bluuebunny/crossref_metadata_2025 huggingface dataset:
Coupled-channel scattering calculations of ICN(Ã–X̃) photodissociation using ab initio potentials
Authors: Bowman Joel M. , Mayrhofer Rudolph C. , Amatatsu Yoshiaki
Published: July 2002

We report coupled-channel scattering calculations of the ICN(Ã–X̃) photodissociation using the recent ab initio potential energy surfaces, diabatic coupling, and transition moments of Morokuma and co-workers [Y. Amatatsu, S. Yabushita, and K. Morokuma, J. Chem. Phys. 100, 4894 (1994)]. We focus on transitions from the ground electronic state to the coupled 1Π1(5A′) and 3Π0+(4A′) states, the 4A″ component of the 1Π1 state, and to the 3A′ component of the 3Π1 state, for ICN in the ground and first excited bend vibrational states. Total photodissociation cross sections to form I(2P3/2) and I*(2P1/2) are calculated over a large range of energies, and compared to experiment. Rotational distributions, i.e., partial cross sections, for the associated CN fragments are also calculated for numerous energies within the absorption profile, and compared with experiment, and the quasiclassical trajectory calculations of Amatatsu et al.
Electron and energy transfer processes of photoexcited oligothiophenes onto tetracyanoethylene and C60
Authors: Janssen R. A. J. , Moses D. , Sariciftci N. S.
Published: July 2002

We present near-steady-state photoinduced absorption (PIA) studies on photoexcited states of a series of oligothiophenes (nT, n=6,7,9,11) in solution. Photoexcitation in solution reveals efficient formation of a metastable triplet state (3nT). In addition, oligothiophene radical cations (nT+.) are formed from the singlet excited state via electron transfer to the medium in solvents of high electron affinity. The reactivity of the 3nT state towards acceptor molecules is investigated. In the presence of C60 an energy transfer reaction is observed which quenches the 3nT state and produces 3C60 via photosensitization. Addition of tetracyanoethylene (TCNE) results in quenching of the 3nT state via electron transfer and efficiently produces the corresponding nT+. radical cations, as evidenced from PIA and light-induced electron spin resonance (ESR).
Vibrationally resolved cross sections for the photoionization of CS2
Authors: Stratmann R. E. , Lucchese Robert R.
Published: July 2002

We have performed vibrationally resolved calculations of the excitation of the symmetric stretch in the photoionization of CS2 leading to the X 2Πg, A 2Πu, B 2Σ+u, and C 2Σ+g states of CS+2. Previous theoretical work has determined that the kπg shape resonance in the (5σu)−1 channel consists mainly of a linear combination of low lying virtual d orbitals on sulfur and is thus essentially atomic in nature. This conclusion was primarily based on the shape of the resonant wave function and the insensitivity of the energy of the resonance to bond stretching. Here, we have determined that the energies of the kπ shape resonances located well above threshold and the σ bound states just below threshold are insensitive to bond length. We have also found nearly constant vibrational branching ratios in all channels and polarization components. This is in qualitative agreement with experimental vibrationally resolved cross sections [S. Kakar, H. C. Choi, and E. D. Poliakoff, J. Chem. Phys. 97, 4690 (1992)] which show nearly constant vibrational branching ratios. Our present results indicate that caution must be exercised when using bond length sensitivity as an exclusive means to determine the existence of shape resonances.
Size dependence of HF vibrational frequency shift for ArnHF (n=1–14) van der Waals clusters via quantum five-dimensional bound state calculations
Authors: Liu Suyan , Bac̆ić Zlatko , Moskowitz Jules W. , Schmidt Kevin E.
Published: July 2002

HF vibrational frequency shifts for ArnHF (n=1–14) clusters have been determined for the first time using quantum 5D bound state calculations. Our results for n=1–4 clusters are in very good agreement with the available experimental data. The size dependence of the redshift is predicted to be very nonmonotonic.
Phase transition in constrained polymer gels
Authors: Suzuki A. , Kojima S.
Published: July 2002

Equilibrium diameter and stress change of cylindrical N-isopropylacrylamide gels of submillimeter diameter were measured as a function of temperature under mechanical constraint along the uniaxial direction. As a result, the transition temperature increased up to 1 °C with increasing the strain, which indicates the possibility of the uniaxial strain-induced phase transition of gels at selected temperatures. The diameter and stress change were quite successfully described by the phenomenological model on the basis of the extended Flory-type free energy. The time course of the change in diameter and stress was also studied after the large steplike elongation was rapidly applied. A gradual increase in diameter during decrease in stress was detected clearly in the swollen phase, which is due to the relaxation of polymer segmental units to approach the favored conformation under the strong deformation.
Zero kinetic energy, pulsed-field ionization spectroscopy of hydrogen iodide
Authors: Pratt S. T.
Published: July 2002

The rotationally resolved, zero kinetic energy, pulsed-field ionization (ZEKE-PFI) spectrum of the HI+ X 2Π1/2, v+=0 level, obtained by double-resonance excitation via the HI F 1Δ2, v=0 level, is reported. The rotational and Λ-doubling constants for the HI+ X 2Π1/2, v+=0 level obtained from the experiment are close to those estimated theoretically by Mank et al. [J. Chem. Phys. 95, 1676 (1991)]. At higher pressures, the dissociative charge transfer reaction HI*+HI→HI++H+I− represents a potentially serious loss mechanism for the high Rydberg states that give rise to the ZEKE-PFI signal. This result is of more general applicability, because it provides evidence that collisions of the Rydberg electron with neighboring molecules can play a significant role in ZEKE-PFI experiments.
Signatures of large amplitude motion in a weakly bound complex: High-resolution IR spectroscopy and quantum calculations for HeCO2
Authors: Weida Miles J. , Sperhac Jeanette M. , Nesbitt David J. , Hutson Jeremy M.
Published: July 2002

The infrared spectrum of the HeCO2 van der Waals molecule is recorded in the region of the CO2 ν3 asymmetric stretch via direct absorption of a tunable Pb–salt diode laser. HeCO2 is formed in a slit jet supersonic expansion; the slit valve and the stagnation gas must be precooled to −35 °C before substantial formation of the complex is observed. Sixty-six rovibrational transitions are recorded by exciting the ν3 asymmetric stretch of the CO2 monomer within the complex. Forty-three of these transitions can be assigned using internally consistent combination differences as a b-type band of a T-shaped asymmetric rotor. There are several indications that large amplitude motion is significant in HeCO2, including the poor quality of the fit to an asymmetric rotor model and the large positive inertial defects of Δ=8.54 and 10.98 uÅ2 in the ground and excited states, respectively. However, a hindered rotor analysis based on these inertial defects demonstrates that the CO2 motion within the complex is far from the free rotor limit. No evidence of predissociation broadening is observed, indicating a lifetime for the complex of τ≳6 ns. Quantum close-coupling calculations which correctly treat both angular and radial degrees of freedom are carried out on the full 2D HeCO2 potential energy surface of Beneventi et al. [J. Chem. Phys. 89, 4671 (1988)]. Comparison of this analysis with the experimental results demonstrates that the theoretical potential is too isotropic in the region of the potential minimum.

Predicted spectra from this model potential, however, indicate that the remaining 17 much weaker HeCO2 transitions are due to a ‘‘hot band’’ excitation out of the first intermolecular bending level, lying 9±2 cm−1 above the ground state. In sharp contrast to the ground vibrational state of HeCO2, an asymmetric rotor model fails qualitatively to characterize the rotational structure for the lowest excited bend. The simple physical reason for this is confirmed by inspection of the quantum wave functions; in the ground state the He atom is localized near the C atom in a T-shaped geometry, whereas in any of the excited bending states the He atom is largely delocalized around the CO2 molecular framework.
Monte Carlo simulation of a lattice model for micelle formation
Authors: Bernardes Américo T. , Henriques Vera B. , Bisch Paulo M.
Published: July 2002

We have investigated the aggregation into micelles of nonionic amphiphiles in water through Monte Carlo simulations of a very simple model system. Amphiphiles and water are modeled, respectively, as three-site and one-site molecules on a square lattice. The model presents characteristic properties of experimental micellization, namely, monomer vs amphiphile concentration with a plateau above a ‘‘critical’’ concentration (cmc) and a distribution of micelle sizes (polydispersity). Results of the numerical experiment were compared with predictions from statistical thermodynamic theory. Sample size and relaxation properties of the simulation have also been analyzed.
High resolution electron energy loss spectroscopy investigation of molecular conformation at the surface of poly(amino acid)s films
Authors: Leclerc G. , Pireaux J. J. , Caudano R. , Hönings C. , Höcker H.
Published: July 2002

Electron-induced vibrational spectra of four poly(amino acid)s—poly-alanine, poly-leucine, poly-methionine, and poly(benzyl-aspartate)—have been recorded. After discussing the surface sensitivity of the technique and the excitation mechanisms of the molecular vibrations, it is concluded that high resolution electron energy loss spectroscopy (HREELS) spectra are dominated by vibrational bands fingerprinting the polymer sidechains. From these observations, suggestions are made on the molecular conformation at the surface of these poly(amino acid)s films.
A new formulation of the Hartree–Fock–Roothaan method for electronic structure calculations on crystals
Authors: Hammes-Schiffer Sharon , Andersen Hans C.
Published: July 2002

We present a general formulation of the Hartree–Fock–Roothaan method of electronic structure calculations for systems subject to periodic boundary conditions and apply this method to crystals. The derivation of the method does not involve any divergent or conditionally convergent infinite series. The final result for the Hartree–Fock energy per unit cell consists of only absolutely convergent series and can be written in a form whose structure is almost identical to that for the nonperiodic Hartree–Fock energy. A Fock matrix that consists of only absolutely convergent series is also defined. An important feature of the method is that the Ewald potential, which has been used in the past to eliminate divergences in series involving the expectation value of the Coulomb interaction, is introduced in a physically reasonable way at an early stage of the formulation of the quantum mechanical problem. In the final result, the Ewald potential is used not only to express the Coulomb energy, but also to express the exchange energy as an absolutely convergent series, thereby eliminating the problem of slow convergence, or lack of convergence, of the series for the exchange energy. The numerical implementation of this method, which is not discussed in this paper, requires calculation of standard one- and two-electron matrix elements of the electronic kinetic energy and the Coulomb interaction, as well as certain easily calculated moments of basis function overlap charge densities. No integrals involving matrix elements of the Ewald potential between basis functions are required for evaluation of either the energy or the Fock matrix. Instead, Ewald interactions must be evaluated only for point multipoles. The methods used here to formulate the Hartree–Fock problem can be extended to formulate Mo/ller–Plesset perturbation theory and coupled cluster theory for crystals.
Infrared spectrum and potential energy surface of He–CO
Authors: Chuaqui Claudio E. , Le Roy Robert J. , McKellar A. R. W.
Published: July 2002

For 3He–CO and 4He–CO van der Waals bimers, fully resolved infrared spectra in the 4.7 μm region near the fundamental band origin of the CO monomer have been measured for the first time. Only a small fraction of the observed lines could be assigned using conventional empirical spectroscopic techniques, and little additional insight was gained from synthetic spectra generated from a published ab initio potential for this system. However, a complete set of unique assignments was made on the basis of comparisons with synthetic spectra generated from a variety of trial potential energy surfaces, and least-squares fits to the observed transition frequencies were used to determine a new anisotropic potential energy surface for this system. This new surface is much deeper and has a much stronger well depth anisotropy than the best previous one, and its predictions of very low temperature microwave line broadening cross sections raise serious questions regarding the methodology for calculating that property.
Initial investigation of the homogeneous nucleation of cesium vapor
Authors: Cha Gyu-Suk , Uchtmann Hermann , Fisk Jeffery A. , Katz Joseph L.
Published: July 2002

Initial measurements of the homogeneous nucleation of supersaturated cesium vapors using an upward thermal diffusion cloud chamber specially designed for use with highly reactive materials are presented. Comparison of the supersaturations measured at higher temperatures (greater than 455 K) with the supersaturations predicted by Classical nucleation theory show for the first time that the nucleation behavior of simple normal metals can be described by Classical theory. This agreement implies that the description of relatively large clusters of simple metal atoms (more than 25 atoms) as small liquid droplets with bulk metallic properties is valid. However, as the temperature decreases below 455 K, an increasingly larger supersaturation than that predicted by theory is required. This deviation from theory is so sharp that by 430 K the difference is an order of magnitude, and below 420 K nucleation cannot be observed. The deviation is discussed in the context of loss of bulk metallic properties with decreasing critical nuclei size. Other possible reasons for a deviation towards larger supersaturations are also considered.
Axial asymmetry in the nuclear magnetic resonance spectra of deuterated methyl groups: An alternative explanation
Authors: Wann Mieng-Hua , Harbison Gerard S.
Published: July 2002

The deuterium nuclear magnetic resonance (NMR) powder spectra of methyl groups at ambient temperatures are invariably averaged by rotation or hopping about the C3 axis. The canonical result for a perfectly threefold symmetric methyl is an axially symmetric Pake doublet whose splitting, in the case of perfect tetrahedral angles around the group, is averaged by a factor of exactly three. However, deuterium NMR spectra of methyl groups are often somewhat axially asymmetric, indicating a significant deviation either of the methyl group, or of its environment, from threefold symmetry. Previous explanations for this phenomenon have hinged on either an additional motional mechanism along a different axis, or on a perturbation of the field gradient around the methyl group by electronegative groups. We propose a simpler explanation; it may arise in a distortion of the methyl geometry itself, so that the group no longer has perfect rotational symmetry. We analyze some experimental examples of distorted methyls, and compare the extent of distortion required by the NMR data with those observed in published neutron structures.
Homogeneous nucleation rates for n-butanol
Authors: Viisanen Y. , Strey R.
Published: July 2002

Homogeneous nucleation rates of n-butanol in argon were measured in an expansion chamber maintaining pressure, temperature, and vapor supersaturation practically constant during nucleation periods of about 1 ms. Tables of the homogeneous nucleation rates J in the range 105<J/cm−3 s−1<109 as function of supersaturation S in the temperature range 225<T/K<265 are presented along with the other experimental measurables. For the temperatures considered the equilibrium vapor pressure of n-butanol and the surface tension are known with sufficient accuracy. Accordingly, a quantitative comparison of the experimentally obtained nucleation rates with the predictions of the classical theory could be performed. The agreement is sufficient. The reported individual J−S curves were obtained at selectable constant temperatures. From the slopes of the isothermal J−S curves the number of molecules in the critical clusters were determined without reference to any specific nucleation theory. Conceiving the clusters as small droplets one finds, using macroscopic liquid density and surface tension, that the Gibbs–Thomson equation provides remarkably good predictions even for droplets with radii down to 10 Å.
On the energy dependence of the steric effect for atom–molecule reactive scattering. II. The reaction Ca(1D)+CH3F(JKM=111)→CaF(2Π)+CH3
Authors: Meijer Anthony J. H. M. , Groenenboom Gerrit C. , van der Avoird Ad
Published: July 2002

The observed increase in the steric effect of the reaction Ca(1D)+CH3F(JKM=111) →CaF(2Π)+CH3 with increasing energy has been tentatively ascribed to a reorientation of the initially oriented CH3F axis, due to anisotropic long-range forces [M. H. M. Janssen, D. H. Parker, and S. Stolte, J. Phys. Chem. 95, 8142 (1991)]. Here we present ab initio calculations and use these to construct a long-range potential. To this potential we fit an isotropic model potential and two anisotropic model potentials. On the long-range potential and on the model potentials we perform classical trajectory calculations, and we compare the two methods presented in the preceding paper [G. C. Groenenboom and A. J. H. M. Meijer, J. Chem. Phys. 101, 7592 (1994)] for the extraction of the steric effect. We conclude that already the attractive isotropic model presented in this paper can account for the observed energy dependence of the steric effect for this reaction via a ‘‘trapping’’ mechanism. We show that although reorientation is possible, it contributes little to the positive energy dependence of the steric effect.
Rovibrational structure of NO+2 and state-to-state dynamics in the high-resolution threshold photoionization of NO2
Authors: Bryant Gregg P. , Jiang Yanan , Martin Margarita , Grant Edward R.
Published: July 2002

Triple-resonant zero-kinetic-energy (ZEKE) photoelectron spectroscopy is employed to characterize the rovibrational structure of NO+2. Relative threshold positions determine vibrational frequencies and rotational constants; while anomalous, and in some cases forbidden, intensities of transitions provide information on rotational and vibrational coupling between the NO+2 core and electronic degrees of freedom associated with Rydberg orbitals of very high principal quantum number. Trends in the strengths of anomalous threshold photoionization features establish that mode selectivity, which has a pronounced effect on the vibrational autoionization dynamics of NO2, also operates to determine the envelope of rotational-line intensities in threshold photoionization. Competition between rotationally and vibrationally mediated coupling leads to a dependence of the rotational profile on the mode of vibrational motion of the core.
Optical interference in second-harmonic generation from a metal surface with molecular adlayers
Authors: Li C. M. , Ying Z. C. , Dai H. L.
Published: July 2002

A strong oscillation pattern was observed in optical second-harmonic generation (SHG) at a Ag(111) surface under pyridine adlayers with variable thickness. This oscillation appears only when s-, but not p-, polarized fundamental light is used. This effect is attributed to the modulation of the radiation efficiency of the fundamental light penetrating into the Ag bulk and of the second-harmonic light radiating out to the vacuum by pyridine overlayers through optical interference. A standard optical interference model has been developed to quantitatively analyze the oscillation patterns and determine the adlayer thickness. It was found that at 90 K the pyridine overlayer growth rate is 1.4 Å per Langmuir exposure.
One-electron density matrices and energy gradients in the random phase approximation
Authors: Ortiz J. V.
Published: July 2002

Energy gradients and effective one-electron density matrices corresponding to excitation energies calculated with the random phase approximation of the polarization propagator are derived. Combination of these results with second-order ground state energies yields final state total energies and their gradients. Geometry optimizations and evaluations of one-electron properties are performed for excited states of formaldehyde.
Comments on the numerical simulations of electrolytes in periodic boundary conditions
Authors: Caillol Jean-Michel
Published: July 2002

Relying on the article of de Leeuw, Perram, and Smith [Proc. R. Soc. London, Ser. A 373, 27 (1980)] we justify the expression of the Hamiltonian actually used in the numerical simulations of electrolyte solutions in a periodic geometry. It involves the itinerant dipole moment Mi of the charges rather than the electric moment MB of the simulation box. The vector Mi is defined as the electric moment of the particles which were in the simulation box at t=0 and may have diffused away in the neighboring cells in the course of the simulation. Some comments on Stillinger–Lovett conditions are included.
Accurate electric dipole and quadrupole moment, dipole polarizability, and first and second dipole hyperpolarizability of ozone from coupled cluster calculations
Authors: Maroulis George
Published: July 2002

Accurate values are reported for the dipole (μ) and quadrupole (Θ) moment, dipole (α) polarizability, and first (β) and second (γ) dipole hyperpolarizability of ozone. Electron correlation effects are accounted for via coupled cluster (CC) theory with double substitutions corrected for the effects of single and triple excitations. Electron correlation brings about a drastic change of the self-consistent field molecular orbital picture of the molecule. The final values of the Cartesian components of the dipole polarizability are, with z as the C2 axis, αxx=30.54, αyy=11.96, and αzz=14.36 e2a20E−1h. The final values of the mean first and second dipole hyperpolarizability are β̄=−31.7 e3a30E−2h and γ̄=325×101 e4a40E−3h, to be compared to self-consistent field values of 20.7 e3a30E−2h and 475 e4a40E−3h, respectively.
Modeling large Gaussian ring polymers
Authors: Koniaris Kleanthes
Published: July 2002

In many types of computer simulation, a ring polymer of length l in a particular solvent is represented as a polygon of N sides of length lp, where N≊l/lp, and lp is the persistence length. There are many established methods to make such polygons, including kink-jump, bead-spring, and dimerization. This paper introduces a very efficient and easy-to-program method, called vector shuffling, which is useful for generating such polygons in the important special case where the quality of the solvent is good, and the polymer is said to be under θ conditions.
Potential energy surfaces for Rh–CO, Rh–OC, Ir–CO, and Ir–OC interactions
Authors: Dai Dingguo , Balasubramanian K.
Published: July 2002

The potential energy surfaces for eight low-lying electronic states of the Rh–CO and Rh–OC complexes and of the Ir–CO and Ir–OC complexes have been studied using the complete active space multiconfiguration self-consistent field method followed by the multireference singles+doubles configuration interaction calculations which included up to 1.6 million configurations. In addition, spin–orbit effects are included through the relativistic configuration interaction method for the Ir–CO complex. It is found that the Rh(2F) and Ir(2F) states react spontaneously with CO to form stable RhCO and IrCO molecules in which the 2Δ ground states are 42.4 and 75.1 kcal/mol more stable than Rh(2F)+CO(1Σ+) and Ir(2F)+CO(1Σ+) states, respectively, in the absence of spin–orbit effects. The RhOC and IrOC complexes in the 2Δ states were found to be less stable than Rh(4F)+CO(1Σ+) and Ir(4F)+CO(1Σ+), respectively. The nature of the chemical bond is discussed in terms of the wave function composition, and the electronic population analysis.
The structure of the HCCCO radical. Rotational spectra and hyperfine structure of monosubstituted isotopomers
Authors: Cooksy A. L. , Watson J. K. G. , Gottlieb C. A. , Thaddeus P.
Published: July 2002

Rotational spectra of six isotopomers of the propynonyl radical—HCCCO, DCCCO, H13CCCO, HC13CCO, HCC13CO, and HCCC18O—have been measured and analyzed, yielding the geometry and leading hyperfine constants of its 2A′ vibronic ground state. The radical is found to have an acetylenic carbon chain with the unpaired electron strongly localized on the carbon atom Cc, with the carbon atom positions labeled according to HCaCbCcO. The geometry, assumed to be fully trans, is given by the parameters θHCC = 168°, θCCC = 163°, θCCO = 136.5°, rHCa = 1.060 Å, rCaCb = 1.219 Å, rCbCc = 1.387 Å, and rCcO = 1.192 Å. The Fermi contact hyperfine constants are ac(H) = −11.593(41) MHz, ac(13Ca) = 35.8(1.4) MHz, ac(13Cb) = 166.2(3.8) MHz, and ac(13Cc) = 347.6(3.2) MHz, where dipolar terms in the proton and 13Ca hyperfine structure have been neglected. No evidence of other HCCCO isomers is found in the spectra, although ab initio calculations identify a second minimum at a cumulenic structure with the unpaired electron localized at atom Ca. Brief investigation of the formation mechanism for HCCCO from C2H2 and CO in our apparatus indicates that roughly half the HCCCO is formed without breaking the CO bond.
Exact quantum mechanical vibrational kinetic energy operator of sequentially bonded molecules in valence internal coordinates
Authors: Császár Attila G. , Handy Nicholas C.
Published: July 2002

The full nonrelativistic quantum mechanical vibrational (J=0) kinetic energy operator for sequentially bonded N-atom molecules, expressed in valence stretch, bend, and torsion internal coordinates, is explicitly given. Certain properties of the operator and its possible applications are discussed.
Temperature dependence of persistent infrared spectral holes for SeH molecules in the GeAsSe glass series
Authors: Till R. , Sievers A. J.
Published: July 2002

Persistent infrared spectral holes have been burned at low temperature into the Se–H stretch mode absorption band of SeH molecules in the GeAsSe-alloy glass series. Eight different compositions have been examined by changing the relative contents of germanium and arsenic covering a range of mean coordination numbers from 2.0 to 2.6. This is the first time that the dephasing time has been examined as a function of the glass coordination number. For all samples a linear dependence between hole width and hole area is found as a function of burn time at constant intensity and temperature, which is then used to extrapolate the hole width to the limit of zero burn time. The temperature dependence of the hole width is found to follow a power law. Both the zero temperature width and the temperature exponent increase with increasing mean coordination number indicating that the vibrational lifetime and the dephasing time may depend on the microscopic topology of the glass rather than on the chemical composition.
Determination of ground state populations and alignment parameters using nonresonant three-photon absorption
Authors: Meyer Henning
Published: July 2002

The polarization dependence of the three-photon absorption signal is described in terms of the matrix elements of the irreducible representation of the three-photon absorption tensor operator for an anisotropic ensemble with cylindrical symmetry probed with identical photons of linear polarization. The formalism is applicable to the extraction of alignment parameters for diatomics as well as polyatomic molecules produced in collisions of unpolarized particles or in the photodissociation with a single photon of linear polarization. Applications to product ensembles of HCl and CO are discussed.
Detection of vibrational-overtone excitation in water via laser-induced grating spectroscopy
Authors: Buntine Mark A. , Chandler David W. , Hayden Carl C.
Published: July 2002

In this paper we describe a method, based on the laser-induced grating technique, for studying the spectroscopy of vibrational overtone-excited gas-phase water. Two phase-coherent visible laser beams whose frequencies are in the range of the third overtone of the OH stretch in water are crossed in the gas-phase sample. As the wavelength of these excitation beams is scanned through individual rovibrational OH overtone transitions, vibrational energy is deposited into the water in a spatially sinusoidal pattern. A fixed-frequency 266 nm probe laser beam is diffracted from the resultant transmission diffraction grating in water. We show that under collision-free conditions, probe laser diffraction is observed from the initially excited grating, which is a necessary condition for using this technique to study the absorption spectroscopy of the vibrationally excited molecules. Under multiple collision conditions, a probe laser wavelength-independent refractive index grating is formed within the bulk sample. In addition, we observe temporal oscillations in the grating diffraction efficiency arising from excitation of standing acoustic waves.
Coupled Hartree–Fock calculations of origin-independent magnetic properties of benzene molecule
Authors: Lazzeretti Paolo , Malagoli Massimo , Zanasi Riccardo
Published: July 2002

Conventional coupled Hartree–Fock approach and two computational methods based on continuous transformation of origin for the electronic current density induced by a magnetic field have been employed to calculate magnetic susceptibilities and nuclear magnetic shieldings of carbon and hydrogen in benzene. Near Hartree–Fock estimates have been obtained by using a basis set of 492 gaugeless contracted Gaussians containing ad hoc polarization functions. Theoretical magnetic susceptibility and nuclear shieldings evaluated in the present work via pointwise coordinate transformation are independent of the origin of the reference system. A series of sum rules for gauge independence of computed results and charge-current conservation has been tested to document the high accuracy of the calculation of magnetic properties.
Density profiles of a model of associating hard spheres in contact with a crystalline surface: An integral equation approach
Authors: Henderson Douglas , Sokol/owski Stefan , Pizio Orest
Published: July 2002

Density profiles of a fluid of associating or chemically reacting hard spheres near a crystalline surface are studied. The model of Cummings and Stell is utilized to provide the description of a bulk associating fluid. The crystal symmetry of the substrate surface plane is assumed to be that of the (100) plane of the face centered cubic lattice. The model of the particle–solid interaction is that proposed by Steele. The effect of association of the particles of the bulk fluid on the density profiles of particles near the crystalline surface is investigated within the hypernetted chain approximation. When the first layer is completed, some dimers tend to be vertically oriented over the adsorption site.
A new time-dependent approach to the direct calculation of reaction rates
Authors: Manthe Uwe
Published: July 2002

A wave packet dynamical approach to the direct calculation of the rate constant of a chemical reaction is presented. Based on the position-flux correlation function of Miller, Schwartz, and Tromp [J. Chem. Phys. 79, 4889 (1983)] a reaction rate operator is introduced, which can be viewed as the thermal analog of the energy-dependent reaction probability operator [J. Chem. Phys. 99, 3411 (1993)]. It is shown that this reaction rate operator has in general only a small number of eigenstates with nonvanishing eigenvalues. These eigenstates can be interpreted as the vibrational ground state and the vibrationally excited states of the activated complex. The eigenstates and eigenvalues can efficiently be computed via an iterative (Lanczos) diagonalization scheme. The number of wave packet propagations required equals approximately the number of relevant states of the activated complex, it is considerably smaller as in previous approaches to the calculation of rate constants based on wave packet dynamics. The new approach is illustrated by three examples: transmission through a one-dimensional (Eckart) potential barrier, the collinear model of the H+H2 reaction, and the H+H2 reaction in its full dimensionality for J=0. For temperatures below 1000 K, in all examples presented, the rate constant can be calculated employing only a single wave packet. This result suggests that the approach can efficiently be applied to problems with a larger number of degrees of freedom.
Spatio-temporal dynamics of oscillatory heterogeneous catalysis: CO oxidation on platinum
Authors: Yamamoto S. Y. , Surko C. M. , Maple M. B. , Pina R. K.
Published: July 2002

Reaction-rate oscillations in the oxidation of carbon monoxide on the surface of platinum catalysts are studied in a continuous flow reactor at atmospheric pressure using infrared imaging. Small-amplitude temperature oscillations (0.2–8 K) result in approximately isothermal conditions, where changes in rate constants, for typical activation energies and temperatures, are small. The catalysts are in the form of platinum thin films on quartz substrates and provide highly repeatable oscillatory behavior. The platinum films are fabricated in the form of annular rings which provide a quasi-one-dimensional geometry in order to simplify comparison to theoretical models. Time-series measurements by means of thermocouples are used to characterize the oscillations. The infrared images show that most oscillations are spatially synchronized to within the 0.25 s time resolution of the experiment. The images also show that ‘‘fine structure’’ oscillations (i.e., small-amplitude, high frequency oscillations superimposed on larger-amplitude waveforms) are associated with spatially desynchronized patterns.
Far-wing excitation study on the transit region of Hg 3P1→3P0 intramultiplet process in collisions with N2 and CO
Authors: Ohmori K. , Kurosawa T. , Chiba H. , Okunishi M. , Ueda K. , Sato Y. , Nikitin E. E.
Published: July 2002

Laser-pump and probe approach has been applied to the far wings of Hg 3P1−1S0 resonance line broadened by collisions with N2 and CO to measure excitation spectra for the formation of Hg(6 3P0) and Hg(6 3P1). The excitation spectra are highly asymmetric with the red wing being much more extended than the blue wing. The absolute ratio of nascent yields of Hg 3P0 to 3P1 is determined as a function of the excitation wave number. From these measurements, it is found, commonly for Hg–N2 and Hg–CO systems, that (a) the nascent product ratio, Hg(3P0)/Hg(3P1), grows on the red-wing surface (the Ã state) with increasing shift, Δν, of the excitation wave number from the line center and finally surpasses unity; (b) the blue-wing surface (the B̃ state) gives mostly Hg(3P1) but has a small chance to give Hg(3P0). Time constant τ0 for the Ã→3P0 process of Hg–N2 is found to change from 17 to 35 ns as the absorption distance Rc between Hg and N2 changes from 3.6 to 4.7 Å. From these values of τ0, the transition probability P(Ã→3P0) for a single approach of Hg–N2 to the turning point region is estimated to be about 3.7×10−5. The transition probability P(B̃→3P0) is about 270 times larger than P(Ã→3P0).

CO is about 20 times more effective than N2 for the B̃→3P0 process. The Rc dependence of τ0 can be qualitatively explained by the vibrational frequencies of the bound Ã state and the Franck–Condon factor between the bound Ã state and the free (repulsive) ã state arising from Hg(3P0)+N2. These findings suggest that the direct Ã→ã transition is realized in these Hg–N2 and Hg–CO collisions. This gives a remarkable contrast to Hg-atom collisions, where the A→3P0 process is parity-forbidden due to the 0+ and 0− characters of the A and a states, respectively. The coupling mechanisms for the Ã→ã and B̃→ã transitions in Hg–N2 collisions are discussed in detail. The theoretical estimate of the Ã→ã transition probability is made to be compared with the experimental value.
Spin–resolved analysis of electronegativity equalization and electron flow in molecules
Authors: Cioslowski Jerzy , Martinov Martin
Published: July 2002

Individual spin contributions to electron flow in molecules are analyzed with concepts of density functional theory. The intra- and interspin components of the bond hardness tensor are rigorously defined and expressed in terms of the second-order response properties. Stability conditions with respect to charge transfer and spin polarization are formulated for closed-shell systems and the relative magnitudes of the bond and spin bond hardnesses are investigated. The energetics of electron flow is illustrated using the examples of the formaldehyde molecule and its radical cation. While the inclusion of electron correlation is found to have only a relatively minor effect on the computed bond hardness tensors, comparison of the data obtained at the unrestricted and restricted open-shell Hartree–Fock levels of theory uncovers the complete uselessness of the latter method in studies of spin–polarized electron flow. Test calculations show that several diatomic molecules suffer from spin–polarization instabilities at the UMP2/6-311++G** level of theory.
Systematic study of the static electrical properties of the CO molecule: Influence of the basis set size and correlation energy
Authors: Luis Josep M. , Martí Josep , Duran Miquel , Andrés José L.
Published: July 2002

The influence of the basis set size and the correlation energy in the static electrical properties of the CO molecule is assessed. In particular, we have studied both the nuclear relaxation and the vibrational contributions to the static molecular electrical properties, the vibrational Stark effect (VSE) and the vibrational intensity effect (VIE). From a mathematical point of view, when a static and uniform electric field is applied to a molecule, the energy of this system can be expressed in terms of a double power series with respect to the bond length and to the field strength. From the power series expansion of the potential energy, field-dependent expressions for the equilibrium geometry, for the potential energy and for the force constant are obtained. The nuclear relaxation and vibrational contributions to the molecular electrical properties are analyzed in terms of the derivatives of the electronic molecular properties. In general, the results presented show that accurate inclusion of the correlation energy and large basis sets are needed to calculate the molecular electrical properties and their derivatives with respect to either nuclear displacements or/and field strength. With respect to experimental data, the calculated power series coefficients are overestimated by the SCF, CISD, and QCISD methods. On the contrary, perturbation methods (MP2 and MP4) tend to underestimate them. In average and using the 6-311+G(3df) basis set and for the CO molecule, the nuclear relaxation and the vibrational contributions to the molecular electrical properties amount to 11.7%, 3.3%, and 69.7% of the purely electronic μ, α, and β values, respectively.
The photodissociation of FNO in the S1 state: Three-dimensional calculation on a new potential energy surface
Authors: Dobbyn Abigail J. , von Dirke Michael , Schinke Reinhard , Fink Reinhold
Published: July 2002

We present three-dimensional wave packet calculations for the photodissociation of FNO in the first excited singlet state S1 using a new ab initio potential surface. While the calculated absorption spectrum agrees satisfactorily with the measured spectrum, the energy dependence of the partial cross sections for particular NO product states is only in fair agreement with experiment. The same is true for the vibrational and rotational state distributions of NO for selected energies. Because of the interference between direct and indirect dissociation, details of the cross sections are highly sensitive to subtleties of the potential surface. Altogether, the new calculation reproduces the available experimental data more satisfactorily than a previous one.
Frequency-dependent viscosity of linear polymer chains. Influence of non-Gaussian effects
Authors: Rey Antonio , Freire Juan J.
Published: July 2002

We have performed Brownian dynamics simulations of Gaussian and semirigid isolated linear chains, with fluctuating hydrodynamic interactions, both with and without intrachain excluded volume interactions. From the simulated dynamic trajectories, we have calculated the stress correlation function. And from this function, after applying a detailed numerical procedure, we have obtained the frequency-dependent complex viscosity. The influence of the rigidity and the excluded volume interactions on this property is discussed as a function of the chain size. While the introduction of semirigid bonds in the chain model creates noticeable differences only in the high frequency region of the viscoelastic moduli, the effect of excluded volume interactions is much more notorious at any frequency, yielding results which are very similar to experimental measurements of dilute polymer solutions in good solvent conditions.
Order–disorder transitions and the method of moments
Authors: Burdett Jeremy K. , Mitchell John F.
Published: July 2002

The method of moments is used to study the electronic driving force for disorder to order transitions. Building on the moments foundation by introducing a mean-field approximation and a Landau-like order parameter, it is shown that ordering is energetically favorable for certain electron counts, minimizing the fourth moment of the crystal density of states. A straightforward method for calculating the moments to arbitrary order is presented, relying only on a knowledge of the low-temperature ordering pattern and a finite number of possible short-range ordering patterns.
Critical phenomena in ionic fluids: A systematic investigation of the crossover behavior
Authors: Narayanan T. , Pitzer Kenneth S.
Published: July 2002

The critical behavior of turbidity for an array of ionic fluids is reported. The ionic fluid is comprised of a low melting organic salt in an organic solvent, and the critical temperature (Tc) refers to the liquid–liquid coexistence. The phase separation is driven, predominantly, by Coulombic interactions in the low dielectric constant (ε) solvents and, primarily, by solvophobic effects in the high ε solvents. For the moderately Coulombic case, the critical exponents of osmotic compressibility and correlation length (γ and ν, respectively), which are deduced from turbidity, exhibit a crossover from their Ising to mean-field values when moved away from the Tc. The crossover region moves closer to Tc as the effective ε of the solvent is decreased. For the solvophobic case, the critical behavior of turbidity can be adequately described by Ising critical exponents without correction-to-scaling terms. In addition, several specific features of this class of ionic fluids are presented. Some potential factors that influence the values of the critical exponents are also discussed.
Quantum dynamics via a time propagator in Wigner’s phase space
Authors: Gro/nager Michael , Henriksen Niels Engholm
Published: July 2002

We derive an expression for a short-time phase space propagator. We use it in a new propagation scheme and demonstrate that it works for a Morse potential. The propagation scheme is used to propagate classical distributions which do not obey the Heisenberg uncertainty principle. It is shown that the simple classical deterministic motion breaks down surprisingly fast in an anharmonic potential. Finally, we discuss the possibility of using the scheme as a useful approach to quantum dynamics in many dimensions. To that end we present a Monte Carlo integration scheme using the norm of the propagator as a part of the sampling function.
Nascent vibrational populations in He*(2 1,3S)+H2, HD, D2 Penning ionization from electron spectroscopy in crossed supersonic molecular beams
Authors: Bevsek H. M. , Dunlavy D. C. , Siska P. E.
Published: July 2002

Penning ionization electron spectroscopy (PIES) in crossed supersonic molecular beams has been used to measure nascent H+2, HD+, and D+2 vibrational populations, line shifts, and their collision energy dependence in the case of H2 (E=1.6, 2.1, and 2.6 kcal/mol), in the title systems. The use of optical spin–state selection and beam modulation enabled estimates of vibrational populations to be obtained out to the dissociation limit, although the low electron energy resolution (90 meV) employed and low counting rates allowed typically only 12 vibrational states for 1S, 9 for 3S, to be resolved. With a few mild exceptions, the following properties were found. The vibrational populations, which are characteristic of the Penning ionization transition state rather than the asymptotic final state, follow Franck–Condon behavior quite closely, more so than do He i photoionization populations, independent of spin and isotope, and nearly independent of E in the range examined. As expected from the E independence, the H2 results compare well with and extend earlier Maxwellian beam PIES populations; isotopomeric populations have not been reported previously. All vibrational peaks are blueshifted (to higher electron energy) by a roughly constant amount for a given E, and the blueshift grows with increasing E. The results suggest that ionization occurs largely on the repulsive part of the incoming potential energy surface, but with little coupling between the translational and vibrational coordinates. A spherical-potential model based on previous experimental estimates of incoming potential surfaces, combined with a Franck–Condon distribution of the reaction flux into product vibration, produces simulated spectra that compare well with experiment both as to intensity envelope and blueshift.
Barriers, thresholds, and resonances: Spectral quantization of the transition state for the collinear D+H2 reaction
Authors: Sadeghi R. , Skodje Rex T.
Published: July 2002

We have analyzed the quantum dynamics of the collinear D+H2 reaction in the region of the transition state on the DMBE potential energy surface. Using the spectral quantization method, the dynamical features of the transition state are mapped out through a sequence of hypothetical Franck–Condon spectra. These spectra are generated by time-dependent wave packet dynamics. A comprehensive analysis of the spectra has revealed three distinct classes of peaks associated with (1) conventional reactive resonances, (2) threshold anomalies, and (3) barrier resonances. Individual peaks in the spectra are assigned through the use of time-independent wave functions which are obtained by Fourier transformation of the wave packet at the peak energies. The positions, lifetimes, and wave functions are extracted for all 23 conventional and barrier resonances. A new analytic line shape formula is developed to fit the spectral peaks of the barrier resonances. The exact quantum analysis is also supplemented with a semiclassical treatment based on periodic orbits. Most of the resonance states are found to be associated with single quantized periodic orbits in the transition state region.