bluuebunny/crossref_metadata_2025 huggingface dataset:
A new He–CO interaction energy surface with vibrational coordinate dependence. I. Ab initio potential and infrared spectrum
Authors: Heijmen Tino G. A. , Moszynski Robert , Wormer Paul E. S. , van der Avoird Ad
Published: July 2002

The intermolecular potential energy surface of the He–CO complex including the CO bond length dependence has been calculated using symmetry-adapted perturbation theory (SAPT). The potential has a minimum of εm=−23.734 cm−1 with Rm=6.53 bohr at a skew geometry (ϑm=48.4°) if the molecular bond length is fixed at the equilibrium value of 2.132 bohr. We have applied the potential in the calculation of bound state levels and the infrared spectrum for the He–CO3 and He–CO4 complexes. The computed ab initio transition frequencies are found to agree within 0.1 cm−1 with experiment. In paper II [J. P. Reid, H. M. Quiney, and C. J. S. M. Simpson, J. Chem. Phys. 107, 9929 (1997)], the potential surface is used to calculate vibrational relaxation cross sections and rate constants.
Potential energy surfaces of NaFH
Authors: Topaler Maria S. , Truhlar Donald G. , Chang Xiao Yan , Piecuch Piotr , Polanyi John C.
Published: July 2002

High-level ab initio calculations of the ground and several excited-state adiabatic potential surfaces of the NaFH system are reported. These calculations were performed by multireference configuration interaction on a large grid of geometries which allowed them to be used for constructing an accurate analytic representation of the NaFH potential surfaces. For the ground and first excited states, using a genetic algorithm, an analytic 2×2 matrix fit was obtained corresponding to a diabatic representation. The off-diagonal coupling was obtained by fitting the energy gap between the surfaces in the region of their avoided crossing, and the diagonal elements were then fit to reproduce the ab initio adiabatic energy at 1530 points. The full fit was used to locate the barrier and the van der Waals well on the ground-state potential surface, the exciplex on the first-excited-state potential surface, and the minimum energy path for the ground-state Na+HF→NaF+H reaction. Additional calculations on the van der Waals and saddle point regions were carried out by a variety of ab initio methods as a check on accuracy. Major topological features of the potential energy surfaces representing higher-than-first excited states were examined.
Diffusion mechanisms of short-chain alkanes on metal substrates: Unique molecular features
Authors: Raut Janhavi S. , Fichthorn Kristen A.
Published: July 2002

We have used transition-state theory to study the diffusion of short n-alkanes (n-butane–n-decane) on a model Pt(111) surface. We have determined diffusion energy barriers, preexponential factors, and characteristic mechanisms by which these molecules diffuse. Our studies reveal novel features associated with molecular mobility, including: Non-nearest-neighbor hops, local minima, conformational correlations, and directional anisotropy induced by molecular orientation. We examine factors that contribute to the relationship between molecular size and mobility. Tracer-diffusion coefficients for these molecules are best described by a heterogeneous lattice model. Diffusion coefficients calculated using this model agree well with those from experimental studies.
Vibrational constants and binding energies for the A 2Π and X 2Σ states of Sr+Kr from photodissociation spectroscopy
Authors: Prekas Dimitris , Feng Bao-Hua , Velegrakis Michalis
Published: July 2002

Two vibrational progressions corresponding to transitions from the X 2Σ1/2 electronic ground state to the excited A 2Π1/2 and A 2Π3/2 spin–orbit states for the Sr+Kr complex in the energy region between 22 000 and 23 750 cm−1 are investigated by laser photofragmentation spectroscopy. From isotope-resolved measurements the absolute numbering of the vibrational numbers is obtained and thus the vibrational constants and the binding energies of the involved electronic states. The results are compared with similar systems and explained in the framework of an electrostatic potential model.
Ab initio molecular-orbital study of the trichlorine radical, Cl3
Authors: Kaledin A. L. , Heaven M. C. , Lawrence W. G. , Cui Q. , Stevens J. E. , Morokuma K.
Published: July 2002

We report a rigorous ab initio study of the ground and low-lying excited-state potential-energy surfaces (PESs) of the Cl3 radical at CASSCF, CASPT2, and MRSDCI levels of theory. The ground state has two Cl⋯Cl2 van der Waals complexes, X̃L and X̃′B. The linear asymmetric minimum (X̃L) is 2Π, with a Cl–Cl distance r=3.90 bohr, and a Cl–M (M: the Cl2 center-of-mass) distance R=8.70 bohr. The bent asymmetric minimum (X̃′B) is of 2A′ symmetry, with r=3.90 bohr, R=6.85 bohr, and the angle between r̂ and R̂, γ=68.4°. Spin–orbit CI (configuration interaction) predicts that the global minimum is linear X̃L (2Π3/2) with a bond dissociation energy of De(Cl2(X)-Cl) of 280 cm−1. Low-lying doublet excited states have only one strongly bound structure, a linear symmetric ÃL (1 2Πg) state with a bond distance of 4.67 bohr. This state is bound by ∼4300 cm−1 with respect to the Cl2(3Πu)+Cl asymptote, and its minimum lies about 8700 cm−1 above the X̃L van der Waals minimum. Transition dipole moment calculations show that the Ã–X̃ transition is fully allowed. Two bound quartet minima were located. The most deeply bound was QD3h (1 4A1′) with a D3h equilibrium geometry (r=5.00 bohr) about 11 300 cm−1 above X̃L. The other state, QC2v (1 4A2) had a C2v equilibrium geometry (r1=4.83 bohr and θ=101.7°) and an energy of about 13 500 cm−1 relative to X̃L. Although Cl3(X̃) is shown to be unstable, the present results support the notion that Cl3 participates in Cl atom recombination processes. However, the energies and transition moments of the low-lying excited states are not consistent with electronic spectra that have been tentatively assigned to Cl3.
Hexapole-selected supersonic beams of reactive radicals: CF3, SiF3, SH, CH, and C2H
Authors: Weibel Michael A. , Hain Toby D. , Curtiss Thomas J.
Published: July 2002

A supersonic corona discharge source was used to produce molecular beams of plasma particles. Neutral, polar components of the plasma mixture were selectively focused by an electrostatic hexapole, thereby “simplifying” the chemical and rotational state composition of the beam. Careful choice of a radical precursor, combined with control of discharge and hexapole voltage allowed the production of pure beams of CF3, SiF3, and SH (purity typically better than 90%), with no noticeable signal arising from undissociated precursor, ions, or other radicals. Focused beams from a hydrocarbon plasma contained a radical mixture of predominantly CH and C2H. Radical beams were characterized by rotationally and translationally cold temperatures (typically TR<20 K and TS<20 K, respectively) and high intensities (typically 1011–1012 cm−2 s−1). Simulated focusing spectra using classical trajectory calculations showed generally good agreement with the experimental data, leading to the first experimental measurement of the permanent electric dipole moment of SiF3 (μ=1.2±0.1 D).
Resonance ionization spectroscopy of cyanogen
Authors: Velazquez Jon , Cool Terrill A.
Published: July 2002

Strong (1+2) resonance-enhanced multiphoton ionization (REMPI) spectra have been recorded for cyanogen resonantly excited by single photon absorption on vibronically allowed transitions to the Ã 1Σu− and B̃ 1Δu electronic states at wavelengths from 205 to 220 nm. A previously unreported Fermi resonance between the ν2 and 4ν5 vibrational modes of the Ã state is observed for the rotationally resolved spectra of transitions that involve excitations of the ν2 mode. New measurements of vibrational and rotational spectroscopic constants are reported for the Ã and B̃ electronic states. A detection limit of 120 ppb measured for (1+2) REMPI of cyanogen is an order of magnitude larger than typical (2+1) REMPI detection limits for other aliphatic compounds.
Integral equations and simulation studies of waterlike models
Authors: Bresme Fernando
Published: July 2002

Central force potentials designed to model water are investigated by means of integral equation theories and computer simulations. We consider an associating neutral model which yields molecules with the geometry of the water molecule and that incorporates an effective pair potential aimed at describing the hydrogen bond interaction. In addition we study a charged version of the Hamiltonian, which provides a realistic description of the properties of liquid water. These models are analyzed in the full association limit, which is obtained by imposing a number of bonds per particle compatible with the water geometry, i.e., 2 and 1 for OH and HH correlations. The structure of the neutral model presents remarkable resemblances with that obtained using realistic models of water. In addition, the atomic Ornstein–Zernike theory along with the hypernetted-chain closure provides an accurate description of the structure of this anisotropic molecular system. The consideration of the full association limit introduces important improvements in the theoretical description of the charged central force model. The performance of this approach in the prediction of thermodynamic, structural, and dielectric properties of liquid water is investigated and the results compared with simulation and experimental data.
Charge transfer and polymer phases in AC60 (A=K, Rb, Cs) fullerides
Authors: Nikolaev Alexander V. , Prassides Kosmas , Michel Karl H.
Published: July 2002

Starting from a microscopic model of interactions in A+C60− crystals, where A+ is an alkali metal ion and C60− the molecular ion, we present a theory of the phase transitions from the orientationally disordered Fm3̄m structure to polymer and dimer phases. The electronic charge transfer is accounted for by the t1u molecular orbitals of C60−. The resulting Coulomb interactions between neighboring C60− and between C60− and the sublattice of alkali metal ions lead to new orientation dependent potentials which have to be added to the intermolecular pair potential and to the crystal field. By studying the orientation dependence of the crystal field and the molecular field, we find that, in comparison with the phase transition Fm3̄m→Pa3̄ known from C60-fullerite, additional channels to a Pmnn structure with subsequent polymerization and dimerization are opened. We study the symmetry of the low temperature phases and compare with experimental results. The present work comprises an extension of the formalism of rotator functions to molecular orbitals.
Binary nucleation of sulfuric acid-water: Monte Carlo simulation
Authors: Kusaka I. , Wang Z.-G. , Seinfeld J. H.
Published: July 2002

We have developed a classical mechanical model for the H2SO4/H2O binary system. Monte Carlo simulation was performed in a mixed ensemble, in which the number of sulfuric acid molecules is fixed while that of water molecules is allowed to fluctuate. Simulation in this ensemble is computationally efficient compared to conventional canonical simulation, both in sampling very different configurations of clusters relevant in nucleation and in evaluating the free energy of cluster formation. The simulation yields molecular level information, such as the shape of the clusters and the dissociation behavior of the acid molecule in the cluster. Our results indicate that the clusters are highly nonspherical as a result of the anisotropic intermolecular interactions and that a cluster with a given number of acid molecules has several very different conformations, which are close in free energy and hence equally relevant in nucleation. The dissociation behavior of H2SO4 in a cluster differs markedly from that in bulk solution and depends sensitively on the assumed value of the free energy fhb of the dissociation reaction H2SO4+H2O→HSO4−⋅H3O+. In a small cluster, no dissociation is observed. As the cluster size becomes larger, the probability of having an HSO4−⋅H3O+ ion pair increases. However, in clusters relevant in nucleation, the resulting ion pairs remain in contact; about 240 water molecules are required to observe behavior that resembles that in bulk solution. If a larger value of fhb is assumed to reflect its uncertainty, the probability of dissociation becomes negligible. A reversible work surface obtained for a condition typical of vapor to liquid nucleation suggests that the rate-limiting step of new particle formation is a binary collision of two hydrated sulfuric acid molecules. The ion pairs formed by dissociation play a key role in stabilizing the resulting cluster. The reversible work surface is sensitive to the assumed value of fhb, thus pointing to the need for an accurate estimate of the quantity either by ab initio calculations or experiments.
Calculating free energies of Lennard-Jones clusters using the effective diffused potential
Authors: Schelstraete Sigurd , Verschelde Henri
Published: July 2002

We use a previously presented method to calculate thermodynamic properties of Lennard-Jones microclusters. We obtain approximate values for the free energy and internal energy by solving a variational equation for the probability distribution with an ansatz of a suitable form. We apply this method to small clusters from five to eight atoms. The results for the calculated quantities are in excellent agreement with earlier calculations.
Tunneling currents in long-distance electron transfer reactions. III. Many-electron formulation
Authors: Stuchebrukhov Alexei A.
Published: July 2002

Many-electron formulation of the method of interatomic tunneling currents introduced in our earlier work [J. Chem. Phys. 104, 8424 (1996); 105, 10819 (1996)] for the description of long-range electron tunneling in large molecules such as proteins or DNA is proposed. The tunneling currents can be used both for calculation of the tunneling matrix element and for the description of the spatial distribution of tunneling pathways at the atomic level of resolution. It is shown that the tunneling currents can be expressed as a matrix element of a certain (current) operator evaluated between two diabatic nonorthogonal one- or multideterminant wave functions of the initial and final states of the electrons in the system. These states can be found in the standard ground state energy minimization calculations. Explicit expressions for the currents in terms of the atomic basis functions and the transformation matrices to molecular orbitals of the donor and acceptor states are given. Thus, the proposed theory provides a method that allows ordinary electronic structure calculations to be utilized for studies of tunneling dynamics in many-electron systems. All electron–electron interactions are included in the expressions for currents at the Hartree–Fock level, so that electron polarization effects arising due to interaction of the tunneling electron and other electrons in the system are taken into account in such a description.
Vibrationally induced formation of NaH in the Na(3p)+H2 collision system: Rate equation model and comparison with experimental results
Authors: Motzkus M. , Pichler G. , Kompa K. L. , Hering P.
Published: July 2002

Nonlinear optical methods (four-wave-mixing spectroscopy) and linear absorption spectroscopy were used to analyze the reaction of sodium hydride (NaH) in the Na(3p)+H2 collision system. To explain the reaction pathway, we developed a rate equation model which reproduced the dynamic and kinetic behavior of the Na(3p)+H2 system at all parameter values chosen in the cell experiment. The concentration of the reactants was varied and different excitation lasers with short (10 ns) and long (1.5 μs) pulses were used. The model describes the chemical formation of NaH by the following two-step process: (I) Na(3p)+H2→Na(3s)+H2(v″>0), and (II) Na(3p)+H2(v″>0)→NaH+H. The rate constant of the final step, which represents a collision of a Na(3p) atom with a vibrationally excited H2 molecule (v″>0), is determined for the first time as k=(1.1±0.3)×10−9 cm3 s−1.
Vibronic structure in the multiple state fluorescence spectrum of C70: A theoretical investigation
Authors: Negri Fabrizia , Orlandi Giorgio
Published: July 2002

The vibronic structure of the fluorescence spectrum of C70 is analyzed on the basis of semi-empirical quantum-chemical calculations followed by modeling of the spectra. Excitation energies of the lowest electronic states of C70 and transition dipole moments are computed with the semi-empirical complete neglect of differential overlap/spectroscopic parametrization (CNDO/S) Hamiltonian combined with configuration interaction calculations which include single and double excitations. Vibronic interactions required to model the structure of the spectra are computed at the same level of theory and the emission spectra of the lowest dipole-forbidden and dipole-allowed excited states of C70 are simulated on the basis of a perturbative expansion of vibronic wavefunctions. The comparison between simulated and observed luminescence spectra indicates that the lowest state responsible for the observed emission is a dipole-forbidden A2′ state which borrows intensity mainly from the lowest two dipole-allowed states of E1′ symmetry. The weakly allowed 1 E1′ state, lying slightly above S1, whose simulated emission shows almost negligible vibronic activity, is assigned to the second emitting state which contributes to the multiple state emission observed for C70.
Multidimensional harmonic inversion by filter-diagonalization
Authors: Mandelshtam Vladimir A. , Taylor Howard S.
Published: July 2002

We present a new method for harmonic inversion in multi-dimensions, i.e., extracting the wave vectors ωk and amplitudes dk from a signal cn=∑kdke−inωk, where n defines the multi-index. The method is an extension of the filter-diagonalization method for 1D signals. As such it enables the harmonic inversion in any small wavevector domain Dω by solving a small generalized eigenvalue problem. The computed ωk and dk can then be used to create a high resolution image F(ω) for ω∈Dω. The method greatly overperforms the conventional Fourier analysis for a model 2D signal containing as many as 10 000 damped sinusoids with moderate amount of noise.
The fast cooling/heating rate effects in devitrification of glasses. II. Crystallization kinetics
Authors: Shneidman V. A. , Uhlmann D. R.
Published: July 2002

An analytical description of devitrification kinetics induced by time-dependent nucleation and growth of crystallites during a quench-heating cycle is proposed. Relevant experimental situations include differential thermal analysis (DTA) or differential scanning calorimetry (DSC). The proposed description involves very few assumptions regarding the temperature dependence of various kinetic parameters, but rather employs measured values of growth rates. It is shown that the conventional description of nucleation based on the steady-state approximation, as a rule, is inapplicable for the description of the DTA/DSC experiment for experimentally reasonable quench/heating rates. The latter is confirmed by the analysis of available experimental data on o-terphenyl and lithium disilicate.
Electron momentum densities of atoms
Authors: Koga Toshikatsu , Matsuyama Hisashi , Inomata Hidenori , Romera E. , Dehesa Jesus S. , Thakkar Ajit J.
Published: July 2002

Spherically averaged electron momentum densities Π(p) are constructed by the numerical Hartree–Fock method for all 103 atoms from hydrogen (atomic number Z=1) to lawrencium (Z=103) in their experimental ground states. We find three different types of momentum densities spread across the periodic table in a very simple manner for the 98 atoms other than He, N, Mn, Ge, and Pd. Atoms in groups 1–6, 13, and 14, and all lanthanides and actinides have a unimodal momentum density with a maximum at p=0, atoms in groups 15–18 have a unimodal momentum density with a local minimum at p=0 and a maximum at p>0, and atoms in groups 7–12 have a bimodal momentum density with a primary maximum at p=0 and a small secondary maximum at p>0. Our results confirm the existence of nonmonotonic momentum densities reported in the literature, but also reveal some errors in the previous classification of atomic momentum densities. The physical origin for the appearance of the three different modalities in Π(p) is clarified by analysis of subshell contributions to momentum densities.
Structure of octadecanol monolayers: An x-ray diffraction study
Authors: Brezesinski G. , Kaganer V. M. , Möhwald H. , Howes P. B.
Published: July 2002

Octadecanol monolayers at the air/water interface are studied by grazing incidence x-ray diffraction. At low lateral pressures one observes the diffraction patterns inherent to the next-nearest neighbor tilt of the molecules. The unit cells are found to be distorted at both low (6 °C) and high (33 °C) temperatures due to tilt of the molecules and ordering of their backbones. Unexpectedly, at an intermediate temperature of 22 °C, the unit cell remains hexagonal in the water plane, despite the tilt of the molecules. This behavior is treated in terms of hydrogen bonding with the water molecules and does not imply the existence of a separate phase. Our diffraction study also does not show any indication of a phase transition S*–L2* corresponding to the singularities in the isotherms within the tilted phase. The tilting phase transitions from untilted phases S and LSII seem to be continuous, since the transition pressures obtained by extrapolation of the tilt angle to zero tilt coincide with the ones determined by Brewster angle microscopy.
A molecular theory for the rank dependence of orientational relaxation in Brownian dipolar lattice
Authors: Taraphder Srabani
Published: July 2002

The rank dependence of orientational relaxation is investigated in model Brownian dipolar lattice using the projection operator formalism. The contribution of long range dipolar interactions to the rate of single particle orientational relaxation is evaluated perturbatively in terms of the polarity parameter of the system. Analytical expressions, exact up to second order, for the general rank dependent, single particle orientational correlation function are presented. It is found that the frequency term in the equation of motion of the single particle orientational correlation function does not depend on dipolar interactions when evaluated up to the second order term. The memory function, on the other hand, derives significant contributions from the dipolar interactions. Relaxation of orientation of rank ℓ is found to become nonexponential due to coupling with those of ranks ℓ−1 and ℓ+1, and this becomes more pronounced in the limit of strong interaction. The theoretical predictions are corroborated well by the known computer simulation results.
Evolution of the third-order molecular polarizability in polyenes: A local view from atomic charge derivatives
Authors: Geskin V. M. , Brédas J. L.
Published: July 2002

The origin and chain length evolution of the static third-order polarizability, γ, are discussed in polyenes up to C80H82 via a local analysis of atomic π-orbital electric field-induced polarizations, i.e., of γ charges and their moments. This conceptually and computationally simple approach allows one to obtain a visual representation of the local contributions to γ. The patterns of these local contributions are observed to change qualitatively as a function of chain length; the end parts of the molecule dominate the third-order response at moderate chain lengths, while the contributions from the inner section are responsible for the slow chain-length saturation.
Adsorption and coadsorption of molecular hydrogen isotopes in zeolites. II. Infrared analyses of H2, HD, and D2 in NaA
Authors: Stéphanie-Victoire Françoise , de Lara Evelyne Cohen
Published: July 2002

The infrared analysis of the induced bands of molecular hydrogen isotopes, adsorbed in NaA zeolite, shows several features; (i) when the amount of adsorbed molecules increases, the bands become complex, showing three components, which can be related to different energetic situation of the molecules in the cavity; (ii) the band frequency is lower than the gas frequency; for each species the shifts are proportional to the inverse square root of the mass (same interaction with the crystal); (iii) the absolute intensity increases when the temperature decreases, this is due to the fact that the molecule remains longer in the vicinity of the adsorption site where the electric field is the highest. Its amplitude is deduced from the absolute intensities measured at the lowest temperature (∼1010 V/m, in agreement with other results).
Phase equilibria in binary polymer blends: Integral equation approach
Authors: Gromov Dmitry G. , de Pablo Juan J.
Published: July 2002

Phase equilibria in polymer blends are studied using the polymer reference interaction model (PRISM) theory and expanded Gibbs ensemble (EGE) simulations. The fluctuation theory of Kirkwood and Buff is generalized to polyatomic fluids and used in conjunction with self-consistent PRISM; the formalism discussed here permits direct calculation of the volume and Gibbs free energy of mixing of polymer blends at constant temperature and pressure. Theoretical predictions for the volume of mixing are in good agreement with constant pressure simulations of binary polymer blends. Miscibility diagrams for polymer blends predicted by the theory are consistent with the results of EGE simulations.
Improved power series expansion for the time evolution operator: Application to two-dimensional systems
Authors: Drozdov Alexander N. , Hayashi Shigeo
Published: July 2002

The power series expansion formalism is used to construct analytical approximations for the propagator of the partial differential equation of a generic type. The present approach is limited to systems with polynomial coefficients. Three typical two-dimensional examples, a Hénon–Heiles anharmonic resonating system, a system–bath Hamiltonian, and a Fokker–Planck chaotic model are considered. All results are in excellent agreement with those of an established numerical scheme in the field. It is found that the power series expansion method accurately describes the dynamics of very anharmonic processes in the whole time domain.
Quantum dynamics of overtone relaxation in 30-mode benzene: A time-dependent local mode analysis for CH(ν=2)
Authors: Minehardt Todd J. , Adcock J. David , Wyatt Robert E.
Published: July 2002

We report the results of a detailed quantum dynamical investigation of energy flow in 30-mode benzene. The CH(ν=2) overtone was examined by propagating an initial bright state in an active space of dimension N=16000 for 9.6 ps. Two sets of initial conditions were investigated: one in which the overtone energy was distributed as in one of the E1u symmetries appropriate for dipole excitation from the ground state and the other in which the overtone energy was localized in a single CH stretching motion. The rate and extent of energy flow out of the CH chromophore is dictated by the choice of initial conditions. Conclusions from the model where energy is localized in a single CH stretch indicate that the meta in-plane CH wags and para out-of-plane CH bend are the major recipients of bursts of energy for t⩽300 fs; the ipso CH stretch retains only 10% of the overtone energy by 9.6 ps. For the E1u model, the initially excited CH stretches retain 30% of the overtone energy in long time; the ipso out-of-plane CH bend is activated before the ipso in-plane CH wag and with less energy, mimicing the results of previous classical investigations. For both models examined, energy is equipartitioned within the CC stretches, in-plane CH wags, and out-of-plane CH bends by 9.6 ps; the magnitude of energy taken up by the CC stretches and in-plane CH wags is the same. CCC in-plane bends and CCCC out-of-plane ring torsions are of relatively minor importance (on the time scale considered) in intramolecular vibrational energy redistribution in benzene.
Atomic nitrogen on steps: A fast x-ray photoelectron spectroscopy study of the NO uptake on Rh(533), Rh(311), and Rh(111)
Authors: Esch F. , Baraldi A. , Comelli C. , Lizzit S. , Kiskinova M. , Cobden P. D. , Nieuwenhuys B. E.
Published: July 2002

The interaction of NO with a flat and two stepped Rh(111) surfaces was studied at different temperatures by monitoring in situ the evolution of the O 1s and N 1s spectra during NO exposure using fast high resolution x-ray photoelectron spectroscopy. The O 1s and N 1s intensities and binding energies were used to fingerprint the types of adsorbed species and to monitor the changes in their coverage and adsorption sites. From the O 1s intensity during uptake on Rh(533) it was determined that the NO initial sticking coefficient is temperature independent between 330 and 490 K. The N 1s spectra revealed the consecutive appearance of two atomic nitrogen species. By comparing the N 1s spectra on Rh(533) with those measured on Rh(111) and Rh(311) these species were attributed to adsorption on terrace sites, NT, and step sites, NS. NT is the only species formed in the initial stage of adsorption, whereas the NS species appears later on. This finding indicates that the NO dissociation occurs on terrace adsorption sites in the low coverage regime (total coverage ⩽0.25 monolayers). The stepped surfaces conserve a high reactivity at higher coverages because the NO dissociation continues on the steps after the terraces have been filled. The abrupt changes in the NO dissociation probability with increasing adsorbate coverage are discussed in terms of destabilization of the NT species by repulsive interactions and of subsequent compression of the adsorbed O and NO.
Substrate induced crystallization of amorphous solid water at low temperatures
Authors: Dohnálek Z. , Ciolli Ryan L. , Kimmel Greg A. , Stevenson K. P. , Smith R. Scott , Kay Bruce D.
Published: July 2002

We show that N2 monolayer desorption from ice surfaces is a quantitative, highly sensitive method for following the surface crystallization kinetics at low temperatures. Vapor deposited water films on a crystalline ice substrate exhibit amorphous growth at temperatures below ∼110 K. The rate of crystallization for these amorphous films is dramatically accelerated compared to the rate of crystallization observed for the amorphous films deposited directly on Pt(111). We find that the crystalline ice substrate acts as a two-dimensional nucleus for the growth of the crystalline phase, thereby accelerating the crystallization kinetics.
Spatio–temporal growth of nematic domains in liquid crystal polymer mixtures
Authors: Chiu Hao-Wen , Kyu Thein
Published: July 2002

Dynamics of phase separation and morphology development in mixtures of a low molar mass liquid crystal (LC) and a polymer have been investigated theoretically in comparison with experimental results. In the theoretical model, the combined free-energy densities of Flory–Huggins theory for isotropic mixing and Maier–Saupe theory for nematic ordering have been incorporated into the time-dependent Ginzburg–Landau equation (type C). The temporal evolution of the structure factor and the emergence of phase-separated liquid crystal domains have been simulated on the basis of an explicit central difference method based on a square lattice (128×128) with a periodic boundary condition. Of particular interest is the observed plateau (or inflection) region in the growth dynamic curve, which may be attributed to the breakdown of the interconnected domains caused by the nematic ordering. This unique behavior has been verified experimentally in terms of the growth of structure factor following several temperature quenches into a nematic+liquid region of the experimental phase diagram of an E7/poly(methyl methacrylate) mixture. Further, the emergence of LC domains in the metastable and unstable nematic–liquid spinodal regions has been investigated theoretically and compared with the reported experimental results.
Development of an intermolecular potential function for interactions in formamide clusters based on ab initio calculations
Authors: Cabaleiro-Lago Enrique M. , Rı́os Miguel A.
Published: July 2002

A potential function for describing interactions between formamide molecules is proposed. The function was developed from ab initio computed molecular properties and intermolecular perturbation theory (IMPT) calculations for formamide dimer involving the 6-311G** basis set. It consists of an electrostatic term that is a function of multipoles distributed over the atoms, an exponential repulsion term obtained by fitting results for the dimer, an induction contribution that is a function of atomic polarizabilities, and a dispersion term based on a London expression that is also dependent on atomic polarizabilities. The results obtained by applying the proposed function to formamide dimer are similar to those found at the MP2/6-311G** level; the function allows one to identify five structures corresponding to as many energy minima. Application of the function to larger clusters revealed that the most attractive minima correspond to planar structures, the most common structural pattern among which is that of the global minimum for the dimer. Based on cooperativeness data for the larger clusters, chained structures seemingly form stronger hydrogen bonds due to increased cooperativeness in interactions between molecules, which may account for the tendency of condensed phases of formamide to adopt open structures.
Ab initio study of structural and cohesive properties of polymers: Polyiminoborane and polyaminoborane
Authors: Abdurahman Ayjamal , Albrecht Martin , Shukla Alok , Dolg Michael
Published: July 2002

Results of Wannier orbital-based Hartree–Fock and various correlated ab initio calculations using 6–31G** basis sets are reported for the two boron–nitrogen polymer systems polyaminoborane [BNH4]∞ and polyiminoborane [BNH2]∞. At the Hartree–Fock level the calculated equilibrium geometries, cohesive energies, polymerization energies, and band structures are virtually identical with those obtained from the standard Bloch orbital-based approach. Electron correlation effects on the investigated ground state properties are discussed within Mo/ller–Plesset second-order perturbation theory and coupled-cluster singles, doubles, and triples theory. For polyaminoborane no bond alternation is found in contrast to previous studies. Correlation corrections to the band structures are considered in second-order Mo/ller–Plesset perturbation theory with third-order localization diagrams included. They lead to a decrease of the fundamental gap of polyaminoborane and polyiminoborane by 40% and 51%, respectively, and reduce the band dispersions.
Theoretical study on the spin-forbidden predissociation reaction of N2O: Ab initio potential energy surfaces and quantum dynamics calculations
Authors: Nakamura Hisao , Kato Shigeki
Published: July 2002

The spin-forbidden predissociation reaction of the ground state N2O is studied by quantum dynamics calculations. Ab initio calculations are carried out to obtain the potential energy surfaces (PES’) of the singlet ground state of N2O and three triplet ones correlating with the asymptote N2+O(3P) and the spin–orbit coupling (SOC) elements among them. The decay rate of individual singlet vibrational state to the A′3 state are estimated by applying Fermi golden rule. For the A′1 state, totally 1692 vibrational eigenstates with the even parity for the total angular momentum J=0 are obtained, and time-dependent wave packet calculations on the triplet PES are performed to obtain the autocorrelation functions whose Fourier transforms provides the decay rates. The resultant decay rates for 887 singlet vibrational states in the energy range 67.3⩽E⩽83.7 kcal/mol are analyzed in terms of a random matrix/transition state theory. Incomplete energy randomization of the vibrational energy in the singlet state even near the singlet state dissociation threshold is concluded from the analyses of calculated decay rate distributions.
A semiclassical study of collision-induced dissociation in He+H2: The effect of molecular rotation
Authors: Sakimoto Kazuhiro
Published: July 2002

Dissociation processes in three-dimensional He+H2 collisions are studied by using a semiclassical approach which treats the relative radial motion in classical mechanics and the other motions (vibration/dissociation and rotation) in quantum mechanics. Centrifugal sudden approximation is assumed to solve the semiclassical equation. The results are compared with previous full quantum mechanical calculations in which infinite order sudden approximation has been applied. The present study shows that the molecular rotation must be accurately taken into account in the dissociation process even at very high collision energies where the energy sudden assumption is usually expected to be satisfied for low molecular rotational states.
A second-order effect causing the layer structure of arsenic
Authors: Yoshizawa Kazunari , Yumura Takashi , Yamabe Tokio
Published: July 2002

The role of second-order perturbations in the interlayer interactions of gray arsenic is discussed. A hypothetical structural change from the germanium (or diamond) to the arsenic structures is considered using a two-layer model in terms of second-order Jahn–Teller distortions. We show from the concept of transition density or transition force that the second-order term originating from mixing of the pseudodegenerate bands near the Fermi level leads to the well-known layer structure of arsenic. The interesting structure of arsenic is a consequence of orbital interactions near the Fermi level.
Landau level of gaseous nitric oxide studied by two-color multiphoton ionization spectroscopy
Authors: Takazawa Ken , Abe Haruo
Published: July 2002

Two-color multiphoton ionization spectra of gaseous nitric oxide via a single Zeeman sublevel in the A 2Σ+ (v=1) state were measured in magnetic fields up to 10 T. The Zeeman effect on high-lying Rydberg states (n⩾28) and Landau levels above an ionization potential corresponding to the v=1 vibrational level of cation were observed. Landau level spacing gradually decreased with increasing energy, reflecting the transition from the quasi-Landau level to the Landau level. The broadness of the band due to the transition to the Landau level is discussed based on ionization mechanism.
Quantum molecular dynamics calculations and experimental Raman spectra confirm the proposed structure of the odd-numbered dimeric fullerene C119
Authors: Lebedkin Sergei , Rietschel Hermann , Adams Gary B. , Page John B. , Hull William E. , Hennrich Frank H. , Eisler Hans-Jürgen , Kappes Manfred M. , Krätschmer Wolfgang
Published: July 2002

A first-principles quantum molecular dynamics (QMD) method and a bond polarizability model, whose parameters were optimized on the basis of C60 data, have been used to calculate theoretical Raman spectra for four possible low-energy isomers of the odd-numbered dimeric fullerene C119 produced by thermolysis of C60 oxides. Comparison of the calculated and experimentally determined spectra provides strong evidence that the structure obtained by thermolysis is indeed the thermodynamically most stable isomer with C2 symmetry, as proposed earlier on the basis of semiempirical molecular modeling and C-NMR13 spectroscopy. This isomer has the structure originally predicted for C119 on the basis of QMD simulations.
Structures and coalescence behavior of size-selected silicon nanoclusters studied by surface-plasmon-polariton enhanced Raman spectroscopy
Authors: Honea E. C. , Ogura A. , Peale D. R. , Félix C. , Murray C. A. , Raghavachari K. , Sprenger W. O. , Jarrold M. F. , Brown W. L.
Published: July 2002

The structures and coalescence behavior of size-selected, matrix-isolated silicon clusters have been studied using surface-plasmon-polariton (SPP) enhanced Raman spectroscopy. The cluster ions were produced in a laser vaporization source, mass selected then deposited into a co-condensed matrix of Ar, Kr or N2 on a liquid He cooled substrate. Raman spectra from monodisperse samples of the smaller clusters studied, Si4, Si6 and Si7, show sharp, well-resolved, vibrations which are in good agreement with predictions based on ab initio calculations. From these comparisons we confirm that Si4 is a planar rhombus, and assign Si6 as a distorted octahedron and Si7 as a pentagonal bypyramid. Si5 depositions down to 5 eV did not reveal a measurable Raman spectrum under our experimental conditions. Evidence for cluster–cluster aggregation (or fragmentation) was observed under some conditions, even for a “magic number” cluster such as Si6. The spectra of the aggregated small clusters were identical to those observed for directly deposited larger cluster “bands,” such as Si25–35. The Raman spectra of the aggregated clusters bear some similarity to those of bulk amorphous silicon. Cluster-deposited thin films were prepared by sublimating the matrix material. Even under these “soft landing” conditions, changes in the Raman spectrum are observed with the thin films showing even greater similarity to amorphous silicon.
Optimal intermediates in staged free energy calculations
Authors: Lu Nandou , Kofke David A.
Published: July 2002

We examine the precision of free energy perturbation (FEP) methods of molecular simulation. We develop a quantitative model of the calculation in terms of the so-called f and g distributions that characterize the energies sampled in a FEP calculation. The model is then re-expressed in terms of the entropy difference between the systems of interest, and the variance of the g distribution. We apply the model to analyze the optimization of multistage insertion free energy calculations, and reach a quantitative criterion for minimizing the overall variance. The analysis indicates that the appropriate heuristic for optimizing the choice of intermediates is to select them to equalize the entropy differences among all stages of the overall FEP calculation. A more accurate criterion is specified, but in practice it differs little from this simpler one. We illustrate our conclusions by conducting two-stage FEP calculations using Monte Carlo simulations on Lennard-Jones (LJ) systems at several temperatures and two densities. The LJ systems differ in the presence of a single LJ particle, and the intermediate is defined to have a single hard sphere of diameter that can be used to optimize the two-stage FEP calculation. The model provides an excellent description of the precision of the FEP calculations.
Molecular-field splitting of the 2p3/2 peak in x-ray photoelectron spectra of second-row atoms: A theoretical study of phosphine and phosphorus trifluoride
Authors: Bo/rve Knut J. , Thomas T. Darrah
Published: July 2002

The theory of molecular-field-induced splitting of the 2p3/2 peak in x-ray photoelectron spectra of second-row atoms is reexamined, using PH3 and PF3 as examples. Existing theory, which includes crystal field and pseudoatomic valence effects, is found to describe correctly the electrostatic field in the 2p region, but, nevertheless, to severely underestimate the magnitude of the resulting molecular-field splitting. Numerical simulations show that core-valence correlation in the core ionized, final states is responsible for amplifying the molecular-field splitting to the extent that it is observable with the present resolution. A detailed analysis based on second-order Mo/ller–Plesset theory is presented of these correlation effects. A qualitative model is developed which predicts that the contribution from core-valence correlation scales with the asymmetry of the valence shell at the atom of interest. This represents an extension of the pseudoatomic valence theory, to take into account not only electrostatic and exchange effects, but also core-valence correlation.
Modified Göuy–Chapman theory for an ion-penetrable charged membrane
Authors: Hsu Jyh-Ping , Kuo Yung-Chih
Published: July 2002

The classic Göuy–Chapman theory is modified to describe the electrical potential distribution in an ion-penetrable charged membrane by taking the sizes of the charged species into account. We show that for a negatively charged membrane, if the density of fixed charge is low, a reverse in electrical potential may occur if cations are smaller than both anions and fixed groups. Also, only one plane of zero charge (PZC) exists. If a membrane is positively charged, there may exist zero to two PZC, depending upon the fixed charge density and the relative magnitudes of anions and fixed groups. Under a critical condition determined mainly by the sizes of the charged species, a unique PZC exists in the inner plane of fixed charge. The present model is capable of explaining why microorganisms can adjust the degree of dissociation of the ionogenic groups in their cell membranes to absorb nutritive ingredients and to shun toxic species by fluctuations in the extent to which specific ions penetrate the membranes.
Discrete dipole approximation for ultraviolet–visible extinction spectra simulation of silver and gold colloids
Authors: Félidj Nordin , Aubard Jean , Lévi Georges
Published: July 2002

In order to understand more deeply the surface enhanced Raman scattering (SERS) effect, this article develops a model, based upon the simulation of the UV–visible extinction spectra which allows the determination of the morphology of metallic particles in silver and gold colloids either unaggregated or in any aggregation state. The main assumptions of this model are (i) light scattering by independent particles or clusters (objects) which enables us to express the total extinction cross section as a suitably weighted sum of cross sections of individual objects (targets); (ii) these targets are supposed to be compact and their individual cross sections are determined either from the Mie theory for spheres or from the discrete dipole approximation (DDA) for objects of any shape and size; (iii) the weight of each individual cross section is determined using a minimization process (simplex method) which looks for the best possible agreement between the experimental and calculated spectrum; (iv) lastly a simple calculation, based on the assumption of compact objects, provides the absorbance (optical density). In the case of unaggregated silver and gold colloids, this model gives a very good agreement between experimental and simulated extinction spectra thus leading to a particle size histogram which is consistent to that determined from transmission electronic microscopy (TEM) measurements. For aggregated colloids, an excellent agreement is still obtained between experimental and simulated band profiles; a slight discrepancy is observed between experimental and calculated intensities which might result from the tendency of DDA to underestimate the individual cross sections and/or from a lack of validity of the compact approximation. The clusters histogram deduced from the simulation process reveals small spheres (unaggregated particles) and elongated objects small compared to the visible light wavelength. This latter result is different from that obtained by TEM data in which the likely occurrence of a further aggregation leads to the observation of large aggregates.
Spectroscopic characterization of the weakly bound Ca(4s4dσ 3D3)⋅Ar[3Σ+] state: Evidence for a substantial maximum in the potential curve at long range
Authors: Leung Allen W. K. , Kaup John G. , Bellert D. , McCaffrey John G. , Breckenridge W. H.
Published: July 2002

The weakly bound Ca(4s4dσ 3D3)⋅Ar[3Σ+] state has been characterized by means of R2PI (Resonant Two-Photon Ionization) spectroscopy, using transitions from the Ca(4s4pπ 3P0)⋅Ar[3Π0−] metastable state prepared in a laser-vaporization/supersonic jet source. Because several of the vibrational levels are above the dissociation limit [to Ca(4s4d 3D3)+Ar(1S0)], it is concluded that there must be a substantial maximum in the Ca(4s4dσ 3D3)⋅Ar[3Σ+] potential curve (>200 cm−1) at large R (>4.0 Å). This is discussed, and shown to be consistent with our earlier ideas of “penetration” of outerlobes of electron density of metal atom excited states by RG (rare-gas) atoms. Perturbations observed, due to possible potential curve crossings with states of different electronic symmetry, are also discussed.