bluuebunny/crossref_metadata_2025 huggingface dataset:
Monte Carlo microsolvation simulations for excited states using a mixed-Hamiltonian model with polarizable and vibrating waters: Applications to the blueshift of the H2CO 1(π*←n) excitation
Authors: Kawashima Y. , Dupuis M. , Hirao K.
Published: July 2002

The previously formulated quantum mechanical molecular mechanical (QM/MM) model applicable to the microsolvated solute excited state, the QM/MM-pol-vib/CAV model, has been combined with a Monte Carlo averaging scheme to derive the averaged properties of the solvated solutes. The methodology was applied to the electronic (1π*←n) excitation of formaldehyde in water. We first performed Monte Carlo MM/MM calculations to generate the water configurations. Then, we chose 400 configurations for the QM/MM excitation energy calculations. Finally, we carried out complete active space self-consistent field calculations to derive the average excitation energy. Several different sizes of water clusters with 23, 54, and 108 water molecules were used. The first solvent shell of the clusters was found to be well structured. We also calculated the shift of the vertical excitation energies and of the dipole moments resulting from microsolvation. The calculated blueshift of the vertical excitation energies using a nonpolarizable MM potential was in the range 2610–2690 cm−1, and using a polarizable MM potential, was in the range 2540–2660 cm−1. Thus the treatment that considered polarization improved the results, although the improvement was not significant. The cluster size dependence has been found to be small which indicates that the outer water molecules have little influence to the solute–solvent interaction. The dipole moments of the ground and excited states showed a significant increase arising from microsolvation. The ground state dipole moment showed larger solvent shifts than the excited state dipole moment. This leads to a decrease in the strength of the hydrogen bond between the oxygen atom of formaldehyde and hydrogen atoms of water after excitation. We analyzed the structures of the solvent configurations that produced both high and low blueshifts. The first solvent shell is proven to play a principal role in the solvent effect.
Propagation of solitons of the Derivative Nonlinear Schrödinger equation in a plasma with fluctuating density
Authors: Ruderman M. S.
Published: July 2002

The propagation of quasi-parallel nonlinear small-amplitude magnetohydrodynamic waves in a cold Hall plasma with fluctuating density is studied. The density is assumed to be a homogeneous random function of one spatial variable. The modified Derivative Nonlinear Schrödinger equation (DNLS) is derived with the use of the mean waveform method developed by Gurevich, Jeffrey, and Pelinovsky [Wave Motion 17, 287 (1993)], which is the generalization of the reductive perturbation method for nonlinear waves propagating in random media. This equation differs from the standard DNLS equation by one additional term describing the interaction of nonlinear waves with random density fluctuations. As an example of the use of the modified DNLS equation, the quasi-adiabatic evolution of a one-parametric DNLS soliton propagating through a plasma with fluctuating density is studied.
Third order mode optically pumped semiconductor laser
Authors: De Rossi A. , Semaltianos N. , Chirlias E. , Vinter B. , Ortiz V. , Berger V.
Published: July 2002

Lasing action on a third order waveguide mode is demonstrated at room temperature under optical pumping, in a specifically designed quantum well laser structure. The AlGaAs heterostructure involves barriers which ensure that the third order waveguide mode has a higher overlap with the single quantum well emitter than the fundamental mode. Third order mode operation of a laser structure opens the way to modal phase matched parametric down conversion inside the semiconductor laser itself. It is a first step towards the realization of semiconductor twin photon laser sources, needed for quantum information experiments.
Excited-state absorption of Cr3+ in Cs2NaScCl6
Authors: Wenger Oliver S. , Güdel Hans U. , Kück Stefan
Published: July 2002

Excited-state absorption (ESA) measurements on Cr3+ doped Cs2NaScCl6 in the 6300–17 500 cm−1 spectral region at 15 K and 300 K are reported. Two ESA bands due to T2g4→4 T1g(4F) and T2g4→4 T1g(4P) transitions are observed. The large bandwidth of the t2g2eg-intraconfigurational T2g4→4 T1g(4F) transition at 15 K is rationalized using a single configurational coordinate model involving the eg Jahn–Teller coordinate. At 15 K the effective peak stimulated emission cross section at 10 870 cm−1 is about 3.5×10−20 cm2, and there is essentially no overlap with ESA at this temperature. At 300 K the ESA cross section is larger than the stimulated emission cross section over the whole range of spontaneous emission.
Particle velocity fluctuations and correlation lengths in dilute sedimenting suspensions
Authors: Bernard-Michel G. , Monavon A. , Lhuillier D. , Abdo D. , Simon H.
Published: July 2002

We measure the instantaneous velocity of particles sedimenting in a three-dimensional container at low particulate Reynolds numbers. We aim at characterizing the main specificities of the particle velocity fluctuations. We obtain the local and instantaneous Eulerian velocity field from particle image velocimetry: a thin Yag laser light sheet (about two diameters thick) illuminates the particles from one side of the cell to the other. Our measurements are therefore spatially localized and, together with the squared cross sections of the cells, these are the two main originalities of our instrumentation. Four different cells and three different particle sizes give access to aspect ratios (cell width W over particle radius a) ranging from about 50 up to 800. We confirm the existence of eddy-like structures for the velocity fluctuations. The structure size is found to be almost independent of the volume fraction Φ for 6.25×10−4<Φ<5×10−2 for a fixed aspect ratio W/a, in seeming contradiction with the results of Segrè et al. [Phys. Rev. Lett. 79, 2574 (1997)]. The velocity fluctuations’ profiles are close to parabolic for the smallest aspect ratios but display a plateau value in the central part of the cell for higher aspect ratios. The unexpected result of our experiments is the large distance of influence of the boundaries: in fact, the plateau values are observed to saturate for aspect ratios larger than a few hundreds for Φ=1×10−2, for example. Moreover, the saturated plateau values scale as φ0.45±0.05, in contrast with the φ1/3 scaling observed in most previous works.
Transient coating flow of a thin non-Newtonian fluid film
Authors: Kim Kyu-Tae , Khayat Roger E.
Published: July 2002

The interplay between non-Newtonian effects, gravity, and substrate topography is examined in this theoretical study for the transient two-dimensional flow of a thin non-Newtonian film. The study is a continuation of the previous work by Khayat and Welke [Phys. Fluids 13, 355 (2001)], which focused on the influence of inertia on a Newtonian film. The fluid emerges from a channel and is driven by a pressure gradient maintained inside the channel. The substrate is assumed to be stationary and of arbitrary shape. The flow is dictated by the thin-film equations of the “boundary layer” type, which are solved by expanding the flow field in terms of orthonormal modes in the transverse direction and using the Galerkin projection, combined with a time-stepping implicit scheme, and integration along the flow direction. Gravity and substrate topography can have a significant effect on transient behavior, but this effect varies significantly, depending on whether the fluid is Newtonian, shear thinning or shear thickening. Wave formation and propagation, as well as steady film flow are examined. It is found that shear-thickening fluids tend to accumulate near the channel exit, exhibiting a standing wave that grows with time. This behavior clearly illustrates the difficulty faced with coating shear-thickening fluids at any level of inertia. The influence of the substrate topography has been explored in the case of undulated substrate.
Drying of acoustically levitated droplets of liquid–solid suspensions: Evaporation and crust formation
Authors: Yarin A. L. , Brenn G. , Kastner O. , Tropea C.
Published: July 2002

Measurements on the drying behavior of droplets of aqueous suspensions of glass beads are carried out in a tube levitator at varying drying conditions. The experimental results show that there exist two distinct drying stages. At stage I the droplet diameter decreases. At stage II it is constant, while solvent evaporation still proceeds, as it follows from the permanent upward motion of the drop toward the pressure node. A theory for describing the evaporation of a liquid/solid suspension droplet in the acoustic field at the first drying stage is developed and validated with the measurement data. The theory solves the mass transfer problem of liquid vapor from the droplet surface. It takes the convective influence of the acoustic field on the mass transfer into account, using results from the earlier work of the authors on the evaporation of acoustically levitated droplets. Comparison of the theory with the experiments supports the idea that at the first drying stage solvent evaporation proceeds from the wetted surface of suspension droplets. The acoustically driven mechanism of the evaporation process is related to the acoustic streaming flow. The present work does not propose any theory for the second drying stage. It presents only some physical estimates, suggesting that stage II begins from a relatively fast precipitation of the suspended particles (glass beads) in the outer layers of the droplet, and crust formation. The estimates also suggest that the further solvent evaporation proceeds through the pores of the crust. Liquid evaporation in the pores, according to the estimates, proceeds according to the ordinary diffusion-driven mechanism, which is a limiting process in this case.
Observations of structured and long-range transport in a large volume dusty (complex) plasma experiment
Authors: Thomas Edward , Amatucci William E. , Compton Christopher , Christy Brian
Published: July 2002

DUPLEX—the DUsty PLasma EXperiment at the Naval Research Laboratory—is a large volume dc glow discharge plasma device. The DUPLEX chamber is a transparent polycarbonate cylinder that is 40 cm in radius and 80 cm in height. Argon dc glow discharge plasmas are generated between a grounded cathode and a biased anode. The anode and cathode are separated by 15 to 20 cm. Clouds of 1 μm diameter alumina microparticles are suspended in the plasma—with the highest density directly above the cathode. However, particles have been detected throughout the entire plasma volume. This article reports on previously unidentified, and possibly unknown, phenomena observed in a dusty plasma. Two specific features will be the focus of this article. First, large microparticle clouds—up to 15 cm in diameter in some locations—with highly complex internal structures are identified. Second, long range—20 to 30 cm distances—periodic transport of microparticles from one region of the plasma to another is identified. It is believed that the large size and nonconducting boundary of the DUPLEX chamber facilitates the observation of these phenomena.
Angular and energy distributions of D2 molecules desorbing from sulfur and oxygen modified V(111) surfaces
Authors: Eibl Christian , Winkler Adolf
Published: July 2002

The energy and angular distribution of deuterium molecules desorbing from a vanadium (111) surface modified either by oxygen or by sulfur has been studied, using time-of-flight spectroscopy. It has been shown that the desorption flux contains two contributions, a thermal and a hyperthermal contribution. The mean translational energy of the hyperthermal part can be described by 〈E〉=8.3⋅kTs and 5.8⋅kTs for the sulfur and oxygen covered V(111) surface, respectively. Interestingly, the mean translational energy of the hyperthermal contribution is independent of the desorption angle. The angular distribution of the hyperthermal desorption flux is forward focused and can be described by cos3.3 θ and cos4.3 θ functions for the sulfur and oxygen modified surface, respectively. From the angular flux distribution and the angle independent mean translational energy of the hyperthermal contribution one can conclude that normal energy scaling does not exist for this adsorption/desorption channel. This is mainly due to the strong geometric corrugation of the modified V(111) surfaces.
A polarizable mixed Hamiltonian model of electronic structure for micro-solvated excited states. I. Energy and gradients formulation and application to formaldehyde (1A2)
Authors: Dupuis M. , Aida M. , Kawashima Y. , Hirao K.
Published: July 2002

We describe an efficient implementation of a polarizable mixed Hamiltonian model of electronic structure that combines Hartree–Fock, Kohn–Sham, or multiconfiguration quantum-chemical wave functions with a polarizable and flexible molecular mechanics potential of water, and that is applicable to micro-solvated electronic excited states. We adopt a direct algorithm for the calculation of the polarization response of the solvent subsystem. The strategy facilitates the calculation of the energy of the system and of the forces with respect to the solute coordinates and the solvent coordinates, including for excited states. This capability opens the way to the determination of optimized, transition structures, force constants, and intrinsic reaction pathways for the solute–solvent system, and to molecular dynamics calculations to account for finite temperature effects. As an illustration we characterize the structure and energy of micro-solvated formaldehyde H2CO in its ground state and in its (π*←n)1 excited state. A novel perpendicular structure is found to be the lowest energy conformation of the H2CO1(π*←n):H2O complex. The all-quantum-chemical results and the mixed Hamiltonian results, with or without solvent polarizability, are in semiquantitative agreement. We comment on the choice of Lennard-Jones parameters associated with a solute excited state. Lennard-Jones parameters that yield good ground state structures and energies with the mixed Hamiltonian model, are found to be too soft for the micro-solvated excited state H2CO in the adiabatic (equilibrium micro-solvation) regime.
A polarizable mixed Hamiltonian model of electronic structure for solvated excited states. II. Application to the blue shift of the H2CO 1(π*←n) excitation in water
Authors: Dupuis M. , Kawashima Y. , Hirao K.
Published: July 2002

We illustrate an application of a polarizable mixed Hamiltonian model of solvation developed in the companion Paper I [J. Chem. Phys. 117, 1242 (2002)] and describe the structure of electronically excited formaldehyde in water. We used Hartree-Fock and multiconfiguration wave functions together with the tip3p, pol1, and pol2 interaction potentials combined with the Bartlett–Shavitt vibrational potential for water. We calculated the structure of H2CO (A11, A23, and A21) micro-solvated with 1 or 2 water molecules and we mimicked the aqueous environment with up to 81 waters with equilibrium solute–solvent configurations. We calculated the vertical and adiabatic excitations energies. The vertical absorption energy shows a blue shift between ∼1000 and ∼2500 cm−1 due to solvation, that is in fact already present in the micro-solvated systems and increases with the degree of solvation. The dipole moments of the ground and excited states show a marked increase with the degree of solvation. The polarizable character of the pol1 and pol2 water potentials has only a minor effect on the magnitude of the shift, even on the vertical excitation shift, with a reduction of ∼100 cm−1 in blue shift. The polarizable mixed model gives a satisfactory description of the formaldehyde–water hydrogen-bond structure and of the energetics. Those are very similar to the all-quantum chemical description when considering ground-state H2CO. For the excited states (A23 and A21) the H2CO---HOH distance in H2CO:1w is calculated to be ∼0.10 Å shorter with the polarizable mixed model than with the all-quantum chemical model, albeit the calculated hydrogen bond energies are in accord with the all-quantum chemical results and smaller than for the ground state. This finding suggests that, at least in the equilibrium solvation regime, the σ and ε Lennard-Jones parameters for the excited states of H2CO should have larger values than those used for the ground state, in accord with simple arguments based on the increased size and polarizability of the molecular excited state.
Simulations of vibrational relaxation in dense molecular fluids. II. Generalized treatment of thermal equilibration between a sample and a reservoir
Authors: Holian Brad Lee
Published: July 2002

The general problem of adiabatic relaxation, or thermal equilibration between a sample and a reservoir initially at two different temperatures, is presented. By thermostatting both the sample and reservoir, a nonequilibrium steady state can be set up to measure this relaxation rate. An accurate treatment of the coupling between sample and reservoir in the steady-state case leads to substantially improved agreement with vibrational relaxation rates obtained from adiabatic equilibration. We show that for large signal to noise ratio, the nonequilibrium simulation can be considerably more accurate than the direct equilibration measurement. We demonstrate two other examples of transport phenomena obtained from thermostatted sample-reservoir driving, namely, thermal conduction and shear flow in a fluid.
A stochastic Lagrangian model for acceleration in turbulent flows
Authors: Pope Stephen B.
Published: July 2002

A stochastic model is developed for the acceleration of a fluid particle in anisotropic and inhomogeneous turbulent flows. The model consists of an ordinary differential equation for velocity (which contains directly the acceleration due to the mean and rapid pressure gradients), and a stochastic model for the remainder of the acceleration, which is due to the slow pressure gradient and to viscosity. In addition to a rapid-pressure model, the stochastic model involves three tensor coefficients. For isotropic turbulence, the model reverts to that previously proposed by Sawford. At high Reynolds number the model is consistent with local isotropy and the Kolmogorov hypotheses, and tends to the generalized Langevin model for fluid-particle velocity. In this case two of the tensor coefficients are known in terms of the Kolmogorov constant C0, while the third is related to the coefficient in the generalized Langevin model. A complete analysis of the model is performed for homogeneous turbulent shear flow, for which there are Lagrangian data from direct numerical simulations. The main result is to establish the one-to-one correspondence between the model coefficients and the primary statistics, namely, the velocity and acceleration covariances and the tensor of velocity integral time scales. The autocovariances of velocity and acceleration obtained from the model are in excellent agreement with the direct numerical simulation (DNS) data. Future DNS studies of homogeneous turbulence can be used to investigate the dependence of the model coefficients on Reynolds number and on the imposed mean velocity gradients. The acceleration model can be used to generate a range of turbulence models which, in a natural way, incorporate Reynolds-number effects.
Thrust stand for ground tests of solid propellant microthrusters
Authors: Orieux S. , Rossi C. , Estève D.
Published: July 2002

A pendulum type balance has been designed and fabricated to characterize solid propellant microthrusters. The thrust measurement system mainly consists of a rigid arm rotating freely around a pivot and a feedback control loop. The deflection of the arm produced by the applied thrust is detected using a high frequency transmitter and an antenna. The principle of the measurement is to compensate exactly the thrust force applied to the pendulum by injecting a current in a coil. The measurement of this current gives a perfect image of the thrust force. This article presents the design and performances of the balance. The thrust force measurement sensitivity is 25 μN and the system bandwidth is 40 Hz. Using this balance, single cylindrical thrusters have been characterized. The results gave less than a 1% error between experiment and theoretical evaluation.
Raman spectra of nanocrystalline carbon nitride synthesized on cobalt-covered substrate by nitrogen-atom-beam-assisted pulsed laser ablation
Authors: Xu Ning , Cui Fuming , Lin Hao , Ying Zhifeng , Wu Jiada , Du Yuancheng , Li Fuming
Published: July 2002

Nanocrystalline carbon nitride films were synthesized on cobalt-covered Si(111) wafers using a nitrogen-atom-beam-assisted pulsed laser ablation method. Raman spectroscopy and other analyses showed that as-synthesized films were constructed primarily from nanometer-sized β-C3N4 and CNx crystallites. Catalyzation by the cobalt in the synthesis process is considered to play an important role in the formation of nanocrystalline β-C3N4. The effect of the experimental conditions on the characters of the as-synthesized films has been discussed.
Crystallization and related magnetotransport properties of amorphous manganite films grown by metalorganic chemical vapor deposition
Authors: Dubourdieu C. , Audier M. , Roussel H. , Sénateur J. P. , Pierre J.
Published: July 2002

La 1−x Sr x MnO 3 films were grown at 450 °C by pulsed liquid-injection metalorganic chemical vapor deposition on silicon substrates. The films were amorphous and exhibited a high resistivity (> 10 kΩ cm). Different kinds of annealing (650 °C for 1, 3, 6, and 10 h, 800 °C for 15 mn, 900 °C for 5 mn, and rapid thermal annealings) were performed in order to achieve crystallization through both variations of nucleation and growth rates. X-ray diffraction and transmission electron microscopy were used to characterize the different microstructures obtained. After annealing, the perovskite La1−xSrxMnO3 (x∼0.3) phase was obtained. The resistivity of the films ranged within two orders of magnitude depending on the annealing temperature and time. The magnetoresistive properties were also recovered and similar to those obtained on as-grown polycrystalline films. The rapid thermal annealing process proved to be efficient: the amorphous films exhibited, after a 90 s flash-annealing, an ordering temperature of 340 K, a resistivity of the order of 1 Ω cm, and a low-field magnetoresistance of 20% at 22 K and 0.2 T.
Formation and stability of small-size bubbles in garnet films
Authors: Ohkoshi M.
Published: July 2002

Anomalous magnetic bubble domains have been detected in garnet films after chopping stripe domains with a high intensity pulsed field. These bubbles have smaller diameters than typically observed bubbles under low bias fields. These small-size bubbles remain stable within a narrow bias field range, above which they collapse and below which they transit irreversibly into larger elliptical bubbles. The contrast exhibited by these bubbles in Faraday images is different from that of larger bubbles, suggesting that the magnetization of the small-size bubbles lies in the film plane.
Structural and magnetic phase transformation in metastable Fe–Cr alloys induced by ion irradiation
Authors: Wieser E. , Reuther H. , Prokert F. , Gorbunov A. , Tselev A. , Pompe W. , Levin A. A. , Meyer D. C. , Paufler P.
Published: July 2002

Unusual metastable paramagnetic phases have been observed in Fe–Cr thin films (thickness about 40 nm) fabricated by pulsed laser deposition. In the present article, x-ray diffraction and Mössbauer spectroscopy have been applied to follow the structural and magnetic phase transformation in these alloys induced by ion irradiation with a projected range positioned in the center of the films. It has been found that the critical dose for the transformation to the more stable body-centered cubic (bcc) structure depends on the initial phase of the film and the ion mass. The initial body-centered tetragonal phase, which forms in the alloys with low Cr content (∼30 at. %), can be completely transformed to the bcc phase already by a dose of 5×1015 Cr/cm2, whereas the primitive orthorhombic phase of roughly equiatomic Fe–Cr alloys is about four times more resistant against ion bombardment. A five times higher Ne ion dose is required to induce the same transformation as by the Cr bombardment. The observed effects are discussed in view of radiation damage caused by the different ions and the grade of affinity of the initial phase to the bcc one.
Mass spectrometric study of double photoionization of HBr molecules
Authors: Alagia Michele , Boustimi Mohamed , Brunetti Brunetto G. , Candori Pietro , Falcinelli Stefano , Richter Robert , Stranges Stefano , Vecchiocattivi Franco
Published: July 2002

The double photoionization of HBr molecules, by synchrotron radiation in the energy range between 25 and 55 eV, has been studied in a mass spectrometric experiment. The HBr2+ and Br2+ product ions have been detected by a photoelectron-photoion-coincidence technique, while the H++Br+ formation, which follows the double ionization of HBr, has been studied by photoelectron-photoion-photoion-coincidence technique. HBr2+ ions are produced with a threshold of 32.4±0.4 eV, while the dissociative channel leading to H++Br+, shows a threshold around 33 eV. The production of H+Br2+ occurs with a threshold of 40.2±0.4 eV. These results appear to be in a fairly good agreement with earlier nonrelativistic calculations of potential energy curves and also with values indirectly obtained from experimental Auger spectra.
Gradient symplectic algorithms for solving the Schrödinger equation with time-dependent potentials
Authors: Chin Siu A. , Chen C. R.
Published: July 2002

We show that the method of factorizing the evolution operator to fourth order with purely positive coefficients, in conjunction with Suzuki’s method of implementing time-ordering of operators, produces a new class of powerful algorithms for solving the Schrödinger equation with time-dependent potentials. When applied to the Walker–Preston model of a diatomic molecule in a strong laser field, these algorithms can have fourth order error coefficients that are three orders of magnitude smaller than the Forest–Ruth algorithm using the same number of fast Fourier transforms. Compared to the second order split-operator method, some of these algorithms can achieve comparable convergent accuracy at step sizes 50 times as large. Morever, we show that these algorithms belong to a one-parameter family of algorithms, and that the parameter can be further optimized for specific applications.
Dependence of turbulent scalar flux on molecular Prandtl number
Authors: Zhou Haibing , Cui Guixiang , Zhang Zhaoshun , Shao L.
Published: July 2002

The dependence of turbulent scalar flux on molecular Prandtl number is studied by direct numerical simulation of statistically stationary isotropic turbulence with uniform mean gradient of temperature. Both Reynolds averaged scalar flux and subgrid scalar flux are investigated at molecular Prandtl numbers ranging from 0.1 to 3.0. In order to consider the Reynolds number effect, two cases of grid resolution are computed, i.e., 1283 and 2563, with the Taylor-scale Reynolds numbers equaling 30 and 50, respectively. The turbulent Prandtl number is used to characterize the turbulent scalar flux. It is found that both Reynolds averaged turbulent Prandtl number (simplified as turbulent Prandtl number hereafter) and subgrid Prandtl number change with molecular Prandtl number significantly. The turbulent Prandtl number has been found to be a linearly reciprocal function of molecular Prandtl number, whereas the subgrid Prandtl number takes a minimum around Pr=1. The appearance of minimum subgrid Prandtl number around Pr=1 can be well understood based on the analysis of transfer spectrum of scalar flux.
Localized relaxation in a glass and the minimum in its orientational polarization contribution
Authors: Johari G. P. , Power G. , Vij J. K.
Published: July 2002

The dielectric permittivity and loss spectra of the glassy state of 5-methyl-2-hexanol obtained by quenching it from the liquid state has been studied. In one experiment, the spectra were studied at different temperatures as the quenched sample was heated at 0.1 K/min from 105.3 to 160.5 K. In the second experiment, the quenched sample was heated from 77 to 131.6 K and kept at that temperature for 14.6 ks. The relaxation rate, fm,β, the dielectric relaxation strength, Δεβ, and the distribution of relaxation time parameters, α and β, for the Johari–Goldstein process were determined. The parameter β was found to be equal to 1 and independent of both the temperature and time, Δεβ initially decreased on increasing the temperature, reached a minimum value at ∼145.6 K, and then increased. The plot of fm,β against the reciprocal temperature decreased in slope and at ∼140 K became linear. This indicates that fm,β increases on structural relaxation. In the course of the annealing at 131.6 K, Δεβ of the quenched sample decreased with time, approaching a plateau value. It is described by an equation, Δεβ(t)=Δεβ(t→∞)+[Δεβ(t=0)−Δεβ(t→∞)]exp[−(t/τ)], where t is the time, and τ (=3.5 ks) is the characteristic time. It is pointed out that contrary to the earlier finding, o-terphenyl shows a β relaxation in the equilibrium liquid state. A consideration of dielectric permittivity arising from small-angle motions of all molecules, which has been suggested as an alternative mechanism for the localized motions seen as β relaxation, indicates that this mechanism is inconsistent with the known increase in the equilibrium permittivity on cooling.
Molecular simulation study of nanoscale friction for alkyl monolayers on Si(111)
Authors: Zhang Luzheng , Jiang Shaoyi
Published: July 2002

Molecular dynamics (MD) simulations were performed to study nanoscale friction for alkyl monolayers terminated with -CH3 (hydrophobic) and -OH (hydrophilic) on Si(111) in the presence of water molecules. Variation of the -OH/-CH3) ratio results in different surface hydrophobicities. To calculate friction in MD simulations, two Si(111) surfaces coated with the alkyl monolayers were slid against each other. It was shown that the friction coefficient decreased quickly for hydrophilic monolayers, whereas it remained almost the same for hydrophobic monolayers, as the number of water molecules (relative humidity) increased. Simulation results are in good agreement with those from scanning force microscopy measurements for both hydrophilic and hydrophobic monolayers. The behavior of water molecules confined between hydrophobic or hydrophilic monolayers was also discussed. A fundamental understanding of nanoscale friction is critical to the design of coatings for microelectromechanical systems.
Dominant diffusing species in the growth of amorphous interlayer between Yb metal thin films and crystalline Si
Authors: Chi K. S. , Chen L. J.
Published: July 2002

The dominant diffusing species in the formation of an amorphous interlayer between Yb thin films and crystalline Si substrates have been determined by a Mo cluster marker experiment. Metal thin films with multilayered structures were deposited on both (111) and (001)Si substrates in an ultrahigh vacuum electron beam evaporation system. The positions of the Mo cluster markers relative to the Si substrates, before and after heat treatment, were determined by high-resolution transmission electron microscopy and energy dispersive analysis of x-ray as well as autocorrelation function analysis. The displacement of the Mo cluster markers in the amorphous interlayer during the Yb–Si interdiffusion indicates that Si atoms constitute the dominant diffusing species during the growth of amorphous interlayer.
Fluid variational theory for pressure dissociation in dense hydrogen: Multicomponent reference system and nonadditivity effects
Authors: Juranek Hauke , Redmer Ronald , Rosenfeld Yaakov
Published: July 2002

In a recent paper, standard hard-sphere variational theory has been applied to pressure dissociation in dense fluid hydrogen (Juranek and Redmer, J. Chem. Phys. 112, 3780 (2000)). The correlation contributions to the dissociation equilibrium were determined from the free energy functional using effective pair potentials and minimization with respect to the hard-sphere reference system. For simplicity, the Berthelot mixing rule was used to determine the H–H2 pair potential (which entails additive effective hard spheres), and single-component reference pair correlation functions were employed for evaluating the correlation integrals. In this paper, we employ multicomponent reference pair correlations, and we study the sensitivity of the results with respect to nonadditivity of the effective hard spheres. We compare our results with available ab initio simulation data.
Evolution of structural order in germanium ion-implanted amorphous silicon layers
Authors: Cheng S. L. , Lin H. H. , He J. H. , Chiang T. F. , Yu C. H. , Chen L. J. , Yang C. K. , Wu D. Y. , Chien S. C. , Chen W. C.
Published: July 2002

High-resolution transmission electron microscopy in conjunction with autocorrelation function analysis have been applied to investigate the evolution of structural order in germanium ion-implanted amorphous silicon (a-Si) layers. A high density of Si nanocrystallites as small as 1 nm in size was detected in as-implanted a-Si layers. The density of embedded nanocrystalline Si was found to diminish in a-Si layers with annealing temperature first then increase. The results are discussed in the context of free energy change with annealing temperature.
Tunneling-assisted impact ionization fronts in semiconductors
Authors: Rodin Pavel , Ebert Ute , Hundsdorfer Willem , Grekhov Igor
Published: July 2002

We discuss a type of ionization front in layered semiconductor structures. The propagation is due to the interplay of band-to-band tunneling and impact ionization. Our numerical simulations show that the front can be triggered when an extremely sharp voltage ramp (∼10 kV/ns) is applied in reverse direction to a Si p+–n–n+ structure that is connected in series with an external load. The triggering occurs after a delay of 0.7 to 0.8 ns. The maximal electrical field at the front edge exceeds 106 V/cm. The front velocity vf is 40 times faster than the saturated drift velocity vs. The front passes through the n-base with a thickness of 100 μm within approximately 30 ps, filling it with dense electron–hole plasma. This passage is accompanied by a voltage drop from 8 kV to a voltage in the order of 10 V. In this way a voltage pulse with a ramp up to 500 kV/ns can be applied to the load. The possibility to create a kilovolt pulse with such a voltage rise rate sets new frontiers in pulse power electronics.
The formation of dimers and trimers in free jet He4 cryogenic expansions
Authors: Bruch Ludwig W. , Schöllkopf Wieland , Toennies J. Peter
Published: July 2002

The formation of dimers, trimers, and tetramers in a free jet cryogenic expansion of He4 atoms has been studied by diffraction from a nanostructure transmission grating. The final average velocities, speed ratios and ambient temperatures of the expansions for source temperatures of 30, 12, and 6 K and source pressures between 0.1 and 80 bar were determined from time-of-flight measurements of the He atoms. The final mole fractions of the He2, He3, and He4 clusters in the beam were determined from the intensities of the corresponding first-order diffraction peaks for the same range of source conditions. For each source temperature, the final mole fractions of these small clusters first rise, pass through a maximum and then decrease with increasing source pressure. The processes leading to the formation of these clusters are simulated with a kinetic model that allows for density and temperature changes in the expanding beam. The best-fit three-body recombination rate constant for dimer formation increases by over three orders of magnitude as the thermal energy decreases from 1 K to 1 mK, in qualitative agreement with recent theories.
Water hexamer clusters: Structures, energies, and predicted mid-infrared spectra
Authors: Losada Martin , Leutwyler Samuel
Published: July 2002

We present an ab initio theoretical study of five low-energy isomers of the water hexamer {Chair, Cage(du)[1], Book, Prism, and Boat}, their intramolecular vibrations, binding energies De and dissociation energies D0. Møller–Plesset second order perturbation calculations using the aug-cc-pVTZ basis set at aug-cc-pVDZ optimized geometries including vibrational zero point energy corrections predict Chair to be the most stable isomer, followed closely by Cage(du)[1] (+0.02 kcal/mol) and Book (+0.05 kcal/mol), while Prism is 0.15 kcal/mol higher. The Boat conformer is least stable at both the De and D0 levels. The main focus is on the intramolecular normal modes of the five isomers. The calculated O–H stretching frequencies and intensities are compared to recent infrared spectra of water hexamer in supersonic jets, liquid-helium droplets and solid para-hydrogen matrices. The IR spectra indicate that Book and Chair are major species in the latter two environments and may also exist in supersonic jets. The (H2O)6 gas phase interconversion equilibria are calculated and predict that the most abundant isomer is Chair below 8 K, Cage between 8–26 K, and Book above 26 K. Several of the low-frequency vibrational modes are identified as low-amplitude precursors of the Chair↔Book↔Cage isomerization pathways.
Harmonic content of electron-impact source functions in inductively coupled plasmas using an “on-the-fly” Monte Carlo technique
Authors: Sankaran Arvind , Kushner Mark J.
Published: July 2002

Electron temperatures in low-pressure (<10s mTorr) inductively coupled plasma (ICP) reactors operating at 10s MHz do not significantly vary during the radio frequency (rf) cycle. There can be, however, considerable modulation of electron-impact source functions having high-threshold energies due to modulation of the tail of the electron energy distributions (EEDs). In many instances, it is convenient to use cycle-averaged values for these quantities in models due to the computational burden of computing and storing spatial and time-dependent EEDs. In this paper an “on-the-fly” (OTF) Monte Carlo technique is described to address these time-dependent plasma parameters. The OTF method directly computes moments of the EEDs during advancement of the trajectories of the pseudoparticles, thereby reducing computational complexities. The method can also be used to directly calculate the harmonic components of excitation, which can subsequently be used to reconstruct the time-dependent source functions. The OTF technique was incorporated into a two-dimensional plasma equipment model to investigate the time dependence of electron-impact source functions in low-pressure ICP systems. We found that even harmonics dominated the source functions for high-threshold processes, and that the harmonic content decreased with increasing frequency and increased with increasing pressure. We also observed axial pulses of excitation and increasing harmonic content at low pressures which are attributed to nonlinear Lorentz force acceleration and nonlocal transport.
Local weak-pairs pseudospectral multireference configuration interaction
Authors: Walter Derek , Szilva Andrew B. , Niedfeldt Keith , Carter Emily A.
Published: July 2002

We present a new reduced scaling multireference singles and doubles configuration interaction (MRSDCI) algorithm based upon the combination of local correlation and pseudospectral methods. Taking advantage of the locality of the Coulomb potential, the weak-pairs approximation of Saebo/ and Pulay is employed to eliminate configurations having simultaneous excitations out of pairs of distant, weakly interacting orbitals. In conjunction with this, the pseudospectral approximation is used to break down the most time-consuming two-electron integrals into a product of intermediate quantities depending on no more than two orbital indices. The resulting intermediate quantities are then used directly in the CI equations to substantially reduce the number of floating point operations required for diagonalization of the Hamiltonian. Additionally, our CI algorithm is based upon the symmetric group graphical approach CI (SGGA-CI) of Duch and Karwowski. For the purpose of developing reduced scaling CI algorithms, this approach has some important advantages. The most important of these advantages are the on-the-fly calculation of integral coupling coefficients and the separation of the spin and spatial parts of the wave function, which simplifies implementation of local correlation approximations. We apply the method to determine a series of binding energies in hydrocarbons and show that the approximate method predicts binding energies that are within a few kcal/mol of those predicted by the analytic nonlocal method. For large molecules, the local pseudospectral method was shown to be over 7 times as fast as the analytic nonlocal method. We also carry out a systematic study on the performance of different basis sets in the weak-pairs method. It was determined that triple-ζ basis sets were capable of recovering only 99.0% of the correlation energy, whereas double-ζ basis sets recovered 99.9% of the correlation energy.
Quantum chemical study of π–π stacking interactions of the bacteriochlorophyll dimer in the photosynthetic reaction center of Rhodobacter sphaeroides
Authors: Wang Yanli , Hu Xiche
Published: July 2002

Intermolecular π–π stacking interactions of the bacteriochlorophyll dimer in the photosynthetic reaction center of the purple bacterium Rhodobacter sphaeroides were analyzed by the second order Møller–Plesset perturbation method using the modified 6-31G*(0.25) basis set with diffuse d-polarization by Hobza and co-workers. MP2/6-31G*(0.25) calculations yield an intermolecular interaction energy of −21.50 kcal/mol for the bacteriochlorophyll dimer. Thus, the attractive nature of the π–π stacking interaction of the bacteriochlorophyll dimer in the photosynthetic reaction center from Rhodobacter sphaeroides is, for the first time, firmly established.
Absolute measurements of neutron yields from DD and DT implosions at the OMEGA laser facility using CR-39 track detectors
Authors: Frenje J. A. , Li C. K. , Séguin F. H. , Hicks D. G. , Kurebayashi S. , Petrasso R. D. , Roberts S. , Glebov V. Yu. , Meyerhofer D. D. , Sangster T. C. , Soures J. M. , Stoeckl C. , Chiritescu C. , Schmid G. J. , Lerche R. A.
Published: July 2002

The response of CR-39 track detectors to neutrons has been characterized and used to measure neutron yields from implosions of DD- and DT-filled targets at the OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)], and the scaling of neutron fluence with R (the target-to-detector distance) has been used to characterize the fluence of backscattered neutrons in the target chamber. A Monte-Carlo code was developed to predict the CR-39 efficiency for detecting DD neutrons, and it agrees well with the measurements. Neutron detection efficiencies of (1.1±0.2)×10−4 and (6.0±0.7)×10−5 for the DD and DT cases, respectively, were determined for standard CR-39 etch conditions. In OMEGA experiments with both DD and DT targets, the neutron fluence was observed to decrease as R−2 up to about 45 cm; at larger distances, a significant backscattered neutron component was seen. The measured backscattered component appears to be spatially uniform, and agrees with predictions of a neutron-transport code. As an additional application of the calibration results, it is shown that the neutron-induced signal in CR-39 used in charged-particle spectrometers on OMEGA can be used to determine DD and DT yields ranging from about 1010 up to 1014. With further improvements in the processing and analysis of CR-39, this upper limit can be increased by at least two orders of magnitude.
Dopant extraction from scanning capacitance microscopy measurements of p-n junctions using combined inverse modeling and forward simulation
Authors: Chim W. K. , Wong K. M. , Teo Y. L. , Lei Y. , Yeow Y. T.
Published: July 2002

This article proposes a more accurate approach to dopant extraction using combined inverse modeling and forward simulation of scanning capacitance microscopy (SCM) measurements on p-n junctions. The approach takes into account the essential physics of minority carrier response to the SCM probe tip in the presence of lateral electric fields due to a p-n junction. The effects of oxide fixed charge and interface state densities in the grown oxide layer on the p-n junction samples were considered in the proposed method. The extracted metallurgical and electrical junctions were compared to the apparent electrical junction obtained from SCM measurements.
Improved light extraction from emitters in high refractive index materials using solid immersion lenses
Authors: Zwiller Valéry , Björk Gunnar
Published: July 2002

Solid immersion lenses (SILs) are optically transparent, truncated spheres, brought in contact with a sample to be imaged. The combination of a conventional optical microscope and a SIL results in a highly effective numerical aperture of the imaging system that can improve the resolution. In addition, when imaging high refractive index samples, such as semiconductors, the light collection efficiency can be increased drastically. We investigate the collection efficiency as a function of the SILs geometry and refractive index, using an analytical expression for the light dispersion through an arbitrarily truncated sphere. The theoretical results are compared to experimental measurements obtained on single quantum dots and are found to be in good agreement.
Undulational instabilities of the columnar phase of diblock copolymers
Authors: Pereira G. G.
Published: July 2002

We consider the problem of a thin film of fluid with a columnar mesostructure (i.e., hexagonal phase of a diblock melt) placed between two hard, flat surfaces. The columns form parallel to the surfaces and the thickness of the thin film is slightly greater than the bulk equilibrium spacing of the columns. We determine the stability of the columns to a Helfrich–Hurault instability where the columns take on an undulational mode and determine the elastic constants and critical strain for the instability.
CO on Pt(111) puzzle: A possible solution
Authors: Grinberg Ilya , Yourdshahyan Yashar , Rappe Andrew M.
Published: July 2002

CO adsorption on the Pt(111) surface is studied using first-principles methods. As found in a recent study [Feibelman et al., J. Phys. Chem. B 105, 4018 (2001)], we find the preferred adsorption site within density functional theory to be the hollow site, whereas experimentally it is found that the top site is preferred. The influence of pseudopotential and exchange-correlation functional error on the CO binding energy and site preference is carefully investigated. We also compare the site preference energy of CO on Pt(111) with the reaction energy of formaldehyde formation from H2 and CO. We show that the discrepancies between the experimental and theoretical results are due to the generalized gradient approximation (GGA) treating different bond orders with varying accuracy. We can therefore expect that GGA results will contain significant error whenever bonds of different bond order are broken and formed.