bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigations of ferroelectric-to-paraelectric phase transition of vinylidenefluoride trifluoroethylene copolymer thin films by electromechanical interferometry
Authors: Urayama Kenji , Kircher Oliver , Böhmer Roland , Neher Dieter
Published: July 2002

Electrostrictive and piezoelectric properties for a 1.3 μm thick film of a vinylidenefluoride/trifluoroethylene copolymer exhibiting a ferroelectric-to-paraelectric phase transition have been investigated as a function of temperature by means of electromechanical interferometry. The electrostriction remarkably increases in the vicinity of the phase transition temperature according to a sharp increase in the dielectric constant. For the unpoled sample in the ferroelectric phase with a polydomain structure in which the local spontaneous polarization is macroscopically cancelled out, the dependence of the electrostriction on the square of the applied electric field is nonlinear in the ferroelectric phase, while it is linear in the paraelectric phase. A theoretical model which takes into account the nonlinear dielectric constant can quantitatively explain the nonlinear contribution to the electrostriction in the vicinity of the ferroelectric–paraelectric phase transition, but it underestimates the contribution in the ferroelectric phase. Interactions within and/or between the ferroelectric domains in the polydomain structure are expected to contribute significantly to the nonlinear electrostriction. For poled samples a pronounced inverse-piezoelectric effect is measured. The achieved polarization and its temperature dependence are almost the same as for thicker films reported in earlier studies, obtained by different experimental techniques.
GaAs/AlGaAs single quantum well p-i-n structures: A surface photovoltage study
Authors: Ashkenasy N. , Leibovitch M. , Rosenwaks Y. , Shapira Yoram , Barnham K. W. J. , Nelson J. , Barnes J.
Published: July 2002

The photovoltage (PV) response of single quantum well p-i-n structures under open circuit conditions has been studied experimentally and numerically. The numerical calculations show a monotonic increase in the PV response with decreasing well width, implying that the ensuing increase in carrier generation rate and band gap governs the PV response. The well layer has been shown to dominate the recombination of excess carriers generated throughout the structure, and their lifetime at the well has been found to be a critical structure parameter. Using a simple semi-empirical model, the effective carrier lifetimes at the well layer/interfaces for the different samples were estimated. The results demonstrate the benefits of using surface photovoltage spectroscopy for characterization and quality control of quantum well structures.
Blue vertical-cavity surface-emitting lasers based on second-harmonic generation grown on (311)B and (411)A GaAs substrates
Authors: Kaneko Yasuhisa , Nakagawa Shigeru , Ichimura Yoshikatsu , Yamada Norihide , Mars Dan E. , Takeuchi Tetsuya
Published: July 2002

We have studied blue vertical-cavity surface-emitting lasers (VCSELs) based on second-harmonic generation (SHG) grown on (411)A and (311)B GaAs substrates in order to investigate suitable substrate orientations for SHG-VCSELs. The comparison among substrate orientations has been made on three parameters, SHG conversion efficiency, transparency current density and gain coefficient. The transparency current density and the gain coefficient are characterized by edge emitting lasers grown on the above substrates. We also discuss the transparency current density and the gain coefficient for (311)A reported previously by Takahashi et al. [M. Takahashi, M. Hirai, K. Fujita, N. Egami, and K. Iga, J. Appl. Phys. 82, 4551 (1997)]. SHG conversion efficiency is 38 and 30% W for SHG-VCSELs grown on (311)B and (411)A substrates, respectively, which is consistent with theory, assuming identical nonlinear coefficients for the A face and B face. Transparency current density for (311)A, (311)B and (411)A is 80, 105 and 60 A/cm2, and gain coefficients for (311)A, (311)B and (411)A are 20, 13 and 18 cm−1, respectively. There are no significant differences between (311)A and (411)A, while those for (311)B are less than those for (311)A. In conclusion, SHG efficiency is dependent on crystal orientation consistent with theory and thus the (311) orientation is preferable. Optical gain is independent of substrate orientation, while it is dependent on substrate face and A face is preferable.
Theoretical analysis of switching times in polarization switching lasers
Authors: Numai Takahiro
Published: July 2002

Switching times for polarization switching lasers are analyzed using two-mode rate equations including carrier dynamics. Two-mode operations are considered in weak, neutral, and strong coupling with a net gain difference between two modes and light output as parameters. It is found that a modulation speed is enhanced with increases in a net gain difference and a light output, and with a decrease in coupling constant. For example, when a net gain ratio α2/α1 is 0.2 and the light output is 20 mW, a modulation speed of 24.3 Gbit/s for nonreturn-to-zero signals is expected.
Comparison study of nucleation and growth characteristics of chemical-vapor-deposited diamond films on CoSi2 (001) and Si(001)
Authors: Gu C. Z. , Jiang X. , Kappius L. , Mantl S.
Published: July 2002

Epitaxial CoSi2 (001) layers, deposited on Si(001) substrates by molecular beam allotaxy, were used as substrates for diamond deposition in order to realize applications. The nucleation and textured growth of diamond films were compared with those on the Si(001) substrates. The results indicate that in a microwave-plasma chamber diamond can be nucleated with a higher density on CoSi2 at lower temperatures using a bias-enhanced method. High-quality [001]-textured diamond films can be synthesized on CoSi2 (001) using the [001]-textured growth conditions. So far, epitaxial growth of diamond on CoSi2 cannot be observed. Statistically, a rotating angle distribution of diamond grains around the [001] axis in a [001]-textured film shows, however, preferred in-plane orientations of 13°, 22°, 45°, and 77° relative to the CoSi2 [011] axis. The structural and chemical analyses show that no Co and Si element diffusion from the CoSi2 substrate into the diamond film can be detected. The films grown on CoSi2 have similar crystal quality to that of epitaxial films deposited directly on Si.
Optical and magneto-optical properties of (CoFe)x(HfO21)1−x magnetic granular films
Authors: Kravets V. G. , Petford-Long A. K. , Kravets A. F.
Published: July 2002

Granular films of composition (CoFe)x(HfO2)1−x with continuous variation of the Co0.5Fe0.5 volume fraction in the range 0.1<x<1 have been prepared by e-beam co-evaporation. The optical properties of the granular films have been determined from transmission and reflection spectra in the 300–1100 nm region. The optical characteristics of the films as a function of x were also determined by the ellipsometrical method at λ=632.8 and 830 nm. The Faraday rotation in the films was determined at λ=632.8 nm. We find that the transmission and reflection spectra and the dielectric function have anomalies near the percolation threshold at x≈0.45. The spectral dispersion of Faraday effects has a peak in the region of the plasma frequency. The Faraday effect is greatly enhanced near xp. We also find that the extraordinary Hall resistivity, measured at room temperature and at 8 kOe of applied field, increases dramatically from 0.2 to about 600 μΩ cm near the percolation threshold. These films also possess large magnetoresistance, whose values are about 4% in maximum for x=0.35–0.45. The calculated optical and magneto-optical parameters and the extraordinary Hall resistivity, described in the framework of the effective medium approximation, are in qualitative agreement with experimental data.
The origin of current gain under illumination in amorphous silicon n-i-p-i-n structures
Authors: Chatterjee P. , Vanderhaghen R. , Equer B.
Published: July 2002

Detailed computer modeling in conjunction with experiments has been used to explain the current gain sometimes observed in amorphous silicon n-i-p-i-n structures under illumination and to specify under what conditions such a gain is possible. Calculations reveal that the origin of the current gain at a given applied voltage, is excess injection of electrons from the contacts as a result of the lowering of the i-layer/n-layer barrier in the forward biased diode on account of photogenerated hole trapping. However to actually obtain this gain, the thickness and the density of states of the partitioning p layer must be small enough, so that all the excess injected electrons do not recombine in the p layer before being collected in the reverse biased diode. We also find that under conditions for which the gain varies linearly with voltage, it is almost independent of the incident light flux. On the other hand, when the barrier height at the p layer (Ec−EF0)max, where Ec is the conduction band edge and EF0 the thermodynamic equilibrium Fermi level, approaches its maximum value corresponding to a thick or highly doped p layer, there is no gain. This is the condition to be fulfilled for a color sensitive device.
Dielectric breakdown in (Pb,La)(Zr,Ti)O3 ferroelectric thin films with Pt and oxide electrodes
Authors: Stolichnov I. , Tagantsev A. , Setter N. , Okhonin S. , Fazan P. , Cross J. S. , Tsukada M.
Published: July 2002

Dielectric breakdown of (Pb,La)(Zr,Ti)O3(PLZT) film capacitors with Pt and SrRuO3 electrodes, in its initial or virgin state and after voltage cycling that causes polarization fatigue, is studied by constant-current measurements. It is shown that for different types of PLZT capacitors, the breakdown onset is controlled by the critical charge flown through the film (charge to breakdown) rather than the voltage applied to the capacitor. The charge to breakdown for the asymmetrical Pt/SrRuO3/PLZT/Pt capacitors is found to be much higher that that for the Pt/PLZT/Pt system. Polarization fatigue caused by bipolar voltage cycling provokes a substantial decrease of charge to breakdown. These results can be interpreted in terms of percolation model used for breakdown in SiO2, where the charge to breakdown is associated with injected-carrier-assisted creation of the critical concentration of defects required for formation of the breakdown paths. In this article we propose two possible mechanisms for interpretation of the observed variation of the PLZT capacitor breakdown performance. One mechanism is based on different contributions of PLZT film grains and grain boundaries to the total current flowing through the system. The second mechanism is related to the different kinetics of generation and relaxation of defects in the band gap of the PLZT film. The decrease of the charge to breakdown induced by polarization fatigue is explained by generation of defects during the fatigue bipolar polarization reversals.
Electroluminescence of different colors from polycation/CdTe nanocrystal self-assembled films
Authors: Gao Mingyuan , Lesser Constanze , Kirstein Stefan , Möhwald Helmuth , Rogach Andrey L. , Weller Horst
Published: July 2002

Water soluble thiol capped CdTe nanocrystals are assembled into ultrathin films in combination with poly(diallyldimethylammonium chloride) (PDDA) by the self-assembly method of layer-by-layer adsorption of oppositely charged polyelectrolytes. Electroluminescent devices, which produce different color emissions, are fabricated by sandwiching CdTe/PDDA films between indium–tin–oxide (ITO) and aluminum electrodes using CdTe nanocrystals of different sizes. It is shown that the electroluminescence (EL) spectra of the CdTe/polymer films are nearly identical to the photoluminescence spectra of the corresponding CdTe nanocrystals in aqueous solutions. The devices produce room-light visible light output with an external quantum efficiency up to 0.1%. Light emission is observed at current densities of 10 mA/cm2 and at low onset voltages of 2.5–3.5 V, which depends on the thickness of the film indicating field-dependent current injection. A variation of the EL efficiency with the size of the CdTe particles is observed and explained by the size dependent shift of the CdTe energy levels with respect to the work function of the electron injecting Al electrode. This is confirmed by the behavior of two-layer devices prepared from two differently sized CdTe particles being spatially separated, i.e., one size CdTe near ITO and the other size CdTe near Al by using the self-assembly method.
Electromechanical response of ionic polymer-metal composites
Authors: Nemat-Nasser Sia , Li Jiang Yu
Published: July 2002

An ionic polymer-metal composite (IPMC) consisting of a thin Nafion sheet, platinum plated on both faces, undergoes large bending motion when an electric field is applied across its thickness. Conversely, a voltage is produced across its faces when it is suddenly bent. A micromechanical model is developed which accounts for the coupled ion transport, electric field, and elastic deformation to predict the response of the IPMC, qualitatively and quantitatively. First, the basic three-dimensional coupled field equations are presented, and then the results are applied to predict the response of a thin sheet of an IPMC. Central to the theory is the recognition that the interaction between an imbalanced charge density and the backbone polymer can be presented by an eigenstress field (Nemat-Nasser and Hori, Micromechanics, Overall Properties of Heterogeneous Materials, 2nd Ed., Elsevier, Amsterdam, 1999). The constitutive parameter connecting the eigenstress to the charge density is calculated directly using a simple microstructural model for Nafion. The results are applied to predict the response of samples of IPMC, and good correlation with experimental data is obtained. Experiments show that the voltage induced by a sudden imposition of a curvature, is two orders of magnitude less than that required to produce the same curvature. The theory accurately predicts this result. The theory also shows the relative effects of different counter ions, e.g., sodium versus lithium, on the response of the composite to an applied voltage or a curvature.
Measurement of stacking fault densities in CoCrPt thin film media using grazing incidence x-ray scattering
Authors: Holloway L. , Laidler H.
Published: July 2002

In this work, synchrotron x-ray measurements are reported on a set of CoCrPt media grown on NiP/Al substrates in varying base pressures. A grazing incidence geometry was employed to eliminate the large background scatter from the underlayers and amorphous substrate. The percentage of stacking faults was found to increase with increasing base pressure, varying from 9% to 30% for samples grown at 10−8 and 10−4 Torr, respectively. We have also found that the in-plane c-axis orientation is destroyed when the sample is grown at the highest base pressure of 10−4 Torr. These structural differences are found to correlate well with the magnetic properties of the media.
Buried word line planarization and roughness control for tunnel junction magnetic random access memory switching
Authors: Sousa R. C. , Soares V. , Silva F. , Bernardo J. , Freitas P. P.
Published: July 2002

Spin dependent tunnel junctions were fabricated on top of buried word lines. This was achieved modifying an existent 1.2 μm complementary metal–oxide–semiconductor backend metalization process. The word lines are 0.2 μm thick and 4 μm wide. Roughness over the buried line was decreased to 0.2 nm (rms), performing a spin-on-glass local planarization step. Tunnel junctions with 9–13 Å plasma oxidized Al barriers were patterned on top of the word line with different aspect ratios from 3×2 μm2 to 7×1 μm2 areas. Magnetoresistance values reach over 20% for top free electrode configurations. In this inverted structure the magnetization of the free electrode can be fully reversed with the field from word line. The word line creates a field of 0.7 Oe per mA. Applying simultaneously two perpendicular fields reduces the threshold for magnetization switching. The fabricated structure can be used to assess the junction switching mechanism for tunnel junction magnetic random access memories.
Nanosized powders of NiZn ferrite: Synthesis, structure, and magnetism
Authors: Albuquerque Adriana S. , Ardisson José D. , Macedo Waldemar A. A. , Alves Maria C. M.
Published: July 2002

The structure and magnetic behavior of nanostructured powders of stoichiometric NiZn ferrite, Ni0.5Zn0.5Fe2O4, synthesized by coprecipitation, are investigated by extended x-ray-absorption fine structure spectroscopy (EXAFS), x-ray diffraction, Mössbauer spectroscopy, and vibrating sample magnetometry. Samples of high purity and high homogeneity were obtained by annealing at relatively low temperatures (300–800 °C) resulting in nanoparticles with average diameter between 9 and 90 nm, as determined by x-ray diffraction. EXAFS was applied to follow Ni, Zn, and Fe cations distribution and the evolution of the short range order of the samples with increasing annealing temperature. Our results show ferrimagnetic NiZn ferrite nanosized powders with high purity, 1:1 Ni to Zn stoichiometric ratio and superparamagnetic behavior. Moreover, the samples exhibit good structural ordering already after heat treatment at 400 °C. Analysis by vibrating sample magnetometry indicated a critical particle diameter for the transition from monodomain to multidomain behavior close to 40 nm.
Coupling between eddy currents and Landau–Lifshitz dynamics
Authors: Mayergoyz I. D. , Serpico C. , Shimizu Y.
Published: July 2002

In the article, self-consistent numerical solution of the magnetic field diffusion equation and Landau–Lifshitz equation is considered for the one-dimensional geometry and for linearly and circularly polarized applied magnetic fields. The effects of the magnetization dynamics on the diffusion process are discussed.
A delay of percolation time in carbon-black-filled conductive polymer composites
Authors: Wu Guozhang , Asai Shigeo , Zhang Cheng , Miura Tadashi , Sumita Masao
Published: July 2002

This work attempts to determine how percolation at an equilibrium state is correlated to percolation under experimental conditions. The dynamic process of forming conductive networks in carbon-black (CB)-filled poly(methyl methacrylate) composites was investigated by real-time tracing the time dependence of electrical resistivity during isothermal treatments. It was observed that the dynamic percolation curves maintains the same shape and shift to a shorter percolation time with increasing annealing temperature and filler concentration. An Arrhenius plot of the shift factor against the annealing temperature shows a linear relationship, irrespective of the filler concentration, and the activation energy of the percolation time is close to the activation energy of the zero-shear-rate viscosity of the polymer matrix. Furthermore, an increase in the thermodynamic interactions between CB and the polymer matrix causes a large reduction in polymer mobility, resulting in an increase in the percolation time. These results lead to the conclusion that percolation is delayed by the bulk mobility of polymer layers surrounding CB particles. An experimental approach for determination of the retardation time is proposed based on theoretical analysis of the dynamic movement of the carbon particles. It is suggested that the difference in the kinetic history with respect to percolation among different composite systems can be eliminated by normalizing the experimental conditions to the same value of retardation time.
Saturation kinetics of the S0 to S2 optical transition in isolated diphenylhexatriene
Authors: Kohler Bryan E. , Spiglanin Thomas A.
Published: July 2002

A three level kinetic model is fitted to fluorescence intensities measured as a function of excitation intensity for the S0 to S2 transition of isolated diphenylhexatriene. Parameters derived in the fit are coupled with experimental observations to bracket the rate of energy redistribution from S2 to vibrationally excited levels of S1 and S0 to between 1010 and 4×1012 s−1. The rate of excitation, which is directly proportional to oscillator strength, is precisely determined in the fit to be 3.7×108 s−1 per μJ at the peak of our 6 ns FWHM laser pulse.
The microwave spectrum of the PF radical in the ground X 3Σ− electronic state
Authors: Saito Shuji , Endo Yasuki , Hirota Eizi
Published: July 2002

The microwave absorption spectrum of the PF radical in the X 3Σ− state was detected by using a source modulation microwave spectrometer. The radical was produced in a free space cell by a dc glow discharge in a mixture of PH3 and CF4. Five rotational transitions with N=1 ← 0 up to 5 ← 4 were observed in the frequency region of 44 to 171 GHz and were least-squares analyzed to determine the rotational constant, the centrifugal distortion constant, the spin–spin coupling constant with its centrifugal distortion term, the spin-rotation coupling constant, and the magnetic hyperfine coupling constants for both phosphorus and fluorine nuclei. A centrifugal correction term was found to be necessary for the c magnetic hyperfine constant of phosphorus in order to achieve a good fitting of the observed spectrum. The present data agree with, but considerably improve the accuracies of ground-state molecular constants which Douglas and Frackowiak derived from high-resolution emission spectra. The spin density and the s character of the unpaired-electron orbital were calculated from the observed magnetic hyperfine coupling constants to be 91.3% and 0.88% for P and 9.1% and 0.17% for F.
Intramolecular dynamics of the unimolecular dissociation of CS2: Analysis of the classical phase space structure
Authors: Kato Shigeki
Published: July 2002

The mechanism of unimolecular dissocation reactions of carbon disulfide (CS2) has been studied theoretically. Analytic forms of potential energy surfaces were constructed using the results of ab initio configuration interaction (CI) calculations. A simplified rotation-vibration Hamiltonian was derived to carry out the classical trajectory calculations and to analyze the classical phase space structure of high vibrationally excited CS2 molecule. It was found that two different types of trajectories, the normal mode type quasiperiodic and the chaotic trajectory, coexist even above the dissociation threshold. The vibrational energy levels, calculated by the semiclassical quantization for the normal mode type trajectories, showed a nearly equal spacing. Based on the results of present calculations, the possibility of mode selective infrared (IR) multiphoton excitations and dissociation were examined.
Observation of activation energies in the chemiluminescent reactions of Sr with Cl2 and Br2 at collision energies below 4 eV
Authors: Meyer H.-J. , Ross U. , Schulze Th.
Published: July 2002

In a molecular beam experiment the chemiluminescence M+X2 → MX*+X in the reactive scattering of strontium atoms and halogen molecules Cl2 and Br2 has been studied below 4 eV collision energy. Relative integral reactive cross sections as a function of collision energy reveal large activation energies. Dynamical aspects of endothermic reactions are discussed. A simple kinematic transformation yields a classical limit for the rotational excitation of the product molecule.
Theory of monolayer physical adsorption. III. Density functional theory of two-dimensional nonuniform fluids and theory of freezing
Authors: Sokolowski Stefan , Steele William
Published: July 2002

Density functional theory is applied to the description of submonolayer physical adsorption of spherically symmetric molecules on an exposed simple crystal face of an inert solid. To support calculations of the thermodynamic functions characterizing adsorption, a method of computation of the one- and two-particle distribution functions of a nonuniform adsorbed fluid is presented. With some knowledge of the structural information for the nonuniform fluid we next describe a theory of freezing of submonolayer films. Theoretical arguments are supplemented by numerical calculations of the freezing of hard spheres on a periodic solid surface. The results obtained are compared with experiments and computer simulations for rare gases on graphite.
The displacement reactions of the triatomic rare gas halide excimers
Authors: Sauerbrey R. , Tittel F. K. , Wilson W. L. , Zhu Y.
Published: July 2002

Rate coefficients are measured for the quenching of Ar2F by Krypton and Xenon and Kr2F by Xenon. The results are compared with a classical calculation for these rate coefficients. (AIP)
Pressure dependence of the 2.3 μm laser-induced fluorescence (LIF) of room temperature PuF6
Authors: Rice W. W. , Barefield J. E.
Published: July 2002

When PuF6 is excited in the 800 nm region, fluorescence peaked at 2.296 μm with less intense peaks at 2.207 and 2.126 μm is observed. Unlike the previous works that reported no self-quenching of the 2.3 μm LIF of PuF6, our data indicate a pressure dependence for the 2.3 μm LIF lifetime with a self-quenching rate constant of 162±4 Torr−1 s−1 and a collision-free lifetime of 218±5 μs. Both fluorescence and excitation spectra are in good agreement with the previously reported absorption spectrum of PuF6.
Collisional quenching of excited iodine atoms (5p5 2P1/2) by Cl2 in a flow system
Authors: Hall G. E. , Arepalli Sivaram , Houston P. L. , Wiesenfeld J. R.
Published: July 2002

Time-resolved infrared emission from photolytically generated I*(2P1/2) has been studied in a slow flow apparatus. The total rate of deactivation of I*(2P1/2) by Cl2 has been measured to be no more than 8×10−15 cm3 molecule−1 s−1, substantially slower than previous reports. Evidence is presented for a very fast (k≂2×10−10 cm3 molecule−1 s−1) relaxation of I* by Cl atoms, which can account for both the earlier and the present observations.
Floquet theory analysis of stable and unstable motion
Authors: Thiele Everett , Goodman Myron F. , Stone James
Published: July 2002

Floquet theory is applied to the stability analysis of linear triatomic molecules having local Morse bonding potentials. We identify conditions for the existence of a symmetric mode and analyze its stability as a function of the energy and symmetric to antisymmetric normal mode frequency ratio. In the stable region we apply a special phase normalization to the Floquet eigenvalues that allows us to identify factors in the Floquet index with a red-shifted generalized antisymmetric mode frequency. Instabilities set in when the ratio of this frequency to the symmetric mode frequency is integer or half-integer. Analytic forms for Poincaré surfaces of section valid for the linearized theory are derived and compared with actual trajectory intersections for both stable and unstable cases. In the stable (quasiperiodic) cases, surfaces corresponding to different sections are ellipses with varying eccentricity but constant area. Hyperbolas are obtained in unstable cases.
Relative atomic radii from scaling of repulsive interatomic force in noble gas van der Waals molecules
Authors: Carlton Terry S.
Published: July 2002

The repulsive contribution Fij(Rij) to the interatomic force of every noble gas atomic pair can be transformed into FArAr by scaling of R alone. The scale factors Q satisfy the combining rule Qij=1/2(Qii+Qjj). Homonuclear Qii constitute a set of relative atomic radii at equal F and are invariant to F. One can use the Qii to partition Rij into contributions from each atom. Changes in the radius of a particular atom are explained in terms of attractive forces, as are differences between Qii and relative radii based on homonuclear equilibrium distances Rm. Values of the Qii are much closer to relative radii at equal atomic electron density or equal repulsive potential than to relative radii based on Rm.
Hydration of the Dy3+ ion in dysprosium chloride solutions determined by neutron diffractiona)
Authors: Annis B. K. , Hahn R. L. , Narten A. H.
Published: July 2002

The coordination of water molecules about the Dy3+ ion in a 2.38 m DyCl3 in D2O solution has been determined by neutron scattering. The information was obtained using samples that were identical except for the isotopic species of the Dy3+ ions. The experiment yields the distribution of the deuterium and oxygen atoms in the first hydration shell. Each Dy3+ ion is surrounded by 7.4±0.5 water molecules. The dysprosium–oxygen separation is 2.37 Å and the dysprosium–deuterium separation is 3.04 Å. The cation–water molecule orientation is closer to planar than that which has generally been observed in other aqueous solutions.
2 1S excitation of helium: A precise distorted wave approach
Authors: Srivastava Rajesh , Kumar Mukesh , Tripathi A. N.
Published: July 2002

The differential and total cross sections are evaluated for 1 1S–2 1S excitation of helium atom by electron impact in distorted wave approximation. The effect of the distortion of incident electron, contribution due to polarization of target and the exchange effect are appropriately taken in both the initial and final channels. Fourier decomposition of interaction potential between the projectile and target is taken and an accurate form factor has been used to calculate the transition matrix. The present results are compared with various distorted wave models as well as other theoretical and experimental results.
Time resolved surface and volume generated photocurrent in a fluid aromatic solution
Authors: Prock Alfred
Published: July 2002

Time resolved photocurrent following a pulse was measured for the system tetraphenylporphyrin solution in fluorobenzene contained between tin oxide semiconductor electrodes. Analysis is presented in terms of surface and volume generation of photocurrent. The one-dimensional diffusion equation for triplet state excitation diffusion to the electrode is modified to include a nonlinear term for bimolecular excitation self-quenching. Numerical computation for selected parameters provides a good fit for both amplitude and shape of photocurrent transients over a range of a factor of 480 in initial excitation density. Both excitation lifetime and the quenching rate constant are evaluated. Results are consistent with the model for extrinsic photocurrent generation based on the precursor being bulk triplet state excitation in such aromatic systems. Additionally, computation demonstrates that the quantum efficiency of extrinsic photocurrent production remains high when electrode separation is comparable with excitation diffusion length, falling by only 30% when the pseudo-first order quenching rate constant is as large as ten times the inverse triplet lifetime.
The microwave and radiofrequency spectrum of H2S⋅Ar
Authors: Viswanathan R. , Dyke T. R.
Published: July 2002

Molecular beam electric resonance spectroscopy has been used to determine the radiofrequency and microwave spectrum of H2S⋅Ar, HDS⋅Ar, and D2S⋅Ar. The electric dipole moment and nuclear hyperfine interactions reflect an angular structure in which the H2S C2 axis is nearly perpendicular to the Ar...S axis, and the H2S and argon are nearly coplanar. The effective argon–H2S center-of-mass distance is 3.977 Å.
Time-resolved detection of Cu atoms during photochemical laser metal vapor deposition
Authors: Marinero E. E. , Jones C. R.
Published: July 2002

A pump/probe laser approach is utilized to investigate in a time-resolved manner the photochemistry of copper coordination complexes in the gas phase. These compounds are metal atom precursors for photochemical laser metal vapor deposition. Dissociation is effected by a KrF excimer laser and the copper atoms are detected via laser induced fluorescence. Evidence is provided that suggests that under excimer laser photolysis, the metal atom generation may proceed via unimolecular decomposition.
Dynamics of the H+D2→HD+D reaction: Dependence of the product quantum state distributions on collision energy from 0.98 to 1.3 eV
Authors: Gerrity Daniel P. , Valentini James J.
Published: July 2002

Measurement of the nascent HD product rotational and vibrational state distributions for the H+D2→HD+D reaction is reported. Ultraviolet photolysis of HI using a pulsed laser at 291 or 280 nm is used to create H atoms in an HI/D2 gas mixture, giving H+D2 collisions with relative energies of 0.98 or 1.1 eV. Pulsed-laser coherent anti-Stokes Raman scattering (CARS) spectroscopy is used to record rotationally and vibrationally resolved Raman spectra of the HD product of the photolytically initiated H+D2 reaction, under effectively single-collision conditions. The HD product quantum state distributions are extracted from these CARS spectra. The present data are combined with results we obtained previously at 1.3 eV collision energy to reveal the collision energy dependence of the product quantum state distributions. We find that at all three collision energies the product distributions can be quite accurately represented by a linear rotational and vibrational surprisal function. The rotational surprisal parameter is large, positive, and nearly constant (θR=3.0–3.5) over this energy range, indicating a strong and consistent dynamical bias against product rotational excitation. The vibrational surprisal parameter, in contrast, varies much more, and is not even a monotonic function of collision energy (λV=2.2, 3.3, and 2.6 at 0.98, 1.1, and 1.3 eV, respectively). This behavior indicates enhancement of the cross section for the vibrationally nonadiabatic reaction channel at certain energies. At all collision energies 69%–73% of the available energy appears as translational energy of the products, with 22%–24% in rotation, and only 4%–7% in vibration. Product quantum state distributions derived from quasiclassical trajectory calculations at 0.98 and 1.3 eV collision energies agree very well with our measurements, indicating that classical mechanics provides an adequate description of the dynamics at these energies, and that the ab initio H3 potential surface used in these calculations must be at least reasonably accurate.
An a
b i
n
i
t
i
o calculation of spectroscopic properties of SiO and HOSi+
Authors: Botschwina Peter , Rosmus Pavel
Published: July 2002

Spectroscopic properties of SiO and HOSi+ have been calculated from hgihly correlated wave functions. HOSi+ is linear with re=0.959 Å and Re=1.534 Å, but the bending potential is extremely flat. The wave numbers of the fundamental vibrations of H16O28Si+ (in cm−1) are predicted to be ν1=3658, ν2=248, and ν3=1110 with an uncertainty of about 20 cm−1. While the dipole moment of HOSi+ is very small (which will make detection of pure rotational transitions of this ion a difficult task), large intensities are predicted for stretching vibrational transitions both in absorption and emission. The proton affinity of silicon monoxide is calculated to be 8.44 eV.
The electronic and electrochemical properties of poly(isothianaphthene)
Authors: Kobayashi M. , Colaneri N. , Boysel M. , Wudl F. , Heeger A. J.
Published: July 2002

The physical and electronic properties of poly(isothianaphthene) PITN, are reported, including initial characterization, electrochemical cyclic voltammetry, spectroscopy, and transport properties. PITN has the smallest energy gap of any known conjugated organic polymer, Eg≂1 eV. This novel conjugated polymer exhibits reversible chemical and electrochemical p-type doping with an associated high contrast color change. After doping, thin films of PITN have very low optical density in the visible portion of the spectrum. Thus, PITN is the first example of a transparent highly conducting polymer (σ∼50 Ω−1 cm−1).
A computational model of the energy of dissociative chemisorption
Authors: Baetzold R. C.
Published: July 2002

A computational surface potential method is developed to describe the energy barrier of dissociative chemisorption on fcc(111) surfaces. The energy at any point on the reaction coordinate, involving simultaneous tipping and bond elongation of a chemisorbed diatomic fragment, is determined by conservation of total bond order. The potential energy profile is determined by a balance between repulsive closed-shell interactions between the free end of the diatomic fragment with the surface atoms bound to the opposite end of the molecule and attraction between other surface atoms with the free end of the diatomic fragment. The heat of chemisorption of the atomic constituents plays a major role in determining the activation barrier of dissociation. There is considerable surface anisotropy in the activation barrier, with bridge sites being most favorable for dissociation and hollow sites least favorable. Activation barriers are directly dependent upon molecular vibration frequency for on-top and bridge sites. Low activation barriers computed for some sites correlate with low molecular vibrational frequencies of surface species. Comparison of computed activation barriers with an analytic formalism gives good correspondence.
Detection of the bending fundamental band of DN+2 by diode laser absorption spectroscopy
Authors: Sears Trevor J.
Published: July 2002

The detection of 28 rotational lines in the bending fundamental band of DN+2 is reported. The spectra were obtained in a modulated discharge of D2 and N2 in a large hollow cathode which was cooled to liquid nitrogen temperature. Analysis leads to accurate values for the band origin and all of the major parameters describing the (0110) state of the ion.
The polymorphic phases of the hexaalkanoyloxytriphenylene liquid crystals, as studied by deuterium NMR spectroscopy
Authors: Goldfarb D. , Lifshitz E. , Zimmermann H. , Luz Z.
Published: July 2002

Deuterium NMR spectra are reported for several specifically deuterated hexaalkanoyloxytriphenylenes in their corresponding liquid crystalline phases. The higher homologs of this series are polymorphic and exhibit a variety of discotic mesophases, including both biaxial (D0 and D1) and uniaxial (D2) columnar phases. The ordering characteristics of these phases are studied using the quadrupolar splittings of the aromatic and aliphatic deuterons. The results show that during the transition from the biaxial D1 to the axial D2 phase the major susceptibility tensor switch orientation, apparently due to strong tilting of the molecules with respect to the columnar axis in the biaxial phase. Characteristic features which appear in the spectra of these phases are interpreted in terms of intercolumnar jumps of mesogen molecules.
Vibrational energy transfer from highly excited anharmonic oscillators: Quasiclassical Monte Carlo trajectory study of Br2–Ar and Br2–Br system
Authors: Koshi M. , Itoh H. , Matsui H.
Published: July 2002

Thermally averaged rate coefficients for vibrational energy transfer of Br2 by Ar and Br atoms are calculated by a quasiclassical Monte Carlo trajectory method. In the Br2–Br system, both the nonreactive and the reactive energy transfer rates are calculated using the potential energy surface derived from a VB-DIM method. The quantum number dependence of the transition rates is investigated over a wide range of initial vibrational levels at T=2000–3500 K. It is found that the vibrational scaling law is very much different from the predictions based on the simple first-order theories of the vibrational energy transfer. A surprisal analysis of the results shows that the surprisals depend linearly on the absolute values of the quantum number difference. The characteristics of the energy transfer in the highly excited vibrational states are discussed.
Two-center pseudostate calculations of He2 dispersion coefficients
Authors: Magnasco Valerio , Figari Giuseppe
Published: July 2002

Starting with the Hartree–Fock wave function for the ground state, the C6,C8, and C10 dispersion coefficients are calculated for He2. (AIP)
Vibrational resonances and IR multiphoton excitation
Authors: Brown Robert C. , Wyatt Robert E.
Published: July 2002

The influence of an internal vibrational resonance on the IR multiphoton excitation of a model molecular system is studied numerically using Floquet theory. It is shown that a classical resonance zone can bottleneck photon absorption. The bottleneck effect is easily understood in terms of the strongly perturbed phase space topology near a classical resonance zone and the corresponding distortions in quantum mechanical eigenstates and eigenvalues of the field-free Hamiltonian.
Solvation thermodynamics of water in aqueous solutions. I. Theory and limiting effect of solutes
Authors: Ben-Naim A.
Published: July 2002

A new set of measures of the limiting effects of a solute on the solvation thermodynamics of water is defined. It is argued that these measures may be used to probe the effect of various solutes on the environment of a water molecule in aqueous solutions. Some numerical examples are presented, and a qualitative comparison with other methods is discussed.
High resolution spectrum of Tl–Hg excimer
Authors: Chilukuri Santaram , Pearce Willard J. , Winans J. Gibson
Published: July 2002

Emission, absorption, and fluorescence spectra have been obtained from a mixture of thallium and mercury vapors. In absorption, a number of narrow band systems near the sharp and diffuse series lines of Tl, extending primarily toward longer wavelengths, were observed. In emission, bands with intensity maxima at 3808, 4296, 4585, 5225, 5385, and 6558 Å have been observed with excitation by a Tesla coil. Most of these systems were also observed in sensitized fluorescence using a cooled mercury arc, or a zinc or an aluminium spark as a source. 7 2S of Tl appears to be the parent atomic state for all the molecular emissions in the visible. The bands at 5225 and 4296 are likely due to HgTl2. The bands are 4585 and 6558 are due to TlHg. Emission and absorption spectra have been recorded with a dispersion of 1.31 Å/mm in the red and 0.873 Å/mm in the ultraviolet. Vibrational structures have been resolved for all the band systems, except for the weak 4296 band, and the molecular constants and potentials derived.
The observation of stimulated emission in the 119 to 149 nm range from HD excited by picosecond 193 nm radiation
Authors: Luk T. S. , Egger H. , Müller W. , Pummer H. , Rhodes C. K.
Published: July 2002

Intense stimulated emission in the 119 to 149 nm region is observed from HD excited by two quanta at 193 nm. For certain transitions, the conversion efficiency approaches 1% so that peak powers of ∼10 MW are produced. Electron collisions with the HD (E,F) level are found to efficiently transfer population to the HD(C) state which produces stimulated emission on several C → X Werner band lines in the vicinity of 120 nm. The data on the pressure dependence of the stimulated signals strongly support the conclusion that, in comparison to H2, HD exhibits a large isotopic dependence on the cross section for rotationally inelastic electron collisions in electronically excited states. The rate constant for this rotationally inelastic process is estimated as ∼2×10−7 cm3/s. This is the first observation of an isotopically sensitive electron collisional process involving an electronically excited level.
Dynamics of nonadiabatic reactions (theory). I. Branching ratios for early and late seams
Authors: Mayne H. R. , Polanyi J. C. , Tully J. C.
Published: July 2002

The 3D classical trajectory surface hopping (TSH) method has been applied in a ‘‘model’’ study of factors governing nonadiabatic reaction, A+BC→AB+C* and →AB+C. In the diabatic approximation the potential-energy surfaces (pes) were a LEPS surface for F+H2 (→AB+C*) and a repulsive pes Vrep (→AB+C). These intersected in the exit valley to give an early or a late seam (E or L, perpendicular to the exit valley). The splitting at the avoided crossing 2ε was adjusted to ε=1.26 or 5.02 kcal/mol. The ratio of reactive cross sections onto the upper and lower adiabatic pes ρ* was investigated for mass combinations H+HL, L+HL, L+HH, and H+LL with E and L seams, and for small and large ε. The effect on ρ* of reapportioning a constant total energy (ETOT=13.84 kcal/mol) between reagent translation T and vibration V was examined for these 16 cases. Since the velocity in the coordinate of separation increased with increased T (yielding increased product translation; ΔT→ΔT′) ρ* also tended to increase with T. The extreme mass combinations H+HL and L+HH exhibited modified ρ* due to markedly differing widths in the entry and exit valley. The strongly skewed pes for H+LL led to multiple crossing of the seam which reduced ρ*. For other mass combinations ρ* was reduced by the inability of the low T′ component of the product to hop across the 2ε gap. In all cases ρ* was an index of the local dynamics at the seam, and hence shed light on the intermediate motions en route to the asymptotic outcome V′, R′, T′.
On the corotating frame and evolution equations in kinetic theory
Authors: Eu Byung Chan
Published: July 2002

The corotating frame evolution equations are obtained for fluxes by means of the kinetic equation. The evolution (constitutive) equations thus obtained are now fully consistent with the evolution equations in continuum mechanics. A pair of rules are given for obtaining the corotating frame versions from the fixed frame evolution equations previously derived from the kinetic equation.
Atom–diatomic molecule kinetic theory cross sections
Authors: Curtiss C. F. , Tonsager M. W.
Published: July 2002

Classical expressions for the generalized or kinetic theory cross sections for gases of diatomic molecules derived earlier are specialized to describe atom–diatomic molecules collisions and developed further. Numerical results based on parameters intended to describe Ar–N2 collisions are then presented and the resulting generalized cross sections are compared with those derived from experimental studies of the Senftleben–Beenakker effect on the viscosity of mixtures of argon and nitrogen. The results indicate that the experimental values may be reproduced using reasonable values of the anisotropy parameters.
Structures of C6H7+ ions formed in unimolecular and bimolecular reactions
Authors: Lias Sharon G. , Ausloos P.
Published: July 2002

The structures, isomerization mechanisms, and reaction kinetics of C6H7+ ions formed in a variety of systems have been studied. The ions formed in the reactions (H2C=C=CH2++H2C=C=CH2) and (HC=CCH3++HC=CCH3), as well as the fragment C6H7+ ions in 1,3-cyclohexadiene, 1,4-cyclohexadiene, trans-1,3,5-hexatriene, 1-methylcyclopentene, 3-methylcyclopentene, and 4-methylcyclopentene exhibit at least two structures under the conditions of an ICR experiment. In each case, one isomer transfers a proton to bases with proton affinity higher than that of benzene demonstrating that this species has the benzenium (protonated benzene) structure. The remaining C6H7+ ions have in some systems (e.g., allene++allene, 1- and 4-methylcyclopentene) a conjugate base with a proton affinity of 202 kcal/mol, while in other systems (e.g., 3-methylcyclopentene, trans-1,3,5-hexatriene) the conjugate base of the nonbenzenium isomer has a proton affinity of 205 kcal/mol. When the C6H7+ ions are formed by fragmentation of a C6H8+ ion (in the cyclohexadienes or trans-1,3,5-hexatriene) which has a well-defined internal energy, it is seen that the fraction of C6H7+ ions exhibiting the benzenium structure decreases with increasing energy. The fraction of ions with the benzenium structure also decreases with increasing internal energy of the (C3H4++C3H4) reaction complex from 0.72 in allene (energy level of separated reactants: 315 kcal/mol) to 0.62 in propyne (energy level of separated reactants: 327 kcal/mol). The energetics of the fragmentation process leading to C6H7+ formation have been examined in a photoelectron-photoion coincidence spectrometer for trans-1,3,5,-hexatriene and 1,3- and 1,4-cyclohexadiene. It is found that the transition state for the fragmentation process (C6H8+→C6H7++H) is effectively the same in all three systems but lies at an energy level higher than (benzenium ion +H) products. Rate constants for reactions of benzenium ions with a variety of organic and inorganic compounds have been determined.