bluuebunny/crossref_metadata_2025 huggingface dataset:
Anisotropic photoconduction in novel photoconductive (oligo phenylene sulfide)–polyimide films cast on a Langmuir–Blodgett polyimide underlayer
Authors: Takimoto Akio , Suzuki Masa-aki , Wakemoto Hirofumi , Osazki Yusuke , Ogawa Hisahito
Published: July 2002

Two methods for preparing highly oriented crystalline polyimide films comprised of oligo-p-phenylene sulfide [–(C6H5S)n−1–C6H4–:n=3,4,5...] monomers have been developed, which show novel photoconductivity. The first method is using modified terminals of the polymer chains for carboxylic anhydrides as crystallization nuclei in order to enhance the crystallization of the (oligo phenylene sulfide)–polyimides (OPS-PIs). The second uses a Langmuir–Blodgett polyimide underlayer to prepare oriented OPS-PI films parallel to the substrate, extending up to several microns. The Langmuir–Blodgett polyimide underlayer introduces an oriented epitaxial crystallization of the OPS-PIs. Anisotropic photocurrent measurements on these films revealed an anisotropic photoconduction mechanism in which carriers were transported perpendicular to the polymer chain axis.
The accurate determination of spectroscopic constants for a dark vibrational state: The Coriolis coupled ν4 and ν6 bands of D11BF2
Authors: Lewis-Bevan Wyn , Stork Wilmer D.
Published: July 2002

The high resolution infrared spectrum of deuterated difluoroborane, DBF2, has been recorded from 700 to 912 cm−1. At low resolution, the only observable bands in this region belong to the c-type, out-of-plane deuterium motion, ν4, of the two B isotopomers. Upon analysis of the ν4 fundamental in D11BF2, a large perturbation was observed due to a ΔK=±1 Coriolis interaction with a dark vibrational state. The dark state was presumed to be the unseen, a-type, in-plane D–B rocking fundamental, ν6. A novel method of upper state energy level interlacing was used to determine A, B+C and T0 for the dark excited state, 61, the band origin, rotational and centrifugal distortion constants for the 41 state, and ξ4,6b (≊2ζ4,6bB). Based upon these constants, ν6 transitions were predicted and located in the region of the perturbation. Experimentally determined Coriolis, rotational, and centrifugal distortion constants are reported. The experimental results are compared with the results from ab initio calculations.
Optical orientation effects on the atom-dimer equilibrium composition of lithium vapor
Authors: He Chun , Bernheim R. A.
Published: July 2002

A shift in the equilibrium distribution between reactants and products in the vapor phase chemical reaction 6Li+7Li=6Li 7Li is caused by the optical orientation of atomic Li spin angular momenta and is shown to be controlled by the degree of atomic orientation. The observations were carried out in a heat-pipe cell operated at 631 K and containing a He buffer gas density of 3.1×1018 cm−3. The shift is similar to that measured for the ortho molecular components in the 2 6Li=6Li2 and 2 7Li=7Li2 gas phase equilibrium reactions.
A perturbation density functional theory for polyatomic fluids. II. Flexible molecules
Authors: Kierlik E. , Rosinberg M. L.
Published: July 2002

The density-functional theory for polyatomic fluids presented earlier [J. Chem. Phys. 97, 9222 (1992)] on the basis of Wertheim’s thermodynamic perturbation theory of polymerization is extended to the case of nonrigid molecules. In this formulation, the free energy in the limit of complete association is expressed as a functional of the full molecular distribution function and the excess contribution is obtained perturbatively from the excess free energy and the structural properties of the reference monomeric fluid at the same density. This defines a set of approximations which can be also related to the general formalism developed by Chandler and Pratt. As a first application, the intermolecular site–site correlations of homonuclear chainlike hard-sphere molecules are calculated within first-order perturbation theory (TPT1). Qualitative agreement with simulation data is found for short chains at high densities but the theory fails for long chains and low densities because of the neglect of the intramolecular excluded volume.
Molecular packing in Langmuir Blodgett films by core level loss spectroscopy
Authors: Sastry Murali , Pal Sipra , Paranjape D. V. , Rajagopal Aparna , Adhi Shubhada , Kulkarni S. K.
Published: July 2002

Molecular packing in lead arachidate [(C19H39COO)2Pb] Langmuir Blodgett monolayer films transferred to glass substrates has been investigated using x-ray photoemission spectroscopy. It was observed that the degree of salt formation was a sensitive function of the subphase pH reaching a maximum at a pH of nearly 5.8. The loss features in the C 1s core level spectra of the films were clearly defined for films consisting of purely metal salt of arachidic acid while they became progressively diffused as the subphase pH was reduced. The blurring of the C 1s loss features has been explained as arising due to disorder introduced into the alkyl chains of lead arachidate molecules by the evaporation of acid molecules under ultrahigh vacuum. This leads to changes in the quasi one-dimensional electronic band structure in such molecular crystal films. A comparison is made with results obtained on cadmium arachidate and arachidic acid monolayer Langmuir Blodgett films. It is thus shown that molecular packing in Langmuir Blodgett films may be investigated using core level loss spectroscopy.
Orientational disorder modeling in mixed-stack molecular crystals
Authors: Rohleder Krzysztof , Luty Tadeusz , Kuchta Bogdan
Published: July 2002

The paper presents a simple model to mimic orientational disorder in molecular solids. The model is based on the concept of split molecule, an object which is a superposition of two molecules in orientations which are supposed to mimic orientational distribution. The split molecule is characterized by a disorder angle, an extra parameter which labels disorder states and allows one to calculate free energy of the crystal. The model is simple for applications illustrated by numerical calculations for charge-transfer crystals A-TCNB and N-TCNB, the family of solids where the onset of orientational disorder is responsible for structural phase transitions.
A method of generating an effective orbital set for configuration interaction calculations
Authors: Huzinaga S. , Miyoshi E. , Sekiya M.
Published: July 2002

It is proposed that early stages of multiconfiguration self-consistent field (MCSCF) type calculations can be exploited to generate good orbital sets for use in the general configuration interaction (CI) calculation.
Electronic distortion in keV particle bombardment
Authors: Bhatia Reena , Garrison Barbara J.
Published: July 2002

The angle resolved velocity distributions of excited (4F7/2) and ground state (4F9/2) Rh atoms ejected from the Rh {100} surface due to keV Ar+ ion bombardment are described with a model that takes into account the local electronic environment. The lifetime of the excitation probability for each excited Rh atom is assumed to depend on the local embedded-atom method (EAM) density. It is thus possible to distinguish between ejected atoms that experience very little difference in their electronic environments. Although most excited atoms that survive with significantly high excitation probabilities originate from the surface layer, it is not uncommon for an atom beneath the surface to eject from a disrupted environment and end up with a high excitation probability. This model improves upon a previous one, where the lifetime was assumed to vary with the height above the original surface.
Monte Carlo simulation of the CaF2
Authors: Montani Ruben A.
Published: July 2002

A Monte Carlo simulation of CaF2 in the canonical ensemble was carried out using the Metropolis algorithm and the Soft-Core and Born–Mayer–Huggins potentials. By studying the behavior of the mean square displacement and the specific heat, the existence of a transition to a superionic phase at about 1470 K was confirmed. A partial occupation of the interstitial sites in this superionic phase was also found.
Exploiting the properties of line-shape singularities in orientationally selected electron spin echo envelope modulation spectra of Cu2+−(15N-imidazole)4 for the determination of hyperfine coupling with the remote imidazole nitrogen
Authors: Dikanov Sergei A. , Spoyalov Andrei P. , Hüttermann Jürgen
Published: July 2002

Orientationally selected two-, three-, and four-pulse electron spin echo envelope modulation (ESEEM) spectra from remote 15N nuclei in a Cu2+-(15N–imidazole)4 complex have been obtained in the microwave frequency region 9.12–9.82 GHz. This range of frequencies has allowed us to reach the conditions corresponding to S and ν⊥ singularities of the lineform of basic transitions. The development of these lineform singularities in orientationally selected spectra is discussed. The knowledge of the magnetic fields corresponding to the singularities was used to find the hyperfine coupling parameters for the remote nitrogen as a=2.44 MHz and T⊥=−0.39 MHz as well as to determine the angle between the axis of the complex and the direction of the largest principal hyperfine tensor element as 40°. Additional geometrical information about the ligand orientation has been obtained from the analysis of shifts of the sum combination harmonic in proton four-pulse ESEEM spectra.
Observation of liquidlike correlations of polyelectrolytes under high-shear conditions
Authors: Reed Wayne F.
Published: July 2002

Angular static light scattering peaks were observed for a polyelectrolyte (Proteoglycan subunit) under elution in pure water in a size exclusion chromatography (SEC) system. The shear rates involves in the SEC experiments were fairly high. The peaks were quite comparable to those found under stationary (non-SEC) conditions, adding strength to the notion that dynamic, liquidlike correlations are the origin of such peaks observed for simple polyelectrolytes at low ionic strength.
Comment on ‘‘Molecular dynamics of the surface tension of a drop’’ [J. Chem. Phys. 9
6, 565 (1992)]
Authors: Brodskaya E. N. , Rusanov A. I.
Published: July 2002

The procedure of the estimation of Tolman’s length δ for small droplets based only on the normal component of the local pressure tensor is proposed. The treatment of the computer simulation data with the help of this method results in the definite positive value of δ for the Lennard-Jones droplets.
Structural optimization in variational quantum Monte Carlo
Authors: Tanaka Shigenori
Published: July 2002

A new computational scheme to simultaneously optimize the electronic and ionic configurations in solids and molecules is presented in the framework of variational quantum Monte Carlo method. This scheme, in which a fictitious Lagrangian to describe the dynamics of electronic variational parameters and ionic coordinates is introduced, is formulated virtually in parallel with the Car–Parrinello method for density-functional theory. The feasibility and usefulness of the proposed scheme are demonstrated by carrying out a structural optimization for water molecule with the aid of steepest-descents technique.
Intermolecular energy-band dispersion in oriented thin films of bis(1,2,5-thiadiazolo)-p-quinobis(1,3-dithiole) by angle-resolved photoemission
Authors: Hasegawa Shinji , Mori Takehiko , Imaeda Kenichi , Tanaka Shoji , Yamashita Yoshiro , Inokuchi Hiroo , Fujimoto Hitoshi , Seki Kazuhiko , Ueno Nobuo
Published: July 2002

Angle-resolved ultraviolet photoemission spectra using synchrotron radiation were measured for oriented thin films of bis(1,2,5-thiadiazolo)-p-quinobis(1,3-dithiole) (BTQBT) on graphite. From the photon energy dependence of normal emission spectra, the energy-band dispersion of π-bands were observed for the highest (HOMO) and next highest (NHOMO) bands. This is the first observation of intermolecular dispersion in a single-component organic molecular crystal. The results demonstrate that the BTQBT molecules have a strong intermolecular interaction, which can be derived from the introduction of a covalent interaction between sulfur atoms in addition to the usual intermolecular interaction by van der Waals forces.
Inelastic electron scattering on C60 clusters
Authors: Yabana K. , Bertsch G. F.
Published: July 2002

We calculate the electronic excitation of C60 by inelastic electron scattering or electron energy loss spectroscopy (EELS). The scattering process is treated in the distorted-wave Born approximation, and the electronic excitations are calculated in a spherical basis model. We find that low energy electrons excite some nonphotoactive modes, in agreement with experiment. Spin triplet modes are poorly excited, even at the lowest electron energies.
Size-corrected theory of homogeneous nucleation
Authors: Zhukhovitskii D. I.
Published: July 2002

A size-corrected thermodynamic model for clusters is proposed which implies that cluster chemical potential is a linear function of the number of molecules at its surface and the total number of molecules. The model is free from the introduction of the surface tension for small clusters and therefore has no limitation in this size range. The expressions for homogeneous nucleation rates have been obtained in the approximations of small and large critical sizes. Calculated nucleation rates are slightly lower than predicted by the classical theory for water vapor and much higher for mercury vapor, in agreement with the experiments.
Microwave spectrum of the HNCN radical in the X̃ 2A″ ground electronic state
Authors: Yamamoto Satoshi , Saito Shuji
Published: July 2002

The rotational spectral lines of the HNCN radical are observed in the millimeter-wave and submillimeter-wave regions. The radical is produced in a glow-discharge plasma of a gaseous mixture of CH4 and N2 at the room temperature. Rotational constants, centrifugal distortion constants, and spin–rotation interaction constants with their centrifugal distortion corrections are precisely determined from observed frequencies of 43 a-type R branch transitions with N=12−11 to N=18−17 and Ka=0 to Ka=3. From the observed spin–rotation interaction constant, εaa, the energy of the Ã 2A′ electronic state is estimated to be 12 000 cm−1, which is comparable to the energy of the corresponding electronic state (Ã 2A1) for the NH2 radical. A preliminary radioastronomical search for the HNCN radical is carried out toward the Galactic center without success.
Existence of a density maximum in extended simple point charge water
Authors: Báez Luis A. , Clancy Paulette
Published: July 2002

Simulations of water using the exnteded simple point charge (SPC/E) model at temperatures between 190 and 330 K were performed using molecular dynamics techniques. A maximum in the density at 1 bar pressure was found to occur at 235 K. The energies and diffusivities are also reported. The SPC/E-modeled water exhibits a glass transition ∼177 K. No crystallization events were observed during the course of the long simulations.
Dissociation dynamics of diatomic molecules embedded in impact heated rare gas clusters
Authors: Raz Tamar , Schek Israel , Ben-Nun M. , Even Uzi , Jortner Joshua , Levine R. D.
Published: July 2002

Molecular dynamics simulations demonstrate facile dissociation of halogen molecules embedded in rare gas clusters upon impact at a surface at collision velocities up to 10 km/s. Two pathways are discerned: a heterogeneous dissociation of the molecule on the surface and a homogeneous mechanism where rare gas atoms which have rebounded from the surface cause the translational–vibrational coupling. The total yield of dissociation of the clustered molecule can reach up to 100%, whereas the yield of dissociation of the bare, vibrationally cold molecule saturates below 40%. A systematic study of the role of different conditions is made possible by not accounting for the atomic structure of the surface. The role of dissipation at the surface is found, however, to be quite important and is allowed for. Larger clusters, clusters of the heavier rare gases and a more rigid surface, all favor the homogeneous mechanism. Evidence for a shock front which, upon the initial impact, propagates into the cluster; the binary nature of the homogeneous dissociation process; and the absence of a dominant cage effect are discussed. A quantitative functional form of the velocity dependence of the yield of dissociation, which accounts for the size of the cluster, the rigidity of the surface and other attributes, is used to represent the data. The physics of the processes within the cluster is dominated by the novel dynamical features made possible when the duration of the atom–molecule collisions is short compared to the vibrational period. This ‘‘sudden’’ regime is sudden with respect to all modes of the nuclear motion and provides a hitherto unavailable tool for examination of reaction dynamics under extreme conditions.
Walking on the free energy hypersurface of the 18-crown-6 ion system using free energy derivatives
Authors: Cieplak Piotr , Pearlman David A. , Kollman Peter A.
Published: July 2002

Molecular dynamics simulations have been used to calculate free energy derivatives with respect to nonbonded parameters for both ions and the host, 18-crown-6 in cation-18-crown-6 complexes. The free energy derivatives have been used to examine the free energy hypersurface for such complexes and to ‘‘predict’’ the optimal cationic ligand for 18-crown-6 complexation in aqueous solution. This method has promise to enable the refinement of the intermolecular interaction parameters for both hosts and guests in aqueous solution. In particular, we show that the optimal ion size for 18-crown-6 is calculated to be between the sizes of Na+ and K+, consistent with the experimental data. More generally, it has potential to be a powerful tool in computer aided molecular design.
Dissociation pathways and binding energies of lithium clusters from evaporation experiments
Authors: Bréchignac C. , Busch H. , Cahuzac Ph. , Leygnier J.
Published: July 2002

The unimolecular dissociation of energy rich lithium cluster ions shows that Li+n dissociate by sequential atom or dimer loss. The binding energies of Li+n (n=4–42) generated in an evaporative ensemble are determined from unimolecular decay, within a well defined time window, and energy constraint. They present a sawtooth behavior vs cluster size less pronounced that it should be from a simple metal model. Odd–even alternation is superimposed on the sawtooth behavior, with odd sized cluster ions being more stable. Cohesive energies per atom of Li+n are deduced from these dissociation energies up to n=40 and from extended photo-induced measurements up to n=95. Cohesive energies per atom of neutral clusters Lin are derived by combining these ionic cohesive energies with the literature ionization potentials. The linearity of the neutral cluster cohesive energy vs the cluster surface to volume ratio permits a volume and a surface energy to be deduced. These values are compared to the bulk values.
Auger electron–ion coincidence experiment on nitrogen molecule excited by electron impact
Authors: Fainelli Ettore , Maracci Francesco , Platania Rosario
Published: July 2002

The first Auger electron–ion coincidence experiment on the nitrogen molecule excited by electron impact is described. The kinetic energy releases of the N++ and the N+ fragments in the 43–72 eV binding energy range have been measured. The experimental results are compared with previous experimental data obtained by using synchrotron radiation and theoretical predictions.
A resonance enhanced multiphoton ionization study of the gerade excited states of Xe2 with a Xe 1S+Xe* 6s[3/2]1 dissociation limit
Authors: Koeckhoven S. M. , Buma W. J. , de Lange C. A.
Published: July 2002

Low-lying gerade electronically excited states of the Xe dimer, viz., 1g and 0+g, with a Xe 1S0+Xe* 6s[3/2]1 dissociation limit have been investigated using two-photon resonance enhanced multiphoton ionization techniques. High resolution is achieved by using this method in combination with a supersonic expansion. Moreover, the mass-resolved ion detection used in the present experiments has led to the observation of both vibrationally well-resolved spectra and continuous features, allowing the bound and dissociative parts of the excited states to be investigated separately. All spectra have been measured under both linearly and circularly polarized excitation conditions, enabling an unambiguous assignment of the signals. On the basis of Franck–Condon calculations information on the potential energy curves of the bound and dissociative parts of the excited states has been obtained.
Association reactions at low pressure. V. The CH+3/HCN system. A final word?
Authors: Anicich Vincent G. , Sen Atish D. , Huntress Wesley T. , McEwan Murray J.
Published: July 2002

The reaction of the methyl cation with hydrogen cyanide is revisited. We have confidence that we have resolved a long standing apparent contradiction of experimental results. A literature history is presented along with one new experiment and a re-examination of an old experiment. In this present work it is shown that all of the previous studies had made consistent observations. Yet, each of the previous studies failed to observe all of the information present. The methyl cation does react with HCN by radiative association, a fact which had been in doubt. The product ions formed in the two-body and three-body processes react differently with HCN. The collisionally stabilized association product formed by a three-body mechanism, does not react with HCN and is readily detected in the experiments. The radiatively stabilized association product, formed by a slow two-body reaction, is not detected because it reacts with HCN by a fast proton transfer reaction forming the protonated HCN ion. Previous studies either ‘‘lost’’ this product in the extremely large protonated HCN signal that is always present when HCN is used, or discounted it for various reasons. We have been able to show by ion cyclotron resonance (ICR) techniques (both FT-ICR and tandem ICR-dempster-ICR) that the radiative association product does react with the HCN to form the protonated HCN ion.
Benchmark calculations with correlated molecular wave functions. VII. Binding energy and structure of the HF dimer
Authors: Peterson Kirk A. , Dunning Thom H.
Published: July 2002

The hydrogen bond energy and geometry of the HF dimer have been investigated using the series of correlation consistent basis sets from aug-cc-pVDZ to aug-cc-pVQZ and several theoretical methods including Mo/ller–Plesset perturbation and coupled cluster theories. Estimates of the complete basis set (CBS) limit have been derived for the binding energy of (HF)2 at each level of theory by utilizing the regular convergence characteristics of the correlation consistent basis sets. CBS limit hydrogen bond energies of 3.72, 4.53, 4.55, and 4.60 kcal/mol are estimated at the SCF, MP2, MP4, and CCSD(T) levels of theory, respectively. CBS limits for the intermolecular F–F distance are estimated to be 2.82, 2.74, 2.73, and 2.73 Å, respectively, for the same correlation methods. The effects of basis set superposition error (BSSE) on both the binding energies and structures have also been investigated for each basis set using the standard function counterpoise (CP) method. While BSSE has a negligible effect on the intramolecular geometries, the CP-corrected F–F distance and binding energy differ significantly from the uncorrected values for the aug-cc-pVDZ basis set; these differences decrease regularly with increasing basis set size, yielding the same limits in the CBS limit. Best estimates for the equilibrium properties of the HF dimer from CCSD(T) calculations are De=4.60 kcal/mol, RFF=2.73 Å, r1=0.922 Å, r2=0.920 Å, Θ1=7°, and Θ2=111°.
Compatibilizing A/B blends with AB diblock copolymers: Effect of copolymer molecular weight
Authors: Israels Rafel , Jasnow David , Balazs Anna C. , Guo Lantao , Krausch Georg , Sokolov Jon , Rafailovich Miriam
Published: July 2002

We use numerical self-consistent field (SCF) calculations to determine the interfacial behavior of AB diblocks in a blend of immiscible homopolymers, A and B. In particular, we compare the compatibilizing effect of relatively short and long AB copolymers. In the calculations, we explicitly take into account the formation of micellar or multilamellar phases. The results show that the interfacial tension can be reduced to zero only if the blocks in the diblock are longer than the corresponding homopolymer. Our two-dimensional SCF calculations reveal that short diblocks form multilamellar structures in the blend, whereas a microemulsion is formed when relatively long copolymers are added to the A/B mixture. These observations are compared with experiments on blends of polystyrene (PS), polymethyl methacrylate (PMMA), and PS-PMMA symmetric diblock copolymers. By measuring the contact angle of PS droplets on the PMMA layer, we can obtain a direct estimate for the interfacial tension at the PS/PMMA interface. The experiments also reveal that negligible interfacial tensions and emulsionlike structures are only obtained with relatively long diblocks.
Vibrational predissociation of HF dimer in νHF=1: Influence of initially excited intermolecular vibrations on the fragmentation dynamics
Authors: von Dirke Michael , Bačić Zlatko , Zhang Dong H. , Zhang John Z. H.
Published: July 2002

Theoretical study of the influence of excited intermolecular vibrations on the total and partial decay widths of HF dimer is reported. Vibrational predissociation (VP) lifetimes and rotational state distributions of HF fragments were calculated for various quasibound states of (HF)2, corresponding to combinations of the intermolecular stretching (ν4) and bending (ν5) vibrations with the ‘‘free’’ (ν1) and ‘‘bonded’’ (ν2) HF stretch fundamentals, for total angular momentum J=1, K=0. The calculations were performed on an ab initio six-dimensional potential energy surface of Quack and Suhm, using a quantum four-dimensional golden rule methodology. The VP lifetimes and product rotational distributions exhibit pronounced dependence on the type of the initially excited intermolecular vibration of HF dimer. The energy deposited in the ν4 intermolecular stretch evolves into the translational energy of the fragments. Excitation of the ν5 intermolecular bending vibration, combined with the ν1 fundamental, is transferred to the product rotational energy. This is in good agreement with the experimental results of Bohac and Miller. We also found that in conjunction with the ν2 fundamental, most of the ν5 bending vibrational energy emerges in the translational energy of the products.
Coherent isotropic averaging in zero-field nuclear magnetic resonance. II. Cubic sequences and time-reversal of spin couplings
Authors: Llor A. , Olejniczak Z. , Pines A.
Published: July 2002

We present a special case of the theory of coherent isotropic averaging in zero-field NMR, given in part I of this work. In a zero external field, combinations of the magnetic-field pulses restricted to π/2 rotations along the three coordinate axes can selectively average internal spin Hamiltonians while preserving the intrinsic invariance of the spectrum with respect to the sample orientation. Compared with the general case, the limits of the allowed scaling factors of first- and second-rank interactions are slightly reduced. For instance, time reversal is possible for second-rank tensors with a −1/5 scaling factor, instead of −1/4 in general. Finite pulse compensations are analyzed and experimental illustrations are given for two optimum time-reversal sequences. The cubic sequences, though less efficient than the icosahedral sequences, are technically more feasible and may be used in zero-field experiments such as decoupling (by rank or nuclear species), time reversal or multipolar experiments (the zero-field equivalent of multiple-quantum NMR).
Nonperturbative approach to femtosecond spectroscopy: General theory and application to multidimensional nonadiabatic photoisomerization processes
Authors: Seidner Luis , Stock Gerhard , Domcke Wolfgang
Published: July 2002

A general nonperturbative approach to calculate femtosecond pump-probe (PP) signals is proposed, which treats both the intramolecular couplings and the field-matter interaction (numerically) exactly. Experimentally as well as in a perturbative calculation it is straightforward to distinguish between different spectroscopic processes through the direction of the wave vector of the emitted radiation. A nonperturbative calculation, on the other hand, yields the overall polarization of the system, which is the sum of all these contributions. We present a general and practical method that allows to extract the individual spectroscopic signals, which are resolved in time, frequency, and direction of the emission, from the overall polarization. We briefly derive the basic expressions for the time- and frequency-resolved PP signals under consideration, and discuss in detail the simplifications that arise when the usual assumptions (i.e., weak laser fields, nonoverlapping pulses, slowly-varying envelope assumption and rotating-wave approximation) are invoked. The computational procedure is illustrated by nonperturbative calculations of the polarizations and PP signals for a one-dimensional shifted harmonic oscillator. To demonstrate the capability of the approach we have evaluated the polarization as well as PP signals for a three-dimensional model system with vibronically coupled potential-energy surfaces, which describes ultrafast nonadiabatic isomerization dynamics triggered by the twisting of a double bond. We consider various wavelengths and pulse durations of the laser fields and study integral and dispersed PP spectra as well as coherent photon-echo signals. It is shown that the time- and frequency-resolved PP signals reflect in real time the disappearance of the reactants and the delayed appearance of the products.
Optical emission studies of laser desorbed C60
Authors: Mitzner Rolf , Campbell Eleanor E. B.
Published: July 2002

The optical emission spectra of laser desorbed C60 have been investigated as a function of laser fluence for desorption with a XeCl excimer laser (pulse length 20 ns). The observed spectra show close similarities to black-body radiation and can be fitted with the Planck black body formula (modified for small particles) thus giving information on the temperature of the desorbed species. The temperatures obtained (2300–3000 K) are in good qualitative agreement with previous, indirect temperature estimates. Spatially and temporally resolved measurements provide additional insight into the desorption mechanisms. An estimate of cooling rates indicates that thermionic electron emission and C2 fragmentation dominate for temperatures above about 3000 K but below this value the dominant cooling mechanism is black-body radiation.
Molecular dynamics simulations of ion clustering and conductivity in NaI/ether solutions. II. Effect of ion concentration
Authors: Payne Vilia Ann , Xu Jian-hua , Forsyth Maria , Ratner Mark A. , Shriver Duward F. , de Leeuw Simon W.
Published: July 2002

Molecular dynamics simulations of sodium iodide dissolved in dimethyl ether or 1,2-dimethoxyethane (glyme) were studied at a range of salt concentrations. The interactions among the species were represented with Lennard-Jones and Coulomb forces. Dimethyl ether and glyme were represented by a rigid three-site model and a six-site model with flexible dihedral angles, respectively. Glyme is demonstrated to be a much better solvent than dimethyl ether, although both are low-dielectric solvents. At the highest concentration studied in glyme, which corresponds to an oxygen/cation ratio of 16:1, free ions make up about 50% of the total ion concentration, and neutral pairs make up about 20%. A quantitative analysis of the species important in conductivity shows that the current is primarily the result of the movement of free ions and the relative movement of ions within loosely bound ion pairs. At higher salt concentrations, many different ionic species can make contributions to the conductivity.
Proximity coupling of boundary waves in liquids
Authors: Gerhard M. , Staude W.
Published: July 2002

We measured the distance-dependent coupling of transverse boundary waves in a system of layers of two immiscible liquids by laser light scattering. The results were explained on the basis of a theoretical treatment of the dynamics of this system. The deviations of the results for very small distances of the two interfaces could be accounted for by the assumption of a small surface viscoelasticity, which is about one order of magnitude smaller than those measured so far with light-scattering techniques. The technique used in this paper might therefore be well suited to study molecular interactions and relaxation processes at liquid boundaries, which are responsible for this viscoelasticity.
Effect of three-body interactions on the nematic ordering of a polymer solution
Authors: Kuznetsov Dmitri V.
Published: July 2002

Onsager-type (Khokhlov–Semenov) theory of the nematic ordering in an athermic system of long polymers, which accounts for two-body interactions only, is extended to take into account three-body interactions. The orientation distribution function, the free energy, the isotropic-nematic coexistence and the spinodal conditions are computed numerically for both persistent wormlike chains and chains with freely joint rigid segments. Even though second virial approximation is known to be asymptotically correct at large values of the polymer segment asymmetry l/d, three-body interactions are shown to cause the nontrivial dependence of the nematic ordering critical concentrations on l/d for the realistic values of l/d. Conditions of applicability of the second virial approximation are analyzed quantitatively.
Dependence of level-resolved energy transfer on initial vibrational level in Li2A1Σu+−Ne collisions
Authors: Gao Yunxiao , Gorgone Peter S. , Davis Scott , McCall Eric K. , Stewart Brian
Published: July 2002

We have investigated collision induced rotational and vibrational energy transfer in the Li2 A1Σu+(vi,ji=30)−Ne system experimentally under single-collision conditions at an effective temperature of 691 K. Over 800 inelastic rate constants have been measured, with the initial vibrational level vi ranging from 2 to 24 and −2≤Δv≤+2. Increasing vi results in a linear increase in the vibrational transition rate constants, which is accompanied by a decrease in the rotationally inelastic transition rate constant. The total inelastic rate constant increases with vi only at the highest values of vi. Net vibrational energy transfer 〈ΔE〉 calculated using rotationally summed rate constants is qualitatively consistent with a simple model. However, explicit inclusion of rotation gives quite different values of 〈ΔE〉. The experimental results are compared with our three-dimensional trajectory calculations on an ab initio potential surface and on a simple repulsive potential surface.
Thermally stimulated desorption of neutral CF3 from CF3I on Ag(111)
Authors: Junker K. H. , Sun Z.-J. , Scoggins T. B. , White J. M.
Published: July 2002

The low temperature thermal chemistry of CF3I on Ag (111) presents an example of competing reaction pathways; molecular desorption vs desorption of radical CF3. Temperature programmed desorption and angle resolved temperature programmed desorption, complemented with Auger electron spectroscopy and low energy electron diffraction, were used to discern the mechanism of the CF3 radical desorption channel. CF3 desorption is limited to the first monolayer of CF3I; 0.75 ML CF3I is the coverage used for angular dependence measurements. At 90 K most of the CF3I adsorbs molecularly to the metal, but also present under these conditions are dissociative adsorption and thermal decomposition channels limited to C–I bond cleavage. The decomposition product, CF3, desorbs as a radical at high temperatures (∼320 K) with the I remaining on the surface until 850 K. At submonolayer CF3I coverages, thermal activation produces a low temperature (100–150 K) radical desorption channel. Results indicate that low temperature CF3 thermal desorption occurs via dissociative electron attachment to molecular CF3I, yielding radical CF3 and adsorbed iodine.
Dynamic curve crossing of an atom interacting with a set of atoms in three dimensions
Authors: Cohen L. Kenneth
Published: July 2002

A general method is presented for computing the probability of an atomic curve crossing in three dimensions (3D). This method enables one to compute this probability totally dynamically, under conditions of random scattering with a cluster of atoms. A 3D Landau–Zener diabatic curve crossing probability calculation is shown how to be built into 3D atom-group of atoms trajectory dynamics calculations. The objective of this method is to mirror real dynamical systems as closely as possible by theory. The atom is assumed to be confined to the cluster of atoms, so that it makes a sequence of attempted crossovers until successfully crossing over, or until curve crossing is no longer physically possible. A set of reference trajectories covering the allowed curve crossing energy range of interest must be obtained. Subsequently, this reference set is used in a runoff procedure yielding the net bonding probability to a subset of the atoms of the cluster. The method is applied to the specific case where the atomic cluster is a solid. More specifically, the loss function of graphite due to 6.0 eV incident oxygen atoms is obtained.
Auger electron–ion coincidence experiment on nitric oxide molecule excited by electron impact
Authors: Fainelli Ettore , Maracci Francesco , Platania Rosario , Avaldi Lorenzo
Published: July 2002

The fragmentation of nitric oxide by electron impact is studied via Auger electron–ion coincidence experiments. The kinetic energy releases of the different fragments have been measured in the 39–70 eV binding energy region of the dication. The experimental data confirm that the three lower lying states of NO2+ are bound states, while all the other states in the studied region are repulsive ones. Experimental evidence is provided that all the repulsive states up to 54 eV binding energy dissociate to the lowest dissociation limit and that dissociative channels leading to the formation of N2+ and O2+ are populated at 62 eV binding energy. The experimental results are compared with previous experimental data obtained with different techniques and with the more recent theoretical predictions.
Chain model of electrorheology
Authors: Martin James E. , Anderson Robert A.
Published: July 2002

We develop a chain model of an electrorheological fluid in steady and oscillatory shear. This model is based on a balance of hydrodynamic and electrostatic forces and focuses on the mechanical stability of chains. With this model we compute the shape and orientation of an unconfined dipolar chain in steady shear as a function of the shear rate and electric field, and show that this leads to the expected shear-thinning viscosity. We then demonstrate that chains confined by electrodes are unstable in some Mason number regimes. The chain model is extended to the case of oscillatory shear by including a kinetic equation for the aggregation and fragmentation of chains. The resultant chain dynamics is found to be strongly nonlinear, as is the rheology. Finally, we consider the effect of local field corrections and long range interactions on the bare dipolar interaction force and derive a self-consistent pair interaction force that demonstrates that the maximum orientation of a chain in shear depends strongly on the dielectric contrast.
A Kohn–Sham study of CH4, C6H6, and O3 using functionals incorporating exact exchange
Authors: Tozer David J.
Published: July 2002

Kohn–Sham density functional theory calculations are presented for CH4, C6H6, and O3 using popular exchange-correlation functionals based on the adiabatic connection method (ACM). The structure and harmonic vibrational frequencies of CH4 and C6H6 are described very well with the ACM functional. In particular, a very accurate description of CH vibrations is obtained, suggesting a possible resolution of this well-known deficiency of conventional functionals. In contrast, the application of ACM functionals to the energetics, structure, and harmonic frequencies of O3 yields results which are rather poor compared to conventional functionals. These observations appear to be closely related to the performance of Hartree–Fock theory for these systems.
Solvation of complex molecules in a polar liquid: An integral equation theory
Authors: Beglov Dmitrii , Roux Benoît
Published: July 2002

A statistical mechanical integral equation theory is developed to describe the average structure of a polar liquid around a complex molecular solute of irregular shape. The integral equation is formulated in three-dimensional Cartesian coordinates from the hypernetted chain (HNC) equation for a solute at infinite dilution. The direct correlation function of the pure solvent used in the theory is taken from the analytical solution of the mean spherical approximation (MSA) equation for a liquid constituted of nonpolarizable hard spheres with an embedded dipole at their center. It is demonstrated explicitly that, in the limit where the size of the solvent particles becomes very small, the present theory reduces to the well-known equations for macroscopic electrostatics in which the solvent is represented in terms of a dielectric continuum. A linearized version of the integral equation corresponds to a three-dimensional extension of the familiar MSA equation. This 3D-MSA integral equation is illustrated with numerical applications to the case of an ion, a water molecule, and N-methylacetamide. The numerical solution, obtained on a discrete three-dimensional cubic grid in Cartesian coordinates, yields the average solvent density and polarization density at all the points x,y,z around the solute. All spatial convolutions appearing in the theory are calculated using three-dimensional numerical fast Fourier transforms (FFT).
Behavior of electronic wave functions near cusps
Authors: Rassolov Vitaly A. , Chipman Daniel M.
Published: July 2002

The behavior of a nonrelativistic many-electron wave function when one electron approaches either a nucleus or another electron is well known to be governed by a cusp condition. This is a consequence of the Coulomb singularity in the potential and provides a specific relation between the wave function and its first derivative at the point of coalescence. It is shown here that the coalescence behavior also uniquely determines the third derivative of the spherically averaged wave function in terms of the lower derivatives. The new relation is valid for any atom, molecule, or electron gas in any smooth external field. Further extensions are also discussed for electron–electron coalescence in electron gas systems and for electron–nucleus coalescence in multiconfigurational self-consistent-field wave functions.
Discharge flow-tube studies of O(3P)+N2H4 reaction: The rate coefficient values over the temperature range 252–423 K and the OH(X 2Π) product yield at 298 K
Authors: Vaghjiani Ghanshyam L.
Published: July 2002

The absolute second-order reaction rate coefficient, k1, for the gas phase reaction, O(3P)+N2H4→products, was studied in a discharge flow-tube apparatus. The reaction was studied under pseudo-first-order conditions in O(3P) concentration (i.e., [N2H4]≫[O(3P)]). The O atoms were generated by a microwave discharge of a suitable precursor gas in He in a fixed side-arm reactor upstream of the flow tube, or in the sliding inner injector of the flow tube. The hydrazine concentration was photometrically measured and introduced into the apparatus in a flow of He via the sliding injector or the fixed side-arm port, respectively. The kinetics of the O-atoms in the reaction was directly followed by 130.2–130.6 nm cw-resonance fluorescence detection of O(3P) at the fixed detector situated downstream of the flow tube. The Arrhenius expression, k1=(7.35±2.16)×10−13 exp[(640±60)/T] cm3 molec−1 s−1, in the temperature range 252–423 K, was fit to the data points. The rate coefficient at room temperature was, within experimental errors, independent of the He buffer gas pressure in the range 1.74 to 8.30 Torr, or the O-atom source reactor. The formation of OH(X 2Π) in the reaction, which can be vibrationally excited (v″≤2), was directly detected by pulsed laser-induced fluorescence. The total yield of OH in the reaction was determined to be (0.15±0.05) at 298 K, of which ∼50% is thought to be produced vibrationally hot. These results suggest that the single-H-atom removal channel is a minor process, in agreement with earlier molecular beam studies in which a direct two-H-atom removal channel was proposed to be the principal reaction mechanism by which O(3P) reacts with N2H4.
Photodissociation of NH2: Two-dimensional potential energy surfaces for the dissociation into NH and H
Authors: Vetter R. , Zülicke L. , Koch A. , van Dishoeck E. F. , Peyerimhoff S. D.
Published: July 2002

Extensive multireference configuration interaction calculations were carried out in order to obtain complete two-dimensional (2D) potential energy surfaces for the amidogen (NH2) radical as functions of both N–H bond lengths keeping the bond angle fixed at its experimental ground state equilibrium value. The eight lowest-lying states (four of each symmetry, A′ and A″) were treated mainly for the purpose of using these surfaces in subsequent studies of the photodissociation dynamics. In analogy with the neighboring dihydrides CH2 and H2O the photodissociation of NH2 into NH+H (hydrogen abstraction) takes place preferentially after excitation of the first two Rydberg s states (3 2A′/2 2A1 and 2 2A″/2 2B1) found closely together at about 7.6 eV. The transition dipole moments connecting the ground state with these two states are large (0.44 a.u. and 0.66 a.u.) in the Franck–Condon region, but the behavior of the potentials in the dissociation channel is quite different. The 3 2A′/2 2A1 state is weakly repulsive whereas the 2 2A″/2 2B1 state is strongly repulsive. This will result in differences in the dissociation dynamics for the two states.

The next higher state which should play a role in the NH2 photodissociation is the 4 2A″/3 2B1 Rydberg s state at 9.4 eV, because of its large transition dipole moment with the ground state (0.36 a.u.). Close to this state, many Rydberg p states were found. Due to the high density of states in the region above 9.0 eV, interactions of these states are expected and should lead to complicated dissociation dynamics. Contrary to CH2, the two low-lying valence states for NH2 are found at lower energies [2.2 eV (1 2A1) and 6.5 eV (1 2B2)], well separated from the first members of the Rydberg series. These states are less important for the photodissociation of NH2, compared with CH2, because the first state is bound and the transition to the other is dipole-forbidden in C2v symmetry. For H2O, the valence states are missing.
Transport properties of a binary gas mixture of molecules with internal energy. I. Shear and volume viscosities
Authors: Singh G. S. , Kumar B.
Published: July 2002

General expressions in terms of the square bracket integrals for the shear and volume viscosities of a classical dilute binary gas mixture of nonspherical molecules have been derived by extending the Taxman classical kinetic theory of transport phenomena within the framework of the Chapman–Enskog second approximation method for a mixture of spherical molecules. The dimensionality of integrals for the binary mixture of rigid spheroidal molecules having C∞v symmetry has been reduced analytically to get ultimately the forms wherein the integrals are to be performed only over the orientational coordinates of two colliding like or unlike spheroids. The general expressions for the nonspherical molecules as well as the final integrals for the spheroidal molecules are shown to be correct. The shear- and volume-viscosity formulas for a classical dilute binary gas mixture of hard sphere molecules have been retrieved as the limiting case of our corresponding expressions for the spheroidal systems.