bluuebunny/crossref_metadata_2025 huggingface dataset:
Fluorescence quenching with exponential distance dependence: Application to the external heavy-atom effect
Authors: Rae M. , Fedorov A. , Berberan-Santos M. N.
Published: July 2003

A model for fluorescence quenching with exponential distance dependence is developed and applied to external heavy-atom quenching. The systems studied are C70-bromobenzene and phenanthrene-iodide, in liquid solutions at room temperature and in rigid glasses at 77 K. The predicted parameter correlation is observed in the two systems, which correspond to two extreme and opposite situations, strong and weak quenching. A detailed analysis of the fitted parameters allows the determination of the effective Bohr radii L and of the intrinsic unimolecular rate constants for quenching at contact, k0. The unusually high value of L for the C70-bromobenzene pair is tentatively attributed to the extended size of the external part of the π orbitals of the fullerene. The room temperature quenching rate constant computed with the determined low-temperature parameters is shown to be in good agreement with the experimental one for both systems studied.
The microscopic origin of optical phonon evolution during water oxidation of Si(100)
Authors: Queeney K. T. , Weldon M. K. , Chabal Y. J. , Raghavachari Krishnan
Published: July 2003

We present the results of a combined theoretical and experimental study of the growth of extended surface vibrational modes during the initial oxidation of Si(100)-(2×1). Controlled incorporation of oxygen into the first two layers of the silicon surface is achieved via sequential water (H2O) exposures and stepwise annealing in ultrahigh vacuum. Transmission infrared spectroscopy is used to monitor the transition from vibrational modes characteristic of isolated Si–O structures, so-called silicon epoxides, to extended modes with the character of transverse optical and longitudinal optical phonons in an extended SiO2 film. Quantum chemical calculations on two-dimer silicon clusters identify these modes as arising predominantly from coupling between species on adjacent dimer units, consistent with the thermodynamic driving force for local oxygen agglomeration. The vibrational signature of this surface is proposed to offer a link between well-characterized local surface Si–O structures and the extended SiOx film that exists between Si(100) and SiO2 after thermal oxidation of silicon.
An algorithm to describe molecular scale rugged surfaces and its application to the study of a water/lipid bilayer interface
Authors: Pandit Sagar A. , Bostick David , Berkowitz Max L.
Published: July 2003

We propose an algorithm for the general description of rugged molecular scale interfacial surfaces. This algorithm was implemented in the description of a phospholipid membrane/water interface with the rugged surface defined by the phospholipid phosphorous atoms. The method allowed us to clearly discern four layered regions of water based upon the water local density as a function of the distance from the membrane surface. Furthermore, the water in each of the layered regions was found to have distinct orientational properties. The classification we make based on density due to our new algorithm is in agreement with that delineated in previous studies based on water orientation. The contribution of the different water regions to the total electrostatic potential reveals the particular way in which each layer’s water polarization contributes to the total dipole potential of the hydrated membrane.
Squeezing molecular thin alkane lubrication films between curved solid surfaces with long-range elasticity: Layering transitions and wear
Authors: Sivebaek I. M. , Samoilov V. N. , Persson B. N. J.
Published: July 2003

The properties of alkane lubricants confined between two approaching solids are investigated by a model that accounts for the curvature and the elastic properties of the solid surfaces. We consider linear alkane molecules of different chain lengths, C3H8, C4H10, C8H18, C9H20, C10H22, C12H26, and C14H30 confined between smooth gold surfaces. In most cases we observe well defined molecular layers develop in the lubricant film when the width of the film is of the order of a few atomic diameters. An external squeezing-pressure induces discontinuous, thermally activated changes in the number n of lubricant layers. We find that with increasing alkane chain length, the transition from n to n−1 layers occurs at higher pressure, as expected based on the increasing wettability (or spreading pressure) with increasing chain length. Thus, the longer alkanes are better boundary lubricants than the shorter ones, and this should result in less wear. We obtain good correlation between our theoretical results and wear experiments.
Theoretical ab initio study of the electronic states of KrH and KrH+: Quantum defect and complex coordinate calculations on the Rydberg states of KrH
Authors: Petsalakis Ioannis D. , Theodorakopoulos Giannoula , Buenker Robert J.
Published: July 2003

Potential energy curves have been calculated for the ground and excited electronic states of KrH and the cation KrH+ by ab initio configuration interaction calculations using effective core potentials for Kr. Quantum defect functions have been determined from the ab initio potentials of the low-lying Rydberg states of KrH and potential energy curves have been generated for higher n (s,p,d) Rydberg states. The resulting bound–bound transition energies are in excellent agreement with experimental data. The interaction of the 5p B 2Π state with the 5s and 5p A 2Σ and C 2Σ+ states and their predissociation by X 2Σ+ has been treated by multistate complex scaling calculations for both KrH and KrD. Much larger predissociation widths are obtained in KrH than in KrD, in agreement with experimental observations.
Physical interpretation and evaluation of the Kohn–Sham and Dyson components of the ε–I relations between the Kohn–Sham orbital energies and the ionization potentials
Authors: Gritsenko O. V. , Braïda B. , Baerends E. J.
Published: July 2003

Theoretical and numerical insight is gained into the ε–I relations between the Kohn–Sham orbital energies εi and relaxed vertical ionization potentials (VIPs) Ij, which provide an analog of Koopmans’ theorem for density functional theory. The Kohn–Sham orbital energy εi has as leading term −niIi−∑j∈Ωs(i)njIj, where Ii is the primary VIP for ionization (φi)−1 with spectroscopic factor (proportional to the intensity in the photoelectron spectrum) ni close to 1, and the set Ωs(i) contains the VIPs Ij that are satellites to the (φi)−1 ionization, with small but non-negligible nj. In addition to this “average spectroscopic structure” of the εi there is an electron-shell step structure in εi from the contribution of the response potential vresp. Accurate KS calculations for prototype second- and third-row closed-shell molecules yield valence orbital energies −εi, which correspond closely to the experimental VIPs, with an average deviation of 0.08 eV. The theoretical relations are numerically investigated in calculations of the components of the ε–I relations for the H2 molecule, and for the molecules CO, HF, H2O, HCN. The derivation of the ε–I relations employs the Dyson orbitals (the ni are their norms). A connection is made between the KS and Dyson orbital theories, allowing the spin-unrestricted KS xc potential to be expressed with a statistical average of individual xc potentials for the Dyson spin–orbitals as leading term. Additional terms are the correction vc,kin,σ due to the correlation kinetic effect, and the “response” vresp,σ, related to the correction to the energy of (N−1) electrons due to the correlation with the reference electron.
Free-energy calculations of elemental sulphur crystals via molecular dynamics simulations
Authors: Pastorino C. , Gamba Z.
Published: July 2003

Free-energy calculations of two crystalline phases of the molecular compound S8 were performed via molecular dynamics (MD) simulations of these crystals. The elemental sulphur S8 molecule model used in our MD calculations consists of a semiflexible closed chain, with fixed bond lengths and intramolecular potentials for its bending and torsional angles. The intermolecular potential is of the atom–atom Lennard-Jones type. Two free-energy calculation methods were implemented: the accurate thermodynamic integration method and an estimation that takes into account the contribution of the zero-point energy and the entropy of the crystalline vibrational modes to the free energy of the crystal. The last estimation has the enormous advantage of being easily obtained from a single MD simulation. Here, we compare both free-energy calculation methods and analyze the reliability of the fast estimation via the vibrational density of states obtained from constrained MD simulations. New results on α– and α′–S8 crystals are discussed.
Quantum/classical studies of O(3P)+Ar⋅HCl collision dynamics
Authors: Wang Lichang , McCoy Anne B.
Published: July 2003

The dynamics of the O(3P)+HCl and Ar–HCl reactions is investigated using a multiple configuration quantum/classical approach. In this work the dynamics of the hydrogen atom is propagated quantum mechanically in the three Cartesian coordinates of the atom, while the dynamics of the other atoms is propagated classically, in a center-of-mass frame. It is found that the introduction of the argon atom affects the reaction probability through two mechanisms. For nearly collinear O+Ar–HCl collisions, the argon atom blocks the transition state for the O+HCl reaction and inhibits the reaction. On the other hand when the collision geometry is such that the oxygen atom does not collide with the argon atom, the reaction probability is increased. These results are analyzed in terms of the shape of the ground state Ar–HCl wave function. The energy transfer dynamics from the oxygen atom and to the argon atom is also investigated.
Photon statistics in the fluorescence of single molecules and nanocrystals: Correlation functions versus distributions of on- and off-times
Authors: Verberk Rogier , Orrit Michel
Published: July 2003

The stream of photons emitted by a single quantum system such as a molecule or a nanocrystal is often statistically characterized by the distribution of delays between consecutive photons, or by the autocorrelation function of the intensity, or by the distributions of on- and off-times. We derive and discuss general relations between their Laplace transforms, addressing the influence of detection yield and background. Our analytical treatment applies to any distribution of delays and to random telegraph signals, including nonexponential distributions. We examine the special case of systems switching between two states characterized by different distributions of delays, where the switching can obey various statistics. We show that the second-order autocorrelation function keeps track of long-time fluctuations which are obviously lost in averaging the distributions of delays. We apply our formalism to random telegraphs, in particular to those with power-law distributions of on- and/or of off-times, which are encountered in the blinking of single semiconductor nanocrystals.
A study of the molecular structure and Renner–Teller effect in the Ã 2Πu–X̃ 2Πg electronic spectrum of jet-cooled boron disulfide, BS2
Authors: He Sheng-Gui , Evans Corey J. , Clouthier Dennis J.
Published: July 2003

The Ã 2Πu–X̃ 2Πg electronic band system of jet-cooled boron disulfide (BS2) has been recorded in the 760–530 nm region by laser-induced fluorescence (LIF) and wavelength resolved emission techniques. The free radical was produced in a pulsed electric discharge jet using a precursor mixture of BCl3 and CS2 in argon. The LIF spectrum consists of a long progression of strong bands involving the ν1′ (B–S stretching) mode and combinations involving 2ν2′ and 2ν3′, as well as many hot bands built on the 211 band. Detailed analysis of the spectrum shows that the ground state exhibits a substantial Renner–Teller effect with ε=−0.2012 and a spin–orbit coupling constant of A=−413.80 cm−1. Angular momentum coupling is negligible in the excited state, which has a much smaller spin–orbit coupling constant of A=−259.15 cm−1. The vibrational frequencies of BS211 are ω1″=542.8, ω2″=285.0, ω3″=1010.5, ω1′=514.1, ω2′=309.7, and ω3′=1543.6 cm−1. Rotational analysis of the 000 band allowed a determination of the effective molecular structures as r0″(BS)=1.6648(4) Å and r0′(BS)=1.7127(5) Å. Ab initio methods have been used to predict the BS2 spectroscopic parameters and good agreement with experiment was found. Our present understanding of the LIF spectrum is in accord with the pioneering analysis of the absorption spectrum of matrix isolated BS2 published by Brom and Weltner [J. Mol. Spectrosc. 45, 82 (1973)] three decades ago.
Monte Carlo simulation of polymer chain with ferromagnetic Ising interaction
Authors: Luo Meng-Bo , Huang Jian-Hua
Published: July 2003

We consider a self-avoiding walking polymer chain with nearest-neighbor ferromagnetic Ising interaction (J>0) on the simple cubic lattice. The magnetic and conformational properties are simulated using Monte Carlo technique with bond-fluctuation model. Spontaneous magnetization is observed at low temperature. We estimate the critical temperature Tc=1.37±0.05(J/kB) and the critical exponents α≈0.42 and ν≈0.80 based on the finite-size scaling law. The values of α and ν are different from those of two- or three-dimensional Ising models, indicating that the phase transition in the Ising polymer chain model belongs to a different universality class. Near Tc, the chain also undergoes a collapse transition from random coil at T>Tc to compacted globule at T<Tc. We conclude that the phase transitions in the Ising polymer chain model result from the cooperation of configurational movement and the ferromagnetic Ising interaction.
A new double-rebridging technique for linear polyethylene
Authors: Banaszak Brian J. , de Pablo Juan J.
Published: July 2003

A variable connectivity, double-rebridging Monte Carlo (MC) technique is developed for simulation of long chain molecules. The method changes the connectivity of inner segments of two chain molecules by making use of a recently proposed inner-chain rebridging scheme [Chen et al., J. Chem. Phys. 113, 11382 (2000)]. The new method yields results consistent with other molecular dynamics and MC methods, but it enhances considerably the rate of equilibration of chain end-to-end vectors for long molecules. The new method is tested for linear polyethylene melts at 600 K. Polyethylene is modeled as linear 200 and 1000 carbon chains, respectively, using the NERD united-atom force-field (Nath, Escobedo, and de Pablo revised united-atom force field) [Nath et al., J. Chem. Phys. 108, 9905 (1998); Mol. Phys. 98, 231 (2000); J. Chem. Phys. 114, 3612 (2001)].
Aggregation of ions in methanol–water solutions of sodium halides
Authors: Hawlicka Ewa , Swiatla-Wojcik Dorota
Published: July 2003

Molecular-dynamics simulations have been performed for NaCl and NaI solutions in methanol–water mixtures to investigate an association of ions. Molecules of water and methanol have been represented by flexible models. Iodide ion was considered as charged Lennard-Jones sphere, whereas interactions of sodium and chloride ions were described by either the sum of Coulomb and Lennard-Jones potential or the potentials derived from ab initio calculations. Analysis of aggregation has been based on radial distribution functions, association constants, geometry and lifetime of the ion clusters. Computed association constants are in good agreement with experimental data. Clustering of either unlike or like ions occurs in all simulated systems and the ratio of free ions to aggregated ions is independent of the solvent composition.
Polarizability anisotropy relaxation in pure and aqueous dimethylsulfoxide
Authors: Skaf Munir S. , Vechi Sérgio M.
Published: July 2003

A molecular dynamics simulation study is presented for the relaxation of the polarizability anisotropy of liquid dimethylsulfoxide (DMSO) and DMSO–water mixtures of DMSO mole fractions xD=0.05, 0.10, 0.25, 0.50, and 0.75. The system’s collective polarizability is computed through a dipolar induction mechanism involving the intrinsic polarizability and first hyperpolarizability tensors for water and DMSO, obtained from ab initio quantum chemical calculations at the MP2/6-311++G(d,p) level. The rotational-diffusion components of the anisotropy relaxation of the pure liquids increase upon mixing to a maximum near 25% DMSO, showing consistency with other dynamical properties of these mixtures. Features of the optical Kerr effect (OKE) nuclear response of liquid water, previously ascribed to hydrogen bonding distortions, show significant enhancement upon addition of DMSO due to the formation of strong DMSO–water H-bonds. The OKE spectrum for DMSO is in close agreement with experimental measurements, but there are discrepancies for pure water in the vicinity of 60 cm−1, pointing to the existence of inaccuracies in our description of OKE sensitive polarizability fluctuations of water. The mixtures OKE spectra feature an enhancement in the high frequency water librational band.
On the perturbation of multiconfiguration wave functions
Authors: Rolik Zoltán , Szabados Ágnes , Surján Péter R.
Published: July 2003

A simple variant of perturbation theory is used to correct reference states of a general multiconfigurational character. The full solution of an active space is not required, and no iterative procedure is applied to construct the resolvent operator. The perturbed wave function is expanded in a complete set of determinants from which the reference function is projected out, and the overlap between projected determinants is handled by an explicit, analytic inversion of the overlap matrix.
A modified fundamental measure theory for spherical particles in microchannels
Authors: Yu Yang-Xin , Wu Jianzhong
Published: July 2003

Canonical-ensemble Monte Carlo simulation and an improved fundamental-measure theory are applied to calculating the structures and chemical potentials of neutral and associating spherical particles confined in rectangular or corrugated microchannels. It is found that the confinement significantly affects the distributions of neutral spheres in the microchannels, especially at high densities or near the confining surfaces. However, for associating particles, the combined effects of packing and association lead to virtually uniform density distributions. The density profiles calculated from the density functional theory agree well with simulation results for neutral hard spheres in both rectangular and corrugated microchannels except when the average packing density inside the channel is near the freezing point.
Generalization of the Nosé–Hoover approach
Authors: Brańka A. C. , Kowalik M. , Wojciechowski K. W.
Published: July 2003

A simple extension of the Nosé–Hoover canonical dynamics based on a more general form of the thermostat kinetic energy term in the Nosé Hamiltonian is considered. With this extension considerable enhancement of chaotic behavior is achieved, particularly for small and stiff systems. The considered deterministic thermostat exhibits most of the desirable properties of a good canonical thermostatting mechanism.
Knudsen self- and Fickian diffusion in rough nanoporous media
Authors: Malek Kourosh , Coppens Marc-Olivier
Published: July 2003

The effect of pore surface roughness on Knudsen diffusion in nanoporous media is investigated by dynamic Monte Carlo simulations and analytical calculations. A conceptual difference is found between the roughness dependence of the macroscopic, transport diffusivity and the microscopic, self-diffusivity, which is reminiscent of diffusion in zeolites, where a similar difference arises due to adsorption effects and intermolecular interactions. Because of the dependence of the self-diffusivity on molecular residence times, self-diffusion may be roughness dependent, while transport diffusion is not. Detailed proofs are given. The differences become significant when the pore surface is rough down to molecular scales, as is the case, e.g., for many common sol–gel materials. Simulations are in good agreement with analytical calculations for several tested rough, fractal pore structures. These results are important for the interpretation of experimental diffusion measurements and for the study of diffusion-reaction processes in nanoporous catalysts with a rough internal surface.
A simple harmonic model as a caricature for mismatch and relaxation effects for ion hopping dynamics in solid electrolytes
Authors: Kunow Magnus , Heuer Andreas
Published: July 2003

We formulate a simple harmonic mean-field model with N+1 particles and analyze the relaxation processes following a jump of one of these particles. Either the particle can jump back (single-particle route) or the other N particles adjust themselves (multiparticle route). The dynamics of this model is solved analytically in the linear response regime. Furthermore, we relate these results to a phenomenological approach by Funke and co-workers (concept of mismatch and relaxation, CMR) which has been successfully used to model conductivity spectra in the field of ion dynamics in solid electrolytes. Since the mean-field model contains the relevant ingredients of the CMR-approach, a comparison of the resulting rate equations with the CMR-equations becomes possible. Generalizations beyond the mean-field case are discussed.
Single molecule vibrational and electronic analyses of the formation of inorganic complexes: CO bonding to Au and Ag atoms on NiAl(110)
Authors: Wallis T. M. , Nilius N. , Ho W.
Published: July 2003

The bonding of CO molecules to single Au and Ag atoms on a NiAl surface was studied with a scanning tunneling microscope. A peak at 35 meV in the vibrational spectra of individual AuCO molecules is assigned to the hindered rotational mode. Vibrational spectra of individual AgCO molecules displayed peaks at 26 meV and 266 meV. These peaks are assigned to the hindered rotational mode and the C–O stretch mode, respectively. Scanning tunneling spectroscopy measurements show an unoccupied electronic resonance about 2.0 eV above the Fermi energy on single Au and Ag atoms. This resonance is quenched when monocarbonyls are formed due to electron donation from the CO ligand into the unoccupied electronic resonance.
An intermediate state representation approach to K-shell ionization in molecules. II. Computational tests
Authors: Thiel Alexander , Schirmer Jochen , Köppel Horst
Published: July 2003

We report on a new implementation and first numerical tests of the fourth-order algebraic–diagramatic construction [ADC(4)] propagator method for K-shell ionization in molecules. The theory, which has been presented in a preceding paper (paper I) [J. Chem. Phys. 115, 10621 (2001)], is based on an intermediate state representation (ISR) or non-Dyson reformulation of the general ADC approach and uses the core–valence separation approximation to specialize the method to the case of core-level ionization. The ISR form offers the possibility to go beyond the finite perturbation-theoretical expressions of the previous (strict) ADC(4) scheme, and several such modifications are considered. The general aim of the present development is to establish a practical “frozen” orbital method that can be applied equally well to systems with and without equivalent core levels. A set of small model calculations has been carried out on the 1s ionization in CO and N2 allowing us to compare the ADC results with those of large-scale configuration interaction treatments and analyze the performance of several ADC(4) modifications in describing the 1s ionization energies as a function of the bond lengths. These studies clearly demonstrate the need for improving the previous (strict) ADC(4) scheme. In the theoretical description of the C1s and N1s ionization, the proposed modifications lead to a substantial improvement, whereas in the case of the O1s ionization further developments appear to be necessary. In another set of calculations using reasonably large basis sets, the improved ADC(4) method is applied to the vibrational structure in the CO C1s and N2 N1s ionization spectra. The results are in very good agreement with the findings of recent high-resolution measurements. An interesting feature is the different behavior of the g and u components of the N1s ionization energy curves of N2 leading to different vibrational line intensity distributions.
Spin–orbit density functional theory calculations for heavy metal monohydrides
Authors: Choi Yoon Jeong , Lee Yoon Sup
Published: July 2003

Spin–orbit density functional theory method implemented in the NWCHEM program package has been employed with the shape-consistent relativistic effective core potentials to calculate spectroscopic constants (bond lengths, frequencies, and dissociation energies) and estimate spin–orbit effects for 6th(Tl–At)- and 7th(113–117)-row element monohydrides. Results calculated with local density approximation and gradient-corrected approximation of the exchange-correlation functional are usually similar to those of other all-electron relativistic density functional approaches. The spin–orbit effects on the spectroscopic constants are in good agreement with previous two-component coupled-cluster singles and doubles with perturbative triples results calculated with same relativistic effective core potentials and basis sets. Spin–orbit density functional theory calculations with extended basis sets and extensive set of functionals for TlH, (113)H, PbH, (114)H, and PbO molecules indicate that there could be substantial variations among functionals and that the hybrid functionals produce the results in excellent overall agreement with empirical measures.
Phase separation dynamics in binary fluids containing quenched or mobile filler particles
Authors: Laradji Mohamed , MacNevin Grant
Published: July 2003

The dynamics of phase separation of binary fluids in the presence of quenched or mobile filler particles, with preferential attraction for one of the two fluid components, is investigated by means of extensive molecular dynamics simulations in two dimensions. When the filler particles are quenched, we found that they lead to a slowing-down of the kinetics that is enhanced as the density of the filler particles is increased. The domain growth in this case is found to follow a crossover scaling form which links domain growth in pure binary mixtures to that in the presence of quenched filler particles. On the other hand, when the filler particles are annealed, systematic simulations for various values of single filler particle mass, μc, and filler particle density, ρc, show that the filler particles only affect the nonuniversal prefactor of the power law. The power law itself remains given by t2/3, characteristic of inertial growth that is typically observed in pure binary fluid mixtures. The prefactor is found to depend on μc as μc−1/3 as expected in phase separating fluid in the inertial regime.
Buckling and persistence length of an amphiphilic worm from molecular dynamics simulations
Authors: den Otter W. K. , Shkulipa S. A. , Briels W. J.
Published: July 2003

A wormlike micelle of coarse-grained amphiphilic molecules is simulated with molecular dynamics. We demonstrate that our worm is inherently stable, i.e., it does not depend on periodic boundary conditions for its continued survival, which sets it apart from some, and perhaps even all, previously simulated worms. The worms are observed to buckle under sufficiently strong compression forces. The persistence length and bending rigidity follow from analyzing the thermal undulations of a tensionless worm. System size dependencies of the elastic modulus of the worm, as reported for amphiphilic bilayers, are eliminated by explicitly calculating the arc length of the worm.
Optical properties of CdSe quantum dots
Authors: Troparevsky M. Claudia , Kronik Leeor , Chelikowsky James R.
Published: July 2003

Ab initio absorption spectra and optical gaps for CdnSen (n=17,26,38) quantum dots are calculated using time-dependent density functional theory within the local density approximation. The spectra computed using the time-dependent local density approximation technique differ from the ones calculated using conventional approaches in both absorption threshold and spectral features. The time-dependent density functional spectra suppress surface contributions in the lower-energy region of the spectra. These contributions come from the Se atoms at the surface with two dangling bonds. The optical gaps calculated using the time-dependent approach are in good agreement with experimental results.
Adsorption hysteresis in ink-bottle pore
Authors: Morishige Kunimitsu , Tateishi Noriko
Published: July 2003

To examine the mechanism of the adsorption hysteresis in ink-bottle pores, we measured the temperature dependence of the adsorption–desorption isotherms of argon, oxygen, and carbon dioxide onto SBA-16 ordered mesoporous material with cagelike pores. The hysteresis loop always shrank with increasing temperature and eventually disappeared at a hysteresis temperature (Th), well below the bulk critical temperature (Tc). When the relative pressures p/p0 of the capillary condensation and evaporation are plotted as a function of reduced temperature T/Tc, all the data including the transition pressures for nitrogen reported previously are represented by a common curve. We also calculated the temperature dependence of the capillary condensation and evaporation pressures of nitrogen under the assumption that adsorption and desorption in an ink-bottle pore may be regarded as the process of the disappearance and formation of a gas bubble in a liquid droplet confined to the pore. A fit between the observed and calculated transition pressures in a wide temperature range was reasonable in light of several assumptions and approximations used. This clearly indicates that the energy barrier for the formation and disappearance of vapor bubbles in the liquid confined to the pores is responsible for the appearance of the adsorption hysteresis and the hysteresis temperature is not concerned with the so-called capillary criticality. At temperatures higher than Th, the reversible capillary condensation takes place, because the energy barrier between a full liquid pore and the vapor coexisting with the liquid film becomes surmountable.
The kinetics of mixing and the fluctuation theorem in ideal mixtures of two-component model fluids
Authors: Baranyai András
Published: July 2003

Using a simple combinatorial model we show that the fluctuation theorem holds for the nonequilibrium transient process of mixing in ideal mixtures of two-component fluids. We show that the fluctuation theorem is very closely connected to the phenomenological formalism of reaction kinetics, in particular, to unimolecular reactions.
Integral equation theories for orientionally ordered fluids
Authors: Paci I. , Cann N. M.
Published: July 2003

Integral equation theories have been quite successful at providing structural information for isotropic fluids. In particular, the expansion method originally proposed by Blum and Torruella [J. Chem. Phys. 56, 303 (1972)] has been pivotal in making these theories feasible for molecular fluids. Recently, integral equation theories have also been applied to nematic phases for simple one-component fluids. In this article, we derive the formalism appropriate for fully, and partially, orientationally anisotropic fluids. Appropriate expansions of the pair and singlet functions are employed to obtain tractable expressions for the three coupled equations which must be solved. The numerical solution of these equations is discussed and simplifications resulting from symmetry are considered in detail. Numerical results are presented for fluids of dipolar ellipsoidal molecules in an external field.
Vibrational energy relaxation in classical fluids. II. High-frequency spectra in liquids
Authors: Teubner Max , Schwarzer Dirk
Published: July 2003

A procedure is outlined to determine high-frequency spectra of classical liquids interacting via Lennard-Jones and similar potentials and applied to the problem of vibrational energy relaxation. The theory is based on analytical expressions derived for spherical particles in gases [Paper I, D. Schwarzer and M. Teubner, J. Chem. Phys. 116, 5680 (2002)] and is extended to the dense liquid phase by considering binary collisions in the potential of mean force. The calculated spectra are in good agreement with those derived from classical trajectory and molecular dynamics simulations.
Molecular dynamics study of the photodissociation and photoisomerization of ICN in water
Authors: Winter Nicole , Chorny Ilya , Vieceli John , Benjamin Ilan
Published: July 2003

The photodissociation and photoisomerization of ICN in water is studied using molecular dynamics simulations. A water–ICN potential energy function that takes into account the different ground and excited state charges and their shift as a function of the reaction coordinate is developed. The calculations include nonadiabatic transitions between the different electronic states and allow for a complete description of the photodissociation leading to ground-state and excited-state iodine and to recombination producing ICN and INC. The calculated UV absorption spectrum, the cage escape probability, the quantum yield of ICN and INC, and the subsequent vibrational relaxation rate of ICN and INC are in reasonable agreement with recent experiments. The trajectories provide a detailed microscopic picture of the early events. For example, it is shown that most recombination events on the ground state involve nonadiabatic transitions before the molecule has a chance to completely dissociate on the excited state, and that the quantum yield for photoisomerization to form INC is statistically determined very early in the photodissociation process.
A microscopic approach to studying colloidal stability
Authors: Sun Zhiwei , Xu Shenghua , Dai Guoliang , Li Yinmei , Lou Liren , Liu Qiusheng , Zhu Ruzeng
Published: July 2003

A novel approach to the investigation of colloidal aggregation in experiments performed at microscopic particle levels by means of artificially induced particle collisions with the aid of optical tweezers is proposed. A physical model describing the artificially induced collisions was suggested. The stability ratios for different electrolyte concentrations (NaCl) are estimated by dividing the total number of particle collisions by the number of collisions leading to permanent doublets. The experimental results under different electrolyte concentrations are compared with zeta potentials and turbidity measurements. In regards to fully dispersed or rapid-coagulated systems, this approach can conveniently provide an accurate judgment of the system’s stability. For general slow-coagulated systems, when the stability ratio is greater than 1.8, our data are in agreement with those obtained from the turbidity measurements. When the stability ratio is less than 1.8, the accuracy of the approach is limited due to the possibility of the imprecise control of the short duration during which the particle pair is held in the optical trap for the induced collisions.
Conformational dynamics of phenylene rings in poly(p-phenylene vinylene) as revealed by 13C magic-angle-spinning exchange nuclear magnetic resonance experiments
Authors: deAzevedo E. R. , Franco R. W. A. , Marletta A. , Faria R. M. , Bonagamba T. J.
Published: July 2003

Poly (p -phenylene vinylene) (PPV) has shown a great potential for electro-optical applications due to its electroluminescent and semiconducting properties. Such properties are directly related with the polymer chain conformation and dynamics. Then, it is important to understand in detail the local chain motions. In this work, three 13C solid-state magic-angle-spinning (MAS) exchange NMR techniques were used to study conformational dynamics of phenylene rings in PPV. The standard 2D MAS exchange experiment was used to identify exchange processes between equivalent and nonequivalent sites. Centerband-only detection of exchange (CODEX) experiments were applied to determine the amplitude of the phenylene ring flips and small-angle oscillations. Additionally, a new version of the CODEX technique, which allows for the selective observation of segments executing exchange between non-equivalent sites, is demonstrated and applied to determine the flipping fractions and the activation energies of the phenylene ring rotations. It was found that, at −15 °C, (26±3)% of the rings undergo 180° flips in the millisecond time scale, with average imprecision of (30±5)° and activation energies of (23±3) kJ/mol. Other (31±10)% of the rings perform only small-angle oscillations with an average amplitude of (9±2)°. These results corroborate previous experimental data and agree with recent ab initio calculations of potential energies barriers in phenylenevinylene oligomers.
Two-chamber lattice model for thermodiffusion in polymer solutions
Authors: Luettmer-Strathmann Jutta
Published: July 2003

When a temperature gradient is applied to a polymer solution, the polymer typically migrates to the colder regions of the fluid as a result of thermal diffusion (Soret effect). However, in recent thermodiffusion experiments on poly(ethylene-oxide) (PEO) in a mixed ethanol/water solvent it is observed that for some solvent compositions the polymer migrates to the cold side, while for other compositions it migrates to the warm side. In order to understand this behavior, we have developed a two-chamber lattice model approach to investigate thermodiffusion in dilute polymer solutions. For a short polymer chain in an incompressible, one-component solvent we obtain exact results for the partitioning of the polymer between a warm and a cold chamber. In order to describe mixtures of PEO, ethanol, and water, we have extended this simple model to account for compressibility and hydrogen bonding between PEO and water molecules. For this complex system, we obtain approximate results for the composition in the warmer and cooler chambers that allow us to calculate Soret coefficients for given temperature, pressure, and solvent composition. The sign of the Soret coefficient is found to change from negative (polymer enriched in the warmer region) to positive (polymer enriched in the cooler region) as the water content of the solution is increased, in agreement with experimental data. We also investigate the temperature dependence of the Soret effect and find that a change in temperature can induce a change in the sign of the Soret coefficient. We note a close relationship between the solvent quality and the partitioning of the polymer between the two chambers, which may explain why negative Soret coefficients for polymers are so rarely observed.
Ab initiostudies of ClOx reactions. VIII. Isomerization and decomposition of ClO2 radicals and related bimolecular processes
Authors: Zhu R. S. , Lin M. C.
Published: July 2003

The isomerization and decomposition of ClOO and OClO radicals and related Cl+O2 and O+ClO reactions have been investigated by ab initio molecular orbital and transition-state theory calculations. The species involved have been optimized at the PW91PW91/6-311+G(3df ) level and their energies refined by single-point calculations with the modified Gaussian-2 method. Predicted bond-dissociation energies of ClOO and OClO, D0(Cl–OO)=4.6 and D0(O–ClO)=58.5 kcal/mol, agree well with experimental values. Calculated rate constants for the Cl+O2→ClOO reaction in 160–1000 K at the high- and low-pressure limits can be expressed by k1∞=1.8±0.1×10−10 cm3 molecule−1 s−1 and k10(He)=1.66×10−19 T−5.34 exp(−675/T) and k10(O2)=1.26×10−16 T−6.22 exp(−943/T) cm6 molecule−2 s−1. For Ar and N2, theory underpredicts k10(M) below room temperature due to significant contributions from the “chaperon” mechanism involving Cl–M complexes. The corresponding rate constants for O+ClO→OClO are predicted to be: k2∞=4.33×10−11 T−0.03 exp(43/T) cm3 molecule−1 s−1 and k20=8.60×10−21 T−4.1 exp(−420/T) cm6 molecule−2 s−1 for 200–1000 K with N2 as the third body. The O+ClO reaction producing Cl+O2 via ClOO was found to be pressure-independent with k3=4.11×10−11 T−0.06 exp(42/T) cm3 molecule−1 s−1. For the dissociation of ClOO, the rate constants are predicted to be: k−1∞=6.17×1015 T−0.46 exp(−2570/T) s−1  and k−10=1.89×107 T−5.88 ×exp(−3280/T) cm3 molecule−1 s−1 for 160–500 K with O2 as the third-body. The corresponding rate constants for OClO dissociation can be given by: k−2∞=1.11×1016 T−0.28exp(−29600/T) s−1 and k−20=1.64×10−47 T11.0 exp(−16700/T) cm3 molecule−1 s−1 for 200–2500 K with N2 as the third body. All of the predicted rate constants, with the exception mentioned above, are in close agreement with the available experimental results.
Vibrational relaxation in I2−(Ar)n (n=1,2,6,9) and I2−(CO2)n (n=1,4,5) clusters excited by femtosecond stimulated emission pumping
Authors: Davis Alison V. , Wester Roland , Bragg Arthur E. , Neumark Daniel M.
Published: July 2003

Vibrational relaxation dynamics in I2−(Ar)n (n=1,2,6,9) and I2−(CO2)n (n=1,4,5) clusters are studied using femtosecond stimulated emission pumping (fs-SEP) in conjunction with femtosecond photoelectron spectroscopy. fs-SEP generates coherently excited I2− within the cluster; results are reported here for excitation energies of 0.57 and 0.75 eV. The time-dependent PE spectra track relaxation of the clustered I2− through coherent intensity oscillations observed at short times (<10 ps) and shifts of the photoelectron spectra that can be seen out to several hundred picoseconds. The relaxation rates depend on the cluster type and excitation energy: the overall time scale in I2−(CO2)n clusters is relatively independent of both, but in I2−(Ar)n clusters the time scale generally increases with cluster size and decreases with excitation energy. The observed dynamics for I2−(CO2) and several of the I2−(Ar)n clusters directly probe the time scale for solvent evaporation.
Phase diagrams classification of the systems with thermoreversible alternating association (the Flory approach)
Authors: Thamm M. V. , Erukhimovich I. Ya.
Published: July 2003

We consider incompressible blends of f1- and f2-functional monomers Af1 and Bf2, capable of forming thermoreversible bonds AB only between unlike monomers with an equilibrium Arrhenius association constant k=k0 exp(−ε/T). Effects of the parameters f1, f2, k0, and ε on the global phase behavior of these systems are analyzed within the conventional Flory approach. An explicit topological classification of all types of phase diagrams is given via building (i) the phase portraits, i.e., separation of the plane (ln k0,ε) into the regions corresponding to topologically similar phase diagrams, and (ii) typical phase diagrams on the (volume fraction; T) plane for all regions of the phase portraits. The phase behavior of the systems with alternating association is found to be rather sensitive to the values of the parameters (ln k0,ε) and diverse. In particular, seven possible types of phase diagrams with topologically different binodals are found. Nontrivial phase behavior in the systems with alternating association includes closed immiscibility loops, triple points, metastable critical points, and even completely metastable phases. The main trend governing all the variety of these phase diagrams is an association-induced increase in the thermodynamic stability of blends of stoichiometric composition. In particular, low-temperature blends of stoichiometric composition turn out to be thermodynamically stable if (−ε/2Θ)>(f1+f2)−1, where Θ is the value of the Θ temperature measured in energetic units.
Towards an anisotropic bead-spring model for ribbonlike polymers: Monte Carlo simulations of liquid benzene
Authors: Lorenz S. , Walsh T. R. , Sutton A. P.
Published: July 2003

We present a Monte Carlo study for liquid benzene in the NVT ensemble. The benzene pair potential is described by an anisotropic Gay–Berne model fitted to ab initio interaction energies of the benzene dimer. We investigated the density and structure of liquid benzene at room temperature through the calculation of the carbon–carbon and molecular center–center radial distribution functions as well as the angular-radial distribution function. The simulated results are compared to experimental data and previous theoretical investigations.
Two-dimensional vibrational spectroscopy of the amide I band of crystalline acetanilide: Fermi resonance, conformational substates, or vibrational self-trapping?
Authors: Edler J. , Hamm P.
Published: July 2003

Two-dimensional infrared (2D-IR) spectroscopy is applied to investigate acetanilide, a molecular crystal consisting of quasi-one-dimensional hydrogen bonded peptide units. The amide-I band exhibits a double peak structure, which has been attributed to different mechanisms including vibrational self-trapping, a Fermi resonance, or the existence of two conformational substates. The 2D-IR spectrum of crystalline acetanilide is compared with that of two different molecular systems: (i) benzoylchloride, which exhibits a strong symmetric Fermi resonance and (ii) N-methylacetamide dissolved in methanol which occurs in two spectroscopically distinguishable conformations. Both 2D-IR spectra differ significantly from that of crystalline acetanilide, proving that these two alternative mechanisms cannot account for the anomalous spectroscopy of crystalline acetanilide. On the other hand, vibrational self-trapping of the amide-I band can naturally explain the 2D-IR response.
The time-of-flight signal in a Gaussian disordered chain
Authors: Freire José A. , da Luz M. G. E.
Published: July 2003

We consider a one-dimensional master equation model for the time-of-flight (TOF) experiment performed on an organic disordered material where the charge transport occurs via thermally activated hops between localized electronic states. From the model we obtain an expression for the average transit time in terms of the site energies and of the forward hopping rates. In the particular case of a blocking cathode we are able to perform the (Gaussian) average over the site energies and to obtain an exact expression for the transit time as a function of the applied field and of the variance of the energy distribution. We also obtain numerically the TOF signal I(t) and show that it exhibits two power-law regimes whose exponents do not sum up to 2, as in the time-dependent-random-walk model by Scher and Montroll. We investigate the dependence of the exponents with the field and with the amount of disorder. Finally, we show how the field dependence of the exact average transit time can be inferred from tR, the time of the transition between the two power-law regimes.
Laser-induced fluorescence, electronic absorption, infrared and Raman spectra, and ab initio calculations of 1,2-dihydronaphthalene: Investigation of the out-of-plane ring modes for the ground and S1(π,π*) excited states
Authors: Autrey Daniel , Arp Zane , Choo Jaebum , Laane Jaan
Published: July 2003

The laser-induced fluorescence spectra and dispersed fluorescence spectra of jet-cooled 1,2-dihydronaphthalene have been analyzed to investigate the ring inversion process in both the S0 and S1(π,π*) excited states. Ultraviolet absorption, infrared, and Raman spectra were also recorded to complement the analyses. Ab initio calculations predict the inversion process to involve four out-of-plane ring motions, and linear combinations of these were made to model the inversion process. The data show the barrier to inversion in the ground state to be 1363±100 cm−1 (the triple-zeta ab initio value is 1524 cm−1). The experimental data indicate that the barrier increases substantially in the excited state, for which the calculated barrier is 1526 cm−1 with a CIS/6-311+G(d) basis set.