bluuebunny/crossref_metadata_2025 huggingface dataset:
Mechanism of topography formation during CO2 laser texturing of silicate glasses
Authors: Bennett Ted D. , Krajnovich Douglas J. , Li Lei , Wan Dujiang
Published: July 2002

A theoretical picture is advanced to explain experimental results of CO2 laser texturing of silicate glasses. Common characteristics among several glass types are found that corroborate observations made by other investigators and establish features of glass laser texture. The principal experimental results include increasing bump height with pulse energy (above some threshold); bump width scales with the bump height; annealing, before or after laser texture, reduces bump heights; and finally, further growth in bump height occurs with a finite number of laser pulses subsequent to the first. Explanations for these results have been unified in terms of a “fictive temperature map” that relates microstructure to the thermal history of the glass. On this map, the glass transition temperature identifies a fictive temperature at which the rate of change of temperature is comparable to the rate of change of the microstructure. Therefore, the time scale imposed by the laser pulse can elevate the transition temperature of the glass, making accessible high fictive temperatures to the heat affected zone. As a result of this description we can offer explanations to important characteristics of glass laser texturing, perhaps most notably the nonlinear threshold dependence of bump height on pulse energy. We can also explain the observed bump growth with multiple laser pulses, the effect of chemical strengthening, and the effect of annealing before or after laser texturing.
First-principles study on electronic and elastic properties of BN, AlN, and GaN
Authors: Shimada Kazuhiro , Sota Takayuki , Suzuki Katsuo
Published: July 2002

We have carried out first-principles total energy calculations to investigate electronic and elastic properties of both zinc-blende and wurtzite BN, AlN, and GaN. We have calculated lattice parameters, elastic constants, deformation potential constants, phonon frequencies at Γ point, Born effective charges, and piezoelectric constants. Lattice parameters are fully relaxed by using the first-principles molecular dynamics method with variable cell shape. The internal strain in a strained crystal is also relaxed by the first-principles molecular dynamics method. The internal strain influences the elastic constants, the deformation potential constants, and the piezoelectric constants effectively. We have calculated the wurtzite deformation potential constants D1–D5 considering the internal strain correction. The piezoelectric constants of wurtzite and also zinc-blende crystals have been calculated using the Berry phase approach and we have found from first principles that those of BN have an inverse sign in contrast to AlN and GaN. Discussions will be given in comparison with results obtained herein with the previous ones.
Improved quantitative mobility spectrum analysis for Hall characterization
Authors: Vurgaftman I. , Meyer J. R. , Hoffman C. A. , Redfern D. , Antoszewski J. , Faraone L. , Lindemuth J. R.
Published: July 2002

We present an improved quantitative mobility spectrum analysis (i-QMSA) procedure for determining free electron and hole densities and mobilities from magnetic-field-dependent Hall and resistivity measurements on bulk or layered semiconductor samples. The i-QMSA technique is based on a fundamentally new approach, which optimizes the fit to the conductivity tensor components and their slopes by making those adjustments in the mobility spectra that result in the greatest error reduction. Empirical procedures for manipulating the mobility spectra are also introduced, with the dual purpose of reducing the error of the fit and simplifying the shape of the spectra to minimize the presence of unphysical artifacts. A fully automated computer implementation of the improved QMSA is applied to representative synthetic and real data sets involving various semiconductor material systems. These results show that, as compared with previous approaches, the presented algorithm maximizes the information that may be extracted from a given data set, and is suitable for use as a standard tool in the characterization of semiconductor material and device transport properties.
Electrical resistivity of sputtered Cu/Cr multilayered thin films
Authors: Misra A. , Hundley M. F. , Hristova D. , Kung H. , Mitchell T. E. , Nastasi M. , Embury J. D.
Published: July 2002

Parallel (in-plane) electrical resistivities of single-layered Cu and Cr films, and Cu/Cr multilayered thin films sputter deposited on Si substrates were evaluated as a function of layer thicknesses ranging from 2.5 to 150 nm in the temperature range of 4–325 K. The resistivity of the multilayers at a given temperature increased and residual resistivity ratio decreased with decreasing layer thicknesses. At 300 K, the resistivity of a 1 μm thick Cu film was approximately equal to the bulk value, but the resistivity of the Cr film was an order of magnitude higher than that of bulk Cr. The microstructures of the multilayers and the single-layered Cu and Cr thin films were characterized by transmission electron microscopy. For layer thicknesses ranging from 2.5 to 150 nm, the multilayers exhibited sharp, planar interfaces between the two phases. The individual Cu and Cr layers were nanocrystalline with near-equiaxed grains in Cu and columnar grains in Cr. The dependence of electrical resistivity on the layer thickness of multilayers is explained using a model that accounts for interface scattering and thin-film resistivities of polycrystalline Cu and Cr.
Low dielectric constant films prepared by plasma-enhanced chemical vapor deposition from tetramethylsilane
Authors: Grill A. , Patel V.
Published: July 2002

Dielectric films have been prepared by radio-frequency plasma-enhanced chemical vapor deposition from mixtures of tetramethylsilane with oxygen. The films have been characterized as-deposited and after annealing at 400 °C to determine the thermal stability of their properties. Rutherford backscattering and forward recoil elastic scattering have been used for determination of the composition of the films. Optical properties were characterized by Fourier transform infrared spectroscopy and measurements of the index of refraction and optical gap. The electrical properties were measured in a Si/insulator/metal configuration. It has been found that the index of refraction decreases and the optical gap and dielectric constant increase with increasing oxygen concentration in the gas feed. While the materials did not show a mass or composition loss after annealing, the annealing resulted in a reduction of the dielectric constant of the films. Dielectric constants as low as 3.1 have been obtained after annealing the film deposited from pure tetramethylsilane.
High resolution study of domain nucleation and growth during polarization switching in Pb(Zr,Ti)O3 ferroelectric thin film capacitors
Authors: Hong Seungbum , Colla E. L. , Kim Eunah , Taylor D. V. , Tagantsev A. K. , Muralt P. , No Kwangsoo , Setter N.
Published: July 2002

The domain nucleation and growth during polarization switching in Pb(Zr,Ti)O3 (PZT) ferroelectric thin film capacitors with Pt top (TE) and bottom electrodes (BE) were studied by means of atomic force microscopy (AFM). The experimental configuration used in this study differs from that conventionally used (AFM tip/PZT/BE) where the AFM tip acts as a positionable TE. A small ac voltage was applied between the electrodes with a step by step increasing dc bias voltage. The induced piezoelectric vibration was detected by the AFM tip, its amplitude and phase determined with the lock-in amplifier. The phase difference between the applied ac voltage and the piezoelectric signal as a function of the x-y position was nearly locked at 0 or 180, representing film regions with parallel (in-phase) and antiparallel (antiphase) polarization direction, respectively. The polarization reversal was induced by application of a step by step increasing dc bias field opposite to the polarization of the prepoled sample. At each bias step 10 μm×10 μm images of the amplitude and the phase were collected. By approaching the coercive field (Ec), a rapid decrease of the amplitude with a small change in phase was observed, first in preferential sites for reversed nucleation, then over the whole studied area. The first regions with reversed polarization were observed at about 3.2 MV/m and the switching was completed at 5.5 MV/m. The results were interpreted using both the forward and the sideways domain growth models that are named by their rate-limiting kinetics. It was concluded that under the assumption of a sufficient resolution of domain imaging in our study, the forward domain growth being the rate limiting mechanism prevails in our PZT thin film.
Mechanism of bump formation on glass substrates during laser texturing
Authors: Shiu Ting-Ruei , Grigoropoulos Costas P. , Cahill David G. , Greif Ralph
Published: July 2002

A quantitative model is proposed to elucidate and predict the dome-shaped surface topography resulting from CO2 laser heating of glass substrates. In the analysis, a permanent structural change in glass is induced by a higher glass transition temperature due to the faster cooling process, with a final topography being determined by the temperature history resulting from the absorbed laser energy. The analysis is validated by experiment, which focuses on the energies which trigger the permanent deformation and induce a dome-shaped topography. The dimensions (maximum height and base area) of the bump show a logarithmic dependence on energy as expected from the theory. Using the constants determined from the experimental data and our analysis, bump profiles over a range of laser fluences are predicted. These two constants provide the information for determining the new glass transition temperature and the threshold energy needed to form a permanent bump. The result also suggests that the topography is mostly determined from the conditions at the end of the laser pulse. The effects of thermally induced stress on the model, and the physics of bump formation in chemically strengthened glass are addressed.
Ultrahigh frequency permeability of sputtered Fe–Co–B thin films
Authors: Klemmer T. J. , Ellis K. A. , Chen L. H. , van Dover B. , Jin S.
Published: July 2002

Amorphous FeCoB alloys can have high resistivity and relatively high saturation magnetization which are desired for ultrahigh frequency devices such as future write heads and wireless inductors. In this study, FeCoB films are observed to have a low easy axis coercivity (1–2 Oe), a field deposited induced anisotropy of ∼35 Oe and 4πMs∼17 500 G when sputtered in a typical configuration. However, samples sputtered in an off axis arrangement are found to have an additional uniaxial anisotropy as a result of the oblique incidence of the atomic flux onto the substrate. This extra anisotropy increases with increasing oblique angle. For these same samples, there is little change in the easy axis coercivity, saturation magnetization, and the Hoffmann structure factor (S∼0.05 ergs/cm2). The obtained large anisotropy fields (>35 Oe) are found to push the ferromagnetic resonance frequency to at least above 1 GHz at the expense of the initial permeability.
Effect of intermixing on self-propagating exothermic reactions in Al/Ni nanolaminate foils
Authors: Gavens A. J. , Van Heerden D. , Mann A. B. , Reiss M. E. , Weihs T. P.
Published: July 2002

Exothermic reactions can self-propagate rapidly in multilayered foils, and the properties of these reactions depend strongly on the heat of reaction, the average atomic diffusion distance, and the degree of intermixing at the layer interfaces prior to ignition. By performing low-temperature anneals on sputter-deposited Al/Ni nanolaminate foils, the thickness of the intermixed region between layers was increased and its effects on the heats and velocities of reactions were measured. The intermixed region consisted of the metastable Al9Ni2 phase while the final phase of the foil was Al3Ni2. Analytical and empirical models were used to predict reaction velocities as a function of bilayer thickness and intermixing thickness, and the predictions are in good agreement with the experimental results. Increasing the average thickness of the intermixed region from 2.4 to 18.3 nm reduced the reaction velocity for all of the foils but was most significant for the foils with bilayer thicknesses less than 25 nm. The results indicate that the reaction velocity can be separated into two distinct regimes. The first regime occurs for thicker bilayers in which the average atomic diffusion distance is large. In this regime, reaction temperatures are high and reducing the bilayer thickness increases the reaction velocity. The second regime occurs for thinner bilayers where reaction velocity is dominated by the reduction in available energy due to intermixing. In this regime, reducing bilayer thickness results in a decrease in reaction velocity.
CuInSe
2
phase formation during Cu2Se/In2Se3 interdiffusion reaction
Authors: Park J. S. , Dong Z. , Kim Sungtae , Perepezko J. H.
Published: July 2002

Diffusion couples based upon Cu2Se/In2Se3 pairings have been examined in order to identify the kinetics of intermediate phase development and the associated phase equilibria. For the diffusion couples annealed at 550 °C for 1.5 h, all phases included in the Cu2Se–In2Se3 pseudobinary phase diagram section developed including the CuInSe2 (CIS) phase. Also, the In6Se7 phase formed for annealing times in excess of 1.5 h at 550 °C, indicating a modification of the diffusion pathway outside the pseudobinary phase diagram section. The growth of the CIS phase formed by reactive diffusion follows parabolic kinetics (x2=kt) with the k value of 3.3×10−8 cm2/s. CIS phase precipitates with a dendritic morphology are also produced within the Cu2Se side of the diffusion couple far from the initial interface, indicating that In is the fast component during interdiffusion. Based upon electron diffraction analysis and simulation of electron diffraction patterns, the dendritic shaped CIS precipitate structure was uniquely determined to be the metastable zinc blende type rather than the stable chalcopyrite-type structure. The structure and orientation relationship between the metastable CIS phase and the Cu2Se matrix satisfy the conditions established for the development of a solid state dendritic morphology.
Snoek’s limit in high-frequency permeability of polycrystalline Ni–Zn, Mg–Zn, and Ni–Zn–Cu spinel ferrites
Authors: Nakamura Tatsuya
Published: July 2002

Three series of polycrystalline spinel ferrite, Ni–Zn ferrite, Mg–Zn ferrite and Ni–Zn–Cu ferrite, were prepared by the usual ceramic sintering method. The complex permeability spectra of each were measured. The low-frequency permeability increases with zinc content. However, the natural resonance frequency shifted down as the zinc content increased. The spectra were also numerically separated into spin rotational and domain wall resonance components. For the spin rotational contribution, which remained in the high frequency region, the static spin susceptibility increased and the spin resonance frequency shifted down with an increase in zinc content. Additionally, it was found that the product of the static spin susceptibility and the spin resonance frequency is proportional to the magnetization, irrespective of the kind of spinel ferrite. These relationships correspond to Snoek’s limitation rule. From these results, it was clarified that polycrystalline ferrites having large magnetization are suitable for high-frequency applications.
Hydrogen chemisorption on transition metal clusters
Authors: Geusic M. E. , Morse M. D. , Smalley R. E.
Published: July 2002

The dissociative chemisorption of H2 on supersonic beams of Nb, Co, and Cu clusters is studied. (AIP)
Infrared spectroscopy at the surface of clusters: SF6 on Ar
Authors: Gough T. E. , Mengel M. , Rowntree P. A. , Scoles G.
Published: July 2002

We have succeeded in obtaining infrared spectra of molecules adsorbed on the surface of clusters. The method is based on the photodissociation spectroscopy technique developed in our laboratory for the study of cluster beams and on a simple but effective way to prepare mixed clusters in which an IR chromophore is attached to the surface of a nonabsorbing host cluster. The possible extension of this technique to the study of molecular spectroscopy at the surface of clusters large enough to simulate crystal and liquid surfaces is also discussed.
Nuclear spin–spin coupling constants evaluated using many body methods
Authors: Sekino Hideo , Bartlett Rodney J.
Published: July 2002

In nuclear spin–spin coupling constant determinations, correlation corrections to the Fermi contact term are significant. In this paper we report the coupling constants calculated for the HD and HF molecules obtained by the infinite-order coupled cluster singles and doubles (CCSD) methods and MBPT(4). These are in good agreement with the experimentally estimated value for the Fermi-contact term. In addition, it is well known that the coupled perturbed Hartree–Fock (CPHF) scheme fails for multiply bonded molecules because the closed shell Hartree–Fock solution is triplet unstable. A CCSD method using ordinary nonrelaxed SCF orbitals is presented in order to circumvent this problem, and illustrated by application to the C2H4 molecule. It is shown that CCSD results based upon ordinary SCF orbitals include effectively all the effect of orbital relaxation and reproduce the experimental values for most of the coupling constants. Unlike previous results, the 3J(H–H) constant is positive in agreement with experiment.
Picosecond measurements of the dissociation rates of the nitric oxide dimer v1=1 and v4=1 levels
Authors: Casassa Michael P. , Woodward Anne M. , Stephenson John C. , King David S.
Published: July 2002

Direct picosecond measurements of the vibrational predissociation rates of the nitric oxide dimer excited to v=1 levels of the symmetric (ν1=1870 cm−1) and antisymmetric (ν4=1789 cm−1) N–O stretching fundamentals are reported. Lifetimes obtained are 880±260 ps for ν1 and 39±8 ps for ν4 excitations.
Local modes of benzene and benzene dimer, studied by infrared–ultraviolet double resonance in a supersonic beam
Authors: Page Ralph H. , Shen Y. R. , Lee Y. T.
Published: July 2002

We used rotational cooling of molecules to ∼5 K by supersonic expansion and state-selective, multilevel saturation spectroscopy to obtain high-resolution spectra of the fundamental and first and second overtone transitions of C–H stretching modes in ground-electronic-state benzene and its dimer. Greatly reduced linewidths (<3 cm−1 FWHM) in the rich spectra show that previously reported spectra have suffered from inhomogeneous congestion. Our observed spectral widths indicate that the vibrational lifetimes of the C–H stretches are at least a few ps, even at the energy of the second overtone (8800 cm−1). The ‘‘local mode’’ picture appears to apply when at least three quanta of C–H stretching motion are present. Spectra of the dimer are similar to those of the monomer but show a red shift of a few cm−1, the appearance of combination bands involving van der Waals vibrational modes, some intensity changes, and a broadening of spectral features that increases with the vibrational energy. The dimer’s predissociation lifetime at ∼3000 cm−1 vibrational energy exceeds ∼3 ps.
Excess electronic states in fluid helium
Authors: Coker D. F. , Berne B. J.
Published: July 2002

We study the nature of the excess electronic states in fluid helium by calculating the excess electron energies and wave functions for a set of configurations of solvent atoms taken from path integral Monte Carlo calculations on the solvated electron system. The eigenvalues and eigenfunctions for the different configurations of the solvent are used to calculate the inhomogeneously broadened density of states and absorption line shape of the excess electron over a range of fluid densities. The predictions of a simple theory of the excess electronic states in fluid helium due to Springett et al. [B. E. Springett, M. H. Cohen, and J. Jortner, Phys. Rev. 159, 183 (1967)] are found to agree quite well with our computer simulation results. This simple theory, however, predicts an inhomogeneously broadened elecronic absorption linewidth which is much narrower than that obtained from simulation. It is found that the RISM-polaron theory of Nichols and Chandler [A. L. Nichols III and D. Chandler, J. Chem. Phys. 87, 6671 (1987)] gives an absorption line shape which is in better agreement with our simulations. We observe a transition from lower energy states in which the electron is bound to density fluctuations in the fluid to the situation where the excess electron scatters through the fluid in continuum states at higher energies.
The rotational Raman spectrum of H2 in water
Authors: Taylor David G. , Strauss Herbert L.
Published: July 2002

The first four rotational Raman lines of H2 dissolved in water were observed. The lines broaden considerably, but do not shift in position, compared to the corresponding lines of gas phase H2. The breadth of the lines varies with J and with the change from H2O to D2O. The H2 rotational motion is thought to interact with the librational motion of the water molecules.
A new model for the thermally induced volume phase transition of gels
Authors: Otake Katsuto , Inomata Hiroshi , Konno Mikio , Saito Shozaburo
Published: July 2002

A theoretical model that considers hydrophobic interaction is proposed for the thermally induced discontinuous shrinkage of hydrogels. In this model, the free energy of a hydrogel is divided into four parts: the elastic free energy of networks formulated according to the theory which takes the limit of elongation into account, the free energy arising from osmotic pressure of dissociated counterions in the gel, the free energy of interactions except for the hydrophobic interaction represented by a virial-type volume interaction equation, and the free energy of the hydrophobic interaction. By the former three terms, the thermally induced swelling of gels and the effects of aspect ratio of segment are expressed. The incorporation of the fourth hydrophobic interaction term makes it possible to explain the thermally induced shrinkage of gels, and suggests the possibility of an explanation of ‘‘convexo’’-type volume phase transition by means of the hydrophobic interaction.
Optical spectra of a planar aggregate and of a lamellar crystal containing it: How are they related?
Authors: Young R. H. , Marchetti A. P. , Newhouse E. I. P.
Published: July 2002

A simple theory is developed to relate the optical absorption spectrum of a two-dimensional dye aggregate, either dispersed in a transparent matrix or deposited on a transparent substrate, to the spectra of a crystal that consists of a stack of such aggregate layers. Spectral data can then provide evidence on whether the structure of such an aggregate is similar to that of a layer in the crystal. A thiapyrylium dye aggregate dispersed in a polymer film is treated as an example.
Coil to rod transitions in Monte Carlo simulations of a short polyelectrolyte. I. New thermal and screening effects
Authors: Brender Chava
Published: July 2002

A Monte Carlo simulation of a single short polyelectrolyte chain immersed in a dielectric continuum solvent with screened Coulombic interactions is described. Two main effects are involved in the coil→rod conformational transitions that have been observed when the temperature is decreased: a new thermal effect and a screening effect. The pure thermal effect is resolved by simulating a bare fully ionized polyion with Coulombic interactions at various T. With decreasing T, the radius of gyration exhibits a crossover from a self-avoiding walk configuration to a rod-like shape. This phenomenon is explained by fundamental thermodynamic considerations. The screening effect at various temperatures, has been investigated for several ‘‘salt’’ concentrations using a screened Coulomb potential instead of a pure Coulomb one. Configurational properties such as the mean square end-to-end distance 〈R2〉, the mean square radius of gyration 〈S2〉, the average of the ratio of the means 〈S2〉/〈R2〉, the mean reduced electrostatic energy 〈UN/NkT〉, and the reduced mean electrostatic energy fluctuations Cν/Nk at various concentrations and temperatures were studied. Our results show that, at low salt concentration where C<0.001 M, the chain behaves like a bare polyelectrolyte chain at all temperatures (no screening at all). At high salt concentration, C>1 M, the chain is fully screened and exhibits a self-avoiding walk configuration at all temperatures, so that the thermal effect cannot be observed. At intermediate salt concentrations, 0.001 M<C<1 M, the chain is partially screened, exhibiting the coil to rod-like transition but with less extension under decreasing temperature. No increase in the effective screening due to temperature decrease is observed under these conditions, despite the decrease in κ−1, the Debye–Hückel screening length.
Raman spectroscopic study of bulk water supercooled to − 33 °C
Authors: Hare D. E. , Sorensen C. M.
Published: July 2002

We present Raman spectroscopic measurements in the OH stretch region of water (2900 to 3800 cm−1) for the temperature range 80 to −33 °C. This latter temperature represents the homogeneous limit of nucleation for our bulk samples. We find that the temperature dependence of both our intensity and depolarization ratio measurements are well described by a two-state model in which both states have frequency dependent depolarization ratios. We argue that the hydrogen bonded mode of the spectrum begins to develop a collective nature at both its low and high frequency ends. This collective nature can explain a breakdown in isosbestic behavior at low temperature. It is suggested that these collective modes may be due to in- and out-of-phase motions of OH oscillators. Their intensity approaches those seen in amorphous solid water as the liquid is cooled to the apparent singular temperature Ts≂−45 °C, for a variety of bulk properties. We use our Raman data to make an estimate of hydrogen bond probability and show that four bonded molecules would percolate near the singularity. We then argue that the collective mode grows because clusters of four bonded molecules grow in extent until at Ts percolation occurs to yield an infinite cluster with the amorphous solid’s collective spectrum.
On the self-poisoning of small particles upon island formation of the reactants in a model for a heterogeneously catalyzed reaction
Authors: Albano Ezequiel V.
Published: July 2002

Assuming the Langmuir–Hinshelwood mechanism, the reaction A+(1/2) B2■AB is studied on both small homogeneous and disordered surfaces by means of computer simulations. The occurrence of a new self-poisoning regime, where for a determined initial condition the substrata could be completely covered for each of the reactants with a certain probability, is observed and discussed. Large fluctuations in the size of adsorbed islands formed by A and B particles would cause the observed behavior. A crossover from the new self-poisoning regime to a steady state with AB production occurs at L≂30±5 and pA≂0.36±0.02 for incipient percolation clusters and at L≂12±2 and pA≂0.51±0.02 on the square lattice, where L and pA are the lattice size and the mole fraction of the specie A in the gas phase, respectively. It is shown that surface diffusion of A particle does not influence the location of the self-poisoning regime.
Multireference coupled-cluster method using a single-reference formalism
Authors: Oliphant Nevin , Adamowicz Ludwik
Published: July 2002

A multireference coupled-cluster singles and doubles method utilizing two reference determinants which differ by a two electron excitation is proposed. One of these determinants is selected as the formal reference determinant. The proposed method includes single-reference coupled-cluster equations truncated after quadruples. These equations are graphically derived using Feynman diagrams. The appropriate restrictions are then placed on the triple and quadruple amplitudes to allow only those amplitudes which correspond to single and double excitations from the second reference determinant.
Thermoelectric power and conductivity of iodine-doped ‘‘new’’ polyacetylene
Authors: Zuzok R. , Kaiser A. B. , Pukacki W. , Roth S.
Published: July 2002

We report measurements of the thermoelectric power and conductivity of ‘‘new’’ polyacetylene doped with iodine. In highly doped samples in which the conductivity extrapolates to finite values at zero temperature, we observe a nonlinearity in the thermopower very similar to that characteristic of the electron–phonon effect in metallic diffusion thermopower. The size of this nonlinearity is unusually small in the most highly conducting samples, as expected when the intrinsic conductivity is very large. For samples with additional sp3 defects or lower doping levels, in which the conductivity follows variable-range hopping behavior, we also find evidence for a hopping contribution in the thermopower.
A theoretical study on the dipole moment of N2O and the weakly bound complexes formed by N2O
Authors: Mogi K. , Komine T. , Hirao K.
Published: July 2002

We studied the structures and stability of several weakly bound complexes formed by N2O theoretically. It is shown that the N2O dipole moment is very sensitive to two bond lengths and to the electron correlation. N2O⋅⋅⋅HF has two minima on the energy hypersurface corresponding to bent NNO–HF and linear FH–NNO structures. The theory predicts an energetic difference of 267 cm−1 between two isomers with bent isomer more stable at the single and double configuration interaction/double zeta polarization (SDCI/DZP). The interconversion path connecting two isomers is also calculated. There is a low energy conversion path between linear and bent complexes. The activation energy between two minima is estimated to be 497 cm−1. The stability and structures of (N2O)2, N2O⋅⋅⋅BH3, N2O⋅⋅⋅NH3 complexes are also examined.
Relativistic self-consistent-field methods for molecules. III. All-electron calculations on diatomics HI, HI+, AtH, and AtH+
Authors: Matsuoka Osamu
Published: July 2002

All-electron relativistic and nonrelativistic self-consistent-field programs have been developed and calculations on diatomics HI, HI+, AtH, and AtH+ have been performed to get the spectroscopic constants. The results of HI, HI+, and AtH agree fairly well with those by the effective-core potential methods. The relativistic contractions of bond lengths of AtH and AtH+ are found to be about 2%. Contrary to the common belief, the harmonic frequencies found from the relativistic calculations are smaller than those of the nonrelativistic calculations.
High pressure nuclear magnetic resonance measurement of spin–lattice relaxation and self-diffusion in carbon dioxide
Authors: Etesse Patrick , Zega James A. , Kobayashi Riki
Published: July 2002

Spin–lattice relaxation time and self-diffusion coefficient in 13CO2 have been measured on the four isotherms 0, 25, 50, and 75 °C at pressures ranging from 10 to 500 bar. The governing relaxation mechanism in this range is shown to be spin–rotation relaxation. Low pressure T1 data are adequately described by Gordon’s theory, while high pressure T1 data agree semiquantitatively with the diffusion model proposed by Hubbard. The low density self-diffusion coefficient behavior is in agreement with the kinetic theory of the dilute gas phase. Strong divergence from Enskog theory is observed in the dense gas and liquid phases. The hard-sphere theory predicts the self-diffusion coefficient within ±2% at densities above 1.5 ρc. The simple hydrodynamic theory predicts the self-diffusion coefficient within ±5% at densities above 1.5 ρc.
A neutron diffraction study of dimethyl sulphoxide–water mixtures
Authors: Soper A. K. , Luzar Alenka
Published: July 2002

Neutron diffraction with hydrogen/deuterium isotope substitution is used to investigate the structure of water in concentrated dimethyl sulphoxide (DMSO) aqueous solutions. Partial structure factors and pair correlation functions involving the hydrogen atom on the water molecule are determined. Water structure is not found to be strongly affected by the presence of DMSO. However, the percentage of water molecules which hydrogen bond to themselves is substantially reduced compared to pure water, with a large proportion of the hydrogens available for bonding associated with the lone pairs on the DMSO. These experimental findings are in good agreement with the assumptions made in the simple mean-field type model for hydrogen-bonded mixtures, developed by Luzar [J. Chem. Phys. 91, 3603 (1989)]. A general scheme for analyzing experimental data on the HH and OH pair correlation functions in terms of coordination numbers is presented. The hydrogen–hydrogen correlation in the solvent (water) is also used to discuss the interparticle correlations between solute (DMSO) particles.
Phonons in liquids: A random walk approach
Authors: Wu Ten-Ming , Loring Roger F.
Published: July 2002

We develop a method to calculate the configuration-averaged density of phonon modes in a liquid. Our strategy is based on the isomorphism between the calculation of the density of modes of a harmonic structure and the determination of transport properties of a random walker on that structure. The density of modes calculation for a fluid in d dimensions is shown to require solution of a random walk, in which a walker with d internal states moves among sites located at the particles of the fluid. We generalize the random walk theory of Gochanour, Andersen, and Fayer to treat this vector random walk, and use this approach to calculate the averaged density of phonon modes in a Lennard-Jones fluid. The calculation agrees well with Monte Carlo simulation results of Seeley and Keyes.
Liquid–vapor interface of water–methanol mixture. I. Computer simulation
Authors: Matsumoto Mitsuhiro , Takaoka Yuji , Kataoka Yosuke
Published: July 2002

Results of molecular dynamics computer simulation are presented for liquid–vapor interface of water–methanol mixture of various compositions at room temperature. The composition dependence of calculated surface tension is typical of aqueous solutions of organic compounds. The outermost surface layer is saturated with methanol even at low bulk concentrations of methanol. The density profile of each component seems oscillatory at some compositions.
Product selectivity of vibrationally mediated photofragmentation of methanol
Authors: Marston C. C. , Weide K. , Schinke R. , Suter H. U.
Published: July 2002

We investigate the photodissociation of CH3OH in the first absorption band (S0→S1) by a two dimensional wave packet study employing the associated internuclear potential energy surfaces obtained from ab initio calculations. The quantum chemical calculations are performed in the complete active space self-consistent field approach including the CH3O and OH bond distances with the CH3–O–H bond angle being fixed. The methyl group is considered as a structureless particle. The nuclear wave functions in the ground and in the excited electronic state are then calculated by using a novel expression for the kinetic energy operator in terms of bond coordinates for fixed bending angle and fixed total angular momentum J=0. The photodissociation of methanol is a very fast process and, in agreement with experiment, leads to a broad and structureless absorption spectrum. Dissociation of the vibrational ground state yields, also in qualitative agreement with experiment, exclusively the products CH3O+H. The main emphasis of the present study, however, is the influence of initial vibrational excitation in the electronic ground state on the fragmentation process. For example, the branching ratio for the chemical channels CH3+OH and CH3O+H is shown to exhibit a drastic dependence on the initial vibrational state of CH3OH(S0) as well as on the energy of the dissociating photon. Thus proper selection of the initial vibrational state and of the dissociating photon frequency can, in principle, be used to exert control of products to obtain branching ratios spanning several orders of magnitude.
A direct algorithm for self-consistent-field linear response theory and application to C60: Excitation energies, oscillator strengths, and frequency-dependent polarizabilities
Authors: Weiss Horst , Ahlrichs Reinhart , Häser Marco
Published: July 2002

We present a direct self-consistent-field (SCF)-type algorithm and its implementation for the computation of linear response properties: excitation energies, oscillator strengths, and frequency-dependent polarizabilities within the time-dependent SCF or random phase approximation. The treatment of singles configuration interaction for electronic excitations and Hartree–Fock instability criteria are covered as special cases. The algorithm is based on proven direct SCF methodology. This, together with full exploitation of molecular symmetry, opens the way to the treatment of large molecules. Applications to C60 strongly support the assignment of the lowest-lying dipole allowed transition to the strong band at 3.8 eV.
Theoretical determination of spectral lines for the Zn atom and the ZnH molecule
Authors: Jamorski Ch. , Dargelos A. , Teichteil Ch. , Daudey J. P.
Published: July 2002

Theoretical calculations of the low lying excited states of the Zn atom and ZnH molecule are reported. An averaged relativistic effective core potential is used for the 1s2 2s2 2p4 3d10 electrons of Zn and the polarization and intrashell correlation effects are treated with a semiempirical core polarization potential. Spin–orbit splittings are incorporated into the correlated electronic states.
Monte Carlo simulation of Lennard-Jones chains
Authors: Li Xiao-Jian , Chiew Yee C.
Published: July 2002

Thermodynamical properties of ‘‘infinitely stiff’’ freely jointed Lennard-Jones chains are obtained through Monte Carlo simulation technique in the canonical (NVT) ensemble. A force-bias sampling scheme is used in the simulation. The compressibility factor, energy, and constant-volume heat capacity of 8-, 16-, and 32-mer Lennard-Jones chains over wide ranges of densities and temperatures are obtained. It is known that the virial pressure of infinitely stiff freely jointed Lennard-Jones chains depends on two- and three-body correlations between segments. We determine, for the first time, these two contributions to the virial pressure of Lennard-Jones chains, and found that their relative magnitudes are comparable with each other. While the three-body term remains positive for all temperatures and densities examined, the two-body term adopts negative values at low temperatures and densities and positive values at high temperatures and densities. Our simulation results reveal the importance of three-body intrachain correlations that arise from chain connectivity and provide a quantitative basis for the assessment and development of theories for chain fluids.
Noble gas clusters as matrices for infrared spectroscopy. From small clusters to the bulk-matrix limit: SF6Arn, SF6Krn, and SF6Xen with 100≲n≲10 000
Authors: Goyal Sandeep , Schutt David L. , Scoles Giacinto
Published: July 2002

It is demonstrated that matrix-like spectroscopy may be carried out in the gas phase using molecular beams of clusters and the conditions under which bulk-matrix-like behavior is achieved are illustrated. At the same time, we obtain information on the structural evolution of noble gas clusters as a function of their size. Infrared spectra for SF6 attached to noble gas clusters of argon, krypton, and xenon were recorded using a free jet cluster source and a laser photofragmentation detection technique. When a dilute mixture of the chromophore in Ar and Kr is expanded at relatively low pressures, the clusters spectra show a feature characteristic of the SF6 solvated in a defective, unannealed matrix. This feature disappears at higher source pressures (larger sizes) at which the chromophore prefers to reside on the surface of the cluster. This can be demonstrated by producing neat clusters and depositing the chromophore on them. However, on producing still larger clusters, a different absorption appears which is accurately located at the same position as the main absorption in a well-annealed matrix of Ar or Kr. This behavior is related to the transition of clusters from a Mackay icosahedral structure, shown to be the most stable for smaller clusters, to the face-centered-cubic (fcc) structure which is observed in the bulk phase. This structural transition occurs at a nozzle stagnation pressure which corresponds to an average cluster size of about 2000 atoms for both Ar and Kr. Scattering studies performed on argon clusters suggest that the fcc-type clusters correspond to the largest sizes in the cluster size distribution present in the beams. A similar structural transition for xenon cluster was not established as the SF6 appears to solvate only slightly in Xe in the size range studied here.
Effective and intermediate Hamiltonians obtained by similarity transformations
Authors: Meissner Leszek , Nooijen Marcel
Published: July 2002

A simple similarity transformation is used to derive equations for effective and intermediate Hamiltonians in a lucid way. Effective and intermediate Hamiltonians based on the wave operator formalism provide only a subset of all eigenvalues while the similarity transform technique divides the eigenvalue problem into two subproblems that can be solved separately. This means that the complete spectrum of the Hamiltonian remains well defined and this proves to be advantageous in the formal analysis and may be useful in many applications. Moreover both left and right hand eigenvectors of the transformed Hamiltonian can be obtained and this allows a convenient evaluation of properties. Rayleigh–Schrödinger and Brillouin–Wigner perturbation expansions of the intermediate Hamiltonians are discussed and a comparison is made of the various possible schemes.
Nematic wetting at the free surface of 4-cyano-4′-n-alkyl-biphenyls
Authors: Kasten H. , Strobl G.
Published: July 2002

The free surface of a homologous series of low molecular weight liquid crystals 4-cyano-4′-n-alkyl-biphenyls (nCB, n=5,6,7,8) is studied by reflection ellipsometry. On approaching the bulk isotropic–nematic transition temperature TNI from the isotropic phase, the growth of a nematic layer at the surface is observed. The director orientation in the layer is identified as homeotropic. The ellipsometric data are analyzed in terms of a perturbation calculation. It is shown that for 7CB and 8CB very close to TNI, where the layer has a thickness d≥50 Å, both d and the order parameter S0 at the surface can be determined. In all other cases only one parameter, which corresponds to the nematic coverage, can be extracted from the data. Results are discussed in terms of a wetting process. Partial wetting by the nematic phase is observed for n=6,7,8. On approaching TNI from lower temperatures where the bulk is in the nematic phase a small increase of excess surface order is measured which is interpreted as a homeotropic nematic wetting layer possessing higher order than the nematic bulk.
Reaction–diffusion description of biological transport processes in general dimension
Authors: Nadler W. , Stein D. L.
Published: July 2002

We introduce a reaction–diffusion system capable of modeling ligand migration inside of proteins as well as conformational fluctuations of proteins, and present a detailed analytical and numerical analysis of this system in general dimension. The main observable, the probability of finding the system in the starting state, exhibits dimension-dependent as well as dimension-independent properties, allowing for sharp experimental tests of the effective dimension of the process in question. We discuss the application of this theory to ligand migration in myoglobin and to the description of gating fluctuations of ion channel proteins.
Polymer chains tethered to impenetrable interfaces: Broadening of relaxation spectra
Authors: Koch Martin , Sommer Jens-Uwe , Blumen Alexander
Published: July 2002

We investigate the dynamics of an ideal polymer chain which is tethered with one of its ends to an impenetrable interface. Due to the geometric restrictions the relaxation of modes perpendicular to the interface differs significantly from that of modes parallel to it. A superposition of both components leads to a marked broadening of the relaxation spectrum, which may find its signature in dielectric measurements. We obtain the relevant dynamical quantities for a polymer chain subject to geometric constraints by use of the Smoluchowski equation combined with a Rayleigh-Ritz variational scheme.
The self-consistent statistical theory of condensation
Authors: Vompe A. G. , Martynov G. A.
Published: July 2002

We propose a statistical approach to the first order phase transitions problem. For this purpose a self-consistent method for the calculation of high-order virial terms together with efficient numeric realization are developed. The method is based on the idea of differential conditions for thermodynamic consistency what makes possible the calculation of up to 30 virial terms. It is shown numerically that the obtained expansion is convergent. In the accordance with the Lee and Yang theorem, the convergency limit of virial series is found to correspond to the condensation curve. The unknown properties of high order virial terms are revealed. We show that near the critical temperature, the remainder of virial series vanishes, with high order virial terms being close to zero and the properties of a fluid being determined by the first 4–5 terms. The results are tested for two model systems; the square well and Lennard-Jones potentials. Our estimates for the phase transition curve are in good agreement with independent numeric simulation data.
Oxygen chemisorption and oxide film growth on Ni{100}, {110}, and {111}: Sticking probabilities and microcalorimetric adsorption heats
Authors: Stuckless J. T. , Wartnaby C. E. , Al-Sarraf N. , Dixon-Warren St. J. B. , Kovar M. , King D. A.
Published: July 2002

Using single-crystal adsorption calorimetry, heat data have been measured for the adsorption of oxygen on the three low-index planes of Ni at 300 K along with corresponding sticking probabilities. New data are presented with coadsorbed potassium on each plane, and temperature-dependent data for O2/Ni{100}. The initial heats of adsorption of oxygen on Ni{100}, {110}, and {111} are 550, 475, and 440 kJ (mol O2)−1, respectively, at 300 K, and the heat is found to drop rapidly with coverage in the chemisorption regime, indicating strong interadsorbate interactions. However, this rapid decline is not seen with coadsorbed potassium, a difference discussed both in terms of electron availability and coadsorbate attractions. The integral heats of adsorption for oxide film formation are 220, 290, and 320 kJ mol−1, respectively. Corresponding sticking probability measurements show initial values, all less than unity, of 0.63, 0.78, and just 0.23, again for the {100}, {110}, and {111} surfaces in that order. The coverage dependence of the sticking probability is consistent in each case with a passivating oxide film four layers thick. Comparable data for Ni{100} obtained using a pyroelectric detector gave good agreement with the conventional results at 300 K. At 410 K, however, the heat-coverage curve was flat up to 0.25 monolayers. Data were also obtained at 90 K. Analysis and Monte Carlo simulation of the temperature-dependent adsorption heat curves indicates that the large drop in adsorption heat with coverage seen at room temperature is consistent with a local second-nearest neighbor adatom–adatom repulsion rather than a long-range electronic effect.