bluuebunny/crossref_metadata_2025 huggingface dataset:
Generalized propagation formulas of arbitrarily high order
Authors: Glasner Moses , Yevick David , Hermansson Björn
Published: July 2002

We present a procedure for expressing the exponential of the sum of any number of noncommutative operators each linear in a parameter α as a product of the exponentials of the individual operators valid to a given order in α. These products then yield simple and easily programmed quantum-mechanical propagation techniques.
Dilute solution behavior of cyclic and linear polyelectrolytes
Authors: Hodgson Donald F. , Amis Eric J.
Published: July 2002

Viscosities and diffusion coefficients of dilute solutions of molecular weight matched cyclic and liner poly(2-vinyl pyridine) samples with molecular weights ranging from 2.8×104 to 1.0×105 have been measured under conditions where the polymer can be charged by protonation. Limiting conditions of high charge density and full electrostatic screening with high acid concentration solutions show expected normal polymer behavior in extrapolations to the intrinsic viscosity and infinite dilution diffusion coefficients. By employing weakly acidic solvents, large reduced viscosities, typical of polyelectrolytes in low salt solutions are observed. A pronounced maximum in reduced viscosity occurs at a concentration which is independent of molecular weight and topology. Values of the reduced viscosity are lower for the cyclic samples over all concentrations investigated. Independent pH titrations are used to calculate polyion charge density and electrostatic screening lengths. Possible origins of the viscosity maxima are explored.
Distribution function theory for inhomogeneous fluids
Authors: Schlijper A. G. , Harris C. K.
Published: July 2002

A theory for inhomogeneous fluids is presented. It is derived by an approach that is inspired by the cluster variation method for lattice systems. A systematic expansion of the free-energy functional is generated, which is then truncated and minimized to obtain integral equations for the density profile and the pair distribution function. The theory can be simplified by an additional approximation that serves to decouple the integral equations. Numerical results from this simplified theory show promising agreement with computer simulations.
Nuclear magnetic resonance line shapes of double and triple quantum coherences of spin 3/2 nuclei
Authors: Eliav Uzi , Navon Gil
Published: July 2002

Nuclear magnetic resonance (NMR) line shapes of double and triple quantum coherences were calculated for spin 3/2 nuclei over a wide range of reorientation times. It is shown that for slow rotating molecules the dynamic shifts of all the coherences are a result of adiabatic averaging of the powder spectra. Line shapes are shown to be single Lorentzians far beyond the range of rotational motion, where Redfield’s theory is valid. Analytical expressions for linewidths and dynamic shifts are given for the whole range of motion for which the line shape is Lorentzian. It is suggested that the high ratio between the dynamic shift and the linewidth for the triple quantum coherence can make it possible to distinguish between sodium nuclei in environments with different rotational mobility and quadrupole interactions.
Reaction of Zn+ with NO2. The gas-phase thermochemistry of ZnO
Authors: Clemmer D. E. , Dalleska N. F. , Armentrout P. B.
Published: July 2002

The homolytic bond dissociation energies of ZnO and ZnO+ have been determined by using guided ion-beam mass spectrometry to measure the kinetic-energy dependence of the endothermic reactions of Zn+ with nitrogen dioxide. The data are interpreted to yield the bond energy for ZnO, D00=1.61±0.04 eV, a value considerably lower than previous experimental values, but in much better agreement with theoretical calculations. We also obtain D00(ZnO+)=1.67±0.05 eV, in good agreement with previous results. Other thermochemistry derived in this study is D00(Zn+–NO)=0.79±0.10 eV and the ionization energies, IE(ZnO)=9.34±0.02 eV and IE(NO2)=9.57±0.04 eV.
Long time wave packet behavior in a curve-crossing system: The predissociation of NaI
Authors: Meier Ch. , Engel V. , Briggs J. S.
Published: July 2002

Stimulated by experiments by Zewail and co-workers [Chem. Phys. Lett. 172, 109 (1990)], we perform quantum calculations on the long time behavior of a wave packet which moves on coupled electronic surfaces of the NaI molecule. Beat structures in the simulated femtosecond pump/probe signal are shown to depend critically on the parameters used in the calculation. The adiabatic motion is compared to the dynamics including the nonadiabatic coupling and the consequences for the signal are explored. The results are compared to a semiclassical analysis of Chapman and Child [J. Phys. Chem. 95, 578 (1991)].
Low temperature relaxation in polypyrrole
Authors: Singh Ramadhar , Tandon R. P. , Panwar V. S. , Chandra Subhas
Published: July 2002

The loss tangent and dc conductivity of electrochemically polymerized BF−4 doped polypyrrole films have been measured in the temperature range 77–300 K. The low temperature relaxation has been observed which shows decrease of relaxation frequency with increase in temperature. The dc conductivity data has been described by a hopping model.
Poisson–Boltzmann integral equation for polyelectrolyte solutions
Authors: Victor J. M.
Published: July 2002

An interaction-site model (ISM) theory, well developed for molecular liquids, is proposed for solutions of flexible polyelectrolytes. A Debye–Hückel limit is considered in details and new screening properties are deduced. Potentials of mean force are reexamined for these complex fluids resulting in a generalization of the well-known Poisson–Boltzmann approximation. A system of nonlinear integral equations is obtained, correctly including counterions condensation, which should therefore fit quantitatively the neutron scattering curves. Physical arguments are given.
Kinetics and spectroscopy of electronically excited BiO obtained by energy transfer from metastable O2. II. Spectroscopic analysis of the A (2Π1/2)–X1(2Π1/2) transition
Authors: Martin F. , Bacis R. , Linton C. , Vergès J. , Mayo I. , Rosenwaks S.
Published: July 2002

The A(2Π1/2)–X1(2Π1/2) transition of BiO was studied via high resolution Fourier transform spectroscopy in the 7550–20 000 cm−1 region. BiO in the A state was obtained in a mixture of Bi vapor and O2(1Δg,1Σ+g). From rotational and vibrational analysis of the spectrum, equilibrium constants for both the X1 ground state and the A excited state have been calculated. The magnetic hyperfine constant of the ground state of BiO has been found to be 0.0928(4) cm−1.
The effects of a nonlinear delayed feedback on a chemical reaction
Authors: Chevalier T. , Freund A. , Ross J.
Published: July 2002

With delay feedback experiments on the minimal bromate oscillator, we show that chemical systems with delay display a variety of dynamical behavior. Using a nonlinear delayed feedback, we induce Hopf bifurcations, period doubling, bifurcations into chaos, and crisis (observed for the first time in a chemical system) into the system, which does not display this behavior without the delay. We test a conjecture [M. Le Berre, E. Ressayre, A. Tallet, H. M. Gibbs, D. L. Kaplan, and M. H. Rose, Phys. Rev. A 35, 4020 (1987)] that the dimension of a chaotic attractor is equal to τ/δf, where δf is the correlation time of the delayed feedback. Using the Grassberger–Procaccia algorithm [P. Grassberger and I. Procaccia, Phys. Status Solidi D 9, 189 (1983)] to calculate the dimensions of the chaotic attractors from the experimental system, we show that the calculated dimensions are less than those calculated by τ/δf. We compare numerical integrations of the proposed mechanism for the minimal bromate oscillator with the experimental results and find agreement of the predicted bifurcation sequence with the experimental observations. The results of this study indicate that with appropriate delay feedback functions, and a sufficiently nonlinear dynamical system, it is possible to ‘‘push’’ a dynamical system into further bifurcation regimes, of interest in themselves, which also yield information on the system without delay.
Mean spherical approximation algorithm for multicomponent multi-Yukawa fluid mixtures: Study of vapor–liquid, liquid–liquid, and fluid–glass transitions
Authors: Arrieta E. , Jedrzejek C. , Marsh K. N.
Published: July 2002

An efficient algorithm is given to find the Blum and Ho/ye mean spherical approximation (MSA) solution for mixtures of hard-core fluids with multi-Yukawa interactions. The initial estimation of the variables is based on the asymptotic high-temperature behavior of the fluid. From this initial estimate only a few Newton–Raphson iterations are required to reach the final solution. The algorithm consistently yields the unique thermodynamically stable solution, whenever it exists, i.e., whenever the fluid appears as a single, homogeneous phase. For conditions in which no single phase can appear, the algorithm will declare the absence of solutions or, less often, produce thermodynamically unstable solutions. A simple criterion reveals the instability of those solutions. Furthermore, this Yukawa-MSA algorithm can be used in a most simple way to estimate the onset of thermodynamic instability and to predict the nature of the resulting phase separation (whether vapor–liquid or liquid–liquid). Specific results are presented for two binary multi-Yukawa mixtures. For both mixtures, the Yukawa interaction parameters were adjusted to fit, beyond the hard-core diameters σ, Lennard-Jones potentials. Therefore the potentials studied, although strictly negative, included a significant repulsion interval. The characteristics of the first mixture were chosen to produce a nearly ideal solution, while those of the second mixture favored strong deviations from ideality. The MSA algorithm was able to reflect correctly their molecular characteristics into the appropriate macroscopic behavior, reproducing not only vapor–liquid equilibrium but also liquid–liquid separations. Finally, the high-density limit of the fluid phase was determined by requiring the radial distribution function to be non-negative. A case is made for interpreting that limit as the fluid–glass transition.
Hypersonic velocity in binary polymer mixtures: Poly(dimethylsiloxane)–poly(ethylmethylsiloxane)
Authors: Fytas G. , Kanetakis J. , Momper B. , Akcasu A. Z.
Published: July 2002

The first Brillouin experiment on the blend poly(dimethylsiloxane)–poly(ethylmethylsiloxane) with critical concentration φPDMS =0.53 and an upper critical solution temperature Tc =58 °C is reported. The theoretical calculation of the sound velocity cL in compressible binary mixture is based on the random phase approximation for a ternary incompressible system. According to this new result, the speed cL, in the absence of dispersion effects, depends mainly on free volume and segment–segment interaction potential and remains finite as the blend approaches Tc. The main experimental features are in agreement with the theoretical predictions.
Rotationally inelastic collisions of Li2(A 1Σ+u) with Ne: Fully a
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Authors: Alexander Millard H. , Werner Hans-Joachim
Published: July 2002

The potential energy surface (PES) for the interaction of Li2(A 1Σ+u) with Ne has been computed using highly correlated multiconfiguration–reference configuration expansions (MRCI) and a large basis set. From the calculated points an analytical fit of the potential was obtained. Particular care was used to ensure a smooth fit to the angular dependence of this highly anisotropic potential. This PES has been used in exact close-coupling (CC) quantum scattering calculations of cross sections for rotationally inelastic collisions. The dependence of the calculated cross sections on velocity, as well as on the initial and final states, is found to be in excellent agreement with the measurements of Smith, Scott, and Pritchard [J. Chem. Phys. 80, 4841 (1984);81, 1229 (1984)]. For comparison, cross sections were also computed within the coupled-states (CS) approximation. At low collision energies the CS results deviate significantly from both the exact CC results and the experimental data.
The quantum mechanics of clusters: The low-temperature equilibrium and dynamical behavior of rare-gas systems
Authors: Rick Steven W. , Leitner D. L. , Doll J. D. , Freeman David L. , Frantz D. D.
Published: July 2002

We consider in the present paper the quantum-mechanical effects on the equilibrium and dynamical behavior of low-temperature rare-gas clusters. Using a combination of ground-state and finite-temperature Monte Carlo methods, we examine the properties of small (2–7 particles) neon clusters. We find that the magnitude of the equilibrium quantum-mechanical effects in these systems is significant. The present studies also suggest that the low-temperature dynamics of these rare-gas systems is appreciably nonclassical.
Theoretical model study of the quantum-mechanically mixed-spin (S=5/2,3/2) ground state in iron (III) porphyrins
Authors: Bominaar E. L. , Block R.
Published: July 2002

After a selective review of some recent studies on structural and spin properties of iron–porphyrin complexes, the role of axial ligands and the porphyrin-structure effect on spin state is presented. It is shown that in crystal-field approximation the structure effects on spin-state differences in five- and six-coordinate complexes can be rationalized within the available x-ray data. In the framework of a vibronic-coupling analysis, added to the ‘‘charge–repulsion’’ model of Saito and Kashiwagi, the possibility of a drastic reduction in the energy needed for triggering the intermediate (S=3/2) to high-spin (S=5/2) transition is numerically demonstrated. In addition, two supplementary quantum-mechanical effects through axial ligands on bond distances and spin-state preferences are described and illustrated by model calculations. The results are found to agree with the experimental data.
Laser-induced fluorescence study of the reactions Cu+X2→CuX+X (X=F, Cl, Br, and I)
Authors: Fang C. C. , Parson J. M.
Published: July 2002

Reactions of a beam of Cu with halogen gases at low pressure have been studied in the single-collision regime using a pulsed tunable dye laser to excite the ground electronic state products to fluorescing states. CuF vibrational state populations could be estimated up to v=25, but for the heavier halides information on only v≤3 was obtainable from the excitation spectra. The vibrational distributions are similar to prior expectations but somewhat colder. Spectral simulations also reveal that the fraction of energy available to products appearing as rotation is considerably below prior predictions. This pattern of energy release may be related to product repulsion, which occurs as the products transform from a strongly bound XCuX intermediate to the singly ionic ground state molecule plus halogen atom.
Direct observations of pair interactions on a metal: Heteropairs on W(110)
Authors: Watanabe Fumiya , Ehrlich Gert
Published: July 2002

Quantitative information about the range and strength of interactions between adatoms on surfaces is still scarce. However, through the use of the field ion microscope it is possible to pinpoint the location of individual metal atoms on a crystal, and therefore to establish by direct observation the probability P(R) of finding two atoms at sites separated from each other by the vector R. If equilibrium is established at the surface, this probability is related to F(R), the free energy of interaction, by P(R)∝exp[−F(R)/kT]; measurements of the probability therefore provide a quantitative estimate of the interaction between adatoms at specified sites on the surface. Such measurements have been carried out with two metal atoms on the most densely packed plane of tungsten, the (110). To minimize the amount of data taking, initial studies have been made on the distribution of an adatom (Pd) mobile at the temperature of equilibration around a chemically different metal atom (either Re or W) stationary at the center of the W(110) plane. These observations reveal a remarkable anisotropy. Palladium atoms are most frequently found at the nearest-neighbor position to the central atom, but the close by second and third-nearest neighbor sites are not populated. It is shown that this is due to a strong orientational dependence of interactions. Quantitative values for the free energy of interaction with the central adatom have been determined for Pd atoms at different sites on the surface. Along the close-packed direction [11̄1], interactions are attractive out to ≊10 Å, and show an oscillatory dependence upon distance; along [001] and [11̄0] they tend to be repulsive for both Re and W as the central atom. These effects are so far not predicted by theory.
Conformations of a polymer boxed into a corner: Role of excluded volume
Authors: Myers Kurt , Nemirovsky Adolfo M. , Freed Karl F. , Wang Zhen-Gang
Published: July 2002

The influence of surface constraints on the conformations of a flexible polymer are investigated. The polymer is modeled as a Gaussian chain, and the influence of repulsive polymer–polymer excluded volume interactions is investigated using renormalization group methods. Various measures of the polymer mean dimensions are computed for chains terminally attached to mutually orthogonal surfaces and for a wide range of polymer excluded volume interactions. Two limiting cases of the polymer–surface interaction are considered—the absorbing boundary condition corresponding to a ‘‘repelling’’ surface and the reflecting boundary condition associated with the ‘‘adsorption theta point,’’ where surface attractions fall just short of adsorbing the polymer.
Picosecond polarization-selective transient grating experiments in sodium-seeded flames
Authors: Fourkas John T. , Brewer Timothy R. , Kim Hackjin , Fayer M. D.
Published: July 2002

Polarization-selective transient grating experiments have been used to study the subnanosecond time scale dynamics of several sodium-seeded, premixed flames. Intensity gratings (in which both excitation beams are of the same polarization) were used to determine excited-state quenching collision rates, while polarization gratings (in which the excitation beams are cross polarized) were used to measure Na diffusion constants and the rates of Na ground state magnetic sublevel population scattering collisions. In addition, the rates of scattering between the 3P1/2 and 3P3/2 excited state levels were measured using an excited state probing scheme.
A simple predictive model for planar vibrations of polycyclic benzenoid hydrocarbons
Authors: Ohno Koichi
Published: July 2002

A very simple force field model is designed to be applied as a predictive means to all kinds of planar benzenoid hydrocarbons to the infinite size without any modifications and extra knowledge except for topological connections of hexagonal rings. The model involves only eight potential parameters including CC, CH, CCC, and CCH stretchings, CC–CCC and CC–CCH bendings, and two constants for conjugation effects which are essential for the description of strongly correlated collective motions of CC bonds. The CC–CC interaction constants in the standard set of parameters were found to be in good agreement with those for benzene experimentally determined by Ozkabak and Goodman and those obtained from scaled quantum mechanical (SQM) calculations by Pulay et al. The standard set of force constants in the present model were shown to be almost completely transferable for the known planar vibrations of typical benzenoids. It is of note that in comparison with the SQM method the present model gives a small difference of ∼16 cm−1 in average for the calculated planar frequencies of benzene and naphthalene. Further applications to large benzenoids including perylene, benzo(g,h,i)perylene, coronene, ovalene, and tribenzo-(a,g,m)coronene demonstrated that the present model predicts not only planar frequencies in an accuracy of ∼20 cm−1 but also vibronic band intensities in good agreement with experiment.
The vibrational spectrum and rotational constants of difluoroethyne FC 3/4 CF. Matrix and high resolution infrared studies and a
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Authors: Bürger H. , Schneider W. , Sommer S. , Thiel W. , Willner H.
Published: July 2002

Infrared spectra of the short-lived difluoroethyne molecule have been recorded in neon and argon matrices between 200 and 5000 cm−1. Fourier transform infrared spectra with a resolution of 0.004 cm−1 have been measured in the gas phase around 1350 cm−1 (ν3, ν2+ν4+ν5, hot bands) and 2150 cm−1 (ν2+ν3, ν1−ν5, hot bands). The high resolution study yields rotational parameters of the ground and all singly excited vibrational states. The interpretation of the experimental data has been guided by ab initio calculations at the SCF (self-consistent-field) level and the correlated MP2 level (Moller–Plesset second order perturbation theory) employing three different large basis sets. The theoretical calculations provide the SCF and MP2 harmonic fields as well as the SCF anharmonic force field of FCCF. The agreement between the available theoretical and experimental results is generally quite good, with the exception of the spectroscopic constants involving the trans-bending mode ν4 where more theoretical work is required. The combined use of theoretical and experimental information leads to an estimate of the equilibrium structure [D∞h, re(CC)=1.1865 Å, re(CF)=1.2832 Å] and to recommended ‘‘best’’ values for the wave numbers of all fundamental vibrations based on the matrix and high resolution infrared data and some ab initio anharmonicity constants. The present study demonstrates the advantages of a combined theoretical and experimental approach to the spectroscopy of short-lived molecules.
Identification and stability of U2O2, U2O3, and U2O4 gaseous oxides molecules
Authors: Guido M. , Balducci G.
Published: July 2002

The U2O3(g) and U2O4(g) species have been for the first time identified by Knudsen cell-mass spectrometry and their heats of formation and atomization energies have been tentatively derived. A redetermination of these quantities for the already known UO3(g) and U2O2(g) molecules has been also attempted.
Activities of copper in the Y–Ba–Cu–O system
Authors: Myers D. L. , Wahlbeck P. G.
Published: July 2002

The vapor pressures of Cu in equilibrium with YBa2Cu3O6.24 were measured as a function of temperature by the Knudsen effusion method using a quadrupole mass spectrometer as the detector. The data were fitted to ln [P(Cu)/bar]=(−66104±4485)(1/T)+(35.360±3.869). The activity of Cu [in YBa2Cu3O6.24(s) ] was calculated as the ratio of the above equilibrium pressure divided by the vapor pressure of pure Cu(s). From the activities, relative chemical potentials were calculated. The vapor pressure of Cu from the superconductor intersects the vapor pressure of Cu from pure Cu(s) at 1290±20 K, which indicates that the superconductor decomposes to metallic Cu at temperatures greater than 1290 K.
Charge fluctuation in reverse micelles
Authors: Bratko D. , Woodward C. E. , Luzar A.
Published: July 2002

Electrostatic interactions in and among reverse micelles in the microemulsion are studied by the grand canonical ensemble Monte Carlo simulation (GCEMC) and by the linear response/Poisson–Boltzmann approximation (LPB). The latter theory is found to reproduce the ion-correlation effects among the micelles seen in the earlier canonical Monte Carlo approach. The open ensemble simulation and the Poisson–Boltzmann (PB) equation are used to study solubilization of simple electrolyte in the microemulsion. In systems with monovalent ions, the PB approximation agrees well with the simulation. Since individual ions are allowed to enter or leave the micelle in our GCEMC simulation, the deviations from the electroneutrality of the droplets are also studied. A good agreement with the LPB theory and with the simple charge fluctuation models considering only the Born solvation energy of charged droplets is observed. For large reverse micelles, the LPB theory and the continuum approach of Eicke and co-workers [J. Phys. Chem. 93, 314 (1989)] proved satisfactory while only the discrete model of Hall [J. Phys. Chem. 94, 429 (1990)] agrees with the simulation for small droplets. The GCEMC results lend support to recent interpretations of electric conductivity of microemulsions. The intermicellar forces due to the correlated charge fluctuation on adjacent droplets are discussed. An approximate expression for the charge fluctuation contribution to the long-ranged pair potential among large droplets with the radius Rm of the form w(r)/kT≂−(1/2)(Rm/r)2 is derived.
Correcting the coupled states approximation using sensitivity analysis
Authors: Eno Larry
Published: July 2002

A sensitivity approach to the correction of the coupled states (CS) approximation, as originally proposed by Eno and Rabitz, is here examined by application. A related approach, developed by McLenithan and Secrest, is also examined. Results from the two approaches, for the rotational scattering of Ne+HD, are compared. We find that each has its preferred domain of applicability.
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o calculations on the four lowest electronic states of AlF+ and AlCl+
Authors: Glenewinkel-Meyer Th. , Müller B. , Ottinger Ch. , Rosmus P. , Knowles P. J. , Werner H.-J.
Published: July 2002

The potential energy curves of the two lowest 2Σ+ (X,B) and the two lowest 2Π (A,C) electronic states of the AlF+ and AlCl+ molecular ions have been calculated using highly correlated multireference configuration interaction (MR-CI) wave functions. It is found that both 2Σ+ states and the C 2Πr state are bound, whereas the A 2Πi state is repulsive. Electronic transition moment functions for all six pairs of states of both ions have also been calculated and used for evaluation of the radiative transition probabilities between bound vibrational states. The calculated charge distributions show that the X and C states are dominated by Coulombic attraction between a doubly charged positive aluminum and the singly charged negative halogen ion. The results provide a new assignment of the photoelectron spectra of the neutral AlF. They are also in good agreement with the recently observed optical B–X emission spectra of both molecular ions and C–X bands of AlCl+. The absence of C–X emission of AlF+ is most likely due to predissociation of the C 2Πr state by the repulsive A 2Πi state.
The relation of fluxes and forces to work in nonequilibrium systems
Authors: Ross John , Harding Robert H. , Wolff Anita N. , Chu Xiaolin
Published: July 2002

Prior work has shown that an excess work is necessary to displace a chemical or physical system from a stationary state, and this excess work determines the stationary distribution of a stochastic birth–death master equation. We derive the augmentation of this master equation for a one-variable system in the presence of external noise. When this noise is much larger than internal noise, but still small compared to macroscopic averages, then the stationary distribution reduces to a form suggested by Landau and Schlögl, which is the integral of the flux of the deterministic kinetic equation. A similar result was obtained on the basis of an assumed Fokker–Planck equation. Hence, in the presence of external forces exceeding in intensity the internal fluctuations, fluxes are proportional to forces without linearization in concentrations.
On the theory of electrostatic interaction of neutral lipid bilayers separated by thin water film
Authors: Leikin S.
Published: July 2002

A simple analytical solution is obtained for the screened Coulombic force between neutral lipid bilayers separated by thin water film within a model formally analogous to the ‘‘spherical model’’ for Ising systems. This model accounts for constraints on the surface charge density fluctuations imposed by the structure of lipid bilayers. The force is attractive at low and repulsive at high temperatures. A relationship between the system parameters and the temperature of the transition from the repulsion to attraction is established. At room temperature predicted force can be repulsive as well as attractive depending on the system parameters and on the assumptions.
Direct kinetic studies of SiH3+SiH3, H, CCl4, SiD4, Si2H6, and C3H6 by tunable infrared diode laser spectroscopy
Authors: Loh S. K. , Jasinski J. M.
Published: July 2002

Gas phase reactions of silyl radical, SiH3, are investigated at room temperature using tunable diode laser flash kinetic spectroscopy. Photolytic generation of silyl at 193 and 248 nm is demonstrated using several different precursor systems. The silyl recombination reaction, SiH3+SiH3→Si2H6, is studied by quantitative measurement of SiH3 and attendant product densities. Analysis yields a refinement of the rate constant, krc=(7.9±2.9)×10−11 cm3 molecule−1 s−1. By modeling silyl densities following photolysis of HCl in SiH4, bimolecular rate constants for H+SiH3 and H+SiH4 are determined to be (2±1)×10−11 and (2.5±0.5)×10−13 cm3 molecule−1 s−1, respectively. Reactions of SiH3 with SiD4, Si2H6, CCl4, and C3H6 (propylene) are studied under pseudo-first-order conditions. Derived upper limits to the rate constants show these reactions to be slow at room temperature. The data demonstrate the reactivity of silyl with open-shell (radical) species and the general inertness of silyl toward closed shell molecules. Under typical chemical vapor deposition conditions, SiH3 is, therefore, a kinetically long-lived species in the gas phase and consequently a potentially important film forming species under plasma and photochemical deposition conditions.
Bonding geometry and bonding character of thiocyanate adsorbed on a Ag(100) surface
Authors: Pacchioni Gianfranco , Illas Francesc , Philpott Michael R. , Bagus Paul S.
Published: July 2002

The adsorption geometry and the nature of the interaction of the SCN molecule at an on-top site of the Ag(100) surface have been investigated using ab initio cluster model wave functions. The SCN anion, SCN−, is a bidentate ligand. If the SCN–Ag bond is ionic, we could expect, by analogy with thiocynate–metal complexes, that SCN could be bound to a metal surface through either the N end or the S end. We show that the chemisorption bond for SCN/Ag is very ionic and that the interaction between chemisorbed SCN and the Ag surface is largely electrostatic. The most important bonding mechanism is the polarization of the Ag surface due to the presence of SCN−. However, we do find that there is a small, but non-negligible, covalent interaction. There is a very small energetic cost to change the angle of SCN with respect to the surface between a perpendicular and a parallel orientation. We contrast this with the case of a covalently bonded adsorbate, CO/Ag(100), where the π bond strongly favors orientations near perpendicular. The flat potential energy curve for bending SCN suggests that the adsorption geometry at high SCN coverage may be largely determined by nonbonding interactions between adjacent adsorbates.
The insignificance of the interference of •CFCl2 radical in the electron spin resonance study of radical cations using the CFCl3 matrix
Authors: Shida T. , Kato T. , Momose T. , Matsushita M.
Published: July 2002

The insignificance of the ‘‘background’’ signal due to the •CFCl2 radical in the ESR study of radical cations using the CFCl3 has been demonstrated by calculating the dipolar broadening of the hyperfine interaction.
Concentration profiles of a colloidal mixture near a charged wall
Authors: González-Mozuelos P. , Medina-Noyola M. , D’Aguanno B. , Méndez-Alcaraz J. M. , Klein R.
Published: July 2002

The structure of a model two-component colloidal mixture in the vicinity of a charged wall is studied in the framework of the Derjaguin–Landau–Verweg–Overbeek potential and the hypernetted-chain approximation for the particle–particle and wall–particle direct correlation functions as the closure for the bulk and wall–particle Ornstein–Zernike equations. It is found that for strongly repulsive walls the structure of the inhomogeneous mixture near the wall only depends on the bulk parameters. For neutral and attractive walls, a monolayer of colloidal particles adjacent to the wall is formed. Depending on the charge asymmetry between the two colloidal species, the composition of this monolayer indicates a preferential electrostatic adsorption of the more highly charged species. For large charge asymmetry between the two species, a distinct segregation effect is predicted.
Picosecond surface restricted transient grating studies of carrier reaction dynamics at n-GaAs(100) interfaces
Authors: Gomez-Jahn L. A. , Miller R. J. Dwayne
Published: July 2002

The surface restricted transient grating technique has been found to be sensitive to the Fermi level pinning surface states at the atomic interface of the native oxide layer of (100) GaAs. The sensitivity to these states is better than 10−4 of a monolayer. The grating signal associated with the surface states is eliminated by photoinduced removal of the oxide layer and hole transfer to Se−2 ions adsorbed to the surface. These results support the assignment of the signal to electronic factors associated with surface state species. The use of space charge field focusing of hole carriers to the surface has enabled a discrimination of the hole carrier reaction dynamics from those of the electron at the surface. In situ studies of interfacial hole transfer to Se−2 ions present at liquid junctions found the hole transfer time to be less than 30 ps. The selective quenching of the hole carrier through interfacial charge transfer has found that the hole carrier contributes to the optical dispersion in the 1 μ region to approximately the same extent as the electron.
Adatom bond dissociation and H–O2 bond formation in the reaction between an adsorbed hydrogen atom and an oxygen molecule: A trajectory dynamics study
Authors: Shin H. K.
Published: July 2002

The collisional dissociation of the H-surface bond and the formation of the H–O2 bond in the O2(gas)/H(ads) collision taking place on a tungsten surface have been studied by classical trajectory methods over the collision energy range of 0.1–2.0 eV. The effects of the interactions between the H atom and higher-order neighbors of the center metal atom are important in the collisional dissociation of adatoms. This many-body interaction leads to an oscillatory dependence of the adatom dissociation probability on the collision energy. The attractive well depth of the O2(gas)/H(ads) interaction is varied between 0.202 to 4.624 eV. At an intermediate range of well depth, energy preferentially transfers into the adatom bond and leads to a large dissociation probability. As well depth increases, energy transfer to O2 becomes significant, thus causing the accumulation of a smaller amount of energy in the adatom bond, so adatom dissociation is less effective.
Fluctuations of thermodynamic quantities calculated from the fundamental equation of thermodynamics
Authors: Yan Zijun , Chen Jincan
Published: July 2002

On the basis of the probability distribution of the various values of the fluctuation and the fundamental equation of thermodynamics of any given system, a simple and useful method of calculating the fluctuations is presented. By using the method, the fluctuations of thermodynamic quantities can be directly determined from the fundamental equation of thermodynamics. Finally, some examples are given to illustrate the use of the method.
A model of defect-mediated transport through amorphous membranes
Authors: Hubbard J. B. , Nguyen T. , Bentz D.
Published: July 2002

We introduce a class of membrane diffusion models which consists of an impermeable slab punctured by transmembrane pores which are constructed by continuous random-walk algorithms. These formally infinite tortuosity models closely resemble low tortuosity models in their steady-state flux and total transient uptake. The sharpest contrast between random and homogeneous membranes is in the stretched exponential (subdiffusive) decay [exp(−tα), α=1/2] of the transient diffusive flux for the random-pore models vs simple exponential decay [exp(−t)] for homogeneous membranes.
Direct observation of interactions between identical adatoms: Ir–Ir and Re–Re on W(110)
Authors: Watanabe Fumiya , Ehrlich Gert
Published: July 2002

Information about pair interactions between adatoms at specified sites on a surface can be readily obtained from the probability P(R) of finding two atoms at a separation R at equilibrium on a surface at temperature T. Observations of two adatoms in such a system at different separations, using the field ion microscope, yield a map of the free energy of interaction F(R), inasmuch as the probability P(R) is proportional to exp[−F(R)/kT]. Only pairs of chemically different metal adatoms have so far been examined in this way, and measurements have therefore been extended to chemically similar atoms. Observations with two iridium adatoms in many different configurations on W(110) reveal behavior similar to that previously found with Re–Pd and W–Pd: interactions extending over long distances, ≥10 Å, showing a pronounced orientational anisotropy. With the iridium pair oriented along the close packed [11̄1] direction, interactions are generally attractive at close distances; along [001] and [11̄0], repulsion is the rule. However, the strength of the attractions is greater than in the heteropairs previously studied. The behavior of rhenium atoms is quite different, in that pair interactions are repulsive at short distances. However, at larger ranges, interactions become attractive and are again dependent upon the orientation of the pair on the surface. The extent to which three-atom effects are important in cohesion has been tested for both iridium and rhenium by comparing the experimentally measured dissociation energy of trimers with values of the cohesion estimated from pair interactions. Many-atom contributions turn out to play a significant role in cohesion, not just for rhenium but also for iridium trimers.
Constant temperature molecular dynamics simulations of Si(100) and Ge(100): Equilibrium structure and short-time behavior
Authors: Weakliem Paul C. , Carter Emily A.
Published: July 2002

The structures of the (100) surfaces of silicon and germanium generally have been interpreted in a static manner in the past. We present molecular dynamics (MD) simulations that show these surfaces to consist of a mixture of rapidly interconverting buckled and unbuckled dimers. Over a time average, the surface is found to have long p(2×1) rows of symmetric, unbuckled dimers, as seen in recent scanning tunneling microscopy images of silicon. However, higher order unit cells are observed in He scattering and low energy electron diffraction experiments at low temperatures. We present a dynamical interpretation of the structure to explain both sets of observations. The simulations have been performed on different size slabs at both constant energy and constant temperature utilizing a new method for effective removal of heat from an exothermic system while retaining the correct dynamics. Several different interaction potentials were analyzed in an attempt to find the most realistic one for simulations of these surfaces. The effect of surface defects and annealing were also investigated. The surface phonon densities of states were calculated and for Si(100) are in good agreement with experiments and other theoretical treatments. Such simulations and structural analyses are reported for the first time for Ge(100).
Creation, migration, and termination of conformational defects in poly(1,4-t
r
a
n
s-butadiene) in form II and in the inclusion complex in perhydrotriphenylene
Authors: Zhan Yongjian , Mattice Wayne L.
Published: July 2002

The histories of the conformational defects created by transitions between anticlinal± states, either singly or correlated as next-nearest neighbors, have been studied using molecular dynamics trajectories computed for a chain of poly(1,4-trans-butadiene) in form II and in the inclusion complex with perhydrotriphenylene. The most likely fate of a newly created conformational defect is immediate reversion to the preferred conformation. The remaining defects can undergo diffusion. The average contour lengths for the paths covered in the diffusion are on the order of ten CH–CH2 bonds for a long poly(1,4-trans-butadiene) in the inclusion complex in perhydrotriphenylene. The diffusion is terminated either by loss of the defect out the end of the chain, or by the annihilation of two defects that encounter one another. The average time between hops is much shorter for the chain in the inclusion complex than for the chain in form II. The time-averaged number of defects in methyl terminated decamers is virtually identical in the two systems. For much longer chains, the poly(1,4-trans-butadiene) in the inclusion complex should contain more defects than the chain in form II.
Translationally and vibrationally activated reaction of CO2 on Si(111)7×7
Authors: Lorraine P. W. , Thoms B. D. , Machonkin R. A. , Ho W.
Published: July 2002

The interaction of CO2 molecules with the Si(111)7×7 surface for translational energies between 0.2 and 1.6 eV and varying vibrational energies has been studied with time-resolved electron-energy-loss spectroscopy (TREELS), temperature-programed desorption, and Auger electron spectroscopy. Energy from the normal component of translational motion has been found to strongly increase the dissociation probability of CO2 on the surface. TREELS has been used to tentatively identify the resulting surface complex as O on a Si adatom with CO bonded in a bridging site to a next-layer Si rest atom. This complex decomposes at 400 K to a surface oxide and gas-phase CO. In addition, vibrational excitation has been found to increase the initial sticking coefficient for normal translational energies less than 0.5 eV.
Vibrational predissociation in argon complexes of 3-amino-s-tetrazine and 3-amino-6-methyl-s-tetrazine: Evidence for extreme mode-selectivity
Authors: Alfano Joseph C. , Martinez Selso J. , Levy Donald H.
Published: July 2002

We have investigated the vibrational predissociation of argon complexes of 3-amino-s-tetrazine (AT) and 3-amino-6-methyl-s-tetrazine (AMT). Twelve vibrational levels of complexes of the form AT–Arn, where n=1, 2, and 3, were examined. The dissociation rate of the binary AT–Ar complex varied from 3×106 s−1 to >4×108 s−1 depending on the initially excited mode, with the fastest rate corresponding to the lowest energy vibration. Even levels with vibrational energies over five times that needed to break the van der Waals bond still showed significant amounts of fluorescence from the unfragmented complex. Photodissociation of the AT–Ar2 complex can yield two different dissociation products, AT and AT–Ar. The ratio of these photoproducts varied greatly among different vibrational modes. The 16b2 level of AT–Ar2 at +462 cm−1 gives primarily the AT photoproduct, while the 16a2 level at +505 cm−1 gives exclusively the AT–Ar photoproduct. The two levels of AT–Ar3 which were studied both appear to show only partial fragmentation. We examined the basic spectroscopy of AMT using vibrationally resolved fluorescence excitation spectroscopy, and assigned a number of vibrational levels in both the ground and excited states. We then used this information to measure the branching ratios and dissociation rates for seven vibrational levels of AMT–Ar. These vibrational predissociation rates show strong qualitative differences from similar data for tetrazine–Ar and aminotetrazine–Ar complexes. Finally, these results were compared to similar work on other tetrazine derivatives to attempt to account for this strong mode-selective behavior.
The determination of the reorientational angle distribution in two-dimensional exchange nuclear magnetic resonance spectroscopy on powder samples
Authors: Grabowski D. , Honerkamp J.
Published: July 2002

The Tikhonov regularization method is applied to two-dimensional nuclear magnetic resonance (NMR) data directly in the time domain in order to estimate the reorientational angle distribution. This method is an appropriate and reliable tool to treat such ill-posed inverse problems formulated as integral equations of the first kind. The parameters needed for a complete specification of the kernel function will also be estimated from the time domain data. Results for simulated and for real data are presented.
The Floquet theory of nuclear magnetic resonance spectroscopy of single spins and dipolar coupled spin pairs in rotating solids
Authors: Schmidt Asher , Vega Shimon
Published: July 2002

The Floquet theory for the description of magic angle sample spinning (MASS) nuclear magnetic resonance (NMR) experiments is introduced. MASS NMR signals of single spin systems and homonuclear two-spin system in the presence of direct and indirect dipolar couplings are evaluated. The single spin system is utilized to develop our Floquet formalism for the MASS NMR experiments and the coupled spin system is discussed to demonstrate its methodology and its applicability. This theoretical approach enables the calculation of the positions, the amplitudes, and line shapes of MASS center and sideband. The diagonalization of the MASS Floquet matrices of the different spin systems provides eigenvalues and eigenvectors which in turn determine the frequencies and intensities of the various bands in the MASS spectra. Their frequency shifts can be explained in terms of anticrossing of Floquet states and their amplitudes can be calculated via the matrix elements of transition–amplitude Floquet operator. New Floquet operators are introduced and the commutation relations between them utilized to enable the evaluation of diagonalization matrices. The Floquet theory is applied to determine the rotational resonance conditions for homonuclear dipolar-coupled spin pairs. It is shown that significant band shifts are expected when degeneracies between Floquet states occur. These shifts endow the centerband and sidebands with powder line shapes from polycrystalline samples. Simulations of experimental line shapes can provide direct and indirect dipolar coupling constants, including their relative signs.

This approach is valid for all possible dipolar coupling strengths as well as for all sample spinning speeds and isotropic and anisotropic chemical shift parameters. The decay of zero-quantum coherences in the coupled spin-systems is also considered in terms of the Floquet approach. The sensitivity of the MASS line shapes close to the rotational resonance conditions with respect to the relative magnitudes of the chemical shift anisotropies of the spins and their direct and indirect coupling constants is examined by the calculation of various MASS spectra. To do so the interaction parameters of the different 13C nuclei in glycine and alanine are used. Experimental results of the weakly dipolar coupled spin pairs in uniformly 13C labeled alanine are also presented and compared with theoretical results.
Monofluorinated hydrogen sulfide (HFS): A definitive theoretical prediction of the infrared spectrum
Authors: Crawford T. Daniel , Burton Neil A. , Schaefer Henry F.
Published: July 2002

High-level ab initio quantum-mechanical methods have been used to study the HFS molecule. The most advanced method involves a triple-zeta plus double polarization plus f functions (TZ2P+f) basis set in conjunction with the single- and double-excitation coupled-cluster method perturbatively including connected triple excitations, i.e., CCSD(T). Two of the three fundamental vibrational frequencies are in decided disagreement with experiment.
Relationship between energy gaps and melting temperatures in various classes of materials
Authors: Aniya M.
Published: July 2002

An interesting relationship between the energy gaps and melting temperatures in a variety of classes of materials is found. The relationship suggests the existence of two different processes for the melting phenomenon which depend on the nature of the chemical bond. By using the bond orbital theory, it is shown that the relationship is correlated with the total energy of the compounds considered. A brief discussion on the melting of superionic conductors is also given.