bluuebunny/crossref_metadata_2025 huggingface dataset:
The adsorbed states of ethylene on Si(100)c(4×2), Si(100)(2×1), and vicinal Si(100) 9°: Electron energy loss spectroscopy and low-energy electron diffraction studies
Authors: Yoshinobu J. , Tsuda H. , Onchi M. , Nishijima M.
Published: July 2002

The adsorbed states of ethylene on the Si(100)c(4×2), Si(100)(2×1), and the Si(100) 9° vicinal surfaces have been studied using high resolution electron energy loss spectroscopy (EELS) and low-energy electron diffraction (LEED). Ethylene is nondissociatively chemisorbed on the Si(100) surface in the wide temperature range between 77 and ∼600 K, and is rehybridized to have a near sp3 hybridization state. The adsorbed structure is proposed in which ethylene is di-σ bonded to two adjacent Si atoms of the dimer at the Si(100) surface. The thermal decomposition of chemisorbed ethylene and the influence of steps on the adsorbed states of ethylene are discussed.
Investigation of the phase diagram and critical fluctuations of the system polyvenylmethylether and d-polystyrene with neutron small angle scattering
Authors: Schwahn D. , Mortensen K. , Springer T. , Yee-Madeira H. , Thomas R.
Published: July 2002

The system PVME/d-PS has been investigated by neutron small angle scattering. By extrapolating the critical scattering, the spinodal was determined and used to calculate the Flory–Huggins parameter χ(φ,T) in the frame of mean field theory. Separating the Gibbs potential of mixing ΔG in a Flory–Huggins term χ(φ,T) and a configurational term for the entropy ΔS, it turns out that χ is independent of the molecular weight, and of the weight distribution, i.e., it is a local or ‘‘segmental’’ quantity. The critical scattering was carefully investigated at the critical concentration. At temperatures Tc−T>2.4 K(Tc=141.9 °C), a mean field behavior of the susceptibility S(Q=0) was clearly observed, with a critical exponent γ=1. However, in a region of Tc−2.4 K≤T<Tc, a crossover occurs to a critical exponent γ≂1.26; it resembles the Ising behavior which is well established for normal liquid mixtures. The function S(Q) also describes the characteristic segmental length σ. Both σ and the range parameter of χ, r0, are independent of molecular weight and weight distribution within experimental errors. The binodal, i.e., the line between the homogeneous and the two-phase region was identified by a change of critical scattering caused by the precipitated domains. The spinodal approaches the binodal near Tc in a characteristic way as predicted for polydisperse systems.
The coil–stretch transition of polymers in external fields
Authors: Mansfield Marc L.
Published: July 2002

We consider the exact statistical mechanical properties of a simple cubic lattice chain with one end anchored at the origin and with all other segments experiencing an external potential of one of two forms: −c‖x‖α (even Hamiltonian) or −c sgn(x)‖x‖α (odd Hamiltonian), for α an arbitrary exponent greater than zero and for c an arbitrary field strength. The problem is exactly soluble numerically for N not too large and for arbitrary α by transfer matrix techniques. In addition, the odd Hamiltonian with α=1 is especially simple to solve, yielding closed form expressions for a number of properties. Both Hamiltonians exhibit a first-order phase transition at c=0 in the limit of large N. The even Hamiltonian exhibits a coil (c<0) to stretch (c>0) transition. The odd Hamiltonian exhibits a left-stretched (c<0) to right-stretched (c>0) transition. For N sufficiently large and for α>1, the entire chain participates in the transition, becoming completely stretched for c only slightly greater than zero. When α<1, the transition is concentrated in one end of the chain. This transition is related to the coil–stretch transition of polymers in elongational flow. This model (unlike real polymers) does not exhibit hysteresis in the position of the transition, in agreement with the generally held belief that such hysteresis is due to hydrodynamic screening in the coil.
Cross sections for He scattering from surface imperfections: Vacancies and CO adsorbates on Pt(111)
Authors: Yinnon A. T. , Kosloff R. , Gerber R. B. , Poelsema Bene , Comsa George
Published: July 2002

Total cross sections for He scattering from isolated imperfections on surfaces are calculated using the Sudden approximation, and in some cases also by a numerically exact, time-dependent quantum-mechanical wave packet method. Systems studied include: CO adsorbates on Pt(111); mono-, di-, and trivacancies on Pt(111). The main results are: (1) the incidence angle and energy dependence of the cross section for He/[Pt(111)+CO] are very sensitive to the CO distance from the Pt plane. Interactions with the adsorbate image have little effect on the cross section. (2) The cross sections for clusters of vacancies are given within 10% or better, by the geometric sum of the monovacancy cross sections, the latter being treated as circles centered at each monovacancy. (3) The dependence of the cross section on the energy is sufficiently sensitive to distinguish between the ‘‘electron density hole’’ and ‘‘electron density hump’’ models for vacancies and vacancy clusters. (4) The Sudden approximation compares well with the exact quantum-mechanical results at typical experimental energies, when the incidence angle is not too far from the normal. These results indicate that experimental measurements of He scattering cross sections as a function of energy and incidence angle, combined with Sudden or wave packet scattering calculations, can provide detailed information on surface defects and their interactions with gas-phase atoms.
Structure of sulfur clusters using simulated annealing: S2 to S13
Authors: Hohl D. , Jones R. O. , Car R. , Parrinello M.
Published: July 2002

The ground state geometries of sulfur clusters S2 to S13 have been calculated using a parameter-free density functional (DF) method, combined with molecular dynamics (MD) and simulated annealing techniques. The results are in good agreement with available experimental data, and should provide reliable predictions where detailed measurements are lacking (n=3,4,5,9). The bonding trends are discussed in detail. The MD-DF approach is particularly valuable in larger clusters, where there are many local energy minima with comparable energies.
Solvent friction in polymer solutions and its relation to the high frequency limiting viscosity
Authors: Morris R. L. , Amelar S. , Lodge T. P.
Published: July 2002

The rotational relaxation of the solvent Aroclor 1248 (A1248) has been examined in solutions containing either polystyrene (PS), 0≤c≤0.269 g/cc, or polybutadiene (PB), 0≤c≤0.264 g/cc, by oscillatory electric birefringence. Measurements were performed at eight temperatures from −17.31 to 25.00 °C. In all cases the normalized solvent relaxation time τ/τ0, where τ0 was the value in neat solvent, was an exponential function of concentration. For PS/A1248, ∂ ln(τ/τ0)/∂c was independent of temperature and equal to 13±1 cc/g, whereas for PB/A1248, ∂ ln(τ/τ0)/∂c increased steadily with temperature and was negative for measurement temperatures of 2.81 °C and below. These observations were qualitatively consistent with a picture of solvent clustering or ordering, which was enhanced by the addition of PS but disrupted by PB. Although some features of the data were also consistent with changes in solution free volume, as indicated by measurements of the composition-dependent glass transition temperature, such considerations were not sufficient to reconcile all the observed behavior. The ratio τ/τ0 was interpreted as representing an effective, average solvent friction function ζ̂(c,T), which could also be used to define an effective solvent viscosity, ηe(c,T)=ηs(T)⋅ζ̂(c,T). It was shown that ∂ ln ζ̂/∂c, also equivalent to an intrinsic effective viscosity [ηe], was very close in magnitude and sign to the intrinsic high frequency limiting viscosity [η′∞] for both PS/A1248 and PB/A1248. This implies that the measured values of

η∞ reflect predominantly polymer-induced changes in solvent friction. Therefore, it may not be necessary to invoke additional sources of energy dissipation, such as chain stiffness or internal viscosity, to describe the high frequency viscoelastic or oscillatory flow birefringence properties of flexible chains. Furthermore, to the extent that η′∞ reflects changes in solvent friction for any polymer/solvent system, measured values of the intrinsic viscosity [η] require reinterpretation, particularly for lower molecular weight polymers. It was also demonstrated that ζ̂(c,T), as inferred from measurements of solvent or probe diffusion in the same systems [von Meerwall, Amelar, Smeltzly, and Lodge, Macromolecules (in press)], was quite different from that reported here, indicating inter alia that the correct method for accounting for changes in local friction in polymer solutions has yet to be established.
Unimolecular dynamics following vibrational overtone excitation of HN3 v1=5 and v1=6: HN3(X̃;v,J,K)→HN(X 3Σ−;v,J,Ω)+N2 (X 1Σ+g)
Authors: Foy B. R. , Casassa M. P. , Stephenson J. C. , King D. S.
Published: July 2002

Excitation of the NH-stretch overtone transitions of HN3 to v1=5 and 6 resulted in predissociation to HN(X) and N2(X) with lifetimes of 80+60−30 and ≤3 ns, respectively. Following excitation of either overtone, the HN fragments were formed predominantly in the symmetric F1, F3 spin–rotation states, with less than 4% population in the antisymmetric F2 levels. Fragment Doppler profiles confirmed that most of the available energy (>96%) went into translational motion.
Molecular dynamics with internal coordinate constraints
Authors: Tobias Douglas J. , Brooks Charles L.
Published: July 2002

The method of Ryckaert, Ciccotti, and Berendsen [J. Comp. Phys. 23, 327 (1977)] for integrating the Cartesian equations of motion of a system with holonomic constraints, has been extended to allow the independent constraint of arbitrary internal coordinates. To illustrate this new methodology, and to investigate the effects of dihedral angle constraints on the equilibrium and dynamical properties of macromolecules, we have carried out parallel sets of molecular dynamics simulations and normal mode analyses of a small dipeptide: one without constraints, and one with a single backbone dihedral angle constrained. We find that the averages and the fluctuations of the energies, and of the internal degrees of freedom are not significantly modified by the constraint. However, in the region between 100 and 1400 cm−1 of the normal mode spectrum, the constraint shifts the frequencies of the modes, and modifies their contributions to the spectra of the internal coordinates. Except for the lowest frequency torsional modes, in which anharmonic effects are significant, the behavior of the molecular dynamics power spectra is similar to that of the normal mode spectra. We also illustrate the use of a dihedral angle constraint, in conjunction with a recently developed thermodynamic perturbation method, to calculate the torsional potential of mean force for gas phase n-butane. These results are in good agreement with those obtained in previous studies, and they suggest that the combined internal coordinate constraint/thermodynamic perturbation method is well suited for computing free energy surfaces. As an application of this combined approach, we investigate the role of thermal bond and angle fluctuations on the relative free energies of the three lowest energy conformers of N-methylalanylacetamide. The results indicate that the contribution to the free energy from these fluctuations can be significant, and that care must be taken when models with rigid bonds and angles are used to calculate free energy surfaces.
Photofragment spectroscopy of cobalt tricarbonylnitrosyl and its trialkylphosphine derivatives
Authors: Georgiou Savas , Wight Charles A.
Published: July 2002

The photodissociation of gaseous Co(CO)3(NO) and Co(CO)2(NO)(PX3), where X=CH3, C2H5, n-C3H7, and n-C4H9 has been investigated near 225 nm by state-selective multiphoton ionization detection of nitric oxide fragments. The observed NO photofragment rotational distributions in v=0 are approximately Boltzmann, and the characteristic temperature decreases with increasing size of the phosphine ligand. The yield of photofragments appearing within the 10 ns detection time scale also decreases with increasing molecular size. An analogous effect is observed for the v=1/v=0 vibrational population ratio of the NO photoproducts. These results show that intramolecular energy transfer is fast compared with scission of the Co–NO bond. A dissociation model is proposed involving initial loss of CO followed by loss of other ligands via an RRKM-like process.
Observation of NH(a 1Δ, v=1) from the H+N3 reaction
Authors: Chen Jing , Quiñones Edwin , Dagdigian Paul J.
Published: July 2002

The observation and preliminary characterization, in a molecular beam scattering experiment, of the NH(a 1Δ) product from the reaction of hydrogen atoms with the azide radical, H(2S)+N3(X̃ 2Πg)→NH+N2(X 1Σ+g), are reported. Three electronic states of NH are energetically accessible [X 3Σ−, a 1Δ, b 1Σ+] in this reaction. The N3 radical was generated by reacting hydrazoic acid with fluorine atoms in a discharge-flow prereactor. A beam of hydrogen atoms was generated by a microwave discharge source using an extended Evenson–Broida cavity in a differentially pumped chamber. The NH(a 1Δ) product was detected in its v=1 level by laser fluorescence excitation in the c 1Π←a 1Δ (0,1) band. The nascent NH(a 1Δ,v=1) rotational distribution was parametrized by a Boltzmann form, and the distribution was fit to a ‘‘temperature’’ of 750±100 K. No Λ doublet specificity was observed in the NH(a 1Δ) product from our experiments.
Coupled cluster approach or quadratic configuration interaction?
Authors: Paldus J. , Čížek J. , Jeziorski B.
Published: July 2002

It is shown that a recently proposed quadratic configuration interaction (QCI) method, when limited to single and double substitutions (QCISD), represents a special case of the single reference coupled cluster approach. When applied to higher levels of substitutions (QCISDT) the method ceases to be size extensive. The relationship of QCISD method with existing coupled cluster approaches is shown in detail.
Structures and stabilities of sulfur clusters
Authors: Raghavachari Krishnan , Rohlfing Celeste McMichael , Binkley John Stephen
Published: July 2002

The geometric structures and relative stabilities of small sulfur clusters (S2–S12) are explored by means of ab initio quantum chemical calculations. The effects of polarization functions and electron correlation are included in these calculations. Open forms are more stable for the small clusters up to S4 whereas ring structures are favored for the larger clusters. The stereochemical aspects of cyclo-S5 and cycle-S6 exhibit features very similar to that of the valence-isoelectronic hydrocarbons cyclopentane and cyclohexane. The calculated geometries of S6–S8 and S10–S12 are all in excellent agreement with the experimentally determined structures. Vibrational frequencies have been computed and are compared with experimental data. The relative stabilities of the different sulfur rings are compared to each other.
Angle-resolved supersonic molecular beam study of the Cl2/GaAs{110} thermal etching reaction
Authors: DeLouise L. A.
Published: July 2002

Angle-resolved supersonic molecular beam scattering and time-of-flight techniques (TOF) are used to probe the dynamics of the Cl2/GaAs{110} thermal etching reaction. TOF spectra are recorded for the unreacted Cl2 and the GaCl3 reaction product as a function of incident translational energy and surface temperature at various final scattering angles. Our results show that the reaction mechanism is precursor mediated. The weakly bound molecular (Cl2)ads species is a key reaction intermediate through which subsequent reaction steps occur. The reaction probability, determined by angle integration of the unreacted Cl2 flux, increases with surface temperature (Ts) reaching a plateau and decreasing slightly between Ts∼425 K and 525 K before increasing sharply again beyond 525 K. This trend is nearly independent of incident kinetic energy. Detailed analysis of the TOF spectra reveal that the dynamical origin of this effect is due in part to a change in the reaction kinetics in which Cl2 desorption competes with product formation. The insights gained in this study are used to formulate plausible explanations for several discrepancies existing in the literature concerning the temperature dependence of the etch rate and of the high temperature product distribution.
n
f Rydberg complexes of NO in a magnetic field, probed by double resonance multiphoton ionization
Authors: Guizard S. , Shafizadeh N. , Horani M. , Gauyacq D.
Published: July 2002

n f (v=1) Rydberg states of NO have been probed by double resonance multiphoton ionization in a 1 T external magnetic field. Due to the nonpenetrating character of the f orbitals, these Rydberg states are very sensitive probes of any external perturbation. As n increases, a decoupling of the angular momentum l of the Rydberg electron from the molecular frame occurs gradually, as the magnetic interaction becomes more and more important with respect to intramolecular forces. Up to n≂15, only the linear Zeeman perturbation has been taken into account. The rotational–electronic structure of the 7f and 15f states has been interpreted theoretically by considering the linear Zeeman perturbation in addition to the Coulombic interaction and the long range interaction due to the quadrupole moment and the polarizability of the ion core. The intensities and line positions of the transitions from the intermediate A 2Σ+,v=1 level to the 7f and 15f levels have been calculated. The alignment of the N, MS, MN Zeeman sublevels of the A state by the two-photon pump excitation from the ground state as well as the polarization of both lasers have been taken into account in the calculations. A good agreement between the observed and the calculated transitions has been obtained. For the 7f levels, the electronic–rotational structure is well described in a coupled case (d) representation. For the 15f levels, the strong coupling of l to the field axis led to the first observation of the Paschen–Back effect in a molecule, within each rotational N+ series, with an accompanying drastic simplification of the spectra. This level is better described in a decoupled case (d) representation corresponding to a moderately strong field regime.
Path-integral study of excitons and biexcitons in semiconductor quantum dots
Authors: Pollock E. L. , Koch S. W.
Published: July 2002

Computational path integral methods have been used to study the excitonic properties of an effective mass quantum dot model. The results are compared to those of variational calculations. The variational method, using a free particle basis, accurately gives the ground and low lying excited state properties for small radii dots where the kinetic energy dominates over the Coulomb interaction. The path integral approach, which is not restricted to small radii dots, gives a thermal average over states. One thermal effect noted for this model is an inversion of the surface charge layer with temperature due to the competition between thermal and Coulomb energies. At high temperature, where Coulomb effects are secondary, the thermal deBroglie wavelength sets the scale for the density variation near a surface and thus the heavier holes approach the surface more closely than the electrons. As the temperature is lowered the now more important Coulomb binding causes the holes to be cushioned away from the surface by the electron cloud. This inversion temperature decreases as the dot radius increases. Both calculations predict increasing biexciton binding energy with decreasing dot size.
Phase transitions, surface structures, and adsorbate bonding in the H/Mo(100) chemisorption system
Authors: Prybyla J. A. , Estrup P. J. , Chabal Y. J.
Published: July 2002

Low energy electron diffraction (LEED) and surface infrared spectroscopy (SIRS) have been used to investigate the phase diagram of the H/Mo(100) chemisorption system, for hydrogen coverages up to saturation in the temperature interval 100 K<T<350 K. A strikingly large number of surface phases are observed as the temperature and coverage are varied. We find that the hydrogen atoms bond in bridge sites under all conditions while each phase is due to a distinct reconstructed geometry of the Mo(100) substrate. Results obtained from the combination of LEED and SIRS show that most of the phase transitions observed for this system are first order, involving regions of coexisting phases. In particular, for low coverages, it is found that formation of dilute hydrogen islands occurs; the effective H–H interactions which lead to this can be explained by means of a substrate-phonon-mediated interaction mechanism. Possible substrate surface structures for each of the observed phases are considered. Finally, the results obtained in this work are compared to those from previous studies of the closely related H/W(100) system.
Hydrodynamic surface modes on concentrated polymer solutions and gels
Authors: Harden J. L. , Pleiner H. , Pincus P. A.
Published: July 2002

We present a coupled two-fluid model for hydrodynamic surface modes on concentrated polymer solutions and polymer gels. This model is used to investigate such surface modes in the limit of strong coupling. The surface mode dispersion relation ω(k) and the structure factor S(k,ω) of thermally induced surface fluctuations are derived in this limit for materials with a general constitutive relation η(ω). We argue for the existence of several types of surface modes, including Rayleigh elastic waves, capillary waves, and overdamped modes resulting from viscous dissipation in the solvent as well as from polymer diffusion in the case of polymer solutions. The properties of surface modes are discussed separately for the cases of polymer gels and concentrated polymer solutions. Detailed predictions for the form of the normal modes ω(k), and the surface mode structure factor S(k,ω) are given and their dependence on relevant material properties are discussed in each case. Possible experimental scenarios are anticipated and discussed. Finally, we discuss some of the limitations of the model presented and future directions to be explored.
Molecular dynamics in perfluoro-n-eicosane. I. Solid phase behavior and crystal structures
Authors: Schwickert Heinz , Strobl Gert , Kimmig Martin
Published: July 2002

Perfluoro-n-eicosane (n-C20F42) passes through three different solid modifications, called M(T<146 K), I(146<T<200 K), and R(T>200 K). Structures of the phases M and R were characterized by x-ray scattering experiments. Crystals are built up by rectangular layers. In modification M layers are set up by arrays of right-handed and left-handed (15/7-) helices in alternate succession. The unit cell is monoclinic (a=9.65 Å, b=5.70 Å, c=28.3 Å, and β=97°; space group Pa). In modification R molecules are highly mobile. There is no ordering with regard to the helix handedness. Molecules resemble cylindrical objects and are packed in a hexagonal dense mode. Regular layer stacking results in crystals with rhombohedral symmetry (a=b=5.70 Å, c=85.00 Å, γ=120°; space group R3̄m).
Orientational ordering in lipid monolayers: A two-dimensional model of rigid rods grafted onto a lattice
Authors: Scheringer M. , Hilfer R. , Binder K.
Published: July 2002

A simple model for lipid monolayers on water surfaces at high spreading pressure is investigated in this work. In this model, the hydrophilic head group of the lipid molecules form a rigid regular triangular lattice, and the hydrophobic alkane chains (assumed to be in an all-trans state) are represented by rigid rods with two angular degrees of freedom (θ, φ). The rods consist of ‘‘effective monomers,’’ and between the effective monomers on neighboring rods a Lennard-Jones interaction is assumed. The model is studied by exact ground-state calculations, mean-field theory, and Monte Carlo simulations. Basic parameters are rod length a and lattice constant b. The ground-state phase diagram shows the following phases: for small b, the rods are oriented perpendicularly to the monolayer plane (no-tilt phase, 〈θ〉=0); for somewhat larger b, a sixfold degenerate uniform-tilt state occurs with all rods tilted towards one of their next-nearest neighbors. For still larger b, the rods are tilted nonuniformly and form a ‘‘striped’’ structure. These unexpected phases do not occur if we allow a rectangular distortion of the lattice. For T>0, the simplest mean-field theory predicts a gradual disordering of the uniform-tilt state via a second-order phase transition. For the transition region, the Monte Carlo results disagree with this picture. Instead they show a strong asymmetric first-order phase transition with pronounced hysteresis. The transition temperature increases with increasing rod length a, qualitatively similar to experiment.
The formation of SO5− by gas phase ion–molecule reactions
Authors: Möhler O. , Reiner T. , Arnold F.
Published: July 2002

A flow tube apparatus was used to investigate the formation of SO5− ions by gas phase ion–molecule reactions. The reactions studied in an N2 buffer gas at 2.5 hPa pressure and room temperature (298 K) included SO2 and O2 reactions with O2−, O3−, CO3−, SO2−, and SO3− as well as their hydrates. Reaction rate constants were measured and the major product channels were identified for most reactions. The free energy changes for the hydration reactions of SO3−, SO4−, and SO5− were derived from equilibrium constant measurements. The present investigations clearly show that SO5− ions are formed in the gas phase by the association of O2 to SO3− and by the switching reaction of SO3−H2O with O2. An effective binary rate constant of 2.0×10−12 cm3 s−1 was measured for the association reaction at 2.5 hPa N2 and the rate constant of the switching reaction was 5.0×10−11 cm3 s−1. Also the reaction of O3−H2O with SO2 probably yields SO5− by a switching process having a rate constant of 1.8×10−9 cm3 s−1. The heat of formation of SO5− was estimated to be less than −715 kJ/mol. The present results have implications to the negative ion chemistry of the atmosphere and are important for measurements of atmospheric SO2 concentrations by chemical ionization mass spectrometry.
Franck–Condon structure of the S→S1 and S→S2 transitions in norbornadiene
Authors: Zgierski Marek Z. , Zerbetto Francesco
Published: July 2002

We report an ab initio study of the geometry and force field of the S0, S1, and S2 electronic states of norbornadiene at the Hartree–Fock/6–31G* and configuration interaction singles/6–31+G levels of theory. The calculated Franck–Condon structures of the S0→S1 and S0→S2 transitions agree well with the structures observed in the optical absorption and energy loss spectra. It is argued that the 47 000 cm−1 absorption band which undergoes changes in the vibronic structure upon going from the gas to the solid phase owes its different activity to the freezing of the angle between the two ‘‘wings’’ of the cyclohexadiene part of the molecule. The complete neglect of differential overlap/spectroscopic parametrization method is used to locate valence states of the title molecule and to calculate vibronic coupling parameters to predict vibronic activity of the nontotally symmetric modes in the spectra. It is demonstrated that the highest frequency C=C stretching mode of the b2 symmetry is the most effective in bringing intensity into the S0→S1 transition. The positions of the five lowest valence states obtained by the complete neglect of differential overlap/spectroscopic parametrization method agree very well with the peaks of bands observed in the electron energy loss spectra.
Collisional vibrational relaxation of a triplet state: Energy-dependent energy loss from T1 pyrazine
Authors: Bevilacqua Thomas J. , Weisman R. Bruce
Published: July 2002

The loss of vibrational energy from gas phase T1 pyrazine molecules has been measured for thermal collisions with helium, argon, H2, SF6, and ground state pyrazine. Triplet pyrazine was prepared with a well defined vibrational energy of 5433 cm−1 through S1→T1 intersystem crossing following optical excitation to the 8a1 level of S1. The time-dependent vibrational energy content of the excited pyrazine molecules was then deduced using a recently developed ‘‘direct’’ method involving the kinetics of subsequent T1→S0 intersystem crossing. For each of the collision partners studied, it was possible to find the average energy lost per gas kinetic collision for donor energies ranging from ca. 2000 to 5433 cm−1. The magnitudes of these energy losses generally increased with the mass and vibrational complexity of the relaxing collision partner. For vibrational energy contents near 5000 cm−1, relaxation of the triplet pyrazine was enhanced by factors of as much as 24 relative to S0 benzene at a similar vibrational energy. In addition, with all collision partners studied the average energy lost per collision showed appparent threshold behavior near 3000 cm−1, increasing by approximately an order of magnitude as the donor’s energy increased from 2500 to 5000 cm−1. The findings of this first quantitative study of triplet relaxation suggest that collisional vibrational energy transfer from organic triplet states may proceed by mechanisms different from those that dominate ground state relaxation.
Combining the discrete variable representation with the S-matrix Kohn method for quantum reactive scattering
Authors: Groenenboom Gerrit C. , Colbert Daniel T.
Published: July 2002

In order to reduce the memory requirements of quantum reactive scattering calculations based on delocalized basis sets, we use a discrete basis in a single interaction region coordinate system, resulting in a sparse Hamiltonian matrix. The resulting set of linear equations is solved via an iterative method which exploits their sparsity. Other important features of our formalism are the use of a truncated grid and distorted waves used to shrink the interaction region, and therefore the basis size. We demonstrate the method and assess its efficiency for the reaction D+H2→DH+H, at a total energy of 0.9 eV and zero total angular momentum (J=0).
Photofragmentation of HF dimer: Quantum dynamics studies on ab initio potential energy surfaces
Authors: Zhang Dong H. , Zhang John Z. H.
Published: July 2002

This is a followup and extension of a previous study [J. Chem. Phys. 98, 5978 (1993)] that reported decay width calculations for vibrational predissociation (VP) of (HF)2 for total angular momentum J=0 using a hybrid potential energy surface. Detailed quantum dynamics calculations of decay widths in (HF)2 are given in the present study for both J=0 and J=1 and results are shown on both the Quack and Suhm surface and the surface of Bunker et al. The lower state of the tunneling doublet in ground vibrational (HF)2 for J=1, K=0 has the − exchange symmetry (tunneling symmetry) in contrast to the + symmetry that the lower state has for J=0. The calculated VP decay widths on two potential energy surfaces (PES) are noticeably different from each other, underscoring the sensitivity of decay widths to details of interaction forces in hydrogen-bonded complexes. The VP dynamics on the PES of Quack and Suhm gives a lifetime differential between ν1 and ν2 excitations that is in close agreement with experiments. The calculations on the PES of Bunker et al. do not exhibit enough sensitivity in lifetime between ν1 and ν2 excitations. In general, theoretical calculations of product rotation distributions are in reasonably good agreement with experimental results. In particular, about 90% of the excess energy in vibrational predissociation of (HF)2 goes into rotations of HF fragments, in good agreement with the recent experimental measurement of Bohac et al.
Thermal expansivity, bulk modulus, and melting curve of H2O–ice VII to 20 GPa
Authors: Fei Yingwei , Mao Ho-kwang , Hemley Russell J.
Published: July 2002

Equation of state properties of ice VII and fluid H2O at high pressures and temperatures have been studied experimentally from 6 to 20 GPa and 300–700 K. The techniques involve direct measurements of the unit-cell volume of the solid using synchrotron x-ray diffraction with an externally heated diamond–anvil cell. The pressure dependencies of the volume and bulk modulus of ice VII at room temperature are in good agreement with previous synchrotron x-ray studies. The thermal expansivity was determined as a function of pressure and the results fit to a newly proposed phenomenological relation and to a Mie–Grüneisen equation of state formalism. The onset of melting of ice VII was determined directly by x-ray diffraction at a series of pressures and found to be in accord with previous volumetric determinations. Thermodynamic calculations based on the new data are performed to evaluate the range of validity of previously proposed equations of state for fluid water derived from static and shock-wave compression experiments and from simulations.
Dynamics of the dissociative adsorption of disilane on Si(100): Energy scaling and the effect of corrugation
Authors: Engstrom J. R. , Hansen D. A. , Furjanic M. J. , Xia L. Q.
Published: July 2002

The reaction of Si2H6 with the Si(100) surface has been examined via supersonic molecular beam scattering techniques. It is found that the reaction probability is most sensitive to the incident translational energy, varying nearly linearly with increasing energy for 〈Etr〉≳1 eV. The effect of incident angle θi is described by a model that accounts explicitly for surface corrugation and assumes that the reaction probability varies with 〈Etr〉 cos2 θloc, where θloc is the incident angle with respect to the local surface normal.
Photoelectron spectroscopy of lithium hydride anion
Authors: Sarkas Harry W. , Hendricks Jay H. , Arnold Susan T. , Bowen Kit H.
Published: July 2002

We present negative ion photoelectron spectra of the smallest stable molecular negative ion, the lithium hydride anion. Photoelectron spectra, recorded using 2.540 eV photons, are reported for the LiH(D) [X 1Σ+]+e−←LiH(D)−[X 2Σ+] transitions of 7LiH− and 7LiD−. Adiabatic electron affinities of 0.342±0.012 eV and 0.337±0.012 eV were determined for 7LiH and 7LiD, respectively. The experimentally determined electron affinities led to anion dissociation energy (D0) values of 2.017±0.021 eV for 7LiH− and 2.034±0.021 eV for 7LiD− relative to their Li[2S1/2]+H−(D−)[1S0] asymptotes. Franck–Condon analyses yielded the following molecular parameters for the ground state of 7LiH−: Be=6.43±0.18 cm−1, re=1.724±0.025 Å, and ωe=920±80 cm−1; and the following parameters for the ground state of 7LiD−: Be=3.62±0.06 cm−1, re=1.724±0.015 Å, and ωe=650±45 cm−1. In addition, we have observed the alkali hydride anions: 7LiH−2, 7LiD−2, Li2D−, NaD−, NaD−2, NaD−3, and NaD−4. No photodetachment signal was observed for the lithium dihydride anion, 7LiD−2, using 2.540 eV photons.
The viscosity of polymer–surfactant mixtures in water
Authors: Chari K. , Antalek B. , Lin M. Y. , Sinha S. K.
Published: July 2002

The viscosity of aqueous solutions of poly(ethyleneoxide) (PEO) in the dilute and semidilute regimes exhibits a maximum as a function of sodium dodecyl sulfate (SDS) concentration. The maximum is shown to correspond to the point at which the polymer coils are saturated with surfactant. From intrinsic viscosity and small angle neutron scattering (SANS), we obtain a value of 0.65 for the excluded volume exponent of PEO at saturation. Carbon-13 nuclear magnetic resonance (NMR) relaxation measurements indicate that the correlation time for local segmental motion for polymer segments attached to micelles is reduced from about 10−11 to 10−9 s. However, only a relatively small fraction of the total coil is in direct contact with the micelles at saturation. A model for the polymer–surfactant complex is presented based on these considerations.
The protonated water dimer: Extensive theoretical studies of H5O+2
Authors: Xie Yaoming , Remington Richard B. , Schaefer Henry F.
Published: July 2002

Ab initio quantum mechanical methods have been applied to the H5O+2 system, for which experiments are beginning to appear. These methods include basis sets up to triple-ζ plus double polarization plus f functions (TZ2Pf ) and levels of correlation up to coupled cluster including single, double, and perturbatively treated connected triple excitations [CCSD(T)]. The potential energy hypersurface is very flat and is sensitive to the level of theory. At the highest level of theory the global minimum is the structure with C2 symmetry, but the transition state with Cs symmetry is only 0.4 kcal/mol higher in energy. Some low energy excursions into C1 symmetry are carefully examined. The theoretical dissociation energy for the H3O+–H2O system is predicted to be 33.4 kcal/mol, and is in good agreement with experimental measurements. The harmonic vibrational frequencies and their infrared intensities at several levels of theory are also reported, and compared to the recent spectroscopic observations in Lee’s laboratory.
Derivation and efficient implementation of the fast multipole method
Authors: White Christopher A. , Head-Gordon Martin
Published: July 2002

The fast multipole method (FMM) of Greengard evaluates Coulomb interactions of point charges with computational requirements that increase linearly with the number of particles. In this work, the central transformations of the FMM are obtained in a very compact manner from simple algebraic manipulations of two addition theorems. The intermediate multipole and Taylor expansions are defined differently from previous work to yield simplified and more efficient transformations. Error estimates are obtained due to the effect of multipole truncation and the use of the multipole to Taylor transformation operator. Efficient implementation of the FMM for potential and forces is discussed, and calculations are presented that probe the accuracy and performance of the method.
Surface properties of diluted aqueous solutions of tert-butyl alcohol
Authors: Gliński Jacek , Chavepeyer Guy , Platten Jean-Karl
Published: July 2002

Surface tension σ of aqueous solutions of tert-butyl alcohol (2-methyl-2-propanol) was measured in the temperature range between 278 and 303 K using Wilhelmy plate method. The surface entropies and enthalpies are calculated. The results are interpreted in terms of changing structure of solution, especially formation of clathratelike hydrates.
Dynamics of a polymer in the presence of permeable membranes
Authors: Yoon Hyoungsoo , Deutsch J. M.
Published: July 2002

We study the diffusion of a linear polymer in the presence of permeable membranes without excluded volume interactions, using scaling theory and Monte Carlo simulations. We find that the average time it takes for a chain with polymerization index N to cross a single isolated membrane varies with N as N5/2, giving its permeability proportional to N2. When the membranes are stacked with uniform spacing d in the unit of the monomer size, the dynamics of a polymer is shown to have three different regimes. In the limit of small d≪N1/2, the chain diffuses through reptation and D∼N−2. When d is comparable to N1/2 the diffusion coefficients parallel and perpendicular to the membranes become different from each other. While the diffusion becomes Rouse-like, i.e. D∼N−1, in the parallel direction, the motion in the perpendicular direction is still hindered by the two-dimensional networks. The diffusion eventually becomes isotropic and Rouse-like for large d≫N.
Spectroscopy of the transition state: Elementary reactions of the hydroxyl radical studied by photoelectron spectroscopy of O−(H2O) and H3O−2
Authors: Arnold Don W. , Xu Cangshan , Neumark Daniel M.
Published: July 2002

The transition state regions of the OH+OH→O(3P)+H2O and the OH+H2O→H2O+OH reactions are studied by photoelectron spectroscopy of the O−(H2O) and H3O−2 anions and their deuterated analogs. The spectra show resolved vibrational progressions attributed to H-atom vibrational motion in the unstable neutral complexes formed by photodetachment. The positions and intensities of the peaks change markedly upon isotopic substitution. One-dimensional Franck–Condon calculations using ab initio potentials for the anion and neutral are used to interpret the peak spacings and intensities, as well as the strong isotopic effects. The results are discussed in the context of previously obtained transition state spectra for heavy+light–heavy reactions.
The role of lateral surface corrugation for the quantum dynamics of dissociative adsorption and associative desorption
Authors: Gross Axel
Published: July 2002

The influence of one-dimensional lateral surface corrugation on the dissociative adsorption and associative desorption has been investigated by quantum dynamical model calculations. The study includes potential energy surfaces where either the adsorption is hindered by high barriers as in the system H2/Cu(111), or where activated as well as nonactivated paths to adsorption exist, which should be relevant for the description of, e.g., H2/Pd(100). In the high-barrier system the dynamics for normal energies below the minimum barrier height is determined by the minimum energy path, whereas at larger energies the specific distribution in the barrier height and location has a strong influence on the adsorption and desorption dynamics. At surfaces where also nonactivated paths to adsorption are present, strong quantum effects due to the wave nature of the hydrogen beam are found although sticking at such surfaces is classically possible at all incident energies. It is shown that effects usually attributed to the existence of a precursor to adsorption, i.e., sticking probabilities decreasing with increasing normal kinetic energy and increasing with increasing incident angle, can also be achieved by strong corrugation.
Nonlinear effects in the hydrogen/deuterium catalytic exchange reaction over Pt{100}
Authors: Dixon-Warren St. J. , Pasteur A. T. , King D. A.
Published: July 2002

The exchange reaction between H2 and D2 has been studied over Pt{100} as a function of surface temperature between 150 and 1100 K using molecular beam techniques. The reactive sticking probability, sr, shows strong hysteresis over this temperature range due to adsorbate-induced restructuring of the surface from hex-R to (1×1) structures. A nonlinear (1×1) island growth power law, rate ∝(θ hexH)4.0, plays a critical role in the dynamics of the exchange process. Hydrogen adsorption below ∼250 K induces incomplete (1×1) island growth. As the temperature is ramped upwards desorption leaves a clean (1×1) surface at ∼400 K, with a high sr(=0.21), but by ∼450 K the surface is converted to a defect-containing hex surface; the defects are characterized by a higher exchange activity than on the hex surface. At 650 K the defects are annealed away, and the surface exhibits the relatively low reactivity [sr=0.06+(4×10−5) Ts] of the stable Pt{100}-hex-R phase. On cooling from 1100 to 150 K, the hex-R structure remains until ∼250 K, when (1×1) island growth causes irreversible hydrogen adsorption to saturation, with a sharp decrease in sr. A kinetic model incorporating these features gives an excellent description of the data.
Reply to ‘‘Comment on ‘The conservation of quantum zero-point energies in classical trajectory simulations’ ’’ [J. Chem. Phys. 103, 1989 (1995)]
Authors: McCormack Drew A. , Lim Kieran F.
Published: July 2002

The use of quasiclassical simulations arises from practical considerations: It is not possible to do quantum simulations for most systems. By necessity, only quantum dynamics can reproduce all quantum properties. The use of restricted energy flows by using pseudoholonomic constraints to conserve ‘‘quantum’’ zero-point energy [J. Chem. Phys. 102, 1705 (1995)] is an attempt to reproduce at least one important quantum phenomenon using a (pseudo)classical method. Future applications for the pseudoholonomic-constraint method are discussed.
Electronic structures of Pd4 and Pt4
Authors: Dai Dingguo , Balasubramanian K.
Published: July 2002

Complete active space multiconfiguration self-consistent field (CAS-MCSCF) followed by multireference configuration interaction computations which included up to 4.1 million configurations and correlated all 40 electrons of Pd4 and Pt4 were made. Relativistic effective core potentials (RECPS) were employed for both Pt and Pd atoms. We found 44 electronic states for Pd4 within the 2.2 eV region and 51 electronic states for Pt4 within 1.2 eV. Two nearly-degenerate electronic states with tetrahedral geometries were found as candidates for the ground states of Pd4 and Pt4 with 3T1 and 1A1 symmetries at the highest level of theory. The metal–metal bond lengths for Pd4 and Pt4 were found to be 2.686 and 2.602 Å for the 3T1 state and 2.696 and 2.595 Å for the 1A1 state, respectively. The atomization energies of Pd4 and Pt4 were computed as 5.63 and 11.8 eV, respectively, suggesting that Pt4 is considerably more bound compared to Pd4. Relativistic effects are attributed to the enhanced stability of Pt4. The Mulliken population analysis reveals enhanced Pt(6s) and reduced Pt(5d) populations for the electronic states of Pt4 while the electronic states of Pd4 exhibit the opposite trend.
Fragment size distribution in cluster impact: Shattering versus evaporation by a statistical approach
Authors: Raz Tamar , Even U. , Levine R. D.
Published: July 2002

The transition from intact clusters to shattered small fragments is discussed for high velocity impact of cold atomic clusters at hard surfaces. The distribution of fragment sizes is computed as one of maximal entropy subject to conservation of matter and energy. The entropy is computed using a graph-theoretical approach for counting the number of possible isomers for a cluster of given size. As the impact velocity is increased, there is an onset of fragmentation into small fragments and the heat capacity is found to be discontinuous at that point. The results are compared with molecular dynamics simulations for rare gas clusters. Throughout the emphasis is on the special conditions that prevail during the ultrafast compression stage of the high energy cluster.
Effects of various halide ions and probe molecules on inelastic Mie scattering from surface enhanced Raman scattering active surfaces: Determination of particle size distributions from band shapes simulation
Authors: Felidj Nordin , Aubard Jean , Lévi Georges
Published: July 2002

Extensive Raman experiments at very low frequencies arising from the scattering of mechanical vibrations of surface roughness features and colloidal particles (inelastic Mie scattering of localized acoustic vibrations) have been carried out at various laser excitation wavelengths. These low frequency bands have been studied either on electrochemically roughened silver electrodes or in colloidal silver sols, in the presence of polarizable molecules (pyridine, benzoic acid, acridine) or only with various salts. A simple model has been built and allows to account satisfactorily for the experimental band shapes. From the fit between experimental and calculated curves, we can approach the size distribution for resonant particles. These distributions display a shift of their maximum toward large size particles and a broadening of their widths when the laser excitation wavelength is turned from violet to red. An unexpected intensity enhancement of these acoustic modes, detected when excitation takes place in the red, cannot be explained as originating solely from electric dipolar plasmon resonance. Likewise, surface enhanced Raman scattering (SERS) spectra, molecular mechanisms via charge transfer complexes and/or dipolar magnetic scattering are invoked in an attempt of explanation.