bluuebunny/crossref_metadata_2025 huggingface dataset:
Radiative recombination in n-type and p-type GaAs compensated with Li
Authors: Gislason H. P. , Yang B. H. , Pétursson J. , Linnarsson M.
Published: July 2002

We report fundamental changes of the radiative recombination in a wide range of n-type and p-type GaAs after diffusion with the group-I element Li. These optical properties are found to be a bulk property and closely related to the electrical conductivity of the samples. In the Li-doped samples the radiative recombination is characterized by emissions with excitation-dependent peak positions which shift to lower energies with increasing degree of compensation and concentration of Li. These properties are shown to be in qualitative agreement with fluctuations of the electrostatic potential in strongly compensated systems. For Li-diffusion temperatures above 700–800 °C semi-insulating conditions with electrical resistivity exceeding 107 Ω cm are obtained for all conducting starting materials. In this heavy Li-doping regime, the simple model of fluctuating potentials is shown to be inadequate for explaining the experimental observations unless the number of charged impurities is reduced through complexing with Li. For samples doped with low concentrations of Li, on the other hand, the photoluminescence properties are found to be characteristic of impurity-related emissions.
Shock-induced amorphization of q-GeO2
Authors: Suresh N. , Jyoti G. , Gupta Satish C. , Sangeeta S. K. Sikka , Sabharwal S. C.
Published: July 2002

Polycrystalline samples of the α-quartz phase of GeO2 (q-GeO2), recovered from peak shock compressions of 5, 6.8, and 10 GPa, have been examined by x-ray diffraction, thermoluminescence, and Raman techniques. The measurements show that q-GeO2 irreversibly amorphizes above 6.8 GPa. The estimates of the shear band temperature suggest that the mechanism of shock-induced amorphization in q-GeO2 is a solid-solid one, in contrast to that in SiO2 quartz which has both solid-solid and fusion-quenched components.
Growth rate and quality variation of homoepitaxial diamond grown at elevated temperatures
Authors: Weimer R. A. , Thorpe T. P. , Snail K. A.
Published: July 2002

Homoepitaxial diamond films were grown at temperatures between 1000 and 1400 °C with an oxy-acetylene torch. The growth rates of the {100} and {111} faces were observed to increase through 1400 °C, while the {110} face did not grow above 1400 °C. The quality of all faces deteriorated significantly between 1300 and 1400 °C, as shown by scanning electron microscopy and Raman spectroscopy. The transparency of a film as measured by Fourier-transform infrared spectroscopy was type-IIa quality with very little C–H absorption.
Al intermediate oxidation states observed by core level photoemission spectroscopy
Authors: Faraci G. , La Rosa S. , Pennisi A. R. , Hwu Y. , Margaritondo G.
Published: July 2002

Aluminum oxidation states in stoichiometric or substoichiometric configuration are studied by core level photoemission spectroscopy on different substrates (SiO2, graphite). They are compared with recent results reported for the interface Si–Aln+O. Three Al oxidation states have been identified and their space distribution (binding energy, intensity, and width) is determined in the region from the interface with the substrate up to the surface of a thick overlayer. The Al2+–O intermediate oxidation state is shown to be confined at the interface; on the contrary, the Al1+–O oxidation state and the stoichiometric oxide (alumina) are present beyond the interface region. From the attenuation of the substrate core level peak, the deposition morphology and the attenuation length of the photoelectrons have been deduced.
Hyperfine magnetic fields and Curie temperature in the Heusler alloy Ni2MnSn at high pressure
Authors: Gavriliuk A. G. , Stepanov G. N. , Sidorov V. A. , Irkaev S. M.
Published: July 2002

The ‘‘hyperfine magnetic field’’- Hhpf at the nuclei of the diamagnetic atoms 119Sn was measured in Heusler alloy Ni2MnSn by Mössbauer absorption spectroscopy for pressures up to 10.8 GPa at room temperature. The pressure dependences of HhpfSn does not show hysteresis and can be fitted by a linear function HhpfSn(P)/HhpfSn(0) =1+kH⋅P with the coefficient kH=−0.095±0.015 GPa−1, where HhpfSn(0)=4.60±0.12T. Diamond anvil cells were used to obtain high pressure at room temperature. The Curie temperature (TC) has been measured (up to 5.2 GPa) in the ‘‘toroid’’ high pressure device using thermal ac susceptibility measurements. That pressure dependence was linear with the coefficient dTC/dP=7.44 K/Gpa [TC(0)=341.3 K]. The compressibility of the sample has been measured by means of resistive strain gauges at hydrostatic pressure up to 9 GPa. The equation (V−V0)/V0=−a⋅P+b⋅P2 has been fitted to the experimental data with the coefficients a=8.64⋅10−3 GPa−1, b=1.13⋅10−4 GPa−2. The drop of the HhpfSn in pressure range 0–10.8 GPa can be explained on the basis of the empirical theory developed by Delyagin, Krylov, and Nesterov [JETP 76, 1049(1980)].
Systematic investigation of the effects of organic film structure on light emitting diode performance
Authors: Joswick M. D. , Campbell I. H. , Barashkov N. N. , Ferraris J. P.
Published: July 2002

We present a systematic investigation of the effects of organic film structure on light emitting diode (LED) performance. Metal/organic film/metal LEDs were fabricated using a five ring, poly(phenylene vinylene) related oligomer as the active layer. The structure of the vacuum evaporated oligomer films was varied from amorphous to polycrystalline by changing the substrate temperature during deposition. The intrinsic properties of the oligomer films and the LED performance were measured. The measured intrinsic film properties include: optical absorption, photoluminescence (PL) spectra, PL lifetime, PL efficiency, and effective carrier mobility. The measured device characteristics include current–voltage, capacitance–voltage, electroluminescence (EL) efficiency, and the contact metal/organic film Schottky barrier heights. The optical absorption and PL properties of the films are weakly dependent on film structure but the effective carrier mobility decreases with increasing crystallinity. The EL quantum efficiency decreases by more than one order of magnitude, the drive voltage at a fixed current increases, and the electron Schottky barrier height increases as the crystallinity of the film is increased. The diode current–voltage characteristic is determined by the dominant hole current and the electroluminescence efficiency is controlled by the contact limited electron injection. These results demonstrate significant effects of organic film structure on the performance of organic LEDs.
On the magnetic field dependence of the critical current in small irregular polygonal Josephson junctions
Authors: Nappi C. , Cristiano R. , Frunzio L. , Pagano S. , Lisitskii M. P.
Published: July 2002

Explicit analytic expressions for the lobe structures of the critical current versus magnetic field are provided for small arbitrarily shaped four-sided Josephson junctions. The orientation of the external parallel magnetic field, in the plane of the junction, is also arbitrary. Several configurations worth noting are considered in detail for rhomboid, triangle, parallelogram, and trapezium junctions, once the dependence from the normalized magnetic field has been introduced.
Metal/Al2O3: A new class of x-ray mirrors
Authors: Morawe Ch. , Zabel H.
Published: July 2002

We have examined sputtered metal/Al2O3 multilayers (metal: Co, Pt, W) with respect to their suitability as x-ray mirrors in high temperature environments. The rf-sputtering technique leads to layered structures with interface roughnesses of only 0.2 nm as confirmed by x-ray scattering and transmission electron microscopy. In situ resistance measurements characterize the percolation process and indicate the lower thickness limits for the chosen materials which lead to minimum modulation periods of ≊2 nm. The samples were characterized after stepwise annealing up to 1000 °C. Reflectivity values of nearly 100% at the first satellite-reflection were found in the case of Pt and W (for λ=0.15418 nm). The most stable W/Al2O3 multilayer reflects 70% of the primary beam even after annealing at 900 °C.
Excitation by electrons and fast neutrals in nitrogen discharges at very high electric field to gas number density ratios
Authors: Stojanović V. D. , Jelenković B. M. , Petrović Z. Lj.
Published: July 2002

A Monte Carlo code for simulation of electron, ion and fast neutral transport was developed to model the spatial distribution of excitation under nonequilibrium conditions at very high electric field to gas number density ratios (E/N) and low currents. The code includes the most detailed representation of electron scattering and transport including the reflection and multiplication at the anode and simulation of ion and fast neutral transport for realistic geometry of the experiments of B. M. Jelenković and A. V. Phelps [Phys. Rev. A 36, 5310 (1987)] and V. T. Gylys, B. M. Jelenković, and A. V. Phelps [J. Appl. Phys. 65, 3369 (1989)]. A good agreement between the simulations and experimental data was achieved by using reasonable data for scattering cross sections from the literature. Simulations confirm the model proposed by A. V. Phelps, B. M. Jelenković and L. C. Pitchford [Phys. Rev. A 36, 5327 (1987)] of transport and excitation kinetics at very high E/N where fast neutral excitation is the dominant process for second positive (2+) band of nitrogen.
Equation of state of beryllium at shock pressures of 0.4–1.1 TPa (4–11 Mbar)
Authors: Nellis W. J. , Moriarty J. A. , Mitchell A. C. , Holmes N. C.
Published: July 2002

High-pressure shock Hugoniot data were measured for Be using strong shock waves generated by underground nuclear explosions. These data and a preliminary theoretical analysis are reported.
Microstructure and optical properties of epitaxial GaN on ZnO (0001) grown by reactive molecular beam epitaxy
Authors: Hamdani F. , Yeadon M. , Smith David J. , Tang H. , Kim W. , Salvador A. , Botchkarev A. E. , Gibson J. M. , Polyakov A. Y. , Skowronski M. , Morkoç H.
Published: July 2002

High quality GaN epilayers have been grown on oxygen and zinc surfaces of ZnO (0001) substrates by reactive molecular beam epitaxy and the effect of the intermediate buffer layer on the structural and optical properties of the GaN films has been investigated. The optical and structural characterization of the GaN epilayers and ZnO substrates were performed using photoluminescence, reflectivity, x-ray double diffraction, atomic force microscopy, and transmission electron microscopy. The optical results indicated that GaN was grown with compressive strain due to the difference in thermal expansion coefficient between GaN and ZnO. The surface roughness has been reduced by using an intermediate low temperature GaN buffer layer. The low temperature photoluminescence spectra of GaN/ZnO epilayers did not reveal any sign of the well-known midgap yellow signal. Linear polarized reflectivity and photoluminescence indicated that GaN epilayer planes were not misoriented with respect to the ZnO substrate planes: this result was confirmed by x-ray double diffraction measurements.
Up-conversion characteristics of Er3+ in transparent oxyfluoride glass–ceramics
Authors: Takahashi Masahide , Izuki Manabu , Kanno Ryoji , Kawamoto Yoji
Published: July 2002

Up-conversion characteristics of Er3+ under 800 nm excitation are clarified for 30SiO2 ⋅15AlO1.5⋅28PbF2⋅22CdF2⋅(5−x)GdF3⋅xErF3 (x=0.3–5) transparent glass–ceramics, which are composed of fluoride nanocrystals and oxide glass matrices. Strong up-conversion emissions at around 525, 550, and 660 nm, and weak up-conversion emissions at around 410, 450, and 490 nm are observed. The intensities of up-conversion emissions at around 550 and 660 nm change remarkably with ErF3 contents from 1 mol % to 5 mol %: emission at 550 nm decreases and one at 660 nm increases with increasing ErF3 content. These features can be explained by Er3+ up-conversion mechanisms which were proposed for ZrF4-based glasses.
Thermally induced structural modifications in sputtered C/Al2O3 multilayers
Authors: Veldkamp M. , Zabel H. , Morawe Ch. , Arnscheidt B.
Published: July 2002

We have prepared C/Al2O3 multilayers by rf and rf magnetron sputtering on sapphire (112̄0) and silicon (100) substrates. We have analyzed their structure by x-ray reflectivity measurements in the as-deposited state and after annealing. The interface roughness is only 0.3 nm. After thermal treatment, the double-layer thickness increases, which is caused by an expansion of the carbon layers. Raman scattering experiments show that this behavior can be explained by a reduction of the number of sp3-coordinated carbon atoms. Conductivity measurements support the structural model. From in situ resistance measurements during the deposition process we derive a percolation threshold of 1.2 nm for carbon and 0.9 nm for Al2O3. Therefore, the smallest double-layer thickness possible would be slightly above 2.1 nm. Both materials remain amorphous during the annealing process up to 1100 °C. The thermal stability of this material combination is extremely high. Annealing at 1100 °C for 1 h not only enhances the reflectivity of the multilayer system, but also prevents further period expansion of the carbon layers during subsequent annealing steps. We also discuss possible applications as normal-incidence mirrors for the C K edge.
Theory of third harmonic generation in random composites of nonlinear dielectrics
Authors: Hui P. M. , Cheung P. , Stroud D.
Published: July 2002

We consider the effective nonlinear susceptibility tensor χ for third harmonic generation (THG) in a nonlinear composite medium in which the components may have nonvanishing second- and third-order nonlinear susceptibilities. We derive an expression for this susceptibility in terms of the positional-dependent second- and third-order susceptibilities within the composite, as well as several factors which describe the local field effect in a corresponding linear medium. We consider both the THG due to the presence of THG susceptibility in the components, and the induced THG due to the presence of second-order nonlinear susceptibilities in the components. The resulting expression can be used to calculate both local field and percolation effects on χ in a wide range of geometries. The general expression reduces to a simple result in the dilute limit, which is similar to that previously derived. An effective medium approximation, which is applicable to the whole range of concentration, is proposed for both the effective second and third harmonic susceptibilities. Results obtained from the general expression and the effective medium approximation are found to be in good agreement with those obtained by numerical simulations for a model system of nonlinear composites consisting of a nonlinear metallic component and a linear insulating component.
Large magnetic fields generated by z-pinch flux compression
Authors: Appartaim R. K. , Dangor A. E.
Published: July 2002

Experiments to generate large magnetic fields by flux compression have been performed in two z-pinch configurations: (i) a novel plasma liner developed from a thin cylindrical surfactant film and (ii) a conventional gas pinch formed in a uniform cylinder of gas. The apparatus, experiments, and the results obtained are described in this article. A comprehensive investigation of the gas pinch shows that an optimum set of conditions exists, for given discharge parameters, for obtaining the highest magnetic field. The scaling of the attainable peak magnetic field with discharge parameters is obtained experimentally and compared with simplified models of the compression. Using a current of 500 kA with τ1/4=2 μs, an initial magnetic field of 0.3 T has been compressed to 38 T, demonstrating a compression ratio of over 120.
Double peak phenomenon in superconducting tunnel junction x-ray detectors
Authors: Ohkubo M. , Fukuda D. , Sakamoto I. , Hayashi N. , Martin J. , Panteleit F. , Huebener R. P.
Published: July 2002

It has been normally observed that superconducting tunnel junction x-ray detectors produce a double peak originating from different absorption events in two electrodes. The double peak is associated with the dynamics of quasiparticles created by x-ray absorption events. In this study we have found that the double peak phenomenon depends on both bias voltage and magnetic history. On the events in the base and counterelectrodes the detector exhibits dissimilar signal–height versus bias-voltage characteristics, which suffer a large change when a small number of Abrikosov vortices corresponding to even a terrestrial magnetic field are frozen during cooling. These observations are explained by a multiple quasiparticle tunneling model with quasiparticle trapping in the vortices.
Colossal magnetoresistance in ultrathin epitaxial La0.75Sr0.25MnO3 films
Authors: Khartsev S. I. , Johnsson P. , Grishin A. M.
Published: July 2002

We present results on magnetotransport measurements of a series of La0.75Sr0.25MnO3 films with thickness ranging from 32 to 3300 Å. Films were fabricated using pulsed laser deposition onto SrTiO3 and LaAlO3 substrates. The substrate-film lattice mismatch causes strain in ultrathin films which diminishes with increasing thickness. Ultrathin films exhibit perfect match of in-plane film and substrate lattice parameters. The inhomogeneity of lattice parameters in thick films is caused by film-substrate mismatch strain and displayed by nonuniform broadening of x-ray diffraction Bragg reflections. The observation of characteristic Kiessig fringes is evidence for very uniform thickness of fabricated films. We found that the magnetotransport properties are strongly controlled by the film thickness and that allowed us to tailor the temperature of the metal-to-semiconductor phase transition in the range of 100–340 K with a change in magnetoresistivity within a factor of 3. We also present comparisons between the thickness dependence and oxygen deficiency in the films.
Effect of annealing on magnetic exchange coupling in CoPt/Co bilayer thin films
Authors: Kim J. , Barmak K. , De Graef M. , Lewis L. H. , Crew D. C.
Published: July 2002

Thin film CoPt/Co bilayers have been prepared as a model system to investigate the relationship between microstructure and exchange coupling in two-phase nanocomposite permanent magnets. The bilayers were prepared by magnetron sputter deposition of near-equiatomic CoPt with a thickness of 25 nm onto oxidized Si wafers. In the as-deposited state, CoPt had the A1 (fcc) structure and was magnetically soft. Before reinsertion into the sputtering chamber for the deposition of 2.8–16.7 nm thick Co layers, the CoPt films were annealed at 700 °C for 120 min to produce the magnetically hard, fully ordered L10 phase. The presence of exchange coupling in the bilayers was verified by magnetic hysteresis and recoil measurements and showed that only for Co thicknesses below 6.3 nm was this layer (in its as-deposited state) coupled through its full thickness to the CoPt layer. Annealing the bilayer samples at 300 and 550 °C for 20 min resulted in improvement of the interlayer magnetic coupling and produced clear differences in the magnetic reversal coherency and the recoil curves. However, for some samples, the improved coupling resulted in a decrease in coercivity, indicating that there is an optimum in the coupling strength for the attainment of high coercivity. Transmission electron microscopy studies of the bilayers in plan view showed that the increased interlayer coupling with annealing was a result of improved granular epitaxy of Co to CoPt.
High-performance alloyed In metal-bonded magnets produced from Zn/Sm2Fe17Nx powders
Authors: Machida K. , Noguchi K. , Nishimura M. , Adachi G.
Published: July 2002

Alloyed In metal-bonded magnets with ρ=∼6.5 g cm−3 were molded from surface-coated fine powders, Zn/Sm2Fe17Nx (x=∼3), under conditions of 1.4 GPa, ∼460 K, and 1.4 MA m−1 by adding an appropriate amount of In metal powder. The resulting In–Zn alloy with an eutectic composition served as the metal binder and the lowered melting point was responsible for improving the effective alignment of the Sm2Fe17Nx particles along a direction of the applied magnetic field for the bonded magnets. The following high-performance magnetic characteristics were attained: (BH)max=∼151 kJ m−3, Br=∼0.96 T, and Hcj=∼0.72 MA m−1. In–Zn metal-bonded Sm2Fe17Nx magnets showed good thermal stability, good oxidation resistance, and high densification due to effective surface coating by the In–Zn eutectic alloy. The temperature dependence of reversible flux losses measured over temperatures from 273 to 393 K provided a thermal coefficient of α(Br)=−0.06 % K−1.
Crystal structure and electrical properties of epitaxial SrBi2Ta2O9 films
Authors: Ishikawa Katsuyuki , Funakubo Hiroshi , Saito Keisuke , Suzuki Toshimasa , Nishi Yuji , Fujimoto Masayuki
Published: July 2002

The crystal structure and electrical properties were investigated for (001)- and (116)-oriented SrBi2Ta2O9 (SBT) thin films deposited on (100) SrTiO3 and (100)SrRuO3∥(100)SrTiO3 substrates, and (110) SrTiO3 and (110)SrRuO3∥(110)SrTiO3 substrates, respectively. Both oriented SBT films were epitaxially grown with high crystal perfection, and twinning existed in the (116)-oriented one. Both oriented SBT films were found to form c/6 lattice displacements to relax the stress. The interface of the (001)-oriented film did not include misfit dislocations, defects, and an interfacial layer, and that of the (116)-oriented film included the lattice strain contrast due to an irregular atomic arrangement. The electrical property of the SBT film shows anisotropy of the ferroelectricity along the c- and a-axis directions; the remanent polarizations of the (001)- and (116)-oriented SBT films were 0 and 10.5 μC/cm2, respectively.
A semiclassical theory in phase space for molecular processes: Formalism based on dynamical characteristic function
Authors: Takatsuka Kazuo , Nakamura Hiroki
Published: July 2002

A new semiclassical theory is formulated by introducing a new distribution function to be propagated in phase space. Primarily this theory is intended to describe molecular collisions such as reactive scattering, but is also applicable to intramolecular processes including bound state problems. This new distribution function is a kind of characteristic function in the terminology of the phase-space-quantum theory developed by Weyl, Wigner, Moyal and others, and is called a dynamical characteristic function (DCF). The equation of motion and some properties of the DCF are presented. The essential feature of our theory is that the semiclassical DCF can be propagated using classical trajectories. This distribution function carried information not only on the density, but also on the phase (action integral). In this sense this theory can be regarded as a kind of phase-space-path-integral formalism. The semiclassical approximation is analyzed from the viewpoint of path integrals. The Wigner distribution function is also discussed in the context of our analysis. As a theoretical application, the semiclassical DCF is shown to reproduce the expression of the periodic orbit theory for bound states. Also, as a simple numerical application, a propagation of the DCF on a nonquadratic (Morse) potential is presented.
Laser-induced charge transfer in the CH6+ quasimolecule
Authors: Errea L. F. , Méndez L. , Riera A.
Published: July 2002

The charge transfer cross section is calculated for C6++CH(1s) collisions, through photon assisted 5gσ–6hσ, 5gσ–4fσ, 5gσ–4fπ, and 5gσ–4dσ transitions. The theory developed by Copeland and Tang, and ourselves, is employed, and the validity of the approximations used is tested. The four processes considered have widely different characteristics with regards to the laser wavelength needed, the collision dynamics and the applicability of back-of-the-envelope estimates based on the Landau–Zener approximation. We point out the relevance of those processes to the impurity diagnostics of magnetically confined fusion plasmas and to the development of short wavelength lasers.
E-T and near-resonant E-V energy transfer in the quenching of the 1.0 and 2.3 μm electronically excited states of PuF6 by H2 and D2
Authors: Rice W. W. , Barefield J. E. , Pack R. T , Dye B. A.
Published: July 2002

The quenching and energy transfer of electronically excited PuF6 by H2 and D2 have been measured using laser-induced fluorescence (LIF) from both the 1.0 and 2.3 μm emitting states of PuF6. Linear Stern–Volmer plots of the single exponential decay data give quenching rate constants (all in units of cm3 molecule−1 s−1) for D2 of (3.7±1.7)×10−15 and (8.7±0.3)×10−16 for the 1.0 and and 2.3 μm states, respectively, and a H2 quenching rate constant of (4.8±0.3)×10−14 for the 1 μm LIF. The 2.3 μm LIF in H2 is not a single exponential decay but a rapid decay followed by an anomalously long tail, which implies almost resonant E⇄V energy transfer and storage. Kinetic modeling yields the following new rate constants: PuF6*+H2→PuF6+H≠2, k=(2.05±0.30)×10−14; H2≠+PuF6→H2+PuF6*, k=(3.1±0.5)×10−14; PuF6*+H2→PuF6+H2, k=(3.1±2.0)×10−15; H2≠+PuF6→H2+PuF6, k=(3.1±3.0)×10−15; where PuF6* is the 2.3 μm emitting electronically excited state of PuF6 and H2≠ is vibrationally excited H2 in the v=1 state.
Rydberg states of the nitrogen moleculea)
Authors: Chung Sunggi , Lin Chun C. , Lee Edward T. P.
Published: July 2002

Rydberg states (1,3Σg+, 1,3Σu+, 1,3Πg, and 1,3Πu) of N2 that converge to the N2+ (X 2Σg+) ion have been calculated. The core orbitals are taken as those of the free N2+(X 2Σg+) ion. The Rydberg electron wave functions are expanded by one-center basis functions which are solution of an electron moving in a sphericalized potential of the N2+(X 2Σg+) ion. This expansion converges rapidly since the basis functions are already a fairly good representation of the Rydberg states especially for the higher ones. The use of one-center expansion allows us to examine the mixing of the basis functions of different angular-momentum quantum number l in the Rydberg states which is found to be generally small. Energy levels are reported for singlet and triplet Rydberg states of the Σ and Π types (Λ=0,1) with l from 0–9 and n through 11. The Rydberg levels with l≥4 are entirely hydrogenic. For lower l the quantum defect varies significantly with n within a given l series. Also a wide range of quantum defect is found for different states. Transitions among the various Rydberg series produce a very rich spectrum in the long-wave infrared region.
General factorization relations and consistency conditions in the sudden approximation via infinite matrix inversion
Authors: Chan C. K. , Hoffman D. K. , Evans J. W.
Published: July 2002

Local, i.e., multiplicative, operators satisfy well-known linear factorization relations wherein matrix elements (between states associated with a complete set of wave functions) can be obtained as a linear combination of those out of the ground state (the input data). Analytic derivation of factorization relations for general state input data results in singular integral expressions for the coefficients, which can, however, be regularized using consistency conditions between matrix elements out of a single (nonground) state. Similar results hold for suitable ‘‘symmetry class’’ averaged matrix elements where the symmetry class projection operators are ‘‘complete.’’ In several cases where the wave functions or projection operators incorporate orthogonal polynomial dependence, we show that the ground state factorization relations have a simplified structure allowing an alternative derivation of the general factorization relations via an infinite matrix inversion procedure. This form is shown to have some advantages over previous versions. In addition, this matrix inversion procedure obtains all consistency conditions (which is not always the case from regularization of singular integrals).
Dynamics of triplet excitons in molecular pairs: Temperature dependence of transverse relaxation time
Authors: Reineker P. , Köhler J. , Schmid U. , Silbey R.
Published: July 2002

For triplet excitons in dimers of differently oriented molecules the temperature dependence of the ESR linewidth is calculated analytically. The temperature of the extrema of the linewidth is discussed as a function of the coupling strength between excitons and phonons.
On quantum trajectories and an approximation to the Wigner path integral
Authors: McLafferty Frank
Published: July 2002

In this paper we suggest an approximation to the Wigner Path integral using quantum trajectories. We discuss the results in terms of Bohm’s theory and Takabayasi’s study of the relation between Bohm and Wigner theory. We discuss how this new approach is an approximation to Bohm’s theory and discuss its validity. We also suggest a method of evaluation and where such an approach would be useful.
EPR spectrum features of the tetrameric copper compounds of formula Cu4OX6L4
Authors: Buluggiu E.
Published: July 2002

The recent experimental observation of S=2 EPR signals from tetrameric Cu(II) complexes having the general formula Cu4OX6L4 is interpreted on the basis of a static quasi-Heisenberg model for the complex, where the anisotropic symmetric and skew-symmetric interactions have been taken into account with the approximation of the second order of the perturbation theory. It is found that the observed g value fits the single-ion mean value g=(gCu∥+2gCu⊥)/3, as expected, and the constant for the cubic zero-field splitting may be simply written as B4=(D2a+D2b+D 2c)/120J0, where J0 is the isotropic exchange coupling constant and Da,b,c are the principal values of the symmetric component of the anisotropic interaction. It is found, in contrast, that both the symmetric and skew-symmetric parts may contribute to the not yet observed S=1 signals. When the model is applied to the case of Cu4OCl6(TPPO)4 the B4 value measured by EPR and the J0 value derived by magnetic susceptibility data seem to support a symmetric exchange component very large compared with the dipolar energy.
Welding of polymer networks
Authors: Prager S. , Adolf D. , Tirrell M.
Published: July 2002

Evolution of mechanical strength of interfaces between surfaces of tethered polymer chaims has been studied as a model for the autoadhesion of cross-linked polymers. Chain motion is modeled as restricted diffusion in a tube. We use Helfand and Pearson’s results for the motion of the end of a tethered chain in a tube. The result is a theory that predicts how much more slowly a tethered arm will cross an interface than a free linear chain. The form of the evolution of crossings with time for a (initial growth ∼t1/2) tethered arm is nearly identical to that for linear chains but retarded due to restrictions on their motion.
Static scattering function for randomly broken chains
Authors: Hammouda Boualem , Garcia Molina Jesus J. , Garcia de la Torre Jose
Published: July 2002

To analyze small angle neutron, x-ray, or static light scattering data from stiff macromolecules in dilute solutions, semiflexible chain models are needed. The randomly broken chain model is used to calculate by numerical simulation the static scattering function. It is then compared to other models.
Fluorescence line narrowing and site-selection experiments in the optical spectra of adsorbed molecules
Authors: Deeg F. W. , Bräuchle Chr.
Published: July 2002

We have recorded the low-temperature fluorescence spectra of resorufin adsorbed on a γ-Al2O3 surface excited by a krypton-ion laser. Our results demonstrate that it is possible to remove the inhomogeneous broadening of the electronic spectra of adsorbed molecules by optical site selection which promises to reveal new details about adsorbate–substrate systems.
A 23Na NMR study of the structure of Na β″-alumina conduction plane at 5 K
Authors: Carduner Keith R. , White David
Published: July 2002

From measurements of the angularly dependent second order quadrupole shift of the central transition of the 23Na spectrum in Na β″-alumina at 5 K, it is possible to conclude that sodium resides in two distinct sites in the conduction plane. Calculations of electric field gradients (efg) consistent with experiment indicate that these sites are located along the miror planes characterizing the conduction directions. Independent calculation of the efg’s with a point charge model reveals (1) that the Na ions in the two sites are shifted from the BR crystallographic positions in the lattice; in one case a very small amount and in the second a much larger shift and (2) the presence of vacant BR positions. The NMR results agree with the extended 2D superlattice of Boilot et al. [Phys. Rev. B 22, 5912 (1980)] used to account for x-ray diffuse scattering and diffraction.
Superconducting and other phases in organic high polymers of polyacenic carbon skeletons. II. The mean field method
Authors: Kimura M. , Kawabe H. , Nishikawa K. , Aono S.
Published: July 2002

Ordered phases such as CDW, SDW, and the singlet superconductivity(SSC) are predicted by means of a mean field theory. The electronic Hamiltonian is linearized by introducing order parameters which are expected to arise, and these order parameters are determined self-consistently. The behaviors of gap, transition temperature, and condensation energy are greatly different from those of BCS theory. The coexistence of the various phases is discussed. Aside from a very special case the single phase is most stable.
Frequency shifts of some pure rotational lines in polar mixtures in the far infrared range. Experiments and theoretical predictions
Authors: Blancher H. , Bachet G. , Coulon R. , Boulet C.
Published: July 2002

Measurements with a far infrared Fabry Perot interferometer of pure rotational line shifts for dipolar mixtures at low pressure for the three gases HF, HCl, and HBr, each of them being successively perturbed by one of the two others, are presented. Theoretical calculations have been performed starting from the formalism of Robert and Bonamy. A good quantitative agreement with experiments has been obtained for all the cases considered by taking into account the noncommutative character of the potential in the calculations of the differential cross section.
Hydrodynamic fluctuations in a chemically reacting fluid
Authors: de la Selva S. M. T. , García-Colín L. S.
Published: July 2002

It is shown that the calculation of the nonlocal in time second moments of hydrodynamic fluctuations in a chemically reacting fluid can be done within the framework of extended irreversible thermodynamics. The results obtained imply that a non-Gaussian noise is associated with the fluctuations of the rate of the reaction and of the diffusive flux. The classical results from linear fluctuating hydrodynamics are recovered as a special case of the general results. For the thermostatic second moment in the fluctuations of the internal energy new terms arise whenever fast variables intervene for their description.
Ionization energies of the transition metal diatomics Cu2, Ag2, Cr2, and Mo2: A Green’s function investigation
Authors: von Niessen W.
Published: July 2002

The vertical valence ionization energies of the first and second row transition metal diatomics Cu2, Ag2, Cr2, and Mo2 in their 1Σ+g ground states are calculated by an ab initio Green’s function method which incorporates the effects of electron correlation and relaxation. For the lowest ionization energy the following values are obtained Cu2: 7.78 eV, Ag2: 6.70 eV; Cr2: 6.76 eV, and Mo2: 6.21 eV. The higher lying valence ionization energies corresponding to removal of d electrons are calculated as well. Some conclusions on the applicability of the calculational methods can be derived.
Statistical geometry of molten alkali halides
Authors: Baranyai András , Ruff Imre
Published: July 2002

The generalized Dirichlet–Voronoi polyhedra were used to characterize the adjacent neighborhood of the ions in computer simulated configuration set of molten RbCl and LiI as examples of binary Coulomb liquids with like and unlike size ions, respectively. The statistical distribution of various features of the DV polyhedra has been studied with the following main results: (i) radial and angular distributions of adjacent neighbors indicate traces of face-centered-cubic symmetry in molten RbCl and wurtzite-like structure in LiI; (ii) the radial internal point distribution function on the DV polyhedra reveal more compact coordination spheres for Cl−, Rb+, and I− ions than for Li+ ions in the corresponding melts; (iii) the volume fluctuation of the DV polyhedra makes it possible to calculate isothermal compressibility coefficients even from (N,V,T) simulation data; (iv) identification of vacancies by seeking for the most protruding vertices of the DV polyhedra provides information on vacancy–vacancy pair correlation functions.
Indirect dephasing model and librovibrational spectra of diatomics trapped in rare gas matrices. Application to the case of CO/Ar
Authors: Allavena M. , Seiti K.
Published: July 2002

In the present work, the theoretical scheme applied to the study of hindered rotation of molecular impurity in rare gas matrices [J. Chem. Phys. 77, 1757 (1982)] is extended to librational motion. The librational motions are considered as the intermediate modes in the process of vibrational dephasing. The effect of libron–phonons coupling is formally calculated for a three dimensional lattice. The case of CO dispersed in Ar is examined within the frame of a simple cubic lattice. Despite the oversimplified model, the computed temperature dependence of the vibrational absorption band adequately reproduces the observed experimental trend.
The effect of molecular vibrations on calculated second virial coefficients
Authors: Refson Keith , Lie George C. , Clementi Enrico
Published: July 2002

The limitation of the rigid molecule approximation in the calculation of the second virial coefficient is examined using two different potential models for steam. Both the MCY and the ST2 potentials have been augmented by allowing bond flex and stretch according to an ab initio internal potential. Second virial coefficients are calculated within the classical approximation by the Monte Carlo method. The calculated coefficients with flexible molecules are lower than in the rigid case by up to 10% for the MCY potential and 8% for the ST2 potential.
Transport properties of polydisperse fluids. I. Shear viscosity of polydisperse hard-sphere fluids
Authors: Xu Jiasai , Stell George
Published: July 2002

The shear viscosity expressions for a polydisperse hard-sphere fluid in both the dilute and dense fluid regions have been obtained as the solution of a set of linear integral equations in the Enskog transport theory. In the monodisperse limit we recover the known Enskog result. Explicit analytical solutions have been obtained for the special case of equal-mass particles. For general mass-size distributions, a simple numerical method has been proposed. Computations have been performed for a mass-size relation of power-law form. The two size distributions we have looked at are the Schulz distribution and the log-normal distribution. It is found that both distributions show similar effects on the shear viscosity. In the dilute-gas region, for each given fractal dimension, there exists a critical variance of the size distribution at which the shear viscosity attains a maximum. In the dense-gas region, the shear viscosity increases as the variance and the density increase except for the low density and the high fractal-dimension region where the shear viscosity attains a minimum at some particular variance. The shear viscosity diverges at a close-packing density that decreases with increasing variance.
Vibrationally mediated photodissociation of t-butyl hydroperoxide: Vibrational overtone spectroscopy and photodissociation dynamics
Authors: Likar M. D. , Baggott J. E. , Crim F. F.
Published: July 2002

Vibrationally mediated photodissociation is a two-photon technique for studying the spectroscopy and photodissociation dynamics of highly vibrationally excited molecules. In these experiments, a highly vibrationally excited t-butyl hydroperoxide (t-BuOOH) molecule, prepared by excitation in the region of the third overtone of the O–H stretching vibration (4νOH), absorbs a second photon to dissociate to OH and t-butoxy fragments, and laser induced fluorescence determines the quantum state populations of the OH fragment. Vibrational overtone excitation spectra, obtained by varying the vibrational overtone excitation wavelength while monitoring a single OH rotational state, are nearly identical to photoacoustic spectra. We fit the coarse structure in the vibrational overtone excitation spectrum in the region of the 4νOH transition and the photoacoustic spectra in the regions of the 5νOH and 6νOH transitions using a spectroscopic model of the interaction of the O–H bond stretching vibration with the torsional vibration about the O–O bond. This analysis determines the barrier to internal rotation of the O–H and t-butoxy groups through the trans configuration and its variation with vibrational excitation. The trans barrier in the ground vibrational state is 275 cm−1 and increases with vibrational excitation to 425, 575, and 680 cm−1 for t-BuOOH molecules with four, five, and six quanta of O–H stretching excitation, respectively. Comparison of the energy disposal in the vibrationally mediated photodissociation with that for direct photolysis at 376 nm, which adds the same amount of energy to the molecule, illustrates the unique dynamics that can occur when vibrational excitation precedes photodissociation. Single-photon photolysis produces fragments with large recoil velocities, while vibrationally mediated photodissociation produces slowly recoiling fragments having substantially more energy in internal excitation.
Analytical approach to molecular liquids. III. The Born solvation free energy of two fixed ions in a dipolar solvent
Authors: Zhou Yaoqi , Friedman Harold L. , Stell George
Published: July 2002

The Born solvation free energy (BSFE) of two ions at a fixed distance from one another in a model polar solvent is obtained via two approaches. In the interaction-site approach, the two ions are modeled as a rigid extended dipolar dumbbell. Analytical expressions for the BSFE for such a dumbbell model in a dipolar dumbbell and a dipolar hard-sphere solvent are obtained under a mean spherical approximation (MSA). In the second approach, a thermodynamic cycle is established such that the BSFE for two ions a fixed distance apart can be expressed in terms of the solvent-averaged potential between the two ions and other known quantities. The results obtained via these two approaches are reasonably consistent, with the thermodynamic-cycle BSFE as a function of distance exhibiting more of the structure one expects to find in a molecular solvent. Both BSFE functions are substantially different from the corresponding continuum-solvent result. When the distance between two ions goes to infinity, our results reduce to earlier results for the single-ion BSFE obtained by us for a dipolar dumbbell solvent and by Chan et al. for a dipolar hard-sphere solvent. The BSFE of two ions in an ionic solution with a dielectric-continuum solvent is also obtained; it is found that the contribution of other ionic particles to the total BSFE is usually negligible if the dielectric constant of the solution is assumed to be unchanged.
Microwave spectrum, structure, dipole moment, and deuterium nuclear quadrupole coupling constants of the acetylene–sulfur dioxide van der Waals complex
Authors: Andrews Anne M. , Hillig Kurt W. , Kuczkowski Robert L. , Legon A. C. , Howard N. W.
Published: July 2002

Thirty-three a- and c-dipole transitions of the acetylene–SO2 van der Waals complex have been observed by Fourier transform microwave spectroscopy and fit to rotational constants A=7176.804(2) MHz, B=2234.962(1) MHz, C=1796.160(1) MHz. The complex has Cs symmetry with the C2H2 and SO2 moieties both straddling an a–c symmetry plane (i.e., only the S atom lies in the plane). The two subunits are separated by a distance Rcm=3.430(1) Å and the C2 axis of the SO2 is tilted 14.1(1)° from perpendicular to the Rcm vector, with the S atom closer to the C2H2. The dipole moment of the complex is 1.683(5) D. The deuterium nuclear quadrupole hyperfine structure was resolved for several transitions in both C2HD⋅SO2 and C2D2⋅SO2. A lower limit for the barrier to internal rotation of the C2H2 was estimated to be 150 cm−1 from the absence of tunneling splittings. The binding energy was estimated by the pseudo-diatomic model as 2.1 kcal/mol. A distributed multipole analysis was investigated to rationalize the structure and binding of the complex.
The calculation of frequency-dependent polarizabilities as pseudo-energy derivatives
Authors: Rice Julia E. , Handy Nicholas C.
Published: July 2002

The definition of frequency-dependent polarizabilities α(−ω;ω), β(−2ω;ω,ω), β(−ω;ω,0), and β(0;ω,−ω) is discussed, and it is argued that the most convenient definitions are as energy derivatives, a pseudo-energy being defined as the expectation value of [H−i(∂/∂t)]. This definition outlines a straightforward procedure for obtaining frequency-dependent polarizabilities for all quantum chemistry methods including those which account for the effects of electron correlation. It is demonstrated at the self-consistent field level of theory that αλμ(−ω;ω) cos ωt may be considered as the derivative of the static dipole moment μλ with respect to the strength Eωμ of a frequency-dependent field Eωμ cos ωt (as is usual), or as the derivative of an appropriately defined frequency-dependent dipole moment μμ cos ωt with respect to a static field E0λ. In this way, polarizabilities may be determined from finite static field calculations on lower-order tensors. Therefore, α(−ω;ω) cos ωt is defined within second-order Mo/ller–Plesset perturbation theory (MP2) as the second derivative of the MP2 energy with respect to one static and one frequency-dependent field.

An analytic expression is given for αλμ(−ω;ω) at the MP2 level of theory. An MP2 frequency-dependent dipole expression is also defined, which if finite static field calculations are applied, gives the same values for αλμ(−ω;ω). MP2 values are reported for α(−ω;ω) of formaldehyde and ammonia for a range of frequency ω=0.01–0.1 a.u. From comparison of the self-consistent field (SCF) and MP2 values of the frequency-dependent contribution to ᾱ(−ω;ω), it is concluded that it is appropriate to use an SCF frequency-dependent correction in conjunction with a static polarizability determined at a higher level of theory in order to obtain an accurate value for ᾱ(−ω;ω) of H2CO in this frequency range. For ammonia, the frequency-dependent contribution to ᾱ(−ω;ω) is more sensitive to electron correlation. Nevertheless, compared to the total polarizability ᾱ(−ω;ω), the error in the frequency-dependent contribution determined using the SCF method is small (∼2% at ω=0.1 a.u.)
Pressure-induced disordering in α-SrZrF6: Implications for the role of the countercation in glassy SrZrF6
Authors: Serghiou George C. , Hammack William S.
Published: July 2002

Reversible pressure-induced changes in the Raman and x-ray scattering of α-SrZrF6 are reported. Above a pressure of ∼10.0 GPa there is a precipitous decline in the Raman intensity; this is a phenomenon associated with the formation of an amorphous state. Energy dispersive x-ray diffraction (EDXD) measurements show that the amorphization is not complete—some residual ordering persists. The results suggest that glassy SrZrF6, prepared at ambient pressure, (1) contains kinked chains of ZrF8 dodecahedra and (2) the medium range order of the glass is determined by the way ZrF8 units are arranged around the strontium countercation.