bluuebunny/crossref_metadata_2025 huggingface dataset:
Trends in Primary Health Care Consumption by the Elderly
Authors: Vožehová Sylvie , Zikmundová Květuše , Zavázalová Helena , Zaremba V. , Vlasák J.
Published: May 2003

Summary: Trends in primary health care consumption were monitored in the elderly (60 or more years of age). Respondents (5162) were picked by random mechanical selection. They were monitored for a complex of characteristics composed of frequency of visits to a GP office, frequency of procedures performed at the GP office, drug prescriptions, frequency (and type) of procedures performed at specialists’ offices, and hospitalization. The study was conducted in 1982, 1992, 1993 and 1996. Results were statistically analysed. There was a statistically significant increase in the consumption of primary health care by the elderly: GP office visits rose from 80 % to 98.8 %, and there was a quantitative change in procedure types. There was also a marked increase in drug prescription: from 77 % receiving prescriptions in 1982 to 97 % in 1996. The highest consumption was of cardiotonics, analgesics and antipyretics, which corresponds to an increasing prevalence of chronic diseases and multimorbidity. The results of our studies underline the important role of primary care physicians in the care of the elderly, who represent 80 % of the clientele of their offices in our country.
Unusual Localizations of Hydatid Disease
Authors: Ç öl C. , Ç öl M. , Lafçi H.
Published: May 2003

Summary: Hydatid disease is endemic in several Mediterranean countries, posing an important health problem for these countries. The hydatid cyst is characterized by cystic lesions with clear boundaries, which can be observed in all parts of the body. Approximately 70 % of hydatid cysts are situated in the liver, followed by the lung (25 %). The kidneys, spleen, bile ducts, mesentery, soft tissue and brain are less frequent sites. We investigated patients who were treated for hydatid disease in our departments in the last 5 years with respect to localization of the disease, symptoms, surgical intervention, length of hospitalization, diameters of the cyst, and classification by Gharbi. In this retrospective and descriptive study, 176 patients are evaluated who were treated for hydatid disease between 1995 and 1999 in our departments. Of these patients, 14 were included with localization other than in the liver and lungs. Fourteen of the patients diagnosed with unusually located hydatid disease were men, six were women. Their mean age was 41.6 ± 20.8 years; the length of hospital stay was 7.07 ± 0.4 days. Overall, 28.6 % of patients with unusually located hydatid cyst had recurrent disease.

The time period since last cyst operation was 5.25 ± 3.5 years. The mean cyst diameter was 96.5 ± 54.5 mm. According to Gharbi's classification, three cases (21.4 %) of the unusually located hydatid cysts were type I, two (14.3 %) type II, and eight (57.1 %) type III. There was only one case of type IV and no cases of type V. Spleen and kidneys are the organs where hydatid disease is most frequently observed after the liver and lung. It can be observed in all parts of the body including the brain, peritoneum, mesenterium, choledochus, pancreas, bone and muscles. The type of treatment is determined by the localization and type of hydatid disease. Surgical treatment for splenic hydatid cysts is splenectomy. The functional kidney should be saved in non‐communicable hydatid disease. Total excision is almost never possible; endocystectomy and drainage procedure should be preferred for hydatid disease of the brain, pancreas and choledochus. Chemotherapy is usually given because of the risk of recurrence; this medical treatment consists in albendazole and mebendazole administration for 3–6 months in the postoperative period.
Trace resetting density matrix purification in O(N) self-consistent-field theory
Authors: Niklasson Anders M. N. , Tymczak C. J. , Challacombe Matt
Published: May 2003

A new approach to linear scaling construction of the density matrix is proposed, based on trace resetting purification of an effective Hamiltonian. Trace resetting is related to the trace preserving canonical purification scheme of Palser and Manolopoulos [Phys. Rev. B 58, 12704 (1999)] in that they both work with a predefined occupation number and do not require adjustment or prior knowledge of the chemical potential. In the trace resetting approach, trace conservation is not strictly enforced, allowing greater flexibility in the choice of purification polynomial and improved performance for Hamiltonian systems with high or low filling. However, optimal polynomials may in some cases admit unstable solutions, requiring a resetting mechanism to bring the solution back into the domain of convergent purification. A quartic trace resetting method is developed, along with analysis of stability and error accumulation due to incomplete sparse-matrix methods that employ a threshold τ to achieve sparsity. It is argued that threshold metered purification errors in the density matrix are O(τΔg−1) at worst, where Δg is the gap at the chemical potential. In the low filling regime, purification derived total energies are shown to converge smoothly with τ2 for RPBE/STO-6G C60 and a RPBE0/STO-3G Ti substituted zeolite. For the zeolite, the quartic trace resetting method is found to be both faster and over an order of magnitude more accurate than the Palser–Manolopoulos method. In the low filling limit, true linear scaling is demonstrated for RHF/6-31G** water clusters, and the trace resetting method is found to be both faster and an order of magnitude more accurate than the Palser–Manolopoulos scheme. Basis set progression of RPBE chlorophyll reveals the quartic trace resetting to be up to four orders of magnitude more accurate than the Palser–Manolopoulos algorithm in the limit of low filling. Furthermore, the ability of trace resetting and trace preserving algorithms to deal with degeneracy and fractional occupation is discussed.
Discretization parameters in fine-grained lattice simulations of linear and branched polymers
Authors: Natarajan Sudarshan , Maranas Janna K.
Published: May 2003

Monte Carlo simulations of polymer chains on a discretized lattice, where each atom occupies multiple lattice sites, have been shown to provide accuracies comparable with off-lattice simulations in one-tenth the computation time. For polyatomic molecules, the main requirement for the level of discretization required is set by the intramolecular geometry of the molecule. The current work provides a method to determine this level of discretization without running full off lattice simulations for comparison of results. Chains are generated off-lattice, placed on lattices of various discretizations, and the resulting errors are analyzed. We apply the method to three cases, united atom polyethylene, united atom polypropylene, and atomistic polyethylene. The atomic diameter to lattice size ratios required are 12 for the united atom cases, and 17 for atomistic polyethylene.
Correlated tensor interactions and rotational-echo double resonance of spin clusters
Authors: Mueller Leonard J. , Elliott Douglas W.
Published: May 2003

Rotational-echo double resonance (REDOR) is an important tool in solid-state NMR for providing heteronuclear distances via the measurement of through-space dipolar couplings. For isolated IS spin pairs, the REDOR dephasing curve reports on a single distance, while in ISN spin clusters the I-spin dipolar dephasing reflects a nonlinear combination of multiple dipolar tensors and their relative orientations. The interpretation of multispin dephasing in REDOR experiments typically requires numerical simulations, offering little insight into the relationship between the spin dynamics and the underlying tensor interactions. Here a novel paradigm for describing correlated-tensor spin dynamics is presented for the REDOR experiment. Significantly, the I-spin dipolar dephasing curve for an ISN spin cluster is found to be reducible to a sum of 2N-1 generalized dephasing curves, corresponding to spatial tensors for single I-spin transitions in the dipolar spectrum. Both simulations and experiments are presented that illustrate the power of this formalism for describing REDOR dephasing in clusters as large as IS12 and for extracting structural parameters. This approach can be extended to other solid state NMR “recoupling” experiments, underscoring the general applicability of correlated-tensor decomposition in NMR.
Ab initio studies of ClOx reactions. VII. Isomers of Cl2O3 and their roles in the ClO+OClO reaction
Authors: Zhu R. S. , Lin M. C.
Published: May 2003

The mechanism for the reaction ClO+OClO has been investigated by ab initio molecular orbital and transition state theory calculations. Nine stable isomers of Cl2O3 (including optical isomers) are located at the B3LYP/6-311+G(3df ) level. The transition states between pairs of isomers are explored and the stability of the isomers and their dissociation mechanisms are discussed. The relative stability predicted by the modified Gaussian-2 (G2M) method at the G2M//B3LYP/6-311+G(3df ) level is ClOCl(O)O>ClOOOCl(C2)>ClOOOCl(Cs)>ClClO3(C3v)>cyc-ClOOCl(O)>ClOOClO. The formation of ClOCl(O)O is dominant at low temperatures, taking place barrierlessly with the second and third order rate constant, k1∞=3.0×10−10 cm3 molecule−1 s−1 and k10=1.1×10−17 T−5.5 exp(−398/T) cm6 molecule−2 s−1 in the temperature range of 200–400 K for N2 as the third-body. Over 500 K, formation of ClOO+ClO becomes dominant and ClOCl+1O2 is also competitive. Their overall rate constants can be given by kClOO=1.0×10−22 T2.8 exp(−78/T) and kClOCl=9.6×10−22 T2.4 exp(−1670/T)cm3 molecule−1 s−1, respectively, in the temperature range of 500–2500 K.
Structure, bonding, and adhesion at the TiC(100)/Fe(110) interface from first principles
Authors: Arya A. , Carter Emily A.
Published: May 2003

Metal carbide ceramics offer potential as protective coatings for steels. Here we report a pseudopotential-based density functional (DFT) investigation of one such coating, wherein we predict the atomic structure, bonding, and the ideal work of adhesion (Wadideal) of the interface between a TiC(100) coating and a bcc Fe(110) substrate. Calibration of the DFT approximations used yields TiC and Fe bulk properties in reasonable agreement with experiment. Subsequent characterization of the low-index TiC and Fe surfaces reveals that all surfaces retain near bulk termination, in agreement with experiment. Stabilities of both TiC and Fe surfaces increase with their packing densities, i.e., (110)<(111)<(100) for TiC and (111)<(100)<(110) for bcc Fe. We estimate that the minimum critical stress required for crack propagation in bcc Fe is 27% larger than that in TiC. The TiC(100)/Fe(110) interface exhibits a lattice mismatch of ∼2.1%, leading to a smooth interface with only a small structural relaxation, except for the ultrathin 1 monolayer (ML) coating. A mixture of metallic and covalent bonding dominates across the interface, due to significant C p-Fe d interaction and somewhat less pronounced Ti d-Fe d mixing; the latter is found to decrease with increasing coating thickness, but reaches a saturation value for 3-ML-thick coating. The asymptotic value of Wadideal for the TiC(100)/Fe(110) interface is predicted to be ∼2.56 J/m2 and is reached for a 3-ML-thick coating of TiC on Fe. This interface strength is considerably smaller than the energy required for cracking TiC or Fe, but may still be strong enough to survive as a coating for steel in extreme environments.
Structure formation in a phase-separating polymer blend with randomly driven particles
Authors: Zhu Yue-jin , Ma Yu-qiang
Published: May 2003

We investigate the phase separation of a binary polymer blend–particle system under fluctuating fields. Our results show that the disordered stochastic field would drive the formation of ordered stripe structures, and the domain growth dynamics is intensively affected by both the blend–particle coupling effect and stochastic motion of driven particles. Particularly, for large fluctuating force and enough particle number, the domain morphology shows the better orientational stripe patterns in two-dimensional case, and the cylindrical structure along the fluctuating force direction in three-dimensional case. In addition, the particle mobility is also one of the crucial factors dominating the domain growth at the late stage.
On rotational dynamics of an NH4+ ion in water
Authors: Chang Tsun-Mei , Dang Liem X.
Published: May 2003

The structural, thermodynamic, and dynamical properties of a single ammonium ion, NH4+, in liquid water were investigated using molecular dynamic simulation techniques. Many-body polarizable model potentials were employed to describe the molecular interactions. The water molecules were found to form well-defined hydration shells with a preferred orientation around the NH4+ ion as expected. We also found that the average water dipole moment is enhanced in the first hydration shell. The translational and rotational motion of NH4+ was examined via the velocity, angular-momentum, and reorientation-autocorrelation functions. The computed rotational diffusion coefficients of the NH4+ ion in water, which were determined from the angular momentum autocorrelation function and the angular mean-square displacement, are 0.093 and 0.067×1012 rad2/s, respectively. These results are in good agreement with the experimental nuclear magnetic resonance value of 0.075×1012 rad2/s.
Quantum-classical dynamics of nonadiabatic chemical reactions
Authors: Sergi Alessandro , Kapral Raymond
Published: May 2003

A reactive flux correlation function formalism for the calculation of rate constants for mixed quantum-classical systems undergoing nonadiabatic dynamics is presented. The linear response formalism accounts for the stationarity of the equilibrium density under quantum-classical dynamics and expresses the rate constant in terms of an ensemble of surface-hopping trajectories. Calculations are carried out on a model two-level system coupled to a nonlinear oscillator which is in turn coupled to a harmonic heat bath. Relevant microscopic species variables for this system include two stable states, corresponding to the ground state adiabatic surface, as well as another species corresponding to the excited state surface. The time-dependent rate constants for the model are evaluated in the adiabatic limit, where the dynamics is confined to the ground Born–Oppenheimer surface, and these results are compared with calculations that account for nonadiabatic transitions among the system states.
Molecular-dynamics simulations and x-ray analysis of dye precipitates in the polyelectrolyte microcapsules
Authors: Saphiannikova Marina , Radtchenko Igor , Sukhorukov Gleb , Shchukin Dmitri , Yakimansky Alexander , Ilnytskyi Jaroslav
Published: May 2003

The precipitate of the Disperse Red-1 dye in bulk and in confined microsized volumes was investigated by x-ray powder diffraction and molecular-dynamics simulations. The diffraction patterns exhibited two different precipitation regimes: In bulk when dye molecules form a distinct crystallite structure and inside polyelectrolyte capsules with a diameter up to 8 μm when the precipitate presumably represents a very fine polycrystalline powder. The latter result was further supported by molecular-dynamics simulations carried out for up to 640 dye molecules in the NVT ensemble. Calculations have also shown that effects of confined geometry and steric restrictions arising due to encapsulated polyelectrolyte molecules can not prohibit dye nucleation and subsequent crystallization. However, nonspecific interactions between Disperse Red-1 and encapsulated polyelectrolyte could cause the onset of heterogeneous nucleation resulting in formation of a fine polycrystalline powder. The formation process was directly observed building configuration snapshots and calculating the Gay–Berne orientational order parameter and radial distribution function resolved parallel and perpendicular to the director. Comparison of powder diagrams derived from simulation data indicates that it is nearly impossible to distinguish a fine polycrystalline powder from the isotropic system.
Combined quantum mechanics and molecular mechanics simulation of Ca2+/ammonia solution based on the ONIOM-XS method: Octahedral coordination and implication to biology
Authors: Kerdcharoen Teerakiat , Morokuma Keiji
Published: May 2003

An extension of the ONIOM (Own N-layered Integrated molecular Orbital and molecular Mechanics) method [M. Svensson, S. Humbel, R. D. J. Froese, T. Mutsubara, S. Sieber, and K. Morokuma, J. Phys. Chem. 100, 19357 (1996)] for simulation in the condensed phase, called ONIOM-XS (XS=eXtension to Solvation) [T. Kerdcharoen and K. Morokuma, Chem. Phys. Lett. 355, 257 (2002)], was applied to investigate the coordination of Ca2+ in liquid ammonia. A coordination number of 6 is found. Previous simulations based on pair potential or pair potential plus three-body correction gave values of 9 and 8.2, respectively. The new value is the same as the coordination number most frequently listed in the Cambridge Structural Database (CSD) and Protein Data Bank (PDB). N–Ca–N angular distribution reveals a near-octahedral coordination structure. Inclusion of many-body interactions (which amounts to 25% of the pair interactions) into the potential energy surface is essential for obtaining reasonable coordination number. Analyses of the metal coordination in water, water-ammonia mixture, and in proteins reveals that cation/ammonia solution can be used to approximate the coordination environment in proteins.
Different models for the calculation of solvent effects on O17 nuclear magnetic shielding
Authors: Cossi Maurizio , Crescenzi Orlando
Published: May 2003

The challenging task of reproducing solvent effects on computed nuclear shieldings requires a careful analysis of different solvation models: We have compared the performances of continuum, cluster, and molecular dynamics approaches for sp3 and sp2 O17 in aqueous solution. The various solvation models have to be combined to reproduce the experimental results satisfactorily; a different behavior is found for the two hybridization states of the oxygen atom. The proposed procedure allows the accurate calculation of solvent effects on the nuclear shielding, and the separation of the main effects contributing to this quantity.
Ultrasound–order director fluctuations interaction in nematic liquid crystals: A nuclear magnetic resonance relaxometry study
Authors: Bonetto F. , Anoardo E. , Kimmich R.
Published: May 2003

Field-cycling nuclear magnetic resonance relaxometry was used to study the spin-lattice relaxation dispersion of selected standard nematic liquid crystals at different temperatures, in presence and absence of a monochromatic ultrasonic field. It was found that sonication changes the relaxation dispersion and, in some cases, it shortens the spin-lattice relaxation time on a broad frequency range. The theoretical interpretation is based on the interaction between the ultrasound and the nematic director field. A model previously developed and tested in preliminary experiments, is now successfully applied in a broader temperature range for different compounds. We found a remarkably good agreement with earlier experiments in pentylcyanobiphenyl (5CB). Nonetheless, new interesting features and stronger effects were observed in p-azoxyanysole (PAA). The analysis of our results clearly suggests a sound-induced enhancement of order director modes.
High harmonic generation spectra of aligned benzene in circular polarized laser field
Authors: Žďánská Petra , Averbukh Vitali , Moiseyev Nimrod
Published: May 2003

We present model calculations of high-order harmonic generation in benzene, aligned in the polarization plane of circular polarized laser field. The resonance states of the system are obtained using complex scaling Floquet approach (i.e., within non-Hermitian quantum mechanics) combined with (t,t′) time propagation method. Our results show that the photo-induced dynamics of the model benzene molecule at the laser wavelength of 800 nm is dominated by a single long-lived resonance state up to the intensity of about 90 TW cm−2. The high-order harmonics emitted by the system obey the selection rules derived in [Phys. Rev. Lett. 80, 3743 (1998)] on the basis of the dynamical symmetry of the system, namely the emitted harmonics possess the frequencies (6±1)ω,(12±1)ω,…, where ω is the incident laser frequency. These symmetry-allowed harmonics are found to be the dominant ones in the spectrum also when the laser polarization deviates from the “ideal” circular one by about 5%. The nonlinear response of the model benzene molecule is found to originate mainly from the field-induced transitions between the bound states, in accordance with the earlier analytical theory. The cut-off position in the calculated high-order harmonic generation spectra depends linearly on the field strength in the studied intensity interval. Our numerical calculations reveal the enhancements of particular high-order harmonics in the plateau region of the spectrum at certain field intensities. We show that these enhancements occur under conditions of avoided crossing of two or several resonance quasi-energies in the complex energy plane.
The nature of bonding in pericyclic and pseudopericyclic transition states: Thermal chelotropic decarbonylations
Authors: Chamorro E.
Published: May 2003

The electron localization function (ELF), a local measure of the Pauli repulsion, is shown like a useful descriptor of bonding at pericyclic and pseudopericyclic transition states. The main differences between these two relevant topologies have been investigated in detail through the examination of well-characterized typical allowed-symmetry thermal decarbonylations. It is shown that results based on the electron fluctuation between the ELF basin populations at the reaction center, provides a consistent description of bonding which complements the traditional Woodward–Hoffmann symmetry-orbital based analysis.
Ab initio studies of neutral and anionic p-benzoquinone–water clusters
Authors: Manojkumar T. K. , Choi Hyuk Soon , Tarakeshwar P. , Kim Kwang S.
Published: May 2003

Since the energy conversion reactions in most biological systems involve the interactions of various negatively charged quinones with water molecules, we have investigated the equilibrium structures, binding energies, and vibrational frequencies of the water monomer/dimer complexes of p-benzoquinones (Q) and its corresponding negatively charged anionic species (Q− and Q2−). The calculations, which include the evaluation of charges and aromaticity, indicate the existence of double hydrogen bonds in the water monomer and water dimer complexes of neutral p-benzoquinone. When two water molecules are complexing with p-benzoquinone, a conformer exhibiting an H-bond between two water molecules (W2Q) is energetically more favored than the conformer WQW in which there is no direct interaction between the water molecules. However, an increase in the negative charge of p-benzoquinone leads to the energetic stabilization of the WQ2−W conformer over the Q2−W2 conformer. The charge transfer Q to water molecules is found to be significant in the case of Q2−, up to −0.23 a.u. in the presence of two water molecules. An increase in the aromaticity of the p-benzoquinone molecule upon its interaction with the water molecules is noted from an analysis of the Nucleus Independent Chemical Shift (NICS) values.
First-principles calculations of structural, electronic, vibrational, and magnetic properties of C60 and C48N12: A comparative study
Authors: Xie Rui-Hua , Bryant Garnett W. , Jensen Lasse , Zhao Jijun , Smith Vedene H.
Published: May 2003

The structural, electronic, vibrational, and magnetic properties of the C48N12 azafullerene and C60 are comparatively studied from the first-principles calculations. Full geometrical optimization and Mulliken charge analysis are performed. Electronic structure calculations of C48N12 show that the highest occupied molecular orbital (HOMO) is a doubly degenerate level of ag symmetry and the lowest unoccupied molecular orbital (LUMO) is a nondegenerate level of au symmetry. The calculated binding energy per atom and HOMO-LUMO energy gap of C48N12 are about 1 eV smaller than those of C60. Because of electron correlations, the HOMO-LUMO gap decreases about 5 eV and the binding energy per atom increases about 2 eV. The average second-order hyperpolarizability of C48N12 is about 55% larger than that of C60. Our vibrational frequency analysis predicts that C48N12 has 58 infrared-active and 58 Raman-active vibrational modes. Two different methods for calculating nuclear magnetic shielding tensors of C60 and C48N12 are compared, and we find that C48N12 exhibits eight C13 and two N15 NMR spectral signals. Our best-calculated results for C60 are in excellent agreement with experiment. Our results suggest that C48N12 has potential applications as semiconductor components, nonlinear optical materials, and possible building blocks for molecular electronics and photonic devices.
Spectroscopic and electric properties of NaH and NaD: Extrapolation to the complete basis set limit
Authors: Taylor Brian K. , Newman Philip R.
Published: May 2003

We report accurate spectroscopic and electric properties for the ground electronic state of NaH and NaD from first principles electronic structure calculations at the coupled cluster singles-and-doubles with noniterative inclusion of connected triples [CCSD(T)] level of theory. A NaH/NaD Born–Oppenheimer potential energy curve is defined by extrapolating CCSD(T) ab initio energies to the complete basis set limit. Potential energy curves obtained using different basis sets and different basis set extrapolation schemes are compared to experimentally determined curves. When compared to experiment, basis set extrapolation yields a potential energy curve that results in highly accurate rotational and vibrational spectroscopic data for both NaH and NaD. An extensive set of spectroscopic parameters are tabulated. Electric properties are computed from CCSD(T) finite-field energies that have been extrapolated to the complete basis set limit. Dipole moment and dipole polarizability curves are reported. These quantities are vibrationally averaged for comparison with experimental values when they become available. Rovibrational transition matrix elements and vibrational lifetimes are also computed.
On the accuracy limits of orbital expansion methods: Explicit effects of k-functions on atomic and molecular energies
Authors: Valeev Edward F. , Allen Wesley D. , Hernandez Rigoberto , Sherrill C. David , Schaefer Henry F.
Published: May 2003

For selected first- and second-row atoms, correlation-optimized Gaussian k functions have been determined and used in the construction of septuple-ζ basis sets for the correlation-consistent cc-pVXZ and aug-cc-pVXZ series. Restricted Hartree–Fock (RHF) and second-order Møller–Plesset (MP2) total and pair energies were computed for H, N, O, F, S, H2, N2, HF, H2O, and (H2O)2 to demonstrate the consistency of the new septuple-ζ basis sets as extensions of the established (aug)-cc-pVXZ series. The pV7Z and aug-pV7Z sets were then employed in numerous extrapolation schemes on the test species to probe the accuracy limits of the conventional MP2 method vis-à-vis explicitly correlated (MP2-R12/A) benchmarks. For (singlet, triplet) pairs, (X+12)−n functional forms with n=(3, 5) proved best for extrapolations. The (mean abs. relative error, std. dev.) among the 73 singlet pair energies in the dataset is (1.96%, 0.54%) and (1.72%, 0.51%) for explicit computations with the pV7Z and aug-pV7Z basis sets, respectively, but only (0.07%, 0.09%) after two-point, 6Z/7Z extrapolations with the (X+12)−3 form. The effects of k functions on molecular relative energies were examined by application of the septuple-ζ basis sets to the barrier to linearity and the dimerization energy of water. In the former case, an inherent uncertainty in basis set extrapolations persists which is comparable in size to the error (≈20 cm−1) in explicit aug-pV7Z computations, revealing fundamental limits of orbital expansion methods in the domain of subchemical accuracy (0.1 kcal mol−1).
Electron-stimulated desorption of D2O coadsorbed with CO2 ice at VUV and EUV energies
Authors: Šiller L. , Sieger M. T. , Orlando T. M.
Published: May 2003

Electron-induced processes in water ice and CO2 ice are important in planetary science. We have measured desorption yields of positive ion products (C+, O+, D+, D3O+, CO+, O2+) produced as a result of exposing pure CO2 and D2O adsorbed on CO2 ices to 10–100 eV (i.e., the vacuum-ultraviolet–extreme-ultraviolet energy region) electron beams. Measurements on pure CO2 ice irradiated with 40-eV electrons suggest that pores in the CO2 ice are responsible for trapping molecular precursors for bimolecular reactions that are responsible for O2+ production. Investigations of pure CO2 ice irradiated with a 100-eV electron beam reveals production of O2+ by a second channel attributed to post-ion molecule collisions that are strongly influenced by low-energy electrons trapped in the ice. The most significant finding is that the D+ and D3O+ yields as a function of both dose and time indicate that the D2O adsorbed on CO2 at 30 K diffuses into the CO2 ice or desorbs into vacuum very rapidly. Both processes are a result of electron irradiation.
Enhancement of photoluminescence in manganese-doped ZnS nanoparticles due to a silica shell
Authors: Ethiraj Anita S. , Hebalkar Neha , Kulkarni S. K. , Pasricha Renu , Urban J. , Dem C. , Schmitt M. , Kiefer W. , Weinhardt L. , Joshi S. , Fink R. , Heske C. , Kumpf C. , Umbach E.
Published: May 2003

Zinc sulphide nanoparticles doped with manganese (ZnS:Mn) have been stabilized using thioglycerol [HSCH2CH(OH)CH2OH] molecules. The nanoparticles (∼1.7 nm) are highly stable and exhibit photoluminescence at ∼600 nm when excited with ultraviolet light. For increasing luminescence and stability the particles are further treated with tetraethylorthosilicate (TEOS)[Si(C2H5O)4] in an aqueous medium, yielding either a disordered silica matrix or spherical core-shell particles of up to ∼900 nm size with strongly enhanced luminescence under certain conditions. Photoluminescence, excitation spectroscopy, transmission electron microscopy, energy dispersive analysis of x-rays, x-ray diffraction, Raman spectroscopy, and x-ray photoelectron spectroscopy measurements have been performed for the characterization of the ZnS:Mn nanoparticles alone, in the silica matrix as well as in spherical silica shells. Among other things, the analysis indicates that the thioglycerol capping has been affected by the coating neither in the silica matrix nor in core-shell particles.
Discrete tunneling current fluctuations in metal–water–metal tunnel junctions
Authors: Boussaad S. , Xu B. Q. , Nagahara L. A. , Amlani I. , Schmickler W. , Tsui R. , Tao N. J.
Published: May 2003

We have studied electron tunneling through water between two metal electrodes supported on a solid substrate and observed random fluctuations in the tunneling current between two discrete levels. The two-level fluctuations persist when changing the concentration and the valency of the ions, and pH of the water solutions. A given two-level fluctuation is, in general, not affected by the applied bias voltage, but it is usually disrupted by changing the width of the tunnel gap. We attribute the discrete conductance fluctuations to random trapping or escaping of a single electron in or from a localized state in the tunnel gap.
Interaction between surfaces with solvophobicity or solvophilicity immersed in solvent: Effects due to addition of solvophobic or solvophilic solute
Authors: Kinoshita Masahiro
Published: May 2003

Integral equation theories with bridge functions incorporated in the closure equations are employed to analyze how the solvent-induced interaction between surfaces is influenced by solute addition to the solvent. The solvent particles interact through a hard-core plus attractive potential. The surfaces are solvophobic or solvophilic, and the solute has rather high solvophobicity or solvophilicity: A total of four combinations of the surface and solute properties are considered. The solute addition always leads to a downward shift, a shift in a more attractive direction, of the surface interaction (except at very small surface separations). The shift becomes more pronounced as the solute solvophobicity or solvophilicity increases and the solute concentration becomes higher. Overall, the solute effects are the smallest when the solute is neither solvophobic nor solvophilic. The physical origins of the shift are discussed in detail by relating the interaction to the structure of the solvent–solute mixture confined between two surfaces.
Ab initio simulation of water interaction with the (100) surface of pyrite
Authors: Stirling András , Bernasconi Marco , Parrinello Michele
Published: May 2003

Car–Parrinello simulations have been performed to study the interaction of water with pyrite (100) surface. The stability and the structural and electronic properties of both the molecular and dissociative adsorptions have been addressed. We found a very strong preference for molecular adsorption on the surface iron sites, in agreement with experiment. The dissociative chemisorption of water is energetically disfavored and is even locally unstable; the dissociated fragments transform back to the stable molecular form in a short molecular dynamics run. The calculations revealed that hydrogen bonding plays an important role in the stabilization of the adsorbed water for both the molecular and the dissociative states. We have shown that water forms a coordinative covalent bond with the surface iron atoms by donating electron to the empty iron dz2 orbitals which are the lowest empty states on the clean surface. At full coverage, the sulfur 3p states thus become the lowest available empty states and therefore the subject of possible electron-transfer reactions.
Excitons, polarons, and laser action in poly(p-phenylene vinylene) films
Authors: Österbacka R. , Wohlgenannt M. , Shkunov M. , Chinn D. , Vardeny Z. V.
Published: May 2003

We have used a multitude of linear and nonlinear cw optical spectroscopies to study the optical properties of water precursor poly(p-phenylene vinylene) (PPV) thin films. These spectroscopies include absorption, photoluminescence, photoinduced absorption and their respective optically detected magnetic resonance, and electroabsorption spectroscopy. We have studied singlet and triplet excitons, polarons, and laser action in PPV films. We found that the lowest-lying absorption band is excitonic in origin. It consists of two absorption components due to a bimodal distribution of the polymer chain conjugation lengths. Electroabsorption spectroscopy unambiguously shows the positions of the lowest-lying odd parity exciton 1Bu at 2.59 eV and two of the higher-lying even-parity excitons, namely, mAg at 3.4 eV and kAg at 3.7 eV. From these exciton energies we obtained a lower bound for the exciton binding energy in PPV, Eb(min)=E(mAg)−E(1Bu)=0.8 eV. The quantum efficiency spectrum for triplet exciton photogeneration consists of two contributions; the intersystem crossing and, at higher energies, singlet fission. From the onset of the singlet fission process at ESF=2ET, we could estimate the energy of the lowest-lying triplet exciton, 1 3Bu, at 1.55 eV, with a singlet–triplet splitting as large as 0.9 eV. From photoinduced absorption spectroscopy we measured the triplet–triplet transition, T→T*, to be 1.45 eV. The quantum efficiency spectrum for polaron photogeneration also consists of two contributions: one extrinsic and the other intrinsic. The latter shows a monotonically increasing function of energy with an energy onset at 3.3 eV. The intrinsic photogeneration process is analyzed with a model of free-electron–hole pair photogeneration, which separate more effectively at higher energy. The carrier generation quantum yield at 3.65 eV is estimated to be 0.5%. The quantum efficiency for photoluminescence, on the other hand, shows one single step-function process, with an onset at 2.4 eV. The photoluminescence spectrum is well structured, showing five phonon side bands with 190 meV separation. We have also studied laser action in PPV thin films and microcavities such as microrings and microdisks. The effective gain spectrum is calculated and the estimated threshold excitation intensity for laser action for the 0-1 transition is found to be in good agreement with the data, with an estimated exciton density of 2×1018 cm−3. Lasing from microring devices shows several narrow waveguide modes, with intermode spacing of 0.45 nm that corresponds to an effective mode refractive index, neff=1.45. The spectral width of the laser modes is resolution limited and gives a lower estimate of the cavity quality factor, Q. For microrings we found Q>5000, which is limited by self-absorption in the polymer film.
Mathematical model of reaction rate oscillations on a chain of nm-sized catalyst particles
Authors: Peskov N. V. , Slinko M. M. , Jaeger N. I.
Published: May 2003

The model of reaction rate oscillations over the surface of nanoparticles embedded into zeolite matrix is numerically investigated. The reaction rate oscillations on each particle are described by a lumped model. The reactions on separate particles interact via the gas diffusion through the pores, which is modeled in the frame of the Maxwell-Stefan approach. The reaction reveals a complex dynamical behavior if a nonhomogeneous distribution of reagent concentrations exists along the chain of particles with a sufficiently large gradient near the ends of the chain.
Nucleation in binary polymer blends: Effects of adding diblock copolymers
Authors: Wang Jiafang , Zhang Hongdong , Qiu Feng , Wang Zhen-Gang , Yang Yuliang
Published: May 2003

The effects of adding AB diblock copolymers to A/B binary blends on the structure and thermodynamics of critical nuclei are studied using the self-consistent field theory. At a fixed ratio of the amount of the two homopolymers, depending on the degree of polymerization and composition of the diblocks, their addition to the blends can either increase or decrease the nucleation free energy barrier relative to the pure A/B blends. The qualitative trend can be deduced from the shift in the coexistence boundary and the spinodal. The distribution of diblock copolymers in critical nuclei depends on the composition of the diblocks and the quench depth. Near the coexistence, symmetric diblocks exhibit surfactant behavior, being highly concentrated on the interface of the critical nuclei. Near the spinodal, they act more like co-solvent with a relatively uniform distribution.
Modeling reactive compatibilization of a binary blend with interacting particles
Authors: Cheng Mary Hongying , Balazs Anna C. , Yeung Chuck , Ginzburg Valeriy V.
Published: May 2003

We use a multiscale computational approach to study reactive compatibilization in an immiscible binary AB blend that contains A-like and B-like interacting molecules. These molecules are modeled as spherical particles that react at the A/B interface to form A-B dumbbells. Through these simulations, we investigate the reaction kinetics and interfacial morphology of the system as a function of time for different densities of reacting molecules and diffusivities of the dumbbells. The results provide insight into the factors that affect the structural evolution of the interface between the incompatible A and B domains. In particular, we find that for sufficiently high densities of reacting molecules, the initially flat interface is unstable at later times. The instability is initiated by a vanishing of the surface tension, but the amplification of the initial instability is determined by the Brownian motion of the dumbbells. The interfacial width (defined as the root-mean-square variation of the interfacial position) grows as t1/2 and the domains form a lamellar structure at long times.
Energy landscapes, ideal glasses, and their equation of state
Authors: Shell M. Scott , Debenedetti Pablo G. , La Nave Emilia , Sciortino Francesco
Published: May 2003

Using the inherent structure formalism originally proposed by Stillinger and Weber [Phys. Rev. A 25, 978 (1982)], we generalize the thermodynamics of an energy landscape that has an ideal glass transition and derive the consequences for its equation of state. In doing so, we identify a separation of configurational and vibrational contributions to the pressure that corresponds with simulation studies performed in the inherent structure formalism. We develop an elementary model of landscapes appropriate for simple liquids that is based on the scaling properties of the soft-sphere potential complemented with a mean-field attraction. The resulting equation of state provides an accurate representation of simulation data for the Lennard-Jones fluid, suggesting the usefulness of a landscape-based formulation of supercooled liquid thermodynamics. Finally, we consider the implications of both the general theory and the model with respect to the so-called Sastry density and the ideal glass transition. Our analysis shows that a quantitative connection can be made between properties of the landscape and a simulation-determined Sastry density, and it emphasizes the distinction between an ideal glass transition and a Kauzmann equal-entropy condition.
Determination of the interaction potential of the ground electronic state of Ne2 by high-resolution vacuum ultraviolet laser spectroscopy
Authors: Wüest A. , Merkt F.
Published: May 2003

A map of the rovibrational energy level structure of the ground electronic state of Ne2 has been determined from a high-resolution spectroscopic measurement of the II 0u+←X 0g+ transition in the vacuum ultraviolet around 16.8 eV. The positions of the six bound rotational levels of the ground and of the three bound rotational levels of the first excited vibrational state were used to determine the interaction potential of the ground electronic state of Ne2. This experimental potential is compared with previous theoretical results.
High-resolution infrared spectra of the OCS–H2, –HD, and –D2 van der Waals complexes in liquid helium droplets
Authors: Grebenev Slava , Sartakov Boris G. , Toennies J. Peter , Vilesov Andrey F.
Published: May 2003

van der Waals complexes consisting of a single OCS molecule with either a single para-H2, normal H2, ortho-D2, or a HD molecule have been prepared inside large ultracold (0.37 K) He4 droplets and even colder (0.15 K) mixed He4/3He droplets. Their infrared (ν=2061 cm−1) spectra were measured using diode laser droplet beam depletion spectroscopy. The well-resolved rotational structure with up to about 18 lines could be very well fitted with the Hamiltonian of a free asymmetric top, with rotational constants which were found to be between 3 and 9 times smaller than for the corresponding free complexes. As recently reported [S. Grebenev et al., J. Chem. Phys. 114, 617 (2001)] the structure of the complex, obtained from the change in the moments of inertia for the three isotopomers, is in very good agreement with ab initio calculations and the high-resolution infrared spectra of the free complexes. These results provide new insight into the interaction of impurity molecules with the superfluid helium environment.
Probing the B22–2A1 conically intersecting electronic states of ClO2 through photodetachment spectroscopy of its negative ion
Authors: Krishnan Gireesh M. , Mahapatra Susanta
Published: May 2003

The photodetachment spectroscopy of the ClO2− ion is studied theoretically with the aid of a time-dependent wave packet method. The theoretical findings in conjunction with the experimental observations unambiguously established the existence of a conical intersection in the excited B22–2A1 electronic manifold of ClO2. The highly diffused vibrational structure of this electronic manifold observed in the experimental recording is attributed to the nonadiabatic effects associated with this conical intersection. In our study we employed the near-equilibrium potential energy surfaces reported by Peterson and Werner [J. Chem. Phys. 96, 8948 (1992)] along the symmetric stretching and bending vibrations of ClO2. The potential energy surfaces along the asymmetric stretching vibration are assumed to be harmonic. The dynamics of the system is studied within a linear vibronic coupling scheme, and the strength of the coupling is explicitly determined by ab initio methods. The effect of the next higher A22 electronic state of ClO2 on the above photoelectron band is also discussed. The nonradiative decay dynamics of the system mediated by the vibronic coupling is examined, and the findings are correlated with the femtosecond time-resolved experiment.
Photoelectron and photoionization spectroscopy of weakly bound aluminum–methylamine complexes
Authors: Li Shenggang , Rothschopf Gretchen K. , Fuller Jason F. , Yang Dong-Sheng
Published: May 2003

Aluminum–methylamine complexes are produced in pulsed molecular beams. Their electronic spectra are obtained using threshold photoionization and zero-electron-kinetic-energy photoelectron spectroscopies and interpreted using density functional and ab initio calculations. The photoelectron spectra reveal ground electronic states and intermolecular and ligand-based vibrations of Al–NHn(CH3)3−n and Al+–NHn(CH3)3−n (n=0–2), adiabatic ionization energies of Al–NHn(CH3)3−n, and a low-lying excited electronic state of Al–NH2CH3. In addition, the spectroscopic measurements and theoretical calculations show strong methyl substitution effects on the ionization energies and metal–ligand binding. Striking spectral differences are discovered between these aluminum complexes and previously studied gallium and indium analogues.
An optical–optical double-resonance study of the Rydberg states of O2. I. The ns and nd (gerade) states excited via single-rotational levels of the b 1Σ0g+ valence state
Authors: Sheard Howard A. , Ridley Trevor , Lawley Kenneth P. , Donovan Robert J.
Published: May 2003

The ns (n=4–9) and nd (n=3–8) Rydberg states of O2 converging on O2+ X 2Π1/2g and X 2Π3/2g have been studied using optical–optical double resonance via single rotational levels of the initially excited b 1Σ0g+ valence state, together with multiphoton ionization. Both ns and nd states show a transition from (Λ,S) coupling to (Ω,ω) coupling as n increases. Transitions to all four components of an ns cluster are observed and rotational linestrengths show that the ns 3Π2,1,0g+←←b 1Σ0g+ transitions borrow intensity from the ns 1Π1g←←b 1Σ0g+ transition.
An optical–optical double-resonance study of the Rydberg states of O2. II. The np and nf (ungerade) states excited via single-rotational levels of the b 1Σ0g+ valence state
Authors: Sjödin A. Marica , Ridley Trevor , Lawley Kenneth P. , Donovan Robert J.
Published: May 2003

The np (n=3–10) and nf (n=4–9) Rydberg states of O2 converging on O2+ X 2Π1/2,g and X 2Π3/2g have been studied between 75 000 and 99 900 cm−1 using optical–optical double resonance with multiphoton ionization. Three-photon excitation from single rotational levels of the initially excited b 1Σ0g+ valence state was used to access these states. The nf states show a strong tendency towards (Ω,ω) coupling for all values of n, whereas the np states appear to be best described by (Λ,S) coupling for n⩽8. The intensities of some of the 5f bands are anomalously high due to accidental resonances with the 3s d 1Π1g Rydberg state at the two-photon level.