bluuebunny/crossref_metadata_2025 huggingface dataset:
Unstable modes in supercooled and normal liquids: Density of states, energy barriers, and self-diffusion
Authors: Keyes T.
Published: July 2002

The unstable mode density of states 〈ρu(ω;T)〉 is obtained from computer simulation and is analyzed, theoretically and empirically, over a broad range of supercooled and normal liquid temperatures in the unit density Lennard-Jones liquid. The functional form of 〈ρu(ω;T)〉 is determined and the ω, T dependence is seen to be consistent with a theory given by us previously. The parameters in the theory are determined and are related to the topological features of the potential energy surface in the configuration space; it appears that diffusion involves a low degree of cooperativity at all but the lowest temperatures. It is shown that analysis of 〈ρu(ω;T)〉 yields considerable information about the energy barriers to diffusion, namely, a characteristic ω-dependent energy and the distribution of barrier heights, gν(E). The improved description of 〈ρu(ω;T)〉 obtained in the paper is used to implement normal mode theory of the self-diffusion constant D(T) with no undetermined constants; agreement with simulation in the supercooled liquid is excellent. Use of a lower frequency cutoff on the contribution of unstable modes to diffusion, in an attempt to remove spurious contributions from anharmonicities unrelated to barrier crossing, yields the Zwanzig–Bassler temperature dependence for D(T). It is argued that the distribution of barriers plays a crucial role in determining the T dependence of the self-diffusion constant.
Tensor analysis of femtosecond Raman induced polarization spectroscopy: Application to the study of rotational coherence
Authors: Morgen M. , Price W. , Ludowise P. , Chen Y.
Published: July 2002

In this paper we present a tensor analysis of Raman induced polarization spectroscopy (RIPS), a variation of time-resolved rotational coherence spectroscopy (RCS), developed in our laboratory to study coherent rotational motions in fluids over a range of densities. Based on the irreducible tensor algebra, the analysis separates the fundamental molecular response in RIPS from the dependence of the signal on geometric factors (i.e., laser polarizations). Explicit formulas for the femtosecond RIPS response of symmetric and asymmetric tops are derived. The results are in good agreement with experimental RIPS spectra of low pressure CO2 and O3. The tensor analysis is also generalized to other types of four wave mixing experiments, allowing detailed comparison of the various frequency- and time-domain pump–probe experiments.
Diffusion effects in stationary chemical production–annihilation processes
Authors: Schulz Michael , Reineker Peter
Published: July 2002

A field theory for chemical processes with a production and an annihilation reaction taking into account the diffusion of the particles during their lifetime, i.e., to a production-diffusion-annihilation process (PDA model), is formulated exactly by using master equations and Poisson transform techniques. From this standpoint, a Langevin equation for the evolution of the PDA model can be determined. This equation corresponds to a generalized Kadar–Parisi–Zhang (KPZ) equation and predicts a scaling behavior in the small production rate regime, which is calculated by using the dynamical renormalization group analyses.
The kinetics of crystal growth and dissolution from the melt in Lennard-Jones systems
Authors: Báez Luis A. , Clancy Paulette
Published: July 2002

The growth and dissolution of implanted fcc nanocrystallites in a subcooled liquid have been studied using nonequilibrium Molecular Dynamics simulation techniques. The initial sizes of the 70 implanted crystals were each close to the critical cluster size of 120 atoms predicted by classical nucleation theory. The simulated system consisted of approximately 13 500 atoms interacting with a Lennard-Jones potential. The time evolution of the crystal sizes was followed in order to obtain statistics from which the kinetics and the probabilities of growth and dissolution were extracted using a Markovian technique. The growing crystallites showed facets that correspond to hexagonally packed layers. Very small crystals adopted an octahedral shape, but this shape was lost as the crystals grew. Although the implanted crystals had an fcc structure originally, the crystals grew in a mixture of hcp and fcc geometries with a larger proportion of the latter. Our analysis showed the critical cluster size for the implants to be 170 atoms, somewhat larger than the value predicted by a Gibbsian nucleation theory. The rate of growth showed minima, suggesting the existence of ‘‘magic numbers’’ in crystal growth from the melt. Homogeneous nucleation was also simulated. The free energies of the subcritical crystallites is smaller than predicted by classical nucleation theory using the properties of the bulk liquid and solid phases. The rate of nucleation is compared with theoretical predictions.
Generalizations of Huggins–Guggenheim–Miller-type theories to describe the architecture of branched lattice chains
Authors: Foreman K. W. , Freed Karl F.
Published: July 2002

We introduce two methods for extending Huggins–Guggenheim–Miller (HGM)-type theories for lattice model polymer chains to describe the dependence of polymer thermodynamic properties on chain architectures (e.g., linear, branched, comb, structured monomer chains), thereby rectifying a half-century old deficiency of these venerable theories. The first approach is based upon a mathematically precise definition of the ‘‘surface fractions’’ that appear in the final HGM random mixing theory. These surface fractions are determined from exact enumerations for short chains, which are found to converge rather rapidly. The approach is illustrated for linear chains, but is readily applied for branched systems. The resultant ‘‘improved’’ HGM theory is tested by parameter-free comparisons with Monte Carlo simulations as well as with Flory–Huggins theory, the original HGM theory, and the lattice cluster theory (LCT). A second improved HGM theory is generated by providing more accurate treatments of the nearest-neighbor pair probabilities that form the basic assumptions and ingredients in, for instance, Guggenheim’s derivation of the HGM theory. The more accurate pair probabilities are obtained from the LCT for branched polymer systems (or chains with structured monomers), and comparisons are again provided with Monte Carlo simulations and previous theories. These comparisons serve to underscore inherent limitations of fundamental assumptions invoked by HGM theories and possible methods for their alleviation. Unfortunately, all simple ‘‘improvements’’ of the HGM theory diminish its accuracy, thereby demonstrating that the apparent successes of the HGM theory emerge from a cancellation of errors.
Multiperturbation approach to potential energy surfaces for polyatomic molecules
Authors: Galvan Donald H. , Abu-Jafar Moh’d , Sanders Frank C.
Published: July 2002

In Z-dependent perturbation theory, the lowest-order wave functions for a polyatomic molecule are not only independent of the nuclear charges, but also of the total number of nuclear centers and electrons in the molecule. The complexity of the problem is then determined by the highest order retained in the calculation. Choosing the simplest possible unperturbed Hamiltonian, we describe an n-electron, m-center polyatomic molecule as n ‘‘hydrogenic’’ electrons on a single center perturbed by electron–electron and electron–nucleus Coulomb interactions. With this H0, the first-order wave function for any polyatomic molecule will be a sum of products of hydrogenic orbitals with either two-electron, one-center or one-electron, two-center first-order wave functions. These first-order wave functions are obtained from calculations on He-like and H+2-like systems. Similarly, the nth-order wave function decouples so that the most complex terms are just the nth-order wave functions of all the p-electron, q-center subsystems (p+q=n+2) contained in the molecule. We illustrate applications of this method with some results, complete through third order in the energy, for H+3-like molecules. These are compared with accurate variational results available in the literature. We conclude that, through this order, this perturbation approach is capable of yielding results comparable in accuracy to variational calculations of moderate complexity. The ease and efficiency with which such results can be obtained suggests that this method would be useful for generating detailed potential energy surfaces for polyatomic molecules.
The structure of n-octadecane thiol monolayers self-assembled on Au(001) studied by synchrotron x-ray and helium atom diffraction
Authors: Li Jun , Liang K. S. , Camillone N. , Leung T. Y. B. , Scoles G.
Published: July 2002

We report on a new, more precise structural characterization of n-octadecane thiol monolayer self-assembled on the UHV-prepared Au(001) surface using a combination of helium atom diffraction, grazing incidence x-ray diffraction, and x-ray reflectivity measurements. Our results show that the equilibrated alkane chains form a two dimensional distorted hexagonal structure with lattice constants a=5.77±0.06 Å, b=4.81±0.02 Å, and α=53.1±0.46°. The alkane chains are tilted 33.5±1.0° from the surface normal and the tilt direction is ∼6.8±1.0° away from the elongated next-nearest-neighbor direction. The Au–thiol interface is found to have an unusual p(1×4) structure formed by an excess of Au atoms, which are believed to be produced in the dereconstruction process of the clean hexagonal Au(001) surface during the chemisorption of thiols. We propose that this Au interfacial layer modulates the height of the alkane chains resulting in a highly corrugated top surface, which forms a c(2×8) structure as observed by atom diffraction. These structural findings reveal a novel interplay between the head group/substrate interaction and the interchain van der Waals interaction which determines not only the alkane chain packing but also the structure of the interfacial layer of the substrate. Our studies also demonstrate that rich, complementary structural information can be obtained about the topmost surface, the chain packing, the interface arrangement, and the substrate of this and similar systems by the combination of atomic beam and synchrotron x-ray characterization techniques.
A stochastic model and a Monte Carlo simulation for the description of CO oxidation on Pt/Sn alloys
Authors: Mai J. , Casties A. , von Niessen W. , Kuzovkov V. N.
Published: July 2002

In this paper we study CO oxidation on a catalyst consisting of a Pt/Sn alloy. On this catalyst the reaction can take place at room temperature. We use for the description two basically different methods: Monte Carlo simulations and a theoretical stochastic ansatz. The stochastic ansatz introduced recently [Mai, Kuzovkov, and von Niessen, Physica A 203, 298 (1994)] is a general method for the description of surface reaction systems including mono- and bimolecular steps. Using the Markovian behavior of these systems we formulate this ansatz in terms of master equations. It turns out that the stochastic ansatz can be used as an interesting and advantageous alternative to the standard Monte Carlo simulations. The particles involved in the reaction system have different tendencies toward building structures on the surface. The coverages show a strong dependency, not only on the composition of the gas phase but also on the initial concentration of the reaction promotor OH and the concentration of Pt sites in the catalyst material. The reaction probability does not influence the qualitative trends of the coverages versus the gas phase concentration of CO.
Multimode Floquet theory for atomic and molecular laser-modified collision processes
Authors: Madsen Dane K. , Xiao Yiwen , Eno Larry
Published: July 2002

We outline a multimode-Floquet-based approach to the analysis of atomic and molecular laser-modified collision processes. The approach depends upon Fourier decomposition of the radiation field and identification of time-independent laser-interaction elements in the Floquet representation. The corresponding time-independent Floquet dynamical equations are solved and S-matrix elements between the various Floquet channels collected to determine the transition probabilities of interest. The approach is examined for laser excitation in a model energy transfer system. Results are compared with ones obtained conventionally, i.e., by explicitly following the time dependence of the field amplitude. We conclude that the Floquet method nicely complements the conventional.
Adatom bond energies and lateral interaction energies from calorimetry: NO, O2, and N2 adsorption on Ni{100}
Authors: Vattuone L. , Yeo Y. Y. , King D. A.
Published: July 2002

Heats of adsorption and sticking probabilities have been measured for NO and O2 on clean and oxygen precovered Ni{100} at room temperature by single crystal adsorption calorimetry (SCAC). Adsorption is initially dissociative for both species and the initial heat of adsorption is 536 kJ/mol for O2 and 426 kJ/mol for NO. From these data the initial heat of dissociative adsorption of N2 on Ni{100} is estimated to be 136 kJ/mol. With increasing coverage the adsorption heat for both NO and O2 decreases due to repulsive interactions between adsorbed adatoms; the magnitude of the pairwise repulsive interactions between oxygen adatoms in nnn sites is estimated to be about 40 kJ/mol by fitting the experimental data for O2 dissociative adsorption with a Monte Carlo simulation. The pairwise repulsive interaction between nitrogen and oxygen and between nitrogen adatoms in nnn sites is obtained as about 100 kJ/mol. Due to the high magnitude of lateral repulsive interactions between adatoms, a critical coverage of NO is reached for which molecular adsorption is energetically favored with respect to dissociative adsorption. This coverage is ∼0.16 ML for a 1:1 O:N adatom coverage decreasing to ∼0.07 ML for 3:1 O:N adatom ratio. For O2 adsorption, the lower magnitude of repulsive interactions allows dissociative adsorption to take place up to a coverage of about 0.3–0.4 ML; above this coverage the bulk oxide is more energetically favorable and an oxide layer is formed. NO dissociative adsorption is precursor mediated, whereas for O2 the precursor plays only a limited role.
Profiles of liquid metal surfaces and interfaces from x-ray reflectivity
Authors: Makov G. , Kornyshev A. A.
Published: July 2002

A versatile model of the surface profile is proposed to include such features as the smearing of the distribution profile near the surface, surface relaxation, long or short range order in the case of solid or liquid metals, respectively. A closed form expression is derived for the spectrum of x-ray reflectivity from metal surfaces which can be described by this model. The spectra generated by a variety of surface profiles are explored by varying the parameters of this model. Particular features of liquid metal spectra are found, and discussed in relation to experimental data. In particular, the model reproduces the recently obtained [Phys. Rev. Lett. 75, 2498 (1995)] reflectivity spectrum of liquid mercury surface, which has two maxima in the range of small and large Q and an intermediate Q ‘‘shoulder.’’ The shoulder is shown to be a fingerprint of the surface relaxation.
Exploring chromium (VI) dioxodihalides chemistry: Is density functional theory the most suitable tool?
Authors: Torrent M. , Gili P. , Duran M. , Solà M.
Published: July 2002

A comparative systematic study of the CrO2F2 compound has been performed using different conventional ab initio methodologies and density functional procedures. Two points have been analyzed: first, the accuracy of results yielded by each method under study, and second, the computational cost required to reach such results. Weighing up both aspects, density functional theory has been found to be more appropriate than the Hartree–Fock (HF) and the analyzed post-HF methods. Hence, the structural characterization and spectroscopic elucidation of the full CrO2X2 series (X=F,Cl,Br,I) has been done at this level of theory. Emphasis has been given to the unknown CrO2I2 species, and specially to the UV/visible spectra of all four compounds. Furthermore, a topological analysis in terms of charge density distributions has revealed why the valence shell electron pair repulsion model fails in predicting the molecular shape of such CrO2X2 complexes.
Nonlinear terms due to many-particle correlation in the density functional theory
Authors: Yoshimori Akira
Published: July 2002

A method of calculating nonlinear terms in free energy is studied for the density functional theory of a molecular system with orientational freedoms. In the method, a free energy functional is expanded into Taylor series of density fields, and then the coefficients are calculated from the cumulant expansion. Using the method, expansion coefficients of third and forth orders are explicitly derived by the three-point and four-point coefficients of the cumulant expansion. For the third-order coefficient, two approximations are compared by calculations of the triplet direct correlation function. The approximation obtained by expansion in terms of pair correlation functions gives abrupt changes in the triplet direct correlation function. The simplified Kirkwood superposition approximation gives slow changes in the function, though its value should be scaled down by a small parameter.
Observation of the lowest 1Δu Rydberg state of acetylene by multiphoton ionization spectroscopy
Authors: Fillion J. H. , Campos A. , Pedersen J. , Shafizadeh N. , Gauyacq D.
Published: July 2002

The 1Δu, 3dπ component of the lowest ungerade 3d+4s Rydberg supercomplex of acetylene has been observed by (3+1) photon ionization spectroscopy of C2H2 and C2D2. In both isotopic species the vibrationless level of this new electronic state is sufficiently long lived to support rotational structure. From the observed isotopic shift and from the rotational and vibrational parameters determined in the present analyses, this state could be unambiguously identified as the lowest Rydberg state of 1Δu symmetry, belonging to the 3d complex of acetylene. The weak intensity of this three-photon 1Δu–1Σ+g electronic transition is discussed in comparison with the other Rydberg transitions within the same supercomplex. Excited vibronic bands in the same energy region allowed to determine the ν1 and ν2 frequencies for some components of this Rydberg supercomplex. The absence of the nearby H̃ 1Πu, 3dδ Rydberg state in the MPI spectra is discussed in terms of predissociation.
High-pressure Brillouin scattering study of dense argon and nitrogen
Authors: Hochheimer Hans D. , Weishaupt Klaus , Takesada Masaki
Published: July 2002

We have measured the Brillouin shift as a function of hydrostatic pressure in dense argon and nitrogen at different constant temperatures. We show that for argon there is a linear relation between the index of refraction and the reduced density. A comparison of the sound velocities determined ultrasonically and from our Brillouin data shows a negative velocity dispersion. This is in agreement with theoretical predictions and measurements of the temperature dependence of the sound velocity along the vapor-pressure equilibrium curve using Brillouin scattering. The fact that the negative velocity dispersion increase with pressure at constant temperature is smaller at lower temperatures is, however, puzzling and cannot be explained within the framework of the above-mentioned theoretical model.
Mobility and solvation of ions in channels
Authors: Lynden-Bell R. M. , Rasaiah Jayendran C.
Published: July 2002

In this paper we present some results from a simulation study of the mobility and solvation of ions and uncharged molecules in aqueous solution in smooth cylindrical channels at room temperature. This ideal system provides a reference system with which to compare the behavior of water and ions in real porous materials such as zeolites, bucky tubes, and biological channels. We find that in channels of radii between 2.5 and 5.5 Å the water molecules form a cylindrical solvation shell inside the channel walls with some evidence of a second shell in the center of the largest channel. Not all protons are involved in hydrogen bonding and a number point toward the walls. We attribute this to the concavity of the surface. When a sodium ion is added it tends to lie in the center of the channel where it can form the most complete solvation shell. Its diffusion rate decreases in smaller channels until it moves too slowly in a channel of 2 Å radius to be detected in our simulations. This decrease is only partly due to an increase in the mean square force on the ion. A range of ions of different sizes were studied in a channel with radius 3 Å. While the smaller of these ions (F−, Na+, and Ca++) lie preferentially in the center of the channel, larger ions (Cl−, I−, and Cs+) penetrate some way into the layer of water inside the wall and methane and ions with the charges turned off move next to the wall. Landau free energy analysis shows that this change is due to the balance between entropy and energy. The behavior in smooth channels is quite the opposite of what has been observed in experimental studies and simulations of Gramicidin (pore radius of 2 Å), where Cs+ lies closer to the center of the channel and Na+ lies off the axis. This difference can be attributed to the specific molecular structure of the gramicidin pores (e.g., the presence of carbonyl groups). As in bulk solutions, the mobilities of the ions in smooth channels increase to a maximum with ion size and decrease with increasing magnitude of the charge on the ion, while uncharged species diffuse much more rapidly and show a monotonic decrease with size. This behavior is related to the characteristics of the fluctuations of the forces on the solute molecules.
Nuclear magnetic resonance microscopy in liquids using the dipolar field
Authors: Robyr P. , Bowtell R.
Published: July 2002

We demonstrate theoretically and experimentally how the dipolar field can be used in nuclear magnetic resonance (NMR) to investigate the structure of heterogeneous liquid systems. Using the Fourier transformed dipolar field and magnetization distribution, a simple relation between the NMR signal generated by the dipolar field and the sample structure can be established. On the basis of this relation, theoretical models for periodic structures have been derived and used to analyze the variation of the NMR signal as a function of the spatial modulation imposed on the magnetization. If the spatial modulations imposed on the transverse and longitudinal magnetizations have the same wavelength, the signal generated by the dipolar field is a continuous function of the modulation wavelength and is sensitive to the structure of the sample. When this condition is not met, diffraction phenomena may be possible in periodic structures. To test the theoretical work, experimental data have been obtained from water surrounding randomly packed microspheres. These data are in agreement with the theoretical predictions and show that a resolution of the order of 10 μm can be achieved for highly mobile systems. For spin bearing molecules, whose self-diffusion coefficient is two orders of magnitude less than that of free water, submicrometer resolution is expected.
Effects of finite grain size on adsorption in two-dimensional microporous media: Pillared clays
Authors: Nagy T. F. , Mahanti S. D.
Published: July 2002

A model incorporating finite size grain effects is developed to explain the observed nonzero Argon adsorption in two-dimensional microporous media (pillared clays) below the percolation threshold. The model assumes that small grains are formed inside the clay particles. Our theoretical results fit well with the observations at the present level of uncertainties in the experimental data. Further improvements of the current model would require more accurate experimental measurements.
Isotropic and anisotropic interaction induced scattering in liquid argon
Authors: Teboul Victor , Le Duff Yves
Published: July 2002

The collision induced scattering (CIS) spectra have been studied for liquid argon at 130 K and 615 amagat. For the first time, isotropic CIS intensities are measured and the depolarized CIS spectrum is obtained up to 370 cm−1. Molecular dynamics simulations are performed for several models of polarizabilities and intermolecular potentials. They show that theoretical polarizabilities deduced from self consistent field calculations are in agreement with both depolarized and isotropic CIS experimental spectral shape.
Mean-field molecular dynamics with surface hopping
Authors: Prezhdo Oleg V. , Rossky Peter J.
Published: July 2002

Molecular dynamics simulations of many degree of freedom systems are often comprised of classical evolutions on quantum adiabatic energy surfaces with intermittent instantaneous hops from one surface to another. However, since quantum transitions are inherently nonadiabatic processes, the adiabatic approximation underlying the classical equations of motion does not hold in the regions where quantum transitions take place, and the restriction to classical trajectories for adiabatic quantum states is an approximation. Alternatives which employ classical paths that account more fully for nonadiabaticity can be computationally expensive and algorithmically complicated. Here, we propose a new method, which combines the surface hopping idea with the mean-field approximation for classical paths. Applied to three test systems, the method is shown to outperform the methods based on an adiabatic force without significant extra effort. This makes it an appealing alternative for modeling complex quantum–classical processes.
Spin diffusion and spin-lattice relaxation in multiphase polymers
Authors: Wang Jiahu , Jack Kevin S. , Natansohn Almeria L.
Published: July 2002

A model which treats spin diffusion and spin-lattice relaxation in multiphase polymers on the same footing is proposed. This new approach allows for more accurate determination of domain sizes and interfacial thickness in the probed polymers. In a poly(styrene-b-isoprene) copolymer example a significant improvement in the agreement between the NMR measurements and the simulated results can be obtained with the incorporation of a spin-lattice relaxation term into the spin diffusion process. The obtained microphase structural parameters are similar to the small angle X-ray scattering results. In addition, the spin-lattice relaxation times (T1) in different domains can be predicted reasonably well based on T1s of the component homopolymers and on the microphase structure in the present model.
Stilbenoid molecules: An experimental and theoretical study of trans-1-(2-pyridyl)-2-(4-pyridyl)-ethylene and the parent molecule
Authors: Melandri Sonia , Maccaferri Gianni , Favero Paolo G. , Caminati Walther , Orlandi Giorgio , Zerbetto Francesco
Published: July 2002

Free-jet absorption millimeter-wave spectroscopy and gradient-corrected density-functional calculations are combined to study trans-1-(2-pyridyl)-2-(4-pyridyl)-ethylene, 2,4-DPE, a stilbenelike molecule. The interplay between experiments and computational theory backed by the large amount of data and modeling already available for stilbene allows us to establish the parameters of the two-dimensional potential of the pyridyl torsions which well reproduce the experimental defect of inertia (−1.295 uÅ2). The large difference between the defects of inertia of 2,4-DPE and stilbene (≈−15 uÅ2) and further computer simulations suggest that the latter may warrant a reinvestigation. The effect of introducing a weakly binding N⋯H interaction in the stilbene frame is discussed and a very simple model based on the ab initio calculations is found to rationalize the energetics and structural differences of the two conformers of 2,4-DPE.
Macromolecular conformational dynamics in torsional angle space
Authors: He Siqian , Scheraga Harold A.
Published: July 2002

A Brownian dynamics treatment in torsional angle space is presented for the simulation of conformational dynamics of macromolecules with fixed bond lengths and bond angles and with an arbitrary intramolecular potential energy function. The advantages of the torsional angle space treatment over similar treatments (Brownian dynamics or molecular dynamics) in atomic coordinate space are that, first, the number of variables is reduced by roughly a factor of 10 and, second, the integration time step size is increased by 3 to 4 orders of magnitude (because, by confining the treatment to the torsional angle space, the time step size is not limited by the fast oscillation modes of covalent bonds but rather by the slow motion of macromolecular segments whose time scale is roughly 3 to 4 orders of magnitude larger than that of bond oscillations). Consequently, the exploration of global conformational relaxation processes becomes computationally possible. The treatment is tested by studying the folding kinetics of off-lattice chains with fixed bond lengths and bond angles and with prescribed sequences. The present treatment is a general purpose one applicable to all macromolecular conformational relaxation processes (e.g., protein folding kinetics, drug/ligand docking on to target proteins, conformational multiple-minima problems, etc.). It serves as a complement to the molecular dynamics or Brownian dynamics treatments in atomic coordinate space.
Energy transfer processes in organized media. III. A two-center model for nonhomogeneous crystals
Authors: Vásquez Sergio O.
Published: July 2002

As part of a series of studies on energy transfer processes between optical centers in condensed matter, an analysis is made of the problem of a crystalline system made up by a majority of regular optical centers and small amounts of defective optical centers. Within the conceptual framework of the crystal model, analytic equations are derived that allow a direct connection between the microscopic study of the interaction and kinetic type equations to describe the relaxation dynamics from excited states. As an example, Ln3+ ions in elpasolite type lattices are used, showing the biexponential character of the decay curves. These results can be used in the study of relaxation from excited states in crystals with heterogeneities due to crystallographic defects, trace impurities, or slightly distorted optical centers.
Ionization energy and electron affinity of small alkali-metal clusters
Authors: Wojciechowski Kazimierz F.
Published: July 2002

The ionization energy and electron affinity of small spherical jellium clusters of alkali-metal are calculated using Brodie’s idea [I. Brodie, Phys. Rev. B 51, 13660 (1995)]. A comparison is made with the results obtained in the framework of the liquid drop model and in the Kohn–Sham equations with the local-spin-density approximation. The comparison with experimental data shows that the results obtained are better than those obtained by the use of the approximations mentioned above.
Inhomogeneous broadening of electronic spectra in organic model systems
Authors: Geissinger Peter , Kohler Bryan E. , Kulikov Serguei G. , Terpougov Victor
Published: July 2002

We studied the inhomogeneous broadening of the electronic transitions of the linear polyene octatetraene doped into mixed crystal matrices of various n-alkanes with added halogenated alkanes. The additional broadening caused by the halogenated alkane molecules can be described at low concentrations by a Lorentzian, whose width depends linearly on the concentration. The analysis of the experimental data showed that the main contribution to the additional inhomogeneous broadening in the investigated systems is not given by the electrostatic and dispersive interactions, but by the fluctuation of the nuclear coordinates of the chromophore.
Theoretical study of the anomalous surface tension properties of liquid crystals
Authors: Martínez-Ratón Y. , Velasco E. , Somoza A. M. , Mederos L. , Sluckin T. J.
Published: July 2002

We study the surface properties of a molecular model of a liquid crystal using a density functional theory. The intermolecular interaction includes a Maier-Saupe term as well as terms which break the orientational symmetry at the interface. These latter contributions prove to play an essential role in the explanation of some of the anomalous surface-tension features observed in experimental systems. In particular, a dramatic surface tension maximum above the clearing point TNI is seen to be associated with increased nematic order close to the nematic-vapor interface. In addition, further reduction of the surface tension with decreasing temperature below TNI is observed to be related either with rapidly increasing surface-enhanced order, in which case there is a surface tension minimum, or with growing surface smectic order above the nematic-smectic transition temperature.
Theoretical study for water structure at highly ordered surface: Effect of surface structure
Authors: Akiyama Ryo , Hirata Fumio
Published: July 2002

The structure of water at an ordered solid surface is investigated by the reference interaction site method (RISM). A RISM equation devised especially for a solute–solvent system in which the solute is a two-dimensional periodic array is employed to formulate the electrode–solution interface. Calculations are carried out for two types of surfaces: flatlike and Au(111)-like structures. The orientation of water molecules at the Au(111)-like surface with various surface charge densities, which are deduced from the correlation functions, is in good agreement with the results of surface-enhanced infrared absorption spectroscopy. On the other hand, the model of the flatlike surface does not give a consistent picture with the experiments. The difference is significant when the walls are negatively charged. In the case of a negatively charged wall, the Au(111)-like model gives about 108° for the angle between the surface normal vector and the two O–H vectors of the water molecules in the first layer, whereas the angles are about 72° and 180° for the flatlike surface. The results demonstrate that the solid surface structure is very important in discussing the structure and orientation of water molecules at the interface.
Accurate universal Gaussian basis set for all atoms of the Periodic Table
Authors: de Castro E. V. R. , Jorge F. E.
Published: July 2002

We have applied a discretized version of the generator coordinate Hartree–Fock method to generate a universal Gaussian basis set for the atoms from H through Lr. In general, for He to Rn the ground state Hartree–Fock total energy results obtained with our universal Gaussian basis set are lower than the corresponding ones obtained by Clementi et al. using larger atom-optimized geometrical Gaussian basis sets. For the atoms from H through Lr we have obtained ground state energy values with at least seven digits of accuracy (except for Pm, with six digits), in comparison with the corresponding ten digits of the numerical Hartree–Fock total energy results.
Two-dimensional scattering of slow molecules by laser beams
Authors: Seideman Tamar , Kharchenko V.
Published: July 2002

Quantum-mechanical and quasiclassical scattering of molecules in a nonperturbative laser field are studied. Analytical expressions for the observables of interest in laser-induced focusing and guiding of molecules are derived that span the quantal-to-classical energy regime and the weak-to-strong interaction range. General conclusions regarding the problem of two-dimensional quantum-mechanical scattering are drawn.
Microcanonical temperature and “heat capacity” computation of Lennard-Jones clusters under isoergic molecular dynamics simulation
Authors: Salian Umesh A.
Published: July 2002

The cumulated distribution of velocity components of atoms of a cluster in a constant energy classical molecular dynamics simulation is found to fit very well with Maxwellian distribution. This enables, with carefully prepared initial configurations, a cluster as small as containing just four atoms to be viewed to constitute a canonical ensemble in itself. In addition, the statistical distribution of velocity components provides an unambiguous, independent and robust method of obtaining the thermodynamic temperature of an isolated system. The temperature thus obtained differs from the conventionally used kinetic temperature by a fixed factor of (3N−6)/(3N), implications of which are discussed. A new procedure of sampling the configurational energy landscape is introduced, and the “heat capacity” curve computed using this method—which is actually the second moment of the potential energy fluctuations as a function of cluster kinetic energy—exhibits its usefulness by clearly demonstrating the two stage melting of Ar55.
Octupolar versus dipolar crystalline structures for nonlinear optics: A dual crystal and propagative engineering approach
Authors: Zyss J. , Brasselet S. , Thalladi V. R. , Desiraju G. R.
Published: July 2002

A new type of crystalline structure for nonlinear optics whereby octupolar symmetry features are displayed at both molecular and crystalline levels is exemplified by the prototype 2,4,6-triaryloxy-1,3,5-triazine (TPOT) crystal and analyzed in terms of both individual molecular responses and crystal packing features. Polarized harmonic light scattering permits the full determination of the molecular β hyperpolarizability tensor and confirms the octupolar trigonal symmetry of the TPOT molecule. An oriented gas model is used to infer therefrom an estimate of the crystalline nonlinear d tensor which is predicted to be of the same order as that of the reference dipolar N-4-nitrophenyl-(L)-prolinol crystal. The concept of optimal packing toward quadratic nonlinear optics, which had been initially introduced in the realm of quasi-one-dimensional structures, is revisited and enlarged to encompass more isotropic uniaxial structures potentially amenable, in the case of octupoles, to larger optimal values than in the one-dimensional case. Moreover, considerations pertaining to phase matching which had been left aside in the earlier one-dimensional optimization framework are now considered and the various type I and type II configurations compared for both one-dimensional and octupolar uniaxial structures. Application perspectives of octupolar structures toward short pulse nonlinear optics are discussed: their structurally built-in polarization independence is outlined as a major asset in contrast with the more traditional one-dimensional structures.
Vuv photolysis of hydrazoic acid: Absorption and fluorescence excitation spectra
Authors: Schönnenbeck G. , Biehl H. , Stuhl F. , Meier U. , Staemmler V.
Published: July 2002

The vuv-absorption of the HN3/DN3 isotopomers and the formation of NH/ND photofragments in the (c 1Π) and (A 3Π) states were studied. Tunable synchrotron radiation and several atomic resonance lines were used as light sources. The absorption spectrum, which shows more features than reported earlier, was analyzed by means of extensive quantum chemical ab initio calculations. The internal energies of the observed NH/ND(c,A) photofragments were estimated as a function of the photolysis wavelength by emission spectroscopy. The fragment NH/ND(c) is formed with a rather constant quantum yield below 147 nm, while the relative production yield of NH/ND(A) increases with decreasing wavelength. Although NH/ND(A) can be formed directly via a spin forbidden process at long wavelengths, it is more efficiently produced by reactions of the three different triplet N2(A,B,B′) states with HN3/DN3. The variation of the vibrational distribution of the NH/ND(A) radicals indicates that various production mechanisms exist.
Morphological variation in a collapsed single homopolymer chain
Authors: Noguchi H. , Yoshikawa K.
Published: July 2002

We studied the thermodynamics in a single homopolymer chain using a multicanonical Monte Carlo simulation. A polymer chain that exhibits an elongated coil state in a good solvent, or at high temperatures, collapses into a condensed state, i.e., coil–globule transition. For flexible polymer chains, as the temperature decreases, the coil state changes into a liquidlike spherical globule, and this liquid state then changes into a solidlike spherical globule; these are similar to the transitions between gas and liquid and between liquid and solid, respectively. For stiff polymer chains, the coil state changes into a crystalline state without the appearance of an intermediate liquidlike state, to give a product with toroidal morphology. For chains intermediate between stiff and flexible, the coil state changes into a state in which toroid and rod shapes coexist, and this state changes into a single solidlike state in which only the rod shape is present. These calculational results correspond well to experimental findings for the products of the collapse of single long DNA chains.
Evaporation and isomerization dynamics leading to the free-jet formation of isotopically labeled (benzene)13: A spectroscopic observation
Authors: Easter David C. , Mellott James , Weiss Todd
Published: July 2002

Isotopically labeled (benzene)13 clusters, (C6H6)(C6D6)12, were generated by supersonic expansion and studied as a function of nozzle-to-laser distance by resonance-enhanced two-photon ionization (R2PI) spectroscopy through the C6H6B2u←A1g601 transition. Because of the spectrum’s simplicity, it serves as a sensitive monitor of the environment and dynamics of the C6H6 chromophore. We report experimental evidence for both evaporation and isomerization dynamics. Initially, the observed (C6H6)(C6D6)12 cluster population undergoes a transition from fluxional to rigid, resulting from the evaporation of a single C6D6 molecule from (C6H6)(C6D6)13. “Solidification” is followed by isomerization, in which the C6H6 moiety migrates from the surface of ordered, rigid clusters to their interior. The “freezing” temperature of (C6H6)(C6D6)12 is inferred to be near 137 K, in good agreement with theoretical simulations [Bartell and Dulles, J. Phys. Chem. 99, 17107 (1995)].
Slow, steady discharge regime for concentrated lead-acid cells with planar, cylindrical, and spherical electrodes
Authors: Yang Yigang , Saslow Wayne M.
Published: July 2002

A recent theoretical study of slow, steady discharge for a lead-acid cell carrying current I, with planar electrodes, predicts a nonzero bulk charge distribution with an associated voltage profile within the electrolyte that is quadratic in space. A second theoretical study finds that the Ohmic voltage resistance R=ΔV/I across the electrolyte differs from the Joule heating resistance RJ=P/I2. The very different diffusion constants of the charge-carrying ions H+ and HSO4− is responsible for the quadratic voltage profile. The presence of changing chemical energies is responsible for the different resistances. In the present work we study this same chemical system for the cylindrical and spherical geometries, with Pb at inner radius a and PbO2 at outer radius b. For the cylindrical case, the voltage varies with radius as the sum of a logarithm and a quadratic. For the spherical case, the voltage varies with the radius as a sum of an inverse r and a quadratic. For both cases, the quadratic is the signature of a uniform nonzero bulk charge distribution. For both cases, R≠RJ. For large enough b/a, in both the spherical and cylindrical cases, we find that R<0; current flows from lower to higher electrical potential. This does not violate energy conservation when chemical as well as electrical energies are included.
Unusual nonequilibrium effects during the growth of two-dimensional crystals at the solid–fluid interface
Authors: Raudino Antonio
Published: July 2002

We propose a simple analytical model to investigate transient nonequilibrium effects during the formation and growth of 2D islands at the interface between a solid substrate and a fluid which contains freely diffusing adsorbable molecules. The model describes a sequence of processes: fast and uniform adsorption of isolated molecules which form an ideal 2D gaslike phase; migration along the fluid–solid interface; nucleation and growth of a 2D condensed phase in the interfacial region. The results clearly indicate the existence of critical values of adhesion energy at which the growth of two-dimensional clusters is maximum. On approaching the thermodynamic equilibrium we recover the standard behavior where the amount of adsorbed molecules steadily increases with the molecule-substrate adhesion energy. The transient anomalous behavior is the result of the competition between local surface concentration (which increases with the adhesion energy) and surface diffusion (which, in a jump diffusion picture, decreases with the adhesion force). The model has been extended to the case of two different adsorbed species which have nonideal mixing in the condensed phase. Transient anomalies were found when the minority component has a stronger adhesion energy than the more abundant one. The theory might qualitatively explain certain puzzling features observed in the adsorption of single proteins or mixtures of proteins onto flat substrates with different hydrophobicity.
Subband analysis of molecular electronic transitions by nonlinear polarization spectroscopy in the frequency domain
Authors: Beenken W. , Ehlert J.
Published: July 2002

Nonlinear polarization spectroscopy in the frequency domain (NLPF) allows a detailed analysis of substructure behind a molecular optical absorption band. Using the χ(3)-approach here we deduce theoretically line shapes of NLPF spectra for systems with more than one optical transition, which contribute to linear as well as excited state absorption. It is possible from such line shapes to decide whether these transitions are independent from each other or belong to a common ground state. We demonstrate further how energy transfer and relaxation paths can be revealed and quantified by a global analysis of NLPF spectra. This is useful to get a comprehensive understanding of ultrafast processes in complex molecular and supermolecular systems.
Response to “Comment on ‘A 250 GHz ESR study of o-terphenyl dynamic cage effects above TC’ ” [J. Chem. Phys. 109, 10523 (1998)]
Authors: Earle K. A. , Moscicki J. K. , Polimeno A. , Freed J. H.
Published: July 2002

We address the points raised by Giordano and Leporini (GL) and show that accounting properly for the nonexponential decay of the rotational correlation function leads to improved agreement with the Stokes–Einstein–Debye (SED) relation above the crossover temperature TC for those probes 3,3′-dimethyloxazolidinyl-N-oxy-2′,3-5α-cholestane (CSL), and perdeuterated 2,2′,6,6′-tetramethyl-4-methyl aminopiperidinyl-N-oxide) (MOTA) that are well-coupled to the viscous modes of o-terphenyl (OTP) when the average relaxation rate 16〈τ〉 is plotted versus 1/T. On the other hand, 2,2′,6,6′-tetramethyl-4-piperidine-N-oxide (PDT) shows simple Arrhenius behavior in this regime, because of weak coupling to the solvent cage, inconsistent with SED, which was clearly shown in our paper. We also suggest that the difference in chemical structure of the PDT probe, studied by us, compared to 2,2′,6,6′-tetramethylpiperidine-N-oxyl (TEMPO), studied by GL, accounts for the difference in the low-temperature relaxation behavior of the two probes.