bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory of Reactive Scattering. IV. Exact Quantum Mechanical Study of Angular Independent and Angular Dependent Models for Three Dimensional Rearrangement Collisions
Authors: Baer Michael , Kouri Donald J.
Published: February 2004

In this paper we consider some three dimensional models for reactive scattering which can be treated analytically using the coupled channel τ operator formalism. The models are characterized by the fact that the potential energy surface governing the interaction of the three particles is in general angular dependent. As in earlier studies, it is again found that the equations can be reduced to a form essentially equivalent to that encountered in collinear reactive scattering. The most important restriction as to the validity of the model is that the mass of one of the atoms be infinite. Numerical calculations of reaction probabilities and cross sections are reported for several types of angular independent and angular dependent potential surfaces. The calculations constitute exact quantum mechanical results.
New Techniques for the Computation of Multiconfiguration Self-Consistent Field (MCSCF) Wavefunctions
Authors: Das G. , Wahl Arnold C.
Published: February 2004

New techniques are described for obtaining MCSCF wavefunctions and energies via the expansion method. These include (i) a simple but generalized algorithm for obtaining symmetrized configurations and the corresponding vector-coupling coefficients for diagonal and off-diagonal matrix elements referred to the symmetry species of a diatomic molecule; (ii) a new iterative scheme which leads to a fast convergence of the MCSCF process provided the starting conditions are properly chosen, and (iii) a method to ensure such proper starting conditions in regard to the form of the initial orbitals.
Interaction of 25 keV Electrons with the Nucleic Acid Bases, Adenine, Thymine, and Uracil. I. Outer Shell Excitation
Authors: Isaacson Michael
Published: February 2004

The energy loss functions — Im(1/ε) of thin films of the nucleic acid bases, adenine (C5N5H5), thymine (C5N2O2H6), and uracil (C4N2O2H4) have been determined in the range 2–50 eV from energy loss experiments with 25 keV electrons at room temperature. An analysis of the electronic structure of these materials in the region of outer shell excitation has been obtained in the form of determination of the dielectric constants ε1 and ε2 from a Kramers—Kronig analysis of the energy loss function. In the spectral region where ultraviolet absorption data exists (<10 eV), the calculated optical absorption coefficient μ is shown to be in good agreement with the optical measurements.
The Structure of Dielectric Fluids. III. Interaction between Impurities and Dielectric Saturation in Polar Fluids
Authors: Nienhuis Gerard , Deutch J. M.
Published: February 2004

The potential of mean force between two impurities in a sample of a fluid of rigid dipoles is studied. Each impurity may be a charge, an electric dipole or an electric quadrupole. It is found that the longest-range part of the potential of mean force obtained from a molecular theory is the same as the interaction energy obtained from macroscopic electrostatics, when the polar fluid is treated as a dielectric continuum, except that the impurity dipole and quadrupole have to be replaced by effective dipole moments. These effective dipole moments arise because of the local polarization of the polar molecules in the surrounding of the impurities, due to the local interactions. Next a pure polar fluid in the presence of an arbitrary external electric field is considered. The one-particle distribution function in the presence of the field is studied to all orders in the field. It is found that both this one-particle distribution function and the dielectric polarization are shape-independent functions of the local average macroscopic field.
Infrared Emission at 2.7 μm from Flowing CO2—N2* Mixtures
Authors: Oettinger P. E. , Horn K. P.
Published: February 2004

A flow tube has been constructed wherein ground-state CO2 and vibrationally excited N2 molecular streams interact and the combined flow is spectrally investigated at selected downstream stations. The close coupling between the lower vibrational levels of N2(X 1Σg+) and the CO2 asymmetric stretch mode (00° v3) allows rapid energy exchange to these levels, followed by internal redistribution among the vibrational states of the various CO2 modes. When emission at the 2.7−μm bands is monitored, it has been observed that such redistribution produces substantial population of the mixed-mode levels associated with this radiation. Comparison of the observed band profiles with analytically derived counterparts has indicated that for an N2 vibrational temperature of 1900°K, CO2 difference bands at 2.7 μm contribute approximately 2.5 times as much radiant energy as is emitted in the 10°1–00°0 and 02°1–00°0 summation bands.
Internal Relaxation in Chemically Reacting Fluids
Authors: Kapral Raymond
Published: February 2004

The coupling of an internal relaxation to both the chemical reaction and diffusion processes is investigated for a fluid mixture. A microscopic approach beginning with a generalized Langevin equation is used to derive relaxation equations for the concentration fluctuations of the species in specified internal states as well as total concentration fluctuations. These equations are used to calculate the matrix of concentration correlation functions which are then related to the intensity of the light scattered from the fluid. The circumstances under which such a generalized hydrodynamics approach must be used are examined. The possibility of extracting information about both the chemical and internal relaxation processes from a measurement of the width of the polarized component of the Rayleigh line is considered.
Rotatory Motion of Dipolar Molecules Constrained by a Steady Electric Field
Authors: Ullman Robert
Published: February 2004

In this paper a calculation has been carried out on the rotatory diffusion of dipolar rodlike molecules in the presence of an electric field. The time correlation method is used for calculations of dynamic susceptibilities. This calculation has been shown to be in good agreement with the dynamic dielectric studies of Block and Hayes. In addition, the decay of electric birefringence (Kerr effect) in the presence of an electric field has been shown to be extremely sensitive to the relative magnitudes of the steady state field and the initially applied sensing field.
Localized Spatial Structures and Nonlinear Chemical Waves in Dissipative Systems
Authors: Herschkowitz-Kaufman M. , Nicolis G.
Published: February 2004

The problem of chemical instabilities and the subsequent emergence of dissipative structures is studied in the case of systems which are maintained spatially nonuniform. A simple model is analyzed numerically and shown to exhibit, for different ranges of values of the parameters, two types of solution of the kinetic equations: a localized steady state dissipative structure and a time-dependent solution describing a non-linear propagating concentration wave. The biological implications of such solutions are briefly discussed.
Diffraction Pattern and Structure of Noncrystalline BeF2 and SiO2 at 25°C
Authors: Narten A. H.
Published: February 2004

The scattering of x rays from noncrystalline samples of BeF2 and SiO2 at 25°C has been measured and analyzed. The diffraction patterns, typical of the atomic arrangement found in liquids, show interference to values of the scattering variable s=16 and beyond. The glasses contain BeF4 (SiO4) tetrahedra joined at the corners so that one anion is common to two tetrahedra. The packing of these tetrahedra is very similar to that found in the crystalline modifications of BeF2 and SiO2, and a model based on the quartz structure describes the diffraction data quantitatively.
Dynamics of β-Quartz Structures of Vitreous SiO2 and BeF2
Authors: Bates John B.
Published: February 2004

The optically active vibrational modes of vitreous SiO2 and BeF2 were calculated using the β-quartz models determined for these materials from recent x-ray diffraction experiments. The assumption of regular β-quartz lattice models with the appropriate unit cell parameters resulted in good agreement between observed and calculated frequencies. The crystal potential functions of SiO2 and BeF2 were derived from a modified valence force field, and several force fields which differed in the type of interaction terms allowed were studied. Calculated mean amplitudes for Si–O and Be–F stretching motion and for O ··· O, Si ··· Si, F ··· F, and Be ··· Be nonbonded displacements were in good agreement with the respective mean amplitudes derived from the x-ray data.
Low-Energy Repulsive Interaction Potential for Helium
Authors: Cantini P. , Dondi M. G. , Scoles G. , Torello F.
Published: February 2004

New measurements of the integral elastic cross section for a 4He velocity selected supersonic molecular beam scattered by a 4He gas target at 2°K are reported. The structure in the velocity dependent cross section arising from the quantum statistics is well resolved and gives precise information about the weakly repulsive potential at values corresponding to the collision energies (5–30 meV). It is shown that the measured property is insensitive to both the attractive and strongly repulsive parts of the potential. The error in the determination of the potential due to the uncertainty of the velocity selector calibration is discussed. A comparison of the présent data with predictions from recently proposed helium potentials is made, and information on the weakly repulsive interaction obtained from this and other experimental data is summarized.
Molecular Zeeman Effect, Magnetic Properties, and Electric Quadrupole Moments in ClF, BrF, ClCN, BrCN, and ICN
Authors: Ewing J. J. , Tigelaar H. L. , Flygare W. H.
Published: February 2004

The molecular Zeeman effect in the following strongly nuclear coupled molecules was analyzed in magnetic fields near 20 000 G: 35ClF, 37ClF, 79BrF, 81BrF, 35Cl12C15N, 37Cl12C15N, 79Br12C15N, 81Br12C15N, and 127I12C15N. The molecular g values, the shielded nuclear g values, magnetic-susceptibility anisotropies, and molecular quadrupole moments are determined for each of the above molecules. The isotope dependence in the molecular g values gives the electric dipole-moment sign in −ClF+ and +ClCN−. The 127I nuclear magnetic shielding anisotropy in 127I12C15N was also determined. The known molecular structures combined with the above Zeeman data were used to determine the paramagnetic- and diamagnetic-susceptibility tensor elements.
Dissociation Energy of the Gaseous TlBi Molecule
Authors: De Maria G. , Malaspina L. , Piacente V.
Published: February 2004

The Knudsen cell-mass spectrometric technique has been used to identify the heteronuclear molecule TlBi. The thermodynamic treatment of the data led to a value of the dissociation energy D0° (TlBi)=28 ± 3 kcal mole−1. A comparison between bond energies in condensed and gaseous IIIb—Vb binary compounds is reported.
Molecular Orbital Theory for Infinite Systems: Hydrogen-Bonded Molecular Crystals
Authors: Bacon J. , Santry D. P.
Published: February 2004

In a previous paper a self consistent field (SCF) perturbation theory of the interaction between two molecules of a dimer was given. We now extend this theory to molecular crystals, with special reference to hydrogen bonded systems. The theory derived is used to calculate the geometry and energy for an infinite chain of HF molecules. The theory is checked by comparing the results obtained from some direct CNDO calculations. Calculations are reported for the three dimensional HF crystal also. According to CNDO theory, the nonpolar structure is the more stable by 2 kcal/mole.
Triplet Correlation in Liquid Argon by Monte Carlo Method: Low Densities
Authors: Krumhansl J. A. , Wang Shein-shion
Published: February 2004

The triplet correlation function for low density fluid argon was obtained by the Monte Carlo method for three different temperatures, assuming a Lennard-Jones-type potential. At low densities the agreement with the prediction of the Kirkwood superposition assumption (SA) was found to be better than 10% for most values. Significant differences between the two were found at small distances, where the SA gives unreliable estimates, and at the peaks of the triplet function, where the SA predicts too large a value.
Series Expansion for the Monomer—Trimer Problem. I. General Formulation and Application to the Square Lattice
Authors: Van Craen J. , Bellemans A.
Published: February 2004

The combinatorial method of Nagle for monomer—dimer mixtures is extended to the monomer—trimer case. It turns out that the method becomes much more sophisticated: the relevant graphs involve two different types of edges and their weights are complicated functions of the natural expansion variable x of the problem (related to the trimer concentration). The method is applied to assemblies of (a) rectilinear, (b) bent, and (c) flexible trimers on the square lattice. The residual entropy of rectilinear trimers at close packing is estimated, SR/k=0.45.
Collisionally Uncoupled Model for the Contribution of Orientational Polarizations to Transport Properties of Dilute Gases. I
Authors: Coope J. A. R. , Snider R. F.
Published: February 2004

A simple collisionally uncoupled model of the contribution of orientational polarizations to the transport properties of dilute gases is proposed. This takes detailed account of the molecular free motions between collisions, and so encompasses a variety of phenomena associated with the decoupling of angular momenta by relatively strong external fields.
Collisionally Uncoupled Model for the Contribution of Orientational Polarizations to Transport Properties of Dilute Gases. II. The Shear Viscosity
Authors: Coope J. A. R. , Snider R. F.
Published: February 2004

The collisionally uncoupled model formulated in Paper I is applied to the calculation of the shear viscosity. The expressions obtained for the internal state contribution to the viscosity coefficients involve a summation, or integration, over the internal state frequency spectrum for each energy shell. The theory is first developed in a general way, and then a number of particular cases are considered. The expressions simplify in the commonly studied case of low magnetic fields, that is in the linear Zeemann region (and, equally, in the case of a linear Stark effect), because the frequency spectrum for each multiplet is then merely the Larmor frequency and its harmonics. As an example of this case, it is shown that the model gives a satisfactory one-parameter description of the transverse viscosity coefficients of oxygen gas in the low pressure limit. The multiplet dependence of orientational relaxation times appears to be an important factor. The systematic choice of symmetry adapted phenomenological coefficients is also discussed, in particular the choice of symmetry adapted viscosity coefficients.
Optical Selection Studies of Radiationless Decay in an Isolated Large Molecule. II. Role of Frequency Changes
Authors: Nitzan Abraham , Jortner Joshua
Published: February 2004

In this paper we provide an extension of the theoretical study of nonradiative decay of a single vibronic level of a large molecule. We have derived theoretical expressions for the dependence of the electronic relaxation rate on the excess vibrational energy in the excited electronic state of a ``harmonic molecule'' which is characterized by displaced and frequency modified potential surfaces. The simple case of displaced potential surfaces was handled by relating the potential surfaces via a simple displacement operator. To handle frequency changes in optical selection we have applied Feynman's operator techniques to disentangle exponential operators which involve nonlinear terms. The role of frequency changes in optical selection experiments was elucidated.
Influence of Variations of the Potential-Energy Surface on Exchange-Reaction Probabilities
Authors: Diestler D. J.
Published: February 2004

Accurate quantum-mechanical reaction probabilities for the collinear reaction H + H2 → H2 + H on a series of parametric Wall—Porter potential surfaces have been determined by solving the relevant time-independent Schrödinger equation using a newly developed close-coupling technique. The effects of variations of (1) the total energy, (2) the classical activation energy (barrier height), (3) the curvature of the reaction path, and (4) the slope and ``width'' of the potential barrier along the reaction path on various attributes of the reaction are found by systematically changing selected parameters of the surface. The reaction attributes of chief interest are (1) the gross structure of the reaction probability curves (as a function of total energy), (2) the effective threshold for reaction, and (3) the degree of vibrational excitation of the product. The various effects induced in the reaction attributes by variations of the potential surface parameters are correlated and discussed. Several qualitative conclusions are drawn.
Error Bounds for R−8 Dispersion Forces between Atoms
Authors: Starkschall George , Gordon Roy G.
Published: February 2004

Error bounds for the coefficient of the R−8 dipole—quadrupole dispersion term describing the interaction between two He atoms have been calculated by constructing oscillator strength and transition frequency spectra which reproduce known dipole and quadrupole oscillator strength sums. The equations obtained are solved by means of linear programming (LP), and the LP solutions are compared with the exact solutions. The effect of uncertainties in the oscillator strength sums is explicitly accounted for in the LP solution.
Millimeter Wave Rotational Spectra of AlCl, AlBr, and AlI
Authors: Wyse F. C. , Gordy Walter
Published: February 2004

Rotational transitions of Al35Cl, Al37Cl, Al79Br, Al81Br, and AlI have been measured in the millimeter wave region. To our knowledge, this is the initial microwave investigation of AlBr and AlI, and for AlI is the first report of any rotational constants. Transitions in excited vibrational states have been measured (up to v=14 for AlI), allowing accurate experimental determination of rotation—vibration interaction terms Y11, Y21, Y31, and Y12 of the Dunham theory. Data analysis yields: for Al35Cl, Be=7313.206(4) MHz, De=7.503(4) kHz, re=2.13011 Å; for Al37Cl, Be=7141.131 (4) MHz, De=7.154(4) kHz, re=2.13011 Å; for Al79Br, Be=4772.825(2) MHz, De=3.3832(11) kHz, re=2.29480 Å; for Al81Br, Be=4742.799(2) MHz, De=3.3408(11) kHz, re=2.29480 Å; and for AlI, Be=3528.5533(8) MHz, De=1.9550(3) kHz, re=2.53709 Å. From nuclear quadrupole couplings the ionic character of the AlI bond is found to be 85%; that of AlBr is found to be ≥87%. From the ionic character of the aluminum monohalides, the electronegativity value of monovalent aluminum is found to be 1.3.
Pressure-Induced Electronic Changes in Compounds of Iron
Authors: Slichter C. P. , Drickamer H. G.
Published: February 2004

We analyze a series of recent experiments in which it has been found that pressure induces changes in charge or spin of iron [i.e., Fe(III) reduces to Fe(II) and in Fe(II) there are spin changes involving either decrease or increase of multiplicity]. In addition to considering the volume difference of the two states at zero pressure, we find it important to include effects both of the difference in compressibility of the two states, and the change in compressibility of a given state with pressure. The theory casts the problem in terms of the changes in Coulomb energy, closed shell repulsions, and both covalent bonding energy and crystal field energy accompanying the change in electronic state. In addition, interactions between converted iron atoms are included by a form of mean field theory and the effects are shown to be significant. Not only is the theory discussed analytically, but also a simple graphical solution is shown which makes it possible to examine readily the qualitative effects of the various parameters. Repulsive interactions spread the conversion over a larger pressure range and may thus explain why so many compounds exhibit rather broad transitions. Attractive and repulsive interactions can lead to cooperative effects. They should account for the reversible yet discontinuous jumps in conversion as a function of temperature previously observed in several phenanthroline and dipyridyl compounds. The possibility of hysteresis is also indicated. Major anomalies in the combined temperature and pressure variation of the conversion are accounted for by including the temperature dependence of the free energy of interaction.
Hydrodynamic Theory of the Angular Velocity Autocorrelation Function
Authors: Berne Bruce J.
Published: February 2004

The angular velocity correlation function is computed using a frequency dependent version of Stokes' Law. The drag torque on a sphere rotating nonuniformly about one of its diagonals is used to compute the relaxation of the angular velocity. It is shown that the asymptotic time dependence of this function goes as t−5/2. Consequences of this persistence in the orientational relaxation of molecules is discussed.
Ab Initio Calculation of the Electronic Structure of Carbon Suboxide
Authors: Sabin John R. , Kim Hyunyong
Published: February 2004

An ab initio LCGO MO SCF calculation with geometry search is carried out on C3O2. The structure is determined to be linear with C–C and C–O bond distances of 1.332 and 1.243 Å, respectively. The electronic configuration is determined, and one-electron properties are calculated. Potential functions are reported and discussed with particular emphasis on the low-frequency bending mode about the central carbon atom.
Microwave Optical Magnetic Resonance Induced by Electrons (MOMRIE) in H2 G(3d 1Σg+)
Authors: Freund Robert S. , Miller Terry A.
Published: February 2004

The G(3d 1Σg+) state of H2 has been investigated using a new form of molecular double resonance spectroscopy, Microwave—Optical Magnetic Resonance Induced by Electrons (MOMRIE). This state is excited by electron bombardment of ground state H2 molecules residing in a microwave cavity in a magnetic field. Because of L-uncoupling effects, the magnetic moment is of the order of one Bohr magneton so its Zeeman levels can be tuned to resonance with X-band microwave frequency radiation by kilogauss magnetic fields. Such resonances are observed via changes in the polarization of the emitted visible radiation when the state decays. In attempting to interpret the MOMRIE spectrum, we have reviewed previous work on the optical energy levels of the 3d complex and have found the Zeeman effect calculated therefrom to be inadequate to match our present experimental precision; this is probably because of perturbations or inaccuracies in the previously reported optical spectrum. From an approximate Zeeman theory we have obtained values for the lifetime, g value, and nonlinear Zeeman effect in the N=1 level of the G(3d 1Σg+) state.
Raman Spectra and Factor Group Analyses of Crystalline Boron Trihalides
Authors: Clark R. J. H. , Mitchell P. D.
Published: February 2004

The Raman spectra of the crystalline boron trihalides have been recorded at ·77°K in the region of the fundamentals. The spectra of boron trichloride, boron tribromide, and boron triiodide have been interpreted in terms of the factor group (C6h) known to pertain for the trichloride; the trifluoride is shown to have a different crystal structure. Halogen isotopic structure to the a1′ and a2″ bands of the trichloride as well as to the a1′ band of the tribromide has been resolved, the separation between the isotopic components of these bands amounting to 4.2, 2.2, and 1.2 cm−1, respectively. The Raman spectra of the trichloride, tribromide, and triiodide as liquids or solutions have also been recorded. The asymmetric stretching mode (e′ symmetry) is the most sensitive fundamental to change of state, being depressed from its liquid phase values by 11–22 cm−1 for each halide; the other three fundamentals only suffer very small shifts on change of state. The infrared spectrum of solid boron triiodide has been measured in the 1000–70 cm−1 region, and one fundamental has been reassigned; the appropriate force constant has been recalculated.
Self—Consistent Molecular Orbital Methods. XII. Further Extensions of Gaussian—Type Basis Sets for Use in Molecular Orbital Studies of Organic Molecules
Authors: Hehre W. J. , Ditchfield R. , Pople J. A.
Published: February 2004

Two extended basis sets (termed 5–31G and 6–31G) consisting of atomic orbitals expressed as fixed linear combinations of Gaussian functions are presented for the first row atoms carbon to fluorine. These basis functions are similar to the 4–31G set [J. Chem. Phys. 54, 724 (1971)] in that each valence shell is split into inner and outer parts described by three and one Gaussian function, respectively. Inner shells are represented by a single basis function taken as a sum of five (5–31G) or six (6–31G) Gaussians. Studies with a number of polyatomic molecules indicate a substantial lowering of calculated total energies over the 4–31G set. Calculated relative energies and equilibrium geometries do not appear to be altered significantly.
Post-Threshold Energy Dependence of the Cross Section for Endoergic Processes: Vibrational Excitation and Reactive Scattering
Authors: Levine R. D. , Bernstein R. B.
Published: February 2004

The essential features of the translational energy dependence or excitation function σ(Etr) for two types of endoergic collisional processes are deduced on the basis of information on the inverse, exoergic processes. Microreversibility is conveniently exploited via the symmetric yield function, Y(E), which is uniquely determined at a given total energy, E. In the case of the vibrational excitation of diatomic molecules by atomic or molecular impact (T → V), use is made of the abundant data on the temperature dependence of the V → T relaxation time τ. The usual semiempirical two-parameter representation of the relaxation process is transformed to yield an explicit functionality for the translational energy dependence of the inelastic excitation cross section σ0 → 1 (Etr), in the adiabatic region. For diatomics whose relaxation is slow (e.g., slow enough to be studied in a shock tube) the vibrational excitation cross section σ0 → 1 is exponentially small in the post-threshold region. These systems would be unsuitable for study by the crossed beam technique; the latter thus complements rather than replaces the more traditional relaxation methods. Also considered are endoergic chemical reactions whose inverse processes involve negligible activation barriers (e.g., ion—molecule reactions, reactive atom—molecule reactions, etc.). In all cases the dominant post-threshold energy dependence of the reactive cross section σR(Etr) for the endoergic process is fixed via the microreversibility constraint, so experimental data in this region do not provide as much information as is available from studies of the exoergic process. The deviation from a simple Arrhenius cross section functionality is determined by the long-range final product interaction.
Determination of the Temperature Dependence of Gaseous Diffusion Coefficients Using Gas Chromatographic Apparatus
Authors: Liner J. C. , Weissman Stanley
Published: February 2004

The diffusion coefficient for three pairs of gases, He–Ar, He–N2, and He4–He3, have been measured using a gas chromatographic technique. The temperature of the measurements extends from 300 to 800°K. It appears that this is the first time an isotopic system has been so investigated by a gas chromatographic method. The present data agree well with other values in the literature.
Dependence of Radiationless Decay Rates in Polyatomic Molecules upon the Initially Selected Vibronic State: General Theory and Application
Authors: Heller Donald F. , Freed Karl F. , Gelbart William M.
Published: February 2004

A general scheme for calculating relative nonradiative decay rates of initially selected vibronic levels in polyatomic molecules is developed. Here we are concerned only with relative rates corresponding to initial vibronic states in some particular electronic manifold decaying into some other particular electronic manifold. Therefore we need only consider the exact density-of-states-weighted-Franck—Condon factor accounting for the vibrational contribution to the over-all nonradiative decay rate. This contribution is calculated approximately in the large molecule (statistical) limit by deriving the appropriate (modified) energy gap law. The contributions of ``special'' modes (e.g., optically excited modes, and modes with large geometry and/or frequency shifts) are partitioned from those of the other modes by techniques similar to ones developed earlier to treat cis—trans isomerism and are calculated explicitly. Excellent agreement with the recent low pressure experimental results of Spears, Abramson, and Rice for the S1→ T1 intersystem crossing rates from individual vibronic states in benzene and perdeuterobenzene is obtained, but only by taking proper account of the effects of geometry and frequency changes of the various vibrational modes involved in the transition. The sharp dependence of the relative nonradiative rates for a progression in an optical mode upon the frequency shift in that mode leads to the possibility of evaluating the vibrational frequencies in an electronic state by measuring the radiationless decay rate into that electronic state (e.g., in the benzene case determining T1 vibrational frequencies from S1→ T1 decay rates). Finally, we comment upon recent low pressure experimental results for β-naphthylamine, monofluorobenzene, and upon some observed temperature dependences of radiationless transition rates in matrices.
Semiclassical Transition Probabilities for Collinear Diatom—Diatom Collisions
Authors: Morse R. I.
Published: February 2004

The semiclassical model of Heidrich, Wilson, and Rapp is extended to include translation—vibration transition probabilities for the collinear collision of diatomic molecules. The results for H2–H2 collisions are compared with quantum mechanical results and found to be in good agreement.
Molecular Structure Effects on the Free-Ion Yields and Reaction Kinetics in the Radiolysis of the Methyl-Substituted Propanes and Liquid Argon: Electron and Ion Mobilities
Authors: Fuochi P. G. , Freeman G. R.
Published: February 2004

The free-ion yields in liquid alkanes increase with increasing molecular sphericity and increasing temperature, in agreement with earlier observations. The Arrhenius temperature coefficient of free-ion formation is 0.8 kcal/mole in propane and 0.7 kcal/mole in 2-methylpropane. The free-ion yield in purified argon at 87°K appears to be Gfi=2.0. Addition of oxygen to liquid argon decreased the free-ion yield, mainly because the efficiencies of electron energy loss processes with the diatomic oxygen molecules were much greater than those with monatomic argon molecules; the oxygen served to decrease the ion—electron separation distance more by de-energizing the electron than by scavenging it before it reached the end of its normal track. In pure liquid oxygen at 87°K, Gfi=0.013. The ion—electron neutralization rate constant equals 8 × 10−5 cm3/ion · sec in pure argon at 87°K and 3.1 × 10−4 cm3/ion · sec in liquid methane at 120°K. The mobility of electrons in liquid methane is 300 cm2/V · sec and decreases in other hydrocarbons with decreasing sphericity of the molecules. The Arrhenius temperature coefficient of electron mobility in 2-methylpropane is 3 kcal/mole. Ion—ion neutralization rate constants in the present liquids were all of the order of 10−9 cm3/ion·sec, and the ion mobilities were all in the vicinity of 10−3 cm2/V · sec.
Electron Drift Mobility in Solid 3-Methylpentane
Authors: Maruyama Yusei , Funabashi Koichi
Published: February 2004

The excess electron mobility in solid 3-methylpentane was measured by photoinjection method. Observed values range 0.02–0.1 cm2/V·sec for 4.2–85°K. The mobility increased with temperature between 35–85°K with an activation energy of 0.01–0.02 eV and was nearly independent of temperature below 35°K. Phonon-assisted hopping and tunneling mechanisms are proposed for temperatures above and below 35°K, respectively.
Collisional Detachment Studies of NO−
Authors: McFarland M. , Dunkin D. B. , Fehsenfeld F. C. , Schmeltekopf A. L. , Ferguson E. E.
Published: February 2004

Rate constants for collisional detachment of NO−, NO− + X → NO + X + e,have been measured from 193 to 506°K for X as He, Ne, H2, NO, CO, CO2, N2O, and NH3. The data fit Arrhenius plots kd=Ae−E/kT with both A and E showing large variations with X. From equilibrium constant considerations, electron attachment rates to NO have been calculated. An estimate for the electron affinity of NO3 is obtained from some auxiliary considerations. Rate constants for the charge-transfer reactions of NO− with O2 and NO2 were found to be 5 and 7.4 × 10−10  cm3sec−1, respectively.
Three-Atom Scattering in Gas-Phase Electron Diffraction: A Tractable Limiting Case
Authors: Bartell L. S. , Wong T. C.
Published: February 2004

The contribution of intramolecular multiple scattering in electron diffraction patterns of gas molecules is investigated by Glauber's method. An analytical expression for three-atom scattering is derived for the limiting case in which the distance between atoms is large compared with atomic radii. This expression accounts well for the discrepancies between observed and conventionally calculated scattered intensities reported by Jacob and Bartell in the case of ReF6. The analogous four-atom scattering is found to be very small.
Formation of Electronically Excited Ions in Electrode Processes: Electroluminescence of Trivalent Rare-Earth Ions in Liquid Solutions
Authors: Heller Adam , French Kenneth W. , Haugsjaa Paul O.
Published: February 2004

Trivalent cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, thulium, and ytterbium electroluminesce in liquid phosphorus oxychloride solutions at room temperature. Trivalent terbium electroluminesces also in dimethyl sulfoxide and propylene carbonate. The emission is due to the known inner ``f'' subshell transitions of the trivalent ions, originating from established metastable states. The electroluminescence spectra of the ions range from the ultraviolet (in the case of Gd3+), through the visible (Ce3+, Pr3+, Sm3+, Eu3+, Tb3+, Dy3+, and Tm3+), to the infrared (Nd3+, Yb3+). The emission originates at the surface of the negative electrode and becomes observable, in the case of green emitting Tb3+, at about 6 V dc. The electroluminescence is explained by the following mechanism: Application of an external potential attracts the Helmholtz and Gouy Chapman layers of solvated cations towards the surface of the cathode. This causes a Schottky lowering of the barrier to electron escape and allows electrons to escape from the cathode into the electrode/electrolyte interface. The electrons are accelerated across the positive ion rich interface and acquire enough energy to excite the luminescent cations by inelastic collisions.
Proton Affinity of Water
Authors: Chong Shuang-Ling , Myers Ransom A. , Franklin J. L.
Published: February 2004

By employing elevated pressures in the ion source of a quadrupole mass filter, it is shown that D3S+ does not transfer a deuteron to D2O and that D3O+ transfers a deuteron to D2S. Studies were carried out in most instances under conditions where the reactant ion undergoes 30–60 collisions before reacting. Thus, most excess energy should have been removed. The transfer of deuteron from D3O+ to D2S appears to go to equilibrium with an equilibrium constant of 14.4 which corresponds to a heat of reaction of 1.8 kcal/mole. From this we deduce the proton affinity of water to be 168 kcal/mole.
Impurity Quenching of Molecular Excitons. I. Kinetic Comparison of Förster—Dexter and Slowly Quenched Frenkel Excitons in Linear Chains
Authors: Pearlstein Robert M.
Published: February 2004

The quenching kinetics of tightly bound excitons for two different one-dimensional models are compared. The quenching of fully incoherent, or Förster—Dexter, excitons is described by a standard master equation, and that of fully coherent, or Frenkel, excitons by an ad hoc linear differential equation whose eigenvalues are complex. Moments of the chain excitation function (probability that excitation remains at time t) are calculated on each model for finite chains with either localized or uniform initial conditions, free-end or periodic boundary conditions, one disruptive or one nondisruptive quencher, and various quencher locations, but with only nearest-neighbor interactions included. The ad hoc equation is treated only in the limit that the quenching is slow enough not to affect the exciton wave functions in first order. In that limit of Frenkel exciton quenching, an analytic expression is given for the mean de-excitation time in the presence of uniform decay processes such as fluorescence. The master equation is treated for arbitrary relative quencher strength with simple analytic expressions for the zeroth moment obtained by an inverse matrix method. Infinite chains with randomly placed disruptive quenchers are also treated. It is shown that for the Förster—Dexter model the nth moment Mn is proportional to N2n+2 for large N (finite chains) or to c−(2n+2) for small c (infinite chains); for the Frenkel model with an incoherent initial condition, Mn ∝ N(3n+2) or c−(3n+2). This implies that kinetic differences between quenching of Frenkel and Förster—Dexter excitons appear only in higher (than zero) moments and not, e.g., in fluorescence yields. It is also concluded that, because of their ``boundary-avoiding tendency,'' Frenkel excitons can be quenched much more slowly in some situations than Förster—Dexter excitons. Finally, the intuitive notion that excited-state energy transfer is a two-step process—consisting of migration to the quencher neighborhood and subsequent quenching—is substantiated for Förster—Dexter excitons but not for Frenkel excitons.
Radiofrequency and Microwave Spectrum of the Hydrogen Fluoride Dimer; a Nonrigid Molecule
Authors: Dyke Thomas R. , Howard Brian J. , Klemperer William
Published: February 2004

The radiofrequency and microwave spectra of the K=0 states of (HF)2, (DF)2, and HFDF have been studied by the molecular beam electric resonance method. A unique hydrogen tunnelling motion involving the breaking and reforming of the hydrogen bond causes a splitting of rotational energy levels for (HF)2 and (DF)2, but not for HFDF. The electric dipole selection rules and nuclear spin statistics for the tunnelling molecules have been derived from a consideration of an extended permutation-inversion group. Rotational constants, tunneling doublings, electric dipole moments, and deuterium quadrupole coupling constants have been determined from the observed spectra of the K=0 states. (HF)2(DF)2HFDF(B+C)/2 (MHz)6504.8±2.06252.194±0.0026500.1±0.1ν (MHz)19 776±121579.877 ±0.004μa  (D)2.987±0.0032.9919±0.00063.029±0.003(eqQ)Da (KHz)···110±8270±30These results are interpreted with a semirigid, nonlinear model of the dimer geometry. The F—F distance is 2.79 ± 0.05 Å, and the end hydrogen fluoride unit is bent from 60 to 70° from the F—F axis. Large amplitude motion of the hydrogen and deuterium atoms occurs. Higher hydrogen fluoride polymers have been studied by mass spectroscopy and electric deflection methods. All were observed to be nonpolar which is interpreted to imply cyclic structures.
Stress Relaxation of Solutions of Flexible Macromolecules
Authors: Hassager Ole , Bird R. Byron
Published: February 2004

For stress relaxation after cessation of steady shearing flow with vx=κ0y, the following relation has been proposed for connecting the normal stress difference τxx − − τyy − during steady state flow and the shear stress τyx + during the relaxation process after t=0: τxx − − τyy −=2κ0 ∫ 0∞ τyx +dt.It is shown here that this relation can be derived for a dilute suspension of flexible macromolecules represented as a set of N beads joined by N — 1 connectors which may be nonlinear springs; equilibrium-averaged hydrodynamic interaction is included in the theory. A generalization of the above formula may be derived for the stress relaxation following any steady homogeneous flow. In the derivation use is made of an expression for the stress tensor which differs from that of Giesekus in that hydrodynamic interaction has been included. From the latter formula it is particularly easy to rederive the Lodge-Wu constitutive equation for the Zimm model with Gaussian springs.
On a Proton Magnetic Resonance Study of H2S and H2Se
Authors: Saffar Z. M. El , Schultz P.
Published: February 2004

The proton magnetic resonance line shape and second moment have been studied in H2S and H2Se at 58°K. The observed second moment values indicate that the lattice of H2S and H2Se is essentially rigid at this temperature. The line shapes have been fitted by the method of least squares to a theoretical lineshape function having the intramolecular H–H distance as an adjustable parameter. The H–H distances obtained are 1.883 ± 0.007 Å for H2S and 2.014 ± 0.011 Å for H2Se.
Viscous Flow Past a Random Array of Spheres
Authors: Childress Stephen
Published: February 2004

The steady, slow motion of a viscous fluid past a random, uniform array of identical nonoverlapping finite spheres is studied. The expected force F acting on any given sphere is obtained by averaging over the ensemble of possible positions of all other spheres. It is shown that, if the Stokes equations hold in the fluid, then for small particle volume concentration c, F has an expansion of the form F=[ 1 + (3 / 2) c1/2 + (135/64)c logc ] F0 + cD · F0 + 0 (c3/2logc),where F0 is the Stokes force on a single isolated sphere in the same flow and D is a tensor which is given in terms of the two-sphere distribution function for the array and the Stokes solution for two spheres oriented arbitrarily in an unbounded flow. A physical model for the solution is presented and compared with the model of Brinkman.
High-Pressure Phase Transformations in Hexagonal and Amorphous Selenium
Authors: McCann D. R. , Cartz L.
Published: February 2004

Hexagonal selenium transforms to a new phase at 140 ± 10 kbar. The c/a ratio of 1.42 at the transformation corresponds to the same c/a ratio for tellurium at its transformation at 40 kbar. It has not been possible to decide unambiguously a unit cell from the diffraction pattern of the new phase. The high-pressure phase is irreversible being retained upon releasing the pressure. Amorphous Se slowly transforms to the crystalline state under pressure and eventually develops the same unknown high-pressure structure as the hexagonal form. The pressure-cycled amorphous material retains its crystalline structure upon releasing the pressure. This structure appears to be identical to the irreversible hexagonal phase. It is thought that the high-pressure structure is superconducting.
Antiferromagnetic Linear Chains in the Crystalline Free Radical BDPA
Authors: Duffy William , Dubach John F. , Pianetta Piero A. , Deck Joseph F. , Strandburg Donald L. , Miedema Andries R.
Published: February 2004

The static magnetic susceptibility from 1.6 to 300°K and the heat capacity in fields of 0 and 5 kG from 0.9 to 10.2°K have been measured on polycrystalline samples of the complex of 1,3-bisdiphenylene-2-phenylallyl with benzene, commonly abbreviated BDPA. The data are shown to correspond to antiferro-magnetic Heisenberg linear chains characterized by a nearest-neighbor exchange integral of −4.6°K. Weak interchain coupling results in a paramagnetic—antiferromagnetic phase transition at 1.695°K in zero field, as evidenced by the sharp heat capacity anomaly at that temperature. Resolution of the heat capacity into a lattice T3 contribution and a magnetic contribution yields a Debye temperature of 52.5°K and a magnetic entropy per mole of 101% of the expected Rln2.
Angular Correlation of Annihilation Photons in Frozen Aqueous Solutions
Authors: Milošević-Kvajić M. , Mogensen O. , Kvajić G. , Eldrup M.
Published: February 2004

Linear-slit angular correlation curves were obtained at about −140°C for frozen aqueous solutions of HF, HCl, HBr, HI, NH3, FeCl2, FeCl3, NaI, H2SO4, NHO3, MnSO4, KMnO4, K2Cr2O7, NaOH, and LiOH. We found no appreciable influence of a 4% concentration of the last seven impurities. Only halide-containing impurities strongly changed the form of the curves; ppm concentrations of HCl and HF could be seen. The main change was a broadening of the part of the curve where the narrow and broad components meet. This fact is interpreted as being caused by trapping of para-positronium in holes in the structure created by the impurities. No evidence of any chemical reactions of positronium and the impurities was found. The annealing of a fast frozen FeCl2 solution was studied. The strongest changes in the curves occurred on annealing from −136 to −100°C.
Linewidths and Spin Densities in the ESR Spectra of Electrogenerated Radical Anions of Sterically Hindered 2,2′-Dinitrobiphenyl and 1,8-Dinitronaphthalene
Authors: Subramanian J. , Narasimhan P. T.
Published: February 2004

The ESR spectra of the radical anions of 2,2′-dinitrobiphenyl and 1,8-dinitronaphthalene generated by electroreduction in anhydrous dimethylformamide (DMF) have been recorded in the temperature range 20 to −80°C. Interesting linewidth effects are observed in the ESR spectra of these two radical anions at lower temperatures. From the linewidth data, the spectral densities for the modulations of isotropic splittings and anisotropic intramolecular interactions have been computed making use of the complete expressions for linewidths derived from relaxation matrix theory and a two-jump model. Using the spectral densities and the measured dynamic frequency shifts, correlation times for modulations of isotropic splittings are evaluated. It is noticed that the two-jump model with out-of-phase correlation of nitrogen splittings in the two nitro groups explains the observed trends in the spectral densities satisfactorily for both the radical anions. The details of the modulating motions are investigated next, and spin densities calculated for some geometries of the radical anions. Experimental splitting constants are compared with those computed from spin density values obtained by McLachlan and UHFAA (unrestricted Hartree—Fock method of Amos and Snyder with annihilation of quartet component) methods. In 2,2′-dinitrobiphenyl radical anion, satisfactory agreement is obtained between experimental and calculated splittings for the geometry in which the two planes of the phenyl rings make an angle of 40° when they are twisted from the trans—planar configuration with respect to the nitro groups. In order to explain the out-of-phase correlation and the observed couplings, it is suggested that the modulating motions are such that when one of the nitro groups is in the plane of the phenyl ring, the other one is out-of-plane. In the case of 1,8-dinitronaphthalene radical anion also, similar results have been obtained thus leading to the model of a ``seesaw'' type of motion of the nitro groups in these radical anions.