bluuebunny/crossref_metadata_2025 huggingface dataset:
A study of the arsenic, black phosphorus, and other structures derived from rock salt by bond-breaking processes. I. Structural enumeration
Authors: Burdett Jeremy K. , McLarnan Timothy J.
Published: February 2003

The structures of arsenic and black phosphorus can be regarded as distortions of the simple cubic or NaCl structure. The presence of more than four electrons per site results in the breaking of three of the six linkages around each atom in the NaCl structure. A generalization of Pólya’s enumeration theorem is proven which allows counting of the 36 possible ways of breaking bonds to produce structures containing trigonal pyramidal coordination at each site and having the same size unit cell as NaCl. Larger cells are considered briefly.
Molecular ionization potential and dissociation energies related to atomic ionization potentials for singly bonded diatomics
Authors: Mucci J. F. , March N. H.
Published: February 2003

Experimental data is used to demonstrate that for singly bonded homonuclear diatomics the molecular ionization potential Im is directly proportional to the atomic ionization potential I, the proportionality constant being 0.896±0.008, with the single exception of H2 for which Im≳I. Hydrogen halides and mixed halides are then studied and IHX is plotted as a function of the difference IH−IX = ΔI of the atomic ionization potentials between hydrogen and halogen X. The plots for HX and the mixed halides are again linear and the interest in the cases where ΔI = 0 is stressed. Neglecting zero-point energy, the difference Im−I for homonuclear molecules is immediately a measure of the dissociation energy difference ΔD between a neutral molecule of the corresponding singly charged positive ion. Using the relation above for Im/I yields ΔD = [0.104±0.008]I and this is shown to be consistent with measurements on F2+ and Cl2+. Finally, it is pointed out that, at least approximately, IH−IX should relate to the corresponding chemical potential difference Δμ, and it is no cause for surprise that cases where Δμ = 0 are especially simple.
A sum rule for an inhomogeneous electrolyte
Authors: Blum Lesser , Henderson Douglas , Lebowitz Joel L. , Gruber Ch. , Martin Ph. A.
Published: February 2003

We obtain a new sum rule for the density profile of an electrolyte in contact with a charged flat hard (nonconducting) wall. This, in turn, strongly implies that the decay of the pair correlation near the wall is not faster than (distance)−d−1, where d is the dimension of the system. (AIP)
The effect of deuteration upon the tunneling rotation of a hindered methyl group in x-irradiated 2,2-dimethyl-d6-5-methyl-1,3-dioxane-4,6-dione crystals: An ENDOR study
Authors: Geoffroy M. , Wang H. C. , Kispert L. D. , Sipe H. J. , Hwang J. S.
Published: February 2003

ENDOR studies are reported of the effect of deuteration on the tunneling rotational frequency of a hindered methyl group in x-irradiated 2,2,5-trimethyl-1,3-dioxane-4,6,-dione. The radical structure is discussed. (AIP)
Proton transfers in hydrogen-bonded systems. VI. Electronic redistributions in (N2H7)+ and (O2H5)+
Authors: Scheiner Steve
Published: February 2003

Electronic rearrangements accompanying transfer of the central proton between the two XHn units of (H3NHNH3)+ and (H2OHOH2)+ are studied using ab initio molecular orbital methods. Electron density difference maps are calculated by subtracting the density of the equilibrium structure (X–H---X) from that of the midpoint geometry (X--H--X) using the split-valence 4-31G basis set. Some of the features revealed by the maps are common to both systems while others indicate significant differences between nitrogen and oxygen. Decomposition of the total electron density into contributions from individual occupied molecular orbitals (MOs) provides insight into the factors responsible for the overall charge migrations. The orbitals of a1 symmetry lead to density shifts in a direction parallel to the H bond axis. Among the features attributed to these MOs are the charge transfer across the H bond from one molecule to the other and characteristic density changes in the lone pair regions of the first-row X atoms. Internal polarizations of the XH bonds of each molecule arise from the density shifts perpendicular to the H bond axis associated with the MOs of non-a1 symmetry. Simple arguments involving electrostatic and covalent effects are used to explain the redistributions observed in the various MOs. Mulliken analyses provide information, complementary to the difference maps, concerning the relative involvement of various atoms and atomic orbitals in the electronic redistributions associated with each MO.
A unimolecular reaction ABC→A+B+C involving three product molecules and a single transition state. Photodissociation of glyoxal: HCOHCO→H2+CO+CO
Authors: Osamura Yoshihiro , Schaefer Henry F. , Dupuis Michel , Lester William A.
Published: February 2003

Following an earlier proposal [Y. Osamura and H. F. Schaefer, J. Chem. Phys. 74, 4576 (1981)], the unimolecular reaction HCOHCO→H2+CO+CO has been examined via nonempirical molecular electronic structure theory. Specifically, the constrained symmetric (point group C2v) transition state for this ABC→A+B+C reaction has been located at several levels of self-consistent-field (SCF) theory. Four different basis sets of contracted Gaussian functions were used: an STO-3G minimum basis, the small split valence 3-21G basis, the standard C(9s 5p/4s 2p) double zeta (DZ) set, and a double zeta plus polarization (DZ+P) basis. Vibrational analyses of the four stationary point structures (all of which are geometrically similar) yield a remarkable variety of results. The STO-3G stationary point has three imaginary vibrational frequencies, 3-21G has one imaginary frequency (and thus is a genuine transition state), while the DZ and DZ+P structures yield two imaginary vibrational frequencies. For the latter two cases, one of the two imaginary vibrations is a very small bending frequency, while the larger frequency clearly connects glyoxal with the three products H2+CO+CO. This suggests the existence of a slightly nonplanar true transition state. To our knowledge such a unimolecular transition state is without precedent. Configuration interaction (CI) suggests that the barrier for this ABC→A+B+C reaction is competitive with that for HCOHCO→H2CO+CO.
The effect of hydrodynamics on the diffusivity of charged macromolecules: Application to BSA
Authors: Dorshow R. , Nicoli D. F.
Published: February 2003

Using the Batchelor/Felderhof hydrodynamic perturbation of the friction factor in the Stokes–Einstein expression for the diffusion coefficient, we have calculated the correction to the latter due to electrostatic interactions between charged spherical particles in a solution of low ionic strength. We obtain an excellent agreement between this theory and the early experimental results of Doherty and Benedek using BSA. Our theoretical fits to the BSA diffusivity data show a substantial improvement over the recent theoretical predictions of Schor and Serrallach, who considered only the thermodynamic (and not the hydrodynamic) corrections to the diffusion coefficient.
Scaling theory for the criticality of fluids between plates
Authors: Fisher Michael E. , Nakanishi Hisao
Published: February 2003

On the basis of a scaling hypothesis for the criticality of fluids, or fluid mixtures, held between parallel plates, we analyze the combined effects of finite wall separation and ’’symmetry breaking’’ boundary conditions at the walls (i.e., adsorption, or preferential adsorption of one component, at the surfaces). The resulting bulk (but ’’two-dimensional’’) critical point is shifted both in temperature and in density or composition (and in the conjugate field variables) by amounts described by scaling. The scaled form of the phase boundary is consistent with results from a low temperature Ising lattice gas model expansion.
Vibrational relaxation of SiD4 in the liquid and plastic crystalline phases
Authors: Wilde Richard E.
Published: February 2003

The spontaneous Raman spectra of the ν1 band of SiD4 in the liquid and plastic crystalline phases have been modeled by a memory-function procedure. It was found to be necessary to go to third order in the memory-function procedure. In order to get agreement between the ν2 reorientational relaxation times and the deuteron magnetic resonance correlation times, anisotropic rotation has been postulated. The anisotropic rotation accounts for an additional slow variable in the vibrational dephasing dynamics. The ν1 vibration is apparently modulated by the rotations. The vibrational–correlation function is reported out to 10 ps.
Quantum beats and dephasing in isolated large molecules cooled by supersonic jet expansion and excited by picosecond pulses: Anthracene
Authors: Lambert Wm. R. , Felker P. M. , Zewail A. H.
Published: February 2003

The modulation depth and lifetime were measured for quantum beats observed in the fluorescence decay of anthracene. (AIP)
Errors in the Wigner approach to quantum dynamics
Authors: Heller Eric J. , Brown Robert C.
Published: February 2003

It is shown that the recent results and error trends of Lee and Scully and Brown and Heller are explicable in terms of the ’’dangerous cross term’’ analysis. Extended quantum state suffer larger errors in the Wigner method.(AIP)
Light scattering spectrum of a suspension of interacting Brownian macromolecules
Authors: Phillies George D. J. , Wills Peter R.
Published: February 2003

The joint effect of direct and hydrodynamic interactions on the dynamic structure S (k,t) of a solution of rigid macromolecules is examined. The initial slope dS/dt and initial curvature d2 S/dt2 of S(k,t) are obtained. The reference frame correction of Kirkwood et al. [J. Chem. Phys. 33, 1505 (1960)] is shown to be wave-vector dependent. Contrary to some previous results, we argue that the initial slope of S(k,t) is partly due to direct interparticle interactions rather than being due entirely to free-particle Brownian motion.
Molecular dynamics and spectra. II. Diatomic Raman
Authors: Berens Peter H. , White Steven R. , Wilson Kent R.
Published: February 2003

This paper and paper I in this series [P.H. Berens and K.R. Wilison, J. Chem. Phys. 74, 4872 (1981)] indicate that infrared and Raman rotational and fundamental vibrational–rotational spectra of dense systems (high pressure gases, liquids, and solids) are essentially classical, in that they can be computed and understood from a basically classical mechanical viewpoint, with some caveats for features in which anharmonicity is important, such as the detailed shape of Q branches. It is demonstrated here, using the diatomic case as an example, that ordinary, i.e., nonresonant, Raman band contours can be computed from classical mechanics plus simple quantum corrections. Classical versions of molecular dynamics, linear response theory, and ensemble averaging, followed by straightforward quantum corrections, are used to compute the pure rotational and fundamental vibration–rotational Raman band contours of N2 for the gas phase and for solutions of N2 in different densities of gas phase Ar and in liquid Ar. The evolution is seen from multiple peaked line shapes characteristic of free rotation in the gas phase to single peaks characteristic of hindered rotation in the liquid phase. Comparison is made with quantum and correspondence principle classical gas phase spectral calculations and with experimental measurements for pure N2 and N2 in liquid Ar. Three advantages are pointed out for a classical approach to infrared and Raman spectra. First, a classical approach can be used to compute the spectra of complex molecular systems, e.g., of large molecules, clusters, liquids, solutions, and solids. Second, this classical approach can be extended to compute the spectra of nonequilibrium and time-dependent systems, e.g., infrared and Raman spectra during the course of chemical reactions. Third, a classical viewpoint allows experimental infrared and Raman spectra to be understood and interpreted in terms of atomic motions with the considerable aid of classical models and of our well-developed classical intuition.
Optical absorption and emission spectra of Cs2NaHoCl6
Authors: Morley J. P. , Faulkner T. R. , Richardson F. S. , Schwartz R. W.
Published: February 2003

Optical absorption and emission spectra are reported for the cubic Cs2NaHoCl6 elpasolite system. Detailed energy and intensity analyses of the high-resolution, variable-temperature spectra allow characterization of the crystal field energy level structure associated with eleven of the Ho3+ term levels. The term levels included in these analyses are 5I8, 5I7, 5F5, 5F4, 5S2, 5F3, 5F2, 3K8, 5G6, 5G5, and 3K7. Intensity calculations are reported for both the pure magnetic dipole transitions and the vibronically induced electric dipole transitions associated with the ν3(t1u), ν4(t1u), and ν6(t2u) vibrational modes of the octahedral (Oh) HoCl63− chromophoric moiety of the Cs2NaHoCl6 system. The electric dipole intensity model used in these calculations includes contributions from both the static-coupling and dynamic-coupling Ho3+-ligand interaction mechanisms. Excellent agreement between observed and calculated intensities is found, and the theoretically calculated intensity results proved crucial to our detailed analysis and assignment of the observed spectra.
Systematics of ground-state potential minima between two main-group atoms or ions
Authors: Hefferlin Ray , Kutzner Mickey
Published: February 2003

Tabulated equilibrium internuclear separations for diatomic molecules and quasimolecules formed from main-group atoms are presented graphically. Periodicities and trends are easily seen; there are also unexpected features. Data for diatomic ions have been mapped onto the surface for neutrals.
Slater transition state calculations of electron affinity of heavy atoms
Authors: Sen K. D. , Schmidt P. C. , Weiss Alarich
Published: February 2003

The spin restricted relativistic Hartree-Fock-Slater transition-state method is used to predict the electron affinity of heavy atoms (Z≳54).(AIP)
Efficient production of Tl (2P3/2) in the photodissociation of TlI at 248 nm
Authors: Schilowitz A. M. , Wiesenfeld J. R. , Young R. H.
Published: February 2003

The branching ratio for production of thallium atoms following laser photolysis of TLI at 248 nm is determined directly. (AIP)
Vibrational dynamics of glassy and molten ZnCl2
Authors: Aliotta F. , Maisano G. , Migliardo P. , Vasi C. , Wanderlingh F. , Smith G. Pedro , Triolo R.
Published: February 2003

Polarized Raman spectra of ZnCl2 were obtained in the liquid phase near the melting point (T = 598 K), and in the glassy phase (T = 293 K). Measurements were performed down to a very low frequency shift (2 cm−1) from the exciting line. Our analysis of the Raman data provides an interpretation of the collision-like contribution in terms of a structural relaxation time in the picosecond range, while the phonon-like contribution gives an effective Raman density of states. These results are also discussed in terms of existing structureal models.
A perturbation theory for diffusion controlled reactions
Authors: Battezzati M. , Perico A.
Published: February 2003

A perturbative theory of diffusion controlled reactions between interacting particles, based on the Smoluchowski equation and boundary conditions, has been given taking as the perturbative parameter the radius of the reaction sink R, measured in units of the length typical of the range of the equilibrium distribution function. The resulting expansion for the first order rate constant has been worked out in three dimensions for strong potentials and turns out to be valid for those potentials whose derivative for R = 0 is zero. In this case it is possible to evaluate the rate constant at any order in R, the results being coincident up to second order with the inverse of the first time passage recent result of Szabo et al.
Precision of diffusion coefficients by the isotope exchange technique
Authors: Nanba Masao , Oishi Y. , Ando Ken
Published: February 2003

The precision of diffusion coefficients determined by the gas–solid isotope exchange technique was analyzed for planar sheet diffusion, rate controlled by the isotope exhange reaction. An objective function was employed to estimate the degree of fit of a theoretical calculation to the experimental diffusion profile as a function of the sample size (l), the diffusion coefficient (D), the surface exchange coefficient (K), and the atomic ratio in the gas and solid phases. The results indicate that the diffusion coefficient can be determined to a high degree of precision when (Kl/D) is greater than 1 and furthermore that when this condition is satisfied, the precision of the determined diffusion coefficient increases with longer diffusion times.
Thermodynamic properties and phase transitions in CO2 molecular clusters
Authors: Etters R. D. , Flurchick K. , Pan R. P. , Chandrasekharan V.
Published: February 2003

The thermodynamic properties of (CO2) N molecular aggregates of size 2 ⩽ N ⩽ 13 have been investigated. These crystallites exhibit well defined orientational order–disorder rotational transitions accompanied by a structural transition into a plastic crystallite phase. In addition, they exhibit melting and disassociation transitions. It is shown that the interpretation of experimental data, based upon dimer properties, depends crucially on these results. Equilibrium structures and orientations are also given.
Brownian motion of a system of coupled harmonic oscillators
Authors: Berkowitz M. , McCammon J. A.
Published: February 2003

The dynamics of a system of coupled Langevin oscillators is considered. Convenient algorithms are derived for computing the displacement and velocity time correlation functions of one of the oscillating particles, and generalized Langevin equations for these functions are developed. Illustrative results indicate that this model should be useful in the description of single-atom librations in globular proteins.
Aromatic proteinaceous surfactants stabilize long-lived gas microbubbles from natural sources
Authors: D’Arrigo Joseph S.
Published: February 2003

Three different types of protein-specific chemical tests were performed on long-lived gas microbubbles derived from aqueous solutions of agarose powder and from filtered aqueous extracts of Hawaiian forest soil. The separate protein-specific tests involved use of either 0.3% (w/v) ninhydrin, 100 μM methylene blue dye, or 0.7–1.0 M 2-hydroxy-5-nitrobenzyl bromide. The chemical test results obtained with each of the two natural substances, i.e., agarose powder and Hawaiian forest soil, were very similar and indicate that the biological surfactants which surround and stabilize long-lived gas microbubbles are proteinaceous compounds that contain, and whose surface activity depend upon, aromatic amino acid residues, particularly tryptophan.
Sodium diffusion in alkali silicate glass by molecular dynamics
Authors: Soules Thomas F. , Busbey Robert F.
Published: February 2003

Sodium ion diffusion in a sodium silicate glass (22Na2O ⋅ 53SiO2) was studied by molecular dynamics. Sodium diffuses by hopping from one interstitial site to another. At high temperatures reliable sodium diffusion coefficients and activation energies are determined. These are in good agreement with those extrapolated from experimental data. It is suggest that simple Born–Mayer potentials to describe ionic bonding can be used for estimating transport properties. Glass forming is followed as the temperature is lowered in simulated cooling schedules.
Path integral solutions of stochastic equations for nonlinear irreversible processes: The uniqueness of the thermodynamic Lagrangian
Authors: Hunt Katharine L. C. , Ross John
Published: February 2003

The conditional propagator associated with a generalized Langevin equation or Fokker–Planck equation is represented as a functional integral over paths, each exponentially weighted according to the time integral of the thermodynamic Lagrangian along the path. A principal result of this paper is the proof that the thermodynamic Lagrangian is unique for single-variable processes or for multivariable processes in flat spaces, contrary to earlier statements that the Lagrangian depends on an arbitrary parameter, often denoted α. Fixing the parameter α is equivalent to choosing a stochastic calculus (α = 0 corresponds to the Itô calculus and α = 1/2 to the Stratonovich calculus), but consistent operations with any stochastic calculus selected lead to the same Lagrangian. The Euler–Lagrange equations derived by a variational minimization of the thermodynamic action (the time integral of the Lagrangian) are also unique; these equations give the most heavily weighted path between any two fixed points in the space of stochastic variables. For linear Gaussian processes, the global path of least action from a point is identical to the phenomenological path. When Laplace’s method can be used to approximate the path integral, the conditional propagator can be written as a product of a thermodynamic factor and a kinetic factor. The thermodynamic factor is large if a process is thermodynamically favorable, but the kinetic factor may be small if large changes in the stochastic variables are required on a short time scale.
Electron beam fluorescence spectrometry of internal state populations in nozzle beams of nitrogen and nitrogen/rare gas mixtures
Authors: Faubel M. , Weiner E. R.
Published: February 2003

Rotational level populations of N2 were measured downstream from the skimmer in beams of pure N2 and in mixtures of N2 with He, Ne, and Ar expanded from room temperature nozzles. The range of p0D was from 5 to 50 Torr cm. The formation of dimers and higher condensates of beam species was monitored during the runs. The effect of condensation energy release on rotational populations and parallel temperatures was readily observed. Two different methods for evaluating the rotational population distributions were compared. One method is based on a dipole-excitation model and the other on an excitation matrix obtained empirically. Neither method proved clearly superior. Both methods indicated nonequilibrium rotational populations for all of our room temperature nozzle expansion conditions. Much of the nonequilibrium character appears to be due to the behavior of the K = 2 and K = 4 levels, which may be accounted for in terms of the rotational energy level spacing. In particular, the overpopulation of the K = 4 level is explained by a near-resonant transfer of rotational energy between molecules in the K = 6 and K = 0 states, to give two molecules in the K = 4 state. Rotational and vibrational temperatures were determined for pure N2 beams from nozzles heated up to 1700 °K. The heated nozzle experiments indicated a 40% increase in the rotational collision number between 300 and 1700 °K.
Analysis of the Auger spectra of CO and CO2
Authors: Kelber J. A. , Jennison D. R. , Rye R. R.
Published: February 2003

The electron-excited gas phase carbon and oxygen Auger spectra of CO and CO2 are compared to the spectra calculated by a one-electron theory. Calculated Auger transition intensities and energies are generally in good agreement with experiment. The disagreement between certain calculated intensities and experiment, however, illustrates those cases where configuration interaction can be expected to provide significant corrections to the one-electron theoretical results. In addition, a comparison of calculated to experimental final state binding energies reveals the existence, in covalent molecules, of localized two-hole final states in which two holes are always on the same site. A simple two-electron theory predicts where such states will occur in the Auger spectra of homonuclear diatomic molecules.
Reactions of Si+ with H2O and O2 and SiO+ with H2 and D2
Authors: Fahey D. W. , Fehsenfeld F. C. , Ferguson E. E. , Viehland L. A.
Published: February 2003

The reaction of Si+ with H2O to form SiOH+ has a measured rate constant of 2.3(−10) cm3 s−1 at 300 K. This is the major loss of Si+ ions in the Earth’s atmosphere above ∼90 km. The three-body reaction of Si+ with O2 (in He) produces stable SiO2+ ions with a rate constant of 1(−29) cm6 s−1 at 300 K. The endothermic binary reaction of Si+ with O2 to produce SiO+ has been measured from threshold to ∼2 eV. The association of Si+ with O2 is the dominant loss process for Si+ ions below ∼90 km in the Earth’s atmosphere and leads to silicon oxidation to SiO2 since a large fraction of the SiO2+ ions are produced in excited states which charge transfer with O2 before relaxation. The reaction of SiO+ ions with H2 (and D2) is found to have a thermal energy rate constant of 3.2(−10) cm3 s−1 [and 2.0(−10) cm−3 s−1] to produce SiOH+ (and SiOD+). This process has been suggested as a step in SiO production in the interstellar medium. The proton affinity of SiO is found to be 8.1±0.7 eV and the dissociation energy of SiO+ to be 4.8 eV and definitely less than 4.9 eV.
An examination of the use of vibrationally adiabatic basis functions for the calculation of molecular collision cross sections
Authors: Eno Larry , Balint-Kurti G. G.
Published: February 2003

Comparison is made between the use of two different types of vibrational basis functions for the expansion of the total wave function in a vibrationally inelastic scattering problem. The calculations are performed within the framework of the sudden approximation for the rotational motion of the molecular fragments. The different basis functions that are compared are a vibrationally adiabatic set and the standardly used set of diabatic vibrational basis functions. The adiabatic vibrational basis functions are chosen so as to approximately diagonalize the matrix representation of the interaction potential at each value of the scattering coordinate. Nevertheless, they permit the formulation of analytic expressions for the nonadiabatic coupling terms of the kinetic energy operator that are present when an adiabatic basis is used. In order to provide a reference against which to judge the two different bases, the sets of coupled differential equations which arise in the rotational sudden approximation are solved for the He+H2 system, and fixed-angle S matrices are calculated at several scattering energies and different values of the total angular momenta. It is shown that if the customary diabatic basis is used in conjunction with first order distorted wave perturbation theory to calculate the fixed-angle S matrices, these do not agree well with the exactly computed S matrices to which they should correspond. In contrast, if an adiabatic vibrational basis is used, the distorted wave approximation yields fixed-angle S matrices which are in good agreement (within 15% or better) with the fully converged exact calculations.
Conditioned averages in chemical kinetics
Authors: Gillespie Daniel T. , Mangel Marc
Published: February 2003

It is argued that the molecular population functions which satisfy the deterministic reaction-rate equations, and the first moments of the solution to the stochastic master equation, can be generally regarded as differently conditioned averages of the same random variables. Although these two kinds of averages usually differ only on the microscopic scale, it is pointed out they can differ macroscopically for chemical oscillators. However, this is not a sign of any inconsistency in the theory because it is fully explained by the different conditionings employed. Both kinds of averages are seen to play legitimate, useful roles in describing the dynamics of a stochastically evolving chemical system.
Factorization relations and consistency conditions in the sudden approximation
Authors: Chan C. , Evans J. W. , Hoffman D. K.
Published: February 2003

Linear factorization relations are derived for the matrix elements of quantum mechanical operators defined on some space ℋ = ℋ1⊕⋅2 which are diagonalizable on ℋ1. The coefficients in these relationships do not depend on the operators per se but do depend on the representations in which the operators are diagonal. The formulation is very general with regard to the nature of the ’’input’’ information in the factorization. With each choice of input information there are associated consistency conditions. The consistency conditions, in turn, give rise to a flexibility in the form of the factorization relations. These relations are examined in detail for the operators of scattering theory which are local in the internal molecular coordinates. In particular, this includes S and T matrices in the energy sudden (ES) approximation. A similar development is given for the square of the magnitude of operator matrix elements appropriately averaged over ’’symmetry classes.’’ In the ES these relations apply to transition cross sections between symmetry classes. In particular, they apply to degeneracy averaged cross sections in situations where the symmetry classes correspond to energy levels.
Excitation and dissociation of hydrogen fluoride by high power HF-laser irradiation under collision dominated conditions: A fluorescence study
Authors: Fürsich M. , Kompa K. L.
Published: February 2003

Hydrogen fluoride at pressures above 10 Torr was excited by a short pulse of a high power hydrogen fluoride laser. Vibrational populations of HF (v = 2⋅⋅⋅7) as well as corresponding vibrational and rotational temperatures were determined for periods up to 2 μs after the excitation pulse by time resolved vibrational overtone fluorescence spectroscopy. High vibrational populations at comparably low rotational temperatures were found at short time delays. After some time a sharp increase of the rotational temperature to ∼6000 K was observed. The experimental data are compared with estimates based on published rate equation calculations. Dissociation rates of HF under these conditions were estimated from the decline in the vibrational temperature at longer times. This yielded a rate constant for HF dissociation as k = 6×1015exp (−527.3/RT) cm3 mol−1 s−1. Time integrated absorption measurements of the HF sample gave mean absorption values of up to 23 photons/molecule. The degree of ionization of the gas could be estimated to be 10−8–10−9.
Bonding between transition metal atoms. A
b i
n
i
t
i
o effective potential calculations of Cu2
Authors: Pelissier Michel
Published: February 2003

Valence Hartree–Fock calculations using an ab inito effective potential and a Gaussian basis set of triple zeta quality followed by extensive CI’s have been carried out for several states of the copper atom and for the ground state of diatomic copper. Correlation effects are determined to obtain a satisfactory agreement with experimental data for Cu2(re = 2.25 vs 2.22 Å; ωe = 265 vs 266 cm−1; De = 15 550 vs 16 500 cm−1). The most striking effect is the 0.1 Å shortening of the bond length induced by the d correlation energy. Important basis set superposition errors are shown to be possible (especially in all electron calculations) and the use of six Gaussian primitives for the d orbitals was required to avoid them.
Effect of direct and indirect exchange interactions on geometries and binding energies of molecular complexes. The dimer (CL2)2
Authors: Jansen Laurens , Block Ruud
Published: February 2003

An analysis is made of the most stable geometry and the energy of interaction for the chlorine dimer, on the basis of a model which was previously used for interpreting hydrogen bonding in the dimers (HF)2 and (H2O)2 and the molecular complex HF⋅H2O. In the model, an analogy is assumed between the phenomenon of indirect exchange (’’superexchange’’) in ionic solids with paramagnetic cations and weak binding energies (≊1 kcal/mole) of molecular complexes. First-order exchange perturbation theory and an ’’effective-electron’’ description of the system are employed. The model predicts a nonplanar structure lacking a center of symmetry. The dimerization energy is estimated to lie between 1.4 and 2.0 kcal/mole. Agreement with experimental information is analyzed. We also briefly discuss the dimer (F2)2.
Prediction of heteronuclear diatomic force constants from atomic shielding factors
Authors: Ohwada Ken
Published: February 2003

The atomic shielding factor is conveniently defined from the molecular shielding factor which is derived by the application of perturbation theory to a homonuclear diatomic molecule, and is applied to predict the force constants of heteronuclear diatomic molecules. Especially, it is stressed that the atomic shielding factors for alkali atoms can be selected to be nearly zero and shown that for molecules (ij) containing alkali atoms (i) there exists a simple relationship Kij = (Nj+1)e2/?3ij, where Kij and ?ij are the force constant and the equilibrium internuclear distance, respectively, e is the electronic charge, and Nj is the number of electrons of atom (j) which are outside a complete shell.
Model grand potential for a nonuniform classical fluid
Authors: Percus J. K.
Published: February 2003

Classical equilibrium statistical mechanics of one-dimensional hard cores in an arbitrary external field is reviewed. The grand canonical potential is shown to be an extension of local thermodynamics in which suitable averages of density and pressure are used. A general three-dimensional system with arbitrary internal and external potentials is modeled in corresponding form, and the external potential represented as a functional of the density. Comparison with assumed bulk data permits evaluation of the required averages. It is shown that the wall equation of state for a wall-bounded fluid is satisfied exactly. Application is also made to the self-maintained density profile of the two-phase fluid at a first order phase transition.
Direct measurements of ΔH(O2) at 1323 K for nonstoichiometric TiO
Authors: Tetot R. , Picard C. , Gerdanian P.
Published: February 2003

The Tian–Calvet microcalorimetric method has been improved in order to determine ΔH (O2), the partial molar enthalpy of mixing of oxygen in oxide, for TiO. A self-calibration for ΔH (O2) determination between O/Ti = 0 and O/Ti = 1.5 could be performed from ΔH °f 1323(Ti2O3).
A new method of deriving electrical double layer equations from electrolyte theories
Authors: Lozada-Cassou Marcelo
Published: February 2003

By assuming that one of the species of a liquid mixture is made of charged, planar walls of infinite extension such that its concentration tends to zero (here called the direct method), an electrical double layer theory is obtained from the Kirkwood and Poirier theory for ionic solutions. It is shown that this double layer theory is equivalent to the theory of Stillinger and Kirkwood. Another electrical double layer theory is obtained from the Kirkwood and Poirier theory by taking the limits of infinite radius and zero concentration in one of the species of a liquid mixture (here called the asymptotic method). It is shown that this theory is also equivalent to the Stillinger and Kirkwood theory and therefore the direct and the asymptotic methods are equivalent. This happens also when the hypernetted chain and mean spherical approximations are considered. Finally, the electrostatic interaction potential between a charged plate and an ion is discussed in view of its importance in the application of the direct and asymptotic methods.
A theoretical equation of state for molecular hydrogen
Authors: Fiorese G.
Published: February 2003

Using a pair potential deduced from theoretical work, Monte Carlo simulations are performed to compute the thermodynamic properties of molecular hydrogen in the fluid and solid phase. Theoretical models are then tested against these results. Some thermodynamic properties are computed from a general form of the equation of state, theoretical Hugoniots in particular. The calculations agree quite well with the measurements.
Rotational tunneling in phase III of CD4
Authors: Prager M. , Press W. , Heidemann A.
Published: February 2003

High resolution inelastic neutron scattering has been used to study rotational tunneling in CD4 III. Measurements have been performed with an energy resolution of δE = 0.36 μeV and energy transfers E of up to EMAX = 36 μeV. The complex spectrum consists of at least eight lines with energies Ei?8 μeV. The data can only be explained assuming a superposition of several single molecule spectra. Four different models with up to five inequivalent sites describe the data almost equally well. These models have been used in an attempt to explain the tunneling spectra of CH4 in a CD4 matrix.
The specific heat anomaly in some ternary liquid mixtures near a critical solution point
Authors: Bloemen E. , Thoen J. , Van Dael W.
Published: February 2003

The anomaly in the specific heat at constant pressure and constant overall concentration Cpx has been investigated near a critical solution point in a ternary system of triethylamine–water–heavy water and in two different mixtures of triethylamine–water–ethanol. Data have been obtained by means of a standard heat pulse technique and with a constant heating rate method. Accurate results could be obtained for ‖TC−T‖/Tc values as small as 10−5. A detailed analysis of the data has been carried out by means of a nonlinear least squares fitting procedure. No indication of exponent renormalization was observed for triethylamine–water–heavy water. Instead, the value for the critical exponent as well as for the universal amplitude ratio are in close agreement with recent values obtained for the binary system triethylamine–heavy water and with the Ising values. For the two critical mixtures of triethylamine–water–ethanol exponent renormalization effects were, however, clearly visible and increased with increasing ethanol concentration. The effect was much more pronounced in the two-phase region. Although effective exponent values smaller than the Ising value αI = 0.110 were obtained, the data were not compatible with αR = −αI/(1−αI), indicating that for the experimental range the full renormalization regime was not yet reached. The experimental observations do not contradict the theoretical estimates of crossover between αI and αR.
An exceptionally large linear electro-optic effect in the organic solid MNA
Authors: Lipscomb G. F. , Garito A. F. , Narang R. S.
Published: February 2003

The dc linear electro-optic effect and the x-ray crystal structure of the organic molecular solid 2-methyl-4-nitroaniline (MNA) have been studied. Of two crystal orientations measured, one polar orientation was found to have an exceptionally large figure of merit for electro-optic phase retardation, comparable to that of LiNbO3. Comparison with second harmonic generation results demonstrates that the linear electro-optic effect in MNA is primarily electronic in nature. The origin of the large macroscopic second order nonlinear optical susceptibility is traced to a single large component of the corresponding microscopic molecular susceptibility.
Theoretical investigation of rotational rainbow structures in X–Na2 collisions using CI potential surfaces. II. Combined rotational–vibrational excitation for X = He scattering
Authors: Müller Wolfgang , Schinke Reinhard
Published: February 2003

Combined rotational–vibrational excitaton cross sections for He–Na2 scattering are reported for collision energies of 0.05?E?0.15 eV. The infinite-order-sudden (IOS) approximation is used to decouple the rotational and the orbital angular momenta, whereas the vibrational degree of freedom is treated exactly by solving vibrationally coupled radial equations. An analytical potential energy surface obtained from 160 ab initio CI energies is used in these scattering calculations. The cross sections for vibrational excitation or de-excitation are extremely small compared to the vibrationally elastic cross sections but increase rapidly with the collision energy. The influence of the target vibration on pure rotational excitation processes is negligibly small for energies considered in this study, such that the rigid-rotor approximation applied previously for the same system is confirmed to be reasonable. Furthermore, the rotational rainbow structures which are characteristic for the He–Na2 system depend only very slightly on whether the molecule is initially in the ground n = 0 or the first vibrationally excited state n = 1. This was in contradiction to the experimental finding of Bergmann et al. [J. Chem. Phys. 72, 4777 (1980)], who reported a dramatic enhancement of the rotational energy transfer for scattering within the n = 1 vibrational state. The rotational rainbow maxima occur at almost equal positions independent of whether the transition is vibrationally elastic or inelastic. A small shift to wider scattering angles in the case of Δn = 1 is observed at lower energies.
Density and temperature dependencies of ortho-positronium annihilation rates in ethane gas
Authors: Sharma S. C. , McNutt Joe D. , Eftekhari A. , Hejl R. A.
Published: February 2003

Results are reported for the density and temperature dependencies of the annihilation rates of o-Ps in ethane gas. The pickoff quenching rate of o-Ps in ethane is observed to vary with temperature at low densities of the gas. The observed o-Ps annihilation behavior with density and temperature of ethane is interpreted in terms of density fluctuations in the gas.
Surface binding of an electronic analog to CO: Infrared evidence for CH3NC chemisorption on Rh/Al2O3
Authors: Cavanagh R. R. , Yates J. T.
Published: February 2003

The chemisorption of methyl isocyanide by Al2O3- supported Rh has been investigated using transmission infrared spectroscopy. Evidence for the absence of dissociation or isomerization upon chemisorption is presented. The identification of various surface binding sites is possible and is in agreement with the site distribution previously demonstrated for such samples using CO. Samples which are exposed to methyl isocyanide following saturation coverage with CO exhibit a 100 cm−1 decrease in the C≡O stretching mode due to the presence of the isocyanide. This shift is interpreted in terms of a σ donor–π acceptor interaction between the isocyanide and CO adsorbates.
Diffusion and surface oxidation mechanism of Pt-coated Ni–W–Zr alloy studied by x-ray photoelectron spectroscopy
Authors: Hirai Yasuharu , Yamamoto Shigehiko
Published: February 2003

Diffusion phenomena of Ni, W, and Zr atoms in Pt film and oxidation of these atoms at the surface have been studied for a Pt-coated Ni–W–Zr alloy system heated in oxygen ambient by using x-ray photoelectron spectroscopy (XPS). Diffusion constants and sticking probabilities for oxygen molecules are obtained for constituent atoms by adopting a diffusion model which takes account of surface oxidation. Diffusion constants obtained are 1.3×10−6 exp(−1.3 eV/kT) for Ni and 3.7×10−8 exp(−1.1 eV/kT) for W. These results indicate that diffusion of Ni and W atoms in Pt film is mainly dominated by grain boundary diffusion. Sticking probabilities of O2 molecules for Ni and W atoms on Pt film are estimated to be on the order of 10−2. The rate of oxidation of Ni and W atoms is restricted, at first, by the velocity of atoms diffusing to the surface, and then by the number of O2 molecules impinging on the surface.
High-resolution J-resolved NMR spectra of dilute spins in solids
Authors: Terao T. , Miura H. , Saika A.
Published: February 2003

A technique for obtaining J-resolved NMR spectra of dilute spins in solids has been developed. It is based on the observation that a combination of magic-angle irradiation and magic-angle spinning removes dipolar broadening, but leaves indirect spin–spin coupling. A preliminary application of this technique to adamantane clearly reveals the AX (J = 121 Hz) and AX (J = 135 Hz) multiplets in the methylene and methyne 13C spectrum, respectively.
Model compound studies on the electronic structure of PMMA
Authors: Bigelow Richard W. , Jaffe Hans H.
Published: February 2003

The core-exciton spectrum and the valence excitation spectrum of PMMA (polymethylmethacrylate) and DMMP 2,2-dimethylmethylpropionate are calculated by using semiempirical all-valence-electron CNDO/s-CI method as reported by Ritsko et al. (Refs.1 and 2). The use of modified oxygen parameter in the computations is specifically discussed.(AIP)
Crystal structures of pyridine and pyridine trihydrate
Authors: Mootz D. , Wussow H.-G.
Published: February 2003

The crystal structure of pyridine is orthorhombic with space group Pna21, a = 1752.4(3), b = 896.9(2), and c = 1135.2(2) pm at 153 K and Z = 16. A low melting crystalline hydrate of pyridine was found by DTA to be a trihydrate. It is orthorhombic, too, with space group Pbca, a = 1244.4(3), b = 1783.2(6), and c = 679.1(2) pm at 223 K and Z = 8. Both structures were determined from single crystal MoKα diffractometer data and refined to unweighted R values of 0.044 for 2769 and of 0.074 for 932 observed independent reflections. Most apparent in the anhydrous structure is an arrangement of the molecular noncrystallogrpahic twofold axes in layers perpendicular to the c-axis direction and separated by c/4. With the ring centers not far from the points of a bcc lattice a ’’molecular coordination number’’ of 14 can be assigned to each of the four independent molecules. Their geometries are very similar and close to mm-C2v symmetry. In the structure of the trihydrate the water molecules are O–H ⋅ ⋅ ⋅ O hydrogen bonded in a new pattern of condensed four, five, and six-membered rings to form buckled layers, separated by b/2, which enclose the pyridine molecules. There is one hydrogen bond O–H ⋅ ⋅ ⋅ N from a water molecule to the pyridine molecule. All hydrogen atoms are ordered.
Complete basis set correlation energies. I. The asymptotic convergence of pair natural orbital expansions
Authors: Nyden Marc R. , Petersson G. A.
Published: February 2003

An expression for the ’’correlation energy’’ of a multiconfiguration wave function is developed using perturbation theory. The asymptotic form of this expression for an N-configuration pair natural orbital expansion is Error(N×N)?(Σμ = 1NCμ)2 (−225/4608)N−1. The asymptotic form attributes the dominant variation in multiconfiguration pair correlation errors to an interference effect between low-lying natural orbitals. Three levels of extrapolation based on the asymptotic convergence of pair natural orbital expansions are examined. The first requires separate calculations with 5 and 14 natural orbitals. When applied to the helium atom, for which E(5) = −2.897 484 and E(14) = −2.901 697, the extrapolated value, E = −2.903 724, is accurate to within 0.05% of the error from the 14 natural orbital wave function (i.e., the absolute accuracy is ≲0.000 001 hartree). The second extrapolation requires separate calculations with 5 and 14 pair MCSCF configurations and is accurate to within 2% of the MCSCF (14) error (i.e., the absolute accuracy is ≲0.000 05 hartree) for the helium isoelectronic series. The third extrapolation requires only the 5-configuration MCSCF calculation. This extrapolation is accurate to ∼10% of the MCSCF (5) error (i.e., the absolute accuracy is ∼0.0005 hartree) for the cases examined, including CH2, Ne, He, and H2. This is comparable to the accuracy of an MCSCF calculation including ten times as many natural orbitals (which would require a factor of ∼104 more computing time).