bluuebunny/crossref_metadata_2025 huggingface dataset:
Critical behavior of simple fluids confined by microporous materials
Authors: Rżysko W. , de Pablo J. J. , Sokolowski S.
Published: July 2002

We have performed Monte Carlo simulations of a three-dimensional quenched-annealed system on a cubic lattice with nearest-neighbor interactions. A small fraction of the lattices sites are blocked, thereby creating a quenched matrix. Histogram reweighting techniques are applied to investigate the critical behavior of the system. We have studied lattice sizes ranging from L=10 to L=18. For each size, we have evaluated the number of matrix replicas necessary to obtain statistically meaningful results. This number, determined by analyzing the convergence of the histograms, ranged from 50 for the smallest system sizes to 200 for the largest sizes. We have evaluated the critical temperature, the fourth cumulant of Binder et al. [K. K. Kaski, K. Binder, and J. D. Gunton, Phys. Rev. B 29, 3996 (1984)], and the critical exponents 1/ν and β/ν. The estimated critical temperature is only slightly lower than that of the three-dimensional Ising model. The simulated critical exponents, however, differ significantly from those for Ising-class three- and two-dimensional systems.
Direct EELS observation of the oxidation states of Sm atoms in Sm@C2n metallofullerenes (74⩽2n⩽84)
Authors: Okazaki Toshiya , Suenaga Kazutomo , Lian Yongfu , Gu Zhennan , Shinohara Hisanori
Published: July 2002

The oxidation states of Sm atoms in various fullerene cages are directly investigated by electron energy-loss spectroscopy. The observed peak positions of the M45 edges of the Sm atom in Sm@C2n [2n=74, 78, 80, 82 (isomers I, III), 84 (isomers I, II, III)] are apparently shifted to the lower binding energy region in comparison with those of a trivalent Sm3+ in Sm2O3. The results indicate that the Sm atoms take +2 valence states irrespective of the fullerene cages. This observation is discussed based on a simple thermochemical cycle model. We also report that the Sm metallofullerenes have been converted to Sm carbide materials under a prolonged electron beam irradiation.
Comparative study of pulsed laser ablated plasma plumes from single crystal graphite and amorphous carbon targets. Part II. Electrostatic probe measurements
Authors: Mayo R. M. , Newman J. W. , Yamagata Y. , Sharma A. , Narayan J.
Published: July 2002

In an ongoing effort to investigate plasma plume features yielding high quality diamond-like carbon films, we have applied plasma plume diagnosis and film characterization to examine plume character distinction from KrF laser ablation of both amorphous carbon (a-C) and single crystal graphite (SCG) targets. The advancing plasma plume produced by these structurally different targets are observed to possess quantitatively similar total heavy particle inventory, ionized fraction, and electron thermal content, yet quite different ion kinetic energy, plume profile, C2 formation mechanism, and concentration of complex molecules. Plume electron temperatures are observed to reside in the range 1–3 eV, with those in SCG plumes ∼10%–30% greater than a-C at all spatial positions downstream of the target. For both target cases, we find Te drop off with position away from the target with radiation as the most likely loss mechanism for these noninteracting plumes propagating in vacuum. Electron density is found to be ∼10%–12% lower near the target in SCG than a-C plumes consistent with mass loss inventory measurements, whereas ion fractions are estimated in the range ∼10%–15% for both target cases. All recorded data support the conclusion that the SCG target plasma plume is populated with heavier, more complex molecules than those in a-C which have been shown to be predominantly comprised of C and C+ under vacuum conditions with the addition of C2 at high fill pressure. A significantly smaller profile peaking factor for SCG plumes supports this conclusion. Less energetic and slightly lower temperature SCG plume conditions are consistent with reduced peaking and more massive plume species. Plasma plumes from SCG targets exhibit laser energy (El) dependent peaking, again consistent with more complex molecules increasingly disassociated with El increase. The El dependence further suggests the potential for control of particle size distribution and plume profile peaking, though not independently. Consistent with this scenario is the observation of harder films produced from SCG targets at lower El. Micro-Raman results indicate strongly heterogeneous films deposited by SCG target ablation even under vacuum conditions further supporting the case for more complex structures with greater hardness. Energy balance estimates indicate that ion kinetic energy dominates the balance and that SCG ablation liberates about twice the number of C12 atoms from the target per unit El. As well, high pressure background fill indicates lesser plume energy attenuation for SCG plumes, again suggesting the presence of higher mass particles.
Long chain-length behaviors of optical nonlinearities of substituted polyenic chains with charged topological defects
Authors: An Z. , Wong K. Y.
Published: July 2002

The long chain-length behaviors of the static polarizability α and second hyperpolarizability γ of species of polyenic chains with various combinations of donor–acceptor substitutions and charged topological defects are studied by ab initio finite-field method. Both α and γ were found to exhibit linear chain-length dependent growth in the long chain-length limit for all combinations. The growth rates are universal, independent of the presence or absence of the substitutions or the charged topological defects. The origin of the universal growth is discussed using a local-view formalism that is based on the local atomic charge derivatives. It is found that this formalism can provide quantitative accounts of the saturated growth of α and γ. The effects on α and γ of a polyenic chain by the presence of a charged topological soliton are found to be additive. This allows the identification and calculation of the polarizability and second hyperpolarizability of a stand-alone charged soliton.
Overdamped Brownian motion in periodic symmetric potentials
Authors: Brańka A. C. , Das A. K. , Heyes D. M.
Published: July 2002

The dynamics of an overdamped Brownian particle in the field of a one-dimensional symmetric periodic potential U(x;α) have been studied by numerical solution of the Smoluchowski diffusion equation and the Langevin equation using the Brownian Dynamics method. The parameter α controls the shape and height of the potential barrier, which ranges from a sinusoidal spatial dependence for low barrier heights (α small) to a near delta-function appearance for barrier heights tending to infinity (α very large). Both the mean square displacement (MSD) dα(t), and the probability density n(x,t|x0), where x0 denotes the initial position, have been calculated. The MSD over a wide time domain has been obtained for a number of values of α. The exact asymptotic (t→∞) form of the diffusion coefficient has been exploited to obtain an accurate representation for dα(t) at long times. The function, dα(t) changes its form in the range α=8–10, with the appearance of a “plateau” which signals a transition in the particle’s Brownian dynamics from a weakly hindered (but continuous) mechanism to essentially jump diffusion. In the limit α→∞, each well of U(x;α) becomes similar to the classical square well (SW), which we have revisited as it provides a valuable limiting case for dα(t) at α≫1. An effective “attraction” of the probability density towards the SW walls is observed for off-center initial starting positions, and it is suggested that this could explain an observed change in the analytic form of the SW MSD, dsw(t), at long times. Two approximate analytic forms for dsw(t) at short times have been derived. The relaxation of the Brownian particle distribution n(x,t|x0) in the initial-well of U(x;α) has been studied.
Effect of higher-silane formation on electron temperature in a silane glow-discharge plasma
Authors: Takai Madoka , Nishimoto Tomonori , Kondo Michio , Matsuda Akihisa
Published: July 2002

Electron temperature measured by an optical-emission spectroscopy shows a strong substrate temperature dependence in a silane glow-discharge plasma. The electron temperature increases with time after turning on the plasma at a low substrate temperature of 150 °C, while it stays constant at a high substrate temperature of 400 °C. The electron temperature is drastically reduced when the source gas silane is diluted with hydrogen at low substrate temperatures. These results suggest that the electron temperature in silane plasma is strongly affected by an electron-attachment process to higher-order silane molecules whose formation reactions show negative activation energies with gas temperature and are also suppressed by the presence of hydrogen molecules.
Nonlinear optical response of Si nanostructures in a silica matrix
Authors: Vijayalakshmi S. , Grebel H. , Yaglioglu G. , Pino R. , Dorsinville R. , White C. W.
Published: July 2002

We provide a systematic study on the nonlinear optical properties of silicon nanocrystals within a fused silica matrix. Nonlinear measurements at various wavelengths exhibited the role of three bands in the visible spectrum. Measurements at various laser pulse durations showed several time constants, which exhibited the role of quantum confined and surface states.
Red electroluminescence in Si+-implanted sol–gel-derived SiO2 films
Authors: Luterová K. , Pelant I. , Valenta J. , Rehspringer J.-L. , Muller D. , Grob J. J. , Dian J. , Hönerlage B.
Published: July 2002

We report on a continuously emitting electroluminescent device fabricated by Si+-ion implantation and subsequent annealing of a SiO2 layer on a silicon substrate. The SiO2 layer with a thickness of 250 nm was prepared by the sol–gel technique. Four different Si+-ion energies and implantation doses were applied in order to obtain a flat Si+-ion profile across the SiO2 film thickness with an atomic Si excess of 5%. Electroluminescence (EL) occurs above a low-voltage threshold (∼5 V, 1 A/cm2) at one bias polarity only even if the device in fact does not exhibit rectifying properties. EL microscopy reveals that EL at 295 K is emitted from a small number of bright spots with diffraction-limited size. EL spectra of individual bright spots were measured using an imaging spectrometer. The wide EL emission band (situated in the red region ∼750 nm) obtained with spatial averaging over the semitransparent indium–tin–oxide contact represents the envelope of these individual contributions. We suggest that the EL is due to electron–hole injection into Si nanocrystals which create several conductive percolation paths across the SiO2 film. Shunting current paths due to defects exist in parallel and are probably the main factor responsible for low EL efficiency (10−5%).
Site switching and restructuring of oxygen adlayers induced by high coverage NO on Co{101̄0} at 100 K
Authors: Gu J. , King D. A.
Published: July 2002

We have employed reflection–absorption IR spectroscopy (RAIRS) as the principal technique as well as low energy electron diffraction (LEED) and temperature programmed desorption (TPD) as diagnostic tools to study the adsorption and reaction of nitric oxide (NO) on oxygen precovered Co{101̄0} surfaces at 100 K. The presence of oxygen adatoms greatly attenuates the occupation of two-fold sites in favor of atop sites, but O adatoms do not show any significant blocking effect for NO adsorption, and the NO coverage is close to 0.5 ML, as found on the clean surface. Beyond a critical coverage of NO, whatever the initial O coverage, a new IR band appears at ∼1876 cm−1 which is indicative of a strong NO–O interaction. The coadsorption of NO with various precoverages of oxygen (0.15–1.0 ML) including three ordered oxygen overlayers, c(2×4) (θO≈0.5 ML), p(2×1) (θO≈0.5 ML), and p(2×1)-g (θO≈1.0 ML), reveals an NO-induced surface restructuring process, in which O adatoms are driven from overlayer to underlayer sites at high NO coverages. This restructuring process increases the O effective diameter to up to ∼10 Å, this being the range over which the NO–O interaction is strong, producing the 1870 cm−1 N–O band.
On the structure of trapped holes in borosilicates
Authors: Shkrob Ilya A. , Tarasov V. F.
Published: July 2002

Danburite (CaB2Si2O8) is a naturally occurring borosilicate mineral that is a crystalline analog of borosilicate glass. Using two-dimensional correlation electron spin echo envelope modulation spectroscopy, we determined the structure of the prevalent radiation-induced defect, boron oxygen hole center, in this mineral. The hole is trapped by nonbridging oxygen in a strongly distorted O3−B–O––BO3− linkage; the precursor of the hole center is a Ca2+ vacancy. These results and computer modeling suggest that in borosilicate glasses, the hole center is the same >B–O⋅ radical as in alkali borate glasses. We suggest that a >B–O− unit near a cation vacancy is the precursor of hole centers in both types of glasses.
Thermally induced voltage offsets in Pb(Zr,Ti)O3 thin films
Authors: Kim Seung-Hyun , Lee Dong-Su , Hwang Cheol Seong , Kim Dong-Joo , Kingon A. I.
Published: July 2002

Voltage offset in the polarization-voltage characteristics of Pb(Zr,Ti)O3 (PZT) capacitors was evaluated by a thermal stress process. A thermally induced voltage shift occurs when heating the sample under either remanence or a saturating bias. It was found that the voltage shifts can, to a large extent, be attributed to the role of charged defects and the defect-dipole alignment throughout the films. PZT film with a high Zr/Ti ratio, i.e., rhombohedral compositions exhibited the best imprint resistance. When these films were doped wit up to 6% La, the imprint resistance was further improved. It was also found that B-site donors were more effective in minimizing the voltage shift than A-site donors. However, dopants with the same charge value as Pb, for example, Ca and Sr, did not affect the thermally induced voltage shifts of the films since they could not reduce the charged defects in the films.
Computing the classical mechanical vibrational echo with the fluctuating frequency approximation
Authors: Williams Ryan B. , Loring Roger F.
Published: July 2002

The vibrational photon echo is an infrared nonlinear spectroscopic measurement probing the time scales of dynamical processes that underlie a linear absorption spectrum. The challenges posed by the quantum mechanical treatment of large anharmonic systems motivate the consideration of this observable within classical mechanics. The rigorous calculation of the classical mechanical vibrational echo requires going beyond a conventional molecular dynamics simulation of trajectories to propagate stability matrix elements, which quantify the sensitivity of classical trajectories to small changes in initial conditions. As an alternative to this procedure, we present an approximate theory of the vibrational echo that avoids the numerical calculation of stability matrix elements. This approach, the fluctuating frequency approximation (FFA), generalizes a well established treatment of linear spectroscopy that models a driven anharmonic oscillator as a harmonic system with a fluctuating frequency. The FFA compares well with numerically exact calculations of the echo for a solvated anharmonic oscillator.
Monte Carlo studies of two dimensional transport in GaN/AlGaN transistors: Comparison with transport in AlGaAs/GaAs channels
Authors: Zhang Yifei , Singh Jasprit
Published: July 2002

In this communication we report results on Monte Carlo transport studies in GaN/AlGaN two dimensional electron gas. In addition to steady state results we examine transit times in channels of different lengths under various bias conditions. The results are compared to those calculated for the GaAs/AlGaAs device. We find that at low electric fields, transit time in the GaN channel can be considerably longer than the time in a GaAs channel. This is attributed to the overshoot effect in the GaAs channel. However, at large electric field transport the transit times in GaN and GaAs channel are found to be comparable.
Electronic structure and polarizabilities of icosahedral fullerenes: A Pariser–Parr–Pople approach
Authors: Ruiz A. , Bretón J. , Gomez Llorente J. M.
Published: July 2002

A Pariser–Parr–Pople approach, complemented with physical consistency criteria based on the expected molecular response to a weak electric field, has been used to predict the electronic level structure and polarizabilities of five icosahedral fullerenes in the range C60–C720. The behavior of the polarizability α as a function of the fullerene size is given by the expression α=0.75Re3, where Re is an effective molecular radius. It is argued that fullerenes would present the maximum polarizability values allowed for carbon shells, as if they were made of graphene.
Effect of electrical annealing on the luminous efficiency of thermally annealed polymer light-emitting diodes
Authors: Lee Tae-Woo , Park O Ok
Published: July 2002

Both high brightness and high efficiency of polymer light-emitting diodes can be obtained by electrical annealing after an appropriate thermal treatment. Light-emitting diodes made of poly [2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) with an Al electrode were used for this study. Thermal annealing after Al deposition dramatically enhanced the luminous efficiency of electroluminescent (EL) devices about 230 times at 350 mA/cm2, and the maximum external quantum efficiency (QE) also approached 1.43% photons/electrons. With the bias voltage annealing after thermal annealing, not only the onset voltage of the device was reduced but also the efficiency was further enhanced. This enhanced luminance and efficiency is attributed to the dipoles and accumulated space charges of polar ionic impurities within the MEH-PPV that induce more electronic charges for bipolar recombination. We obtained an external QE of 1.96% photons/electrons after 170 °C thermal annealing and 15 V bias annealing.
Adiabatic and vertical ionization energies of 1,4-diazabicyclo[2,2,2]-octane measured by zero electron kinetic energy spectroscopy and Rydberg extrapolation
Authors: Watkins Mark J. , Cockett Martin C. R.
Published: July 2002

We have used a combination of high-n Rydberg spectroscopy and zero electron kinetic energy (ZEKE) spectroscopy to measure the adiabatic and vertical ionization energies of 1,4-diazabicyclo[2,2,2]-octane to high precision. A comparison of the two methods revealed that where extended and unperturbed Rydberg series can be observed, the precision with which the ionization energy of a molecule can be measured is up to six times better via Rydberg extrapolation than with high resolution ZEKE spectroscopy. The difference in precision derives, not from any inherent shortcoming in the resolution achievable with ZEKE spectroscopy, but from errors that arise in evaluating the field ionization redshift. Accurate vertical ionization energies of 59 049.0±0.1 and 58 889.9±0.1 cm−1 were obtained for ionization to the 24+1 and 25+1 vibrational levels in the ion. A value for the adiabatic ionization energy of 58 033.3±0.3 cm−1 has been measured by ZEKE spectroscopy for the first time. The precision with which we have been able to account for the field ionization redshift has been improved by calibrating the ZEKE spectrum against the ν25 Rydberg spectrum. The ZEKE spectra presented here were recorded using a double inverted pulse sequence with field strengths as low as 140 mV cm−1. The result was exceptionally well-resolved spectra revealing the rotational contour of each ZEKE band. For the 24+1 and 25+1 bands the spectra revealed a strong central Q branch, with weaker P and R branches, consistent with a propensity for angular momentum transfer to the Rydberg electron rather than within the core. In contrast to what is commonly observed in ZEKE spectroscopy, where rotational autoionization often results in branches associated with negative ΔJ appearing with enhanced intensity, the R branch appears with significantly enhanced intensity compared to the very weak P branch.
Coulomb and centrifugal barrier bound dianion resonances of NO2
Authors: Andersen L. H. , Bilodeau R. , Jensen M. J. , Nielsen S. B. , Safvan C. P. , Seiersen K.
Published: July 2002

New short-lived resonance states of NO22− have been studied by bombarding NO2− anions by low-energy, mono-energetic electrons at the ASTRID heavy-ion storage ring. Storage for several seconds before the measurement ensures full vibrational relaxation of NO2− target anions. The dianion resonances were identified by the detection of resonances in the cross section for formation of neutral NO2. Two resonances were observed: The one of lowest energy is assigned to be the ground state of NO22− based on an ab initio calculation. This state may be held by a Coulomb barrier alone. A second state of significantly higher energy is argued to be held by a combined Coulomb and centrifugal barrier. Finally, a new scheme in which electron recombination may create stable dianions is proposed.
On the adequacy of the ten-dimensional model for the wall layer
Authors: Podvin Bérengère
Published: July 2002

In this paper, we provide a numerical validation of the ten-dimensional Proper Orthogonal Decomposition-based model constructed by Aubry et al. [J. Fluid Mech. 192, 115 (1988)] for the wall region of the turbulent boundary layer. Under certain conditions, this model was shown to display intermittent features highly reminiscent of the experimental observations of the bursting process in the wall layer, which makes it a potential key player in understanding and possibly controlling the dynamics of wall-bounded flows. In the same spirit as in our previous study [Podvin and Lumley, J. Fluid Mech. 362, 121 (1998)], we carried out a numerical simulation of a channel flow with relatively small horizontal dimensions which matched those in the 10-D model. The closure hypotheses used to build up the model were confronted with numerical results. Time histories of the modes in the model were compared to those of the simulation. Emphasis was put on identifying long-term characteristics such as a “mean” intermittency period. Our model, quite similar to Aubry’s, was found to display the same heteroclinic cycles under conditions consistent with the numerical experiment. The intermittency period in the model was found to agree well with that found in the simulation. However, the well-ordered character of 10-D bursts is significantly different from the simulation. To try and understand this discrepancy, we simulated a model with streamwise modes (32-D) and found evidence of increasing complexity in the bursts displayed.
Predicting wetting behavior from surface adsorption data using Cahn theory
Authors: Dobbs Harvey
Published: July 2002

We examine Cahn theory as a quantitative method for predicting wetting behavior. Whenever a first-order wetting transition involves a state of low adsorption, it is necessary that this state be described accurately. However, we show that Cahn theory in its simplest form is not appropriate in such circumstances. By examining a discrete lattice-gas model, a modified Cahn theory is derived which provides a better description of the low adsorption regime. We apply both standard and modified Cahn theories to the adsorption of n-alkanes on water. Whereas there is a discrepancy between the temperature of the first-order wetting transition predicted by the standard Cahn theory, and the temperature of the first-order thin-thick transition found experimentally with which it is identified, this discrepancy is removed in the modified theory.
Orientations and phase transitions in liquid crystals consisting of short linear polymer chains
Authors: Brostow Witold , Walasek Janusz
Published: July 2002

The system of semiflexible linear polymer liquid crystal (PLC) macromolecules is studied. Each macromolecule constitutes an alternating chain of flexible (F) and rigid (LC) sequences. The distribution function of the chain conformations is factorized in three terms. The Gibbs distribution is used for anisotropically interacting LC sequences; products of the Dirac delta functions represent F sequences modeled by linear chains of freely jointed statistical segments; connections of LC and F sequences in a linear chain are controlled by the Dirac delta functions with a proper argument. The general formula for the Helmholtz function A for arbitrary types of anisotropic interactions between LC sequences and for an arbitrary number of statistical segments per flexible part of linear chain obtained by the present authors [J. Chem. Phys. 105, 4367 (1996)] is applied in numerical calculations performed for some special cases. The cases selected here are (a) the Maier and Saupe mean-field limit formula for LC+LC interactions; and (b) the linear approximation with respect to the inverse value of the total length of the chain flexible part. Mostly one deals with very large (infinite) chain length with Gaussian behavior and earlier we have done this also. In this work we investigate the non-Gaussian case with a finite number of statistical segments per chain. The resulting modifications of the phase diagrams and phase transition points are discussed taking also into account results of the numerical calculations.
Electro-optic modulator for high resolution Brillouin scattering measurements
Authors: Caponi S. , Dionigi M. , Fioretto D. , Mattarelli M. , Palmieri L. , Socino G.
Published: July 2002

A wide band electro-optic modulator is presented which allows the frequency calibration of Brillouin spectra over the frequency range from a few MHz up to ≈40 GHz, a range spanning Brillouin light scattering from condensed matter. Measurements of both frequency and the linewidth of Brillouin peaks relative to longitudinal acoustic modes of water using the electro-optic modulator as a frequency reference are presented.
Local and nonlocal approximation for a simple quantum system
Authors: Palenberg M. A. , Silbey R. J. , Warns C. , Reineker P.
Published: July 2002

We apply two commonly used approximation schemes to the problem of a symmetric dimer with dynamic diagonal disorder. Since the exact analytical solution for this simple dissipative quantum system is known, we are in position to test the range of validity and the quality of the approximations and to derive guidelines for the application of the approximations to more complex systems.
Spectroscopic signatures of bond-breaking internal rotation. II. Rotation-vibration level structure and quantum monodromy in HCP
Authors: Jacobson Matthew P. , Child Mark S.
Published: July 2002

The rotation-vibration level structure of ground electronic state HCP is investigated at vibrational energies approaching and exceeding that of the linear CPH saddle point. With respect to energies above the saddle point, we investigate possible spectroscopic manifestations of strong Coriolis interactions between the hindered, bond-breaking internal rotation of the hydrogen about the CP core and the rotation of the molecule in the space-fixed axis system. With respect to energies below the saddle point, we provide new interpretations, from quantum and semiclassical points of view, of previously observed anomalously large B (rotational) and g22 (energy dependence on the vibrational angular momentum) constants for the large-amplitude pure bending states of HCP (referred to elsewhere as “isomerization” or saddle node states). We also predict similar anomalies in other spectroscopic constants, including the “centrifugal distortion” constant D and the “rotational l-resonance” parameter q2. These changes in the effective spectroscopic rotation-vibration constants are shown to be a direct consequence of the spherical pendulum topology of the HCP bend/internal rotor system, which is associated with a phenomenon called quantum monodromy, defined as the absence of a smoothly valid set of quantum numbers for all states. Our semiempirical model for the HCP bend/internal rotor mode is derived using principles of semiclassical inversion and provides new insights into the breakdown in the ability of rovibrational effective Hamiltonians to model highly vibrationally excited states of HCP.
Intermolecular potential energy surfaces and spectra of Ne–HCN complex from ab initio calculations
Authors: Murdachaew Garold , Misquitta Alston J. , Bukowski Robert , Szalewicz Krzysztof
Published: July 2002

Ab initio calculations of five two-dimensional intermolecular potential energy surfaces of the Ne–HCN dimer have been performed using the symmetry-adapted perturbation theory and the supermolecular method at different levels of electron correlation. A basis set of spdf-symmetry orbitals (including midbond functions) was used. HCN was assumed linear with interatomic distances fixed at their vibrationally averaged 〈r−2〉−1/2 values. Fits to all calculated potential energy surfaces were obtained in the form of angular expansions incorporating the ab initio asymptotic coefficients. It has been found that high-order correlation effects are very important for Ne–HCN and contribute about 20% to the well depth. All of the five surfaces feature a global minimum at the linear Ne–HCN geometry and a narrow and relatively flat valley surrounding HCN. Rovibrational calculations on the surfaces yielded rotational spectra and a rotational constant whose relative differences from their experimental counterparts range from 2% to 12% depending on the method used to obtain the surface. This large sensitivity of spectral quantities to relatively modest differences between the potentials is related to the unusual shape of the potential well.
Quantum dynamics simulation of the ultrafast photoionization of Li2
Authors: Pesce Lorenzo , Amitay Zohar , Uberna Radoslaw , Leone Stephen R. , Ratner Mark , Kosloff Ronnie
Published: July 2002

The Li2 species offers an ideal system to compare experimental pump/probe ultrafast photoionization with quantum dynamical calculations on well characterized potential energy surfaces. The present work utilizes the best available potential energy surfaces and appropriate quantum dynamical methods to analyze the photoionization and dynamics of a wave packet prepared in the E 1Σg+ shelf state of lithium dimer. A direct comparison between calculated (ab initio) and measured quantum dynamics is made for signals obtained with different laser pulse shapes, intensities, and chirps, and the validity of the theoretical model is considered, as well as the applicability and failure of perturbation theory. The results illustrate the high sensitivity of the time-dependent pump/probe ionization transient signals to the detailed modeling of both the pump and probe stages. They also show some of the inadequacies of the current potential surfaces and dipole moment matrix elements of lithium dimer.
Preference of cluster isomers as a result of quantum delocalization: Potential energy surfaces and intermolecular vibrational states of Ne⋯HBr, Ne⋯HI, and HI(Ar)n (n=1–6)
Authors: Slavı́ček Petr , Roeselová Martina , Jungwirth Pavel , Schmidt Burkhard
Published: July 2002

Intermolecular vibrational states are calculated for Ne⋯HBr, Ne⋯HI, and HI(Ar)n (n=1–6) complexes using potential energy surfaces constructed by accurate ab initio methods. Potentials of rare gas–hydrogen halide clusters exhibit two collinear minima, one corresponding to hydrogen lying between the heavy atoms, and the other to hydrogen facing away from the rare gas atom. The relative depths of the two minima are a result of a subtle balance between polarization and dispersion interactions. Moreover, due to a large quantum delocalization in the hydrogen bending (librational) motion the relevance of a particular stationary point on the potential energy surface is only limited. It is more appropriate to discuss the isomers in terms of vibrationally averaged structures. For Ne⋯HBr the potential minimum and the vibrationally averaged structure correspond to the same isomer with hydrogen between neon and bromine. However, for Ne⋯HI the global minimum corresponds to the Ne–IH collinear geometry, while the vibrationally averaged structure has hydrogen between the heavy atoms. In the case of HI(Ar)n we show that one can flip between the two isomers by adding argon atoms, which reconciles the seemingly contradictory experimental results obtained for the photodissociation of HI⋯Ar on one side, and of large HI(Ar)n clusters on the other side.
S
1
–S
2
vibronic coupling in cis-1,3,5-hexatriene. II. Theoretical investigation of absorption and resonance Raman spectra
Authors: Woywod Clemens , Livingood William C. , Frederick John H.
Published: July 2002

A wave packet prepared on the 1 1B1 potential-energy surface of cis-1,3,5-hexatriene (CHT) is characterized by a very short lifetime of ≈20 fs in this state. We present here model calculations of the excited-state dynamics of CHT that are consistent with the experimentally determined population decay time scale and yield an accurate description of the absorption, preresonance and resonance Raman (RR) spectroscopy of the 1 1B1 state. The greater diffuseness and complexity of the free jet 1 1B1 absorption band of CHT as compared to the 1 1Ag→1 1Bu transition of trans-1,3,5-hexatriene can be explained by a faster optical dephasing rate and more densely spaced vibronic level structure in the S2 state of the cis isomer primarily due to the presence of two very active low-frequency S1–S2 coupling modes, ν30 and ν31. The first measurement of the one-photon 1 1A1→2 1A1 transition of CHT has been reported only ten years ago and the S1 state has since been thoroughly studied by different techniques. The simulations of the excitation and RR emission profiles of the 2 1A1 state performed for this work are shown to be in quantitative agreement with the observed spectra. One of the most important and controversial questions arising from the spectroscopic information about the 2 1A1 state concerns the nature of the intensity carrier for the one-photon S0→S1 excitation process. It can be shown that the oscillator strength for one-photon transitions into the 2 1A1 vibronic manifold is exclusively borrowed from the electronic 1 1B1 configuration. One model Hamiltonian is defined for the representation of wave packet motion in the 1 1A1, 2 1A1, and 1 1B1 states and the nuclear coordinate space comprises eight dimensions. The relevant normal modes are either of a1 or b1 symmetry, i.e., only first-order intrastate or S1–S2 vibronic coupling effects are considered, and have been selected based on the electronic structure information compiled in the preceding paper.
Feedout and Richtmyer–Meshkov instability at large density difference
Authors: Velikovich Alexander L. , Schmitt Andrew J. , Gardner John H. , Metzler Nathan
Published: July 2002

The feedout process transfers mass perturbations from the rear to the front surface of a driven target, producing the seed for the Rayleigh–Taylor (RT) instability growth. The feedout mechanism is investigated analytically and numerically for the case of perturbation wavelength comparable to or less than the shock-compressed target thickness. The lateral mass flow in the target leads to oscillations of the initial mass nonuniformity before the reflected rippled rarefaction wave breaks out, which may result in RT bubbles produced at locations where the areal mass was initially higher. This process is determined by the evolution of hydrodynamic perturbations in the rippled rarefaction wave, which is not the same as the Richtmyer–Meshkov (RM) interfacial instability. An exact analytical formula is derived for the time-dependent mass variation in a rippled rarefaction wave, and explicit estimates are given for the time of first phase reversal and frequency of the oscillations. The limiting transition from the case of RM perturbation growth at large density difference (low ambient density behind the rear surface) to the case of feedout (zero density) is studied, and it is shown that the latter limit is approached only if the ambient density is extremely low, less than 1/1000 of the preshock target density.
Statistical analysis of native contact formation in the folding of designed model proteins
Authors: Tiana Guido , Broglia Ricardo A.
Published: July 2002

The time evolution of the formation probability of native bonds has been studied for designed sequences which fold fast into the native conformation. From this analysis a clear hierarchy of bonds emerge: (a) local, fast forming highly stable native bonds built by some of the most strongly interacting amino acids of the protein; (b) nonlocal bonds formed late in the folding process, in coincidence with the folding nucleus, and involving essentially the same strongly interacting amino acids already participating in the fast bonds; (c) the rest of the native bonds whose behavior is subordinated, to a large extent, to that of the strong local and nonlocal native contacts.
Spatial distribution of the absolute densities of CFx radicals in fluorocarbon plasmas determined from single-path infrared laser absorption and laser-induced fluorescence
Authors: Nakamura Masayuki , Hori Masaru , Goto Toshio , Ito Masafumi , Ishii Nobuo
Published: July 2002

The spatial distribution of the absolute density of CFx (x=1–3) radicals and their translational temperatures in an electron cyclotron resonance (ECR) plasma generated from a tetrafluorocarbon (C4F8) gas were examined using infrared diode-laser absorption spectroscopy (IRLAS) without a multiple reflection cell, namely, single-path IRLAS. Furthermore, we have developed a method of measuring CF and CF2 radical densities using single-path IRLAS combined with laser-induced fluorescence (LIF) spectroscopy. This method enables us to measure the spatial distribution of absolute radical densities with high accuracy, because of the IRLAS infrared laser beam and the LIF ultraviolet laser beam having identical paths. Under all the conditions studied, a spatially hollow distribution of the CF2 radical density is formed; the CF2 radical density in the vicinity of the chamber wall is much higher than that in the plasma. However, the spatial distribution of the CF radical density differs greatly from that of the CF2 radical density. The translational temperatures of CF and CF2 radicals are evaluated to be ∼700 K. On the basis of the measured results, we clarify the mechanisms of the formation of the spatial distribution, and conclude that the hollow distribution of the CF2 radical density is not caused by radical generation from the chamber wall, rather, the dominant mechanism for the formation of this distribution is the electron-impact dissociation of C4F8 gas in the ECR region and diffusion from the upper part of the plasma chamber under the present plasma conditions where the flux of ions incident to the chamber wall is low.
Interface profiles in a dimerizing system
Authors: Peery Travis B. , Evans Glenn T.
Published: July 2002

The gas–liquid density profile and surface tension are calculated for a fluid possessing hard sphere repulsion, van der Waals-like dispersion, and sticky-spot attraction. The sticky spot allows monomers to dimerize and the consequences of which are investigated. Association is treated with Wertheim’s thermodynamic perturbation theory and the direct correlation functions are derived by means of the Ornstein–Zernike equations for associating systems. The primary effect of association is to shift the critical temperatures and densities from those of a nonassociating fluid. While there are small, nonmonotonic shifts in the correlation length and surface tension, accompanied by an interface composition change, these effects on the density profile seem to be subsumed in the use of the proper T/Tc.
Transport anisotropy in microcrystalline silicon studied by measurement of ambipolar diffusion length
Authors: Švrček V. , Pelant I. , Kočka J. , Fojtı́k P. , Rezek B. , Stuchlı́ková H. , Fejfar A. , Stuchlı́k J. , Poruba A. , Toušek J.
Published: July 2002

We have studied charge transport anisotropy in microcrystalline silicon (μc-Si:H) by comparing diffusion length measured parallel to the substrate by steady stage photocarrier grating and perpendicular to the substrate by surface photovoltage method (SPV). We have developed a SPV evaluation procedure which allowed us to exclude the effect of light scattering at the naturally rough surface of the μc-Si:H. The procedure allows us to deduce not only the diffusion length, but also the depth of the depletion layer at the surface and recombination coefficients at both top and bottom interfaces of the film. With growing μc-Si:H film thickness the size of the crystallites increases, leading to higher roughness and thus also light scattering. At the same time density of grain boundaries decreases, resulting in an increase of the diffusion length and of the surface depletion layer depth. For all samples the diffusion length perpendicular to the substrate was several times higher than the diffusion length parallel to it, clearly confirming previous indication of the transport anisotropy resulting from the measurements of coplanar and sandwich conductivity.
Ion bombardment as the initial stage of diamond film growth
Authors: Liao Meiyong , Qin Fuguang , Zhang Jianhui , Liu Zhikai , Yang Shaoyan , Wang Zhanguo , Lee Shuit-Tong
Published: July 2002

It is believed that during the initial stage of diamond film growth by chemical-vapor deposition (CVD), ion bombardment is the main mechanism in the bias-enhanced-nucleation (BEN) process. To verify such a statement, experiments by using mass-separated ion-beam deposition were carried out, in which a pure carbon ion beam, with precisely defined low energy, was selected for investigating the ion-bombardment effect on a Si substrate. The results are similar to those of the BEN process, which supports the ion-bombardment-enhanced-nucleation mechanism. The formation of sp3 bonding is based on the presumption that the time of stress generation is much shorter than the duration of the relaxation process. The ion-bombarded Si is expected to enhance the CVD diamond nucleation density because the film contains amorphous carbon embedded with nanocrystalline diamond and defective graphite.
Mesoscopic approach to the soft breakdown failure mode in ultrathin SiO2 films
Authors: Miranda Enrique , Suñé Jordi
Published: July 2002

We present an analytic model for the soft breakdown failure mode in ultrathin SiO2 films based on the conduction theory through quantum point contacts. The breakdown path across the oxide is represented by a three-dimensional constriction in which, due to the lateral confinement of the electron wave functions, discrete transverse energy levels arise. In the longitudinal direction, such levels are viewed by the incoming electrons as effective potential barriers, which can be treated using the one-dimensional tunneling formalism. In addition, it is shown that our mesoscopic approach is also consistent with the hard breakdown conduction mode.
Nitrogen doping of tetrahedral amorphous carbon films: Scanning tunneling spectroscopy
Authors: Bhattacharyya Somnath , Walzer K. , Hietschold M. , Richter F.
Published: July 2002

Amorphous nitrogenated carbon films with nitrogen atomic concentration between 12% and 29% were deposited using a filtered cathodic vacuum arc and a Kaufman-type ion source. The surface topography of the samples has been investigated by scanning tunneling microscopy in ultrahigh vacuum, showing that the roughness of the film surface decreases with nitrogen concentration. Scanning tunneling spectroscopy is employed to understand the role of nitrogen in the change of the surface microstructure and electronic structure near the Fermi level. The tunneling current (I)–bias voltage (V) curve is flat at low bias regions indicating a finite gap for the sample with low (12%) nitrogen concentration. An increase of tunneling current and its nonlinearity along with the decrease of energy gap occurs in the samples with increase of N concentration. The observed surface density of states [(dI/dV)/(I/V)] has been fitted as a square-root function of bias voltage. An improvement of the quality of these fits in the films with the increase of nitrogen concentration suggests that a depletion of defect density of states near the Fermi level (EF) takes place. These analyses could be attributed to the modification of the structure of amorphous carbon by a large concentration of nitrogen.
Towards an analytical formula for the eigenvalues of the Aharonov–Bohm annular billiard
Authors: Fendrik A. J. , Sánchez M. J.
Published: July 2002

We derive an asymptotic formula for the eigenvalues of the Aharonov–Bohm annular billiard (ABAB) that improves and corrects previous estimates. Employing semiclassical arguments we relate the limitations of the procedure to the topology of the classical phase space of the system.
Forced Rayleigh scattering studies of tracer diffusion in a nematic liquid crystal: The relevance of complementary gratings
Authors: Spiegel Daniel R. , Thompson Alexis L. , Campbell Wesley C.
Published: July 2002

We have employed forced Rayleigh scattering (FRS) to study the diffusion of an azo tracer molecule (methyl red) through a nematic liquid crystal (5CB). This system was first investigated in an important study by Hara et al. [Jpn. J. Appl. Phys. 23, 1420 (1984)]. Since that time, it has become clear that the presence of complementary ground-state and photoproduct FRS gratings can result in nonexponential profiles, and that complementary-grating effects are significant even when “minor” deviations from exponential decay are observed. We have investigated the methyl red/5CB system in order to evaluate the possible effects of complementary gratings. In the isotropic phase, we find that the presence of complementary gratings results in a nonmonotonic FRS signal, which significantly changes the values inferred for the isotropic diffusion coefficients. As a result, the previously reported discontinuity at the nematic/isotropic transition temperature (TNI) is not present in the new data. On the other hand, in the nematic phase, the new experiments largely confirm the previous observations of single-exponential FRS decay and the non-Arrhenius temperature dependence of the nematic diffusion coefficients close to TNI. Finally, we have also observed that the decrease in the diffusion anisotropy with increasing temperature can be correlated with the 5CB nematic order parameter S(T) over the full nematic temperature range.
Near-field photocurrent imaging of the optical mode profiles of semiconductor laser diodes
Authors: Guenther T. , Malyarchuk V. , Tomm J. W. , Müller R. , Lienau C. , Luft J.
Published: July 2002

The potential of near-field photocurrent spectroscopy for direct imaging of mode profiles of submicron-sized waveguides in optoelectronic devices is demonstrated. The technique combines the submicron spatial resolution of near-field optics with tunable laser excitation, allowing for selective investigation of the waveguide properties of the device structure. Experiments on InGaAs/AlGaAs high-power laser diodes with different waveguide designs provide direct visualization of the effect of the waveguide design on (i) the number of guided modes and (ii) the spatial profile of both fundamental and higher-order modes. The technique thus provides a sensitive tool for nondestructive in situ analysis of waveguide properties in optoelectronic devices.