bluuebunny/crossref_metadata_2025 huggingface dataset:
Vacancy-assisted domain growth in amphiphilic systems: Monte Carlo simulation
Authors: Liu Ji-wen , Ma Yu-qiang
Published: July 2002

We study the phase separation of amphiphilic systems with vacancies using Monte Carlo simulation of Larson models. We try to clarify the effect of vacancy on the domain growth of a microemulsion at different quench temperatures, and obtain the phase-separating morphology, the time evolution of the structure factor, and the characteristic length scale of the system. It is found that for the deep quench case, vacancies tend to locate at interfaces and accelerate phase separation. However, for shallow quenches an amount of vacancies migrates into the bulk phase and domain growth kinetics assisted by vacancies at interfaces is balanced by the lowering interface tension of vacancies. This has been attributed to the interplay between vacancy-mediated segregation dynamics and reduction of driving force for the phase separation through vacancies at interface, depending on the quench temperatures. On the contrary, all the amphiphile molecules are still deposited onto interfaces even for high temperatures because of the chain properties of the amphiphiles.
Characterization of internal MHD modes in Princeton beta experiment modification by ECE fluctuation radiometry
Authors: Zolfaghari A. M. , Woskov P. P. , Luckhardt S. , Kaita R.
Published: July 2002

Localized measurements of internal magnetohydrodynamic (MHD) activity in the Princeton Beta Experiment-Modification (PBX-M) have been made using a heterodyne radiometer. The radiometer measures third harmonic optically gray electron cyclotron emission (ECE) from the center to the outer edge of the plasma on eight radially localized frequency channels from 112–128 GHz. The radiometer uses a focusing Gaussian optics system, which provides a vertical resolution of approximately 6 cm throughout the plasma. The radial resolution of the diagnostics is limited by the relativistic broadening to approximately 2–3 cm. Long wavelength (k<0.5 cm−1) temperature and density fluctuations can therefore be detected using this diagnostic. A MHD mode’s characteristics such as pressure fluctuations and mode amplitude can be derived from the coherent fluctuations it causes in the ECE signal using an analytical theory developed by Luckhardt et al. (MIT Plasma Fusion Center Report No. PFC-JA/91-16, 1991). Using this analysis, pressure fluctuations (δP/P) have been calculated for high n-number MHD modes in high-βp PBX-M plasmas.
Adjoint operators and perturbation theory of black holes
Authors: Cartas-Fuentevilla R.
Published: July 2002

We present a new approach for finding conservation laws in the perturbation theory of black holes which applies for the more general cases of non-Hermitian equations governing the perturbations. The approach is based on a general result which establishes that a covariantly conserved current can be obtained from a solution of any system of homogeneous linear differential equations and a solution of the adjoint system. It is shown that the results obtained from the present approach become essentially the same (with some differences) to those obtained by means of the traditional methods in the simplest black hole geometry corresponding to the Schwarzschild space–time. The future applications of our approach for studying the perturbations of black hole space–times in string theory is discussed.
The coil–globule transition for a polymer chain confined in a tube: A Monte Carlo simulation
Authors: Sotta P. , Lesne A. , Victor J. M.
Published: July 2002

The behavior of a grafted polymer chain confined in a tube is investigated within a scaling theory substantiated with biased Monte Carlo simulations of a self-avoiding walk (SAW) on a cubic lattice. All the statistical and thermodynamic properties of the chain follow from the knowledge of the joint distribution P(z,m) giving the probability to observe a length z and a number of contacts m, in a model where the energy of the chain in a given configuration is proportional to m. The analysis is based on the factorization of P(z,m) into the a priori distribution P(z) and the conditional probability P(m|z) of finding m contacts given that the chain length is z. P(m|z) is well-approximated by a Gaussian distribution. Taking the variance 〈m2〉−m̄2 of this distribution into account, we obtain a nonmean-field expression for the free energy of the confined chain. We show that the coil–globule transition of the confined chain is independent of its size but depends on the pore diameter. Contrary to free, unconfined chains, it is always a continuous transition.
Evidence for orientational tunneling of CO intercalated in C60: A nuclear magnetic resonance study
Authors: Tomaselli M. , Knecht D. W. , Holleman I. , Meijer G. , Meier B. H.
Published: July 2002

We characterize the low-temperature dynamics of CO intercalated in C60 using NMR spectroscopy. CO in C60 is found to be dynamically inhomogeneous below 30 K: The CO13 line shapes reflect a dynamic disorder to static disorder transition, with only quantum tunneling among equivalent orientations in a local S6 symmetry potential remaining. The increased hindrance of the CO motion cannot be reconciled with common expectations of a homogeneous, thermally activated jumplike reorientation process, but is well accounted for in a model of orientational pinning due to asymmetric distortions of the cage potential.
Novel photofragment spectrometer for the investigation of molecular three-body decay
Authors: Braun M. , Beckert M. , Müller U.
Published: July 2002

We have developed a novel photofragment spectrometer to investigate the fragmentation of neutral molecules into two, three, and more neutral fragments. The parent molecules are laser prepared in a fast beam. The photofragments are detected in coincidence by a time- and position-sensitive detector which consists of multichannel plates and delay line anodes. We have developed new electronic pulse routers which allow us to measure consecutive fragment hits with dead times as low as a few nanoseconds. A new algorithm has been developed to determine for each triple hit the momentum vectors of the three fragments from the measured arrival time differences and positions. To demonstrate the performance of the spectrometer, we have investigated two- and three-body decay processes of laser-prepared triatomic hydrogen molecules H3 into H+H2 fragment pairs as well as into three hydrogen atoms H(1s)+H(1s)+H(1s). For two-body decay of vibrationless H3 3s2A1′(N=1, K=0), we have found that the resolution in the kinetic energy release spectra is far better than that achieved in previous investigations. With the new spectrometer, kinematically complete investigations of the breakup of a neutral molecule into three neutral fragments have been performed. The final state distributions in the three body decay of laser-prepared H3 molecules show pronounced structures which give insight into the complex molecular decay mechanisms.
Double-contact ion-molecule binding: Infrared characterization of the ionic H bonds to formic acid in the I−⋅HCOOH complex
Authors: Robertson William H. , Kelley Jude A. , Johnson Mark A.
Published: July 2002

We report mid-IR predissociation spectra of the I−⋅HCOOH⋅Arm(m=1–4) ion-acid complexes. The spectra are consistent with a planar structure where both hydrogens are engaged in ionic H bonds. Upon binding to the ion, the OH stretching fundamental displays a much more dramatic redshift (792 cm−1) than that of the CH stretch (99 cm−1), giving rise to a complex series of bands in the 2750–2950 cm−1 region. The contributions of the CH and OH stretches to the spectrum are isolated by recording spectra of the I−⋅DCOOH and I−⋅HCOOD species, which reveal that the OH stretching vibration is accompanied by combination bands involving soft modes while the CH stretch spectrum is dominated by a single feature. Some of the complexity in the I−⋅HCOOH spectrum arises from a strong Fermi resonance interaction between the v=1 level of the OH stretch and an overtone or combination band involving CH motion. We compare this behavior to that of the previously reported I−⋅CH3OH and I−⋅H2O complexes.
Atomic indentation and friction of self-assembled monolayers by hybrid molecular simulations
Authors: Leng Yongsheng , Jiang Shaoyi
Published: July 2002

This paper focuses on the atomic indentation and friction properties of self-assembled monolayers (SAMs) by a novel hybrid molecular simulation approach. By introducing a sliding dynamics for the tip-cantilever assembly in atomic force microscopy (AFM) and a fast molecular dynamics relaxation algorithm for SAMs, we simulate the scanning process of the assembly over SAMs in the time scale of AFM experiments. For the atomic indentation of SAM surfaces, we find that elastic modulus is chain-length independent, and has a value of 20±10 GPa. However, under shear, effective shear modulus is found to be chain-length dependent, which explains the SAM chain-length dependence of friction observed in AFM experiments. The calculated surface energy of methyl terminated SAMs is consistent with many experimental results.
A theoretical investigation of the temperature dependence of the optical Kerr effect and Raman spectroscopy of liquid CS2
Authors: Ji Xingdong , Ahlborn Heather , Space Brian , Moore Preston B.
Published: July 2002

The ambient pressure, temperature dependent optical Kerr effect (OKE) spectral density of CS2 has been calculated by way of time correlation function (TCF) and instantaneous normal mode (INM) methods and compared with corresponding experimental OKE data [R. A. Farrer, B. J. Loughnane, L. A. Deschenes, and J. T. Fourkas, J. Chem. Phys. 106, 6901 (1997)]. Over this temperature range the viscosity of CS2 varies by more than a factor of 5, and the molecular dynamics (MD) spectroscopic methods employed do an excellent job in capturing the associated changes in molecular motions that lead to the observed spectroscopy. The resulting TCF spectra are also in very good agreement with experimental measurements at all temperatures, and this is remarkable considering the range of conditions considered. When compared in the reduced Raman spectrum form, where the INM spectral density is proportional to the squared polarizability derivative weighted density of states (DOS), the INM results agree very well with the TCF results, and the low frequency OKE feature corresponding to rotational reorientation is suppressed in this form. Interestingly, the INM signal includes a significant contribution from the imaginary INM’s at all the temperatures considered, and these contributions are crucial to the agreement between INM and TCF results. Furthermore, the INM approximation to the signal (OKE or reduced Raman) demonstrates that the contribution (spectral density) of the real INM’s remains nearly unchanged over the temperature range considered, while the imaginary contribution grows with increasing temperature. The signal from the imaginary INM’s is therefore deduced to be responsible for a large part of the temperature dependence of the OKE spectral density. Finally, the molecular motions that contribute to the OKE signal are analyzed using INM methods.
Time-dependent simulation of helix traveling wave tubes
Authors: Freund H. P. , Zaidman E. G.
Published: July 2002

A time-dependent nonlinear analysis of a helix traveling wave tube (TWT) is presented for a configuration where an electron beam propagates through a three-dimensional tape helix surrounded by a conducting wall. The effects of dielectric- and vane-loading are included as is efficiency enhancement by tapering the helix pitch and external focusing by means of either a uniform solenoidal magnetic field or a periodic field produced by a periodic permanent magnet stack. Dielectric-loading is described under the assumption that the gap between the helix and the wall is uniformly filled by a dielectric material. Vane-loading describes the insertion of an arbitrary number of vanes running the length of the helix. The electromagnetic field is represented as a superposition of waves in a vacuum tape helix. The propagation of each wave as well as the interaction of each wave with the electron beam is included by allowing the amplitudes of the waves to vary in z and t. The field equations are solved in conjunction with the three-dimensional Lorentz force equations for an ensemble of electrons. Collective effects are included by both a finite-difference grid formulation of Poisson’s equation and an analysis based upon a modal superposition of solutions of the Helmholtz equation. The time-dependent simulation is compared with a steady-state simulation for a model corresponding to a TWT built at Northrop–Grumman Corporation.
Alkali–helium exciplex formation on the surface of helium nanodroplets. I. Dispersed emission spectroscopy
Authors: Reho J. , Higgins J. , Callegari C. , Lehmann K. K. , Scoles G.
Published: July 2002

Dispersed emission spectra collected upon the 4 2P3/2,1/2←4 2S1/2 optical excitation of K atoms attached to helium nanodroplets include broad, structured, red-shifted features which are shown to be due to K*He exciplex formation, paralleling our former observation of Na*He [J. Reho, C. Callegari, J. Higgins, W. E. Ernst, K. K. Lehmann, and G. Scoles, Discuss. Faraday Soc. 108, 161 (1997)]. The exciplex formation is demonstrated by the agreement obtained in comparing the K*He A(1) 2Π→X(1) 2Σ emission spectra with the predictions derived from available ab initio potential energy surfaces. Recent analysis of both exciplex emissions also points to the possibility of triatomic (Na*He2 and K*He2) exciplex formation for a small fraction of the alkali atoms. The lack of fluorescence quenching, which is present when the spectra are taken in bulk liquid helium, is due to the surface location of the alkali atoms on the helium droplets that allows the nascent Na*He and K*He exciplexes to desorb from the droplet and emit as isolated molecules.
Investigation of the plasma jet formation in X-pinch plasmas using laser interferometry
Authors: Mitchell I. H. , Aliaga-Rossel R. , Saavedra R. , Chuaqui H. , Favre M. , Wyndham E. S.
Published: July 2002

A two-frame Mach–Zender interferometer is used to investigate the dynamics of X pinches formed from two 10 μm aluminum wires at current levels of 100 kA. Particularly, the columns of plasma that form on the interelectrode axis of the X pinch are studied quantitatively. It is demonstrated that the plasma which forms these columns does not come solely from expansion of the corona from the limbs of the X pinch but rather predominantly from the crossing point region. The results suggest that the plasma column is indeed a jet which consists of several components.
Modification of electron transport due to dynamic screening of the Coulomb interaction
Authors: Zheng Jian , Yu C. X.
Published: July 2002

Dynamic screening of the Coulomb interaction is included to calculate electron transport coefficient (electron thermal conductivity) in the fluid limit for an unmagnetized plasma. Calculated results show that the dynamic screening effect can reduce electron thermal conductivity and may be important for low-Z plasma with very large electron–ion temperature ratio (Te/Ti≳103).
The Kelvin–Helmholtz instability in a plasma with negatively charged dust
Authors: Luo Q. Z. , D’Angelo N. , Merlino R. L.
Published: July 2002

The effect of negatively charged dust on the Kelvin–Helmholtz (parallel velocity shear) instability is investigated experimentally in a magnetized cesium plasma. The dust generally has a stabilizing effect on the instability, although, in some cases, the addition of negatively charged dust into the plasma results in a slight increase in the instability fluctuation amplitude. The results are in general agreement with theoretical predictions.
Effects of ion size and valence on ion distribution in mixed counterion systems of rodlike polyelectrolyte solution. I. Mixed-size counterion systems with same valence
Authors: Nishio Takuhiro , Minakata Akira
Published: July 2002

The influence of the counterion size is investigated in the mixed counterion systems of the salt-free polyelectrolyte solution in a cylindrical cell model. The mixtures of two species of the counterion having the same valence and different size are simulated systematically by means of the Monte Carlo method in the primitive model of the rodlike polyelectrolyte solution. The results of the free fractions and the selectivity coefficients are compared with the numerical solutions of the Poisson–Boltzmann equation. The observed differences between both methods are explained in terms of the ion–ion correlations and the effect of finite ion size.
Status of our understanding and modeling of x-ray coupling efficiency in laser heated hohlraums
Authors: Dattolo E. , Suter L. , Monteil M-C. , Jadaud J-P. , Dague N. , Glenzer S. , Turner R. , Juraszek D. , Lasinski B. , Decker C. , Landen O. , MacGowan B.
Published: July 2002

Recent changes in the manner of performing hohlraum drive experiments have significantly advanced the ability to diagnose, understand and control the x-radiation flux (or drive) inside a laser heated hohlraum. Comparison of modeling and data from a very broad range of hohlraum experiments indicates that radiation hydrodynamics simulation codes reproduce measurements of time dependent x-radiation flux to about ±10%. This, in turn, indicates that x-ray production and capsule coupling in ignition hohlraums will be very close to expectations. This article discusses the changes to experimental procedures and the broad variety of measurements and tests leading to these findings.
Dielectric relaxation and steady-state leakage in low-temperature sputtered (Ba, Sr)TiO3 thin films
Authors: Werner M. C. , Banerjee I. , Zhang R. , McIntyre P. C. , Tani N. , Tanimura M.
Published: July 2002

Dielectric relaxation and steady-state leakage currents were studied over a range of bias voltages for Pt-electroded capacitors in which the 50-nm-thick (Ba, Sr)TiO3 dielectric layer was sputter deposited at 480 °C. A pronounced polarity dependence in the current–voltage characteristic of the capacitors was observed. Dielectric relaxation in the films showed a Curie–von Schweidler time dependence (J=J0t−n) for intervals of 3–200 s duration after application of a voltage step. The relaxation current density was found to depend linearly on electric field for fields up to 700 kV/cm and nonlinearly at higher fields. In addition to the Curie–von Schweidler relaxation currents, a time-dependent leakage current was detected under high voltage bias conditions. An empirical model developed to describe leakage currents in these films is presented.
An experiment to measure the equilibrium charge of dust particles embedded in a plasma
Authors: Kakati M. , Saikia B. K. , Chakraborty M. , Goswami K. S. , Bujarbarua S.
Published: July 2002

The average equilibrium charge of submicron sized silver particles, collected during a finite time of transit through a magnetically confined plasma is reported. The fine, almost spherical particles with uniform size are produced in an evaporation chamber. The average charge of the particles was found to be about 1.4×102 electronic charges. This result is compared with theoretical predictions.
Highly erbium-doped zinc–oxide thin film prepared by laser ablation and its 1.54 μm emission dynamics
Authors: Komuro Shuji , Katsumata Tooru , Morikawa Takitaro , Zhao Xinwei , Isshiki Hideo , Aoyagi Yoshinobu
Published: July 2002

Erbium-doped ZnO (ZnO:Er) thin films were fabricated by the KrF excimer laser ablation technique, which is a useful and simple technique to dope Er atoms on the order of 1020 cm−3 into a host material. As-prepared ZnO:Er films showing strong c-axis orientation with a hexagonal crystalline structure indicate a low electrical resistivity of 6.4×10−3 Ω cm. The sharp and intense photoluminescence (PL) at 1.54 μm originating from the intra-4f transition in the Er3+ ions as well as PL in UV region from the ZnO host were observed even at room temperature. Significant distinction arising from the different Er emission centers responsible for the 1.54 μm emission cannot be found in the temperature dependence between the ZnO:Er and Si:Er film as a reference, except for the PL spectrum feature and main PL peak position. This result suggests the existence of Er emission centers in ZnO:Er and Si:Er films that are different from each other. The details of Er-related 1.54 μm emission dynamics of ZnO:Er films have been investigated for the different excitation conditions, where the Er3+ ions have been excited either through a carrier-mediated process in the ZnO host, or through direct pumping into the 4f energy level of the Er3+ ions. There is no change in the 1.54 μm PL spectrum feature in spite of the different excitation conditions, whereas a sensible change can be seen in the rise time of the 1.54 μm emission. The shorter rise time of the 1.54 μm emission observed for indirect excitation implies an excitation efficiency superior to the direct excitation of Er3+ ions. This result indicates that the ZnO:Er thin films are expected to be a promising infrared optoelectronic materials candidate for carrier injection devices because of the high electrical conductivity and high excitation efficiency of the Er3+ ions of an electron–hole-mediated process resulting from ZnO host excitation.
Response to “Comment on ‘Surface diffusion potential energy surfaces from first principles”’ [J. Chem. Phys. 114, 1051 (2001)]
Authors: Ge Q. , King D. A.
Published: July 2002

The gist of the comment from Graham and Toennies [J. Chem. Phys. 114, 1051 (2001)] is that we should not have generalized our results for the surface diffusion potential energy surface of CO on Pt{110} to systems for which diffusion barriers had been obtained using quasielastic helium atom scattering (QHAS). In this response, we give a brief description of the surface diffusion process, indicating why our new description can be generalized, and we then show new data for CO on Cu{100} and Ni{110} surfaces which strongly support our original conclusion. In particular, our new data demonstrate considerably larger diffusion barriers than those obtained by QHAS, and anisotropies which are in the opposite sense to those reported using QHAS for Ni{110}. Our original concerns about the evaluation of QHAS data are strongly supported by our new results.
A theoretical study of HArF, a newly observed neutral argon compound
Authors: Runeberg Nino , Pettersson Mika , Khriachtchev Leonid , Lundell Jan , Räsänen Markku
Published: July 2002

Computational results up to the CCSD(T)/aug-cc-pV5Z level are presented as support for the newly observed argon containing compound, hydrido argonfluoride (HArF). The molecule is calculated to be linear with R(H–Ar)=132.9 pm and R(Ar–F)=196.9 pm. The calculated vibrational frequencies, corrected for anharmonicity and matrix effects, are 462 (Ar–F stretch), 686 (bend) and 1916 cm−1 (Ar–H stretch). These are in good agreement with the corresponding experimentally observed frequencies of 435.7, 687.0, and 1969.5 cm−1 for the matrix isolated species [Nature 406, 874 (2000)]. Including corrections for the finite basis set as well as for the zero-point energy, the new molecule is stable by 0.15 eV compared to the dissociated atoms. HArF is further stabilized by an additional barrier of 0.18 eV, arising from the avoided crossing between the states corresponding to the ionic (HArδ+)(Fδ−) equilibrium structure and the covalent (HAr⋅)(F⋅) dissociation limit. The dissociation of HArF via bending into the thermodynamic ground-state system, consisting of Ar and HF, is protected by a 1.0 eV barrier.
An experimental and theoretical investigation of the thiophene/aluminum interface
Authors: Blyth R. I. R. , Mittendorfer F. , Hafner J. , Sardar S. A. , Duschek R. , Netzer F. P. , Ramsey M. G.
Published: July 2002

The adsorption of thiophene and 2, 2′-bithiophene on Al(111) has been studied using thermal desorption spectroscopy (TDS), angle-resolved UV photoemission (ARUPS), and work function measurements. Ab initio density functional theory calculations have been performed for thiophene on Al(111). Both thiophene and bithiophene bond only very weakly to Al(111), as indicated by TDS and calculations of the thiophene absorption energy, which is found to be only 0.54 eV. There is no indication of π-bonding in either the ARUPS data or the calculations. The calculated S–Al distance, 3.7 Å, is much greater than either measured or calculated S–metal distances for covalent bonding. The bonding is shown to be almost entirely electrostatic, with a small contribution from the sulfur lone pair. This is in direct contrast to calculations for Al–thiophene complexes which show covalent bonds between the Al atoms and the thiophene α carbons. The calculations show the molecule to be essentially flat, with a tilt angle of only 2° of the molecular plane relative to the surface. There is some indication of self-assembly of bithiophene on Al(111).
σ to π conformational transition: Interactions of the water trimer with π systems
Authors: Tarakeshwar P. , Kim Kwang S. , Brutschy B.
Published: July 2002

We investigate the interaction of the water trimer with the aromatic π systems (benzene, toluene, fluorobenzene, and p-difluorobenzene) at the second-order Møller–Plesset level of theory using both the 6-31+G* and aug-cc-pVDZ basis sets. The minimum-energy structures, binding energies, and the harmonic vibrational frequencies were calculated for all these systems. The minimum-energy structures reveal that the water trimer has only a π type of interaction with benzene and toluene, but has both a π and σ type (H-bond with the fluorine atom) of interaction with fluorobenzene and p-difluorobenzene. The binding energies reveal that, contrary to what has been observed in previous theoretical studies, the strength of the interaction of these π systems with the water trimer is greater than with the water dimer. A good agreement is obtained between the calculated and experimentally measured vibrational frequencies of these complexes. An analysis of the calculated OH stretching modes reveals some unusual facets of the π and σ type of interaction observed in the fluorobenzene and p-difluorobenzene complexes. Thus one observes that the OH red-shift associated with the π type of interaction is larger than with the σ type of interaction. This is particularly interesting because in both the water monomer and water dimer complexes, the OH red-shift associated with the interaction of the water cluster with fluorobenzene and p-difluorobenzene is due to the formation of the H-bond with the fluorine atom. Thus the increase in the size of the water cluster seems to favor the π type of interaction over the σ type of interaction. This inference is also supported by the magnitude of the two-body terms associated with the π and σ type of interaction in both the fluorobenzene and p-difluorobenzene complexes.
Photodissociation dynamics of CBrClF2 at 157.6 nm. I. Experimental study using photofragment translational spectroscopy
Authors: Yokoyama Atsushi , Yokoyama Keiichi , Takayanagi Toshiyuki
Published: July 2002

Photofragment translational spectroscopy of CBrClF2 at 157.6 nm was carried out using a crossed laser-molecular beams technique. Detected species are Br, Cl, and CF2. From the analyses of time-of-flight (TOF) spectra for these three species, the molecules were found to dissociate competitively through CBrF2+Cl, CClF2+Br, and CF2+Br+Cl channels with the branching ratio of 1.0:1.6−0.22+0.13:0.87−0.18+0.13. All of the CClF2 and CBrF2 radicals were found to dissociate spontaneously to produce Cl or Br+CF2, respectively. The angular distributions of these secondary products were found to be anisotropic. These fast secondary reactions are discussed on the basis of the calculated dissociation rates and rotational frequencies.
Reactivity and electronic structure of aluminum clusters: The aluminum–nitrogen system
Authors: Leskiw B. D. , Castleman A. W. , Ashman C. , Khanna S. N.
Published: July 2002

The stability of anionic aluminum–nitrogen clusters has been examined and Al2N−, Al3N2−, Al5N2−, Al6N3−, Al8N3−, and Al9N2− are found to be particularly stable. Theoretical density functional calculations on neutral and anionic AlnN (n=1–8) clusters were performed and the stability and reaction energetics with oxygen examined. Clusters requiring less than 5.7 eV to remove an electron and an Al atom are shown to be resistant to the reaction with oxygen.
Rotational spectra and structures of the Ar3–H2O and Ar3–H2S symmetric tops
Authors: Arunan E. , Emilsson T. , Gutowsky H. S. , Dykstra C. E.
Published: July 2002

Rotational spectra of several isotopomers of Ar3–H2O and Ar3–H2S tetramers were obtained with a Balle–Flygare Fourier transform microwave spectrometer. Both were found to be symmetric tops, the former being an oblate and the latter a prolate one. The rotational constants B, Dj, and DJK were determined to be 1172.1323(1) MHz, 7.199(1) kHz, and −5.545(2) kHz for the H2O and 819.0385(1) MHz, 3.346(1) kHz, and +3.145(2) kHz for the H2S containing tetramer. Substitution analysis with the rotational constants of various isotopomers led to an Ar–Ar distance of 3.848 (3.865) Å and an Ar-c.m.(H2X) distance of 3.675 (4.112) Å for H2O (H2S) complexes. The angle between the C2 axis of the H2X and the C3 axis of the tetramer is estimated to be 74° for H2O and 13° for the H2S complex. No evidence for any excited tunneling/internal rotor states was found for either of the tetramer. MMC calculations show that the equilibrium geometry has the H2X positioned above the plane of the Ar3 with both the protons pointing towards one Ar each. The barrier for the “pseudorotation” in which the protons hop between the argons is determined to be about 6 (8) cm−1 only for H2O (H2S) making the H2X moiety very mobile and effectively making both the tetramers symmetric tops. Rigid body diffusion quantum Monte Carlo (RBDQMC) calculations with the MMC potential have been carried out for vibrational analysis.
Laser pointing stabilization and control in the submicroradian regime with neural networks
Authors: Breitling F. , Weigel R. S. , Downer M. C. , Tajima T.
Published: July 2002

The possibility of controling the pointing stability of a slowly pulsed Ti:Sapphire laser system by lowpass filters and artificial neural networks (NN) is investigated by performing time series analysis and computer simulations on experimentally measured datasets. The simulations show that at pulse repetition rates of 20 Hz it is possible to use a feedforward algorithm to reduce the angular standard deviation from 0.7 to 0.3 μrad. The properties and advantages of NN methods such as automatic adaptation characteristics of a time series are discussed.
Many-particle effects in Ge quantum dots investigated by time-resolved capacitance spectroscopy
Authors: Kapteyn C. M. A. , Lion M. , Heitz R. , Bimberg D. , Miesner C. , Asperger T. , Brunner K. , Abstreiter G.
Published: July 2002

Hole emission from self-organized Ge quantum dots with a diameter of ∼70 nm in a Si matrix is investigated by time-resolved capacitance spectroscopy [deep level transient spectroscopy (DLTS)]. A complex DLTS signal is observed and explained in terms of thermally activated emission from localized many-particle states. In particular, a gradually decreasing activation energy is found with increasing hole population. A qualitative understanding of the DLTS signal and the observed activation energies is achieved in terms of many-particle states determined by quantization and Coulomb charging.
Locally designed pulse shaping for selective preparation of enantiomers from their racemate
Authors: Hoki K. , Ohtsuki Y. , Fujimura Y.
Published: July 2002

We present a method for the design of laser fields to control a selective preparation of enantiomers from their racemate. An expression for two components of the laser pulses [EX(t) and EY(t)] propagating along the Z axis is derived using a locally optimized control theory in the density operator formalism. This expression was applied to a selective preparation of (R-, L-) enantiomers from preoriented phosphinotioic acid (H2POSH) at low temperatures. The target operator was set for the populations to be localized in one side of the double-well potential. First, a simple one-dimensional model was treated. Then, a two-dimensional model in which a free rotation around the preoriented torsional axis is included was briefly considered. In the one-dimensional model, almost complete preparation of the enantiomers was obtained. The optimal electric field consists of a sequence of two linearly polarized pulses with the same phases but with different magnitudes. This means that the resultant electric field is linearly polarized with the polarization for obtaining the R-form nearly parallel to its S–H bond. The optimal electric field transfers the L-form into the R-form while suppressing the reverse process. In the two-dimensional model, the enantiomer selective preparation is controlled by a sequence of circularly polarized pulses.
Lateral oxidation kinetics of AlAsSb and related alloys lattice matched to InP
Authors: Mathis S. K. , Lau K. H. A. , Andrews A. M. , Hall E. M. , Almuneau G. , Hu E. L. , Speck J. S.
Published: July 2002

The lateral oxidation kinetics of AlAs0.56Sb0.44 on InP substrates have been investigated to understand the antimony segregation process during oxidation. Oxidation layers were grown between GaAsSb buffer and cap layers on InP substrates by molecular beam epitaxy. Oxidation temperatures between 325 and 500 °C were investigated for AlAsSb layer thicknesses between 100 and 2000 Å. At low oxidation temperatures (Tox⩽400 °C), the process is reaction limited with a linear dependence of oxidation depth on time. At intermediate oxidation temperatures (400<Tox<450 °C), the oxidation process becomes diffusion limited. At high oxidation temperatures, the oxidation process is termed self-limiting since at 500 °C the process stops entirely after oxidation times on the order of 5 min and distances of 40 μm. It is shown that the antimony float layer lags the oxidation front by a temperature-dependent distance, which suggests that the antimony may change the structure of the oxide at the front and cause self-limiting behavior. The oxidation kinetics of AlxGa1−xAsSb and AlxIn1−xAsSb have also been investigated. Antimony segregation is not suppressed during oxidation of Ga-containing layers and AlInAsSb quaternary alloys do not oxidize laterally at measurable rates in the range 400–525 °C. SiNx cap layers deposited after growth and before oxidation do not affect the Sb segregation or oxidation rate, but do smooth the cap surface by preventing uneven Sb metal segregation to the cap/oxide interface.
A multifacet mechanism for the OH+HNO3 reaction: An ab initio molecular orbital/statistical theory study
Authors: Xia W. S. , Lin M. C.
Published: July 2002

The mechanism for the OH+HNO3 reaction has been studied by ab initio molecular orbital calculations at the G2M(cc3) level of theory. Four complexes and four transition states have been found and confirmed by intrinsic reaction coordinate analyses. The commonly assumed six-membered ring complex formed by hydrogen bonding of the OH radical with HNO3, –ON(O)OH…OH–, was found to be stable by 8.1 kcal/mol; its decomposition producing NO3+H2O was predicted to have a barrier of 11.6 kcal/mol. A five-membered ring complex, –ON(O)OH…O(H)–, with the H atom of the OH radical placed out of the ring plane, was found to have a stability of 5.3 kcal/mol; it fragments to form NO3+H2O with a barrier of 6.6 kcal/mol. Two additional complexes, which are the mirror image of each other with a 7.4 kcal/mol binding energy, were found to be related to the OH exchange reaction with a 13.3 kcal/mol barrier above the complexes. The direct abstraction process producing H2O2 and NO2 was predicted to have a large barrier of 24.4 kcal/mol, insignificant to atmospheric chemistry. The rate constant has been calculated at 200–1500 K and 0–760 Torr. The results show that the reaction has strong pressure and tunneling effects below room temperature. In addition, the rate constants for the decay of OH and OD (in OD+DNO3) evaluated by kinetic modeling compare reasonably well with experimental data below room temperature. The unusually pronounced kinetic isotope effect observed experimentally, kH/kD⩾10, could be accounted for by the combination of the greater tunneling rate in the H system and the large redissociation rate of stabilized complexes in the D system. The rate constant predicted for the production of H2O and NO3 in the temperature range 750–1500 K can be effectively represented by the expression k=1.45×10−23 T3.5 exp(+839/T) cm3/s.
Probe diagnostics of electronegative plasmas with bi-Maxwellian electrons
Authors: Stamate E. , Ohe K.
Published: July 2002

A method to detect the parameters of electronegative plasmas with bi-Maxwellian electrons using a Langmuir probe is proposed by applying the test function. The radial model for the positive ion collection is extended and found to be more appropriate for density detection than the orbital motion limited model. The parameters of a diffused Ar/SF6 plasma are obtained by cylindrical and spherical probes for various Ar/SF6 ratios over a pressure range from 0.5 to 10 mTorr. The density ratio of bulk to hot electrons decreases from about 50 in Ar plasma to less than one for Ar/SF6, of which the SF6 flow is 0.05 standard cubic centimeters per minute. The hot electron group cannot be neglected for density ratios of negative ions to electrons lower than 100, though approximation of Maxwellian electrons is valid for larger density ratios.
Spectral blueshift and improved luminescent properties with increasing GaSb layer thickness in InAs–GaSb type-II superlattices
Authors: Ongstad Andrew P. , Kaspi Ron , Moeller Charles E. , Tilton Michael L. , Gianardi Donald M. , Chavez Joseph R. , Dente Gregory C.
Published: July 2002

We describe the photoluminescence spectroscopy (PL) and Fourier transform infrared absorbance spectroscopy characterization of a large set of InAs/GaSb type-II strained layer superlattice (SLS) samples. The samples are designed to probe the effect of GaSb layer thickness on the optical properties of the SLS, while the InAs-layer thickness is held fixed. As the GaSb layer thickness is increased, we observe a spectral blue shift of the PL peaks that is accompanied by an increase in intensity, narrower linewidths, and a large reduction in the temperature sensitivity of the luminescence. These effects occur despite a significant reduction in the electron-hole wave function overlap as the GaSb layer thickness is increased. In addition, we compare the results of empirical pseudopotential model (EPM) calculations to the observed blueshift of the primary band gap. The EPM calculations are found to be in very good agreement with the observed data.
A quantum-mechanical study of the dynamics of the O(1D)+H2→OH+H insertion reaction
Authors: Honvault Pascal , Launay Jean-Michel
Published: July 2002

We have studied the quantum dynamics of the O(1D)+H2(X 1Σg+)→ OH(X 2Π)+H(2S) reaction at low collision energies. The hyperspherical method has been used in a time-independent formalism. We present rotational and vibrational distributions. OH vibrational distributions are found to be noninverted. Differential cross sections are almost symmetric in the forward and backward directions, with a slight preference for forward scattering. These features are consistent with the formation of an intermediate complex.
Electroluminescence in Si/SiO2 layer structures
Authors: Heikkilä L. , Kuusela T. , Hedman H.-P.
Published: July 2002

Electroluminescence spectra have been studied on samples consisting of a single SiO2 or poly Si layer or SiO2/Si layer pair grown either on silicon or metal substrates. The samples were designed for determining the conditions and the location where electroluminescence originates in SiO2/Si structures. An intermediate gold layer was used to isolate the active layers from the silicon substrate. The results indicate that significant electroluminescence emission can be observed only in those samples that have SiO2 p-type crystalline Si interface. The most promising explanation of this phenomenon is that there are defect levels in the thin silicon oxide layer near the interface.
Quotient of a loop group and Witten genus
Authors: Léandre Rémi
Published: July 2002

We define a Dirac–Ramond operator over the quotient (LG)/H of a loop group by a subgroup H of the compact Lie group G. We state the conjecture that its equivariant Index with respect of the natural circle action over the quotient of the loop group is equal to the Witten genus of the homogeneous manifold G/H. We motivate the conjecture by a short time argument which allows to come back to a Dirac–Ramond operator at the manner of Taubes on a limit model where all the computations can be done by hand.
Focusing method and apparatus for high-resolution projection patterning
Authors: Popovici D. , Meunier M.
Published: July 2002

A novel focusing method and apparatus for determining the position of the image plane of a coherently illuminated object is presented. The principle of this method is based on the light distribution in the defocused image plane of a luminous knife-edge object. The radiation energy extending beyond the geometrical image is a function of the relative position of the projection objective and its minimum indicates the best location of the image plane. The photodetected energy can be used as a feedback signal to focus various optical systems in applications such as high-resolution laser patterning. Preliminary results obtained by means of a 193 nm excimer laser radiation and a reflecting objective are also reported.
Decay pathways and dissociation energies of copper clusters, Cun+ (2⩽n⩽25), Cun2+ (15⩽n⩽25)
Authors: Krückeberg S. , Schweikhard L. , Ziegler J. , Dietrich G. , Lützenkirchen K. , Walther C.
Published: July 2002

The fragmentation pathways and dissociation energies of copper cluster cations, Cun+ and Cun2+, are determined by multiple-collision induced dissociation. For singly charged clusters, an odd–even staggering is observed throughout the investigated size range, 2⩽n⩽25, where the odd-size clusters have a higher dissociation energy than the average value of their even-size neighbors. The odd–even effect decreases with increasing cluster size. In small clusters it manifests itself by dimer evaporation of the odd-size clusters with n=3,5,11 and possibly n=7, while for all other cluster sizes dissociation by neutral monomer evaporation is observed. The clusters of size n=3, 9, 15, and 21 show particularly high dissociation energies and thus indicate electronic shell closures for n=2, 8, 14, and 20 atomic valence electrons. These results are compared with recent density functional theory calculations. The investigations on singly charged clusters are complemented by studies on doubly charged Cun2+, n=15–25. These clusters decay by either neutral monomer evaporation, or, in the case of Cu162+, by the emission of a trimer ion, Cu3+. Their dissociation energies show the behavior expected by analogy with the singly charged clusters with the same number of atomic valence electrons, ne=n−z, where z denotes the charge state.
Ab initio investigation of the adsorption of benzene in mordenite
Authors: Demuth Th. , Benco L. , Hafner J. , Toulhoat H. , Hutschka F.
Published: July 2002

The adsorption of benzene in pure-silica and acid mordenite has been investigated using density functional theory, both in the local density approximation and including generalized-gradient corrections. Benzene acts as a test molecule for studying the interaction between an aromatic hydrocarbon and a zeolite. Different adsorption modes have been studied for several acid sites. All configurations have been optimized by minimizing the total energy with respect to all lattice parameters and to the atomic coordinates. The strength of the adsorption is directly correlated to the local structural distortion of the zeolitic framework, especially of the acid site. Only if strong adsorption occurs, the molecule itself is slightly deformed. Furthermore, an analysis of the differential charges has been performed, indicating substantial polarization effects for the acid site as well as for the molecule. Finally, the red-shift of the OH-stretching frequencies due to the adsorption has been calculated.
Single shot twin x-ray microscopic imaging using simultaneously produced laser-plasma x-ray sources
Authors: Chakera J. A. , Arora V. , Kumbhare S. R. , Naik P. A. , Ganesan V. , Gupta P. D.
Published: July 2002

A simple technique of single shot x-ray microscopic imaging is reported using two simultaneously produced laser-plasma x-ray sources. Twin x-ray images are recorded by placing the sample in a proximity gap with a photoresist and exposing it to two equivalent point x-ray sources. Characterization of the technique is carried out using a carbon fiber as a test sample and x-ray emission (hν⩾1 keV) from laser produced copper plasmas. A spatial resolution of ∼300 nm is observed, consistent with the geometrical parameters of the imaging system. This method may be useful in obtaining a single shot comparison of spectral and temporal characteristics of a sample by using different x-ray filters or plasma targets for the two imaging paths and a temporal delay in generation of the two x-ray sources, respectively.
Development of low-temperature and high vacuum atomic force microscope with freeze–fracture function
Authors: Nakamoto Keiichi , Mooney C. B. , Iwatsuki Masashi
Published: July 2002

A high vacuum low temperature atomic force microscope (AFM) for the direct observation of biological freeze–fracture samples has been developed. This AFM has an integrated vacuum system and a freeze–fracture mechanism inside the vacuum chamber. It is possible to observe the fractured sample surface without exposing the freshly fractured surface to the ambient atmosphere. The design is described in detail. A sample temperature up to −175 °C is achieved, with the temperature fluctuation less than 0.1 °C. Temperature control is possible from −175 to 0 °C for deep etching. An ultimate vacuum pressure of 8.7×10−6 Pa has been achieved. Freeze-fractured human red blood cell has been examined, and fine structure within the cell has been observed. It is demonstrated that this is convenient and useful for the direct observation of a biological freeze–fractured surface.
Energetics and dissociative photodetachment dynamics of superoxide–water clusters: O2−(H2O)n, n=1–6
Authors: Luong A. K. , Clements T. G. , Sowa Resat M. , Continetti R. E.
Published: July 2002

The dissociative photodetachment of O2−(H2O)n=1–6 was studied at 388 and 258 nm using photoelectron–multiple-photofragment coincidence spectroscopy. Photoelectron spectra for the series indicate a significant change in the energetics of sequential solvation beyond the fourth water of hydration. Photoelectron–photofragment kinetic energy correlation spectra were also obtained for O2−(H2O)1–2, permitting a determination of the first and second energies of hydration for O2− to be 0.85±0.05 and 0.70±0.05 eV, respectively. The correlation spectra show that the peak photofragment kinetic energy release in the dissociative photodetachment of O2−(H2O) and O2−(H2O)2 are 0.12 and 0.25 eV, respectively, independent of the photon and photoelectron kinetic energies. The molecular frame differential cross section for the three-body dissociative photodetachment: O2−(H2O)2+hν→O2+2H2O+e− is also reported. The observed partitioning of momentum is consistent with either a sequential dissociation or dissociation from a range of initial geometries.