bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlation between the radiation-induced intrinsic 4.8 eV optical absorption and 1.9 eV photoluminescence bands in glassy SiO2
Authors: Skuja L. , Tanimura K. , Itoh N.
Published: July 2002

The controversial relationship between the intrinsic 1.9 eV photoluminescence (the R band), the 2.0 and 4.8 eV excitation/optical absorption bands is examined for 11 specimens with different OH, Cl, and O2 concentrations and with different irradiation histories. The direct relation is found between the intensities of the R-band luminescence measured with the 4.0 eV (IUV) or resonance (1.94 eV) excitation (Ires), although it is slightly superlinear, Ires∝(IUV). The deviation from the exactly constant ratio between the band intensities in different glass samples is attributed mainly to the inhomogeneous broadening effects. Also, nonzero polarization degree, P=−1.5±0.5%, in the R-band maximum region is reconfirmed upon linearly polarized 4.8 eV excitation, substantiating a close relation between the 4.8 eV absorption band and the R band. These results suggest strongly that both of the two absorption bands at 2.0 and 4.8 eV are originated from a single center, and quantitatively agree with the recently proposed optical transition scheme in the nonbridging oxygen hole center.
The transformation of graphitic onions to diamond under electron irradiation
Authors: Banhart Florian
Published: July 2002

Diamond crystals nucleate in the centers of spherical concentric-shell graphitic particles where high pressure prevails under electron irradiation at specimen temperatures above 900 K. The diamond crystals grow under further irradiation until the graphitic particles have wholly transformed to diamond. Hence, the conversion of the graphitic structure to diamond starts at high pressure and proceeds at decreasing, possibly even at zero, pressure. The experiment is carried out in a transmission electron microscope which enables us to monitor this phase transformation in situ on an atomic scale. It is shown that the graphite/diamond system under electron irradiation is highly dissipative. This suggests that the phase transformation at low pressure occurs via self-organization under nonequilibrium conditions.
Variation of the magnetic moment and strain in epitaxial Cu/Ni/Cu sandwiches
Authors: Lee Jaeyong , Lauhoff G. , Hope S. , Daboo C. , Bland J. A. C. , Schillé J. Ph. , van der Laan G. , Penfold J.
Published: July 2002

We have studied the variation of the strain and the magnetic moment in epitaxial fct Ni(001) film structures using a combination of reflection high energy electron diffraction (RHEED) and x-ray magnetic circular dichroism. A 30 Å Cu/Ni step-wedge (30, 60, 90, and 150 Å)/600 Å Cu buffer/Si(001) structure has been used to study the Ni thickness-dependent strain and magnetic moment. A decreased magnetic moment with decreasing Ni thickness is observed. A 30 Å Cu/50 Å Ni/step-wedged (600, 1000, 1500, and 2000 Å) Cu buffer/Si(001) was used to vary the degree of strain as determined by in situ RHEED measurements in the same Ni film by varying the Cu buffer layer thickness. Our study shows a possible correlation between the magnetic moment and strain.
Optical transitions of Er3+ ions in ZnCl2-based glass
Authors: Shojiya M. , Takahashi M. , Kanno R. , Kawamoto Y. , Kadono K.
Published: July 2002

Optical absorption and emission properties of Er3+ ions in ZnCl2-based glasses are investigated. The electric dipole line strengths for various transitions were estimated from the measured optical absorption spectra. The spontaneous emission probabilities for various transitions were predicted using the Judd–Ofelt theory. Among Ωt (t=2, 4, 6) phenomenological intensity parameters, the Ω2 value is larger than that measured for ZrF4-based glass. On the contrary, the Ω4 and Ω6 values are comparatively small. Multiphonon relaxation rates follow a so-called “energy-gap law” which is given by WMPR=C exp(−αΔE). The C value is quite smaller than that of the ZrF4-based glass by about two orders of magnitude. This result is attributed to the very low-phonon-energy of the chloride glass matrix. On the other hand, the α value is almost the same as those of other glass hosts. Because of the low-phonon-energy and the host-independent α value, the electron–phonon coupling constant ε of the ZnCl2-based glass is extremely large compared with those of oxide and fluoride glasses. It was also found that several emissions due to transitions between the excited 4 f-levels are detectable, which are hardly observed in other glass hosts. Radiative quantum efficiency is definitively high even for emitting levels of which the energy-gap to the next-lower level is small.
Electrical resistance changes of germanium during solidification under high pressure
Authors: He D. W. , Zhang F. X. , Yu W. , Zhang M. , Liu Y. P. , Wang W. K.
Published: July 2002

Changes in electrical resistance of germanium were investigated during melting and cooling under high pressure. It was found that the electrical resistances of the samples dropped abruptly at an early stage of the solidification above ∼4 GPa. This result shows a metallic liquid to metallic solid transition of germanium when cooling above ∼4 GPa as compared to a metallic liquid to nonmetallic solid transition at pressures below ∼4 GPa. The mechanism of the phase evolution in the solidification process of germanium at high pressures is briefly discussed.
Magneto-electro-elastic Eshelby tensors for a piezoelectric-piezomagnetic composite reinforced by ellipsoidal inclusions
Authors: Huang Jin H. , Chiu Ya-Hui , Liu Hsien-Kuang
Published: July 2002

The magneto-electro-elastic Eshelby tensors that represent the stress, electric displacement, and magnetic induction in an inclusion resulting from the constraint of the surrounding matrix of piezomagnetic-piezoelectric composites are presented. The tensors provide the basis for analysis of the magneto-electro-elastic response of the composites and have numerous applications such as to the study of defects, overall properties of multiphase composites, and fracture mechanics, etc. For a three-dimensional transversely isotropic piezoelectric/piezomagnetic composite containing spheriodal inclusions, the number of nonzero Eshelby tensors is 41 while among them only 17 are independent. These independent tensors can be divided into three categories. The first category consists of those tensors related to the elastic response, the second involves those related to the piezomagnetic response, while the third includes elastic and magnetic interactive terms. Moreover, the magneto-electro-elastic Eshelby tensors are obtained in the closed forms when both constituents of the composite are transversely isotropic and shapes of the inclusion are elliptic, rod shaped, penny shaped, and ribbon like.
Linear and nonlinear optical properties of KNbO3 ridge waveguides
Authors: Pliska Tomas , Fluck Daniel , Günter Peter , Beckers Lutz , Buchal Christoph
Published: July 2002

Ridged channel waveguides in KNbO3 were produced using He+ ion implantation, photolithographic masking, and subsequent Ar+ ion sputtering. We investigated the linear and nonlinear optical characteristics of the waveguides. The effective mode indices are derived from the refractive index profiles using the effective index method. The losses are investigated as a function of wavelength and of the geometrical parameters channel width and ridge height. A minimum loss of 2 dB cm−1 is measured at a wavelength of 0.633 μm. We investigated the power handling capabilities at visible and near-infrared wavelengths. Second-harmonic generation in these waveguides is studied both theoretically and experimentally with regard to its dependence on the guide fabrication parameters. Phase-matching configurations for blue light second-harmonic generation are evaluated on the basis of the dispersion of the effective mode indices. Overlap integrals are calculated on the basis of the field distributions derived from the refractive index profiles. The minimum effective guide cross section is 25 μm2. A continuous-wave second-harmonic output power of 14 mW at 438 nm was obtained with an in-coupled fundamental power of 340 mW in a 0.73 cm long waveguide, yielding a normalized internal conversion efficiency of 25% W−1 cm−2 (13% W−1). This corresponds to an improvement by a factor of 3 compared to the best results reported for KNbO3 waveguides up to now.
Molecular dynamics Monte Carlo simulations of grain boundary electron transport in n-silicon
Authors: Joshi R. P. , Wood R. F.
Published: July 2002

Electron transport in Si low-angle bicrystals is analyzed by a novel Monte Carlo molecular dynamics simulation scheme. The effect of discrete charges at the grain boundaries is studied and compared to results from one-dimensional treatments. The average grain boundary charge density strongly influences transport, and a field-dependent threshold effect is predicted. Details of the internal charge arrangement are shown to be quite important at low fields and/or high grain boundary charge densities. Substantial increases in current conduction are predicted at lower temperatures over the thermionic emission model. Finally, analyses of interacting grain boundaries indicate site-correlation effects and a strong dependence of conductivity on the separation distance.
Atomic hydrogen cleaning of InP(100) for preparation of a negative electron affinity photocathode
Authors: Elamrawi K. A. , Hafez M. A. , Elsayed-Ali H. E.
Published: July 2002

Atomic hydrogen cleaning is used to clean InP(100) negative electron affinity photocathodes. Reflection high-energy electron diffraction patterns of reconstructed, phosphorus-stabilized, InP(100) surfaces are obtained after cleaning at ∼400 °C. These surfaces produce high quantum efficiency photocathodes (∼8.5%), in response to 632.8 nm light. Without atomic hydrogen cleaning, activation of InP to negative electron affinity requires heating to ∼530 °C. At this high temperature, phosphorus evaporates preferentially and a rough surface is obtained. These surfaces produce low quantum efficiency photocathodes (∼0.1%). The use of reflection high-energy electron diffraction to measure the thickness of the deposited cesium layer during activation by correlating diffraction intensity with photoemission is demonstrated.
Dynamics of laser produced carbon plasma expanding in a nonuniform magnetic field
Authors: Neogi A. , Thareja R. K.
Published: July 2002

The laser produced carbon plasma expanding in a nonuniform magnetic field in vacuum (P<10−3 Torr) has been studied by emission spectroscopy and fast photography using intensified charged coupled device. The laser ablated plume is observed to break into two lobes along the vertical direction. The temporal profiles of C I transition at 399.7 nm, C II transition at 426.7 nm, C III transition at 465.0 nm, and C IV transition at 580.1 nm show three peaks (fast, intermediate, and slow) for each of these two lobes. The temporal profile of the species at the outermost boundary of the plume shows oscillations.
Effect of laser intensity on the microstructural and mechanical properties of pulsed laser deposited diamond-like-carbon thin films
Authors: Tabbal M. , Mérel P. , Chaker M. , El Khakani M. A. , Herbert E. G. , Lucas B. N. , O’Hern M. E.
Published: July 2002

Diamond-like-carbon (DLC) thin films have been deposited at room temperature on Si substrates by ablation of a graphite target using a KrF excimer laser at intensities ranging from 0.9×108 W/cm2 to 6.0×109 W/cm2. The microstructure of the films was studied by x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The macroscopic properties were evaluated by measurement of their optical constants using in-situ laser reflectometry and their hardness using the continuous stiffness measurement technique. Analysis of the XPS C 1s core level spectra of the DLC films shows that their sp3 hybridized carbon atom content increases with laser intensity up to a maximum value of about 60% obtained at 7.0×108 W/cm2. At higher laser intensities, the sp3 content appears to stabilize at about 53%. Such an evolution of the sp3 content can be understood in terms of the subsurface carbon ions implantation model which has been proposed for ion beam deposited films. On the other hand, Raman analysis indicates that an increase in laser intensity leads to the establishment of some long range order of the sp2 domains in the deposited layers. The extinction coefficient k of the deposited layers was found to be correlated to their sp3 content. Finally, it is shown that hardness values as high as 47 GPa can be obtained and that hardness is also correlated to the sp3 content of the films.
Application of the Preisach and Jiles–Atherton models to the simulation of hysteresis in soft magnetic alloys
Authors: Pasquale M. , Bertotti G. , Jiles D. C. , Bi Y.
Published: July 2002

This article describes the advances in unification of model descriptions of hysteresis in magnetic materials and demonstrates the equivalence of two widely accepted models, the Preisach (PM) and Jiles–Atherton (JA) models. Recently it was shown that starting from general energy relations, the JA equation for a loop branch can be derived from PM. The unified approach is here applied to the interpretation of magnetization measured in nonoriented Si–Fe steels with variable grain size 〈s〉, and also in as-cast and annealed Fe amorphous alloys. In the case of NO Fe–Si, the modeling parameter k defined by the volume density of pinning centers is such that k≈A+B/〈s〉, where the parameters A and B are related to magnetocrystalline anisotropy and grain texture. The value of k in the amorphous alloys can be used to estimate the microstructural correlation length playing the role of effective grain size in these materials.
Photoconductivity of nitrogen-modified hydrogenated tetrahedral amorphous carbon
Authors: Ilie A. , Harel O. , Conway N. M. J. , Yagi T. , Robertson J. , Milne W. I.
Published: July 2002

The changes in the photoconductivity of hydrogenated tetrahedral amorphous carbon (ta-C:H) with nitrogen incorporation were studied. Low level nitrogen incorporation improves the photoconductivity, by shifting the Fermi level upwards in the band gap. Films with a photosensitivity of about 200 at room temperature under white light illumination of 35 mW/cm2 were obtained; thus is the highest value so far reported for diamond-like carbons. At high temperatures, photoconductivity is controlled by nonradiative recombination through gap states, whereas at low temperatures it occurs by energy-loss hopping in the band tails. Nitrogen addition does not create extra charge defect recombination centers. Low temperature photoconductivity allows the direct determination of the localization radius of the band tail states. This radius varies from 2–3 Å in ta-C:H to 9 Å in ta-C. This illustrates how hydrogen can increase state localization and the photoluminescence efficiency in amorphous carbons.
Crystal and magnetic structures of LaNi5−xMnx
Authors: Tai C. Y. , Marasinghe G. K. , Pringle O. A. , James W. J. , Chen M. , Yelon W. B. , Dubenko I. , Ali N. , I’Héritier Ph.
Published: July 2002

The crystallographic and magnetic properties of LaNi5−xMnx with x=1, 1.5, and 2, have been investigated using x-ray and neutron diffraction, vibrating sample magnetometer and superconducting quantum interference device measurements. All the samples crystallize in the hexagonal CaCu5-type (P6/mmm) structure. Manganese atoms preferentially occupy the 3g site in the LaNi5 structure. The bulk magnetization of the LaNi5−xMnx powder samples decreases rapidly as nickel is replaced by manganese. A ferrimagnetic model is applied to describe the magnetic structure of the LaNi5−xMnx samples for x=1.5 and 2. A ferromagnetic model gives the best fit of the neutron diffraction data for the LaNi4Mn sample.
CoPt and FePt thin films for high density recording media
Authors: Christodoulides J. A. , Huang Y. , Zhang Y. , Hadjipanayis G. C. , Panagiotopoulos I. , Niarchos D.
Published: July 2002

Highly anisotropic face-centered-tetragonal (fct) CoPt and fct FePt nanoparticles have been prepared and embedded in a C or BN matrix by cosputtering from pure Co50Pt50, BN, and Fe50Pt50, C solid targets using a tandem deposition mode. The as-made films show a disordered face-centered-cubic structure which is magnetically soft and have low coercivity (<20 Oe). Magnetic hardening occurs after heat treatment at elevated temperatures, which leads to increase in coercivity to values up to 15 kOe at room temperature. The hardening is due to the formation of nanoparticles with the highly anisotropic fct with anisotropy K>107 erg/cm3. Transmission electron microscope studies showed FePt particles embedded in C matrix with a size increasing from below 3 nm in the as-made state to about 8 nm in the optimum annealed state. These results are very promising and make these materials potential candidates for high-density magnetic recording.
Reduced threshold ultraviolet laser ablation of glass substrates with surface particle coverage: A mechanism for systematic surface laser damage
Authors: Kane D. M. , Halfpenny D. R.
Published: July 2002

A recent study of ultraviolet laser cleaning of silica glass surfaces contaminated with medium density alumina particles has shown a systematic type of laser-induced surface damage. This is characterized as a pit which increases in diameter and depth with increasing irradiating fluence. The damage pit occurs at lower fluence levels when smaller contaminant particles are used. The pits are due to laser ablation of the particle coated glass surface. The threshold for this laser ablation is well below that for a sample of the glass in its uncontaminated condition and the efficiency of ablation is estimated as being about 1000 times greater than that for silica. This laser damage is difficult to visualize by standard optical microscopy and could easily go undetected in laser cleaning studies. It may have general implications in the application of the new laser cleaning technologies to a variety of surfaces, including dielectrics and semiconductors, where medium to high densities of contaminant particles are present. It is also significant in suggesting processes by which glass may be rendered more readily machinable by laser ablative methods.
Localized electronic states related to O2 intercalation and photoirradiation on C60 films and C70 films
Authors: Habuchi Hitoe , Nitta Shoji , Han Daxing , Nonomura Shuichi
Published: July 2002

Deep localized electronic states are created by O2 intercalation into C60 films and C70 films, which causes the Fermi level to shift down to the middle of gap. The states act as a trap level for charge carriers and as nonradiative recombination centers. It seems that prepared C60 films and C70 films have a shallow localized state. The shallow state is located at ∼0.2 eV under the conduction band and affects the electrical and optical properties. Furthermore, the photoirradiation of C60 films and C70 films causes polymerization of the O2-free sample and oxidization of the O2-intercalated sample. The quasistable electronic states at room temperature are created as a result of photo-oxidization of C60 films. C60 oxides create deep localized electronic states which cannot disappear under thermal annealing. The photoluminescence intensity of O2-free samples increases with photoirradiation for 1 h. It is found for the first time that this increase occurs along with a decrease of localized state density.
Kinetics of unimolecular decomposition on surfaces: Methanol on Ni(110)
Authors: Richter Lee J. , Ho W.
Published: July 2002

The decomposition of methanol on a clean Ni(110) surface has been studied with temperature programmed reaction spectroscopy, low energy electron diffraction and temperature programmed electron energy loss spectroscopy: a new technique for the characterization of surface dynamic properties. Adsorption of methanol at 170 K results in the formation of a methoxy (CH3O) species. Near 270 K the methoxy decomposes into adsorbed hydrogen and carbon monoxide. For initial methoxy coverages greater than 0.20 monolayer (ML) the decomposition is rate limited by the desorption of product hydrogen. For initial methoxy coverages less than 0.20 ML the abstraction of the first methoxy hydrogen is rate limiting. This step is modeled by first order kinetics with an activation energy of 16 kcal mol−1 and a preexponential factor of 1012 s−1. A normal kinetic isotope effect (kH/kD>1) is observed confirming that C–H bond scission is the rate limiting step at low coverages.
Inelastic processes in ion/surface collisions: Scattered ion fractions and VUV photon emission for Ne+ and Ar+ collisions with Mg and Y surfaces
Authors: Rabalais J. Wayne , Chen Jie-Nan , Kumar R. , Narayana M.
Published: July 2002

Time-of-flight (TOF) scattering spectra and vacuum ultraviolet (VUV) photon emission spectra resulting from 1–10 keV Ne+ and Ar+ ions impinging on magnesium and yttrium surfaces and the corresponding oxidized and hydroxylated surfaces have been measured. Measurements of the scattered neutrals plus ions and neutrals only are used to calculate scattered ion fractions Y+ for the single scattering collisions. The Y+ values rise steeply at low ion energies E0 (∼1–2 keV), reaching values of 70% and 38% at 10 keV for Ne+/Mg and Ar+/Y, respectively, and are very sensitive to adsorbate coverage. The dominant photon emission observed from the clean metals in the VUV range 30–200 nm is 1,3P → 1S resonance radiation from the excited neutral projectile atoms; emission was also observed from excited H and O for adsorbate covered surfaces. A model is developed for electronic transitions in keV ion/surface collisions which considers Auger and resonant transitions along the ion trajectory and electron promotions in the quasidiatomic molecule of the close encounter. By making an assumption of equality in the close encounter, the model can be fitted to the experimental data, allowing determination of ionization PI and neutralization PN probabilities as a function of the distance of approach. The results show that electron promotions within MO’s of the collision complex formed during encounter are significant, if not dominating, processes in keV ion surface collisions.
X-ray diffraction analysis of molten AlCl3–NaCl system
Authors: Takahashi Setsuko , Maruoka Kyouichi , Koura Nobuyuki , Ohno Hideo
Published: July 2002

The way in which structure of molten AlCl3–NaCl mixtures changes with AlCl3 concentration and temperature has been investigated by x-ray diffraction analysis. It was confirmed that AlCl4 tetrahedra are the fundamental structural units in the mixture of 50–70 mol % AlCl3. The nearest-neighbor distance of the Al–Cl, Na–Cl, and Cl–Cl pairs were 2.13, 2.80, and 3.48 Å, respectively. It was considered that the variation of the Q ⋅ i(Q) curves with AlCl3 concentration is due to the change of the mole ratio of AlCl−4 to Al2Cl−7 and the effect of the Na–Cl pair. It was supposed that AlCl−4 is tetrahedral and Al2Cl−7 is constructed from two corner-sharing AlCl4 tetrahedra. Both the concentration of Al2Cl−7 and the coordination number of Cl around an Na increased with increasing AlCl3 concentration at constant temperature, but decreased with increased temperature. The structure of the melt changed with variation of temperature at constant AlCl3 concentration.
Structure and ion dynamics of silver borate glasses: A 19Ag NMR study
Authors: Villa Marco , Chiodelli Gaetano , Magistris Aldo , Licheri Giovanni
Published: July 2002

An analysis of the 109Ag isotropic chemical shift δ in the glasses of the AgI:Ag2O:B2O3 system is presented. In these glasses, δ covers a range of ∼500 ppm but does not depend linearly upon the fraction of I− anions X=[I]/[Ag]. This means that, when AgI is added to the borate network, structures are preferentially formed in which silver is coordinated to both I− and negatively charged borate groups. We discuss also the 109Ag chemical shift range covered by the different forms of AgI and the dependence of δ upon the BO4 units and non-bridging-oxygen (NBO’s) concentrations. We analyze the relationship between spatial compositional fluctuations and NMR–MAS width of the 109Ag line in fast silver conducting glasses. The spin-lattice relaxation of 109Ag and 11B, as well as some of the 109Ag line narrowing results, are quantitatively interpreted by assuming a distribution of Ag+ jump times and by discussing how the different nuclear interactions are affected by the diffusion of silver. We conclude that the NMR evidence does not support the microscopic picture of cation diffusion in vitreous electrolytes suggested by the weak electrolyte theory.
The photoionization cross sections of molecular chlorine
Authors: Samson J. A. R. , Angel G. C.
Published: July 2002

The total absorption and total ionization cross sections of molecular chlorine have been measured between 150 and 1037 Å. Over the range 150 to 850 Å the partial cross section for the production of singly charged molecular ions has been determined. In addition, the dissociative photoionization cross section was both measured directly and derived from other measurements. The cross section for the production of doubly charged molecular ions was also ascertained between 150 and 450 Å. All the cross sections presented are absolute. The data confirm the molecular Cooper minimum seen in both one other experimental measurement and theoretical calculations and show good overall qualitative agreement with both previous experiment and theory.
Theory for externally contracted configuration interaction energy gradients
Authors: Lee Timothy J.
Published: July 2002

A derivation of the analytic energy gradient based upon a multireference externally contracted configuration interaction (MRCCI) wave function is presented. Computational efficiency was the major consideration in deciding upon the approach chosen. The final set of equations which define the gradient are computationally efficient, and avoid writing to disk or transforming any derivative integrals. It is also shown that the MRCCI gradient, evaluated in this way, does not require solving any coupled-perturbed configuration interaction (CPCI) equations. Thus, the formulation presented here demonstrates that the MRCCI gradient is much less expensive than the uncontracted multireference CI (MCSCF-CI) energy gradient. A general set of equations for an internally contracted CI energy gradient is also presented.
Study of chemical reactions by surface second harmonic generation: p-Nitrophenol at the air–water interface
Authors: Bhattacharyya Kankan , Sitzmann E. V. , Eisenthal K. B.
Published: July 2002

Chemical reaction at the interface of a liquid and air has been studied using the technique of surface second harmonic generation. The reaction investigated was the acid–base equilibrium of p-nitrophenol at the air–water interface.
A theoretical and experimental study of interfacial tension of immiscible polymer blends in solution
Authors: Broseta Daniel , Leibler Ludwik , Ould Kaddour Latifa , Strazielle Claude
Published: July 2002

The interfacial properties of demixed solutions of polymer blends are studied. We develop a theory of interfacial concentration profiles taking into account excluded volume effects which are important in solutions. The predictions differ considerably from those obtained by classical mean-field approaches, especially concerning the interfacial thickness. The theory allows us to calculate the interfacial thickness and interfacial tension without any adjustable factor. We measured interfacial tension of demixed poly(styrene)–poly(dimenthylsiloxane) mixtures dissolved in toluene as a function of molecular weight of polymers and monomer concentration. A very good agreement with theoretical predictions is found.
Experimental and theoretical studies of CO vibrational relaxation by He atoms
Authors: Wickham-Jones C. T. , Williams H. T. , Simpson C. J. S. M.
Published: July 2002

Rate constants for the vibrational relaxation of 12C16O(v=1) and 13C18O(v=1) by 4He and 3He over an ab initio potential surface have been calculated with a vibrational close-coupling, rotational infinite order sudden (VCC-IOS) technique. Improved experimental measurements of the relaxation of 12C16O(v=1) by 4He and new measurements of the deactivation of 13C18O(v=1) by 4He and 3He all over the temperature range 300–80 K have been carried out with a laser fluorescence method. These rate constants and existing ones for the deactivation of 12C16O by 4He over the temperature range 2300–580 K and by 3He over the temperature range 300–80 K are compared with the theoretical rate constants. The rate constants vary by over six orders of magnitude but the calculations reproduce the experimental values well. These results give evidence for the accuracy of the potential energy surface and the (IOS) technique.
Study on liquid–vapor interface of water. I. Simulational results of thermodynamic properties and orientational structure
Authors: Matsumoto Mitsuhiro , Kataoka Yosuke
Published: July 2002

Molecular dynamics simulations have been carried out for liquid–vapor interface of water and also of Lennard-Jones system. Surface tension and surface excess energy are calculated, from which surface excess entropy is evaluated. These thermodynamic quantities suggest the existence of some liquid-structural change near the surface of water, which is not seen in Lennard-Jones system. For water, orientational structuring near the surface is studied and two types of orientation are found. In the vapor side, a water molecule has a tendency of projecting one hydrogen atom toward the vapor, and in the liquid side, a molecule prefers to lie down on the surface with both hydrogen atoms slightly directed to the liquid. From these results, surface potential χ can be evaluated to be about +0.16 V at T=300 K, which confirms recent experimental results. The ellipticity coefficient is also discussed and the assumption often used in analysis of experimental results of ellipsometry is found to be inadequate for water due to the structural change. These observed orientational orderings are qualitatively consistent with results of molecular theory of surface recently developed and also with simulational results of water near hydrophobic walls.
Diffusion of a flexible polymer chain in a disordered medium
Authors: Loring Roger F.
Published: July 2002

A theory is presented of the long wavelength dynamics of a flexible polymer chain without excluded volume interactions in a medium composed of randomly distributed static scatterers. The model is applicable to the motion of a polymer chain in a random porous solid. The theory is based on an approximate resummation of an expansion of the frequency-dependent friction coefficient in powers of scatterer density. If the chain is short or the scatterer density is low, the diffusion constant is inversely proportional to the chain length (molecular weight), in accordance with the Rouse model. Above a critical value of the chain length or scatterer concentration, the chain undergoes hindered diffusion with a diffusion constant that is smaller than the Rouse value. If the scatterer size is much smaller than the polymer radius of gyration, this diffusion constant has an inverse quadratic dependence on chain length, as predicted by the reptation model. If the scatterer size is large compared to the radius of gyration, the diffusion constant depends on the inverse cube of the chain length. A similar crossover is predicted for the time dependence of the mean squared displacement of a long chain. At short times, the chain undergoes Rouse diffusion, and at long time it exhibits hindered diffusion. An intermediate time regime of anomalous diffusion is predicted.
The effects of triple and quadruple excitations in configuration interaction procedures for the quantum mechanical prediction of molecular properties
Authors: Lee Timothy J. , Remington Richard B. , Yamaguchi Yukio , Schaefer Henry F.
Published: July 2002

The importance of including triple and quadruple excitations (relative to a single Hartree–Fock determinant) in ab initio electronic structure configuration interaction (CI) theory is investigated for several small molecules [HF, N2, CO, H2O, NH3, (3B1) CH2, and (1A1) CH2]. Specifically the effects of these high order electron correlations on equilibrium molecular geometries, dipole moments, harmonic vibrational frequencies, and infrared intensities are reported. Triple and quadruple excitations are generally found to affect the dipole moment, in an absolute sense, only slightly. In some cases, infrared intensities show a medium to large dependence on higher excitations. Molecular geometries, and subsequently the harmonic vibrational frequencies, however, are significantly more dependent upon these higher excitations. Quadruple excitations are found to be significantly more important than triple excitations for all closed shell systems except for CO, where the relative importance of triples to quadruples is roughly 2:3 in predicting for vibrational frequencies. On the other hand, it is found that triples and quadruples are of nearly equal importance for 3B1 CH2. The equilibrium bond length and molecular properties of the multiply bonded species CO and N2 show a larger absolute dependence on the higher than double excitations. Several additional levels of theory [e.g., all singles, doubles, and quadruples (CISDQ)] have been applied to HF and N2 in a more detailed investigation of the structure of the CI Hamiltonian matrix. It is concluded that only a very small subset of the triply and quadruply excited configurations account for nearly all of the higher excitation effects on harmonic frequencies.
The hydration of Dy3+ and Yb3+ in aqueous solution: A neutron scattering first order difference study
Authors: Cossy Cédric , Barnes Adrian C. , Enderby John E. , Merbach André E.
Published: July 2002

The coordination of water molecules around the metal ion in acidified 1 m DyCl3, 1.0 m Dy(ClO4)3, 0.3 m Dy(ClO4)3, and 1.0 m Yb(ClO4)3 solutions in D2O have been determined by the neutron diffraction difference technique. A coordination number of eight is found for the two ions, which depends neither on the counterion, nor on the concentration. The near metal–oxygen and metal–deuterium distances are, respectively, 2.39±0.02 and 3.03±0.02 Å for dysprosium and 2.33±0.02 and 2.98±0.02 Å for ytterbium. A general discussion concerning the hydration of the lanthanide ions is given.
The molecular structure and vibrational spectrum of the cyclopropenyl cation, C3H+3, and its deuterated isotopomers
Authors: Lee Timothy J. , Willetts Andrew , Gaw Jeffrey F. , Handy Nicholas C.
Published: July 2002

The equilibrium structure, harmonic vibrational frequencies, infrared intensities, anharmonic constants, vibration–rotation interaction constants and quartic and sextic centrifugal distortion constants of C3H+3, the cyclopropenyl cation, and its deuterated isotopomers have been determined via purely ab initio quantum-mechanical methods. Two one-particle basis sets have been employed in conjunction with second-order M≂ller–Plesset perturbation theory (MP2), singles and doubles configuration interaction (CISD), and singles and doubles coupled cluster (CCSD). The best estimate of the harmonic frequencies is obtained from MP2 with a triple zeta plus double polarization (TZ2P) basis set. The anharmonic analysis has been determined via second-order perturbation theory using a double zeta plus polarization (DZP) self-consistent-field (SCF) full quartic force field. A generalization of formulas for the anharmonic analysis of D3h symmetric tops is discussed. The complete quartic force field in symmetry internal coordinates is given. Additionally, the anharmonic constants, vibration–rotation interaction constants and quartic and sextic centrifugal distortion constants for C3H+3 and C3D+3 are reported. Predictions of the fundamental vibrational frequencies for C3H+3 and all its deuterated isotopomers are reported. At the TZ2P MP2 level of theory the equilibrium structure of cyclopropenyl cation is Re(C–C)=1.3647 Å, Re(C–H)=1.0753 Å. Coupling the TZ2P MP2 harmonic frequencies with the DZP SCF anharmonic corrections, the infrared active fundamentals of C3H+3 are predicted to occur at 3136, 1289, 939, and 773 cm−1, with those of C3D+3 predicted to occur at 2346, 1243, 684, and 567 cm−1.
Ultrafast vibrational dynamics of a photoexcited metalloporphyrin
Authors: Rodriguez Juan , Holten Dewey
Published: July 2002

The ultrafast photodynamics of four-coordinate nickel(II) porphyrins in noncoordinating solvents has been studied using femtosecond time resolved optical spectroscopy. Unambiguous evidence has been found for the formation of a metastable metal (d,d) excited state in less than 350 fs following excitation of the macrocycle. However, the transient absorption spectrum of this ligand-field electronic excited state continues to evolve and reaches the steady-state form only after about 20 ps. This spectral behavior and the attendant complex kinetics can be modeled phenomenologically in terms of a broad distribution of the (d,d) excited states evolving to a narrower distribution. The dynamics are associated with vibrational relaxation. Intramolecular and intermolecular contributions to this process are considered.
Theory of dynamics of entangled rod-like polymers by use of a mean-field Green function formulation. II. Rotational diffusion
Authors: Teraoka Iwao , Hayakawa Reinosuke
Published: July 2002

Rotational motion of entangled rod-like polymers in solutions is theoretically treated by using a mean-field Green function formulation for the rotational diffusion in an effective perturbation potential representing the hindrance effect by neighboring rods. A similar procedure to Part I of this series yields an explicit expression of the effective diffusion constant Dr as a function of the rod length and the polymer concentration for solutions in a wide range of concentrations from dilute to semidilute. In the high concentration limit, the present result is consistent with that obtained from the tube model of Doi–Edwards.
The structure of CO on the Pt(112) stepped surface—a sensitive view of bonding configurations using electron stimulated desorption
Authors: Henderson M. A. , Szabó A. , Yates J. T.
Published: July 2002

The structure of chemisorbed CO on the steps of Pt(112) {Pt[3(111)×(001)]} was monitored as a function of coverage by the digital electron stimulated desorption-ion angular distribution method (ESDIAD), digital low-energy electron diffraction (LEED), and temperature-programmed desorption (TPD). The ESDIAD method applied to the desorption of an electronically excited, neutral CO species (the metastable a3 π-CO state) avoids the influence of image potential effects on the trajectory of desorbing species, yielding true desorption angles, equivalent to Pt-CO bond orientations. CO adsorbs exclusively on step sites at low coverage with a 20° ‘‘downstairs’’ tilt from the [112] direction (designated ‘‘0°’’). LEED indicates (2×n) order (i.e., two-fold order along the steps but no order up/down the steps) exists when the steps are half-filled (about 0.19 ML). At 0.24 ML, CO is still adsorbed only on the step sites but one-dimensional CO–CO repulsions between nearest neighbors result in CO tilting along the steps. Terrace CO (+13°) adsorption is observed above 0.24 ML before all the step sites fill. All step CO molecules reorient with new tilt angles up (0°) and down (−38°) the steps when the steps sites saturate (0.53 ML), and (3×1) order is observed by LEED. The surface reorders at saturation (0.79 ML) to (2×1) order. Changes in CO desorption energies, pre-exponential factors and sticking coefficients with coverage in the literature for Pt[n(111)×(001)] surfaces can be directly correlated with structural transformations on the partially filled steps due to CO–CO repulsions.
Theoretical study of the force field of the lowest singlet electronic states of long polyenes
Authors: Negri Fabrizia , Orlandi Giorgio , Zerbetto Francesco , Zgierski Marek Z.
Published: July 2002

The force field of all-trans decapentaene, dodecahexaene, tetradecaheptaene, and hexadecaoctaene have been analyzed in the diabatic and adiabatic representations for the 1A−g and 2A−g states by an extended Pariser–Parr–Pople Hamiltonian. The adiabatic frequencies have been computed and found to agree with the existing experimental frequencies. In particular, the calculations reproduce well the observed increase of the frequency of the Franck–Condon active ag C=C stretch upon excitation to the 2A−g state. This mode has been found to be increasingly localized on the CC oscillators of the central moiety as the polyenic chain gets longer. This, together with the alternate signs of the adjacent oscillators, is the reason of its Franck–Condon activity. The effects of electronic excitation and vibronic interactions on the force constant matrix are discussed.
The role of reference frames in the molecular theory of diffusive transport in solutions
Authors: Raineri Fernando O. , Friedman Harold L.
Published: July 2002

Starting with the Green–Kubo formulas for the phenomenological coefficients of irreversible thermodynamics, the molecular expression for the distinct diffusion coefficient Dd[R],αβ in any internal reference frame R is derived. It follows that Dd[R],αβ is given by the time integral of a time correlation function (tcf) Λd[R],αβ(t) whose dynamical variables are the velocities of two different particles of species α and β, each relative to a microscopic reference velocity ŵR . These relative dynamic variables have the welcome feature of being orthogonal to the total momentum of the system so that Λd[R],αβ(t) vanishes at long times. Hence the divergence of the barycentric distinct diffusion coefficients, suggested by their usual interpretation in terms of a time correlation function that lacks an explicit dependence on the reference frame, is resolved. The influence of the reference frame on the behavior of the barycentric distinct tcf ’s is analyzed. In great measure this influence is exposed by considering initial values of the tcf ’s; in turn the initial values are decisive in establishing the sign of the distinct diffusion coefficients in ideal or nearly ideal mixtures. Finally, by applying new linear relations connecting the self and distinct tcf ’s, a complete description of some of the barycentric distinct tcf ’s may be given in the limiting cases of a pure fluid and a tracer in a host liquid.
The effects of steps, coupling to substrate vibrations, and surface coverage on surface diffusion rates and kinetic isotope effects: Hydrogen diffusion on Ni
Authors: Truong Thanh N. , Truhlar Donald G.
Published: July 2002

We have applied canonical variational transition state theory with semiclassical transmission coefficients to investigate the dynamical effects of metal motions, surface defects (namely, steps), and surface coverage on the diffusion of H on the Ni(100) surface. We have used the embedded diatomics-in-molecules (EDIM) method to represent the hydrogen–hydrogen, hydrogen–metal, and metal–metal interactions. The roles of metal motions, surface defects, and coverage in the overall diffusion rates and kinetic isotope effects are discussed in detail, and comparisons with experimental data are made.
A
b i
n
i
t
i
o studies of polyenes. I. 1,3-butadiene
Authors: Guo Hong , Karplus Martin
Published: July 2002

The potential energy function about the C–C single bond for the ground state 1,3-butadiene has been derived from ab initio calculations at both the Hartree–Fock (HF) level with 6-31G, 6-31G*, and 6-311G** basis sets and the second-order Mo/ller–Plesset perturbation (MP2) level with 6-31G* basis set with the complete geometry optimizations at each of 15 fixed CCCC dihedral angles; the total energies and optimized geometries for the s-trans, gauche, and s-cis conformers were also determined at MP2 level with 6-311G* basis set and the third-order Mo/ller–Plesset perturbation (MP3) level with 6-31G* basis set. The second stable conformer of the butadiene is predicted to be a gauche structure from all the calculations with a CCCC dihedral angle between 35° and 40° and a barrier of 0.5–1.0 kcal/mol to the s-cis transition state, and the theoretical torsional potentials are in good agreement with the experimental potential function of trans–gauche–gauche case derived by Durig et al.; by contrast, the theoretical torsional components differ significantly from the experimental results obtained from a trans–cis model. Vibrational frequencies and force field for s-trans and gauche conformers of 1,3-butadiene are determined at the Hartree–Fock and MP2 levels with 6-31G, 6-31G*, 6-311G, and 6-311G* basis sets. The mean absolute percentage deviations of the calculated frequencies from the experimental values (not corrected for anharmonicity) are ∼10%–13% and 3%–6% for the Hartree–Fock and MP2 methods, respectively. The effects of polarization functions and electron correlation on the force fields are studied, and the additivity of correlation and d function effects are discussed. Comparisons are made with other force fields, including experimental and previous ab initio results.
Determination of the quadrupole moment of nitrogen from the dielectric second virial coefficient
Authors: Huot J. , Bose T. K.
Published: July 2002

Dielectric virial coefficients of nitrogen have been measured at 323.15 K. By combining the dielectric second virial coefficient with the refractivity second virial coefficient we determined the quadrupole moment of nitrogen to be (1.47±0.07)×10−26 esu cm2. In calculating the molecular quadrupole moment from the dielectric second virial coefficient, effects due to various interaction energy terms were taken into consideration. These energy terms are due to quadrupole–quadrupole interaction, quadrupole-induced dipole interaction, reaction fields of induced moments and anisotropic effects.
Dynamic light scattering, mutual diffusion, and linear viscoelasticity of polymer solutions
Authors: Wang C. H.
Published: July 2002

A theoretical study of the dynamic light scattering spectrum from a binary solution consisting of a polymer and a small molecular weight solvent is carried out. The study is based upon a modified hydrodynamic equation previously derived from statistical mechanics by Bearman and Kirkwood for a multicomponent solution. The result shows that mutual diffusion arising from the osmotic pressure fluctuation and the viscoelasticity affecting mechanical properties of the polymer solution are, in general, mixed. The extent of mixing depends on the frequency and a coupling parameter β, which is proportional to the difference between the partial specific volume of the polymer and that of the small molecular solvent.

At low frequency the coupling is negligible, and one obtains a diffusion equation with the mutual diffusion coefficient dependent only on the osmotic modulus. At the frequency in the plateau stress modulus region, the result predicts a cooperative diffusion coefficient which consists of both the osmotic modulus and the longitudinal elastic modulus of the polymer solution, provided that β=1. The result is equivalent to that previously obtained by de Gennes, assuming a transient gel network model for the semidilute polymer solution. The present calculation also provides new results for the polymer solution in which the transient gel network model is not applicable. The overall light scattering spectral density is calculated in terms of the autocorrelation functions of the local pressure and the local concentration, as well as cross correlation functions between them. The result shows that as the polymer concentration is gradually increased toward the bulk state, the concentration fluctuation spectrum losses its importance, and the pressure (or density) fluctuation spectrum dominates the overall light scattering spectral density.