bluuebunny/crossref_metadata_2025 huggingface dataset:
Production and magnetotransport properties of CrO2 films
Authors: Ranno L. , Barry A. , Coey J. M. D.
Published: July 2002

CrO 2 films were deposited on Al2O3 and TiO2 by high-pressure decomposition of CrO3. They are metallic, with a residual resistivity ρ0 in the range 0.2–10 μΩ m. The resistivity below Tc varies as ρ0+αT2+βT7/2 as expected for a correlated fully spin-polarized d band. At Tc the resistivity changes slope but the sign remains positive. There is a small negative magnetoresistance (∼−0.5%/ T) in a wide temperature range below Tc. Coercivity of the films varies according to the substrates from 7 mT for Al2O3 (110) to 15 mT for TiO2 (110).
Sr
2
SbMnO
6
:
A new semiconductor ferroelectric
Authors: Foster M. C. , Nielson R. M. , Abrahams S. C.
Published: July 2002

Sr 2 SbMnO 6 is predicted to be ferroelectric with Curie temperature Tc=450(175) K on the basis of a 1990 structure determination. Calorimetry reveals a stepwise heat capacity change at Tc=431(5) K, indicative of a phase transition close to first order. A further small calorimetric anomaly occurs reproducibly at 743(10) K, consistent with an earlier report of a higher-temperature phase transition to cubic symmetry. The dielectric permittivity ε′ increases sharply at Tc=466(10)K in the range 0.1–100 kHz; ε′ rises ∼3 orders of magnitude at 0.1 kHz, ∼1.0 order at 100 kHz, before a stable maximum is reached at T>Tc. The conductivity σ of Sr2SbMnO6 at 295 K is 0.23(5) S m−1, too high for dielectric hysteresis measurement. The linear dependence of ln σ on 1/T between 295 K and Tc in this semiconductor ferroelectric corresponds to a band-gap EG=0.50(2) eV; above Tc, EG increases abruptly to 0.70(2) eV.
Ge
x
Si
1−x
infrared detectors I. Absorption in multiple quantum well and heterojunction internal photoemission structures
Authors: Strong R. , Misra R. , Greve D. W. , Zalm P. C.
Published: July 2002

We report absorption measurements on two types of long-wave infrared detector structures. Both types were grown by ultrahigh vacuum chemical vapor deposition, and were characterized by multiple analytic techniques. In both multiple quantum well (MQW) and heterojunction internal photoemission (HIP) structures, it is found that free-carrier absorption is dominant for normally incident radiation. The measured absorption is fit well by the classical expression for free-carrier absorption, with scattering times of about 10−14 s (MQW) and 5×10−15 s (HIP). The measured absorption is used to evaluate the responsivity that results when all carriers energetically able to surmount the barrier are collected. Based on this analysis, higher responsivity is predicted for HIP detectors, largely because of the greater density of initial states. The responsivity obtained in practice depends upon the photoconductive gain (MQW detectors) or the escape probability (HIP detectors). The escape probability for HIP detectors is measured in Part II.
Structure of the GaAs/InP interface obtained by direct wafer bonding optimised for surface emitting optical devices
Authors: Patriarche G. , Jeannès F. , Oudar J.-L. , Glas F.
Published: July 2002

Wafer bonding is increasingly used to fabricate heterostructures which cannot be obtained by conventional epitaxial growth because they involve highly mismatched materials. We have optimised the GaAs/InP bonding process in order to realise long wavelength microcavity devices combining advantageously high reflectivity AlGaAs Bragg reflectors with an InP-based active material. The bonded interface has been studied by transmission electron microscopy. Three dislocation networks are found at the interface. The first one accommodates both the lattice mismatch and the inevitable twist between the two crystals whereas the second one accommodates the tilt resulting from the slight vicinality of the initial surfaces. Our experiments strongly suggest that the third network forms upon cooling the structure. Between the dislocation lines, the crystal structure is perfectly reconstructed across the interface, with no amorphous materials present, except for cavities which occupy a few percent of the bonded area. The geometry and distribution of these cavities can be studied by imaging the first dislocation network. In light of these results, we discuss the mechanisms operating during the formation of the interface.
Radical assisted metalorganic chemical vapor deposition of CdTe on GaAs and carrier transport mechanism in CdTe/n-GaAs heterojunction
Authors: Niraula Madan , Aoki Toru , Nakanishi Yoichiro , Hatanaka Yoshinori
Published: July 2002

The growth and carrier transport mechanism in CdTe on the GaAs substrate is reported. Epitaxial layers of CdTe were grown on n- and semi-insulating GaAs substrates by the hydrogen radical assisted metalorganic chemical vapor deposition technique at a low pressure. Dimethylcadmium and diethyltelluride were used as the source materials. The growth was carried out in the substrate temperature range of 150–300 °C. The grown films have high resistivity in the order of 107 Ω cm for the entire growth range. Applicability of this heteroepitaxial CdTe layer on n-GaAs as an x-ray detector was then investigated. The carrier transport mechanism of the CdTe/n-GaAs heterojunction was studied by means of current–voltage measurements at different temperatures. The forward current was characterized by multitunneling capture-emission current and space charge limited current. The reverse current was considered as the generation current from the heterojunction interface states through the analysis of capacitance–voltage measurement. It is found that for devices using minority carriers, this heterojunction alone is not useful because of the high concentration of interface states. A suitable modification, like an isotype heterojunction between GaAs and CdTe before forming the p-n junction, seems to be necessary.
Ultrafast wide range all-optical switch using complex refractive-index changes in a composite film of silver and polymer containing photochromic dye
Authors: Sasaki Kyoichi , Nagamura Toshihiko
Published: July 2002

An all-optical ultrafast switch which operates at a wide wavelength range has been demonstrated based on photoinduced changes of real and/or imaginary parts of complex refractive index in a composite thin film of silver and polymer containing photochromic spiropyran dye. Very fast switching ON and OFF of a reading beam within 20 ns in a 400–800 nm range was achieved at the incident angle of guided-wave mode using a photochromic spiropyran-doped polystyrene thin film and a ns pulsed UV or visible laser as a writing beam. These responses were caused by the changes of complex refractive index due to photochromism. No applied power was required to hold the switched state. An image was also written to the probe beam by irradiation through a pattern.
A fallacious argument in the finite time thermodynamics concept of endoreversibility
Authors: Sekulic D. P.
Published: July 2002

Finite time thermodynamics is a well established field of applied thermodynamics. The key assumption of this approach is the validity of the concept of endoreversibility. In this article, a hypothesis is formulated, and subsequently formally proved, that this concept rests on a fallacious argument. The concept of endoreversibility is inherently inconsistent with the postulated set of assumptions because the internal reversibility of a thermal system appears to be contradictory to an existence of external finite area heat exchangers that communicate with the endoreversible internal part across the finite temperature gaps. The irreversibilities contributed by the system components are inherently interconnected. As a consequence, the maximum power efficiency between the given temperature levels TL and TH>TL, as predicted by finite time thermodynamics assuming that the heat input into the system is free to vary, i.e., 1−(TL/TH)1/2, is incorrect. In addition, the magnitude of this figure of merit may be even smaller compared to the efficiency of a real system.
High frequency behavior of soft magnetic wires using the giant magnetoimpedance effect
Authors: Ciureanu P. , Britel M. , Ménard D. , Yelon A. , Akyel C. , Rouabhi M. , Cochrane R. W. , Rudkowski P. , Ström-Olsen J. O.
Published: July 2002

We have investigated the high frequency properties of several amorphous and polycrystalline wires mounted as inner conductors in coaxial lines. A static magnetic field was applied along the wire axis. The impedance spectra of the wires, measured using a network analyzer, show peaks in the real part of the impedance, which shift to higher frequency with the strength of the static field, a behavior typical of ferromagnetic resonance. The theoretical resonance condition predicts a straight line on an f02−H0 plot, where f0 is the resonance frequency and H0 is the resonant field, whose slope depends only on the saturation magnetization, Ms, of the material. All our wires obey this relation, and the values of Ms calculated from the slopes are in good agreement with those measured directly using a vibrating sample magnetometer.
Nickel induced crystallization of amorphous silicon thin films
Authors: Jin Zhonghe , Bhat Gururaj A. , Yeung Milton , Kwok Hoi S. , Wong Man
Published: July 2002

Nickel (Ni) induced crystallization of amorphous silicon (a-Si) has been studied by selective deposition of Ni on a-Si thin films. The a-Si under and near the Ni-covered regions was found to be crystallized after heat treatment at 500 °C from 1 to 90 h. Micro-Auger electron spectroscopy revealed that a large amount of Ni stayed in the region under the original Ni coverage, but no Ni was detected either in the crystallized region next to the Ni coverage or in the amorphous region beyond the front of the laterally crystallized Si. X-ray photoelectron spectroscopy revealed a nonuniform Ni distribution through the depth of the crystallized film under the original Ni coverage. In particular, a Ni concentration peak was found to exist at the interface of the crystallized Si and the buried oxide. It was found that a layer of 5-nm-thick Ni could effectively induce lateral crystallization of over 100 μm of a-Si, but the lateral crystallization rate was found to decrease upon extended heat treatment. Transmission electron microscopy analysis showed that the crystallized film under the Ni coverage was composed of randomly oriented fine grains, while that outside the Ni coverage was mainly composed of large (110)-oriented grains. A unified mechanism is proposed to explain the Ni induced crystallization of a-Si and possible reasons for the reduction in the lateral crystallization rate are discussed.
Planar-type tunnel junctions with Bi2Sr2CaCu2O8+δ high-temperature superconducting electrodes and Bi2Sr2YCu2O8+δ artificial barriers
Authors: Baca E. , Chacón M. , Lopera W. , Gómez M. E. , Prieto P. , Heiras J. , Di Leo R. , Romano P. , Cucolo A. M.
Published: July 2002

We performed a detailed study of the structure and transport properties of Bi2212/22Y2/2212 planar-type tunnel junctions. Both high-temperature superconducting electrodes and semiconducting barriers are highly epitaxial thin films deposited onto SrTiO3 single-crystal (001) substrates. Deposition of the films was carried out by a high oxygen pressure dc-sputtering technique, which produces high-quality epitaxial thin films, as determined by x-ray diffraction, lattice resolution transmission electron microscopy, and Rutherford backscattering. Critical temperatures for the superconducting electrodes of 85 K were determined by transport measurements (ρ and χ versus T). A study of resistivity as a function of temperature of the semiconducting barriers was performed. Clear quasiparticle tunneling indicating a gap structure at about 30–35 mV, a zero-bias peak, as well as linear and flat background at high voltages have been observed. For junctions with very thin barriers weak-link-type behavior was observed. An analysis of the I–V curves for these junctions has been made based on the resistively shunted junction model.
Optical studies of GaInP(ordered)/GaAs and GaInP(ordered)/GaP/GaAs heterostructures
Authors: Kwok S. H. , Yu P. Y. , Zeman J. , Jullian S. , Martinez G. , Uchida K.
Published: July 2002

We report on a detailed optical study of emission from a series of GaInP (ordered)/GaAs heterostructures. Some of these structures contain one or two thin (∼2 nm) layers of GaP between the GaInP and GaAs layers. A so-called “deep emission” band at 1.46 eV is observed in all our samples. However, at high excitation power, an emission above the band gap of GaAs (previously identified as quantum well emission) emerges only in structures where GaP layers are inserted on both sides in between the GaAs well and its GaInP barriers. From the pressure dependence we have identified the deep emission peak as due to donor–acceptor pair transitions at the GaAs/GaInP interface. The insertion of GaP layers between the GaInP (ordered) and GaAs layers helps to suppress the defects which contribute to this deep emission. By applying pressure to the sample which exhibits quantum well emission we have determined its band alignments. We show that the GaP layers form two effective barriers for confining electrons within the GaAs well. However, the magnetic field dependence of the quantum well emission reveals that the electrons form only a quasi-two-dimensional gas inside the GaAs well.
Imprint failures and asymmetric electrical properties induced by thermal processes in epitaxial Bi4Ti3O12 thin films
Authors: Park B. H. , Hyun S. J. , Moon C. R. , Choe Byung-Doo , Lee J. , Kim C. Y. , Jo W. , Noh T. W.
Published: July 2002

Epitaxial Bi4Ti3O12 (BTO) thin films were deposited on MgO(001) substrates using Pt layers as top and bottom electrodes. In spite of the apparently symmetric capacitor structure, polarization–voltage measurements revealed strong imprint failures and current–voltage measurements showed rectifying behaviors. Imprint pulse tests with a 5 V dc bias and post-annealing treatments suggested that the asymmetric behaviors should be due to interfacial states induced by thermal processes. To probe the interfacial states, capacitance–voltage (C–V) measurements were performed. By fitting the C–V data with a model which describes the Pt/BTO/Pt structure as a series circuit composed of three capacitors, built-in voltages at the top and the bottom interfaces could be determined. Difference in the built-in voltages could explain the imprint failures and the rectifying behaviors. The interfacial states in the BTO capacitors were controlled by varying electrode materials.
Supercurrent switching in three- and four-terminal Josephson junctions
Authors: Ilhan H. Tolga , Bagwell Philip F.
Published: July 2002

Control of the Josephson current by varying a gate current has recently been demonstrated in both four-terminal and three-terminal junctions. We show that, when the gates are weakly coupled to the Josephson junction, the Josephson current versus gate current (or versus gate voltage) relation is the same for both the four- and three-terminal geometries. At low temperature, the supercurrent switches abruptly as a function of the gate voltage, but only slowly as a function of the gate current.
Structure and formation process of the glass film on grain-oriented silicon steel using aluminum nitride as an inhibitor
Authors: Fujii Hiroyasu , Yamazaki Shuichi , Masui Hiroaki , Shiozaki Morio , Nagashima Takeo , Kobayashi Hisashi
Published: July 2002

The oxide film formed during secondary recrystallization annealing of grain-oriented 3% Si steel using aluminum nitride as an inhibitor was investigated by x-ray diffraction analysis (XRD), glow discharge optical emission spectrometry (GD-OES), and quantitative chemical analysis (CA) in order to clarify the structure and the formation process. Spinel (MgAl2O4) was identified in addition to forsterite (Mg2SiO4) by XRD. From the consideration on the basis of the element distribution of Mg, Si, Al in the glass film and the quantitative correlation between the amount of spinel and that of forsterite, it was concluded that spinel was formed by the substitution reaction of Si in forsterite film for Al generated by decomposition of AIN.
Three-dimensional growth mechanism of cosmo-mimetic carbon microcoils obtained by chemical vapor deposition
Authors: Motojima Seiji , Chen Quiqin
Published: July 2002

Cosmo-mimetic carbon microcoils with a three-dimensional helical/spiral structure similar to deoxynucleic acid, having coil diameters of 3–6 μm and coil lengths of 5–8 mm could be obtained by the catalytic pyrolysis of acetylene. A three-dimensional growth model of the carbon coils, based on the anisotropy for the carbon deposition among three crystal faces, is proposed. The microcoiling morphology is formed by the rotation of a Ni catalyst grain, from which six fibers grow and coalesce with each other, followed by the formation of two fibers, and these two fibers entwine in the same direction and at the same speed of about one cycle per second around the coil axis.
Electronic structure of fluorescent lamp cathode surfaces: BaO/W(001)
Authors: Almanstötter Jürgen , Fries Torsten , Eberhard Bernd
Published: July 2002

The BaO/W interaction is responsible for the emission properties of barium–oxide coated tungsten electrode coils in fluorescent lamps. The electronic structure of the BaO/W(001) interface is investigated by first-principles calculations within the local-density approximation of the density functional theory using the full-potential linearized augmented plane wave method. Results are presented for the total density of states (DOS), the atom- and orbital-resolved partial DOS and charge density distributions. Partial covalent character in the W–O and W–Ba bonding is shown. The main contribution to chemical bonding is caused by the tungsten d states and the adsorbate valence states, which interestingly also involve barium d states. This leads to a stabilization of the adsorbed configuration, with respect to cathode operation temperatures. The calculated work function is in agreement with experimental data.
Excitation and propagation of negative-potential solitons in an electronegative plasma
Authors: Sheridan T. E. , Lonngren K. E.
Published: July 2002

The excitation, propagation, and interaction of planar negative-potential solitary waves in a plasma containing positive ions, negative ions, and electrons (i.e., an electronegative plasma) are studied using a hybrid simulation with kinetic, particle ions, and Boltzmann electrons. Solitary waves are launched into the plasma when the potential on an initially unbiased electrode is stepped downward. They then propagate self-consistently through the ambient plasma. Results are shown for excitation and propagation as well as for overtaking collisions. During overtaking collisions, the solitons preserve their shape and speed, though they are not necessarily described by the Korteweg–deVries theory. These solitons may provide a useful diagnostic of the negative ion concentration in electronegative plasmas.
Generalized moment expansion for Brownian relaxation processes
Authors: Nadler Walter , Schulten Klaus
Published: July 2002

The generalized moment expansion has previously only been used to provide an effective algorithm for the approximation of the time dependence of observables connected with reactive Brownian processes. We extend this algorithm to describe the relaxation of observables in nonreactive processes. The wide applicability of the method is demonstrated for various examples: equilibrium correlation functions like autocorrelation functions and dynamic structure factors, particle number correlation functions monitoring diffusive redistribution, and barrier crossing problems.
Slow spinodal decomposition in binary liquid mixtures of polymers
Authors: Izumitani Tatsuo , Hashimoto Takeji
Published: July 2002

Isothermal demixing process of binary polymer mixtures of SBR (styrene–butadiene random copolymer) and polybutadiene at deep quench depths was investigated by time-resolved light scattering technique. The results indicated that the systems undergo extremely slow spinodal decomposition of the type as adequately characterized by Cahn’s linearized theory in the early stage and in the small q regime (q≲qmax≂105 cm−1) covered in this experiment where q is wave number of growing fluctuations and qmax is the q value having maximum growth rate. The spinodal decomposition studied in this work was that in the diffusion-control regime, and in the slowest case of the decomposition, the early stage was found to extend up to about 80 min, corresponding to the reduced time τ about 2.7. The shortest reduced time achieved in this experiment is about 0.03.
Natural bond orbital analysis of molecular interactions: Theoretical studies of binary complexes of HF, H2O, NH3, N2, O2, F2, CO, and CO2 with HF, H2O, and NH3
Authors: Reed Alan E. , Weinhold Frank , Curtiss Larry A. , Pochatko David J.
Published: July 2002

The binary complexes of HF, H2O, NH3, N2, O2, F2, CO, and CO2 with HF, H2O, and NH3 have been studied by ab initio molecular orbital theory and natural bond orbital (NBO) analysis. Most of the complexes involving N2, O2, F2, CO, and CO2 are found to have both hydrogen-bonded and non-hydrogen-bonded structures. The NBO analysis provides a consistent picture of the bonding in this entire family of complexes in terms of charge transfer (CT) interactions, showing the close correlation of these interactions with the van der Waals penetration distance and dissociation energy of the complex. Contrary to previous studies based on the Kitaura–Morokuma analysis, we find a clear theoretical distinction between H-bonded and non-H-bonded complexes based on the strength of CT interactions. Charge transfer is generally stronger in H-bonded than in non-H-bonded complexes. It plays an intermediate role in non-H-bonded CO2 complexes which have been studied experimentally. However, the internal rotation barrier (1 kcal mol−1) of the H2O⋅⋅⋅CO2 complex is found to be primarily of electrostatic origin with only a small (π-type) CT contribution. The role of electrostatic interactions, effect of electron correlation, and comparison of theory with experiment are also discussed.
Static dipole polarizabilities of Li2, Na2, and K2
Authors: Müller Wolfgang , Meyer Wilfried
Published: July 2002

Static dipole polarizability functions (α∥ and α⊥) and thermally averaged isotropic polarizabilities (α) of Li2, Na2, and K2 in their electronic ground states have been obtained from all-electron SCF+valence CI calculations which include core polarization effects by use of an effective potential. As a function of internuclear separation, α⊥ shows a monotonic increase in all three cases while α∥ exhibits a pronounced maximum at about 1.5 Re. For T=300 K, we find α(Li2)=214, α(Na2)=262, and α(K2)=477 a.u. Our results, to which we attribute an uncertainty of 2%, support the measured value for the average polarizability of Li2 [R. W. Molof et al., J. Chem. Phys. 61, 1816 (1974)] but disagree with their results for Na2 and K2. For these dimers, Molof ’s mean experimental values appear to be in error by 33% and 18%, respectively.
Eigenstate-free, Green function, calculation of molecular absorption and fluorescence line shapes
Authors: Yan Yi Jing , Mukamel Shaul
Published: July 2002

Green function techniques are used to develop a simple and efficient method towards the calculation of optical absorption, excitation, and dispersed fluorescence spectra of large harmonic polyatomic molecules. The molecular line shapes are expressed in terms of Fourier transforms of appropriate correlation functions which may be explicitly evaluated. Closed expressions are derived for a general harmonic molecule with two electronic states including equilibrium displacements, frequency changes, and Dushinsky rotation, within the Condon approximation. A simple method for extracting the complete set of parameters characterizing the ground and the electronically excited states from supersonic beam absorption and emission spectra is presented. Detailed calculations are performed for a model system with ten vibrational modes, and the sensitivity of the various experimental observables to Dushinsky rotation is analyzed.
Breakdown of the Debye theory of bulk ion recombination
Authors: Tachiya M.
Published: July 2002

We have investigated how the rate of diffusion-controlled bulk recombination between electrons and positive ions is influenced by the mean free path of electrons. By use of the Monte Carlo method previously developed we have calculated the rate constant as a function of the mean free path. For mean free paths negligibly short compared with the Onsager length, the rate constant increases linearly with increasing mean free path, in agreement with the Debye theory of bulk ion recombination. However, as the mean free path further increases, the rate constant goes through a maximum and then decreases. The present result quantitatively explains recent experimental data on electron–ion recombination rate constants in liquid methane in which the mean free path of electrons is comparable with the Onsager length.
Structure of bimetallic clusters. Extended x-ray absorption fine structure (EXAFS) of Pt–Re and Pd–Re clusters
Authors: Meitzner G. , Via G. H. , Lytle F. W. , Sinfelt J. H.
Published: July 2002

Extended x-ray absorption fine structure (EXAFS) studies were conducted on catalysts containing platinum and rhenium, or palladium and rhenium, on alumina. The atomic ratio of rhenium to either platinum or palladium was close to one in the catalysts investigated. The metallic entities in the catalysts were characterized by analyses of the EXAFS associated with the LI absorption edge of platinum, the LIII edge of rhenium, and the K absorption edge of palladium. It was concluded that Pt–Re and Pd–Re bimetallic clusters are present in the catalysts. However, the clusters have regions rich in rhenium and other regions which are rich in either platinum or palladium. Exposure of Pt–Re clusters to sulfur has little influence on their structure.
Annihilation of singlet excitons in J aggregates of pseudoisocyanine (PIC) studied by pico- and subpicosecond spectroscopy
Authors: Sundström V. , Gillbro T. , Gadonas R. A. , Piskarskas A.
Published: July 2002

The excited-state dynamics of J aggregates of PIC have been studied by means of picosecond and subpicosecond absorption spectroscopy as well as integrated fluorescence yield measurements. The results of these measurements show that both the lifetime and the fluorescence yield are strongly dependent on excitation pulse intensity. At relatively high light intensity (1014 photons cm−2 pulse−1) the S1 lifetime is essentially pulse limited (<1 ps) and the fluorescence yield is very low. Upon decreasing the light intensity a gradual increase of the excited-state lifetime and fluorescence yield is observed. At very low excitation intensity (1010 photons cm−2 pulse−1) a single exponential lifetime of 400 ps is observed. At intermediate intensities the excited-state decay is strongly nonexponential. The observed intensity dependence of the excited-state dynamics is attributed to efficient exciton–exciton annihilation between the highly mobile singlet excitons. By applying an expression for bimolecular exciton annihilation, derived and used for photosynthetic antenna systems [Paillotin et al., Biophys. J. 25, 513 (1979)], and treating the energy migration as a hopping motion, information about aggregate size and exciton hopping rate was obtained.
Fourier transform linewidths measurements in NH3 in the vibrational ground state
Authors: Aroui H. , Picard-Bersellini A. , Broquier M. , Blanchard S. , Billing G. D.
Published: July 2002

We have measured linewidths of several pure rotational transitions in the vibrational ground state in the far infrared (80–200 cm−1 ). The experiments were conducted by using a Fourier transform spectrometer and gases (NH3 diluted by H2 or He) contained in a 15 cm absorption cell at room temperature. The aim of our study is to discuss the vibrational effect by comparing our measurements with those done in the infrared, 3 and 6 μm regions (ν3 and ν4 bands) and to find evidence for vibrational dependence. The measured pressure broadening cross sections were typically found to be 26 Å2 for the NH3 –H2 collisions and 16 Å2 for the NH3 –He collisions, by comparing these values with those obtained for the ν4 mode, 35 Å2 for the H2 perturber and 17 Å2 for the He perturber, we see a vibrational effect for the NH3 –H2 collisions but not significant for the NH3 –He ones. Our measurements are in good agreement with the theoretical calculations done by Billing by using the semiclassical coupled states method.
Full configuration-interaction study of the ionic–neutral curve crossing in LiF
Authors: Bauschlicher Charles W. , Langhoff Stephen R.
Published: July 2002

Full configuration-interaction (FCI) calculations are used to assess the relative ability of methods for truncating the n-particle expansion in describing the ionic–neutral curve crossing between the two lowest 1Σ+ states of LiF. While the FCI calculations yield a smooth dipole moment function, MRCI calculations based on CASSCF orbitals optimized for the lowest state at all r values yield a discontinuous dipole moment function. However, when the orbitals are optimized using a state-averaged CASSCF procedure, with equal weights for the ionic and neutral solutions, both the CASSCF and MRCI dipole moment functions are smooth and in reasonable agreement with the FCI. No single-reference-based method is found to work satisfactorily. Potential curves for the lowest two 1Σ+ states are determined in both the adiabatic and diabatic representations using a large ANO Gaussian basis set and a state-averaged CASSCF/MRCI treatment of electron correlation.
A molecular dynamics study of the low temperature structure and dynamics of ethane monolayers physisorbed on the graphite basal plane
Authors: Moller Michael A. , Klein Michael L.
Published: July 2002

The three known low-temperature structures of ethane monolayers physisorbed on the graphite basal plane (hereafter called S1, S2, and S3) have been examined by molecular dynamics calculations with intermolecular and molecule–surface potentials based on atom–atom interactions. The structure of the low density S1 phase is shown to be strongly influenced by the corrugation of the graphite surface. The intermediate density S2 phase is incommensurate with a slightly different structure than found experimentally. By contrast, the structure of the high density S3 phase, in which all of the molecules stand erect, appears to be independent of surface corrugation. The dynamical behavior of the three surface phases has been probed by examining power spectra calculated from the hydrogen atom trajectories. The resulting spectra for the S1 and S3 phases agree reasonably well with experimental inelastic incoherent neutron scattering data but not as well in the case of the S2 phase.
Small-angle light scattering of phase separation structures in polymer blends
Authors: Wang Z.-Y. , Konno M. , Saito S.
Published: July 2002

The correlation function presented by Debye et al. for characterizing the random two-phase structure in solids is further extended by considering an effect of partial ordering to describe the statistical features in phase separation structures of polymer blends. The new correlation function is in the form γ(r)=exp(−r/a0 )cos(q0 r) with two parameters. The scattering intensity distribution calculated from this function can predict a maximum whose sharpness and position are characterized by a0 and q0 . Good agreement was achieved between the calculated intensity and measured small-angle light scattering curve from phase-separated poly(methyl methacrylate)/poly(vinyl acetate) blends.
Oxidation reactions at variably sized transition metal centers: Fe+n and Nb+n +O2 (n=1–3)
Authors: Loh S. K. , Lian Li , Armentrout P. B.
Published: July 2002

Cross sections for the reactions of Fe+n and Nb+n (n=1–3) with O2 are measured as a function of kinetic energy over a range of 0 to >10 eV. In all systems, analysis yields insight into the kinetics and thermochemistry of the oxidation processes. Nb+n reaction with O2 exothermically near the Langevin–Gioumousis–Stevenson close-collision limit, driven by formation of strong NbO+ and NbO bonds. Fe+n are less reactive, although oxidation becomes progressively more facile as the size of the reactant increases from Fe+ to Fe+3. In contrast to the Nb+n systems, Fe+n (n=2,3) react at elevated energies by simple cluster fragmentation processes. Quantitative limits are established for ionic and neutral cluster oxide bond dissociation energies. Cross sections for formation of MnO+ from reaction of M+n (Fe+3, Nb+2, and Nb+3) are observed to have both an exothermic and an endothermic feature. Since there is only one chemical pathway to form this product, it is suggested that there are activation barriers to formation of favorable reaction intermediates. A similar suggestion is required to explain product branching ratios involving metal dioxides which run counter to thermodynamic predictions.
The crystal structure of 1,1-cyclobutanedicarboxylic acid at 20 K
Authors: Santarsiero B. D.
Published: July 2002

The structure is disordered at room temperature, with atom C3 (Cγ) of the cyclobutane ring occupying sites on each side of the plane of the other three ring atoms. At 20 K, the structure is ordered. The space group is P21/c with Z=4; the unit cell dimensions at 20 K are a=5.983(2) Å, b=10.374(4) Å, c=10.917(3) Å, β=101.19(3)°, V=664.8(4) Å3. The average Cα–Cβ bond length, 1.560 Å, is longer than the average Cβ–Cγ bond length, 1.548 Å, and the internal ring angles are all acute, with that at Cα, the smallest, 88.52°, those at Cβ intermediate, average 89.11°, and that at Cγ, the largest 89.35°. The cyclobutane ring deviates only slightly from mm symmetry, with Cγ displaced from the CαCβ plane by 0.40 Å. The average C=O and C–OH bond lengths in the carboxylate groups are 1.223 and 1.323 Å, with each acid group hydrogen bonded to another acid group through an inversion center (O⋅⋅⋅O contacts, 2.65 Å, on average). Examination of subtle differences in structural parameters suggests that substituent effects on cyclobutane are substantially reduced from those observed in substituted cyclopropanes.
Kinetics of swelling and shrinking of gels
Authors: Li Yong , Tanaka Toyoichi
Published: July 2002

The kinetics of swelling and shrinking of gels is theoretically generalized and experimentally studied. A new relation, in addition to the differential equation developed by Tanaka and Fillmore, is formulated to solve the kinetics of gels having arbitrary shape. Using our new theory, we provide explicit solutions for long cylinder and large disk gels. These solutions predict that the effective diffusion constants of long cylinder and large disk gels are 1.5 and 3 times smaller than that of a spherical gel. The relaxation times of long cylinder and large disk gels with small shear modulus μ are approximately 2.0 and 5.7 times longer than that of a sphere, where the diameters of the cylinder and the sphere and the thickness of the disk are the same. The theory also concludes that the experimentally measured kinetics of a long cylinder and disk gels along the z axis and the radial axis are the same. The results have been excellently confirmed by the experiments.
A
b i
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i
t
i
o characterization of the S1–S2 conical intersection in pyrazine and calculation of spectra
Authors: Seidner L. , Stock G. , Sobolewski A. L. , Domcke W.
Published: July 2002

Potential-energy functions of the three lowest singlet states of pyrazine have been computed on the self-consistent field, second-order Mo/ller–Plesset (SCF/MP2) level as a function of ab initio determined ground-state normal coordinates. The electronic-structure calculations confirm the existence of a conical intersection of the S1(nπ*) and S2(ππ*) surfaces near the minimum of the latter surface. A vibronic-coupling model involving four spectroscopically active vibrational modes is constructed on the basis of the ab initio data. Absorption, resonance-Raman, fluorescence, and femtosecond time-resolved pump–probe spectra are computed for this model using previously developed methods. The results are compared with experimental data where available.
Molecular beam Stark spectroscopy of the C 1Σ+–X 1Σ+ (0,0) band of yttrium monochloride
Authors: Simard Benoit , James Andrew M. , Hackett Peter A.
Published: July 2002

The yttrium monochloride molecule (YCl) has been produced in a free jet molecular beam apparatus by chemical reaction in a laser-produced plasma. The origin band of the C 1Σ+–X 1Σ+ system of YCl was probed at the sub-Doppler resolution of 120 MHz using a ring dye laser to excite fluorescence. Spectra due to the two isotopomers 89Y35Cl and 89Y37Cl were obtained, and molecular constants determined. The following bond lengths were derived (89Y35Cl):r0(X)=2.384 78(36) Å, r0(C)=2.460 50(40) Å (2σ error bounds). Results for the ground state are in good agreement with those recently reported by Xin et al. [J. Mol. Spectrosc. 148, 59 (1991)]. The permanent electric dipole moments for both the X and C states were determined by performing molecular beam Stark spectroscopy on the lines P(1) and R(0), respectively. Values of 2.587(29) D (X state) and 3.258(36) D (C state) were obtained. Results are compared with ab initio predictions of the molecular parameters, and a molecular orbital interpretation of the bonding is presented.
Accurate fit of the two lowest excited-state potential-energy surfaces for doublet HeH2+
Authors: Aguado Alfredo , Suárez Cristina , Paniagua Miguel
Published: July 2002

Diabatic potential-energy functions for the two lowest excited states of the doublet HeH2+ system which fit published ab initio data [McLaughlin and Thompson, J. Chem. Phys. 70, 2748 (1979)] at a quantitative level have been obtained, similar to that obtained previously for the ground state (root-mean-square error about 1 kcal/mol), and without any quantity of ad hoc character, preserving the accuracy of the ab initio points. The corresponding adiabatic potential-energy functions are obtained by solving a 2×2 determinant that uses the diabatic energies and an interaction term fitted to the lowest eigenvalue to obtain a global root mean square as low as possible (0.93 kcal/mol). The lowest adiabatic potential-energy function satisfies the criteria needed to be used in full three-dimensional scattering calculations for the collisionally-induced predissociation charge-exchange reaction He++H2→He+H++H and, using the ground-state potential-energy surface also, the collision-induced dissociation reaction H2+(v)+He→He+H++H and the exoergic reaction He++H2→HeH++H.
Steady shear viscosity of weakly aggregating polystyrene latex dispersions
Authors: de Rooij R. , Potanin A. A. , van den Ende D. , Mellema J.
Published: July 2002

In order to describe the steady shear behavior of weakly aggregating dispersions a microrheological model is developed. This model combines the concept of fractal aggregation in shear flow with the transient network modeling originally developed for polymer dynamics. The former accounts for the hydrodynamic stress in the aggregates, while the latter is employed to calculate the stress component arising from the forces transmitted by chains of particles combining the neighboring aggregates into a network. The contribution of this network is shown to be significant for steady shear behavior at low volume fractions of solid and low shear rates. The validity of the model is tested by fitting it to experimental data on a well characterized weakly aggregating polystyrene latex dispersion. In this way a number of relevant parameters, among which the fractal dimension df of the aggregates and the capture efficiency of particle chains belonging to neighboring aggregates, are determined. The best agreement of the calculations with the experiments is found at small capture efficiencies and rather high fractal dimensions, i.e., 2.0≤df≤2.3. Hence, combining the steady-shear viscosity measurements with the microrheological interpretation one obtains a tool for probing the structure of weakly aggregating dispersions.
The kinetic and internal energy of NO from the photodissociation of nitrobenzene
Authors: Galloway Douglas B. , Glenewinkel-Meyer Thomas , Bartz Jeffrey A. , Huey L. Gregory , Crim F. Fleming
Published: July 2002

Laser induced fluorescence probing of the nitric oxide fragment determines the distribution of rotational and vibrational energies of NO produced in the 226 and 280 nm photolysis of nitrobenzene. Combining these results with kinetic energy measurements using vacuum ultraviolet photoionization to detect the fragment gives a detailed view of the energy release in the photolysis. Boltzmann distributions describe the rotational state populations at both photolysis wavelengths. The rotational temperature of NO from the 226 nm photolysis is (3700±350) K, corresponding to an average rotational energy of (0.32±0.03) eV, and that of NO from the 280 nm photolysis is (2400±200) K, corresponding to an average rotational energy of (0.20±0.03) eV. We observe no vibrationally excited NO and place an upper limit of 10% on the fraction of nitric oxide produced in any one vibrationally excited state. Two different limiting models, impulsive energy release and statistical energy redistribution, both correctly predict much more rotational than vibrational excitation, but neither completely describes the observed internal and kinetic energies. The impulsive model finds more NO rotational and translational energy, but much less phenoxy fragment internal energy than we observe. The statistical model does better for the NO rotation and phenoxy fragment internal energy, but underestimates the translational energy substantially. A combination of these two types of behavior provides a physical picture that qualitatively explains our observations. It is likely that statistical energy redistribution occurs during the approach to the transition state for isomerization of nitrobenzene to phenyl nitrite and impulsive energy release dominates during the subsequent rupture of the CO–NO bond.
The adsorption of CO on Rh(100): Reflection absorption infrared spectroscopy, low energy electron diffraction, and thermal desorption spectroscopy
Authors: de Jong A. M. , Niemantsverdriet J. W.
Published: July 2002

The adsorption of CO on Rh(100) has been investigated as a function of temperature and CO pressure, with reflection absorption infrared spectroscopy (RAIRS), low energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS). At low coverages, most CO adsorbs linearly, up to 0.5 monolayers (ML), where a c(2×2) LEED pattern is observed. The saturation coverage at 300 K is 0.75 ML, associated with a p(4√2×√2)R45° LEED pattern, and adsorption of CO in at least three states, namely two linear modes and one bridged mode. At temperatures well below 300 K and relatively high exposures, CO is compressed into a dense hexagonal overlayer structure with a c(6×2) LEED pattern and a saturation coverage of 0.83 ML, in which the majority of the CO is bonded linearly. The activation energy of desorption for CO from Rh(100) in the limit of zero coverage is 131±4 kJ/mol and the preexponential factor is (4±3)×1016 s−1. We compare our results with the literature of CO on Rh(100), which shows contradictory results on the vibrational spectra, LEED structures, and saturation coverages of CO.
Structure of the ammonium radical from a rotationally resolved photoelectron spectrum
Authors: Signorell R. , Palm H. , Merkt F.
Published: July 2002

High-resolution zero-kinetic-energy photoelectron spectroscopy has been used to record the transition between the lowest bound state (3s 2A1) of the perdeuterated ammonium radical (ND4) and the X̃ 1A1 ground vibronic state of the perdeuterated ammonium ion (ND4+). The spectra obtained are the first rotationally resolved photoelectron spectra ever measured for a tetrahedral molecule. The analysis of the rotational structure is accompanied by a description of the observed symmetry selection rules and propensity rules for core rotational angular momentum changes that characterize the photoionization process. Rotational constants (B0=2.8560±0.0037 cm-1 and B0+=2.9855±0.0037 cm−1) and centrifugal distortion constants (D0=(4.78±1.4)×10−5 cm-1 and D0+=(4.77±1.5)×10−5 cm−1) have been determined for the 3s 2A1 state of ND4 and the X̃ 1A1 state of ND4+, respectively. The ionic rotational constant is in good agreement with the value B0+=2.9787±0.0029 cm-1 determined indirectly by Crofton and Oka (J. Chem. Phys. 86, 5983 (1987)) from the measurement of allowed transitions of the ν3 vibrational band of ND4+. The neutral rotational constant differs markedly from the ab initio value B0=3.0407 cm-1 of Havriliak and King (J. Am. Chem. Soc. 105, 4 (1983)) used by Alberti, Huber and Watson (J. Mol. Spectrosc. 107, 133 (1984)) as input data to fit the rotational structure of the Schüler band of ND4. The adiabatic ionization potential of ND4 is determined to be 37490.7±1.5 cm-1 (4.64826±0.00019 eV). The large changes in core rotational angular momentum that accompany the removal of the photoelectron may be caused by the Cooper minimum in the s→p photoexcitation/photoionization channel recently predicted by Smith and Chupka [Chem. Phys. Lett. 250, 589 (1996)] to lie in the vicinity of the ionization threshold.
Thermodynamics of the Tl–O system and Tl-superconductors
Authors: Wahlbeck P. G. , Myers Dwight L.
Published: July 2002

Experiments have been performed to determine vaporization reactions in the Tl–O system. When the starting sample was pure Tl2O3(c), the vapor was initially rich in oxygen compared to the starting Tl2O3(c) composition. When the extent of vaporization of the sample was between 30% and 80%, the residue composition was Tl4O3(c). The congruently vaporizing composition was concluded to be in the Tl4O3(c) single phase region. This conclusion supports the thermodynamic data for thallium oxides which was published by Holstein. Vapor pressure data for the two phase region with Tl-superconductors Tl-2223(c) and Tl-1223(c), which we measured previously and reported as activities, have been reevaluated.
The diffusion of oxygen on W(110) revisited
Authors: Nahm T.-U. , Gomer R.
Published: July 2002

The diffusion of oxygen on the (110) plane of tungsten has been reexamined via the field emission fluctuation method, using a digital correlation apparatus. Measurements were carried out over a coverage range of 0.13⩽O/W⩽0.79. Where overlap exists activation energies agree well with previous work; there is qualitative but not quantitative agreement with prefactors D0. Mean-square fluctuations show very little temperature dependence. The present results show that E rises almost linearly from 14 kcal/mol at O/W⩽0.2 to 23 kcal/mol at O/W=0.4, then shows a slight dip at O/W=0.56 and then rises again to 27 kcal/mol at O/W=0.79. D0=4×10−8 cm2/s at O/W=0.13, and rises by six orders of magnitude to 5×10−2 cm2/s at O/W=0.79. It is postulated that the increases in E and D0 with coverage have their origin in polaronic effects, to wit the participation of substrate atoms in the diffusion process and a progressive stiffening of the substrate by O atoms as coverage increases.