Diffusion of nanoparticles in dense fluids

The diffusion process of a single spherical nanoparticle immersed in a fluid solvent is studied by molecular dynamics simulations. When the nanoparticle mass stays constant, it is shown that, at short times, the decay of the nanoparticle velocity autocorrelation function is strongly modified when the particle diameter increases. It is also shown that, at large times, the characteristic algebraic decay induced by the hydrodynamic correlations between the solvated particle and the solvent presents a scaling behavior depending on the particle diameter.     

Ion-induced nucleation in polar one-component fluids

We present a Ginzburg-Landau theory of ion-induced nucleation in a gas phase of polar one-component fluids, where a liquid droplet grows with an ion at its center. By calculating the density profile around an ion, we show that the solvation free energy is larger in gas than in liquid at the same temperature on the coexistence curve. This difference much reduces the nucleation barrier in a metastable gas.     

Rotational relaxation of polar molecules

The method of obtaining rotational energy relaxation times from experimental thermal conductivities using the Wang Chang-Uhlenbeck theory of transport coefficient for polyatomic gases is considered. For polar gases the method turns out to be useful only if serious calculations of the inelastic collisions involved are performed. Using a semiclassical, partly statistical collision treatment the results for the rotational energy relaxation numbers for halogen hydrides taking dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interactions into account are presented. It is characteristic for the results that hardly any temperature dependence or isotope effects are observed, a behaviour different from earlier investigations.     

Polarizabilities of interacting polar molecules

Collision-induced polarizability in the electro-optical Kerr effect is shown to be the dominant contributor to the second Kerr virial coefficient B_K of dipolar gases. It gives a positive B_K and may be orders of magnitude larger than the contribution due to the intrinsic anisotropy in the polarizability of the free molecules, thus resolving a long-standing discrepancy between experiment and theory. The collision-induced contribution provides a reasonable fit to the observed B_K for the fluoromethanes if a simple Stockmayer-type potential is used.     

Optical manipulation of charged microparticles in polar fluids

In this study, we report a systematic study of the response of a charged microparticle confined in an optical trap and driven by electric fields. The particle is embedded in a polar fluid, hence, the role of ions and counterions forming a double layer around the electrodes and the particle surface itself has been taken into account. We analyze two different cases: (i) electrodes energized by a step‐wise voltage (DC mode) and (ii) electrodes driven by a sinusoidal voltage (AC mode). The experimental outcomes are analyzed in terms of a model that combines the electric response of the electrolytic cell and the motion of the trapped particle. In particular, for the DC mode we analyze the transient particle motion and correlate it with the electric current flowing in the cell. For the AC mode, the stochastic and deterministic motion of the trapped particle is analyzed either in the frequency domain (power spectral density, PSD) or in the time domain (autocorrelation function). Moreover, we will show how these different approaches (DC and AC modes) allow us, assuming predictable the applied electric field (here generated by plane parallel electrodes), to provide accurate estimation (3%) of the net charge carried by the microparticle. Vice versa, we also demonstrate how, once predetermined the charge, the trapped particle acts as a sensitive probe to reveal locally electric fields generated by arbitrary electrode geometries (in this work, wire‐tip geometry).     

Energy balance in dense microemulsions

A water-in-oil microemulsion composed of water, AOT and decane with volume fraction φ=0.50 and molar ratio X=40.8 was analysed by DSC. The percolation and the bicontinuous transitions as well as the melting endotherms and the freezing exotherms were measured. The main attention was focussed on the system energy balance. It was found that, by freezing the samples after the occurrence of the percolative transition, the total heat released is significantly less than the heat absorbed in the melting endotherms. A simple geometrical model was used as an analysis tool of the aforementioned energy difference. Since the system studied exhibits a percolative transition of dynamic type, on approaching the percolation threshold temperature (T≤T _p) and a static percolation for T≥T _p, the structural change from the connecting water-droplet-cluster to a connecting water channel was schematised in the model as a change from a sphere-necklace to a water-cylindrical channel of equal volume and equal length. The surface energy associated with the formation of the two different geometrical surfaces was evaluated and the amount of saved energy compared with the experimentally measured one.     

Entanglement of polar molecules in pendular states

In proposals for quantum computers using arrays of trapped ultracold polar molecules as qubits, a strong external field with appreciable gradient is imposed in order to prevent quenching of the dipole moments by rotation and to distinguish among the qubit sites. That field induces the molecular dipoles to undergo pendular oscillations, which markedly affect the qubit states and the dipole-dipole interaction. We evaluate entanglement of the pendular qubit states for two linear dipoles, characterized by pairwise concurrence, as a function of the molecular dipole moment and rotational constant, strengths of the external field and the dipole-dipole coupling, and ambient temperature. We also evaluate a key frequency shift, △ω, produced by the dipole-dipole interaction. Under conditions envisioned for the proposed quantum computers, both the concurrence and △ω become very small for the ground eigenstate. In principle, such weak entanglement can be sufficient for operation of logic gates, provided the resolution is high enough to detect the △ω shift unambiguously. In practice, however, for many candidate polar molecules it appears a challenging task to attain adequate resolution. Simple approximate formulas fitted to our numerical results are provided from which the concurrence and △ω shift can be obtained in terms of unitless reduced variables.     

Dielectric permittivity profiles of confined polar fluids

The dielectric response of a simple model of a polar fluid near neutral interfaces is examined by a combination of linear response theory and extensive molecular dynamics simulations. Fluctuation expressions for a local permittivity tensor epsilon(r) are derived for planar and spherical geometries, based on the assumption of a purely local relationship between polarization and electric field. While the longitudinal component of epsilon exhibits strong oscillations on the molecular scale near interfaces, the transverse component becomes ill defined and unphysical, indicating nonlocality in the dielectric response. Both components go over to the correct bulk permittivity beyond a few molecular diameters. Upon approaching interfaces from the bulk, the permittivity tends to increase, rather than decrease as commonly assumed, and this behavior is confirmed for a simple model of water near a hydrophobic surface. An unexpected finding of the present analysis is the formation of "electrostatic double layers" signaled by a dramatic overscreening of an externally applied field inside the polar fluid close to an interface. The local electric field is of opposite sign to the external field and of significantly larger amplitude within the first layer of polar molecules.     

Molecular kinetics of cluster formation in the dense fluids

In this paper we report on molecular dynamics (MD) simulations of the cluster formation kinetics in dense systems. We suggest a cluster identification procedure for dense systems that takes into account the interactions between particles. Problems of cluster existence in unsaturated compressed gas are considered and cluster formation in saturated compressed gas is studied. Molecular dynamics models of adsorbed gas condensation on the surface and the kinetics of the adsorption layer formation have been considered.     

Picosecond observations of electron solvation in dilute polar fluids

Electron solvation has been studied in dilute polar fluids in order to quantify the role of the fluid in the proposed mechanisms of electron trapping and solvation. In a series of dilute alcohol-alkane systems, the picosecond evolution of the absorption spectrum is shown to be a sensitive function of the local liquid structure and dynamics. A solvation mechanism is outlined which correlates the absorption and mobility data from neat and dilute polar fluids.     

Concentration fluctuations in binary mixtures of model polar fluids

We present calculations of the mean square concentration fluctuations, S_α(0), for binary mixtures of model polar fluids. The assumed pair interactions are taken to be of the forms of a hard core plus either dipole-dipole, quadrupole-quadrupole or dipole- quadrupole terms. The calculations are carried out within a mean field approximation. We have considered in some detail the interplay between size differences and the difference in the strength and range of the potentials in deciding how S_α(0) deviates from the ideal behaviour.     

The sound velocities in dense fluids from distribution functions

The sound velocities and adiabatic compressibilities in dense fluids have been evaluated using three known analytical expressions for radial distribution functions (RDFs). Using such approach not only tests the power of distribution functions theory in predicting the sound velocities and adiabatic compressibilities, but also specifies better expressions in determining these properties. To calculate these quantities, the variation of RDF with density and temperature is required. Therefore, we should have analytical expressions which explicitly present RDF as a function of temperature, density and interparticle distance. It is shown that if an expression is used which properly presents RDFs as a function of interparticle distance, density and temperature, it is possible to calculate sound velocities and adiabatic compressibilities from distribution function theory.     

Nonconformal interaction models and thermodynamics of polar fluids

In this work, we develop a simple potential model for polar molecules which represents effectively and accurately the thermodynamics of dilute gases. This potential models dipolar interactions whose nonpolar part is either spherical, as in Stockmayer (SM) molecules, or diatomic, as for 2-center Lennard-Jones molecules (2CLJ). Predictions of the second virial coefficient for SM and polar 2CLJ fluids for various dipole moments and elongations agree very well with results of recent numerical calculations by C. Vega and co-workers (Phys. Chem. Chem Phys. 2002, 4, 3000). The model is used to predict the critical temperature of Stockmayer fluids for variable dipole moment and is applied to HCl as an example of a real polar molecule.     

Adsorption of polar molecules on krypton clusters

The formation process of binary clusters has been studied using synchrotron based core level photoelectron spectroscopy. Free neutral krypton clusters have been produced by adiabatic expansion and doped with chloromethane molecules using the pickup technique. The comparison between the integrated intensities, linewidths, and level shifts of the cluster features of pure krypton and of chloromethane-krypton clusters has been used to obtain information about the cluster geometry. We have shown that most of the chloromethane molecules remain on the surface of the clusters.     

Electronic relaxation of small molecules in a dense medium

In this paper we present a theoretical study of radiationless transitions in a small molecule embedded in a dense inert medium. Two extreme situations of the molecule-medium coupling were considered, involving the case of zero displacements of the medium modes between the two electronic states (i.e. the Shpolskii matrix) and the limit of strong molecule-medium coupling. The Fourier transform of the non radiative decay probability of a small molecule in a Shpolskii matrix involves exponential damping, while for the strong coupling situation Gaussian damping is involved. In the case of the Shpolskii matrix the decay rate of a small molecule can be expressed in terms of an infinite series where each term corresponds to a product of an (intramolecular) Poisson distribution and a (medium induced) Lorentzian distribution. The Lorentzian widths were explicitly expressed in terms of the vibrational relaxation widths. The Robinson-Frosch formula can be obtained for the extreme case of near degeneracy in a Shpolskii matrix. In the limit of strong molecule-medium coupling the decay rate of a small molecule can be recast in terms of an infinite sum where each term involves a superposition of a Poisson distribution and a Gaussian distribution. The medium induced Gaussian distribution is determined by intramolecular phonon broadening. We have elucidated some new features of the electronic relaxation of a small molecule in a dense medium pertaining to the problem of off-resonance intramolecular coupling which modifies the energy gap law and the deuterium isotope effect.

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Zusammenfassung Strahlungslose Übergänge in einem kleinen Molekül, das von einem dichten inerten Medium umgeben ist, werden untersucht, wobei zwei Grenzfälle bei der Kopplung Molekül/Medium zugrunde gelegt werden: keine Verschiebungen der Medium-Bewegungen beim Übergang (d.h. der Shpolskii-Matrix) einerseits und starke Kopplung Molekül/Medium andererseits. Die Fouriertransformierte für die Wahrscheinlichkeit des strahlungslosen Zerfalls eines kleinen Moleküls in Form einer Shpolskii-Matrix schließt exponentielle Dämpfung ein, wohingegen bei starker Kopplung die Dämpfung einer Gauss-Funktion entspricht. Im ersteren Fall läßt sich der Zerfall als unendliche Reihe von Produkten einer intramolekularen Poisson-Verteilung mit einer vom Medium induzierten Lorentz-Verteilung formulieren, wobei die Lorentz-Breite explizit mittels der Schwingungsrelaxationsbreiten angegeben wird. Die Robin-Frosch-Formel ergibt sich für den Grenzfall der Fastentartung der Shpolskii-Matrix. Bei starker Molekül-Medium-Kopplung laßt sich der Zerfallsverlauf als unendliche Summe von Überlagerungen von Poisson- und Gaussverteilungen angeben. Dabei wird die Medium-induzierte Gauss-Verteilung durch die intramolekulare Phononen-Verbreiterung bestimmt. In diesem Zusammenhang zeigten sich einige neue Gesichtspunkte für die elektronische Relaxation kleiner Moleküle in dichten Medien, wie z. B. das Problem von Nicht-Resonanz bei intramolekularer Kopplung, wo der Satz vom Energie-Sprung und der Deuterium-Isotopie-Effekt modifiziert werden müssen.