bluuebunny/crossref_metadata_2025 huggingface dataset:
Modelling the bacterial photosynthetic reaction center. V. Assignment of the electronic transition observed at 2200 cm−1 in the special-pair radical-cation as a second-highest occupied molecular orbital to highest occupied molecular orbital transition
Authors: Reimers Jeffrey R. , Shapley Warwick A. , Hush Noel S.
Published: July 2003

Primary charge separation in photoexcited photosynthetic reaction centers produces the radical cation P+ of a bacteriochlorophyll dimer known as the special-pair P. P+ has an intense electronic transition in the vicinity of 1800–5000 cm−1 which is usually assigned to the interchromophore hole-transfer excitation of the dimer radical cation; in principle, this spectrum can give much insight into key steps of the solar-to-electrical energy-conversion process. The extent to which this transition is localized on one-half of the dimer or delocalized over both is of utmost importance; an authoritative deduction of this quantity from purely spectroscopic arguments requires the detailed assignment of the observed high to medium resolution spectra. For reaction centers containing bacteriochlorophylls a or b, a shoulder is observed at 2200 cm−1 on the low-energy side of the main hole-transfer absorption band, a band whose maximum is near 2700 cm−1. Before quantitative analysis of the hole-transfer absorption in these well-studied systems can be attempted, the nature of the processes leading to this shoulder must be determined. We interpret it as arising from an intrachromophore SHOMO to HOMO transition whose intensity arises wholly through vibronic coupling with the hole-transfer band. A range of ab initio and density-functional calculations are performed to estimate the energy of this transition both for monomeric cations and for P+ of Blastochloris viridis, Rhodobacter sphaeroides, Chlorobium limicola, Chlorobium tepidum, Chlamydomonas reinhardtii, Synochocystis S.6803, spinach photosystems I and II, Heliobacillus mobilis, and finally Heliobacterium modesticaldum, with the results found to qualitatively describe the available experimental data. Subsequent papers in this series provide quantitative analyses of the vibronic coupling and complete spectral simulations based on the model developed herein.
Modelling the bacterial photosynthetic reaction center. VI. Use of density-functional theory to determine the nature of the vibronic coupling between the four lowest-energy electronic states of the special-pair radical cation
Authors: Reimers Jeffrey R. , Shapley Warwick A. , Rendell Alistair P. , Hush Noel S.
Published: July 2003

It is now over ten years since the first FTIR spectra were recorded of the radical cation of the special-pair, a dimer of bacteriochlorophyll molecules that forms the primary electron donor responsible for primary charge separation in bacterial photosynthesis. While spectra of this type promise to reveal much concerning the role of the special pair electron donor in photosynthesis, attempts to model and interpret them have been limited by poor knowledge of the vibrationally specific aspects of the electron–phonon coupling and have thus been restricted to crude model calculations only. We develop techniques through which density-functional theory can be employed to evaluate most of the unknown properties. This includes symmetric-mode displacements, antisymmetric-mode vibronic coupling constants, and interstate electronic couplings evaluated for interactions between the four lowest-energy states of the special-pair cation radical: the ground state, the primary hole-transfer state, and states involving these two combined with SHOMO to HOMO transitions. Geometry optimizations are performed for all four states of the dimer while vibrational analyses are obtained for the first two; vibronic coupling constants are extracted from analysis of stolen infrared transition moments using Herzberg–Teller theory. Quantitatively, these results are employed in the subsequent paper in this series to simulate the observed spectra. Qualitatively, these results indicate that: (1) vibronic coupling occurs through a large number of antisymmetric modes of the dimer rather than through a small number of strongly active modes, (2) the role of symmetric vibrational motions of the dimer is only minor, (3) that the active symmetric modes are significant in number and low in frequency, (4) that vibronic coupling between the hole-transfer state and the SHOMO to HOMO state is relatively weak and influences spectra only near resonance, and (5) that the calculated electronic couplings are qualitatively realistic and may provide an explanation for the much weaker coupling observed in chlorophyll-containing reaction centers.
Kinetic Monte Carlo simulation of the effect of coalescence energy release on the size and shape evolution of nanoparticles grown as an aerosol
Authors: Mukherjee D. , Sonwane C. G. , Zachariah M. R.
Published: July 2003

We develop a kinetic Monte Carlo algorithm to describe the growth of nanoparticles by particle–particle collision and subsequent coalescence. The unique feature of the model is its ability to account for the exothermic nature of particle coalescence events and to show how the resulting nonisothermal behavior can be used to change the primary particle size and the onset of aggregation in a growing nanoaerosol. The model shows that under certain conditions of gas pressure, temperature, and particle volume loadings, the energy release from two coalescing nanoparticles is sufficient to cause the particle to exceed the background gas temperature by many hundreds of degrees. This in turn results in an increase in the microscopic transport properties (e.g., atomic diffusivity) and drive the coalescence process even faster. The model compares the characteristic times for coalescence and collision to determine what conditions will lead to enhanced growth rates. The results, which are presented for silicon and titania as representative nanoparticle systems, show that increasing volume loading and decreasing pressure result in higher particle temperatures and enhanced sintering rates. In turn, this results in a delay for the onset of aggregate formation and larger primary particles. These results suggest new strategies for tailoring the microstructure of nanoparticles, through the use of process parameters heretofore not considered as important in determining primary particle size.
The dynamics of vortices on S2 near the Bradlow limit
Authors: Baptista J. M. , Manton N. S.
Published: July 2003

The explicit solutions of the Bogomolny equations for N vortices on a sphere of radius R2>N are not known. In particular, this has prevented the use of the geodesic approximation to describe the low energy vortex dynamics. In this article we introduce an approximate general solution of the equations, valid for R2≳N, which has many properties of the true solutions, including the same moduli space CPN. Within the framework of the geodesic approximation, the metric on the moduli space is then computed to be proportional to the Fubini–Study metric, which leads to a complete description of the particle dynamics.
Accurate measurement of 1.5 μm lifetime of Er3+ in LiNbO3 crystals and waveguides
Authors: Zhang De-Long , Pun E. Y. B.
Published: July 2003

Two optimized methods for accurately measuring the 1.5 μm lifetime of Er3+ in LiNbO3 crystals and waveguides are proposed. The first method, suitable for measurement of bulk material, was suggested by forming an annealed proton exchanged sandwich structure and fluorescence was collected through an adjustable aperture. This excitation collection scheme was used to investigate bulk Er-doped LiNbO3 crystals with eight different Er3+ doping levels, three different crystal thicknesses, and three different cut orientations. The 1.5 μm lifetime was investigated in three configurations: with both a sandwich and an aperture, with a sandwich but without an aperture, and without not only a sandwich but also without an aperture. In addition, the effect of the aperture was also studied. The second method, suitable for a waveguide structure, was proposed on the basis of transversely gathering the fluorescence aided by an outcoupling fiber. It was optimized by comparing three different fluorescence collection schemes. Using this method, the lifetimes of some Ti:Er:LiNbO3 strip waveguides were measured. In addition, the effect of radiation trapping on the polarized fluorescence spectrum was also investigated. The experimental results are discussed and summarized.
On the Q-ball profile function
Authors: Ioannidou T. A. , Vlachos N. D.
Published: July 2003

We use analytic and numerical methods to obtain the solution of the Q-ball equation of motion. In particular, we show that the profile function of the three-dimensional Q-ball can be accurately approximated by the symmetrized Woods–Saxon distribution.
Magnetoresistance of electrodeposited iron–cobalt–nickel–copper multilayers
Authors: Huang Q. , Young D. P. , Podlaha E. J.
Published: July 2003

Giant magnetoresistance (GMR) is observed in electrodeposited FeCoNiCu/Cu multilayers with Cu layer thickness varying from 1.0 to 2.5 nm at a fixed alloy layer thickness of 2.0 nm. A maximum GMR occurred at a copper layer thickness of 1.8 nm, which corresponds to the second maximum peak reported for similar multilayers fabricated by vapor deposition. The saturation magnetic field was considerably larger than that for the vapor deposited counterparts. The GMR of multilayers with a thicker alloy layer was sensitive to the magnetic field history, indicating the onset of anisotropic magnetoresistance.
Synthesis of highly oriented, single-crystal silicon nanoparticles in a low-pressure, inductively coupled plasma
Authors: Bapat Ameya , Perrey Christopher R. , Campbell Steven A. , Barry Carter C. , Kortshagen Uwe
Published: July 2003

Single-crystal nanoparticles of silicon, several tens of nm in diameter, may be suitable as building blocks for single-nanoparticle electronic devices. Previous studies of nanoparticles produced in low-pressure plasmas have demonstrated the synthesis nanocrystals of 2–10 nm diameter but larger particles were amorphous or polycrystalline. This work reports the use of an inductively coupled low-pressure plasma to produce single-crystal silicon nanoparticles with diameters between 20 and 80 nm. Electron microscopy studies confirm that the nanoparticles are highly oriented diamond-cubic silicon.
Near-infrared photoluminescence in titania: Evidence for phonon-replica effect
Authors: Montoncello F. , Carotta M. C. , Cavicchi B. , Ferroni M. , Giberti A. , Guidi V. , Malagù C. , Martinelli G. , Meinardi F.
Published: July 2003

The photoluminescence of rutile and anatase TiO2 has been investigated in the range of 360–890 nm at several temperatures. An unexpected intense near-infrared (1.53 eV) photoluminescence band was recorded for both phases. At low temperatures, the resulting bands form a substructure of equally spaced peaks irrespective of the phase. The spectra were interpreted in the framework of the single-configuration-coordinate model as the phonon-replica effect, originating from ionization of oxygen vacancies.
Electron capture behaviors of deep level traps in unintentionally doped and intentionally doped n-type GaN
Authors: Cho H. K. , Kim C. S. , Hong C.-H.
Published: July 2003

In n-type GaN films grown on sapphire substrates by metal-organic chemical vapor deposition such as unintentionally GaN and intentionally Si-doped GaN and In-doped GaN, the electron capture behaviors were investigated by deep level transient spectroscopy with various filling pulse durations. Two distinct deep levels E1 and E2 were typically observed in unintentionally doped n-type GaN. After optimized growth of undoped GaN, deep level E1 disappears. With increasing Si doping, the trap concentration of deep level E2 is increased. However, In doping in n-type GaN growth was found to suppress the formation of deep level E2. The electrons captured at the traps E1 and E2 were found to depend logarithmically on the duration time of the filling pulse. From an analysis of a model involving barrier-limited capture rate, it can be concluded that deep level E1 is associated with linear line defects along dislocation cores while deep level E2 is related to point defects.
Mechanism for Ohmic contact formation of oxidized Ni/Au on p-type GaN
Authors: Jang Ho Won , Kim Soo Young , Lee Jong-Lam
Published: July 2003

The mechanism for Ohmic contact formation of oxidized Ni/Au on p-type GaN was investigated using three-dimensional secondary ion mass mapping and synchrotron photoemission spectroscopy. Annealing under O2 ambient caused the preferential outdiffusion of Ni to the contact surface to form NiO, leading to the final contact structure of NiO/Au/p-GaN. Ga atoms were dissolved in the Au contact layer and the oxygen atoms incorporated during annealing promoted the outdiffusion of Ga atoms from the GaN layer, leaving Ga vacancies below the contact. The drastic reduction of contact resistivity by the oxidation annealing could be attributed to the formation of Ga vacancies, which plays a role in increasing the net hole concentration and lowering the Fermi level position.
Fictive-temperature dependence of structural relaxation in silica glass
Authors: Kakiuchida Hiroshi , Saito Kazuya , Ikushima Akira J.
Published: July 2003

To clarify the factors that determine the structural relaxations in silica glass, the influence of fictive temperature on the relaxation process was investigated. It was found that the relaxations at different heat-treatment temperatures are enhanced with a rise in the fictive temperature. The structural relaxation is not simply the Maxwell process that is determined only by the heat-treatment temperature. Since the density of silica glass increases with a rise in the fictive temperature, the mechanism of the aforementioned enhancement cannot be understood by the free volume theory, which has often been utilized for explaining structural relaxations in many glass formers. The present results reveal the significance of the fictive temperature in understanding the structural relaxation phenomenon in silica glass.
In situ monitoring of size distributions and characterization of nanoparticles during W ablation in N2 atmosphere
Authors: Landström L. , Márton Zs. , Arnold N. , Högberg H. , Boman M. , Heszler P.
Published: July 2003

Nanoparticles were generated by pulsed laser ablation of tungsten in a N2 ambient at atmospheric pressure. Size distributions and concentrations were monitored in situ versus laser fluence, repetition rate, and ablated spot size, by a differential mobility analyzer and a particle counter. The multishot ablation threshold was determined to be ∼6 J/cm2 for the laser used (ArF excimer, λ=193 nm). Mostly small, nonlognormally, distributed particles (<20 nm in diameter) were generated below the ablation threshold (φth), and the relative concentration of larger particulates (>20 nm in diameter) increased above φth. Modeling of the temperature and ablated depth dependence on fluence showed that the formation of clusters below φth could not be assigned to a thermal process, but are connected to particle condensation from a photochemically desorbed thin layer. X-ray diffraction and x-ray photoelectron spectroscopy analysis performed on polydisperse nanoparticles revealed an amorphous phase of the particles, and the elemental composition was found to be WN0.3.
Highly stable polarization gratings in photocrosslinkable polymer liquid crystals
Authors: Ono Hiroshi , Emoto Akira , Takahashi Fumihiro , Kawatsuki Nobuhiro , Hasegawa Takako
Published: July 2003

Highly stable polarization gratings have been prepared in photocrosslinkable polymer liquid-crystal films by the use of two orthogonally polarized He–Cd laser beams and subsequent annealing. The resulting pure polarization gratings exhibiting thermal stability up to 150 °C diffract the beam and convert the polarization state at the same time according to the theoretical expectation.
Characterization of GaN Schottky barrier photodetectors with a low-temperature GaN cap layer
Authors: Lee M. L. , Sheu J. K. , Lai W. C. , Su Y. K. , Chang S. J. , Kao C. J. , Tun C. J. , Chen M. G. , Chang W. H. , Chi G. C. , Tsai J. M.
Published: July 2003

By using organometallic vapor phase epitaxy we have prepared i-GaN/low temperature (LT) GaN/Ni/Au (sample A) and i-GaN/Ni/Au (sample B) Schottky barrier ultraviolet (UV) photodiodes (PDs). It was found that we could significantly reduce leakage current and achieve a much larger photocurrent to dark current contrast ratio by introducing a LT GaN on top of the conventional nitride-based UV PDs. With an incident light wavelength of 350 nm and a −1 V reverse bias, it was found that the measured responsivity was around 0.1 and 0.37 A/W for samples A and B, respectively. Furthermore, it was found that the operation speed of sample A is slower than that of sample B due to the highly resistive LT GaN layer induced large RC time constant.
The Davey–Stewartson I equation on the quarter plane with homogeneous Dirichlet boundary conditions
Authors: Fokas A. S.
Published: July 2003

Dromions are exponentially localized coherent structures supported by nonlinear integrable evolution equations in two spatial dimensions. In the study of initial-value problems on the plane, such solutions occur only if one imposes nontrivial boundary conditions at infinity, a situation of dubious physical significance. However, it is established here that dromions appear naturally in the study of boundary-value problems. In particular, it is shown that the long time asymptotics of the solution of the Davey–Stewartson I equation in the quarter plane with arbitrary initial conditions and with zero Dirichlet boundary conditions is dominated by dromions. The case of nonzero Dirichlet boundary conditions is also discussed.
Enhancement of implantation energy using a conducting grid in plasma immersion ion implantation of dielectric/polymeric materials
Authors: Fu Ricky K. Y. , Tian Xiubo , Chu Paul K.
Published: July 2003

Plasma immersion ion implantation (PIII) is conducted on insulating materials using a conducting grid to enhance the ion implantation energy. The biased grid that is connected to the sample holder enshrouds the insulating specimens, and ions from the overlying plasma are implanted through the grid into the samples. The implantation voltage is applied to the grid via the sample platen so problems associated with PIII of insulating materials such as capacitance and charging (and secondary electrons) effects can be greatly alleviated. In the work reported here, we investigate the efficacy of the grid approach. Secondary ion mass spectrometry is used to determine the nitrogen depth profiles. Simulation indicates that for insulating specimens that are plasma implanted without the conducting grid, the maximum nitrogen ion energy is only about 23 keV for an applied voltage of 40 kV while it improves to 30 keV in the presence of the grid. The experimental results are consistent with the surface potentials derived from theoretical modeling of the charging effects. To further improve the ion implantation energy, more effective grid dimension scaling and surface shielding, more optimal separation between the conducting grid and insulator surface, as well as better confinement of the secondary electrons are required.
Microcanonical thermodynamic properties of helium nanodroplets
Authors: Lehmann Kevin K.
Published: July 2003

The density of states and other thermodynamic functions of helium nanodroplets are calculated for a microcanonical ensemble with both energy and total angular momentum treated as conserved quantum numbers. These functions allow angular momentum conserving evaporative cooling simulations. As part of this project, a recursion relationship is derived for the reduction to irreducible representations of the nth symmetric power of the irreducible representations of the rotation group. These give the distribution of total angular momentum states generated by putting multiple quanta into a ripplon or phonon mode of the droplet, each of which is characterized by a angular momentum quantum number.
Hard sphere fluids in random fiber networks
Authors: Schmidt Matthias , Brader Joseph M.
Published: July 2003

We investigate an annealed hard sphere fluid in contact with a rigid, random fiber network modeled by quenched, vanishingly thin hard needles. For this model a quenched-annealed density functional theory is presented that treats arbitrary spatially inhomogeneous situations, in particular anisotropic and spatially varying needle distributions. As a test case we consider the structure of the hard sphere fluid at the surface of an isotropic fiber network and find good agreement of the theoretical density profiles with our computer simulation results. For high needle densities the surface acts like a rough impenetrable wall. In the limit of infinite needle density the behavior near a smooth hard wall is recovered. Results for the partition coefficient agree well with existing data.
Free energy formalism for polymer adsorption: Self-consistent field theory for weak adsorption
Authors: Blokhuis Edgar M. , Skau Karl Isak , Avalos Josep B.
Published: July 2003

Polymer adsorption has been widely investigated in the context of self-consistent mean-field theories. As a further simplification, the “ground state dominance approximation” is often made, treating the polymer chains as infinitely long. For short polymers, or not so concentrated polymer solutions, corrections to ground state dominance may be important, however. In this work, we discuss analytical solutions to the full self-consistent field equations, valid for any chain length, in the limit of weak adsorption. We show how the resulting equations may be put into a free energy functional formalism, in analogy to the de Gennes–Lifshitz free energy for infinitely long polymer chains. Analytical expressions are derived for polymer density profiles, surface tension and the interaction potential between two planar, polymer-adsorbing surfaces. Particular attention is paid to the distal ordering of the polymer coils that shows up as oscillations in the polymer density profile and interaction potential at the scale of the polymer’s radius of gyration.
The role of the basis set: Assessing density functional theory
Authors: Boese A. Daniel , Martin Jan M. L. , Handy Nicholas C.
Published: July 2003

When developing and assessing density functional theory methods, a finite basis set is usually employed. In most cases, however, the issue of basis set dependency is neglected. Here, we assess several basis sets and functionals. In addition, the dependency of the semiempirical fits to a given basis set for a generalized gradient approximation and a hybrid functional is investigated. The resulting functionals are then tested for other basis sets, evaluating their errors and transferability.
Capillary nozzles for liquid-jet laser-plasma x-ray sources
Authors: de Groot J. , Johansson G. A. , Hertz H. M.
Published: July 2003

We describe a method to fabricate tapered glass nozzles suitable for liquid-jet-target generation in laser-plasma soft x-ray and extreme ultraviolet sources. In the method, a tapered nozzle is formed as an integral part of a flexible capillary glass tubing. The method makes use of inert materials, extending the possible choice of target liquids compared to current nozzles. It also provides flexibility as regards nozzle diameter and pressure, thereby allowing optimization of the target size and extending the range of applicability for the liquid-jet-target laser plasmas.
Robust and inexpensive thermal vaporizer for low-vapor pressure liquids
Authors: Xiao Z. G.
Published: July 2003

A thermal vaporizer has been developed to vaporize liquid organometallic compound precursors for plasma-enhanced chemical vapor deposition. The organometallic precursors include titanium (IV) isopropoxide, tetrakis(dimethylamino)titanium, zirconium 2-methyl-2-butoxide, zirconium t-butoxide, and bis(ethylbenzene)chromium. Steady gas flows were achieved at temperature ranges of 140 °C to 220 °C for the five liquid precursors. The vaporizer is robust and very easy to rebuild.
Real-time atomic absorption mercury continuous emission monitor
Authors: Zamzow Daniel S. , Bajic Stanley J. , Eckels David E. , Baldwin David P. , Winterrowd Chris , Keeney Robert
Published: July 2003

A continuous emission monitor (CEM) for mercury (Hg) in combustor flue gas streams has been designed and tested for the detection of Hg by optical absorption. A sampling system that allows continuous introduction of stack gas is incorporated into the CEM, for the sequential analysis of elemental and total Hg. A heated pyrolysis tube is used in the system to convert oxidized Hg compounds to elemental Hg for analysis of total Hg; the pyrolysis tube is bypassed to determine the elemental Hg concentration in the gas stream. A key component of the CEM is a laboratory-designed and -assembled echelle spectrometer that provides simultaneous detection of all of the emission lines from a Hg pen lamp, which is used as the light source for the optical absorption measurement. This feature allows for on-line spectroscopic correction for interferent gases such as sulfur dioxide and nitrogen dioxide, typically present in combustion stack gas streams, that also absorb at the Hg detection wavelength (253.65 nm). This article provides a detailed description of the CEM system, the characteristics and performance of the CEM, and the results of field tests performed at the Environmental Protection Agency-Rotary Kiln at Research Triangle Park, NC.
Characteristics of ultrathin HfO2 gate dielectrics on strained-Si0.74Ge0.26 layers
Authors: Lee Je-Hun , Maikap S. , Kim Doh-Y. , Mahapatra R. , Ray S. K. , No Y. S. , Choi Won-Kook
Published: July 2003

The structural and electrical characteristics of HfO2 gate dielectrics along with the interfacial layers formed on strained-Si0.74Ge0.26 films have been investigated. The polycrystalline HfO2 film with a physical thickness of ∼4.0 nm and an amorphous Hf–silicate interfacial layer with a physical thickness of ∼4.5 nm have been observed by high-resolution transmission electron microscopy and time-of-flight secondary ion mass spectroscopy. The electrical properties have been studied using metal–oxide–semiconductor (MOS) structures. A dielectric constant of 26 for HfO2 film and 8.0 for Hf–silicate interfacial layer have been calculated from the accumulation capacitances of the capacitors. These dielectrics show an equivalent oxide thickness as low as 0.6 nm for HfO2 and 2.2 nm for the Hf–silicate layers. The fabricated SiGe MOS capacitors show a low leakage current density of ∼6.5×10−7 A/cm2 at a gate voltage of −1.0 V, breakdown field of 6.5 MV/cm, and moderately low interface state density of 5.5×1011 cm−2 eV−1.
Pressure control of properties of pulsed laser deposited carbon and carbon nitride films
Authors: Szörényi Tamás , Fogarassy Eric
Published: July 2003

Carbon and carbon nitride films have been deposited by ArF excimer laser ablation of identical graphite targets in the same system, in argon and nitrogen atmospheres, respectively. Based on the comparative analysis of apparent growth rates, deposition rates of the constituting elements and mass densities, a consistent description of film formation in the 10−5–200 Pa pressure domain is presented. The results reveal that the formation, composition, and microstructure of carbon and carbon nitride films fabricated in this process window are governed by gas phase processes. Below 5 Pa the film building blocks are mainly atoms and molecules resulting in compact, dense films. At higher pressures increasing contribution of clusters to film growth leads to films of increasing porosity ready to trap (mainly water) molecules from the surrounding atmosphere.
Effects of molecular distribution on the fluorescence transfer: Exact results for slab geometry
Authors: Klushin Leonid , Tcherkasskaya Olga
Published: July 2003

Fluorescence energy transfer between donor and acceptor molecules in nonuniform molecular distributions is considered. In particular, we investigate the effect of the molecular distributions on the time-resolved fluorescence intensity decay of the donor and provide exact analytical expressions for the ensemble-averaged donor decays when the acceptor molecules are distributed uniformly within a slab of a finite thickness. The effect of the donor distribution on the energy transfer is also examined. Importantly, the derived formulas display both asymptotic limits for the two- and three-dimensional molecular distributions, and the crossover between them. Analytical results show very good agreement with numerical lattice model simulations. On this basis, we introduce new procedures for the energy transfer data analysis that provide a direct estimation of the slab thickness and the acceptor concentration from a single time-resolved fluorescence measurement.
Velocity redistribution of excited atoms by radiative excitation transfer. I. Experimental demonstration by photodissociation of Na2 and field-free imaging
Authors: Kaufmann O. , Ekers A. , Bergmann K. , Bezuglov N. , Miculis K. , Auzinsh M. , Meyer W.
Published: July 2003

Photodissociation of state-selected sodium molecules, Na2(X 1Σg+,v″)+hν→Na2*(B 1Πu)→Na*(3p3/2)+Na(3s1/2) has been studied theoretically and experimentally using a novel “field-free” ion imaging design. The experiment uses a supersonic Na/Na2 beam in combination with the stimulated Raman adiabatic passage technique to prepare Na2 molecules in selected rovibronic levels of the electronic ground state. The Na(3p3/2) fragments are photoionized (or excited to high Rydberg states) in a permanently field-free reaction zone. The fragments enter the ion optics because of the flow velocity of the beam and are focused onto a position sensitive detector, which provides an energy resolution of about 50 meV. The measured anisotropic photofragment angular distributions reflect the alignment of the molecules prior to dissociation and are well explained by the anisotropic nature of the photodissociation by polarized laser light. The measured images show not only the expected relatively fast photodissociation fragments, but also the efficient formation of slow Na(3p3/2) atoms. Fast and slow refer to the atomic velocity relative to the center of-mass of the dissociating molecule. The ratio of the numbers of slow atoms and fast photofragments is 0.16 and 0.22 for the dissociation of Na2 from levels v″=17 and v″=23, respectively. Several models are analyzed to explain the observations. Calculations show that the dramatic velocity redistribution is caused by radiation trapping: the excitation is efficiently radiatively transferred from the fast Na(3p) photofragments to the abundant Na(3s) atoms from the primary beam, whereby the hyperfine splitting of the 3s state must be taken into account. Analytical formulas describing this mechanism show a ratio of slow to fast Na(3p) atoms of 0.13 for v″=17 and 0.19 for v″=23, which is in very good agreement with the experimental observations.
Formation of fine near-field scanning optical microscopy tips. Part II. By laser-heated pulling and bending
Authors: Lazarev Alexander , Fang Nicholas , Luo Qi , Zhang Xiang
Published: July 2003

We have developed a simplified heating and pulling method for formation of near-field scanning optical microscopy probing tips from optical fibers. Laser power and continuous pulling force are two key processing parameters investigated. We found a narrow working range of laser power of 1.85–1.95 W and the pulling force of 0.5–0.9 lb, with the optimum conditions of 1.90 W and 0.74 lb. Tips with short taper (∼300 μm), small apex (∼50 nm), and large aperture cone angles (∼45°) have been achieved. The as-prepared tips are subsequently bended by pulsed laser heating and metal coating. Digitized bending from 10° to 90° can be achieved by linearly adjustment of the laser dose. The fabricated tips have shown good light guiding.
Optimization of a rod pinch diode radiography source at 2.3 MV
Authors: Menge P. R. , Johnson D. L. , Maenchen J. E. , Rovang D. C. , Oliver B. V. , Rose D. V. , Welch D. R.
Published: July 2003

Rod pinch diodes have shown considerable capability as high-brightness flash x-ray sources for penetrating dynamic radiography. The rod pinch diode uses a small diameter (0.4–2 mm) anode rod extended through a cathode aperture. When properly configured, the electron beam born off of the aperture edge can self-insulate and pinch onto the tip of the rod creating an intense, small x-ray source. Sandia’s SABRE accelerator (2.3 MV, 40 Ω, 70 ns) has been utilized to optimize the source experimentally by maximizing the figure of merit (dose/spot diameter2) and minimizing the diode impedance droop. Many diode parameters have been examined including rod diameter, rod length, rod material, cathode aperture diameter, cathode thickness, power flow gap, vacuum quality, and severity of rod–cathode misalignment. The configuration producing the greatest figure of merit uses a 0.5 mm diameter gold rod, a 6 mm rod extension beyond the cathode aperture (diameter=8 mm), and a 10 cm power flow gap to produce up to 3.5 rad (filtered dose) at 1 m from a 0.85 mm x-ray on-axis spot (1.02 mm at 3° off axis). The resultant survey of parameter space has elucidated several physics issues that are discussed.
Methods of single-molecule fluorescence spectroscopy and microscopy
Authors: Moerner W. E. , Fromm David P.
Published: July 2003

Optical spectroscopy at the ultimate limit of a single molecule has grown over the past dozen years into a powerful technique for exploring the individual nanoscale behavior of molecules in complex local environments. Observing a single molecule removes the usual ensemble average, allowing the exploration of hidden heterogeneity in complex condensed phases as well as direct observation of dynamical state changes arising from photophysics and photochemistry, without synchronization. This article reviews the experimental techniques of single-molecule fluorescence spectroscopy and microscopy with emphasis on studies at room temperature where the same single molecule is studied for an extended period. Key to successful single-molecule detection is the need to optimize signal-to-noise ratio, and the physical parameters affecting both signal and noise are described in detail. Four successful microscopic methods including the wide-field techniques of epifluorescence and total internal reflection, as well as confocal and near-field optical scanning microscopies are described. In order to extract the maximum amount of information from an experiment, a wide array of properties of the emission can be recorded, such as polarization, spectrum, degree of energy transfer, and spatial position. Whatever variable is measured, the time dependence of the parameter can yield information about excited state lifetimes, photochemistry, local environmental fluctuations, enzymatic activity, quantum optics, and many other dynamical effects. Due to the breadth of applications now appearing, single-molecule spectroscopy and microscopy may be viewed as useful new tools for the study of dynamics in complex systems, especially where ensemble averaging or lack of synchronization may obscure the details of the process under study.
High-pressure discharges with imposed stability resulting in reduced power requirements
Authors: Brooks N. H. , Jensen T. H. , Moeller C. M.
Published: July 2003

High-pressure plasma discharge columns that are long and straight, as well as uniform in the axial direction, have been generated in a steady state. These discharges, stabilized by rotation of the containment envelope about a horizontal axis, have reduced heat transport to the wall due to the absence of buoyancy-driven radial convection and the elimination of eddies caused by sheared flow. These “rotating tube discharges” may take the form of electric arcs powered with dc or low-frequency ac; or of electrodeless plasmas, heated by electromagnetic waves at rf or microwave frequencies.
Effect of the addition Y2O3 on persistent spectral hole burning in europium doped sodium borate glass
Authors: Kalluru Rajamohan R. , Schoolfield Elizabeth , Rami Reddy B.
Published: July 2003

Persistent hole burning is observed in a europium doped sodium borate glass from 10 to 100 K. Hole burning efficiency increased 18× when 5% yttrium oxide was added to the glass mixture. Hole burning was observed up to 300 K in Eu3+, Y3+ Co-doped glass.
Bound state properties and astrophysical applications of negatively charged hydrogen ions
Authors: Frolov Alexei M. , Smith Vedene H.
Published: July 2003

Highly accurate energies and other bound state properties of the negatively charged hydrogen ions, 1H−, D−, T−, and ∞H−, are presented. The computed expectation values include the electron–nucleus and electron–electron delta-functions and cusp values, triple delta-functions and some other geometrical and kinematical properties. All variational energies and bound state properties determined in this study are significantly more accurate than results known from earlier computations in the literature. In particular, the total energies obtained in this study are −0.527 751 016 544 377 196 566 8 a.u. (the ∞H− ion), −0.527 649 048 202 999 950 354 0 a.u. (the T− ion), −0.527 598 324 686 477 854 280 5 a.u. (the D− ion) and −0.527 445 881 114 178 893 391 1 a.u. (the 1H− ion). The method used in this study is a fully nonadiabatic variational approach. A few important astrophysical applications are discussed for the H− ions, including their photodetachment.
Energy transfer in collisions of peptide ions with surfaces
Authors: Laskin Julia , Futrell Jean H.
Published: July 2003

Time- and energy-resolved surface induced dissociation (SID) of a singly protonated octapeptide des-Arg1-bradykinin was used to study the effect of physical properties of the SID target on the efficiency of translational to vibrational energy transfer (T→V) in collisions of peptide ions with surfaces. Four SID targets of varying chemical composition and stiffness were examined in this work: self-assembled monolayers of 1-dodecane thiol (HSAM) and its fluorinated analog (CF3(CF2)9C2H4SH — FSAM) on gold, a 300 nm thick layer of lithium fluoride (LiF) on a polished titanium surface, and a 2 μm carbon vapor deposited diamond layer on a titanium surface. A RRKM-based modeling approach was utilized to extract internal energy distributions deposited into the precursor ion upon collisions with different surfaces. We found that the percent of T→V transfer increases in the order: HSAM (10.1%), LiF (12.0%), diamond (19.2%), FSAM (20.5%). Furthermore, the width of the energy deposition function (EDF) is affected by the properties of the SID target. Collisions of peptide ions with the HSAM surface results in deposition of relatively narrow internal energy distributions with the width of the EDF increasing in the order: HSAM<FSAM<LiF<Diamond. The results demonstrate that surface stiffness has a major effect on the width of the EDF, while the average energy deposited into the ion is mainly affected by the mass of the chemical moiety representing an immediate collision partner for the ion impacting the surface.
Defect hydrogen vibrations in various phases deuterium ice
Authors: Li J. C. , Wang Y. , Dong S. L. , Zhang P. , Kolesnikov A. I.
Published: July 2003

The inelastic incoherent neutron scattering spectra of D2O mixed with a small amount of H2O (5% by weight) high density amorphous (hda) ice, ice-VIII, and ice-II have been measured on HET spectrometer at Rutherford Appleton Laboratory (UK). The hydrogen atom in D2O ice lattice has three distinguished vibrations: two modes normal to the O–––H bond at lower frequency and a stretching mode along the O–H bond at higher frequency. For different ice phases these frequencies are different, it was found that the lower defect mode is at ∼97 meV for ice-II, at about 95 meV for hda-ice and ice-VIII, and they are all lower than the value of 105 meV for ice-Ih. The O–H stretching modes are at 415 meV for ice-II, at 418 meV for hda-ice, and at 425 meV for ice-VIII, which all are much larger than the value for ice-Ih, 406 meV. It was also found that O–D stretching modes in D2O ice-VIII is centered at ∼320 meV which is significantly higher than the corresponding value of ∼305 meV for ice-Ih.
Sputtered stoichiometric TeO2 glass films: Dispersion of linear and nonlinear optical properties
Authors: D’Amore Franco , Di Giulio Massimo , Pietralunga Silvia M. , Zappettini Andrea , Nasi Lucia , Rigato Valentino , Martinelli Mario
Published: July 2003

We report on the optical characterization of TeO2 amorphous films deposited by radio-frequency (rf) reactive sputtering. X-ray diffraction, electronic microscopy, and Rutherford backscattering analysis have been performed in order to assess the structural and compositional properties of the samples. The linear optical characterization has been performed by variable angle spectroscopic ellipsometry at wavelengths between 260 and 1700 nm, leading to determination of the dispersion of the complex refractive index for sputtered tellurium dioxide. In the spectral range of transparency, between 1700 and 500 nm, refractive index values ranging from 2.05 to 2.16 have been obtained. The dispersion of the complex third-order nonlinear optical coefficient of the sample has been evaluated in the near-infrared spectral range by third harmonic generation measurements. The out-of-resonance value of the χ(3) coefficient for TeO2 is in the range between 1.2×10−12 and 1.4×10−12 esu. The conjugation of optical transparency, high nonlinear refractive index and the possibility of growing films of good optical quality, make rf sputtered TeO2 a promising material for the implementation of devices for the processing of optical communication signals.
Point defect distributions and their electrical effects on (Ba,Sr)TiO3/Pt thin films
Authors: Wang Ruey-Ven , McIntyre Paul C.
Published: July 2003

Deuterium annealing effects on the electrical properties of (Ba,Sr)TiO3 (BST) thin films were investigated using electrical measurements and x-ray photoelectron spectroscopy (XPS) band bending analysis as a function of Pt surface coverage. In combination with the results from secondary ion mass spectroscopy (SIMS) depth profiling, these results suggest that the majority of deuterium defects incorporated during low temperature D2/N2 anneals is electrically inactive. Therefore, there are quite likely other contributors, in addition to charged interstitial deuterium defects, to the observed leakage current degradation after deuterium/hydrogen forming gas annealing, such as oxygen vacancies formed under reducing D2 annealing conditions and annealing-induced changes in the BST/Pt interface state density. The XPS band bending analysis provides information on the electrostatic potential difference across BST thin films with a Pt bottom electrode. With this boundary condition as input, simulations based on a previously developed equilibrium point defect model and the static (annealing time independent) deuterium depth profiles obtained by SIMS analysis have been performed to predict equilibrium defect distributions across BST thin films.
Three-dimensional functional model proteins: Structure function and evolution
Authors: Blackburne Benjamin P. , Hirst Jonathan D.
Published: July 2003

The mapping of phenotype onto genotype for a set of functional model proteins is accomplished by exhaustive enumeration on a three-dimensional diamond lattice. Chains of up to 25 monomers are investigated and their evolution characterized. The model is used to investigate the origins of designability. Highly designable functional model protein structures possess contact maps that have a relatively little commonality with other physically allowed contact maps. Although the diamond lattice has the same coordination number as the square lattice, differences between three-dimensional and two-dimensional functional model proteins are observed. One difference is the lower frequency of structures of low designability on the three-dimensional lattice. In other respects, the conclusions drawn from previous studies using the square lattice remain valid in three dimensions. For example, we observe the tendency for longer chains to form larger networks of sequences with greater stability to mutation. We identify various topographical characteristics of the landscapes: evolutionary bottlenecks bridge otherwise unconnected networks, and hub sequences allow rapid movement between the different neutral networks. The diversity of landscapes that arises from even a minimalist model suggests that real proteins have a rich variety of evolutionary landscapes.
Observation of ion wave decay products of Langmuir waves generated by stimulated Raman scattering in ignition scale plasmas
Authors: Geddes C. G. R. , Kirkwood R. K. , Glenzer S. H. , Estabrook K. , Cohen B. I. , Young P. E. , Joshi C. , Wharton K. B.
Published: July 2003

Thomson scattering has been used to measure the time resolved spectrum of ion wave decay products from two instabilities which can limit the growth of stimulated Raman scattering (SRS). This experiment detected ion wave decay products far above the thermal level and demonstrates that SRS produced Langmuir waves undergo the Langmuir decay instability in ignition relevant plasmas. Product waves of the electromagnetic decay instability were not detected.
Below bulk-band-gap photoluminescence at room temperature from heavily P- and B-doped Si nanocrystals
Authors: Fujii Minoru , Toshikiyo Kimiaki , Takase Yuji , Yamaguchi Yasuhiro , Hayashi Shinji
Published: July 2003

Photoluminescence (PL) properties of heavily P- and B-doped Si nanocrystals (nc-Si) are studied. By simultaneously doping two types of impurities, nc-Si exhibit strong PL at around 0.9 eV at room temperature. The temperature quenching of the PL is very small. Although the PL peak energy is very close to that of dangling-bond related PL previously observed, all of the observed properties, i.e., decay dynamics, degree of temperature quenching, etc., are apparently different. The transition between donor and acceptor states in nc-Si is the possible origin of the low-energy PL.
Two-region model of CH4, CO, CO2, and H2O formation in graphite due to simultaneous irradiation by O+ and H+
Authors: Chen Allen Y. K. , Haasz A. A. , Davis J. W.
Published: July 2003

Based on our experimental studies of the simultaneous O+ and H+ irradiation of graphite, symbolically represented as the O+-H+→C reaction system, we have developed a semiempirical kinetic model for describing the reactions leading to the formation of the reaction products CH4, CO, CO2, and H2O. The model builds on previously proposed concepts for methane formation via intermediate precursor generation and a “two-region” description of the graphite microstructure. These concepts have been extended to include intermediate precursor-containing steps for the formation of CO, CO2, and H2O, and the breakup of CH4 by the incident ions. Model predictions of the flux and energy dependence of the CH4, CO, and CO2 yields were generally in good agreement with experimental results for the H+→C and O+→C reactions. For the O+-H+→C reaction, the model agrees quite well with the flux ratio dependence of the H2O yield and the corresponding reductions in the CO, CO2, and CH4 yields.