bluuebunny/crossref_metadata_2025 huggingface dataset:
On the positive charge and interface states in metal-oxide-semiconductor capacitors
Authors: Meinertzhagen A. , Petit C. , Yard G. , Jourdain M. , El Hdiy A.
Published: July 2002

We have studied the defects created in p-metal-oxide-semiconductor capacitors by Fowler– Nordheim injection. This injection has been performed either from the gate or from the substrate. We have shown that the oxide keeps no memory of the trapped holes created by an injection from the gate, once they have been neutralized. Nevertheless, we think that the corresponding traps are stress created by a mechanism similar to that which creates the slow states. The trapped hole annihilation has no influence on the number of interface states or slow states present after stress in the oxide. The increase of the interface state density with injected charge depends on whether the injection is from the gate or from the substrate. In both cases, the increase of the interface density differs from that of the trapped holes and slow states which suggests that the formation mechanisms of trapped holes and slow states are not the same as those of interface states.
Electron density effects in the modulation spectroscopy of strained and lattice-matched InGaAs/InAlAs/InP high-electron-mobility transistor structures
Authors: Dimoulas A. , Davidow J. , Giapis K. P. , Georgakilas A. , Halkias G. , Kornelios N.
Published: July 2002

The effects of the channel electron density on the interband optical transitions of strained (x=0.6 and 0.65) and lattice-matched (x=0.53) InxGa1−xAs/In0.52Al0.48As/InP high-electron-mobility transistor structures have been investigated by phototransmittance at room temperature. Analysis of the ground and first excited transitions for low and high densities, respectively, enabled a separate estimation of the electron densities occupying each one of the first two subbands. It was found necessary to include the modulation of the phase-space filling in the analysis of the spectra, especially for the samples with a high electron density, in which case this modulation mechanism becomes dominant.
Temporal and spatial evolution of C2 in laser induced plasma from graphite target
Authors: Harilal S. S. , Issac Riju C. , Bindhu C. V. , Nampoori V. P. N. , Vallabhan C. P. G.
Published: July 2002

Laser ablation of graphite has been carried out using 1.06 μm radiation from a Q-switched Nd:YAG laser and the time of flight distribution of molecular C2 present in the resultant plasma is investigated in terms of distance from the target as well as laser fluences employing time resolved spectroscopic technique. At low laser fluences the intensities of the emission lines from C2 exhibit only single peak structure while beyond a threshold laser fluence, emission from C2 shows a twin peak distribution in time. The occurrence of the faster velocity component at higher laser fluences is explained as due to species generated from recombination processes while the delayed peak is attributed to dissociation of higher carbon clusters resulting in the generation of C2 molecule. Analysis of measured data provides a fairly complete picture of the evolution and dynamics of C2 species in the laser induced plasma from graphite.
Alternating current magnetic susceptibility measurements in La1−xSrxCoO3 (x⩽0.30) below 300 K
Authors: Mira J. , Rivas J. , Sánchez R. D. , Señarís-Rodríguez M. A. , Fiorani D. , Rinaldi D. , Caciuffo R.
Published: July 2002

A dynamical magnetic susceptibility study on La1−xSrxCoO3 samples obtained by a coprecipitation method is reported for x⩽0.30. For x<0.20 differences between zero field cooled and field cooled dc magnetic susceptibility appear at a temperature Ta below the Curie point, TC≈240 K, as well as a magnetic freezing point at a temperature Tb<Ta. Alternating current magnetic susceptibility shows two maxima at these temperatures, which is interpreted as the evidence of different interaction processes between ferromagnetic clusters mediated by a matrix of changing spin-state Co3+ ions. For x⩾0.20 an unique blocking process seems to take place due to the percolation of the ferromagnetic clusters, making unimportant the contribution of the matrix.
Si(001):B gas-source molecular-beam epitaxy: Boron surface segregation and its effect on film growth kinetics
Authors: Kim H. , Glass G. , Spila T. , Taylor N. , Park S. Y. , Abelson J. R. , Greene J. E.
Published: July 2002

B-doped Si(001) films, with concentrations CB up to 1.7×1022 cm−3, were grown by gas-source molecular-beam epitaxy from Si2H6 and B2H6 at Ts=500–800 °C. D2 temperature-programed desorption (TPD) spectra were then used to determine B coverages θB as a function of CB and Ts. In these measurements, as-deposited films were flash heated to desorb surface hydrogen, cooled, and exposed to atomic deuterium until saturation coverage. Strong B surface segregation was observed with surface-to-bulk B concentration ratios ranging up to 1200. TPD spectra exhibited β2 and β1 peaks associated with dideuteride and monodeuteride desorption as well as lower-temperature B-induced peaks β2* and β1*. Increasing θB increased the area under β2* and β1* at the expense of β2 and β1 and decreased the total D coverage θD. The TPD results were used to determine the B segregation enthalpy, −0.53 eV, and to explain and model the effects of high B coverages on Si(001) growth kinetics. Film deposition rates R increase by ⩾50% with increasing CB>̃1×1019 cm−3 at Ts⩽550 °C, due primarily to increased H desorption rates from B-backbonded Si adatoms, and decrease by corresponding amounts at Ts⩾600 °C due to decreased adsorption site densities. At Ts⩾700 °C, high B coverages also induce {113} facetting.
Efficiency enhancement in a Cherenkov laser by a proper permittivity variation
Authors: Hirata A. , Shiozawa T.
Published: July 2002

With the aid of particle simulation, we discuss efficiency enhancement in a single-pass Cherenkov laser by varying the permittivity of a dielectric sheet loaded on a parallel plate waveguide. For the analysis of the problem, a two-dimensional model for the Cherenkov laser is considered which is composed of a planar relativistic electron beam and a parallel plate waveguide, one plate of which is loaded with a dielectric sheet. In order to maintain the synchronism between an electron beam and an electromagnetic wave, the permittivity of the dielectric is properly varied in accordance with the variation in the average velocity of the electron beam. The result of numerical simulation shows that the efficiency of energy transfer from the electron beam to the electromagnetic wave is greatly enhanced by properly varying the permittivity of the dielectric sheet.
Fabrication of a polymer composite with periodic structure by the use of ultrasonic waves
Authors: Saito Mitsunori , Daian Tetsuya , Hayashi Kiyohiko , Izumida Shin-ya
Published: July 2002

A polymer composite was fabricated by solidification of a particle suspension in ultrasonic standing waves. Particles of various materials and shapes can be arranged periodically at the nodes of standing waves. A polysiloxane resin is suitable as a host material of the composite, since it is synthesized easily from a solution and it yields a small periodic structure even at a low ultrasonic frequency due to the low sound velocity. This method is useful for fabrication of bulk materials with periodic microstructures. Optical diffraction of a laser beam was observed in a polymer composite made of a polysiloxane resin and acrylic particles.
Wafer fused p-InP/p-GaAs heterojunctions
Authors: Salomonsson F. , Streubel K. , Bentell J. , Hammar M. , Keiper D. , Westphalen R. , Piprek J. , Sagalowicz L. , Rudra A. , Behrend J.
Published: July 2002

This article reports on the fabrication and characterization of wafer fused heterojunctions between p-InP and p-GaAs. Secondary ion mass spectroscopy was used to characterize doping profiles across the interface as well as the interface contamination with oxygen or carbon. The crystalline quality of the fused material was characterized using cross section and plan-view transmission electron microscopy. The electrical properties of the fused interface were studied as a function of various doping elements such as Be and Zn in InP or Zn and C in GaAs as well as for different acceptor concentrations in GaAs. Finally, the electrical characteristics were analyzed using a numerical model that includes thermionic emission and tunneling across the heterobarrier.
Electromagnetic induction in anisotropic half-space and electromagnetic stress model
Authors: Zhou Jianwei , Dover William D.
Published: July 2002

An analytical solution to the electromagnetic induction problem involving both electrical anisotropy and magnetic anisotropy has been derived. The method of solution can be extended to electromagnetic induction problems in infinite plates and layered media. A theoretical model for measuring magnetic field change under a uniaxial stress based on anisotropic electromagnetic induction has also been developed. An experiment designed to measure the effect of mechanical stress on magnetic permeability has been carried out. Theoretical predictions of relative magnetic field change are in good agreement with the experimental data.
Dynamics of laser produced carbon plasma expanding in low pressure ambient atmosphere
Authors: Neogi A. , Mishra A. , Thareja R. K.
Published: July 2002

The laser produced carbon plasma expanding in vacuum (P<10−3 Torr) and low pressure ambient atmosphere (P<10−2 Torr and P=10−1 Torr) has been studied by emission spectroscopy and fast photography using intensified charged coupled device (ICCD). The double peak structure is observed in the temporal profile of C I transition 2p3 D30−8f F(5/2) at 399.7 nm, C II transition 3d D2−4fF20 at 426.7 nm, and C III transition 3s S2−3pP20 at 465.0 nm as the plasma expands in ambient atmosphere. The stratification of the plasma into slow and fast components is imaged using ICCD. A compression wave model is proposed to explain the occurrence of a double peak structure.
Exchange-correlation induced energy-level shift in quantum wells with strain
Authors: Kim Mi-Ra , Kim Cheol-Hoi , Han Baik-Hyung
Published: July 2002

The energy-level shift due to exchange-correlation effects for a quasi-two-dimensional InxGa1−xAs/InGaAsP quantum well systems is studied as a function of the sheet carrier density for different biaxial compressive strain. We have calculated the leading-order self-energy within the random-phase approximation, by solving a 4×4 Luttinger-Kohn Hamiltonian for the consideration of the valence-band nonparabolicity of strained layer. We have found that the correlation energy, as well as the exchange energy significantly contributes to the energy-level shifts and these shifts grow with the increases of the biaxial compressive strain and the sheet carrier density. Our calculations show that for the biaxial compressive strain of 0.47%–1.02% the negative shift ratios of the band-gap energy are about 3.9%–4.6% and 7%–8.4% at sheet carrier densities of ns=1×1012 cm−2 and ns=1×1013 cm−2, respectively.
Piezoresistive effect in AlN/GaN short range superlattice structures
Authors: Gaska R. , Bykhovski A. D. , Shur M. S. , Kaminskii V. V. , Soloviov S. M.
Published: July 2002

We report on a strong piezoresistive effect in AlN–GaN short-range superlattices. The measured static gauge factor varies from 30 to approximately 90 depending on the superlattice composition. These values are substantially larger than the values of the gauge factor reported for GaN–AlN–GaN semiconductor–insulator–semiconductor structures. The measured data are in good agreement with the results of the calculation accounting for the piezoelectric effect. Our results demonstrate a high potential of AlN–GaN-based materials for the development of piezoelectric and piezoresistive sensors.
Origins of perpendicular magnetic anisotropy in Ni/Pd multilayer films
Authors: Jeong Jong-Ryul , Kim Young-Seok , Shin Sung-Chul
Published: July 2002

Room-temperature perpendicular magnetic anisotropy (PMA) was observed in Ni/Pd multilayer films having 5.1–11 Å Ni and constant 5.7 Å Pd sublayer thicknesses, prepared by dc magnetron sputtering at 7 mTorr Ar sputtering pressure. The magnetoelastic anisotropy determined from delicate in situ stress and ex situ magnetostriction coefficient measurements was found to contribute positively to the observed PMA in those samples and its magnitude was nearly comparable to the surface anisotropy.
Transmission electron microscopy–investigation of the microstructure of rapidly quenched Co80B20 alloys
Authors: Zern A. , Kleinschroth I. , Gonzalez A. , Hernando A. , Kronmüller H.
Published: July 2002

Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and magneto-optical Kerr effect have been used to characterize different crystallization states of melt-spun Co80B20 alloys. The microstructures have been prepared by isothermal annealing at temperatures of TA=325–600 °C for different annealing times. Samples annealed for ta=30 min show an onset of the crystallization at annealing temperatures TA less than 325 °C with the primary precipitation of both face-centered-cubic Co and hexagonal close-packed Co embedded in the amorphous matrix. By HRTEM it is revealed that Co grains with ordered as well as highly distorted lattices are formed, whereby a characteristic feature of the latter are stacking faults. The following step is the formation of orthorombic Co3B with an average grain size of 7.6 μm and of nanocrystalline Co grains, whereby still some amorphous phase is present. Fully crystallized samples were obtained for annealing temperatures higher than 412 °C (ta=30 min). The decomposition of the large-grained Co3B in nanocrystalline, body-centered tetragonal Co2B and Co starts at temperatures of TA>400 °C. The volume fraction and average grain size of the remaining Co3B decreases with increasing annealing temperature, whereas the Co2B grains show the opposite behavior. After a long time annealing for seven days at temperatures higher than 550 °C, the microstructure consists of fcc-Co grains with facet-like morphology and Co2B, but no Co3B have been detected.
Hole confinement and energy subbands in a silicon inversion layer using the effective mass theory
Authors: Rodrı́guez Salvador , López-Villanueva Juan A. , Melchor I. , Carceller J. E.
Published: July 2002

We present a study of the main features of a two-dimensional hole gas confined near a Si–SiO2 heterointerface. Starting from the framework of the effective mass theory, we were able to separate the Luttinger Hamiltonian into two 3×3 matrices using a semiaxial approximation and still retaining the warped shape of the isoenergetic surfaces in the kx−ky plane and the coupling of heavy, light, and split-off holes. This allows us to solve iteratively and simultaneously the Schrödinger and Poisson equations in the case of an inversion layer of holes in a P-channel metal–oxide–semiconductor structure for different applied gate biases. We have obtained the energy subbands and the main characteristics of the inversion layer. The form of the energy subbands suggests that the use of parabolic bands should be seriously questioned, and that even the use of a unique effective mass in each subband is not a realistic assumption. Furthermore, our results show that the character of the subbands becomes mixed as k∥ separates from zero, and that the complete dispersion characteristics must be considered in hole studies.
Measurement of strain in Al–Cu interconnect lines with x-ray microdiffraction
Authors: Solak H. H. , Vladimirsky Y. , Cerrina F. , Lai B. , Yun W. , Cai Z. , Ilinski P. , Legnini D. , Rodrigues W.
Published: July 2002

We report measurement of strain in patterned Al–Cu interconnect lines with x-ray microdiffraction technique with a ∼1 μm spatial resolution. Monochromatized x rays from an undulator were focused on the sample using a phase fresnel zone plate and diffracted light was collected by an area detector in a symmetric, angle dispersive x-ray diffraction geometry. Measurements were made before and after the line sample was stressed for electromigration. Results show an increase in inter- and intra-grain strain variation after the testing. Differences in strain behavior of grains with (111) and (200) crystallographic planes parallel to the substrate surface were observed. A position dependent variation of strain after the testing was measured whereas no such dependence was found before the testing.
Director tilting of liquid crystals on photoisomerizable polyimide alignment layers doped with homeotropic surfactant
Authors: Park Byoungchoo , Han Ki-Jong , Jung Youngyi , Choi Hyun-Hee , Hwang Ha-Keun , Lee Sooman , Jang Sei-Hum , Takezoe Hideo
Published: July 2002

Director tilting of liquid crystal (LC) molecules on photosensitive alignment layers has been investigated. The layers, polyimides with photoisomerizable groups in their main chains, were irradiated with unpolarized near-UV (UPUV) light at oblique incidence. The irradiated film was found to align LC molecules homogeneously with a negligible pretilt angle. It was found that the pretilt angle can be generated successfully by introducing surfactant lecithin and decreasing the incidence angle of the UPUV light. The LC alignment and electro-optical properties of the LC cells with the photoalignment layers were demonstrated to be suitable for twisted nematic LC, vertical alignment LC, and surface-stabilized ferroelectric LC devices.
Magnetic properties of nanocrystalline γ-Fe–Ni–N nitride systems
Authors: Panda R. N. , Gajbhiye N. S.
Published: July 2002

Nanocrystalline γ-Fe0.75Ni0.25–N, γ-Fe0.5Ni0.5–N, and γ-Fe0.25Ni0.75–N nitrides have been synthesized by borohydride reduction followed by nitridation at 773 K using NH3(g). γ-Fe0.75Ni0.25–N, γ-Fe0.5Ni0.5–N and γ-Fe0.25Ni0.75–N nitrides crystallize in fcc cubic structures having unit cell parameters: a=3.588, 3.556 and 3.534 Å, respectively. X-ray diffraction and electron microscopic studies reveal the ultrafine nature of the particles. Chemisorption of atmospheric oxygen results in the formation of nonmagnetic oxide and/or a complex oxynitride layer at the surface of the ultrafine particles. The reduction of room temperature saturation magnetization is attributed to fine particle size effects and the formation of nonmagnetic oxide and/or an oxynitride layer at the surface of the ultrafine particles. The change in Curie temperature Tc in γ-Fe–Ni–N nitride systems compared to their corresponding bulk Fe–Ni alloys is related to the presence of concentration inhomogeneities in the nitride materials. Mössbauer study of γ-Fe0.75Ni0.25–N, γ-Fe0.5Ni0.5–N and γ-Fe0.25Ni0.75–N nitride particles exhibit the coexistence of ferromagnetic and superparamagnetic fractions and corroborate the observed magnetic properties.
Lifetime and switching characteristics of a high-current multichannel pseudospark
Authors: Naweed A. , Kiefer J. , Röhner M. , Neff W.
Published: July 2002

A radial multichannel pseudospark switch is integrated into a plasma focus system designed to generate soft x rays to determine switch durability in a long-term test. The operating parameters correspond to a repetition rate of up to 2 Hz, stand-off voltages of 10 kV and peak currents of 200 kA. Only minimal electrode erosion is witnessed after a serial test of 180 000 shots where the electrode surfaces are inspected after every 25 000 shots. The good condition of electrode surfaces suggests the possibility of an additional 200 000 shots, implying that for the given parameter range the lifetime of the multichannel pseudospark is at least four times longer than that of traditional high-current switches such as spark gaps. In a comparative test the performance of the multichannel pseudospark switch is evaluated against a multichannel spark gap. The results of these investigations show that the low-erosive pseudospark is well suited to substitute for spark gaps in high-current applications.
Time-resolved measurements of ion energy distributions and optical emissions in pulsed radio-frequency discharges
Authors: Wang Yicheng , Benck Eric C. , Misakian Martin , Edamura Manabu , Olthoff James K.
Published: July 2002

In pulse-modulated inductively coupled plasmas generated in CF4:Ar mixtures, a transition between a capacitive coupling mode (E mode) and an inductive coupling mode (H mode) was observed. For a pulsed plasma in a 50%CF4:50%Ar volume mixture with a peak rf power of 200 W at 13.56 MHz and a modulation frequency at 500 Hz with a duty cycle of 95%, the E→H mode transition occurs repetitively 0.75 ms after each rf pulse is applied. This long delay in the mode transition allows us to perform not only time-resolved measurements of optical emission and electrical characteristics but also time-resolved measurements of ion energy distributions at the grounded electrode. These measurements show that a relatively short rf off period can severely perturb the equilibrium plasma state and its recovery may take much longer than the rf off period.
Texture development of blanket electroplated copper films
Authors: Lingk C. , Gross M. E. , Brown W. L.
Published: July 2002

The transition from sputtered Al to electroplated Cu interconnects for future microelectronic devices has led to an interest in understanding the relationships between the microstructure and texture of Cu that might impact electrical performance, similar to what has been done for Al. Electroplated Cu undergoes a recrystallization at room temperature that is related to the presence of organic and inorganic additives in the plating bath. As plated, the Cu grains are small (approx. 0.1 μm) and equiaxed, but over a period of hours to days, recrystallization results in grains several microns in size. We observe a significant weakening of the strong as-plated (111) texture by x-ray diffraction pole figure measurements and an increase in the level of randomness. We propose that multiple twinning is the leading mechanism for this phenomenon.
An investigation of La1−xSrxMnO3 near x=0.5
Authors: Patil S. I. , Bhagat S. M. , Ogale S. B. , Shu Q. Q. , Lofland S. E. , Troyanchuk I. O.
Published: July 2002

We have done a systematic study of the ac susceptibility, dc magnetization, and ferromagnetic resonance in a set of polycrystalline samples of La1−xSrxMnO3 with nominal x values varying from 0.46 to 0.53 at 0.01 intervals. Apart from noting the remarkable changes attendant on small variations in x, it is found that at low T, one has coexistence of ferromagnetic and antiferromagnetic phases. The resonance data are interpreted to provide a measure for the size of the ferromagnetic regions.
Structural and electronic differences between deuterated and hydrogenated amorphous silicon
Authors: Shih An , Yeh Jiun-Lin , Lee Si-Chen , Yang T. R.
Published: July 2002

Both as-deposited and plasma-treated hydrogenated and deuterated amorphous silicon (a-Si:H and a-Si:D) films are prepared by plasma-enhanced chemical vapor deposition. Their structural and electronic characteristics are studied and compared. It is found that the deuterium concentration of a-Si:D is higher than the hydrogen concentration of a-Si:H in as-deposited samples, but is less in plasma-treated samples, which is consistent with the photoluminescence spectra. Besides, the proportionality constants relating the integrated stretching mode absorption to the concentrations of Si–D and Si–H bonds were determined, i.e., 1×1020 for Si–D and 6.9×1019 for Si–H, by means of secondary ion mass spectrometry.
Influence of vibrational motion on solid state line shapes and NMR relaxation
Authors: Henry Eric R. , Szabo Attila
Published: July 2002

The influence of vibrational motion on bond lengths and quadrupole constants obtained from dipolar and quadrupolar solid state line shapes is considered. It is shown that such motions average both the magnitude and the orientation of the intrinsic interaction tensor. Explicit expressions for the effective coupling constants that can be conveniently evaluated using the results of a normal mode analysis are derived. When the vibrationally averaged interaction tensor is axially symmetric, it is shown that the effect of vibrational motion on relaxation can be rigorously incorporated into an effective coupling constant which is formally identical to the one that determines the line shape. Illustrative calculations for several alkanes, in both the gas and solid phases, are presented. The relative contributions of stretching and bending vibrations and the effect of anharmonicity on the effective C–H bond lengths and deuterium quadrupole constants are examined. The influence of vibrational averaging on the magnitude of the dipolar coupling is shown to be essentially independent of the nature of the molecule and its environment and to be quite small (i.e., for C–H bonds, the coupling constant decreases by 3% and hence the effective bond length increases by 1%). Vibrational averaging of the orientation of the dipolar interaction vector, on the other hand, depends on the size of the molecule and its environment because of the predominant role played by low frequency bending and torsional modes. The implication of these results for the value of the effective internuclear distance that should be used for the interpretation of the dipolar relaxation experiments is considered.
Atom condensation on an atomically smooth surface: Ir, Re, W, and Pd on Ir(111)
Authors: Wang S. C. , Ehrlich Gert
Published: July 2002

The distribution of condensing metal atoms over the two types of sites present on an atomically smooth Ir(111) has been measured in a field ion microscope. For Ir, Re, W, and Pd from a thermal source, condensing on Ir(111) at ≊20 K, the atoms are randomly distributed, as expected if they condense at the first site struck.
Investigation of the effects of molecular association on diffusion in binary liquid mixtures at the infinite dilution limit
Authors: Erkey C. , Alhamid K. A. , Akgerman A.
Published: July 2002

The limiting mutual diffusion coefficients of primary alcohols (C1–OH to C14–OH) in n octane and 1 octanol and of normal alkanes (C1 to C14) in 1 octanol were measured using the Taylor dispersion technique in the temperature range 298–374 K. The data were interpreted using the rough hard sphere theory. The combined effects of translational–rotational coupling and hydrogen bonding were extracted from experimental data for each solute–solvent pair at each temperature. For diffusion of alcohols in n octane, the polarity of the solute molecules tends to increase the amount of coupling. For diffusion of alkanes and alcohols in 1 octanol, the deviations from smooth hard sphere behavior showed a strong temperature dependency indicating the effects of hydrogen bonding. For diffusion of alcohols in 1 octanol, stronger temperature dependency of the deviations compared to alkane solutes indicate strong interactions between solute alcohol molecules and 1 octanol molecules.
A molecular dynamics study of the isomerization of CH3NC in a dense argon fluid
Authors: Marks A. J. , Murrell J. N. , Stace A. J.
Published: July 2002

A molecular dynamics study has been made of the isomerization of CH3NC in a dense argon fluid. The model potential energy surface for CH3NC treats the CH3 group as a heavy atom, but otherwise gives a good representation of the remaining stationary points. Batches of ≂150 trajectories were followed for the molecule, together with up to eight argon atoms contained in a soft-walled spherical vessel. The temperature of the system was defined in terms of mean kinetic energies, and the radius of the vessel was used to define the argon density. The total energy was chosen to give typical isomerization times of 10−11 s. First-order rate constants were determined for each batch of trajectories for solvent densities ranging from 9 to 80 mol dm−3. At low densities the rate constants increase linearly with inert gas density, but reach a plateau at between 35 and 70 mol dm−3. Such behavior is in agreement with Lindemann theory. The interpolated value of k∞ is 2.2×1011 s−1, which agrees well with an experimental value derived from an Arrhenius extrapolation to an effective temperature appropriate for the system. Above 70 mol dm−3 the calculated rate constants increase rapidly with pressure, and this is attributed to the effect a negative volume of activation has on the isomerization process.
Diffusion-controlled reactions in entangled polymer systems
Authors: O’Shaughnessy Ben
Published: July 2002

Intramolecular reactions in entangled polymer systems are studied in the ‘‘reptation’’ or ‘‘tube’’ model. In the diffusion-controlled limit the reaction rates are strongly modified by the entanglement constraints and depend sensitively on the positioning of the reactive groups. For two internally positioned groups the fraction of polymers which have reacted after time t,R(t), scales as the volume explored by a group for times small compared to ts where ts is the relaxation time of the portion of polymer connecting the groups. This leads to R(t)∼t3/4 followed by R(t)∼t3/8 for t<ts . For times larger than ts but less than the longest relaxation or reptation time τrep, R(t) scales as the distance moved by a group along its tube: R(t)∼t1/4 followed by R(t)∼t1/2. For the longest times, t>τrep , the unreacted fraction [1−R(t)] decays exponentially with a reaction rate constant which is much less than τ −1rep. These results motivate new direct microscopic experimental tests of the reptation model.
Minimum total energy calculations for conjugated polymer chains
Authors: Rossi Giuseppe
Published: July 2002

Results from a set of minimum energy calculations performed for the Su Schrieffer Heeger Hamiltonian for trans-polyacetylene are presented. We fix the occupation numbers of the electronic states and look for the bond geometry which minimizes the total energy. The usual nonlinear solutions (polarons, solitons) are recovered for appropriate sets of occupation numbers of the electronic states. We study the spectrum of excited states and the properties of the model in the case of arbitrary doping. The situation where the original SSH Hamiltonian is modified in such a way as to account for the intrinsic periodicity of the monomeric units of conjugated polymers without a degenerate ground state is also examined.
Force between colloidal particles in a Lennard-Jones fluid
Authors: Henderson Douglas , Plischke Michael
Published: July 2002

We calculate the force between hard spheres of several diameters at infinite dilution in a Lennard-Jones fluid on the basis of the Percus–Yevick solution of the Ornstein–Zernike equation. In particular, we study the region of the phase diagram near the critical point of the solvent. We find, in contrast to recent results for sticky spheres, that the solvent mediated force is attractive and monotonically increasing in the critical region. At high solvent density the force, while still attractive, oscillates as function of separation.
Effects of bound pairs on the low-density transport properties of gases
Authors: Curtiss C. F.
Published: July 2002

In some derivations of the Boltzmann equation, the equation which describes the time evolution of the distribution function in the phase space of the atoms of a low-density gas, it has been assumed that all of the trajectories in the phase space of the pairs of atoms are such that transforming back sufficiently far in time leads to separated atoms. In a recent development, it has been shown that if this is not true the ‘‘bound-state’’ trajectories lead to an addition to the collision integral which, in turn, leads to corrections to the values of the low-density transport coefficients. In the present alternate development, it is shown that the ‘‘bound-state’’ trajectories may be joined to trajectories in the ‘‘unbound region’’ through the introduction of a complex time and analytic continuation. With this generalization of the concept of trajectories, one may introduce the ‘‘molecular chaos assumption’’ in the usual manner, and the ‘‘collision integral’’ may be generalized to include the bound-state region of the phase space. This leads to ‘‘corrections’’ to the low-density limiting values of the transport coefficients. These effects of the bound pairs of atoms on the low-density transport coefficients of a gas of atoms which interact according to a Lennard-Jones potential are investigated numerically. It is found that the effects become important at temperatures, scaled in the usual manner by ε/k, below about unity. Specifically, it is found that at a scaled temperature of unity, corresponding to a temperature in the range of about 100–200 K, the effect is to lower the self-diffusion coefficient by about 3.7% and the viscosity and thermal conductivity by about 0.7%. The effects become significantly greater at lower temperatures.
Interaction between an atom and a mesoscopic helicoidal system. Fourier-transform analysis
Authors: Alvira E. , Breton J. , Delgado V. , Girardet C.
Published: July 2002

A Fourier-transform analysis (FTA) of the interaction potential between an atom and a helicoidal periodic distribution of atoms is presented. The accuracy of this procedure is tested with respect to the standard pairwise potential sum techniques. It is shown that the convergence of FTA is quite good, mainly when the probe atom lies inside the helicoidal cavity and when the matter density on the helix remains reasonably large. Criteria are given for the applicability of the FTA to a helicoidal atomic distribution.
Transport and relaxation cross sections for He–N2 mixtures: A test of a multiproperty interaction
Authors: Gianturco F. A. , Sanna N. , Serna S.
Published: July 2002

Full quantum calculations of several transport and relaxation cross sections have been carried out for the title system, using the multiproperty potential energy surface obtained earlier by Gianturco et al. [Chem. Phys. 109, 417 (1986)] which has been recently shown to be one of the most reliable descriptions of the He–N2 interaction. A rigorous close coupling (CC) treatment of the scattering problem turns out to be the only realistic choice for evaluating viscomagnetic relaxation data and for anisotropy-dependent nonequilibrium properties. A comparison of the present calculations with classical trajectory (CT) estimates is carried out for several transport cross sections as function of both the collision energy and the temperature of the mixture. It is clearly shown by the present results that classical treatments markedly differ from the quantum calculations at collision energies which are comparable with the average well depth of the chosen interaction. On the other hand, CT and CC values are nearly coincident at higher collision energies for all the computed properties. Thus, their combined use in evaluating the temperature dependence of diffusion and viscosity coefficients reveals the high quality of the present multiproperty interaction vis à vis the experimental findings examined in this work.
The 14 μm bands of BF3–CO complexes: Isotope-dependent chaotic fine structure
Authors: Lee Gang-Ho , Takami Michio
Published: July 2002

Infrared absorption spectra of jet-cooled four isotopic 10,11BF3–12,13CO complexes are studied in the 14 μm region. The spectra of the 10BF3–12CO and 10BF3–13CO complexes are parallel bands with unresolved K-structure characteristic of a heavy symmetric top molecule. On the other hand, the spectra of the 11BF3–12CO and 11BF3–13CO complexes exhibit a complicated fine structure that cannot be interpreted by an ordinary semirigid molecular model. All of the observed bands are shifted to the red by 28–30 cm−1 from the ν3 band origin of uncomplexed BF3, indicating a considerable increase in bond energies by the vibrational excitation. Possible origins of the anomalous fine structure are discussed.
General formula evaluation of the electron-repulsion integral (ERI) and its derivatives over Gaussian-type orbitals. II. ERI evaluation improved
Authors: Ishida Kazuhiro
Published: July 2002

The general recurrence relation (RR) of paper I [J. Chem. Phys. 95, 5198 (1991)] can be improved. This improved RR, with the use of the Head–Gordon–Pople (HGP), is able to take the contraction of Gaussian-type orbitals (GTO’s) efficiently into account for the electron-repulsion-integral (ERI) evaluation. With the use of the RR by Hamilton and Schaefer and by Lindh, Ryu, and Liu (HSLRL), this improved RR makes the evaluation more rapid. Furthermore, the calculation of the roots and weights of the Rys polynomial can be improved. As a result, an extremely rapid code can be obtained for a vector computer. For example, the measured time is 135 ns per one ERI for [HH‖HH] shell block of the uncontracted GTO’s, which is 16 times faster than that of paper I. When the degree of contraction K=2 for (HH‖HH) shell block of the contracted GTO’s, the measured time is 40 ns per one primitive ERI. It is confirmed that the present method is most efficient for the ERI evaluation of GTO’s higher than or equal to f. It is found that both the HGP and the HSLRL RR’s are numerically unstable in their bad cases. Several procedures to avoid their instabilities are discussed.
Dynamics of polymeric brushes: End exchange and bridging kinetics
Authors: Johner A. , Joanny J. F.
Published: July 2002

We study some dynamical aspects of grafted polymer layers in a solvent. The structure of the brush is described in a θ solvent by the self-consistent mean field theory and in a good solvent using scaling laws. The Rouse–Zimm model including hydrodynamic interactions is used for the dynamic properties. A given chain end explores the entire thickness of a free grafted layer in a time Te proportional to the cube of the thickness. The exploration time is much larger than the Rouse time that characterizes the relaxation of the fluctuations of the chains conformations. In a θ-solvent we give a detailed study of the relaxation of the density of a few labeled chain ends towards its equilibrium value. The bridging kinetics between a grafting plate and a plate adsorbing the free ends is also discussed. When adsorption proceeds, an exclusion zone grows in the vicinity of the adsorbing plate. To cross the exclusion zone and adsorb, a chain end must overcome an energy barrier. The typical adsorption time is the first passage time through this barrier. Except in the very late stages where the fraction of chains η forming bridges saturates at its equilibrium value, the energy barrier against adsorption increases as a power law of η and the bridging fraction increases very slowly (logarithmically) with time. Both weak bridging where only a small fraction of chains form bridges and total bridging where all the chains form bridges are studied.
Electron-energy-loss spectroscopy of condensed butadiene and cyclopentadiene: Vibrationally resolved excitation of the low-lying triplet states
Authors: Swiderek P. , Michaud M. , Sanche L.
Published: July 2002

Low-energy electron-energy-loss spectra of 1,3-butadiene and 1,3-cyclopentadiene deposited on a thin film of solid argon are measured at a temperature of 15 K. This new method allows us to resolve vibrational structure within the low-lying triplet bands. The vibrational frequencies observed in the first triplet band of butadiene are in agreement with theoretical predictions under the assumption of a metastable planar structure for the lowest triplet state (1 3Bu). The lowest observable vibronic band of the second triplet transition of butadiene is located at 4.63 eV. The 0–0 transition to the first triplet state of cyclopentadiene is resolved and located at 2.555 eV. Concerning the second triplet band of cyclopentadiene, we find evidence from a comparison with optical spectra that the onset lies in the energy range from 4.55 to 4.7 eV. This is the first likely observation of T2 for a cis-diene.
Methane tunneling in disordered solid argon–nitrogen phases
Authors: Langel W. , Prager M. , Fleger H.-W. , Knözinger E. , Lauter H.-J. , Blank H. , Carlile C. J.
Published: July 2002

Methane has been matrix isolated in solid Arc(N2)1−c, 0≤c≤1. Neutron spectra and diffractograms from identical samples are reported. In addition vapor-deposited argon and its mixtures with methane and nitrogen were studied by low temperature x-ray powder diffraction. The diffractograms and spectra were interpreted in terms of five different types of sites (I–V) in the vapor-deposited matrix after deposition at temperatures of 7–25 K. Shifts and broadenings of the methane tunneling spectra are caused by these sites: (I) Methane in regular fcc substitutional sites of solid pure argon or pure nitrogen in thermodynamic equilibrium shows nearly free rotation. The effective hindrance potential in nitrogen is lower than that in argon. In earlier IR-absorption work a site splitting of the vibrations was assigned as a splitting of the J=1 level of methane. INS shows that the latter is smaller than 0.030 meV. (II) Due to the nonequilibrium conditions during vapor-deposition, stacking faults and grains with hcp symmetry are formed both in pure argon and krypton, but not in pure nitrogen. Rather sharp lines in the methane spectra at 0.6 meV neutron energy transfer are assigned to such sites. (III) Argon and nitrogen form solid mixtures with an fcc structure in the argon rich phase (c≥0.45). By the addition of nitrogen the concentration of stacking faults and grains with hcp structure is enhanced. Both the peaks at 0.9 and at 0.6 meV are seen without major shift or broadening compared to the spectra of methane in pure argon. (IV) In nitrogen rich mixtures (c≤0.45) broad features in the energy range below 1 meV are observed. They are fitted by a simple mean field model.

The fit shows that replacing of nitrogen next neighbors of a methane molecule by argon atoms strongly enhances the hindrance potential in this concentration range. (V) A major amount of the argon and krypton matrices is strongly distorted and has a very small coherence length. It is speculated that distorted zones are present as small clusters or on surfaces and small angle grain boundaries. The corresponding trapping sites for methane have a very low symmetry. A broad distribution of tunnel transitions around the elastic line occurs in samples which are, to a significant amount, composed of distorted lattices.
The effect of initial velocity on rotational energy transfer in 13CH3F
Authors: Song Quan , Schwendeman R. H.
Published: July 2002

The dependence on initial velocity of collisionally-induced rotational transitions in CH3F has been studied by infrared–infrared four-level double resonance experiments in which the offset of the pump laser was varied in order to vary the velocity component in the direction of the pump beam of the excited molecules. For this purpose, the frequency of the carbon dioxide pump laser was stabilized to a saturation dip (Lamb dip) in the Stark spectrum in a CH3F sample in a cell outside the laser cavity, which provided a tunability of ∼30 MHz for each laser line. It was found that the rms change in a velocity component during collisionally-induced transitions is roughly proportional to the rms relative speed of the colliding molecules. Velocity dependence of the rates of rotational energy transfer has also been observed. Classical elastic scattering theory has been used to calculate the one-dimensional collision kernel at different initial values of the component of the velocity of the active molecule in the direction of the beam. The results of this calculation do not agree with the qualitative features of the observation.
Molecular-beam spectrum of the 970 cm−1 Fermi triad of CF3CH3
Authors: Fraser G. T. , Pine A. S. , Domenech J. L. , Pate B. H.
Published: July 2002

A tunable microwave-sideband CO2 laser is used with an electric-resonance optothermal spectrometer to investigate the infrared spectrum of CF3CH3 near 970 cm−1. A Fermi-coupled triad of states is observed, resulting from the interactions of 2ν6+ν11 and ν5+ν12 with the fundamental vibration, ν10, which is assumed to carry the oscillator strength in this region. The high resolution (∼3 MHz) of the spectrometer allows the observation of tunneling splittings associated with the ν6 torsional vibration. These splittings are used to identify the torsional character of the states observed. At the normal-mode level the ν10 and ν5+ν12 states are found to be nearly degenerate and interacting by an anharmonic matrix element of ∼3 cm−1. The lower-energy component of this diad exhibits torsional splittings of up to 400 MHz due to an anharmonic coupling of 0.70 cm−1 with the lower energy 2ν6+ν11 state which has an intrinsic tunneling splitting of ∼800 MHz. A fourth state, 3ν6+ν12, which has a still larger zeroth-order tunneling splitting, may also be affecting the torsional splittings of the observed states. The present investigation illustrates the utility of using resolved torsional splittings to unravel complex vibrational couplings in molecules.
A high repetition rate, picosecond hard x-ray system, and its application to time-resolved x-ray diffraction
Authors: Anderson T. , Tomov I. V. , Rentzepis P. M.
Published: July 2002

A new technique for generation of hard x-ray picosecond pulses is presented. Excitation of a x-ray diode with 10 ps ultraviolet light pulses produces characteristic x-ray pulses with duration in the range of 10–100 ps at a repetition rate of 300 Hz. The x-ray pulses are synchronized with picosecond accuracy to the optical pulses of the laser system as well to other ultrafast phenomena. Results of picosecond time-resolved x-ray diffraction are reported.
Diatomic molecules, rotations, and path-integral Monte Carlo simulations: N2 and H2 on graphite
Authors: Marx D. , Sengupta S. , Nielaba P.
Published: July 2002

The rotational motion of homonuclear diatomic molecules confined to two dimensions at finite temperatures is discussed within the framework of path-integral Monte Carlo (PIMC) techniques. For single rotators the symmetry restriction on the total wave function coupling nuclear spin and rotations of these diatomic molecules is carried over to PIMC for fermionic and bosonic diatomic molecules. Three experimentally relevant quantum statistical averages are formulated, and quantum effects due to discrete level spacing and exchange are separated with the help of these averages. The method is applied to single N2 and H2 rotators adsorbed on graphite in the frozen-in crystal field which is due to the commensurate (√3×√3)R30° ‘‘2-in’’ herringbone phase. Contrary to H2, exchange effects are negligible for N2 in the relevant temperature range. The resulting sign problem for certain combinations of molecule and averaging procedure is discussed. PIMC simulations of the phase transition from the translationally √3-ordered and orientationally disordered phase to the herringbone phase were carried out for complete N2 monolayers without a symmetry restriction on the wave function. Due to dispersive quantum fluctuations, transition temperature and ground-state order parameter are depressed by roughly 10% as compared to classical MC simulations of the same realistic model. In addition, the PIMC results are compared to quasiharmonic and quasiclassical approximations. The quasiharmonic treatment yields the correct order parameter suppression, the quasiclassical simulation the lowering of the transition temperature, but only the full quantum PIMC simulations describe the entire temperature range of interest correctly.