bluuebunny/crossref_metadata_2025 huggingface dataset:
Atomic structure factors from a molecular integral equation theory: An application to homonuclear diatomic fluids
Authors: Martín C. , Lombardero M. , Alvarez M. , Lomba E.
Published: July 2002

We present a novel approach for the theoretical determination of atomic structure factors (or site–site distribution functions) based on the calculation of the molecular pair distribution function by integral equations (reference hypernetted chain approximation). The results are compared with experimental structure factors and computer simulation results for homonuclear diatomic fluids (N2, Cl2 and Br2) which are modeled by means of two center Lennard-Jones potentials. The proposed method leads to a surprisingly good agreement with experimental data, within the obvious limitations that stem from intrinsic inadequacies of the model interaction potential. Comparison with RISM integral equation results evidences the superiority of the molecular integral equation approach.
Dynamic simulation of fiber suspensions in shear flow
Authors: Yamamoto Satoru , Matsuoka Takaaki
Published: July 2002

A new method is presented to simulate the motion of concentrated fiber suspensions in shear flow at low Reynolds numbers without Brownian motion. The hydrodynamic interaction among fibers is considered in a particle simulation method (PSM), in which a fiber is modeled by arrays of spheres. The motion of each constituent sphere of a fiber, which are dispersed into a unit cell with periodic boundaries, is followed to predict the microstructure and the rheological properties. The hydrodynamic interaction is decomposed into two parts, intra- and interfiber ones. In the former, the many-body problem is solved by calculating the mobility matrix for each fiber to obtain the hydrodynamic force and torque exerted on each sphere. In the latter, only the near-field lubrication force is considered between spheres of one fiber and another. The validity of this approximate treatment was first examined for the sphere dispersed system. The simulated microstructure and the rheological properties were in very good agreement with results of both experiments and the Stokesian dynamics. The methodology was then applied to concentrated rigid and flexible fiber suspensions. The overshoot of suspension viscosity was observed at the early stage for rigid fiber suspensions, but not for flexible ones. This was because of the transient change of the microstructure from the flow-directional orientation to the planar orientation of rigid fibers. The normal stress was calculated for the flexible fiber suspension and clearly showed that the elasticity of fiber suspensions was due to the deformation of fibers. The proposed simulation method can predict the effect of such parameters as the aspect ratio, flexibility, and volume fraction of fibers on the microstructure and the rheological properties of fiber suspensions.
Multiphoton ionization of Ag2: Assessment of a new excited electronic state and resolution of the Ag2 ionization potential
Authors: Winstead C. B. , Paukstis S. J. , Walters J. L. , Gole J. L.
Published: July 2002

A previously unknown excited electronic state of Ag2 has been observed using mass selected resonant two photon ionization. The initial rovibrational level of this state observed via a forbidden transition near 46 870 cm−1 lies in near perfect double resonance with the lower energy A 1Σ+u(v′=3)←X 1Σ+g(v′=0) transition. The double resonance leads to an anomalously large single color ionization signal near the A(v′=3)←X(v″=0) transition wavelength (426.7 nm). Symmetry selection rules allow an identification of the new state symmetry as 1g or 0+g [Hund’s case (c)]. The presence of this state is demonstrated to have a significant impact on previous measurements of the ionization potential of Ag2. Two color resonant two photon ionization spectroscopy of the Ag2 A1Σ+u state is implemented using both excimer laser and Nd:YAG laser fourth harmonic ionization. The results of these experiments yield a new measurement of the ionization potential of Ag2, while demonstrating the importance of field ionization effects.
Two-photon spectroscopy of HI in the 69 600–73 600 cm−1 region
Authors: Pratt S. T. , Ginter M. L.
Published: July 2002

Several new electronic states in HI have been identified from resonance-enhanced multiphoton ionization (REMPI) spectra in the 69 600–73 600 cm−1 region. These results have been combined with previously unpublished single-photon absorption data and, in selected wavelength regions, with REMPI spectra of DI to provide a more complete description of the excited electronic structure of HI up to ∼9.2 eV above the ground state.
Light intensity effects on diffusion-influenced fluorescence quenching in a hard-sphere liquid: Molecular dynamics simulation and the many-body Smoluchowski equation approach
Authors: Bandyopadhyay T.
Published: July 2002

Molecular dynamics simulation of a model fluorescence quenching reaction A*+B→B (A* is fluorophore and B is quencher molecule) in a hard-sphere liquid where the fluorophore is excited for the first time or reexcited shortly after a bimolecular quenching process is carried out. The effects of light intensity on the temporal behavior of the fluorescence quenching kinetics is obtained by a summed form of an appropriate convolution integral using the simulation data. The convoluted results are compared with the recently developed general theoretical framework for the quenching kinetics where the exciting light pulse has a short but finite duration. The theory is based on hierarchy of phenomenological kinetic equations involving reactant molecule distribution functions. The alone effect of potential of mean force is examined and the radiation boundary condition is considered. Improvements over the simplest version of the Smoluchowski theory has been found. Considering the error introduced due to truncation of repeated excitation of A at the level of first repeated excitation (after a bimolecular process) the agreement between theory and simulation is excellent under certain limiting time profile of the exciting light pulse.
Studies of the 193 nm photolysis of diethyl ketone and acetone using time-resolved Fourier transform emission spectroscopy
Authors: Hall Gregory E. , Metzler H. W. , Muckerman James T. , Preses Jack M. , Weston Ralph E.
Published: July 2002

We have observed the infrared emission from the products of the 193 nm photolysis of diethyl ketone (3-pentanone) in comparison with acetone (2-propanone) using time-resolved Fourier transform spectroscopy. In the photolysis of diethyl ketone, two bands are apparent: The first, spanning the region 1950 to 2250 cm−1, is assigned to CO rovibrational transitions; the other band, spanning the region 2800 to 3400 cm−1 and not exhibiting resolved line structure, is assigned to the ethyl radical. Spectral simulations of the CO bands under conditions of minimal, but not negligible, relaxation produce a lower bound for the nascent CO rotational temperature of ∼2100 K. The CO vibrational population distribution varies slowly over the ∼80 μs time spanned by our experiment. Both the rotational and vibrational energies of CO exceed statistical partitioning in the dissociation of acetone. In comparison to the case of acetone, absolute energies in CO vibration and rotation decrease only modestly for diethyl ketone, corresponding to a dramatic increase in the excess above the statistically partitioned energies. Several simple dissociation models are compared to these results. None is fully satisfactory.
Photofragmentation dynamics of Mg2(CO2)+1,2
Authors: Ding L. N. , Kleiber P. D. , Cheng Y. C. , Young M. A. , ONeil S. V. , Stwalley W. C.
Published: July 2002

We have investigated the photofragmentation spectroscopy of Mg2(CO2)+1,2 in a reflectron time-of-flight mass spectrometer. For the linear bimolecular complex Mg2(CO2)+, we have observed three distinct molecular absorption bands, in the red, the green, and the near uv spectral regions. In each band both Mg+ and Mg+2 fragments are observed, although with different action spectra. In the uv band, we also observe reactive fragmentation to form the chemical product Mg2O+. As an initial step in investigating the dissociation mechanism and the dynamical effects which determine the final state branching, we have carried out preliminary calculations of the low lying Mg2(CO2)+ potential energy curves in collinear geometry. Photofragmentation of the complex Mg2(CO2)+2 occurs over a broad spectral range in the visible. We have found that the addition of the second solvent molecule dramatically enhances the solvent cage effect, as evidenced by a large increase in the Mg+2/Mg+ branching ratio throughout this spectral region.
Electronic friction of physisorbed molecules
Authors: Persson B. N. J. , Volokitin A. I.
Published: July 2002

We calculate the electronic friction force acting on physisorbed molecules sliding on metal surfaces, where the metal is treated within the jellium model. We find that the contribution to the friction from the repulsive (Pauli repulsion) and attractive (van der Waals) adsorbate–substrate interactions are of similar magnitude. For small inert molecules and atoms such as C2H6 and Xe the calculated electronic friction (η∼108 s−1) is in good agreement with those deduced from surface resistivity and quartz crystal microbalance measurements. We discuss recent inelastic helium–atom scattering measurements for saturated hydrocarbons (hexane, octane and decane) on Cu(100) and show that the damping of the parallel adsorbate vibrations is dominated by the electronic friction while the phononic friction dominates for the perpendicular vibrations.
Unitary group based open-shell coupled cluster approach and triplet and open-shell singlet stabilities of Hartree–Fock references
Authors: Li Xiangzhu , Paldus Josef
Published: July 2002

The performance of recently introduced state-selective coupled-cluster method, based on the unitary group approach to the many-electron correlation problem and enabling a properly spin-adapted treatment of open-shell states, is examined for open-shell singlet and triplet states having unstable restricted Hartree–Fock solutions that serve as a reference. The study employs an ab initio model at the double-zeta-plus-polarization level of approximation describing the dissociation of the methylene radical in its lowest triplet and open-shell singlet states into the CH and H fragments in their ground states. It is shown that the spin restricted Hartree–Fock equations have different solutions in the bonding and nonbonding regions, which coexist in a small transition region, where the corresponding potential energy curves intersect, and which is determined by the vanishing of the lowest root of the stability problem for each solution. These instabilities are then responsible for a slight discontinuity in the potential energy curves in the transition region when the correlation effects are accounted for. It is shown that the overall performance of the unitary group based coupled cluster method gives very satisfactory results at a moderate cost. Finally, the possible pitfalls of the spin orbital coupled cluster method employing the unrestricted Hartree–Fock reference are illustrated for the triplet ground state dissociation model of methylene.
Many-body correlations in the interaction-induced light scattering from liquid CS2
Authors: Stassen H. , Steele W. A.
Published: July 2002

Additional information concerning the interaction-induced contribution to the intensities in depolarized light scattering spectra of liquid carbon disulfide is presented here. This was obtained from molecular dynamics computer simulations of relevant time-correlation functions (TCFs) along the gas-liquid coexistence curve. The interaction-induced part of the polarizability was evaluated assuming a first-order dipole-induced dipole mechanism. The simulated spectral TCFs for this system show coupled rotation-translation dynamics that are due in part to the nonspherical molecular shape of CS2 and in part to the orientation dependence of the dipole-induced polarizability Π. By writing Π in the usual way as the sum of a diagonal orientation-independent term plus a second-rank tensor in the polarizability anisotropy, the TCFs can be split into terms with varying degrees of orientation dependence. In addition, the collective TCFs were broken down into their component 2-, 3- and 4-body parts. The component parts of both the pure interaction-induced (II) and the cross (CR) TCFs between the II polarizabilities and the orientation (OR) variable were evaluated. At the highest densities, it is shown that the cancellation of II TCFs at short times is not as complete as for other simple molecules even though there is nearly a perfect cancellation of the 2- and 4-body correlations by negative 3-body terms in the long time limit.
Conformational properties of polyelectrolyte brushes: A Monte Carlo and self-consistent-field study
Authors: Chen Hao , Zajac Richard , Chakrabarti Amitabha
Published: July 2002

We present and compare results from Monte Carlo and self-consistent-field studies of a grafted brush of polyelectrolyte chains (of length N), interacting via a screened-Coulombic potential. We examine the brush under a variety of grafting and screening conditions and investigate the various régimes of this system. The agreement between Monte Carlo and self-consistent-field methods is quite good. When the effective interaction strength is reasonably large, and the screening is strong, the polyelectrolyte brush behaves like a neutral brush with an effective excluded volume parameter, in agreement with previous scaling studies. For weaker screenings, we find and describe some region where the behavior of the brush height H resembles the H∼N3 prediction made by Pincus, and more recently by von Goeler and Muthukumar.
Vibrational hyperpolarizabilities and the Kerr effect in CH4, CF4, and SF6
Authors: Shelton D. P. , Palubinskas J. J.
Published: July 2002

The hyperpolarizabilities γ of CH4, CF4, and SF6 were measured by the dc Kerr effect at wavelengths from 457.9 to 1092 nm. Vibrational hyperpolarizabilities γv were obtained by combining these measurements with electric-field-induced second harmonic generation (ESHG) measurements. The vibrational contribution to the hyperpolarizability ranges from 6% to 35% of the total. At high optical frequency the difference between γv for Kerr and γv for ESHG is approximately constant, and has values 18, 31, and 51×10−63 C4 m4 J−3 for CH4, CF4, and SF6, respectively. The experimental results are in good quantitative agreement with the results of recent ab initio calculations of the frequency dependence of γv for CH4, except for a small but non-negligible discrepancy at high frequency.
Optimal Kinoshita wave functions with half-integer powers
Authors: Koga Toshikatsu
Published: July 2002

Kinoshita wave functions for helium and helium-like atoms have been generalized by using half-integer powers for constituent terms. An extensive optimization has been performed for these powers as well as the exponent and mixing coefficients. The optimal functions have been constructed for the number of terms N=10, 20, 30, 50, 100 and the atomic number Z=1 (H−)−10 (Ne8+). It is demonstrated that the use of half-integer powers dramatically improves the accuracy of the Kinoshita function: In the case of He, for example, the optimal 100-term function gives −2.903 724 377 033 hartrees, which is only 1×10−12 hartrees higher than the most accurate literature value. The high accuracy of the optimal Kinoshita functions with half-integer powers has been also confirmed for the first two excited states of the helium atom.
Geometric phase effects in H+O2 scattering. I. Surface function solutions in the presence of a conical intersection
Authors: Kendrick Brian , Pack Russell T
Published: July 2002

The general vector potential (gauge theory) approach for including geometric phase effects in accurate 3D quantum scattering calculations in hyperspherical coordinates is presented. A hybrid numerical technique utilizing both the DVR (discrete variable representation) and the FBR (finite basis representation) is developed. This method overcomes the singular behavior of the vector potential terms giving accurate surface function solutions to the complex Hermitian nuclear Schrödinger equation. The hybrid DVR/FBR technique is applied explicitly to HO2 for zero total angular momentum. The resulting complex surface functions include the geometric phase effects due to the C2v conical intersection. The O2 permutation symmetry is implemented to give real double-valued surface functions which exhibit both even and odd symmetry. The surface function eigenvalues are compared to calculations without the geometric phase. The results indicate that geometric phase effects should be significant for H+O2 scattering even at low energies.
Ultrafast molecular dynamics controlled by pulse duration: The Na3 molecule
Authors: Reischl B. , de Vivie-Riedle R. , Rutz S. , Schreiber E.
Published: July 2002

Laser pulses of moderate intensities with durations of either 1.5 ps or 120 fs were employed to excite the Na3 molecule to its electronic B state. Using a pump–probe technique the temporal evolution of the two-photon ionization signal could be resolved in real time. Different vibrational modes of the excited trimer are detected selectively. While the ps laser pulses yield preferential excitation of the slow pseudorotational mode with a period of 3 ps, the use of ∼10 times shorter pulses allows the trimer’s symmetric stretch mode with a 310–320 fs period for the first 5 ps to be observed. These complementary experimental results can be explained to a great extent by quantum dynamical simulations of the pump–probe experiments. The calculations are performed on three-dimensional ab initio potential energy and transition dipole surfaces. Thus all three vibrational degrees of freedom of the Na3 molecule are included in the theoretical treatment. The time-dependent wave-packet dynamics elucidate the effect of ultrafast state preparation on the molecular dynamics. Extensive theoretical calculations manifest the possibility of initiating the molecular dynamics dominantly in selected modes during a certain time span by variation of the pump–pulse duration.
Dopant location in SF6He39,40
Authors: McMahon Michele A. , Barnett Robert N. , Whaley K. Birgitta
Published: July 2002

Recent quantum Monte Carlo studies of doped helium clusters have yielded different results for the location of the SF6 impurity, despite good agreement on helium density profiles, thus raising the question of wave function bias on structural properties. We present here a systematic analysis of the effect of the trial function on variational and diffusion Monte Carlo (VMC and DMC) results for the ground state of SF6HeN (N=39 and 40). Four different sets of wave functions are used, together with isotropic pairwise potentials. Use of a two-peak term in the He–SF6 wave function to describe the extensive helium structuring induced by the impurity greatly improves the VMC energies and helium densities. For all of the wave functions, the impurity SF6 distribution has its maximum at the cluster center in both VMC and DMC. This result agrees with the conclusion previously presented by Barnett and Whaley, but it contradicts the recent DMC result of Chin and Krotscheck. To explain this discrepancy, we analyze the amount of sampling necessary to ensure a reliable description of the SF6 density. We find that considerably more sampling is required for the impurity than for the helium density or the energy. Similar considerations are shown to affect the extent of structure seen in pure HeN density profiles.
Decoupling of lithium and proton self-diffusion in supercooled LiCl:7H2O: A nuclear magnetic resonance study using ultrahigh magnetic field gradients
Authors: Feiweier Thorsten , Isfort Olaf , Geil Burkhard , Fujara Franz , Weingärtner Hermann
Published: July 2002

Self-diffusion coefficients of lithium ions and water protons (DLi and DH) in the glass-forming electrolyte LiCl:7H2O have been measured by nuclear magnetic resonance spin-echo experiments using ultrahigh static magnetic field gradients up to 184 T m−1. The measurements were complemented by measurements of 7Li and 1H spin-lattice relaxation times. The data cover the temperature range from 313 K down to 173 K, i.e., 34 K above the glass transition temperature Tg=139 K. In this range DLi and DH change over five orders of magnitude. The self-diffusion data exhibit a strong non-Arrhenius temperature dependence which is typical for fragile glass formers. In the supercooled regime the ratio of the self-diffusion coefficients DH/DLi increases gradually with decreasing temperature, reflecting a decoupling of these diffusive modes. These results are discussed in relation to the behavior of the viscosity, electrical conductance and reorientational correlation time of water in this temperature range. It is found that lithium ion diffusion is closely coupled to these other transport processes, while proton diffusion begins to decouple at T<1.5 Tg. Additionally, an analysis of 1H and 7Li magnetic relaxation rates 1/T1 is given. It is found that the intermolecular modes causing 1H–1H dipolar relaxation and 7Li quadrupolar relaxation also decouple from the viscosity. The results are discussed in the framework of similar phenomena observed with other fragile glasses and, more specifically, of structural changes known to occur in supercooled LiCl:H2O systems.
Inelastic neutron scattering studies of hydrogen bonding in ices
Authors: Li Jichen
Published: July 2002

Using high resolution inelastic neutron scattering measurements of the phonon density of states of ice, two separate molecular optic bands at about 28 and 37 meV for ice Ih and ice Ic have been observed, which is the direct result of the very high flux of pulsed neutron source and excellent energy resolution of the inelastic instruments on ISIS at the Rutherford–Appleton Laboratory. In order to reproduce the measured phonon density of states, a new lattice dynamic model has been proposed that two interaction strengths among the H bonds in ice Ih (and ice Ic) associated with the proton configurations have to be assumed. These two force constants are randomly distributed in the network of ice, having a ratio of almost 1:2. This model is capable of reproducing almost every aspect of the measured spectra, the two molecular peaks in 28 and 37 meV in particular and may provide insight into the complex nature of H bonding in ice and water. The large differences in the force constants between the strong and weak H bonds in a disordered ice structure as required by the model would affect the physical and chemical properties of ice and water and could have broader implications. In this paper, we illustrate a complete measurement for all possible recovered high-pressure forms of ice, including ice I, II, IX, V, VI, VII, VIII, and high-/low-density amorphous ice. These measurements demonstrate that the two well-separated molecular bands are associated with the local configurations of protons, which have quite different interaction strengths.
Lateral interactions and corrugation in physisorption systems: CH4/Cu(100)
Authors: Wei D.-H. , Skelton D. C. , Kevan S. D.
Published: July 2002

We present and analyze isothermal adsorption and desorption measurements for methane physisorbed on a Cu(100) surface to characterize the interplay between molecule–surface and molecule–molecule interactions. A 2D gas–liquid phase boundary for the methane layer is observed and partially mapped in coverage and temperature. Comparing our measured critical temperature of 66 K ±2 K to previous work on related systems implies an attractive well between physisorbed molecules of ∼130 K. This reduction compared to the gas phase well can be adequately understood in terms of dielectric screening of the dispersion interaction. By contrast, analyzing the low-coverage adsorption and desorption isotherms using a quasiequilibrium model and imperfect gas statistics, we deduce a lateral attractive intermolecular potential well of only 40–50 K. This drastic reduction of the attractive well from the gas phase value is not consistent with image charge screening, and is difficult to reconcile with the observed critical temperature. This discrepancy is discussed in terms of surface corrugation which dictates different positions for molecule–surface and molecule–molecule potential energy minima.
On the theoretical investigation of vibronic spectra of ethylene by ab initio calculations of the Franck–Condon factors
Authors: Mebel Alexander M. , Chen Yit-Tsong , Lin Sheng-Hsien
Published: July 2002

The vibronic spectra of ethylene have been studied using ab initio molecular orbital methods. Geometries of the singlet π–π*, π–3s, and π–3p excited electronic states have been optimized at the CIS and CASSCF levels of theory with the 6-311(2+)G* basis set. Vertical and adiabatic excitation energies, calculated by the multireference configuration interaction (MRCI) and equation-of-motion coupled cluster (EOM-CCSD) methods are in quantitative agreement with experiment. Vibrational frequencies and normal coordinates for the ground and excited states are used for the calculations of vibrational overlap integrals and Franck–Condon factors, taking into account distortion, displacement, and normal mode mixing (up to four modes). Major features of the observed absorption spectrum of ethylene have been interpreted on the basis of the computed Franck–Condon factors. The role of each electronic state in the spectra has been clarified; the π–3s transition corresponds to the distinct intensive peaks in the 57 000–61 000 cm−1 energy region, the less intensive distinct bands in the interval of 62 000–65 000 cm−1 are due to the π–3pσ states and the π–π* peaks constitute the continuum underlying the spectrum. The theoretical vibronic spectrum is in qualitative agreement with the experimental one, except of some details. Possible reasons for the discrepancies between theory and experiment are also discussed.
Bending dynamics from acetylene spectra: Normal, local, and precessional modes
Authors: Rose John P. , Kellman Michael E.
Published: July 2002

The bending dynamics of acetylene are analyzed starting from spectroscopic fitting Hamiltonians used to fit experimental spectra. The possibility is considered of a transformation in the dynamics from normal to local bending modes, as well as a new kind of correlated bending motion called precessional modes. The spectroscopic fitting Hamiltonian of C2H2 is discussed with particular attention to the coupling interactions present due to Fermi and Darling–Dennison resonances. It is argued that for analysis of experiments in which the energy is initially placed in the bends, many couplings can be neglected. Of the remaining couplings, that responsible for the primary pathway of energy transfer out of the bends is a single Darling–Dennison coupling between the bends. A Hamiltonian containing this coupling alone is analyzed to isolate the bending dynamics involved in the primary energy transfer pathway. The anharmonic modes born in bifurcations from the low-energy normal modes are determined from analysis of the classical form of the Hamiltonian. In addition to the usual normal modes, local and precessional modes are found. Precessional modes have relative phases of π/2 or 3π/2, with one local bend fully extended while the other has maximal velocity. Sets of levels or ‘‘polyads’’ with the same total number of bend quanta are plotted in phase space on the polyad phase sphere, allowing a determination of the normal, local, or precessional character of a given quantum state. It is determined that local modes are found in the experimentally observed bend polyads with P≥14, and precessional modes are found in the polyads P≥20. Polyads are classified on the molecular catastrophe map according to their structure of normal, local, and precessional modes. Energy level spacing patterns within a polyad, shown previously to be characteristic of phase space bifurcation structure, are determined and correlated with the phase sphere. A diabatic correlation diagram analysis, previously applied to H2O, is suggested to extend the analysis here of normal, local, and precessional bending states to the full multiresonance, chaotic spectral fitting Hamiltonian.
Study on ‘‘regularity’’ of barrier recrossing motion
Authors: Komatsuzaki Tamiki , Nagaoka Masataka
Published: July 2002

A method to scrutinize ‘‘regularity’’ of barrier recrossing dynamics of chemical reactions in the vicinity of the transition state is developed by using Lie canonical perturbation theory (LCPT). As an example, the recrossing dynamics of a four-degrees of freedom Hamiltonian regarded as a model of proton transfer reaction of malonaldehyde is investigated. It is shown that the second order LCPT is essential to describe frequent saddle recrossings whose total number of crossings is greater than three, and reproduces the time-dependent transmission coefficient. It is found that the local recrossing dynamics can be regarded as quasiperiodic and a well-defined reaction coordinate along which no barrier recrossings occur, can be extracted in the phase space by using the second order LCPT Hamiltonian. We also formulate a new transition state theory which allows us to estimate the reaction rate constant taking account of the barrier recrossing effect if the recrossings are near-integrable in the short time but long enough to determine the final state of the recrossing dynamics.
An electron spin resonance investigation of vanadium dioxide (51V16O2 and 51V17O2) and 51V17O in neon matrices with preliminary assignments for VO3 and V+2: Comparison with ab initio theoretical calculations
Authors: Knight Lon B. , Babb Robert , Ray Matthew , Banisaukas T. J. , Russon Larry , Dailey Robert S. , Davidson E. R.
Published: July 2002

The first spectroscopic characterization of the VO2 radical is reported along with new results for V17O and tentative assignments for the VO3 and V+2 radicals. These vanadium radicals were investigated in neon matrices at 4 K by electron spin resonance utilizing conventional high temperature vaporization and pulsed laser ablation generation methods. A detailed ESR study of VO2 showed it to be nonlinear with a 2A1 ground state; the g tensor analysis reveals the presence of an excited electronic state (2B1) approximately 1 eV above the ground state. This excited state prediction and the observed nuclear hyperfine interactions (A tensors) for 51V and 17O were compared with theoretical results obtained from various ab initio computational methods. Ab initio calculations with an extended basis set were performed at various levels of theory including UHF, ROHF, CAS-SCF, and MR-SDCI (multireference single and double configuration interaction). While UHF calculations of the hyperfine interaction were grossly in error, the better levels of theory gave qualitative agreement with experiment and provided an aid to interpretation. VO2 is predicted to be a bent 2A1 state, correlating with the linear 2Δ configuration having the odd electron predominantly in the V 3d orbital. VO3 is predicted to be planar C2v, with the odd electron in a b2 orbital localized in the oxygen in-plane n-type p orbitals.
Heterogeneous and homogeneous dynamics in a simulated polymer melt: Analysis of multi-time correlation functions
Authors: Heuer A. , Okun K.
Published: July 2002

In general, nonexponential relaxation can result either from a heterogeneous (superposition of different exponential processes) or a homogeneous scenario (identical intrinsically nonexponential processes). In the present paper for the first time a strict definition of both scenarios is formulated. A procedure is presented which allows to estimate the heterogeneous and homogeneous contributions to the nonexponentiality, i.e., the type of relaxation, from comparing a two-time and a three-time correlation function. On this basis the type of relaxation is calculated analytically for the Rouse model. The relaxation of the Rouse model turns out to be mainly homogeneous. Furthermore, the type of relaxation is calculated for the dynamics of a simulated polymer melt above the glass transition. The polymer model used in this work is the well-known bond fluctuation model. For large lengthscales and high temperatures the relaxation of the polymer melt is mainly homogeneous in quantitative agreement with the Rouse model. However, for short length scales and/or low temperatures the dynamics of the polymer melt contains significant heterogeneous contributions. This is a direct indication of the relevance of local intermolecular interactions for the polymer dynamics. Furthermore the fluctuations within the heterogeneous rate distribution are analyzed. The time scale of fluctuations turns out to be of the same order as the relaxation process itself.
Photoionization spectroscopy of ionic metal dimers: LiCu and LiAg
Authors: Brock L. R. , Knight A. M. , Reddic J. E. , Pilgrim J. S. , Duncan M. A.
Published: July 2002

Electronic spectra are reported for the heteronuclear metal dimers LiCu and LiAg, with resonant one-color two-photon ionization (R2PI). The dimers are produced in a pulsed supersonic molecular beam by laser vaporization of either a copper or silver rod coated with a thin film of vacuum deposited lithium metal. A total of twelve excited electronic states for LiCu and seven for LiAg are observed. Analysis of the vibrational progressions yields ground and excited state vibrational frequencies and dissociation energies for both LiCu and LiAg. In addition, selected vibronic bands are rotationally resolved. This data, together with that obtained by Morse and co-workers for LiCu [J. Chem. Phys. (to be published)], gives bond lengths for LiCu and LiAg (r0″=2.26 and 2.41 Å, respectively). The bond lengths for LiCu and LiAg are significantly shorter than expected by comparison to the homonuclear diatomics Li2 and Cu2 or Ag2. Dissociation energies in the heteronuclear dimers are also much greater than the mean of the corresponding homonuclear dimer values. These trends indicate that ionic character plays a leading role in the ground-state bonding.
Reinvestigation of the HSiCl electronic spectrum: Experimental reevaluation of the geometry, rotational constants, and vibrational frequencies
Authors: Harper Warren W. , Clouthier Dennis J.
Published: July 2002

The ground and first excited singlet states of monochlorosilylene have been reexamined using pulsed discharge jet and laser induced fluorescence techniques. HSiCl and DSiCl have been produced by an electric discharge through SiHCl3 and SiDCl3 vapor in argon. The 000 band rotational constants of four isotopomers of HSiCl and the harmonic force fields for both states have been combined to obtain the following estimates of the equilibrium structures: r′′(SiCl)=2.067(3) Å, r′′(SiH)=1.525(5) Å, θ′′(HSiCl)=96.9(5)°, r(SiCl)=2.040(3) Å, r(SiH)=1.532(8) Å, and θ(HSiCl)=118.1(5)°. Previous anomalies in the ground-state structure and the excited-state vibrational frequencies have been resolved. The radiative lifetime of the 480–400 nm (Ã 1A′′–X̃ 1A) band system of HSiCl has been measured to be 432±20 ns.
Repulsive interactions of potassium on Re(001)
Authors: Verhoef R. W. , Zhao W. , Asscher M.
Published: July 2002

Repulsive interactions of potassium on Re(001) were investigated using temperature programed desorption (TPD), work-function measurements, and optical second-harmonic generation (SHG). The TPD and work-function results were used for the first time to evaluate critically the validity of available electrostatic models for explaining simultaneously both sets of data. The activation energy for desorption in the limit of zero coverage was determined to be 68.4±0.1 kcal/mol, decreasing monotonically with increasing potassium coverage to 22.1±0.1 kcal/mol at a full monolayer (ML) coverage. The TPD data were fit well by a depolarization model which includes the adsorbate–surface distance. The work function decreases monotonically upon potassium adsorption until reaching a minimum value of 4.35 eV below the work function of the bare rhenium surface at a coverage of approximately 0.5 ML, after which the work function increased to an ultimate value of 3.00 eV below the work function of Re(001). The work function of the bare Re(001) surface was determined to be 5.26±0.05 eV. The work-function data were fit using a model which considers the change in work function assuming local contributions by the substrate and by the adsorbate, using the same parameters obtained from fitting the TPD data. SHG measurements were consistent with previous alkali–transition metal systems, including a characteristic resonantlike signal enhancement at a potassium coverage of 0.3–0.4 ML. The SHG data taken during potassium adsorption and desorption complement information obtained by TPD and work-function measurements.
Fluctuations and growth of a cubic network observed in a nonionic surfactant system
Authors: Imai Masayuki , Kato Tadashi , Schneider Dieter
Published: July 2002

We have investigated stability of a cubic lattice having a periodic-minimal surface in a nonionic surfactant system using a time resolved small-angle neutron scattering technique. The cubic lattice transformed from a lamellar phase showed polycrystalline scattering patterns and a strong fluctuation of the diffraction intensity as a function of time. The intensity distribution analysis indicates that the domain coarsening proceeds until 2000 min and then the coarsening is suppressed. At a critical temperature, such fluctuating peaks suddenly grow to a monocrystalline pattern showing no intensity fluctuations. On the contrary, in the case of the cubic lattice transformed from a hexagonal phase, the polycrystalline diffraction peaks also showed fluctuating behavior, but in this case, the domain coarsening is suppressed and the scattering intensity fluctuates around an equilibrium value. We consider that the fluctuations arise from the polycrystalline cubic network trapped in a metastable state.
The gas-phase infrared spectra of anthracene-h10 and anthracene-d10
Authors: Cané Elisabetta , Miani Andrea , Palmieri Paolo , Tarroni Riccardo , Trombetti Agostino
Published: July 2002

The IR spectra of anthracene-h10 and -d10 have been recorded for the first time in the gas phase from 450 to 3200 cm−1 with a resolution of 0.2 cm−1, using a multipass cell heated to 100°C. For the assignment of vibrational bands we have evaluated the theoretical spectrum using density functional theory (DFT) and scaled self consistent field force fields. We found that both methods reproduce the sequence of the experimental frequencies to a good accuracy, allowing in most cases consistent and unambiguous assignments. The relative intensities of C14H10 and C14D10 have also been measured and compared to theory.
Improving harmonic vibrational frequencies calculations in density functional theory
Authors: Stratmann R. Eric , Burant John C. , Scuseria Gustavo E. , Frisch Michael J.
Published: July 2002

Using a previously introduced weight scheme, microbatching, and grid compression [R. E. Stratmann, G. E. Scuseria and M. J. Frisch, Chem. Phys. Lett. 257, 213 (1996)], we significantly speed up the numerical integration of the exchange-correlation contribution to the Coupled-Perturbed Kohn–Sham equations. In addition, we find that the nature of the integrand is such that it is possible to employ substantially fewer grid points in the quadrature and to use the Gaussian very Fast Multipole Method (GvFMM) with very short multipole expansions for the Coulomb contribution, with negligible loss in accuracy. As a representative example, the computational demand for the exchange-correlation portion of a coronene (C24H12) frequency calculation with a 3-21G basis is reduced by more than one order of magnitude. The overall speed up achieved in this calculation is between a factor of 4 to 6, depending on the specific functional. We also present sample calculations using polarized bases, gradient-corrected functionals, and on even larger systems (C54H18 and C96H24), to illustrate the various effects and improvements that we have accomplished.
Determination of the active space in molecular dynamics by a time-dependent wave operator method
Authors: Jolicard Georges , Tuckey Philip A. , Billing Gert Due
Published: July 2002

The time-dependent wave operator formalism is used to describe the quantum dynamics of molecular systems. It is shown that the wave function, if correctly normalized at each moment of time, is the solution of an “instantaneous eigenvalue equation.” This result is verified in the case of an harmonic oscillator with a linear perturbation. It is further shown that the resulting time-dependent “eigenvalue” can be used to obtain the eigenvalues of the Floquet eigenvectors participating in the dynamics, and constituting the target space of the system. This is illustrated by a numerical example, concerning the photodissociation of the H2+ ion in a continuous electromagnetic field.
Photodissociation of HCl at 193.3 nm: Spin–orbit branching ratio
Authors: Zhang J. , Dulligan M. , Wittig C.
Published: July 2002

HCl was photodissociated by ultraviolet (uv) radiation at 193.3 nm. Time-of-flight spectra of the hydrogen atom fragment provided the spin–orbit state distribution of the chlorine fragment, [Cl(2P1/2)]/[Cl(2P3/2)]=0.69±0.02, in excellent agreement with recent theoretical studies. The H atom angular distribution studied by changing the uv photolysis laser polarization confirmed a dominant A 1Π←X 1Σ+ electronic transition in the photoexcitation process (β=−1.01±0.04 and β*=−0.94±0.07).
Spin relaxation by diffusion on biaxial rods
Authors: Gustafsson Stefan , Halle Bertil
Published: July 2002

A theoretical framework is presented for the analysis of orientation and frequency dependent spin-relaxation rates from nuclei diffusing on biaxial rodlike aggregates in rectangular and other amphiphilic liquid crystals. With the aid of eigenfunction expansion and integral transform techniques, the problem of calculating the spectral density functions for surface diffusion on a biaxial rod can be reduced to quadrature, without the need to specify the rod geometry. The general results are applied to ribbonlike and elliptic rods, yielding analytical results. The calculations demonstrate that spin relaxation is a considerably more sensitive probe of interface curvature than the static line shape. The relaxation anisotropy, in particular, can discriminate between closely similar rod geometries with different curvature distributions.
Correlation potentials and functionals in Hartree-Fock-Kohn-Sham theory
Authors: Chan Garnet K-L. , Tozer David J. , Handy Nicholas C.
Published: July 2002

We compute molecular Hartree-Fock-Kohn-Sham correlation potentials from ab initio coupled-cluster densities via a modified Zhao, Morrison and Parr [Phys. Rev. A, 50, (1994) 2138] scheme involving exact exchange. We examine the potential for several small systems, and observe complex structure. By fitting a functional expansion to our potentials we obtain a closed-shell functional which is an improvement over other pure correlation functionals in Hartree-Fock-Kohn-Sham calculations. The leading term in our functional is dependent on the number of electrons. Our results lead us to question the utility of correlation defined within the Hartree-Fock-Kohn-Sham scheme, and to consider alternative partitionings of the exchange-correlation energy.
Equilibrium and non-equilibrium phase transitions in copolymer polyelectrolyte hydrogels
Authors: English Anthony E. , Tanaka Toyoichi , Edelman Elazer R.
Published: July 2002

A continuum model of polyelectrolyte hydrogel phase transitions has been developed and used to analyze the equilibrium and non-equilibrium swelling transitions observed in copolymer N-isopropylacrylamide (NIPA), acrylic acid (AAc), and acrylamide (AAm) hydrogels. The contribution of the dominant forces to the overall swelling response has been illustrated experimentally by systematically varying both the hydrogel and bath composition. We have interpreted the equilibrium swelling pattern theoretically by separately examining the role of charge, acid dissociation, and solvent mediated effects. The non-equilibrium swelling response to changes in bath ionic strength has been examined in terms of competing coupled ion, diffusion reaction, polymer network mechanisms, and the potential role of film boundary effects. We have obtained qualitative agreement between our theory and measurements and have critically examined both the strengths and limitations of our model.
Some properties of the lower electronic states for nonlinear He3+ clusters
Authors: Gianturco F. A. , de Lara-Castells M. P. , Schneider F.
Published: July 2002

Accurate, highly correlated calculations have been carried out for the ground electronic state and for a few of the lower excited electronic states, two of which are discussed in this work, of the trimer ionic helium cluster. Both linear asymmetric and nonlinear, symmetric, and asymmetric, configurations have been considered over a rather broad range of nuclear geometries. The results confirm the experimentally found [Chem. Phys. 102, 2773 (1995)] fragmentation patterns involving He and He+ fragments only and further suggest a range of possible nuclear geometries from which nonadiabatic couplings could also lead to He2+ fragments, albeit with a lower probability than the former channels.
Effect of degree of cross-linking on spatial inhomogeneity in charged gels. I. Theoretical predictions and light scattering study
Authors: Shibayama Mitsuhiro , Ikkai Fumiyoshi , Shiwa Yasuhiro , Rabin Yitzhak
Published: July 2002

Analysis of the Rabin–Panyukov theory of scattering from polyelectrolyte gels shows that the theory predicts an anomalous cross-link density (CD) dependence of amplitude of spatial inhomogeneity in charged polymer networks. By increasing the interaction parameter or by decreasing the degree of ionization, one can pass from a regime in which the calculated structure factor is a monotonically increasing function of CD, to one in which an inflection in the CD dependence is predicted. These theoretical predictions were experimentally verified by light scattering experiments on a series of weakly charged polymer gels of N-isopropylacrylamide-acrylic acid copolymer. The ensemble average scattered intensity, a measure of the spatial inhomogeneity, increased with CD for χ<χc and decreased with CD for χ>χc, where χ and χc are the interaction parameter and the critical value of χ, respectively.
Phase diagrams of a binary smectic-A mixture
Authors: Chiu Hao-Wen , Kyu Thein
Published: July 2002

A variety of smectic phase diagrams involving smectic-isotropic and smectic-nematic-isotropic transitions have been calculated based on a combination of Flory–Huggins (FH) theory for isotropic mixing and Maier–Saupe–McMillan (MSM) theory for smectic-A ordering of liquid crystals (LC). To describe the mesophase transitions, two nematic order parameters and two smectic order parameters have been coupled through the normalized orientation distribution and partition functions. Flory–Huggins interaction parameter (χ) for isotropic mixing and the coupling term involving the nematic interaction parameter (ν) and the McMillan smectic interaction parameter (α) for phase transitions of liquid crystals have been incorporated in the calculation. The predictive capability of the present combined FH/MSM model for determining the coexistence regions of a binary smectic-A mixture has been demonstrated by critically testing with a reported smectic phase diagram.
High-resolution laser spectroscopy of methane clusters trapped in solid parahydrogen
Authors: Momose Takamasa , Katsuki Hiroyuki , Hoshina Hiromichi , Sogoshi Norihito , Wakabayashi Tomonari , Shida Tadamasa
Published: July 2002

Clusters of methane are isolated in solid parahydrogen. The vibrational spectral region of the ν4 fundamental of methane molecule is surveyed with a Fourier transform infrared spectrometer and a high-resolution difference-frequency infrared laser system. More than 200 sharp absorption lines are discovered whose linewidth is as narrow as 0.007 cm−1 (200 MHz). The spectrum indicates that the rovibrational levels of the clusters of small sizes are well quantized in solid parahydrogen.