bluuebunny/crossref_metadata_2025 huggingface dataset:
Intermolecular energy transfer involving an iridium complex studied by a combinatorial method
Authors: Suljovrujic E. , Ignjatovic A. , Srdanov V. I. , Mitsumori T. , Wudl F.
Published: August 2004

A recently developed combinatorial method utilizing angular dependence of evaporation rate was used to create compositional spread thin film libraries of Tris(2-pyridin-2-yl-indolizino[3,4,5-ab] isoindole-C1, N′)iridium(III) [Ir(pin)3] and 4,4′-N,N′-dicarbazol-biphenyl (CBP) composite, with the molar fraction of Ir(pin)3 complex varying in the 0.0003<χIr(pin)3<0.96 range. Spatially correlated absorbance and photoluminescence (PL) measurements were used to quantify luminescence quenching of photo-excited CBP molecules by the Ir(pin)3 complex as a function of the average distance between two molecules. It was found that the CBP→Ir(pin)3 energy transfer proceeds by the Förster mechanism with the Förster radius of 30 Å. The CBP×Ir(pin)3 composite has the highest photoluminescence quantum efficiency ≈0.95, for χIr(pin)3=0.03 and is characterized by a structured green emission (λmax=538 nm) originating from the ligand-centered (π-π*)3 state of the Ir(pin)3 complex. On the contrary, the PL spectra of Ir(pin)3 bulk are characterized by a weak red emission (λmax=673 nm) attributed to the lowest metal-to-ligand charge transfer state. A statistical analysis based on a binomial distribution indicates that the emission from the (π-π*)3 state is quenched in Ir(pin)3 molecules that are in a direct contact with each other.
Binding of propene on small gold clusters and on Au(111): Simple rules for binding sites and relative binding energies
Authors: Chrétien Steeve , Gordon Mark S. , Metiu Horia
Published: August 2004

We use density functional theory (DFT) to investigate the bonding of propene to small gas-phase gold clusters and to a Au(111) surface. The desorption energy trends and the geometry of the binding sites are consistent with the following set of rules. (1) The bond of propene to gold is formed by donation of electron density from the highest occupied molecular orbital (HOMO) of propene to one of the low-lying empty orbitals [denoted by LUMO1, LUMO2, … (LUMO–lowest unoccupied molecular orbital)] of the gold cluster. (2) Propene binds to a site on the Au cluster where one of the low-lying LUMOs protrudes in the vacuum. Different isomers (same cluster, but different binding sites for propene) correspond to sites where different low-lying LUMOs protrude in space. (3) The desorption energy of the lowest energy isomer correlates with the energy of the lowest empty orbital of the cluster; the lower the energy of that LUMO, the higher the desorption energy. (4) If the lowest-lying LUMO protrudes into space at two nonequivalent sites at the edge of a cluster, propene binds more strongly to the site with the lowest coordination. These rules are consistent with the calculated bond energies and geometries for [Aun(C3H6)]q, for n=1−5 and n=8 and q=−1, 0, +1. Based on them we have made a number of predictions that have been confirmed by DFT calculations. The bond of propene to gold is strengthened as the net charge of the cluster varies from −1, to zero, to +1. Compared to a gas-phase cluster, a cluster on a support binds propene more strongly if the support takes electron density from the cluster (e.g., a Au cluster on a gold surface) and more weakly if the support donates electron density to the cluster (e.g., a Au cluster on an oxygen vacancy on an oxide surface).
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electronic relaxation dynamics probed via time-resolved photoelectron imaging
Authors: Bragg Arthur E. , Verlet Jan R. R. , Kammrath Aster , Neumark Daniel M.
Published: August 2004

Anion time-resolved photoelectron imaging has been used to investigate the electronic relaxation dynamics of C6− following excitation of the C̃ 2Πg←X̃ 2Πu and 2 2Πg←X̃ 2Πu 000 transitions at 607 and 498 nm, respectively. Analysis of evolving photodetachment energy distributions reveals differing relaxation pathways from these prepared states. Specifically, the C̃ 2Πg 00 level relaxes on a time scale of 620±30 fs to vibrationally hot (∼2.0 eV) anion ground state both directly and indirectly through vibrationally excited levels of the intermediate-lying Ã 2Σg+ state that decay with a time scale of 2300±200 fs. In contrast, the 2 2Πg 00 level relaxes much more quickly (<100 fs) to vibrationally hot (∼2.5 eV) anion ground state directly and with transient population accumulation in the Ã 2Σg+, B̃ 2Σu+, and C̃ 2Πg electronic levels, as determined by spectral and time-scale analyses. This work also presents the experimental observation of the optically inaccessible B̃ 2Σu+ state, which is found to have an electronic term value of 1.41±0.05 eV.
Transport coefficients of the Lennard-Jones model fluid. I. Viscosity
Authors: Meier Karsten , Laesecke Arno , Kabelac Stephan
Published: August 2004

In an extensive computer simulation study, the transport coefficients of the Lennard-Jones model fluid were determined with high accuracy from equilibrium molecular-dynamics simulations. In the frame of time-correlation function theory, the generalized Einstein relations were employed to evaluate the transport coefficients. This first of a series of four papers presents the results for the viscosity, and discusses and interprets the behavior of this transport coefficient in the fluid region of the phase diagram. Moreover, the kinetic-kinetic, kinetic-potential, and potential-potential viscosity contributions are resolved over the whole range of fluid states, and their characteristic dependence on temperature and density is described. Finally, an additional analysis of the shear-stress correlation functions reveals aspects of the momentum-transport mechanisms on the molecular scale.
Structure and thickness-dependent lattice parameters of ultrathin epitaxial Pr2O3 films on Si(001)
Authors: Schroeder T. , Lee T.-L. , Zegenhagen J. , Wenger C. , Zaumseil P. , Müssig H.-J.
Published: August 2004

Pr 2 O 3 grown heteroepitaxially on Si(001) is a promising candidate for applications as a high-k dielectric in future silicon-based microelectronics devices. The technologically important thickness range from 1to10nm has been investigated by synchrotron radiation-grazing incidence x-ray diffraction. The oxide film grows as cubic Pr2O3 phase with its (101) plane on the Si(001) substrate in form of two orthogonal rotation domains. Monitoring the evolution of the oxide unit-cell lattice parameters as a function of film thickness from 1to10nm, the transition from almost perfect pseudomorphism to bulk values is detected.
Sign change of the Soret coefficient of poly(ethylene oxide) in water/ethanol mixtures observed by thermal diffusion forced Rayleigh scattering
Authors: Kita Rio , Wiegand Simone , Luettmer-Strathmann Jutta
Published: August 2004

Soret coefficients of the ternary system of poly(ethylene oxide) in mixed water/ethanol solvent were measured over a wide solvent composition range by means of thermal diffusion forced Rayleigh scattering. The Soret coefficient ST of the polymer was found to change sign as the water content of the solvent increases with the sign change taking place at a water mass fraction of 0.83 at a temperature of 22 °C. For high water concentrations, the value of ST of poly(ethylene oxide) is positive, i.e., the polymer migrates to the cooler regions of the fluid, as is typical for polymers in good solvents. For low water content, on the other hand, the Soret coefficient of the polymer is negative, i.e., the polymer migrates to the warmer regions of the fluid. Measurements for two different polymer concentrations showed a larger magnitude of the Soret coefficient for the smaller polymer concentration. The temperature dependence of the Soret coefficient was investigated for water-rich polymer solutions and revealed a sign change from negative to positive as the temperature is increased. Thermodiffusion experiments were also performed on the binary mixture water/ethanol. For the binary mixtures, the Soret coefficient of water was observed to change sign at a water mass fraction of 0.71. This is in agreement with experimental results from the literature. Our results show that specific interactions (hydrogen bonds) between solvent molecules and between polymer and solvent molecules play an important role in thermodiffusion for this system.
Structure and stability of the defect fullerene clusters of C60: C59, C58, and C57
Authors: Lee Sang Uck , Han Young-Kyu
Published: August 2004

We have performed density functional calculations for the structures and stabilities of various isomers of the defect fullerene clusters of C60: C59, C58, and C57. The C59_5-8, C58_5-5-7, and C57_4-5-9 clusters were calculated to be the most stable isomers of the C59, C58, and C57 clusters, respectively. There are obvious relationships between structure and stability of the defect fullerene clusters. First, an unsaturated carbon atom favors being located at a 6-membered ring rather than a 5-membered ring. Second, the most stable isomers prefer to have newly formed 5-membered rings, rather than newly formed 4-membered rings.
Calculation of exchange-correlation potentials with auxiliary function densities
Authors: Köster Andreas M. , Reveles J. Ulises , del Campo Jorge M.
Published: August 2004

The use of Hermite Gaussian auxiliary function densities from the variational fitting of the Coulomb potential for the calculation of exchange-correlation potentials is discussed. The basic working equations for the energy and gradient calculation are derived. The accuracy of this approximation for optimized structure parameters and bond energies are analyzed. It is shown that the quality of the approximation can be systematically improved by enlarging the auxiliary function set. Average errors of 0.5 kcal/mol are obtained with auxiliary function sets including f and g functions. The timings for a series of alkenes demonstrate a substantial performance improvement.
A derivation of the mixed quantum-classical Liouville equation from the influence functional formalism
Authors: Shi Qiang , Geva Eitan
Published: August 2004

We show that the mixed quantum-classical Liouville equation is equivalent to linearizing the forward-backward action in the influence functional. Derivations are provided in terms of either the diabatic or adiabatic basis sets. An application of the mixed quantum-classical Liouville equation for calculating the memory kernel of the generalized quantum master equation is also presented. The accuracy and computational feasibility of such an approach is demonstrated in the case of a two-level system nonlinearly coupled to an anharmonic bath.
Wetting transitions of ionic solutions
Authors: Denesyuk N. A. , Hansen J.-P.
Published: August 2004

Cahn’s phenomenological theory of wetting of a solid substrate by a saturated vapor is generalized to the case where the substrate is charged and the wetting film contains counterions, with or without added salt. The electrostatic contribution to the grand potential associated with these ions is calculated within a nonlinear Poisson-Boltzmann theory. In the salt-free case, when the wetting film includes only counterions released by the substrate, the wetting transition is always first order, regardless of its nature in a neutral system. When salt is present, other wetting scenarios may arise, depending on the salt concentration and substrate surface charge. Over a restricted range of salt concentrations, a wetting scenario similar to that of prewetting, is predicted to occur along the liquid-vapor coexistence line. This scenario includes a discontinuous wetting transition between microscopic and mesoscopic film thicknesses, followed by a continuous divergence of the film thickness at higher temperatures.
A prefactor free semiclassical initial value series representation of the propagator
Authors: Zhang Shesheng , Pollak Eli
Published: August 2004

A new class of prefactor free semiclassical initial value representations (SCIVR) of the quantum propagator is presented. The derivation is based on the physically motivated demand, that on the average in phase space and in time, the propagator obey the exact quantum equation of motion. The resulting SCIVR series representation of the exact quantum propagator is also free of prefactors. When using a constant width parameter, the prefactor free SCIVR propagator is identical to the frozen Gaussian propagator of Heller [J. Chem. Phys. 75, 2923 (1981)]. A numerical study of the prefactor free SCIVR series is presented for scattering through a double slit potential, a system studied extensively previously by Gelabert et al. [J. Chem. Phys. 114, 2572 (2001)]. As a basis for comparison, the SCIVR series is also computed using the optimized Herman–Kluk SCIVR. We find that the sum of the zeroth order and the first order terms in the series suffice for an accurate determination of the diffraction pattern. The same exercise, but using the prefactor free propagator series needs also the second order term in the series, however the numerical effort is not greater than that needed for the Herman–Kluk propagator, since one does not need to compute the monodromy matrix elements at each point in time. The numerical advantage of the prefactor free propagator grows with increasing dimensionality of the problem.
Effect of the excitation pulse carrier frequency on the ultrafast charge recombination dynamics of donor-acceptor complexes: Stochastic simulations and experiments
Authors: Fedunov Roman G. , Feskov Serguei V. , Ivanov Anatoly I. , Nicolet Olivier , Pagès Stéphane , Vauthey Eric
Published: August 2004

The influence of the excitation pulse carrier frequency on the ultrafast charge recombination dynamics of excited donor-acceptor complexes has been explored both theoretically and experimentally. The theoretical description involves the explicit treatment of both the optical formation of the nuclear wave packet on the excited free energy surface and its ensuing dynamics. The wave packet motion and the electronic transition are described within the framework of the stochastic point-transition approach. It is shown that the variation of the pulse carrier frequency within the absorption band can significantly change the effective charge recombination dynamics. The mechanism of this phenomenon is analyzed and a semiquantitative interpretation is suggested. The role of the vibrational coherence in the recombination dynamics is discussed. An experimental investigation of the ultrafast charge recombination dynamics of two donor-acceptor complexes in valeronitrile also is presented. The decays of the excited state population were found to be highly nonexponential, the degree of non-exponentiality depending on the excitation frequency. For one complex, the charge recombination dynamics was found to slow down upon increasing the excitation frequency, while the opposite behavior was observed with the other complex. These experimental observations follow qualitatively the predictions of the simulations.
Experimental and theoretical studies of charge transfer and deuterium ion transfer between D2O+ and C2H4
Authors: Liu Li , Cai Xiaohui , Li Yue , Richards O’Grady Elizabeth , Farrar James M.
Published: August 2004

The charge transfer and deuterium ion transfer reactions between D2O+ and C2H4 have been studied using the crossed beam technique at relative collision energies below one electron volt and by density functional theory (DFT) calculations. Both direct and rearrangement charge transfer processes are observed, forming C2H4+ and C2H3D+, respectively. Independent of collision energy, deuterium ion transfer accounts for approximately 20% of the reactive collisions. Between 22 and 36 % of charge transfer collisions occur with rearrangement. In both charge transfer processes, comparison of the internal energy distributions of products with the photoelectron spectrum of C2H4 shows that Franck-Condon factors determine energy disposal in these channels. DFT calculations provide evidence for transient intermediates that undergo H/D migration with rearrangement, but with minimal modification of the product energy distributions determined by long range electron transfer. The cross section for charge transfer with rearrangement is approximately 103 larger than predicted from the Rice-Ramsperger-Kassel-Marcus isomerization rate in transient complexes, suggesting a nonstatistical mechanism for H/D exchange. DFT calculations suggest that reactive trajectories for deuterium ion transfer follow a pathway in which a deuterium atom from D2O+ approaches the π-cloud of ethylene along the perpendicular bisector of the C–C bond. The product kinetic energy distributions exhibit structure consistent with vibrational motion of the D-atom in the bridged C2H4D+ product perpendicular to the C–C bond. The reaction quantitatively transforms the reaction exothermicity into internal excitation of the products, consistent with mixed energy release in which the deuterium ion is transferred in a configuration in which both the breaking and the forming bonds are extended.
Electron delocalization by polar molecules: Interaction of Na atoms with solid ammonia films studied with MIES and density functional theory
Authors: Borodin A. , Höfft O. , Kempter V. , Ferro Y. , Allouche A.
Published: August 2004

The interaction of Na and NH3 on tungsten was studied with metastable impact electron spectroscopy under UHV conditions. NH3(Na) films were grown at 90(±10) K on tungsten substrates and exposed to Na(NH3). No Na-induced reaction involving NH3 takes place. At small Na exposures a Na-induced shift of the NH3 spectral features is seen, in parallel with a decrease of the surface work function. At larger exposures three 3sNa-related spectral structures are seen, two of them at energetic positions different from that found for Na on metals or semiconductors. The main additional peak is attributed to delocalized Na species. A small additional feature is attributed to simultaneous ionization and excitation of partially ammoniated Na2 species. The results are compared with density functional theory calculations which suggest that the 3sNa emission at small exposures appears to originate mainly from delocalized 3sNa electrons; they are located far from the Na species and become stabilized by solvent molecules. When depositing NH3 molecules onto Na films, metalliclike Na patches and delocalized Na species coexist. The delocalization of 3sNa is seen up to T=130 K where the NH3 species desorb.
Multilayer adsorption of water at a rutile TiO2(110) surface: Towards a realistic modeling by molecular dynamics
Authors: Kornherr Andreas , Vogtenhuber Doris , Ruckenbauer Matthias , Podloucky Raimund , Zifferer Gerhard
Published: August 2004

We present a model combining ab initio concepts and molecular dynamics simulations for a more realistic treatment of complex adsorption processes. The energy, distance, and orientation of water molecules adsorbed on stoichiometric and reduced rutile TiO2(110) surfaces at 140 K are studied via constant temperature molecular dynamics simulations. From ab initio calculations relaxed atomic geometries for the surface and the most probable adsorption sites were derived. The study comprises (i) large two-dimensional surface supercells, providing a realistically low concentration of surface oxygen defects, and (ii) a water coverage sufficiently large to model the onset of the growth of a bulk phase of water on the surface. By our combined approach the influence of both, the metal oxide surface, below, and the bulk water phase, above, on the water molecules forming the interface between the TiO2 surface and the water bulk layer is taken into account. The good agreement of calculated adsorption energies with experimental temperature programmed desorption spectra demonstrates the validity of our model.
A grand canonical Monte Carlo study of capillary condensation in mesoporous media: Effect of the pore morphology and topology
Authors: Coasne B. , Pellenq R. J.-M.
Published: August 2004

We study by means of Grand Canonical Monte Carlo simulations the condensation and evaporation of argon at 77 K in nanoporous silica media of different morphology or topology. For each porous material, our results are compared with data obtained for regular cylindrical pores. We show that both the filling and emptying mechanisms are significantly affected by the presence of a constriction. The simulation data for a constricted pore closed at one end reproduces the asymmetrical shape of the hysteresis loop that is observed for many real disordered porous materials. The adsorption process is a quasicontinuous mechanism that corresponds to the filling of the different parts of the porous material, cavity, and constriction. In contrast, the desorption branch for this pore closed at one end is brutal because the evaporation of Ar atoms confined in the largest cavity is triggered by the evaporation of the fluid confined in the constriction (which isolates the cavity from the gas reservoir). This evaporation process conforms to the classical picture of “pore blocking effect” proposed by Everett many years ago. We also simulate Ar adsorption in a disordered porous medium, which mimics a Vycor mesoporous silica glass. The adsorption isotherm for this disordered porous material having both topological and morphological defects presents the same features as that for the constricted pore (quasicontinuous adsorption and steep desorption process). However, the larger degree of disorder of the Vycor surface enhances these main characteristics. Finally, we show that the effect of the disorder, topological and/or morphological, leads to a significant lowering of the capillary condensation pressure compared to that for regular cylindrical nanopores. Also, our results suggest that confined fluids isolated from the bulk reservoir evaporate at a pressure driven by the smallest size of the pore.
Diagrammatic kinetic theory for a lattice model of a liquid. I. Theory
Authors: Feng Edward H. , Andersen Hans C.
Published: August 2004

We present a diagrammatic formalism for the time correlation functions of density fluctuations for an excluded volume lattice gas on a simple d-dimensional hypercubic lattice. We consider a multicomponent system in which particles of different species can have different transition rates. Our theoretical approach uses a Hilbert space formalism for the time dependent dynamical variables of a stochastic process that satisfies the detailed balance condition. We construct a Liouville matrix consistent with the dynamics of the model to calculate both the equation of motion for multipoint densities in configuration space and the interactions in the diagrammatic theory. A Boley basis of fluctuation vectors for the Hilbert space is used to develop two formally exact diagrammatic series for the time correlation functions. These theoretical techniques are generalizations of methods previously used for spin systems and atomic liquids, and they are generalizable to more complex lattice models of liquids such as a lattice gas with attractive interactions or polymer models. We use our formalism to construct approximate kinetic theories for the van Hove correlation and self-correlation function. The most simple approximation is the mean field approximation, which is exact for the van Hove correlation function of a one component system but an approximation for the self-correlation function. We use our first diagrammatic series to derive a two site multiple scattering approximation that gives a simple analytic expression for the spatial Fourier transform of the self-correlation function. We employ our second diagrammatic series to derive a simple mode coupling type approximation that provides a system of equations that can be solved for the self-correlation function.
First-principles string molecular dynamics: An efficient approach for finding chemical reaction pathways
Authors: Kanai Y. , Tilocca A. , Selloni A. , Car R.
Published: August 2004

A recently proposed approach, called “string method,” allows us to find minimum energy pathways connecting two metastable states of a system [W. E et al., Phys. Rev. B 66, 052301 (2002)]. So far this approach has been only used with empirical force field parametrizations of the atomic potential energy surface or in the context of macroscopic continuum models. Here we show that the string method can be efficiently combined with first-principles molecular dynamics to provide an accurate description of chemical reaction pathways and barriers. We illustrate the first-principles string molecular dynamics by applying it to the study of a surface chemical reaction, for which extensive experimental and theoretical works are available, namely, the adsorption of H2 on the reconstructed Si(100) surface.
Extremely narrow peaks in predissociation of sodium dimer due to rovibronic coupling
Authors: Narevicius Edvardas , Moiseyev Nimrod , Sadeghpour H. R. , Cederbaum Lorenz S.
Published: August 2004

In sodium dimer the 2 3Πg, 3 3Πg, and 4 3Σg+ electronic states are coupled; the coupling of the two Πg3 states is due to vibrational motion while the nonadiabatic interaction between the Σg+3 and the Πg3 states—in particular, the 3 3Πg state—is mediated by rotational interaction. The resulting vibronic problem is studied in some detail. The bound vibrational states of the 3 3Πg and 4 3Πg+ states lie in the dissociation continuum of the 2 3Πg state and become resonances due to the prevailing nonadiabatic coupling. The resonances are calculated using the complex scaling method and the available ab initio adiabatic potential energy curves. It is demonstrated that the resonances associated with rotational nonadiabatic coupling are narrower by several orders of magnitude than those that emerge from the vibrational nonadiabatic coupling. The predissociation cross section is computed and compared with experiment.
Biomolecule-directed assembly of nanoscale building blocks studied via lattice Monte Carlo simulation
Authors: Chen Ting , Lamm Monica H. , Glotzer Sharon C.
Published: August 2004

We perform lattice Monte Carlo simulations to study the self-assembly of functionalized inorganic nanoscale building blocks using recognitive biomolecule linkers. We develop a minimal coarse-grained lattice model for the nanoscale building block (NBB) and the recognitive linkers. Using this model, we explore the influence of the size ratio of linker length to NBB diameter on the assembly process and the structural properties of the resulting aggregates, including the spatial distribution of NBBs and aggregate topology. We find the constant-kernel Smoluchowski theory of diffusion-limited cluster–cluster aggregation describes the aggregation kinetics for certain size ratios.
Quantum Monte Carlo study of helium clusters doped with nitrous oxide: Quantum solvation and rotational dynamics
Authors: Moroni Saverio , Blinov Nicholas , Roy Pierre-Nicholas
Published: August 2004

Dynamical and structural properties of small HeN4-N2O complexes have been analyzed using ground-state and finite-temperature Monte Carlo simulations. The effective rotational constants resulting from the ground-state calculations are in excellent agreement with the results of a recent spectroscopic study [Y. Xu et al., Phys. Rev. Lett. 91, 163401 (2003)]. After an initial decrease for cluster sizes up to N=8, the rotational constant increases, signaling a transition from a molecular complex to a quantum-solvated system. Such a turnaround is not present in the rotational constants extracted from the finite-temperature Monte Carlo calculations, performed for Boltzmann statistics, thus highlighting the importance of exchange effects to explain the decoupling between a solvated dopant and the helium motion.
Development of density functionals for thermochemical kinetics
Authors: Boese A. Daniel , Martin Jan M. L.
Published: August 2004

A density functional theory exchange-correlation functional for the exploration of reaction mechanisms is proposed. This functional, denoted BMK (Boese-Martin for Kinetics), has an accuracy in the 2 kcal/mol range for transition state barriers but, unlike previous attempts at such a functional, this improved accuracy does not come at the expense of equilibrium properties. This makes it a general-purpose functional whose domain of applicability has been extended to transition states, rather than a specialized functional for kinetics. The improvement in BMK rests on the inclusion of the kinetic energy density together with a large value of the exact exchange mixing coefficient. For this functional, the kinetic energy density appears to correct “back” the excess exact exchange mixing for ground-state properties, possibly simulating variable exchange.
Propagators for the time-dependent Kohn–Sham equations
Authors: Castro Alberto , Marques Miguel A. L. , Rubio Angel
Published: August 2004

In this paper we address the problem of the numerical integration of the time-dependent Schrödinger equation i∂tφ=Ĥφ. In particular, we are concerned with the important case where Ĥ is the self-consistent Kohn–Sham Hamiltonian that stems from time-dependent functional theory. As the Kohn–Sham potential depends parametrically on the time-dependent density, Ĥ is in general time dependent, even in the absence of an external time-dependent field. The present analysis also holds for the description of the excited state dynamics of a many-electron system under the influence of arbitrary external time-dependent electromagnetic fields. Our discussion is separated in two parts: (i) First, we look at several algorithms to approximate exp(Â), where Â is a time-independent operator [e.g., Â=−iΔtĤ(τ) for some given time τ]. In particular, polynomial expansions, projection in Krylov subspaces, and split-operator methods are investigated. (ii) We then discuss different approximations for the time-evolution operator, such as the midpoint and implicit rules, and Magnus expansions. Split-operator techniques can also be modified to approximate the full time-dependent propagator. As the Hamiltonian is time dependent, problem (ii) is not equivalent to (i). All these techniques have been implemented and tested in our computer code OCTOPUS, but can be of general use in other frameworks and implementations.
On the role of coupling in mode selective excitation using ultrafast pulse shaping in stimulated Raman spectroscopy
Authors: Malinovskaya S. A. , Bucksbaum P. H. , Berman P. R.
Published: August 2004

The coherence of two coupled two-level systems, representing vibrational modes in a semiclassical model, is calculated in weak and strong fields for various coupling schemes and for different relative phases between initial state amplitudes. A relative phase equal to π projects the system into a dark state. The selective excitation of one of the two, two-level systems is studied as a function of coupling strength and initial phases.
Zero kinetic energy photoelectron spectroscopy of p-amino benzoic acid
Authors: He Yonggang , Wu Chengyin , Kong Wei
Published: August 2004

We report studies of supersonically cooled p-amino benzoic acid using one-color resonantly enhanced multiphoton ionization and two-color zero kinetic energy (ZEKE) photoelectron spectroscopy. With the aid of ab initio and density functional calculations, vibrational modes of the first electronically excited state S1 of the neutral species and those of the cation have been assigned, and the adiabatic ionization potential has been determined to be 64 540±5 cm−1. A common pattern involving the activation of five vibrational modes of the cation is recognizable among all the ZEKE spectra. A propensity of Δv=0, where v is the vibrational quantum number of the intermediate vibronic state from S1, is confirmed, and the origin of this behavior is discussed in the context of electron back donation from the two substituents in the excited state and in the cationic state. A puzzling observation is the doublet splitting of 37 cm−1 in the ZEKE spectrum obtained via the inversion mode of the S1 state. This splitting cannot be explained from our density functional calculations.
Polarization and bonding of the intrinsic characteristic contours of hydrogen and fluorine atoms of forming a hydrogen fluoride molecule based on an ab initio study
Authors: Yang Zhong-Zhi , Zhao Dong-Xia , Wu Yang
Published: August 2004

The spatial changing feature of the shapes and sizes of the system consisted of one hydrogen atom and one fluorine atom of forming a hydrogen fluoride molecule is investigated. We give formalism of the potential acting on an electron in a molecule and derive its concrete expression in Hartree–Fock self-consistent molecular orbital theory including configuration interaction. The program of calculating the potential acting on an electron in a molecule is programmed and compiled in the framework of the MELD program package. We formulate briefly the approach of the molecular intrinsic characteristic contour (MICC) which is defined in terms of the classical turning points of electronic motion. The MICC for a molecular system is intrinsic and can be calculated by means of an ab initio CI method. Then, the polarization and bonding features of the intrinsic characteristic contours of hydrogen and fluorine atoms forming a hydrogen fluoride molecule are presented and discussed from ab initio calculations. Furthermore, electron density distribution as an added dimension has been demonstrated on the changing MICC and thus the vivid polarization and bonding features for a chemical process have been shown. It seems that at the early stage (internuclear distance Ind=5.0–20.0 a.u.) the fluorine atom gives more enthusiastic with the sensitive and expanded polarization to welcome coupling with the hydrogen atom while the latter has little response even “shy” with shrinking a bit its size at the beginning of putting the two atoms into a system and it is only around the critical point, the contact point (Ind=4.73 a.u.), that both of them stretch their hands and arms to meet and then fuse together.
A theoretical study of solvent effects on the (n→π*)1 electron transition in acrolein
Authors: Martı́n M. E. , Muñoz Losa Aurora , Fdez.-Galván I. , Aguilar M. A.
Published: August 2004

The 1(n→π) electron transition of acrolein in liquid water was studied theoretically by using the averaged solvent electrostatic potential/molecular dynamics method. The model combines a multireference perturbational treatment in the description of the solute molecule with molecular dynamics calculations in the description of the solvent. We demonstrate the importance of the solvent electron polarization, bulk solvent effects, and the use of relaxed geometries in solution on the calculated solvent shift. It is also shown that the inclusion of the dynamic correlation does not change the solvent shift although it must be used to reproduce the transition energy.
Universal consequences of the presence of excluded volume interactions in dilute polymer solutions undergoing shear flow
Authors: Kumar K. Satheesh , Prakash J. Ravi
Published: August 2004

The role of solvent quality in determining the universal material properties of dilute polymer solutions undergoing steady simple shear flow is examined. A bead-spring chain representation of the polymer molecule is used, and the influence of solvent molecules on polymer conformations is modelled by a narrow Gaussian excluded volume potential that acts pairwise between the beads of the chain. Brownian dynamics simulations data, acquired for chains of finite length, and extrapolated to the limit of infinite chain length, are shown to be model independent. This feature of the narrow Gaussian potential, which leads to results identical to a δ-function repulsive potential, enables the prediction of both universal crossover scaling functions and asymptotic behavior in the excluded volume limit. Universal viscometric functions, obtained by this procedure, are found to exhibit increased shear thinning with increasing solvent quality. In the excluded volume limit, they are found to obey power law scaling with the characteristic shear rate β, in close agreement with previously obtained renormalization group results. The presence of excluded volume interactions is also shown to lead to a weakening of the alignment of the polymer chain with the flow direction.
Theoretical study of the electronic state and H-elimination reactions for solvated magnesium cluster ions
Authors: Daigoku Kota , Hashimoto Kenro
Published: August 2004

The potential-energy curves of the ground and low-lying excited states for Mg+NH3 along the N-H distance were examined by the ab initio configuration interaction method. The photoinduced hydrogen elimination reaction found by the recent experiment is considered to occur via the ground-state channel. The geometries, energetics, and electronic nature of the ground-state Mg+(NH3)n and MgNH2+(NH3)n−1 (n=1–6) were also investigated by second-order Møller-Plesset perturbation theory and compared with those of the corresponding hydrated species. In contrast to Mg+(H2O)n, the successive solvation energies of Mg+(NH3)n become as large as those of MgNH2+(NH3)n−1 containing the Mg2+-NH2− core for n=5 and 6, because of the growing one-center ion-pair state with the Mg2+ and the diffuse solvated electron. As a result, the solvation energies of the MgNH2+(NH3)n−1 are insufficient to overcome the huge endothermicity of Mg+(NH3)→MgNH2++H, even at these sizes, which is responsible for no observation of the H-loss products, MgNH2+(NH3)n−1.
Asymmetric line shape in the emission spectra of conjugated polymer thin films: An experimental signature of one-dimensional electronic states
Authors: Cury L. A. , Guimarães P. S. S. , Moreira R. L. , Chacham H.
Published: August 2004

The photoluminescence (PL) properties of thin films of the conjugated polymer [poly(2,5-bis(2′-ethyl-hexyl)-1,4-phenylenevinylene] have been investigated. At low temperatures the PL spectra show a narrow peak for the electronic transition and a series of well defined vibronic sidebands, which clearly reveal the electron coupling with two different vibronic modes. The purely electronic transition peak is observed to be very asymmetric so that it cannot be adjusted by a single Lorentzian or Gaussian function. In order to understand and explain this asymmetry we have considered a model where the purely electronic transition line shape is partially generated by a broadened square-root singularity representing one-dimensional electron states, and partially by localized (zero-dimensional) states. The localized states are assumed to be those very close to the band edges and are represented in our model by a single Gaussian function. Numerical Franck–Condon analysis was performed, resulting in a very good agreement between the theoretical and the experimental emission spectra. This procedure has confirmed the one-dimensional character of the electron states as the basis for the understanding of the purely electronic line shape asymmetry in the PL spectra of conjugated polymers at low temperatures.
The effects of oxygenation on the optical properties of dimethyl-dithienothiophenes: Comparison between experiments and first-principles calculations
Authors: Raganato M. F. , Vitale V. , Della Sala F. , Anni M. , Cingolani R. , Gigli G. , Favaretto L. , Barbarella G. , Weimer M. , Görling A.
Published: August 2004

Modifications of the optical properties of dimethyl-dithienothiophenes due to the oxygen functionalization of the central sulfur atom are investigated. We have measured the absorption, photoluminescence (PL) and PL excitation spectra, the PL quantum efficiencies, and the PL decay times. These experimental results are interpreted and compared with first-principles time-dependent density-functional theory calculations, which predict, for the considered systems, excitation and emission energies with an accuracy of 0.1 eV. It is found that the oxygenation strongly changes optical and photophysical properties. These effects are related to the modifications of the energetically lowest-unoccupied molecular orbital and the energetically second highest occupied one, which change the relative position of the two lowest singlet and triplet excited states.
A density functional theory study of the coadsorption of water and oxygen on TiO2(110)
Authors: Zhang Changjun , Lindan Philip J. D.
Published: August 2004

The behavior of adsorbed water on oxides is of fundamental interest in many areas. Despite considerable attention received recently, our understanding of water chemistry is still short of needs and expectations, particularly on the topic of the coadsorption of water and other species. In this study we carry out density functional theory calculations to investigate the coadsorption of water and oxygen on the TiO2(110) surface. We show that oxygen exerts profound influences on the water adsorption, altering the mechanism of water dissociation. On the one hand, the possible dissociation route along [−110] is prohibited due to the weakening of the H bond between water and the lattice bridging oxygen in the presence of the coadsorbed oxygen, and on the other hand the coadsorbed oxygen induces dissociation along [001]. These results lead to a consistent interpretation of experiments. Furthermore, several possible final states and the related formation mechanisms are discussed in detail.
Structures and vibrations of Nb3O and Nb3O−: A density functional study
Authors: Calaminici Patrizia , Flores-Moreno Roberto , Köster Andreas M.
Published: August 2004

Density functional calculations of neutral and anionic niobium trimer monoxides are presented. The calculations were performed employing scalar quasirelativistic effective core potentials. In order to test the accuracy of the used effective core potentials in the framework of density functional theory the pulsed field ionization–zero electron kinetic energy photoelectron spectrum of Nb3O was simulated and compared to experiment. Different isomers of Nb3O and Nb3O− were studied in order to determine the ground state structures. For both neutral and anionic systems a planar C2v structure with an edge-bound oxygen atom was found as a ground state. Equilibrium structure parameters, harmonic frequencies, and adiabatic electron affinity are reported. The calculated electron affinity and frequencies are in good agreement with the available experimental data obtained recently from vibrationally resolved negative ion photoelectron spectroscopy.
Delayed rejection variational Monte Carlo
Authors: Bressanini Dario , Morosi Gabriele , Tarasco Silvia , Mira Antonietta
Published: August 2004

An acceleration algorithm to address the problem of multiple time scales in variational Monte Carlo simulations is presented. After a first attempted move has been rejected, the delayed rejection algorithm attempts a second move with a smaller time step, so that even moves of the core electrons can be accepted. Results on Be and Ne atoms as test cases are presented. Correlation time and both average accepted displacement and acceptance ratio as a function of the distance from the nucleus evidence the efficiency of the proposed algorithm in dealing with the multiple time scales problem.
Average electron radii in many-electron atoms
Authors: Koga Toshikatsu
Published: August 2004

In many-electron atoms, the average electron radius 〈r〉 represents the mean distance of a single electron from the nucleus when all the interelectronic interactions are averaged. If the electron-electron interaction is explicitly considered, the average radius 〈r〉 splits into two different radii, inner radius 〈r<〉 and outer radius 〈r>〉. For the 102 atoms He through Lr in their ground states, the radii 〈r<〉 and 〈r>〉 are systematically examined at the Hartree-Fock limit level. The effect of electron correlations on 〈r<〉 and 〈r>〉 is also discussed for the He atom and its isoelectronic ions.
Pseudospectral approach to relativistic molecular theory
Authors: Nakajima Takahito , Hirao Kimihiko
Published: August 2004

The efficient relativistic Dirac-Hartree-Fock (DHF) and Dirac-Kohn-Sham (DKS) methods are proposed by an application of the pseudospectral (PS) approach. The present PS-DHF/DKS method is a relativistic extension of the PS-HF/KS method of Friesner, though we aim at higher numerical accuracy by elimination of superfluous arbitrariness. The relativistic PS-DHF/DKS method is implemented into our REL4D programs. Several PS applications to molecular systems show that the relativistic PS-DHF/DKS approach is more efficient than the traditional approach without a loss of accuracy. The present PS-DKS method successfully assigns and predicts the photoelectron spectra of hexacarbonyl complexes of tungsten and seaborgium theoretically.
Structure of inhomogeneous polymer solutions: A density functional approach
Authors: Patra Chandra N.
Published: August 2004

The structure of polymer solutions confined between surfaces is studied using a density functional theory where the polymer molecules have been modeled as a pearl necklace of freely jointed hard spheres and the solvent as hard spheres. The present theory uses the concept of universality of the free energy density functional to obtain the first-order direct correlation function of the nonuniform system from that of the corresponding uniform system, calculated through the Verlet-modified type bridge function. The uniform bulk fluid direct correlation function required as input has been calculated from the reference interaction site model integral equation theory using the Percus-Yevick closure relation. The calculated results on the density profiles of the polymer as well as the solvent are shown to compare well with computer simulation results.
A time correlation function theory of two-dimensional infrared spectroscopy with applications to liquid water
Authors: DeVane Russell , Space Brian , Perry Angela , Neipert Christine , Ridley Christina , Keyes T.
Published: August 2004

A theory describing the third-order response function R(3)(t1,t2,t3), which is associated with two-dimensional infrared (2DIR) spectroscopy, has been developed. R(3) can be written as sums and differences of four distinct quantum mechanical dipole (multi)time correlation functions (TCF’s), each with the same classical limit; the combination of TCF’s has a leading contribution of order ℏ3 and thus there is no obvious classical limit that can be written in terms of a TCF. In order to calculate the response function in a form amenable to classical mechanical simulation techniques, it is rewritten approximately in terms of a single classical TCF, BR(t1,t2,t3)=〈μj(t2+t1)μi(t3+t2+t1)μk(t1)μl(0)〉, where the subscripts denote the Cartesian dipole directions. The response function is then given, in the frequency domain, as the Fourier transform of a classical TCF multiplied by frequency factors. This classical expression can then further be quantum corrected to approximate the true response function, although for low frequency spectroscopy no correction is needed. In the classical limit, R(3) becomes the sum of multidimensional time derivatives of BR(t1,t2,t3). To construct the theory, the response function’s four TCF’s are rewritten in terms of a single TCF: first, two TCF’s are eliminated from R(3) using frequency domain detailed balance relationships, and next, two more are removed by relating the remaining TCF’s to each other within a harmonic oscillator approximation; the theory invokes a harmonic approximation only in relating the TCF’s and applications of theory involve fully anharmonic, atomistically detailed molecular dynamics (MD). Writing the response function as a single TCF thus yields a form amenable to calculation using classical MD methods along with a suitable spectroscopic model. To demonstrate the theory, the response function is obtained for liquid water with emphasis on the OH stretching portion of the spectrum. This approach to evaluating R(3) can easily be applied to chemically interesting systems currently being explored experimentally by 2DIR and to help understand the information content of the emerging multidimensional spectroscopy.