bluuebunny/crossref_metadata_2025 huggingface dataset:
Optically induced rotation of dissymmetrically shaped fluorinated polyimide micro-objects in optical traps
Authors: Higurashi E. , Ohguchi O. , Tamamura T. , Ukita H. , Sawada R.
Published: July 2002

Fluorinated polyimide micro-objects (6–7.5 μm cross-sectional radius) fabricated using reactive ion etching have been both optically trapped and simultaneously rotated in both high and low relative–refractive index surrounding media. Symmetrical micro-objects with a low relative–refractive index were optically trapped by exerting optical radiation pressure through their center openings by using a strongly focused trapping laser beam. Micro-objects were both trapped and rotated by the radiation pressure when the horizontal cross sections of these objects showed dissymmetry (that is, not bilateral but rotational symmetry). In the case of micro-objects with a high relative–refractive index, the pressure is exerted on the outer walls. For objects with a low relative–refractive index, the pressure is exerted on the inner walls. The rotation speed versus optical power (typically, 0.4–0.7 rpm/mW) and the axial position of the laser focal point were investigated for high relative–refractive index micro-objects. The optically induced torque generated by a TEM01* (doughnut)-mode laser beam was found to be greater than that generated by a TEM00-mode laser beam.
Mosaic structure in epitaxial thin films having large lattice mismatch
Authors: Srikant V. , Speck J. S. , Clarke D. R.
Published: July 2002

Epitaxial films having a large lattice mismatch with their substrate invariably form a mosaic structure of slightly misoriented sub-grains. The mosaic structure is usually characterized by its x-ray rocking curve on a surface normal reflection but this is limited to the out-of-plane component unless off-axis or transmission experiments are performed. A method is presented by which the in-plane component of the mosaic misorientation can be determined from the rocking curves of substrate normal and off-axis reflections. Results are presented for two crystallographically distinct heteroepitaxial systems, ZnO, AlN, and GaN (wurtzite crystal structure) on c-plane sapphire and MgO (rock salt crystal structure) on (001) GaAs. The differences in the mosaic structure of these films are attributed to the crystallographic nature of their lattice dislocations.
Thermal oxide on CdSe
Authors: Masson D. P. , Lockwood D. J. , Graham M. J.
Published: July 2002

X-ray photoelectron spectroscopy (XPS) has been used to characterize the thin thermal oxide film grown on single crystal CdSe(0001) and polycrystalline CdSe by exposure to O2 (dry air) at 350 °C. SeOx species, where x=2,3, are clearly identified by a 5 eV shift of the Se 3d3/2,5/2 peaks to higher binding energy. A very weak shift to lower binding energy is observed for the Cd peaks. The positions of the Cd and O peaks do not match those found for the known cadmium oxides, CdO and CdO2. Instead, it is proposed that the Cd bound oxygen atoms occupy substitutional Se sites. The presence of Cd bound oxygen can also be inferred from the intensities of the SeOx, Cd, and O peaks. Raman spectroscopy confirms the existence of O in Se substitutional sites. Angle-resolved XPS is used to determine the thickness of the oxide and the relative amount of SeOx and Cd bound oxygen. The XPS data are consistent with an 8–9 Å thick oxide where ∼60% of the oxygen is bound to Se and ∼40 is bound to Cd. The data show that the oxide structure contains two layers; a passivation layer made of the SeOx species and, underneath, a layer containing oxygen in Se substitutional sites.
Evidence for proton motion in the recovery of light-induced degradation in amorphous silicon solar cells
Authors: Carlson D. E. , Rajan K.
Published: July 2002

The light-induced degradation of amorphous silicon solar cells can be reversed by the application of a strong electric field in the dark, and the rate of reversal increases with field strength, temperature, and light intensity. The activation energy for annealing the degradation in the dark is reduced from about 1.34 eV under open circuit conditions to 1.16 eV by applying a strong reverse bias. When the degraded cells are exposed to intense illumination in addition to a strong reverse bias, the activation energy for the recovery of the performance decreases to about 0.77 eV. Both the light-induced degradation and the reversal of the degradation can be explained by a model based on proton motion within a metastable defect complex.
Effect of coumarin on blue light-emitting diodes based on carbazol polymers
Authors: Gautier-Thianche Emmanuelle , Sentein Carole , Lorin André , Denis Christine , Raimond Paul , Nunzi Jean-Michel
Published: July 2002

Light-emitting diodes made of a single spin-coated layer of poly(9-vinylcarbazole) have been doped with coumarin-515 dye and the effect of the dye concentration on emission and electrical characteristics has been studied. Two different regimes have been identified. For low concentrations, the apparent hole injection barrier rises, holes are trapped, and mobility decreases. External quantum efficiency increases with concentration. At concentrations larger than 10 wt % coumarin aggregates, photoluminescence yield drops and electroluminescence yield increases up to 0.1% photons per electron. As a conclusion, using coumarin dye in a single-layer diode improves electron–hole injection and recombination balance more than using an additional hole-blocking layer.
Time-resolved scanning Kerr microscopy of ferromagnetic structures (invited)
Authors: Freeman M. R. , Hiebert W. K. , Stankiewicz A.
Published: July 2002

Time-resolved microscopy enables valuable new measurements of the dynamics of resonance and relaxation in a range of magnetic systems. An overview of the scope of applications to ferromagnetic microstructures is presented. These include observations of ferromagnetic resonance and spatially nonuniform modes of oscillation, studies of magnetization reversal, and characterizations of the speed of magnetic recording devices.
Magnetic relaxation in Ga0.6Mo2S4 spinel
Authors: Taniyama T. , Nakatani I.
Published: July 2002

Magnetization measurements on Ga0.6Mo2S4 cluster compound of modified spinel were performed at temperatures from 2 to 30 K. Temperature irreversibility of the magnetization develops just below the ferromagnetic transition temperature. Relaxation isotherms of the thermoremanent magnetizations can be described by a superposition of a stretched exponential and a constant term, and shows distinctive aging effects. This implies the presence of the orientational spin freezing in the ferromagnetic regime of the Ga0.6Mo2S4.
Ultraviolet photoabsorption measurements in alkali iodide and caesium bromide evaporated films
Authors: Boutboul T. , Breskin A. , Chechik R. , Braem A. , Lion G.
Published: July 2002

Ultraviolet–photon absorption constants were measured for thin CsI, KI, RbI, NaI and CsBr evaporated films, in the 140–200 nm spectral range. The constant measured for CsI is consistent with literature data; the absorption data for the other materials are given here for the first time. The absorption lengths for all materials investigated vary between 10 and 40 nm. Post-evaporation thermal treatment of the films, known to enhance the photoemission properties of CsI, NaI and CsBr films, had no effect on their photoabsorption properties.
Percent dissociation of Cl2 in inductively coupled, chlorine-containing plasmas
Authors: Malyshev M. V. , Donnelly V. M. , Kornblit A. , Ciampa N. A.
Published: July 2002

The percent dissociation of Cl2 was determined for two configurations of a commercial transformer-coupled plasma (TCP) reactor (LAM Research Alliance metal etcher), using Cl2 and BCl3/Cl2 feed gases, during slow etching of SiO2 covered Si wafers. Emission from Cl2 at 305 nm was recorded as a function of TCP source power, along with emission from 1% Ar and Xe, added as part of an equal mixture of the five rare gases. Absolute Cl2 number densities were determined from the Cl2-to-rare gas emission intensity ratios. The Cl2 percent dissociation increases with power, reaching 70% between 1 and 2 mTorr at the highest power (900 W, 0.080 W/cm3). The percent dissociation decreases with increasing pressure between 1 and 10 mTorr. Decreasing the gap between the TCP window and the wafer chuck from 11 to 6.5 cm decreases dissociation at pressures between 0.5 and 2 mTorr, and increases dissociation slightly at 10 mTorr. The percent dissociation as a function of power, and for the most part as a function of pressure and gap, is reproduced by a zero-dimensional model that includes electron-impact dissociation and dissociative attachment of Cl2, and diffusion-controlled recombination of Cl at the walls. Addition of BCl3 to Cl2 increases the percent dissociation of Cl2, most likely due to a passivation of the chamber walls by adsorbed BClx, lowering the Cl-atom recombination coefficient.
Combined transmission electron microscopy and cathodoluminescence studies of degradation in electron-beam-pumped Zn1−xCdxSe/ZnSe blue-green lasers
Authors: Bonard Jean-Marc , Ganière Jean-Daniel , Vanzetti Lia , Paggel Jens J. , Sorba Lucia , Franciosi Alfonso , Hervé Denis , Molva Engin
Published: July 2002

We explored degradation in electron-beam-pumped Zn1−xCdxSe/ZnSe laser structures by combining cathodoluminescence (CL) measurements in a scanning electron microscope with transmission electron microscopy. The rate of degradation, measured as the decrease of the emitted CL intensity under electron bombardment, depends critically on the threading dislocation density and on the strain in the quantum well. Degradation occurs via the formation of dark spot defects, which are related to bombardment-induced networks of dislocation loops in the quantum well. These degradation defects often initiate where threading dislocations cross the quantum well. We propose a self-supporting dislocation climb mechanism activated by nonradiative recombination to explain the formation and propagation of the degradation defects.
Statistics and microphysics of the fracture of glass
Authors: Katz J. I.
Published: July 2002

The tensile strength of fused silica fibers is believed to approach its intrinsic value at low temperature, and modern experiments indicate very small, perhaps unmeasured, intrinsic dispersion in this strength. The application of classical “weakest link” models to this problem in an attempt to determine the number and therefore the nature of the failure sites is considered. If the skewness as well as the dispersion (Weibull modulus) of failure strengths are measured it may be possible to determine both the number of sites and the distribution of their strengths. Extant data are not sufficient, but calculated skewnesses for comparison with future data are presented.
Transport mechanisms in porous silicon
Authors: Ray A. K. , Mabrook M. F. , Nabok A. V. , Brown S.
Published: July 2002

The current transport mechanism through porous silicon (PS) films fabricated from 8 to 12 Ω cm p-type silicon (p-Si) substrates has been investigated using current–voltage I(V) measurements on metal/PS/p-Si/metal devices in the temperature range of 77–300 K. The characteristics for all devices show a rectifying behavior with ideality factor very close to unity. A value of 0.7 eV is obtained for the barrier height at the interface between PS and bulk p-Si at room temperature and the barrier height is found to increase with rising temperature. A band model is proposed in order to explain the observed characteristics.
Polarity sensitive electrooptical response in a nematic liquid crystal–polymer mixture
Authors: Bartolino R. , Scaramuzza N. , Lucchetta D. E. , Barna E. S. , Ionescu A. Th. , Blinov L. M.
Published: July 2002

A polarity sensitive electrooptical effect has been observed in nematic liquid crystal–polymer mixtures where a gradient of the polymer network density is created by strongly absorbed ultraviolet light. The effect is accounted for by a model based upon consideration of the space charge distribution caused by the polymerization process. The effect may be of interest for practical applications in optical shutters and electrically controlled windows.
Electron irradiation effects on the intersubband transitions in InGaAs/AlGaAs multiple quantum wells
Authors: Manasreh M. O. , von Bardeleben H. J. , Mousalitin A. M. , Khokhlov D. R.
Published: July 2002

Intersubband transitions in InGaAs/AlGaAs multiple quantum wells were studied under the influence of electron irradiation using the optical absorption technique. The intensity of the intersubband transition was dramatically decreased in samples irradiated with 2 MeV electron beams and doses higher than 1×1017 cm−2. This reduction was interpreted as being due to the trapping of the two-dimensional electrons gas in the quantum wells by the irradiation-induced defects. The total integrated area of the intersubband transition in irradiated samples was studied as a function of temperature. The results show that two irradiation-induced traps are involved in capturing and then releasing the confined electrons as the temperature is lowered to 77 K and then increased to 300 K.
Thermal analysis of a laser pulse for discrete spot surface transformation hardening
Authors: Woodard Paul R. , Dryden John
Published: July 2002

Conventional laser transformation hardening is achieved by scanning a laser beam over a surface to raise the temperature above the austenitizing temperature and allowing self quenching to form martensite. A new approach, developed at the National Research Council of Canada’s Integrated Manufacturing Technologies Institute, uses discrete pulses to create a series of hardened “spots.” Unfortunately, existing transformation hardening models do not apply to this case due to the traditional assumption of a scanning velocity. For this reason, new predictive tools must be developed. An analytical solution to the axisymmetric heat conduction under a Gaussian energy distribution is presented which can be used to calculate the temperature distribution from a short laser pulse. The method is also used to calculate the size of a transformation hardened spot under the incident energy of a stationary laser.
Analysis of the role of mobility-lifetime products in the performance of amorphous silicon p-i-n solar cells
Authors: Asensi J. M. , Merten J. , Voz C. , Andreu J.
Published: July 2002

An analytical model of an amorphous silicon p-i-n solar cell is presented to describe its photovoltaic behavior under short-circuit conditions. It has been developed from the analysis of numerical simulation results. These results reproduce the experimental illumination dependence of short-circuit resistance, which is the reciprocal slope of the I(V) curve at the short-circuit point. The recombination rate profiles show that recombination in the regions of charged defects near the p-i and i-n interfaces should not be overlooked. Based on the interpretation of the numerical solutions, we deduce analytical expressions for the recombination current and short-circuit resistance. These expressions are given as a function of an effective μτ product, which depends on the intensity of illumination. We also study the effect of surface recombination with simple expressions that describe its influence on current loss and short-circuit resistance.
The effect of as-implanted damage on the microstructure of threading dislocations in MeV implanted silicon
Authors: Bourdelle K. K. , Eaglesham D. J. , Jacobson D. C. , Poate J. M.
Published: July 2002

The dose dependence of as-implanted damage and the density of threading dislocations formed after MeV implants into Si is measured. The role of the damage and amorphization in the evolution of dislocation microstructure is assessed. As-implanted damage is analyzed by Rutherford backscattering spectroscopy and channeling. Defect etching is used to delineate threading dislocations in near-surface regions of annealed (900 °C, 30 min) samples. For a variety of implants with 1.1 μm projected range (600 keV B, 1 MeV P, and 2 MeV As) we observe a sharp onset for formation of threading dislocations with a peak in dislocation density at a dose of about 1×1014 cm−2, this dose depends on the ion mass. With a further increase in dose, the dislocation density decreases. This decrease, however, is drastically different for the different ions: sharp (4–5 orders of magnitude) reduction for P and As implants and slow decline for B implant. The sharp decrease in the density of threading dislocations at higher doses is correlated with the onset of amorphization observed by channeling for P and As implants. Our data for low-temperature implants provide conclusive proof that a reduction in the dislocation density for P and As implants is a result of amorphization.
An analytic model for the prediction of incoherent shaped charge jets
Authors: Kelly R. J. , Curtis J. P. , Cowan K. G.
Published: July 2002

It is now a well recognized fact that the jet from a shaped charge can be overdriven in the sense that the fastest moving particles are not produced as a cohesive mass of material. Rather, the tip material may be produced as a number of discrete particles which possess different nonzero radial components of velocity and hence spread out from the axis of symmetry of the charge. Such a jet is classed as incoherent and when this incoherency occurs the jet’s target penetration capability is invariably degraded. This physical phenomenon is the subject matter of this article. Several experimental results using common shaped charge materials are presented first. An analytic model which predicts the jet speed at the transition point between a coherent and incoherent state is then described. This model is based on the assumption that a stagnant core with circular boundaries exists in the flow region. Further, the flow field is assumed to be compressible with circular streamlines. The Murnaghan equation of state is used to relate the pressure and density in the flow region where the jet is produced. It is postulated that the transition between a coherent and incoherent state occurs when the circular flow becomes wholly supersonic. The critical Mach number for coherency is shown to be approximated to high accuracy by a simple formula depending on the collapse angle of the flow and the incoming flow speed. Excellent agreement between the model predictions and the experimental data is demonstrated.
Patterning of silicon surfaces with noncontact atomic force microscopy: Field-induced formation of nanometer-size water bridges
Authors: Garcı́a Ricardo , Calleja Montserrat , Rohrer Heinrich
Published: July 2002

Nanometer-size water bridges have been used to confine the oxidation of silicon surfaces with a noncontact atomic force microscope. The formation of a water bridge between two surfaces separated by a gap of a few nanometers is driven by the application of an electrical field. Once a liquid bridge is formed, its length and neck diameter can be modified by changing the tip-sample separation. The liquid bridge provides the ionic species and the spatial confinement to pattern Si(100) surfaces in noncontact force microscopy. The method is applied to write arrays of several thousands dots with a periodicity of 40 nm and an average width of 10 nm.
High intensity femtosecond laser deposition of diamond-like carbon thin films
Authors: Qian F. , Craciun V. , Singh R. K. , Dutta S. D. , Pronko P. P.
Published: July 2002

Hydrogen-free diamond-like carbon (DLC) films have been deposited with a 100 fs (FWHM) Ti:sapphire laser beam at intensities I in the 1014–1015 W/cm2 range. The films were studied with scanning probe microscopy, variable angle spectroscopic ellipsometry, Raman spectroscopy, and electron energy loss spectroscopy. DLC films with good scratch resistance, excellent chemical inertness, and high optical transparency in the visible and near infrared range were deposited at room temperature. As the laser intensity was increased from 3×1014 to 6×1015 W/cm2, the films showed an increased surface particle density, a decreased optical transparency (85%→60%), and Tauc band gap (1.4→0.8 eV), as well as a lower sp3 content (60%→50%). The time-of-flight spectra recorded from the laser plume exhibited a double-peak distribution, with a high energy suprathermal ion peak preceding a slower thermal component. The most probable ion kinetic energy showed an I0.55 dependence, increasing from 300 to 2000 eV, when the laser intensity was varied from 3×1014 to 6×1015 W/cm2, while the kinetic energy of suprathermal ions increased from 3 to over 20 keV and showed an I0.33 dependence. These high energy ions are believed to have originated from an electrostatic acceleration field established by suprathermal electrons which were formed by resonant absorption of the intense laser beams.
Reactive magnetron sputtering of CNx films: Ion bombardment effects and process characterization using optical emission spectroscopy
Authors: Vlček Jaroslav , Rusňák Karel , Hájek Václav , Martinů Ludvı́k
Published: July 2002

CN x films were deposited on Si(100) substrates at a substrate temperature of 600 °C using dc magnetron sputtering of a high-purity graphite target in pure nitrogen. The film characteristics were primarily controlled by the pressure, p, (0.05–5 Pa), the discharge current on the magnetron target, Im, (0.5–3 A), and the rf induced negative substrate bias voltage, Ub, (−300 to −1200 V). The films, typically 1–2 μm thick, were found to be amorphous, and they possessed the N/C atomic concentration ratio up to 0.35, the hardness up to 40 GPa, the elastic recovery up to 85%, and good adhesion and promising tribological properties. The energy and flux of ions bombarding the target and the growing films were evaluated on the basis of the discharge characteristics measured for both the dc magnetron discharge and the rf discharge in the deposition zone. Optical emission spectroscopy was used to study the behavior of significant atomic and molecular species, such as N, N2, N+, N2+, CN, C, and C2, near the substrate and in the plasma bulk. The measurements indicated that both dc and rf discharges are mutually coupled. We confirmed that CN radicals and N atoms are the principal precursors for reactive magnetron sputtering of the CNx films. Good correlation between the N/CN concentration ratio in the deposition zone and the N/C atomic ratio in the films was found over a wide range of the investigated process parameters. We have shown that the CNx films have a high hardness only when the energy and flux of the nitrogen ions are sufficiently high for effective incorporation of nitrogen in the layers; such conditions require the pressures lower than 0.5 Pa and the Ub values between −500 and −700 V. It has been proved that the frequently used deposition parameters such as Im, Ub, p, and Ts, do not generally suffice to satisfactorily characterize the film growth process.
Threading dislocation reduction mechanisms in low-temperature-grown GaAs
Authors: Mathis S. K. , Wu X.-H. , Romanov A. E. , Speck J. S.
Published: July 2002

In these studies, we have investigated the role of low-temperature growth in the reduction of threading dislocation (TD) densities in large mismatch heteroepitaxy. Low- and high-temperature (LT) and (HT) GaAs growths on highly mismatched substrates were used to find the mechanism of enhanced TD reduction in LT grown (250 °C) GaAs. LT templates have symmetric (equal) TD subdensities on the {111}A and {111}B planes, whereas HT templates have asymmetric TD subdensities. A model based on TD reactions was applied to the experimental results and confirmed the beneficial role of symmetric TD subdensities in LT GaAs TD reduction. A ductile-to-brittle transition in dislocation behavior was observed at ∼400 °C.
The structural, chemical, and electronic properties of a stable GaS/GaAs interface
Authors: Cao X. A. , Hu H. T. , Dong Y. , Ding X. M. , Hou X. Y.
Published: July 2002

A stable GaS passivating layer was deposited on GaAs using α-Ga2S3 powder as a single-source precursor. Both good crystal quality and clean GaS/GaAs interface were achieved. Electron-energy-loss spectra showed that the sulfide material has a band gap of 3.0 eV. The valence band discontinuity of the heterostructure was determined to be 1.9 eV from a series of ultraviolet photoelectron spectra with increasing deposition thickness. Al/GaS/GaAs metal-insulator-semiconductor structures exhibited typical high frequency capacitor versus voltage (C–V) behavior with very small loop hysteresis. The C–V curves showed no aging after 20 months.
Diagnostics of chlorine inductively coupled plasmas. Measurement of electron temperatures and electron energy distribution functions
Authors: Malyshev M. V. , Donnelly V. M.
Published: July 2002

The goal of the study that begins with this paper is the creation of a base set of parameters (densities and temperatures or energy distributions of all charged and neutral species) in a chlorine transformer-coupled plasma, measured with the same commercial plasma source over an extensive range of pressure and power. Electron temperatures Te and electron energy distribution functions (EEDFs) are reported as a function of pressure (1–20 mTorr) and power (10–1000 W) during slow etching of SiO2-covered Si wafers. Te values are obtained both by trace rare gases optical emission spectroscopy and Langmuir probe methods. EEDFs are obtained with the Langmuir probe. A zero-dimensional (global) model with revised rate coefficients is used to compute Te from both minimum (pressure, total plasma density, gas temperature, and wall recombination coefficient) and maximum (all experimentally measured parameters necessary to calculate Te) sets of input parameters.
Phase transition of Potts model on a frustrated 3D lattice
Authors: Puha I. , Diep H. T.
Published: July 2002

We report here on Monte Carlo simulations of the q-state Potts model on a fully frustrated cubic geometry. Most investigations so far were restricted to the case of Ising, XY, or Heisenberg spins. Nothing is known about the fully frustrated Potts model and on the disorder effects in such systems. The effects of frustration and weak disorder on the q=3 and q=4 fully frustrated Potts system are studied in the present work.
A low energy method for the deposition of boron nitride thin films
Authors: Hobbs K. R. , Coombe R. D.
Published: July 2002

Boron nitride (BN) thin films are deposited via the dissociation of B(N3)3 in a stream of excited N2. Experiments suggest the energy carrier responsible for dissociation of the azide is electronically excited N2(A3Σu+). Films grown on silicon and sapphire substrates at 200–300 °C are reported, with an average growth rate of ∼1.5 nm/min. X-ray photoelectron analyses reveal that the films contain boron and nitrogen with B:N≈1:1. Infrared absorption spectra show the films grown on silicon contain mainly sp2-bonded BN. X-ray diffraction patterns of the films indicate that they are amorphous. The results suggest that the energy released by dissociation of the B(N3)3 parent is available for the formation of nearest neighbor B–N chemical bonds, but not for the formation of long range crystalline structure.
Morphology of liquid microstructures on chemically patterned surfaces
Authors: Darhuber Anton A. , Troian Sandra M. , Miller Scott M. , Wagner Sigurd
Published: July 2002

We study the equilibrium conformations of liquid microstructures on flat but chemically heterogeneous substrates using energy minimization computations. The surface patterns, which establish regions of different surface energy, induce deformations of the liquid–solid contact line. Depending on the geometry, these deformations either promote or impede capillary breakup and bulge formation. The contact angles of the liquid on the hydrophilic and hydrophobic regions, as well as the pattern geometry and volume of liquid deposited, strongly affect the equilibrium shapes. Moreover, due to the small scale of the liquid features, the presence of chemical or topological surface defects significantly influence the final liquid shapes. Preliminary experiments with arrays of parallel hydrophilic strips produce shapes resembling the simulated forms. These encouraging results provide a basis for the development of high resolution lithography by direct wet printing.
Interaction of wide band gap single crystals with 248 nm excimer laser irradiation. VI. The influence of thermal pretreatment on laser desorption of positive ions from a water-containing ionic crystal (CaHPO4⋅2H2O)
Authors: Kawaguchi Y. , Dawes M. L. , Langford S. C. , Dickinson J. T.
Published: July 2002

Morphological changes during dehydration of single crystals containing lattice H2O (hydrates) can be dramatic, leading to surfaces that are rough, polycrystalline, and laden with defects. Previous studies have shown that ionic crystal surfaces containing point defects readily desorb energetic positive ions under UV excimer laser irradiation. In this work, we examine the consequences of exposing thermally pretreated single crystal brushite (CaHPO4⋅2H2O) to 30 ns pulses of 248 nm laser irradiation and show that the surface modifications strongly enhance the emission of Ca+ and other positive ions.
Adiabatic electronic energies and nonadiabatic couplings to all orders for system of three identical nuclei with conical intersectiona)
Authors: Thompson Todd C. , Mead C. Alden
Published: July 2002

One of us has previously developed a technique for obtaining the leading terms in a perturbation expansion of the Born–Oppenheimer energies and derivative couplings for the X3 system near the conical intersection at a C3v configuration. In a preceding paper, these leading terms have been utilized to study various aspects of the adiabatic approximation for both model and real systems of this type. In the present article, this technique is generalized and extended to all orders, yielding rigorous functional forms for both energies and derivative couplings. In particular, the ‘‘nonremovable’’ part of the derivative coupling, which cannot be transformed away by going to a diabatic basis, is explicitly exhibited. Convenient approximations for both removable and nonremovable couplings are also obtained. These should facilitate the estimation of the effect of the different couplings in various situations.
Low temperature measurements of the rotational relaxation in D2 free jets
Authors: Kern Klaus , David Rudolf , Comsa George
Published: July 2002

The rotational relaxation of molecular deuterium in a free jet expansion has been studied between 78 and 473 K by pseudorandom time-of-flight technique. In the temperature range 78–308 K a decrease of the rotational collision number Zrot with increasing temperature is observed as predicted theoretically by Raff and by Rabitz and Lam. Also reported are room temperature data of Zrot for H2 and HD expansions. The number of collisions required to relax H2, D2, and HD at room temperature is in the ratio 10:5:1.
Photoacoustic signal in a cylindrical resonator: Theory and laser experiments for CH4 and C2H6
Authors: Karbach A. , Hess P.
Published: July 2002

The theory of the photoacoustic signal is developed for acoustic resonances in a cylindrical resonator. Measurements were performed to determine the signal strength in the pressure range between 1 and 760 Torr for CH4 and C2H6. For CH4 the optical power absorbed in the cavity is nearly constant but vibrational relaxation varies the signal in the investigated pressure region. C2H6 shows a strong pressure dependence of the optical power absorbed, however, V–R,T relaxation is so fast that in this case no relaxational variation of the signal is observed in the range of ν/p values studied. As expected, the strength of the acoustic signal is proportional to optical power deposition in this case. For CH4, the influence of V–V and V–R,T exchange processes on the photoacoustic signal is studied in detail. The ratio of heat generated by fast V–V processes to heat generated by slow V–R,T transfer is determined. Using the relaxation time 5.2 ns bar obtained by laser-induced fluorescence experiments for double quantum V–V exchange between the stretching levels ν1,ν3 and the bending levels ν2,ν4 a value of 33 ns bar is derived for single quantum V–V exchange between these manifolds.
Computer simulation of molecular liquid mixtures. I. A diatomic Lennard-Jones model mixture for CO2/C2H6
Authors: Fincham D. , Quirke N. , Tildesley D. J.
Published: July 2002

Molecular dynamics simulations of model anisotropic molecules have been carried out which are intended to represent the molecular mixture CO2/C2H6. By paying careful attention to the methodology of the simulation it has been found possible to reduce the statistical and systematic errors to such an extent that the pressure-composition curve can be predicted for the mixture. Note that for the experimental system the difference between the maximum pressure of the mixture and the saturated vapor pressure of the pure components differ by about 10 atm. The model mixture displays an azeotrope in qualitative agreement with experiment. We believe this is the first time azeotropy has been observed by molecular dynamics. Detailed results are presented for the saturated liquid structure as a function of composition.
Quantum studies of inelastic collisions of O2(X 3Σ−g) with He: Polarization effects and collisional propensity rules
Authors: Corey Gregory C. , Alexander Millard H. , Schaefer Joachim
Published: July 2002

We investigate rotationally inelastic cross sections of O2(X 3Σ−g) with He at a collision energy of 27 meV. Theoretical cross sections obtained from close-coupled (CC) calculations are compared with results from the infinite-order sudden (IOS) approximation. Both the CC and IOS fine-structure state-resolved cross sections exhibit a strong ΔN=ΔJ Fi conserving collisional propensity. An analysis of the general expression for state-resolved cross sections in terms of spin-independent tensor opacities clearly establishes, without the introduction of dynamical approximations, the direct connection between this propensity rule and the collisional propensity for the conservation of the orientation of the nuclear rotational angular momentum vector N. In the low-N limit, Fi changing O2–He collisions are much more strongly depolarizing than collisions that conserve the Fi symmetry level. This enhanced collisional depolarization of an initial distribution of the total molecular angular momentum vector J is related to the collisional reorientation of N that must occur in Fi changing transitions.
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o SCF-CI calculation on free base porphin and chlorin; theoretical analysis on intensities of the absorption spectra
Authors: Nagashima Umpei , Takada Toshikazu , Ohno Kimio
Published: July 2002

A b initio self-consistent field and configuration interaction (SCF-CI) calculations with a minimal basis set are carried out on a few lower π→π* states of free base porphin and free base chlorin. Excitation energies and oscillator strength of these states are calculated and are compared with observed values. For the two lowest states of porphin and chlorin, the calculated excitation energies and oscillator strength are in good agreement with the observed ones. In particular, a characteristic of chlorin spectra is reproduced by the calculation. Namely, the oscillator strength of the lowest transition is several times larger than that of porphin but the oscillator strength of the second transition is similar to that of porphin. Such differences of the oscillator strength between chlorin and porphin are explained by an interchange of the order of two highest occupied orbitals and that of the two lowest unoccupied orbitals. Those differences are caused by a change of orbital shapes of chlorin due to the addition of two hydrogen atoms to the porphin skeleton.
Chemical exchange effects in the NMR spectra of rotating solids
Authors: Schmidt Asher , Smith Steven O. , Raleigh Daniel P. , Roberts James E. , Griffin Robert G. , Vega Shimon
Published: July 2002

We present a theoretical analysis based on Floquet theory describing the effect of intermediate rate exchange on line shapes in magic angle sample spinning (MASS) spectra. As a test case, 13C spectra were obtained of dimethyl sulfone (DMS) from 25–55 °C. DMS exhibits an axially symmetric powder pattern at 25 °C which becomes asymmetric due to 180° flips about the twofold axis bisecting the CH3–S–CH3 angle. As the temperature is increased, the principal effect on the MASS spectrum is a pronounced broadening of both the center and sidebands. Calculated spectra fit the experimental spectra well and provide information on the rate constants and the activation energy for the exchange process. These results extend the line shape methods successfully used to study molecular motion in static samples to include high resolution MASS.
Overtone and combination stretching vibrational bands of hydrogen peroxide, ethene, and propadiene
Authors: Halonen Lauri
Published: July 2002

Symmetry allowed Coriolis and H22 operators have been constructed for the OH stretching vibrational states of hydrogen peroxide and for the CH (CD) stretching vibrational states of ethene and propadiene. The behavior of the coefficients of these terms has been investigated at a special local mode limit with no kinetic or potential energy coupling between the bond oscillators and with special constraints on the molecular geometry and the atomic masses. It is found that the Coriolis coefficients disappear but that the H22 coefficients remain important at this limit. For the stretching vibrational overtones and combinations the effects of quartic anharmonic resonance terms, Darling–Dennison resonance terms, on effective Coriolis and H22 constants are investigated. Finally the effects of the tunneling motion of H2O2 on the rotational energy level structure are discussed.
Exact and approximate solutions of the Peierls–Hubbard model
Authors: Schmidt W. , Schreiber M.
Published: July 2002

The exact eigenvalues for the ground state and the excitation spectra were obtained for a system with two electrons on two sites, taking into account the transfer energy, the Coulomb repulsion on the same site, and the intersite correlation energy. The vibrational modes couple linearly to the electronic density. We discuss the interplay of these interactions resulting in different phases of distorted and undistorted eigenstates with corresponding localized and delocalized electronic wave functions and accompanying charge transfer. Several simple approximation schemes generalizable to larger systems are evaluated. The comparison with the exact results shows that their validity depends strongly on the system parameters. The according limits of applicability are discussed.
On the CH bond dipole moment in alkanes
Authors: Lazzeretti Paolo , Zanasi Riccardo , Raynes William T.
Published: July 2002

The Thomas–Reiche–Kuhn sum rule, within the acceleration gauge for the transition dipole moment, is used to partition the total number of electrons in a molecule and to define atomic populations, which can be related to corresponding experimental estimates of atomic polar tensors from IR intensities. As a bond dipole moment can only be defined for diagonal atomic polar tensors, it is shown that the assumption of a C–H bond moment, transferable from molecule to molecule in the alkane series, is physically unreliable. From experimental IR intensities of methane we infer that, for the equilibrium geometry, μC–H =0.339 D, directed C−H+. Accordingly, it is argued that the theoretical bond dipole moments, estimated for the C–H bond in methane via localization procedures of SCF wave functions, are questionable, as they predict the opposite polarity. Finally, a resolution of the electric dipole moment into atomic contributions is suggested.
NMR line shape analysis for two-site exchange in rotating solids
Authors: Schmidt Asher , Vega Shimon
Published: July 2002

The theory for the analysis of magic angle spinning (MASS) NMR spectral line shapes of spin systems in the solid state undergoing two-site chemical exchange is presented. The theoretical approach which is based on the Floquet formalism is valid for the whole range of exchange rates—from slow to fast, as well as for any sample spinning rate. The effect of the exchange on the widths and positions of the center and sidebands is discussed. For fast and slow exchange rates expressions for these parameters are derived using perturbation theory. Spectral line fitting between calculated and experimental spectra can provide information on the rate constants and the activation energies for the exchange processes together with relative structural parameters of the sites involved, thus extending the line shape analysis methods to include MASS.
The libration–vibration bands of the ammonium ion in ammonium tetraphenylborate: Evidence for nearly free rotation
Authors: Roberts Malcolm P. , Clavell-Grunbaum Dolores , Strauss Herbert L.
Published: July 2002

The N–H stretching and H–N–H bending regions of the infrared spectrum of the title compound (ATPB) have been examined with care. The stretching bands show rotation-like structure that we attribute to rotational motion of NH+4 in a very low barrier. The barrier has the symmetry of a tetrahedral molecule in a tetrahedral site and is characterized by the dimensionless parameter β=1. The J=0 and J=1 levels are below the barrier and the J=2 levels are above it. Slow interconversion of nuclear spin species is seen in keeping with the low barrier model. The spectra of the ATPB with some NH3D+, with a high concentration of K+ and with deuterated phenyl groups have been examined, and show spectra in keeping with the model.
The effect of heteroatomic substitutions on the band gap of polyacetylene and polyparaphenylene derivatives
Authors: Lee Yong-Sok , Kertesz Milkos
Published: July 2002

The electronic structures of polyparaphenylene (PPP), polyacetylene (PA), and their derivatives with small energy gaps have been studied by the Hückel and MNDO crystal orbital methods. The effect of nuclear relaxation and heteroatomic substitution on the energy gaps (Eg) have been taken into account by compelete geometry optimization using periodic boundary conditions as opposed to earlier cluster based calculations. Calculations were done on the following polyacetylene derivatives: polypyrrole (PPy), polythiophene (PT), polyisothianaphthene (PITN), poly (5,5’-bithiophene methenyl) (PBTM, 11, X=S, R=H, m1=m2=1), and poly (5,5’-bipyrrole methenyl) (PBPM, 11, X=NH, R=H, m1=m2=1). Energetics and band gaps for the two isomeric forms, the quinoid and aromatic structures of PPy and PT, are discussed and critically compared with previous calculations. Perturbational molecular orbital theory is invoked to explain the narrower Eg for PITN, PBTM, and PBPM, relative to that of PA. Calculations for PPP derivatives {m-polybenzo[b]thiophene (PBT,5b), polybenzo[b,f]thieno[3,4-c]thiophene (PBTT, 5a, X=S), and polybenzo[b,f]pyrrolo[3,4-c]pyrrole (PBPP, 5a, X=NH)} show that the Eg of some of these polymers is substantially smaller than that of PPP. Comments on ways to stabilize structures with desired small energy gaps are made. A correlation of the Eg with heteroatom perturbation and geometrical relaxation is given. It is found that Eg is controlled not by aromatic vs quinoid contributions, but by the geometrical and heteroatomic effects on the frontier orbitals of the polymer.
Dissociative adsorption of alkanes on Ni(100): Comparison with molecular beam results
Authors: Sault Allen G. , Goodman D. Wayne
Published: July 2002

The dissociative adsorption of ethane, propane, and n-butane on Ni(100) has been investigated at pressures of 0.1–0.001 Torr and temperatures between 350 and 500 K. Activation energies for dissociation are found to be 9.5 kcal/mol for ethane, 3.8 kcal/mol for propane, and 3.1 kcal/mol for n-butane. Dissociative sticking probabilities increase with increasing carbon chain length. Comparison of the present results with the results of a recent molecular beam study of alkane dissociation on Ni(100) show very poor agreement. The sticking probabilities measured here are all orders of magnitude higher than those predicted from the molecular beam study. The discrepancy is attributed to the behavior of alkane molecules with very low normal kinetic energies, which have sticking probabilities below the limits of detection of the beam experiment. C2 H6 and C2 D6 have identical dissociative sticking probabilities indicating that quantum tunneling of hydrogen is not an important step in ethane dissociation. Dissociation is proposed to occur via a trapped molecular precursor. The measured alkane dissociation rates compare favorably with hydrogenolysis and steam reforming rates.
Theoretical study of the first and second negative systems of N+2
Authors: Langhoff Stephen R. , Bauschlicher Charles W.
Published: July 2002

The potential curves for the X 2Σ+g, B 2Σ+u, C 2Σ+u, (1) 2Δu, (2) 2Δu, 4Πu, and 2Σ−u states of N+2 are determined using extended Gaussian basis sets and large CASSCF/multireference CI wave functions. The radial dependence of the electronic transition moments for the first and second negative systems of N+2 are determined, and used to compute Einstein coefficients and radiative lifetimes. Based on the calculated crossing points between the C 2Σ+u , 4Πu, and 2Σ−u states, the predissociation in the C 2Σ+u state for v′≥3 appears to first involve a spin–orbit induced crossing to the bound 2Σ−u state, followed by predissociation of this state by the 4Πu state, which crosses it on the outer limb (just above v′=3 of the C state). This is consistent with the accidental predissociation mechanism suggested by Lorquet and Lorquet.
Optimization of a distributed Gaussian basis set using simulated annealing: Application to the adiabatic dynamics of the solvated electron
Authors: Sprik Michiel , Klein Michael L.
Published: July 2002

A molecular dynamics technique is introduced for the simulation of the adiabatic dynamics of an excess electron coupled to a classical many-body system. The instantaneous ground state wave function of the electron is represented by a superposition of distributed Gaussian basis functions, each with equal amplitude. We present generalized equations of motion for the coupled system, which optimize the positions and widths of the Gaussians by simulated annealing. The condition of equal amplitude ensures the aggregation of the Gaussians in regions of finite electron probability density and hence yields a particularly efficient representation of localized ground states. The method is applied to an electron solvated in liquid ammonia and results for equilibrium properties are compared to quantum path integral calculations. New results for the dynamics are discussed in the light of mobility measurements.