bluuebunny/crossref_metadata_2025 huggingface dataset:
Fast measurement of picoamp plasma flows using trapped electron clouds
Authors: Kabantsev A. A. , Driscoll C. F.
Published: October 2004

We demonstrate that magnetized electron clouds can diagnose picoamp ion currents (or equivalent neutralized plasma flows) on a kHz time scale. This could be used to measure the dynamics of neutral plasma losses to the walls, e.g., along divertor field lines. In essence, a current passing through an electron cloud in a Penning trap transfers angular momentum to the cloud, driving an easily measured orbital “diocotron” instability (from ion currents) or orbital damping (from electron currents). With neutralized plasma flows, the predominant effect is from the lower velocity (i.e., higher density) charge species. Experiments with electron, ion, and neutralized currents have fully characterized this collective (collisionless) electrostatic interaction, and demonstrate the picoamp and kHz resolutions.
X-ray calibration of the time resolved crystal spectrometer SXDHR-1t of the Ligne d’Intégration Laser
Authors: Reverdin C. , Morlens A. S. , Angelier B. , Bourgade J. L. , Boutin J. Y. , Briat M. , Charles G. , Duval A. , Estadieu A. , Cholet C. , Gontier D. , Husson D. , Jacquet H. P. , LeBreton J. P. , Lidove G. , Marchet B. , Marmoret R. , Maroni R. , Millier P. , Raimbourg J. , Remond C. , Rosch R. , Soullié G. , Stemmler P. , Troussel P. , Ulmer J. L. , Villette B. , Wrobel R.
Published: October 2004

The time resolved crystal x-ray spectrometers called SXDHR-lt of the Ligne d’Intégration laser is presented. It is necessary to calibrate all x-ray sensitive elements of diagnostics before using them in laser matter interaction experiments. In particular, crystals need to be calibrated. Measurements of the integrated coefficient of reflection of a beryl cylindrical crystal used in this spectrometer were performed with synchrotron radiation and with an x-ray tube and are presented. A test of the homogeneity of the reflection of the crystal was also performed. Aging or accidental pollution of x-ray diagnostics installed around target chambers is always possible. This happened to the DMX broadband spectrometer installed on the OMEGA Laser Facility at the University of Rochester (Laboratory for Laser Energetics) and this changed the spectral sensitivity of its channels. The evolution over time of the x-ray sensitivity needs to be critically assessed and if necessary x-ray sensitive elements will need to be recalibrated.
Evaluation of site-site bridge diagrams for molecular fluids
Authors: Vatamanu J. , Cann N. M.
Published: October 2004

The presence of bridge functions in formally exact integral equation theories is the primary obstacle preventing the extraction of exact fluid structure from these theories. The bridge functions are typically neglected but in many fluids their impact may be significant. Each bridge function can be subdivided into bridge diagrams, which are well defined but difficult to evaluate. The calculation of bridge diagrams for the Chandler–Silbey–Ladanyi (CSL) integral equation theory [D. Chandler, R. Silbey, and B. Ladanyi, Mol. Phys. 46, 1335 (1982)] is the subject of this paper. In particular, we evaluate the diagrams required to yield an exact theory up to the first power in density [O(ρ1)] and provide algorithms that remain feasible for any molecule. Further, the bridge diagrams are evaluated and compared with the f-bond and h-bond formulations. Exact bridge diagrams are numerically evaluated for several chiral molecules, for two polar dimers, and for SPC/E water [W. L. Jorgensen, J. Chandrasekhar, J. D. Madura, R. W. Impey, and M. L. Klein, J. Chem. Phys. 79, 926 (1983)]. The quality of the diagrams is assessed in two ways: First, the predicted interatomic distributions are compared with those obtained from Monte Carlo simulations. Second, the connectivity constraints are evaluated and the errors in satisfying these exact relationships are compared for the f-bond and h-bond formulations. For apolar fluids, a clear improvement in CSL theory is evident with the inclusion of O(ρ0) and O(ρ1) diagrams. In contrast, for polar fluids, the inclusion of bridge diagrams does not lead to improvement in the structural predictions.
Energy transfers and spectral eddy viscosity in large-eddy simulations of homogeneous isotropic turbulence: Comparison of dynamic Smagorinsky and multiscale models over a range of discretizations
Authors: Hughes Thomas J. R. , Wells Garth N. , Wray Alan A.
Published: October 2004

Energy transfers within large-eddy simulation (LES) and direct numerical simulation (DNS) grids are studied. The spectral eddy viscosity for conventional dynamic Smagorinsky and variational multiscale LES methods are compared with DNS results. Both models underestimate the DNS results for a very coarse LES, but the dynamic Smagorinsky model is significantly better. For moderately to well-refined LES, the dynamic Smagorinsky model overestimates the spectral eddy viscosity at low wave numbers. The multiscale model is in good agreement with DNS for these cases. The convergence of the multiscale model to the DNS with grid refinement is more rapid than for the dynamic Smagorinsky model.
Ab initio rate constants from hyperspherical quantum scattering: Application to H+C2H6 and H+CH3OH
Authors: Kerkeni Boutheïna , Clary David C.
Published: October 2004

The dynamics and kinetics of the abstraction reactions of H atoms with ethane and methanol have been studied using a quantum mechanical procedure. Bonds being broken and formed are treated with explicit hyperspherical quantum dynamics. The ab initio potential energy surfaces for these reactions have been developed from a minimal number of grid points (average of 48 points) and are given by analytical functionals. All the degrees of freedom except the breaking and forming bonds are optimized using the second order perturbation theory method with a correlation consistent polarized valence triple zeta basis set. Single point energies are calculated on the optimized geometries with the coupled cluster theory and the same basis set. The reaction of H with C2H6 is endothermic by 1.5 kcal/mol and has a vibrationally adiabatic barrier of 12 kcal/mol. The reaction of H with CH3OH presents two reactive channels: the methoxy and the hydroxymethyl channels. The former is endothermic by 0.24 kcal/mol and has a vibrationally adiabatic barrier of 13.29 kcal/mol, the latter reaction is exothermic by 7.87 kcal/mol and has a vibrationally adiabatic barrier of 8.56 kcal/mol. We report state-to-state and state-selected cross sections together with state-to-state rate constants for the title reactions. Thermal rate constants for these reactions exhibit large quantum tunneling effects when compared to conventional transition state theory results. For H+CH3OH, it is found that the CH2OH product is the dominant channel, and that the CH3O channel contributes just 2% at 500 K. For both reactions, rate constants are in good agreement with some measurements.
High sensitivity neutron detector for Z
Authors: Ruggles L. E. , Porter J. L. , Simpson W. W. , Vargas M. F. , Zagar D. M. , Hartke R. , Buersgens F. , Symes D. R. , Ditmire T.
Published: October 2004

We have developed, calibrated, and tested a high sensitivity neutron detector that can be operated in the harsh x-ray bremsstrahlung environment that exists in experiments conducted on the 20 MA Z z-pinch facility located at Sandia National Laboratories in Albuquerque, New Mexico. The detector uses a scintillator coupled to a microchannel-plate photomultiplier tube detector and extensive x-ray shielding.
X-ray polarization spectroscopy for measurement of anisotropy of hot electrons generated with ultraintense laser pulse
Authors: Inubushi Yuichi , Nishimura Hiroaki , Ochiai Masayuki , Fujioka Shinsuke , Izawa Yasukazu , Kawamura Tohru , Shimizu Seiji , Hashida Masaki , Sakabe Syuji
Published: October 2004

Polarization spectroscopy is a useful diagnostic method to measure directly the anisotropy of hot electron velocity distribution function inside ultrahigh intense laser produced plasmas. Polarization of a Cl–He α line (2.79 keV) from double-layer targets, which consist of polystyrene and chlorinated polystyrene, was observed with a toroidally curved crystal spectrograph. It was found that the line from the target surface is polarized more parallel to the surface, whereas that from a deeper region is more perpendicular to it. The polarization of emitted x rays corresponds to the shape of velocity distribution function of hot electrons. The anisotropic shape of hot electron velocity distribution function depending on depth of a target was clarified for the first time using this diagnostic method.
Solvation dynamics in supercritical fluids: Equilibrium versus nonequilibrium solvent response functions
Authors: Egorov S. A.
Published: October 2004

We present a theoretical study of solvation dynamics in supercritical fluids. Molecular dynamics simulations show a significant difference between equilibrium and nonequilibrium solvent response functions, especially pronounced at medium and low solvent densities. We propose an analytical theory for the nonequilibrium solvation function based on the generalized nonlinear Smoluchowski-Vlasov equation. The theory is shown to be in good agreement with simulation, providing an accurate description of the nonequilibrium time-dependent solvent density profile around the solute over a wide range of supercritical solvent densities. The nonequilibrium solvent response function is shown to reflect gradual solvent clustering around the excited solute.
Active beam spectroscopy diagnostics for ITER: Present status (invited)
Authors: Malaquias A. , von Hellermann M. , Tugarinov S. , Lotte P. , Hawkes N. , Kuldkepp M. , Rachlew E. , Gorshkov A. , Walker C. , Costley A. , Vayakis G.
Published: October 2004

An overview is given of the present design of the active beam spectroscopy diagnostics for the ITER. Present spatial resolution and signal-to-noise indicate that, in principle, all proposed measurements are possible covering the entire plasma minor radius. Calculations show that the mirror based periscopes and the impurity coating of the first mirror affect the signal strength and the polarization characteristics of the measured spectra having an impact in the measurement accuracy. On-line calibration techniques and methods to access the first mirror status are addressed.
Nonpolar a-plane p-type GaN and p-n Junction Diodes
Authors: Chakraborty Arpan , Xing H. , Craven M. D. , Keller S. , Mates T. , Speck J. S. , DenBaars S. P. , Mishra U. K.
Published: October 2004

Growth and electrical characteristics of Mg-doped p-type nonpolar (112¯0) a-plane GaN films, grown on (11¯02) r-plane sapphire substrates via metalorganic chemical vapor deposition, were investigated as a function of growth rate, the ammonia to trimethylgallium flow ratio (V/III ratio), and the growth temperature. The electrical conductivity of the films exhibited a strong dependence on the growth parameters. Secondary-ion-mass-spectroscopy measurements indicated that more Mg was incorporated at higher growth rate and at lower growth temperatures. The Mg concentration in the films increased linearly with the Mg flow. A maximum hole concentration of 6.8×1017cm−3 was achieved at room temperature for a Mg concentration of 7.6×1019cm−3, corresponding to 0.9% ionization. Further increase in the Mg concentration resulted in increased surface roughness as well as a significant decrease in the hole concentration. p-n junction diodes were fabricated using nonpolar a-plane GaN, and the current-voltage characteristics of these diodes showed a sharp turn-on at ∼3V.
Structure, surface excess and effective interactions in polymer nanocomposite melts and concentrated solutions
Authors: Hooper J. B. , Schweizer K. S. , Desai T. G. , Koshy R. , Keblinski P.
Published: October 2004

The Polymer Reference Interaction Site Model (PRISM) theory is employed to investigate structure, effective forces, and thermodynamics in dense polymer-particle mixtures in the one and two particle limit. The influence of particle size, degree of polymerization, and polymer reduced density is established. In the athermal limit, the surface excess is negative implying an entropic dewetting interface. Polymer induced depletion interactions are quantified via the particle-particle pair correlation function and potential of mean force. A transition from (nearly) monotonic decaying, attractive depletion interactions to much stronger repulsive-attractive oscillatory depletion forces occurs at roughly the semidilute-concentrated solution boundary. Under melt conditions, the depletion force is extremely large and attractive at contact, but is proceeded by a high repulsive barrier. For particle diameters larger than roughly five monomer diameters, division of the force by the particle radius results in a nearly universal collapse of the depletion force for all interparticle separations. Molecular dynamics simulations have been employed to determine the depletion force for nanoparticles of a diameter five times the monomer size over a wide range of polymer densities spanning the semidilute, concentrated, and melt regimes. PRISM calculations based on the spatially nonlocal hypernetted chain closure for particle-particle direct correlations capture all the rich features found in the simulations, with quantitative errors for the amplitude of the depletion forces at the level of a factor of 2 or less. The consequences of monomer-particle attractions are briefly explored. Modification of the polymer-particle pair correlations is relatively small, but much larger effects are found for the surface excess including an energetic driven transition to a wetting polymer-particle interface. The particle-particle potential of mean force exhibits multiple qualitatively different behaviors (contact aggregation, steric stabilization, local bridging attraction) depending on the strength and spatial range of the polymer-particle attraction.
Anomalous viscoelasticity near the isotropic-nematic phase transition in liquid crystals
Authors: Jose Prasanth P. , Bagchi Biman
Published: October 2004

Recent optical Kerr effect experiments have shown that orientational relaxation of nematogens shows a pronounced slow down of the response function at intermediate times and also a power law decay near the isotropic-nematic (I-N) transition. In many aspects, this behavior appears to be rather similar to the ones observed in the supercooled liquid near-glass transition [Cang et al., J. Chem. Phys. 118, 9303 (2003)]. We have performed molecular dynamics simulations of model nematogens (Gay-Berne with aspect ratio 3) to explore the viscoelasticity near the I-N transition and also investigated the correlation of viscoelasticity (if any) with orientational relaxation. It is found that although the viscosity indeed undergoes a somewhat sharper than normal change near the I-N transition, it is not characterized by any divergencelike behavior (like the ones observed in the supercooled liquid). The rotational friction, on the other hand, shows a much sharper rise as the I-N transition is approached. Interestingly, the probability distribution of the amplitude of the three components of the stress tensor shows anisotropy near the I-N transition—similar anisotropy has also been seen in the deeply supercooled liquid [Phys. Rev. Lett. 89, 25504 (2002)]. Frequency dependence of viscosity shows several unusual behaviors: (a) There is a weak, power law dependence on frequency [η′(ω)∼ω−α] at low frequencies and (b) there is a rapid increase in the sharp peak observed in η′(ω) in the intermediate frequency on approach to the I-N transition density. These features can be explained from the stress-stress time correlation function. The angular velocity correlation function also exhibits a power law decay in time. The reason for this is discussed.
Linear scaling computation of the Fock matrix. VII. Parallel computation of the Coulomb matrix
Authors: Gan Chee Kwan , Tymczak C. J. , Challacombe Matt
Published: October 2004

We present parallelization of a quantum-chemical tree-code [J. Chem. Phys. 106, 5526 (1997)] for linear scaling computation of the Coulomb matrix. Equal time partition [J. Chem. Phys. 118, 9128 (2003)] is used to load balance computation of the Coulomb matrix. Equal time partition is a measurement based algorithm for domain decomposition that exploits small variation of the density between self-consistent-field cycles to achieve load balance. Efficiency of the equal time partition is illustrated by several tests involving both finite and periodic systems. It is found that equal time partition is able to deliver 91%–98% efficiency with 128 processors in the most time consuming part of the Coulomb matrix calculation. The current parallel quantum chemical tree code is able to deliver 63%–81% overall efficiency on 128 processors with fine grained parallelism (less than two heavy atoms per processor).
Theoretical investigation of the linear and second-order nonlinear susceptibilities of the 3-methyl-4-nitropyridine-1-oxyde (POM) crystal
Authors: Guillaume Maxime , Botek Edith , Champagne Benoı̂t , Castet Frédéric , Ducasse Laurent
Published: October 2004

The linear and nonlinear optical properties of the 3-methyl-4-nitropyridine-1-oxyde (POM) crystal have been evaluated using semiempirical quantum chemistry techniques. The scheme includes (i) the evaluation of the polarizability and first hyperpolarizability of increasingly large one-dimensional, two-dimensional, and three-dimensional clusters of POM, (ii) the use of the time-dependent Hartree–Fock approach to determine the static and dynamic responses in combination with semiempirical Austin model 1 Hamiltonian, (iii) the assessment, for the POM monomer and dimer, of the electron correlation effects using second-order Møller–Plesset perturbation theory with several basis sets containing polarization and diffuse functions, (iv) the assessment of the validity of the multiplicative scheme and its application to get effective polarizability and first hyperpolarizability of the POM unit cell in the crystal, (v) the use of a sum-over-states approach to attribute the first hyperpolarizability to a dominant charge-transfer excited state, and (vi) comparison with experimental data as well as with calculated values obtained using the oriented gas approximation.
A theoretical study of the staggered and eclipsed forms of the dinuclear complex Mn Re(CO)10
Authors: Palmer Michael H. , Camp Philip J. , Tanjaroon Chakree , Keck Kristen S. , Kukolich Stephen G.
Published: October 2004

Two possible conformers of the dinuclear complex Mn Re(CO)10, each of C4v symmetry, with eclipsed and staggered conformations, have been analyzed theoretically. Using both the B3LYP and BP86 density functionals we find that the staggered form is lower in energy. A determination of the B3LYP potential energy surface as a function of the Mn-Re distance is presented for both conformers. The computed bond lengths, bond angles, and rotational constant for the staggered conformation compare favorably with the results from microwave experiments. The harmonic frequencies for the staggered structure have been determined using several basis sets, with both analytical and finite difference methods. These unscaled vibrational frequencies, together with their intensities for both infrared and Raman activity, are used to assign the three most intense experimental IR and Raman bands, and in particular, the νCO region. The lowest A2 vibration was calculated to occur at 41 cm−1 in the staggered conformer; this frequency becomes imaginary in the (saddle point) eclipsed form. Several fundamentals remain to be observed experimentally.
Structural and electronic properties of small beryllium clusters: A theoretical study
Authors: Srinivas Sudha , Jellinek Julius
Published: October 2004

Geometric structures and electronic properties of small beryllium clusters (Ben, 2⩽n⩽9) are investigated within the gradient-corrected density functional theory. The computations are performed with the Becke exchange and Perdew-Wang correlation functionals. Both low and high multiplicity states are considered. A predominance of higher multiplicity states among the low-energy isomers of the larger clusters is found. An analysis of the variations in the structural and electronic properties with cluster size is presented, and the results are compared with those of earlier studies.
Circular dichroism effect for linear molecules induced by a resonant circularly polarized pumping optical field
Authors: Zheng Ren-hui , Chen Dong-ming , Wei Wen-mei , He Tian-jing , Liu Fan-chen
Published: October 2004

In this paper we propose and discuss the laser-induced circular dichroism (LICD) effect, which is expected to occur in linear molecules pumped by a strong circular resonant light beam. The effect is to be detected via the absorption of a weak circularly polarized probe beam on another transition. Analogous to the external magnetic field in magnetic circular dichroism the resonant circular polarized pumping optical field can induce the nonzero antisymmetric rotational polarizabilities of a linear molecule, and cause the LICD effect. LICD contains three distinct contributions from M-dependent splittings of the sublevels |JM〉 due to the ac Stark effect, from the differences of Boltzmann statistical distributions among the ground state sublevels |JM〉 due to the ac Stark splittings, and from the changes of occupation probability in rotational sublevels |JM〉 due to the pumping effect. The fundamental formulas for the above three terms of LICD have been deduced by the density matrix method. As an example, the LICD for CO molecules have been calculated. The results indicate that in comparison with the rotationally resolved magnetic vibrational circular dichroism experiment, LICD may be measurable and form a basis of a different kind of CD spectroscopy.
Comment on “Electrostatics in periodic slab geometries. I” [J. Chem. Phys. 117, 2496 (2002)]
Authors: Bródka A.
Published: October 2004

Arnold and co-workers reported a new method for summation of Coulomb interactions in a system with slab geometry. They derived the electrostatic layer correction term, which constitutes the unwanted interaction with the layer replicas in the three-dimensional summation method. It has been shown that the method can be obtained from the Ewald-type method worked out for a system with two-dimensional periodicity.
The growth of perylene on Ru(0001)
Authors: Honying Mao , Han Huang , Qiao Chen , Richardson N. V. , Yue Wu , Jianhua Zhang , Haiyang Li , Pimo He , Shining Bao
Published: October 2004

The structure of perylene adsorbed on Ru(0001) surface has been studied by ultraviolet photoemission spectroscopy (UPS) and low-energy electron diffraction. An ordered p(4×4) structure is observed from a monolayer (about 4 Å thickness) of the perylene on Ru(0001) surface. UPS measurements show the molecular features, from the perylene multiplayer, between 2 and 10 eV below the Fermi level. Angle-resolved ultraviolet photoemission spectroscopy measurements suggest that the perylene molecular plane is parallel to the substrate. Temperature dependent UPS measurements show that the perylene multilayer is stable on Ru(0001) surface up to 125 °C. The desorption of the multilayer and the decomposition of the monolayer are observed above 125 °C.
Effective potential in density matrix functional theory
Authors: Nagy Á. , Amovilli C.
Published: October 2004

In the previous paper it was shown that in the ground state the diagonal of the spin independent second-order density matrix n can be determined by solving a single auxiliary equation of a two-particle problem. Thus the problem of an arbitrary system with even electrons can be reduced to a two-particle problem. The effective potential of the two-particle equation contains a term vp of completely kinetic origin. Virial theorem and hierarchy of equations are derived for vp and simple approximations are proposed. A relationship between the effective potential up of the shape function equation and the potential vp is established.
Irradiation-induced improvement in crystal quality of epitaxial Ag∕Si(111) films
Authors: Takahiro Katsumi , Kawatsura Kiyoshi , Nagata Shinji , Yamamoto Shunya , Naramoto Hiroshi
Published: October 2004

It has been found that 0.5MeV Si+ irradiation at −150°C greatly improves the crystal quality of epitaxially grown Ag films on Si(111) substrates. The improvements include the decrease in the population of twinning grains and the decrease in mosaic spread in the films. To clarify the mechanism of the irradiation-induced improvement in crystal quality (IIICQ), polycrystalline Ag films with [111] preferred orientation were also irradiated at −150°C. Grain growth in a lateral direction was clearly observed in such Ag films using x-ray diffraction (XRD) analysis. It is evident that the atomic rearrangements occur at grain boundaries due to low-temperature irradiation. On irradiation with 0.5MeV Si ions at −150°C the cross section for the grain growth, estimated by XRD analysis, is about 1.8×10−16cm2, very close to that achieved with IIICQ (1.9×10−16cm2) estimated by Rutherford backscattering spectroscopy/channeling analysis. This result indicates that the mechanism of the IIICQ for the epitaxial Ag∕Si(111) films is very similar to that of the ion bombardment enhanced grain growth.
Extrapolating bound state data of anions into the metastable domain
Authors: Feuerbacher Sven , Sommerfeld Thomas , Cederbaum Lorenz S.
Published: October 2004

Computing energies of electronically metastable resonance states is still a great challenge. Both scattering techniques and quantum chemistry based L2 methods are very time consuming. Here we investigate two more economical extrapolation methods. Extrapolating bound states energies into the metastable region using increased nuclear charges has been suggested almost 20 years ago. We critically evaluate this attractive technique employing our complex absorbing potential/Green’s function method that allows us to follow a bound state into the continuum. Using the Πg2 resonance of N2− and the Πu2 resonance of CO2− as examples, we found that the extrapolation works suprisingly well. The second extrapolation method involves increasing of bond lengths until the sought resonance becomes stable. The keystone is to extrapolate the attachment energy and not the total energy of the system. This method has the great advantage that the whole potential energy curve is obtained with quite good accuracy by the extrapolation. Limitations of the two techniques are discussed.
Divide-and-conquer local correlation approach to the correlation energy of large molecules
Authors: Li Wei , Li Shuhua
Published: October 2004

A divide-and-conquer local correlation approach for correlation energy calculations on large molecules is proposed for any post-Hartree-Fock correlation method. The main idea of this approach is to decompose a large system into various fragments capped by their local environments. The total correlation energy of the whole system can be approximately obtained as the summation of correlation energies from all capped fragments, from which correlation energies from all adjacent caps are removed. This approach computationally achieves linear scaling even for medium-sized systems. Our test calculations for a wide range of molecules using the 6-31G or 6-31G** basis set demonstrate that this simple approach recovers more than 99.0% of the conventional second-order Møller-Plesset perturbation theory and coupled cluster with single and double excitations correlation energies.
Relaxation time scales in collective dynamics of liquid alkali metals
Authors: Mokshin Anatolii V. , Yulmetyev Renat M. , Hänggi Peter
Published: October 2004

In this paper the investigation of the dynamical processes of liquid alkali metals is executed by analyzing the time scales of relaxation processes in liquids. The obtained theoretical dynamic structure factor S(k,ω) for the case of liquid lithium is found to be in excellent agreement with the recently received inelastic x-ray scattering data. The comparison and interrelation with other theories are given here. Finally, an important part of this paper is the confirmation of the scale uniformity of the dynamic processes in liquid alkali metals predicted by some previous molecular dynamic simulation studies.
Dissociation energies of six NO2 isotopologues by laser induced fluorescence spectroscopy and zero point energy of some triatomic molecules
Authors: Michalski G. , Jost R. , Sugny D. , Joyeux M. , Thiemens M.
Published: October 2004

We have measured the rotationless photodissociation threshold of six isotopologues of NO2 containing N14, N15, O16, and O18 isotopes using laser induced fluorescence detection and jet cooled NO2 (to avoid rotational congestion). For each isotopologue, the spectrum is very dense below the dissociation energy while fluorescence disappears abruptly above it. The six dissociation energies ranged from 25 128.56 cm−1 for N1416O2 to 25 171.80 cm−1 for N1518O2. The zero point energy for the NO2 isotopologues was determined from experimental vibrational energies, application of the Dunham expansion, and from canonical perturbation theory using several potential energy surfaces. Using the experimentally determined dissociation energies and the calculated zero point energies of the parent NO2 isotopologue and of the NO product(s) we determined that there is a common De=26 051.17±0.70 cm−1 using the Born-Oppenheimer approximation. The canonical perturbation theory was then used to calculate the zero point energy of all stable isotopologues of SO2, CO2, and O3, which are compared with previous determinations.
Statistical mechanical theory for the structure of steady state systems: Application to a Lennard-Jones fluid with applied temperature gradient
Authors: Attard Phil
Published: October 2004

The constrained entropy and probability distribution are given for the structure that develops in response to an applied thermodynamic gradient, as occurs in driven steady state systems. The theory is linear but is applicable to gradients with arbitrary spatial variation. The phase space probability distribution is also given, and it is surprisingly simple with a straightforward physical interpretation. With it, all of the known methods of equilibrium statistical mechanics for inhomogeneous systems may now be applied to determining the structure of nonequilibrium steady state systems. The theory is illustrated by performing Monte Carlo simulations on a Lennard-Jones fluid with externally imposed temperature and chemical potential gradients. The induced energy and density moments are obtained, as well as the moment susceptibilities that give the rate of change of these with imposed gradient and which also give the fluctuations in the moments. It is shown that these moment susceptibilities can be written in terms of bulk susceptibilities and also that the Soret coefficient can be expressed in terms of them.
Multidimensional infrared spectroscopy of the N–H bond motions in formamide
Authors: Park Jaehun , Ha Jeong-Hyon , Hochstrasser Robin M.
Published: October 2004

The heterodyned two-dimensional (2D) IR spectra and equilibrium dynamics of the N–H stretching motion of DCONHD in deuterated formamide, DCOND2, were studied with 80 fs pulses at 3 μm. The time evolution of the heterodyned 2D IR spectra, pump-probe spectra, and photon echo peak shift demonstrate that interstate dynamics is occurring by relaxation of the original N–H excitation. The N–H vibrational frequency correlation function can be expressed as a sum of three exponentials with correlation times 0.24 ps, 0.8 ps, and 11 ps. The intermediate component is attributed to motions of the N–H⋯O unit involving only slight angular variations of the N–H bond. The slow component is attributed to the structure breaking and making. The anisotropy decay confirmed that the significant angular N–H bond motion occurs on the 11 ps time scale. The fast component, which is the least well determined, might correspond to the modulation of the H-bond distance without angular motion. The correlation coefficient between the pumped and relaxed state distributions was +0.51, implying that the excited state phase memory is only slightly diminished by the relaxation of the N–H excitation. The relaxed modes are concluded to be local to the driven N–H mode.
Current driven electromagnetic wave amplification by double quantum wire superlattice
Authors: Aizin G. R. , Horing N. J. M. , Mourokh L. G. , Kovalev V. M.
Published: October 2004

We analyze the electrodynamic interaction of an incident terahertz electromagnetic wave with a current-carrying lateral double-quantum-wire superlattice (at normal incidence). The superlattice (in the x-y plane) is taken to consist of two parallel quantum-wire sublattices, each of period d, shifted with respect to each other by distance Δ in the transverse y direction. The parallel quantum wires of the sublattices are all oriented in the x direction. The two sublattices are taken to carry equal steady currents in opposite directions, and are coupled by Coulomb forces alone, with tunneling neglected. We recently showed that quasi-one-dimensional plasmons of such double-quantum-wire superlattice systems become unstable when the electron drift velocity falls between the phase velocities of the acoustic and optical plasmon modes of the Coulomb-coupled wire subsystems. Here, we employ a random phase approximation for plasmon dispersion taken jointly with the full system of Maxwell equations to describe the electrodynamic interaction of the incident terahertz electromagnetic radiation with the superlattice electron system. Coupling of the electromagnetic wave with the plasmon excitations is provided by introducing a metal grating with the grating stripes oriented perpendicular to the quantum wires. We have determined the transmission, absorption, and reflection coefficients for an incident terahertz electromagnetic wave propagating through the grating-superlattice system, demonstrating that amplification of the terahertz electromagnetic radiation occurs in the region of plasma instability. Our numerical calculations show that this effect occurs at experimentally achievable drift velocities in GaAs-based structures.
Kinetics of a polysoap collapse
Authors: Lee Nam-Kyung , Abrams Cameron F.
Published: October 2004

We study the dynamics of collapse of a polysoap by means of large-scale molecular dynamics simulation and scaling arguments. A polysoap consists of a hydrophilic backbone and hydrophobic side chains attached at regular intervals along the backbone. In selective solvent conditions, the hydrophobic components aggregate, forcing the hydrophilic backbone to form loops anchored at the surface of the core, ultimately forming a micelle. The kinetics of polysoap collapse includes two major mechanisms: (1) early aggregation of the hydrophobic side chains controlled by first-order kinetics whose rate constant is given by a contact probability and (2) coalescence into larger clusters which requires activation to overcome energy barriers due to excluded volume repulsions between intermediate micelle coronas. In the late stage, the energy barrier is increasing as p3/2, with p the number of aggregated side chains in an intermediate micelle. The corresponding late-stage rate constant decays exponentially as ∼exp(−p3/2).
Structural analysis of porphyrin molecular squares using molecular mechanics and density-functional methods
Authors: Miljacic Ljubomir , Sarkisov Lev , Ellis Donald E. , Snurr Randall Q.
Published: October 2004

“Molecular squares” formed from Re(CO)3Cl corners and porphyrin sides have potential applications as hosts for catalytic sites and as building blocks for membranes. In these materials, knowledge of the conformations of the squares is important. Molecular-mechanics (MM) and density-functional (DF) calculations have been used iteratively in this work to find the minimum-energy configurations of several porphyrin molecular squares. MM predicts that the steric and torsional interactions at connecting junctures of the square framework determine the overall geometry. Torsional degrees of freedom around these junctures were therefore analyzed using DF methods, giving further insight and helping choose among MM force-field options. Single-point DF calculations on the entire squares showed that the energy and conformation of the entire square could be reliably obtained by performing DF calculations on the critical elements of the square and then piecing them together. This “piecewise” strategy allows for both the major torsional motions and the most important local relaxations of large supramolecular species such as molecular squares.
Mechanisms and dynamics of the metastable decay in Ar2+
Authors: Fedor J. , Głuch K. , Parajuli R. , Matt-Leubner S. , Echt O. , Scheier P. , Märk T. D.
Published: October 2004

A detailed experimental as well as theoretical investigation of the properties of the metastable dissociation Ar2+→Ar++Ar is presented. The mass-analyzed ion kinetic energy (MIKE) scan technique has been performed using a three sector field mass spectrometer. The possible mechanisms of the metastability of Ar2+ have been examined and the observed decay process is assigned to the II(1/2)u→I(1/2)g bound to continuum radiative transition, in agreement with earlier work. The calculation of the theoretical shape of the kinetic energy release distribution of fragment ions allowed us to construct the theoretical MIKE peak and compare it with the raw experimental data. The accuracy of various sets of potential energy curves for Ar2+ is discussed, as well as the way of production of the metastable Ar2+[II(1/2)u] electronic state by electron impact. Excellent agreement between the experimental data and theoretical model has been observed.
Correlated geminal wave function for molecules: An efficient resonating valence bond approach
Authors: Casula Michele , Attaccalite Claudio , Sorella Sandro
Published: October 2004

We show that a simple correlated wave function, obtained by applying a Jastrow correlation term to an antisymmetrized geminal power, based upon singlet pairs between electrons, is particularly suited for describing the electronic structure of molecules, yielding a large amount of the correlation energy. The remarkable feature of this approach is that, in principle, several resonating valence bonds can be dealt simultaneously with a single determinant, at a computational cost growing with the number of electrons similar to more conventional methods, such as Hartree-Fock or density functional theory. Moreover we describe an extension of the stochastic reconfiguration method, which was recently introduced for the energy minimization of simple atomic wave functions. Within this extension the atomic positions can be considered as further variational parameters, which can be optimized together with the remaining ones. The method is applied to several molecules from Li2 to benzene by obtaining total energies, bond lengths and binding energies comparable with much more demanding multiconfiguration schemes.
Surface wave propagation in an ideal Hall-magnetohydrodynamic plasma jet in flowing environment
Authors: Sikka Himanshu , Kumar Nagendra , Zhelyazkov Ivan
Published: October 2004

The behavior of the Hall-magnetohydrodynamic (Hall-MHD) sausage and kink waves is studied in the presence of steady flow. The influence of the flow both inside and outside the plasma slab is taken into account. The plasma in the environment is considered to be cold and moves with the different flow velocity outside the slab. In the limit of parallel propagation, dispersion relation is derived to discuss the propagation of both the modes. Numerical results for the propagation characteristics are obtained for different Alfvénic Mach number ratios inside and outside the slab. It is found that the dispersion curves for both surface modes, namely, the sausage and kink ones in cold plasma show complexities in their behavior in terms of multivalued portions of the curves. These multivalued portions correspond to the different normalized phase velocities for the same value of Alfvénic Mach number. In contrast to the conventional MHD surface waves which are assumed to be pure surface waves or pseudosurface waves, surface waves are obtained which are bulk waves for very small dimensionless wave numbers, then turn to leaky waves and finally transform to pure surface waves for values of dimensionless wave number greater than one.
Vapor flows in the continuum limit in the presence of a small amount of noncondensable gas
Authors: Taguchi Satoshi , Aoki Kazuo , Takata Shigeru
Published: October 2004

Steady flows of a vapor around its condensed phase of arbitrary shape, on the surface of which evaporation and condensation of the vapor may take place, are considered in the presence of a small amount of a noncondensable gas. By a systematic asymptotic analysis of the Boltzmann system, the present authors have derived the fluid-dynamic system describing such flows in the continuum limit in the case where the amount of the noncondensable gas is infinitesimal compared with that of the vapor [K. Aoki, S. Takata, and S. Taguchi, Eur. J. Mech. B/Fluids 22, 51 (2003)]. In the present study, the fluid-dynamic system is applied to some physical problems, and it is demonstrated with concrete examples that such a tiny amount of the noncondensable gas causes dramatic changes of the overall vapor flows.
Shock formation in the presence of entropy gradients in fluids exhibiting mixed nonlinearity
Authors: Muralidharan Srevatsan , Sujith R. I.
Published: October 2004

The nonlinear steepening of a finite amplitude disturbance in a quiescent gas with very high specific heat in its near-critical regime is analyzed. The atypical phenomenon of rarefaction shocks are found to occur in the region where the nonlinearity parameter is negative. The undisturbed medium is assumed to be at rest with entropy gradients (temperature gradients). The steepening of the wave front in such a nonhomentropic medium, where a variation in the nonlinearity parameter is present, is investigated using the technique of wave front expansion. A calorically imperfect gas governed by an arbitrary equation of state is considered. An exact closed form solution is obtained for the evolution of the slope of the disturbance. In particular, results have been discussed for a van der Waal’s fluid in its gaseous phase in the near-critical region. The distortion of both compression and rarefaction wave forms are examined and the corresponding shock formation distances are calculated.
Drift of rigidly rotating spirals under periodic and noisy illuminations
Authors: Zhang Hong , Wu Ning-Jie , Ying He-Ping , Hu Gang , Hu Bambi
Published: October 2004

Under the weak deformation approximation, the motion of rigidly rotating spirals induced by periodic and noisy illuminations are investigated analytically. We derive an approximate but explicit formula of the spiral drift velocity directly from the original reaction-diffusion equation. With this formula we are able to explain the main features in the periodic and noisy illuminations induced spiral drift problems. Numerical computations of the Oregonator model are carried out as well, and they agree with the main qualitative conclusions of our analytical results.
Oxygen vacancies on TiO2 (110) from first principles calculations
Authors: Oviedo J. , San Miguel M. A. , Sanz J. F.
Published: October 2004

We have carried out a systematic study of oxygen vacancy formation on the TiO2 (110) surface by means of plane-wave pseudopotential density-functional theory calculations. We have used models with the mean number of vacancies per surface unit cell being θ=0.25 and θ=0.5. The study comprises several kind of vacancies within the outermost layers of the surface. The use of a suitable set of technical parameter is often essential in order to get accurate results. We find that the presence of bridging vacancies is energetically favored in accordance to experimental data, although the formation of sub-bridging vacancies might be possible at moderate temperatures. Surprisingly, the spin state of the vacancy has little influence on the results. Atomic displacements are also analyzed and found to be strongly dependent on the particular arrangement of vacancies.
Dynamic interfacial effect of electroosmotic slip flow with a moving capillary front in hydrophobic circular microchannels
Authors: Yang Jun , Lu Fuzhi , Kwok Daniel Y.
Published: October 2004

Miniaturization of chemical analysis using microfabrication is an emerging technology. The use of polymeric materials as opposed to conventional glass substrate is also a promising alternative. As most polymeric materials are hydrophobic relative to glass, we describe here the implication for the loading process of electroosmotic flow (EOF) when a three-phase (solid–liquid–vapor) contact line exists. The presence of these interfaces can result in a large Laplace pressure that resists EOF and hence hinders its flow performance. This effect depends on the phenomenological contact angle at the solid–liquid interface. In our model for EOF, we considered simultaneously the presence of an electric double layer, liquid slips via a weaker solid–liquid interaction and Laplace pressure across a liquid–vapor interface.
Optical properties of Cu nanoclusters supported on MgO(100)
Authors: Del Vitto Annalisa , Sousa Carmen , Illas Francesc , Pacchioni Gianfranco
Published: October 2004

The vertical transitions of Cu atoms, dimers, and tetramers deposited on the MgO surface have been investigated by means of ab initio calculations based either on complete active space second-order perturbation theory or on time-dependent density functional theory. Three adsorption sites have been considered as representative of the complexity of the MgO surface: regular sites at flat (100) terraces, extended defects such as monoatomic steps, and point defects such as neutral oxygen vacancies (F or color centers). The optical properties of the supported Cu clusters have been compared with those of the corresponding gas-phase units. Upon deposition a substantial modification of the energy levels of the supported cluster is induced by the Pauli repulsion with the substrate. This causes shifts in the optical transitions going from free to supported clusters. The changes in cluster geometry induced by the substrate have a much smaller effect on the optical absorption bands. On F centers the presence of filled impurity levels in the band gap of MgO results in a strong mixing with the empty levels of the Cu atoms and clusters with consequent deep changes in the optical properties of the color centers. The results allow to interpret electron energy loss spectra of Cu atoms deposited on MgO thin films.
Compressed CO2-enhanced solubilization of 1-butyl-3-methylimidazolium tetrafluoroborate in reverse micelles of Triton X-100
Authors: Li Junchun , Zhang Jianling , Han Buxing , Wang Yong , Gao Liang
Published: October 2004

We carried out the first study about the effect of a compressed gas on the properties of reverse micellar solutions with ionic liquid (IL) polar cores. And the properties of compressed CO2/cyclohexane/1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4])/Triton X-100 (TX-100) system were investigated at 288.2, 293.2, 298.2, 308.2 K and different pressures by using phase behavior measurement, small-angle x-ray scattering, and UV-Vis techniques. The concentration of the surfactant in the solution was 0.3 mol/l (M). It was found that compressed CO2 could enhance solubilization of the IL in the reverse micelles considerably at suitable pressures, and formation of the reverse micelles could be controlled easily by pressure. Increase of CO2 pressure resulted in decrease of the micellar sizes at fixed [bmim][BF4]-to-surfactant molar ratios (w), and the size of the reverse micelles increased with the increase of w values. The polarity of the IL cores increased continuously with increasing w value.