bluuebunny/crossref_metadata_2025 huggingface dataset:
Teratomic recombination of excited RbXe*
Authors: Carrington C. G. , Gallagher Alan
Published: February 2004

Free Rb(5 2S) is optically excited to Rb*(5 2P1/2) in the presence of Xe gas, and the RbXe*(A 2Π1/2)-RbXe(X 2Σ1/2) fluorescence is measured as a function of xenon pressure. The teratomic recombination process, Rb* + 2Xe→RbXe* + Xe, and other collision processes compete with radiative decay. The radiative rate is known, so collisional rates can be inferred from the data. The molecular spectrum is observed as a continuum, whose intensity profile yields the bound-state vibrational distribution at each xenon pressure. The low-pressure limit of this distribution yields the teratomic recombination rate as a function of vibrational state, and the total recombination rate constant kf. The recombination rate into a bound state increases with increasing binding energy, but less rapidly than the increase in equilibrium population. The transition from this initially formed distribution to the high-pressure equilibrium distribution is reported. The ratio kf/Keq=2.4 × 10−11 cm3/sec and the equilibrium constant Keq≃3.4 × 10−21 cm3, corresponding to the experimental temperature of 300 K, are deduced from the data. The A 2Π1/2 and X 2Σ1/2 potentials are also obtained from analysis of the spectrum in the low-pressure and high-pressure limits.
Isothermal compressibility and partial molal volume for polyatomic liquids
Authors: Gubbins K. E. , O'Connell J. P.
Published: February 2004

For dense polyatomic liquids (ρ > 2ρc), the anisotropic part of the intermolecular forces has remarkably little influence on the isothermal compressibility or on the partial molal volume for the case of mixtures; this is also the case for dilute gases at high reduced temperatures. Such behavior is shown to be predicted by a perturbation treatment in which these properties for the polyatomic fluid are expanded about the corresponding values for a fluid with isotropic intermolecular forces. For dense liquids (or dilute, high temperature gases), the perturbation terms in such series usually become negligible compared to the zeroth order term. This leads to corresponding states relationships for these properties, which have the same functional form as for fluids composed of simple isotropic molecules, such as argon. This prediction is confirmed by a comparison of isothermal compressibilities for polyatomic liquids with the values for argon at the corresponding state condition.
Initial susceptibility study of a magnetic transition in the two dimensional system (CH3NH3)2MnCl4. Absorption and dispersion in the neighborhood of the critical temperature
Authors: Gerstein B. C. , Chang K. , Willett Roger D.
Published: February 2004

Magnetic susceptibility measurements on the layer structured system bis(methylammonium) tetrachloromanganate(II) revealed a transition centered at 45.3°K, and a broad maximum in the susceptibility at 80°K. The critical exponents for the sharp transition were found to be γ′ =2.39(7), T < Tc, and γ=2.90(13), T > Tc. The susceptibility was found to be field dependent below Tc, with (∂χ/∂H)T > 0.
Mechanism of chemical instability for periodic precipitation phenomena
Authors: Flicker Michele , Ross John
Published: February 2004

A mechanism of chemical instability of autocatalytic reactions coupled with diffusion is proposed for spatially periodic precipitation phenomena (such as Liesegang rings or bands). We postulate that precipitation is preceded by colloid formation; electric double layers surround the colloid particles, and at sufficient concentration of colloid, autocatalytic growth of particles and formation of new particles occur owing to an increased product of ion concentrations within the double layer with an increased colloid concentration. A linear stability analysis of the reaction-diffusion equations for this mechanism predicts (1) formation of macroscopic inhomogeneities (spatial structures) due to the onset of instability in the absence of imposed external gradients and prior to the appearance of any visible bands; (2) ranges of the characteristic length scales and time scales of the initial structure pattern; (3) phase relations of concentrations of ionic species and colloid; (4) variation of characteristic length scales with initial concentrations, and rate and diffusion coefficients of ions and colloid; (5) a mechanism for secondary structure of precipitation bands; (6) the possibility of revert structure (decreasing distance between bands away from the origin of an imposed gradient); and (7) the suggestion that the imposition of a concentration gradient, as in a Liesegang experiment, polarizes and may alter the shape of the periodic precipitation structure but is not the basic cause of that structure. The quantitative nature of the structure requires solution of the nonlinear reaction-diffusion equations which is not attempted. The predictions of the proposed mechanism are in substantial agreement with experiments. We report on some experiments which provide evidence for items (1), (6), and (7); for the remaining items we refer to experiments reported in the literature. We conclude with a critique of the Ostwald theory of Liesegang rings, which does not predict revert spacing nor secondary structure. We provide additional experimental evidence against the requirements of this theory of an imposed concentration gradient that in time brings about supersaturation followed by precipitation.
On the aggregation of mixtures of long and short chain molecules
Authors: Brady George W.
Published: February 2004

The results of a series of experiments on the aggregation of long ( C20−) and short (C2−) chain alkanes in mixtures of the two are presented. By labelling the two types of chain with atoms of different scattering power, the interaction and location of each can be identified. It is found that the short chains, when in large concentration, group into satellite appendages of the long chain aggregates; the resultant aggregation schemes are quite complex. From an x-ray viewpoint, the technique allows the scattering to be put on an absolute basis such that it can be resolved into its individual components without the use of Fourier transform methods.
Stochastic models of the interconversion of three or more chemical species
Authors: Bartis James T. , Widom B.
Published: February 2004

The phenomenological rate equations for reactions in which three or more chemical species are simultaneously interconverting are derived from a microscopic stochastic model. Particular attention is focused on the establishment of long chemical relaxation times, and on an important orthogonality property which guarantees that the principle of detailed balancing is obeyed. By developing a quantum mechanical analog, the mathematical origins of both of the above properties are related to a resonance phenomenon associated with three or more wells separated by high energy barriers. The quantum analog is itself equivalent to a stochastic master equation, the rate constants of which are analytically determined. These are shown to contain the expected Arrhenius factors and to obey the principle of the independent coexistence of reactions.
Translational and rotational diffusion in liquids. I. Translational single-particle correlation functions
Authors: Chandler David
Published: February 2004

This is the first of two articles which together present a theory for translational and orientational single-particle correlation functions in molecular liquids. The theory is based on the assumption that the single-particle motion in a liquid is determined primarily by the harsh repulsive parts of the intermolecular forces. It is shown that the dynamics due to the repulsive forces can be related to the particle motion in a model rough hard sphere fluid. The dynamics of the model hard sphere system is studied within the context of a binary collision expansion. An approximate formula is derived for the translational self-correlation function of the rough hard sphere fluid. The formula is computationally convenient. Arguments are given which indicate that the formula is accurate. In particular, it is shown that the single-particle distribution function obtained from the approximation is positive; the function is well behaved in the hydrodynamic limit, the approximation becomes exact at low densities; the first three time derivatives of the distribution function at t = 0 are exact for all densities; and the velocity autocorrelation function obtained from the distribution function is the Enskog result when the hard spheres are perfectly smooth. A quantitative criterion is proposed to determine the rough hard sphere model which is most closely related to a real liquid.
Translational and rotational diffusion in liquids. II. Orientational single-particle correlation functions
Authors: Chandler David
Published: February 2004

This is the second of two articles concerned with the translational and rotational motion in molecular liquids. In this paper, orientational single-particle correlation functions are considered. By applying the approximations developed in Paper I of this series, Gordon's J diffusion model theory is derived. The analysis does not require one to adopt the unrealistic views of the liquid state that are usually attributed to that theory. The range of validity of the J diffusion model approximation is ascertained. Further, a microscopic expression is derived for the relaxation time which Gordon introduced phenomenologically. The expression leads naturally to a simple method for calculating τJ. These predictions are tested against results obtained from recent computer simulations of liquid N2. The comparison shows that the theory is qualitatively accurate.
Kinetic theory of polyatomic liquids. I. The generalized moment method
Authors: Theodosopulu Mary , Dahler John S.
Published: February 2004

It is demonstrated that one can successfully generalize the moment method of dilute gas kinetic theory to a dense fluid composed of polyatomic molecules. The basic premise of the theory is Bogoliubov's assumption of the existence of a functional relationship between the pair and singlet distribution functions. The theory is rendered explicit by selecting for this functional the generalization to polyatomic fluids of a relationship first obtained by Severne and subsequently modified and generalized by Prigogine, Nicolis, and Misguich and by Davis. This leads to a closed set of approximate moment equations from which are extracted explicit, computationally tractable formulas for the characteristic relaxation times and frequency dependent transport coefficients associated with the transport of momentum, energy, and molecular internal angular momentum or spin.
Chain ordering in liquid crystals. I. Even-odd effect
Authors: Marčelja S.
Published: February 2004

In the liquid crystalline phases of many organic materials molecular end chains take part in the ordering process. For a single chain, this ordering is calculated by exact summation over all conformations of a chain in the molecular field due to the neighboring molecules. The nematic orientational ordering of a system where molecules consist of rigid parts and chains is then obtained within a self-consistent molecular field approximation. The results are in very good agreement with the experimental phase diagrams for homologous series of liquid crystals, and for the first time provide the explanation for the well known even-odd effect in isotropic-nematic transition temperatures. In agreement with observations, the corresponding transition entropies exhibit both the increase with increasing end-chain length and the even-odd variations. The decrease of order along the chain is also calculated and compared with the available data.
On the assignment of the lowest triplet state of 1,4-dihalobenzenes. MODR spectrum of the 3B1u state of p -dichlorobenzene in a p -dibromobenzene host crystal
Authors: Kothandaraman G. , Yue H. J. , Pratt D. W.
Published: February 2004

A careful examination of the orientational dependence of the microwave-optical double resonance (MODR) spectrum of the lowest (π,π*) triplet state of p -dichlorobenzene (DCB) in a host crystal of p -dibromobenzene at liquid helium temperatures yields accurate values of the principal components of the zero-field (ZF) splitting tensor (X = ∓ 0.0987, Y = ± 0.0780, and Z = ± 0.0207 cm−1) and the g tensor (gxx = 1.986, gyy = 2.001, and gzz = 2.002). Formally forbidden transitions involving simultaneous electron/nuclear spin flips are also observed at high microwave power levels, and these are attributed to a transferred hyperfine interaction between the guest triplet state and the bromine atoms of the host. When combined with the results of previous PMDR studies on this system, these experiments show conclusively that the ordering of the ZF levels of the lowest triplet state of DCB is τy > τz≫τx and that the orbital symmetry of this state is B1u. This is the state derived from the accepted assignment of the lowest triplet state of benzene itself.
A theoretical analysis of the shock compression experiments of the liquid hydrogen isotopes and a prediction of their metallic transition
Authors: Ross Marvin
Published: February 2004

Shock compression experiments in which the liquid hydrogen isotopes were compressed as much as sevenfold in density to pressures of the order of 900 kbar are analyzed to obtain an effective intermolecular potential. Because temperatures as high as 7000°K are generated during the shock process, this potential is most accurate in the highly repulsive, small separation region, and is thereby well suited to calculate the properties of dense molecular hydrogen in the region of the metallic transition. By equating the Gibbs free energies of the metal and molecular phases, we calculate the pressure and density of the metallic transition. Unfortunately, the large experimental error and the sensitivity of the transition to the free energy allows us only to estimate a lower bound of 2 to 3 Mbar for the transition pressures.
Pressure dependence of the dielectric constant of H2O and D2O
Authors: Srinivasan K. R. , Kay Robert L.
Published: February 2004

The static dielectric constant of H2O and D2O have been measured at 10, 25, and 40°C at pressures up to 3 kbar by a bridge method. An all-glass, three-terminal, Kay-Vidulich-type dielectric cell was designed specifically for these high pressure measurements and was used in conjunction with a bridge that was equipped with a capacitance-inductance free, conductance balancing network. The structurally significant quantities gKμ2 and gK, the Kirkwood correlation factor, were evaluated from the experimental data and the Fröhlich equation. Their pressure dependence and the structural implications are analyzed in detail.
Oxidation of cyanogen. I. Ignition behind reflected shocks
Authors: Lifshitz Assa , Scheller Karl , Bass Davy
Published: February 2004

The ignition of cyanogen in cyanogen-oxygen-argon mixtures, and the distribution of reaction products before and after ignition, were studied behind reflected shocks in a single pulse shock tube. The measurements covered the temperature range 1400–2000°K at pressures P5 varying from roughly 5 to 24 atm. The composition of the reaction mixtures varied from 3.0 to 9.0% C2N2 and 3.0 to 10.5% O2. Under these conditions, the observed induction times ranged between 15 and 450 μsec. A least squares analysis of approximately 150 tests yielded the following empirical relation: τ = 10−13.97 exp(34 700/RT)[C2N2]−1.01 [O2]−0.21[Ar]+0.22,where τ is given in seconds and concentrations are in moles per cubic centimeter. An additional twenty-three tests were run, in which 1% hydrogen was added to a stoichiometric mixture of 8.6% C2N2 and 8.6% O2. The ignition delay times under these conditions were considerably shorter and the temperature dependence was found to be much weaker, 23.7 compared to 34.7 kcal/mole in the absence of hydrogen. In a number of tests, samples were taken from the shock tube and examined for product distribution. These were taken before and after ignition and analyzed by mass spectrometry. In stoichiometric mixtures, the main products before ignition are N2, CO, and some CO2, which disappears after ignition. In mixtures with excess oxygen, the products following ignition are CO2, N2, and some CO.
Use of biorthogonal orbitals in perturbation calculation of molecular interactions from a multiconfigurational unperturbed state. II
Authors: Gouyet J. F.
Published: February 2004

It has been shown in a previous paper that molecular interactions can be diagrammatically expanded using biorthogonal orbitals and non-Hermitian operators so that all intramolecular terms cancel. The purpose of this paper is to extend this method to the general case where degenerate and quasidegenerate configurations appear both in each subsystem and in the supersystem. We also study in more detail the development of the wavefunctions and the consequences on varied observables.
Ionizing collisions of fast alkali atoms with Cl2, Br2, and O2
Authors: Kashihira N. , Schmidt-Bleek F. , Datz S.
Published: February 2004

A time-of-flight apparatus is described for measuring the inelastic energy loss incurred by fast (< 50–350 eV) alkali atoms in ionizing collisions with diatomic molecules. The inelastic energy losses were found to be nearly energy independent for collisions between K and Cs with Cl2, Br2, and O2, and it is furthermore shown that these energy losses are from about 8 to 10 eV, irrespective of the reactants. The collisions between alkali atoms and halogen molecules (Cl2 and Br2) yield as intermediate alkali ions (M+) and excited molecular ions, (X2−) in the 2Σg state, followed by dissociation to the ground-state halogen ions and excited halogen atoms. In the case of oxygen, excited molecular ions in the 2πu state are formed and these species either remain in the bound state of the 2πu or decay to vibrationally excited molecular states (bΣg+, aΔg). The excitation functions (change of cross sections with incident energy variations) are interpreted in terms of curve crossings near or inside the equilibrium distance of the M−X2 potential surfaces. A comparison of the results obtained at high energies (20–200 eV c.m.) is made to those known from collision energies between 1 and 20 eV (c.m.) and a composite excitation function for the three most probable processes over the whole energy range is constructed.
Quantum theory of a collinear collision of a particle with a harmonic oscillator
Authors: Liu William S.
Published: February 2004

The unitary theory of a collinear collision of a particle with a harmonic oscillator is studied. A comparison is made between the unitary results and the exact calculations of Secrest and Johnson. It is shown that the unitary theory of inelastic scattering, which inclusdes the effect of the distortion of the wavefunction, can lead to accurate estimates of both single- and multiple-transition probabilities and is superior to other improved quantum theories. The indirect transitions are important even for the low energy case, where the probabilities are low. The indirect transitions can reduce the probabilities for single-quantum jumps and significantly increase the results for multiple-quantum jumps of the distorted wave perturbation theory.
Radiative and nonradiative transition probabilities for the 1B2 state of aniline in the low pressure vapor phase
Authors: Ware William R. , Garcia Alberto M.
Published: February 2004

Fluorescence decay studies are described for the 1B2 state of aniline excited to several approximately single vibronic levels in the collision-free vapor phase. The decay is found to be strictly a single exponential for both 0.8 and 2.0 Å bandwidth excitation into either the strong bands or into some of the valleys between strong bands. The radiative transition probability is independent of excitation energy for states corresponding to the intense absorption bands, up to an excess energy of about 2000 cm−1. The nonradiative transition probability increases slowly with energy in a manner analogous to that observed for benzene. Environmental effects on the radiative and nonradiative transition probabilities are also discussed.
Resonance fluorescence involving optically active continua
Authors: Mukamel Shaul , Jortner Joshua
Published: February 2004

In this paper we utilize the T matrix formalism of scattering theory for the study of the cross sections specifying optical absorption, resonance fluorescence elastic photon scattering, resonance Raman scattering, and photodissociation for molecules undergoing direct photodissociation or predissociation into a continuum, which carries oscillator strength from the ground state. We have demonstrated that for the special case of radiative interactions with a single molecular continuum explicit expressions can be derived for the Green's function and for the reaction operator, incorporating radiative interactions with one photon states to infinite order. From the complete solution for elastic photon scattering from a dissociative continuum we conclude that the direct radiative corrections are of the order of the ``radiative Lamb shift'' for the continuum states, and are negligible. The elastic photon scattering cross sections can be expressed in terms of a complex Hilbert transform of the Franck-Condon transition density, while the absolute photon scattering quantum yield is [inverted lazy s] 10−7. This treatment has been extended for the study of resonance Raman scattering from a dissociative continuum, where the scattering cross sections can be expressed in terms of absolute squares of complex Hilbert transforms of the product of two Franck-Condon vibrational overlaps for bound-continuum transitions. No selection rules for the final vibrational state are exhibited. Finally, we have derived a general solution for photon scattering for a discrete molecular level coupled to an optically active dissociative continuum. The absorption cross section is finite at the interference dip, being determined by the interference function for the continuum states. The quantum yield for resonance fluorescence exhibits a sharp maximum reaching a value of unity at the interference dip where the quantum yield for dissociation vanishes.
ICR study of nonreactive ion-molecule collision rate constants for Cl− and Cr(CO)5−
Authors: Dymerski Paul P. , Dunbar Robert C. , Dugan John V.
Published: February 2004

An ion cyclotron resonance line-broadening technique has been used to determine experimental momentum transfer rate constants for systems of Cl− and Cr (CO)5− ions in a selected group of isotropic and anisotropic nonpolar and polar gases between 10−4 and 10−2 torr. The nonpolar molecule systems are compared to rate constants predicted by the Langevin and Mason-Schamp ion mobility theories, and the polar molecule systems are compared to the Langevin theory and to rate constants scaled from numerical calculations. In the polarization force dominated limit, rate constants for both Cl− and Cr(CO)5− in isotropic nonpolar gases give good agreement with rate constants predicted by the Langevin potential (r−4), while the anisotropic systems require corrections for the anisotropy of the neutral polarizability. Cr(CO)5− in the ``light'' gases (He, Ne, H2, Ar) shows marked deviations from the Langevin polarization limit predictions. Despite used of the hard-sphere corrected Langevin potential only fair agreement with experiment is found in this area: The adjustable parameter γ of the Mason-Schamp (12–6–4) potential function was calculated for each Cr(CO)5−-neutral pair at 300°K. Most of the systems studied gave γ>0.5. In terms of the MS potential function these results suggest large r−6 contributions to the potential function. Both Cl− and Cr(CO)5− in the polar gases (HCN, CH3CN, CH3F) show significant deviations from the locked-dipole theory suggesting that the ion-dipole locking potential is only 50% effective. Appropriately scaled numerical calculations give satisfactory agreement with experiment and thus give support to this result.
Use of contracted basis functions in algebraic variational scattering calculations
Authors: Abdallah Joseph , Truhlar Donald G.
Published: February 2004

Schemes for the use of contracted basis functions in algebraic variational electron scattering calculations are presented. These schemes are tested by computing partial cross sections for the scattering of electrons by atomic hydrogen for several contracted and uncontracted basis sets. The results indicate that the use of contracted functions is an effective means of circumventing lengthy algebraic computations without much loss of accuracy. One contraction scheme provides a method intermediate in accuracy and volume of computations required between the distorted wave method and the close coupling method. Another provides a method intermediate in accuracy and volume of computations required between an M-channel close coupling calculation and an (M + M′)-channel close coupling calculation where M′ is the number of channels in which the number of contracted functions is insufficient for convergence.
Atmospheric pressure pulsed HF chemical laser
Authors: Chen Hao-Lin , Taylor Raymond L. , Wilson Jack , Lewis Paul , Fyfe Walter
Published: February 2004

Experiments are described on a pulsed chemical laser operating on the reaction between hydrogen and fluorine at pressures above the second explosion limit. A laser energy density of 80 J/liter-atm is obtained for a mixture of 0.1 F2/0.1 H2/0.8 He mole fraction at 1.1 atm total pressure. This performance corresponds to a chemical efficiency of 4% and an over-all electrical efficiency of 1.3% for the particular photolysis geometry employed. Various diagnostic experiments are developed to measure the initial fraction of F2 dissociated by the photolysis, the temporal extent of the over-all reaction, and measurements of medium homogeneity. These data together with measurements of over-all laser performance are compared with calculations from a comprehensive theoretical model which includes the photolysis process, chemical kinetic reactions, vibrational energy transfer processes, and stimulated emission. The major features of this laser appear to be understood and described by the existing kinetic rate data. However, certain important effects, such as the influence of rotational nonequilibrium and optical output coupling, remain to be fully understood.
Exciton surface states in molecular crystals
Authors: Hoshen Joseph , Kopelman Raoul
Published: February 2004

A new Hamiltonian partition method, used previously for cooperative excitations in molecular crystals, has been adopted for the treatment of surface exciton states in molecular crystals. The formation of surface excitons depends on the relative magnitude of the exciton transfer integrals, J terms, as compared to the environmental shift integrals, D terms. This was established for a sample calculation on a simple cubic molecular crystal. It was found that when the absolute value of the nearest neighbor D term exceeds the corresponding J term two localized states emerge for each value of a two dimensional wavevector. The two localized states are degenerate in the limit of an infinitely thick crystal. The localization of the surface states increases with an increase in the magnitude of the D term.
Charge-conserving integral approximations for ab initio quantum chemistry
Authors: Wilhite D. L. , Euwema R. N.
Published: February 2004

Three charge-conserving integral approximations are presented that either zero or economically calculate the less important integrals that arise in quantum chemical calculations on molecular systems and in solids. One of these is based upon the use of Gaussian basis functions. Results are presented for test calculations on glycine, glycylglycine, and diamond.
Gamma-ray directional correlation study of bovine carbonic anhydrase
Authors: Richer L. , Graf G. , Glass J.
Published: February 2004

Two perturbed gamma-gamma directional correlation experiments have been performed to examine internal fields and correlation times for the active region and active site in bovine carbonic anhydrase (BCA-B). A sample of BCA-B with 133Ba implanted in the active region was prepared and time-dependence of the anisotropy of the 356–81 keV cascade of 133Cs was measured at room temperature. Time-differential and time-integrated behavior of the perturbation factor indicates a possible time-dependent interaction with a relaxation constant λ22 = 5×107 sec−1. Using a previously reported value of rotational correlation time of BCA-B, the perturbation factor is found to be caused by a time-dependent electric quadrupole interaction with an electric field gradient having a time-averaged value of 1 V/Å2 at the nuclear site of 57Co. In a second experiment, BCA-B was demetallated and the zinc ion at the active site replaced by 57Co with no appreciable loss of specific activity. Time-differential measurements of the anisotropy of the 122–14 keV case in 57Fe showed that the anisotropy goes rapidly to zero. The time-integrated perturbation factor gives λ22 = 7×107 sec−1 at the active site.
Rate constant for O(3P) +CF3Br from 800 to 1200 °K
Authors: Frankiewicz Theodore C. , Williams Frederick W. , Gann Richard G.
Published: February 2004

The rate constant for the reaction of oxygen atoms with CF3Br (halon 1301) was measured at low pressures in a heated discharge-flow reactor. The halon concentration was followed gas chromatographically. The likely primary reaction products are CF3 and OBr radicals. Variation of experimental parameters and correlated numerical solutions of the total rate equations indicate that secondary reactions are not significant. The reaction dynamics fit an Arrhenius plot with A = 1.5 ± 0.5 × 10−11 cm3/molecule·sec and Ea = 13.5 ± 1.0 kcal/mole. The application of these studies to flame inhibition indicates that, in diffusion flames at least, CF3Br+O(3P) is a candidate as the inhibition initiating reaction.
Zero-phonon absorption transitions in S2− and Se2− molecules dissolved in potassium iodide crystals
Authors: Vella G. J. , Rolfe John
Published: February 2004

Very weak zero-phonon absorption lines were detected and measured in KI crystals containing S2− or Se2− impurities at 2°K, using a newly developed sensitive absorption apparatus. Each impurity caused the same type of absorption spectrum, consisting of two upper-state progressions corresponding to transitions to two sets of vibrationally excited levels in the excited state. Quantum numbers for these levels were determined by isotope shift measurements, and a complete set of spectroscopic constants was calculated. By combining these results with previously reported emission results for the same system a possible energy level scheme was worked out, and the crystal field splittings in both ground and excited states calculated.
Analysis of the 13C chemical shift tensor in CO, Ni(CO)4, and Fe(CO)5 and its relationship to π-back bonding
Authors: Mahnke H. , Sheline R. K. , Spiess H. W.
Published: February 2004

Components of the 13C shielding tensor. σ are presented for CO, Ni(CO)4, and Fe(CO)5. These shift components are separated into diamagnetic and paramagnetic parts using theoretical estimates for the former and spin rotation constants for the latter. The paramagnetic part of the perpendicular component of the chemical shift σp is shown to be mainly responsible for the large isotropic shift between free CO and metal bound CO. This fact is interpreted in terms of π-back bonding in the metal carbonyls.
Kinetic theory of condensation and evaporation. II
Authors: Cipolla J. W. , Lang H. , Loyalka S. K.
Published: February 2004

The macroscopic temperature and pressure jumps that occur at a condensate surface during a nonequilibrium phase change are calculated using a variational principle on the linearized Boltzmann equation. When evaluated for the BGK collisional model, the macroscopic jumps have values within 1% of a recent numerical calculation. Furthermore, the microscopic density and temperature jumps are estimated using an ansatz for the distribution function of molecules impinging on the surface. The jumps obtained in this way are correct to within 5% of the numerical values when evaluated for the BGK model. Also a simple procedure is described by which the jumps occuring at a surface for which the mass accommodation (condensation) coefficient σ≠1 may be calculated from those for which σ=1. In addition the connection of this kinetic theory with the thermodynamics of irreversible processes is made and the elements of the appropriate Onsager matrix are expressed in terms of the macroscopic jumps for arbitrary values of σ.
Perturbation corrections to Koopmans' theorem. I. Double-zeta Slater-type-orbital basis
Authors: Chong Delano P. , Herring F. Geoffrey , McWilliams Denis
Published: February 2004

Ordinary third-order Rayleigh-Schrödinger perturbation theory is used to evaluate vertical ionization potentials of closed-shell molecules. The method is tested with double-zeta calculations on H2O, CH2, NH3, H2CO, and F2O. The results are compared with the ΔSCF method and discussed. On the basis of our calculations, we confirm the suggested alternative assignment for the photoelectron spectrum of F2O, instead of the original assignment based on Koopmans' theorem.
DVMO (directed-valence molecular orbital) method: A rotationally invariant approximate MO method preserving directed-valence effects
Authors: Coffey Patrick , Van Wazer John R.
Published: February 2004

The necessity for the results of an approximate MO method to be invariant under a rotation of the molecular coordinate system does not require that directed valence effects (e.g., the difference between σ and π p orbitals) be neglected in the one-center Fock-matrix elements. The formalism for a new semiempirical MO method, DVMO (directed valence molecular orbital), is presented. The method is parameterized and tested on a number of molecules, demonstrating its ability to predict one-electron properties.
Time-resolved fluorescence kinetics and 1B1(1Δg) vibronic structure in tunable ultraviolet laser excited SO2 vapor
Authors: Brus L. E. , McDonald J. R.
Published: February 2004

A frequency doubled, tunable dye laser has been used to excite fluorescence from the first excited singlet state of SO2 (2600–3250 Å). Two different groups of rovibronic states, with different collision-free lifetimes and bimolecular quenching rates, are observed throughout the region. The major group of states has exceptionally long collision-free lifetimes (80–600 μsec), which are up to a factor of 20 longer than previously reported. The minor group, which has not been observed by previous workers, has ∼50 μsec lifetimes and a bimolecular quenching rate that is an order of magnitude above the gas kinetic rate. No evidence for collisional equilibration between the groups is found. The exceptionally long lifetimes invalidate the results of previous experiments, from which it was concluded that the first excited singlet state undergoes unimolecular radiationless transitions. The major group is discussed in terms of extensive Renner rovibronic perturbation between an excited, quasilìnear 1B1 state and the ground 1A1 state. The minor group may result from an overlapping, vibronically induced, 1A2←1A1 transition.
Spin-orbit contribution to the zero-field splitting in CH2
Authors: Langhoff Stephen R.
Published: February 2004

The spin-orbit contribution to the zero-field splitting in methylene is calculated at bond angles of 135° and 180° using the correct microscopic Hamiltonian. The two lowest states of 1A1 symmetry as well as the lowest states of 1,3A2 and 1,3B2 symmetry are considered in the second-order perturbation treatment. Configuration-interaction wavefunctions obtained using a double-zeta-plus polarization quality basis of contracted Gaussian-lobe functions are used to describe each state. Spin-orbit contributions to the splitting parameter, D, of 0.023 and 0.0245 cm−1 are obtained at 135° and 180°, respectively. Configuration interaction is found to reduce the spin-orbit contribution at 135° by about 50%. Combining these results with the results for the spin-spin dipole contribution to D gives D=0.807 cm−1 and E=0.049 cm−1 at 135° which is slightly higher than the current experimental value of D=0.76±0.02 cm−1. The increase in the value of the parameter D from 0.807 cm−1 at 135° to 0.934 cm−1 at 180° is observed to come almost entirely from increased spin-spin contributions at the larger angle.
Localized molecular orbitals for polyatomic molecules. I. A comparison of the Edmiston-Ruedenberg and Boys localization methods
Authors: Kleier Daniel A. , Halgren Thomas A. , Hall John H. , Lipscomb William N.
Published: February 2004

The Edmiston-Ruedenberg (ER) and Boys localization methods are compared for a number of boron hydrides and carboranes. The Boys localization equations are solved explicitly for the two-orbital case and an iterative procedure for the multidimensional case is presented. The Boys analogs of Taylor's [J. Chem. Phys. 48, 2385 (1968)] ER equations are derived and the second-order term is converted into a symmetrized quadratic form which is analogous to the ER equations of Switkes et al. [J. Am. Chem. Soc. 92, 3847 (1970)]. A limited second-derivative test is employed in order to determine the nature of the convergence, and a transformation based on it is defined which successfully resolves structural ambiguities to which the standard two-orbital transformation procedure is subject. The Boys and ER localization methods are in at least qualitative agreement for all molecules except 1,2-C2B4H6, where the Boys localization yields a structure with fractional bonds and no open three-center B–C–B bonding. Previous ER localizations of 1,2-C2B4H6 resulted in two open B–C–B three-center bonds and no fractional bonding.
Low temperature magnetic susceptibility of uranium and rare earth tungsten oxide bronzes
Authors: Thomas Jack F. , Sienko M. J.
Published: February 2004

Magnetic susceptibilities of uranium tungsten bronzes, UxWO3, and their thorium-diluted counterparts, UxTh0.10−xWO3, have been measured by the Faraday method over the range 1.6–300°K. The sharp, dilution-independent antiferromagnetic behavior previously reported was not confirmed. Weiss constants observed showed a weak decline with dilution. Susceptibilities of light rare earth bronzes (RE=Pr, Nd) deviated at low temperatures from Curie-Weiss behavior, probably because of changes in population of intra-atomic levels. For the heavy rare earth bronzes (RE=Gd, Tb, Dy, Ho, Er), very small Weiss constants were observed. Near independence of the Weiss constants of conduction electron density and rare earth magnetic content suggests that the apparent Curie-Weiss behavior is mainly a manifestation of crystal field interactions, not an indication of magnetic exchange effects.
The calculation of equilibrium mole fractions of polar-polar, nonpolar-polar, and ion dimers
Authors: Calo J. M. , Brown J. H.
Published: February 2004

Methods are developed for the calculation of equilibrium concentrations of polar-polar, nonpolar-polar, and ion dimers. These methods are based on the work of Stogryn and Hirschfelder, who derived the relative contribution of bound and metastably bound double molecules to the reduced second virial coefficient (B*b and B*m). A unique approach for reducing the non-spherically-symmetric Stockmayer potential for polar molecules to the Lennard-Jones 6–12 form φ*=4[r*−12−(1+Ā)r*−6] is presented, where Ā is an average dipole orientation parameter and a monotonic decreasing function of temperature. Estimates of polar-polar equilibrium dimer concentrations are obtained from analytical expressions for B*b and B*m derived from this potential. Nonpolar-polar dimer concentrations are calculated with a similar approach using Ānp (a temperature independent constant) and consideration of the second virial coefficient of gas mixtures. Ion dimers are treated via a pseudo-second virial coefficient approach. Tables for Ā, B*b(T*), and B*m(T*) are given as a function of reduced temperature and dipole moment for polar molecules. Sample numerical calculations are also presented for all the types of dimers discussed, and compared with other results where possible.
Wavelength dependence of the quantum yield of fluorescence and photoionization of indoles
Authors: Steen H. B.
Published: February 2004

The fluorescence quantum yields φf (λx) of L-tryptophan and indole were measured relative to that of Rhodamine B as functions of the excitation wavelength λx between 200 and 300 nm. Two different experimental geometries produced identical results, indicating that experimental artifacts were negligible. In aqueous solution at 296 K, φf (λx) increased by 45% for both solutes in going from 230 to 260 nm. Outside this wavelength region, φf (λx) was constant to within 2%. Similar results, although with a somewhat smaller shift in φf (λx), were obtained for tryptophan in an equivolume mixture of ethylene glycol and water EG/H2O at both 296 and at 113 K, the latter temperature being below the glass transition temperature of this medium. The emission spectra and the phosphorescence to fluorescence ratios were independent of λx, indicating that the shift in φf (λx) was not due to intersystem crossing from higher excited states. The shift in φf (λx) was accompanied by a considerable increase in the quantum yield of photoionization of tryptophan in aqueous solution below 260 nm. Hence, it is concluded that photoionization from higher states competes favorably with internal conversion to the fluorescent singlet state. For indole in methanol at 296 K and in EG/H2O at 113 K, φf (λx) was independent of λx to within 2%.
Atomic force constants and the heat capacities of materials at elevated temperatures
Authors: King William T.
Published: February 2004

Atomic force constants for C, Si, and Ge in diamond, graphite, and crystalline silicon and germanium are determined by the analysis of the heat capacities of these materials. These force constants are then compared with those found in simple molecular analogs of these crystals. The dependence of the atomic force constants upon crystal and molecular structure is analyzed in terms of an ``atoms in molecules'' model. Using this model and approximate electron densities, determined by either Slater-type atomic orbitals or the Thomas-Fermi theory, the magnitudes of the atomic force constants were estimated and compared with their experimental values.
van der Waals constants for alkaline earth-noble gas pairs
Authors: Hyman H. A.
Published: February 2004

The long-range R−6 interaction between alkaline earth and noble gas atoms is considered from both classical and quantum-mechanical viewpoints. Intra-atomic electron correlation effects are shown to be important for these systems. The relevant van der Waals coefficients are calculated with the use of an accurate, correlated wavefunction in the case of Be and from oscillator strength sum rules for the other alkaline earth atoms.
Time-dependent Nernst-Planck and Poisson equations: Initial phase of action potential
Authors: Meyer J. P. , Kostin M. D.
Published: February 2004

The time-dependent Nernst-Planck and Poisson equations are solved numerically for a membrane between aqueous solutions containing several ionic species. Explicit results are given for the initial phase of the action potential as a function of time following excitation of the membrane. It is shown that the Nernst-Planck and Poisson equations predict that a rapid increase in the permeability of the membrane to sodium ions can cause a rapid increase in the potential.
Products' translational energy and c.m. angular distribution vs collision energy for the Rb + CH3I reaction
Authors: Ureña A. González , Bernstein R. B.
Published: February 2004

An analysis has been made of the laboratory angular distributions of RbI from the crossed beam reaction of Rb with CH3I obtained by Litvak et al. over the range of relative translational energy, Ē, from 2.8 to 36.6 kcal mol−1. Although velocity analysis data were not available, the product flux distributions contain sufficient information to allow deduction of the essential features of the center-of-mass (c.m.) angular and recoil energy distributions, with the assumption that they are uncoupled. The c.m. differential (solid angle) reaction cross section is found to be ``backward-peaked'' at low energies, shifting ``forward'' with increasing Ē. The deduced product recoil energy distribution is consistent with the results of previous velocity analysis experiments at low Ē for the reactions of both Rb and K with CH3I. The average translational energy of the products, Ē′, increases approximately linearly with increasing collision energy: Ē′ ≃ 15 + 0.5 Ē (in kcal mol−1). The intercept is in good agreement with the impulsive photodissociation model of Herschbach.
Studies of polyelectrolyte solutions containing mixtures of mono- and divalent counterions. II. Heat of dilution
Authors: Dolar D. , Škerjanc J.
Published: February 2004

The heats of dilution of aqueous solutions containing mixtures of polystyrenesulfonic acid and its magnesium salt have been measured at 25°C in the concentration range 0.1–0.001M. From these data the enthalpy difference between two concentrations has been obtained as a function of the equivalent fraction of the acid. The curve representing this function has a sigmoidal shape. On the basis of the cell model with cylindrical symmetry, the electrostatic contribution to the enthalpy of the polyelectrolyte solution containing a mixture of mono- and divalent counterions has been calculated. A reasonable agreement between theory and experiment has been found.
Microwave spectrum of tert-butyl mercaptan
Authors: Valenzuela Edward A. , Woods R. Claude
Published: February 2004

The spectrum of the ground vibrational state of t-butyl mercaptan from 8 to 40 GHz has been measured and assigned using a computer controlled spectrometer. The spectrum consists of many long Q-branch series arising from the perpendicular component of the dipole moment and also parallel R-branch groupings at intervals of B+C. The molecule is a near prolate symmetric top with energy level splittings of several gigahertz due to the internal rotation about the C–S bond. A computer program utilizing the IAM in a form suited for the intermediate to low barrier case has been written and used to obtain a satisfactory fit of the observed spectrum to the simple rigid top-rigid frame Hamiltonian with six adjustable parameters: A, B, C, ρ, β, and Δ0. The barrier obtained (1742 cal/mole) is significantly larger than the previously determined microwave value for methyl mercaptan and is also greater than the recent far infrared estimates for t-butyl mercaptan. Measurements of the Stark effect for several transitions has permitted determination of the magnitudes and (because of interference effects between the internal rotation and Stark perturbations) the relative sign of the two components of the dipole moment. The direction of the dipole moment vector thus obtained is consistent with the only physically reasonable choice in this molecule.
Square-well potential. V. Energy analysis of the Yvon-Born-Green equation
Authors: Lincoln William W. , Tago Yoshio , Luks Kraemer D.
Published: February 2004

The Yvon-Born-Green equation, in θ-expanded form for the square-well potential (R = 1.85), was solved at 38 densities, and the results were used in deriving pressure predictions via analysis of the energy equation of state. Compressibility factors, the coexistence envelope, and the critical point location and critical exponents are presented. These properties, derived from energy analysis, are closer to experimental values for simple fluids than those derived from the pressure equation in earlier studies.