bluuebunny/crossref_metadata_2025 huggingface dataset:
Spatial distributions of absolute densities of argon metastable state 3p54s in a Gaseous Electronics Conference Reference Cell
Authors: Augustyniak Edward , Filimonov Serguei , Borysow Jacek
Published: July 2002

The spatial distributions of the 3p54s metastable state of Ar have been measured in radio frequency argon plasmas in a Gaseous Electronics Conference Reference Cell by means of high resolution absorption spectroscopy. A single-mode, tunable diode laser has been used to obtain absorption profiles at 750 and 801 nm. Spatial profiles across the gap between electrodes have been measured with 1 mm resolution for various pressures (0.15–1.0 Torr) and plasma powers (1–320 W). The densities of 3p54s metastable were obtained from integrated absorption profiles.
Analysis of scanning probe used for simultaneous measurement of tunneling current and surface potential
Authors: Oyama Yutaka , Majima Yutaka , Iwamoto Mitsumasa
Published: July 2002

The scanning probe microscope used for the simultaneous measurement of tunneling current and surface potential (TSM) is analyzed. Using an electric image method, the capacitance formed between the vibrating tip and plane sample is calculated, assuming that the vibrating tip is a sphere electrode with a radius r. The formula expressing the displacement current and the capacitance is obtained. The resolution of the TSM is discussed, and concluded that the lateral resolution of the displacement current mode is better than one fifth of the tip radius r, e.g., about 1 μm for r=5 μm. Finally, the distance between the tip and plane sample, and the tip radius were experimentally determined by comparing the theoretical relationship between the displacement current and the distance.
Anomalous microwave properties of high-Tc superconducting microstrip antennas around the critical temperature
Authors: Chung Dong-Chul , Han Byoung-Sung , Lee Young Hee , Hyun Ok-Bae , Choi Hyo-Sang
Published: July 2002

We report the peculiar microwave properties of high-Tc superconducting (HTS) microstrip antennas without impedance matching circuits near the critical temperature (Tc), where impedance mismatch is usually expected. The superconducting thin films used in this report were YBa2Cu3O7−x films (Tc=88 K) with various thicknesses deposited on MgO substrates by the pulse laser deposition technique. At the temperature of 86 K the reflection coefficient rapidly drops down below −62.52 dB and the standing wave ratio approaches almost unity 1.0015 at the resonant frequency of 11.812 GHz. This implies that a HTS microstrip antenna somehow tunes itself for the optimal performance at a specific temperature, indicative of possible exemption from impedance matching circuits.
Nonthermal mixing mechanism in a diffusion-cooled hot-electron detector
Authors: Semenov Alex D. , Gol’tsman Gregory N.
Published: July 2002

We present an analysis of a diffusion-cooled hot-electron detector fabricated from clean superconducting material with low transition temperature. The distinctive feature of a clean material, i.e., material with large electron mean free path, is a relatively weak inelastic electron scattering that is not sufficient for the establishment of an elevated thermodynamic electron temperature when the detector is subjected to irradiation. We propose an athermal model of a diffusion-cooled detector that relies on suppression of the superconducting energy gap by the actual dynamic distribution of excess quasiparticles. The resistive state of the device is caused by the electric field penetrating into the superconducting bridge from metal contacts. The dependence of the penetration length on the energy gap delivers the detection mechanism. The sources of the electric noise are equilibrium fluctuations of the number of thermal quasiparticles and frequency dependent shot noise. Using material parameters typical for A1, we evaluate performance of the device in the heterodyne regime at terahertz frequencies. Estimates show that the mixer may have a noise temperature of a few quantum limits and a bandwidth of a few tens of GHz, while the required local oscillator power is in the μW range due to ineffective suppression of the energy gap by quasiparticles with high energies.
Reconstructed (12, 2, 7) Si surface structure observed by scanning tunneling microscopy
Authors: Kawamura Tsutomu , Kanzawa Tomohide , Kojima Shiro , Rozgonyi George A.
Published: July 2002

A series of reconstructed high-index Si surfaces, with angle β from the [111] to [11̄0] direction varying between 10° and 35° (in increments of 5°) were systematically studied by ultrahigh vacuum scanning tunneling microscopy. The reconstruction of the (12, 2, 7) surface with β=30° as a typical example of the series was described in detail. Fourier transforms revealed a (2×2) structure of the terrace, the splitting lattice on the step wall, and diffuse streaks from the kinks. The results were compared with other high-index surfaces of the β series. The reconstructed terrace structures were separated into two groups by the boundary between β=20° and 25°. With regard to the reconstructed step wall structures, a remarkable characteristic pattern of these high-index surfaces was clarified, namely that they are composed of several splitting lattices. The degree of splitting proved to be closely proportional to the effective dangling bond density. The diffuse streaks from the kinks were also observed as a common phenomenon of this series.
Field effect on the impact ionization rate in semiconductors
Authors: Redmer R. , Madureira J. R. , Fitzer N. , Goodnick S. M. , Schattke W. , Schöll E.
Published: July 2002

Impact ionization plays a crucial role for electron transport in semiconductors at high electric fields. We derive appropriate quantum kinetic equations for electron transport in semiconductors within linear response theory. The field-dependent collision integral is evaluated for the process of impact ionization. A known, essentially analytical result is reproduced within the parabolic band approximation [W. Quade et al., Phys. Rev. B 50, 7398 (1994)]. Based on the numerical results for zero field strengths but realistic band structures, a fit formula is proposed for the respective field-dependent impact ionization rate. Explicit results are given for GaAs, Si, GaN, ZnS, and SrS.
Electron microscopy study of interfacial reaction between eutectic SnPb and Cu/Ni(V)/Al thin film metallization
Authors: Liu C. Y. , Tu K. N. , Sheng T. T. , Tung C. H. , Frear D. R. , Elenius P.
Published: July 2002

The wetting reaction between molten eutectic SnPb solder and a sputtered trilayer Cu/Ni(V)/Al thin film metallization was studied using cross-sectional transmission electron microscopy and scanning electron microscopy. Reaction temperatures were from 200 to 240 °C and reaction times ranged from 1 to 40 min. The initial reaction products were Cu6Sn5 and Cu3Sn. The latter transforms to the former after an annealing greater than 1 min at 220 °C. The Cu6Sn5 grains adhere well to the Ni(V) surface and no spalling of them was observed, even after 40 min at 220 °C. This surprising result indicates that the Cu/Ni(V)/Al or Cu6Sn5/Ni(V)/Al is a stable thin film metallization for low temperature eutectic SnPb solder direct chip attachment to organic substrates. Additionally, Kirkendall voids accompanied Cu3Sn formation, yet the voids disappear when the Cu3Sn transforms to Cu6Sn5.
Electron field emission from diamond-like carbon, a correlation with surface modifications
Authors: Grant Peter , Py Christophe , Mößner Claudia , Blais Alexandre , Tran Hue , Gao Mae
Published: July 2002

We report a series of experiments characterizing the emission obtained from near-amorphous carbon deposited by excimer laser ablation. We found that vacuum arc discharge and material transfer are responsible for morphology modifications that greatly enhance emission. When the morphology of the materials are well controlled, we find that our carbon has a work function one half that of silicon.
Theoretical and experimental behavior of monolithically integrated crystalline silicon solar cells
Authors: Keller S. , Scheibenstock S. , Fath P. , Willeke G. , Bucher E.
Published: July 2002

A concept for the fabrication of monolithically integrated silicon solar cells is presented. The concept is based on standard Si wafer technology and does not use thin-film approaches. A key feature is isolation trenches dividing the wafer into several unit solar cells. Due to the imperfect isolation between unit cells defined on the same conductive wafer, some device aspects deviating from an ordinary series connection of solar cells arise. For the theoretical description, a model proposed by Valco et al. [G. J. Valco, V. J. Kapoor, J. C. Evans, Jr., and A. T. Chin, in Proceedings of the 15th IEEE Photovoltaic Specialists Conference, Orlando, FL (1981), p. 187] has been generalized by using a two-diode concept for the unit cells and by weakening the assumption of identical unit cells. The model was used to simulate the cell performance in dependence on light intensity, isolation resistance, cell area, and number of unit cells. As a result, general design rules for these truly monolithically integrated solar cells are given. The theoretical predictions could be partially confirmed by experimental prototypes. The best cell with a total area of 21 cm2 and six unit cells exhibits an open-circuit voltage of 3.43 V and a conversion efficiency of 10.9% under 100 mW/cm2 AM1.5G illumination and standard reporting conditions.
Numerical study of conductance distribution in granular metal films
Authors: Chan Man-Chung , Pakhomov A. B. , Zhang Zhao-Qing
Published: July 2002

We study the shape of distribution F(G) for the conductance G between a point on the surface of a metal–insulator nanocomposite film and the conducting substrate. Random resistor networks with both metallic and tunneling bonds included are used to model nanocomposite films. Our simulation results show explicitly that the shape of F(G) is determined mainly by the connectivity of metal particles and the maximum tunneling distance in the composite. By applying our results to the available experimental data on granular NiFe–SiO2, we find important implications for the understanding of microscopic conduction mechanisms near the metal–insulator transition.
Anisotropic fundamental absorption edge of KTiOPO4 crystals
Authors: Dudelzak A. , Proulx P.-P. , Denks V. , Mürk V. , Nagirnyi V.
Published: July 2002

The fundamental absorption edge of KTiOPO4 (KTP) crystals has been investigated with the polarization spectroscopy technique in the temperature range of 80–300 K. The Urbach rule parameters, E0, α0, σ0, and hω, have been determined for the polarizations along each of the KTPs three crystallographic axes. A direct estimation of the KTP energy gap based on Urbach parameters has been made. A strong anisotropy of the gap has been found. The origin of the lowest energy electron excitations in KTP crystals and the possibility of self-trapping are discussed.
X-ray diffraction investigation of the low temperature thermal expansion of porous silicon
Authors: Faivre C. , Bellet D. , Dolino G.
Published: July 2002

Using high resolution x-ray diffraction, the porous silicon lattice parameter was measured in vacuum, as a function of temperature in the range 90–300 K, showing that the thermal expansion of porous silicon is larger than that of bulk silicon. We then estimate the differential thermal expansion coefficient δα of p+-type porous silicon samples of various porosity (from 60% to 80%). Between 90 and 250 K, δα is constant as a function of the temperature but increases linearly with the sample porosity. The porous silicon thermal expansion is related to the temperature variation of the silicon nanocrystallite surface stress.
Hydrogen plasma passivation of GaAs on Si substrates for solar cell fabrication
Authors: Soga T. , Jimbo T. , Wang G. , Ohtsuka K. , Umeno M.
Published: July 2002

The effects of hydrogen plasma treatment and postannealing on GaAs solar cells on Si substrates have been investigated. It is found that postannealing temperature is an important parameter to obtain GaAs on Si with a long minority carrier lifetime. The minority carrier lifetime is increased and the deep level concentration is decreased by the hydrogen plasma treatment. Even after 450 °C postannealing with the complete recovery of the shallow impurity level, the minority carrier lifetime is still longer and the deep level concentration is lower than those of the as-grown sample. It means that the defects in GaAs on Si are passivated by hydrogen. The efficiency of GaAs-on-Si solar cell (air mass 0, 1 sun) is improved from 16.6% (as-grown) to 18.3% by the hydrogen plasma passivation and to 17.2% by the hydrogen plasma passivation and postannealing at 450 °C.
Optical properties and heterojunction band alignment in fully coherent strain-compensated InAsxP1−x/GayIn1−yP multilayers on InP(001)
Authors: Beaudoin M. , Desjardins P. , Aı̈t-Ouali A. , Brebner J. L. , Yip R. Y.-F. , Marchand H. , Isnard L. , Masut R. A.
Published: July 2002

The low temperature optical absorption spectra of fully coherent strain-compensated InAsxP1−x/GayIn1−yP (0.15⩽x⩽0.27; 0.09⩽y⩽0.13) multilayers grown on InP(001) by organometallic vapor phase epitaxy were measured using a Fourier transform infrared interferometer. All spectra show well-resolved excitonic transitions. The analysis with a solution to the Schrödinger equation in the envelope function formalism using the Bastard–Marzin model allows to determine accurately that the strained conduction band offset δEc equals (776±4)x meV, or 77%±2% of the total strained band gap difference. The heterojunction alignment is of type I for both heavy- and light-hole bands but the light-hole confinement is very shallow. The heavy- and light-hole exciton binding energies obtained from the analysis are ≃9 and 6–7 meV, respectively.
High spatial density nanocrystal formation using thin layer of amorphous Si0.7Ge0.3 deposited on SiO2
Authors: Yoon Tae-Sik , Kwon Jang-Yeon , Lee Dong-Hoon , Kim Ki-Bum , Min Seok-Hong , Chae Dong-Hyuk , Kim Dae Hwan , Lee Jong Duk , Park Byung-Gook , Lee Hwack Joo
Published: July 2002

The process to make nanocrystals with an average size <5 nm and a spatial density >1012/cm2 was proposed using agglomeration and partial oxidation of thin amorphous Si0.7Ge0.3 layer deposited in between the SiO2 layers by low pressure chemical vapor deposition. The reason to use an amorphous layer is to make it possible to deposit a thin continuous layer with a thickness of less than 5 nm. Si0.7Ge0.3 alloy layer was used to control the spatial density of the nanocrystals by using selective oxidation of Si in Si0.7Ge0.3 alloy layer. The single electron memory, similar to a flash type memory device was fabricated using these Si0.7Ge0.3 nanocrystals. The Coulomb blockade effect could be clearly observed at room temperature with a threshold voltage shift of about 2.4 V, which demonstrated the formation of nanocrystals with a high spatial density.
Influence of temperature on magnetism in a quantum confined system
Authors: Qiao Bi , Ruda H. E. , Baihua Qi
Published: July 2002

In this article, we present new formulas to describe the relation between temperature and average spin in a quantum confined system by using a Green’s function approach. We focus on a lattice of quantum dots, each of which confines several electrons on the sites of the confined crystal lattice under the influence of an external magnetic field. Our treatment makes use of the Heisenberg model, subject to a transverse magnetic field. We show that at low temperatures, the temperature dependence of magnetic moment differs from a 3/2 power law, and present a unique signature for temperatures approaching or beyond the phase transition temperature.
Quantum transport in mesoscopic devices: Current conduction in quantum wire structures
Authors: Haque A. , Khondker A. N.
Published: July 2002

A theory based on the Keldysh formalism is developed to study carrier transport in inhomogeneous quantum effects devices that operate at higher temperatures under large applied bias voltages. The scattering rates due to dissipative processes within devices are estimated self-consistently from the nonequilibrium particle density and the density of states. Unlike many existing models, the present model guarantees the conservation of the current and the number of particles in active devices. We have applied our model to study carrier transport in GaAs quantum wire devices and report several interesting results. It is found that a sudden increase in the polar-optical phonon scattering rates may result in a negative current at some critical energies when the bias voltage is positive. At low temperatures, the conductance of quantum wires shows quantized steps as a function of the applied bias voltage. Moreover, a negative differential conductance (NDC) is observed in the current–voltage characteristics of devices containing a single tunnel barrier. Such NDC disappears in the presence of strong inelastic scattering. Our results show that it is not possible to simulate many of the novel transport effects without explicitly incorporating the appropriate energy and the position dependences of the scattering rates.
Size effects on ferroelectricity of ultrafine particles of PbTiO3
Authors: Jiang B. , Peng J. L. , Bursill L. A. , Zhong W. L.
Published: July 2002

High resolution transmission electron microscopy is used to image the nanostructure of ultrafine ferroelectric lead titanate particles ranging from 20 to 2000 nm in diameter. The crystal structure, surface morphology, domain–wall structure, as well as surface reconstruction under a 400 KeV electron beam are studied. High resolution images and selected area diffraction patterns showed that all the particles had tetragonal structure; the c/a ratio and domain size both decreased with decreasing particle size and the particles became monodomain when their diameter was less than 20 nm. A domain wall width of 14 Å was deduced from strain contrast shown by 90° domain walls. There is no evidence of amorphous surface layers; however surface reconstruction of lead titanate particles under the electron beam irradiation may produce small particles identified as face-centred PbO. The relationship between the physical properties and the observed nanostructures is discussed.
Generation of Frenkel defects in heavily arsenic doped silicon: A first-principles study
Authors: Xie Jianjun , Chen S. P.
Published: July 2002

The generation of Frenkel defects (a self-interstitial and a vacancy) in heavily As doped Si is investigated theoretically based on first-principles total energy calculations. We find that it is much easier to generate a self-interstitial and a vacancy close to substitutional As atoms than in pure Si, due to the lower energy cost. The As atom binds strongly with the vacancy, but does not bind with Si self-interstitial and other As atoms. We have considered several different reactions such as Si5→Si4V+I, AsSi4→AsSi3V+I, As2Si3→As2Si2V+I, As3Si2→As3SiV+I, and As4Si→As4V+I. The theoretical results are in good agreement with experimental observations.
Amorphous carbon films deposited from carbon ions extracted from a discharge in fullerene vapor
Authors: Maiken E. B. , Taborek P.
Published: July 2002

Amorphous carbon (a-C) films with physical properties ranging from diamond like to graphite like were deposited at rates of 1 μm/h under 10−6 Torr vacuum by extracting a molecular carbon ion beam from a pure carbon discharge in fullerene vapor. With ion energies fixed near 300 eV, substrate temperature and beam neutralization were the main process parameters controlling film properties. a-C stress and density increased with decreasing deposition temperature, ranging to greater than 3 GPa and 2.9 g/cm3, respectively. Room temperature conductivity ranged from 10 to 10−11(Ω cm)−1, with cool substrates and a well neutralized ion beam producing the most insulating films. a-C conductivity increased irreversibly after imposition of electric fields exceeding 108 V/m through the film planes. The high conductivities of films deposited without a beam neutralizer are attributed to phase changes resulting from dielectric breakdown driven by electric fields induced by surface charging. Conductivity data obtained for films deposited at different temperatures are discussed in the context of activated conduction, variable range hopping, and multiphonon assisted hopping transport processes.
Tunnel magnetoresistance versus micromagnetism in magnetic tunnel junctions
Authors: Tiusan C. , Dimopoulos T. , Ounadjela K. , Hehn M. , van den Berg H. A. M. , da Costa V. , Henry Y.
Published: July 2002

The impact of the micromagnetic configuration within the ferromagnetic layers on transport properties of hard/soft magnetic tunnel junctions is presented. An artificial ferrimagnetic (AFi) trilayer structure is used as a magnetically hard subsystem. Fluctuations in magnetization in the AFi affect the resistance of the tunnel junctions and are fully reflected in the shape and amplitude of the tunnel magnetoresistance signal.
Effects of laser fluence, size, and shape of the laser focal spot in pulsed laser deposition using a multielemental target
Authors: Tyunina M. , Leppävuori S.
Published: July 2002

Two-dimensional distributions of thickness and of composition of the deposit produced by the room temperature pulsed laser ablation of lead zirconate titanate in vacuum were studied experimentally as a function of laser fluence, of size, and of elongation of the rectangular laser focal spot. The flip over and the elliptical shape of the deposit were observed. Increase in laser fluence, increase in elongation, and decrease in size of the spot resulted in a stronger broadening of the thickness profiles. The deposit was lead deficient, with the lead profiles “inverse” to the thickness profiles. Excess and/or nominal content of zirconium and of titanium were observed with the profiles resembling those of the thickness. The thickness profiles were in general formal agreement with the model of adiabatic expansion of the monoelemental plume. An additional broadening of the profiles was ascribed to the spatial distribution of the composition in the deposit. The behavior of the composition was qualitatively analyzed in terms of sorption of ablated species at the substrate. Good agreement between experimental observations and the conclusions of the sorption analysis suggests a determining role of the plume-substrate interaction in the deposition process.
Luminescence probing of crystallization in a polymer film
Authors: Granlund T. , Pettersson L. A. A. , Andersson M. R. , Inganäs O.
Published: July 2002

We report studies of a thin film multilayer stack including a highly emissive substituted polythiophene, poly[3-(2,5-dioctylphenyl)thiophene]. Analysis of the photoluminescence spectra revealed an inhomogeneous polymer film. X-ray diffraction studies attribute the existence of an inhomogeneous film as originating from crystallization of the polymer. We used the interference effect of light to detect the region of crystallization in the film. Photoluminescence and absorption were redshifted upon crystallization and displayed an enhanced vibronic structure. Comparison between calculated and measured photoluminescence shows that the crystallization starts from the top of the film and not from the supporting substrate.
Optical nonlinearity of water in oil microemulsion near percolation
Authors: Vicari L.
Published: July 2002

Anomalous strong nonlinear optical behavior of a heterogeneous two-component system is experimentally demonstrated and briefly discussed. The material is a three component microemulsion of water microspheres coated by a monomolecular layer of sodium-bis-di-ethylsulfosuccinate (AOT) immersed in an aliphatic oil (Decane): the three-component Water/AOT/Decane. Traditional nonlinear optics mechanisms, like thermal blooming, thermal diffusion, and electrostriction are shown to be insufficient to account for this nonlinearity. Both its amount and its dynamic require the consideration of a new effect. We retain anisotropic cluster formation and consequent rotational and diffusive dynamic as the main reasons for the new phenomenon.
Kinetic, crystallographic, and optical studies of GaN and AlxGa1−xN thin films grown on Si(111) by pulsed reactive crossed-beam laser ablation using liquid alloys and N2 or NH3
Authors: Willmott P. R. , Antoni F. , Döbeli M.
Published: July 2002

High quality GaN and AlxGa1−xN films have been grown by reactive crossed-beam pulsed laser deposition at 248 nm (KrF) using liquid Al–Ga alloy targets and a synchronous gas pulse on 7×7 reconstructed Si(111) substrates. The growth kinetics and their effect on the optical properties were studied in detail. The growth rates were lower and the optical properties were superior for GaN films grown with N2 than when NH3 was used as the nitriding source. The ratio of Ga to Al in the alloy was reproduced in AlxGa1−xN films when using NH3, but was in general lower when using N2. These findings can be explained by the lower reaction rate of Ga with N2 and the high vapor pressure of Ga at the adopted substrate temperatures.
Strain relaxation by pit formation in epitaxial SiGe alloy films grown on Si(001)
Authors: Di Gaspare L. , Palange E. , Capellini G. , Evangelisti F.
Published: July 2002

We have investigated the pit formation and evolution in compositionally graded SiGe thick films heteroepitaxially grown on Si(001) surfaces. Besides the pits clearly related to impurities and interface defects, a class of pits intrinsic to the strained-layer growth was found. We demonstrate that a mechanism of self-organization drives the nucleation and the growth evolution of these intrinsic pits. Two stable and one metastable pit shapes have been identified that can be related to the alloy film thickness and pit dimension.
High-velocity waveguide acoustic modes in solids
Authors: Darinskii A. N. , Weihnacht M.
Published: July 2002

It is shown that a layered waveguide can support the propagation of nonattenuated acoustic modes of which the phase velocity exceeds the velocity of both the quasitransverse bulk waves in the medium that surrounds the waveguide. These high-velocity modes appear at a special choice of materials for the external medium and the waveguide. Numerical computations are performed for a number of structures. A comparison is given of numerical results with approximate analytical estimations. A criterion for the existence of high-velocity waveguide modes at small values of the thickness/wavelength ratio is derived.
A fractal analysis of adsorption processes by pillared swelling clays
Authors: Van Damme H. , Fripiat J. J.
Published: July 2002

We have applied the concept of fractal dimension D of solid surfaces, developed by Pfeifer and Avnir [J. Chem. Phys. 79, 3566 (1983)] to adsorption on pillared clays, namely, on layer lattice silicates in which the layers are propped apart by large organic or inorganic cations. D was determined either from the radius dependence of the saturation coverages of various yardstick molecules on the same pillared clay, or, alternatively, from the saturation coverages of one type of yardstick molecule (nitrogen) on various surfaces covered with pillars of increasing size. Both methods yield D≂2, or slightly below 2, within experimental error, in the range ∼4–16.5 Å. This confirms the common assumption that mica-type surfaces are molecularly smooth, and shows that the distribution of pillars in the pillared clays that we examined is close to homogeneity. The relation between D, microporosity, and molecular sieving is briefly discussed.
On the coupling of electron and vibrational energy distributions in H2, N2, and CO post discharges
Authors: Gorse C. , Capitelli M. , Ricard A.
Published: July 2002

A coupled solution of the Boltzmann equation, of the vibrational master equation, and of the plasma chemistry describing the dissociation process has been performed in H2 post discharges in the μs and ms regimes. The results in H2 show that the superelastic vibrational gain tends to compensate both the inelastic and elastic (including rotational) energy losses, thereby yielding a quasistationary situation characterized by an electron ‘‘temperature’’ smaller than the vibrational temperature θ1(Te<θ1). Results for N2 and CO show that the correlation Te∼θ1 does exist only at moderate θ1 values, when the vibrational distribution is mainly concentrated on the first vibrational levels. At higher θ1 values, Te>θ1 in N2 and CO as a result of the deviation of the actual vibrational distributions of these species from the Boltzmann one.
A second order multiconfiguration SCF procedure with optimum convergence
Authors: Werner Hans-Joachim , Knowles Peter J.
Published: July 2002

An MCSCF procedure is described which is based on the direct minimization of an approximate energy expression which is periodic and correct to second order in the changes in the orthonormal orbitals. Within this approximation, the CI coefficients are fully optimized, thereby accounting for the coupling between orbital rotations and CI coefficients to higher order than in previous treatments. Additional transformations among the internal orbitals and their associated one- and two-electron integrals are performed which amounts to treating the rotations among internal orbitals to higher than second order. These extra steps are cheap compared to the four index transformation performed in each iteration, but lead to a remarkable enhancement of convergence and overall efficiency. In all calculations attempted to date, convergence has been achieved in at most three iterations. The energy has been observed to converge better than quadratically from the first iteration even when the initial Hessian matrix has many negative eigenvalues.
Computer experiments on aqueous solutions. VII. Potential energy function for urea dimer and molecular dynamics calculation of 8 mol % aqueous solution of urea
Authors: Tanaka Hideki , Nakanishi Koichiro , Touhara Hidekazu
Published: July 2002

Molecular dynamics calculation (MD) has been carried out for an aqueous solution of urea at 298.15 K and with experimental density value at ordinary pressure by the use of constant temperature technique developed previously. The total number of molecules is 216, of which 17 are urea. The mole fraction of urea in the solution is thus 0.078. For water–water and water–urea interactions, the MCY potential and previously determined potential have been used. A new urea–urea pair potential is determined by ab initio LCAO SCF calculations for more than 750 different dimer configurations with an STO-3G basis set and subsequent multiparameter optimization of the MO data to a 12-6-3-1 potential energy function. The MD calculation is extended up to 76 000 time steps and final 44 000 time steps (17.6 ps) are used to calculate both static and dynamic properties. Among other information, the following results are important and interesting: (1) Urea molecules exhibit appreciable self-association, (2) such association is possible with strong hydrogen bondings and this is in sharp contrast with the association of t-butanol through hydrophobic interaction studied previously, (3) there is no evidence that urea destroys structure of water, and (4) the decrease of self-diffusion coefficient of water in urea solution is appreciably smaller than that in the case of t-butanol solution.
Structural and energetic effects of truncating long ranged interactions in ionic and polar fluids
Authors: Brooks Charles L. , Pettitt B. Montgomery , Karplus Martin
Published: July 2002

The effects of Coulomb potential truncation schemes used in computer simulations of ionic and polar fluids are examined by use of integral equation techniques. A renormalized HNC type equation capable of describing both ionic and polar molecular fluids with truncated interactions is derived and applied to several model systems of interest. Good agreement is found between the integral equation results and Monte Carlo simulations of the same potential for dilute solutions of ions in a dielectric continuum. Very large effects on the distribution functions result from truncation of the electrostatic interaction in dilute systems. Even in comparatively dense systems, unrealistic pair correlations near the cutoff distance result from some of the proposed truncation schemes. The effect of Coulomb potential truncation for a molecular model of pure water is also studied. Significant errors appear in the second neighbor region for commonly used truncation schemes; a simple switching function that zeros the potential and its first derivative yields results closest to the Coulomb potential without truncation.
The experimental vibrational spectra, vibrational assignment, and normal coordinate analysis of thiirane-h4 and -d4 and c
i
s- and t
r
a
n
s-1,2-dideuteriothiirane: A
b i
n
i
t
i
o theoretical IR spectra of thiirane, thiirene, and isotopically substituted derivatives
Authors: Allen W. D. , Bertie John E. , Falk M. Victor , Hess B. A. , Mast George B. , Othen David A. , Schaad L. J. , Schaefer H. F.
Published: July 2002

This paper reports three independent studies. In the first study, the infrared band shapes and relative intensities of gaseous thiirane-d4 (ethylene sulfide-d4, C2D4S), the Raman spectrum of liquid thiirane-d4, and infrared spectra of gaseous cis- and trans-1, 2-dideuteriothiirane, (CHD)2S, are reported for the first time. The vibrational spectra of C2H4S, C2D4S, and some bands of cis-(CHD)2S are assigned from the symmetry analysis, group frequencies, infrared band shapes, and Raman polarization data. The frequencies so assigned are used to derive a modified valence force field, (MVFF), which reproduces them well, allows the remaining fundamental frequencies of cis-(CHD)2S to be found, and allows the spectrum of trans-(CHD)2S to be assigned. The MVFF is then further refined to optimize the fit to the 46 assigned frequencies of the four molecules. Twenty four nonzero force constants fit the 46 frequencies with an average error of 0.4%. The assignment is thus well based and self-consistent. In

the second study, ab initio SCF calculations of optimum geometry, vibrational frequencies, and IR intensities of thiirane, thiirene, and a number of isotopically substituted derivatives are reported for the 6-31G*, 3-21G, and STO 3G bases. The force constants of thiirane from the 6-31G* basis are in good agreement with those of the MVFF when allowance is made for the fact that some were constrained to zero in the MVFF. The potential energy distributions from the ab initio and normal coordinate calculations agree well, with the former confirming some defects in the latter. The 6-31G* force constants multiplied by 0.80 reproduced the 46 observed frequencies with an average error of 1.4%. For thiirene and isotopic derivatives, the 6-31G* IR spectra are in much better agreement with experiment than previous results with smaller bases. In particular, significantly higher frequency C–S stretches are predicted with the 6-31G* basis. Nevertheless, a few discrepancies remain between experiment and the 6-31G* SCF results. In the third study, vibrational frequencies and IR intensities of thiirene and isotopic derivatives were evaluated at the CISD level of theory using a standard DZP basis set. In the DZP-CISD thiirene spectrum, the B1 C–H out-of-plane bend and its position relative to the A1 C–S stretch differ significantly from the 6-31G* SCF results, giving confirmation of the experimental assignments. However, the same DZP-CISD force constants predict that two low-frequency bands of thiirene-d1 are assigned incorrectly. No other significant discrepancies between theory and experiment remain for the thiirene species.
Theory of the interaction of light with large inhomogeneous molecular aggregates. II. Psi-type circular dichroism
Authors: Keller David , Bustamante Carlos
Published: July 2002

A theory of the polymer and salt induced (psi)-type circular dichroism observed in DNA aggregates is presented. Using the main formalism developed in the previous paper to treat the interaction of light and large, dense molecular aggregates, it is shown that the anomalously large signals observed in the circular dichroism of certain molecular aggregates result from: (a) the presence of a long-range chiral structure in the aggregate; (b) delocalization throughout the entire particle of the light-induced excitations in the chromophores. This delocalization and the resulting ‘‘collective response’’ of the chromophores in the aggregate is favored in particles having a three-dimensional packing. It is shown that to describe adequately the internal field in these aggregates, intermediate and radiation coupling mechanisms should be taken into account in addition to the regular dipole–dipole interactions. Furthermore, no dipole approximation in the exponentials of the form eik⋅x are made. It is shown that in these circumstances, one of the circular polarizations of the light can exchange energy more efficiently than the opposite polarization. This gives rise to a circular dichroism signal whose magnitude is proportional to the overall size and long-range chiral nature of the aggregates. The theory is applied to two cases: (1) to the dimer ApA when it is shown (for the case of this small system) to reduce to the classical theories of DeVoe and Tinoco, and (2) for a toroidal aggregate of DNA of 3000 Å diameter with an internal chiral structure, as found by Haynes et al. in polylysine–DNA condensates. Good qualitative agreement with the observed spectra is found. The theory represents the first successful attempt to explain the physical origin of the psi-type CD effect. Useful information regarding the chiral structure of the aggregates can be inferred from the theory.
Electron affinities of the alkali halides and the structure of their negative ions
Authors: Miller Thomas M. , Leopold Doreen G. , Murray Kermit K. , Lineberger W. C.
Published: July 2002

Photoelectron spectra are reported for the MX (X̃ 1Σ+)+e−←MX−(X̃ 2Σ+) transitions of ten alkali halide anions at 488 nm. Adiabatic electron affinities (±0.010 eV) are determined to be 0.593 (LiCl), 0.520 (NaF), 0.727 (NaCl), 0.788 (NaBr), 0.865 (NaI), 0.582 (KCl), 0.642 (KBr), 0.728 (KI), 0.543 (RbCl), and 0.455 eV (CsCl). Fundamental vibrational frequencies, equilibrium bond lengths, and dissociation energies are also reported for the anion 2Σ+ ground states. An observed linear correlation of electron affinities with α/r2 (α=metal atom polarizability) is used to predict the electron affinities of the remaining alkali bromides and iodides, as well as related alkali salts. A simple electrostatic model for the alkali halide anions is also presented which enables the accurate (±0.1 eV) calculation of electron affinities.
A new mechanism for K promotion of surface reactions: N2 on K-precovered Fe(111)
Authors: Whitman L. J. , Bartosch C. E. , Ho W.
Published: July 2002

The adsorption of N2 on K-precovered Fe(111) at 74 K has been studied with low energy electron diffraction (LEED), thermal desorption spectroscopy (TDS), and high resolution electron energy loss spectroscopy (HREELS). The presence of low precoverages of K (<1.4×1014 cm−2) dramatically enhances the sticking coefficient of α1-N2, the π-bonded precursor to dissociation, and causes an increase in the maximum population of this species. We conclude that the effects of K on this system are primarily mediated by long range interactions; we have modeled the nonlocal K-induced changes of the adsorption and desorption of N2 for temperatures ≤430 K and found that by slightly decreasing the γ-N2 adsorption energy and increasing the α1-N2 adsorption energy we can quantitatively account for both the increase of the α1-N2 sticking coefficient at 74 K and the increase of the dissociative sticking coefficient at 430 K previously reported by Ertl, Lee, and Weiss [Surf. Sci. 114, 527 (1982)]. The promoted α1 state has an N–N stretch frequency less than 20 cm−1 lower than that of unpromoted α-N2, as expected for a weak long range interaction with K, indicating that the N–N bond is not appreciably perturbed. This is significant, as the catalytic effects of K have been previously attributed to bond weakening in the dissociation precursor. At higher K precoverages, local N2–K interactions dominate, characterized by broad N–N vibrations at 1600–1800 cm−1. The HREELS spectra of H2 and H2O, common vacuum contaminants, are also reported for adsorption on a K-precovered surface at 83 K.
IR spectra of carboxylic acids in the gas phase: A quantitative reinvestigation
Authors: Maréchal Y.
Published: July 2002

FTIR spectra of various isotopic forms of formic and acetic acids in the gas phase have been recorded in the range 500–4000 cm−1. The way for separating spectra due to monomers and to H-bonded cyclic dimers is described. A careful measurement of pressure and a precise control of temperature make it possible to measure intensities related to one molecule for all bands appearing in this region. It allows to measure the effects of H bonds on intensities of all bands. The comparison of intensities of νs (O–H↘⋅⋅⋅O) and νC=O bands between H-bonded and D-bonded dimers is particularly interesting as it confirms the existence of an anomalous isotope effect, which we propose to attribute, after analysis, to a nonadiabatic transfer of intensities between electronic and protonic transitions favored by the particular ring structure of these cyclic dimers (pseudo-Jahn–Teller effect). It might explain why simple double-well potentials have up to now failed to describe experimental results concerning transfers of protons through H bonds and it stresses the role that one may attribute to ring structures in describing dynamical properties of H bonds. The particular bandshape of νs is analyzed using a peeling-off procedure which allows, in a low resolution approximation, to eliminate features due to Fermi resonances. It allows to measure the magnitude of factor group splittings of νs modes, which had been scarcely performed before. Peeled-off spectra then appear as classical spectra of a rapid motion (νs ) modulated by low-frequency intermonomer modes of H bonds. Qualitative attributions of their submaxima is given in terms of a transition in νs accompanied by transitions of intermonomer modes. Stretching as well as bending intermonomer modes are shown to have an active part in the modulation of νs.
Dynamic disorder in methylammoniumtrihalogenoplumbates (II) observed by millimeter-wave spectroscopy
Authors: Poglitsch A. , Weber D.
Published: July 2002

The temperature-dependent structure of crystalline methylammoniumtrihalogenoplumbates (II)—CH3NH+3PbX−3 (X=Cl, Br, I)—as determined by x-ray diffraction, is compared with measurements of the temperature-dependent complex permittivity at frequencies of 50–150 GHz. The dielectric measurements reveal a picosecond relaxation process which corresponds to a dynamic disorder of the methylammonium group in the high-temperature phases of the trihalogenoplumbates.
The A 2Π–X 2Σ+ red and B 2Σ+–X 2Σ+ violet systems of the CN radical: Accurate multireference configuration interaction calculations of the radiative transition probabilities
Authors: Knowles Peter J. , Werner Hans-Joachim , Hay P. Jeffrey , Cartwright David C.
Published: July 2002

Potential energy, dipole moment, and electronic transition moment functions have been calculated for the X 2Σ+, A2Π, and B 2Σ+ states of the CN radical using internally contracted CASSCF-CI electronic wave functions (configuration interaction with complete active space self-consistent-field reference functions) and large basis sets. All molecular orbitals which can be formed from the atomic 2s and 2p orbitals were included in the active space. The effect of adding δ orbitals to the active space was found to be small. The largest calculations included up to 987 reference configurations and were equivalent to uncontracted MR-CI calculations with 11.8 million configurations. These calculations are the most accurate that have been carried out to date in terms of the size of the basis set and the treatment of electron correlation effects. Using the theoretical transition moment functions and RKR potential energy functions, radiative lifetimes of the A 2Π and B 2Σ+ states have been derived. The lifetimes of the A state vary between 11.2 μs for v′=0 and 5.3 μs for v′=10. The lifetime of the B, v′=0 state is calculated to be 60.7 ns. These values are estimated to be accurate within 5% and are compared to previous experimental and theoretical data. The most recent experimentally measured lifetimes differ by 20% to 35% from the calculated values, while our results are in close agreement with other theoretical studies. Einstein coefficients of spontaneous emission and oscillator strengths for A–X, B–X, and B–A transitions are tabulated. Dipole moment functions have also been determined, and radiative transition probabilities between vibrational levels of the ground state are presented.
A method for two-electron Gaussian integral and integral derivative evaluation using recurrence relations
Authors: Head-Gordon Martin , Pople John A.
Published: July 2002

An efficient method is presented for evaluating two-electron Cartesian Gaussian integrals, and their first derivatives with respect to nuclear coordinates. It is based on the recurrence relation (RR) of Obara and Saika [J. Chem. Phys. 84, 3963 (1986)], and an additional new RR, which are combined together in a general algorithm applicable to any angular momenta. This algorithm exploits the fact that the new RR can be applied outside contraction loops. It is shown, by floating point operation counts and comparative timings, to be generally superior to existing methods, particularly for basis sets containing d functions.
Infrared spectroscopy of carbo-ions. IV. TheA 2Πu–X 2Σ+gelectronic transition of C−2
Authors: Rehfuss Brent D. , Liu Di-Jia , Dinelli Bianca M. , Jagod Mary-Frances , Ho Wing C. , Crofton Mark W. , Oka Takeshi
Published: July 2002

The infrared spectrum of the A 2Πu←X 2∑+g electronic transition of C−2 has been observed under high resolution and analyzed. Three bands (v′←v)=(0,0), (1,1) and (0,1) have been observed; the first two bands were observed by using the difference laser frequency system in the frequency range of 3960–3780 cm−1 and the last by using a diode laser in the frequency range of 2210–2120 cm−1. A gas mixture of 50 mTorr of acetylene and 7 Torr of He was used for the ac discharge in an air-cooled and a water-cooled multiple inlet–outlet discharge tube. The simplicity of the optimum gas mixture suggested that C−2 is produced directly by simple dissociative electron attachment of acetylene. Altogether 103 absorption lines have been observed and accurately measured. Most of them are P, Q, and R form branches of allowed F1↔F1, F2↔F2 transitions although some forbidden F1↔F2 transitions and O and S transitions have also been measured. The hot bands (1,1) and (0,1) have been observed with intensity which is less than that for the (0,0) by a factor of only 2 or 3, in spite of the fact that the Franck–Condon factors of these transitions are lower than that for the (0,0) by 2.0 and 3.2, respectively, indicating that the vibrational temperature of C−2 in the He plasma is extremely high. All observed transitions are simultaneously fit to the formula for a 2Πu←2∑+g transition. The formulation of Brown and Watson has been used for the Hamiltonian for the 2Πu state. Molecular constants for C−2 in the two electronic states have been determined from the least-squares fitting. The constants are compared with those of other species isoelectronic to C−2. A short discussion is given about the astrophysical implication of the spectrum.