bluuebunny/crossref_metadata_2025 huggingface dataset:
The electronic spectroscopy and molecular structure of the HPCl free radical: A potential III–V semiconductor growth intermediate
Authors: Tackett Brandon S. , He Sheng-Gui , Evans Corey J. , Clouthier Dennis J. , Judge Richard H.
Published: July 2003

The Ã 2A′–X̃ 2A″ electronic spectra of jet-cooled HPCl and DPCl have been obtained for the first time using the pulsed electric discharge technique with a precursor mixture of PCl3 and H2 or D2. From a combination of laser-induced fluorescence and wavelength resolved emission spectra, all of the vibrational frequencies in the ground and excited states of both isotopomers have been measured and vibrational force fields have been determined. Rotational analyses of the 000 bands of both isotopomers showed small doublet splittings characteristic of an asymmetric top molecule with a single unpaired electron. From the rotational constants and the force fields, estimated equilibrium structures were derived with r″(PH)=1.4158(23) Å, r″(PCl)=2.0388(23) Å, θ″=95.02(27)°, and r′(PH)=1.4067(20) Å, r′(PCl)=2.0050(2) Å, and θ′=115.53(12)°. The experimental data firmly establish that the observed spectra and those previously obtained by chemiluminescence techniques [Bramwell et al., Chem. Phys. Lett. 331, 483 (2000)] are due to the HPCl free radical.
Modeling of temporally complex breathing patterns during Pd-catalyzed CO oxidation
Authors: Nekhamkina O. , Digilov R. , Sheintuch M.
Published: July 2003

A mathematical model is formulated to account for experimental infrared thermography observations of spatiotemporal patterns during catalytic oxidation of CO over Pd supported on a glass-fiber disk-shaped cloth in a continuous reactor with feed flowing perpendicular to and through the disk. The model predicts the following observed features: (a) The sustained pattern that the system exhibits is a breathing motion in which a hot spot expands and contracts continuously. This motion emerges due to the imposed cold-edge boundary condition and a qualitative analysis of the experiments supports this suggestion and rules out other mechanisms. (b) The emerging temporally complex patterns can be classified as mixed-mode oscillations with a large relaxation-type conversion peak superimposed with several smaller peaks. (c) The mathematical mechanism that accounts for the change in the number of smaller peaks with varying operating conditions (the reactor temperature) could be characterized as period adding. The mathematical model is based on a published oscillatory kinetics model, and is coupled here with an enthalpy and gas-phase balances with two adjusted parameters. Numerical simulations map the simple and the complex oscillations domains and characterizes the transition between them as either period doubling or period adding.
Dose dependence of surface plasmon resonance of a Ti–SiO2 nanoparticle composite
Authors: Zhao J. P. , Chen Z. Y. , Rabalais J. W.
Published: July 2003

The linear optical absorption properties of a titanium nanoparticle composite formed by implantation of low energy Ti+ into single crystal SiO2 are reported. Evolution of a surface plasmon resonance (SPR) induced by the formation of a metal nanoparticle composite is studied as a function of ion dose. At an implantation temperature of 25 °C, the threshold dose for the appearance of the SPR is ∼3.5×1016 ions/cm2, indicating that spontaneous nucleation and clustering of titanium nanoparticles in SiO2 occurs at ∼3.2×1022 ions/cm3 peak concentration of implants. The average particle size and volume fraction are dependent on the ion dose. The correlation of the optical response, i.e., the intensity and frequency of the observed SPR, with the nucleation/clustering of the metal nanoparticles is discussed on the basis of Mie scattering theory and Maxwell Garnett theory.
Equation-of-motion coupled cluster method with full inclusion of connected triple excitations for electron-attached states: EA-EOM-CCSDT
Authors: Musiał Monika , Bartlett Rodney J.
Published: July 2003

We extend the full triples equation-of-motion (EOM) coupled cluster (CC) method to electron attached states. Proper factorization of the three- and four-body parts of the effective Hamiltonian makes it possible to achieve for the EA-EOM part a scaling no higher than nocc2nvir5. The method is calibrated by the evaluation of the valence vertical electron affinities for the C2 and O3 molecules for several basis sets up to 160 basis functions. For C2, EA-EOM-CCSDT gives 3.24 eV at the extrapolated basis limit, while the experimental adiabatic EA is equal to 3.27±0.008 eV. For O3 the agreement is ∼1.9 eV compared to an adiabatic value of 2.1 eV.
Integral expressions satisfied by the gradient of density functional potentials
Authors: Joubert D. P.
Published: July 2003

It is shown that the product of the gradient of the density functional correlation potential and the charge density and the product of the gradient of the sum of the exchange and Hartree potentials and the charge density integrate to zero. The integral of the product of the charge density and the gradient of exchange potential is equal to an integral expression of a product of occupied Kohn–Sham orbitals and the gradient of the mutual Coulomb interaction. It is also shown that the expectation value of the gradient of the mutual Coulomb interaction is zero for the ground state of a many-electron system.
Nine-dimensional quantum molecular dynamics simulation of intramolecular vibrational energy redistribution in the CHD3 molecule with the help of coupled coherent states
Authors: Shalashilin Dmitrii V. , Child Mark S.
Published: July 2003

A previously developed method of coupled coherent states (CCS) is applied to the simulation of intramolecular vibrational energy redistribution in the CHD3 molecule. All nine modes are taken into account within a fully quantum approach. Emphasis is placed on convergence with respect to the number of coherent states in relation to the desired propagation time, which was taken to be sufficient to resolve Fermi resonance splitting of ∼100 cm−1 at an excitation energy of ∼16 000 cm−1. Fermi-resonance beatings of energy between C–H stretch and two C–H bends as well as slow energy flow to the rest of the molecule are reproduced. Due to the use of Monte Carlo grids the CCS technique scales extremely well with the number of modes and allows fully quantum molecular dynamics simulations of polyatomic systems.
Photodissociation dynamics of azulene
Authors: Lin Ming-Fu , Huang Cheng-Liang , Lee Yuan T. , Ni Chi-Kung
Published: July 2003

Photodissociation of azulene at 193 nm was studied in a molecular beam using multimass ion imaging techniques. Most of the azulene molecules excited by 193 nm photon quickly relax to the ground electronic state through internal conversion, then isomerize to naphthalene, and eventually dissociate through the H atom elimination channel with a rate of 5.1×104 s−1. A small amount of azulene entering different isomerization channels was found. The effect of dissociation in the energy transfer experiments using azulene as a vibrationally highly excited molecule and the existence of azulene in an interstellar medium is discussed.
Selectively observing the amplitude modulation under magic angle sample spinning
Authors: Liu Yun , Leu Gabriela , Singer S. , Sen Pabitra N. , Cory David G.
Published: July 2003

Magic angle sample spinning (MASS) averages the inhomogeneous magnetic field due to the susceptibility contrast in porous media by modulating the local magnetic field (frequency modulation). Molecular diffusion introduces a homogeneous broadening, which modulates the amplitude of the signal (amplitude modulation). The depth of amplitude modulation is determined by the interplay of molecular diffusion and MASS averaging and contains rich information on local magnetic fields, through which the spatial structure of the sample may be obtained. In this paper, we present two methods to quantify the amplitude modulation: a phase suppressed method and a more conventional two-dimensional (2D)-exchange method. The phase suppressed method directly observes the amplitude modulation in the time domain. The approximate equations are derived to extract the physical information from the time domain data. The conventional 2D-exchange spectrum contains both the frequency modulation and the amplitude modulation terms. The amplitude modulation introduces cross peaks in the 2D spectrum. The integration of the 2D spectrum along lines parallel to the main diagonal line will give a one-dimensional spectrum, that is the Fourier transform of the amplitude modulation.
Thermal electron emission from the hot electronic subsystem of vibrationally cold C60
Authors: Hansen K. , Hoffmann K. , Campbell E. E. B.
Published: July 2003

We compare theoretical results on statistical electron emission from electronically hot but vibrationally cold C60 with recent experimental results involving excitation with ultrashort laser pulses. Both photoelectron spectra and ion yields of C60 as well as fragment ions are compared with the predictions of the statistical electron emission model. Quantitative agreement is obtained between the calculations and the experimentally measured photoelectron spectra, total ion yields and singly-doubly charged branching ratio. The electron-phonon coupling time is fitted to a few hundred femtoseconds, consistent with independent measurements. The data allow a determination of the thermal properties of the electron system, which are consistent with the theoretical input. The data also allow a fit of the averaged photon absorption cross section.
The role of atomic quadrupoles in intermolecular electrostatic interactions of polar and nonpolar molecules
Authors: Torii Hajime
Published: July 2003

For the purpose of getting insight into the reason for the anomalous vibrational frequency shifts observed in some usually used solvents for a mode that has a large dipole derivative, the role of atomic quadrupoles in intermolecular electrostatic interactions is studied for some halogen-containing molecules (CX4, HX, and X2 with X=F, Cl, and Br), CH4, CO2, and CS2. From the fitting to the electrostatic potentials around the molecules, large atomic quadrupoles are obtained for the chlorine, bromine, and sulfur atoms, suggesting that the atomic quadrupolar effect is important for electrostatic interactions around covalently bonded atoms on the third and higher rows in the periodic table. Taking the case of the chlorine atoms as an example, the electron densities inside the atoms in CCl4, HCl, and Cl2 are examined. It is found that these electron densities are highly anisotropic. This anisotropy in electron densities is reasonably explained by the forms of the occupied molecular orbitals, and is considered to be the electronic structural origin of the large atomic quadrupoles.
Determination of the nucleus size from the growth probability of clusters
Authors: ter Horst Joop H. , Kashchiev Dimo
Published: July 2003

Exact and approximate general expressions for the growth and decay probabilities of a cluster in one-component nucleation are given. A method is proposed for a model-independent determination of the nucleus size with the help of data for the dependence of the cluster growth probability on the cluster size. Monte Carlo simulation data for the cluster growth probability are obtained for homogeneous one-component nucleation of droplets in Lennard-Jones vapors. From these data, the size of the nucleus droplet and the Zeldovich factor are determined by the newly proposed method.
Coexistence curves for melts of lattice polymers with structured monomers: Monte Carlo computations and the lattice cluster theory
Authors: Buta Dorel
Published: July 2003

The influence of monomer structure on the vapor–liquid coexistence curves of lattice polymer melts is investigated through Monte Carlo calculations. The extended lattice model used in the computations has been introduced as the underlying model for the lattice cluster theory (LCT), a thermodynamic theory for polymer mixtures in the liquid state. The LCT model is essentially a bare-bones united atom model defined on a cubic lattice. The computational method employed is a variant of the density of states Monte Carlo algorithm. Chain architectures analogous to those of polyethylene, polypropylene, polyethylethylene, and polyisobutylene are considered. The computational results are compared with the coexistence curves predicted by the LCT. Qualitative and quantitative disagreements between the theoretical and the computed values are observed, casting doubts on the reliability of the LCT in the study of the phase behavior of melts and solutions of polymers with structured monomers.
Influence of salt on the structure of polyelectrolyte solutions: An integral equation theory approach
Authors: Hofmann T. , Winkler R. G. , Reineker P.
Published: July 2003

We investigate the influence of monovalent saltions on the structural properties of polyelectrolyte solutions using an integral equation theory. In this approach all species of the solution (polyions, counterions, and positively and negatively charged saltions) are treated explicitly leading to a four-component system. The polymer-reference-interaction-site model for this system, together with the reference-Laria–Wu–Chandler closure is solved numerically. We demonstrate that addition of salt leads to a screening of the Coulomb interaction, which is well captured by the Debye–Hückel potential with a salt density-dependent screening length, by discussing various correlation functions. Furthermore, we show that for an appropriate range of parameters, such as density or Bjerrum length, a shell of equally charged saltions exists in the vicinity of the polyion. The effective potential between two monomers reflects attraction among the equally charged polyions with a pronounced dependence on the salt concentration.
Dependence on sphere size of the phase behavior of mixtures of rods and spheres
Authors: Urakami Naohito , Imai Masayuki
Published: July 2003

By the addition of chondroitin sulfate (Chs) to the aqueous suspension of tobacco mosaic virus (TMV), the aggregation of TMV occurs at very dilute TMV concentration compared with the addition of polyethylene oxide (PEO). The difference of physical behavior between Chs and PEO is the chain conformation in solution. The Chs chain has a semirigid nature, whereas the PEO chain has a flexible nature. In this study, the Chs and PEO chains are simplified to spherical particles having different size, and we use the spherocylinder model for TMV particle. The effect of the sphere size on the phase behaviors in the mixtures of rods and spheres is investigated by Monte Carlo simulations. By the addition of small spheres, the system transforms from the miscible isotropic phase to the miscible nematic phase. On the other hand, by the addition of large spheres, the system changes from the miscible isotropic phase to the immiscible nematic phase through the immiscible isotropic phase. The different phase behaviors between the small and the large spheres originate from the difference of overlapping volume of the depletion zone. In addition, we perform the Monte Carlo simulations in the case that semirigid chains are used as the Chs chain models. The same phase behaviors are observed as the mixtures of rods and large spheres. Thus the sphere model captures the phase behaviors of rod and polymer mixture systems.
Two-color photoassociation spectroscopy of the lowest triplet potential of Na2
Authors: de Araujo Luı́s E. E. , Weinstein Jonathan D. , Gensemer Stephen D. , Fatemi Fredrik K. , Jones Kevin M. , Lett Paul D. , Tiesinga Eite
Published: July 2003

We have performed a type of Autler–Townes spectroscopy to locate a number of rovibrational-hyperfine levels of the a 3Σu+ potential, the lowest triplet potential of the Na2 dimer. The spectroscopy starts with the photoassociation of ultracold atoms in a magneto-optical trap. We have measured the binding energies of over 100 individual states spanning the vibrational levels v=8–15 of this potential (binding energies up to 27 cm−1). We obtain a typical accuracy of 15 MHz and a typical resolution of 20 MHz, improving on the 10 GHz accuracy and 30 GHz resolution previously available for the vibrational states v<12. Vibrational, rotational, and hyperfine structures are resolved. Additionally, we have been able to resolve the magnetic electron–electron spin–spin dipole splitting of a number of these hyperfine levels. The measured rotational and hyperfine structures show good agreement with theoretical calculations. An analysis of the remaining discrepancies indicates where possible refinements to the potentials can be made. We also observe evidence for the presence of second-order spin–orbit coupling.
Ab initio derived analytical fits of the two lowest triplet potential energy surfaces and theoretical rate constants for the N(4S)+NO(X 2Π) system
Authors: Gamallo P. , González Miguel , Sayós R.
Published: July 2003

This work presents two new analytical fits of the ground potential energy surface (PES) (3A″) and the first excited PES (3A′) involved in the title reaction, considering the N-abstraction (1) and the O-abstraction (2) reaction channels, and the reverse reaction (−1). The PESs are derived from ab initio electronic structure calculations by means of second-order perturbation theory on a complete active-space self-consistent-field wave function (CASPT2 method). Stationary points and extensive grids of ab initio points (about 5600 points for the A″3 PES and 4900 points for the A′3 PES) were fitted along with some diatomic spectroscopic data to better account for the experimental exoergicity. Thermal rate constants were calculated (200–5000 K) for all mentioned reaction processes by means of the variational transition-state theory with the inclusion of a semiclassical tunneling correction. Excellent agreement with the experimental data was observed for reaction (1) and its reverse, within all the temperature range, substantially improving the results derived from previous analytical PESs. The contribution of the A′3 PES to the reaction rate constant (k1) was small even at high temperatures (e.g., only 10.8% at 2500 K). Moreover, the main contribution to reaction rate constant (k2) was due to the A′3 PES, differing from what happens for reaction (1). The O-abstraction reaction channel accounts for a 3.0% of the total reaction (k=k1+k2) at 5000 K, consistent with the very limited experimental information available.
Spatial correlation in a Lotka–Volterra reaction diffusion mechanism
Authors: Schmidt Hansen Jesper , Toxvaerd Søren , Præstgaard Eigil L.
Published: July 2003

Here we study a derivative of the Lotka–Volterra reaction diffusion mechanism using the framework of molecular dynamics. First, we perform a series of simulations in one and two dimensions and we find that there exists a critical correlation length in both cases. This means that for domain lengths below this critical correlation length the system will show spatial homogeneous oscillations, whereas for systems with domain lengths larger than the critical correlation length spatial concentration gradients will emerge and the temporal oscillations will be wiped out. We then show, as a main result, that the critical correlation length is smaller in two dimensions than in one dimension, which is in contrast to what is found in, for example, the Poincaré model.
Water’s Tg-endotherm, sub-Tg peak of glasses and Tg of water
Authors: Johari G. P.
Published: July 2003

In our studies of Tg of amorphous forms of water, it had been ascertained that the observed endotherm had the characteristics of glass softening and not those of a sub-Tg peak, but an explicit discussion was not provided. To prevent this Tg-endotherm from being mistaken as part of a sub-Tg peak, we use our earlier data to explain why the endotherm corresponds to water’s Tg of 136 K, and not to a sub-Tg peak.
Theoretical study of electronic and solvent reorganization associated with a charging process of organic compounds. I. Molecular and atomic level description of solvent reorganization
Authors: Sato Hirofumi , Kobori Yasuhiro , Tero-Kubota Shozo , Hirata Fumio
Published: July 2003

The electronic and solvation structures of N,N-dimethylaniline in acetonitrile are examined by means of the ab initio reference interaction site model self-consistent-field theory coupled with the method to evaluate nonequilibrium solvation free energy developed by Chong et al. [J. Phys. Chem. 99, 10 526 (1995)]. The key quantities characterizing the solvation process—the free-energy profile governing the solvent fluctuations and solvent reorganization—are evaluated from first principles. A new scheme, which enables us to partition solvent reorganization into atomic contributions in the solute molecule, is proposed and used to analyze the process at the atomic level. We found that the linear response approximation holds well and the overall observable λs is not much affected by solute geometry, while the individual atomic contribution is significantly changed, especially by the wagging motion of the amino group.
Constant-number Monte Carlo simulation of aggregating and fragmenting particles
Authors: Friesen W. I. , Dabros T.
Published: July 2003

The constant-number Monte Carlo method introduced by Matsoukas and co-workers for simulating particulate systems is applied to the kinetics of aggregating and fragmenting particles. The efficiency of this approach is increased by incorporating a modified version of Gillespie’s full-conditioning algorithm for selecting an aggregation or fragmentation event. After the steps comprising the algorithm are outlined, it is validated by simulations for several aggregation and fragmentation kernels for which the population balance equations can be solved exactly. The results agree very well with the analytical expressions except for those kernels that give rise to a gelation transition, such as the product kernel kij=ij. In this case, the simulation data are accurate below the transition time tg, but deviate significantly above tg. The accuracy of the simulation method in describing gelling kernels, including those of the form kij=(ij)ω, is also investigated. For a strongly gelling kernel, tg is accurately predicted by maxima in the time derivative of the second moment of the particle mass and the time dependence of the number of size classes in the simulation. Gel formation is simulated by setting a threshold size g above which particles have properties of the gel in the Stockmayer or Flory models. The Stockmayer model can be accurately simulated for a value of g that depends on the number of particles in the simulation. Simulation of the Flory model is less successful; results are obtained more efficiently by using the conventional constant-volume Monte Carlo method.
High-pressure stability, transformations, and vibrational dynamics of nitrosonium nitrate from synchrotron infrared and Raman spectroscopy
Authors: Song Yang , Hemley Russell J. , Liu Zhenxian , Somayazulu Maddury , Mao Ho-kwang , Herschbach Dudley R.
Published: July 2003

The properties of nitrosonium nitrate (NO+NO3−) were investigated following synthesis by laser heating of N2O and N2O4 under high pressures in a diamond anvil cell. Synchrotron infrared absorption spectra of NO+NO3− were measured at pressures up to 32 GPa at room temperature. Raman spectra were obtained at pressures up to 40 GPa at room temperature and up to 14 GPa at temperatures down to 80 K. For both lattice and intramolecular vibrational modes, a smooth evolution of spectral bands with pressure indicates that NO+NO3− forms a single phase over a broad range above 10 GPa, whereas marked changes, particularly evident in the Raman spectra at low temperature, indicate a phase transition occurs near 5 GPa. NO+NO3− could be recovered at atmospheric pressure and low temperature, persisting to 180 K. The Raman and IR spectroscopic data suggest that the NO+NO3− produced by laser heating of N2O followed by decompression may differ in structure or orientational order–disorder from that produced by autoionization of N2O4.
Nuclear magnetic shielding and chirality. I. The shielding tensor of Xe interacting with Ne helices
Authors: Sears Devin N. , Jameson Cynthia J. , Harris Robert A.
Published: July 2003

Chirality and, in particular, induced chirality is investigated using Xe interacting with chirally perturbed Ne helices. The full nuclear magnetic shielding tensors are calculated and physical implications are discussed.
Nuclear magnetic shielding and chirality. II. The shielding tensor of a naked spin in Ne helices
Authors: Sears Devin N. , Jameson Cynthia J. , Harris Robert A.
Published: July 2003

We continue our investigation of the nuclear magnetic shielding tensors of Xe@Nen helix complexes (I). Here we replace Xe by a naked spin. The full shielding tensor of Ne8 helix diastereomers is calculated and compared with the equivalent Xe@Ne8 diastereomers.
Nuclear magnetic shielding and chirality. III. The single electron on a helix model
Authors: Sears Devin N. , Jameson Cynthia J. , Harris Robert A.
Published: July 2003

In this third paper of the triptych we use the Tinoco–Woody model of an electron on a helix as our chiral system [I. Tinoco, Jr. and R. W. Woody, J. Chem. Phys. 40, 160 (1964)]. Diastereomerism is achieved by varying the pitch of the helix. The full nuclear magnetic shielding tensor of a naked spin is determined with various subtleties explicated. The results are compared to the Ne helix diastereomers.
Instantaneous nonvertical electronic transitions with shaped femtosecond laser pulses: Is it possible?
Authors: Henriksen Niels E. , Møller Klaus B.
Published: July 2003

In molecular electronic transitions, a vertical transition can be induced by an ultrashort laser pulse. That is, a replica of the initial nuclear state—times the transition dipole moment of the electronic transition—can be created instantaneously (on the time scale of nuclear motion) in the excited electronic state. Now, applying pulse shaping via the modulation of the phases of each spectral component of an ultrashort pulse, it is tempting to ask whether it is also possible to induce instantaneous nonvertical transitions to bound electronic states, provided that the phases of each spectral component of the pulse are set to appropriate values at the discrete frequencies corresponding to the energy levels of the potential. We analyze the problem in the weak-field limit, and show that such a phase requirement cannot be encoded into an ultrashort pulse. This result is equivalent to the statement that it is not possible to move matter faster than the time associated with the natural (field-free) dynamics of the system.
A realistic multi-sheeted potential energy surface for NO2(2A′) from the double many-body expansion method and a novel multiple energy-switching scheme
Authors: Varandas A. J. C.
Published: July 2003

We report a new multi-sheeted double many-body expansion potential energy surface that reproduces most known topological features of the title system. Near spectroscopic accuracy is conveyed to the ground-state sheet of 2A1 (1 2A′ in Cs) symmetry in the vicinity of the minimum by merging it with a spectroscopically determined Taylor-series-expansion-type form via a novel multiple energy-switching scheme. A high energy ridge for C2v insertion of N(4S) into O2(X 3Σg−) has also been imposed to mimic the result of accurate ab initio complete active space self-consistent field and second-order perturbation theory on CAS wave function calculations carried out for such geometries. This ridge decreases for Cs geometries yielding a minimum barrier height for the N+O2 reaction of 0.273 eV at a bent N–O–O structure defined by RNO=3.107a0, ROO=2.513a0, and ∠NOO=113.5 deg. Both the location and height of this barrier are in good agreement with existing ab initio calculations and the recommended values. Another salient feature is a shallow minimum on the Ã 2B2 potential energy surface that is separated from the absolute X̃ 2A1 minimum by a conical intersection. Such a feature is accurately predicted by the newly reported ab initio calculations and well mimicked by the global double many-body expansion/energy-switching potential energy surface here reported. This is therefore commended both for spectroscopic and reactive dynamics studies on the title system. A final comment in relation to the conical intersection and the energy-switching scheme goes to the expected accuracy of current approaches for spectroscopically determined effective single-valued forms.
Massive thermostatting in isothermal density functional molecular dynamics simulations
Authors: Windiks René , Delley Bernard
Published: July 2003

This paper demonstrates the excellent temperature control and rapid equipartioning of the kinetic energy of the massive generalized Gaussian moment thermostat (MGGMT, one thermostat is coupled to each degree of freedom) in isothermal density functional molecular dynamics (MD) simulations on the Born–Oppenheimer potential energy surface. The MGGMT is implemented in the DMoL3 approach and, as far as we know, it is the first time in literature that the MGGMT is combined with density functional methods. The performance of the MGGMT approach is illustrated with MD simulations of the iron porphyrin–imidazole–carbon monoxide [FeP(Im)(CO)] complex and compared with constant energy MD simulations on the same system. Both MD approaches lead to similar average structures of the FeP(Im)(CO) complex. The examination of the frequency distribution functions reveals that the structural dynamics are not seriously affected by the dynamics of the parameters introduced by the MGGMT. The equipartitioning rates in the MGGMT simulations are significantly faster than in the constant energy simulation. We recommend the MGGMT approach as an very efficient equilibration technique in MD simulations and it emerges as a useful technique for, e.g., simulated annealing and nonequilibrium MD simulations.
Conformational distribution functions extracted from residual dipolar couplings: A hybrid model based on maximum entropy and molecular field theory
Authors: Stevensson Baltzar , Sandström Dick , Maliniak Arnold
Published: July 2003

This paper describes a new approach for analysis of residual dipolar couplings (RDCs). The method, which focuses on construction of the conformational distribution function, is applied to 4-n-pentyl-4′-cyanobiphenyl in the nematic phase. The RDCs are calculated from a trajectory generated in a molecular dynamics (MD) simulation, based on a realistic atom–atom interaction potential. Computer simulation is an attractive method for investigating theoretical models for partially ordered systems since the answer is provided: we know the true orientational order and molecular structure. Our new approach is based on two models that have been frequently used for interpretations of dipolar couplings in liquid crystals: the additive potential (AP) model and the maximum entropy (ME) method. These models suffer, however, from serious limitations: the AP model requires a priori knowledge of the functional form of the torsional potential, whereas the ME approach gives the flattest possible distribution, which results in an incorrect description of systems with low orientational order. The procedure presented here (which we call APME) does not require knowledge of the functional form of the intramolecular potential and is applicable to weakly ordered systems. This makes the APME model a potentially useful tool for investigations of conformations in biomacromolecules dissolved in dilute aqueous liquid crystals. In the investigation reported in the present study, the results from the APME analysis are in excellent agreement with the true molecular structure in the MD simulation. The estimation of the validity range indicates that the APME approach is applicable to weakly ordered systems as well as to conventional nematic mesophases.
Crossed-beam scattering of F+CD4→DF+CD3(νNK): The integral cross sections
Authors: Zhou Jingang , Lin Jim J. , Shiu Weicheng , Pu Shih-Chieh , Liu Kopin
Published: July 2003

The title reaction was investigated in a crossed-beam experiment. A (2+1) resonance-enhanced multiphon ionization technique was used to interrogate the internal-state distributions of the CD3 product at three different collision energies. Only the ν2 (umbrella) mode excitation was observed. Its distribution changes from a monotonically declined distribution at low energy to a slightly inverted one at higher collision energy. Although the rotational excitations of CD3 were small, a strong preference for K=0 was found, indicative of the dominance of the tumbling rotation motion of the CD3 product. The vibration-resolved excitation functions were also measured for ν2=0–3. A reaction barrier of 0.5 kcal/mol was deduced.
The interaction of oxygen with small gold clusters
Authors: Varganov Sergey A. , Olson Ryan M. , Gordon Mark S. , Metiu Horia
Published: July 2003

Presented in this work are the results of a quantum chemical study of oxygen adsorption on small Aun and Aun− (n=2,3) clusters. Density functional theory (DFT), second order perturbation theory (MP2), and singles and doubles coupled cluster theory with perturbative triples [CCSD(T)] methods have been used to determine the geometry and the binding energy of oxygen to Aun. The multireference character of the wave functions has been studied using the complete active space self-consistent field method. There is considerable disagreement between the oxygen binding energies provided by CCSD(T) calculations and those obtained with DFT. The disagreement is often qualitative, with DFT predicting strong bonds where CCSD(T) predicts no bonds or structures that are bonded but have energies that exceed those of the separated components. The CCSD(T) results are consistent with experimental measurements, while DFT calculations show, at best, a qualitative agreement. Finally, the lack of a regular pattern in the size and the sign of the errors [as compared to CCSD(T)] is a disappointing feature of the DFT results for the present system: it is not possible to give a simple rule for correcting the DFT predictions (e.g., a useful rule would be that DFT predicts stronger binding of O2 by about 0.3 eV). It is likely that the errors in DFT appear not because of gold, but because oxygen binding to a metal cluster is a particularly difficult problem.
Simulation of dense amorphous polymers by generating representative atomistic models
Authors: Curcó David , Alemán Carlos
Published: July 2003

A method for generating atomistic models of dense amorphous polymers is presented. The generated models can be used as starting structures of Monte Carlo and molecular dynamics simulations, but also are suitable for the direct evaluation physical properties. The method is organized in a two-step procedure. First, structures are generated using an algorithm that minimizes the torsional strain. After this, an iterative algorithm is applied to relax the nonbonding interactions. In order to check the performance of the method we examined structure-dependent properties for three polymeric systems: polyethyelene (ρ=0.85 g/cm3), poly(L,D-lactic) acid (ρ=1.25 g/cm3), and polyglycolic acid (ρ=1.50 g/cm3). The method successfully generated representative packings for such dense systems using minimum computational resources.
Modeling of ultrafast electron-transfer processes: Validity of multilevel Redfield theory
Authors: Egorova Dassia , Thoss Michael , Domcke Wolfgang , Wang Haobin
Published: July 2003

The capability of multilevel Redfield theory to describe ultrafast photoinduced electron-transfer reactions is investigated. Adopting a standard model of photoinduced electron transfer in a condensed-phase environment, we consider electron-transfer reactions in the normal and inverted regimes, as well as for different values of the electron-transfer parameters, such as reorganization energy, electronic coupling, and temperature. Based on the comparison with numerically exact reference results, obtained using the self-consistent hybrid method, we discuss in some detail the advantages and shortcomings of two different versions of Redfield theory, which employ the time-dependent and stationary Redfield tensor, respectively. The results of the study demonstrate that multilevel Redfield theory, if applied in the appropriate parameter regime, is well suited to describe the ultrafast coherent dynamics of photoinduced electron-transfer reactions.
Fractional reaction-diffusion equation
Authors: Seki Kazuhiko , Wojcik Mariusz , Tachiya M.
Published: July 2003

A fractional reaction-diffusion equation is derived from a continuous time random walk model when the transport is dispersive. The exit from the encounter distance, which is described by the algebraic waiting time distribution of jump motion, interferes with the reaction at the encounter distance. Therefore, the reaction term has a memory effect. The derived equation is applied to the geminate recombination problem. The recombination is shown to depend on the intrinsic reaction rate, in contrast with the results of Sung et al. [J. Chem. Phys. 116, 2338 (2002)], which were obtained from the fractional reaction-diffusion equation where the diffusion term has a memory effect but the reaction term does not. The reactivity dependence of the recombination probability is confirmed by numerical simulations.
General features of the energy landscape in Lennard-Jones-like model liquids
Authors: Angelani L. , Ruocco G. , Sampoli M. , Sciortino F.
Published: July 2003

Features of the energy landscape sampled by supercooled liquids are numerically analyzed for several Lennard-Jones-like model systems. The properties of quasisaddles (minima of the square gradient of potential energy W=|∇V|2), are shown to have a direct relationship with the dynamical behavior, confirming that the quasisaddle order extrapolates to zero at the mode-coupling temperature TMCT. The same result is obtained either analyzing all the minima of W or the saddles (absolute minima of W), supporting the conjectured similarity between quasisaddles and saddles, as far as the temperature dependence of the properties influencing the slow dynamics is concerned. We find evidence of universality in the shape of the landscape: plots for different systems superimpose into master curves, once energies and temperatures are scaled by TMCT. This allows to establish a quantitative relationship between TMCT and potential energy barriers for Lennard-Jones-like systems, and suggests a possible generalization to different model liquids.
Dielectric permittivity and electric modulus in Bi2Ti4O11
Authors: Liu Jianjun , Duan Chun-Gang , Yin Wei-Guo , Mei W. N. , Smith R. W. , Hardy J. R.
Published: July 2003

Frequency and temperature dependences of dielectric permittivity and electric modulus of pure and Ba-doped Bi2Ti4O11 were studied in the ranges of 10−1–106 Hz and −150–350 °C, respectively. We found that the antiferroelectric phase transition temperature of Bi2Ti4O11 decreases with Ba doping. In the permittivity studies, we also observed dielectric relaxation peaks shift to higher temperature with increasing frequency. Furthermore, in the electric modulus formalism, conducting peaks were uncovered above 150 °C in addition to the dielectric relaxation peak. We discussed the mechanisms for the dielectric relaxation and conduction processes based on TiO6 octahedra distortion and a space-charge model.
Kramers’ turnover theory for diffusion of Na atoms on a Cu(001) surface measured by He scattering
Authors: Guantes R. , Vega J. L. , Miret-Artés S. , Pollak Eli
Published: July 2003

The diffusion of adatoms and molecules on a surface at low coverage can be measured by helium scattering. The experimental observable is the dynamic structure factor. In this article, we show how Kramers’ turnover theory can be used to infer physical properties of the diffusing particle from the experiment. Previously, Chudley and Elliot showed, under reasonable assumptions, that the dynamic structure factor is determined by the hopping distribution of the adsorbed particle. Kramers’ theory determines the hopping distribution in terms of two parameters only. These are an effective frequency and the energy loss of the particle to the bath as it traverses from one barrier to the next. Kramers’ theory, including finite barrier corrections, is tested successfully against numerical Langevin equation simulations, using both separable and nonseparable interaction potentials. Kramers’ approach, which really is a steepest descent estimate for the rate, based on the Langevin equation, involves closed analytical expressions and so is relatively easy to implement. Diffusion of Na atoms on a Cu(001) surface has been chosen as an example to illustrate the application of Kramers’ theory.
Condensed-phase effects on absolute cross sections for dissociative electron attachment to CFCs and HCFCs adsorbed on Kr
Authors: Lu Q.-B. , Sanche L.
Published: July 2003

We present measurements of absolute dissociative electron attachment (DEA) cross sections to CFCl3, CHF2Cl, and CH3CF2Cl adsorbed on the surface of Kr as a function of electron energy (0–10 eV). The DEA cross sections are measured to be ∼7.2×10−15 cm2 at ∼0 eV, ∼4.2×10−16 cm2 at 0.65 eV, and ∼7.8×10−16 cm2 at 0.89 eV for CFCl3, CHF2Cl, and CH3CF2Cl, respectively. This cross section is similar to the gas-phase value for CFCl3, while for the latter two molecules, it is orders of magnitude higher than the gaseous values. These results can be explained by considering the changes in the survival probability of the anion resonance and in the electron capture probability due to the decrease of the nuclear wave function overlap in the Franck–Condon region.
Adsorption structure of acetylene on Ge(001): A first-principles study
Authors: Cho Jun-Hyung , Kleinman Leonard
Published: July 2003

The adsorption of acetylene on the Ge(001) surface is investigated by first-principles density-functional calculations within the generalized gradient approximation. We find that the di-σ structure is energetically favored over the end-bridge, r-bridge, and p-bridge structures, while the paired end-bridge structure is the most stable. Especially, our calculated adsorption energy for the p-bridge structure is −0.03 eV, indicating that this structure is not stable on Ge(001). This result does not support the conclusion drawn from a recent scanning tunneling spectroscopy (STM) and temperature programmed desorption experiment where the most populated adsorption configuration was assigned to the p-bridge structure. Our calculated energetics and STM simulation of various adsorption configurations are consistent with the two observed configurations being the di-σ and paired end-bridge structures.
Li ion transport in an intercalated polymer electrolyte
Authors: Arun N. , Vasudevan S. , Ramanathan K. V.
Published: July 2003

Lithium ions solvated by polyethylene-oxide (PEO) have been confined by intercalation within the galleries of an insulating, inorganic layered solid CdPS3. The dc conductivity of this confined polymer electrolyte Cd0.75PS3Li0.5(PEO) displays a distinct change in the mechanism of conduction with temperature, exhibiting a crossover from an Arrhenius temperature dependence at low temperatures to an non-Arrhenius, Vogel–Tamman–Fulcher behavior at higher temperatures. We use 2H, 7Li, and 13C nuclear magnetic resonance (NMR) in combination with infrared spectroscopy to probe Li ion mobility as well as segmental motion of the intercalated PEO. Within the galleries of Cd0.75PS3Li0.5(PEO) both rigid and mobile fractions of the intercalated polymer are present with the equilibrium fraction of the mobile species increasing with temperature. The 7Li (I=3/2) NMR of the confined polymer electrolyte exhibits an unusual behavior—the appearance of quadrupolar satellites at high temperatures where the conductivity values are appreciable. The results signify the solvation of Li ions by mobile segments of the intercalated PEO. Further proof of this association is seen in the 13C NMR as well as infrared spectra, both of which show evidence of the complexation of Li ions by ether linkages of the interlamellar PEO at high temperature and the absence of such an association at lower temperature when polymer motion is absent.
The configurational phase transitions of flexible polymers in planar mixed flows near simple shear
Authors: Woo N. J. , Shaqfeh Eric S. G.
Published: July 2003

We examine the configurational phase transitions of flexible polymer chains in planar mixed flows. We distinguish three distinct phases: the coiled state (C) in which the flow is unable to deform the chain significantly from its equilibrium configuration, the tumbling state (T) in which the chain undergoes a tumbling motion and the chain fluctuations are comparable to the full chain contour length as in shear flow, and the fully stretched state (S) in which the chain is fully stretched in its steady configuration and the chain fluctuations are small compared to the full chain contour length as in extensional flow. As the mixed flow parameter α changes, the chain undergoes changes in configuration from C to T to S. We consider separately the phase transitions from C to T (C-T) and from T to S (T-S). We use physical arguments and supporting Brownian dynamics simulations to derive the range in the parameter α (as a function of the flow strength Wi and chain molecular weight N) over which such phase transitions occur in the high flow strength limit.
Experimental and theoretical characterization of a C2H2O2+ cation in solid argon
Authors: Dong Jian , Miao Lei , Zhou Mingfei
Published: July 2003

Laser ablation of transition metals with concurrent codeposition of C2H2/O2/Ar mixtures at 11 K produced metal independent absorptions at 1493.1 cm−1. On the basis of isotopic shifts and splittings, enhancement in doping with electron trapping gas, and quantum chemical frequency calculations, the band is assigned to the O–O stretching vibration of the C2H2O2+ cation, which was predicted to have a 2A″ ground state with a nonplanar Cs symmetry.
Conformation-assisted fluctuation of density and kinetics of nucleation in polymer melts
Authors: Tan Hongge , Miao Bing , Yan Dadong
Published: July 2003

The phase separation kinetics of density and conformation in polymer melts is studied by the linearized time-dependent Ginzburg–Landau equations. In the present model of the free energy density, there are two order parameters which are the density and the conformation. A new nucleation mechanism is proposed, in which the fluctuation in density and the fluctuation in conformation are coupled by the mixed derivative term and the cross gradient term of these two parameters. These terms are important factors for the phase separation kinetics, since the density and the conformation further each other and induce the phase separation both in density and in conformation, which finally induce the nucleation in polymer melts. The structure factors for both density and conformation are calculated. The relevant small x-ray scattering and depolarized light scattering experimental results are compared to test this model.
Near-surface characterization of amorphous carbon films by neutron reflectivity
Authors: Johnson J. A. , Woodford J. B. , Erdemir A. , Fenske G. R.
Published: July 2003

A high-density top-surface layer of approximately 30 Å was found in a form of diamondlike carbon, “near-frictionless carbon,” developed at Argonne National Laboratory. Three diamondlike-carbon films were prepared by plasma-enhanced chemical vapor deposition with different hydrogen-to-methane ratios in the plasma. Complementary films were prepared with deuterated methane and deuterium, in the same ratios. Results from neutron relectivity experiments are presented, along with model comparisons, and a hypothesis is postulated.