bluuebunny/crossref_metadata_2025 huggingface dataset:
The two-state linear curve crossing problems revisited. II. Analytical approximations for the Stokes constant and scattering matrix: The Landau–Zener case
Authors: Zhu Chaoyuan , Nakamura Hiroki
Published: July 2002

Based on the exact solution of the linear curve crossing problems reported in the previous paper of this series, approximate analytical solution is discussed here for the same sign of slopes of the diabatic potentials (the Landau–Zener case). A new general method is proposed for connecting wave functions along Stokes lines in the complex plane. Two new compact analytical formulas for reduced scattering matrix are derived and compared with others. The whole range of the two parameters which effectively represent the coupling strength and the collision energy is divided into five regions, in each one of which the best recommended formulas are proposed. The new formulas proposed here are simple and explicit functions of the two parameters and thus useful for practical application. Especially, a simple and compact formula which works better than the conventional Landau–Zener formula is obtained for nonadiabatic transition probability for one passage of crossing point. Furthermore, in a region near the crossing point at intermediate coupling strength where no analytical approximation works well, certain fitting formulas are provided for the Stokes constant.
Nonexponential behavior in the vibrational predissociation dynamics of I2(B,ν)–Ar13
Authors: Li Zhiming , Borrmann Angela , Martens Craig C.
Published: July 2002

The vibrational predissociation dynamics of an Ar13 cluster containing a vibrationally excited I2 molecule in its B electronic state are studied by classical trajectory simulation. The kinetics of the loss of the first Ar atom from the cluster induced by vibrational relaxation of the diatomic exhibits unusual nonexponential behavior, characterized by an instantaneous rate that increases with time. The simulation results are modeled by a hybrid statistical model, which explicitly takes into account the slow relaxation of the impurity while assuming rapid redistribution of energy among the low frequency cluster modes. Minor deviations between the simulation and theory are observed, and indicate nonstatistical effects in the ‘‘solvent’’ degrees of freedom.
Second order optical nonlinearity in octupolar aromatic systems
Authors: Joffre Manuel , Yaron David , Silbey Robert J. , Zyss Joseph
Published: July 2002

We investigate the second-order nonlinear optical properties of octupolar molecules, as opposed to traditional charge transfer dipolar molecules. We consider in particular the case of hexasubstituted aromatic rings, trianiline, trinitrobenzene, and tri-amino-,tri-nitro-benzene (TATB).
Collision-induced dissociation of Ti+n (n=2–22) with Xe: Bond energies, geometric structures, and dissociation pathways
Authors: Lian Li , Su C.-X. , Armentrout P. B.
Published: July 2002

The kinetic energy dependence of the collision-induced dissociation (CID) of Ti+n (n=2–22) with Xe is studied by using a guided ion beam mass spectrometer. Examination of the CID cross section behavior over a broad collision energy range demonstrates that Ti+n clusters dissociate exclusively by sequential loss of Ti atoms. Bond energies of ionic titanium clusters, D0(Ti+n−1–Ti), are determined from measurements of the CID thresholds. D0(Ti+n−1–Ti) are found to change significantly as a function of cluster size, with local maxima at n=7, 13, and 19. This pattern of highly stable cluster ions suggests that titanium cluster ions favor icosahedral structures.
Ar 2p spectroscopy of free argon clusters
Authors: Rühl E. , Heinzel C. , Hitchcock A. P. , Baumgärtel H.
Published: July 2002

Total electron and total and partial ion yield spectra of Ar clusters (with average size up to 600±200) in the region of Ar 2p excitation have been measured using synchrotron radiation and time-of-flight mass spectrometry. As the average cluster size increases, the x-ray absorption spectrum changes systematically from that of atomic Ar to that of solid Ar. The shape of the Ar 2p3/2→4s region is found to be a sensitive monitor of the cluster sizes present in a molecular beam of Ar clusters. Extended x-ray absorption fine structure (EXAFS) is detected in the spectra of the larger clusters. There is a strong correlation between the intensity of the components of the Ar 2p3/2→4s signal associated with clusters and the intensity of the Fourier filtered first shell Ar 2p EXAFS signal. A low amplitude, high frequency fine structure is observed in the Ar 2p continuum of the heaviest clusters which corresponds closely to that observed in solid Ar. This signal develops with cluster size more slowly than the Ar 2p EXAFS and 4s exciton signals.
Low-energy elastic electron scattering by tetrafluoromethane (CF4)
Authors: Winstead Carl , Sun Qiyan , McKoy Vincent
Published: July 2002

We report cross sections for electronically elastic electron scattering by CF4 from 1 to 40 eV, calculated within the static-exchange approximation using the Schwinger multichannel method. Although the static-exchange approximation does not give results that are accurate in detail below 20 eV, it is useful in understanding resonant features in the elastic and vibrationally inelastic cross sections. Above 20 eV, where the static-exchange approximation is more reliable, we derive a dissociation cross section in fair agreement with experiment by subtracting our result from the measured total cross section. We compare our integral and differential cross sections with the results of recent elastic and vibrationally inelastic scattering experiments.
Vibrational and rotational relaxation times of solvated molecular ions
Authors: Li M. , Owrutsky J. , Sarisky M. , Culver J. P. , Yodh A. , Hochstrasser R. M.
Published: July 2002

Infrared pump–probe and infrared polarization spectroscopy have been used to measure the vibrational relaxation times (T1) of the antisymmetric stretching mode and the reorientation times (TR) for N3−, NCS−, and NCO− in D2O and/or methanol. For N3−, experiments were also conducted in H2O and hexamethyl–phosphamide (HPMA) solutions. The rapid vibrational relaxation and slow reorientation observed demonstrate strong coupling between the ions and the solvents. Longer vibrational relaxation and shorter reorientation times measured for NCS− reveal weaker solvent interactions that may be due to the importance of the charge distribution and the form of the normal coordinate. A comparison of the T1 and TR times in different solvents permits a determination of the relative interaction strengths for the solvents investigated. The relatively weaker coupling of N3− in the aprotic solvent HMPA demonstrates the importance of hydrogen bonding in strong solvent interactions in ionic solutions. The experimental results are compared with recent molecular dynamics simulations of ionic solutions.
Electronic and vibrational spectra of matrix isolated anthracene radical cations: Experimental and theoretical aspects
Authors: Szczepanski Jan , Vala Martin , Talbi Dahbia , Parisel Olivier , Ellinger Yves
Published: July 2002

Radical cations of anthracene have been formed by vapor phase electron impact followed by trapping in an argon matrix at 12 K. Visible/ultraviolet and infrared absorption spectra of the anthracene cations in an argon matrix have been run. Significant differences in the infrared band intensities between neutral and cationic anthracene have been observed. The effects of photolysis and added CCl4 have been studied and their influence on the infrared band intensities correlated with known visible bands attributable to the anthracene cation. Theoretical calculations using Pariser–Parr–Pople and intermediate neglect of differential overlap methodologies with high level multireference perturbation configuration interaction, specifically modified for spectroscopic applications, have been performed. Both approaches predict the previously observed photoelectron spectrum well. For the optical absorption, the match with the experimental spectrum is also good, but there are notable differences between the two predictions. Ab initio restricted open shell Hartree–Fock calculations for the cation vibrational frequencies show excellent correspondence with the observed infrared bands. The theoretical infrared intensities show some remarkable differences between the neutral and ionized species. For example, the CH in-plane bending modes and CC in-plane stretching vibrations are predicted to increase by several orders of magnitude upon ionization. Experimental infrared intensities have been estimated and certain bands do show a significant increase in the cation band intensities over the neutrals. The implication of these findings for the hypothesis that polycyclic aromatic hydrocarbon cations are responsible for the unidentified infrared emission bands from interstellar space is discussed briefly.
Classical dynamics of hydrogen bonded systems: Water clusters
Authors: Vegiri A. , Farantos S. C.
Published: July 2002

The minimum energy structures and the classical dynamics of (H2O)n clusters, with n=3–6,8, have been investigated with the potential function of Cieplak, Kollman, and Lybrand. The potential is the sum of a pairwise additive part plus a polarization term calculated iteratively from the permanent and induced dipole moments. It is found that at the minimum energy structures the polarization energy makes a contribution of about 13% in the total energy. Caloric curves and rms fluctuations of the oxygen–oxygen bond lengths show that the tetramer, with a tetragonal minimum geometry, and the octamer, with a cubic minimum geometry, are more stable clusters than the pentamer and hexamer. The geometries of absolute minima for the trimer, tetramer, and octamer agree with those predicted by other potentials. For the pentamer and hexamer the present potential predicts floppy type structures instead of cyclic ones, which are supported by other functions and ab initio calculations. The dynamics of water clusters is chaotic with a characteristic time for forgetting the initial conditions equal to the period of the stretching mode of the hydrogen bond. Phenomena of coexistence appear for the hexamer and larger clusters for which classes of minima of different geometrical type exist. For all studied clusters the power spectra of the oxygen velocity autocorrelation function are characterized by two frequency bands; one around 30 cm−1, torsional, and one at 200 cm−1, stretching. Generally, no significant influence on the dynamics of the clusters is found by the inclusion of polarization interaction, although for the hexamer and octamer polarization energy results in interchanging the geometries of absolute minima.
The production and photodissociation of iron–sulfur cluster ions
Authors: Yu Zhongde , Zhang Nan , Wu Xiaojun , Gao Zhen , Zhu Qihe , Kong Fanao
Published: July 2002

Iron–sulfur cluster ions FenSm+ (n=1–13, m=1–13) were produced by direct laser ablation on a solid sample containing a mixture of iron and sulfur powder. UV photodissociation of the cluster ions was studied with a tandem time-of-flight mass spectrometer. It was found that all the cluster ions with compositions of m=n, m=n−1, or m=n+5 were relatively more abundant, indicating that they were stable. The photodissociation results of the FenSm+ ions showed that, for parent ions with n≪m, the main channels were sequential losses of neutral S atoms until n∼m, while for parent ions with n∼m, the main product ions had compositions of smaller m=n or m=n−1. From these experimental results, it is proposed that the FenSn+ cluster ions might have structures similar to those of the FenS*n cores in iron–sulfur proteins, while the FenSm+ (m≳n) cluster ions could be considered to have structures with the FenSn+ cores surrounded by some peripheral S atoms.
Simulated annealing study of neutral and charged clusters: Aln and Gan
Authors: Jones R. O.
Published: July 2002

Density functional calculations with simulated annealing have been performed for clusters of aluminum Aln and gallium Gan up to n=10. There are many local minima in the energy surfaces, with a rich variety of structures and spin multiplicities. With increasing cluster size we find transitions from planar to nonplanar structures at n=5, and to states with minimum spin degeneracy at n=6. Isomers (n≥5) with buckled planar structures reminiscent of the layers in crystalline α-gallium are generally less stable than ‘‘three-dimensional’’ isomers. All structures show regular patterns of bond and dihedral angles. Systematic differences between Al and Ga clusters—bonds in the latter are shorter and bond angles closer to 90°—can be understood in terms of atomic properties. Trends in binding and ionization energies are compared with experiment and with the predictions of other calculations.
Configurational entropy of microemulsions: The fundamental length scale
Authors: Reiss Howard , Ellerby H. Michael , Manzanares José A.
Published: July 2002

Phenomenological models have been quite successful in characterizing both the various complex phases and the corresponding phase diagrams of microemulsions. In some approaches, e.g., the random mixing model (RMM), the lattice parameter is of the order of the dimension of an oil or water domain and has been used as a length scale for computing a configurational entropy, the so-called entropy of mixing, of the microemulsion. In the central and material section of this paper (Sec. III), we show that the fundamental length scale for the calculation of the entropy of mixing is of the order of the cube root of the volume per molecule—orders of magnitude smaller than the dimension of such a domain. This length scale is specifically the scale for the configurational entropy—not that which measures either the curvature of the interface, the ‘‘granularity’’ of the microemulsion, or the persistence length. Furthermore, we demonstrate, in general, that mixing entropy, evaluated in configuration space as opposed to phase space, will not be physically correct unless it is made to be consistent with the phase space evaluation. Following this core section, we give a one-dimensional illustration of the problem (Sec. IV), and discuss the consequences of our general result with respect to the RMM (Sec. V). The RMM not only seriously underestimates the entropy of mixing but exhibits a dependence on composition that is qualitatively very different from the correct dependence. Furthermore, for oil or water rich compositions of the microemulsion, the correct mixing entropy reinforces effects that would normally be attributed to bending energy, i.e., it destabilizes the system.
Critical temperature and concentration versus molecular weight in polymer blends: Conformations, fluctuations, and the Ginzburg criterion
Authors: Hol/yst Robert , Vilgis T. A.
Published: July 2002

We use the field-theoretical methods to derive the self-consistent one-loop equations (Hartree approximation) for the location of the critical point in binary mixtures of polymers. The small parameter in the loop expansion is the inverse of the square root of the number of segments (monomers) in a polymer chain 1/√N. Both the ideal chain conformation and fluctuation corrections to the Flory–Huggins mean field result are taken into account. For symmetric mixtures, the critical temperature Tc is shown to deviate from its mean field value (∼N) as √N, while the critical concentration remains unchanged in comparison to its mean field value φ̄c=1/2. Since the fluctuations tend to disorder the system, the real value of the critical temperature is lower than its mean field value. For asymmetric A,B mixture (NA≳NB), the critical concentration of A monomers φ̄c is shown to be larger than its mean field value. In the limit of NA→∞, the critical temperature attains its mean field value, even for finite NB. However, we estimate that for NA=NB∼104, the correction to the Flory–Huggins parameter at the critical point may still be of the order of 10%, although it depends on the details of the system. The explicit formulas for Tc and φ̄c as functions of NA and NB are given. The role of the upper wave vector cutoff in these formulas is emphasized and its proper estimate is given. The loop expansion, viewed as an expansion in a small parameter, is correct as long as both NA and NB are much larger than unity and breaks down when any of these quantities becomes of the order of unity. The calculation of the Ginzburg region in the temperature-concentration plane is also given. It is based on the analysis of the scattering intensity. The comparison with the earlier estimates of the Ginzburg criterion (for temperature) is made.
A highly accurate interatomic potential for argon
Authors: Aziz Ronald A.
Published: July 2002

A modified potential based on the individually damped model of Douketis, Scoles, Marchetti, Zen, and Thakkar [J. Chem. Phys. 76, 3057 (1982)] is presented which fits, within experimental error, the accurate ultraviolet (UV) vibration-rotation spectrum of argon determined by UV laser absorption spectroscopy by Herman, LaRocque, and Stoicheff [J. Chem. Phys. 89, 4535 (1988)]. Other literature potentials fail to do so. The potential also is shown to predict a large number of other properties and is probably the most accurate characterization of the argon interaction constructed to date.
On the Munn–Silbey approach to nonlocal exciton–phonon coupling
Authors: Zhao Yang , Brown David W. , Lindenberg Katja
Published: July 2002

A canonical transformation method originally proposed by Munn and Silbey is used to partially diagonalize a model Hamiltonian which incorporates both local and nonlocal exciton–phonon coupling. At the heart of the method is a secular elimination principle which poses a difficult self-consistency problem. A limited form of this self-consistency problem was solved in an approximate fashion by primarily analytical methods in the original work of Munn and Silbey. We take a numerical approach, solving the general self-consistency problem to desired accuracy. Among the differences between our findings and those of the original work are polaron binding energies much larger and Debye–Waller factors much smaller than originally anticipated.
Molecular dynamics study of the curvature correction to the surface tension
Authors: Haye M. J. , Bruin C.
Published: July 2002

The first order curvature correction to the surface tension of a drop is given by Tolman’s length. Up until now, no accurate estimates of this length existed for realistic fluids. Recently Blokhuis and Bedeaux proposed a new relation that expresses Tolman’s length as an integral over the pair distribution function of a planar liquid–vapor interface. We have used this relation to obtain estimates of Tolman’s length from molecular dynamics simulations of a Lennard-Jones liquid–vapor interface. We found it to be positive and small for a range of temperatures running from the triple-point temperature to the critical temperature.
The effect of harmonic conformational trajectories on protein fluorescence and lifetime distributions
Authors: Alcala J. Ricardo
Published: July 2002

Proteins change their structure constantly due to their conformational flexibility and dynamic nature. The structure of the molecule follows a trajectory in conformational space determined by the hierarchy of conformational substates of the protein. These conformational trajectories may differ in the ground and excited states. The molecule constant structural changes exposes the fluorescent protein residues to continuously mutating environments in a predetermined sequence given by the molecule conformational path. The absorption extinction coefficient and the emission decay rate of a given fluorescent residue, dependent upon the instantaneous localized environment of the residue, are modulated by the conformational dynamics of the molecule. A simple harmonic conformational trajectory describes the behavior of traditional, time invariant, fluorescence lifetime distributions with temperature. The lifetime distribution describes at any given time the distribution of excited residues in the protein conformational space. Consequently, dynamic conformational trajectories require the use of time dependent lifetime distributions. The evolution of uniform lifetime distributions modulated by harmonic conformational trajectories are illustrated. The results presented here open the way to the description of complex conformational dynamics in terms of structural harmonic fluctuations.
The thermal roughening of liquid surfaces and its effect on gas–liquid collisions
Authors: King Mackenzie E. , Saecker Mary E. , Nathanson Gilbert M.
Published: July 2002

Collisions of inert gases with a perfluorinated liquid over an 85 K range reveal how gas–liquid energy transfer depends on the temperature of the liquid. At higher temperatures, thermal accommodation of impinging Ne, Ar, and Xe atoms grows at the expense of prompt inelastic scattering. The experiments suggest that hotter liquids possess rougher surfaces, which promote multiple collisions that dissipate the atom’s incident energy and which may momentarily trap gases in gaps created by incommensurate packing of the surface molecules. The fraction of energy transferred during a high energy impulsive collision remains almost unaltered by changes in the liquid’s temperature, implying that impulsive encounters within the scattering plane are dominated by single collision events.
Aluminum clusters. A comparison between all electron and model core potential calculations
Authors: Martínez Ana , Vela Alberto , Salahub Dennis R. , Calaminici Patrizia , Russo Nino
Published: July 2002

Density-functional calculations using gradient-corrected functionals have been performed for aluminum clusters up to Al4 (neutral and cationic) using the linear-combination-of-Gaussian- type-orbitals approach. Two different types of calculations (all electron and model core potential) were performed to obtain full geometry and spin optimization. A comparison between both methods and with other experimental and theoretical values has been done. Bond distances, equilibrium geometries, atomization energies, ionization potentials, and harmonic frequencies are in good agreement between them. The nature of the ground state of Al2 was examined in detail. This work shows that all electron and model core potential calculations generate the same results.
A computer simulation method for the calculation of chemical potentials of liquids and solids using the bicanonical ensemble
Authors: Swope William C. , Andersen Hans C.
Published: July 2002

We present a method for the calculation of the chemical potential of a liquid or solid using molecular dynamics or Monte Carlo computer simulations of the material. Like several previously presented methods, it is based on the insertion and deletion of particles from the system. It is based on what we call a ‘‘bicanonical ensemble,’’ which is an equilibrium statistical mechanical ensemble characterized by a temperature T and a volume V and in which all systems have either N or N−1 particles. The chemical potential is simply related to the fraction of the systems of the ensemble that have N particles. It is possible to construct algorithms, combining either molecular dynamics motion or Monte Carlo moves with the insertion and deletion of particles, such that the set of states generated by the algorithm constitute a Markov chain whose stationary distribution is that of the bicanonical ensemble. The key to the success of the method is a feature that leads to a high success rate for the insertion and deletion steps, even at high densities, thus giving good statistics for the chemical potential. The method is applied to a two-dimensional liquid of particles with a pairwise additive inverse 12th power repulsive potential, for which it gives accurate results for the chemical potential for the entire equilibrium liquid range of densities. The method is applicable and accurate for the two-dimensional solid of the same particles, and so it can be used in studies of the phase diagram for two-dimensional materials.
Vibrational frequencies of transition metal chloride and oxo compounds using effective core potential analytic second derivatives
Authors: Russo Thomas V. , Martin Richard L. , Hay P. Jeffrey , Rappé Anthony K.
Published: July 2002

The application of analytic second derivative techniques to quantum chemical calculations using effective core potentials is discussed. Using a recent implementation of these techniques, the vibrational frequencies of transition metal compounds are calculated including the chlorides TiCl4, ZrCl4, and HfCl4, the oxochlorides CrO2Cl2, MoO2Cl2, WO2Cl2, and VOCl3, and the oxide OsO4. Results are compared to previous calculations and with experimental results.
Ground and excited states of Xe+2 observed by high resolution threshold photoelectron spectroscopy of Xe2
Authors: Lu Y. , Morioka Y. , Matsui T. , Tanaka T. , Yoshii H. , Hall R. I. , Hayaishi T. , Ito K.
Published: July 2002

Threshold photoelectron spectra of the xenon dimer have been observed with a resolution of 2 meV in the wavelength range 920–945 Å and 1022–1112 Å using the penetrating field technique and synchrotron radiation. Threshold photoelectron bands associated with transitions to the A2 Σ+1/2u, B2 Π3/2g, C2 Π3/2u, C2 Π1/2u, and D2 Σ+1/2g states of Xe+2 have been identified. Vibrational structure associated with the C2 Π1/2u state has been observed for the first time and a new value of the D2 Σ+1/2g state ionization potential is reported.
Rumpled graphite basal plane as a model heterogeneous carbon surface
Authors: Bakaev V. A.
Published: July 2002

A new model is suggested for the heterogeneous surfaces of nongraphitized carbon adsorbents. It may be called the rumpled graphite basal plane (RGBP). The atomic structure of RGBP can be obtained by squeezing a graphite basal plane in a molecular dynamics computer simulation under a random distribution of initial atomic velocities. The empirical Tersoff potential describes the carbon–carbon interactions. The degree of squeezing is chosen to reproduce the main features of the x-ray interference function of nongraphitized carbon blacks. Grand canonical ensemble Monte Carlo simulation of the isotherms of adsorption of N2 on RGBP reproduce experimental isotherms on these carbon blacks reasonably well, especially in the BET region of relative pressures.
Photoionization spectroscopy of yttrium clusters: Ionization potentials for Yn and YnO (n=2–31)
Authors: Knickelbein Mark
Published: July 2002

The photoionization spectra of yttrium clusters Yn, and their monoxides YnO have been recorded for n=2–31. As has been observed for other transition metal clusters, the vertical ionization potentials decrease rapidly yet nonmonotonically to ∼10 atoms, beyond which a more gradual decrease takes place. The ionization potentials of the cluster monoxides were found to be similar to those of the corresponding bare clusters except for n≤10 where differences of 0.1–0.3 eV were observed in some cases. The absence of magic numbers or odd–even alternations is taken as evidence that threshold photoionization occurs from a localized 4d orbital rather than a delocalized band derived from 5s valence electrons.
Ionic distribution around simple DNA models. I. Cylindrically averaged properties
Authors: Montoro J. C. Gil , Abascal J. L. F.
Published: July 2002

Properties depending on the radial ionic concentration profiles are calculated by Monte Carlo simulation for several simple B-DNA models in the presence of added (monovalent) salt up to 2.5 M concentration. The models include both homogeneously and discretely charged polyions. Besides, the effect of hard and soft repulsive forces is considered. A novel model which represents the DNA grooved structure in a simplified manner is introduced. From a methodological point of view, special attention is paid to the treatment of long-range forces along the axial direction. Exact formulas for discretely charged polyelectrolytes are used. Regarding the density profile results, it is concluded that the main effect is not due to the discreteness of the positions of the charges, i.e., homogeneously charged models lead to properties not significantly different from discretely charged ones. A similar statement holds for the comparison between hard and soft models. Nevertheless, the inclusion of the grooved shape of DNA modifies this behavior. A double hump in the concentration profile function is brought about by the coupling between repulsive and coulombic forces in the grooved model. It is shown that not only this but also other properties of full atomic models of DNA are adequately predicted by our simplified grooved model. Finally, at high concentrations of added salt, it is seen that the condensed ionic cloud overneutralizes the polyelectrolyte charge. This charge reversal phenomenon, which is observed in all the models studied, has not been previously observed due to the high salt concentration required.
Generalized relativistic effective core potential. I. Numerical calculations for atoms Hg through Bi
Authors: Tupitsyn I. I. , Mosyagin N. S. , Titov A. V.
Published: July 2002

A generalization of the shape-consistent Relativistic Effective Core Potential (RECP) method is developed for the case of pseudospinors that may have nodes. Both valence and outermost core pseudospinors are included in the Generalized RECP generation procedure. The numerical potentials have been constructed and the calculations of transition energies and some other properties have been made for the atoms Hg through Bi in the jj-coupling scheme in order to study the treatment of relativistic effects in the scope of the two-component Generalized RECP approach. As compared with the other shape-consistent RECP variants, significant improvement is reached when 5s, 5p, and 5d shells are included in the pseudovalence space.
Dynamics of H2 dissociation on Cu(100): Effects of surface defects
Authors: Xie Jianjun , Jiang Ping , Zhang Kaiming
Published: July 2002

The dissociative adsorption of H2 on both perfect and imperfect Cu(100) surfaces is studied by using a mixed quantum-classical method. The six-dimensional potential energy surface (PES) is obtained via a simplified embedded-atom method (EAM). The effects of the surface vacancy and surface impurity (here a substitutional Ni atom is considered) on the dissociation of H2 are taken into account. The variation of activation barriers for H2 dissociation near a surface defect with different pathways and different molecular orientations is investigated. Dissociation probability Pd is computed as a function of incident translational energy Ei for different vibrational and rotational states. It is found that the presence of both vacancy and impurity atom Ni enhances the dissociation of hydrogen molecule on Cu(100) surface. Meanwhile, the general variation trend of Pd with the rotational quantum number J is unchanged, i.e., Pd first decreases and then increases as J is increased.
Glass transition and atomic structures in supercooled Ga0.15Zn0.15Mg0.7 metallic liquids: A constant pressure molecular dynamics study
Authors: Jin Zhao-Hui , Lu Ke , Gong Yan-Dong , Hu Zhuang-Qi
Published: July 2002

Glass transition in supercooled Ga0.15Zn0.15Mg0.7 metallic liquid has been simulated by making use of a constant-pressure molecular dynamics technique via the pairwise interatomic potentials calculated from a self-consistent nonlocal model pseudopotential theory. The structures of liquids and glasses are analyzed through careful examinations of the pair distributions functions, structure factors, and the local ordering units, in comparing with the Zn0.3Mg0.7 and Ga0.3Mg0.7 binary cases. It demonstrates that binary Ga0.3Mg0.7 liquids and glasses show relatively stronger compound formation ability than Zn0.3Mg0.7. Although the partial substitution of Zn by Ga in Zn0.3Mg0.7 alloy leads no significant change in the glass transition temperature, it can produce considerable changes in both chemical and topological short-range orders. Chemically, there is a strong phase separation tendency between Ga and Zn atoms, the Zn–Mg heterocoordination preference can be suppressed to some extent by a stronger Ga–Mg compound formation tendency. Topologically, Ga0.15Zn0.15Mg0.7 alloy first appears to be similar to Ga0.3Mg0.7 at the high-temperature region during cooling, then it behaves more like that of Zn0.3Mg0.7 alloy at the low-temperature region. The addition of Ga also induces a restraint to the five-fold symmetry accompanied by an enhancement of the short-range order characterized by 1422- or 1311-type atomic bonded pairs. These results may provide qualitative explanations to some experimental observations on crystallization products and measured transport properties of the GaxZn0.15−xMg0.7 glasses. This study also provides further understandings of glass transition mechanisms and structural properties for the much more complicated multicomponents glass-forming systems that go beyond both the monatomic and the binary cases.
Stress overshoot in a model particle gel
Authors: Whittle Martin , Dickinson Eric
Published: July 2002

Brownian dynamics simulation is used to investigate the large strain rheology of a model three-dimensional particle gel. The gel is formed from soft spherical particles incorporating flexible surface-to-surface bonds which restrict the angular reorganization of fractal aggregates resulting in a stable, percolating, network structure. Extending earlier work, bond breakage is introduced to the model allowing the study of structural disruption in response to large deformations. The accompanying interparticle stress response reveals a distinctive stress overshoot characteristic of viscoelastic materials. Hookean and non-Hookean bonding interactions have been studied, and for both cases the stress maximum scales as a power law close to the square root of the strain-rate, while the strain at which the stress maximum occurs is relatively invariant. The limiting aggregate size at high strain-rates is dependent on the bond force as well as the energetic barrier to breakage.
Equation of state of alkanes II
Authors: Toxvaerd S.
Published: July 2002

The pressure in condensed fluids of pentane and decane is calculated by molecular dynamics for temperatures from room temperature to 673 K and pressures up to 350 MPa using the anisotropic united atom (AUA) model [Toxvaerd, J. Chem. Phys. 93, 4290 (1990)], and compared with other recently published united atom models (UA). The pressure for the AUA model agrees well with experimentally obtained pressures whereas the UA model fails outside the region of moderate pressure and density. The impact of the torsion potential and the intermolecular potential on structure, thermodynamics, and self-diffusion is investigated for fluids of decanes at high and moderate densities. A time reversible and numerical stable implementation of Gauss’ principle of least constraint (of bond lengths) is described in the Appendix. The constrained molecular dynamics is performed without any adjustment or rescaling of the bond lengths.
Hydrogen chemisorption and thermal desorption on the diamond C(111) surface
Authors: Su C. , Song K.-J. , Wang Y. L. , Lu H.-L. , Chuang T. J. , Lin J.-C.
Published: July 2002

Temperature programmed desorption (TPD) and low energy electron diffraction (LEED) were utilized to study the interaction of atomic hydrogen with single crystal diamond C(111) surface. From isotherm and isostere analysis of TPD spectra acquired at various sample heating rates ranging from 0.6 K/s to 30 K/s, the kinetic parameters were extracted. It is found that molecular hydrogen desorption from the C(111) surface exhibits the first-order kinetics. This result is confirmed by no apparent shift in peak temperatures of TPD spectra for hydrogen coverage above 0.2 ML. At lower coverage regime, the isothermal desorption experiment also indicates the first-order desorption kinetics. A nearly coverage-independent activation energy of (3.7±0.1) eV and a prefactor of (9.5±4.0)×1013 s−1 are obtained except at relatively low coverages (below ∼0.2 ML). In addition, the half-order LEED spots intensity decreases linearly with increase of the hydrogen coverage and drops to zero at ∼0.5 ML. These results are interpreted with a model that during adsorption hydrogen atoms segregate to form metastable, highly hydrogenated domains from where hydrogen atoms recombine and desorb concertedly as the substrate surface is heated up. The comparison of this work with the hydrogen adsorption and desorption on silicon surfaces is also discussed.
Understanding all of water’s anomalies with a nonlocal potential
Authors: Cho Chul Hee , Singh Surjit , Robinson G. Wilse
Published: July 2002

Though water in its liquid state is of central importance to a vast number of problems in the sciences, in engineering, and in health and environmentally related areas, it remains a mysterious and misunderstood material. It seems to violate the very basis of our 20th century scientific training—starting from first principles. For this reason, one of the most important of all scientific problems remains in a chaotic state of understanding, stifling progress in many other fields. Many scientists feel that this old problem must surely have been solved, or for other reasons is simply not worth worrying about, and continue to visualize water in incorrect ways. However, it is noteworthy that not until our own recent work was any one of the roughly ten anomalous properties of this liquid understood at a molecular level. In fact, it is rather easy to show that every intermolecular potential model currently used for water has the same basic flaw—too great a dominance by Coulomb terms. This flaw prevents such models from providing a realistic picture of water’s anomalous properties. Since water is more and more becoming considered not simply a solvent but an integral part of chemical and biological systems, it is now urgent to determine its true character. One has first to create a “working model” that is consistent with water’s anomalies. Next, because the “first principle” understanding of water extends only from the monomer to nanoscale clusters, a new intermolecular potential, whose consequences extend beyond nanoscale, is proposed. The success of this new potential in helping to explain all the anomalies supports the view that our new “working model” can provide a valid molecular-level description of this material.
Rapidly convergent iteration methods for quantum optimal control of population
Authors: Zhu Wusheng , Botina Jair , Rabitz Herschel
Published: July 2002

A family of new iteration methods is presented for designing quantum optimal controls to manipulate the transition probability. Theoretical analysis shows that these new methods exhibit quadratic and monotonic convergence. Numerical calculations verify that for these new methods, within very few steps, the optimized objective functional comes close to its convergent limit.
Density functional analysis of 13C and 1H chemical shifts and bonding in mercurimethanes and organomercury hydrides: The role of scalar relativistic, spin-orbit, and substituent effects
Authors: Kaupp Martin , Malkina Olga L.
Published: July 2002

Relativistic and substituent effects on 13C NMR chemical shifts in mercurimethanes and on 1H shifts in organomercury hydrides have been studied by density functional calculations, comparing quasirelativistic and nonrelativistic effective-core potentials for mercury. The positive shift increments in the 13C shifts as a function of HgCl or HgCN substituents in the mercurimethanes CHn(HgX)4−n (X = Cl, CN; n=0–4) are due to scalar relativistic effects. The relativistic effects for a given structure and the influence of the relativistic Hg–C bond contraction partly oppose each other, in contrast to results obtained recently for 17O shifts in oxo complexes. These differences are due to different types of metal orbitals involved in bonding, mainly of 6s-character for the mercury compounds but predominantly of 5d-character for the oxo complexes. Remaining discrepancies between computed and experimental 13C shifts of CH3HgX for more electropositive substituents X=CH3, SiH3 and particularly between computed and experimental 1H shifts in organomercury hydrides RHgH (R=CH3, C2H5, C2H3, C6H5, C6F5), appear to be largely due to spin-orbit coupling, as indicated by preliminary calculations of spin-orbit corrections to the chemical shifts. The spin-orbit contributions are almost entirely due to a σu(X–Hg–Y) → π⋆(Hg 6px,y)-type coupling and affect exclusively the shift tensor components perpendicular to the X–Hg–Y axis. The magnitude of the spin-orbit corrections correlates well with the inverse of the energy differences between the corresponding Kohn–Sham MOs. Thus spin-orbit coupling probably accounts in part for the increase of the 13C shifts in CH3HgX with decreasing electronegativity of X, and for similar trends of the 1H shifts in organomercury hydrides. In addition to the chemical shift results, analyses of the molecular and electronic structures of the mercurimethanes reveal interesting counter examples to Bent’s rule.
The adiabatic rotation approximation for rovibrational energies of many-mode systems: Description and tests of the method
Authors: Carter Stuart , Bowman Joel M.
Published: July 2002

We extend the vibrational self-consistent field method (VSCF), and two types of state mixing [denoted VSCF-CI and V-CI (configuration interaction)], to include an approximate, adiabatic treatment of overall rotation. In this approach, the asymmetric-top rotational Hamiltonian is diagonalized in an “instantaneous” principal axis system, and the resulting coordinate-dependent rotational energy is added to the exact Hamiltonian of the nonrotating system to form an effective Hamiltonian for the rotation/vibrational energies. The energy eigenvalues of that Hamiltonian are then obtained by the VSCF approach and/or variational, state-mixing methods. In this present formulation for many-mode systems, we use the general Watson Hamiltonian, and also a hierarchical representation of the many-mode potential described previously [S. Carter, S. Culik, and J. M. Bowman, J. Chem. Phys. 107, 10458 (1997)]. This approach, at the VSCF, and VSCF-CI and V-CI levels is tested against recent exact calculations of vibrational/rotation energies of HO2 and H2O. HO2 is an approximate prolate symmetric top, which is a favorable case for the approximate treatment of rotation, whereas H2O is a highly asymmetric top with large rotation constants, and represents an unfavorable case for the method.
Fermi resonance distortion of the Ru–CO stretching mode of CO adsorbed on Ru(001)
Authors: Jakob P.
Published: July 2002

A Fermi resonance of the Ru–CO stretching mode and the combination band of the hindered rotation+hindered translation mode of adsorbed CO on Ru(001) has been detected using infrared absorption spectroscopy. The Fermi resonance has been unambiguously identified through isotopic substitution experiments at various CO coverages. Clear evidence for a delocalized nature of the Fermi resonance is found.
Quantum mechanical and quasiclassical simulations of molecular beam experiments for the F+H2→HF+H reaction on two ab initio potential energy surfaces
Authors: Castillo J. F. , Hartke B. , Werner H.-J. , Aoiz F. J. , Bañares L. , Martı́nez-Haya B.
Published: July 2002

Laboratory (LAB) angular distributions (AD) measured in molecular beam experiments by Lee and co-workers in 1985 and very recently by Keil and co-workers for the prototypic F+H2 reaction have been simulated using new quantum mechanical (QM) and quasiclassical trajectory (QCT) state-resolved differential cross sections (DCS) calculated on the ab initio potential energy surfaces (PES) by Stark and Werner (SW) and by Hartke, Stark and Werner (HSW); the latter PES includes spin-orbit coupling corrections added to the entrance channel of the former. The simulations of the 1985 LAB ADs performed using the new QM calculations on the SW PES show a very good agreement with the experimental results for all final vibrational states of the HF product. The inclusion of spin-orbit coupling corrections in the ab initio HSW PES does not seem to improve the agreement between theoretical and experimental results. As for the simulation of the recent experiments of Keil and co-workers, the LAB ADs are very well reproduced by the QM and QCT results on both the SW and HSW PESs with the exception of the negative signal measured at LAB scattering angles of about −8°, arising from HF scattering into the forward hemisphere for the v′=1, j′=5,6,7 states. This peak cannot be accounted for by either of the QM and QCT calculations on any of the two PESs.
Microscopic hydration of the fluoride anion
Authors: Cabarcos Orlando M. , Weinheimer Corey J. , Lisy James M. , Xantheas Sotiris S.
Published: July 2002

A combined experimental and theoretical investigation of the step-wise hydration of the fluoride ion has been performed in order to characterize the details of its solvation at the microscopic level. The comparable anion–water and water–water interactions pose a challenging experimental/theoretical problem due to competing intermolecular forces in these small ionic clusters. Vibrational spectra of size-selected F−(H2O)3−5 in the O–H stretching region, coupled with high level ab initio calculations, have been used to analyze the spectra and assign the structures of these species. The interaction between the fluoride anion and water plays the dominant role, resulting in internally solvated clusters. The microhydration of fluoride ion is thus qualitatively different from the other halide ions.
Effect of rotation and vibration on nuclear magnetic resonance chemical shifts: Density functional theory calculations
Authors: Cromp Benoit , Carrington Tucker , Salahub Dennis R. , Malkina Olga L. , Malkin Vladimir G.
Published: July 2002

The effect of rotation and vibration on the nuclear magnetic resonance (NMR) shielding constants was computed for HF, F2, N2, CO, and HBr. The shielding constants for H, C, N, O, and F nuclei were calculated using sum-over-states density functional perturbation theory (SOS-DFPT). Diatomic ro-vibrational states were calculated from a discrete variable representation using Morse potentials and potential curves calculated with density functional theory. Our ro-vibrational corrections to shielding constants for HF, CO, F2, and N2 molecules are in good agreement with experimental data and CCSD(T) calculations. These results together with satisfactory first and second derivatives of the shielding constants with respect to interatomic distances confirm that the shielding surfaces produced by the SOS-DFPT method are of good accuracy, providing reassurance of the use of these methods for more complex systems. The unusual temperature dependence of the hydrogen chemical shift in HBr and a first attempt to include both relativistic spin-orbit and ro-vibration effects are discussed.
Structural study of supercritical water. II. Computer simulations
Authors: Matubayasi Nobuyuki , Wakai Chihiro , Nakahara Masaru
Published: July 2002

The proton chemical shift of supercritical water is analyzed by computer simulations with emphasis on its relationship to the number of hydrogen bonds per water molecule and the dipole moment of a water molecule. The chemical shift is shown to be proportional to the number of hydrogen bonds, and the dipole moment of a water molecule at supercritical states is estimated within the simple point charge (SPC)-like and TIP4P-like frameworks of the water intermolecular potential model. The dipole moment can then be used to construct an effective potential model suitable for simulating supercritical water. The radial and orientational correlations in supercritical water are examined using the effective potential model.
Fourier-transform infrared observation of SiCn chains. I. The ν4(σ) mode of linear SiC9 in Ar at 10 K
Authors: Ding X. D. , Wang S. L. , Rittby C. M. L. , Graham W. R. M.
Published: July 2002

The linear SiC9 cluster has been detected for the first time in Fourier-transform infrared spectra observed when the products from the laser evaporation of Si/C rods were trapped in Ar at ∼10 K. Comparison of C13 isotopic shift measurements with the results of density functional theory calculations (DFT) confirms the identification of the ν4(σ) mode of SiC9 at 1935.8 cm−1.