long-lived (milliseconds) molecular negative ions is detected at 1.2 eV, but not at thermal energy. A huge increase in the electron detachment time as compared with the reference para-benzoquinone (40 µs) is ascribed to the presence of the isoprene side chains. Elimination of a neutral CH3 radical is found to be the most intense decay detected on the microsecond time scale. The results give some insight into the timescale of electron-driven processes stimulated in living tissues by high-energy radiation and are of importance in prospective fields of radiobiology and medicine.

Radicals are prevalent in gas-phase environments such as the atmosphere, combustion systems, and the interstellar medium. To understand the properties of the processes occurring in these environments, it is helpful to study radical reaction systems in isolation-thereby avoiding competing reactions from impurities. There are very few methods for generating a pure beam of gas-phase radicals, and those that do exist involve complex setups. Here, we provide a straightforward and versatile solution. A magnetic radical filter (MRF), composed of four Halbach arrays and two skimming blades, can generate a beam of velocity-selected low-field-seeking hydrogen atoms. As there is no line-of-sight through the device, all species that are unaffected by the magnetic fields are physically blocked; only the target radicals are successfully guided around the skimming blades. The positions of the arrays and blades can be adjusted, enabling the velocity distribution of the beam (and even the target radical species) to be modified. The MRF is employed as a stand-alone device-filtering radicals directly from the source. Our findings open up the prospect of studying a range of radical reaction systems with a high degree of control over the properties of the radical reactants.

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in J. Chem. Phys. 153, 104506 (2020) and may be found at https://doi.org/10.1063/5.0015371., The Lennard-Jones potential is taken as a basis to study the structure and dynamics of the face centered cubic (fcc) solid along an isochore from low temperatures up to the solid/fluid transition. The Z method is applied to estimate the melting point. Molecular dynamics simulations are used to calculate the pair distribution function, numbers of nearest neighbors, and the translational order parameter, analyzing the weakening of the fcc-symmetry due to emerging premelting effects. A range of dynamic properties, such as the mean-squared displacement, non-Gaussian parameter, Debye–Waller factor, and vibrational density of states, is considered for the analysis of the solid state. All of these parameters clearly show that bulk mobility is activated at about 2/3 of the melting temperature, known as the Tammann temperature. This indicates that vibrational motion of atoms is not maintained exclusively in the entire stable solid state and that collective atomic motion constitutes a precursor of the melting process.

We report on an experimental investigation of the low-energy stereodynamics of the energy transfer reactions Ne(3P2) + X, producing Ne(1S) + X+ and [Ne–X]+ (X = N2 or CO). Collision energies in the range 0.2 K–700 K are obtained by using the merged beam technique. Two kinds of product ions are generated by Penning and associative ionization, respectively. The intermediate product [Ne–X]+ in vibrationally excited states can predissociate into bare ions (X+). The experimental ratio of the NeX+ and X+ product ion yields is similar for both molecules at high collision energies but diverge at collision energies below 100 K. This difference is explained by the first excited electronic state of the product ions, which is accessible in the case of CO but lies too high in energy in the case of N2.

Colloids that interact via a short-range attraction serve as the primary building blocks for a broad range of self-assembled materials. However, one of the well-known drawbacks to this strategy is that these building blocks rapidly and readily condense into a metastable colloidal gel. Using computer simulations, we illustrate how the addition of a small fraction of purely repulsive self-propelled colloids, a technique referred to as active doping, can prevent the formation of this metastable gel state and drive the system toward its thermodynamically favored crystalline target structure. The simplicity and robust nature of this strategy offers a systematic and generic pathway to improving the self-assembly of a large number of complex colloidal structures. We discuss in detail the process by which this feat is accomplished and provide quantitative metrics for exploiting it to modulate self-assembly. We provide evidence for the generic nature of this approach by demonstrating that it remains robust under a number of different anisotropic short-ranged pair interactions in both two and three dimensions. In addition, we report on a novel microphase in mixtures of passive and active colloids. For a broad range of self-propelling velocities, it is possible to stabilize a suspension of fairly monodisperse finite-size crystallites. Surprisingly, this microphase is also insensitive to the underlying pair interaction between building blocks. The active stabilization of these moderately-sized monodisperse clusters is quite remarkable and should be of great utility in the design of hierarchical self-assembly strategies. This work further bolsters the notion that active forces can play a pivotal role in directing colloidal self-assembly., Comment: Supplemental Material available here: https://aip.scitation.org/doi/suppl/10.1063/5.0016514

Energy transfer between vibrational modes can be quite facile, and it has been proposed as the dominant mechanism for energy pooling in extreme environments such as nonthermal plasmas and laser cavities. To understand such processes, we perform quasi-classical trajectory studies of CO(v) + CO(v) collisions on a new full-dimensional potential energy surface fit to high-level ab initio data using a neural network method and examine the key vibrational energy transfer channels. In addition to the highly efficient CO(v + 1) + CO(v - 1) channel, there exists a significant, sometimes dominant, CO(v + 2) + CO(v - 2) channel for large v states at low collision energies. The latter is shown to stem from the substantially increased interaction between highly vibrationally excited CO, which has a much larger dipole moment than at its equilibrium bond length. Finally, the vibrational state-specific cross sections and their energy dependence on the thermal range are predicted from a limited dataset using Gaussian process regression. The relevance of these results to plasma chemistry and laser engineering and the recently observed flipping of highly vibrationally excited CO adsorbates on a cold NaCl surface is discussed.

We examine the symmetry-breaking effect of fixed constellations of particles on the surface-directed spinodal decomposition of binary blends in the presence of particles whose surfaces have a preferential affinity for one of the components. Our phase-field simulations indicate that the phase separation morphology in the presence of particle arrays can be tuned to have a continuous, droplet, lamellar, or hybrid morphology depending on the interparticle spacing, blend composition, and time. In particular, when the interparticle spacing is large compared to the spinodal wavelength, a transient target pattern composed of alternate rings of preferred and non-preferred phases emerges at early times, tending to adopt the symmetry of the particle configuration. We reveal that such target patterns stabilize for certain characteristic length, time, and composition scales characteristic of the pure phase-separating mixture. To illustrate the general range of phenomena exhibited by mixture-particle systems, we simulate the effects of single-particle, multi-particle, and cluster-particle systems having multiple geometrical configurations of the particle characteristic of pattern substrates on phase separation. Our simulations show that tailoring the particle configuration, or substrate pattern configuration, a relative fluid-particle composition should allow the desirable control of the phase separation morphology as in block copolymer materials, but where the scales accessible to this approach of organizing phase-separated fluids usually are significantly larger. Limited experiments confirm the trends observed in our simulations, which should provide some guidance in engineering patterned blend and other mixtures of technological interest.

Passive tracers in the active bath express fascinating behaviors. However, most studies are restricted to dilute active baths. Here, we use 2D simulation of suspensions consisting of active Brownian particles and a passive disk-shaped tracer to investigate tracers' diffusive behaviors in a wide range of volume fractions. Due to the competition between the thermal noise and collisions with active particles, tracers express a first transition from the normal diffusion to the superdiffusion at a short time scale and recur to normal diffusion at a long time scale. At a low volume fraction, infrequent active collisions retard the first transition of smaller tracers. At a high volume fraction, active particles with high activity aggregating around tracers induce a bimodal probability distribution function of tracer displacements during superdiffusion. Considering the enhancement of diffusion, the non-dimensional enhanced diffusivity increases asymptotically with the Peclet number. The asymptotic line gives an upper limit of non-dimensional enhanced diffusivity of tracers. Cases with lower enhanced diffusion have a high volume fraction and a low active velocity that indicates the inhibition of concentrated active baths. With the high negentropic work of these cases, the inhibition is explained as the change of the configuration of active baths for introducing tracers.

We derive a matrix formalism for the simulation of long range proton dynamics for extended systems and timescales. On the basis of an ab initio molecular dynamics simulation, we construct a Markov chain, which allows us to store the entire proton dynamics in an M × M transition matrix (where M is the number of oxygen atoms). In this article, we start from common topology features of the hydrogen bond network of good proton conductors and utilize them as constituent constraints of our dynamic model. We present a thorough mathematical derivation of our approach and verify its uniqueness and correct asymptotic behavior. We propagate the proton distribution by means of transition matrices, which contain kinetic data from both ultra-short (sub-ps) and intermediate (ps) timescales. This concept allows us to keep the most relevant features from the microscopic level while effectively reaching larger time and length scales. We demonstrate the applicability of the transition matrices for the description of proton conduction trends in proton exchange membrane materials.

Perturbations of fluid media can give rise to non-equilibrium dynamics, which may, in turn, cause motion of immersed inclusions or tracer particles. We consider perturbations ("activations") that are local in space and time, of a fluid density which is conserved, and study the resulting diffusiophoretic phenomena that emerge at a large distance. Specifically, we consider cases where the perturbations propagate diffusively, providing examples from passive and active matter for which this is expected to be the case. Activations can, for instance, be realized by sudden and local changes in interaction potentials of the medium or by local changes in its activity. Various analytical results are provided for the case of confinement by two parallel walls. We investigate the possibility of extracting work from inclusions, which are moving through the activated fluid. Furthermore, we show that a time-dependent density profile, created via suitable activation protocols, allows for the conveyance of inclusions along controlled and stable trajectories. In contrast, in states with a steady density, inclusions cannot be held at stable positions, reminiscent of Earnshaw's theorem of electrostatics. We expect these findings to be applicable in a range of experimental systems. The phenomena described here are argued to be distinct from other forms of phoresis such as thermophoresis., NSF (DMR-1708280)

Density functional theory calculations are reported that elucidate the initial steps toward Au ad-island nucleation on c(2 × 2)-Cl covered Au(001) surfaces, which is relevant for Au electrodeposition in Cl− containing electrolytes. The atomic geometry of (Auad)n adatom structures for n ≤ 4, as well as their stability as a function of Cl chemical potential, has been determined. The electrolyte, however, has not been accounted for in the computation. We find a weakly bonded (AuadCl2)-chain as the most stable structure in the case of Cl chemical potentials such that Cl vacancies are suppressed. In the range of Cl chemical potential, where Cl vacancies occur in equilibrium and bind to an (modified) Auad induced ad-structure, the formation of a dimer from two monomers is accompanied by an energy gain between 0.06 and 0.23 eV. For Auad trimer and tetramer formation, the calculations suggest a similar tendency. This suggests that on c(2 × 2)-Cl covered Au(001) surfaces, the Au ad-island nucleation is supported by the presence of vacancies in the Cl adlayer.

Dielectric relaxation measurements, in combination with density determinations, on tetramethyl tetraphenyl trisiloxane (DC704) over an unusually broad range of temperatures and pressures revealed a state-point dependency in its density scaling exponent. This is the first unambiguous experimental demonstration of a breakdown of density scaling in a nonassociated glass-forming material, and unanticipated for DC704, among the "simplest" of liquids, having a constant breadth of the relaxation dispersion and a Prigogine-Defay ratio near unity characteristic of approximate single-parameter systems. We speculate that the anomalous behavior has origins in the large value of its scaling exponent and relative flexibility of the chemical structure.

We use a computational model to investigate the emergence of interaction forces between pairs of intruders in a horizontally vibrated granular fluid. The time evolution of a pair of particles shows a maximum of the likelihood to find the pair at contact in the direction of shaking. This relative interaction is further studied by fixing the intruders in the simulation box where we identify effective mechanical forces and torques between particles and quantify an emergent long range attractive force as a function of the shaking relative angle, the amplitude, and the packing density of grains. We determine the local density and kinetic energy profiles of granular particles along the axis of the dimer to find no gradients in the density fields and additive gradients in the kinetic energies.

Ultraslow radiative cooling lifetimes and adiabatic detachment energies for three astrochemically relevant anions, Cn- (n = 3-5), are measured using the Double ElectroStatic Ion Ring ExpEriment (DESIREE) infrastructure at Stockholm University. DESIREE maintains a background pressure of approximate to 10(-14) mbar and temperature of approximate to 13 K, allowing storage of mass-selected ions for hours and providing conditions coined a "molecular cloud in a box." Here, we construct two-dimensional (2D) photodetachment spectra for the target anions by recording photodetachment signal as a function of irradiation wavelength and ion storage time (seconds to minute time scale). Ion cooling lifetimes, which are associated with infrared radiative emission, are extracted from the 2D photodetachment spectrum for each ion by tracking the disappearance of vibrational hot-band signal with ion storage time, giving 1e cooling lifetimes of 3.1 +/- 0.1 s (C3-), 6.8 +/- 0.5 s (C4-), and 24 +/- 5 s (C5-). Fits of the photodetachment spectra for cold ions, i.e., those stored for at least 30 s, provide adiabatic detachment energies in good agreement with values from laser photoelectron spectroscopy on jet-cooled anions, confirming that radiative cooling has occurred in DESIREE. Ion cooling lifetimes are simulated using a simple harmonic cascade model, finding good agreement with experiment and providing a mode-by-mode understanding of the radiative cooling properties. The 2D photodetachment strategy and radiative cooling modeling developed in this study could be applied to investigate the ultraslow cooling dynamics of a wide range of molecular anions.

Accurate analysis of pair correlations in condensed matter allows us to establish relations between structures and thermodynamic properties and, thus, is of high importance for a wide range of systems, from solids to colloidal suspensions. Recently, the interpolation method (IM) that describes satisfactorily the shape of pair correlation peaks at short and at long distances has been elaborated theoretically and using molecular dynamics simulations, but it has not been verified experimentally as yet. Here, we test the IM by particle-resolved studies with colloidal suspensions and with complex (dusty) plasmas and demonstrate that, owing to its high accuracy, the IM can be used to experimentally measure parameters that describe interaction between particles in these systems. We used three- and two-dimensional colloidal crystals and monolayer complex (dusty) plasma crystals to explore suitability of the IM in systems with soft to hard-sphere-like repulsion between particles. In addition to the systems with pairwise interactions, if many-body interactions can be mapped to the pairwise ones with some effective (e.g., density-dependent) parameters, the IM could be used to obtain these parameters. The results reliably show that the IM can be effectively used for analysis of pair correlations and interactions in a wide variety of systems and therefore is of broad interest in condensed matter, complex plasma, chemical physics, physical chemistry, materials science, and soft matter.

The diffusive transport of biased Brownian particles in a two-dimensional symmetric channel is investigated numerically considering both the no-flow and the reflection boundary conditions at the channel boundaries. Here, the geometrical confinement leads to entropic barriers which effectively control the transport properties of the particles. We show that compared to no-flow boundary conditions, the transport properties exhibit distinct features in a channel with reflection boundary conditions. For example, the nonlinear mobility exhibits a nonmonotonic behavior as a function of the scaling parameter $f$, which is a ratio of the work done to the particles to available thermal energy. Also, the effective diffusion exhibits a rapidly increasing behavior at higher $f$. The nature of reflection, i.e., elastic or inelastic, also influences the transport properties firmly. We find that inelastic reflections increase both the mobility and the effective diffusion for smaller $f$. In addition, by including the short range interaction force between the Brownian particles, the mobility decreases and the effective diffusion increases for various values of $f$. These findings, which are a signature of the entropic nature of the system, can be useful to understand the transport of small particles or molecules in systems such as microfluidic channels, membrane pores, and molecular sieves., Comment: 17 Pages, 7 Figures, Manuscript Accepted in 2019

We test the Localization Model (LM) prediction of a parameter-free relationship between the α-structural relaxation time τα and the oxygen ion diffusion coefficient DO with the Debye-Waller factor ⟨u2⟩ for crystalline UO2 under superionic conditions where large anharmonic interactions lead to non-Arrhenius relaxation and high ion mobility. As in a previous study of structural relaxation in Cu–Zr metallic glass materials having a range of compositions, we find that the LM relationship between the picosecond atomic dynamics (“fast” beta relaxation) and the long-time structural relaxation time and oxygen ion diffusion coefficient holds to an excellent approximation without any free parameters over the full range of temperatures and pressures investigated in our simulations.

By using classical density functional theory, we study the effects of interionic nonhard sphere neutral interaction (NHSNI) and solvent crowding on curve of differential electrical capacitance (Cd) vs surface charge strength |σ| of an electrical double layer (EDL) formed inside a cylindrical pore electrode. Main new findings are summarized as follows. (i) Increasing the intercounterion neutral attractive interaction helps in raising the Cd − |σ| curve and vice versa; the lowering or enhancing effect continues until rather high |σ|, such as |σ| = 1 C m−2. (ii) Attractive NHSNI between coion and counterion lowers the Cd − |σ| curve and vice versa; the lowering or enhancing effect quickly becomes very weak with |σ|. (iii) The lowering or enhancing effect strength of the intercounterion NHSNI potential range is negatively correlated with the bulk electrolyte concentration, whereas the correlation becomes positive for the potential range of the NHSNI between coion and counterion. (iv) Whether the bulk concentration is high or low, one stronger intercounterion attractive NHSNI tends to induce one more obvious peak of the Cd − |σ| curve at higher |σ|; only after this peak, the intercounterion NHSNI does not influence the Cd − |σ| curve obviously. However, the peak does not come with one intercounterion repulsive NHSNI and any kind of NHSNI between coion and counterion. (v) The solvent crowding induced by its granularity generally raises the Cd curve when |σ| is low and moderate, but the enhancing effect tends to become unobservable with further rising of |σ|. The above findings are explained reasonably by analyzing the ions local distributions in the EDL, their adsorption capacities, and the peak of change rate of the total ion adsorption capacity with |σ|.

Liquid oxygen is an example for a quadrupolar medium—a dense fluid made of nonpolar molecules carrying a significant quadrupolar moment. In this work, we present a method for the computation of the macroscopic quadrupolarizability of such a quadrupolar liquid. As a first step, the quadrupole moment and the molecular quadrupolarizability of O2 are calculated from first principles. Next, we apply a model generalizing Onsager’s dielectric cavity theory to compute the macroscopic quadrupolarizability of liquid oxygen under a wide range of conditions. Literature data for the density and dielectric permittivity of oxygen are used to determine the cavity size independently.Liquid oxygen is an example for a quadrupolar medium—a dense fluid made of nonpolar molecules carrying a significant quadrupolar moment. In this work, we present a method for the computation of the macroscopic quadrupolarizability of such a quadrupolar liquid. As a first step, the quadrupole moment and the molecular quadrupolarizability of O2 are calculated from first principles. Next, we apply a model generalizing Onsager’s dielectric cavity theory to compute the macroscopic quadrupolarizability of liquid oxygen under a wide range of conditions. Literature data for the density and dielectric permittivity of oxygen are used to determine the cavity size independently.

Transport of liquid mixtures through porous membranes is central to processes such as desalination, chemical separations, and energy harvesting, with ultrathin membranes made from novel 2D nanomaterials showing exceptional promise. Here, we derive, for the first time, general equations for the solution and solute fluxes through a circular pore in an ultrathin planar membrane induced by a solute concentration gradient. We show that the equations accurately capture the fluid fluxes measured in finite-element numerical simulations for weak solute–membrane interactions. We also derive scaling laws for these fluxes as a function of the pore size and the strength and range of solute–membrane interactions. These scaling relationships differ markedly from those for concentration-gradient-driven flow through a long cylindrical pore or for flow induced by a pressure gradient or an electric field through a pore in an ultrathin membrane. These results have broad implications for transport of liquid mixtures through membranes with thickness on the order of the characteristic pore size.Transport of liquid mixtures through porous membranes is central to processes such as desalination, chemical separations, and energy harvesting, with ultrathin membranes made from novel 2D nanomaterials showing exceptional promise. Here, we derive, for the first time, general equations for the solution and solute fluxes through a circular pore in an ultrathin planar membrane induced by a solute concentration gradient. We show that the equations accurately capture the fluid fluxes measured in finite-element numerical simulations for weak solute–membrane interactions. We also derive scaling laws for these fluxes as a function of the pore size and the strength and range of solute–membrane interactions. These scaling relationships differ markedly from those for concentration-gradient-driven flow through a long cylindrical pore or for flow induced by a pressure gradient or an electric field through a pore in an ultrathin membrane. These results have broad implications for transport of liquid mixtures through m...

Type II clathrate hydrates (CHs) were studied by thermal and dielectric measurements. All CHs amorphize, or collapse, on pressurization to 1.3 GPa below 135 K. After heating to 160 K at 1 GPa, the stability of the amorphous states increases in a process similar to the gradual high density to very high density amorphous ice (HDA to VHDA) transition. On a subsequent pressure decrease, the amorphized CHs expand partly irreversibly similar to the gradual VHDA to expanded HDA ice transformation. After further heating at 1 GPa, weak transition features appear near the HDA to low density amorphous ice transition. The results suggest that CH nucleation sites vanish on heating to 160 K at 1 GPa and that a sluggish partial phase-separation process commences on further heating. The collapsed CHs show two glass transitions (GTs), GT1 and GT2. GT1 is weakly pressure-dependent, 12 K GPa-1, with a relaxation time of 0.3 s at 140 K and 1 GPa; it is associated with a weak heat capacity increase of 3.7 J H2O-mol-1 K-1 in a 18 K range and an activation energy of only 38 kJ mol-1 at 1 GPa. The corresponding temperature of GT2 is 159 K at 0.4 GPa with a pressure dependence of 36 K GPa-1; it shows 5.5 times larger heat capacity increase and 4 times higher activation energy than GT1. GT1 is observed also in HDA and VHDA, whereas GT2 occurs just above the crystallization temperature of expanded HDA and only within its ∼0.2-0.7 GPa stable pressure range.

We investigate the effect of the Schmidt number (Sc) on phase separation dynamics of two immiscible fluids in a two-dimensional periodic box using dissipative particle dynamics. The range of Sc investigated spans liquid-liquid separation processes. Phase separation is characterized by a domain size r(t), which typically follows a power law tβ in time t, where β is a characteristic exponent corresponding to the coarsening mechanism at play. The phase separation dynamics is studied for strongly (deep quench) separating mixtures. We consider cases of critical (ϕ ∼ 0.5) and off-critical (ϕ < 0.5) mixtures of fluids A and B for both ScA = ScB and ScA ≠ ScB. In all cases, the growth dynamics slow down with the increasing Schmidt number of either fluid. We observe the power law exponent β = 0.5 for symmetric (ScA = ScB) critical mixtures and β = 0.33 for all other cases. However, for off-critical mixtures, the exponent is 0.33 irrespective of the Schmidt number ratio of the two fluids. We explain these results from an analysis of the competition between diffusive effects vis-a-vis dynamical forces.

Molecular dynamics simulations reveal anomalous short- and medium-range ordering with increasing temperature in network-forming ionic liquids (NIL) consisting of alkyl-diammonium cations with long side chains of 6 carbon atoms and citrate anions (NIL 5-6). This effect is weaker, and only a short-range order is observed in equivalent systems with side chains shortened to 3 C atoms (NIL 5-3). The short-range ordering can be attributed to volume expansion during heating, but the intermediate range order requires volume expansion as well as an increase in temperature. We find that the cross (cation-anion) interactions are the major contributors to the observed trend and the development of complex 3D correlations in the two-particle correlation functions. The simulations suggest that the above phenomenon can be correlated to local trapping of cation molecules in a variety of configurations at lower temperatures where molecular shape distributions show great variability; as temperature increases, the distribution of molecular radii of gyration becomes narrower, enabling the increased ordering.

A chain-revised Groot-Warren equation of state (crGW-EOS) was developed and tested to describe systems of homo-oligomeric chains in the framework of dissipative particle dynamics (DPD). First, thermodynamic perturbation theory is applied to introduce correction terms that account for the reduction in pressure with an increasing number of bonds at constant bead number density. Then, this EOS is modified by introducing a set of switching functions that yields an accurate second virial coefficient in the low-density limit. The crGW-EOS offers several improvements over the revised Groot-Warren equation of state and Groot-Warren equation of state for chain molecules. We tested the crGW-EOS by using it to predict the pressure of oligomeric systems and the B2 virial coefficient of chain DPD particles for a range of bond lengths. Additionally, a method is developed for determining the strength of cross-interaction parameters between chains of different compositions and sizes and for thermal and athermal mixtures. We explored how different levels of coarse-graining affect the upper-critical solution temperature.

We introduce an approximation to the short-range correlation energy functional with multide-terminantal reference involved in a variant of range-separated density-functional theory. This approximation is a local functional of the density, the density gradient, and the on-top pair density, which locally interpolates between the standard Perdew-Burke-Ernzerhof correlation functional at vanishing range-separation parameter and the known exact asymptotic expansion at large range-separation parameter. When combined with (selected) configuration-interaction calculations for the long-range wave function, this approximation gives accurate dissociation energy curves of the H2, Li2, and Be2 molecules, and thus appears as a promising way to accurately account for static correlation in range-separated density-functional theory., The Journal of Chemical Physics, In press

Two independent methods, namely, Binary-encounter Bethe (BEB) and complex scattering potential-ionization contribution (CSP-ic) methods, are employed to calculate the total ionization cross section (Qion) for cyclic organic molecules from ionization threshold to 5 keV for which there is a paucity of data in the literature. The Qion calculated with the (BEB/ωB97X) combination is found to give good agreement with the experimental results, the CSP-ic method, and the Qion calculated from Irikura orbital energies. The Qion for most of the targets are calculated for the first time over such a wide energy range. Hence, to check the consistency and reliability of the present data, we have also computed the static polarizability for all the targets and the variation of maximum ionization cross section (Qion,max) with polarizability is studied. A linear relationship is observed between these quantities indicating the consistency and reliability of the present Qion data. The targets studied are important for industrial applications, radiation biology, and astrophysics.

A new mean-field theory is developed to treat a binary mixture of nanoparticles imbedded in a polydisperse polymer solution. The theory is based on a many-body polymer-mediated potential of mean force (PMF) between the particles and remains accurate even in the protein regime, where the particles' diameters cannot necessarily be considered large compared to the polymer radius of gyration. As implemented here, the theory is strictly valid for dilute to semi-dilute polymer solutions near the theta temperature (the so-called theta regime) or when the range of the PMF is strongly affected by the polymer size. For non-adsorbing particles, this is the same regime where the celebrated Asakura-Oosawa (AO) model is often used. Unlike the traditional AO model, however, our approach includes polymer flexibility and is accurate in the protein regime. We use the theory to calculate phase diagrams for a binary mixture of unequal-sized particles, both adsorbing and non-adsorbing. To test the theory, we carry out comparisons with simulations and obtained good quantitative agreement, which gives support to its accuracy. On the other hand, the oft-used approach assuming pairwise-additive potentials of mean force produce quantitatively (and sometime qualitatively) different phase diagrams.

We report molecular-dynamics (MD) simulation results for the surface free energy of a hard-sphere fluid at cylindrical and spherical hard walls of different radii. The precision of the results is much higher than that in our previous study [B. B. Laird et al., Phys. Rev. E 86, 060602 (2012)], allowing us to estimate the size of deviations from the predictions of Morphometric Thermodynamics (MT). We compare our results to the analytical expressions for the surface energy as a function of wall radius R and fluid density derived from the White Bear II variant of the density functional theory, as well as to the leading terms of the virial expansion. For the cylindrical wall, we observe deviations from MT proportional to R−2 and R−3, which are consistent with the available virial expressions. For the spherical wall, while the precision is not sufficient to detect statistically significant deviations from MT, the MD results indicate the range of densities for which the truncated virial expansions are applicable.We report molecular-dynamics (MD) simulation results for the surface free energy of a hard-sphere fluid at cylindrical and spherical hard walls of different radii. The precision of the results is much higher than that in our previous study [B. B. Laird et al., Phys. Rev. E 86, 060602 (2012)], allowing us to estimate the size of deviations from the predictions of Morphometric Thermodynamics (MT). We compare our results to the analytical expressions for the surface energy as a function of wall radius R and fluid density derived from the White Bear II variant of the density functional theory, as well as to the leading terms of the virial expansion. For the cylindrical wall, we observe deviations from MT proportional to R−2 and R−3, which are consistent with the available virial expressions. For the spherical wall, while the precision is not sufficient to detect statistically significant deviations from MT, the MD results indicate the range of densities for which the truncated virial expansions are applicable.

Vibrational transitions, induced by collisions between rare-gas atoms and molecules, play a key role in many problems of interest in physics and chemistry. A theoretical investigation of the translation-to-vibration (T-V) energy transfer process in argon atom and sulfur dioxide molecule collisions is presented here. For such a purpose, the framework of the quasi-classical trajectory (QCT) methodology was followed over the range of translational energies 2 ≤ Etr/kcal mol-1 ≤ 100. A new realistic potential energy surface (PES) for the ArSO2 system was developed using pairwise addition for the four-body energy term within the double many-body expansion. The topological features of the obtained function are compared with a previous one reported by Hippler et al. [J. Phys. Chem. 90, 6158 (1986)]. To test the accuracy of the PES, additional coupled cluster singles and doubles method with a perturbative contribution of connected triples calculations were carried out for the global minimum configuration. From dynamical calculations, the cross sections for the T-V excitation process indicate a barrier-type mechanism due to strong repulsive interactions between SO2 molecules and the Ar atom. Corrections to zero-point energy leakage in QCT were carried out using vibrational energy quantum mechanical threshold of the complex and variations. Rate coefficients and cross sections are calculated for some vibrational transitions using pseudo-quantization approaches of the vibrational energy of products. Main attributes of the title molecular collision are discussed and compared with available information in the literature.

The effect of deuteration on the thermal conductivity κ of water, crystalline ice, and amorphous ices was studied using the pressure induced amorphization of hexagonal ice, ice Ih, to obtain the deuterated, D2O, forms of low-density amorphous (LDA), high-density amorphous (HDA), and very-high density amorphous (VHDA) ices. Upon deuteration, κ of ice Ih decreases between 3% and 4% in the 100-270 K range at ambient pressure, but the effect diminishes on densification at 130 K and vanishes just prior to amorphization near 0.8 GPa. The unusual negative value of the isothermal density ρ dependence of κ for ice Ih, g = (d ln κ/d ln ρ)T = −4.4, is less so for deuterated ice: g = −3.8. In the case of the amorphous ices and liquid water, κ of water decreases by 3.5% upon deuteration at ambient conditions, whereas κ of HDA and VHDA ices instead increases by up to 5% for pressures up to 1.2 GPa at 130 K, despite HDA’s and VHDA’s structural similarities with water. The results are consistent with significant heat transport by librational modes in amorphous ices as well as water, and that deuteration increases phonon-phonon scattering in crystalline ice. Heat transport by librational modes is more pronounced in D2O than in H2O at low temperatures due to a deuteration-induced redshift of librational mode frequencies. Moreover, the results show that κ of deuterated LDA ice is 4% larger than that of normal LDA at 130 K, and both forms display an unusual temperature dependence of κ, which is reminiscent of that for crystals (κ ∼ T−1), and a unique negative pressure dependence of κ, which likely is linked to local-order structural similarities to ice Ih.

Measurements of elastic differential cross sections for electron scattering from acetonitrile (CH3CN) have been performed utilizing a crossed electron-molecular beam experiment and with the relative flow method, for the incident electron energy range of 0.7 eV-30 eV and the scattering angle range of 10°-130°. These differential cross sections have been used to calculate the elastic integral and momentum-transfer cross sections, revealing a π * resonance located around 3 eV. The elastic differential cross sections are compared with available theoretical models using the R-matrix method and the Schwinger multichannel method.

We have studied photodetachment of the amidogen anion NH2− as a function of photon energy near the detachment threshold. The detachment spectrum is obtained over the energy range of 6190–6355 cm−1 from the loss rate of the anions from a cryogenic radiofrequency multipole ion trap. By modeling all accessible rotational state-to-state photodetachment transitions, we can assign rotational state-specific thresholds to the measured spectrum. In this way, we have determined the electron affinity of NH2 to be 6224 ± 1 cm−1.We have studied photodetachment of the amidogen anion NH2− as a function of photon energy near the detachment threshold. The detachment spectrum is obtained over the energy range of 6190–6355 cm−1 from the loss rate of the anions from a cryogenic radiofrequency multipole ion trap. By modeling all accessible rotational state-to-state photodetachment transitions, we can assign rotational state-specific thresholds to the measured spectrum. In this way, we have determined the electron affinity of NH2 to be 6224 ± 1 cm−1.

A coupled-channel Schrodinger equation model of predissociation in the B Σu−3 state of S2 is developed and optimized by comparison with recent photoabsorption spectra of the B Σu−3−X Σg−3(v,0) bands for 11 ≤ v ≤ 27, covering the energy range 35 800–41 500 cm−1. All bands in this range exhibit varying degrees of diffuseness, with corresponding predissociation linewidths Γ ≈ 4–60 cm−1 full-width at half-maximum. Model comparisons with both low-temperature (T = 370 K) and high-temperature (T = 823 K) spectra indicate, for many bands, significant dependence of the linewidth on both the rotational quantum number J and the fine-structure component Fi. Just as in the analogous case of O2, the B(v)-state predissociation in S2 is caused principally by spin-orbit interaction with 3Πu, 1Πu, 5Πu, and Σu+3 states. The inner-limb crossing with B″3Πu is responsible for the predissociation of B(v = 11) and provides a significant slowly varying contribution for B(v ≥ 12). The outer crossings with the 1Πu, 5Πu, and 2 Σu+3 states are responsible for oscillatory contributions to the predissociation widths, with first peaks at v = 13, 20, and 24, respectively, and the 5Πu contribution dominant. Prior to the photodissociation imaging study of Frederix et al. [J. Phys. Chem. A 113, 14995 (2009)], which redefined the dissociation energy of S2, the prevailing paradigm was that only the 1Πu interaction was responsible for the B(v = 11–16) predissociation: this view is not supported by our model.A coupled-channel Schrodinger equation model of predissociation in the B Σu−3 state of S2 is developed and optimized by comparison with recent photoabsorption spectra of the B Σu−3−X Σg−3(v,0) bands for 11 ≤ v ≤ 27, covering the energy range 35 800–41 500 cm−1. All bands in this range exhibit varying degrees of diffuseness, with corresponding predissociation linewidths Γ ≈ 4–60 cm−1 full-width at half-maximum. Model comparisons with both low-temperature (T = 370 K) and high-temperature (T = 823 K) spectra indicate, for many bands, significant dependence of the linewidth on both the rotational quantum number J and the fine-structure component Fi. Just as in the analogous case of O2, the B(v)-state predissociation in S2 is caused principally by spin-orbit interaction with 3Πu, 1Πu, 5Πu, and Σu+3 states. The inner-limb crossing with B″3Πu is responsible for the predissociation of B(v = 11) and provides a significant slowly varying contribution for B(v ≥ 12). The outer crossings with the 1Πu, 5Πu, and 2 Σu+3 ...