bluuebunny/crossref_metadata_2025 huggingface dataset:
Defect dominated charge transport in amorphous Ta2O5 thin films
Authors: Fleming R. M. , Lang D. V. , Jones C. D. W. , Steigerwald M. L. , Murphy D. W. , Alers G. B. , Wong Y.-H. , van Dover R. B. , Kwo J. R. , Sergent A. M.
Published: July 2002

Ta 2 O 5 is a candidate for use in metal–oxide–metal (MOM) capacitors in several areas of silicon device technology. Understanding and controlling leakage current is critical for successful implementation of this material. We have studied thermal and photoconductive charge transport processes in Ta2O5 MOM capacitors fabricated by anodization, reactive sputtering, and chemical vapor deposition. We find that the results from each of these three methods are similar if one compares films that have the same thickness and electrodes. Two types of leakage current are identified: (a) a transient current that charges the bulk states of the films and (b) a steady state activated process involving electron transport via a defect band. The transient process involves either tunneling conductivity into states near the Fermi energy or ion motion. The steady state process, seen most commonly in films <300 Å thick, is dominated by a large number of defects, ∼1019–1020 cm−3, located near the metal–oxide interfaces. The interior of thick Ta2O5 films has a substantially reduced number of defects. Modest heating (300–400 °C) of Ta2O5 in contact with a reactive metal electrode such as Al, Ti, or Ta results in interfacial reactions and the diffusion of defects across the thickness of the film. These experiments show that successful integration of Ta2O5 into semiconductor processing requires a better understanding of the impact of defects on the electrical characteristics and a better control of the metal–Ta2O5 interface.
Influence of the nature of the buffer on the coupling and the transport properties in Co/Ru/Co sandwiches
Authors: Colis S. , Dinia A. , Deck D. , Schmerber G. , Da Costa V.
Published: July 2002

A series of Co30 Å/Ru5 Å/Co30 Å sandwiches has been prepared by ion beam sputtering on a glass substrate using different buffer layers in order to optimize the antiferromagnetic (AF) coupling strength and the giant magnetoresistance (GMR) signal. The best GMR of about 1.7% is obtained for a Co/Ru/Co sandwich deposited on the Fe50 Å/Co5 Å/Cu30 Å buffer. On the basis of the GMR and exchange coupling results, the two more interesting buffers have been chosen for a detailed analysis. Two additional series have been grown with varying Ru spacer layer thickness on these two buffers: 150 Å Ru and Fe50 Å/Co5 Å/Cu30 Å buffer layers. For the two series, oscillations in magnetoresistance and AF indirect exchange coupling have been observed as a function of the Ru thickness with the same period of about 10 Å. The first AF peak is observed at a thickness of 6 Å Ru for the series grown on Ru150 Å buffer and 4 Å for the series grown on a Fe50 Å/Co5 Å/Cu30 Å buffer layer. At this first peak, both GMR and AF coupling are largely enhanced with values JAF=−2.6 erg/cm2 and a GMR of 1.34% for the Fe50 Å/Co5 Å/Cu30 Å buffer compared to JAF=−0.75 erg/cm2 and a GMR of 0.08% for the Ru150 Å buffer. Atomic force microscopy and x-ray diffraction studies have also been performed in order to understand the origin of the observed increase in GMR and AF exchange coupling between the two series.
Relaxation behavior, at very low frequencies, of glassy polymers containing aliphatic-aromatic side groups in their structures
Authors: Saez-Torres P. , Sanchis M. J. , Diaz-Calleja R. , Guzman J. , Riande E.
Published: July 2002

The curves depicting the temperature dependence of the global depolarization curves of poly(2-[naphthylacetyl)oxy]ethyl acrylate), poly(2-[naphthylacetyl)oxy]ethyl methacrylate), and the model compound of the side groups of these polymers, 2-(acetyloxy)-ethyl-(2-naphthylacetate), present ostensible glass-rubber relaxations located at −45, 34, and 53 °C, respectively. These values are very close to the respective calorimetric glass transition temperatures of these materials. As these substances exhibit a weak dielectric activity in the glassy state, it was possible to determine the components of the complex dielectric permittivity at very low frequencies from partial depolarization curves obtained on these materials in this state. Possible mechanisms responsible for the subglass relaxations are suggested from the comparison of the relaxation behavior of the model compound and the polymers.
Conductive atomic force microscopy study of InGaN films grown by hot-wall epitaxy with a mixed (Ga+In) source
Authors: Iwata Futoshi , Chu Shucheng , Sasaki Akira , Ishino Kenji , Ishida Akihiro , Fujiyasu Hiroshi
Published: July 2002

The local morphology and electrical properties of InGaN films grown using a hot-wall epitaxy system coupled with a simple mixed source (metallic gallium and indium) were investigated by means of conductive atomic force microscopy. The photoluminescence (PL) wavelengths of two types of samples grown at different substrate temperatures (685 and 660 °C) show different wavelength peaks (400 nm for the former and 420 nm for the latter), even though these samples have almost the same In contents as deduced from x-ray diffraction results. The sample grown at the higher substrate temperature exhibits a very flat morphology with both a roughness of about 3 nm and a low conductive homogeneous distribution. On the other hand, the sample grown at the lower substrate temperature shows a rougher topography and inhomogeneous conductive distribution. The longer PL peak and the inhomogeneous conductive distribution were attributed to the In fluctuation of the InGaN films.
On the validity of Smoluchowski’s equation for cluster–cluster aggregation kinetics
Authors: Ziff Robert M. , McGrady E. D. , Meakin Paul
Published: July 2002

Large-scale computer simulations of cluster–cluster aggregation of particles undergoing Brownian motion have been performed to investigate the kinetic properties of the coagulation process. It is shown that the mean-field approach of the Smoluchowski equation accurately describes the kinetics if the fractal geometry of the clusters is taken into account. The kinetic rate kernels that enter into that equation are deduced explicitly from the simulations.
Quantum Monte Carlo study of the thermodynamic properties of argon clusters: The homogeneous nucleation of argon in argon vapor and ‘‘magic number’’ distributions in argon vapor
Authors: Freeman David L. , Doll Jimmie D.
Published: July 2002

The thermodynamic properties of clusters of argon atoms are studied by a combination of classical and quantum mechanical Monte Carlo methods. The argon atoms are represented by Lennard-Jones interactions and internal energies, free energies, and entropies are calculated as a function of temperature and cluster size. For the argon system quantum effects and anharmonicity corrections are found to be simultaneously important for a temperature range from 15 to 20 K. By examining local minima in the free energy of formation of argon clusters as a function of cluster size, magic numbers in the Boltzmann mass distribution are observed at n=7, 13, and 19 under some conditions of temperature and pressure. In some cases magic numbers are predicted in the quantum and not in the classical calculation. The entropy changes associated with cluster growth are found to be insensitive to cluster size. Quantum corrections are calculated to nucleation rates and found to be very important at low temperatures.
The S1(n, π*) states of acetaldehyde and acetone in supersonic nozzle beam: Methyl internal rotation and C=O out-of-plane wagging
Authors: Baba Masaaki , Hanazaki Ichiro , Nagashima Umpei
Published: July 2002

Fluorescence excitation spectra of CH3CHO, CH3CDO, (CH3)2CO, and (CD3)2CO have been observed in an Ar supersonic nozzle beam. Vibrational analyses have been performed for vibronic bands in the region at wavelengths longer than 313 nm. The 0–0 bands of the S1(n, π*) states were located at 29 771, 29 813, 30 435, and 30 431 cm−1, respectively. The spectra could be analyzed taking the C=O out-of-plane wagging and the CH3 internal rotation as active modes. By fitting a double minimum potential function to the observed vibrational levels, it has been shown that these molecules are pyramidally distorted in the S1(n, π*) state with barrier heights to inversion of 541, 578, 468, and 480 cm−1, respectively. Similar analyses using the Mathieu function gave threefold potential functions for the methyl internal rotation with barrier heights to rotation of 691, 645, 740, and 720 cm−1 for CH3CHO, CH3CDO, (CH3)2CO, and (CD3)2CO, respectively. High resolution measurements of rotational envelopes have shown that the out-of-plane polarization dominates in the acetone spectrum. This result, together with a detailed investigation of the vibronic intensity borrowing mechanism, has led us to conclude that the second order interaction dominates in which the methyl torsion and the C=O out-of-plane wagging are active. The origin of the methyl rotational barrier in the S1 state is discussed on the basis of our recent ab initio calculations. The hyperconjugative interaction is suggested to be important in determining the barrier.
Raman optical activity of molecules adsorbed on metal surfaces: Theory
Authors: Efrima S.
Published: July 2002

The circular intensity differences (CID) in a Raman Optical Activity experiment for molecules adsorbed on metal surfaces are derived. Statistical averages are computed for the cases of complete orientational randomness and of adsorption with an axis fixed normal to the surface, and random orientation in the plane parallel to the surface. Unusually large CID values are expected provided that: (i) the electric field near the surface changes over distances much smaller than the wavelength of the incident light, and (ii) the gradients of the field have different phases than the field itself, or (ii′) the molecular polarizability tensors have an imaginary part. These conditions should be met in realistic systems, especially in systems that exhibit surface enhanced Raman scattering.
Self-diffusion of interacting micelles: FRAPP study of micelles self-diffusion
Authors: Chatenay D. , Urbach W. , Messager R. , Langevin D.
Published: July 2002

We have studied self-diffusion in DTAB micellar systems at three different salinities with fluorescence photobleaching experiments. In order to characterize interactions in these systems, we have also performed light scattering experiments. Despite of the strong variation of the interactions with salinities, almost no salinity variation in self-diffusion coefficients vs droplets volume fraction curves has been observed. A clear difference between the friction coefficients involved in self- and mutual diffusion have been evidenced.
The quartic force field of H2O determined by many-body methods. II. Effects of triple excitations
Authors: Bartlett Rodney J. , Cole Samuel J. , Purvis George D. , Ermler Walter C. , Hsieh Hsiuchin C. , Shavitt Isaiah
Published: July 2002

A b initio coupled cluster and many-body perturbation theory methods that include triple excitation effects are applied to the determination of the quartic force field of the water molecule using an extended Slater-type basis set. Predictions of fundamental, overtone, and combination vibrational frequencies, rotational constants, and vibration–rotation coupling constants are reported for H2O and its isotopomers. The best predicted harmonic frequencies for the stretching modes of H2O are accurate to 3 cm−1, while the bending mode has an error of 28 cm−1. The mean absolute error for all frequencies reached by two quanta is 0.6%, while the anharmonic constants xi j have a mean absolute error of less than 3%. The important role of triple excitation effects in the surface determination is discussed, and is compared with the effects of quadruple excitations.
Property evaluation and orbital relaxation in coupled cluster methods
Authors: Salter E. A. , Sekino Hideo , Bartlett Rodney J.
Published: July 2002

Molecular electronic properties such as dipole moments, polarizabilities and hyperpolarizabilities and quadrupole moments and polarizabilities, and spin properties such as hyperfine splitting constants and nuclear magnetic coupling constants are predicted by ab initio coupled cluster (CC) methods for a variety of molecules. We compare the results of property evaluation using orbitals that have been allowed to relax in the presence of the perturbation and results obtained using nonrelaxed orbitals. It is demonstrated numerically, and proven formally, that the coupled cluster singles and doubles (CCSD) model using nonrelaxed orbitals is able to include most of the relaxation effects for the evaluation of first- and second-order properties. Thus there is little reason to perform coupled (perturbed) Hartree–Fock calculations as a precursor to correlated CCSD calculations of such properties.
Spin–orbit and electronic autoionization in HCl
Authors: Lefebvre-Brion H. , Dehmer P. M. , Chupka W. A.
Published: July 2002

The high resolution relative photoionization cross section for HCl is reported in the wavelength region 720–975 Å. In addition, results of calculations are presented for spin–orbit autoionization in the energy region between the X 2Π3/2 and X 2Π1/2 ionization thresholds and for electrostatic autoionization in the energy region containing Rydberg series converging to the A 2Σ+ excited ionic state. For the case of spin–orbit autoionization in the region just above the X 2Π3/2 ionic threshold, it is found that the energy positions of the first peaks are approximately correct if the ionization potential is modified by ≊80 cm−1; however the intensities of the peaks are strongly perturbed, most probably by high lying vibrational levels of the V 1Σ+ valence state. In the wavelength region 780–930 Å, two progressions of Rydberg states converging to the A 2Σ+ excited ionic state and autoionized by electrostatic interaction are also studied by ab initio multichannel quantum defect theory. Qualitative agreement with the experimental data is obtained by a slight adjustment of some quantum defects, the other parameters being those obtained by ab initio calculations. Comparison with the absorption spectra of both HCl and DCl is also made. Progression I of Terwilliger and Smith [J. Chem. Phys. 63, 1008 (1975)] is reassigned to the (A 2Σ+) 3dπ 1Π state and progression II to the (A2Σ+) 3pπ 1Π state on which is superimposed the (A 2Σ+) 3dσ 1Σ+ state. The large difference between the quantum defects of dπ and dσ states precludes the existence of a 3d complex.
Off-lattice Monte Carlo simulations of polymer melts confined between two plates
Authors: Kumar Sanat K. , Vacatello Michele , Yoon Do Y.
Published: July 2002

In this paper, we present the results of Monte Carlo simulations of the static properties of polymer melts confined between hard walls. The simulations are conducted in the canonical ensemble with a method that is a combination of reptation and crankshaft motions. 1250 polymer chains each comprising of 100 connected beads are placed in a box which allows for the simulation of a typical polymer melt confined between two hard plates at a separation of 51 bead diameters. Noncovalently bonded beads are assumed to interact with an empirical 6-12 Lennard-Jones potential which has parameters chosen to simulate a polyethylene melt at 400 K. From the analysis of the simulation results we show the existence of two relevant length scales in the problem. Single-chain statistics are perturbed by the wall, and this effect is screened out only after one proceeds to a distance comparable to twice the unperturbed radius of gyration of the polymer chain. However, many-chain statistics, i.e., packing and orientation of chain segments, are screened out as soon as one proceeds about three times the bead size from the wall. The simulation also allows for the study of the conformations of chains near the wall, and we observe that chains near the surface are flattened into nearly two-dimensional structures. The interface therefore corresponds to a region where chain configurations gradually evolve from this nearly two-dimensional structure to the unperturbed, three-dimensional Gaussian configurations in the bulk.
Rate constant of the gas phase reaction of SO3 with H2O
Authors: Wang Xiuyan , Jin Y. G. , Suto Masako , Lee L. C. , O’Neal H. E.
Published: July 2002

The rate constants for the reaction of SO3 with H2O in He and in N2 were measured at total pressures from 1–10 Torr in a flow tube at room temperature. The concentration of SO3 was monitored by photofragment emission produced by 147 nm excitation. Dependencies of apparent reaction rates on wall conditions and reaction tube sizes were investigated. At total He pressures of 1–10 Torr, a value of (5.7±0.9)×10−15 cm3 /s was obtained for the upper limit of the homogeneous gas phase reaction rate constant. This rate value is more than two orders of magnitude lower than the previously published value, but it is consistent with the theoretical calculation provided in this paper.
Determination of transition moment directions in molecules of low symmetry using polarized fluorescence. II. Applications to pyranine, perylene, and DPH
Authors: van Gurp M. , van Heijnsbergen T. , van Ginkel G. , Levine Y. K.
Published: July 2002

The application of angle-resolved fluorescence depolarization techniques to the simultaneous determination of the directions of the transition moments and orientational order of dye molecules incorporated into uniaxially stretched polymer films is discussed and illustrated. The directions of the transition moments of pyranine molecules are determined in a general way, on exciting the molecules in two distinct electronic transitions and monitoring the fluorescence emission under the same conditions. This assumes that the fluorescence emission occurs via the same transition moment for every excited absorption band. Experiments on perylene molecules show that the directions of the transition moments can also be determined from measurements utilizing a single electronic absorption band. In this case the orientational distribution of the molecules is assumed to have the form given by the maximum entropy method. Finally, it is shown the directions of the transition moments of DPH molecules in unstretched polymer films can be determined with this technique. It appears that these molecules are oriented to a very small, yet measurable, extent in unstretched films.
Fermi resonance structure in the CH vibrational overtones of CD3CHO
Authors: Amrein A. , Hollenstein H. , Quack M. , Zenobi R. , Segall J. , Zare R. N.
Published: July 2002

Gas-phase fundamental and CH and CO overtone spectra (700–17 500 cm−1) of 2,2,2-trideuteroacetaldehyde were recorded using FTIR and laser photoacoustic techniques. The Fermi resonance structure in the overtone spectra of the coupled CH stretching and in-plane CH bending vibrations is analyzed with a tridiagonal Hamiltonian, yielding a large effective coupling constant, ‖k′sbb‖ =93 cm−1, corresponding to subpicosecond redistribution times. No coupling between the out-of-plane CH bending mode and the Fermi resonance system is apparent. This study presents the first detailed analysis of the anharmonic couplings in the CH chromophore at an sp2 carbon atom. The in-plane CH bending vibration couples in a manner similar to the CH(sp3) bending vibrations, whereas the out-of-plane bending vibration is decoupled, similar to the CH(sp) bending vibrations.
Correlated dynamic percolation: Many bond effective-medium theory
Authors: Granek Rony , Nitzan Abraham
Published: July 2002

We study the diffusion (and conductivity) associated with the random walk of noninteracting particles on a disordered lattice characterized by bond disorder, temporal rearrangement, and spatial correlations. This paper extends previous works on dynamic bond percolation processes to situations where spatial correlations in the rearrangement process are important. Many bond effective-medium theory is used to obtain the effective diffusion coefficient Deff(ω) in such systems. The resulting Deff(ω) depends on the frequency through combinations of the form ω−i/τj where τj are characteristic relaxation times associated with the rearrangement process. We analyze in detail a model combining single bond renewal with a two bond exchange process. The resulting DC (ω=0) diffusion coefficient shows a new percolation threshold for the bond exchange model (in the absence of single bond renewal which eliminates the threshold altogether), and a crossover between the different limiting behaviors is seen as the different kinds of renewal process are switched on and off. Implications for ionic transport in polymeric ionic conductors are discussed.
Activation of methane dissociation on a Pt(111) surface
Authors: Luntz A. C. , Bethune D. S.
Published: July 2002

This paper reports detailed molecular beam measurements of the dissociative chemisorption probability for methane on a Pt(111) surface. We find large increases in the dissociative chemisorption probability S0 with increases in Ei cos2 θi (the normal component of translational energy), Ev (the vibrational energy of the incident methane), and Ts (surface thermal energy). The comparable activation of the reaction by addition of any of these three forms of energy cannot be accounted for by any single model for C–H bond activation proposed to date. A large kinetic isotope effect is also observed, with S0 decreasing significantly for CD4 relative to CH4.
Near Hartree–Fock quality GTO basis sets for the first- and third-row atoms
Authors: Partridge Harry
Published: July 2002

Energy-optimized, near Hartree–Fock quality GTO basis sets are reported for the first-row (Li to Ne) and third-row (K to Kr) atoms. The most accurate basis sets reported for the first row are (18s13p) sets which are within 4 μEH of the numerical Hartree–Fock (NHF) results. For B to Ne basis sets with more than 15s functions are quadruple zeta in the valence space. For the first-row transition metal atoms the (20s12p9d) basis sets are triple zeta in the valence space and are approximately equivalent to Clementi and Roetti’s accurate STO sets. Supplementing the (20s12p9d) basis sets optimized for the lowest state with the 4s23dn occupation with a diffuse d function gives SCF energy separations to the 4s13dn+1 and 3dn+2 states which are within 100 μEH of the NHF results. The most accurate basis sets for the transition metal atoms are within 30 μEH of the NHF results. In addition, energy optimized sets are reported for He(3P), Li(2P), and Be(3P).
Competition between autoionization and predissociation in the HCl and DCl molecules
Authors: Lefebvre-Brion H. , Keller F.
Published: July 2002

Experimental photodissociation cross sections for HCl and DCl have been recently measured in the autoionization energy region. These cross sections are obtained from the fluorescence excitation spectra of the fragments in the visible [White et al., J. Chem. Phys. 87, 6553 (1987)] or VUV [Frohlich et al. (to be published)] energy region. The peaks observed between 770–900 Å in the photodissociation cross sections do not appear in the same place as that observed in the photoionization cross sections. A theoretical explanation of this fact is proposed. The Rydberg states with a A 2 Σ+ core are predissociated due to the spin–orbit interaction, by Rydberg states belonging to series converging to the repulsive a 4 Π state of HCl+ . Using an unified MQDT treatment of autoionization and predissociation processes, the photoionization and photodissociation cross sections are calculated and compared with the experimental results. It is concluded that the states which are predissociated do not have the same symmetry as the states which are autoionized by the continuum of the X 2 Π state. The photodissociation cross section in HCl represents only a few percent of the photoionization cross section. It is predicted that for the heavier molecule HBr, the photodissociation cross section would be more than 10% of the photoionization cross section.
Monte Carlo simulation of hard chain–hard sphere mixtures in slitlike pores
Authors: Yethiraj Arun , Hall C. K.
Published: July 2002

The structure of mixtures of hard chains (modeled as a pearl necklace of freely jointed hard spheres) and hard spheres in slitlike pores is studied using a canonical ensemble Monte Carlo method. Simulation results for the density profiles in pores with wall separations varying from two to ten hard sphere diameters are presented at overall volume fractions of 0.12 and 0.34, and the effect of pore size, chain concentration, and chain length on the structure of the chains is investigated. It is found that the chains are depleted at the wall at the lower density, but enhanced at the wall (relative to the center of the pore) at the higher density; this depletion increases as the chain length is increased. It is found that the enhancement of monomers and depletion/enhancement of chains at the wall becomes more marked as the pore size is increased. As all the pores we study are integer multiples of the bead diameter, we do not observe the oscillatory variation of the wall density with wall separation which is expected at high densities. The fluid is uniform in the middle of the larger pores, but is not uniform anywhere in the smaller pores. At the same total packing fraction, increasing the mole fraction of the monomers tends to remove the chains from the wall and also make the oscillations in the chain profile more pronounced. The density of end sites of the chain at the wall is higher than it is for middle sites. In the smaller pores, the chains are found to be flattened in the direction perpendicular to the walls; in large pores the fluid in the middle of the pore is essentially uniform.
From micelles to microemulsion droplets: Size distributions, shape fluctuations, and interfacial tensions
Authors: Borkovec Michal
Published: July 2002

We present a theoretical description of the aggregation equilibrium of nonswollen and water-swollen micelles in oil (or vice versa). While we specialize on spherical, noninteracting aggregates, we treat the competition between phase separation, dissolution in monomers and formation of micelles with variable extent of swelling. The present model is based on an interfacial free energy of the surfactant monolayer which includes stretching and bending contributions. This free energy allows one to desribe both, the macroscopic interface in a two-phase system and the internal interface surrounding the aggregates on equal footing. The extent of swelling of the micelles at the CMC is determined by the bending energy of the saturated surfactant monolayer. Increasing the splay modulus or decreasing the spontaneous curvature favors swollen micelles (microemulsion droplets) and ultralow interfacial tensions. We present explicit results for the size distributions of the aggregates and interfacial tensions. Considering shape fluctuations of the aggregates we conclude that spherical micelles may be stabilized by small interfacial tension alone, while stability of strongly swollen microemulsion droplets requires a finite splay modulus of the monolayer.
Simulation of a monolayer of alkyl thiol chains
Authors: Hautman Joseph , Klein Michael L.
Published: July 2002

Molecular dynamics simulations have been used to study the structure and dynamics of monolayers of long-chain molecules on a metallic substrate. The system consisted of 90 molecules held at a fixed surface density and periodically replicated in the plane of the surface. Two models have been studied as alternative representations of the admolecule–surface interactions in layers formed by the self-assembly of alkyl thiol [SH(CH2)15CH3] molecules onto a gold substrate. The principal difference between the two models is that in one, the S–C bond is required to lie nearly parallel to the substrate surface. After lengthy equilibration at room temperature, both models yield monolayers in which the chains are aligned and tilted with respect to the surface normal. Although the tilt angle is different for the two models, the thickness of the monolayers is the same and the influence of the ‘‘modified-headgroup’’ on the detailed structure of the film is confined to the region closest to the metal surface. There are striking differences in the chain rotational dynamics of the two models which are likely amenable to experimental verification.
A second-order algorithm for the simulation of the Brownian dynamics of macromolecular models
Authors: Iniesta A. , García de la Torre J.
Published: July 2002

Most recent works on Brownian dynamics simulation employ a first-order algorithm developed by Ermak and McCammon [J. Chem. Phys. 69, 1352 (1978)]. In this work we propose the use of a second-order algorithm in which the step is a combination of two first-order steps, like in the second-order Runge–Kutta method for differential equations. Although the computer time per step is roughly doubled, the second-order algorithm is more efficient than the previous one because a given accuracy in the results can be achieved with less than half the number of steps. The new algorithm also allows for longer time steps without divergence. The advantage of the new procedure is illustrated in the simulation of four macromolecular systems: A quasirigid dumbbell, a semiflexible trumbbell, a semiflexible hinged rod, and a Gaussian polymer chain.
Picosecond photofragment spectroscopy. IV. Dynamics of consecutive bond breakage in the reaction C2F4I2→C2F4+ 2I
Authors: Khundkar Lutfur R. , Zewail Ahmed H.
Published: July 2002

Picosecond photofragment spectroscopy of the ultraviolet (UV) photodissociation of 1,2-diiodotetrafluoroethane reveals consecutive breaking of the two C–I bonds. Spin–orbit excited (I*) atoms show a prompt rise, in agreement with a direct mode dissociation of the first bond. Ground-state (I) atoms show a biexponential buildup, one component being fast (≤1 ps) while the other component is slow (30–150 ps depending on total energy), characteristic of the second bond breaking. The transient behavior of I atoms changes with the available energy. These results are interpreted in terms of a two step model involving a weakly bound radical. Simulations of transient behavior of I atoms, based on estimated internal energy distributions from the primary step and a model for dissociation rates as a function of energy, suggest that surface crossings are relevant to the dynamics and that the quantum yield of I atoms varies with excitation energy.
Electronic energies, geometries, and vibrational frequencies of the ground and low-lying excited states of the boron trimer
Authors: Hernandez Ramon , Simons Jack
Published: July 2002

A b initio electronic structure calculations on several low-lying valence states of B3 have been carried out using correlation-consistent polarized valence double-zeta basis sets and complete-active-space self-consistent field treatments of electron correlation. Stable triangular structures, linear structures, and Jahn–Teller unstable structures are all observed. Equilibrium geometries, energies, and local harmonic vibrational frequencies are determined for various locally stable structures of several states. Several of the local minima are found to be unstable or metastable when zero-point vibrational energy is included; as a result, fluxional species are predicted to occur for certain B3 states. Transition states connecting local minima are found and characterized in terms of energies, geometries, and local harmonic vibrational frequencies. The ground state of B3 is predicted to have an equilateral triangular structure and to be of 2A′1 symmetry in the D3h point group. Dipole and vibronically allowed vertical transition energies to other states are also considered.
Intermolecular potential of H2O⋅⋅⋅H2 in the van der Waals region. An a
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i
t
i
o study
Authors: Zhang Q. , Chenyang L. , Ma Y. , Fish F. , Szczȩśniak M. M. , Buch V.
Published: July 2002

The fourth-order Mo/ller–Plesset perturbation theory is used to evaluate the intermolecular potential of the H2O⋅⋅⋅H2 system with special emphasis on the van der Waals well region. When interacting with H2O, the H2 molecule can act either as a proton donor or as a proton acceptor. In the minimum energy configuration (−197 cm−1), H2 approaches the O atom collinearly with the C2 axis of H2O. In the secondary attractive region (−184 cm−1), H2 forms a T-shaped structure with the O–H bond of H2O (the H2 axis is perpendicular to the H2O plane). Other attractive areas of the potential are also examined. The origins of anisotropy of the interaction potential are studied by dissecting the interaction energy into its components—electrostatic, exchange repulsion, dispersion, deformation, etc. The potential energy surface is highly anisotropic, due largely to electrostatic interactions.
Dynamical disorder: Passage through a fluctuating bottleneck
Authors: Zwanzig Robert
Published: July 2002

A model is proposed for a rate process that is controlled by passage through a fluctuating bottleneck. The rate of passage is proportional to the cross-sectional area of the bottleneck. The radius of the bottleneck fluctuates in time according to a Langevin equation with a decay constant λ. Two consequences of the model are (1) decay is not exponential at short times, but changes to exponential at long times. (2) In the limit of small λ, the resulting effective rate constant at long times is proportional to λ1/2. Predictions of the model are qualitatively consistent with experimental observations on the viscosity dependence of ligand motion in myoglobin.
The nonclassical entropy flux in extended thermodynamics
Authors: Nettleton R. E.
Published: July 2002

An extended thermodynamics, originally a modification of rational thermodynamics, with nonclassical entropy flux, Js, is compared with early work based on the postulational structure of de Groot, with classical Js. Consistency between them and with physical and statistical models requires using the same classical Js in both.
Monte Carlo study of light scattering by linear polyelectrolytes
Authors: Reed Christopher E. , Reed Wayne F.
Published: July 2002

The light scattering function P(Θ) of charged isolated threefold rotational isomeric state model polymers was simulated by Metropolis Monte Carlo in the Debye–Hückel approximation. Number of chain units N (up to 250), and ratio (from 0.5 to 64) of Debye screening length κ−1 to bond length D were varied at a bond angle θ of 90°; θs of 70° and 45° were also investigated. Charge was fixed by setting the ‘‘Manning parameter’’ to 1. Because the properties of linear polyelectrolytes have been interpreted in terms of electrostatic excluded volume and electrostatic persistence lengths, similar chains with hard sphere repulsion were simulated, up to N=400, as were nearly wormlike chains with no repulsion. Overall, a function suggested by Noda et al. described the P(Θ) of both hard sphere chains and electrostatic repulsion chains with κ−1/D of 0.5 moderately well. However, first order renormalization group (RG) calculations correctly predict an intersection between this P(Θ) and that for Gaussian random coils, while the Noda et al. function does not. Requirements for experimental observation of excluded volume effects in polyelectrolytes are discussed. The P(Θ) of chains with large κ−1/D was not fit well by that of nonexcluded volume wormlike chains with the persistence lengths predicted by theory, possibly because excluded volume effects were not small. Linear fits to a log–log plot of 1/P(Θ) vs the square of the scattering vector times the radius of gyration gave the expected slopes, but not intercepts, as predicted by first order RG, probably because u was not large enough.
Mixed quantum wave packet/classical trajectory treatment of the photodissociation process ArHCl→Ar+H+Cl
Authors: García-Vela A. , Gerber R. B. , Imre D. G.
Published: July 2002

The photodissociation of HCl in the cluster Ar...HCl by an extremely short pulse was studied using a hybrid quantum mechanical/classical approach. In this method, the H atom is treated quantum mechanically, the heavy atoms classically, and the time-dependent self-consistent-field (TDSCF) approximation is used to couple the quantum with the classical modes. The results are compared with those of classical trajectory calculations. On the whole, good qualitative agreement is found between the results of the (partly quantum) hybrid method and the pure classical ones. However, quantum interference effects of quantitative significance are found both in the angular and in the kinetic energy distribution of the H atom product. These effects, and resonances that contribute to the process, are analyzed in terms of wave packets obtained for the H atom in the hybrid method. The usefulness and applicability of the hybrid method are discussed in the light of the results.
Photon bunching in the fluorescence from single molecules: A probe for intersystem crossing
Authors: Bernard J. , Fleury L. , Talon H. , Orrit M.
Published: July 2002

Fluorescence photons emitted by a single molecule trapped in a solid at low temperature are correlated in time. A simple theory of the correlation pattern is presented, including in the same formalism photon antibunching from coherent Rabi oscillations and photon bunching due to incoherent quantum transitions between electronic levels. The correlation method is applied to single pentacene molecules in a para-terphenyl crystal. A clear photon bunching allows us to determine the ISC rates for each molecule. We attribute the scatter of our results for the transition rate between excited singlet and triplet states, and the difference with the average value taken from the literature, to molecular distortions induced by crystal defects in our small sample. We conclude that the correlation method associated to single molecule spectroscopy has a great potential to study dynamical processes on intermediate time scales in condensed matter.
Multipolar electrolyte solution models. IV. Thermodynamic perturbation theory
Authors: Eggebrecht John , Ozler Pelin
Published: July 2002

Expressions for the equilibrium thermodynamic properties of a model electrolyte solution are obtained from the Taylor series expansion of the Helmholtz free energy in electrostatic interaction parameters. Rational polynomials, which are explicit in temperature, density, and composition, for ion–ion, ion–solvent, and solvent–solvent components of the free energy are used to extrapolate these series to low temperatures and finite ion concentrations. Comparisons of predicted internal energies, Helmholtz free energies, and compressibility factors are made with the results of Monte Carlo computer simulation. Predictions for the solutions are comparable in accuracy to those for the pure solvent. An expression for the solute concentration dependent dielectric response of the solution is developed and tested by comparison with simulation and experiment. This function introduces observed saturation with increasing ionic concentration to the ion–solvent and solvent–solvent contributions to the energies.
Electron energy loss and x-ray photoemission study of electron inelastic scattering in cadmium arachidate Langmuir Blodgett films
Authors: Sastry Murali , Pal Sipra , Rajagopal Aparna , Kundu Manisha , Kulkarni S. K.
Published: July 2002

The inelastic scattering of electrons in Langmuir–Blodgett films of cadmium arachidate [(C19H39COO)2Cd] deposited on glass substrates has been studied by reflection electron energy loss spectroscopy (REELS) and by monitoring the loss features of the C 1s and O 1s core levels by x-ray photoemission spectroscopy (XPS). REELS studies with a primary beam of energy 95 eV revealed some vibrational modes of the cadmium arachidate molecules and features due to transition of valence electrons to the conduction band. Similar structures were also identified in the C 1s and O 1s loss spectra and interpreted as reflecting the density of states of the conduction band, in agreement with earlier findings using secondary electron emission spectroscopy. In contrast to the monolayer film, the 5 ML film showed reduced order as inferred from the lack of features in the loss spectra.
Perturbation theory approach to dynamical tunneling splitting of local mode vibrational states in ABA molecules
Authors: Stuchebrukhov A. A. , Marcus R. A.
Published: July 2002

We introduce a point of view for treating the dynamical tunneling splitting of symmetric local mode vibrational states in ABA molecules (A=H typically) which is the one we have employed in treating the vibrational spectroscopy of CH overtones in molecules such as (CX3)3YCCH. Namely, the vibrational coupling corresponding to the dynamical tunneling in semiclassical mechanics via many intermediate off-resonance weak transitions between initial and final states can be treated by a standard high-order perturbation theory. We apply that method to the present simpler problem of tunneling splittings in ABA molecules, and compare the results with those of exact diagonalization, the semiclassical method, and the periodic orbit quantization. Of all the approximate methods, the perturbation theory was found to provide the best approximation to the results of exact diagonalization for the system treated. The relationship between these three methods and application to the problem of vibrational relaxation in polyatomic molecules with tunneling mechanism of intramolecular vibrational relaxation is discussed.
The OBeH2 hypersurface: Local and global minima, transition states, and reaction paths
Authors: Valtazanos Petros , Nicolaides Cleanthes A.
Published: July 2002

The lowest singlet potential energy surfaces of OBeH2 and its isomers are calculated, using geometry optimization techniques at the Fermi–Sea-multiconfiguration-self-consistent-field level. The reaction paths going from one isomer to the others are mapped and the appropriate transition states are located and verified. It is shown that, although the transition from BeO+H2 to OBeH2 is virtually barrierless, it can be achieved by an approach of molecular hydrogen to BeO from the Be side at right angles only. It is also shown that, should OBeH2 be formed, the transition to other, energetically lower points on the potential energy surface, involves twisting of the H2 moiety, thus making subsequent formation of linear H–O–Be–H the only option. The energy release upon this isomerization is 71 kcal/mol, while the activation barrier is 10 kcal/mol.
Molecular engineering implications of rotational invariance in quadratic nonlinear optics: From dipolar to octupolar molecules and materials
Authors: Zyss Joseph
Published: July 2002

Molecules and assemblies therefrom with strictly vanishing multipolar-like tensorial susceptibilities of given orders are defined in general, following group theoretical prescriptions. This approach leads, in the case of quadratic nonlinear optical properties, to a new class of molecules with strictly vanishing dipole moments and dipolar components of the quadratic hyperpolarizability tensor β. The remaining nonvanishing irreducible β component, referred to as the octupolar component, had not been previously considered in the perspective of molecular engineering and optimization, as proposed in this work. The adequate tensorial framework for depicting such an approach is derived for the various point-symmetry classes and a vectorial representation introduced to depict the full anisotropic nature of nonlinear polarizabilities. It permits a more general and adequate scaling of molecules and materials in terms of their efficiencies, while previous molecular classifications, strongly biased by the electric field second-harmonic (EFISH) solution experiment, focus almost exclusively on the dipolar component of β. Various molecular engineering routes meant at enhancing the octupolar β component are proposed and illustrated by specific examples. The molecular quantum implications of the existence of octupolar nonlinearities are discussed with three-level systems shown to replace the inadequate traditional two-level model. Finally, identical tensorial symmetry considerations, applied consistently to molecular assemblies (χ(2) susceptibility) and interacting light beam (F(2) cubic field tensor) evidence the relevance of circularly polarized beams to probe octupolar assemblies, the ellipticity of the outgoing beam in the case of second-harmonic generation depending on the ratio of the octupolar over dipolar susceptibility components.
A diffusion Monte Carlo algorithm with very small time-step errors
Authors: Umrigar C. J. , Nightingale M. P. , Runge K. J.
Published: July 2002

We propose modifications to the simple diffusion Monte Carlo algorithm that greatly reduce the time-step error. The improved algorithm has a time-step error smaller by a factor of 70 to 300 in the energy of Be, Li2 and Ne. For other observables the improvement is yet larger. The effective time step possible with the improved algorithm is typically a factor of a few hundred larger than the time step used in domain Green function Monte Carlo. We also present an optimized 109 parameter trial wave function for Be which, used in combination with our algorithm, yields an exceedingly accurate ground state energy. A simple solution to the population control bias in diffusion Monte Carlo is also discussed.
Analytical treatment of magnetic field effects in contact geminate recombination of radical ion pairs in solutions of arbitrary polarity
Authors: Shokhirev N. V. , Zharikov A. A. , Krissinel E. B.
Published: July 2002

Geminate recombination of radical ion pairs in the presence of magnetic field is considered on the assumption of a contact reaction and exchange and diffusion character of the reagents motion. Analytical expressions for recombination probabilities for arbitrary magnitude of interparticle Coulomb interaction are obtained. For the case of high magnetic fields, the influence of a medium polarity and viscosity, the initial separation in a pair and the scavenging reactions rate on recombination probability and CIDNP is studied.
Dielectric relaxation in water-tert-butanol mixtures. The water rich region
Authors: Fioretto D. , Marini A. , Massarotti M. , Onori G. , Palmieri L. , Santucci A. , Socino G.
Published: July 2002

Measurements of dielectric relaxation in water-tert-butanol mixtures have been performed by means of a frequency domain coaxial technique in the range 0.03–3 GHz, at 5 °C and for an alcohol mole fraction X ranging from 0 to 0.1. Experimental data give evidence for two relaxation processes whose relative amplitude and relaxation time depend on alcohol concentration. A two state model involving hydration and bulk water mutually exchanging at a finite rate is used to interpret dielectric data in the water rich region (X<0.02). An anomalous behavior, evidenced at X*≊0.045, is interpreted as the occurrence of a self-aggregation process involving part of the alcohol molecules in the solution.
Examination of the surface second harmonic response from noble metal surfaces at infrared wavelengths
Authors: Wong E. K. L. , Richmond G. L.
Published: July 2002

Contributing factors to the second harmonic response from Ag(111) and Au(111) surfaces have been examined in the long wavelength limit where nonresonant conditions prevail. The rotational anisotropy in the nonlinear response is found to persist for both metals in the absence of surface resonances and to show convergent behavior in both the relative phase and intensity as the incident energy is lowered from resonant to nonresonant conditions. The phase difference between the in-plane and out-of-plane responses in the long wavelength limit can be described by linear Fresnel theory. Whereas the tensor elements corresponding to the out-of-plane response are found to dominate, the in-plane contributions are found to be non-negligible.