bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory for photoluminescence from SiO2 films containing Si nanocrystals and Er ions
Authors: Qin G. , Qin G. G. , Wang S. H.
Published: July 2002

In this article, the reported experimental phenomena of energy transfer from Si nanocrystals (nc-Si) to Er ions inside the SiO2 surrounding the nc-Si or at the nc-Si/SiO2 interface, and the origin of the characteristic double photoluminescence (PL) peaks in the SiO2 film containing nc-Si and Er ions [Fujii et al. Appl. Phys. Lett. 71, 1198 (1997)] are explained by means of the quantum confinement–luminescence center model. The theoretical results show that the PL peak at 0.805 eV is caused by a recombination process outside the nc-Si, i.e. the electron–heavy-hole pairs tunnel into the SiO2 barrier, are absorbed, and then recombine radiatively in the Er ions. The PL peak at 1.53 eV most probably originates mainly from another type of defects or impurities in the SiO2 barrier or at the nc-Si/SiO2 interface. The experimental results, that as the concentration of Er ions increases the intensity of PL peak at 0.805 eV increases while the intensity of PL peak at 1.53 eV decreases, have been explained.
Depth profiling of thermally inhomogeneous materials by neural network recognition of photothermal time domain data
Authors: Glorieux C. , Li Voti R. , Thoen J. , Bertolotti M. , Sibilia C.
Published: July 2002

Thermal conductivity depth profiles of thermally inhomogeneous materials are retrieved from the time dependence of the surface temperature after a flash illumination. A neural network method, which is trained to recognize the correlation between depth profiles and the surface temperature on the basis of many examples, is employed. Depth profiles retrieved from simulated noisy signals are shown and the average reconstruction errors are analyzed for different circumstances.
Raman studies of heavily carbon doped GaAs
Authors: Seon M. , Holtz M. , Duncan W. M. , Kim T. S.
Published: July 2002

Raman spectroscopy is used to study GaAs heavily doped with carbon. Hole concentrations in these samples range from 2.3×1019 to 1×1020 cm−3. Three main Raman features are investigated: the longitudinal-optic (LO) phonon mode, the substitutional carbon-at-arsenic local-vibrational mode, and the coupled plasmon–LO phonon present due to the interaction between the LO phonon and the free carriers. Only one allowed phonon-like coupled mode is observed due to the large plasmon damping and high effective carrier masses. The coupled mode is seen to systematically redshift as carrier concentration increases. This behavior is described by a model which includes the effects of high hole concentrations on the dielectric function and an additional shift in the optic phonon we tentatively attribute to carbon size effect. The local vibrational mode intensity is found to be directly proportional to the carrier concentration p. Interestingly, the local mode intensity shows good correlation with that of the coupled plasmon–LO phonon mode as a function of p. The ratio of the coupled plasmon–LO phonon mode intensity to that of the LO phonon is found to be directly proportional to the carrier concentration.
ZnSe/GaAs band-alignment determination by deep level transient spectroscopy and photocurrent measurements
Authors: Souifi A. , Adhiri R. , Le Dantec R. , Guillot G. , Uusimaa P. , Rinta-Möykky A. , Pessa M.
Published: July 2002

Using deep level transient spectroscopy and photocurrent measurements we have investigated Schottky contacts formed on p-isotype Zn(SSe)/GaAs heterostructures grown by molecular beam epitaxy on p-GaAs(100) substrates. A deep level located at 0.6 eV above the ZnSe valence band is observed in agreement with literature data for p-type ZnSe, and is used as a reference level for the understanding of photocurrent transitions in the 0.8–3.0 eV energy range. The threshold energies obtained on a series of Zn(SSe)/GaAs samples are explained in terms of absorption processes from the ZnSe and GaAs valence bands, and from the nitrogen acceptor level and a deep level of the ZnSe layers located at 0.1 and 0.6 eV above the valence band maximum, respectively. These absorption processes towards the ZnSe and GaAs conduction bands have been finally used to give the values of the conduction and valence band offsets at p-ZnSe/p-GaAs interface. Our experimental data gives ΔEc=0.25±0.03 eV and ΔEv=1.00±0.05 eV in agreement with literature data for Zn-rich interfaces.
Surface acoustic modes in thin films on anisotropic substrates
Authors: Maznev A. A. , Akthakul Ariya , Nelson Keith A.
Published: July 2002

Propagation of surface acoustic modes on the (001) and (111) surfaces of Si coated by a thin isotropic overlayer is studied theoretically and experimentally. It is shown that when a surface acoustic wave (SAW) coexists with a pseudosurface wave (PSAW) of the uncoated substrate, the second-order acoustic mode of the film/substrate system originates from a PSAW and the first-order one from a SAW. The polarization pattern of either mode varies from Rayleigh type (saggital plane polarization) to Love type (horizontal polarization) depending on the propagation direction and the product of the wave vector q and film thickness d. It is also shown that the isolated off-symmetry pure mode point within the PSAW branch disappears at some critical qd value. Experimentally, surface acoustic modes of Ti-coated Si wafers are measured with the impulsive stimulated thermal scattering (ISTS) technique based on laser generation and detection of acoustic waves at a specified wave vector. ISTS data are shown to be determined by the surface elastodynamic Green function G13(ω,g). The measurements, only sensitive to Rayleigh-type waves, detect either the first- or the second-order mode, or both, depending on the qd and the observation angle.
Microstructural investigation of oxidized Ni/Au ohmic contact to p-type GaN
Authors: Chen Li-Chien , Chen Fu-Rong , Kai Ji-Jung , Chang Li , Ho Jin-Kuo , Jong Charng-Shyang , Chiu Chien C. , Huang Chao-Nien , Chen Chin-Yuen , Shih Kwang-Kuo
Published: July 2002

The microstructure of oxidized Ni/Au films on p-GaN was examined to elucidate the formation of a low resistance ohmic contact to p-GaN with a field-emission gun transmission electron microscope in conjunction with composition analyses. The p-GaN/Ni/Au samples were heat treated at 500 °C in air mainly composed of a mixture of crystalline NiO, Au, and amorphous Ni–Ga–O phases. Small voids adjacent to the p-GaN film were also observed. The as-deposited Au film converted into discontinuous islands containing small amounts of Ni that connect with p-GaN. NiO formed a continuous film at the surface that covers the Au islands and the amorphous Ni–Ga–O phases. Moreover, NiO partially contacts p-GaN as well as Au islands and the amorphous Ni–Ga–O phase. The orientation relationship of the crystalline NiO, Au-rich islands, and p-GaN film was identified as NiO(111)//Au(111̄)//GaN(0002) and NiO[11̄0]//Au[11̄0]//GaN[112̄0]. The results suggested that Ni atoms diffuse through the Au layer onto the surface and react with oxygen to form NiO, whereas Au atoms diffuse towards the inside to form a Au–Ni alloy. The microstructural examination indicated that the crystalline NiO and/or the amorphous Ni–Ga–O phases may significantly affect the low resistance ohmic contact to p-GaN.
Seed layer dependence of room-temperature recrystallization in electroplated copper films
Authors: Ueno Kazuyoshi , Ritzdorf Tom , Grace Scott
Published: July 2002

Room-temperature recrystallization (self-annealing) of electroplated copper (Cu) films is investigated using three kinds of seed/barrier layers with nontexture and (111) texture. The as-plated films have almost the same texture as the seeds. The texture changes during self-annealing depend on the seed texture. The (111) texture of the self-annealing films increases for the film deposited on the nontexture seed layer, but decreases for the film deposited on (111) texture seed layers. For all the plated films, tensile stress increases after self-annealing, which corresponds to the film shrinkage. Recrystallization rate of the Cu film plated on the nontexture seed layer is higher than the one on the (111) texture seed. It is postulated that the reduction of grain boundary energy is the major driving force for film self-annealing and grain growth.
Metalorganic vapor-phase epitaxy-grown AlGaN materials for visible-blind ultraviolet photodetector applications
Authors: Omnès F. , Marenco N. , Beaumont B. , de Mierry Ph. , Monroy E. , Calle F. , Muñoz E.
Published: July 2002

Low-pressure metalorganic vapor-phase epitaxy growth conditions of AlxGa1−xN epilayers on c-oriented sapphire have been optimized for aluminum mole fractions x lying in the 0–0.35 range both on GaN and AlN nucleation layers, with a view to application in visible blind UV photodetectors. Good structural, electrical, and optical properties were obtained for undoped and n-type doped AlGaN alloys on (0001)-oriented sapphire substrates. A typical full width at half maximum of 670–800 arc s is measured for the (0002) x-ray double-diffraction peak in the ω mode of 1-μm-thick AlGaN epilayers grown on a GaN nucleation layer. Room-temperature electron mobilities up to 90 cm2/V s are measured on n-type (1018 cm−3) AlGaN epilayers. The low-temperature photoluminescence (T=9 K) performed on nonintentionally doped AlGaN epilayers with low-Al content (10% and 14%) exhibits reproducibly a sharp exciton-related peak, associated with two phonon replica and does not exhibit any low-photon energy transitions. Optical transmission as well as absorption coefficient measurements using photothermal deflection spectroscopy have been used to study the variation of the T=300 K energy gap of AlGaN with the aluminum concentration. Visible-blind AlGaN(Si)-based photoconductors and Schottky barrier photodiodes with good operating characteristics have been fabricated with these materials.
Electrical properties of thin SiON/Ta2O5 gate dielectric stacks
Authors: Houssa M. , Degraeve R. , Mertens P. W. , Heyns M. M. , Jeon J. S. , Halliyal A. , Ogle B.
Published: July 2002

The electrical characteristics of metal–oxide–semiconductor capacitors with SiON/Ta2O5 gate dielectric stacks with thin Ta2O5 layers (6–10 nm) are investigated. From the field and temperature dependence of the current of the gate stacks, it is shown that the main conduction mechanism at low bias is tunneling through the stack and that Poole–Frenkel conduction in the Ta2O5 layer becomes important at larger bias and temperature. From the analysis of the data in the high voltage and temperature range, taking into account the field distribution in both layers, the refractive index n of Ta2O5 and the energy level φB of traps involved in Poole–Frenkel conduction are found to be 2.3 and 0.85 eV, respectively. It is also shown that the gate current density of the stack is reduced by one to three orders of magnitude as compared to SiO2 layers with equivalent electrical thickness (2.5–3 nm). The temperature acceleration effect on the time-dependent dielectric breakdown is shown to be much reduced in the SiON/Ta2O5 stack as compared to SiO2 layers with equivalent electrical thickness.
Relaxation of positive charge during bidirectional electric stress on metal–oxide–silicon capacitors
Authors: El-Hdiy A. , Ziane Dj
Published: July 2002

Relaxation phenomena of positive charges, created in a metal–oxide–silicon capacitor by bidirectional Fowler–Nordheim electron injections under constant current, have been studied and their kinetics have been quantitatively characterized. After creation, positive charges are neutralized under electron injections at constant current. The gate bias shift caused by the neutralization process has an exponential dependence on time during relaxation irrespective of stress field polarity and stress duration. Analysis of data assuming nonelectron detrapping gives capture cross section values in the range of 3.5–6.7×10−16 to 2.1×10−15 cm2. The compilation of data given in literature and our present results shows that the main responsible defect in the oxide is the amphoteric traps negatively charged near the cathode and positively charged near the anode.
Shear strength measurements in a tungsten alloy during shock loading
Authors: Millett J. C. F. , Bourne N. K. , Rosenberg Z. , Field J. E.
Published: July 2002

Lateral stress measurements in a tungsten alloy, in combination with known Hugoniot data, have been used to find the shear strength of this material, and its variation with longitudinal shock stress, up to 14 GPa. Results show that the shear strength increases significantly with increasing stress. Prior to this work, there has been disagreement in the literature on the effect of shock stress on the shear strength of tungsten and its alloys. The present work agrees with the data obtained by Zhou and Clifton [J. Appl. Mech. 64, 487 (1997)] who used pressure shear. However, the range of stresses studied has been greatly extended.
Ion-assisted pulsed laser deposition of aluminum nitride thin films
Authors: Lu Y. F. , Ren Z. M. , Chong T. C. , Cheong B. A. , Chow S. K. , Wang J. P.
Published: July 2002

Aluminum nitride (AlN) thin films were deposited by nitrogen-ion-assisted pulsed laser ablation of an AlN target. A KrF excimer laser with a pulse duration of 23 ns and a wavelength of 248 nm was used as a light source for the ablation. A nitrogen ion beam with energies in a range of 200–800 eV is used to assist the deposition. The nitrogen ion implantation can compensate the possible loss of nitrogen species in the ablated plasma and can effectively assist the deposition by providing energetic nitrogen ions. Raman and Fourier transform infrared spectroscopy measurements were used to characterize the deposited thin films. The influences of the substrate temperature and the ion energy on the electronic and structural properties of the deposited thin films were studied.
Influence of strain on microwave dielectric properties of (Ba,Sr)TiO3 thin films
Authors: Chang Wontae , Gilmore Charles M. , Kim Won-Jeong , Pond Jeffrey M. , Kirchoefer Steven W. , Qadri Syed B. , Chirsey Douglas B. , Horwitz James S.
Published: July 2002

Epitaxial Ba1−xSrxTiO3 (BST) thin films have been deposited onto (100)MgO and LaAlO3 substrates using pulsed-laser deposition. Thick (>1 μm) Ag interdigitated capacitors capped with a thin protective layer of Au have been deposited on top of the BST films using electron-beam deposition. The capacitance (C) and dielectric quality factor (Q=1/tan δ) of the structure has been measured at microwave frequencies (1–20 GHz) as a function of electric field (E⩽67 kV/cm) at room temperature. In epitaxial BST films, either high dielectric tuning (4:1), which is defined as {[C(0)−C(E)]/C(0)}×100, or high dielectric Q (∼100–250) was observed but not both at the same time. Film strain was observed by x-ray diffraction and is closely related to the dielectric properties as limiting the ability to obtain both high tuning and high dielectric Q in epitaxial BST thin films. A thin BST buffer layer was used to relieve the strain in the films. In strain-relieved films, both dielectric tuning and dielectric Q were increased after annealing. A theoretical analysis of the strain effect of the films is presented based on Devonshire thermodynamic theory.
Polarization insensitive electro-optic polymer modulator
Authors: Donval A. , Toussaere E. , Hierle R. , Zyss J.
Published: July 2002

Electro-optic waveguiding polymer devices are very attractive for optical communication systems as a result of simple and potentially low-cost fabrication procedures. Several functions, such as high speed modulation or switching, have been already demonstrated. However, their polarization sensitive response precludes any actual use in optical telecommunication systems. The traditional uniform poling procedure, a prerequisite processing step for electro-optic polymer based devices whereby an electric field is applied at the vicinity of the glass transition temperature resulting in statistical polar orientation of the chromophores, is responsible for polarization sensitivity. By way of different coplanar or sandwich electrode configurations, any desired prerequisite modulation axis can, however, be imprinted onto the material resulting in the possibility of balanced TE/TM polarization efficiencies. Monolithic integration of both electrode configurations within a Mach–Zehnder modulator is shown to result in an original polarization insensitive electro-optic polymer amplitude modulator for integrated optics. Design procedures including modeling, fabrication, as well as performance testing of the integrated sandwich and transverse electrode configurations device are discussed. The electrode configuration is optimized in view of ensuring both efficient poling conditions and maximal overlap of the modulation field with the active layer cross section.
Electrical properties of semiconducting barium vanadate glasses
Authors: Sen S. , Ghosh A.
Published: July 2002

The preparation of vanadate glasses containing barium oxide and their electrical properties in the temperature range of 80–500 K have been reported in this article. Analysis of the electrical properties has been made in the light of different hopping models. The multiphonon assisted hopping model of small polarons in the nonadiabatic regime, proposed by D. Emin [Phys. Rev. Lett. 32, 303 (1974)] have been observed to describe the temperature dependence of the conductivity data of these glasses over the entire temperature range of measurement. The parameters obtained from the fits of the experimental data to this model are reasonable and consistent with the glass composition. J. Schnakenberg’s model [Phys. Status Solidi 28, 623 (1968)] is also consistent with the temperature dependence of the conductivity data. Mott’s optical phonon assisted hopping model at high temperatures provides smaller values of the localization length. However, Mott’s variable range hopping model is valid at low temperatures.
Spin-flip rate of excitonic magnetic polarons in Cd1−xMnxTe/Cd1−yMgyTe quantum wells
Authors: Debnath M. C. , Shen J. X. , Shirado E. , Souma I. , Sato T. , Pittini R. , Oka Y.
Published: July 2002

The lifetime of excitons in Cd1−xMnxTe/Cd1−yMgyTe single quantum wells was investigated by time-resolved photoluminescence spectroscopy. Spin-flips between dark and bright excitons remarkably change the excitonic lifetime yielding a two-exponential decay in the photoluminescence intensity. The temperature dependence of the two decay times shows that the spin-flip rate depends strongly on the formation of excitonic magnetic polarons. By forming polarons, the spin-flip rate from dark excitons to bright excitons is reduced effectively by the spin- barriers of the polarized Mn ions, which extends significantly the luminescence decay time.
Interdependence of the electrical and optical properties of liquid crystals for phase modulation applications
Authors: Guralnik I. R. , Belopukhov V. N. , Love G. D. , Naumov A. F.
Published: July 2002

The electrical capacitance and conductance of nematic liquid crystal (LC) cells were measured in combination with their optical properties as functions of applied voltage magnitude and frequency. A single experimental system was used in order to determine the correlation between these characteristics. These parameters are crucial for understanding and optimizing the performance of modal LC devices. Both ordinary and dual frequency LCs were investigated. For the latter type, Cole-Cole diagrams show a Debye type frequency dispersion in a limited range of 4–9 kHz. Also, a phenomenological theory of the measured parameters was developed and is in good agreement with experimental data. Two examples are discussed, and they illustrate the importance of taking into account the equivalent conductance of LC cells.
Bulk noise processes and their correlation to structural imperfections in magnesium-doped p-type GaN grown on sapphire
Authors: Rice A. K. , Malloy K. J.
Published: July 2002

We investigate the noise properties of p-type Mg-doped GaN using low frequency noise spectroscopy. The epitaxial GaN:Mg films were grown on a sapphire substrate by metalorganic chemical vapor deposition in different laboratories. Generation–recombination (g–r) noise and one-over-f (1/f) noise are observed for temperatures above 250 K. The magnitude of the 1/f noise exceeds the g-r noise magnitude for frequencies less than 30 Hz, and the 1/f noise level is characterized by high values of the Hooge parameter, α≈1–150, indicating a high level of structural imperfection. In addition, the integrated noise power spectral density divided by the voltage squared in the frequency range of 1 to 30 Hz, correlates strongly with the structural imperfection of the sample as measured from the asymmetric rocking curve (ω scan) full width at half maximum. The generation-recombination noise is related to a high concentration trap level with an activation energy of 120±25 meV and a repulsive barrier that is possibly associated with the Mg dopant.
A humidity-sensing model for metal–insulator–semiconductor capacitors with porous ceramic film
Authors: Li G. Q. , Lai P. T. , Huang M. Q. , Zeng S. H. , Li B. , Cheng Y. C.
Published: July 2002

A new physical model for a humidity-sensitive metal–insulator–semiconductor (MIS) capacitor with porous thin film is proposed. The model is used to determine water adsorption isotherm and calculate pore-size distribution of the film. The relative-humidity dependences of the capacitance and resistance of the film were measured. The effects of frequency on the dielectric constant of the film and the current of the MIS structure under various relative humidities were investigated.
Strain determination in PbEuTe/PbTe multi-quantum wells
Authors: Abramof E. , Rappl P. H. O. , Ueta A. Y. , Motisuke P.
Published: July 2002

A series of Pb1−xEuxTe/PbTe multi-quantum well (MQW) samples were grown on (111) cleaved BaF2 substrates by molecular beam epitaxy. The Eu content was maintained at x∼0.05–0.06 and the PbTe well width was varied from 23 to 206 Å. The samples were characterized structurally by high resolution x-ray diffraction in the triple axis configuration. The ω/2Θ scans of the (222) Bragg reflection showed very well resolved satellite peaks up to the tenth-order for all samples indicating that sharp interfaces were obtained. Reciprocal space mapping around the (224) lattice point indicated that the MQW structure tended to the free-standing condition. The (222) ω/2Θ scans were calculated by dynamical theory of x-ray diffraction and compared to the measured ones. Using the in-plane lattice constant as the main fitting parameter, the strain in the PbTe well inside the MQW structure was obtained as a function of its width. It decreased monotonically from an almost fully strained layer to 26% of strain relaxation as the PbTe well increased from 23 to 206 Å.
A large vibrational enhancement in the reaction of CN(v″=1, 2)+NO
Authors: Li Xuechu , Sayah Nahid , Jackson William M.
Published: July 2002

A large vibrational enhancement of the rate constant has been observed in the two-body reaction of CN(v″=1 and 2) with NO. NO also has a high efficiency as a third body in the termolecular recombination reaction between CN and NO. The rate constant obtained in this study for the bimolecular reaction between NO and CN (X 2 ∑ +) radicals in v″=0,1, and 2 are 1.6 ± 0.3×10−13, 5.6±0.8×10−11, and 6.2±1.0×10−11 cm3 molecule−1 s−1, respectively. The termolecular recombination reaction between CN(v″=0) and NO using Ar as a third body is 6.0±0.6×10−31 cm6  molecule−2 s−1, whereas when NO is the third body, the rate constant is 3.2±0.3×10−29 cm6 molecule−2 s−1.
The STO 3G MO structure and internal rotational potential of benzophenone
Authors: Schaefer Ted , Penner Glenn H.
Published: July 2002

Extensive geometry-optimized STO 3G MO computations yield C2 symmetry and 32° for the twist angles of the phenyl groups in benzophenone. For the planar molecule the internal rotational barrier is 33.7 kJ/mol and is 24.6 kJ/mol for a twist angle of 90°. The expectation value of the twist angle is therefore very near 32° at 298 K; implying that, in the crystal, packing forces cause only a minor perturbation of the conformation, the angle being 30° in the solid. An energy profile for non-C2 symmetry is also computed.
Reductions in dynamical theories
Authors: Grmela Miroslav
Published: July 2002

Can dynamical equations arising in nonequilibrium statistical mechanics be formulated in such a way that they become manifestly compatible with other, more macroscopic, dynamical theories? We show that the bracket formulation introduced previously answers the question if the more macroscopic theory is equilibrium thermodynamics. An extension of the bracket formulation, called a dynamic bracket formulation, is introduced in this paper. It provides an answer to the question also if the more macroscopic theories are dynamical theories. The dynamical bracket formulation reveals also a thermodynamics-like structure that is naturally associated with more macroscopic dynamical theories.
Statistical thermodynamics of molecular organization in mixed micelles and bilayers
Authors: Szleifer I. , Ben-Shaul A. , Gelbart W. M.
Published: July 2002

The conformational and thermodynamic characteristics of molecular organization in mixed amphiphilic aggregates of different compositions and geometries are analyzed theoretically. Our mean-field theory of chain conformational statistics in micelles and bilayer membranes is extended from pure to mixed aggregates, without invoking any additional assumptions or adjustable parameters. We consider specifically binary aggregates comprised of long-chain and short-chain surfactants, packed in spherical micelles, cylindrical rods, and planar bilayers. Numerical results are presented for mixtures of 11- and 5-carbon chain amphiphiles. The probability distribution functions (pdfs) of the (different types of) chains are determined by minimizing the conformational free energy, subject to packing constraints which reflect the segment density distribution within the hydrophobic core. In order to analyze the relative thermodynamic stabilities of mixed aggregates of different compositions (long/short chain ratios) and different geometries, the aggregate’s free energy is expressed as a sum of conformational, surface, and mixing contributions. The conformational free energy is determined by the pdfs of the chains and the surface term is modeled in terms of the ‘‘opposing forces’’ operative at the hydrocarbon–water interface. An interesting coupling between these terms arises from the special geometric (surface/volume) limitations associated with packing short and long chains in a given ratio within a given aggregate. In particular, it is found that the minimal area per surfactant head group in a mixed spherical micelle is significantly lower than that in a pure micelle (similarly, though less drastically so, for cylindrical micelles). The most important qualitative conclusion of our thermodynamic analysis is that the preferred aggregation geometry may vary with composition. For example, we find that under certain conditions (areas per head group, chain lengths) the preferred micellar geometry of pure long or short-chain aggregates is that of a planar bilayer, whereas at intermediate compositions spherical micelles are more stable. Our analysis of chain conformational properties provides quantitative

information on the extent of long (or short) chain distortion attendant upon chain mixing. For example, the results for bond order parameter profiles and segment density distributions reveal enhanced stretching of the long chain towards the central regions of the hydrophophic core as the fraction of short chains is increased.
Cluster expansion of the wave function. Valence and Rydberg excitations and ionizations of benzene
Authors: Kitao O. , Nakatsuji H.
Published: July 2002

The SAC (symmetry-adapted cluster) expansion and the SAC-CI theory are applied to the calculations of the valence and Rydberg excitations and the ionizations of benzene. The active space consists of 80 MO’s including both π and σ spaces; viz., 35π+45σ for the valence excitations and ionizations and 29π+51σ for the Rydberg excitations. For the lower six valence excited states, 3B1u(T1), 3E1u(T2), 1B2u(S1), 3B2u(T3), 1B1u(S2), and 1E1u(S3), generated from the π(e1g)→π*(e2u) transitions, the excitation energies are reproduced to within 0.5 eV of the experimental values with the average discrepancy being 0.34 eV. The first three states are explainable within the π electron space. For the other three states, the σ-reorganization effect is very important; viz., 0.6, 0.7, and 0.8 eV for the T3, S2, and S3 states, respectively. The polarization

d π function also works to lower the T3 and S2 states by 0.3 and 0.4 eV, respectively. The total lowerings from the SDT π-CI of Hay and Shavitt to the 35π+45σ SAC-CI are about 1 eV for these three states. With including the σ-reorganization effect, the diffuse nature of the S3 state, a typical V state, decrease from 〈x2〉=62 (π SAC-CI) to 41 (π+σ SAC-CI) a.u., in comparison with the ground state value of 30 a.u. Accordingly, the oscillator strength changes from 0.61 (π SAC-CI) to 1.03 (π+σ SAC-CI). The first systematic theoretical study is given for the Rydberg excitations including both π and σ states. With the aid of the recent experimental studies due to the MPI (multiphoton ionization) spectroscopy, the lower Rydberg excited states are almost completely identified. The SAC-CI results agree with the experimental values to within 0.3 eV for all the Rydberg states studied here. The σ-reorganization effect on the Rydberg transitions is about 0.3 eV. The SAC-CI calculation further gives satisfactory results for the outer and inner valence ionizations and their satellite peaks. The theoretical ionization spectrum well reproduces the general trends of the observed ESCA spectrum.
Thermodynamics far from equilibrium: Reactions with multiple stationary states
Authors: Ross John , Hunt Katharine L. C. , Hunt Paul M.
Published: July 2002

We present a thermodynamic analysis of global validity for effectively one-variable, irreversible chemical systems with multiple steady states. A hypothetical reaction chamber is held at constant temperature and volume and is connected by selectively permeable membranes to reservoirs of reactant(s) and product(s), each at constant selected pressures. An appropriate free energy function, which yields criteria of evolution to equilibrium for the composite system of reaction chamber and reservoirs, is a hybrid of Gibbs and Helmholtz free energies. The one variable in the reaction chamber is the pressure of a chemical intermediate which varies in time according to a given reaction mechanism. With the hybrid free energy, the kinetics for a given mechanism, and a concept of instantaneous indistinguishability of systems with different mechanisms, we establish a thermodynamic driving force, or species-specific affinity, for the intermediate. The species-specific affinity vanishes at steady states, and upon its differentiation we obtain necessary and sufficient conditions for the stability of steady states and for critical points. The integral of the species-specific affinity globally provides valid Liapunov functions for the evolution of the intermediate. These results are independent of the number of steady states of the system, and they hold both near to and far from equilibrium. For a large class of mechanisms with a single intermediate, the integral of the species-specific affinity appears in the irreversible part

of the time-dependent transition probability of the single-variable Master equation and in its stationary solution. Hence for these mechanisms we obtain a direct interpretation of the stochastic results in terms of thermodynamic quantities. The time rate of change in the pressure of the intermediate multiplied by its species-specific affinity yields a species-specific term in the dissipation. The total system dissipation (or entropy production) is not in general a minimum at a nonequilibrium steady state, but the species-specific term is minimized at every such state. The expression of the stationary solution of the master equation in terms of the species-specific affinity provides a generalization of the Einstein relation for the probability of equilibrium fluctuations to far-from-equilibrium conditions. The functional form of the species-specific term in the dissipation parallels a form that appears in Boltzmann’s H theorem for the momentum relaxation of a dilute gas.
Localized and canonical atomic orbitals in self-interaction corrected local density functional approximation
Authors: Pederson Mark R. , Lin Chun C.
Published: July 2002

The self-interaction corrected (SIC) version of the local spin density (LSD) approximation has been applied to the first two rows of the periodic table with particular emphasis on local orbital choice. These are the first SIC–LSD calculations for atomic systems that account for all nonspherical corrections and are based on a rigorous variational theory. The resulting total energies and orbital energies are improved in comparison to experiment and restore a desirable trend which is found in Hartree–Fock theory. We demonstrate that with a proper treatment of the SIC–LSD off-diagonal Lagrange multipliers, the viral theorem is satisfied at self-consistency.
On the stability of the infinite dimensional fluid of hard hyperspheres: A statistical mechanical estimate of the density of closest packing of simple hypercubic lattices in spaces of large dimensionality
Authors: Bagchi Biman , Rice Stuart A.
Published: July 2002

We report an analysis of the bifurcation of the solution to the nonlinear equation for the inhomogeneous singlet density in a system of hard hyperspheres; the instability examined corresponds to the liquid-to-simple hypercubic lattice transition. We propose that in the limit d→∞ the continuous bifurcation which occurs is at the maximum achievable density in a simple hypercubic lattice. Extension of this result to 1<d<∞ leads to estimates of the closest packing densities of simple hypercubic lattices in d dimensions. An examination of the liquid-to-simple hypercubic lattice transition for particles with a Gaussian pair repulsion leads to the identification of that transition with the onset of absolute instability, i.e., the spinodal of the liquid.
A theoretical study on the mechanism of internal conversion of S1 benzene
Authors: Kato Shigeki
Published: July 2002

The mechanism of internal conversion of S1 benzene has been studied theoretically. The MCSCF and MCSCF-CI methods were employed to calculate the potential energy surfaces of ground and excited states. It was found that the S1 potential surface crosses with the ground state surface near the saddle point of the benzene–prefulvene isomerization reaction. A simple theoretical model, based on the reaction path concept and the calculated potential surface characteristics, was used to calculate the rate of internal conversion as a function of excess vibrational energy of S1 benzene. The results obtained were that: (a) the internal conversion occurs through the tunneling mechanism at a low excess energy range, E<3700 cm−1; and that (b) both the surface hopping transition and the reaction to prefulvene are accessible at a high energy range, E>3700 cm−1. A new theoretical explanation for the channel three decay was proposed on the basis of the results of present calculations.
Mechanisms of spatial self-organization in isothermal kinetic oscillations during the catalytic CO oxidation on Pt single crystal surfaces
Authors: Eiswirth M. , Möller P. , Wetzl K. , Imbihl R. , Ertl G.
Published: July 2002

The rate of catalytic CO oxidation on Pt(100) and (110) surfaces at low pressures (≤10−4 Torr) and under isothermal conditions may exhibit sustained temporal oscillations which are coupled with periodic transformations of the surface structures between reconstructed and nonreconstructed phases, the latter exhibiting higher oxygen sticking coefficients and hence higher reactivity. With Pt(100) the two surface phases exhibit a much larger difference in reactivity (=oxygen sticking coefficient) than with Pt(110), which effect accounts for the qualitative differences in the oscillatory behavior: if two of the control parameters (say pO2, T) are kept fixed, the third (pCO) may be varied with Pt(100) over a fairly wide range without leaving the oscillatory region. Minor (<1%) fluctuations of the partial pressures associated with the varying reaction rate are hence without any noticeable effect. Coupling between surface reaction and diffusion causes wave propagation of the surface phase transformations and therefore spatial self-organization, as demonstrated by scanning LEED experiments. With Pt(110), on the other hand, the oscillatory region is very narrow. In this case mass transport through the gas phase as caused by the small pressure variations associated with the reaction lead to synchronization between different parts of the surface. Computer simulations with the cellular automaton technique confirm qualitatively the experimental findings and support the conclusions reached.
Photodissociation of vibrationally excited water in the first absorption band
Authors: Weide Klaus , Hennig Steffen , Schinke Reinhard
Published: July 2002

We investigate the photodissociation of highly excited vibrational states of water in the first absorption band. The calculation includes an ab initio potential energy surface for the Ã-state and an ab initio X̃→Ã transition dipole function. The bending angle is fixed at the equilibrium value within the ground electronic state. Most interesting is the high sensitivity of the final vibrational distribution of OH on the initially prepared vibrational state of H2 O. At wavelengths near the onset of the absorption spectrum the vibrational state distribution can be qualitatively understood as a Franck–Condon mapping of the initial H2 O wave function. At smaller wavelengths final state interaction in the excited state becomes stronger and the distributions become successively broader. Our calculations are in satisfactory accord with recent measurements of Vander Wal and Crim.
Electronic energy transport and trapping on fractals
Authors: Pines Dina , Huppert Dan
Published: July 2002

The kinetics of the excitation quenching by acceptors in the presence of excitation migration over the donors is determined on fractals. We have used the Burshtein approach to determine the dynamics of the excitation hopping mechanism. The results were compared with the experimental study of electronic energy transfer of a two-component system [rhodamine 6G (R6G) as the donor and malachite green (MG) as the acceptor] adsorbed on a silica gel 200 surface. Samples ranging from high acceptor–low donor concentrations (the Förster limit) to the opposite regime were studied. The one-parameter fitting procedure yields a fractal dimension of 2.3±0.03. The same dimension has been previously obtained by us in both depolarization measurements and in direct energy transfer experiments.
A stimulated emission pumping study of jet-cooled methyl glyoxal
Authors: Reid S. A. , Kim H. L. , McDonald J. D.
Published: July 2002

Stimulated emission pumping (SEP) spectroscopy has been used to investigate rovibrational level mixing in the ground singlet state of methyl glyoxal. A low resolution (0.35 cm−1) SEP survey spectrum allows accurate frequency assignments for 15 normal vibrations and seven hindered rotor levels. High resolution (0.05 cm−1) SEP spectra are reported for selected vibrational bands, including the fundamental vibrations of the symmetric carbonyl stretch and symmetric carbonyl bend. A van der Waals interaction between the neighboring carbonyl oxygen and methyl hydrogens is found to influence mixing of the carbonyl vibrations, affirming the results of previous studies.
Phase equilibria in copolymer/homopolymer ternary blends: Molecular weight effects
Authors: Broseta Daniel , Fredrickson Glenn H.
Published: July 2002

The phase behavior of ternary blends of an A–B random copolymer with two homopolymers (A and B) is investigated within the Flory–Huggins lattice theory. We restrict consideration to the formation of (isotropic) liquid phases. For compositionally symmetric systems in which the two homopolymers have equal molecular weights, two different topologies are found for the phase diagrams according to the length of the copolymer relative to the homopolymers. Namely, upon lowering the temperature, a three-liquid-phase region emerges either continuously via a tricritical point if the copolymer is long enough, or discontinuously otherwise. This change in phase behavior, an entropy of mixing effect, occurs when the copolymer length is a fraction 2/5 of the homopolymer molecular weight. The properties of the (symmetric) tricritical points are discussed, as well as the phase behavior of systems in which the copolymer is not symmetrical in composition and/or the two homopolymers differ in size. This Flory–Huggins approach should be valid for blends containing random copolymers, but also, at high enough temperatures, for blends that contain block copolymers, the temperature range of validity being broader for smaller block copolymers. The critical behavior of systems containing block copolymers is described by the simple Flory–Huggins theory over a range delineated by isotropic Lifshitz points. These Lifshitz points are located within the random phase approximation.
The effect of water models on the interaction of the sodium–chloride ion pair in water: Molecular dynamics simulations
Authors: Dang Liem X. , Rice Julia E. , Kollman Peter A.
Published: July 2002

We present a comparative study of the potentials of mean force (PMF) of the sodium–chloride ion pair in the simple point charge (SPC and SPC/E) water models using thermodynamic perturbation theory and the molecular dynamics technique. In agreement with previous studies, the PMF’s display two minima corresponding to the contact and solvent-separated ion pairs. However, the contact and solvent-separated regions are of comparable importance for the SPC water model, whereas the solvent-separated region is favored over the contact region for the SPC/E water model.
Frequency dependence of catalyzed reactions in a weak oscillating field
Authors: Robertson Baldwin , Astumian R. Dean
Published: July 2002

The frequency dependence of the average rates of reactions catalyzed by one or more catalysts in a weak oscillating field is derived. The average rates are sums of Lorentzian curves whose characteristic frequencies are the inverse relaxation times of the normal modes of the kinetic system and whose amplitudes are quadratic in the field. The signs of the Lorentz amplitudes can be either positive or negative, so the rates versus frequency can have a variety of shapes, including frequency windows. One can get relaxation times and amplitudes by measuring steady-state rates as a function of the frequency of the field. The theory is applied to determine the Lorentz amplitudes and characteristic frequencies of ion transport rates catalyzed by Na+–K+–ATPase in erythrocytes.
Characterization of the intrachain charge-generation mechanism of electronically conductive poly (3-methylthiophene)
Authors: Sun Z. W. , Frank Arthur J.
Published: July 2002

The charge carriers and band structure that evolve upon doping of electrochemically grown poly (3-methylthiophene) (PMeT) films are characterized as a function of film preparation conditions by optical absorption, infrared, and electron spin resonance measurements. Regardless of the solvent (acetonitrile or propylene carbonate) or dopant anion (ClO−4, PF−6, BF−4, or CF3SO−3 ) used in preparing these films, bipolarons are found to play a prominent role in the charge-generation mechanism of PMeT at doping levels above 2 mol %. At low doping concentrations (<2 mol %), structural disorder effects in the polymer are evident. The amount of structural disorder depends on the nature of the solvent and dopant anion. The binding energy of bipolarons to dopant anions was evaluated and found to be independent of the identity of the dopant anion. Solvent-dependent shifts in the bipolaron gap states and the binding energy are observed. These results are interpreted in terms of current theories of charge-carrier production in conductive polymers.
The shape of linear and star polymers with and without excluded volume
Authors: Bishop Marvin , Clarke Julian H. R. , Rey Antonio , Freire Juan J.
Published: July 2002

The shape of isolated linear and star polymers, with and without excluded volume, has been determined by Monte Carlo simulations using a pivot algorithm. Stars with 3, 4, 5, and 6 arms are studied. The high efficiency of the pivot algorithm has allowed us to obtain accurate data for long chains. Good agreement is found with existing analytical expressions for nonexcluded volume chains and with other computer simulations for excluded volume chains.
Monte Carlo simulations for the behavior of multiblock copolymers at a penetrable interface
Authors: Balazs A. C. , Siemasko C. P. , Lantman C. W.
Published: July 2002

We use Monte Carlo simulations to examine the behavior of a single multiblock AB copolymer near the penetrable interface between two immiscible fluids, A and B. We determine the values of χAB, the interaction energy between comonomer and fluid, that are necessary to localize the chain at the fluid–fluid boundary and investigate the dependence of this value on the block length. Once localized at the interface, we calculate how the conformation of the multiblock copolymer is perturbed from its random coil dimensions. We then compare the interfacial behavior of the multiblock chain for a range of χAB values and for different block lengths. Furthermore, we compare these results with the properties of various diblocks at such an interface. Finally, we examine the behavior of several multiblocks at the fluid–fluid surface. The findings of the simulations are helpful in contrasting the compatibilizing activity of diblock and multiblock copolymers.
The origin of glassy dynamics in the 2D facilitated kinetic Ising model
Authors: Butler Scott , Harrowell Peter
Published: July 2002

Monte Carlo simulation data from the two-dimensional facilitated kinetic Ising model proposed by Fredrickson and Andersen is examined with the goal of understanding the processes responsible for the characteristic features of glassy dynamics. The spatial distribution of spin flip rates is found to be highly nonuniform with pockets of rapidly relaxing spins surrounded by kinetically locked domains. The slow relaxation of these latter domains, which gives rise to the characteristic long time tail of the linear response function, is due to the action of rare clusters of spins which are able to propagate their influence throughout the sample. An analytic expression is derived for the density of these active sites which is found to fully account for the non-Arrhenius temperature dependence of the relaxation time in this model. The consequences of these results for both theories and experiments in structural glasses are discussed.