bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotational temperature measurements of excited and ground states of C2(d 3Πg−a 3Πu) transition in a H2/CH4 915 MHz microwave pulsed plasma
Authors: Duten Xavier , Rousseau Antoine , Gicquel Alix , Leprince Philippe
Published: July 2002

The rotational temperature of a low energy (0.09 eV), C2 Swan band state (a 3Πu), obtained by white light absorption, is compared to the rotational temperatures of three electronic excited states [C2(d 3Πg),CH(A2Δ) and CN(B 2Σ+)] in a high power, H2/CH4 microwave plasma used for diamond deposition. All temperatures are measured at 50 mbar, and both continuous (as a function of microwave power) and pulsed (as a function of time after the pulse) modes of operation are investigated. The rotational temperature of C2’s excited state is found to be higher than that of the low energy state (assumed equal to the gas temperature), indicating that the excitation of C2 is to a large extent the result of chemical reactions (chemiluminescence) rather than electronic excitation. The rotational temperatures of CH(A 2Δ) and CN(B 2Σ+) excited states are also higher than that for C2’s low energy state temperature.
Dynamic electromagnetic response of three-dimensional Josephson junction arrays
Authors: Oppenländer J. , Häussler Ch. , Schopohl N.
Published: July 2002

We present a theoretical study on the dynamical properties of three-dimensional arrays of Josephson junctions. Our results indicate that such superconducting networks represent highly sensitive three-dimensional superconducting quantum interference devices (3D SQUIDs) having some major advantages in comparison with conventional planar SQUIDs. The voltage response function of 3D SQUIDs is directly related to the vector character of external electromagnetic fields. The theory developed here allows the three-dimensional reconstruction of a detected external field including phase information about the field variables. Applications include the design of magnetometers, gradiometers, and particle detectors.
Origin of nonlinear surface impedance and intermodulation distortion in YBa2Cu3O7−x microstrip resonator
Authors: Hoshizaki H. , Sakakibara N. , Ueno Y.
Published: July 2002

We present measurements of the nonlinear surface impedance Zs and the third-order intermodulation distortion (IMD) of a YBa2Cu3O7−x microstrip resonator, which is initially in the as-processed state and subsequently annealed at 450–600 °C. Zs of the as-processed sample in the high-dissipated-power range is explained in terms of the vortices in weak links. In the low-dissipated-power range, outputs of IMD are changed by annealing the resonator, and the minimum IMD generation is obtained with a sample annealed at 500 °C. By comparing the IMD generation of the samples with the crystalline structure measured by x-ray diffraction and transmission electron microscopy, it is shown that IMD of the YBa2Cu3O7−x film is generated at lattice distortions of the a−b plane in grains and grain boundaries.
Device model investigation of bilayer organic light emitting diodes
Authors: Crone B. K. , Davids P. S. , Campbell I. H. , Smith D. L.
Published: July 2002

Organic materials that have desirable luminescence properties, such as a favorable emission spectrum and high luminescence efficiency, are not necessarily suitable for single layer organic light-emitting diodes (LEDs) because the material may have unequal carrier mobilities or contact limited injection properties. As a result, single layer LEDs made from such organic materials are inefficient. In this article, we present device model calculations of single layer and bilayer organic LED characteristics that demonstrate the improvements in device performance that can occur in bilayer devices. We first consider an organic material where the mobilities of the electrons and holes are significantly different. The role of the bilayer structure in this case is to move the recombination away from the electrode that injects the low mobility carrier. We then consider an organic material with equal electron and hole mobilities but where it is not possible to make a good contact for one carrier type, say electrons. The role of a bilayer structure in this case is to prevent the holes from traversing the device without recombining. In both cases, single layer device limitations can be overcome by employing a two organic layer structure. The results are discussed using the calculated spatial variation of the carrier densities, electric field, and recombination rate density in the structures.
Thermal degradation of spin valve multilayers caused by Mn migration
Authors: Takiguchi Masafumi , Ishii Satoru , Makino Eiji , Okabe Akihiko
Published: July 2002

We have investigated the thermal degradation of spin valve multilayers caused by annealing at 250–300 °C, and determined that it significantly depended on selection of antiferromagnetic layers. Spin valves with a CuAu-I type ordered PtMn antiferromagnetic layer exhibited better thermal stability than those with a disordered Rh17–Mn83 or an Ir20–Mn80 layer. Comparing the magnetoresistance ratio dR/R of those spin valves after annealing at 280 °C for 5 h, that of PtMn spin valves was more than 60% higher than the others, even though the same stack of layers was used, except the antiferromagnetic layer. Such an excellent thermal stability of PtMn spin valves was attributed to less Mn migration over the existing layers after annealing. This indicated that Mn impurities in ferromagnetic layers such as Co90–Fe10 or Ni81–Fe19 deleteriously affected the spin-dependent scattering of conduction electrons.
Zero-field birefringence in magnetic fluids: Temperature, particle size, and concentration dependence
Authors: Bakuzis A. F. , Da Silva M. F. , Morais P. C. , Olavo L. S. F. , Skeff Neto K.
Published: July 2002

The temperature, particle size, and particle concentration dependence of the zero-field birefringence were investigated using γ-Fe2O3 and MnFe2O4 ionic magnetic fluids in the range of 290–350 K. Upon heating a sample from below its characteristic temperature Tc, which depends upon the particle size and particle concentration, the zero-field birefringence goes critically down to zero. The experimental data are successfully explained when the following two points are considered in the model describing magnetic birefringence in magnetic fluids. First, dimers rather than monomers are responsible for the zero-field birefringence. Second, thermal disruption of the dimer structure follows a critical behavior. A theory for the zero-field birefringence is developed using classical statistics to evaluate the orientational order tensor component Szz. Finally, the zero birefringence signal at zero field, as found in magnetic fluids containing surfactant on the particle surface, is discussed within the dimer model.
Sweep rate-dependent magnetization reversal in epitaxial Fe/GaAs(001) and Fe/InAs(001) thin films
Authors: Lee W. Y. , Xu Y. B. , Gardiner S. M. , Bland J. A. C. , Choi B. C.
Published: July 2002

We present the magnetization reversal dynamics of epitaxial Fe thin films grown on GaAs(001) and InAs(001) studied as a function of field sweep rate in the range 0.01–160 kOe/s using magneto-optic Kerr effect. For 55 and 250 Å Fe/GaAs(001), we find that the hysteresis loop area A follows the scaling relation A∝Ḣα with α=0.03–0.05 at low sweep rates and 0.33–0.40 at high sweep rates. For the 150 Å Fe/InAs(001) film, α is found to be ∼0.02 at low sweep rates and ∼0.17 at high sweep rates. The differing values of α are attributed to a change of the magnetization reversal process with increasing sweep rate.
Intrinsic magnetic behavior of single and multilayered metal-evaporated films
Authors: Bottoni G. , Candolfo D. , Cecchetti A.
Published: July 2002

Metal-evaporated CoNi tapes constituted of one layer and of three layers are studied. In these obliquely deposited films, the angle of the inclined columns on the tape plane conditions the magnetic properties of the medium. The increase of the angle enhances the magnetization, but decreases the coercivity. Multilayered thin films have large magnetization and contemporarily high coercivity. The magnetostatic interactions are positive, or magnetizing, in the triple-layered tape and negative, or demagnetizing, in the single-layered tape. This explains the larger magnetization and remanence of the multilayered tape. The stronger shape anisotropy, due to the larger magnetization, makes smaller the angle of the easy axis on the plane in triple-layered tape, notwithstanding the larger inclination of the columns. The analysis of the magnetization reversal mode indicates that in the triple-layered tape the reversal mechanism is partly coherent rotation, while in the single layered it is more probably incoherent. Such evolution of the basic process of magnetization explains the large coercivity of the multilayered tape.
Dynamic hysteresis behavior in epitaxial spin-valve structures
Authors: Lee W. Y. , Samad A. , Moore T. A. , Bland J. A. C. , Choi B. C.
Published: July 2002

We report the dynamic hysteresis behavior of epitaxial single ferromagnetic fcc NiFe(001), fcc Co(001) layers, and fcc NiFe/Cu/Co(001) spin-valve structures investigated as a function of field sweep rate in the range of 0.01–270 kOe/s using the magneto-optic Kerr effect. The hysteresis loop area A is found to follow the scaling relation A∝Ḣα with α∼0.13 and ∼0.02 at low sweep rates and ∼0.70 and ∼0.30 at high sweep rates for 60 Å NiFe and 40 Å Co single magnetic layer structures, respectively. For the single and double spin valves, the “double-switching” behavior which occurs at low sweep rates transforms to “single switching” at ∼154 and ∼192 kOe/s, respectively. Our results provide direct experimental evidence that the magnetic anisotropy strength affects dynamic hysteresis scaling in ultrathin magnetic films.
Magnetoacoustics in low dimensional spin systems in pulsed magnetic fields
Authors: Wolf B. , Schmidt S. , Schwenk H. , Zherlitsyn S. , Lüthi B.
Published: July 2002

We present results on sound wave propagation in pulsed fields up to 50 T for CsCuCl3 and (VO)2P2O7. Sound velocity and attenuation exhibit anomalies at the field induced spin transitions at low temperatures. In CsCuCl3 we observe step-like anomalies in sound velocity and pronounced peaks in attenuation at the saturation field (34 T at 1.5 K) and broader anomalies at the commensurate-incommensurate transition (14–17 T). In (VO)2P2O7 we observe a strong anomaly at the critical field when the lowest triplet branch crosses the singlet ground state (27 T at 1.6 K). All these experiments give important information on the spin state in high magnetic fields of these low dimensional spin systems and on the spin-phonon coupling.
Thermal stability of α-Fe2O3/CoFe/Ru/CoFe-based spin valves
Authors: Kawawake Yasuhiro , Sugita Yasunari , Satomi Mitsuo , Sakakima Hiroshi
Published: July 2002

Magnetoresistive (MR) properties of α-Fe2O3-based spin valves with and without a CoFe/Ru/CoFe synthetic antiferromagnet (SAF) were studied at elevated temperatures and after the annealing in different conditions. The pinning field of the spin valve consisting of α-Fe2O3(20 nm)/CoFe(2 nm)/Ru(0.7 nm)/CoFe(2 nm)/Cu(2 nm)/ CoFe(1 nm)/NiFe(5 nm)/Ta(3 nm)was larger than 300 Oe at 200 °C, which was about five times as much as that of the spin valve consisting of α-Fe2O3(20 nm)/CoFe(2 nm)/Cu(2 nm)/CoFe(1 nm)/NiFe(5 nm)/ Ta(3 nm). The MR ratio and sheet resistance change in the α-Fe2O3(20 nm)/CoFe(2 nm)/Ru(0.7 nm)/CoFe(2 nm)/Cu(2 nm)/ CoFe(0.5 nm)/NiFe(3 nm)/Ta(3 nm)film were 9.8% and 2.6 Ω/□, respectively, after annealing at 270 °C for 3 h. The simple spin-valve structure with the same composition also showed good thermal stability of MR properties against annealing at 200 °C without an applied magnetic field.
Ion dynamics model for collisionless radio frequency sheaths
Authors: Bose Deepak , Govindan T. R. , Meyyappan M.
Published: July 2002

Full scale reactor model based on fluid equations is widely used to analyze high density plasma reactors. It is well known that the submillimeter scale sheath in front of a biased electrode supporting the wafer is difficult to resolve in numerical simulations, and the common practice is to use results for electric field from some form of analytical sheath model as boundary conditions for full scale reactor simulation. There are several sheath models in the literature ranging from Child’s law to a recent unified sheath model [P. A. Miller and M. E. Riley, J. Appl. Phys. 82, 3689 (1997)]. In the present work, the cold ion fluid equations in the radio frequency sheath are solved numerically to show that the spatiotemporal variation of ion flux inside the sheath, commonly ignored in analytical models, is important in determining the electric field and ion energy at the electrode. Consequently, a semianalytical model that includes the spatiotemporal variation of ion flux is developed for use as boundary condition in reactor simulations. This semianalytical model is shown to yield results for sheath properties in close agreement with numerical solutions.
Defects, structure, and chemistry of InP–GaAs interfaces obtained by wafer bonding
Authors: Sagalowicz L. , Rudra A. , Kapon E. , Hammar M. , Salomonsson F. , Black A. , Jouneau P.-H. , Wipijewski T.
Published: July 2002

We have examined the crystallographic structure of GaAs/InP interfaces obtained by wafer fusion following different procedures. Plan-view and cross-sectional transmission electron microscopy reveal that the interface is not only composed of a regular array of two sets of edge dislocations and is more complex than generally supposed. If a twist is created due to misalignment of the two substrates, the dislocations are not edge dislocations but also have a screw component. Dislocations for which the Burgers vectors have a component normal to the interface are also present. Those dislocations probably result from steps and some of them accommodate the tilt between the two substrates. Inclusions and voids as well as a low number of volume dislocations are present in all the samples. The observed volume dislocation density near the interface lies in the 105–107 cm−2 range and these volume dislocations may be associated with thermal mismatch. The origin of all these defects is discussed.
Permeability and magnetoimpedance in Fe77.5Si7.5B15 wire under torsional stress
Authors: Ryu G. H. , Yu S. C. , Kim C. G. , Nahm I. H. , Yoon S. S.
Published: July 2002

The complex permeability spectra and the magnetoimpedance (MI) are measured as a function of torsional stress and torsion angle in Fe77.5Si7.5B15 amorphous wire. The permeability spectra show irreversible and reversible relaxation with relaxation frequencies of 40 and 300 kHz, respectively. The static permeability resulted from the irreversible magnetization decreases with torsion angle for a negative direction, but increases with a positive angle. The MI profile shows one symmetric peak for the positive torsion angle, which is same MI behavior as in the case of zero torsion. However, there is a hysteresis in the MI profile for negative torsion, that is, there are two asymmetric peaks for decreasing field and one peak for increasing field.
Magnetic force microscopy study of interactions in 100 nm period nanomagnet arrays
Authors: Hwang M. , Abraham M. C. , Savas T. A. , Smith Henry I. , Ram R. J. , Ross C. A.
Published: July 2002

The magnetic behavior of two different 100 nm period arrays of Ni pillars has been characterized using vibrating sample magnetometry and magnetic force microscopy. The samples have similar aspect ratios of approximately two but different particle dimensions, leading to differences in the strength of magnetostatic interactions. The experimental results were compared with a simulation based on an Ising-like simple interaction model.
Magnetic characteristics of Fe–N films prepared by reactive ion beam sputtering with a nitrogen bombardment process
Authors: Iwatsubo S. , Naoe M.
Published: July 2002

Fe–N films were prepared by reactive ion beam sputtering with a nitrogen bombardment at the ratio in arrival rate of nitrogen ions to sputtered iron atoms of 0.1, of which the value was nearly equal to the Fe16N2 composition. The voltage for nitrogen bombardment VA was varied in the range between 80 and 2000 V. The nitrogen content of the Fe–N films was in the range of 8–13 at. %. The phase of the films changed from α′ to α+γ′ at VA higher than 400 V. The saturation magnetization 4πMS of the films took the large value in the range of 22–23.5 kG at VA lower than 200 V. 4πMS with the α+γ′ phase was smaller than pure Fe ones. The films exhibited a large perpendicular magnetic anisotropy field to 3.4 kOe with an increase of VA. After the films of α′ phase prepared at VA of 100 V were postannealed, the films appeared at the γ′ phase from the annealing temperature TA of 400 °C. 4πMS of the films decreased gradually with an increase of TA in the range higher than 300 °C and was smaller than that of pure Fe at TA of 400 °C. These results indicate that the energy of the nitrogen bombardment should impact significantly the phase of the films by the thermal effects of the bombardment energy and the energy below 200 eV may be suitable for preparing the Fe–N films with large 4πMS.
Contributions to switching field in NiO–Co–Cu-based spin valves
Authors: Chopra Harsh Deep , Yang David X. , Chen P. J. , Egelhoff W. F.
Published: July 2002

The present study shows that the often observed and undesirable coercivity enhancement in the free Co layer of spin valves result from magnetostatic coupling between domain walls in the free Co layer and high coercivity NiO-pinned Co layer. The observed escape fields, in excess of intrinsic coercivity of Co single film, required to overcome magnetostatic coupling between domain walls are in agreement with theoretically predicted values.
Looping mechanism in Er-doped fluoroindogallate glasses
Authors: Bell M. J. V. , de Sousa D. F. , Nunes L. A. O.
Published: July 2002

In this work we report a room temperature green upconversion in Er-doped fluoroindogallate glasses, pumped at 833 nm. The rise time and stationary intensity of the S43/2+2H11/2→4I15/2 Er3+ transition as a function of laser intensity were monitored. Some possible pumping mechanisms are discussed in detail, and it is concluded that the main pumping route may be described in terms of the looping mechanism. It is also shown that cross relaxation among neighboring pairs of Er3+ ions is near the threshold value to achieve the photon avalanche region.
Evidence of photo- and electrodarkening of (CdSe)ZnS quantum dot composites
Authors: Rodrı́guez-Viejo J. , Mattoussi H. , Heine J. R. , Kuno M. K. , Michel J. , Bawendi M. G. , Jensen K. F.
Published: July 2002

We present a study of the kinetics of photoluminescence (PL) and cathodoluminescence (CL) degradation of semiconductor quantum dot composites, formed by highly luminescent (CdSe)ZnS core-shell nanocrystals embedded in a ZnS matrix. The photoluminescence and cathodoluminescence spectra indicate that both emissions originate from the same near band-edge state of the nanocrystals. We observe a strong decrease in the PL and CL intensities with time. Photoluminescence experiments carried out at high laser fluences (0.5–10 mJ/cm2 per pulse) show that the PL intensity decay with time depends on the size of the nanocrystals and the nature of the surrounding matrix. For instance, close-packed films showed a much slower decay than composite films. The cathodoluminescence intensity degradation is enhanced at lower temperatures. Partial recoveries of the CL signal have been achieved after thermal annealing at temperatures around 120 °C, which indicates that activation of trapped carriers can be induced by thermal stimulation. We attribute the CL and PL decay in the composite films to photo- and electroionization of the nanocrystals, and subsequent trapping of the ejected electrons in the surrounding semiconductor matrix.
Thermal stress, optical distortion, and birefringence in a heated cylindrical trigonal crystal rod
Authors: Greninger Charles E. , Rodriguez Sergio E.
Published: July 2002

Expressions are developed here for the relationships among the strains in rectangular and polar coordinates. Using these relationships it is a simple matter to convert the thermally induced stresses/strains and their optical effects developed earlier for the rectangular trigonal crystal rod to the cylindrical trigonal crystal rod. For a temperature variation that satisfies T=T(r), these expressions give simple solutions that appear much like expressions developed earlier for isotropic materials except the coefficients are functions of the stiffness and photoelastic matrix elements of the material. An example for a lithium niobate rod is given.
Room temperature study of low temperature grown Al0.3Ga0.7As/GaAs multiple quantum wells by modulation reflectance
Authors: Lai C. Y. , Hsu T. M. , Lin C. L. , Wu C. C. , Lee W. C.
Published: July 2002

We studied the subband energy levels in AlGaAs/GaAs multiple quantum wells grown at different temperatures (200–600 °C) by using the piezoreflectance and photoreflectance in room temperature. Under subsequent 30 s rapid-thermal annealing at different temperatures (600–1000 °C), we observed a large energy blueshift in samples with growth temperature below 400 °C. This blueshift energy may be attributed to the modification of quantum wells caused by gallium vacancy enhanced Al–Ga interdiffusion. The energy blueshifts were analyzed by solving Fick’s second law for Al diffusion in quantum wells, obtaining an effective activation energy of 0.75 eV. We discuss our results using the nonequilibrium diffusion equation and comparing them with other experiments.
The construction of orientation-dependent crystal growth and etch rate functions. Part I. Mathematical and physical aspects
Authors: van Suchtelen J. , van Veenendaal E.
Published: July 2002

For mathematical analysis and computer simulation of the shape evolution of crystals, we need a continuum description of crystal growth or etching, rather than the conventional atomistic description. This allows the mathematical integration of the interface process with other transport steps that are usually also described by continuum equations, like diffusion, viscous flow, and chemical reactions. For this reason we need a function R(n,T,C,p,…): the growth or etch rate as a function of the surface orientation n and of experimental variables such as temperature, composition, pressure, etc. of the parent phase. In this article we describe a logical construction method for such growth or etch rate functions. The virtue of our method is that the n variable covers the full unit sphere, i.e., all minima due to different crystal facets are expressed in the R function. The orientation dependence of the growth or etch rate of interfaces (three dimensional) and of steps on a facet (two dimensional) is described in a way which is logically based on the kink/step motion (KSM) growth model. The building blocks of the growth/etch rate function are the elementary KSM functions, plus a number of constants which are to be determined by a parameter-fitting procedure but do have an obvious physical meaning. For instance, for each face a roughening parameter enters into the function, expressing the effect of the roughening transition for this face. This compares favorably with a Fourier series or spherical harmonics expansion for which the constants that appear have no specific relevance for the growth/etch mechanism. In this article we introduce the mathematical toolbox which is required for the “nonlinear network” formalism and we use this formalism for the construction of growth/etch rate functions. In Part II we work out a practical case and compare a set of accurately measured etch rate data (silicon crystals in concentrated KOH solutions) with a network etch rate function.
Alloy scattering in GaAs/AlGaAs quantum well infrared photodetector
Authors: Fu Y. , Willander M.
Published: July 2002

Surface kinetic processes and postgrowth treatments make the vertical and lateral interdiffusions of the Al atoms across GaAs/AlGaAs heterointerfaces inevitable. The increased degree of the Al interdiffusion has been clearly reflected in the spectra of dark current and photocurrent from postgrowth-treated GaAs/AlGaAs multiple-quantum-well-based infrared photodetectors. The Al diffusion length can be characterized from the cutoff wavelength of the photodetector. It has been shown that by including only the increased density of thermal-excited carriers, the theoretical expectation of the dark current enhancement due to the Al diffusion is more than what was observed experimentally. The discrepancy between carrier-density consideration and measurements about the dark current is explained by further considering the enhanced alloy scattering due to the Al interdiffusion and thus the reduced mobility of carriers at excited states. Furthermore, the reduced carrier mobility explains the decreased photocurrent while theoretically the absorption coefficient increases following the increases of the Al diffusion length.
Characterization of ZrO2/Y2O3 laser ablation plasma in vacuum, oxygen, and argon environments
Authors: Voevodin A. A. , Jones J. G. , Zabinski J. S.
Published: July 2002

Laser ablation deposition of highly oriented yttria stabilized zirconia (YSZ) films is important for various technological applications and depends critically on the selection of background environment, with low pressure oxygen being the most common choice. Here, the spatial-temporal distribution of YSZ plume chemistry, excitation states, and energy was determined for ablations in vacuum, low pressure O2, and low pressure Ar, using fluorescence analyses, element specific imaging techniques, and time-of-flight experiments. It was found that an Ar background considerably promotes excitation and ionization of zirconium during the first 1–3 μs after the laser strike. There is much less zirconium excitation in an O2 background, where a large fraction of atomic oxygen with a broad spatial distribution was found. ZrO and YO molecules were observed in both environments. Their highest concentrations were in the O2 background, where fluorescence from these molecules near the substrate lasted for 2–5 μs. Neutral species in YSZ plumes were fitted to Maxwellian type velocity distributions with a shifted center of mass. Kinetic energies derived from the fitted data were reduced by about a factor of 2 in Ar and O2 backgrounds compared to in vacuum. This was not observed for Zr1+ species, which maintained about 100–120 eV mean kinetic energy nearly independently of the background. The ionization of Zr in the presence of Ar, the high velocity of ionized Zr atoms relative to the rest of the plume, the generation of molecular ZrO, YO, and atomic oxygen in the presence of O2 are potentially important for chemistry and structure control of YSZ films.
Vacuum ultraviolet emission characteristics from He–Ne–Xe gas discharge in an alternating current plasma display panel cell
Authors: Seo Jeong Hyun , Jeong Heui Seob , Lee Joo Yul , Yoon Cha Keun , Kim Joong Kyun , Whang Ki-Woong
Published: July 2002

We measured the time integrated vacuum ultraviolet (VUV) emission spectra of He–Ne–Xe gas mixture from a surface type alternating current (ac) plasma display panel cell. The measured emission lines are the resonance line (147 nm) from Xe*(1s4), the first continuum (150 nm) and the second continuum (173 nm) from Xe dimer excited states. The relative intensities of VUV spectral lines from Xe* and Xe2* are dependent on the He/Ne mixing ratio as well as the Xe partial and total pressure. The intensity of 147 nm VUV increases with the Ne content increase and Xe2* molecular emission increases with the He content increase. Infrared (IR) spectra and the time variation of VUV were measured to explain the reaction pathway and the effect of the mixing ratio of He/Ne on the spectral intensity. A detailed study for the decay time shows that the decay time of 147 nm has two time constants and the radiation of 150 and 173 nm results mainly from Xe*(1s5). The IR spectra shows that the contribution from Xe**(>6 s) to Xe*(1s5) and Xe*(1s4) in He–Xe is different from that of Ne–Xe. The change of IR intensity explains the spectral intensity variations of He–Xe and Ne–Xe discharge.
Rotational- and vibrational-state dependence of transverse relaxation rate constants of OCS
Authors: Matsuo Yukari , Minowa Tatsuya , Komatsu Haruko , Shimizu Tadao
Published: July 2002

The sensitivity of a microwave coherent transient spectrometer has been improved by two orders of magnitude. Rotational transitions in the ground and the vibrational excited states of ν2, ν1, and 2ν2 of OCS are systematically investigated to obtain the transverse relaxation rate constants. The calculation using the Anderson–Tsao–Curnutte theory gives a satisfactory explanation of the observed rotational- and vibrational-state dependence of the rate constants.
On the densities and numbers of rovibronic states of a given symmetry species: Rigid and nonrigid molecules, transition states, and scattering channels
Authors: Quack Martin
Published: July 2002

A simple interpretation of the decomposition of the densities of states in the classical mechanical limit according to the regular representation of the permutation group and—under certain circumstances—the molecular symmetry group is presented. The relationship between recent results on densities of states of a given point group symmetry species and previous results based upon the molecular symmetry group is established. Some restrictions are discussed as well as the definition and application of symmetry numbers for the densities of states and partition functions of nonrigid and rigid molecules and transition states.
Increased power output and resonance effects in a thermal engine driven by a first or second order model reaction
Authors: Escher C. , Kloczkowski A. , Ross J.
Published: July 2002

In the preceding article we show that in a model of a chemically driven thermal engine the dissipation may be decreased and the power output and efficiency may be increased by an appropriate variation of external constraints which couple to nonlinearities in the reaction mechanism. Here we investigate these possibilities, and resonance phenomena, for two cases: an engine driven by a first order reaction and one driven by a second order reaction. The reactions may be exothermic or endothermic. We discuss the properties and mechanisms which lead to an increased power output: the unperturbed stationary state must be sufficiently far from equilibrium that LeChatelier’s principle is not obeyed; high frequency perturbations are favorable in the exothermic case, whereas an increase in power output of an endothermic reaction requires low frequency perturbations. With a linear response analysis we find resonances in power output for both first order and second order reactions when the system approaches a Hopf-type instability. The evaluation of the actual shape of the resonance curves, of course, requires the inclusion of the full nonlinearities which is left to a future numerical investigation.
Reaction path curvature effects in the isomerization of the methoxy radical
Authors: Colwell Susan M. , Handy Nicholas C.
Published: July 2002

The reaction path Hamiltonian is used to investigate the isomerization CH3O→CH2OH, the reaction path and necessary properties being determined by ab initio methods. In this paper a careful analysis of the effects of the curvature of the path on the temperature dependent rate constants is presented; curvature affects the log of these rate constants by about 5%.
Velocity dependence of the cross section for Penning and associative ionization of argon atoms by metastable neon atoms
Authors: Aguilar-Navarro A. , Brunetti B. , Rosi S. , Vecchiocattivi F. , Volpi G. G.
Published: July 2002

Relative cross sections for Penning and associative ionization in Ne*(3P2,0)–Ar collisions have been measured, in a crossed beam experiment, as a function of the collision velocity, in the thermal energy range. The total ionization cross sections have been analyzed, together with other experimental results, obtaining a best fit resonance width function. The analysis of the associative to Penning ionization cross section ratios shows that, in the high collision energy range, the ionization occurs predominantly through the 2Σ1/2 ground state of NeAr+ ion. Some considerations on the role played by the interaction anisotropy in these ionization processes are reported.
A model calculation for the ground state of 4He tetramer
Authors: Elminyawi I. , Levinger J. S.
Published: July 2002

We use the following approximation to the potential between two 4He atoms—V(r)=4420 exp(−r2/1.69)−1302 exp(−r2/2.44) (V in K, r in Å). We use the hyperspherical harmonics expansion, with Fabre’s optimal subset, to solve for the ground state of the 4He tetramer. The Schrödinger equation is transformed into an infinite set of second order coupled differential equations (CDE), which we truncate to M equations. We test our program for solving CDE in two appendices. We find that the energy for our model is less than −4.3 K.
Deuterium spin relaxation and spectral densities in the liquid crystal 4-n-pentyloxybenzylidene-4′-n-heptylaniline-2, 3, 5, 6,-d4
Authors: Dong Ronald Y. , Sridharan K. R.
Published: July 2002

We report on the measurements of the deuterium Zeeman and quadrupolar spin-lattice relaxation times and the deuterium spin–spin relaxation time in the nematic, smectic A and C phases of a ring-deuterated 4-n-pentyloxybenzylidene-4′-n-heptylaniline. The spectral densities of motion J0(0), J1(ω0), and J2(2ω0) at ω0/2π=14.73 MHz were separated from the above relaxation times. A simple ‘‘rotational diffusion in a cone’’ model was used to account for the observed increase in J1(ω0) and the decrease in J2(2ω0) in the nematic phase as the temperature was decreased. The model of rotational diffusion in a restoring potential was also examined. Our data can be understood qualitatively using the strong collision model or semiquantitatively using the cone model. The restricted rotational diffusion in a cone model supports the molecular reorientations being in the intermediate motion regime (ω0τ≂1). Additional information on Jp(ω) such as its frequency dependence are required to check the various assumptions used in the above motional models.
Vibrational nonequilibrium dissociation of Br2 in collisions with Ar and Br atoms
Authors: Itoh H. , Koshi M. , Asaba T. , Matsui H.
Published: July 2002

The collision induced dissociation processes in dilute mixtures of Br2–Ar and Br2–Br are investigated. Vibrational transition rates and dissociation rates from individual vibrational levels are derived from quasiclassical Monte Carlo trajectory calculations where only vibrational nonequilibrium effects are considered. The full transition rate matrix is completed by a moment analysis of the trajectory results. From the steady state solution of vibrational-level master equations, it is found that steady state populations in excited vibrational states are largely depleted but the nonequilibrium dissociation rates are about 30%–75% less than the equilibrium rates. The low activation energy observed in the previous experiments in the dissociation process, Br2+Br→Br+Br+Br cannot be explained by the present analysis.
A model for stirring effects on transitions in bistable chemical systemsa)
Authors: Kumpinsky Enio , Epstein Irving R.
Published: July 2002

Experimental and model studies are reported of nucleation induced transitions between bistable states of a chemical reaction in a stirred tank reactor. In both systems studied, BrO−3–Br−–Mn2+ and ClO2−–I−, the rate of stirring has a marked effect upon the transition in one direction but not in the other. In both cases, good qualitative agreement is found between the experimental results and those of a simple, two-compartment reactor model.
A potential-energy surface study of the 2A1 and low-lying dissociative states of the methoxy radical
Authors: Jackels Charles F.
Published: July 2002

Low-lying bound and dissociative states of the methoxy radical have been studied at C3v conformations using accurate ab initio quantum chemical techniques. A double-zeta quality basis set augmented with polarization and diffuse functions has been used throughout. The Ã 2A1 excited state is found to have an equilibrium conformation with RCO=1.614 Å, RCH=1.091 Å, and ΘHCO=102.6°, and the three a1 harmonic frequencies have been calculated (3239, 1308, and 733 cm−1). The following excitation energies have been estimated: Ã 2A1 ← X̃ 2E vertical excitation, 4.21 eV; Ã 2A1 → X̃ 2E vertical deexcitation, 3.52 eV; and the Ã–X̃ system origin, 3.96 eV. The surface intersections of the Ã 2A1 state with three low-lying dissociative states (4A2, 2A2, and 4E) have been characterized. The threshold for predissociation has been estimated as 4.47 eV, corresponding to the 2A1–4A2 intersection. The rate of methoxy photolysis is estimated to be too small to warrant inclusion in atmospheric models.
Intensities in local mode overtone spectra: Dichloromethane and deuterated dichloromethane
Authors: Mortensen O. Sonnich , Ahmed M. Khalique , Henry Bryan R. , Tarr Allan W.
Published: July 2002

The liquid phase overtone spectrum of CD2Cl2 is measured in the CD-stretching regions corresponding to ΔvCD=2 to 4, and the peaks are assigned in terms of the local mode model. Oscillator strengths are determined from these spectra and from the previously measured overtone spectra of CH2Cl2. A general theoretical description is developed for the overtone intensities in these molecules. The theory considers the functional dependence of the dipole moment on local coordinates, as determined from CNDO calculations and numerical differentiation techniques. Morse oscillator wave functions are used to describe the vibrational state within a symmetrized local mode description, and matrix elements of these wave functions are evaluated over powers of the local CH/CD coordinate. It is evident that any description of overtone intensities in terms of harmonic oscillator wave functions would be totally inadequate. Vibrational mixing of the symmetrized Morse oscillator product states is detemined from a harmonic coupling model. The theory predicts that vibrational mixing is much more important as a source of intensity for combinations than off-diagonal terms in the local coordinate expansion of the electric dipole moment. Significant contributions to overtone intensity arise from terms involving the first, second, and third derivatives of the dipole moment. The linear and quadratic terms always appear with opposite sign. Comparison of the calculated and experimental oscillator strengths reveals that, although the calculated oscillator strengths are too small, they account reasonably well for the observed exponential fall off in overtone intensity with increasing v, and the greater intrinsic overtone intensity of CH2Cl2 as compared to CD2Cl2. The calculations also account for several detailed features of the individual spectra.
The effects of surface geometry and island formation on alkali-promoted surfaces: The coadsorption of CO and K on Ni(110)
Authors: Whitman Lloyd J. , Ho W.
Published: July 2002

The room temperature coadsorption of CO and low coverages of K on Ni(110) has been studied with LEED, thermal desorption spectroscopy, and high resolution electron energy loss spectroscopy (HREELS). The results indicate that two distinct CO species coexist on the surface. A strongly perturbed species is observed that exhibits behavior commonly attributed to alkali-promoted surfaces: a higher thermal desorption temperature and a lower C–O stretch frequency of 1660 cm−1. A very weakly perturbed species is observed that behaves in nearly identical fashion to CO adsorbed on the clean surface. We propose that the appearance of the distinct alkali-promoted species in HREELS is the result of the formation of ordered surface structures of fixed alkali:CO stoichiometry, and suggest that this relationship may be a general one.
Microscopic theory of the transient grating experiment
Authors: Loring Roger F. , Mukamel Shaul
Published: July 2002

We present the first fully microscopic theory of the generation of the transient grating signal from a crystal of interacting molecules. We derive a general expression for the signal, measured at a time immediately after a short probe pulse, in terms of the exciton Green function. This expression agrees with previous calculations of the grating signal, in which the experiment was treated as a diffraction process. It is also shown that the temporal profile of the signal pulse is, in general, different from that of the probe pulse. This result is inconsistent with the usual diffraction picture. Our treatment of this experiment as a transient four-wave mixing process rather than as a diffraction process allows us to derive an expression for the signal in terms of equilibrium correlation functions of the dipole operator. Using this formulation, we discuss the relationship of this experiment to other nonlinear spectroscopic techniques.
Excluded volume effects for polymers in presence of interacting surfaces: Chain conformation renormalization group
Authors: Nemirovsky A. M. , Freed Karl F.
Published: July 2002

The chain conformational space renormalization group method is extended to consider excluded volume effects in polymer chains interacting with surfaces. The general theory is illustrated primarily by considering a system with a single impenetrable flat interface. The presence of boundaries, while breaking the translational invariance of the full-space theory, introduces a number of novel theoretical features into the renormalization group treatment. A parameter δ is introduced to describe the strength of the polymer chain–surface interaction, and previous expansions in powers of δ or δ−1 are not required. We evaluate several moments of the end-vector distribution such as 〈zn〉, 〈‖ρ‖2〉, etc. to first order in the excluded volume. Our work differs essentially from previous studies because the full dependence on the polymer–surface interaction parameter δ is retained to all orders, the crossover dependence on excluded volume is incorporated and the generalized crossover (i.e., excluded volume dependent) exponents are corrected through second order. Previous results, such as power law exponents, scaling forms for distributions, end-vector distributions in the absence of excluded volume, etc., are obtained simply as particular limiting cases upon the values of the excluded volume and the interaction parameter δ.
Coaxial measurement of the translational energy distribution of CS produced in the laser photolysis of CS2 at 193 nm
Authors: McCrary V. R. , Lu R. , Zakheim D. , Russell J. A. , Halpern J. B. , Jackson W. M.
Published: July 2002

Carbon disulfide (CS2) photolysis was investigated in the gas phase using an argon fluoride (ArF) laser at 193 nm. The coaxial time-of-flight (TOF) distributions of CS radicals produced in the photolysis have been measured. Photochemical fragments have been observed with translational energies below 3 kcal/mol. The vibrational distribution of the CS fragments was also probed by laser induced fluorescence (LIF), and these measurements confirm that significant amounts of CS radicals are produced in vibrational levels greater than v″=6. From a computer simulation of the experimental LIF data, a vibrational distribution was also obtained. Vibrational levels up to v″=12 were found to be populated in a bimodal distribution, which peaks at v″=4, and extends to v″=12. There was a significant amount of rotational excitation of nascent CS produced in high vibrational levels of the ground state. The disjoint translational energy and CS vibrational energy distributions can be used to obtain an estimate of the S(3P) to S(1D) ratio of 0.66.
Conformational equilibrium of 1,2-dichloroethane in methylchloride. A Monte Carlo simulation of the differential g
a
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h
e–a
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t
i solvation
Authors: Bigot B. , Costa-Cabral B. J. , Rivail J. L.
Published: July 2002

A Monte Carlo simulation of a solution of 1,2-dichloroethane in methyl chloride has been performed in the NPT ensemble. The solution serves as a model to study the differential solvation of the gauche and anti conformers of a polar solute in a polar solvent. The expected conformational change induced by the solvation in favor of the gauche conformer is well reproduced in the simulation. To explain it, the r−12, r−6, and r−1 contributions of the intermolecular potential functions (TIPS format) to the solute–solvent and solvent–solvent energies have been recorded during the simulation. The mean r−1 contributions are computed to be less than one-third of the sum of the r−12 and r−6 contributions; however, the computed difference of 2±0.5 kcal/mol between the gauche and anti solute–solvent mean energy is totally accounted for by the difference in the r−1 contributions. The analysis of the r−1 energy pair distribution shows that the main part of this difference comes from changes in the interaction of the solute with the nearest solvent neighbors. The gauche conformer shows at one and the same time more attractive and more repulsive interactions than the anti conformer. When the pure liquid, the anti and gauche solutions are compared, it is found that the solvent molecules experience, on the mean, nearly the same solvent–solvent interactions in the three cases.
Vibrational relaxation in liquid hydrogen and deuterium bromide—The influence of self and distinct correlation functions on the isotropic and anisotropic Raman line shape
Authors: Oehme Karl-Ludwig , Rudakoff Georg , Klostermann Klaus
Published: July 2002

Both, the isotropic and anisotropic Raman line shapes of orthobaric liquid hydrogen and deuterium bromide are measured at 295 K and close to the triple point (185 K). Concentration dependent studies in HBr/DBr mixtures allow an experimental separation of the resonant transfer mechanism from other line broadening mechanisms. It is found that the anisotropic spectra are nearly unaffected by isotopic dilution, whereas the isotropic ones show a blue shift and a narrowing with increasing isotopic dilution. The overall contribution of the transfer mechansim to the line broadening becomes most pronounced at the lower temperature. The results are discussed in terms of a recent Raman line shape theory of Bratos and Tarjus.
An MCSCF study of the low-lying states of t
r
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s-butadiene
Authors: Aoyagi Mutsumi , Osamura Yoshihiro , Iwata Suehiro
Published: July 2002

A b initio MCSCF gradient method is applied to explore the potential surfaces of the low-lying excited states of 1, 3-trans-butadiene. The determination of the equilibrium geometries and the force constants shows that there are several local minima for the 1 3Bu (T1), 2 1Ag (S1), and 1 1Bu (S2) states. Since each excited state has a different electronic character, its potential energy surface is complicated with respect to the rotation of C–C double bond and C–C stretching motions. The second 1Ag state is found to have four minima with low symmetry (Ci, C2, and C1). A planar local minimum with C2h symmetry is found on the 1 1Bu surface. The comparable planar minimum was found for the 1 1Bu state of trans-hexatriene. This is the first ab initio confirmation on the experimental analysis for the planar structure of the state. The present full π space MCSCF calculation gives not only reasonable transition energies but also detailed structures for low-lying states.
The planar dumbbell fluid
Authors: Talbot J. , Tildesley D. J.
Published: July 2002

The thermodynamic and structural properties of the planar dumbbell fluid are examined. The Reference Interaction Site Model (RISM) theory in two dimensions is used to calculate the site–site radial distribution functions for a range of elongations and densities. The results are compared with Monte Carlo simulations. This theory also provides a route to the thermodynamic properties. Scaled particle theory, the y expansion of Barboy and Gelbart, and a median potential are also applied to the problem of predicting the thermodynamic properties. The relative merits of the theories are discussed.