bluuebunny/crossref_metadata_2025 huggingface dataset:
Development of multicathode high flux metal ion plasma sources in Korea
Authors: Kim Do-Yun , Lee Eui-Wan , Lee Myoung-Bok
Published: September 2004

Multicathode high flux metal plasma ion sources were self-developed and its performance was proved to be appropriate for the high-purity ion implantation and thin-film deposition. As key results of self-design, a bipolar pulse power supply with a peak voltage of 250 V, a repetition rate of 20 Hz, and a pulse width of 100 μs showed an output current of 2 kA and an average power of 2 kW and the operational plasma flux of multicathode ion source was well sustained even at an ion current of about 5 A. A high-voltage pulse generator was employed as a trigger power supply producing a peak voltage of 12 kV, peak current of 50 A, and stable repetition rate of 20 Hz.
Effect of hydrodynamic interactions on the evolution of chemically reactive ternary mixtures
Authors: Good Kevin , Kuksenok Olga , Buxton Gavin A. , Ginzburg Valeriy V. , Balazs Anna C.
Published: September 2004

We investigate the structural evolution of an A/B/C ternary mixture in which the A and B components can undergo a reversible chemical reaction to form C. We developed a lattice Boltzmann model for this ternary mixture that allows us to capture both the reaction kinetics and the hydrodynamic interactions within the system. We use this model to study a specific reactive mixture in which C acts as a surfactant, i.e., the formation of C at the A/B interface decreases the interfacial tension between the A and B domains. We found that the dynamics of the system is different for fluids in the diffusive and viscous regimes. In the diffusive regime, the formation of a layer of C at the interface leads to a freezing of the structural evolution in the fluid; the values of the reaction rate constants determine the characteristic domain size in the system. In the viscous regime, where hydrodynamic interactions are important, interfacial reactions cause a slowing down of the domain growth, but do not arrest the evolution of the mixture. The results provide guidelines for controlling the morphology of this complex ternary fluid.
Template copolymerization near a patterned surface: Computer simulation
Authors: Berezkin Anatoly V. , Solov’ev Maxim A. , Khalatur Pavel G. , Khokhlov Alexei R.
Published: September 2004

We perform a Monte Carlo simulation of irreversible template copolymerization near a chemically heterogeneous surface with a regular distribution of discrete adsorption sites that selectively adsorb from solution one of the two polymerizing monomers and the corresponding chain segments. In the polymerization model, the chain propagation process is simulated by adding individual monomers to the end of growing macroradical. We focus in this paper on the influence of polymerization rate, adsorption energy, and the distance between adsorption sites on the chain conformation and the primary sequence of the resulting two-letter (AB) copolymers and, specifically, on the coupling between polymerization and adsorption. The conditions for the realization of conformation-dependent copolymerization are formulated. For this regime, we observe the formation of a quasiregular copolymer with two types of alternating sections. One of them contains randomly distributed A and B segments. The second one consists mainly of strongly adsorbed A segments. It is found that the average length of the random sections is proportional to the distance between the nearest neighbor adsorption sites. The average length of the A-rich sections is determined by the “adsorption capacity” of adsorption site. By varying the strength of the effective monomer-substrate interaction and the distribution of adsorption sites on the substrate, the copolymers with different surface-induced primary sequences can be designed and synthesized in a controlled fashion.
Controlling of explicit internal signal stochastic resonance by external signal
Authors: Li Ya Ping , Wang Pin , Li Qian Shu
Published: September 2004

Explicit internal signal stochastic resonance (EISSR) is investigated in a model of energy transduction of molecular machinery when noise is added to the region of oscillation in the presence of external signal (ES). It is found that EISSR could be controlled, i.e., enhanced or suppressed by adjusting frequency (ωe) and amplitude (A) of ES, and that there exits an optimal frequency for ES, which makes EISSR strength reach the maximum. Meanwhile, a critical amplitude (Ac) is found, which is a threshold of occurrence of EISSR. Finally, the difference and similarity between EISSR and IISSR (implicit internal signal stochastic resonance) are discussed.
Spin–orbit ab initio study of alkyl halide dissociation via electronic curve crossing
Authors: Ajitha D. , Wierzbowska M. , Lindh R. , Malmqvist P. A.
Published: September 2004

An ab initio study of the role of electronic curve crossing in the photodissociation dynamics of the alkyl halides is presented. Recent experimental studies show that curve crossing plays a deterministic role in deciding the channel of dissociation. Coupled repulsive potential energy curves of the low-lying n-σ* states are studied including spin–orbit and relativistic effects. Basis set including effect of core correlation is used. Ab initio vertical excitation spectra of CH3I and CF3I are in agreement with the experimental observation. The curve crossing region is around 2.371 Å for CH3I and CF3I. The potential curves of the repulsive excited states have larger slope for CF3I, suggesting a higher velocity and decreased intersystem crossing probability on fluorination. We also report the potential curves and the region of curve crossing for CH3Br and CH3Cl.
On the structure and physical origin of the interaction in H2⋯Cl− and H2⋯Br− van der Waals anion complexes
Authors: Lukeš Vladimı́r , Ilčin Michal , Laurinc Viliam , Biskupič Stanislav
Published: September 2004

The ab initio three-dimensional potential energy surface (PES) for the weak interaction of hydrogen molecule with bromine anion is presented. The surface was obtained by the supermolecular method at the coupled cluster with single and double excitations and noniterative correction to triple excitations (CCSD(T)) level of theory. Our calculations indicate the van der Waals (vdW) system for the linear orientation at R=3.37 Å with a well depth of De=660.1 cm−1. The presented PES reveals also transition state for the perpendicular orientation at R=4.22 Å with a barrier of 607.1 cm−1. The physical origin of the stability of vdW H2⋯Br− structure with respect to the H2⋯Cl− one was analyzed by the symmetry adapted perturbation theory based on the single determinant Hartree–Fock (HF) wave function. The separation of the interaction energy shows that the dispersion forces play slightly more important role in the stabilization of the vdW system with Br− than with Cl−.
Structural properties of charge-stabilized ferrofluids under a magnetic field: A Brownian dynamics study
Authors: Mériguet G. , Jardat M. , Turq P.
Published: September 2004

We present Brownian dynamics simulations of real charge-stabilized ferrofluids, which are stable colloidal dispersions of magnetic nanoparticles, with and without the presence of an external magnetic field. The colloidal suspensions are treated as collections of monodisperse spherical particles, bearing point dipoles at their centers and undergoing translational and rotational Brownian motions. The overall repulsive isotropic interactions between particles, governed by electrostatic repulsions, are taken into account by a one-component effective pair interaction potential. The potential parameters are fitted in order that computed structure factors are close to the experimental ones. Two samples of ferrofluid differing by the particle diameter and consequently by the intensity of the magnetic interaction are considered here. The magnetization and birefringence curves are computed: a deviation from the ideal Langevin behaviors is observed if the dipolar moment of particles is sufficiently large. Structure factors are also computed from simulations with and without an applied magnetic field H: the microstructure of the repulsive ferrofluid becomes anisotropic under H. Even our simple modeling of the suspension allows us to account for the main experimental features: an increase of the peak intensity is observed in the direction perpendicular to the field whereas the peak intensity decreases in the direction parallel to the field.
Confirmation of the metastability of HF (v=1) in helium nanodroplets
Authors: Lindsay C. M. , Lewis W. K. , Miller R. E.
Published: September 2004

Infrared laser spectroscopy is reported for HF in helium nanodroplets. The purpose of the present study is to resolve a discrepancy in the literature, namely, between the previous optothermal study, which indicates the HF does not vibrationally relax in the droplets, with an earlier mass spectrometer experiment which showed laser induced depletion. The resolution of this issue is that the depletion signal arises from the pickup of another molecule after an infrared photon has been absorbed by the HF monomer. In the case of pickup of an N2 molecule, the formation of the N2-HF (v=1) complex then facilitates vibrational relaxation, causing the laser induced depletion. Simulations of this effect provide quantitative agreement with the observed behavior.
Angular distributions for the F+H2→HF+H reaction: The role of the F spin-orbit excited state and comparison with molecular beam experiments
Authors: Tzeng Yi-Ren , Alexander Millard H.
Published: September 2004

We report quantum mechanical calculations of center-of-mass differential cross sections (DCS) for the F+H2→HF+H reaction performed on the multistate [Alexander–Stark–Werner (ASW)] potential energy surfaces (PES) that describe the open-shell character of this reaction. For comparison, we repeat single-state calculations with the Stark–Werner (SW) and Hartke–Stark–Werner (HSW) PESs. The ASW DCSs differ from those predicted for the SW and HSW PES in the backward direction. These differences arise from nonadiabatic coupling between several electronic states. The DCSs are then used in forward simulations of the laboratory-frame angular distributions (ADs) measured by Lee, Neumark, and co-workers [J. Chem. Phys. 82, 3045 (1985)]. The simulations are scaled to match experiment over the range 12°<Θlab<80°. As a natural consequence of the reduced backward scattering, the ASW ADs are more forward and sideways scattered than predicted by the HSW PES. At the two higher collision energies (2.74 and 3.42 kcal/mol) the enhanced sideways scattering of HF v′=2 products bring the ASW ADs in very good agreement with the experiment. At the lowest collision energy (1.84 kcal/mol), the simulations, for all three sets of PESs consistently underestimate the sideways scattering. The residual disagreements, particularly at the lowest collision energy, may be due to the known deficiencies in the PESs.
A computational study of the dihydrogen bonded complexes HBeH⋯HArF and HBeH⋯HKrF
Authors: McDowell Sean A. C.
Published: September 2004

We report an ab initio computational study of the properties of two linear dihydrogen-bonded complexes of HBeH with the recently discovered rare gas compounds HArF and HKrF at the MP2(full)/6-311++G(2d,2p) level of theory. The HBeH⋯HArF and HBeH⋯HKrF complexes were found to have zero-point energy corrected binding energies of 27 and 12 kJ mol−1, respectively. Large red shifts of the H-Rg vibrational stretching frequency in both complexes were also predicted. The electron density rearrangement of the rare gas compounds on complexation was also examined. We also consider the relative stabilities of D-containing isotopomers of the complexes by comparison of their computed zero-point vibrational energies.
Core excitations of naphthalene: Vibrational structure versus chemical shifts
Authors: Minkov I. , Gel’mukhanov F. , Friedlein R. , Osikowicz W. , Suess C. , Öhrwall G. , Sorensen S. L. , Braun S. , Murdey R. , Salaneck W. R. , Ågren H.
Published: September 2004

High-resolution x-ray photoelectron emission (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectra of naphthalene are analyzed in terms of the initial state chemical shifts and the vibrational fine structure of the excitations. Carbon atoms located at peripheral sites experience only a small chemical shift and exhibit rather similar charge-vibrational coupling, while the atoms in the bridging positions differ substantially. In the XPS spectra, C-H stretching modes provide important contributions to the overall shape of the spectrum. In contrast, the NEXAFS spectrum contains only vibrational progressions from particular C-C stretching modes. The accuracy of ab initio calculations of absolute electronic transition energies is discussed in the context of minute chemical shifts, the vibrational fine structure, and the state multiplicity.
Statistical mechanics of worm-like polymers from a new generating function
Authors: Carri Gustavo A. , Marucho Marcelo
Published: September 2004

We present a mathematical approach to the worm-like chain model of semiflexible polymers. Our method is built on a novel generating function from which all the properties of the model can be derived. Moreover, this approach satisfies the local inextensibility constraint exactly. In this paper, we focus on the lowest order contribution to the generating function and derive explicit analytical expressions for the characteristic function, polymer propagator, single chain structure factor, and mean square end-to-end distance. These analytical expressions are valid for polymers with any degree of stiffness and contour length. We find that our calculations are able to capture the fully flexible and infinitely stiff limits of the aforementioned quantities exactly while providing a smooth and approximate crossover behavior for intermediate values of the stiffness of the polymer backbone. In addition, our results are in very good quantitative agreement with the exact and approximate results of five other treatments of semiflexible polymers.
Geometric view of the thermodynamics of adsorption at a line of three-phase contact
Authors: Djikaev Y. , Widom B.
Published: September 2004

We consider three fluid phases meeting at a line of common contact and study the linear excesses per unit length of the contact line (the linear adsorptions Λi) of the fluid’s components. In any plane perpendicular to the contact line, the locus of choices for the otherwise arbitrary location of that line that makes one of the linear adsorptions, say Λ2, vanish, is a rectangular hyperbola. Two of the adsorptions Λ2 and Λ3 then both vanish when the contact line is chosen to pass through any of the intersections of the two corresponding hyperbolas Λ2=0 and Λ3=0. There may be two or four such real intersections. It is found most surprisingly, and confirmed in a numerical example, that Λ1(2,3), the adsorption of component 1 in a frame of reference in which the adsorptions Λ2 and Λ3 are both 0, depends on which intersection of the hyperbolas Λ2=0 and Λ3=0 is chosen for the location of the contact line. This implies that what had long been taken to be the line analog of the Gibbs adsorption equation is incomplete; there must be additional, previously unanticipated terms in the relation, consistent with the invariance of the line tension to choice of location of the contact line. It is then not Λ1(2,3) by itself but a related expression containing it that must be invariant, and this invariance is also confirmed in the numerical example. The presence of the additional terms in the adsorption equation is further confirmed and their origin clarified in a mean-field density-functional model. The supplemental terms vanish at a wetting transition, where one of the contact angles goes to 0.
Solvation dynamics in triton-X-100 and triton-X-165 micelles: Effect of micellar size and hydration
Authors: Kumbhakar Manoj , Nath Sukhendu , Mukherjee Tulsi , Pal Haridas
Published: September 2004

Dynamic Stokes’ shift measurements using coumarin 153 as the fluorescence probe have been carried out to study solvation dynamics in two nonionic micelles, viz., triton-X-100 (TX-100) and triton-X-165 (TX-165). In both the micelles, the solvent relaxation dynamics is biexponential in nature. While the fast solvation time τs1 is seen to be almost similar for both the micelles, the slow solvation time τs2 is found to be appreciably smaller in TX-165 than in TX-100 micelle. Dynamic light scattering measurements indicate that the TX-165 micelles are substantially smaller in size than that of TX-100. Assuming similar core size for both the micelles, as expected from the similar chemical structures of the nonpolar ends for both the surfactants, the Palisade layer is also indicated to be substantially thinner for TX-165 micelles than that of TX-100. The aggregation number of TX-165 micelles is also found to be substantially smaller than that of TX-100 micelles. Fluorescence spectral studies of C153 dye in the two micelles indicate that the Palisade layer of TX-165 micelles is more polar than that of TX-100 micelles. Fluorescence anisotropy measurements indicate that the microviscosity in the Palisade layer of TX-165 micelles is also lower than that of TX-100 micelles. Based on these results it is inferred that the structure of the Palisade layer of TX-165 micelles is quite loose and have higher degree hydration in comparison to that of TX-100 micelles. Due to these structural differences in the Palisade layers of TX-165 and TX-100 micelles the solvation dynamics is faster in the former micelles than in the latter. It has been further inferred that in the present systems the collective response of the water molecules at somewhat away from the probes is responsible for the faster component of the solvation time, which does not reflect much of the structural changes of the micellar Palisade layer. On the contrary, the slower solvation time component, which is mainly due to the single particle response arising from water molecules adjacent to the probe in the micellar Palisade layer, is largely affected by the structural changes in the micellar Palisade layer.
The transition-state region of the O(3P)+O2(3Σg−) potential energy surface
Authors: Schinke Reinhard , Fleurat-Lessard Paul
Published: September 2004

New electronic structure calculations for the transition-state region of the lowest ozone potential energy surface are reported. A two-dimensional potential energy surface in the asymptotic channel is calculated with the O2 bond distance being fixed. The calculations are performed at the multireference average quadratic coupled cluster level of theory using full-valence complete active space self-consistent field wave functions and the augmented correlation consistent polarized V6Z atomic basis set. The general shape of the potential energy surface as predicted in earlier studies, that is, a narrow transition state below the O+O2 asymptote, is confirmed by the present calculations. The transition state is 181 cm−1 below the asymptote and 72 cm−1 above the van der Waals–like minimum. The changes in the O+O2→O3* capture cross section and rate constant when the new potential energy surface is employed are investigated by means of classical trajectory calculations.
A semiempirical generalized gradient approximation exchange-correlation functional
Authors: Keal Thomas W. , Tozer David J.
Published: September 2004

We describe our attempts to improve upon the quality of the KT1 and KT2 generalized gradient approximation (GGA) exchange-correlation functionals [T. W. Keal and D. J. Tozer, J. Chem. Phys. 119, 3015 (2003)], through the introduction of additional gradient-corrected exchange and correlation terms. A GGA functional, denoted KT3, is presented, which maintains the high quality main-group nuclear magnetic resonance shielding constants obtained with KT1 and KT2; results are 2–3 times more accurate than conventional GGA and hybrid functionals. For the extensive range of systems considered in this study, KT3 also provides atomization energies, ionization potentials, electron affinities, proton affinities, bond angles, and electronic polarizabilities that are comparable to, or that surpass, those of the best present-day GGAs. Furthermore, it provides equilibrium molecular bond lengths and diatomic harmonic vibrational wave numbers that are as accurate as those from the best hybrid functionals. Further improvements are required in the description of classical chemical reaction barriers.
The importance of spin-orbit coupling and electron correlation in the rationalization of the ground state of the CUO molecule
Authors: Infante Ivan , Visscher Lucas
Published: September 2004

We present calculations at the relativistic coupled cluster theory that predict the Σ0+1 ground state of CUO to lie 58.2 kJ/mol below the first excited state, Φ23. This can be contrasted with the outcome of earlier density functional theory and complete active space second order perturbation theory (CASPT2) calculations that both predicted a Φ23 ground state upon inclusion of spin-orbit coupling in the calculations. Our result gives further justification to the interpretation of the measured frequency shifts of this species in various noble gas matrices as being caused by significant interaction between the uranium and the heavier noble gas atoms.
A time-dependent wave packet study of the vibronic and spin-orbit interactions in the dynamics of Cl(2P)+H2→HCl(X̃ 1Σg+)+H(2S) reaction
Authors: Ghosal Subhas , Mahapatra Susanta
Published: September 2004

We investigate the vibronic and spin-orbit (SO) coupling effects in the state-selected dynamics of the title reaction with the aid of a time-dependent wave packet approach. The ab initio potential energy surfaces of Capecchi and Werner [Science 296, 715 (2002)] have been employed for this purpose. Collinear approach of the Cl(2P) atom to the H2 molecule splits the degeneracy of the P2 state and gives rise to Σ2 and Π2 electronic states. These two surfaces form a conical intersection at this geometry. These states transform as 1 2A′, 1 2A″, and 2 2A′, respectively, at the nonlinear configurations of the nuclei. In addition, the SO interaction due to Cl atom further splits these states into 2Σ1/2, 2Π3/2, and 2Π1/2 components at the linear geometry. The ground-state reagent Cl(2P3/2)+H2 correlates with 2Σ1/2 and 2Π3/2, where as the SO excited reagent Cl*(2P1/2)+H2 correlates with 2Π1/2 at the linear geometry. In order to elucidate the impact of the vibronic and SO coupling effects on the initial state-selected reactivity of these electronic states we carry out quantum scattering calculations based on a flux operator formalism and a time-dependent wave packet approach. In this work, total reaction probabilities and the time dependence of electronic population of the system by initiating the reaction on each of the above electronic states are presented. The role of conical intersection alone on the reaction dynamics is investigated with a coupled two-state model and for the total angular momentum J=0 (neglecting the electronic orbital angular momentum) both in a diabatic as well as in the adiabatic electronic representation. The SO interaction is then included and the dynamics is studied with a coupled three-state model comprising six diabatic surfaces for the total angular momentum J=0.5 neglecting the Coriolis Coupling terms of the Hamiltonian. Companion calculations are carried out for the uncoupled adiabatic and diabatic surfaces in order to explicitly reveal the impact of two different surface coupling mechanisms in the dynamics of this prototypical reaction.
Microwave-generated low-frequency plasma wave excited in the periphery of the evanescent layer
Authors: Raychaudhuri Santwana , Kamal-Al-Hassan Md. , Yugami Noboru , Ito Hiroaki , Nishida Yasushi
Published: September 2004

The observation of microwave-created low-frequency electrostatic waves from the evanescent layer in a pulsed plasma discharge and effects of a weak magnetic field on its propagation characteristics are reported. The creation of the electrostatic wave depends on the density variations along the direction of microwave propagation. It is observable when a part of the microwave is reflected and another part of it eventually gets absorbed in the resonance-absorption layers (ɷ=ɷpe) on two sides of the evanescent layer. The velocity of the wave is one order of magnitude higher than that of the ion-acoustic wave. Under the application of a very weak magnetic field (affecting only the electrons) perpendicular to the direction of propagation of both the excited plasma waves and the microwaves, the waves are still observable only with altered velocity and characteristics. At higher magnetic fields the electrostatic waves are completely subdued under a continuous instability independent of presence or absence of the microwaves.
Ab initio calculations for the photoelectron spectra of vanadium clusters
Authors: Li Shen , Alemany M. M. G. , Chelikowsky James R.
Published: September 2004

We report ab initio calculations for the electronic and structural properties of Vn, Vn−, and Vn+ clusters up to n=8. We performed the calculations using a real-space pseudopotential method based on the local spin density approximation for exchange and correlation. This method assumes no explicit basis. Wave functions are evaluated on a uniform grid; only one parameter, the grid spacing, is used to control convergence of the electronic properties. Charged states are easily handled in real space, in contrast to methods based on supercells where Coulombic divergences require special handling. For each size and charge state, we find the lowest energy structure. Our results for the photoelectron spectra, using the optimized structure, agree well with those obtained by experiment. We also obtain satisfactory agreement with the measured ionization potential and electron affinity, and compare our results to calculations using an explicit basis.
Novel isomers of hexasulfur: Prediction of a stable prism isomer and implications for the thermal reactivity of elemental sulfur
Authors: Wong Ming Wah , Steudel Yana , Steudel Ralf
Published: September 2004

High-level ab initio molecular orbital calculations were employed to explore the potential energy hypersurface of hexasulfur, S6. Twelve isomeric structures of S6 have been identified: two unbranched rings (chair and boat), one trigonal prism of D3h symmetry, two singly branched rings (S5=S), three triplet chains, one singlet chain, and three doubly branched rings (S=S4=S). The prism structure is essentially a cluster of three S2 molecules connected via a six-center π*-π*-π* interaction. It is by 51 kJ mol−1 less stable than the lowest-energy chair form. The reactions to generate the boat, the prism, and the singly branched isomers from the chair form are predicted to have lower barriers than the ring opening reaction of cyclo-S6, which requires an activation energy of 149 kJ mol−1. The prism and singly branched isomers are found to be more reactive species than the chair form and they are potential sources of S2 in chemical reactions involving elemental sulfur.
Truncation of the correlation consistent basis sets: An effective approach to the reduction of computational cost?
Authors: Mintz Benjamin , Lennox Kristin P. , Wilson Angela K.
Published: September 2004

The systematic reduction of commonly used basis sets as a means to reduce computational cost is examined for a small test set of molecules, which includes H2, CH4, NH3, H2O, HF, and HCN. Coupled cluster with single, double, and quasiperturbative triple excitations calculations were performed using both the correlation consistent basis sets, and a set of systematically reduced basis sets to examine both the impact of the reduction upon the accuracy of the structures and energies, and the computational cost savings achieved. The effect of several truncation scenarios upon basis set convergence is also examined. Overall, for the systems studied, a reduction can occur which preserves the well-established systematic convergence behavior of the correlation consistent basis sets.
Calculation of binary magnetic properties and potential energy curve in xenon dimer: Second virial coefficient of Xe129 nuclear shielding
Authors: Hanni Matti , Lantto Perttu , Runeberg Nino , Jokisaari Jukka , Vaara Juha
Published: September 2004

Quantum chemical calculations of the nuclear shielding tensor, the nuclear quadrupole coupling tensor, and the spin-rotation tensor are reported for the Xe dimer using ab initio quantum chemical methods. The binary chemical shift δ, the anisotropy of the shielding tensor Δσ, the nuclear quadrupole coupling tensor component along the internuclear axis χ∥, and the spin-rotation constant C⊥ are presented as a function of internuclear distance. The basis set superposition error is approximately corrected for by using the counterpoise correction (CP) method. Electron correlation effects are systematically studied via the Hartree–Fock, complete active space self-consistent field, second-order Møller–Plesset many-body perturbation, and coupled-cluster singles and doubles (CCSD) theories, the last one without and with noniterative triples, at the nonrelativistic all-electron level. We also report a high-quality theoretical interatomic potential for the Xe dimer, gained using the relativistic effective potential/core polarization potential scheme. These calculations used valence basis set of cc-pVQZ quality supplemented with a set of midbond functions. The second virial coefficient of Xe nuclear shielding, which is probably the experimentally best-characterized intermolecular interaction effect in nuclear magnetic resonance spectroscopy, is computed as a function of temperature, and compared to experiment and earlier theoretical results. The best results for the second virial coefficient, obtained using the CCSD(CP) binary chemical shift curve and either our best theoretical potential or the empirical potentials from the literature, are in good agreement with experiment. Zero-point vibrational corrections of δ, Δσ, χ∥, and C⊥ in the ν=0, J=0 rovibrational ground state of the xenon dimer are also reported.
Ab initio study of the reactions of Ga(2P, S2, and P2) with silane
Authors: Pacheco-Sánchez J. H. , Luna-Garcı́a H. , Castillo S.
Published: September 2004

The interactions of Ga(2P:4s24p1, S2:4s25s1, and P2:4s25p1) with SiH4 are studied by means of Hartree–Fock self-consistent field (SCF) and multiconfigurational SCF followed by extensive variational and perturbational second-order multireference Møller–Plesset configuration by perturbation selected by iterative process calculations, using relativistic effective core potentials. The Ga atom in its P2(4s25p1) state can spontaneously insert into the SiH4. The Ga atom in its S2(4s25s1) state is inserted into the SiH4. In this interaction the 3 2A′ potential energy surface initially attractive becomes repulsive after meeting the 2 2A′ surface linked with the Ga(2P:4s24p1)+SiH4 fragments. The two A′2 curves (2 2A′ and X 2A′) derived from the interaction of Ga(2P:4s24p1) atom with silane molecule are initially repulsive. The 2 2A′ curve after an avoided crossing with the 3 2A′ curve goes down until it meets the X 2A′ curve. The 2 2A′ curve becomes repulsive after the avoided crossing with the X 2A′ curve. The X 2A′ curve becomes attractive only after its avoided crossing with the 2 2A′ curve. The lowest-lying X 2A′ potential leads to the HGaSiH3X 2A′ intermediate molecule. This intermediate molecule, diabatically correlated with the Ga(2S:4s25s1)+SiH4 fragments, which lies 1.5 kcal/mol above the ground state reactants leads to the GaH+SiH3 or H+GaSiH3 products through the dissociation channels. These products are reached from the HGaSiH3 intermediate without activation barriers. This work shows that the Ga atom at its first excited state in the presence of silane molecules in gas phase leads to the formation of SiH3 radicals, H atoms, GaH hydrides, as well as gallium silicide molecules.
Observations on the eigenspectrum of the linearized Doi equation with application to numerical simulations of liquid crystal suspensions
Authors: Gopinath Arvind , Armstrong Robert C. , Brown Robert A.
Published: September 2004

We present a simple linear stability analysis of the diffusion equation for nematic polymers that delivers the equilibrium bifurcation diagram for rigid rod, excluded volume potentials. Symmetry properties of the diffusion equation yield insight into invariant subspaces of solutions and allow for interpretation of recently published simulation results.
Using molecular similarity to construct accurate semiempirical electronic structure theories
Authors: Janesko Benjamin G. , Yaron David
Published: September 2004

Ab initio electronic structure methods give accurate results for small systems, but do not scale well to large systems. Chemical insight tells us that molecular functional groups will behave approximately the same way in all molecules, large or small. This molecular similarity is exploited in semiempirical methods, which couple simple electronic structure theories with parameters for the transferable characteristics of functional groups. We propose that high-level calculations on small molecules provide a rich source of parametrization data. In principle, we can select a functional group, generate a large amount of ab initio data on the group in various small-molecule environments, and “mine” this data to build a sophisticated model for the group’s behavior in large environments. This work details such a model for electron correlation: a semiempirical, subsystem-based correlation functional that predicts a subsystem’s two-electron density matrix as a functional of its one-electron density matrix. This model is demonstrated on two small systems: chains of linear, minimal-basis (H-H)5, treated as a sum of four overlapping (H-H)2 subsystems; and the aldehyde group of a set of HOC-R molecules. The results provide an initial demonstration of the feasibility of the approach.
Relativistically corrected hyperfine structure constants calculated with the regular approximation applied to correlation corrected ab initio theory
Authors: Filatov Michael , Cremer Dieter
Published: September 2004

The infinite-order regular approximation (IORA) and IORA with modified metric (IORAmm) is used to develop an algorithm for calculating relativistically corrected isotropic hyperfine structure (HFS) constants. The new method is applied to the calculation of alkali atoms Li–Fr, coinage metal atoms Cu, Ag, and Au, the Hg+ radical ion, and the mercury containing radicals HgH, HgCH3, HgCN, and HgF. By stepwise improvement of the level of theory from Hartree–Fock to second-order Møller–Plesset theory and to quadratic configuration interaction theory with single and double excitations, isotropic HFS constants of high accuracy were obtained for atoms and for molecular radicals. The importance of relativistic corrections is demonstrated.
Finite size scaling for the atomic Shannon-information entropy
Authors: Shi Qicun , Kais Sabre
Published: September 2004

We have developed the finite size scaling method to treat the criticality of Shannon-information entropy for any given quantum Hamiltonian. This approach gives very accurate results for the critical parameters by using a systematic expansion in a finite basis set. To illustrate this approach we present a study to estimate the critical exponents of the Shannon-information entropy S∼(λ−λc)αS, the electronic energy E∼(λ−λc)αE, and the correlation length ξ∼|λ−λc|−ν for atoms with the variable λ=1/Z, which is the inverse of the nuclear charge Z. This was realized by approximating the multielectron atomic Hamiltonian with a one-electron model Hamiltonian. This model is very accurate for describing the electronic structure of the atoms near their critical points. For several atoms in their ground electronic states, we have found that the critical exponents (αE,ν,αS) for He (Z=2), C (Z=6), N (Z=7), F (Z=9), and Ne (Z=10), respectively, are (1, 0, 0). At the critical points λc=1/Zc, the bound state energies become absorbed or degenerate with continuum states and the entropies reach their maximum values, indicating a maximal delocalization of the electronic wave function.
Polymer translocation through a nanopore induced by adsorption: Monte Carlo simulation of a coarse-grained model
Authors: Milchev Andrey , Binder Kurt , Bhattacharya Aniket
Published: September 2004

Dynamic Monte Carlo simulation of a bead-spring model of flexible macromolecules threading through a very narrow pore in a very thin rigid membrane are presented, assuming at the cis side of the membrane a purely repulsive monomer-wall interaction, while the trans side is attractive. Two choices of monomer-wall attraction ε are considered, one choice is slightly below and the other slightly above the “mushroom to pancake” adsorption threshold εc for an infinitely long chain. Studying chain lengths N=32, 64, 128, and 256 and varying the number of monomers Ntrans (time t=0) that have already passed the pore when the simulation started, over a wide range, we find for ε<εc (nonadsorbing case) that the translocation probability varies proportional to ctrans=Ntrans(t=0)/N for small ctrans, while for ε>εc a finite number Ntrans(t=0) suffices that the translocation probability is close to unity. In the case ε<εc, however, the time it takes for those chains to get through the pore to complete the translocation process scales as τ∝N2.23±0.04. This result agrees with the suggestion of Chuang, Kantor, and Kardar [Phys. Rev. E 65, 011802 (2001)] that the translocation time is proportional to the Rouse time, that scales under good solvent condition as τRouse∝N2ν+1, with the excluded-volume exponent ν≈0.59 in d=3 dimensions. Our results hence disagree with the suggestions that the translocation time should scale as either N2 or N3. For ε>εc, we find that the translocation time scales as τ∝N1.65±0.08. We suggest a tentative scaling explanation for this result. Also the distribution of translocation times is obtained and discussed.
Wavelength dependence of first molecular hyperpolarizability of a dendrimer in solution
Authors: Tai Oliver Y.-H. , Wang C. H. , Ma H. , Jen Alex K.-Y.
Published: September 2004

The frequency dependence of the first molecular hyperpolarizability of a dendrimer incorporated with thiophene-stilbene based charge-transfer chromophores is investigated by using a nanosecond 1907 nm laser and a number of wavelengths ranging from 1160 to 1760 nm emitted from an optical parametric amplifier pumped by a 1 kHz 130 fs Ti:sapphire laser. The measured hyperpolarizabilities are compared with those calculated from the charge-transfer absorption spectrum involving a Kramers–Kronig transformation scheme. The Kramers–Kronig transformation analysis provides a satisfactory account of the dispersion of the first molecular hyperpolarizability over the entire excitation wavelength range measured. The Kramers–Kronig technique extends the Oudar–Chemla two-level model previously proposed for the first molecular hyperpolarizability and it can be used in the nonresonance as well as the resonance region where the Oudar–Chemla model fails. The Kramers–Kronig transformation scheme allows a consistent intrinsic hyperpolarizability β0 to be obtained from the measured βHRS using different excitation wavelengths for the dendrimer. The comparison of β0 for the dendrimer, which contains three chromophores, with that of corresponding monomer chromophore suggests that the chromophores inside the dendrimer are independent. This gives the evidence of the site isolation effect of the dendrimer and substantiates the larger macroscopic optical nonlinearity recently obtained for the dendrimer.
Theoretical study on isotope and temperature effect in hydronium ion using ab initio path integral simulation
Authors: Tachikawa Masanori , Shiga Motoyuki
Published: September 2004

We have applied the ab initio path integral molecular dynamics simulation to study hydronium ion and its isotopes, which are the simplest systems for hydrated proton and deuteron. In this simulation, all the rotational and vibrational degrees of freedom are treated fully quantum mechanically, while the potential energies of the respective atomic configurations are calculated “on the fly” using ab initio quantum chemical approach. With the careful treatment of the ab initio electronic structure calculation by relevant choices in electron correlation level and basis set, this scheme is theoretically quite rigorous except for Born–Oppenheimer approximation. This accurate calculation allows a close insight into the structural shifts for the isotopes of hydronium ion by taking account of both quantum mechanical and thermal effects. In fact, the calculation is shown to be successful to quantitatively extract the geometrical isotope effect with respect to the Walden inversion. It is also shown that this leads to the isotope effect on the electronic structure as well as the thermochemical properties.
A regularized inverted perturbation approach method: Potential energy curve of the 4 1Σu+ state in Na2
Authors: Grochola A. , Kowalczyk P. , Jastrzebski W. , Pashov A.
Published: September 2004

We describe a modification of the inverted perturbation approach method allowing to construct physically sensible potential energy curves for electronic states of diatomic molecules even when some parts of the potential are not adequately characterized by the experimental data. The method is based on a simple regularization procedure, imposing an additional constraint on the constructed potential curve. In the present work it is applied to the double minimum 4 1Σu+ state of Na2, observed experimentally by polarization labeling spectroscopy technique.
Phase-space invariants for aggregates of particles: Hyperangular momenta and partitions of the classical kinetic energy
Authors: Aquilanti Vincenzo , Lombardi Andrea , Sevryuk Mikhail B.
Published: September 2004

Rigorous definitions are presented for the kinematic angular momentum K of a system of classical particles (a concept dual to the conventional angular momentum J), the angular momentum Lξ associated with the moments of inertia, and the contributions to the total kinetic energy of the system from various modes of the motion of the particles. Some key properties of these quantities are described—in particular, their invariance under any orthogonal coordinate transformation and the inequalities they are subject to. The main mathematical tool exploited is the singular value decomposition of rectangular matrices and its differentiation with respect to a parameter. The quantities introduced employ as ingredients particle coordinates and momenta, commonly available in classical trajectory studies of chemical reactions and in molecular dynamics simulations, and thus are of prospective use as sensitive and immediately calculated indicators of phase transitions, isomerizations, onsets of chaotic behavior, and other dynamical critical phenomena in classical microaggregates, such as nanoscale clusters.
A three-dimensional electron spin resonance microscope
Authors: Blank Aharon , Dunnam Curt R. , Borbat Peter P. , Freed Jack H.
Published: September 2004

An electron spin resonance (ESR) imaging system, capable of acquiring three-dimensional (3D) images with a resolution of ∼10×10×30 μm in a few minutes of acquisition, is presented. This ESR microscope employs a commercial continuous wave ESR spectrometer, working at 9.1 GHz, in conjunction with a miniature imaging probe (resonator+gradient coils), gradient current drivers, and control software. The system can acquire the image of a small (∼1.5×1.5×0.25 mm) sample either by the modulated field gradient method, the projection reconstruction method, or by a combination of the two. A short discussion regarding the resolution of the modulated field gradient method in two-dimensional (2D) and 3D imaging is given. Detailed descriptions of the various system components are provided, along with several examples of 2D and 3D images that demonstrate the capabilities of the system.
Thermal contact conductance between solid interfaces under low temperature and vacuum
Authors: Xiao Youming , Sun Heng , Xu Lie , Feng Haidong , Zhu Hongmei
Published: September 2004

In this article, an experimental device used for thermal contact conductance tests under low temperature and vacuum was demonstrated in detail, which may be used to simulate the real environment of some solid materials, such as stainless steel, or aluminum in a space satellite. Thermal contact conductance experiments of stainless steel, of aluminum, and between stainless steel and aluminum were made for 0.02 Pa and 100–330 K. The relationships of thermal contact conductance to the contact loading pressure and to the temperature were measured. It is demonstrated that the thermal contact conductance between stainless steel, aluminum, between stainless steel, and aluminum contacts rises with the temperature increase of the contact interface, as well as the loading pressure increase of the contact interface. The thermal contact conductance of aluminum is larger than that of stainless steel for the same temperature and same loading pressure of the contact interface. The thermal contact conductance between stainless steel and aluminum is close to that between aluminum contacts at the same temperature and same loading pressure of the contact interface.
Generalized simulated tempering realized on expanded ensembles of non-Boltzmann weights
Authors: Kim Jae Gil , Fukunishi Yoshifumi , Kidera Akinori , Nakamura Haruki
Published: September 2004

A generalized version of the simulated tempering operated in the expanded ensembles of non-Boltzmann weights has been proposed to mitigate a quasiergodicity problem occurring in simulations of rough energy landscapes. In contrast to conventional simulated tempering employing the Boltzmann weight, our method utilizes a parametrized, generalized distribution as a workhorse for stochastic exchanges of configurations and subensembles transitions, which allows a considerable enhancement for the rate of convergence of Monte Carlo and molecular dynamics simulations using delocalized weights. A feature of our method is that the exploration of the parameter space encouraging subensembles transitions is greatly accelerated using the dynamic update scheme for the weight via the average guide specific to the energy distribution. The performance and characteristic feature of our method have been validated in the liquid-solid transition of Lennard-Jones clusters and the conformational sampling of alanine dipeptide by taking two types of Tsallis [C. Tsallis, J. Stat. Phys. 52, 479 (1988)] expanded ensembles associated with different parametrization schemes.
Vibrational spectroscopy and intramolecular dynamics of 1-butyne
Authors: Portnov Alexander , Rosenwaks Salman , Bar Ilana
Published: September 2004

Photodissociation of jet-cooled vibrationally excited 1-butyne, C2H5C≡C–H, coupled with mass spectrometric detection of H photofragments, facilitated measurements of action spectra and Doppler profiles, expressing the yield of the ensuing fragments versus the vibrational excitation and UV probe lasers, respectively. Both the action spectra and the simultaneously measured room temperature photoacoustic spectra in the 2ν1, 3ν1, and 4ν1 C–H acetylenic stretch regions exhibit unresolved rotational envelopes with significant narrowing of the former due to temperature-related change in the rotational structure. The narrowing of the action spectrum in the 3ν1 region exposed a resonance splitting, implying intramolecular vibrational energy redistribution (IVR) time of ≈1 ps. Asymmetric rotor simulation of the band contours provided the rotational constants and estimates for the homogeneous broadening arising from IVR to the bath vibrational states. The homogenous linewidth of 4ν1 is anomalously narrower than that of 2ν1 and 3ν1, indicating a longer lived 4ν1 state despite the increasing background state density, suggestive of a lack of low-order resonances or of mode-specific coupling with the bath states. The Doppler profiles indicate that the H photofragments are released with low average translational energies, pointing to an indirect dissociation process occurring after internal conversion (IC) to the ground electronic state or after IC and isomerization to butadiene.