bluuebunny/crossref_metadata_2025 huggingface dataset:
A hierarchy of models for the dynamics of polymer chains in dilute solution
Authors: Perico Angelo , Ganazzoli Fabio , Allegra Giuseppe
Published: July 2002

The dynamics of a hierarchy of sophisticated polymer models described by an interbond potential of quadratic form is derived in the optimized Rouse–Zimm (ORZ) approximation introduced by Bixon and Zwanzig. The resulting quadratic ORZ configurational potential for a generalized rotational isomeric states (RIS) model preserves the maximum of local details and of degree of stiffness compatible with a universal description of polymer dynamics. The present matrix-based RIS–ORZ description has analogies with Allegra’s scalar Fourier approach in terms of the generalized characteristic ratio for chains with periodic boundary conditions. The hierarchy of models amenable to quantitative calculations includes the more popular models used in polymer statistics as the freely jointed and freely rotating chain as well as the more sophisticated models with interdependent rotational states. Comparisons are made in the latter case between the present results and those obtained analytically with the simplifying assumption of a conformationally periodic chain. The relaxation spectra are explicitly calculated for polyethylene, isotactic polystyrene, isotactic polypropylene, and polydimethylsiloxane, which display large differences in stiffness among random coil polymers.
CASSCF/CI calculations of electronic states and potential energy surfaces of PtH2
Authors: Balasubramanian K.
Published: July 2002

Complete active space MCSCF followed by MRSDCI (multireference singles and doubles configuration interaction) calculations are carried out on the electronic states of PtH2. Spin–orbit interaction is introduced using a relativistic configuration interaction scheme on PtH+ whose d orbital Mulliken population is close to that of the d population of PtH2 and thus enables calculation of spin–orbit splittings for the electronic states of PtH2. The bending potential energy surfaces of the 1A1 and 3A1 states are obtained. The 1A1 surface has a bent minimum and dissociates almost without a barrier into Pt(1S0) and H2, while the 3A1 state has a large (∼55 kcal/mol) barrier to dissociation into Pt(3D3)+H2. The ground state of PtH2 is a bent 1A1 state (θ=85°).
Laser photoelectron spectroscopy of CrH−, CoH−, and NiH−: Periodic trends in the electronic structure of the transition-metal hydrides
Authors: Stevens Miller Amy E. , Feigerle C. S. , Lineberger W. C.
Published: July 2002

The laser photoelectron spectra of CrH−, CoH−, and NiH− and the analogous deuterides are reported. The spectra are interpreted using a qualitative description of the electronic structure for the hydrides. This model is used to assign off-diagonal transitions in the photodetachment to low-spin states of the neutrals, and diagonal transitions to high-spin states of the neutrals. These data are used to identify the high-spin states of CoH and NiH; several other states of CrH, CoH, and NiH are also identified. Periodic trends in the bond lengths, vibrational frequencies, and electronic excitation energies for the MnH through NiH molecules are examined. Electron affinities are reported for CrH (0.563±0.010 eV), CoH (0.671±0.010 eV), and NiH (0.481±0.007 eV), and the corresponding deuterides.
Valence quantum Monte Carlo with a
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o effective core potentials
Authors: Hammond Brian L. , Reynolds Peter J. , Lester William A.
Published: July 2002

Effective-core potentials (ECP’s) obtained from ab initio methods are implemented in molecular quantum Monte Carlo (QMC). The theory is presented, and applied to the calculation of electron affinities (EA) of Li and Na, the ionization potential (IP) of Mg, the binding energies (De) of NaH and Na2, and the potential energy curve of Na2. In all cases ECP–QMC results are found to be as accurate as previous all-electron results. In particular, the calculated quantities (vs experimental values) are (in eV): EA(Li)=0.611±0.020 (0.620), EA(Na)=0.555±0.021 (0.546), IP(Mg)=7.637±0.026 (7.646), De (NaH) =1.954±0.073 (1.971), and De (Na2)=0.746±0.020 (0.747). In addition, the statistical precision obtained surpasses that which can be readily achieved in all-electron QMC calculations on these systems.
Molecular theories of confined fluids
Authors: Vanderlick T. K. , Scriven L. E. , Davis H. T.
Published: July 2002

Density profiles and normal pressures predicted by three different approximate density functional free energy theories and the Fischer–Methfessel approximation to the Yvon–Born–Green (YBG) equation are compared with computer simulation results for fluids confined between planar walls. All models require as input a homogeneous fluid equation of state. Comparisons are made using two mean-field equations of state, one based on a Clausius hard-sphere reference fluid and the other based on a Carnahan/Starling hard-sphere reference fluid. The simplest and oldest of the models, the generalized van der Waals model, becomes unphysical at high mean pore densities. The Carnahan/Starling version of Tarazona model agrees best overall with the simulations. This model represents a systematic improvement on the generalized van der Waals model and is computationally the most complicated of all models examined. The YBG and generalized hard-rod models are not as accurate as the Tarazona model, but they capture the qualitative trends observed in the simulations. Both of these models are intuitive extensions of the exact theory of one-dimensional hard rods, and are computationally much simpler than the Tarazona model.
CO adsorption site occupations on Fe(111) vs coverage and temperature: The kinetics of adsorption and reaction
Authors: Whitman L. J. , Richter L. J. , Gurney Bruce A. , Villarrubia J. S. , Ho W.
Published: July 2002

The CO adsorption site occupations on Fe(111) have been determined as a function of coverage and temperature using temperature programmed desorption (TPD) and time-resolved electron energy loss spectroscopy (TREELS). CO does not adsorb in a thermally equilibrated distribution among the four adsorption sites at 100 K; selective adsorption occurs due to different coverage and temperature dependent sticking probabilities. Upon heating, the nonequilibrium distribution relaxes via CO site changes. At ≈250 K and moderate coverage, CO adsorbed in the on-top site changes to the bridge-like shallow hollow site. A heating rate variation analysis yielded an activation barrier E=4.5±0.5 kcal mol−1 and effective first order preexponential ν1=101.5±0.5 s−1 for this process. A similar site change from the deep hollow to the shallow hollow occurs at low coverage near 160 K. Dissociation occurs at ≈300 K when all CO are adsorbed in the shallow hollow site. With low coverages the dissociation reaction occurs with E=20±5 kcal mol−1 and ν1=1011±2 s−1. The resulting adsorbed atomic C and O recombinatively desorb at ≈760 K with E≈48 kcal mol−1 and ν2≈0.1 cm2 s−1. At higher coverages, in addition to dissociation, some of the CO in the shallow hollow desorb, with E≈32 kcal mol−1 and ν1≈1017 s−1.
Critical wetting in two-component polymer blends
Authors: Cohen Scott M. , Muthukumar M.
Published: July 2002

We derive the free energy of a bulk two-component polymer blend in the presence of a planar surface, and use these results to map out the region of critical wetting in parameter space. Unlike previous investigations of wetting, our free energy has terms containing gradients of the composition profile at the surface, and we find that these terms have important consequences for critical wetting. In particular, the single gradient term causes the region of parameter space in which it is possible to find critical wetting to be greatly reduced. Although, as discussed in our concluding remarks, we find that the random phase approximation fails for a polymer blend near a surface, our conclusions concerning critical wetting are valid for a free energy which includes gradient terms of the general type considered here.
Adsorption of oxygen molecule in NaA zeolite: Isotherms and infrared measurements
Authors: Soussen-Jacob Janine , Tsakiris Jean , Cohen De Lara Evelyne
Published: July 2002

Isotherm and infrared induced band of O2 have been studied in the temperature range 93–193 K. At low temperature and low coverage, two components of the fundamental vibrational band appear on both sides of the gas frequency. Their intensities with respect to T and to the number of adsorbed molecules have been measured in order to understand the interaction of O2 with NaA zeolite. The frequency shift of each component has been related to the orientation of the molecule, from considerations about the interaction potential.
The time evolution of the velocity distribution of hydrogen atoms in a bath gas
Authors: Park Jeunghee , Shafer Neil , Bersohn Richard
Published: July 2002

In this work we probe an ensemble of energetic hydrogen atoms dilutely dispersed in a bath gas which is at thermal equilibrium. A recently developed laser-induced fluorescence technique allows a quantitative measurement on a nanosecond time scale of the shape of a Doppler-broadened absorption line of hydrogen atoms. A nonequilibrium distribution of H atoms is generated by photodissociation, and the line shape is then measured at a flxed time delay as a function of buffer gas pressure. The buffer gases used are He, Ar, Kr, Xe, O2, and N2. A technique perfected by Shizgal is used to integrate the linearized Boltzmann equation for a hard-sphere potential. The experimental results for the decay of the kinetic energy are fit with cross sections of 10±2, 9±2, 12±2, and 16(+8−4) Å2 for He, Ar, Kr, and Xe, respectively.
The a
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o model potential method. First series transition metal elements
Authors: Seijo Luis , Barandiarán Zoila , Huzinaga Sigeru
Published: July 2002

In the ab initio core model potential method the Coulomb and exchange core operators are represented accurately and the valence basis set is the only component of the method which is optimized (following the variational principle) in an atomic valence restricted Hartree–Fock (RHF) calculation. In this paper we present the nonrelativistic Ar-like and Mg-like core model potentials and valence Gaussian basis sets for the first series transition metal elements. The pilot RHF molecular calculations on ScO, MnO, CuO, and ScS show that freezing the 3s orbital is safe all along the transition series but the same is true for the 3p orbital only towards the end of the series. A good agreement exists between the all-electron and model potential results if the same type of valence part of the basis set is used: 0.01 Å in Re and 25 cm−1 in ν̄e.
Potential energy surfaces of the low-lying states of N2O++ and photodissociation mechanisms
Authors: Levasseur N. , Millié P.
Published: July 2002

A b initio calculations of the potential energy surfaces of the first low-lying states of N2O++ (3Σ−, 1Δ, 1Σ+, and 3Π) have been performed. Photodissociation mechanisms have been investigated. The dissociation into NO++N+ appears to be a direct process for the first three lowest states. On the other hand, for the 1Δ and 1Σ+ states, high and wide potential barriers are found for the dissociation into N+2+O+ . The 3Σ− state seems to be the most likely state to dissociate along this direction. The predissociation by the 3Π state appears to be an unlikely process. From these theoretical results, the main experimental features are explained. In addition, a simple valence-bond model predicting the bond formation between two positively charged ions is proposed.
CH3ONO predissociation by ultrashort laser pulses: Population transients and product state distribution
Authors: Engel Volker , Metiu Horia
Published: July 2002

We use a two-dimensional potential energy surface for CH3ONO to model the behavior of the molecule interacting with an ultrashort laser pulse. All the results presented here are obtained by solving accurately the time-dependent Schrödinger equation; when possible we also develop a simple model to explain the exact observations. We calculate the lifetimes of the predissociative resonances, the transient populations induced by semi-infinite pulses, and the time evolution of the product state distribution after excitation with a very short pulse. The behavior of the excited state population created by the pulse is explained in terms of the interference between the amplitudes representing photon absorption at different times when the pulse–molecule interaction is not zero. The time evolution of the final state distribution is explained in terms of interference between the wave packets at different times from the interaction region.
Reactive quenching of two-photon excited xenon atoms by Cl2
Authors: Bruce M. R. , Layne W. B. , Meyer Enno , Keto J. W.
Published: July 2002

Total binary and tertiary quench rates have been measured for the reaction Xe (5p5np, np′, n=6,7)+Cl2 at thermal temperatures. Xenon atoms are excited by state-selective, two-photon absorption with an ultraviolet laser. The time-dependent fluorescence from the excited atom in the infrared, visible, and from XeCl* (B) product near 308 nm have been measured with subnanosecond time resolution. The decay rates are measured as a function of Cl2 pressure to 20 Torr and Xe pressure to 400 Torr. The measured reaction rates (k2∼10−9 cm3 s−1) are consistent with a harpoon model described in a separate paper. We also measure large termolecular reaction rates for collisions with xenon atoms (k3∼10−28 cm6 s−1). Total product fluorescence has been examined using a gated optical multichannel analyzer. We measure unit branching fractions for high vibrational levels of XeCl* (B) with very little C state fluorescence observed.
Predissociation channels of the C 3Πg Rydberg state of O2
Authors: Friedman R. S. , Du M. L. , Dalgarno A.
Published: July 2002

A theoretical formulation is presented of the predissociation of vibrational levels of the C 3Πg Rydberg state of O2 in terms of an inhomogeneous driven equation which takes account of couplings between the Rydberg state and the lowest two 3Πg valence states and the coupling between the valence states. The branching ratios for decay to the end products O(3P)+O(1D) and O(3P)+O(3P) are obtained and it is found that by a suitable choice of potential energy curves and coupling interactions agreement can be obtained with the experimental data. Utility of the Landau–Zener model is investigated and it is shown to be highly successful in describing the decay paths of the low-lying vibrational levels but it fails for the high-lying levels because of the occurrence of quantal interference between the channels. It is demonstrated that because of flux conservation the total width of the Rydberg vibrational state is independent of the coupling between the valence states.
Molecular-beam optical Stark spectroscopy of ScO
Authors: Shirley Jeffrey , Scurlock Chris , Steimle Timothy
Published: July 2002

The molecular-beam optical Stark spectrum of the A 2∏(v=1)–X 2∑+(v=0) band system of ScO has been recorded and analyzed. The experimentally determined permanent electric dipole moments of the A 2∏3/2, A 2∏1/2, and X 2∑+ states are 4.06(3), 4.43(2), and 4.55(8) D, respectively. The determined hyperfine parameters for the A 2∏ state are a=135(1) MHz, d=177(2) MHz, and eQq0=−83(12) MHz. The spectroscopic parameters are interpreted in terms of plausible models for the electronic nature of the A 2∏ and X 2∑+ states and are compared with theoretical values.
Fluorescence-based intracavity laser spectroscopy and the electronic structure of NiH
Authors: Hill Ernest J. , Field Robert W.
Published: July 2002

We describe a fluorescence-based continuous-wave (cw) laser intracavity spectrometer which provides tunable cw laser light of high intensity (greater than 200 W/cm2) over a large spatial region (greater than 3×10−2cm3), greatly increasing the amount of observable side fluorescence and facilitating saturation of weak optical transitions as compared to extracavity spectroscopy. Using examples (saturation dip spectra of the hyperfine structure of the rare isotopomer 61NiH and dispersed fluorescence spectra terminating in previously unobserved low-lying states of 58NiH), we illustrate the capability of this spectrometer to rapidly and efficiently gather information crucial to the verification of electronic structure models.
Electronic structure, binding energies, and interaction potentials of transition metal clusters
Authors: Cheng H.-P. , Ellis D. E.
Published: July 2002

The linear combination of atomic orbitals discrete variational method is used to investigate the electronic structure and interatomic interactions of small transition metal particles in the local density theory. Binding energy curves to two-, three-, and four-atom clusters of Fe, Ni, and Pt are calculated at different geometries, and some Fe–Ni clusters are also studied. We thus determined effective pairwise potentials, and three- and four-body effects in an energy expansion by fitting to the binding-energy vs distance data. Use of these data in generating effective potentials for molecular dynamics simulations is discussed. Relativistic effects are estimated for the Pt systems.
Dynamics of precursor-mediated chemisorption
Authors: Doren Douglas J. , Tully John C.
Published: July 2002

We have developed theoretical models of precursor-mediated (nondissociative) molecular chemisorption and used the stochastic classical trajectory method to simulate experiments capable of determining whether a precursor is present. The simulations employ empirical many-dimensional potentials and include the full effects of surface vibrations, coupling among molecular degrees of freedom, and coupling between the molecule and surface. We find that coupling between molecular rotational and translational modes strongly affects the experimentally observable quantities. As a result, reasoning based on the usual one-dimensional picture of a precursor is unreliable. The most notable effect of a precursor is the strong rotational polarization it induces in desorbing molecules. We discuss the origins of rotational polarization and conclude that with no precursor, polarization will be weak and opposite in sign to that of the precursor case. The sticking probability as a function of incident energy and surface temperature can also distinguish whether precursor mediation is important but, again, inferences from one-dimensional models can be misleading. We also find that equilibrium diffusion rates for precursor molecules are faster than for chemisorbed molecules. However, the equilibrium process is relatively slow and until it is complete, chemisorbed molecules may well move farther across the surface than precursors.
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o potential energy surfaces of Ar–NH3 for different NH3 umbrella angles
Authors: Bulski M. , Wormer P. E. S. , van der Avoird A.
Published: July 2002

Four ab initio potential energy surfaces of the van der Waals system argon–ammonia are computed for the following four different HNH (‘‘umbrella’’) angles of ammonia: 100°, 106.67°, 113.34°, and 120°. These potentials have been obtained by Heitler–London short-range calculations and from multipole-expanded dispersion and induction long-range contributions. A Tang–Toennies-like damping is applied to the long-range energy. Each surface is given analytically in the form of a spherical harmonic expansion through l=7, where the expansion functions depend on the polar angles of the argon atom with respect to the principal axes of NH3. The expansion coefficients are represented by functions depending on the distance between the monomers. The potential for the equilibrium HNH angle 106.67° is applied to the computation of interaction virial coefficients in which quantum effects through ℏ2 are included.
Nanocrystallites vibration modes of CdSxSe1−x semiconductors in glasses: Size determination by Raman scattering
Authors: Champagnon B. , Andrianasolo B. , Duval E.
Published: July 2002

Nanocrystallites eigenmodes of CdSxSe1−x semiconductors are observed by low frequency inelastic scattering and the diameters ranging between 2.5 nm and 9 nm are determined. This new application of Raman scattering is a powerful tool to study quantum size effects in these nonlinear optical materials.
Multidimensional intermolecular dynamics from tunable far-infrared laser spectroscopy: Angular-radial coupling in the intermolecular potential of argon–H2O
Authors: Cohen R. C. , Saykally R. J.
Published: July 2002

Five new vibration–rotation tunneling states of Ar–H2O [the Σ and Π(111) and the Σ and Π(212) internal rotor states and the n=1, Π(101) stretching-internal rotor combination level] have been accessed by tunable far-infrared laser spectroscopy. The measured vibrational band origins of transitions to these states are within 2% of predictions made from an anisotropic three-dimensional intermolecular potential surface (denoted AW1) derived from a nonlinear least-squares fit to previous far-infrared spectral data [J. Phys. Chem. 94, 7991(1990)]. This provides strong evidence that the AW1 intermolecular potential surface incorporates much of the essential physics of the intermolecular forces which bind the cluster. However, larger deviations from the predictions are found in the observed rotational term values. A detailed analysis of these deviations clearly demonstrates the need for even stronger angular-radial coupling in the Ar–H2O intermolecular potential than the already substantial coupling present in the AW1 surface. Specifically, the presently observed Σ(111) state and the n=1, Σ(000) state are found to be approximately 65:35 mixtures of the basis states which represent pure stretching and internal rotation. The Σ(212) level is found to be mixed just as strongly with n=2, Σ(101). The formalism for accurately deperturbing vibration–rotation–tunneling states coupled by Coriolis interactions used in the above analysis is presented.
The OH vibrational spectrum of liquid water from combined a
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o and Monte Carlo calculations
Authors: Hermansson Kersti , Knuts Sören , Lindgren Jan
Published: July 2002

The infrared vibrational OH stretching spectrum of isotopically isolated HDO molecules in liquid water has been calculated by ab initio methods at the MP2 level for a number of geometrical configurations taken from a Monte Carlo simulation. Each vibrating water molecule with its environment was described by a pentamer supermolecule, surrounded by a large number of point charges representing polarized water molecules. The anharmonic stretching potentials (MP2 force constants up to fifth order) for 40 uncoupled OH water vibrators were calculated. The average computed re distance found for liquid water is 0.01 Å longer than the free-water value. The frequencies were obtained by solving the one-dimensional Schrödinger equation variationally for each OH potential curve. Using the squared dipole moment derivatives, which vary by a factor of 7 over the frequency band, the density-of-states histograms were converted to intensities. The resulting computed average frequency downshift is ∼260 cm−1, compared to ∼310 cm−1 (experimental), with a bandwidth in good agreement with experiment. The remaining discrepancy between theoretical and experimental frequency shifts is to a large part due to the charge transfer within the water clusters. This charge transfer gives rise to an electrostatic field which, at the site of the vibrating H atom, counteracts the downshift induced by the other environmental effects. The agreement between experiment and theory is very satisfactory when this charge transfer effect is corrected for or when point-charge embedded heptamer clusters are considered.
Excitation of the nitro group in nitromethane by electron transfer
Authors: Lobo R. F. M. , Moutinho A. M. C. , Lacmann K. , Los J.
Published: July 2002

In this paper we report on complementary measurements on ion-pair formation in collisions between K atoms and CH3NO2 molecules. The experiments were performed in a c.m. energy range from 20 up to 300 eV. Double differential cross sections were obtained by measuring the K+ ion yield as a function of the scatter angle and as a function of the post-collision laboratory energy. On the other hand, relative total partial cross sections for the formation of CH3NO−2, NO−2, and O− were measured in the same energy range. The experimental results lead to the conclusion that in this energy range electron transfer takes place to three ionic states of CH3NO−2, a dominantly repulsive 2A1 state and two 2B1 states with relatively deep potential wells.
Dielectric relaxation dynamics of water and methanol solutions associated with the ionization of N,N-dimethylaniline: Theoretical analyses
Authors: Ando Koji , Kato Shigeki
Published: July 2002

The solvation dynamics associated with the ionization of N,N-dimethylaniline (DMA) in water and methanol solutions has been studied theoretically. Potential energy surfaces of DMA and DMA+ were computed by ab initio molecular orbital (MO) methods. Intermolecular pair potential functions between DMA and H2O were developed with the aid of the electron distributions of DMA and H2O and the results of MO calculations for the DMA–H2O system. Potential functions between DMA and MeOH were also determined empirically using the parameters for DMA–H2O interaction. Equilibrium and nonequilibrium molecular dynamics calculations were carried out for the DMA–water and DMA–methanol solutions. The simulation results were analyzed comparing two solvents in order to obtain a realistic molecular model for the solvation dynamics of DMA in polar solvents. The solvation coordinate was defined by the potential energy difference between neutral and cation states and free energy curves along it were constructed using the umbrella sampling method. They were found to be well described by parabolas and nonlinear effects such as the dielectric saturation were not observed. The fluctuation–dissipation relation was also examined. It was found that the present systems follow the linear response to a reasonable approximation. In order to provide a kinematic foundation for the choice of the solvation coordinate, the generalized Langevin equation (GLE) for the motion along the solvation coordinate is derived utilizing the reaction path model originally developed to describe photochemical processes in the gas phase. The mechanism of the dielectric relaxation dynamics was discussed on the basis of the quantities in the GLE deduced from the molecular dynamics (MD) calculations.
Calculation of the complete-active-space self-consistent-field potential-energy surface, the dipole moment surfaces, the rotation–vibration energies, and the vibrational transition moments for C3(X̃ 1Σ+g)
Authors: Jensen Per , Rohlfing Celeste McMichael , Almlöf Jan
Published: July 2002

The present work reports an ab initio calculation of the potential-energy surface and corresponding dipole moment surfaces for the C3 molecule employing the full valence complete-active-space self-consistent-field method with a very large basis set. On the basis of the ab initio results we have calculated the rotation–vibration energies in the lower part of the energy spectrum with the MORBID (Morse oscillator rigid bender internal dynamics) method, and we find that the present ab initio potential-energy surface reproduces qualitatively the variation of the bending energy-level pattern with excitation of the stretching modes. We conclude from the ab initio results obtained in the present work, and from a fitting to experimental data carried out with the morbid computer program, that the equilibrium structure of C3 is linear. We have used the ab initio dipole moment calculated in the present work to compute the vibrational transition moments for selected vibrational transitions of 12C3, obtaining transition moments of 0.44 and 0.35 D, respectively, for the ν2 and ν3 bands. The ab initio dipole moment values have further been used to calculate the line strengths, integrated absorption coefficients, and peak absorption coefficients of the low-J transitions in the ν2 and ν3 bands; the results of the theoretical intensity calculation are in keeping with recent experimental observations.
Nonlinear optical processes in short polyenes: Configuration interaction description of two-photon absorption and third-harmonic generation
Authors: Shuai Zhigang , Beljonne D. , Brédas J. L.
Published: July 2002

We use a multireference determinant single–double configuration interaction approach within a Pariser–Parr–Pople Hamiltonian (based on long-range hopping integrals and the use of the Ohno formula) to investigate linear polyenes containing from four to sixteen carbons. We calculate the low-lying excited states, the two-photon absorption spectrum, and the third-harmonic generation (THG) response. The mAg state, essential to the cubic nonlinear optical response, is found to saturate as the 6Ag state when chain length increases; for the longer polyenes, another high-lying Ag state also becomes important. We analyze the length dependence of the static third-order susceptibility χ(3); indication of the beginning of saturation behavior is found. Focusing our attention to the two-photon resonance peak present in the free-electron laser THG measurements on polyacetylene, we conclude that the experimental data can be explained within the strongly correlated electron one-dimensional model used in this work, in addition to the weakly interacting model successfully exploited in a previous study.
Random-walk approach to mapping nodal regions of N-body wave functions: Ground-state Hartree–Fock wave functions for Li–C
Authors: Glauser William A. , Brown Willard R. , Lester William A. , Bressanini D. , Hammond Brian L. , Koszykowski M. L.
Published: July 2002

Despite the widespread acceptance of the relevance of the nodes of one-body electronic wave functions (atomic or molecular orbitals) in determining chemical properties, relatively little is known about the corresponding nodes of many-body wave functions. As an alternative to mapping the nodal surfaces present in the ground states of many-electron systems, we have focused instead on the structural domains implied by these surfaces. In the spirit of Monte Carlo techniques, the nodal hypervolumes of a series of atomic N-body Hartree–Fock level electronic wave functions have been mapped using a random-walk simulation in 3N dimensional configuration space. The basic structural elements of the domain of atomic or molecular wave functions are identified as nodal regions (continuous volumes of the same sign) and permutational cells (identical building blocks). Our algorithm determines both the relationships among nodal regions or cells (topology) as well as the geometric properties within each structural domain. Our results indicate that ground-state Hartree–Fock wave functions generally consist of four equivalent nodal regions (two positive and two negative), each constructed from one or more permutational cells. We have developed an operational method to distinguish otherwise identical permutational cells. The limitations and most probable sources of error associated with this numerical method are discussed as are directions for future research.
Product rotational distributions and specific opacity functions for the reaction Ba +HI → BaI (v=0,4,8,12,16,18) +H
Authors: Tsekouras A. A. , Leach C. A. , Kalogerakis K. S. , Zare R. N.
Published: July 2002

The reaction Ba+HI→BaI(v)+H was studied under beam-gas, single-collision conditions with an average center-of-mass collision energy of 13 kJ mol−1. BaI (v) rotational distributions were recorded for v=0, 4, 8, 12, 16, and 18 by means of selectively detected laser-induced fluorescence of the BaI C 2Π–X 2Σ+ band system. Each rotational distribution exhibits a maximum toward its high energy end and the range of rotational states becomes narrower as product vibration increases. Because the kinematic constraint causes almost all reagent orbital angular momentum to appear in product rotation, the principle of angular momentum conservation provides the means for determining specific opacity functions from the rotational distributions and the reagent relative velocity distribution. The specific opacity functions are narrow functions of the impact parameter. The peak values decrease smoothly from approximately 4.5 Å for v=0 to 1.5 Å for v=18, indicating a strong correlation between impact parameter and product vibrational state such that Ba+HI collisions with small impact parameter produce BaI with large vibrational excitation.
Spectroscopic studies of surface and subsurface hydrogen/metal systems
Authors: Lynch D. L. , Rick Steven W. , Gomez M. A. , Spath B. W. , Doll J. D. , Pratt L. R.
Published: July 2002

Recent experiments on the H/Ni(111) system have demonstrated that high-resolution electron-energy-loss spectra of subsurface absorbate species can be observed. We report here molecular-dynamics simulations for both the H/Ni(111) and H/Pd(111) systems. The necessary atomic forces are obtained from embedded atom method (EAM) potentials. From such calculations we have obtained the power spectra and compare our results to the available experimental data. These calculations reasonably reproduce the observed shifts upon embedding the H subsurface and we comment on the possibility of subsurface absorbates interfering with surface adsorbate assignments. Lastly, we illustrate the sensitivity of our results to the parametrization of the EAM potential.
Measurement of circular dichroism in rotationally resolved photoelectron angular distributions following the photoionization of NO A 2Σ+
Authors: Leahy David J. , Reid Katharine L. , Park Hongkun , Zare Richard N.
Published: July 2002

The photoionization process NO A 2Σ+ (v=0, N=22)→NO+ X 1Σ+ (v+=0, N+)+e− is studied with sufficient photoelectron energy resolution that the photoelectron angular distributions (PADs) associated with individual rotational levels N+ of the ion are determined. By ionizing with left and right circularly polarized light and observing the change in the rotationally resolved PADs, we can deduce all dynamical information, including the signs of the relative phase shifts of the photoelectron partial waves. This information constitutes the first complete description of the photoionization of a molecule. We discuss the consistency of our dynamical parameters with the Rydberg series of NO. We present a general formalism for (1+1′) resonance-enhanced multiphoton ionization (REMPI) PADs for rotationally resolved ion states using linearly polarized light for excitation and elliptically polarized light for ionization. Based on the dynamical parameters determined from our fit, we use this formalism to predict the total system state, i.e., three-dimensional PADs and polarization of ion rotational levels following photoionization.
CN radical reactions with hydrogen cyanide and cyanogen: Comparison of theory and experiment
Authors: Yang D. L. , Yu T. , Lin M. C. , Melius C. F.
Published: July 2002

Absolute rate constants were obtained for CN radical reactions with HCN and C2N2 employing the method of laser photolysis/laser induced fluorescence. The rate constants were found to be temperature dependent in the range 300–740 K and pressure independent in the range 100–600 Torr. The rates for CN+HCN may be described well, in units of cm3/s, by k(HCN)=2.50×10−17T1.71 exp(−770/T), which includes the shock tube results of Szekely et al. at 3000 K [Int. J. Chem. Kinetics 15, 1237 (1983)]. The measured rates for CN+C2N2 may be described well by k(C2N2)=2.19×10−21T2.70 exp(−325/T). Rice–Ramsperger–Kassel–Marcus (RRKM) theory calculations employing transition state parameters predicted by the BAC-MP4 method were able to account for the effects of temperature and pressure on both reactions.
Dynamics of surface-aligned photochemistry (theory). II. Localized H-atom scattering in the HBr(ad)/LiF(001)+hν system
Authors: Barclay V. J. , Jack D. B. , Polanyi J. C. , Zeiri Y.
Published: July 2002

The angular and energy distributions of 1.1 and 2.6 eV H-atom scattering from a LiF(001) surface are compared for two different sources of hot H-atoms (1) localized H-atoms produced by the photolysis of HBr molecules adsorbed on the LiF surface; and (2) H-atoms from a beam at the same energy and angle of approach to the LiF surface as in (1). These distributions are the results of classical stochastic trajectories carried out on a 7×7 slab of LiF(001). The calculated angular distributions are compared with experimental distributions [E. B. D. Bourdon et al., J. Chem. Phys. 95, 1361 (1991)]. The computed effects of changing H-atom energy and surface temperature are also reported; localized scattering becomes increasingly nonspecular at lower photon energies, and broader at higher surface temperatures.
Field dependent isotropic shifts in solid state nuclear magnetic resonance: A Floquet treatment
Authors: Augustine Matthew P. , Zilm Kurt W. , Zax David B.
Published: July 2002

The apparent field dependence observed for some isotropic shifts in high resolution solid state NMR is reinvestigated using the Floquet formalism. For 13C–1H systems, second order dipolar shifts are derived that largely agree with the experimental observations made by VanderHart and his theoretical treatment while resolving some difficulties in the use of a secular approximation to handle an inherently time-dependent problem. Additional second order terms are shown to give rise to both frequency shifts and line broadening which can be significant in some spin pairs. This line broadening is found to arise from a second order coupling of the rf field with the I–S dipolar interaction.
Impulsive effects of phase-locked pulse pairs on nuclear motion in the electronic ground state
Authors: Cina J. A. , Smith T. J.
Published: July 2002

The nonlinear effects of ultrashort phase-locked electronically resonant pulse pairs on the ground state nuclear motion are investigated theoretically. The pulse-pair propagator, momentum impulse, and displacement are determined in the weak field limit for pulse pairs separated by a time delay short on a nuclear time scale. Possible application to large amplitude vibrational excitation of the 104 cm−1 mode of α-perylene is considered and comparisons are made to other Raman excitation methods.
Screened intermolecular forces and covalent bond forces in polymer melts
Authors: Gao J. , Weiner J. H.
Published: July 2002

The intermolecular nonbonded forces exerted on a particular mer of a chain in a polymer melt are screened by adjacent mers on the same chain. As a result, the time average of this intermolecular force, referred to a coordinate system bearing a fixed relationship to the covalent bonds attached to the mer in question, is nonzero. These screened forces are studied here by molecular dynamics simulation of a model melt corresponding to the model for polyethylene (PE model) of Takeuchi and Roe. If the macromolecule in a particular conformation is visualized as a long curved cylinder, the screened forces are distributed over its surface, including its capped ends, directed inwards and approximately normal to the surface. These forces are relatively insensitive to the melt pressure p. They are present and have the same character, even when p=0. The character of these forces helps clarify the simulation result that the backbone covalent bonds are in compression at liquidlike densities.
On the validity of the ‘‘inverted snowball’’ picture of solvation dynamics
Authors: Papazyan Arno , Maroncelli Mark
Published: July 2002

In this article we examine the validity of the ‘‘inverted snowball’ picture of solvation dynamics using results of simulations of an idealized model solvent. The solvent consists of point dipoles which undergo rotational Brownian motion while fixed to the sites of a simple cubic lattice. In this system we observe a much more complex distance dependence of the solvation response than assumed in the inverted snowball picture. These results indicate that the intermolecular correlations responsible for solvation do not allow for any useful spatial decomposition of the response and so point to the inadequacy of such a description for understanding solvation dynamics.
Test of theory for long time dynamics of floppy molecules in solution using Brownian dynamics simulation of octane
Authors: Chang Xiao-yan , Freed Karl F.
Published: July 2002

Tests are introduced of one basic approximation inherent in a recent theory for long time dynamics of flexible proteins and polymers in solution. The approximation in question concerns the neglect of memory functions, which for these systems involve a memory function matrix whose properties are not readily modeled without input from simulations. The memory function matrix affects the dynamics on all time scales, but our focus is on those portions influencing mainly the long time dynamics, which is not amenable to simulation for interesting complicated systems. Thus the tests are made on a simple, yet nontrivial system for which long time simulations are possible and provide the most stringent test of the parameter free theory. The test involves Brownian dynamics simulations of united atom models for single octane and pentadecane dynamics in a structureless solvent. The octane case, for instance, yields a 7×7 memory function matrix whose properties are more complex than those of the one-dimensional Kramers model recently studied. More importantly, our computations determine those necessary ingredients of the memory function matrix for describing long time dynamics of flexible large molecules in solution. We compare the theoretical and simulation computations of the bond vector time autocorrelation functions. The leading approximation without memory functions fares remarkably well despite the frequent conformational transitions occurring during the Brownian dynamics trajectories. This approximation systematically leads to faster decay than the simulations due to the neglect of the long time frictional influence of the memory function matrix. We consider computations of these memory functions using both the Mori continued function formalism, and a generalization of the matrix methods introduced to treat the one-dimensional Kramers model. A procedure is developed to obtain the long time influence of the memory function matrix. This procedure improves agreement between theory and simulation and displays good convergence towards the simulation results at the longer times (≳100 ps) for which we are interested.
The microwave rotational spectrum of the van der Waals complex Kr–N2
Authors: Jäger Wolfgang , Xu Yunjie , Heineking Nils , Gerry M. C. L.
Published: July 2002

Pure rotational transitions in the vibrational ground state of the van der Waals complex Kr–N2 have been measured using a cavity microwave Fourier transform spectrometer. For the isotopomers 84Kr–14N2, 86Kr–14N2, 84Kr–15N2, 86Kr–15N2, 82Kr–15N2, and 83Kr–15N2 a-type transitions with Ka=0 and 1 have been measured, confirming a C2v symmetry of the complex. A value for the 14N nuclear quadrupole coupling constant in free nitrogen 14N2 has been estimated from an analysis of the nuclear hyperfine structure in complexes of 14N2. 15N spin–rotation hyperfine structure could be observed and analyzed in the 21,2–11,1 and 21,1–11,0 rotational transitions of 84Kr–15N2. Rotational transitions of 83Kr–15N2 showed a nuclear hyperfine splitting due to 83Kr with nuclear spin 9/2 which has been attributed to the weak bonding with 15N2. For 84Kr–14N15N and 86Kr–14N15N, only transitions involving energy levels with Ka=0 have been found.
Photoionization dynamics of the NO A 2Σ+ state deduced from energy- and angle-resolved photoelectron spectroscopy
Authors: Park Hongkun , Zare Richard N.
Published: July 2002

The direct photoionization process NO A 2Σ+ (v=0, N=13)→NO+ X 1Σ+ (v+=0, N+)+e− is studied by energy- and angle-resolved photoelectron spectroscopy by employing two-color resonance-enhanced multiphoton ionization (REMPI) via excitation of the NO A–X(0–0) R21(11.5) transition. The photoelectron angular distributions (PADs) associated with individual rotational levels N+ of the ion are determined. Combined analysis of the newly obtained PADs and those reported earlier for the processes NO A 2Σ+ (v=0, N≥20)→NO+ X 1Σ+ (v+=0, N+)+e− via P21+Q1 branch excitation shows that the photoionization dynamics is independent of the rotational quantum number of state to be ionized and of the spin state of the photoelectron. Quantitative comparison of our results with threshold photoelectron measurements provides strong evidence that ionization in the pulsed-electric-field threshold technique is not via direct photoionization.
Quantum corrections to the classical path theory
Authors: Billing Gert D.
Published: July 2002

We discuss an effective numerical method for obtaining the best trajectory to be used in the classical path approach. The method is based upon a time dependent variational method, which yields from first principles the classical path equations. In its simplest formulation the classical path equations contain an optimal momentum determined self-consistently. A variational method, which satisfies detailed balance is also discussed.
Low-J rotational spectra, internal rotation, and structures of several benzene–water dimers
Authors: Gutowsky H. S. , Emilsson T. , Arunan E.
Published: July 2002

Low J (0–4) rotational transitions have been observed for the benzene–water dimer of which high J (≥4) transitions were reported recently by Blake [Science 257, 942 (1992)]. Our experiments used a modified Balle/Flygare Fourier transform microwave spectrometer, with a pulsed supersonic nozzle as the sample source, and examined a variety of isotopic species in the ground and first excited internal rotor states (m=0 and 1). The dimers of the parent C6H6 benzene with H2O, HDO, D2O, and H218O have symmetric top spectra characteristic of two coaxial rotors with a symmetric top frame and a very low effective V6 barrier. The dimers of H2O and D2O with the 13C and D monosubstituted benzenes have asymmetric top spectra of which only the m=0 state was assigned. However, doublets in the m=1, J=0→1 transitions show that there is a V2 term of ∼0.5 MHz in their barriers. A substitution analysis was made of the rotational constants found for the m=0 state of the dimers with H218O, D2O, and the 13C and D monosubstituted benzenes. It shows that the oxygen is at the a axis of the dimer, well outside (0.48 Å) the hydrogens. However, the C2 axis of the H2O is not coincident with the a axis but is at an angle β of 37° to it, rotated so that the two hydrogens are equivalent. The sixfold axis of the benzene corresponds to the a axis, there is little or no tilt (γ) of the benzene. The c.m. (C6H6) to c.m. (H2O) distance R is 3.329 Å. The closely spaced hyperfine structure from the proton–proton magnetic dipole interaction and the deuterium quadrupole interaction was resolved for several dimers and transitions, principally J=0→1 and 1→2. The results demonstrate effective nuclear equivalence in dimers with H2O and D2O. Also, the symmetries found for their nuclear spin functions correlate with the lowest rotational levels of free water, the m=0 state with 000 and m=1 with 101 and 111. For the m=1, K=0 transitions of C6H6–H2O the correlation is with 111 and for the K=±1, with 101. These assignments are reversed for C6H6–D2O.
The (2+1) multiphoton ionization spectrum of jet-cooled CS2 between 54 000 and 58 000 cm−1
Authors: Couris Stelios , Patsilinakou Evdokia , Lotz Mark , Grant Edward R. , Fotakis Costas , Cossart-Magos Claudina , Horani Marcel
Published: July 2002

The (2+1) multiphoton ionization (MPI) spectrum of jet-cooled CS2 has been recorded in the range 345–370 nm, corresponding to a two-photon frequency between 54 000 and 58 000 cm−1. The origin bands of the ...2π3g4sσg 1Πg and 3Πg←X̃ 1Σ+g Rydberg transitions have been observed at 351.04 and 355.15 nm (vacuum 56 974 and 56 315 cm−1), respectively. A third, broader and stronger, band is observed at ∼365 nm corresponding to a two-photon transition at ∼54 700 cm−1, which is suggested to belong to the valence 5σu3πu 1Πg state. The position of the two-photon band at 351 nm establishes it as the one-photon electric-dipole forbidden origin of the 800 cm−1 progression observed in the ultraviolet absorption spectrum. The first-photon frequency of this transition falls at a level corresponding to the ã 3A2 valence state, so that the (2+1) MPI spectrum is supplemented by numerous transitions associated with the (1+2) MPI spectrum of the 3A2←X̃ 1Σ+g transition.