bluuebunny/crossref_metadata_2025 huggingface dataset:
Absorption and dispersion in the O2 microwave spectrum at atmospheric pressures
Authors: Smith Earl W.
Published: February 2003

Calculations are performed for absorption and phase dispersion at various frequencies within the 60 GHz band of O2 from low pressures where the spectral lines are isolated, to atmospheric pressures where they merge to form a continuum band. A perturbation theory proposed by Rosenkranz was tested and found to be valid for pressures up to 100 kPa (1 atm). The ’’line coupling coefficients’’, which describe the transfer of excitation from one radiating state to another, are also studied and various methods for evaluating these coefficients are analyzed and compared with experimental data. It is found that dispersion measurements are extremely sensitive to these coefficients and an experimental procedure for systematically measuring them is outlined; it is shown that such measurements can provide a very sensitive test for theoretical calculations of inelastic transition amplitudes.
Halogenated benzene radical cations and ground state degeneracy splitting by asymmetric substitution
Authors: Bondybey V. E. , Vaughn C. R. , Miller Terry A. , English J. H. , Shiley R. H.
Published: February 2003

The absorption and laser induced fluorescence of several halogenated benzene radical cations were studied in solid Ne matrices. The spectra of 1,2,4-trifluorobenzene, 1,3-dichloro-5-fluorobenzene, and 1-chloro-3,5-difluorobenzene radical cations are observed and analyzed. Studies of fluorescence polarization and a photoselection technique were used to examine the splitting of the degeneracy of the benzene cation ground state by asymmetric subsitution.
Collision dynamics of three interacting atoms: Vibrational–rotational excitation in atom–diatom hyperthermal collisions
Authors: Beard Lyntis H. , Micha David A.
Published: February 2003

We describe atom–diatom collisions at hyperthermal energies within the theory of three-body scattering. We show how one can treat angular momentum coupling and rotational excitation to low order in a multiple-collision expansion. The emphasis here is in the use of accurate two-atom transition amplitudes and form factors, and on impulse and peaking approximations with simple interaction potentials. We present results for differential cross sections as functions of final rotational quantum numbers jf for Li+–CO and Li+–N2 at several center of mass angles in the eV region of collision energies. Form factors are also shown for varying jf, and are discussed in terms of simple overlap models in momentum space.
On a model ecosystem showing competitive phenomena
Authors: Iwamoto Kazutoshi , Senō Manabu
Published: February 2003

Competition within an ecosystem consisting of two similar subsystems is studied in detail. The procedure for analysis is based upon a multiplicity of steady states, and it was developed for the investigation of competition in a model chemical system. A spruce budworm system is analyzed as an example. Our results are quite different from those of a well-known ecological theory of L. B. Slobodkin.
The classical limit of the quantum mechnical expressions for the kinetic theory cross sections
Authors: Curtiss C. F.
Published: February 2003

The integral expressions for the various kinetic theory cross sections describing collisions between nonvibrating diatomic molecules, based on the generalized phase shift development of molecular scattering theory, which were developed in a previous paper are considered further. It is shown that in the classical limit the expressions can be written in forms similar to the usual expressions for the special case of spherical molecules. The spherical harmonic Yl(χ,0) where χ is the angle of deflection, which appears in the expressions for the spherical case, is replaced by a properly coupled sum of products of three spherical harmonics and additional integrations over two sets of Euler angles arise.
Variation of chemical shielding with intermolecular interaction and rovibrational motion. V. 15N in N2
Authors: Jameson Cynthia J. , Jameson A. Keith , Wille S. , Burrell Patricia M.
Published: February 2003

The 15N resonance frequency in 15N2 gas has been measured as a function of density and temperature. The effect of intermolecular interactions, which is observed as a density dependence, is about the same magnitude as that for 13C in two interacting CO molecules, but smaller than previously observed for 15N nuclei in other molecules. The temperature dependence in the zero-pressure limit [σ0(T)−σ0(300) =−(0.85±0.10)×10−3 (T−300) ppm] originates from centrifugal distortion. The effect of vibrational averaging contributes negligibly to the temperature dependence in the range 220–380 K. Comparison with previous theoretical calculations of 15N shielding in N2 as a function of internuclear separation shows reasonably good agreement with the observed temperature dependence in the zero-pressure limit. The results for the N2 system are compared with the isoelectronic CO molecule for which more precise 13C NMR measurements are also reported here.
Resonance Raman excitation profiles of lycopene
Authors: Hoskins L. C.
Published: February 2003

The resonance Raman spectrum of lycopene has been examined in acetone solvent and excitation profiles of the three fundamentals ν1, ν2, and ν3 have been determined. The excitation data and the visible spectrum have been analyzed using two-mode and three-mode vibrational models, with the two-mode model involving virtual states of ν1 and ν2 giving the best fit to the data. This mode mixing or Duskinsky effect was not observed for β-carotene. The single-mode and three-mode theories which have been used to explain the corresponding data for β-carotene are shown to be inconsistent with the experimental data of lycopene. Equations for calculating excitation profiles and visible spectra are given.
High-field LMR spectra of ClO2 tuned by the avoidance of Zeeman level crossings
Authors: Uehara Hiromichi , Hakuta Kohzo
Published: February 2003

Laser magnetic resonance of the ν1 band of the stable ClO2 radical has exhibited clearly resolved spectra at high fields. Many of these were well described by a new-type Zeeman tuning due to the avoidance of Zeeman level crossings between ΔN=±1 levels, induced by the spin–rotation interaction. An extensive description of the high-field spectra was given. From the analysis, molecular constants for the (100) state of 35ClO2 were determined as ν1=945.5921(22) cm−1, A=1.7355(4) cm−1, B=0.3303(8) cm−1, and C=0.2762(7) cm−1.
Dynamic structure factor of a dilute polymer solution: Generalized mean field approximation
Authors: Ou Jane Jane , Dahler John S. , Jhon Myung S.
Published: February 2003

A simple, systematic numerical procedure is given for calculating the dynamic structure factor of polymeric solutions. As an example, specific calculations are performed for the Rouse chain. The numerical results are in good agreement with those of Pecora’s exact series summation for both small and large wavelengths.
High energy chlorine for chlorine substitution involving Walden inversion in gaseous enantiomers
Authors: To Kar-Chun , Rack E. P. , Wolf A. P.
Published: February 2003

The reactions of 34mCl- for cl substitution in gaseous 2(S)-(+)- a and 2(R)-(−)-chloro-1-propanol are reported at various system pressures and in the presence of neon moderator and a radical scavenger. (AIP).
A useful interpolation between static and motional narrowing limits for vibrational dephasing
Authors: Oxtoby David W.
Published: February 2003

We suggest a useful ansatz for the correlation function of frequency fluctuations leading to dephasing. It is shown that the ansatz is well behaved at both short and long times, allows analytical calculation of the vibrational correlation function and the corresponding isotropic Raman spectrum, and gives an excellent fit to molecular dynamics simulation results for liquid nitrogen.
Cross-relaxation rates and effective correlation distances for two randomly distributed dilute spin species
Authors: Korb J.-P. , Maruani J.
Published: February 2003

An expression of the cross-relaxation rates of two randomly distributed dilute spin species is derived. These rates provide information of the correlation distances between the interacting spin species. (AIP)
Microwave, infrared, and Raman spectra, vibrational assignment, normal coordinate analysis, and barrier to internal rotation of dimethylphosphine-d3
Authors: Durig J. R. , Hudson S. D. , Jalilian M. R. , Li Y. S.
Published: February 2003

Microwave spectra of dimethylphosphine-d3 and -d6 have been obtained and assigned in the range from 18 to 40 GHz. The six independent rotational constants along with the nine previously determined ones are used to obtain a least-squares fit of the eight structural parameters as follows: r(CP)=1.853±0.006 Å, r(CHα)=1.090±0.020 Å, r(CHβ)=1.086±0.010 Å, r(PH)=1.418±0.020 Å, uHαCP=109.1±1.8 °, uHβCP=110.7±1.0 °, uCPC=99.76 °±0.10 °, and uHPC=96.5 °±1.2 °. The barrier to internal rotation of the methyl group has been determined to be 823.6 cm−1 (2.35 kcal/mole) and 816.7 cm−1 (2.34 kcal/mole) by using two different methods of analyzing the excited state splitting data from the rotational transitions. The infrared spectra (80–3200 cm−1) and Raman spectra (10–3200 cm−1) of gaseous, liquid, and solid dimethylphosphine-d3 have been obtained and the spectra interpreted in detail. Similar information was also obtained for the -d4 species and an assignment of the P–H bending modes is given. The torsional data obtained from the low frequency Raman and far infrared spectra of gaseous dimethylphosphine-d3 have been used to determine the potential function based on the semirigid model and the effective barrier to internal rotation was found to be 832.3 cm−1 (2.38 kcal/mole). V60 and V06 were found to be −28.5 cm−1 (−0.081 kcal/mole) and −37.2 cm−1 (−0.11 kcal/mole), respectively. both the sine–sine (V′33) and cosine–cosine (V33) coupling terms were found to be zero. Spectra of the -d0 species were also recorded and these torsional data were also analyzed on the basis of the semirigid model. The barrier to internal rotation was found to be 810.1 cm−1 (2.32 kcal/mole) with both the cosine–cosine and sine–sine coupling terms being significant. A normal coordinate analysis has been carried out using seven diagonal force constants and 14 interaction constants in order to fit 61 fundamental frequencies. Revised assignments are provided for the PH bending and CH3 rocking motions. Extensive coupling was found for the normal modes in the 700–1000 cm−1 spectral region.
Surface-enhanced Raman spectroscopy of CO adsorbed on colloidal silver particles
Authors: Abe H. , Manzel K. , Schulze W. , Moskovits M. , DiLella D. P.
Published: February 2003

The surface-enhanced Raman spectrum of CO adsorbed on colloidal silver particles of about 100 Å diameter is reported. The spectrum resembles that observed for CO adsorbed on vapor-deposited silver. Only three strong Raman lines appear, at 2113, 160, and 64 cm−1. These are assigned, respectively, to the CO stretching, Ag–C stretching, and AgCO bending vibrations. The excitation profile of the 2113 cm−1 line follows closely the plasmon absorption spectrum of the silver colloid, increasing as one goes from the red to the violet; that of the 160 cm−1 line shows a more gradual increase in the same direction. The two types of behavior are accounted for by means of a model which considers the enhancement of the Raman signal to be due to coupling of the adsorbate vibration with the conduction–electron surface plasma mode through a charge transfer from the adsorbate to the metal, synchronous with the vibration. The two types of excitation profile are generated by assuming that for the AgC vibration, positive displacement of the normal coordinate produces charge injection into the metal while for the CO vibration, charge is withdrawn as the CO bond is elongated.
Configurations and dynamics of real chains. I. Polyethylene
Authors: Allegra G. , Ganazzoli F.
Published: February 2003

The quadratic approximation for the intramolecular potential of a real polyethylene chain is equivalent with a model of ideal springs connecting all atom pairs; the more remote the two atoms along the chain, the weaker the spring. Within the same approximation, the joint probability distribution of atomic coordinates is Gaussian, and all possible averages may be easily derived by it. As for the dynamic behavior, if the observation time t is such that t/to is roughly between 5 and 5×103, to being the relaxation time of the single backbone atom, the chain behaves quite stiffer than the equivalent bead-and-spring model. The half-peak time of the dynamic structure factor obeys an approximate power law t1/2⋅Qβ=const. [Q=4π sin (ϑ/2)/λ] where β is close to 3 both in the coherent and in the incoherent case. (Values of β around 3 are frequently observed, and are usually associated with the effect of hydrodynamic interaction.) Different ranges of observation times correspond to different regimes, and their effect on the dynamic structure factor is discussed.
High-pressure mechanical instability in close-packed Hooke’s-law crystals
Authors: Ladd Anthony J. C. , Hoover William G.
Published: February 2003

Lattice dynamics and molecular dynamics are used to study close-packed crystals with pairwise-additive Hooke’s-law interparticle potentials. Lattice dynamics describes a mechanical soft-mode instability at high pressure. In two dimensions molecular dynamics reveals that a thermodynamic transition, from the triangular close-packed lattice to the square lattice, occurs before the soft-mode instability density is reached. Similar phenomena occur in the three-dimensional close-packed lattices.
Calculation of voltage fluctuations at the Gunn instability
Authors: Keizer Joel
Published: February 2003

Recent measurements of voltage fluctuations near the threshold of the Gunn instability in GaAs by Kabashima, Yamazaki, and Kawakubo yield a Gaussian distribution of fluctuations and an anomalous increase in the variance. These results confirm the qualitative predictions of the fluctuation–dissipation theory of molecular fluctuations. In this note, a quantitative calculation of the fluctuations below threshold is given. Only experimentally determined parameters are involved, and the results are in good agreement with experiment.
Solutions to the Smoluchowski equation for problems involving the anisotropic diffusion or absorption of a particle
Authors: Sano Hisatake
Published: February 2003

The escape probabilities or the recombination probabilities in diffusion-controlled reactions are obtained for some special systems which are not centrosymmetric. Specifically, we calculate (1) the escape probability of an electron from N fixed cations, (2) the recombination probability of a pair of radicals in an anisotropic medium, (3) the recombination probability of a radical with each of two statics sinks, and obtain (4) approximate expressions for the recombination probability of a radical with N static sinks in particular configurations. As a by-product, we obtain the bulk reaction rate for each system. Of special note is our result for the first system listed above. With the proviso that the cation radii are vanishingly small, we find the escape probability to be equal to the reciprocal of the Boltzmann factor for the system. This result factors into a product of isolated geminate ion pair escape probabilities. No competitive effects due to the geometric distribution of the cations appear.
Surface structures and phase diagram for the H/W(001) chemisorption system
Authors: Barker R. A. , Estrup P. J.
Published: February 2003

The chemisorption of hydrogen on tungsten (001) induces displacive rearrangements (’’reconstruction’’) of the substrate surface, depending on the temperature T and the adsorbate coverage ϑ. By means of LEED and other techniques, the surface phase diagram in the T–ϑ plane (ϑ<0.7, T<500 K) has been studied. The predominant structure is (√2×√2) which is stabilized by hydrogen in the region ϑ<0.3, but is transformed to an incommensurate structure at higher coverages. Adsorption below ?200 K produces no ordered phase. The LEED patterns from a terraced surface exhibit reduced symmetry and reveal the direction of the lattice distortion; the atomic displacements are found to be along 〈10〉 for the H-induced structures, in contrast to the 〈11〉 displacements on clean W(001). The substrate rearrangement is believed to be a major cause of the observed coverage dependence of the desorption energy and other adsorbate properties.
Photolytic spectroscopy of simple molecules. II. The production of 6P atoms from the X 1Σg state of Cs2
Authors: Collins C. B. , Lee F. W. , Anderson J. A. , Vicharelli P. A. , Popescu D. , Popescu Iovitzu
Published: February 2003

In this work the two-photon technique for the study of selective photolysis described in the preceding article was used for the measurement of the relative cross sections for the photolysis of Cs2 into each of the energetically possible product channels that could have been reached at wavelengths in the range from 445 to 640 nm. The resulting dispersion curves showed three relatively broad absorption bands, each leading to a different distribution of population among the product states. A relatively broad band in the blue region of the spectrum was found to yield an inverted branching ratio for dissociation which favored the more highly excited product by a factor of better than 50 to 1. This particular result implied that continuous wave, stimulated emission should be obtained from the 5 2D3/2→6 2P1/2 transition of atomic cesium with adequate photolytic pumping of Cs2 at blue wavelengths. An analysis of the complete photolysis spectrum obtained in this work indicated that both dissociation and predissociation mechanisms were involved. These results led to the construction of approximate potential curves for a new 3Σu state correlated with Cs(6 2P3/2) and new 1Σu and 3Σu states correlated with the 5 2D states of atomic cesium.
A study of the high Rydberg state and ion–molecule reactions of carbon disulfide using the molecular beam photoionization method
Authors: Ono Y. , Linn S. H. , Prest H. F. , Gress M. E. , Ng C. Y.
Published: February 2003

The internal energy effects and the energetics of the ion/molecule reaction of carbon disulfide CS+2⋅⋅⋅CS2→CS+3+CS, S+2+2CS, C2S+3+S, and CS+4+C have been investigated in detail by photoionization of the neutral van der Waals carbon disulfide dimer. Using the relative Franck–Condon factors obtained by photoelectron spectroscopy, the relative reaction probabilities for the formation of various product channels of the above reactions with CS2+ in the ? 2Σu+ and ? 2Σg+ states were derived from the analysis of the photoionization efficiency (PIE) data for CS+3, S+2, C2S+3, and CS+4. From the measured threshold energies for CS+3, C2S+3, and CS+4, the heat of formation at 0 K for CS+3, C2S+3, and CS+4 are deduced to be 291.7±1.7, 318±3, and 246±6 kcal/mol, respectively. By comparing the peak heights of the autoionization resonances in the PIE curves for CS+3, S+2, and C2S+3, and normalizing these to the known absorption cross sections of the corresponding Rydberg levels for CS2, the relative reaction probabilities for the formation of various product channels of the chemi-ionization processes CS*2(V,n)⋅⋅⋅CS2→CS+3+CS+e, C2S+3+S+e, S+2+2CS+e, and (CS2)+2+e as a function of Rydberg level (n) were determined. The branching ratios of these product channels for the reactions of CS*2(V, n=4 or 5) with CS2 were also measured. This experiment has demonstrated that molecular beam photoionization studies of van der Waals dimers is an excellent method for the quantitative study of the chemistry of excited molecules, especially for molecules in short-lived radiative states.
Fluorescence quenching in micelles: A theoretical model for the intramicellar first order quenching rate constant
Authors: Van der Auweraer M. , Dederen J. C. , Geladé E. , De Schryver F. C.
Published: February 2003

Considering the fact that in a micellar medium solubilized molecules are mostly present near the surface, a solution of Fick’s diffusion law on a spherical surface is obtained to calculate the rate of diffusion controlled reactions in a micelle. It is shown that transients are small. Corrections are applied for the radial distribution of probe and quencher. The model is extended to reactions slower than diffusion controlled.
Exciton–phonon coupling in a dimer: An analytic approximation for eigenvalues and eigenvectors
Authors: Friesner Richard , Silbey Robert
Published: February 2003

An analytic approximation method is introduced to study the eigenvalues and eigenvectors of the dimer exciton–phonon Hamiltonian with linear coupling. The transition from quasilocalized to quasidelocalized states as a function of n, the number of vibrational quanta, is discussed for various coupling strengths. A discussion of the applicability of this method to other problems is given.
Momentum eigenfunctions in the complex momentum plane. IV. The construction of local potential functions. Perturbation series
Authors: Lassettre Edwin N.
Published: February 2003

Expansion of exchange amplitudes, in inverse powers of a momentum, as a means of generating local exchange potentials has been studied further. The first term in such expansions varies inversely as the square of the momentum. Huo [J. Chem. Phys. 67, 5133 (1977)] obtained a local exchange potential by replacing a momentum by its average value obtained from a semiclassical model. The Xα potential with α=1 was obtained. The present research shows that mometum squared should be averaged and this leads to the Xα potential with α=20/27. The potentials were tested numerically for beryllium and although the potential with α=20/27 was superior in some instances to that with α=1, this was not always the case. In fact, for general all around use, α=1 provided best results. This apparent contradiction has been resolved by extending the expansion of exchange amplitude by one more term and using the semiclassical model to obtain the averages. The Xα potential is again obtained but with a α=80/81, a value differing negligibly from unity. Using the known behavior of exchange amplitudes in the complex momentum plane, a method is developed for testing the convergence of expansions in inverse powers of the momentum. The power series expansions are shown to be divergent. This provides a plausible explanation for the infinities at r=0 observed by Huo and shows that inclusion of more and more terms cannot lead to an exact amplitude. Although divergent, the series are asymptotic, i.e., they hold more accurately as the momentum increases as long as only a few terms are used. Therefore, the formulas obtained by Huo are approximately correct since she showed that the average momenta are large. Some different method must be found, however, for improving the accuracy. Two expansions in series of orthogonal polynomials are developed for which the first term gives a local exchange potential. The convergence region of each series is determined.
A theoretical study of the effects of vibration on the reaction O3+NO→O2+NO2
Authors: Chapman Sally
Published: February 2003

A series of model potential energy surfaces for the reaction O3+NO→O2+NO2 is described. Nuclear geometry is unconstrained: The surfaces are nine dimensional. However, the possible interaction of more than one electronic surface has not been included. The model surfaces include various exoergicities, transition state geometries, and overall reaction topologies. Classical trajectories have been used to describe the dynamics on these surfaces. A variety of initial conditions, including selective excitation of vibrational modes of the ozone molecule, has been included. While translational and vibrational energy have distinctive effects on the reaction, no mode-specific enhancement of the reaction cross section was observed for any of the model functions. This is interpreted to result from the high degree of coupling of the internal coordinates to the reaction path and to the multidimensional curvature of the reaction coordinate. The results suggest caution in using prior studies of triatomic exchange reactions to predict behavior of these larger systems or as a guide in inferring information about the details of the potential energy function from the observed reaction properties. The relationship of these model studies to the actual O3+NO system is discussed.
Cluster dynamics: A classical trajectory study of A*n→A
n−1+A
Authors: Brady John W. , Doll Jimmie D. , Thompson Donald L.
Published: February 2003

The dynamics of the dissociation of quasibound clusters of atoms have been studied using classical trajectories with a Metropolis-type sampling procedure for averaging over the dynamical variables for fixed initial energy values. The case A*4→A3+A was examined for mass and potential parameters appropriate for Ar. Calculations were carried out for pairwise Lennard-Jones potentials. The computed decay rates are in good agreement with those reported previously [J. Chem. Phys. 71, 2467 (1979)] where the quasibound clusters were collisionally formed from A and internally cold A3.
Structural studies of (tetrathiotetracene)2(iodide)3 at room temperature and low temperatures
Authors: Lowe-Ma C. , Williams R. , Samson S.
Published: February 2003

(Tetrathiotetracene)2(iodide)3, (TTT)2I3, is metallic and can be crystallized with varying amounts of disorder. The crystal structures of highly disordered, h.d., (TTT)2I3 and (TTT)2I3 with significant lower disorder, l.d., have been solved. For (TTT)2I3 (l.d.) the basic structure, defined primarily by the TTT molecules, was refined in the space group Cmca. The doubled cell, which includes data from the strongest diffuse layer line, was refined in Pmc21. All iodide sites in (TTT)2I3 (l.d.) have less than full occupancy, indicating that the contents of cells vary and suggesting the presence of species such as I3−, I2, and I−. The crystal structure of (TTT)2I3 (h.d.) was investigated at room temperature (∼294 °K), 164, 74, and 19 °K. At all four temperatures the symmetry of the basic structure, defined primarily by the TTT molecules, remains orthorhombic. The basic TTT structure at all four temperatures was successfully refined in the space group Cmca. During slow cooling the diffuse layer lines were carefully monitored. Even with slow cooling the iodide chains do not three dimensionally order, and no distortions occur in the basic TTT structure down to 19 °K. A model of the iodide chains is presented which explains the positions and intensities of the diffuse layer lines and the absence of three-dimensional ordering at low temperatures.
Exchange interactions in tropylium bis(1,2-dicyanoethylene dithiolato) nickelate (III)
Authors: Manoharan P. T. , Noordik J. H. , de Boer E. , Keijzers C. P.
Published: February 2003

The compound tropylium bis[1,2-dicyanoethylenedithiolato] nickelate (III), i.e., (C7H7)+ [Ni(mnt)2]−, has been prepared and its crystal structure determined; the space group is P21/c, Z=2, the unit cell axes are a=6.364, b=7.458, c=17.991 Å, and β=91.22 °. The Ni(mnt)2− and C7H7+ ions form segregated stacks in the direction of the b axis. This is in contrast with the structure of the analogous thiete complex (C7H7)+ [NiS2C2 (CF3)2]−, in which crystals alternating stacks of cations and anions are present. The susceptibility appears to be dominated by a strong one-dimensional, isotropic, antiferromagnetic exchange coupling (J0=−4.4 cm−1). However, the angular dependence of the EPR linewidth at room temperature has to be explained with an additional anisotropic exchange interaction of 0.26 cm−1. Furthermore, Q-band EPR measurements in the ab plane reveal a very small interstack exchange interaction (0.015 cm−1 at room temperature) which increases by a factor of 4 upon lowering the temperature to 8 K and which might be responsible for a possible long range order at low temperature.
The effect of direct interactions on Brownian diffusion
Authors: Van den Broeck C. , Lostak F. , Lekkerkerker H. N. W.
Published: February 2003

The effect of direct interactions between suspended particles on their diffusion coefficient is investigated starting from the generalized Einstein relation. It is shown that an attractive potential added to the hard core repulsion leads to a decrease of the diffusion coefficient, whereas a repulsive term has the opposite effect. Simple examples of attractive and repulsive potentials are considered in some detail. Using these results the possibility to obtain information on the interaction potential between suspended particles from their diffusion coefficient is discussed.
A kinetic theory for polymer melts. I. The equation for the single-link orientational distribution function
Authors: Curtiss C. F. , Bird R. Byron
Published: February 2003

In this series of papers we show how a kinetic theory of undiluted polymers can be developed using the Curtiss–Bird–Hassager phase-space formulation. The polymer molecule is modeled as a Kramers freely jointed bead–rod chain. The objective is to obtain a molecular-theory expression for the stress tensor from which the rheological properties of polymer melts can be obtained. This development is put forth as an alternative to the Doi–Edwards theory; using a very different approach, we have rederived some of their results and generalized or extended others. In this first paper we develop the partial differential equation for the chain configurational distribution function, and then proceed to get the equation for the orientational distribution function for a single link in the chain. A modification of Stokes’ law is introduced that includes a tensor drag coefficient, characterized by two scalar parameters ζ (the friction coefficient) and ε (the link tension coefficient). In addition, to describe the increase of the drag force on a bead with chain length, at constant bead density, a ’’chain constraint exponent’’ β is used, which can vary from zero (the Doi–Edwards limit) to about 0.5. Solutions to the partial differential equation for the single-link distribution function are given in several forms, including an explicit series solution to terms of third order in the velocity gradients.
A kinetic theory for polymer melts. II. The stress tensor and the rheological equation of state
Authors: Curtiss C. F. , Bird R. Byron
Published: February 2003

An expression for the stress tensor of an undiluted polymer is derived by starting with the phase-space kinetic theory of Curtiss, Bird, and Hassager. The final formula contains two integrals, the first of which is the Doi–Edwards expression; the second integral, multiplied by the ’’link tension coefficient’’ ε, is new, and consequently a rheological equation of state is obtained whose properties are different from those of the Doi–Edwards theory. As illustrations, the constants in the retarded-motion expansion are determined to third order, and the relaxation modulus of linear viscoelasticity is obtained.
Mobilities of N+ ions in helium and argon
Authors: Fahey D. W. , Fehsenfeld F. C. , Albritton D. L.
Published: February 2003

The ion mobility of N+ ions in helium and argon is measured. The N+ ions were formed in a low-pressure, electron impact source containing N2.
Dynamical correlations and chaos in classical Hamiltonian systems
Authors: Kosloff Ronnie , Rice Stuart A.
Published: February 2003

An improved local criterion is proposed for the onset of dynamical chaos in a classical mechanical Hamiltonian system. This criterion is based on the observation that it is only neighboring trajectories which diverge in the direction perpendicular to the flow in phase space that contribute to chaotic motion. The necessary condition for such divergence of neighboring trajectories is the existence of a region where the curvature of the potential energy surface perpendicular to the trajectory is negative. It is shown, for the Hénon–Heiles model system, that the lowest critical energy for the onset of chaos is the same as obtained from the Brumer–Duff–Toda–Corjan–Reinhardt criterion, but that the regions of prescribed negative curvature are only a small fraction of the accessible energy surface. Consequently, the BDTCR criterion for onset of chaotic motion is augmented by the condition that the trajectory cross the critical locus in a manner which permits penetration of one of the regions where the surface curvature is negative perpendicular to the trajectory. Given that the regions of local instability are only a fraction of the accessible energy surface, the relationship between local instability and global dynamical chaos is discussed, and a mapping of the trajectory into a macrostate occupation number representation proposed. That mapping is used to generate an upper bound for the Kolmogorov entropy of the system. The upper bound so obtained is only close to the Kolmogorov entropy when the sequential occupation of macrostates by a trajectory consists of successive uncorrelated transitions. For the Hénon–Heiles model it is shown that correlation in macrostate occupation along a trajectory is long lived, so that even when there is dynamical chaos on the long time scale there can be quasiperiodic motion on the short time scale. The relationship of the new results to the characterization of intramolecular energy transfer is briefly discussed.
Beam–gas study of chemiluminescence from single collisions of Group IIIa metals with F2
Authors: Schwenz R. W. , Geiger L. C. , Parson J. M.
Published: February 2003

Chemiluminescence (CL) resulting from single collisions of the Group IIIa metals with F2 has been studied in a beam–gas arrangement. Computer simulation of the wavelength scans yields relative rates of formation of the various vibrational and spin–orbit states. Population inversions are observed in the vibrational distributions for excited electronic states. The vibrational distributions are consistent with previously reported values for the metal fluoride bond strengths. Absolute CL cross sections on the order of 0.02 Å2 for GaF and 0.05 Å2 for InF are measured. The results suggest that the energy release occurs early in the course of the reaction, and that at least one nonadiabatic process occurs with high efficiency.
Interpretation of ES, CS, and IOS approximations within a translational–internal coupling scheme. III. Exact and CS molecule–molecule cross sections
Authors: Snider R. F. , Coombe D. A. , Parvatiyar M. G.
Published: February 2003

Properties of exact and centrifugal sudden translational–internal reduced S matrices are given for general molecule–molecule collisions. The structure of energy-dependent and kinetic cross sections appropriate for linearized kinetic theory are described, both exactly and within the CS approximation. Connection with analogous total-J coupling scheme expressions is made in the Appendix.
Hyperthermal molecular beam scattering: K–O2 ion/neutral product angular, energy, and branching ratio analysis
Authors: Young C. E. , Sholeen C. M. , Wagner A. F. , Proctor A. E. , Pobo L. G. , Wexler S.
Published: February 2003

Angular distributions of K+ ions and K atoms from collisions of a beam of hyperthermal K atoms with a cross beam of thermal O2 molecules were determined in the range from 18 to 38 eV (lab). A pronounced rainbow was observed in the ion-pair channel at a reduced angle of 240 eV deg. A smaller rainbow was also found in the neutral channel at 220 eV deg. Energy loss distributions for both product species were also determined at 28 eV (lab) by time-of-flight measurements. Both K+ ions and K atoms from K+O2 were detected concurrently. A pseudorandom pulsing method was used to increase the duty cycle. The TOF spectra indicated overlapping energy-loss distributions corresponding to ground and excited state formation for both the neutral and ion products. Excited state to ground state branching ratios for both the neutral and ion products were determined as a function of reduced angle. The neutral branching ratio shows a pronounced peak at ∼180 eV deg., while the ion ratio increases rapidly above 200 eV deg. From these ratios the differential cross sections were calculated for neutral K in which either K or O2 is electronically excited. These cross sections are similar to the differential cross sections of K+ ions. The neutral to ion branching ratios, also determined, exhibit a broad minimum in the vicinity of the rainbow angle. Differential cross sections for neutral and ion scattering were integrated to give the total neutral/ion branching ratio of ∼4. A preliminary analysis of the experimental differential cross sections has been performed via an atom–atom model. Although the model has substantial and expected deficiencies, the analysis strongly indicates that the quartet surfaces emanating from the neutral ground electronic asymptote must be substantially more repulsive than the corresponding doublet surface. An analysis of the low velocity features in the TOF measurements of K indicates production of the lowest electronically excited state of O2 (i.e., 1Δg) as well as electronically excited states of K. A similar analysis of the TOF spectra of K+ suggests the occurrence of autoionization from the neutral ground state surfaces to the K+(1S0)+O2(1Δg) surface.
Correlation of the isotope chemistry of hydrogen, carbon, and oxygen with molecular forces by the WIMPER (2) method
Authors: Bigeleisen Jacob , Ishida Takanobu , Lee Myung W.
Published: February 2003

The WIMPER (2) approximation to the logarithm of the reduced partition function ratio is used to develop analytical expressions for the contribution of individual molecular vibrational force constants to the logarithm of the reduced partition function ratio of isotopic molecules. The method inherently accounts properly for contributions from quadratic terms of the form βpqxpxq, which are counted twice in any numerical method. The zero order (diagonal F–diagonal G) approximation leads to values of ln(s/s′)f0 which are always larger than the exact value when the site of isotopic substitution is a central atom. In the latter case the first order correction (j=1 term) is always negative because of the sign of the off- diagonal G matrix element. As a result of the convergence properties of the WINIMAX polynomials the sign of the j=1 correction to ln(s/s′)f0 usually determines the final sign of the correction to ln(s/s′)f0 associated with any off-diagonal F matrix element. Calculations are given for the contribution of each force constant to ln(s/s′)f, ln(s/s′)f0, and the j=1 and j=2 correction terms for D/H, 13C/12C, and 18O/16O isotopic substitutions. Molecules studied include H2O, CO2, H2CO, CH4, C2H4, C2H6, and C6H6. The contributions for off-diagonal F matrix elements for D/H substitution are found to be small in agreement with previous work. The largest effects associated with off-diagonal F matrix elements arise in the torsion of ring structures. These are illustrated in detail by numerical evaluation of each of the terms contributing to ln(s/s′)f for 13C/12C for the out-of-plane vibrations in benzene. Even in the latter case the WIMPER (2) method leads to approximate values of ln(s/s′)f within 3.6% of the exact value. When the j=2 correction is significant, it is usually associated with the coupling of internal coordinates through quadratic cross terms in the kinetic energy (g2ij fii fjj) or the coupling of an internal coordinate (gii fii) with a cross term in either the kinetic energy (gij) or potential energy (fij). It is shown by numerical calculation and through an analysis of the ground state contribution to ln(s/s′)f0 that both carbon and deuterium isotope effects at 300 °K associated with C–H stretching motions and H–C–H bending motions can be represented as simple functions of the square roots of the respective force constants. The proportionality constants are calculated a priori in good agreement with detailed calculations from a series of molecules with C–H bonds. The latter affords a simple method of calculation of lnc(u0) values directly from C–H stretching and H–C–H bending force constants.
Effects of periodic boundary conditions on equilibrium properties of computer simulated fluids. II. Application to simple liquids
Authors: Pratt Lawrence R. , Haan Steven W.
Published: February 2003

The theory of the previous paper is used to predict anomalous size effects observed for computer simulated liquid Ar. The theoretical results for the boundary condition induced anisotropy of two-particle correlations are found to be large, and in excellent agreement with the computer experimental data of Mandell for densities near the Ar triple point density. The agreement is less good at higher densities.
The effect of localized modes on radiationless electronic transitions. I. Diatomic impurities in a crystal
Authors: Venzl Gerd , Fischer Sighart F.
Published: February 2003

The nonradiative relaxation of a diatomic homonuclear molecule dissolved in an infinite linear chain of inert host atoms is studied in the harmonic approximation. The coupling to the molecular vibration as well as to other local modes and the frequency-dependent coupling to the host vibrations are determined by only one parameter related to the bond length change of the diatomic under consideration. It is demonstrated that the saddle-point method as it is usually applied is not valid if the frequencies of the localized vibrations are sufficiently far above the continuum. An improved method is presented which treats only the coupling to the continuum by the stationary phase method. The calculated rates show a rich structure including strong nonmonotonous variations with the energy gap and activated behavior as a function of temperature with small activation energies even in the weak coupling limit. The limit in which one-phonon processes dominate is discussed. Applications to Cl2 and O2 in argon matrices are presented.
The effect of localized modes on radiationless electronic transitions. II. Dependence on impurity concentration
Authors: Venzl Gerd , Fischer Sighart F.
Published: February 2003

The nonradiative decay of electronically excited impurities in a linear crystal is studied. For isolated isotropic impurities lighter than the host atoms [i.e., ε=−(ΔM/M)≳0] localized modes exist which, however, cannot accept energy in the relaxation process since the Franck–Condon factors vanish due to parity selection rules. This is not the case if there is a second impurity at a finite distance. The effect of the localized modes on the nonradiative rate is studied as a function of the impurity distance, the relative mass change, and the temperature. For low temperatures and weak coupling the rates may be enlarged by several orders of magnitude for neighboring impurities. This is important for increased impurity concentration. The time-dependent decay is in general nonexponential, and in the simplest case a superposition of two exponentials.
Paramagnetic resonance, magnetic susceptibility, and antiferromagnetic exchange in a Cr5+ paramagnet: Potassium perchromate (K3CrO8)
Authors: Dalal N. S. , Millar J. M. , Jagadeesh M. S. , Seehra M. S.
Published: February 2003

EPR and magnetic susceptibility of potassium perchromate (K3CrO8) have been studied from liquid helium temperatures to 400 K. EPR spectra of dilute aqueous solutions of K3CrO8 yielded giso=1.9712±0.0005, and 53Aiso=(18.6±0.5)×10−4 cm−1. The (positive) sign for 53Aiso, deduced here from the dependence of the linewidths of hyperfine components on nuclear spin quantum numbers, constitutes the first direct experimental measurement of the (negative) sign of the unpaired electron spin density at a Cr5+ nucleus. EPR spectra of powdered K3CrO8 doped in (diamagnetic) K3NbO8 and of K3CrO8 crystals yielded g∥=1.9428±0.0005, g⊥=1.9851±0.0005, A∥(53Cr)=+(36.3±0.5)×10−4 cm−1 and A⊥(53Cr)=+(11±1)×10−4 cm−1, the ∥ direction coinciding with the crystal c axis. These parameters establish that the unpaired electron orbital is the metal dx2–y2 orbital, with considerable charge transfer from the peroxy ligands. In concentrated samples, the EPR line shape is Lorentzian, providing direct evidence for electron spin exchange between Cr5+ ions. Temperature dependence of magnetic susceptibility χ over 1.4 to 343 K follows the law χ(cm3/mol)=[2.7933/(T+2.7)]+χd, where χd=−1.5×10−4(cm3/mol) is estimated to be the diamagnetic contribution. From the Curie–Weiss temperature of 2.7 K, an exchange coupling J=1.35 K between the Cr5+ ions is estimated. The calculated exchange-narrowed EPR linewidth is consistent with the experimental value of 15 G observed for K3CrO8. No magnetic or structural phase transition was detected in the range of 1.4 to 400 K; hence K3CrO8 crystals can serve as useful internal standard for spin concentration measurements over a wide temperature range, especially for free radicals.
Viscosity of carbon dioxide and ethane
Authors: Iwasaki Hiroji , Takahashi Mitsuo
Published: February 2003

The viscosity of carbon dioxide was measured at temperatures between 298.15 and 323.15 K at pressures up to 145 bar, and the viscosity of ethane at temperatures between 298.15 and 348.15 K at pressures up to 130 bar, both experimental ranges of which include critical regions of sample gases, respectively. The measurements were carried out by the oscillating disk viscometer, combining with a local determination of the density at the position of the oscillating disk. The viscosities of both substances exhibit an anomalous increase near the critical point. The anomalous increase of viscosity was examined for the following relation along the critical isochore, that is, Δη/?=A ln[(T-Tc)/Tc]+B, where Δη:anomalous viscosity, ?:background viscosity, T:temperature in K, and Tc:critical temperature in K. This relation was satisfied for the temperature range of (T-Tc)/Tc<0.0033 for both substances.
An EPR study of triplet excitons in crystalline anthracene–tetracyanobenzene: Exciton dynamics, long-range order parameter, and critical exponent
Authors: Park J. M. , Reddoch A. H.
Published: February 2003

We report new measurements of the EPR spectra of triplet excitons in the charge-transfer complex anthracene–tetracyanobenzene. The spectra show exchange phenomena arising from exciton motion over a lattice undergoing an order–disorder phase transition. The interpretation of these spectra requires a four-site exchange model from which we estimate the rates of interchain and intrachain motion for the excitons, as well as the temperature dependence of the long-range order parameter and the critical exponent β. This value β=0.34, the first for an order–disorder transition in a molecular crystal, is in agreement with the predictions of renormalization group theory.
Dynamics of interspecies hydrogen bonding of a fluorinated alcohol and a nitroxide free radical
Authors: Borah Babul , Bates Richard D.
Published: February 2003

The combined use of (1) the magnetic field dependence of solvent nuclear relaxation times induced by paramagnetic solutes and (2) the dynamic nuclear polarization of the same solvent nuclei is reported in a study of the dynamics of interactions between trifluoroethanol and 4-oxo-2,2,6,6-tetramethyl piperidino-oxy free radical. The –OH proton behavior is dominated by dipolar coupling with the spin-label unpaired electron that is characterized by rotational diffusion of the complex with a correlation time of 5×10−11 s and an interaction distance of 2.6 Å. The interaction with the –CH2 protons and –CF3 fluorine nuclei is weaker, dominated by dipolar coupling with the free radical, and characterized by a shorter correlation time.