bluuebunny/crossref_metadata_2025 huggingface dataset:
Bonding and backbonding in NiCO: A MCSCF study
Authors: Bagus Paul S. , Roos Björn O.
Published: February 2003

The complete active space SCF (CASSCF) procedure is used to study the mature of the ρ-electron correlation in the ground state of linear NiCO. This correlation leads to significant changes in the electron distribution compared to the SCF wave function. A particularly important feature is that the ρ back-donation from Ni to CO is much larger when this correlation is included (∼0.6 electrons) than in the SCF case where the ρ population on CO is only 0.16 electrons larger than in a free CO molecule. The CASSCF results are compared to a previous generalized valence bond study of NiCO. (AIP)
Bonding in the Mg4 cluster. An example of chemical bonding originating from electron correlation effects
Authors: Chiles Richard A. , Dykstra Clifford E. , Jordan Kenneth D.
Published: February 2003

The Mo4 cluster has been examined with SCF and correlated SCEP and CEPA-2 treatments. It is found that little or no binding is obtained when either correlation effects of atomic 3d functions in the basis set are excluded. However, calculations including correlation effects and utilizing a basis set containing d functions demonstrated chemical bonding and yield A cluster formation energy of about 20 kcal/mole.
Collision induced intramolecular energy transfer: The He–1B2 aniline system
Authors: Cameron Stewart M. , Vandersall Mark , Rice Stuart A.
Published: February 2003

Measurements of rates of intramolecular vibrational energy transfer in room temperature collisions of helium with 1B2 aniline show that the overall pattern of energy transfer is nearly the same and about one half as efficient as for collisions with argon.
Magnetic relaxation study of solid ammonia–boron trifluoride complex
Authors: Yamauchi Jun , McDowell Charles A.
Published: February 2003

Proton and fluorine magnetic resonance absorption, and spin–lattice relaxation time measurements, have been carried out on crystalline ammonia–boron trifluoride complex in the temperature ranges from 4.2 to 300 K in the continuous wave method, and from 89 to 373 K using the pulse technique. The second moment results indicated that the BF3 motion is hindered at ∼200 K, as previously reported, while the NH3 group is considerably mobile at 77 K but an additional motion occurs at 4.2 K. The magnetization decay of 1H and 19F after a 180 ° pulse showed intrinsically nonexponential behavior. The T1 data, taken as the initial decay time constant, are compared with approximate theoretical estimates including three dissimilar spins. For the 19F relaxation at high temperature, a set of three differential rate equations was reduced to a pair of two differential equations in which the effect of the third atom is taken into account in the diagonal coefficients. The temperature dependence was almost completely predicted by this treatment. Three dipolar contributions 19F–19F, 19F–11B, and 19F–1H were found to be 1.244, 2.420, and 0.222×109 s−2 for the relaxation constants, respectively. A discrepancy between theory and experiment was observed in the 1H relaxation data at high temperature and for 19F at low temperature, both of which can not be explained by assuming a two-dissimilar-spins system and require the consideration of magnetic polarization in the third atom.
Distribution of charged particles near a charged hard wall in a uniform background
Authors: Totsuji Hiroo
Published: February 2003

A formula for the contact value of the density due to Henderson et al. is extended to the case where there exists a fixed uniform background, and it is shown that in this case the formula gives an inequality and the asymptotic behavior for the potential difference across the double layer as a function of the surface charge density on the wall. Correction to the Gouy–Chapman theory is given in terms of the bulk pressure and a simple approximation for the case of one component whose results satisfy these exact relations is proposed.
Lattice gas of hard-core tetrahedral molecules on a bcc lattice
Authors: Dougherty Anne M. , Huckaby Dale A.
Published: February 2003

Using the grand canonical ensemble, series approximations have been obtained to both the thermodynamic properties and order parameters for a bcc lattice gas of hard-core tetrahedral molecules. A molecule in the model can occupy a lattice site in either of the two orientations in which the molecular bonds point toward neighboring lattice sites. An analysis of the Padé aproximants to the series indicates an order–disorder transition, which was previously proved to exist, occurs at a number density ρ = 0.44ρ0, a pressure P = 0.83ρ0kT, and an activity z = 1.2, where ρ0 is the number density at closest packing.
Elastic constants and molecular association of cyano-substituted nematic liquid crystals
Authors: Schad Hp. , Osman M. A.
Published: February 2003

The dielectric and elastic properties of cyano-substituted nematic liquid crystals are investigated. The results are discussed in terms of molecular properties and molecular association.
Diffusion-influenced reactions and normal solutions of the Boltzmann equation
Authors: Schell Mark , Kapral Raymond
Published: February 2003

The influence of the boundary layer in the vicinity of a reactive sink on the chemical rate coefficients of diffusion-influenced reactions is studied using kinetic theory. It is argued that normal solutions to the Boltzmann equation break down at first order in P−1e, where Pe is the thermal Peclet number, for the reaction problem. When the effects of the kinetic boundary layer are consistently treated to first order in P−1e the rate coefficient which follows from the diffusion equation with the radiation boundary condition is obtained. The importance of properly treating terms of order P−1e is discussed and parallels with Stokes’ law calculations are described.
Interaction potentials for Br(2P)+Ar, Kr, and Xe (1S) by the crossed molecular beams method
Authors: Casavecchia Piergiorgio , He Guozhong , Sparks Randal K. , Lee Yuan T.
Published: February 2003

Angular distributions of Br(2P3/2,1/2) scattered off Ar, Kr, and Xe (1S0) in the thermal energy range were measured in crossed molecular beams experiments. Interaction potentials for the X1/2 and I3/2 states are derived by using an approximate elastic scattering analysis, which neglects interstate coupling, as previously done for F–Xe, Kr, Ar, Ne and Cl–Xe. While the Br–Xe X1/2 potential (ε = 0.645 kcal/mole, rm = 3.80 Å) shows a stronger interaction than the interaction potential of Kr–Xe, the Br–Kr (ε = 0.460 kcal/mole, rm = 3.90 Å) and Br–Ar (ε = 0.380 kcal/mole, rm = 3.73 Å) X1/2 potentials are closer to those of the corresponding rare gas pairs. The I3/2 potential for all three systems is found to have a shallower ε, a slightly larger rm, and a more repulsive wall than the 1Σ+ potential of the corresponding rare gas pair. The origin of these interactions is discussed and an attempt to examine the rare gas halides ground state bonding is presented.
A study of l changing in Xe (n
f)–neutral collisions at thermal energies
Authors: Higgs C. , Smith K. A. , Dunning F. B. , Stebbings R. F.
Published: February 2003

In this paper we report a study of thermal collisions between Xe (nf) Rydberg atoms and Xe, Kr, Ar, CO2, CO, and N2. The dominant collision mechanism is, in all cases, observed to be l changing: no evidence of n changing or ionization is apparent. Comparison of the experimentally derived cross sections with those obtained using a theoretical model in which effects due to the Rydberg core are ignored appears to lead to satisfactory results only for Xe, Kr, and CO2, whereas when core effects are included satisfactory agreement between experiment and theory is found in all cases.
Simultaneous calculation of several interacting electronic states by generalized Van Vleck perturbation theory
Authors: Kirtman Bernard
Published: February 2003

We present an accurate many-body perturbation theory for the simultaneous calculation of an arbitrary set of interacting electronic states based on the generalized Van Vleck method. This treatment yields an effective Hamiltonian for the interacting states (or model space) which is Hermitian and energy-independent. A special provision is made for external states that are energetically adjacent to the model space. The perturbation formulas for the effective Hamiltonian matrix contain ordinary Rayleigh–Schrödinger type energy denominators which include zeroth-order splitting between the various interacting states. These energy denominators factor in all orders just as they do in the nondegenerate problem. A convenient diagrammatic representation of the perturbation expansion is developed. In carrying out the expansion there is some freedom associated with the orthonormality condition for states in the model space. We make the simplest possible choice with regard to evaluation of the perturbation formulas. This leads to a fully linked cluster expansion through fourth order. In fifth-order unlinked clusters begin to appear off-diagonal. They are few in number and very easily calculated. To illustrate the treatment we consider a model space containing only single substitution configurations. For a space of dimension d one has to compute on the order of d2 off-diagonal matrix elements of the effective Hamiltonian. However, the computational effort to obtain the entire set is roughly comparable to that required for a single diagonal element. Furthermore, the most extensive perturbation sums that occur in the diagonal elements are exactly the same for each state and need be computed only once. These sums also appear in an ordinary Rayleigh–Schrodinger ground state calculation.
Formaldehyde: Electronic structure calculations for the S and T1 states
Authors: Adams George F. , Bent Gary D. , Bartlett Rodney J. , Purvis George D.
Published: February 2003

Many-body perturbation theory (MBPT) and coupled-cluster method calculations are reported for the S0(X 1A1) and T1(a 3A″) electronic states of formaldehyde. The structural parameters for the S0 minimum (RCH = 1.102 Å, RCO = 1.211 Å, HCH = 116.2°) and the T1 minimum (RCH = 1.085 Å, RCO = 1.327 Å, HCH = 118°, ’’out-of-plane’’ angle = 37° 12′) agree well with experimentally deduced values. Calculated heats of reaction for dissociation to radical products and molecular products agree well with literature values. The energy barriers for dissociation to molecular products and rearrangement to hydroxycarbene are presented. Vertical and adiabatic transition energies are reported for S0→T1, while a vertical transition energy for S0→S1 is reported.
Convenient and accurate discretized path integral methods for equilibrium quantum mechanical calculations
Authors: Schweizer Kenneth S. , Stratt Richard M. , Chandler David , Wolynes Peter G.
Published: February 2003

In the path integral representation of quantum theory, a few body quantum problem becomes a classical many body problem. To exploit this isomorphism, it becomes necessary to develop methods by which degrees of freedom can be explicitly removed from consideration. The interactions among the remaining relevant variables are described by effective interactions. In this paper, we present a general methodology to carry out the reduction in numbers of degrees of freedom. Certain path integral algorithms are shown to correspond to reference systems for the full isomorphic classical many body problem. The correspondence allows one to determine systematic corrections to the algorithms by low order perturbation approximations familiar in the theory of simple classical fluids. We show how to use discretized path integrals to compute rigorous upper and lower bounds to the free energy for nontrivial quantum systems, and we discuss how to optimize the upper bounds with variational theories. Several illustrative examples are provided including treatments of a tunneling system that is strongly coupled to a nonadiabatic heat bath. The examples demonstrate that even for strongly perturbed systems in which the quantum energy scales are as large as 40kBT, the methods discussed herein provide a practical route for obtaining equilibrium properties with both high accuracy and analytical simplicity.
Vertex models for the antiferroelectric transition in squaric acid
Authors: Stilck J. F. , Salinas S. R.
Published: February 2003

We consider two-dimensional 4- and 12-vertex models on a square lattice for the phase transition in the layered hydrogen-bonded antiferroelectric crystals H2C4O4 and its deuterated isomorph. An exact solution for the 4-vertex model, which strictly obeys the ’’ice-rules’’, leads to a vanishing free-energy. The 12-vertex model, which takes into account the single ionized configurations, gives a second order phase transition in the framework of the Bethe approximation. The high-temperature entropy of the 12-vertex model is calculated exactly and agrees with the value predicted by the Bethe approximation. An exact calculation of the partition function for a modified 12-vertex model, with a more degenerate ground state, leads to a well behaved thermodynamic potential. Also, we add a mean-field interlayer interaction to the 12-vertex model for predicting the correct low-temperature ordering in squaric acid. Some thermodynamic quantities are explicitly calculated for this model.
Tracer diffusion in polyatomic liquids. II
Authors: Chen S. H. , Davis H. T. , Evans D. F.
Published: February 2003

Tracer diffusion measurements taken over a wide range of temperature (25.0–142.7° C) are reported for nine solutes (Ar, CH4, Kr, Xe, R4 Sn: R = Me, Et, n-Pr, n-Bu) in cyclohexane. The measurements were made in a Taylor dispersion apparatus modified for pressurization and temperature control. Rough-hard-sphere theory, in which the diffusion coefficient is the product of a constant rotational factor, Enskog’s diffusivity of hard spheres, and a computer-generated backscattering coefficient, agrees well with the observed results. The theory furnishes new insights into the origins of the energy of activation of diffusion and explains trends not previously understood.
Dynamical and optical properties of the ethylene crystal: Self-consistent phonon calculations using an ‘‘a
b i
n
i
t
i
o’’ intermolecular potential
Authors: Luty T. , van der Avoird A. , Berns R. M. , Wasiutynski T.
Published: February 2003

Using an intermolecular potential from ab initio calculations, we have calculated the structure and solved the lattice dynamics problem of the ethylene crystal in the self-consistent phonon formalism. The anharmonic effects, as included by this formalism, systematically improve the optical mode frequencies, in comparison with experiment, but the corrections to the harmonic frequencies are still substantially too small. The crystal structure and its pressure dependence are well represented. From the phonon polarization vectors we have also evaluated the Raman scattering and infrared absorption intensities of the optical modes, applying a scheme which takes into account the mutual polarization of the molecules (the ’’local field corrections’’). The Raman intensities agree quite well with experiment; the infrared intensities are less realistic, probably due to the neglect of intermolecular overlap effects in the intensity calculations. Using an empirical atom–atom potential for hydrocarbons instead of the ab initio potential, the assignment of the optical lattice modes by their calculated frequencies was not fully consistent with the Raman intensity ratios obtained from their polarization vectors.
Production and relaxation cross sections for the shear viscosity SBE. II. IOSA results for the N2–He system
Authors: Liu W.-K. , McCourt F. R. , Fitz D. E. , Kouri D. J.
Published: February 2003

Infinite-order sudden approximation (IOSA) calculations of the phenomenological production and relaxation cross sections governing the magnetoviscosity effect have been undertaken for the system N2 at infinite dilution in He for the purpose of testing the usefulness of the IOSA procedure. Three l-labeling schemes (IL, FL, and AVL) have been employed and the results obtained for each of the three types of cross sections occuring in the viscomagnetic effect have been compared. For the relaxation cross sections σT( j′j‖Ek), σ(1)η( j′j‖Ek), and σ(2)η( j′j‖Ek), the IL/FL and AVL results differ by less than 5% on the average, but for the production cross sections σTη( j′j‖Ek), the three schemes IL, FL, AVL give widely differing results (one order of magnitude and sign). Thermally averaged phenomenological cross sections at 77.5 K were obtained for three potentials of the MSV type: one obtained from beam experiments and two using the beam scattering isotropic part and modified anisotropy parameters. For one of the modified anisotropic potentials giving reasonable agreement of the cross section σT calculated using the IOSA procedure and obtained from experiment, initial close-coupled calculations were performed at a kinetic energy of 54 cm−1. The IOSA AVL procedure gave results for j, j′ = 1, 3 differing by as much as 30% for σT( j′j‖Ek), and σ(κ)η( j′j‖Ek) and by as much as a factor of 2 for σTη( j′j‖Ek). Tentatively, it appears that in order to calculate production cross sections with reasonable confidence and accuracy, it will be necessary either to modify the CSA and IOSA methods or to use the more accurate CC procedure.
Infinite order sudden approximation for reactive scattering. II. Computational tests for H+H2
Authors: Kouri D. J. , Khare V. , Baer M.
Published: February 2003

A reactive scattering infinite order sudden approximation presented earlier is applied to the H+H2 exchange reaction. Detailed results for both differential and integral reactive cross sections are presented and compared with accurate close coupling results obtained earlier by Kuppermann and Schatz. Qualitative agreement is obtained for detailed state-to-state cross sections and more quantitative agreement is demonstrated for cross sections summed over final states.
The isotope exchange reactions H++D2⇄HD+D+ and D++H2⇄HD+H+ in the temperature range 200–300 K
Authors: Henchman M. J. , Adams N. G. , Smith D.
Published: February 2003

Forward and reverse rate coefficients have been measured in the temperature range 200–300 K for the two reactions H++D2⇄HD+D+ and D++H2⇄HD+H+. Equilibrium constants derived therefrom agree with theoretical van’t Hoff plots calculated from statistical mechanics and confirm the temperature calibration of the SIFT apparatus used. It is suggested that these reactions can be used as kinetic thermometers to measure independently the temperature of ion–molecule reaction cells. The system provides a particularly clear example of the role of statistical factors in chemical kinetics, 1 for the forward reactions and 1/2 for their reverse reactions; and the system illustrates further the relationship between statistical factors in kinetics, symmetry numbers in statistical mechanics, and the corresponding thermodynamic entropy changes. Constraints upon the temperature dependence of the rate coefficients are derived from consideration of thermodynamics and collision dynamics, and the data are seen to conform to these over a limited temperature range. A further trend is suggested by the data, supporting previous observations of isotope-exchange reactions—the rate coefficients of the exoergic reactions decrease with increasing temperature—and may be described in terms of the partitioning of the system according to the number of states available to the products and the original reactants. It is suggested that this should be a general result for reactions where the exoergicity is comparable to the temperature of measurement.
Radiationless decay of vibronically coupled electronic states. III. Strong coupling and its effect on triplet decay in aromatic hydrocarbons
Authors: Siebrand Willem , Zgierski Marek Z.
Published: February 2003

Radiationless transitions from an optically prepared state to the ground state are studied on a model consisting of three electronic states and four modes of vibration. Two ’’promoting’’ modes induce transitions from the two excited states; they are taken harmonic and treated in the weak-coupling approximation. One mode couples the two excited states; its coupling is allowed to be strong enough to lead to anharmonic adiabatic potentials. The fourth mode is a displaced harmonic oscillator acting as an accepting mode, a role it shares with the coupling mode if the coupling is strong enough. The model differs from that studied in part II [J. Chem. Phys. 72, 1641 (1980)] in allowing stronger coupling as well as indirect (second-order) transition via the upper electronic state. Solutions are obtained by numerical methods that avoid the use of the Condon approximation. Radiationless decay rate constants are calculated for cold and hot bands and related to the energy distribution between the accepting modes. They are plotted as a function of energy separations, displacements, and coupling strengths. The results which confirm and extend the conclusions of part II are applied to radiationless triplet decay in benzene, naphthalene, and their perdeuterated analogs. It is concluded that in the two benzenes, in naphthalene-d8, but not in naphthalene-h8, the mode coupling T1(3La) with T(3Ba) is the dominant accepting mode. This result is used to account for anomalies in the deuterium effect and energy-gap dependence of triplet decay rate constants in aromatic hydrocarbons.
NMR study of molecular motion in some molecules containing t-butyl groups
Authors: Yamauchi Jun , McDowell Charles A.
Published: February 2003

Continuous wave and pulse NMR measurements were carried out on phenol derivatives containing t-butyl groups [tri-t-butylphenol (TTBP), methylene-bis(2,6-di-t-butylphenol) (MBBP), and the galvinoxyl radical] to investigate the molecular motions of methyl and t-butyl groups. From second moment measurements we distinguish three temperature regions characteristic of different forms of molecular motions. Between the rigid-lattice limit (plateau region III) and plateau region I at high temperature where both methyl and t-butyl motions are present, there appeared shoulders in the second moment curve. The shoulders in the second moments and relaxation minimum at low temperature are satisfactorily explained by the motional model which includes nonequivalent methyl motions; two methyls in each t-butyl group at the ortho positions and all methyls in the t-butyl group at the para positions are responsible for these phenomena, while one methyl motion in each t-butyl group at the ortho positions and the entire motion of the t-butyl group are considerably restricted at low temperature. The relaxation mechanism at high temperature is discussed in terms of the nonequivalent methyl motions, assuming that one methyl group rotates slowly at a rate almost equal to that of the t-butyl group motion. The short relaxation time of the galvinoxyl radical is explained in terms of the electron–nuclear interactions and almost equal-weight contributions from the dipolar and scalar (hyperfine) interactions were found to account fully for the experimentally measured values.
Sublevel phosphorescence spectra of pyrazine. Interpretation of ‘‘anomalous’’ behavior of the radiative rate constant ratios for totally symmetric vibrations and the reinvestigation of the triplet state geometry
Authors: Kokai Fumio , Azumi Tohru
Published: February 2003

The phosphorescence spectra from the three individual triplet spin sublevels of pyrazine in a single crystal of benzene are obtained. The radiative rate constants are determined for individual vibronic bands of the individual sublevel phosphorescence spectra. The nonradiative decay rate constants are also determined for individual sublevels. The mechanisms of the radiative and nonradiative transitions from the sublevels are discussed, and are satisfactorily interpreted within a conventional perturbation theory. For vibronic bands involving totally symmetric skeletal vibrations ν1 and ν8a, the ratios of the radiative rate constants are markedly different from the ratio observed at the 0–0 band. This is interpreted as due to the importance of vibronic coupling due to these totally symmetric vibrations and the smallness of the displacement of the potential energy surfaces between the triplet and the ground states. This interpretation, which is achieved within a framework of the Born–Oppenheimer and Herzberg–Teller approximations, substantially differs from the interpretation given for an analogous observation by Breiland and Harris. The vibrational structures of the sublevel phosphorescence substantiate our earlier conclusion that the triplet state of pyrazine is distorted along the ν4(b2g) coordinate leading to the point group of C2h[C2(y)].
Prediction of absolute infrared intensities for the fundamental vibrations of H2O2
Authors: Rogers Jerry D. , Hillman John J.
Published: February 2003

Absolute infrared intensities are predicted for the vibrational bands of gas-phase H2O2, using a hydrogen atomic polar tensor transferred from the hydroxyl hydrogen atom of CH3OH. These predicted intensities are compared with intensities predicted using a hydrogen atomic polar tensor transferred from H2O. The predicted relative intensities agree well with published spectra of gas-phase H2O2, and the predicted absolute intensities are expected to be accurate to within at least a factor of 2. Among the vibrational degrees of freedom, the antisymmetric O–H bending mode ν6 is found to be the strongest with a calculated intensity of 60.5 km mole−1. The torsional band, a consequence of hindered rotation, is found to be the most intense fundamental with a predicted intensity of 120 km mole−1. These results are compared with the recent absolute intensity determinations for the ν6 band.
Study of hydracids trapped in monatomic matrices. III. Pressure effects on the vibrational frequencies of the aggregates and evolution of the charge transfer
Authors: Maillard Daniel , Schriver André , Fondere Françoise , Obriot Jacques , Girardet Claude
Published: February 2003

The modifications induced by pressure on the vibration–orientation spectra of the various hydracid polymers in rare gas matrices are discussed from an experimental and theoretical point of view. The main experimental results obtained are the blue shifts of the monomer and one of the dimer bands and the red shifts of all the other poylmeric bands under increasing pressure. The interpretation in terms of the intermolecular potential is based on a hydrostatic pressure model which contracts the lattice parameter and shortens the intermolecular distance between hydracids. When pressure increases, the angular geometries of the polymers determined at P = 0 are nearly unchanged although the overlap of the electronic clouds of two neighboring molecules becomes more and more important. On the contrary, the potential derivatives with respect to the internal vibrational coordinates are considerably amplified by a charge transfer process due to this overlap. The quasicontinuous study of this evolution with the pressure enables a quantitative discussion of this little known phenomenon in weakly hydrogen bonded species. A model of the molecular parameter derivatives modification due to this charge transfer is proposed and found in quantitative agreement with experiment.
X-ray study of the ‘‘rotator’’ phase of the odd-numbered paraffins C17H36, C19H40, and C21H44
Authors: Doucet J. , Denicolo I. , Craievich A.
Published: February 2003

We present here a study of the ’’rotator’’ phase displayed by the odd-numbered normal paraffins CnH2n+2 with n ranging from 17 to 21. A structural model of the rotator phase can be deduced from x-ray experiments performed both on single domains and on powder samples. Its structure very much looks like the crystalline phase structure, i.e., the molecules are packed within layers forming a bilayer structure with the molecular axes oriented perpendicularly to the layer planes. The comparison of the space group Ccmm with the steric dimensions of the molecules implies the appearance of an orientational disorder of the molecules around their long axes which is the main characteristic of the crystalline→rotator phase transition. Such a conclusion is in agreement with the dynamical measurements which state a uniaxial rotation of the molecules.
The liquid–vapor interface of a binary liquid mixture
Authors: Beaglehole D.
Published: February 2003

Cahn’s model for a binary liquid mixture in contact with an external wall is applied to the liquid–vapor interface. The concentration profile, the adsorption, and the surface tension are calculated using the regular solution free energy function for solutions of near critical composition and dilute solutions, and using a nonclassical free energy function for critical composition solutions. The calculation is in good agreement with recent measurements of the adsorption of the cyclohexane–aniline system.
The photoelectron spectrum of Xe3 by the photoelectron–photoion coincidence technique
Authors: Poliakoff E. D. , Dehmer P. M. , Dehmer J. L. , Stockbauer Roger
Published: February 2003

The technique of photoelectron–photoion coincidence spectroscopy is used to obtain the photoelectron spectrum of the Xe3 trimer in the energy region of the first ionization potential. The results show that the first vertical ionization potential of Xe3 is 11.47(9) eV versus 11.85 eV in Xe2. This experiment demonstrates for the first time the feasibility of obtaining a photoelectron spectrum of a cluster contained in a mixture of n-mers. This measurement yields insight into the electronic structure changes occurring as free atoms combine to form solids and will provide additional information as the studies are extended to higher n-mers.
Quadrature method of evaluating overlaps using Morse oscillators
Authors: Ramjee M. , Rao M. L. P. , Rao D. V. K. , Rao P. T.
Published: February 2003

Using Morse oscillators, the overlap integrals have been evaluated by Gauss–Laguerre quadrature method for the B 1Σ+–X 1Σ+ band system of BH radical observed in sunspot spectra. The Bv values for a few vibrational levels of X 1Σ+ and B 1Σ+ states of the BH radical have been evaluated using Gauss–Laguerre quadrature method to test the accuracy of the method.
Comparison of photoconductivity and optical spectra for trapped electrons in 3-methylpentane glass
Authors: Funabashi T. , Okabe N. , Kimura T. , Fueki K.
Published: February 2003

Photoconductivity spectrum of trapped electrons in 3-methylpentane glass is reported. The optical absorption spectrum from ref. 7 is compared with the photoconductivity spectrum. The assignments of the optical spectrum are discussed. (AIP)
The structure of the Ar ⋅ CH3Cl van der Waals molecule
Authors: DeLeon R. L. , Muenter J. S.
Published: February 2003

Eleven new radio frequency and microwave transitions have been observed in Ar ⋅ CH3Cl. Combining this data with the previously unassigned microwave spectrum [J. Chem. Phys. 71, 4189 (1979)] produced a satisfactory fit of the data. The structure obtained places Ar 3.68 (1) Å from the CH3 Cl center of mass in an orientation perpendicular to the symmetry axis. Ar ⋅ CH3 Cl represents the first contradiction of simple donor–acceptor models for predicting the structure of van der Waals molecules.
Complex self-consistent-field calculations on shape resonances in electron–Mg and electron–Ca scattering
Authors: McCurdy C. William , Lauderdale Jack G. , Mowrey Richard C.
Published: February 2003

The complex self-consistent-field (CSCF) method has been used to compute the complex resonance energies and Hartree–Fock wave functions for the low energy 2P shape resonances in electron–magnesium and electron–calcium atom scattering and for the 2D shape resonance in electron–calcium scattering. Comparison with experiment indicates that correlation effects are important in determining the lifetimes of these resonances. It is also demonstrated that it is possible to find a solution of the CSCF equation in some cases by using a real Hamiltonian (no complex coordinates) and only real basis functions.
Electronic structure of the Cu+ impurity center in sodium chloride
Authors: Chermette H. , Pedrini C.
Published: February 2003

The multiple-scattering Xα method is used to describe the electronic structure of Cu+ in sodium chloride. Several improvements are brought to the conventional Xα calculation. In particular, the cluster approximation is used by taking into account external lattice potential. The ’’transition state’’ procedure is applied in order to get the various multiplet levels. The fine electronic structure of the impurity centers is obtained after a calculation of the spin–orbit interactions. These results are compared with those given by a modified charge-consistent extended Hückel method (Fenske-type calculation) and the merit of each method is discussed. The present calculation produces good quantitative agreement with experiment concerning mainly the optical excitations and the emission mechanism of the Cu+ luminescent centers in NaCl.
Theoretical studies of positron–molecule complexes
Authors: Kurtz Henry A. , Jordan Kenneth D.
Published: February 2003

LCAO–MO SCF Hartree–Fock theory is employed to investigate the properties of several positron–molecule systems. Three types of molecules are studied: polar molecules with dipole moments greater than 1.625 D, molecules with large quadrupole moments, and nonpolar molecules with high polarizabilities. For those systems which bind positrons, we compare the positron binding energies with the electron affinities. The stability of the positron complexes are examined both with respect to detachment of a positron and detachment of positronium.
A
b i
n
i
t
i
o extended Hartree–Fock calculations
Authors: Claxton T. A.
Published: February 2003

A b initio calculations are reported for the isotropic hyperfine coupling constants for sigma (vinyl and ethynyl) and nonconjugated pi (methyl) radicals using the unrestricted, projected, and extended Hartree–Fock methods. It is found that quartet state annihilation procedures closely approximate the results obtained by full projection of the UHF wave function using an STO-3G basis set. The extended Hartree–Fock method is discussed to explain why it overestimates the spin polarization mechanism. Possible alternatives to the UHF method are discussed.
Restricted Hartree–Fock wave functions for positron/anion [A−;e+] and positron/atom [A;e+] systems
Authors: Patrick Allan J. , Cade Paul E.
Published: February 2003

Numerical restricted Hartree–Fock wave functions are reported for positron/anion systems [A−;e+] with A = Li, Be, B, C, N, O, and F for positrons in 1s+, 2s+, 2p+, and 3d+ orbitals. The nature of the positronic orbitals, the modification in the electronic orbitals, and implications are considered. Positron affinities of the A− systems and the stability of [A−;e+] relative to A+Ps are examined. The corresponding positron/atom systems [A;e+] are also considered in an iterative fashion. It is found that RHF calculations suggest that neutral atoms do not bind a positron.
Sampling methods for Monte Carlo simulations of n-butane in dilute solution
Authors: Bigot Bernard , Jorgensen William L.
Published: February 2003

Sampling procedures for enhancing the convergence of Monte Carlo simulations of dilute solutions have been analyzed and applied to modeling n-butane in a Lennard-Jones liquid at 25 °C. The efficiency of the procedures is compared for determining structural and thermodynamic properties of the solution including the trans–gauche equilibrium for the solute. Preferential sampling of the solute plus umbrella sampling with ’’chopped’’ rotational barriers is found to be a viable means for studying conformational equilibria in solution.
Random elastic networks. I. Computer simulation of linked stars
Authors: Eichinger B. E.
Published: February 2003

Results of computer simulations of random networks with spatial constraints are reported. An algorithm has been devised to produce networks from δ-branched stars with Gaussian arms, the nodes of the stars being sited at lattice points. Structural statistics in the form of the percentage of loops, unreacted ends, single and multiple chains, and average junction functionality are given. The eigenvalue spectra of the Kirchhoff matrices of the nets are analyzed in terms of the density function and moments. The spectra for small λ are different from those for random regular nets formed without constraints, in that small eigenvalues are much more numerous when constraints are imposed. Network collapse remains a problem. Viscoelastic relaxation time spectra are also considered, and in one case the spectrum exhibits the beginnings of a rubbery plateau.
Dynamics of fluctuations and spinodal decomposition in polymer blends. II
Authors: Pincus P.
Published: February 2003

We reconsider the initial growth process of the unstable concentration fluctuations when a polymer blend of A and B chains (NA = NB) is rapidly quenched into the unstable region. We correct an earlier result and now predict (i) that the unstable wavelength 2π/? is on the order of R0 (ideal chain radius) for the case of strong segregation, and (ii) the corresponding growth rate is proportional to the melt reptation diffusion constant. These results are consistant with the phase separation kinetics observed in polystyrene–poly(vinyl methyl ether) blends by Nishi, Wang, and Kwei.
Photochemical reaction of phenyl azide studied by ultraviolet photoelectron spectroscopy
Authors: Harada Yoshiya , Takahashi Takashi , Mutai Kiyoshi
Published: February 2003

The photochemical process in a phenyl azide (C6H5N3) film prepared on a substrate held at 77 K was studied by ultraviolet photoelectron spectroscopy. In the HeI spectrum of the phenyl azide film, on irradiation, the first and third bands related to the azide moiety disappeared, while the bands due to the C6H5N skeleton became gradually broader. This observation was interpreted in terms of the photodecomposition process of phenyl azide via nitrene, which then undergoes intermolecular insertion reactions. (AIP)
Multiphoton ionization of iodine and benzene: Photoelectron and ion kinetic energy distributions
Authors: Miller John C. , Compton R. N.
Published: February 2003

We have investigated electron and ion kinetic energy distributions following resonantly enhanced multiphoton ionization of I2 and benzene. The photoelectron spectra demonstrate that n-photon absorption to produce the parent ion in its ground electronic state is the major ionization pathway. Subsequent fragmentation must involve additional photon absorption by the parent ion. The measured kinetic energy distribution of I+ from I+2 is in agreement with that predicted by energy conservation. For benzene, all of the fragment ions have identical, near thermal kinetic energy (0–0.05 eV) distributions implicating a statistical redistribution of the excess energy, rather than sequential absorption and dissociation cycles.
Interpretation of infrared intensities of ethylene by parametric methods
Authors: Jona P. , Gussoni M. , Zerbi G.
Published: February 2003

An interpretation of infrared intensities of ethylene and its deuterated derivatives is presented in this paper; different kinds of parameters, namely, molecular dipole derivatives, electro-optical parameters, polar tensors and effective charges, derived from infrared intensity data are compared. From this comparison and from a further comparison with the quantum mechanical calculations the most likely signs of dipole moment derivatives ∂M/∂Qi seem to be (+ −− ++); this choice of signs leads to a positive equilibrium bond dipole moment μCH0 (C−–H+) and to a decreasing of μCH when the bond is stretched. These results, however, need a further check on other molecules containing the vinyl group before being considered final. In this paper it is also shown that electro-optical parameters are very useful for interpretative purposes.
Comparison of Brownian dynamics with photon correlation spectroscopy of strongly interacting colloidal particles
Authors: Gaylor Kevin , Snook Ian , van Megen William
Published: February 2003

The Brownian dynamics computer simulation method is applied to a dilute system of charged spheres dispersed in a very dilute electrolyte. The parameters of this model system are chosen to match those of an aqueous dispersion of highly charged and strongly interacting polystyrene spheres, on which a number of photon correlation spectroscopy studies have recently been made. The structure factor and the electric field autocorrelation functions calculated by the Brownian dynamics method agree with the experimental data provided that allowances are made for the effects of polydispersity and multiple scattering. A few commonly used approximations for analyzing the dynamic properties of many particle systems are also examined.
The Bloch equation for multiphoton absorption. I. Derivation
Authors: Kay Kenneth G.
Published: February 2003

A Bloch equation describing infrared multiphoton absorption in an isolated polyatomic molecule is derived from first principles. The molecule is divided into a ’’system’’ mode which interacts directly with the laser field and a ’’bath’’ consisting of the remaining modes which interact with each other and the system mode via intramolecular vibrational coupling. In addition to describing the evolution of the system, the derived equation keeps track of changes in the bath state and the resulting changes in the bath–system interaction which occur as the bath gains energy. Unlike the master (or rate) equation for optical pumping, the Bloch equation is valid for arbitrary ratios W/Ω of the intramolecular relaxation rate W/h/ to the Rabi frequency of the system mode Ω/h/. The equation derived differs from certain Bloch equations previously proposed on phenomenological grounds by the appearance of off-diagonal coupling terms. These terms may significantly reduce the vibrational dephasing rate and thus affect net pumping rates and optical line shapes for vibrationally excited molecules.
Examining the accuracy of the infinite order sudden approximation using sensitivity analysis
Authors: Eno Larry , Rabitz Herschel
Published: February 2003

A method is developed for assessing the accuracy of scattering observables calculated within the framework of the infinite order sudden (IOS) approximation. In particular, we focus on the energy sudden assumption of the IOS method and our approach involves the determination of the sensitivity of the IOS scattering matrix SIOS with respect to a parameter which reintroduces the internal energy operator ?0 into the IOS Hamiltonian. This procedure is an example of sensitivity analysis of missing model components (?0 in this case) in the reference Hamiltonian. In contrast to simple first-order perturbation theory a finite result is obtained for the effect of ?0 on SIOS. As an illustration, our method of analysis is applied to integral state-to-state cross sections for the scattering of an atom and rigid rotor. Results are generated within the He+H2 system and a comparison is made between IOS and coupled states cross sections and the corresponding IOS sensitivities. It is found that the sensitivity coefficients are very useful indicators of the accuracy of the IOS results. Finally, further developments and applications are discussed.
Reaction of N2+ beams with aluminum surfaces
Authors: Taylor J. Ashley , Rabalais J. Wayne
Published: February 2003

Reactions of N2+ ion beams with the surface of clean polycrystalline aluminum over the energy range of 300–4000 eV are studied by the techniques of x-ray and UV photoelectron spectroscopy (XPS and UPS) and Auger electron spectroscopy (AES). The reaction produces a nitride layer which the depth–concentration profiles reveal to be a superposition of two distributions: the first is a continuous AlN phase at the near surface to a depth of ∼20 Å and the second is a region of lower nitrogen concentration tailing deeper into the Al substrate where a complete AlN phase has not been established. The AlN film is characterized by the agreement of the binding energies of the N 1s (397.3 eV) and Al 2p (74.1 eV) lines, of the kinetic energy of the Al KLL line (1338.9 eV), and of the Auger parameter (1463.0 eV) with a known standard of AlN. The N/Al ratio initially increases linearly with the nitrogen ion dose at low concentrations and finally reaches a steady state condition, determined by the opposing rates of nitridation and sputtering by impinging ions, at a dose of ∼1×1016 ions cm−2. The reaction cross section is found to vary from 2.0×10−16 to 4.7×10−17 cm2 over the ion energy range of 0.3–1.0 keV. A reaction mechanism which is supported by these results is discussed.
Raman scattering from polymerizing styrene. II. Intensity changes as a function of conversion
Authors: Sears W. M. , Hunt J. L. , Stevens J. R.
Published: February 2003

The Raman spectrum of thermally polymerizing styrene has been recorded for temperatures of 100 and 70 °C. The intensities of the ν9 (C = C stretch mode) and ν14 (C–H bending mode) in the vinyl group are used as an indicator of the percent conversion from monomer to polymer. These intensities, normalized to that of ν24 (phenyl breathing mode), are compared to the depolarized Rayleigh intensity of polymerizing styrene. It is suggested that there is very little orientational correlation between the polymer and monomer molecules.
Raman band shapes and dynamics of molecular motion for SF6 in the supercritical dense fluid region
Authors: Żerda T. W. , Schroeder J. , Jonas J.
Published: February 2003

The Raman reorientational correlation functions and the reorientational correlation times τϑ for the ν1, ν2, and ν5 bands of SF6 have been obtained in the temperature range 253 – 473 °K and at pressures up to 1865 bar. The plot of τϑ(Raman) vs τJ (obtained from the NMR relaxation times) clearly indicates that the reorientational relaxation of SF6 follows the Fokker–Planck–Langevin (FPL) model in the low density region. The experimental correlation functions were compared with correlation functions computed with the extended diffusion (ED) model and the FPL model. Both models reproduce the experimental correlation functions with the same accuracy; the values of angular momentum correlation times τJ used to produce the functions are approximately the same and close to the τJ values obtained from NMR 19F spin–lattice relaxation times. However, the fitting procedure, i.e., comparison of experimental correlation functions with the theoretical ones, does not allow an unambiguous conclusion as to which of the models describes the molecular reorientation of SF6. Another important finding of this study is the observation of a nonlinear behavior of the plot of τϑ vs η/T (η is the viscosity) Hynes et al. Detailed analysis of the ν1 band is presented and it is shown that vibrational–rotational coupling is responsible for the ν1 band shape.
Resonance state lifetimes from stabilization graphs
Authors: Simons Jack
Published: February 2003

The stabilization method has been used by many researchers for computing energies and, in some cases, widths of metastable states arising in electron–atom and electron–molecule interactions. In the present work, the stabilization technique is exploited in a manner which makes a close connection with the so-called complex corrdinate rotation method. The primary result is an expression which permits the resonance lifetimes to be computed directly ’’on the back of an envelope’’ from the stabilization graph which is obtained in all stabilization calculations. Several illustrative calculational results are presented.(AIP)
Collision induced linewidth of the first overtone band of HF–X (X = H2, D2, N2, and CO2) systems
Authors: Mehrotra Suresh C. , Nair K. P. R.
Published: February 2003

The Murphy and Boggs theory as extended by Mehrotra and Boggs to rotational and vibrational levels is applied to compute the collision induced linewidths of the first overtone band of HF–X (X = H2, D2, N2, and CO2) systems. The results are compared with experments of Merdith et al. The J dependence of the linewidth is well explained when the perturbers are N2 and CO2, but the agreement is poor for the light perturbers H2 and D2.