bluuebunny/crossref_metadata_2025 huggingface dataset:
Structures of vinylidene fluoride oligomer thin films on alkali halide substrate
Authors: Noda Kei , Ishida Kenji , Horiuchi Toshihisa , Matsushige Kazumi , Kubono Atsushi
Published: July 2002

Structures and crystal transformation of the newly synthesized vinylidene fluoride (VDF) oligomer with large electric dipoles evaporated on KCl (001) at various substrate temperatures have been investigated by an energy dispersive–grazing incidence x-ray diffraction system, Fourier transform infrared spectroscopy, and atomic force microscope (AFM). It was revealed that the molecules grow epitaxially and are influenced greatly by forces of the crystal surface field in terms of van der Waals or electrostatic potentials, and found that the phase transformation from form II (α phase) to form I (β phase) is induced by raising the temperature of the substrate from 50 to 80 °C, accompanying the alternation in the crystal axes on the substrate from the a axis of form II to the polar b axis of the form I crystal. This fact suggests that the molecular chain of VDF oligomers aligns their c axes along the 〈110〉 row of K+ or Cl− with the aid of electrostatic interaction under enough thermal movement. Moreover, in the transformation process, a pair of “rod-like” crystals, suggesting ferroelectric activity, were observed by AFM.
Light-illumination-induced transformation of electron traps in hydrogen-implanted n-type silicon
Authors: Tokuda Yutaka , Shimada Hitoshi , Ito Akira
Published: July 2002

Light-illumination-induced transformation of electron traps in 170 keV hydrogen-implanted n-type silicon with a dose of 2×1010 cm−2 has been studied with deep level transient spectroscopy for fabricated gold Schottky diodes. In addition to the well-known vacancy-related traps, two hydrogen-related E(0.32) and E(0.49) traps are produced by hydrogen implantation. It is observed that the concentration of vacancy-related traps decreases with illumination of light with a photon energy above the band-gap energy in zero-biased Schottky diodes, while that of hydrogen-related traps increases. No effects of light illumination are found in reverse-biased diodes. It is suggested that the vacancy–oxygen pair is transformed into the E(0.32) trap by adding hydrogen which is liberated from shallow-level, hydrogen-containing defects. The hydrogen liberation is thought to occur through induced instability of them by changing their charge states with electron capture.
Carrier depletion by defects levels in relaxed In0.2Ga0.8As/GaAs quantum-well Schottky diodes
Authors: Chen J. F. , Wang P. Y. , Wang J. S. , Tsai C. Y. , Chen N. C.
Published: July 2002

An increase in leakage current accompanied by a drastic carrier depletion is found for InGaAs/GaAs Schottky diodes when the InGaAs thickness is larger than its critical thickness. Due to drastic carrier depletion, free-carrier concentration around the InGaAs region for relaxed samples cannot be obtained from capacitance–voltage data but from resistance–capacitance time constant effect observed in capacitance–frequency measurement. A trap at 0.33 to 0.49 eV is observed for relaxed samples by deep-level transient spectroscopy. The resistance caused by carrier depletion has an activation energy close to that of the trap, supporting that the carrier depletion is caused by capture from the trap.
Deposition mechanism of hydrogenated amorphous silicon
Authors: Robertson John
Published: July 2002

The surface and subsurface processes occurring during the growth of hydrogenated amorphous silicon (a-Si:H) are analyzed to understand how dangling bond defects and weak bonds form. It is found that the abstraction and addition of adsorbed SiH3 radicals gives a surface defect density which decreases continuously with decreasing temperature with no minimum near 250 °C. Hence it cannot be the process that defines defect densities in the bulk. Hydrogen elimination to create the bulk Si–Si network occurs because the chemical potential of hydrogen causes the expulsion of hydrogen from the bulk. Hydrogen elimination is the rate-limiting step at lower temperatures, as its diffusion is slow. The difficulty of eliminating hydrogen leads to the formation of weak bonds. Weak bonds arise at higher deposition temperatures from thermal disorder. The dangling bond defects arise from weak bonds by the defect pool process, and this process must continue at lower temperatures than normal in the growth zone. Plasma processes which dehydrogenate the surface layers, such as ion bombardment, can lower weak bond densities.
Superparamagnetic nanocrystalline CuFe2O4
Authors: Roy S. , Ghose J.
Published: July 2002

Cubic CuFe2O4 was prepared by coprecipitation in a polymer matrix. Crystallite size and particle size of the prepared samples, determined from x-ray data and transmission electron microscopy analyses respectively, are in the range of nanocrystalline materials. Cation distribution of the nanocrystalline samples was determined from x-ray diffraction data. Cuoct2+ content in the nanocrystalline samples is lower than the critical Cuoct2+ required for cooperative distortion to tetragonal symmetry and hence all the samples are cubic. Field cooled and zero field cooled magnetization measurements show superparamagnetic behavior of the nanocrystalline ferrite samples.
Photoluminescence of donor–acceptor carbazole-based molecules in amorphous and powder forms
Authors: Adès D. , Boucard V. , Cloutet E. , Siove A. , Olivero C. , Castex M. C. , Pichler G.
Published: July 2002

We present absorption and photoluminescence features of four samples of carbazole molecules substituted with various electron–acceptor groups. These molecules named 1-(N-ethylcarbazolyl)-2-substituted-2-cyanovinylene contain in their structure the electron–donor carbazole nucleus and cyanovinylene bearing either another nitrile function, an ethylester, a phenyl, or a para-nitrophenyl groups. It is shown that depending on the strength of the donor–acceptor internal charge transfer, both the absorption and emission spectra are more or less redshifted. It is found that the ethyl-ester derivative displays the best relative photoluminescence efficiency among all the samples and its peak is measured at 490 nm when taking amorphous thin film. The microcrystalline powder form of the same material exhibits spectral narrowing and shift of the peak emission. We obtain further narrowing of the emission band and further redshifting of the emission when we illuminate, transversely, a glass capillary containing the crystalline sample by an ultraviolet light-emitting diode.
Anisotropic electrical transport in epitaxial La2/3Ca1/3MnO3 thin films
Authors: Amaral V. S. , Lourenço A. A. C. S. , Araújo J. P. , Pereira A. M. , Sousa J. B. , Tavares P. B. , Vieira J. M. , Alves E. , da Silva M. F. , Soares J. C.
Published: July 2002

Epitaxial thin films of La0.62±0.05Ca0.33±0.02MnO3−δ were grown by laser ablation on SrTiO3. On (100) substrates the films grow with the larger c axis perpendicular to the plane. The films deposited on (110) SrTiO3 grow with both the c (long) axis and a (or b, short) axis in the plane of the film. The electrical resistivity (ρ) and the magnetoresistance (Δρ/ρ) show crystalline anisotropy. The resistivity ratio between the a and c axis is constant (0.8) from 10 K up to 120 K and decreases to 0.77 between 125 and 225 K, shows a small peak anomaly at Tc (257 K), and is almost constant in the paramagnetic phase. This temperature dependence is associated with anisotropic local lattice distortions. The magnetoresistance anisotropy (Δρ/ρ∥−Δρ/ρ⊥) with the applied field in the plane of the film, is small at low temperatures, peaks close to Tc, and is slightly larger for measurements along the a axis. The contributions of domain rotation and magnetocrystalline anisotropy to the anisotropic magnetoresistance associated with spin–orbit coupling are discussed.
Quantum refrigerators in quest of the absolute zero
Authors: Kosloff Ronnie , Geva Eitan , Gordon Jeffrey M.
Published: July 2002

The second and third laws of thermodynamics can be used to establish a fundamental bound for the maximum possible cooling rate in approaching the absolute zero of temperature. In modeling the behavior of the molecular refrigerators geared toward attaining ultralow temperatures, only quantum mechanical, as opposed to classical physics, models can be admissible. As a simple model, we analyze a three-level quantum refrigerator, and in particular its irreversible thermodynamic performance as absolute zero is approached.
Effects of processing on the characteristics of SrBi2Ta2O9 films prepared by metalorganic decomposition
Authors: Li Aidong , Wu Di , Ling Huiqin , Yu Tao , Wang Mu , Yin Xiaobo , Liu Zhiguo , Ming Naiben
Published: July 2002

SrBi 2 Ta 2 O 9 (SBT) films were prepared by metalorganic decomposition technique. The films were obtained by spin-on pyrolysis of the precursor solutions on various substrates and then annealed at 550–850 °C in dry oxygen, wet oxygen, or Ar. Effects of precursor solution concentration, anneal temperature, anneal atmosphere, substrate and Ar ion sputtering on the microstructure, morphology and electrical properties were investigated by means of x-ray diffraction, scanning electron microscope, atomic force microscope, x-ray photoelectron spectroscopy, and electrical measurements. The results indicated the grain size, and the remnant polarization (Pr) increased with increasing the anneal temperature up to 800 °C and the significant hysteresis loop could be obtained only after anneal above 700 °C. At 850 °C, the pyrochlore phase and other secondary phases were observed along with the SBT phase, leading to the decreasing Pr and dielectric constant. In addition, the development of crystalline phase and electrical properties were affected by anneal atmosphere. When annealed in Ar at 750 °C, the layered SBT structure was destroyed with evident Bi loss. As though the original structure could be restored basically by subsequent adequate O2 anneal, the electrical properties were deteriorated seriously due to the shorted capacitor. Wet oxygen anneal evidently deformed the morphology and the hysteresis loop. This was attributed to the effect of possible produced slight H2 due to the reaction between H2O and residual carbonaceous in films under Pt catalyst. Substrates also played an important role on film crystallinity. The films deposited on Si, SrTiO3, and crystal quartz at 750 °C had a layered perovskite polycrystalline structure while the films on fused quartz exhibited poor crystalline nature even after 800 °C anneal and films on NaCl showed (200)-predominant orientation of SBT phase with some pyrochlore. In addition, Ar ion sputtering might lead to the severe Bi and oxygen deficiency in the surface of the sputtered film.
Statistics of the entanglement of polymers: Concentration effects
Authors: Rubinstein Michael , Helfand Eugene
Published: July 2002

Central concepts of the reptation model of polymer relaxation by disentanglement are the confining tube, the primitive path, and the entanglement net. These are given more precise definitions and are expounded upon here. The entanglement net is regarded as the set of primitive paths of the molecules. The scaling relation connecting distance between entanglements and concentration is derived by a self-consistency argument. Explicit results on probability distributions characterizing unentangled loops and primitive path lengths are derived. The polymers are modeled as random walks in a regular entanglement net. Results are presented as a function of the spacing between entanglements.
Photoabsorption cross section of acetylene in the EUV region
Authors: Wu C. Y. Robert , Judge D. L.
Published: July 2002

Using synchrotron radiation as a continuum background, the absolute total absorption cross section of C2H2 has been measured using a double ionization chamber. The cross sections range from 2.5 to a maximum of 35 Mb in the 175–740 Å region. Two new Rydberg series have been identified and apparently converge to the B̃ 2Σ+u state of C2H+2 at 18.71 eV. The observed Rydberg states are tentatively assigned to (2σu)−1 nsσg 1Σ+u and (2σu)−1 ndσg 1Σ+u, respectively. The present cross section data have been used in the analysis of various sum rules including the TRK sum rule.
Statistical ensembles and molecular dynamics studies of anisotropic solids. II
Authors: Ray John R. , Rahman Aneesur
Published: July 2002

We have recently discussed how the Parrinello–Rahman theory can be brought into accord with the theory of the elastic and thermodynamic behavior of anisotropic media. This involves the isoenthalpic–isotension ensemble of statistical mechanics. Nosé has developed a canonical ensemble form of molecular dynamics. We combine Nosé’s ideas with the Parrinello–Rahman theory to obtain a canonical form of molecular dynamics appropriate to the study of anisotropic media subjected to arbitrary external stress. We employ this isothermal–isotension ensemble in a study of a fcc→ close-packed structural phase transformation in a Lennard-Jones solid subjected to uniaxial compression. Our interpretation of the Nosé theory does not involve a scaling of the time variable. This latter fact leads to simplifications when studying the time dependence of quantities.
Investigation of ribbon lyotropic structures by deuterium magnetic resonance
Authors: Chidichimo G. , Golemme A. , Doane J. W.
Published: July 2002

Deuterium magnetic resonance is used to study the shape and size of ribbon-like aggregates as well as the contrasting molecular ordering which is shown to occur in different regions of the aggregate. Elongated ribbon-like aggregate structures are formed from a ternary mixture of potassium palmitate/potassium laurate/water. Deuterium spectral patterns from various selectively deuterated sites of the potassium palmitate compound are analyzed in terms of a simple model for the ribbon structure. The results indicate that the order parameter S of the terminal methyl position is enhanced by a factor of 2 in the curved cylindrical edge of the ribbon over its value in the uncurved central lamellar-like region. The size of the ribbon model is also determined. At lower temperature, the ribbons transform into cylindrical aggregates on a rectangular lattice. Deuterium magnetic resonance of water is shown to be sensitive to the details of the lattice structure.
Observation of the infrared spectrum of methyl cation CH+3
Authors: Crofton Mark W. , Kreiner Welf A. , Jagod Mary-Frances , Rehfuss Brent D. , Oka Takeshi
Published: July 2002

The vibration-rotation infrared spectrum of the ν3 fundamental band of the methyl cation CH+3 has been observed using a difference frequency laser system. The analysis of the spectrum demonstrates the D3h symmetry of the ion. This work marks the beginning of high resolution spectroscopy of simple polyatomic hydrocarbon cations.
Recombinative desorption dynamics: Molecular hydrogen from Cu(110) and Cu(111)
Authors: Kubiak G. D. , Sitz G. O. , Zare R. N.
Published: July 2002

The rotational and vibrational distributions of H2 and D2 recombinatively desorbing from clean Cu(110) and Cu(111) surfaces following atomic permeation are studied using multiphoton ionization combined with time-of-flight mass spectrometry. Rotational distributions are found to be non-Boltzmann and to possess mean rotational energies which are 80%–90% of the surface temperature, Ts. These distributions are identical to within the experimental accuracy for H2 and D2 and also for desorption from the (110) and (111) faces. Moreover, the ortho and para nuclear spin modifications of both isotopes are statistically populated. In contrast, the vibrational population ratio, Pv″=1/Pv″=0, is found to be as much as 100 times greater than the ratio corresponding to a Boltzmann vibrational population at Ts. Specifically, the Pv″=1/Pv″=0 ratio for H2 (D2) is 0.052±0.014 (0.24±0.20) desorbing from Cu(110), and 0.084±0.030 (0.35±0.20) desorbing from Cu(111). For comparison the Boltzmann-at-Ts ratios would be 0.0009 for H2 and 0.0063 for D2 at T=850 K. Simple models are discussed which attempt to account for the qualitative trends of these results. Detailed balance arguments applied to the vibrational distributions measured in recombinative desorption are unable to predict correctly the dissociative adsorption probability as a function of vibration, indicating that these two processes are dynamically different for this system.
Instability in the infinite dimensional hard sphere fluid
Authors: Klein W. , Frisch H. L.
Published: July 2002

We derive equations for the single particle distribution function and the k dependent structure factor for hard sphere fluids in the infinite dimension (d=∞) limit. Consistent with an earlier prediction that hard spheres will have mean field behavior in the d→∞ limit we find a Kirkwood instability which is interpreted as the mean field spinodal of the supercooled liquid.
Statistical mechanics in homopolypeptides. I. Theoretical approach for titration process in α helix
Authors: Vila J.
Published: July 2002

We can view the titration processes in α helix conformation as a one-dimensional lattice where any site can be either charged (occupied) or uncharged (empty) as a lattice gas. Using this representation, we assume that the characteristic repulsive nature of the Coulombian interaction between charged groups is taken into account for a repulsive pair potential of Kac type: φ(x)=ν exp(−γx) where ν>0. Furthermore, the long range character of the Coulombian interaction will be saved by obtaining the grand partition function in the limit γ→0. Through this analysis we can obtain an analytical expression for the dissociation constant pK, degree of ionization α, and ionization rate dα/dpH. On the other hand, we find good agreement between the theoretical and experimental curve when we plot α vs pH. It is important to remark that the model does not have fitting parameters. Finally, we compare this result with those obtained in other models and then we mention the possibility of applying this analysis to the study of the transition region in denaturation processes induced by pH change.
Lattice vibrations and infrared absorption of ice Ih
Authors: Marchi Massimo , Tse John S. , Klein Michael L.
Published: July 2002

Lattice dynamics calculations have been carried out for ice Ih using the SPC rigid-molecule effective pair potential whose parameters were fitted to the properties of water. We employ a periodically replicated nonpolar sample of 128 molecules whose constituent molecules are arranged according to the ice rules. Long range dipole–dipole interactions are handled using the Ewald method. The calculated density of states in the translational and librational regions accords well with available experimental data. However, the frequency of the longitudinal optic mode is a little low as is the position of whole librational band. Although polarization (induction) effects are not explicitly included in the calculation of the vibrational frequencies and their associated eigenvectors, they are invoked to evaluate the quantities which determine the infrared absorption, namely, derivatives of the crystal dipole moment with respect to the normal coordinates. The overall features of the infrared spectrum for translational modes are well described by including induction effects due to the molecular dipole and quadrupole fields. The same approach is used to calculate the dielectric constant and the energy loss spectrum. The latter calculations lead to a value of 10 cm−1 for the longitudinal optic–transverse optic mode splitting.
The analytic configuration interaction gradient method: Application to the cyclic and open isomers of the S3 molecule
Authors: Rice Julia E. , Amos Roger D. , Handy Nicholas C. , Lee Timothy J. , Schaefer Henry F.
Published: July 2002

The theory for the ab initio evaluation of potential energy gradients at the configuration interaction level of accuracy is presented, with special attention to the treatment of the various types of orbitals (frozen core, active, frozen virtual) which may arise. The new method has been used for a study of the D3h and C2v geometries of S3. SCF and CISD calculations predict the D3h structure to be lower than the C2v by 9.3 and 2.3 kcal/mol, respectively, whereas CASSCF and MRCISD predict the C2v structure to be the lowest by 8.9 and 8.2 kcal/mol, respectively, using good basis sets. These calculations support the prediction of Carlsen and Schaefer that both forms should be observable experimentally.
Chemical reactions in liquids: Molecular dynamics simulation for sulfur
Authors: Stillinger Frank H. , Weber Thomas A. , LaViolette Randall A.
Published: July 2002

A combination of two-atom and three-atom interactions has been selected to represent the structural chemistry of sulfur. This model potential exhibits divalency (bond saturation) and leads to the known preference for Sn molecules to form puckered ring structures. Using this representation of the interactions, molecular dynamics calculations have been performed for 1000 sulfur atoms at the experimental liquid density. Short-range order has been calculated for the low-temperature liquid consisting of S8 cyclic molecules, and agrees qualitatively with the (imprecise) available measurements. At elevated temperatures the cyclic S8 molecules in the simulation begin to break open, and their subsequent chemical reactions yield primarily linear polymeric species. A metastable reaction intermediate in the polymerization process has been identified, a ‘‘tadpole’’ consisting of a diradical chain attached weakly to an S8 ring.
Theoretical calculation of ionic solution properties
Authors: Caillol J. M. , Levesque D. , Weis J. J.
Published: July 2002

A model system of electrolyte solution is studied by molecular dynamics simulation. The results show how the polarizability of the molecules and the ratio of the molecular diameters of the ions and solvent molecules affect the properties of the system. The computation of the frequency dependent dielectric constant and conductivity in terms of correlation functions of the electrical current and microscopic polarization is discussed. A general solution of this problem is given for systems of arbitrary shape composed of nonpolarizable ions and solvent molecules. Three particular cases are considered in detail: the infinite system; a spherical system in contact with a dielectric and conducting continuum; a system with periodic boundary conditions. The zero frequency limit of the dielectric constant and conductivity is investigated.
On the relationship between pairwise fluctuations and thermodynamic derivatives
Authors: Panagiotopoulos A. Z. , Reid R. C.
Published: July 2002

General expressions are derived for the pairwise fluctuations of thermodynamic properties in multicomponent systems, in terms of thermodynamic derivatives. The expressions are valid for an open system close to thermodynamic equilibrium in contact with an energy and mass reservoir. The system is defined by an arbitrary extensive constraint. Fluctuations involving the entropy S, the volume V, and the number of molecules Ni of each component, as well as their conjugate variables, are expressed as second-order derivatives of the energy U and Legendre-transformed functions. Fluctuations involving U itself are obtained in the entropy representation of the fundamental equation.
Effect of friction on electron transfer: The two reaction coordinate case
Authors: Onuchic José Nelson
Published: July 2002

Electron transfer is a very important reaction in many biological processes such as photosynthesis and oxidative phosphorylation. In many of these reactions, most of the interesting dynamics can be included by using two reaction coordinates: one fast (local high frequency vibration modes) and one slow (outersphere modes such as solvent polarization). We report a model to describe this problem, which uses path integral techniques to calculate electron transfer rates, and also to obtain the Fokker–Planck equations associated with this model. Different limiting cases lead to qualitatively different results such as exponential or nonexponential time decay for the donor survival probability. Conditions for the validity of the adiabatic or the nonadiabatic limits will be discussed. Application of this model to real systems is proposed, in particular for a porphyrin rigidly linked to a quinone, which is a very interesting model compound for primary events of photosynthesis. This model can also be used for other multicoordinate biological reactions such as ligand binding to heme proteins. Also, in the concluding part of Sec. III, we discuss the important limit where the fast vibronic mode is much faster than all the other nuclear modes coupled to the problem. In this limit the fast mode ‘‘renormalizes’’ the electronic matrix element, and this considerably simplifies the treatment of the problem, reducing it to coupling only to the slow modes.
The electrostatic interactions in van der Waals complexes involving aromatic molecules
Authors: Price S. L. , Stone A. J.
Published: July 2002

The minima in the electrostatic energy, for accessible orientations, have been located for the s-tetrazine and benzene dimers and the 1:1 complexes of s-tetrazine with hydrogen chloride, water, acetylene, and benzene, and of benzene with acetylene, anthracene, and perylene. The minima give reasonably successful predictions of the structures of these van der Waals molecules, demonstrating the importance of the electrostatic interactions in these systems. The electrostatic energy was calculated using sets of distributed multipoles obtained from ab initio wave functions of the monomers. This method is contrasted with empirical point charge and central multipole models for the electrostatic energy. It is shown that the simple models for the electrostatic interactions can give qualitatively misleading results for aromatic systems.
Visible absorption and magnetic-rotation spectroscopy of 1CH2: The analysis of the b̃ 1B1 state
Authors: Petek Hrvoje , Nesbitt David J. , Darwin David C. , Moore C. Bradley
Published: July 2002

The b̃1B1←ã 1A1 spectrum of CH2 radical has been recorded with Doppler-limited resolution in the 15 600–18 650 cm−1 wavelength region by laser flash-kinetic absorption spectroscopy. Singlet methylene is produced by photolysis of ketene at 308 nm. Assignments for 477 transitions originating from 1A1(0,0,0) and (0,1,0) levels with J≤8 and Ka≤5 are reported from some 10 000 lines observed. Term values are given for these 1A1 levels and 1B1(0,v2,0) levels with 13≤v2≤17. Magnetic-rotation signals were observed for 60% of the 424 lines studied. This anomalous Zeeman effect is explained by singlet–triplet mixing in both ã and b̃ states. Extensive singlet–triplet mixing occurs for 1B1 levels with Ka≠0; this mixing is allowed in second order by Renner–Teller coupling of b̃ 1B1 with ã 1A1 and by spin-orbit coupling via ã 1A1 to X̃ 3B1. The consequent shifts prevent determination of accurate rovibrational structural parameters for the 1B1 state.
Effect of dielectric friction on the viscosity B coefficient for electrolyte in methanol–water mixture
Authors: Ibuki K. , Nakahara M.
Published: July 2002

The viscosity B coefficient for electrolyte solution is obtained in a convenient analytical form on the basis of the Hubbard–Onsager electrohydrodynamic equation. Our dielectric friction theory is successful in predicting the general increase in the B coefficient for the small ion like Li+ with increasing the methanol content in the methanol–water mixture; the early dielectric friction theory by Clark predicts too sharp a rise and the Einstein theory, which completely neglects the dielectric friction, predicts no solvent effect. As limitations of the continuum model, our theory cannot explain the minimum of the B coefficients obtained for the larger ions in the water-rich region as well as their negative values.
Line mixing in a N2-broadened CO2 Q branch observed with a tunable diode laser
Authors: Gentry Bruce , Strow L. Larrabee
Published: July 2002

Line-mixing effects have been observed in the infrared Q branch of the (1110,0310)I–0000 band of CO2 at 2076 cm−1. A tunable diode laser spectrometer was used to record spectra of CO2 broadened by N2 and O2 at total pressures ranging from 100 to 720 Torr. The observed absorption coefficients are up to 65% lower than those calculated using an isolated Lorentzian line approximation. A simple energy gap scaling law is used to determine the off-diagonal relaxation matrix elements from the known pressure-broadening coefficients. The spectra calculated using these matrix elements reproduces the observed absorption coefficients to within several percent.
Distribution function of two cavities and Percus–Yevick direct correlation functions for a hard sphere fluid in D dimensions: Overlap volume function representation
Authors: Rosenfeld Yaakov
Published: July 2002

A one-parameter (‘‘smeared diameter’’) overlap volume function representation of the distribution function of two cavities and of the Percus–Yevick direct correlation functions, for a hard-sphere fluid in D dimensions, is analyzed and found to be very effective.
Structure and dynamics of high-pressure TIP4P water
Authors: Reddy M. Rami , Berkowitz M.
Published: July 2002

Constant temperature molecular dynamics (N, V, T ensemble) is performed on 216 TIP4P water molecules at T=268 and 298 K and densities ρ=0.999, 1.083, and 1.149 g/cc. The main structural effect from the increase of the density is found to be, at both temperatures, weakening of tetrahedral order implied by the disappearance of the second peak in the radial distribution function. The calculated diffusion coefficient reproduces the experimentally observed anomalous behavior at T=268 K. It increases at ρ=1.083 g/cc and then decreases at ρ=1.149 g/cc. For T=298 K the diffusion coefficient decreases with increase of density, which also is in agreement with the experiment.
Infrared spectroscopy of carbo-ions. III. ν3 band of methyl cation CH+3
Authors: Crofton Mark W. , Jagod Mary-Frances , Rehfuss Brent D. , Kreiner Welf A. , Oka Takeshi
Published: July 2002

The infrared spectrum of the degenerate ν3 vibration–rotation band of methyl cation CH+3, one of the most fundamental carbo-ions, has been observed and analyzed. The spectrum was observed in the frequency range of 3240–2960 cm−1 using the difference frequency laser system as the tunable coherent infrared source. A gas mixture of He:H2:CH4∼700:20:1 with a total pressure of ∼7 Torr was used for the ac discharge in an air-cooled and a water-cooled multiple inlet–outlet discharge tube. The velocity modulation method together with noise subtraction and the unidirectional multiple passing of the beam was used for high sensitivity. The spectral lines of CH+3 appeared amid many other lines of C2H+2 and C2H+3 but their widths and their response to chemical conditions gave a good initial clue for the identification. Over 200 absorption lines have been assigned to CH+3 and the isotopic species 13CH+3. The spectral pattern clearly shows that the equilibrium molecular structure is a planar, equilateral triangle as expected from Herzberg’s Rydberg spectrum of CH3 and from ab initio theory. The spectrum has been analyzed by using the symmetric rotor Hamiltonian in a degenerate vibrational state with various A1–A2 splittings and l resonance. The spectral lines have been least-squares fit with a standard deviation of 0.0058 cm−1, and led to 21 vibration–rotation constants of CH+3. Several analytical expressions have been derived among these constants to determine the structure and potential constants. Some perturbation has been noticed and discussed. Also quite a few lines which are definitely due to carbocations with one carbon atom have been left unassigned. The prominence of the CH+3 spectrum in hydrocarbon discharges confirms the importance of this species in the ion–molecule reaction scheme in the laboratory and in space which has already been noted by many people in the field. A simple discussion is given about the formation and destruction of CH+3 as well as other carbocations.
Simulation of the rotational Brownian dynamics of a simple, segmentally flexible model: The elastic trumbbell
Authors: Díaz F. G. , García de la Torre J.
Published: July 2002

The rotational dynamics of a segmentally flexible model, the elastic trumbbell, has been studied using a Brownian dynamics simulation technique. The model is composed of three beads jointed by two bonds. A potential that is quadratic in the bending angle describes the partial flexibility of the model. The correlation analysis of the simulated trajectories gives rotational correlation functions, of which we have preferentially studied those of the second Legendre polynomial of the reorientation angle of the bond and end-to-end vectors. The relaxation times derived from these correlation functions are compared with those predicted by other treatments of the rotational dynamics of segmentally flexible macromolecules.
A
b i
n
i
t
i
o theoretical study on the reactions of a hydrogen molecule with small platinum clusters: A model for chemisorption on a Pt surface
Authors: Nakatsuji H. , Matsuzaki Y. , Yonezawa T.
Published: July 2002

Reactions of a hydrogen molecule with small platinum clusters Ptn (n=1,2,3) are studied theoretically by ab initio methods. This provides a cluster model study for hydrogen chemisorption on a Pt surface. The results suggest that the Pt atom and the linear Pt3 cluster will react with H2 and dissociatively adsorb it, making two Pt–H bonds, whereas the Pt2 cluster will not react with H2 because of a high energy barrier. The dissociative adsorption of a hydrogen molecule occurs at a side-on, on-top site of a surface Pt atom and molecular adsorption does not seem to occur. Essentially only one Pt atom is involved in the initial adsorption process. These findings are in contrast to the palladium case previously reported. Almost no energy barrier exists for the hydrogen migration from one Pt atom to an adjacent one, with a preference being shown for one H atom on each Pt atom rather than two H atoms on one Pt atom. The heat of adsorption, the stability of the catalytic surface, etc., are best represented by the Pt3–H2 system. Spin–orbit coupling effect is examined for the Pt–H2 system and found to be small for the singlet A1 state which is the most important state for the dissociative adsorption. This effect is important, however, to obtain natural potential curve in the large Pt–H2 separation.
Light-scattering studies of a ternary mixture: Comparison of field variables for critical phenomena description
Authors: Sidebottom D. L. , Sorensen C. M.
Published: July 2002

We present static turbidity and dynamic light-scattering measurements near the critical double point of a mixture of secondary butyl alcohol, tertiary butyl alcohol, and water. At the critical double point of this mixture an upper two-phase miscibility loop merges with a lower two-phase region. When temperature is used as a field variable to describe the critical behavior of the turbidity and correlation length of the mixture, nonuniversal behavior is seen in both the critical exponents and amplitudes. If, however, the tBA concentration difference from the critical curve is used, universality is found in both exponents and amplitudes. We thus term this concentration difference to be the relevant parameter for proper description of the critical behavior and, following Griffiths and Wheeler [Phys. Rev. A 2, 1047 (1970)], show this relevant parameter was determined by it never having tangential paths of approach to the critical curve.
Solubility of xenon in perfluoroalkanes: Temperature dependence and thermodynamics
Authors: Kennan Richard P. , Pollack Gerald L.
Published: July 2002

Measurements are reported of Ostwald solubility (L) and mole fraction solubility (x2) for Xe gas, in the form of the radioisotope 133Xe, in three liquid perfluoroalkanes from 5–25 or 5–50 °C. Results at 25.0 °C are L(133Xe in n-C6F14)=2.11±0.01, L(133Xe in C7F16) =1.95±0.01, and L(133Xe in C8F18)=1.85±0.01. From these data the chemical potentials on the number density scale and mole fraction scale, respectively, Δμ0ρ2 and Δμ02, are calculated as functions of temperature. The corresponding average enthalpy and entropy of solvation, on both scales, are also determined. Results of scaled particle theory and of surface energy theory are compared with experimental thermodynamic quantities. The comparisons are suggestive but better data on solvent properties are needed for a definitive comparison.
The a
b i
n
i
t
i
o model potential representation of the crystalline environment. Theoretical study of the local distortion on NaCl:Cu+
Authors: Barandiarán Zoila , Seijo Luis
Published: July 2002

In this paper we formulate a model potential approach to take into account the crystalline environment within the Hartree–Fock–Roothaan formalism. The formulation is based on the assumption that the theory of separability of many-electron systems may be applicable to the group of electrons within a reference cluster and the groups of electrons on a set of external lattice sites which, in turn, can be represented according to the ab initio model potential method. The characteristics of the model potentials permit to analyze the contributions to the cluster energies and wave functions of different environmental effects, such as point-charge and charge-density Coulomb interactions and quantum interactions (exchange and orthogonality). The formalism is applied to the SCF calculation on the ground state of the octahedaral CuCl5−6 cluster (all-electron calculation) embedded in a NaCl lattice which is represented by 118 model-potential ions and 604 point-charge ions. The calculation reveals that (i) the quantum interactions between the CuCl5−6 cluster and the rest of the lattice play an important role in determining the Cu–Cl distance and (ii) a considerable local distortion around the Cu+ impurity is predicted in which the Cl− ions move towards the Cu+ impurity about 0.1 Å. These results are in qualitative agreement with recent EXAFS studies; however, the predicted distortion is smaller than the one suggested by the EXAFS measurements.
The production and spectroscopy of excited sulfur atoms from the two-photon dissociation of H2S
Authors: Steadman Jhobe , Baer Tomas
Published: July 2002

Ground (3PJ) and excited state (1D2, 1S0) sulfur atoms have been produced by the two-photon photodissociation of H2S in the wavelength range 285–316 nm. The resultant sulfur atoms undergo 2+1 resonance enhanced multiphoton ionization (REMPI) at selected laser wavelengths. The initial and final ion states in the 2+1 REMPI process have been determined through the analyses of the REMPI spectrum (laser wavelength scan) and the REMPI-PES taken at the atomic resonances. The intermediate states accessed in the two-photon excitations have been assigned. Several transitions which originate from the 1S0 excited state of atomic sulfur have been observed. The intermediate states of these transitions lie above the ionization continuum and are depleted competitively by autoionization and further photon absorption. Extremely pure state selection of ground and excited sulfur atomic ions is demonstrated at several of the two-photon resonances. Evidence of configuration interaction between the 6p 3P1, 6p 5P1, and 4p 1P1 levels is presented.
Shape-resonant and many-electron effects in the S 2p photoionization of SF6
Authors: Ferrett T. A. , Lindle D. W. , Heimann P. A. , Piancastelli M. N. , Kobrin P. H. , Kerkhoff H. G. , Becker U. , Brewer W. D. , Shirley D. A.
Published: July 2002

The core-level photoexcitation and photoionization of SF6 were studied in the vicinity of the resonances below and above the S 2p threshold. The decay channels of the S 2p→6a1g discrete excitation were characterized, with decay leading mostly to valence-shell satellites. The S 2p continuum data show an oscillatory asymmetry parameter β(S 2p) near threshold that is virtually identical to β(Si 2p) in SiF4. It also resembles—but differs from—theoretical curves for β(S 2p) in atomic sulfur and in SF6. Data at the feature assigned as an eg shape resonance indicate strong multielectron properties for this state, because a resonance in the S 2p satellite is observed at the same photon energy as the main-line resonance. We propose a unified model which generally includes configuration interaction both in the continuum-state manifold and between discrete doubly excited states and the continua, to explain this unexpected satellite behavior. Finally, the S(L2,3VV) Auger electron asymmetry parameter shows no significant deviation from zero near the t2g and eg shape resonances.
General dispersion formulas for molecular third-order nonlinear optical properties
Authors: Bishop David M.
Published: July 2002

For the electronic optical hyperpolarizabilities for molecules γω∥=γZZZZ(−ωσ;ω1, ω2,ω3) and γω⊥=γZXXZ(−ωσ;ω1, ω2,ω3), where Z and X are laboratory axes and ωσ=ω1+ω2+ω3, it is demonstrated that the following relationships exist: (1) γω∥/γ0∥ =1+Aω2L+⋅⋅⋅ , where ω2L=ω2σ+ ω21+ω22+ω23 and A is frequency independent; (2) γω⊥/γ0⊥=1+Bω2L+ ⋅⋅⋅ , where B=p+qa and p and q are frequency independent and a=(ωσω3−ω1ω2)/ω2L ; (3) (1)/(3) (γω∥/γω⊥)=1+Cω2L +⋅⋅⋅ , where C=r(1−6a) and r is frequency independent. In particular, for the four nonlinear optical processes: Kerr (K), degenerate four-wave mixing (D), electric-field-induced second-harmonic generation (E), and third-harmonic generation (T), the ratios (in the same order) are for B, 1:(1+k/2):(1+k/3):(1+k/6) (where k is frequency independent) and for C, 1:(−2):(−1):0.
Using multistate dynamical corrections to compute classically exact diffusion constants at arbitrary temperature
Authors: Voter A. F. , Doll J. D. , Cohen J. M.
Published: July 2002

A method is presented for computing the classically exact, surface or bulk diffusion constant of a point defect at arbitrary temperature. The thermal diffusion constant is expressed using the squared jump length averaged over all possible final states to which the atom can jump. The rate constants that weight this sum are computed using transition state theory and molecular dynamics within a recently developed many-state dynamical corrections formalism. While these rate constants are valid only in the rare-event regime (i.e., at low temperature), it is shown that for a periodic lattice of equivalent binding sites, the resulting diffusion contants is valid at any temperature for which the lattice sites remain well defined. It is thus possible to compute classically exact surface or bulk diffusion constant for an arbitrary interatomic potential, without the time scale limitations of direct molecular dynamics.
Vibrationally resolved shape resonant photoionization of N2O
Authors: Kelly L. A. , Duffy L. M. , Space B. , Poliakoff E. D. , Roy P. , Southworth S. H. , White M. G.
Published: July 2002

We report a vibrationally resolved dispersed fluorescence study of 7σ−1 shape resonant photoionization in N2O. Previous work indicates that there are two shape resonances in the εσ channel, and our results demonstrate that the lower energy shape resonance (hνexc≊20 eV) results in non-Franck–Condon vibrational branching ratios over a wide range. Moreover, the cross section curves for alternative vibrational modes behave differently, and we show that the symmetric stretch exerts a greater influence on resonance behavior than the asymmetric stretching vibration. Such comparisons of vibrationally resolved partial cross section curves provide qualitative insights into the shape resonant photoionization dynamics for polyatomic targets. The higher energy shape resonance is apparently masked by the presence of the dominant nonresonant επ channel. In addition to information on the photoionization dynamics, these results also provide spectroscopic data on the ionic potential surfaces. Specifically, we obtain ratios of Franck–Condon factors for N2O+ (A→X) transitions which are useful for testing calculated properties of the ion.
Vibrationally resolved photoelectron studies of the 7σ−1 channel in N2O
Authors: Ferrett T. A. , Parr A. C. , Southworth S. H. , Hardis J. E. , Dehmer J. L.
Published: July 2002

We present vibrationally resolved photoelectron studies of the 000, 100, 200, and 001 modes of the Ã state (7σ−1) of N2 O+ in the 17.4–26 eV photon-energy range. The vibrational branching ratios σ(100)/σ(000) and σ(001)/σ(000) agree very well with fluorescence measurements by Kelly et al. and qualitatively with recent theoretical predictions of Braunstein and McKoy. The large non-Franck–Condon variations in the σ(100)/σ(000) and σ(200)/σ(000) branching ratios are associated with a predicted 7σ→εσ shape resonance near 20 eV. Overall, the vibrational branching ratios imply lower resonant energies for the stretching modes (100 and 200) and a similar resonant energy for the asymmetric stretch (001), compared with the 000 mode. The vibrational asymmetry parameters (β) display a strong variation with energy which is qualitatively reproduced by theory; however, the experimental values for β(100) and β(001) exhibit additional structure around 20 eV. When combined with theory and recent fluorescence data, these results help to demonstrate a correlation of shape resonance energy with overall molecular length (RN–N+RN–O); this important result implies a resonant state which is localized on the entire triatomic molecular frame rather than on the N–N or N–O components.
Highly correlated single-reference studies of the O3 potential surface. I. Effects of high order excitations on the equilibrium structure and harmonic force field of ozone
Authors: Stanton John F. , Lipscomb William N. , Magers David H. , Bartlett Rodney J.
Published: July 2002

The potential energy surface of ozone in the vicinity of the equilibrium geometry is investigated by single-reference many-body perturbation theory (MBPT) and coupled-cluster (CC) methods. As expected from the known inadequacies of the independent-particle picture of O3, analysis of the CCSDT-1 wave function reveals considerable mixing between the [core⋅⋅⋅]4b226a211a22 and [core⋅⋅⋅]4b226a212b21 configurations. Smaller, but still significant, contributions come from other configurations involving redistribution of electrons within the out-of-plane π orbital framework. As expected, the equilibrium structure and harmonic force field computed at the SCF level of theory are in considerable error. When allowance is made for electron correlation effects, the discrepancies between theory and experiment for the equilibrium structure and totally symmetric force field are significantly reduced, and the MBPT(4), CCSD, CCSD+T(CCSD) and CCSDT-1 results are in reasonable agreement with accepted values. Asymmetric stretching frequencies, however, are found to oscillate wildly [2373 cm−1 at MBPT(2), 1547 cm−1 at MBPT(4), and 680 cm−1 at CCSDT-1] about the experimental value of 1089 cm−1, suggesting that the Cs subspace of the potential surface poses a great challenge to theory. A possible cause of this behavior is discussed, along with implications for future studies of the global O3 potential surface.
Ground state spectroscopic and thermodynamic properties of AlO−, SiN−, CP−, BS−, BO−, and CN− from Mo/ller–Plesset perturbation theory
Authors: Peterson Kirk A. , Woods R. Claude
Published: July 2002

Rotational and vibrational constants in the ground states of the 22 electron negative ions AlO−, SiN−, CP− and BS− and their 14 electron analogs BO− and CN− have been calculated at the MP4SDQ level with two large Gaussian basis sets. Identical calculations have also been carried out on the well known isoelectronic neutral species and used to correct the ab initio values of re and ωe of the ions. Calculations of the heavy atom distances of HCP, HBS, HBO, and HCN were performed to provide additional calibrations for the bond lengths of CP−, BS−, BO−, and CN−, respectively. Singles and doubles CI have been used to obtain equilibrium dipole moments and electric field gradients, from which quadrupole coupling constants were derived. Dissociation energies with expected accuracies of at least 5 kcal mol−1 were calculated by MP4SDTQ for the negative ions and were used to derive electron affinities for the radical precursors. Proton affinities of the negative ions and H–AB dissociation energies of the triatomic species were calculated at the MP4SDQ level.