bluuebunny/crossref_metadata_2025 huggingface dataset:
Free energy in the statistical theory of fluids
Authors: Kiselyov O. E. , Martynov G. A.
Published: July 2002

Explicit formulas for the excess chemical potential and Helmholtz free energy have been derived in different ways making use of a functional integration technique. The method employed is an extension of the Kirkwood ‘‘charging’’ procedure and involves a new way of renormalizing cluster series. The formulas require only the correlation functions for the system under investigation. New and old ways of calculating the free energy of a simple fluid are compared and discussed. Numerical results for the various closures are presented for hard and Lennard-Jones sphere fluids. A bridge functional of the first kind has been introduced and some approximations for this have been proposed and tested.
Brownian dynamics study of surface adsorption of a linear polymer in different regimes
Authors: Bishop Marvin , Clarke Julian H. R.
Published: July 2002

Brownian dynamics computer simulations are used to investigate the properties of isolated, terminally attached polymers interacting with a surface. Three regimes of the polymer chains are studied: excluded volume, θ, and self-collapsed. It is found that chains are progressively adsorbed to the surface as the surface attraction is increased. The mean-square radius of gyration power law exponent for excluded volume chains changes from 1.2 (the three-dimensional excluded volume value) to 1.5 (the two-dimensional excluded volume value) when the chains are completely adsorbed and the exponent for the θ chains changes from 1.0 (the three-dimensional θ value) to 1.5 in agreement with previous lattice simulations. The collapsed chain exponent remains at 2/3 throughout the transition. The density profile as a function of the distance from the surface, the asphericity, and computer graphics snapshots of typical configurations reveal more details about the polymer structure. The self-collapsed chain forms a multilayered structure on the surface.
Level crossing in liquids involving intermolecular electronic to vibrational energy transfer
Authors: Paige M. E. , Harris C. B.
Published: July 2002

The A′/A state lifetime of I2 in various deuterated hydrocarbon solvents has been determined from picosecond transient absorption spectra. The lifetime observed in the deuterated solvents is 2.0 times as long as that which has been measured in the corresponding undeuterated solvent. A mechanism involving intermolecular electronic to vibration energy transfer from the electronically excited I2 to the solvent is proposed to explain this effect and the overall trend of shorter lifetime with increasing density of CH stretching modes.
A density functional theory of polymer phase transitions and interfaces. II. Block copolymers
Authors: McMullen William E. , Freed Karl F.
Published: July 2002

We extend our previous density functional theory of homopolymers to block copolymers. The constraints on the relative number densities of the different types of monomers comprising the block copolymers alter the ideal free energy compared to that of homopolymers and of polymer blends. As in our previous work, the second-order functional derivatives of the nonideal free energy with respect to monomer densities are simply related to monomer–monomer direct correlation functions. When applied to incompressible diblock copolymers, this formalism reduces to quasi-one-component form and reproduces the Landau theory of near symmetric diblock copolymers. For homogeneous liquids, we recover the Flory–Huggins ideal free energy of mixing of block copolymers. The present theory, however, permits the treatment of compressible systems and therefore of more strongly first-order microphase separations. It also provides a rigorous formulation for developing improved density functional models for block copolymer systems.
Amplitude method for multichannel resonances
Authors: Sun Y. , Du M. L. , Dalgarno A.
Published: July 2002

An amplitude method is described for determining the partial and total widths and the energies of resonances in multichannel processes in which resonance states may decay into many channels. The method is tested by application to a model problem and by a study of the predissociation of the C 3Πg Rydberg state of O2.
Integral equation theory of ionic solutions
Authors: Ichiye Toshiko , Haymet A. D. J.
Published: July 2002

A new closure to the Ornstein–Zernike equation is proposed for ionic liquids such as electrolytes. The closure is investigated numerically for a model electrolyte consisting of charged soft spheres in a uniform dielectric medium. The new closure, which we call the ionic Percus–Yevick (IPY) closure, may be viewed as a prescription for the so-called ‘‘bridge function,’’ which is approximated by zero in the well-known hypernetted-chain (HNC) closure. Compared to the results of Monte Carlo simulations, the pair correlation functions predicted from this new closure for 2:2 electrolytes at low concentrations are in much better agreement than those predicted by the HNC closure. In particular, whereas the HNC approximation predicts incorrectly a peak in the pair correlation functions for like charges at an interionic separation of about two diameters at low concentrations, the new IPY closure predicts correctly no such peak. At higher concentrations, both the HNC and IPY closures yield correlation functions which are close to those calculated from Monte Carlo simulations, with the new IPY closure being more accurate for unlike charges and the HNC closure being slightly more accurate for like charges.
Kinetics and spectroscopy of electronically excited BiO obtained by energy transfer from metastable O2. I. Dependence of excitation on [O2]
Authors: Mayo I. , Bachar J. , Rosenwaks S. , Martin F. , Bacis R. , Verges J.
Published: July 2002

Excitation of electronic states of the BiO molecule in a mixture of Bi vapor and O2(1Δ,1Σ) was monitored as a function of the density of the O2 singlet states. Emission in the 300–900 nm wavelength region was studied under low resolution and assigned to the A(2Π1/2), B(4Σ) and C(2Δ3/2) states of BiO. The intensity of the emission from the A(v≥14), B and C states increased while that from A(v≤12) decreased with increasing O2(1Δ) density suggesting that the excitation mechanism for these higher levels involves multiple collisions with O2(1Δ). Emission from BiO C and from several A,v′ and B,v′ states has been observed here for the first time. In addition, emission in the 1400–1600 nm region was observed and studied for the reaction of Bi and low density of O2(1Δ) via Fourier transform spectroscopy and is tentatively assigned to previously unobserved low lying 1/2 and 3/2 states of BiO. It is suggested that these states may play a crucial role in the excitation mechanism of the higher states.
High frequency viscous relaxation from the Brillouin spectra of n-pentanol
Authors: Di Fabrizio E. , Mazzacurati V. , Nardone M. , Nucara A. , Ruocco G. , Signorelli G.
Published: July 2002

Brillouin spectral line shapes of n-pentanol have been measured from −75.5 °C (close to the melting point) up to 115 °C (close to the boiling point) using a grating monochromator. The instrument, owing to its very high resolution, contrast, and free spectral range, has been preferred to the usually employed Fabry–Perot interferometer. The spectra, fitted with a single relaxation generalized hydrodynamic theory, support the existence of a high frequency relaxation process with an activation energy of about 9 KJ/mol. This relaxation process, which should be related to isomeric rotational relaxation, is well separated at low temperature from the structural relaxation observed at lower frequencies in ultrasonic experiments.
A new formula for determining differential cross sections in the vibronic semiclassical description of atom–molecule collisions
Authors: Grimbert D. , Sidis V. , Sizun M.
Published: July 2002

A simple approximation applied to the eikonal expression of the scattering amplitude provides a new formula for the determination of differential cross sections for atom–molecule collisions described within the vibronic semiclassical framework. The derived formula achieves considerable savings in the corresponding numerical work and extends the well-known expansion over Legendre polynomials to nonspherical interactions. Results obtained with the new formula on vibrational excitation and vibronic charge exchange in the H++O2 collision at Ecm=23 eV are found to be in excellent agreement with the calculations of Sizun et al. (J. Phys. Chem. Lett.).
Calculation of hyperfine coupling constant by symmetry adapted cluster expansion configuration interaction theory. II. Anisotropic constants
Authors: Momose Takamasa , Yamaguchi Makoto , Shida Tadamasa
Published: July 2002

Following the previous work on the isotropic hyperfine coupling constants (HFCCs) of polyatomic radicals the symmetry adapted cluster expansion–configuration interaction (SAC–CI) theory is applied to calculate anisotropic HFCCs also. The results are compared with available experimental data from diatomic to polyatomic radicals such as the vinoxy. For radicals consisting of only the first row atoms Dunning’s double zeta (DZ) basis set is shown to be adequate, but for those containing the second row atoms inclusion of polarization functions is required. Compared with the isotropic HFCC the calculation of the anisotropic HFCC is less formidable. However, ignorance of electron correlation causes serious disagreements with experimental data.
Decomposition mechanisms of SiHx species on Si(100)-(2×1) for x=2, 3, and 4
Authors: Gates S. M. , Greenlief C. M. , Beach D. B.
Published: July 2002

Silane adsorption at a surface temperature of 150 K and the surface decomposition of SiH3 and SiH2 have been investigated on the Si(100)-(2×1) surface using static secondary ion mass spectrometry (SSIMS) and temperature programmed desorption (TPD). Silane dissociatively chemisorbs at 150 K to form SiH3 and H. At saturation, the combined coverage of these two is approximately 0.4 groups/1st layer Si atom (0.2 SiH4 adsorbed/1st layer Si atom). Using SiH4, the surface coverage of SiH3 species is varied, and the coverage-dependant kinetics of SiH3 decomposition are examined using temperature programmed SSIMS. Changes in SiH4 exposure and source of SiH3 (di- vs monosilane) cause changes in surface SiH3 stability. The stability changes are interpreted as due to blocking of empty sites (dangling bonds, db) required for SiH3 decomposition to SiH2 and H. It is shown here that the decomposition temperature of SiH3 can vary from 200 to 600 K, depending on the dangling bond coverage (θdb). Subsequently, evidence for a coverage dependant change in the decomposition mechanism of SiH2 is presented. Two mechanisms for SiH2 decomposition are experimentally distinguished, and competition of these is governed by θdb. We conclude that both the mechanism and rate of decomposition of surface SiH3 and SiH2 are dependant on the local surface environment near these species.
Collision-induced double resonance studies of HN+2 and HCN
Authors: Pursell C. J. , Weliky D. P. , Oka T.
Published: July 2002

We have investigated collision-induced rotational transitions of HN+2 and HCN using infrared-microwave four-level double resonance spectroscopy. These two isoelectronic molecules were studied in collisions with He, Ar, and N2. For all cases studied, we have observed that the collision-induced rotational transitions exhibit collisional ‘‘selection rules.’’ The selection rules can be explained using the symmetry properties (i.e., parity) of the dominant terms in the interaction potential. This represents the first observation of selection rules for rotational energy transfer of a molecular ion. This study has allowed us to directly compare the difference between ion–neutral and neutral–neutral collisions which cause rotational transitions. We have experimentally observed that ion–neutral and neutral–neutral collisions differ because of the presence of the Langevin force in the ion–neutral interaction potential, which has two unique effects. The Langevin force produces a charge-induced dipole in the collision partner which is parallel to the ion’s electric field. This charge-induced dipole interacts with the electric charge of the molecular ion which creates an attractive force between the ion and neutral. This interaction therefore decreases the ion–neutral distance and produces strong collisions which randomizes the rotational states. The second effect occurs when the molecular ion has a permanent electric moment. The charge-induced dipole in the collision partner will interact with an electric moment of the molecular ion creating a long-range interaction. For HN+2, a molecular ion with a permanent dipole moment, this interaction produces ‘‘dipole-type’’ collisional selection rules.
Fourier transform infrared spectrum of well ordered solid dinitrogen tetroxide
Authors: Givan A. , Loewenschuss A.
Published: July 2002

The full infrared spectrum of well ordered N2O4(D2h) thin films is reported. Improved sample crystallinity is reflected in a fine structure of the internal mode bands that is less complicated than previously reported, and in a well resolved phonon spectrum. New assignments, notably for the v10(B2u) internal deformation mode, are suggested and the symmetry classification of the phonon bands is substantiated.
Thermochemistry of the gaseous thorium bromides
Authors: Hildenbrand D. L. , Lau K. H.
Published: July 2002

The sublimation of ThBr4(s) was studied by the torsion-effusion and mass spectrometric methods in the range 600–750 K; only monomeric ThBr4 vapor was observed, and the pertinent thermodynamic parameters were derived. The gaseous lower-valent species ThBr3, ThBr2, and ThBr were identified in the bromination of Th(s) at 1500 to 2000 K, and several reaction equilibria involving these species were studied by mass spectrometry over wide temperature ranges for second law analysis. Reaction enthalpies, bond dissociation energies, and standard enthalpies of formation were derived from the results. Unlike ThF4 and ThCl4, the sublimation entropy indicates that ThBr4 may have a distorted tetrahedral structure. Bond dissociation energies and related thermodynamic properties are discussed and compared with values in other thorium and uranium halides.
Energy dependence of the relaxation of highly excited NO2 donors under single collision conditions: Vibrational and rotational state dependence and translational recoil of CO2 quencher molecules
Authors: Chou James Z. , Flynn George W.
Published: July 2002

The rotational state distribution and velocity recoil of CO2 bath molecules excited to the asymmetric stretch (ν3=2349 cm−1) vibrational level by collisions with high energy NO2 molecules have been determined. The vibrational excitation process occurs with very little rotational or translational excitation of the CO2 and its probability is independent of NO2 internal energy over the range 22 200–16 600 cm−1.
The upper ionization potentials of F−, LiF−2, BeF−3, BO−2, AlO−2, and NO−3 ions calculated by Green’s function method
Authors: Zakrzewski V. G. , Boldyrev A. I.
Published: July 2002

Upper ionization potentials of F−, LiF−2, BeF−3, BO−2, AlO−2, and NO−3 ions were calculated by Green’s functions [outer valence Green’s function (OVGF) and algebraic diagrammatic construction, ADC(3)] methods. It was shown that electron relaxation and electron corrections were important for calculations of ionization potentials of superhalogen anions and electron affinity of superhalogen radicals.
Multiple transition states in chemical reactions. II. The effect of angular momentum in variational studies of HO2 and HeH+2 systems
Authors: Song Kihyung , Chesnavich Walter J.
Published: July 2002

The original Wardlaw–Marcus method for the calculation of N(E,K,R) was applied to the HO2 and HeH+2 systems. The angular momentum averaged N(E,R) curves obtained by integrating N(E,K,R) over K using two different limits for K are performed. The results with Kmax calculated along the reaction coordinate show an excellent agreement with the results from the direct calculation of N(E,R). The integration over K was also performed using the minimum of Kmax(K‡) value along the reaction coordinate, which is consistent with the fact that the system with higher K value cannot react due to the centrifugal barrier. This method produced considerably low N(E,R) curves at extremely large and small R values. However, all three results were in good agreement in the transition state region. The individual N(E,K,R) curves support the behavior of the N(E,R) curves. It was pointed out that the shape of the high-K N(E,K,R) curves plays an important role in the final transition states of N(E,R). From these results, it was suggested that the modified method is more adequate to the bimolecular association reactions than the unimolecular decomposition reactions.
Asymmetric-to-centrosymmetric structure change of molecules in squaric acid crystal: Evidence for pressure-induced change of correlated proton potentials
Authors: Moritomo Y. , Koshihara S. , Tokura Y.
Published: July 2002

High-pressure effects on vibrational modes have been investigated in crystals of squaric acid H2C4O4 (H2SQ) and its deuterated analog (D2SQ) which show two-dimensional networks of hydrogen bonds. Specific vibrational modes in Raman and infrared spectra were observed to anomalously disappear with the application of pressures above 30 kbar in H2SQ, but not in D2SQ, indicating that the H2SQ molecule in crystal shows the asymmetric (C1h)-to-centrosymmetric (C4h) structure change. This is possibly caused by pressure-induced deformation of the correlated proton potential essentially from the double-well to single-well type.
Comment on: Resonance structure in the energy dependence of state-to-state differential scattering cross sections for the D+H2(v,j)→HD(v′,j′)+H reaction
Authors: Continetti Robert E. , Zhang John Z. H. , Miller William H.
Published: July 2002

During quantum reactive scattering calculations for the title reaction a pronounced resonance structure became apparent in the energy dependence of state−to−state differentialscattering calculations. This resonance structure is explained.(AIP)
High-pressure nuclear-magnetic-resonance study of carbon-13 relaxation in 2-ethylhexyl benzoate and 2-ethylhexyl cyclohexanecarboxylate
Authors: Adamy S. T. , Grandinetti P. J. , Masuda Y. , Campbell D. , Jonas J.
Published: July 2002

Natural abundance carbon-13 spin-lattice relaxation times and 13G–1H nuclear Overhauser enhancement (NOE) times of 2-ethyl hexylbenzoate (EHB) and 2-ethyl hexylcyclohexanecarboxylate (EHC) have been measured along isotherms of −20, 0, 20, 40, and 80 °C at pressures of 1–5000 bars using high-pressure, high-resolution NMR techniques. The ability to use pressure as an experimental variable has allowed us to study a wide range of molecular motions from extreme narrowing into the slow motional regime. In addition, the high-resolution capability even at high pressure permits the measurement of 13C and NOE for each individual carbon in the molecules studied. Relaxation in both molecules is successfully analyzed in terms of a model assuming a Cole–Davidson distribution of correlation times. The comparison of parameters used in the model demonstrates the increased flexibility of the EHC ring over the EHB ring and also shows how the presence of the flexible ring contributes to the increased over-all mobility of the EHC molecule. The analysis of molecular reorientations in terms of activation volumes also indicates that EHB motion is highly restricted at low temperature.
Interaction of silicon cluster ions with ammonia: Annealing, equilibria, high temperature kinetics, and saturation studies
Authors: Jarrold Martin F. , Ijiri Yumi , Ray Urmi
Published: July 2002

The results of extensive studies of the chemical reactions of size selected silicon cluster ions (containing up to 70 atoms) with ammonia are described. At room temperature all clusters react at close to the collision rate and collisional annealing of the clusters does not influence their reactivity. At temperatures slightly above room temperature (∼400 K) it is possible to establish an equilibrium. Binding energies of ammonia to the silicon clusters of ∼1 eV were determined from measurements of the equilibrium constants as a function of temperature. These small binding energies indicate that molecular adsorption occurs at close to room temperature. Saturation experiments reveal that ammonia only binds molecularly to a small number of sites on the clusters. In contrast, on bulk silicon surfaces at room temperature, rapid dissociative chemisorption occurs until all the surface dangling bonds are saturated. At temperatures above ∼470 K another process, probably dissociative chemisorption, becomes important. Absolute rate constants were measured for clusters with 30–70 atoms at a temperature of 700 K where the dissociative chemisorption process dominates. The sticking probabilities at this temperature are between 10−3 and 10−5, two to four orders of magnitude smaller than on bulk silicon at 700 K.
Spin-polarized density functional calculations based on local scaling transformations
Authors: Koga Toshikatsu , Kryachko Eugene S.
Published: July 2002

The recently proposed method for practical density functional calculations is extended to the spin-decomposed electron densities. From given α- and β-spin electron densities ρα(r) and ρβ(r), local scaling transformations are used to generate their possible parent wave function Ψρ({xi}), where xi=(ri,σi) is the combined position-spin coordinate of electron i. The energy E[ρα,ρβ] associated with the given spin-decomposed densities is then defined as the Hamiltonian expectation value over the generated wave function Ψρ({xi}), and the densities ρα and ρβ are determined so as to minimize the energy functional E[ρα,ρβ]. An illustrative application is given for the ground-state Li atom within the unrestricted Hartree–Fock framework.
The analysis of muonium hyperfine interaction measurements of thermal rate constants for addition reactions
Authors: Duchovic Ronald J. , Wagner Albert F. , Turner Ralph Eric , Garner David M. , Fleming Donald G.
Published: July 2002

A new model is developed for the analysis of muon spin relaxation (μSR) measurements of muonium (Mu) reaction rates with other chemical species. The kinetics component of the model is specifically developed for addition reactions but can be extended to direct reactions. The polarization component of the model assumes that the adduct formed possesses a single, effective spin precession frequency. The complete model is solved to obtain an analytic expression for the μSR signal as a function of time. Both a time-ordered sequence method and a Boltzmann spin equation method give equivalent solutions. These solutions are analyzed under various kinetic conditions. It is concluded that the coefficient controlling the decay of the μSR signal is closely related to the adduct formation rate constant, i.e., the high pressure limit of the apparent bimolecular addition rate constant. In the most favorable case, the decay constant gives the adduct formation rate constant directly at all pressures of buffer gas.
Predissociation of the c 1Π state of NH (ND): The role of dipolar spin–spin coupling
Authors: Parlant Gérard , Dagdigian Paul J. , Yarkony David R.
Published: July 2002

The lifetimes of the NH(c1 Π, v′=0,1) and ND(c 1Π, v′=0) levels are considered. Both radiative c 1Π→(a 1Δ, b 1Σ+) decay and radiationless decay are investigated. It is shown that a direct coupling of the c1Π state to the dissociative 15Σ− state, of magnitude approximately 1 cm−1, explains the known predissociation of the NH (c 1Π, v′=1) level. In this case the theory predicts that in the v′=0 manifold of NH even the low J′ rotational levels will also be predissociated. Predissociation of the v′=0 level in NH was not previously anticipated. The origin of the c 1Π∼1 5Σ− perturbation is considered. It is shown that this coupling originates principally from the dipolar spin–spin interaction, with a smaller contribution from second order spin–orbit coupling. Because the predissociation results from quantum mechanical tunneling, it is of reduced importance in ND.
Benchmark calculations of thermal reaction rates. I. Quantal scattering theory
Authors: Chatfield David C. , Truhlar Donald G. , Schwenke David W.
Published: July 2002

The thermal rate coefficient for the prototype reaction H+H2→H2+H with zero total angular momentum is calculated by summing, averaging, and numerically integrating state-to-state reaction probabilities calculated by time-independent quantum-mechanical scattering theory. The results are very carefully converged with respect to all numerical parameters in order to provide high-precision benchmark results for confirming the accuracy of new methods and testing their efficiency.
Determination of the equation of state and heat capacity of argon at high pressures from speed-of-sound data
Authors: ten Seldam C. A. , Biswas S. N.
Published: July 2002

A method is developed for the simultaneous determination of the equation of state and heat capacity of a gas over a wide range of pressure and temperature. This is based on stepwise construction of pT isochores starting from one experimental reference pV isotherm and using speed-of-sound data of the gas as a function of pressure and temperature. The application of the method is illustrated by computing the density and heat capacity of argon at temperatures from 148.15 to 298.15 K and at pressures from 100 to 1000 MPa. In the region of overlap, the calculated densities are found to be in good agreement with those obtained previously from direct measurements.
Amplified spontaneous emission over the XeF(D→X) transition in solid Kr
Authors: Kunttu H. , Lawrence W. G. , Apkarian V. A.
Published: July 2002

XeF doped solid Kr represents a novel solid state exciplex laser. A net gain in excess of 100 cm−1 and a superradiant beam of 5 mrad divergence are observed over the XeF(D→X) transition at 301 nm, when free standing crystals of XeF doped Kr are optically pumped near the XeF(D←X) absorption maximum at 260 nm. Superlinear dependence of gain on pump intensity and low divergence of the amplified beam are taken as evidence for self-focusing. The XeF(D) state is effectively isolated from the lower manifold of charge transfer states in solid Kr, which includes XeF(B,C) and (KrXe)+F−.
A theory of He diffraction and resonance scattering from Cu(115) by the complex coordinate method
Authors: Engdahl Erik , Moiseyev Nimrod , Maniv Tsofar
Published: July 2002

Our method for computing specular elastic scattering intensities of atomic beams from a periodically corrugated solid surface, is generalized to include nonspecular diffraction intensities, and is applied to the scattering of a helium beam from the Cu(115) surface for resonance as well as off-resonance incident angles. Using a simple corrugated Morse potential with a one-dimensional corrugation function and reasonable values of its parameters, a remarkable agreement with the experimental resonance line shapes over the entire angular range is obtained for both the specular and a nonspecular diffraction channel. The line shapes are found to be extremely sensitive to the corrugation strength parameter, but quite insensitive to the shape of the surface corrugation.
Peroxy and cyclic isomers of NO2 and NO−2
Authors: Meredith Cynthia , Davy Randall D. , Quelch Geoffrey E. , Schaefer Henry F.
Published: July 2002

Ab initio molecular electronic structure methods have been used, in conjunction with large basis sets, to investigate the possible existence of isomers of the conventional C2v open-chain NO2 and its anion, NO−2. Equilibrium geometries have been optimized and harmonic vibrational frequencies obtained for both the ‘‘peroxy’’ and cyclic isomers of these species at the self-consistent-field (SCF), single and double excitation configuration interaction (CISD), coupled cluster including all single and double substitutions (CCSD), and complete active space (CAS) SCF levels of theory utilizing both double-zeta plus polarization (DZ+P) and quadruple-zeta plus double polarization (QZ+2P) basis sets. For comparison, ‘‘normal’’ (C2v open-chain) NO−2 was also studied. Diffuse functions were added to these basis sets to provide a better description of the electron densities of the anionic isomers. The four peroxy and cyclic structures investigated were predicted to be relatively high-lying (≥60 kcal mol−1) minima at most levels of theory considered. Moreover, interconversion between the C2v and Cs open-chain forms appears unlikely both for NO2 and NO−2.
Hopf bifurcation in chemical kinetics
Authors: Nielsen K. , Hynne F. , So/rensen P. Graae
Published: July 2002

We study Hopf bifurcations in chemical reaction systems for their potential use in quantitative experimental analysis of the kinetics of oscillatory reactions. Three models of the Belousov–Zhabotinsky reaction are investigated as examples. For these we have determined and characterized the sub- and supercritical Hopf bifurcations in their dependence on parameters of the models. For supercritical bifurcations we calculate a number of parameters that can be used for quantitative comparison of models and experiment. In particular, we calculate expansion coefficients of the flow rate, the frequency of oscillation, and a Floquet exponent; the small parameter of the expansions is the square of the amplitude of the fundamental Fourier component of the oscillations. We also calculate quenching amplitudes from an adjoint eigenvector of the Jacobi matrix of the kinetics. They determine the conditions under which the small amplitude oscillations can be quenched by addition of the species participating in the reaction. The model properties are compared with experiment. They show qualitative, but not quantitative agreement.
The ultraviolet photoelectron spectrum of SO−
Authors: Polak Mark L. , Fiala Brenda L. , Ervin Kent M. , Lineberger W. C.
Published: July 2002

The ultraviolet photoelectron spectrum (hν=3.531 eV) of SO− shows transitions to the three lowest lying electronic states of SO neutral. The electron affinity of SO was determined to be 1.125(5) eV. The harmonic vibrational frequency and the spin-orbit splitting of the SO− electronic ground state (2Π) were also determined. Franck–Condon analyses of the SO vibrational progressions were used in conjunction with the molecular constants of the well-characterized SO neutral molecule to determine the equilibrium bond length of SO− [re=1.570(5) Å].
Density, structural lifetime, and entropy of H-bond cages promoted by monohydric alcohols in normal and supercooled water
Authors: Bulone D. , Donato I. D. , Palma-Vittorelli M. B. , Palma M. U.
Published: July 2002

Density data for aqueous solutions of monohydric alcohols down to supercooling are presented and combined with data concerning viscosity of the same systems, and with available data on pure water. Two conceptually different families of longer-lived, high-connectivity H-bond structures can be sorted out in the irregular, frequently restructured network of H bonds: spontaneous structures, as in pure water, and alcohol-induced structures. Molar volumes for both are obtained and compared, allowing microscopic conclusions which agree quantitatively with available thermodynamic data. For the three alcohols studied, the well-known negative excess entropy of mixing shows a strict proportionality to the fraction of water molecules sorted out in our study as taking statistically part in alcohol-promoted cages. The remarkable proportionality extends to all alcohols, all concentrations, and all temperatures studied. Apparent (and expected) geometric distortions of alcohol-promoted cages do not affect this proportionality. However, they can be related to disturbing effects on the singular behavior of several properties of cold and supercooled water. These results are further combined with the only available quantitative data on the modulation by alcohols of the hydrophobic contribution to the functional conformational switching of a biomolecule. This allows for the first time deriving, from experiments on a protein solution, the statistical number of water molecules and associate entropy change directly involved in a specific protein function (oxygen uptake/release by human hemoglobin HbA in the specific case). Compared to the bare protein, this functional unit is largely stabilized as a consequence of the remarkably higher dimensionality in its phase space.
The effects of reactant vibrational, fine structure, and collision energy on the reactions of OCS+ with C2H2: Complementary studies of reactions in the [C2H2+OCS]+ system
Authors: Yang Baorui , Chiu Yu-hui , Anderson Scott L.
Published: July 2002

Resonance-enhanced multiphoton ionization has been used to create state-selected OCS+ ions, which are then reacted with C2H2 in a guided-beam tandem mass spectrometer. OCS+ can be produced with excitation in all three of its vibrational modes, in either the upper or lower fine structure electronic state. Absolute cross sections for all product channels (C2H+2, C2HnS+(n=1, 2), and S+) are reported as a function of collision energy and vibrational state in the range from 0.06–4.5 eV. Different modes of nuclear motion have markedly different effects on reactivity and branching ratios. Production of C2H2S+, is the major chemical reaction channel, and its formation is strongly inhibited by collision energy, but only weakly affected by vibrational and fine structure state. The cross section for charge transfer (CT) shows vibrational effects that change with collision energy. For collision energies below 0.3 eV, CT is enhanced by all forms of nuclear motion, while at higher energies CT is weakly enhanced by C–S stretching, strongly enhanced by C–O stretching, and inhibited by bending. Both C2HS+ and S+ are minor channels, which turn on at higher collision energies. They are weakly affected by vibrational energy and fine structure state. These results are compared with those from our complementary study [T. M. Orlando, B. Yang, Y. Chiu, and S. L. Anderson, J. Chem. Phys. 92 7356 (1990)] of the other charge state of the [C2H2+OCS]+ system: reactions of C2H+2 with OCS. This allows comparison of the effects of 12 different reactant internal energy states on the same product channels.
Theoretical study of hyperfine coupling constants in ethyl radical
Authors: Chipman Daniel M.
Published: July 2002

Spin densities that determine hyperfine splitting constants are calculated for ethyl radical from ab initio electronic wave functions. The most important direct and spin polarization contributions are obtained from single-excitation configuration interaction wave functions, in conjunction with recently developed contracted Gaussian basis sets designed specifically for spin density determination. Facile out-of-plane bending at the α-carbon center leads to a significant vibrational correction and temperature dependence for the α-carbon splitting. Coupling of torsion about the CC bond with bending at the α-carbon is found to have only a small effect on the hyperfine constants. For isotropic Fermi contact interactions, agreement with experiment is better than 10% for both carbons and for the α-hydrogens. The larger 28% error found for the β-hydrogens is attributed primarily to the missing effects of electron correlation. Anisotropic dipolar hyperfine constants are also evaluated and agree well with experiment wherever comparison is possible.
Onsager theorem and relation between solvation dynamics and electron transfer kinetics
Authors: Helman A. B. , Keyes T.
Published: July 2002

A direct analytical relation between solvation dynamics and the electron transfer reaction rate is established. Both the time dependence of solvation dynamics and the reaction rate constant are experimentally measurable. If the effects of spatial dispersion of the solvent are not essential then one can expect that solvation dynamics is universal. The favored condition for the observation of the relation between the two phenomena is discussed.
The structure of a model ionic melt in a planar slit
Authors: Bratko D. , Blum L. , Bhuiyan L. B.
Published: July 2002

The structure of the primitive model fused salt in planar slit is studied by Monte Carlo simulation. The density profile of the ionic melt in the slit with respect to the walls displays oscillations similar to those observed with simple liquids. At the pore width of about seven ionic diameters, the pressure exerted on the walls is comparable to the virial pressure of the bulk melt. The calculated surface energy of 210+20 mN m−1 is close to the experimental value for molten potassium chloride at similar conditions.
Curvature elasticity of diblock copolymer monolayers
Authors: Wang Zhen-Gang , Safran S. A.
Published: July 2002

We study the curvature elastic properties of monolayers of diblock copolymers adsorbed at the interface of two incompatible solvents which are also selective solvents for the two blocks. At saturation, the interfacial free energy is minimized with respect to contributions from the chain conformation free energy, the interfacial tension, and the two-dimensional translational entropy of the chains. For a curved interface, this minimization leads naturally to curvature elasticity. The three elastic coefficients, the spontaneous radius of curvature, the bending modulus, and the Gaussian bending modulus, as functions of the molecular weights, the interfacial tension, the interaction parameters, etc., are obtained for a number of cases. Our study employs the theory for grafted chains recently developed by Milner et al. to obtain the chain conformation free energy which takes into account the nonuniformity of the chain-end distribution. This improvement not only affects the overall prefactor of the free energy but it changes the relative values of the three elastic coefficients as well. We consider the cases of both a swollen monolayer and a monolayer consisting of a melt of copolymer chains, as well as an interesting case where one of the blocks is in the swollen condition and the other block is in the melt condition. Because the chains in the melt and the swollen conditions have distinctively different scaling behaviors, the mixed case displays some features that are different from either the swollen and the melt cases.
Gaussian basis set techniques for calculating electronic resonance positions and widths: Analytic continuation and complex stabilization methods
Authors: Isaacson Alan D.
Published: July 2002

Several variational methods are applied to the calculation of the position and width of the lowest 1S resonance state of H−, which is the simplest physical example of an electronic Feshbach resonance. These methods include two different versions of the analytic continuation of stabilization graphs that enforce the correct branch-point structure and two versions of the complex-stabilization approach, one that stabilizes the complex resonance energy with respect to the exponents of the complex orbital(s) and one that stabilizes it with respect to both the real and complex orbital exponents. The calculations involve medium-, large-, and very-large-sized basis sets of Gaussian orbitals and full configuration interaction (CI). The use of the same basis sets with the various methods allows for detailed comparisons among them. Although the sensitivity of the results to the fit parameters prevents true convergence, reliable estimates of the position and width of this resonance (about four-figure accuracy in the position and two-figure accuracy in the width) are obtained both from a version of the analytic continuation of stabilization graphs that employs one eigenvalue of a real, Hermitian Hamiltonian matrix but enforces the correct branch-point structure and from a complex-stabilization approach that involves complex basis functions and a non-Hermitian Hamiltonian matrix. In the former approach, we find that the results are less accurate when two eigenvalues of the Hamiltonian matrix are employed in the analytic continuation, possibly due to interactions with excited resonance states. For the latter approach, we show that good results can be obtained with basis sets containing a single complex orbital if the resonance energy is also stabilized with respect to an analytic continuation of the real orbital exponents, but that there is no advantage in using two complex orbitals with close exponents.
Calculation of the nuclear spin–spin coupling constants. VI. Many-body perturbation theoretic calculation of electron correlation effect
Authors: Fukui H. , Miura K. , Matsuda H.
Published: July 2002

The Fermi-contact contributions to the nuclear spin–spin coupling constants are computed for four first-row hydrides, i.e., HF, H2O, NH3, and CH4 using the finite perturbation many-body perturbation theory (FP-MBPT) through the second order in the electron correlation correction. The calculated values of JXH (X=O,N,C) yield reasonable results, but the calculation for JFH and the geminal JHH (in H2O, NH3, and CH4) presents too large values.
Ultrafast absorption spectroscopy of photodissociated CF2Br2: Details of the reaction mechanism and evidence for anomalously slow intramolecular vibrational redistribution within the CF2Br intermediate
Authors: Gosnell T. R. , Taylor A. J. , Lyman J. L.
Published: July 2002

Ultrafast time-resolved absorption spectroscopy in the hard ultraviolet has been used to investigate the photodissociation of gas-phase CF2Br2 photolyzed at 248 nm. The broadband spectra obtained in the 250–265 nm region have shown that absorption of a single photon activates a two-step sequential elimination of the molecule’s two bromine atoms, leaving the product CF2 radical in the ground or first-excited vibrational state of its ν2 bending mode. The spectra also demonstrate the direct detection of the vibrationally hot CF2Br intermediate species itself. We interpret the ∼6 ps time scale over which the diffuse CF2Br spectrum evolves as evidence for slow intramolecular vibrational redistribution within this molecule.
Theoretical study of the excited states of the Kr*2 excimer: Potential curves including a
b i
n
i
t
i
o spin–orbit coupling
Authors: Audouard E. , Spiegelmann F.
Published: July 2002

The potential energy curves for all the adiabatic Ω states dissociating into Kr+Kr, Kr+Kr*(5s,5s′), Kr+Kr*(5p), and some higher states are calculated using multireference perturbative CI algorithms (CIPSI/CIPSO) with relativistic averaged and spin–orbit core pseudopotentials. A partially diabatic representation of the potential curves rationalizes the complex features of the adiabatic states induced by numerous avoided crossings. A systematic comparison is carried out with extensive experimental data, including absorption from the ground state, fluorescence data, and transient absorption spectroscopy.
Orbiting in inelastic scattering
Authors: Sisak Michael , Secrest Don
Published: July 2002

An investigation is made of several manifestations of the orbiting of two scattering partners. One focus is the effect on the l-average coupled states approximation and its first-order perturbational improvement, the l-average recoupled states approximation. Both methods are shown to fail completely when applied to m-dependent and degeneracy averaged differential and integral cross sections for inelastic collisions of an atom and a rigid rotor when there is significant orbiting behavior. A series of calculations is made for the He–SiO system scattering at 2.25, 9.01, 18.0, and 27.0 meV. The gradual improvement of both approximations with energy is clearly demonstrated. Additionally, close coupling differential and integral cross sections are shown to be extremely sensitive to slight energy changes in the orbiting regime. Moreover, single energy results in this regime are shown to differ significantly from results averaged over a molecular beam’s energy distribution.
Temperature dependence of chlorine adsorption on argon surfaces in the zero coverage limit: A molecular dynamics study
Authors: Trayanov A. L. , Prisant M. G.
Published: July 2002

Molecular dynamics calculations have been carried out on the adsorption of chlorine on an Ar(111) surface. The calculations have been performed at the zero coverage limit for a single chlorine molecule impinging on the surface. The dependence of adsorption dynamics on the temperature of the argon solid has been investigated. At 15 K, no diffusional motion of the halogen is seen after it lands on the surface: it remains fully exposed on the surface. Motion of the halogen consists of an up and down vibrational motion as well as a hindered rotational motion. At 60 K, the halogen executes diffusional motion on the surface after landing: it is neither trapped during the 50 ps time period of our simulations nor does it escape the surface. At 70 K, a temperature close to the melting point of the solid, vacancies are created in the upper most level of the solid. These vacancies and initial diffusional motion on the surface permit inclusion of the halogen adsorbate into the first layer of the argon.
Electron spin resonance of the boron trimer
Authors: Hamrick Y. M. , Van Zee R. J. , Weltner W.
Published: July 2002

The 11B3 molecule, observed in neon, argon, and krypton matrices at 4 K has D3h symmetry and a 2A′1 ground state. the isotropic hyperfine tensor indicates ∼15% s hybridization of the in-plane p-bonded orbital of the unpaired spin and rapid rotation of the trimer in the solids.
A simplified transformation of the rotational–vibrational Hamiltonian to eliminate a Coriolis term and perturbation theory for higher order terms
Authors: Krishnan Mangala S. , Carrington Tucker
Published: July 2002

A quantum mechanical canonical transformation, applied previously to eliminate a Coriolis term from the rotational–vibrational Hamiltonian of a polyatomic molecule [J. Chem. Phys. 94, 461 (1991)], is simplified to permit the computation of rotational energy levels for highly excited vibrational states. An approximate matrix representation of the transformed Hamiltonian is presented which is shown to be very accurate for a two-mode model of formaldehyde. The quantum dynamics of the two nearly degenerate vibrational modes, strongly coupled by a Coriolis term, is studied using the approximate representation. It is shown that the second order Coriolis term and quartic anharmonic terms in the potential can be treated effectively as perturbations using the transformed Hamiltonians as the zeroth-order model.
Intermolecular contributions to the stress tensor of polymeric systems
Authors: Curtiss C. F.
Published: July 2002

The statistical expression for the stress tensor of a polymeric system may be written as a sum of two terms—the single molecule term, which involves the distribution functions in the phase spaces of single molecules and the intermolecular term, which involves the distribution functions in the configuration spaces of pairs of molecules. The evaluation of the first of these two contributions involves the solution of the ‘‘diffusion equation’’ in the configuration space of a single molecule and has been discussed extensively. Here, the solution of the analogous equation in the configuration space of a pair of molecules and the evaluation of the intermolecular term is discussed. The resulting expression involves ‘‘matrix elements’’ of operators in the configuration space of the two molecule system with respect to the eigenfunctions of a Hermitian operator. In particular, an expression for the intermolecular contribution to the frequency-dependent complex viscosity is obtained. As a limiting case, a system consisting of spherical molecules is considered and numerical results are obtained for liquid argon at 89 K. It is predicted that non-Newtonian behavior would occur at frequencies of the order of 1011 s−1.