bluuebunny/crossref_metadata_2025 huggingface dataset:
Quasiequilibrium Treatment of Gas–Solid Reactions. I. Evaporation Rates of Volatile Species Formed in the Reaction of O2 with W, Mo, and C
Authors: Batty J. C. , Stickney R. E.
Published: February 2004

A quasiequilibrium treatment of heterogeneous reactions is used to compute the rates of evaporation (desorption) of volatile species formed in the reaction of gaseous O2 with solid tungsten, molybdenum, and graphite under conditions of low pressure and high temperature. The results are compared with existing mass–spectrometric data for the O–W and O–Mo reactions, and the agreement is surprisingly close at the highest temperatures in view of the extreme simplicity of the theoretical model. Significantly better agreement is obtained over the entire temperature range (1400–3100°K) when an empirical method is introduced to account for the fact that not all of the molecules impinging upon the solid surface attain complete thermodynamic equilibrium with both the solid and adsorbate phase. It appears that the present treatment correctly predicts both the temperature dependence and the pressure dependence of the evaporation rate of each volatile species, as well as predicting that the proportions of the species will be those of an equilibrium mixture. The principal advantages of the quasiequilibrium treatment over existing kinetic treatments of gas–solid reactions are (1) the only empirical parameter is the equilibration or sticking probability, and (2) results are obtained without assuming microscopic models of the adsorbate phase and the reaction mechanisms.
Phonon Curves and Frequency Spectrum for Hydrogen-Bonded Systems: Solid HF and DF
Authors: Tubino Riccardo , Zerbi Giuseppe
Published: February 2004

A few hydrogen-bonded systems can be considered as polymers; hence, dispersion curves ν(k) and frequency distribution g(ν) can be calculated. In the present paper molecular vibrations of a single chain of hydrogen-bonded HF and DF are studied throughout the first Brillouin zone. k = 0 fundamental modes are calculated. From ν(k and g(ν) it is possible to propose a better interpretation of the previously reported inelastic neutron scattering spectrum. The limitations of the model used are discussed and suggestions for a more refined approach of the problem are given.
Statistical Model for the Vibrational Deactivation of Molecular by Atomic Oxygen
Authors: Breig E. L.
Published: February 2004

A theoretical investigation is performed of the transfer of vibrational energy from a highly excited diatomic molecule to a free atom at thermal velocities under the special condition of a deep attractive minimum in the intermolecular potential. The model utilizes a Monte Carlo statistical analysis of classical particle trajectories, with calculations performed for selected semiempirical forms of the interaction potential. Data relevant to the ozone stable configuration are applied to the O–O2 system, with primary emphasis upon the role of the latter in atmospheric reactions which produce the OH airglow emission. A deep attractive potential well is found to be the principal parameter leading to rapid vibrational deactivation of O2, with cross sections of the same order as kinetic cross sections and with the direct collision process of an importance comparable to the atom-interchange mechanism. The fundamental concept of the “intermediate complex” is favored to best describe the details of similar reaction processes which are associated with such strong attractive forces.
Fluctuation Form of Transport Theory
Authors: Gass David
Published: February 2004

Einstein-type expressions for transport coefficients are obtained by introducing Onsager's regression hypothesis for fluctuations into the statistical-mechanical expressions for the fluxes. Liouville operator formalism is used to convert the fluxes into a general fluctuation form. Introduction of a local equilibrium distribution function and linearization in the macroscopic gradients yield the Einstein-type equations. An extension of the results to interference effects is given. The fluctuation form of the fluxes is converted into one in which the nonequilibrium distribution function is used. The latter expressions form the basis for the theories of Kirkwood, Rice and Allnatt, and others. An application is made to particles interacting via a square-well potential.
Solid Rotator Phases in Nearly Spherical Molecules. Dielectric Constant and Proton Magnetic Resonance Measurement
Authors: Segel S. L. , Mansingh Abhai
Published: February 2004

We have investigated the proton magnetic resonance and dielectric constant properties of six nearly spherical molecules. In three of the molecules, 2-chloro-2-methylpropane, 2-methyl-2-propanethiol, and 2,2-dimethylpropanal, marked increases in the dielectric constant were observed to coincide with marked reductions in the magnetic resonance linewidth at temperatures well below the melting point. These transitions were ascribed to rotator phases in the solid state. We have also observed that diffusion either accompanies or immediately follows (with increasing temperature) the solid rotator transition. In three similar molecules, trimethyl-chlorosilane, 3,3-dimethyl-2-butanone, and 2,2-dimethyl-propane nitrile, a similar phenomenon occurs only at the melting point, implying that solid rotator phases do not occur in these materials. We do observe a transition in the nitrile below the melting point but are able to preclude ascribing it to a rotator phase. We also describe a fast-passage technique for recording the proton magnetic resonance.
CF4: Crystal Structure and Solid Phase Diagram with Ar
Authors: Greer Sandra C. , Meyer Lothar
Published: February 2004

The crystal structure of solid CF4 and the temperature–composition solid phase diagram of CF4 with Ar have been studied by x-ray powder diffraction. The diffraction pattern of α-CF4, stable from 0–76.2°K, has been indexed with a monoclinic unit cell where a = 8.71 Å, b = 4.16 Å, c = 15.27 Å, and β = 150.7°, at 40°K. The α-CF4 unit cell is C-centered with four molecules per unit cell and is probably in space group Cc with molecules centered at (0, 0, 0) and (12, 12, 0). The reflections of β-CF4, which is stable from 76.2°K to the melting point at 89.5°K, could not be indexed, but it was established that the structure is not cubic and is probably also monoclinic. The CF4–Ar solid phase diagram shows a eutectic from about 12% Ar to essentially 100% Ar. The eutectic temperature is 70 ± 1°K, coinciding with the solidus line in the region of the eutectic.
Statistical Phase-Space Theory of the Reaction C+ + D2 Including Threshold Behavior
Authors: Truhlar Donald G.
Published: February 2004

The quantum-mechanical statistical phase-space theory of Pechukas, Light, and Nikitin is used to calculate the cross sections for the endothermic reaction C+ + D2→CD+ + D in the energy range 0–4 eV. The calculated cross section is about two times larger than the experimental results of Maier (1967) and has almost the same energy dependence below 2 eV. The statistical-theory threshold law is predicted to be valid only for a region less than 0.1 eV from threshold. Two different types of dependence of the reaction probability on incident impact parameter are emphasized, and one is used to explain the magnitude of the predicted cross section in this case. The intramolecular isotope effect in C+ + HD collisions is studied and the CH+ / CD+ product ratio is predicted to be 0.92 at 1 eV.
Enthalpies of Mixing of Some Binary Fused-Salt Mixtures Involving the Chlorides of Transition Metals, Calcium, Magnesium, and Cadmium
Authors: Papatheodorou G. N. , Kleppa O. J.
Published: February 2004

The integral enthalpies of mixing of the following binary fused-salt mixtures have been determined calorimetrically: (a) MnCl2–FeCl2, MnCl2–CoCl2, FeCl2–CoCl2 at 810°C; MnCl2–CdCl2, FeCl2–CdCl2 at 690°C; (b) CaCl2–MjCl2; and (c) MgCl2–MjCl2 with Mj = Mn, Fe, Co, at 810°C. The results are discussed with respect to the following points: (1) comparison with the binary alkaline-earth chloride systems; (2) comparison with the existing theories for the enthalpies of mixing of fused salts; and (3) effect of possible covalent or covalent-bridging interactions between the ions in the pure components and in the mixture. The results are interpreted in terms of a cyclic process which allows a separation of the enthalpy of mixing into two principal parts. One part is negative and is associated with ionic and dispersion interactions, as in the binary alkali halides and in some of the binary alkaline-earth chlorides. The other part is positive and is attributed to the effect of covalent interactions between the ions on the enthalpy of mixing.
Energetic Metastable Decompositions
Authors: Smyth K. C. , Shannon T. W.
Published: February 2004

A study of the shape of metastable peaks observed in a mass spectrometer was carried out. The shape of the peak is related to the distribution of kinetic energies released in the decomposition. For Gaussian-shaped metastable peaks the kinetic-energy distribution is approximately Boltzmann, while for flat-topped metastable peaks the energy distribution is reversed, with the most probable energy greater than the average energy released. The 31+ − 29+ transition which is accompanied by a flat-topped metastable peak was examined in detail. Molecular-orbital calculations reveal that the reaction may involve tunneling through a barrier formed by the crossing of an attractive and repulsive state. The transition state is examined in detail and compared to the semi-ion pair model for a four-center elimination reaction.
Upper and Lower Bounds to Long-Range Intermolecular Potentials
Authors: McQuarrie D. A. , Terebey J. , Shire S. J.
Published: February 2004

The moment method developed by Gordon is used to calculate upper and lower bounds to the coefficient of R−8 in the expansion of the interatomic potentials of the systems, H–H, H–He, and He–He.
New Type of Triplet Species in the TCNQ Ion-Radical Salts
Authors: Brown I. M. , Jones M. T.
Published: February 2004

New triplet states in the ion-radical salt φ3AsCH3+(TCNQ)2− over and above the regular triplet exciton state are reported. These states are characterized by the properties of their ΔM = 1 spectra. At least one of the triplet states is an excited state with a singlet–triplet splitting of 2.5 cm−1. The maximum values of the zero-field splittings and the limited angular dependence of the spectra appear to indicate that the triplets arise from two unpaired electrons delocalized over sites located on different adjacent TCNQ chains. The lack of hyperfine structure is interpreted to indicate that there may be some motion associated with one or all of the triplet entities, but that this motion, if it exists, is slower than that of the regular exciton.
Symmetry Property of the Critical Point Responsible for Combination Bands in the OH Stretching Region of Ca(OH)2
Authors: Oehler O. , Günthard Hs. H.
Published: February 2004

The complicated band structure of Ca(OH)2 in the OH stretching region can only be understood by assuming combinational transitions occuring in critical points at k≠0. According to a former paper, the bands closest to the center of the band pattern are due to combinations of an internal OH stretching mode with acoustical modes. Using symmetry considerations, it is shown in this paper which symmetry property the critical points responsible for such transitions must have. Points with little group symmetry C2h were found as the most probable ones.
Excess Ultrasonic Attenuation and Volume Viscosity in Liquid Methane
Authors: Singer James R.
Published: February 2004

Ultrasonic attenuation in liquid methane has been measured at temperatures between 94 and 146°K with pressures to 87 kg/cm2. The measured attenuation greatly exceeds that accounted for by the dissipation due to shear viscosity and thermal conductivity. This shows a considerable need for the volume viscosity in the attenuation equation of Navier–Stokes. The excess attenuation observed is attributed to the “intrinsic” volume viscosity in liquid methane. The measured attenuation divided by the frequency squared varies from 0.605 × 10−16 cm−1 sec2 at a temperature of 94°K and pressure of 1.65 kg/cm2 to 1.90 × 10−16 cm−1 sec2 at a temperature of 145.6°K and pressure of 8.8 kg/cm2. The intrinsic volume viscosity ranges from 2.26 × 10−3 g/cm·sec to 1.88 × 10−3 g/cm·sec over the same temperature and pressure variations stated above. It is not believed that the internal degrees of freedom in methane are contributing an extra term to the measured attenuation as if by some relaxation mechanism.
Quantum Vibrational Transition Probabilities in Atom–Diatomic Molecule Collisions. II Linear and Multistep Intermolecular Potentials
Authors: Cheung Agnes S. , Wilson David J.
Published: February 2004

Three methods for the calculation of vibrational transition probabilities in colinear atom–diatomic molecule collisions are outlined. These are based on the approximation of the intermolecular potential by terms which permit the solution of the Schrödinger equation by separation of variables in each of several regions into which the configuration space of the system is divided. Boundary conditions between the regions lead to systems of linear equations the solutions to which yield quantities from which the transition probabilities are easily obtained. Plots of transition probabilities for various repulsive intermolecular potentials are shown and discussed.
Analysis of the Contribution of Internal Vibrations to the Statistical Weights of Equilibrium Conformations of Macromolecules
Authors: Gō Nobuhiro , Scheraga Harold A.
Published: February 2004

The statistical weights of equilibrium conformations of macromolecules contain contributions from internal vibrations. An analysis of such vibrations, in the absence and presence of solvent, is presented from a quantum-statistical-mechanical point of view. Several classical approximations to the quantum-mechanically correct expression, with different degrees of accuracy, are derived. In all of these approximations, all internal degrees of freedom of the polymer are divided into two classes: hard (bond stretching and bond angle bending) and soft (torsional rotations around single bonds, i.e., variation of dihedral angles). Since the hard variables oscillate many times before the soft variables change in value by an appreciable amount, the hard variables can be treated effectively as parameters (i.e., not as independent variables), which are (i) functions of the instantaneous values of the soft variables, or (ii) simply constants, the latter treatment being less accurate. In treatment (i) the molecule is regarded as flexible, whereas in treatment (ii) bond lengths and bond angles are assumed to be rigidly fixed, while the dihedral angles can change. In both treatments, the sum of the zero-point energies corresponding to the hard degrees of freedom must be added to the soft-mode part of the energy unless the errors in the calculation of the latter are ≥0.1 kcal/unit, which is the usual magnitude for the change in zero-point energies for various conformations. The soft degrees of freedom are treated classically, i.e., the statistical weight is given by integration of the Boltzmann factor over phase space. The integration over the momentum space of the soft variables yields a conformation-dependent term, proportional to ln detG, which may be called the kinetic entropy (G being a coefficient matrix for the kinetic energy of a polymer in the canonical expression for the Hamiltonian). A practical method is given for the calculation of detG, and it is applied to a simple example. The result shows that the dependence of ln detG on the coordinates is usually not negligible. For states with small conformational fluctuations (helical polymer structures and globular proteins), the result of treatment (ii) can be used as a perturbational step to proceed to treatment (i). Further approximations, necessary for treating random-coil states (ones with large conformational fluctuations), are discussed. The effect of solvent on the statistical weights is also discussed.
Modified Random-Walk Problem and the Unwinding of DNA
Authors: Vitta Paul B.
Published: February 2004

The random-walk problem is modified by letting the length d and the duration τ of the walker's step change as a function of the distance r from the origin. The theory is developed by taking into consideration the variation of both d and τ with r. This is accomplished by replacing d and τ with their respective Gaussian averages over r. A situation like this arises in the problem of the unwinding of DNA, when the problem is considered as one of rotational Brownian motion. In that case, the angular velocity increases as the DNA rod shortens because of unwinding. The effect of this variation is to reduce the unwinding time by a factor of 4/9 from that calculated previously on the basis of a constant angular velocity. Other improvements on previous calculations are made.
Second-Order Perturbation of the Velocity Distribution in a Fast Gas-Phase Reaction
Authors: Morris B. M. , Present R. D.
Published: February 2004

The chemical-kinetic Boltzmann equation for a single gas is solved to second order in order to determine the nonequilibrium correction to the reaction rate for fast-activated reactions (with activation energy ε*) and for free-radical recombinations resulting from the perturbation of the Maxwell distribution. The Enskog method is followed and the first- and second-order perturbation functions are represented by Sonine polynomial series. Retaining the first two polynomials in each series (second subapproximation), we find that the second-order correction to the reaction rate is negligible when ε* / kT = 10, but it is comparable to the first-order correction when ε* / kT = 0, 2, and 5. The Sonine polynomial series for the total perturbation function in second order converges satisfactorily for the fastest reactions, although this is not true for the series representing the second-order part of the perturbation. Spurious negative values of the distribution function that appear in first order for ε* / kT = 0 and 5 are eliminated in second order. The nonequilibrium correction to the reaction rate, as determined in first order and to the second subapproximation, is not greatest when ε* = 0, but has its maximum value when ε* / kT≃5 and is then about 8% (this was checked by the variational method). The second-order result is that the nonequilibrium correction still has a maximum close to ε* / kT = 5, where the correction is about 6%, and the reason for the maximum is discussed. Parallel calculations for free-radical reactions show that the nonequilibrium corrections are utterly negligible.
Optical-Absorption Spectra of Ketyl Radicals and Radical Anions of Some Pyrimidines
Authors: Hayon E.
Published: February 2004

The pulse radiolysis of aqueous solutions of various pyrimidines has been studied under conditions where only solvated electrons are made to react with them. Transient optical-absorption spectra in the uv range have been obtained which provide information on the site of attack of eaq− on the pyrimidine molecule. These spectra have been identified as the ketyl radicals of the C2 and C4 carbonyls, based on examination of substituted pyrimidine derivatives and making use of the keto–enol tautomerism which these molecules undergo. At pH5.0eaq− add at both the C2 and C4 positions, and the intermediates are rapidly protonated to give the corresponding ketyl radicals. The ketyl radicals dissociate to the corresponding ketyl radical anions with a pK ∼ 7.0–7.5. In alkaline solutions, on ionization of one of the chromophores the ketyl radical anion of the other nondissociated carbonyl is formed. The ketyl radicals were found to have an extinction coefficient of ∼1000 M−1·cm−1 and to decay by a second-order process with 2k ∼ 109 M−1·sec−1.
Transient Species Produced in the Photochemistry of 5-Bromouracil and Its N-Methyl Derivatives
Authors: Langmuir M. E. , Hayon E.
Published: February 2004

The flash photolysis of aqueous solutions of 1-methyl-5-bromouracil, 3-methyl-5-bromouracil, and 1, 3-dimethyl-5-bromouracil has been studied as a function of pH, in the presence and absence of oxygen and nitrous oxide. In neutral solutions, the main primary photolytic process is the formation of Br atoms and the corresponding debrominated uracil radical. Two transient species with maxima at ∼ 320 and 420 nm are formed and are suggested to be the ketyl radicals of the corresponding barbituric acid at the C2 and C4 positions, respectively. These ketyl radicals have as precursor the debrominated uracil radical. Results obtained from pulse radiolysis are used in support of this assignment. In alkaline solutions, in addition to debromination, photoionization of the anions of 5-bromouracil and of 3-methyl-5-bromouracil at the C2 position occurs with the formation of eaq− and the C2–alkoxy radical. The anion of 1-Me-5-BU at the C4 position and 1, 3-dimethyl-5-BU do not photoionize. Transient spectra obtained in the flash photolysis of 5-bromouracil atpH 13.0 are given and discussed.
Optical Rotation of Helical Polymers: Periodic Boundary Conditions
Authors: Loxsom F. M.
Published: February 2004

A general expression for the optical rotation of large helical polymers is developed. The assumption of periodic boundary conditions is shown to be valid. A major term in the optical rotation is found to be a direct result of the helical symmetry as reflected in the correct dipole selection rules.
Relaxation Rates of Alkali Metal and Chloride Nuclei in Aqueous Solution. Quadrupole Moment of 133Cs
Authors: O'Reilly D. E. , Peterson E. M.
Published: February 2004

Relaxation rates of 7Li+, 23Na+, 87Rb+, 133Cs+, and 35Cl− in aqueous salt solutions have been measured at 297°K at various salt concentrations and extrapolated to infinite dilution. Relaxation rates increased in proportion to viscosity (except for 7Li+) in deuterium oxide solutions, demonstrating that the relaxation mechanism is via the electric quadrupole moment of the solute nuclei interacting with fluctuating electric-field gradients produced by the solvent molecules. A general expression for the relaxation rate is given; it was found that the correlation time for the solute-nucleus–solvent-molecule interaction is a slowly varying function of ionic radius of the solute nucleus. The nuclear electric quadrupole moment (Q) of 133Cs is calculated from the relaxation rate of 133Cs in CsI solutions and the observed mechanism of the relaxation process. The calculated value of | Q | = (2.3 ± 0.3) × 10−27cm2 is somewhat smaller than the previously reported value derived from the nuclear optical double resonance technique.
Monte Carlo Simulation of the Unwinding of a DNA-like Helix. II. The Effect of Interstrand Forces
Authors: Simon Eliot M.
Published: February 2004

A Monte Carlo technique is used to simulate the kinetics of the unwinding of DNA. Use is made of a high-speed computer to solve the equations of motion for the elements of a macromolecule originally set in a helical configuration. Suitable boundary conditions help simulate excluded volume and van der Waals interactions between opposite strands. The results show an exponential relaxation unwinding process with a terminal relaxation time preceded by a short-lived transient. The model of unwinding shows a rapid end unwinding followed by a progressive unwinding inward from the outer portions of the original helix. The unwinding time is seen to be critically dependent upon the interstrand potential energy.
Variational Approach to the Equilibrium Thermodynamic Properties of Simple Liquids. I
Authors: Mansoori G. A. , Canfield F. B.
Published: February 2004

A variational technique which is based on two different inequalities for the Helmholtz free energies is used to calculate the equilibrium thermodynamic properties of simple fluids. A system with hard-sphere potential function is used as the reference system. Helmholtz free energy of the original system is calculated by variation around the Helmholtz free energy of the reference system, and the other thermodynamic properties are calculated from free energy. By choosing a hard-sphere reference system, it is possible to calculate the equilibrium thermodynamic properties of fluids from very low densities to densities close to solid, and from high temperatures in the gas phase to low temperatures in the liquid phase, in the ranges where experimental and machine-calculated data are available. It is shown that the present variational technique is a better approach to the prediction of the equilibrium thermodynamic properties of liquids and vapor–liquid phase transition than any other approach so far developed. While the variational calculation based on a hard-sphere reference system does not predict the liquid–solid phase transition, it is argued that this might be due to the neglect of the orientation, or ordering in the formulation of the working inequality for fluids.
Variational Approach to Melting. II
Authors: Mansoori G. A. , Canfield F. B.
Published: February 2004

A variational approach to the equilibrium thermodynamic properties of an original system based upon an inequality for the Helmholtz free energy of that system is introduced. A system with molecules obeying the cell model of Lennard-Jones and Devonshire, and having a harmonic-oscillator-type potential function inside their cells, is used for a reference system to produce the inequality for the Helmholtz free energy of the original system. Optimization upon this inequality indicates that a variational calculation based on a reference system with highly ordered structure, as the cell model, predicts the properties of the solid phase better than the liquid phase. Also, it shows that by an ordered-structure reference model, it is possible to predict the liquid–solid phase transition. Equilibrium thermodynamic properties of the solid phase and liquid–solid phase equilibria are calculated and are compared with the machine-calculated and the experimental data.
Electrostatics of Diffuse Anisotropic Interfaces. I. Planar Layer Model
Authors: Buff Frank P. , Goel Narendra S.
Published: February 2004

In problems involving the electrostatic potential due to charges in the presence of a diffuse interface, for simplicity it is assumed that the interface is an abrupt one. We have investigated the effects of relaxing this assumption. By a procedure related to the method of invariant imbedding, we have formulated, within the regime of macroscopic continuum theory, the calculation of the electrostatic potential for a charge in the presence of an anisotropic diffuse layer. For distances far from the interface, the leading term in the image potential is the same as that for an abrupt interface, but referred to an origin which is determined by the components of the dielectric tensor. The next term arising from an induced quadrupole moment is negligible in representative cases. Close to the interface, divergences in the image potential can only be avoided if the values and first derviatives of the dielectric parameter are continuous. These results are illustrated by model dielectric profiles for the diffuse layer for which analytic expressions are derived for the electrostatic potential. It is found that as the dielectric profile becomes smoother, the value of image potential decreases.
Light Scattering from Reactive Fluids. III. Intensity Calculation
Authors: Blum L.
Published: February 2004

Using standard perturbation theory for degenerate matrices, an expansion for the eigenvectors corresponding to the thermodynamic fluctuation modes in a chemically reactive fluid is obtained. The expansion is valid for small scattering angles, that is, vanishing spatial gradients. These eigenvectors give rather complicated expressions for the intensities of the individual lines, making it rather impractical to get these intensities. We have therefore evaluated partial sums of integrated intensities, which depend only on thermodynamic equilibrium parameters and stoichiometric coefficients. We evaluate explicitly the ratio of the intensity contribution due to the chemical reactions and diffusion processes (Berne–Frisch approximation). For the case of the reaction AB⇆A + B, the intensity is proportional to the net change in the dielectric polarizability in the reaction, in accordance with the result obtained by Berne, Deutch, Hynes, and Frisch for a simpler case. Our method is, however, general and can be used for any number of components and reactions.
Kinetics of a Sequence of First-Order Reactions
Authors: Ninham Barry , Nossal Ralph , Zwanzig Robert
Published: February 2004

Solutions are obtained for the finite set of coupled rate equations ∂Ci / ∂t = αi,i−1Ci−1 + αi,iCi + αi,i+1Ci+1 (i = 0, ···, N), where αi,j are given in general as αi,i−1 = A, αi,i+1 = B, αi,i = − (A + B), except that α0,0 = − α1,0 = − a, αNN = − αN−1,N = − b, α0,−1 = αN,N+1 = 0. Asymptotic expressions are given for the approach to equilibrium as a function of the various rate parameters and the chain length N. For large N, we find that if A < B, the eigenvalue spectrum approaches a continuum, and the approach to equilibrium is described by a simple relaxation time λ1 ≃ (A1/2 − B1/2)2. However, if A(1 − a / A)2 > B, the system exhibits a peculiar eigenvalue spectrum, and the relaxation is characterized by two distinct and well-separated relaxation times, λ1 and λ2 = − a{1 − B[A(1 − a / A}2]−1).
Phosphorescence Spectrum, Vibronic Analysis, and Lattice Frequencies of the Naphthalene Molecule in a Deuteronaphthalene Crystal
Authors: Hanson David M.
Published: February 2004

The phosphorescence spectrum of naphthalene showing 94 transitions between 21 224 and 16 880 cm−1 has been obtained. These data help resolve uncertainties in the vibrational assignments, suggest the presence of localized lattice vibrations in isotopic mixed crystals, and may indicate the relative importance of singlet–triplet coupling mechanisms in naphthalene and some halogenated naphthalene derivatives. The data have been used to identify the corresponding absorption and phosphorescence of the pure crystal and might be used in explaining temperature, environmental, and isotope effects on the radiative or radiationless transition probabilities of the naphthalene molecule.
Atomic and Molecular Calculations with the Pseudopotential Method. V. Two-Valence-Electron Energy Values and Oscillator Strengths for Some (ns)2 Ground States and (nsml) Singly Excited States
Authors: McGinn George
Published: February 2004

The pseudopotential method is applied to a number of two-valence-electron ground states and singly excited states. For the (ns)2 ground-state configurations of Li−, Na−, K−, Rb−, Cu−, Ag−, Be, Mg, Ca, Zn, and Al+ a two-electron pseudopotential equation equivalent to the Hartree–Fock equation is solved. For Be and Ca a number of singly excited states of the form (nsml) are calculated using a further approximation where the nonexcited valence electron is “frozen.” The S1,3 and P1,3 states with m = 2−4 for Be and m = 4−6 for Ca and also the D1,3 states with m = 3−5 for Ca are computed. The transition integrals and f values using the calculated values for all allowed transitions are given.
Semiclassical Methods in Inelastic Scattering
Authors: Cross R. J.
Published: February 2004

A hierarchy of related semiclassical approximations is derived for treating molecular inelastic scattering. Starting from the coupled Schrödinger equations of motion, the radial wavefunctions are expressed as ui(r) = u1i(r)Xi(r) + u2i(r)Yi(r), where u2i and u2i are two linearly independent approximate solutions for elastic scattering. A set of coupled first-order equations for Xi and Yi is obtained. By using simple WKB wavefunctions for u1 and u2 and by neglecting several terms, the coupled time-dependent equations and several previous semiclassical approximations can be readily obtained. Inclusion of the neglected terms yields improved accuracy for these approximations. Use of modified WKB wavefunctions valid near the classical turning point extends these approximations to cases involving large energy transfer where the inelastic cross sections are sensitive to the region near the turning point. In the sudden and “classical” limits of the internal molecular motions the coupled equations in X and Y can be decoupled by using simple transformations. For cases close to these limits a set of coupled equations is obtained giving the deviation from the sudden or “classical” limit.
Polyatomic SCF Calculations Utilizing Anisotropic Basis Sets of Slater-type Orbitals
Authors: Switkes Eugene , Stevens Richard M. , Lipscomb William N.
Published: February 2004

Fully optimized SCF wavefunctions using minimum Slater-type basis sets have been obtained for BH3, NH3, C2H2, C2H4, HCN, and H2CO. Calculations employing anisotropic minimum basis sets are reported for NH3, H2O, HCN, and H2CO. The lower SCF energies of the calculations for anisotropic sets are primarily due to a reduction in electron–electron repulsion. Improvements in calculated dipole moments occur when independent p-orbital components are used to describe lone pairs and covalent bonds. A geometry-optimized calculation for H2O is also reported.
Dependence of Generalized Oscillator Strengths of Ethylene on Momentum Transfer
Authors: Miller Kenneth J.
Published: February 2004

Generalized oscillator strengths are calculated as a function of scattering angle or K, the momentum transfer, for seven electronic transitions in ethylene in order to characterize each type of transition. The valence transitions Ag1 → 1B1u[π → π*] and Ag1 → 1B1g[σ → π*], and the Rydberg transitions Ag1 → 1B1g[π → σ*], 1Ag → 1B2g[π → σ*], 1Ag → 1B3u[π → σ*], 1Ag → 1B3g[σ → σ*] and Ag1 → 1B3u[π → σ*] are studied. Characteristic minima in f(K) for the Rydberg transitions appear. This behavior is attributed to the presence or absence of nodes in the molecular orbitals. It is suggested that this phenomenon be used as a probe of the type of molecular orbitals which are involved in a particular transition as well as the location of optically forbidden transitions. Qualitative agreement is obtained with experimental results for all but the π → π* transition, which does display a minimum. This is probably due to configurational mixing with Hartree–Fock Rydberg states.
Nuclear Magnetic Resonance and Magnetic Susceptibility of Pr3Al11, Nd3Al11, and EuAl4
Authors: van Diepen A. M. , Buschow K. H. J. , de Wijn H. W.
Published: February 2004

The Knight shifts of the 27Al NMR in Pr3Al11, Nd3Al11, and EuAl4 have been measured between 86 and 300°K, and have been found to be different for inequivalent Al sites. The results are discussed in terms of the Ruderman–Kittel–Kasuya–Yosida theory. The magnetic susceptibilities have been measured in the temperature region from 300 down to 4.2°K, and found to deviate from Curie–Weiss laws only for temperatures lower than 40°K.
Infrared Spectra of Crystalline Cyclopentane and Cyclopentane-d10
Authors: Schettino V. , Marzocchi M. P. , Califano S.
Published: February 2004

Infrared spectra of crystalline cyclopentane and cyclopentane-d10 have been measured at different temperatures between 4000 and 200 cm−1. Spectroscopic evidence shows that form I (− 120°C) and form II (− 140°C) have the same degree of disorder as the liquid. In the ordered crystal form III (− 180°C) the molecule has a rigid bent conformation. By the mixed-crystal technique the nature of the observed splittings has been established. This has permitted us to show that the site symmetry of the molecule in form III is C1, and packing concepts together with the spectroscopic data led to the choice of C2h5 as the possible space group of the lowest-temperature form.
Hartree–Fock Approximation to the (ns2)1S Autoionizing States of Helium
Authors: Ramaker D. E. , Stevenson R. B. , Schrader David M.
Published: February 2004

As a first step in estimating the utility of the Hartree–Fock method for calculating positions and widths of autoionizing levels of atoms and molecules, an investigation of the helium problem has been initiated and is reported here. Wavefunctions and energies for the ten lowest (ns2)1S states are calculated by numerically integrating the Hartree–Fock equation for solutions corresponding to energy levels above the ground state. Except for n = 1 all these states are autoionizing, and each is probably the lowest bound state for each closed channel for n = 2(1)10. The energies of the ns2 states are found to be between the nth and (n − 1)th thresholds for n = 2(1)6 but for n > 6 the calculated levels fall below the (n − 1)th threshold. The method of numerical integration used is described, and the wavefunctions are reported in terms of an expansion in a basis determined by a least-squares curve-fitting method.
Collision-Induced Absorption Spectrum of Gaseous Oxygen at Low Temperatures and Pressures. II. The Simultaneous Transitions Δg1 + 1Δg ← 3Σg− + 3Σg− and Δg1 + 1Σg+ ← 3Σg− + 3Σg−
Authors: Blickensderfer Roger P. , Ewing George E.
Published: February 2004

The collision-induced absorption spectra have been measured at room temperature and at 87°K for bands in the Δg1 + 1Δg ← 3Σg− + 3Σg− and Δg1 + 1Σg+ ← 3Σg− + 3Σg− simultaneous electronic systems for oxygen. The binary absorption coefficients were found to increase with decreasing temperature for Δg1 + 1Δg ← 3Σg− + 3Σg−. The band shapes for this system suggest that the Hamiltonian which is responsible for intensity borrowing depends on the angular orientation of the O2 molecules in the collision pair since ΔK = 0, ± 2, ± 4 selection rules are needed to account for the Δν1 / 2 ∼ 200 cm−1 bandwidth. The relative intensity of the (1–0) and (0–0) bands indicates that the interaction Hamiltonian is also strongly modulated by the vibrational coordinates of O2. The frequency shift of this simultaneous transition indicates that the intermolecular distance parameter for Δg1···1Δg is 3% larger than for Σg−3···3Σg−. The unusual band shape for the Δg1 + 1Σg+ ← 3Σg− + 3Σg− band is interpreted in terms of an exiton interaction for the Δg1···1Σg>+ combination. Although bound state (O2)2 molecules undoubtedly exist at low temperatures these data provide no unambiguous spectroscopic evidence of their presence.
NMR Study of Internal Motions in Certain Iron Group Fluosilicates
Authors: Rangarajan G. , Ramakrishna J.
Published: February 2004

NMR study of ferrous fluosilicate hexahydrate indicated the presence of motion of both proton and fluorine nuclei. Only a single narrow line was observed for protons for any arbitrary orientation of a single crystal with respect to the applied magnetic field. This can be interpreted in terms of a phase-correlated flip motion of the interproton vectors between two disordered orientations or in terms of a hindered rotation of the Fe(H2O)6 octahedron about the fourfold axes, together with the flip motion. The fluorine second moment indicated that the SiF6 octahedron also is undergoing reorientation. The temperature variation of the powder linewidth showed a transition around 195°K and led to rather low values for the potential barriers hindering the motions. No significant temperature variation of the linewidth was observed for hexahydrated cobalt fluosilicate in the temperature range between 90°K and room temperature. Similar observations in a powder sample of tetrahydrated copper fluosilicate also showed the presence of internal motions. The linewidth transition in this case took place at about 220°K and was found to be rather abrupt. The potential barrier for the motion was found to be relatively high.
Ultrasonic Dispersion Observed by Optical Diffraction. III. Allene and Carbon Suboxide
Authors: Hancock J. Kent , Decius J. C.
Published: February 2004

The optical diffraction method has been used to measure sound velocities in allene over the range 0.3–33 MHz/atm and in carbon suboxide from 1.1–40 MHz/atm. In allene the lower fourth of the dispersion region has been covered, and we find all the vibrational degrees of freedom relax together at τ = 5.2 nsec at 300°K. Carbon suboxide has been found to be doubly dispersive with all of the vibrational specific heat with the exception of the lowest bend (ν7 = 63 cm−1) relaxing at τ1 = 48 nsec at 300°K. The high-frequency relaxation associated with ν7 could not be observed but on the basis of our data we estimate τ2 ≤ 1 nsec. The subject of multiple relaxation in gases is discussed in detail and includes a survey of gases known to exhibit this behavior.
Threshold Reactions for the Formation of Ion–Dipole Pairs as a Method for Obtaining Monoenergetic Polarized Molecular Beams
Authors: Baz A. I. , Gol'danskii V. I.
Published: February 2004

The energy dependence of the cross sections of an endothermal reaction a+b→I+D (I, ion; D, dipole; e.g., Na++Cl2→Cl++NaCl) near its threshold is analyzed. When the product dipolar molecule leaves the reaction center so that the end having the same charge as that of the ion I (Na in our example) is leading, the cross section σ1 for the reaction increases rapidly from the threshold and soon attains the geometrical value. When the end having a charge opposite to that of I (Cl in our example) is leading, the cross section (σ2) increases but slowly, remaining much smaller than σ1 over an energy range of several electron volts. Therefore the reaction a+b→I+D in this region near the threshold can be used for obtaining monoenergetic beams of polarized dipole molecules D. The effect of the threshold of the above-mentioned reaction on excitation functions of other processes like a+b→c+d (including the a−b elastic scattering) is also treated.
Calculation of Predissociation Rates of the B 3Π+u State of I2
Authors: Chutjian Ara
Published: February 2004

A comparison of the purely radiative decay rates of vibrational levels of the B 3Π0+u state of I2 with total decay rates obtained from several independent lifetime measurements has indicated a significant amount of spontaneous predissociation of the B state. We present here a calculation of the observed relative predissociation rates by means of a model consisting of a repulsive 1u state which is coupled to the υ′J′ levels of the B state by rotational and electronic angular-momentum terms neglected in the zeroth-order Hamiltonian. The heterogeneous predissociation rate between a bound υ′J′ level and a continuum level at the same energy is calculated from Kronig's theory using exact wavefunctions for both levels. The dependence of this rate on the rotational level J′ is taken into account, together with the fractional population of the υ′J′ levels which were excited in the measurement of the total decay rates. Excellent agreement is found between the calculated and observed relative predissociation rates at regions about υ′ of 14, 25, and 50 of the B state for one position of the repulsive state lying very near the repulsive wall of the B state over the energy range 17 300–20 000 cm−1. The position of this state relative to the B 3Π0+u and A 3Π1u states agrees well with Mulliken's prediction of the location of the Π1u1 state of I2.
Dissociation Energy of Li2 from Laser-Excited Fluorescence
Authors: Velasco R. , Ottinger Ch. , Zare R. N.
Published: February 2004

The fluorescence spectra B 1Πu − X 1Σg+ of the molecular species 6Li2, 6Li7Li, and 7Li2, excited by the cw lines of the argon ion laser, have been observed and analyzed. Based on a short Birge–Sponer extrapolation of the vibrational levels of the ground state, the dissociation energy for the Li2 molecule has been determined to be D00 = 1.026 ± 0.006 eV. This value, combined with the dissociation limit of the upper state determined by Loomis and Nusbaum, proves that there is a hump of about 0.12 ± 0.04 eV above the asymptote of the potential curve of the Li2 B 1Πu state. Improved rotational and vibrational constants of the ground state of Li2 have also been obtained. A new technique is described which utilizes collision-induced rotational transfer to facilitate the υ′, J′ assignment of the excited levels.
Oxygen Isotope Fractionation in Divalent Metal Carbonates
Authors: O'Neil James R. , Clayton Robert N. , Mayeda Toshiko K.
Published: February 2004

Equilibrium fractionation factors for the distribution of 18O between alkaline-earth carbonates and water have been measured over the temperature range 0–500°C. The fractionation factors α can be represented by the equationsCaCO3–H2O, 1000 lnα = 2.78(106 T−2)− 3.39,SrCO3–H2O, 1000 lnα = 2.69(106 T−2)− 3.74,BaCO3–H2O, 1000 lnα = 2.57(106 T−2)− 4.73.Measurements on MnCO3, CdCO3, and PbCO3 were made at isolated temperatures. A statistical-mechanical calculation of the isotopic partition function ratios gives reasonably good agreement with experiment. Both cationic size and mass are important in isotopic fractionation, the former predominantly in its effect on the internal vibrations of the anion, the latter in its effect on the lattice vibrations.
Simple Bond-Charge Model for Symmetric Stretching Vibrations of XYn Molecules
Authors: Borkman Raymond F.
Published: February 2004

A bond-charge model, previously applied to homonuclear and heteronuclear diatomic species has been extended to symmetric XYn polyatomic molecules. The chief theoretical tool is the scaled form of the molecular virial theorem. Using this theorem, we relate the force constant for symmetric breathing vibrations to model expressions for the electronic kinetic and potential energies. As in the diatomic case, the kinetic energy is modeled as a particle in a box and the potential energy as a classical system of point charges. Using experimental force constants and bond lengths as input we have evaluated the model bond-charge and path-length parameters for 30 polyatomic species. The bond-charge parameter q is reasonably interpreted as a measure of bond order while the free-electron path-length parameter ν is a measure of atomic core radii. Values of q and ν are given for ground-state and electronically excited-state molecules as well as charged molecular ions, in symmetries D∞h, D3h, C2υ, C3υ, and Td. The path-length parameter ν is shown to correlate with position of the constituent atoms in the periodic chart. It is then shown that polyatomic ν values can be predicted accurately from ν values of homonuclear diatomic molecules. This latter result permits the possibility of predicting force constants from input of bond lengths only, and some results of this type are presented.
Spatial Distributions of the Vapor Species in Alkali Halide Effusion Beams
Authors: Winterbottom W. L.
Published: February 2004

Studies of the individual polymeric species in alkali halide effusion beams have shown that the monomer and dimer spatial distributions are not equivalent. Under molecular flow conditions, the spatial distribution of molecules emerging from an effusion channel must characterize to some extent the nature of the vapor–solid interactions that occur in the transport process. On the basis of a coupled surface- and vapor-transport model the experimental results can be reproduced quantitatively by assuming the nature of the surface interactions of monomer and dimer differ. The monomer spatial distribution is predictable for a species that is chemisorbed and characterized by a long mean free diffusion path in the adsorbed state. For the dimer, the distribution suggests a heterogeneous reaction sequence involving a dissociative adsorption and a reconstitution from adsorbed monomer.
Error Bounds to Expectation Values of One-Electron Operators Using Hartree–Fock Wavefunctions
Authors: Alexander Millard H.
Published: February 2004

The properties of Hartree–Fock wavefunctions are used to derive a number of new expressions for upper and lower bounds to the true expectation value of an arbitrary one-electron operator. These expressions depend on the prior knowledge of upper and lower bounds to the overlap between the Hartree–Fock and true wavefunctions. A new extrapolation method is presented to obtain accurate bounds to these overlaps. A critical study is made of the effectiveness and accuracy of these new error bounds as applied to the systems He, He2, LiH, and Be.