bluuebunny/crossref_metadata_2025 huggingface dataset:
Refrigerator with phonon filters: An application of the phonon deficit effect in superconducting tunnel junctions
Authors: Melkonyan G. G. , Kröger H. , Gulian A. M.
Published: September 2003

Nonequilibrium phenomena in thin solid films can result in cooling effects. These types of effects were predicted theoretically a while ago, and only recently were demonstrated experimentally in superconductor-insulator-normal metal (SIN) tunnel junctions. Since then, there is a growing interest in tunneling effects for the purpose to develop on-chip refrigerators. Thin film devices have the advantage of being extremely compact, operate in a continuous mode, dissipate little power, and can easily be integrated in cryogenic detectors. Currently these refrigerators can generate cooling in the order of 100 mK in an environment of 0.3–0.5 K. There are reasons to expect that this performance can be enhanced but a fundamental investigation of underlying principles is required. One of the outcomes of this type of analysis is the phonon deficit effect. In this article we investigate the phonon deficit effect in thin film superconductor-insulator-superconductor and SIN tunnel junctions. Depending on circumstances, the phonon absorption spectra of such tunnel junctions have spectral windows of phonon emission and/or absorption. We demonstrate that the phonon deficit mechanism can stand for the experimental results with SIN tunnel junctions. In addition, application of the theory of this effect allows us to propose using phonon filters to select the phonon absorption windows and thus to enhance the cooling effect. We discuss a particular superlattice design of corresponding phonon filters.
Electrothermal failure of metallized film capacitor end connections–computation of temperature rise at connection spots
Authors: Qi Xiaoguang , Boggs Steven
Published: September 2003

Self-clearing metallized film capacitor technology offers the highest energy density among high voltage capacitor technologies. The primary limitation of this technology under pulse discharge conditions is failure of the plasma sprayed end connections. Transient nonlinear finite element analysis with coupled electric and thermal fields is applied to quantify the temperature rise around a contact spot along the end connection as a function of discharge current, contact spot size, time and distance between contact spots along the contact edge. An analytic approximation is developed to compute these parameters which agrees to within a few percent with the finite element analysis.
Complementary infrared and transmission electron microscopy studies of the effect of high temperature–high pressure treatments on oxygen-related defects in irradiated silicon
Authors: Londos C. A. , Potsidi M. S. , Misiuk A. , Ratajczak J. , Emtsev V. V. , Antonaras G.
Published: September 2003

Czochralski-grown silicon samples subjected to high temperature–high pressure (HTHP) treatments in the range of 900 °C were irradiated with fast neutrons. Transmission electron microscopy measurements revealed the presence of oxygen precipitates (SiOx) and dislocation loops. The purpose of this work is to investigate the effect of these defects on the annealing behavior of neutron-irradiated induced oxygen-related defects, mainly the VO and the VO2 centers. To this end, infrared spectroscopy measurements were employed to monitor the conversion of the VO center (828 cm−1) to the VO2 center (890 cm−1 band) during isochronal anneals. In the untreated samples this conversion occurs around 300 °C. In our studies, we found that the annealing temperature (Tann) of the VO centers is lower than 300 °C. The value of Tann depends on the particular HTHP pretreatment. Actually, as a result of the precipitation process silicon self-interstitials (SiIs) are emitted and a number of them is bound at the Si/SiOx interface. This region acts as a source of SiIs and upon their liberation the reaction VO+SiI→Oi is activated. The temperature at which this reaction becomes significant depends on the degree of binding of the SiIs at the interface and the number of the SiIs available to participate, which in turn depend on the particular HTHP pretreatment. Thus, if the reaction VO+SiI→Oi, precedes the reaction VO+Oi→VO2, the Tann of the VO defect will be determined mainly by the former reaction.
Electrical transport within polymeric amorphous carbon thin films and the effects of ion implantation
Authors: Khan R. U. A. , Silva S. R. P.
Published: September 2003

Here, we will discuss the electrical transport processes that occur within thin films of polymeric amorphous carbon, which possess a high resistivity of 1015 Ω cm, a Tauc band gap of 2.6 eV, and a low defect density of 1017 spins cm−3. Using current versus voltage measurements, we have shown that the current is space-charge-limited, with a mobility value of typically 10−10 cm2 V−1 s−1. The implantation of boron ions at doses below 6×1014 cm−2 results in an increase in mobility of more than one order of magnitude. Above this dose, the conductivity increases by five orders of magnitude, and the transport mechanism changes to a Frenkel–Poole type conduction process. At an intermediate dose of 2×1015 ions cm−2, the current versus voltage characteristic exhibits strong hysteresis. The observed hysteresis effects could be removed from one polarity by implanting only through the first half of the film. The hysteresis is likely to be due to the trapping of holes at one or both of the interfaces, resulting in the modification of the space-charge within the polymeric amorphous carbon film.
Magnetic insulation of a space-charge dominated flow
Authors: Lahav A. , Berezovsky V. , Schächter L.
Published: September 2003

We report experimental results of magnetic insulation of a space-charge dominated electron flow in a low energy vacuum diode with ferroelectric cathode. Although in the absence of the magnetic field the high current densities are measured well above the estimated space-charge limiting current, the diode is shown to be insulated by a relatively low magnetic field controlled primarily by the anode voltage. A model which accounts for the two-dimensional nature of the electrons flow in the diode has been developed and it reveals the microscopic picture of the flow. From the space-charge dominated vicinity of the cathode, electrons leave the small emitting area towards the large radius anode ring, along a trajectory that is parallel to the applied magnetic field. Only in the close vicinity of the anode plane, their trajectory bends towards the ring. Good agreement between the experimental data and theory was found.
Optical antennas: Resonators for local field enhancement
Authors: Crozier K. B. , Sundaramurthy A. , Kino G. S. , Quate C. F.
Published: September 2003

Electromagnetic field enhancement in optical antenna arrays is studied by simulation and experiment at midinfrared wavelengths. The optical antennas are designed to produce intense optical fields confined to subwavelength spatial dimensions when illuminated at the resonant wavelength. Finite difference time domain (FDTD) method simulations are made of the current, charge, and field distributions in the antennas. The influence of antenna shape, length, and sharpness upon the intensity of the optical fields produced is found. Optical antennas arrays are fabricated on transparent substrates by electron beam lithography. Far-field extinction spectroscopy carried out on the antenna arrays shows the dependence of the resonant wavelength on the antenna length and material. The FDTD calculated and experimentally measured extinction efficiencies of the optical antennas are found to be in good agreement.
Thermal conductance through molecular wires
Authors: Segal Dvira , Nitzan Abraham , Hänggi Peter
Published: September 2003

We consider phononic heat transport through molecular chains connecting two thermal reservoirs. For relatively short molecules at normal temperatures we find, using classical stochastic simulations, that heat conduction is dominated by the harmonic part of the molecular force-field. We develop a general theory for the heat conduction through harmonic chains in three-dimensions. Our approach uses the standard formalism that leads to the generalized (quantum) Langevin equation for a system coupled to a harmonic heat bath, however the driving and relaxation terms are considered separately in a way that leads directly to the steady-state response and the heat current under nonequilibrium driving. A Landauer-type expression for the heat conduction is obtained, in agreement with other recent studies. We used this general formalism to study the heat conduction properties of alkane. We find that for relatively short (1–30 carbon molecules) the length and temperature dependence of the molecular heat conduction results from the balance of three factors: (i) The molecular frequency spectrum in relation to the frequency cutoff of the thermal reservoirs, (ii) the degree of localization of the molecular normal modes and (iii) the molecule–heat reservoirs coupling. The fact that molecular modes at different frequency regimes have different localization properties gives rise to intricate dependence of the heat conduction on molecular length at different temperature. For example, the heat conduction increases with molecular length for short molecular chains at low temperatures. Isotopically substituted disordered chains are also studied and their behavior can be traced to the above factors together with the increased mode localization in disordered chain and the increase in the density of low frequency modes associated with heavier mass substitution. Finally, we compare the heat conduction obtained from this microscopic calculation to that estimated by considering the molecule as a cylinder characterized by a macroscopic heat conduction typical to organic solids. We find that this classical model overestimates the heat conduction of single alkane molecules by about an order of magnitude at room temperature. Implications of the present study to the problem of heating in electrically conducting molecular junctions are pointed out.
Kinetic phase transitions in the reaction CO+O→CO2 on Ir(111) surfaces
Authors: Wehner S. , Baumann F. , Ruckdeschel M. , Küppers J.
Published: September 2003

The oxidation of CO on Ir(111) surfaces was investigated under UHV conditions in the temperature range 360 K to 700 K by CO2 rate measurements utilizing mass spectroscopy. Steady-state CO2 rates were measured at constant total CO+O2 gas flux and variable gas composition (YCO=Y, YO2=1−Y) using mass flow controllers which allowed changes in the CO/O2 gas composition down to 0.1%. Between 360 K and 450 K the CO2 rates initially increase proportional to Y (T<400 K) or to Y1.5 (420 K<T<450 K) and exhibit a sudden drop to a negligible value at a temperature-dependent critical value Y*. The rate drop indicates a kinetic phase transition induced by CO poisoning of the surface. This behavior is similar to the features described by the ZGB and more recently developed lattice gas (LG) models of the CO+O reaction on surfaces. However, in contrast to the ZGB model but in accordance with LG models and experimental results on other platinum metal surfaces, no oxygen poisoning was observed at small Y, i.e., the surface was reactive even at the lowest attainable values of Y. Between 450 K and 530 K the initial CO2 rates remain proportional to Y1.5 up to critical Y* values but the kinetic phase transition softens due to the onset of CO desorption. Accordingly, CO poisoning is not complete and the CO2 rates do not attain the zero level beyond the transition. Above 530 K a kinetic phase transition is no longer seen since substantial CO desorption prevents poisoning, in accordance with conclusions from LG modeling. The kinetic phase transitions, their dependence on Y and temperature, and the measured CO2 rates can be excellently reproduced by simple kinetic modeling.
The interactions of Na, NO, and H2O on the graphite (0001) surface
Authors: Gleeson M. A. , Mårtensson K. , Kasemo B. , Chakarov D. V. , Reibke R. , Hasselbrink E.
Published: September 2003

The reactions of Na and NO on the (0001) surface of graphite and the influence of coadsorbed water on these reactions have been studied by thermal desorption spectroscopy. The products of the NO+Na reactions are dependent on the partial concentrations in the initial coadsorbed layer. For NO:Na dose ratios less than 1:1, N2 formation is dominant. For higher NO doses, this is superseded by N2O formation. In all cases sodium is oxidized by the NO. This leads to carbonate formation, which subsequently decomposes to release CO2 and CO. The addition of H2O at low coverages to the Na+NO system complicates the reactions. It results in ammonia formation by two independent mechanisms. The formation of NH3 is strongly dependent on the water dose. In addition to generating NH3, coadsorbed water alters the sodium oxidation pathway resulting in an enhanced formation of CO2 at certain coverages. Large H2O coverages block the NO reaction pathways by forming an inert “hypermetalated” hydroxide overlayer. The surface composition of this hydroxide is of the type Na2OH or Na3OH. Decomposition of this overlayer results in the desorption of a significant fraction of stable Na2OH molecules.
Reproducing kernel Hilbert space interpolation methods as a paradigm of high dimensional model representations: Application to multidimensional potential energy surface construction
Authors: Ho Tak-San , Rabitz Herschel
Published: September 2003

A generic high dimensional model representation (HDMR) method is presented for approximating multivariate functions in terms of functions of fewer variables and for going beyond the tensor-product formulation. Within the framework of reproducing kernel Hilbert space (RKHS) interpolation techniques, an HDMR is formulated for constructing global potential energy surfaces. The HDMR tools in conjunction with a successive multilevel decomposition technique provide efficient and accurate procedures for reducing a multidimensional interpolation problem to smaller, independent subproblems. It is shown that, when compared to the conventional tensor-product approach, the RKHS–HDMR methods can accurately produce smooth potential energy surfaces over dynamically relevant, nonrectangular regions using far fewer ab initio data points. Numerical results are given for a reduced two-level RKHS–HDMR of the C(1D)+H2 reactive system. The proposed RKHS–HDMR is intimately related to Gordon’s blending-function methods for multivariate interpolation and approximation. The general findings in the paper and the successful illustration provide a foundation for further applications of the techniques.
A theoretical study of vibrational mode coupling in H5O2+
Authors: Dai Jixin , Bačić Zlatko , Huang Xinchuan , Carter Stuart , Bowman Joel M.
Published: September 2003

The vibrational mode coupling in the protonated water dimer is investigated by performing two types of quantum calculations of the vibrational levels of H5O2+ and D5O2+, utilizing the OSS3(p) potential energy surface by Ojamaë et al. [L. Ojamaë, I. Shavitt, and S. J. Singer, J. Chem. Phys. 109, 5547 (1998)]. One is four-dimensional (4D), treating only the central O⋯H(D)+⋯O moiety. Three of the four modes considered, the asymmetric stretch and the two bends, are largely the vibrations of the central proton, while the fourth mode is essentially the O⋯O stretching vibration. The vibrational levels of O⋯H(D)+⋯O are calculated rigorously, as fully coupled (FC), and also in an adiabatic (3+1)D approximation, where the proton asymmetric stretch is treated as adiabatically separated from the other three degrees of freedom. The second set of calculations, designated VCI, is full-dimensional, 15D; it is performed by the code MULTIMODE, which does configuration interaction (CI) calculations using a basis determined from a vibrational self-consistent field Hamiltonian. The FC 4D and 15D VCI calculations give very similar fundamental frequencies of the two bending modes of the central proton, as well as the O⋯O stretch. They differ substantially only for the fundamental of the proton asymmetric stretch, the VCI value being about 25% lower than the FC 4D result. This shows that the asymmetric stretch is strongly coupled to the vibrations outside the O⋯H(D)+⋯O fragment, in contrast to the two proton bending modes and the O⋯O stretching vibration. The FC 4D and 15D VCI calculations predict the same frequency ordering of the four vibrational modes of the O⋯H(D)+⋯O moiety, and are in excellent agreement with respect to the H–D shift of the fundamentals of the shared proton modes. The adiabatic (3+1)D treatment is not quantitatively accurate, yielding fundamental frequencies of the proton vibrational modes which are considerably different from the FC 4D results. Our results have potentially significant implications for the assignment of the bands associated with shared proton vibrations in the recently reported infrared multiphoton photodissociation spectrum of the protonated water dimer.
Collective probabilities algorithm for surface hopping calculations
Authors: Bastida Adolfo , Cruz Carlos , Zúñiga José , Requena Alberto
Published: September 2003

General equations that transition probabilities of the hopping algorithms in surface hopping calculations must obey to assure the equality between the average quantum and classical populations are derived. These equations are solved for two particular cases. In the first it is assumed that probabilities are the same for all trajectories and that the number of hops is kept to a minimum. These assumptions specify the collective probabilities (CP) algorithm, for which the transition probabilities depend on the average populations for all trajectories. In the second case, the probabilities for each trajectory are supposed to be completely independent of the results from the other trajectories. There is, then, a unique solution of the general equations assuring that the transition probabilities are equal to the quantum population of the target state, which is referred to as the independent probabilities (IP) algorithm. The fewest switches (FS) algorithm developed by Tully is accordingly understood as an approximate hopping algorithm which takes elements from the accurate CP and IP solutions. A numerical test of all these hopping algorithms is carried out for a one-dimensional two-state problem with two avoiding crossings which shows the accuracy and computational efficiency of the collective probabilities algorithm proposed, the limitations of the FS algorithm and the similarity between the results offered by the IP algorithm and those obtained with the Ehrenfest method.
Ab initio calculations of the adsorption and reaction of acetonitrile on Si(001)
Authors: Cho Jun-Hyung , Kleinman Leonard
Published: September 2003

The adsorption and reaction of acetonitrile (CH3CN) on the Si(001) surface are investigated by first-principles density-functional calculations within the generalized gradient approximation. We find that two di-σ configurations in which CH3CN bonds on top of a Si dimer and across the ends of two adjacent Si dimers in the same dimer row have adsorption energies of 1.43 and 1.35 eV, respectively. On the other hand, our calculated adsorption energy for the cross-row bridging structure (in which CH3CN adsorbs between two dimer rows) is 0.52 eV, much smaller than that (0.91 eV) of a precursor state where the N atom bonds to the down-buckled atom of the Si dimer. This result does not support the conclusion drawn from a combination of temperature programmed desorption spectroscopy and density functional cluster calculations, where one of the two observed desorption states was assigned to the cross-row bridging structure. Based on our results for the energetics of various adsorption configurations and the energy profile of the reaction pathways, we provide a reinterpretation for the observed desorption states.
Stabilization of silver nanoparticles in nonanoic acid: A temperature activated conformation reaction observed with surface enhanced Raman spectroscopy
Authors: Henneke Dale E. , Malyavanatham Gokul , Kovar Desiderio , O’Brien D. T. , Becker M. F. , Nichols William T. , Keto J. W.
Published: September 2003

Silver nanoparticles were synthesized by ultraviolet (λ=248 nm) laser ablation of an aerosol of micron-sized source particles entrained in nitrogen. As a result of thermionic electron emission and photoionization, nanoparticles produced in this manner were highly charged. The resulting aerosol was primarily composed of nanometer sized particles. The charged nanoparticles were deflected by an electric field that was perpendicular to the aerosol flow. Deflected nanoparticles were deposited directly into n-nonanoic acid flowing along the negative collection electrode. Suspensions of nanoparticles collected in this manner were dark gray in color and were found to be flocculated. When the suspensions were heated to temperatures above 75 °C, a color change from gray to clear was observed. Ultraviolet/visible extinction spectroscopy was performed on each suspension following annealing at different temperatures and times. By modeling the absorption decrease as a first order reaction, a good fit for the data was found. Analysis by dynamic light scattering (DLS) showed that the initial mean flocculent size of the gray suspensions was 602 nm. DLS analysis of the suspensions taken at different annealing intervals showed that the flocculent size decreased, but maintained a narrow size distribution until the size shrank below the instrument resolution limit. The reduction in flocculent size coincided with the observed color change, and an irreversible transition to a deflocculated primary nanoparticle suspension is observed. Surface enhanced Raman scattering is used to confirm that the reaction results from a change in the orientation of the nonanoic molecule on the surface of the nanoparticle.
Generalized Taylor–Aris dispersion analysis of spatially periodic lattice Monte Carlo models: Effect of discrete time
Authors: Dorfman Kevin D. , Slater Gary W. , Gauthier Michel G.
Published: September 2003

The use of a continuous-time approximation in the generalized Taylor–Aris dispersion analysis of periodic lattice Monte Carlo models results in a calculation scheme which is only valid for small applied fields (or fine lattices). In the present contribution, the source of this restriction is identified via a Gram–Charlier expansion and the original moment-matching scheme is adapted to a discrete-time model, thereby resulting in an analytical scheme for computing the mean velocity and dispersivity for strong fields/coarse grids.
Unified theory of reversible target reactions
Authors: Agmon Noam , Popov Alexander V.
Published: September 2003

We discuss two fundamental reversible diffusion influenced reactions: (i) A+B⇌C and (ii) A+B⇌C+D. In the pseudo-unimolecular case, we prove that reaction (i) is a special case of (ii), which thus constitutes a unified reversible problem of multiparticle kinetics. For static A and C (the “target” limit), we suggest to treat this problem as follows. First we generalize the Smoluchowski theory to reaction (ii). In Laplace space, we combine this with a power-law theory, determining the combination coefficient so that the unified theory reduces correctly in all known limits. We also show how to rewrite it in the time domain, with the generalized Smoluchowski theory as the leading term. Comparison with Brownian simulations shows near perfect agreement for both versions of our theory under all conditions.
Interplay of computer simulations and x-ray absorption spectra in the study of the bromide hydration structure
Authors: Merkling Patrick J. , Ayala Regla , Martı́nez José M. , Pappalardo Rafael R. , Sánchez Marcos Enrique
Published: September 2003

X-ray absorption spectra (EXAFS and XANES) were generated from snapshots of a Monte Carlo (MC) simulation of a bromide ion aqueous solution and from model structures. The MC simulation relies on a recently developed and tested polarizable potential based on ab initio potential energy surfaces. A comparison with the experimental K-edge Br spectrum of a 0.3 M YBr3 aqueous solution was performed. XANES spectra are reproduced acceptably only if statistical fluctuations are included, which is performed in this work by using snapshots from computer simulation. As expected, single scattering BrO contributions are dominant in the case of the EXAFS region. Due to this fact, Br− in water is a good model system for studying the influence of the distribution of distances on the determination of structural parameters. Then, a parallel study of the data analysis procedure of the experimental EXAFS spectrum and those theoretically computed from the structures supplied by the MC simulation, was carried out. The shape of the distribution function and its asymmetry must be taken into account in a practical way to obtain a more accurate determination of the BrO first-shell distance. A further refinement consists in using the computer simulation to extrapolate the BrO distance from the experimental EXAFS spectrum. In this way, a BrO distance of 3.44±0.07 Å and a coordination number of 6±0.5 were determined.
Chain and local dynamics of polyisoprene as probed by experiments and computer simulations
Authors: Doxastakis M. , Theodorou D. N. , Fytas G. , Kremer F. , Faller R. , Müller-Plathe F. , Hadjichristidis N.
Published: September 2003

The dynamics of designed short polyisoprene (PI) chains in the melt is investigated on a wide temperature window using dielectric relaxation spectroscopy and pulsed field gradient nuclear magnetic resonance (NMR). At high temperatures, molecular dynamics (MD) simulations performed using two different models (an explicit atom model and a united atom one) capture very well the dynamic properties documented experimentally. Structures pre-equilibrated with end-bridging Monte Carlo are used as initial configurations for MD runs at different temperatures, providing predictions for the temperature dependence of the dynamics of this bulk PI. Local dynamics is unique, independently of the probe (dielectric relaxation, dynamic light scattering, nuclear magnetic resonance, neutron scattering), although mean correlation times are significantly affected, to different extents, by librations. Chain dynamics over the molecular weight and temperature range studied can be described well by the Rouse model, as shown by both experimental data and a normal mode analysis on simulation trajectories. Deviations from the Rouse model emerge for the high modes at short times; still, this model offers a rather accurate picture.
Long lived photoexcitation dynamics in a dendronically substituted poly(fluorene)
Authors: Pogantsch Alexander , Wenzl Franz P. , Scherf Ullrich , Grimsdale Andrew C. , Müllen Klaus , List Emil J. W.
Published: September 2003

Polyfluorenes are possibly the most attractive class of conjugated polymers for blue polymer light emitting devices. Derivatives with dendron side chains have been found to significantly improve the color stability of the devices. Using a quasi-cw photoinduced absorption technique we present a qualitative and quantitative analysis of lifetimes and deactivation mechanisms for the triplet excited states in a dendronically substituted derivative. The triplet lifetimes increase by one order of magnitude in the dendronic derivative compared to those in a standard poly(dialkylfluorene), which has been characterized for comparison. The temperature dependence of triplet lifetimes is interpreted in terms of migration assisted deactivation of triplet excitations. We conclude that the dendrons inhibit the Dexter-transfers of triplet excitations between the polymer chains and so strongly hinder triplet migration.
Configuration selection in the simulations of the crystallization of short polyethylene chains in a free-standing thin film
Authors: Xu Guoqiang , Lin Heng , Mattice Wayne L.
Published: September 2003

The configuration of thin films of short polyethylene chains during the crystallization has been investigated using a recently developed Monte Carlo method on a high coordination lattice, which bridges the coarse-grained and the fully atomistic simulations. Thin films select their own crystal configurations, in which the chains have different orientations with respect to the surface normal. Two kinds of crystal structures, with and without grain boundary in the center of the thin films, in which all the chains are parallel to the free surface, have been found after a deep quench from an equilibrated thin film above the melting temperature in the previous simulations [J. Chem. Phys. 116, 2277 (2002)]. However, another crystal configuration shows up after a quench from the thin film formed by increasing the periodic boundary conditions in one direction from a homogeneous melt. In this configuration all the chains are parallel with each other and to the z axis. This configuration was found both in the experiments and in the previous molecular dynamics simulation of a free-standing thin film of a short alkane. It is demonstrated here that the initial configuration does affect the final structure in the simulation of the crystallization of the free-standing thin film. The current configuration has slightly lower enthalpic and entropic energies and higher melting point than the previous ones. The similarities and differences between three configurations during the crystallization, annealing, and melting processes will be discussed.
Transverse nuclear spin relaxation due to director fluctuations in liquid crystals. II. Second-order contributions of the fluctuating director
Authors: Frezzato Diego , Moro Giorgio J. , Kothe Gerd
Published: September 2003

Recently, we have introduced a slow-motional theory for transverse nuclear spin relaxation due to director fluctuations [D. Frezzato, G. Kothe, and G. J. Moro, J. Phys. Chem. B 105, 1281 (2001)]. This method is now generalized to second-order contributions of the fluctuating director. We consider the specific case in which the director is aligned orthogonal to the magnetic field. By exploiting the Gaussian character of director fluctuations, the stochastic Liouville equation for the coupled spin and director dynamics is solved in terms of a characteristic function whose time dependence is determined by a nonlinear integral equation. A convenient solution of the integral equation is obtained by decomposing the characteristic function according to the relaxation rates of the director fluctuations. In a first application, we evaluate the free induction decay and the corresponding absorption spectrum for quadrupolar probe nuclei in nematic liquid crystals. It is shown that the transverse magnetization is well represented by a monoexponential decay, i.e., a Lorentzian lineshape in the frequency domain. Explicit relations are derived for the linewidths and frequency shifts under slow-motional conditions where the Redfield theory cannot be applied anymore.
Transverse nuclear spin relaxation due to director fluctuations in liquid crystals. III. A slow-motional theory for the angular dependence in pulsed experiments
Authors: Frezzato Diego , Kothe Gerd , Moro Giorgio J.
Published: September 2003

In the previous article, we have proposed a slow-motional theory for second-order effects of director fluctuations on transverse spin relaxation of quadrupolar nuclei in liquid crystals [D. Frezzato, G.J. Moro, and G. Kothe, J. Chem. Phys. 119, 6931 (2003), preceding paper]. This methodology is now generalized to arbitrary orientations of director and magnetic field. The characteristic functions are evaluated for the free induction decay and the echo intensities in Carr–Purcell–Meiboom–Gill (CPMG) multipulse sequences. From the solution of the corresponding integral equations, the relative magnitude of first and second-order contributions can be assessed. This enables a complete characterization of the angular and pulse spacing dependent transverse relaxation rates observed in CPMG multipulse experiments.
Brownian dynamics simulations on a hypersphere in 4-space
Authors: Nissfolk Jarl , Ekholm Tobias , Elvingson Christer
Published: September 2003

We describe an algorithm for performing Brownian dynamics simulations of particles diffusing on S3, a hypersphere in four dimensions. The system is chosen due to recent interest in doing computer simulations in a closed space where periodic boundary conditions can be avoided. We specifically address the question how to generate a random walk on the 3-sphere, starting from the solution of the corresponding diffusion equation, and we also discuss an efficient implementation based on controlled approximations. Since S3 is a closed manifold (space), the average square displacement during a random walk is no longer proportional to the elapsed time, as in R3. Instead, its time rate of change is continuously decreasing, and approaches zero as time becomes large. We show, however, that the effective diffusion coefficient can still be obtained from the time dependence of the square displacement.
ZEKE photoelectron spectroscopy of the silver- and copper-ammonia complexes
Authors: Miyawaki Jun , Sugawara Ko-ichi
Published: September 2003

The single-photon zero kinetic energy (ZEKE) photoelectron spectra of the silver- and copper-ammonia 1:1 complexes have been observed in order to investigate the vibrational structures of their corresponding ions. The adiabatic ionization potentials (I.P.) of AgNH3 (47 580 cm−1) and CuNH3 (46 468 cm−1) decrease from those of the free metal atoms by 1.68 and 1.97 eV, respectively. The intermolecular stretching frequencies of the ionized complexes were determined to be 375 cm−1 for Ag+NH3 and 470 cm−1 for Cu+NH3. These observations indicate that the Cu+–NH3 binding is stronger than the Ag+–NH3 binding, consistent with the previous collision induced dissociation experiments. The binding energy of CuNH3 is found to be larger than that of AgNH3 in the neutral state as well from the observed I.P. shifts and the binding energies of the ionized complexes. This is also consistent with the smaller redshift of the origin band of CuNH3 on deuteration and the Franck–Condon intensity patterns observed in the ZEKE spectra. The stronger Cu–NH3 bonding in the neutral complex is attributed to the more efficient sd hybridization in the Cu atom.
Kinetics of diffraction gratings formation in a polymer matrix containing azobenzene chromophores: Experiments and Monte Carlo simulations
Authors: Pawlik Grzegorz , Mitus Antoni C. , Miniewicz Andrzej , Kajzar Francois
Published: September 2003

The physical processes accompanying an illumination of azobenzene chromophores embedded in a polymer matrix with spatially modulated and linearly polarized light are studied experimentally and simulated using the Monte Carlo (MC) method. The MC simulations base on a simple three-channel kinetic model of trans-cis type photoisomerization cycles in azobenezene molecules. The method focus on studying the kinetics of diffraction grating recording and erasing under various conditions of illumination, light polarization and recording in the presence or absence of a background light. By modeling the “macroscopic” light diffraction efficiency one gets insight into the role played by microscopic processes (angular hole burning, angular redistribution, rotational diffusion) up to the high-temperature limit, characterized by a high mobility of polymer chains. The obtained qualitative agreement between experimental data and the modeling confirms that the main processes underlying grating recording and erasing are well identified. The experimentally observed two-exponential functional dependence of diffraction efficiency η as function of time is well explained by the MC modeling.
van der Waals states in ozone and their influence on the threshold spectrum of O3(X 1A1). I. Bound states
Authors: Grebenshchikov Sergy Yu. , Schinke Reinhard , Fleurat-Lessard Paul , Joyeux Marc
Published: September 2003

Threshold spectra of several isotopomers of ozone are studied using accurate quantum mechanical calculations and an ab initio potential energy surface. Shallow van der Waals minima in the dissociation channels, separated from the deep main wells by an 80 cm−1 high barrier, are shown to accommodate long progressions of assignable states. As a result, dense vibrational spectrum of ozone near dissociation is dominated by van der Waals-type states for all studied isotope compositions.
Reducing the master equations for noisy chemical reactions
Authors: Shibata Tatsuo
Published: September 2003

A reduced description for noisy chemical reactions in small systems is presented. By applying the projection operator formalism to the chemical master equation, we show that even when the number of molecules of an intermediate chemical species is small, its elimination from the description is possible provided that its characteristic time scale is short. The resulting master equation of the reduced system includes additional terms of a diffusive kind. The characteristic time of the eliminated chemical species contributes toward the fluctuations of the downstream chemical species, as well as the dispersion of the eliminated component does.
Femtosecond photon echo measurements of electronic coherence relaxation between the X(1Σg+) and B(3Π0u+) states of I2 in the presence of He, Ar, N2, O2, C3H8
Authors: Comstock Matthew , Lozovoy Vadim V. , Dantus Marcos
Published: September 2003

Photon echo and reverse transient grating measurements of the loss of electronic coherence for molecular iodine are presented. Systematic measurements of the coherence decay rate were made as a function of buffer gas. From the dependence of decay rate on numerical density, we calculated experimental cross sections of decoherence. These values range from 135 Å2 for helium to 1170 Å2 for I2. We find Lennard-Jones parameters for the long-range interactions responsible for decoherence which can be modeled by dispersion forces.
Hydrogen content and density in nanocrystalline carbon films of a predominant diamond character
Authors: Hoffman A. , Heiman A. , Akhvlediani R. , Lakin E. , Zolotoyabko E. , Cyterman C.
Published: September 2003

Nanocrystalline carbon films possessing a prevailing diamond or graphite character, depending on substrate temperature, can be deposited from a methane hydrogen mixture by the direct current glow discharge plasma chemical vapor deposition method. While at a temperature of ∼880 °C, following the formation of a thin precursor graphitic film, diamond nucleation occurs and a nanodiamond film grows, at higher and lower deposition temperatures the films maintain their graphitic character. In this study the hydrogen content, density and nanocrystalline phase composition of films deposited at various temperatures are investigated. We aim to elucidate the role of hydrogen in nanocrystalline films with a predominant diamond character. Secondary ion mass spectroscopy revealed a considerable increase of the hydrogen concentration in the films that accompanies the growth of nanodiamond. It correlates with near edge x-ray adsorption spectroscopy measurements, that showed an appearance of spectroscopic features associated with the diamond structure, and with a substantial increase of the film density detected by x-ray reflectivity. Electron energy loss spectroscopy showed that nanocrystalline diamond films can be deposited from a CH4/H2 mixture with hydrogen concentration in the 80%–95% range. For a deposition temperature of 880 °C, the highest diamond character of the films was found for a hydrogen concentration of 91% of H2. The deposition temperature plays an important role in diamond formation, strongly influencing the content of adsorbed hydrogen with an optimum at 880 °C. It is suggested that diamond nucleation and growth of the nanodiamond phase is driven by densification of the deposited graphitic films which results in high local compressive stresses. Nanodiamond formation is accompanied by an increase of hydrogen concentration in the films. It is suggested that hydrogen retention is critical for stabilization of nanodiamond crystallites. At lower deposition temperatures an excess of hydrogen in the deposited layers helps to prevent the densification of the films and accumulation of microstresses and consequently the films maintains its graphitic character. At higher temperatures the hydrogen content in the films is relatively low and the film maintains its graphitic character.
Emission and reflection spectra from AlxGa1−xN/GaN single heterostructures
Authors: Reynolds D. C. , Hoelscher J. , Litton C. W. , Collins T. C. , Fitch R. , Via G. D. , Gillespie J. , Crespo A. , Jenkins T. J. , Saxler A.
Published: September 2003

Emission and reflection spectra from AlGaN/GaN single heterostructures grown on SiC substrates were investigated. Two-dimensional electron gas (2DEG) transitions were observed in both emission and reflection. The transitions are sharp, associated with the excited state of the 2DEG, reflect the conservation of the K-selection rule, and are excitonlike. The transitions are also associated with both the A- and B-valence bands. To verify the origin of the reflection and emission spectra, the top AlGaN layer was removed by reactive ion etching. After etching, only the excitonic reflection and emission spectra associated with GaN were observed.
Color center formation in soda-lime glass with femtosecond laser pulses
Authors: Lonzaga J. B. , Avanesyan S. M. , Langford S. C. , Dickinson J. T.
Published: September 2003

We show that exposure of soda-lime glass to ultrafast laser pulses at 800 nm causes coloration (darkening). We have characterized this coloring with time-resolved measurements of the transmission of 633 nm light through the glass during laser exposure. Reverse processes (partial bleaching) operate on time scales of μs to seconds. The competition between coloration after the femtosecond pulse and the subsequent transmission recovery limits the darkening that can be achieved at a given femtosecond pulse energy and repetition rate. The response of soda-lime glass to 400 and 267 nm ultrafast pulses is quite similar, although much lower pulse energies are required for darkening. We argue that darkening is due to absorption processes that produce mobile charge carriers, which then interact to produce trapped hole centers (H3+) that absorb strongly at 633 nm. Trapped electrons (that form E centers) are the likely cause of the accompanying loss of transmission in the near ultraviolet. Finally, we show that diffraction gratings can be rapidly and easily produced in this material using holographic methods.
Displacement and polarization switching properties of piezoelectric laminated actuators under bending
Authors: Hayashi Keisuke , Shindo Yasuhide , Narita Fumio
Published: September 2003

The displacement and polarization switching properties of piezoelectric laminated actuators have been investigated theoretically, numerically, and experimentally. A laminated beam theory solution is developed for the piezoceramic/metal/piezoceramic actuator, and the effects of electric fields on the displacements of the actuators are analyzed. The mathematical procedure of the present analysis is simple and clear, and the resulting solutions for the displacement of the cantilever laminated beam actuator are highly accurate. A nonlinear three-dimensional finite element analysis is also employed. Bending tests are used to validate the numerical predictions, using laminated actuators made with ferroelectric lead zirconate titanate piezoelectric layers and Fe-48% Ni host material. Good agreement is obtained between the tests and predictions.
A permeation theory for single-file ion channels: Concerted-association/dissociation
Authors: Nelson Peter Hugo
Published: September 2003

Motivated by atomistic simulations and x-ray analysis of the KcsA ion channel, a recent permeation model [P. H. Nelson, J. Chem. Phys. 117, 11396 (2002)] is modified to include association/dissociation via concerted motion of all n ions in the selectivity filter. The generalized kinetic model is consistent with several microscopic mechanisms, suggesting that experimentally determined parameters represent an ensemble average over all possible permeation mechanisms.
Density functional calculations, using Slater basis sets, with exact exchange
Authors: Watson Mark A. , Handy Nicholas C. , Cohen Aron J.
Published: September 2003

We report the extension of our Slater-type orbital (STO) basis code for density functional theory (DFT) calculations to include “exact exchange,” so that Hartree–Fock and hybrid DFT calculations may be performed. Exchange integrals are evaluated through the insertion of a resolution of the identity, using the same auxiliary STO basis set which was used in our original generalized gradient approximation DFT studies. Three-center two-electron integrals are evaluated by quadrature. A scheme for introducing diatomic auxiliary STO basis sets is introduced, which enables much larger calculations to be performed. We also report an extension to the evaluation of second-order molecular properties using these functionals. Calculations for excitation energies to valence and Rydberg states are included.
Approximate one-matrix functionals for the electron–electron repulsion energy from geminal theories
Authors: Cioslowski Jerzy , Pernal Katarzyna , Buchowiecki Marcin
Published: September 2003

A simple extension of the antisymmetrized product of strongly orthogonal geminals theory produces a “JK-only” one-matrix functional for the electron–electron repulsion energy of a closed-shell system that is exact for two-electron singlet ground states, size-extensive, and incorporates some intergeminal correlation and thus dispersion effects. The functional is defined only for one-matrices with occupation numbers that can be arranged into sets with elements that sum up to two. Its possible generalizations are discussed.
Study of intramolecular electron transfer with a two-state model based on the orbital deletion procedure
Authors: Mo Yirong , Wu Wei , Zhang Qianer
Published: September 2003

Carbocations H2C-bridge-CH2+ are often used as models for donor-bridge-acceptor complexes to study the role of bridges in the electron transport process. In an attempt to delineate the electron delocalization effect from the bridge to the positively charge terminal in the unrestricted Hartree–Fock (UHF) wave functions which are often used for diabatic states to compute the electronic coupling energy, we propose to employ an orbital deletion procedure (ODP) to generate the strictly localized wave functions for the initial (A) and final (B) diabatic states in the electron transfer process in the carbocations of H2C-bridge-CH2+. The electronic coupling energy VAB can be subsequently computed with the two diabatic states by solving a 2×2 secular equation. The comparison of our results with previous theoretical studies based on the widely adopted charge-localized UHF wave functions and Koopmans’ theorem in the case of positively charged 1,3-dimethylenebicyclo[1.1.1]pentane reveals that charge-localized UHF wave functions overestimates the electronic coupling VAB compared with our method and the Koopmans’ theorem. A further study incorporating four water molecules suggests that the aqueous solution has very limited effect on VAB in the positively charged 1,3-dimethylenebicyclo[1.1.1]pentane. To demonstrate the applicability of the current two-state model based on the ODP strategy, we also examined the electron transport across strain-free linear alkyl chains (CH2)n (n=1–8) and linear π-conjugated bridges (CH=CH)n (n=1–5).