bluuebunny/crossref_metadata_2025 huggingface dataset:
A Monte Carlo simulation of the hydrophobic interaction
Authors: Pangali C. , Rao M. , Berne B. J.
Published: February 2003

The hydrophobic interaction between two apolar (Lennard–Jones) spheres dissolved in a model of liquid water (ST 2 water) is simulated using the force-bias Monte Carlo technique recently devised by the authors. Importance sampling techniques are devised and used to give a relatively accurate determination of the potential of mean force of the two apolar spheres as a function of their separation. This determination shows that there are two relatively stable configurations for the spheres. In one configuration each member of the pair sits in its own water cage with one water molecule fitting between them. There is a free energy barrier separating this from the other stable configuration which is such that no water molecule sits between the spheres. This conclusion is shown to be quantitatively consistent with the recent semiempirical theory of Pratt and Chandler and is in disagreement with some previous Monte Carlo studies.
Gasdynamic and infrared spectroscopic measurements in a condensing flow of a sulfur hexafluoride–argon mixture
Authors: Wu Benjamin J. C. , Laguna Glenn A.
Published: February 2003

Homogeneous nucleation and condensation of SF6 in supersonic nozzle flows have been investigated experimentally by static pressure measurements and infrared spectroscopy. The application of high resolution diode laser spectroscopy has allowed us to determine the concentration of SF6 monomers in the precondensation supersaturated region for the first time. The gasdynamic measurements show that disturbances of the flow caused by SF6 condensation are very weak. The spectroscopic results confirmed the belief, based on the classical kinetics of nucleation, that monomers are not appreciably depleted prior to the onset of condensation by homogeneous nucleation.
Empirical triatomic potential energy surfaces defined over orthogonal bond order coordinates
Authors: Agmon N. , Levine R. D.
Published: February 2003

A new simple but realistic empirical representation of collinear triatomic potential energy surfaces is proposed and examined. The approach is based on the introduction of a novel system of orthogonal coordinates defined in terms of the bond orders of the two bonds. The potential surface is represented as an interpolation between the two asymptotic diatomic potentials along these coordinates. In addition to the two diatomic potentials the potential energy surface contains two empirical constants: A coordinate scale factor (which enters into the definition of the bond order via the Pauling relation) and an energy scale factor (which is roughly constant for a series of surfaces with a common central atom).
Calculation of equilibrium geometries and ionization energies of sodium clusters up to Na8
Authors: Flad Jürgen , Stoll Hermann , Preuss Heinzwerner
Published: February 2003

Using a combination of two methods, a pseudopotential scheme and a density-functional approximation for the correlation energy, equilibrium geometries of clusters Nan up to n=8 have been studied. The structures of small sodium clusters do not show any resemblance to the metallic lattice. Experimental ionization energies can be reproduced with a maximum error of 3%; the alternancy in their dependence on n is described appropriately and is explained by means of structures composed of Na2 units.
Series representation of the equation of state for hard particle fluids
Authors: Barboy Boris , Gelbart William M.
Published: February 2003

In most perturbation theory or mean field studies of the structural and thermodynamic properties of fluids, the reference system is taken to be a fluid of hard core particles which reflect the correct shape and size of the constituent molecules. As an alternative to the several existing scaled particle and integral equation approaches, we propose in this paper a simple series representation of the thermodynamic functions: Our basic expansion parameter is y=?/(1−?), where ? is the dimensionless packing factor (i.e. hard core volume times the number density). The coefficient of yn is obtained directly from the first n virial coefficients, with generalization to multicomponent systems being straightforward. Truncated forms of this series representation of the pressure are evaluated for hard spheres, dumbells, spherocylinders, and hard sphere mixtures (including nonadditivity). These results are compared with available Monte Carlo and molecular dynamics calculations, Percus–Yevick theory, Padé approximants, various versions of scaled-particle theory and truncated forms of the virial series. It is concluded that a three-term truncation of the y expansion provides a relatively simple and accurate description of the thermodynamics of hard-core reference systems.
Kinematics of He+–CO2 dissociative charge exchange
Authors: McMillian M. R. , Coplan M. A.
Published: February 2003

The masses, energies and angular distributions of the product ions resulting from the dissociative charge exchange collision between He+ and CO2 have been measured at incident He+ energies of 8.7, 13.8, 60, and 200 eV. At the lower He+ energies the angular distributions are characteristic of complex formation with a significant amount of momentum transfer from He+ to the products. At the higher He+ energies there is little momentum transfer and the energy of the initially formed state of CO2+ is close to the recombination energy of He+. This state has been tentatively identified as the previously unobserved 2Φu. From the energy and angular distributions, a model for the dissociation of this state is developed. The model is consistent with previous optical measurements on the system and agrees with branching ratios predicted by the quasiequilibrium theory of mass spectra.
Observation and atomization energies of the gaseous uranium carbides, UC, UC2, UC3, UC4, UC5, and UC6 by high temperature mass spectrometry
Authors: Gupta Satish K. , Gingerich Karl A.
Published: February 2003

Gaseous carbides of uranium, UC, UC2, UC3, UC4, UC5, and UC6 have been observed in a mass spectrometric investigation of the Knudsen cell effusate from a thorium–uranium–rhodium–graphite system at high temperatures. Partial pressures of the carbide molecules were measured as a function of temperature in the 2300–2700° K range. Second and third law methods were employed to determine enthalpy changes for the reactions of the type U(g)+nC(graphite) =UCn(g), n=1 to 6, and for additional reactions with graphite involving two gaseous carbide species and various homogeneous gas phase reactions. The experimental enthalpies when combined with thermodynamic data taken from literature yielded the following atomization energies, ΔH°at,298, and standard heats of formation, ΔH°f,298:
Raman spectra of solid CH4 under pressure. I. Phase transition between phases II and III
Authors: Fabre D. , Thiéry M. M. , Vu H. , Kobashi K.
Published: February 2003

Raman spectra of the intramolecular vibrations in solid CH4 have been observed at 4.2 K under pressure. The observed ν1−ν3 vibration–rotation band structures in phase II agree well with previous experimental and theoretical data. The ν4 band was detected in both phases II and III. A remarkable change of each spectrum was found at 0.4 kbar, indicating the II–III phase transition. The rotational states of methanes in phase III have been deduced from the observed data. It is concluded that in phase III there are at least three kinds of sites with different site symmetries.
The MTXαR method. III. A study of the ground states of H2, LiH, and NaH
Authors: McAdon Mark H. , Konowalow Daniel D.
Published: February 2003

Potential energy curves for H2, LiH, and NaH are calculated with both the standard MSXα method and the newly developed MTXαR method. Many calculated results for LiH and NaH depend sensitively on the ratio of the muffin-tin atomic sphere radii. This parameter is optimized variationally, and the optimized potential curves for both LiH and NaH are in qualitative and semiquantitative agreement with experiment. Previously, the MTXαR method was found to be superior to the standard MSXα method in the calculation of potential curves for Li2, Na2, and NaLi. For H2, LiH, and NaH, the two methods yield potential curves of similar quality. The muffin-tin potential curves for LiH are comparable to one obtained from an overlapping spheres calculation but are superior to those obtained in LCAO–SCF–Xα calculations which avoid the muffin-tin approximation.
Vibronic dephasing of anharmonic molecules. I. Theory and its application to the separability of intra- and intermolecular processes
Authors: Diestler D. J. , Zewail A. H.
Published: February 2003

A quantum-mechanical theory of vibronic dephasing of impurity molecules in condensed media is presented. An expression for the dephasing time is derived that exhibits explicit dependences upon microscopic properties of both impurity and medium. The expression, which contains the temperature dependence and the cross section for dephasing, is used to predict qualitatively and semiquantitatively some features of vibronic spectral lines in solids. Of particular interest are systems in which the influence of intramolecular properties (of the impurity) upon dephasing is separable from that of intermolecular properties (of the medium). The distinction between vibrational and purely electronic effects on vibronic dephasing is also emphasized.
Vibronic dephasing of anharmonic molecules. II. Impurity molecules isolated in low-temperature matrices
Authors: Diestler D. J. , Zewail A. H.
Published: February 2003

The quantum-mechanical theory of vibronic dephasing presented in the first paper of this series is applied to the case of a diatomic impurity dissolved in a solid rare-gas host. An explicit expression for the pure dephasing rate T2′−1 is derived in terms of microscopic properties of the impurity and host, and the effects of variations in the parameters characterizing these properties are investigated. The expression for T2′−1 is applied specifically to the system Cl2/Ar in order to relate the results to those of previous classical-trajectory calculations and of experimental measurements. The significance of anharmonicity in the intramolecular potential curve (of the impurity) is demonstrated.
The ?2Πu→?2Πg emission and the photoelectron spectrum of dicyanodiacetylene radical cation
Authors: Kloster-Jensen E. , Maier J. P. , Marthaler O. , Mohraz M.
Published: February 2003

Gas phase emission spectrum of dicyanodiacetylene radical cation has been detected. The band system which lies in the 630–770 nm wavelength region was separated from the overlapping emission bands of the cyanogen radical by recording time resolved spectra. The He(Iα) photoelectron spectrum of dicyanodiacetylene is also presented and the inferred ionization energies, vibrational frequencies and assignments are given. From the latter data the emission spectrum is assigned as the ?2Πu→?2Πg electronic transition of dicyanodiacetylene radical cation. The frequencies of the four Σ+g vibrational fundamentals for the cationic ground state have been deduced from the emission spectrum.
The effect of an oxidant on the optical properties of particles produced in an inert gas containing Na vapor
Authors: Hecht J. , West W. P. , Broida H. P.
Published: February 2003

The effects of an oxidant on the optical properties of particles formed in an inert gas containing Na vapor are reported. The oxidant used was NO. Light scattering measurements are reported. (AIP).
Simple expressions for polarized Raman scattering by a uniaxial assembly of partially oriented symmetrical molecules
Authors: Joná? Ivan , Michl Josef
Published: February 2003

The expression describing the symmetric molecular scattering tensor is discussed in terms of orientation of the molecule. (AIP)
Convergence characteristics of Monte Carlo–Metropolis computer simulations on liquid water
Authors: Mezei Mihaly , Swaminathan S. , Beveridge David L.
Published: February 2003

Very long (∼5000 K) Monte Carlo computer simulations are reported for liquid water described in terms of the analytical potential functions of Matsuoka, Clementi, and Yoshimine and Rahman and Stillinger’s empirical ST2 potential. The convergence characteristics of both realizations are fully developed in terms of internal energy, heat capacity molecular distribution functions, and structural indices. A hierarchy in the calculated properties emerges with respect to the degree of computational effort required to obtain reproducible results. Mean energy and radial distribution functions are the most accessible quantities. Fluctuation properties such as heat capacity require roughly twice as many configurations to stabilize as simple orientational averaged quantities. The structural changes over the equilibrated segments of the realization were examined in terms of quasicomponent distribution functions and found to be small in chemical terms.
The application of Hartree–Fock and configuration interaction calculations to transition metal nitrosyls
Authors: Fenske Richard F. , Jensen J. Robert
Published: February 2003

This is a study of the Hartree–Fock single configuration wave function as a reasonable description of the electronic structure of molecules or molecular fragments containing transition metals. For the linear species MnNO and CoNO, intentional reduction of the electronic symmetry from C∞v to C2v resulted in an asymmetric charge distribution which was physically meaningless but lower in calculated total energy than the proper Hartree–Fock result. Configuration interaction was required to restore the cylindrically symmetric charge density. An analysis of the origin of this observed saddle point in the Hartree–Fock energy surface suggests such situations will tend to arise when the lower virtual orbitals are in energetic proximity to the higher occupied orbitals and when the virtual orbital or orbitals delocalize the electron density within the species. The implications of the effects observed in the triatomic species are examined with respect to the known molecules Mn(CO)4(NO) and Co(CO)3(NO). In these latter systems, the orbital degeneracies are removed by the geometries of the species and not by a calculational artifact. The effects on the Hartree–Fock results, however, are quite analogous as clearly evidenced by the configuration interaction results.
Semiclassical Gaussian basis set method for molecular vibrational wave functions
Authors: Davis Michael J. , Heller Eric J.
Published: February 2003

We present theory and numerical results for a new method for obtaining eigenfunctions and eigenvalues of molecular vibrational wave functions. The method combines aspects of the semiclassical nature of vibrational motion and variational, ab initio techniques. Localized complex Gaussian wave functions, whose parameters are chosen according to classical phase space criteria are employed in standard numerical basis set diagonalization routines. The Gaussians are extremely convenient as regards construction of Hamiltonian matrix elements, computation of derived properties such as Franck–Condon factors, and interpretation of results in terms of classical motion. The basis set is not tied to any zeroth order Hamiltonian and is readily adaptable to arbitrary smooth potentials of any dimension.
On some approximations in applications of Xα theory
Authors: Dunlap B. I. , Connolly J. W. D. , Sabin J. R.
Published: February 2003

An approximate Xα functional is proposed from which the charge density fitting equations follow variationally. LCAO Xα calculations on atomic nickel and diatomic hydrogen show the method independent of the fitting (auxiliary) bases to within 0.02 eV. Variational properties associated with both orbital and auxiliary basis set incompleteness are used to approach within 0.2 eV the Xα total energy limit for the nitrogen molecule.
Theory of electrical conduction in organic molecular crystals. II. Characteristic effects of electric field and defect scattering on temperature-independent mobilities
Authors: Sumi H.
Published: February 2003

The model proposed by the present author [J. Chem. Phys. 70, 3775 (1979)] explained successfully the almost-temperature-independent electron mobilities in the c′ direction of anthracene and napthalene in the high temperature region. The present work extends the model to reproduce observed varieties in the temperature dependence of the c′ mobilities below ambout 130 °K under experimentally realistic conditions of high electric fields and nonvanishing defect scattering of electrons. It is ascribed to defect-scattering effects that the observed temperature dependence of the electron mobility along the (a,b) plane of anthracene becomes gradual towards low temperatures. It is predicted that for kBT ≲h/ω2 (the energy of rotational vibations which induce an additional transfer integral of electron in the c′ direction) the drift velocity in the c′ direction should become saturated with electric field, F, when the consequent energy difference, eFa, between neighboring (a,b) planes exceeds h/ω2. It is also predicted that for kBT≲h/ω1 (the Debye cut-off energy of acoustic lattice vibrations which scatter electrons within a single (a,b) plane) the c′ drift velocity should show marked deviation from a simple linear relationship with the electric field even when eFa is smaller than h/ω2 and here it should be sensitive to defect scattering as by impurities. Numerical calculation was performed for h/ω1?50 cm−1 and h/ω2?125 cm−1, where eFa=h/ω2 occurs at a field strength about 2×105 V/cm.
Crossed beam studies of the dynamics of electronic energy transfer: Quenching of Na(3p 2P3/2) atoms by N2, O2, CO, and NO molecules
Authors: Silver Joel A. , Blais Normand C. , Kwei George H.
Published: February 2003

The dynamics of quenching of electronically excited Na atoms by the diatomic molecules N2, O2, CO, and NO has been studied using crossed molecular beam techniques. Distributions in both laboratory scattering angle and recoil velocity provide information on the quenching process and the disposal of energy. For N2, O2, and CO, we observe a direct process which occurs in less than one rotational period of the collision system; all the molecules are scattered forward, along their original direction. For N2 and CO, an average of 30%–40% of the available energy is transferred to internal energy of the diatomic molecules. A comparison with data from infrared absorption studies of the Na*–CO system shows that very little of this internal energy is taken up by rotational modes of the CO molecule. For O2, a greater fraction of the electronic energy is partitioned into internal modes but the details of this partitioning are further complicated by the accessibility of the low lying O2 excited electronic states. For NO, quenching proceeds via the formation of a collision complex with a mean lifetime of one rotational period (≳0.5 ps). The angular distribution shows that rotational excitation is again low and therefore, in this case, the recoil velocity distributions can be transformed directly into vibrational state distributions. Of the 2.24 eV of available energy, 30%–40% is partitioned into translation and 60–70% is partitioned into vibration, while less than 5% is partitioned into rotation. Thus, as for the Na*–CO system, the detailed energy partitioning in the Na*–NO system is completely determined. These results are compared with results from other experiments and with predictions from several available theoretical models.
Least squares calculations of electro-optical parameters from infrared intensities of CH4 and C2H6 and their deuterated derivatives
Authors: Gussoni Mariangela , Abbate Sergio , Zerbi Giuseppe
Published: February 2003

In the frame of electro-optical theory of valence, bond parameters called ’’electro-optical parameters’’ are shown to be able to represent the electrical properties of the same groups of atoms in a different, though chemically similar, environment. In this paper, a set of electro-optical parameters derived from least squares fittings of the infrared intensities of methane, ethane, and deuteroderivatives is shown satisfactorily to account simultaneously for all the experimental intensity data available for these molecules.
Moment theory of electron drift and diffusion in neutral gases in an electrostatic field
Authors: Lin S. L. , Robson R. E. , Mason E. A.
Published: February 2003

We develop a rigorous kinetic theory of electron (or light ion) transport processes in neutral gases in an electrostatic field. The theory does not depend upon the usual approximation of the electron velocity distribution by the first two terms of an expansion in spherical harmonics. We use a modification of the moment method developed by Viehland and Mason for ion transport. There are no restrictions on the field strength or cross sections, and both elastic and inelastic collisions are considered. The smallness of the ion–molecule mass ratio simplifies the evaluation of the matrix elements and allows calculations to be carried to very high orders of approximation if necessary. Three examples (in all of which the differential cross section is assumed isotropic) are treated in numerical detail to establish the main features of the present theory: electrons in a rigid-sphere gas, in argon, and in methane. When only elastic collisions occur, the two-term approximation is adequate, even if the cross section has a deep Ramsauer minimum. However, inelastic collisions spoil the accuracy of the two-term approximation but tend to improve the convergence of the moment calculations. This is illustrated by the example of electrons in methane, which shows negative differential conductivity. The moment method is also used to derive simple approximate formulas for estimating the effect of inelastic collisions and the accuracy and range of validity of the two-term approximation. It is shown, for example, that a modified generalized Einstein relation for the longitudinal diffusion coefficient gives reasonable results even for electrons in methane.
Photoelectron spectroscopy of ethylene, isobutylene, trimethylethylene, and tetramethylethylene at variable angle
Authors: Mintz Donald M. , Kuppermann Aron
Published: February 2003

Using a HeI line 58.4 nm source lamp, photoelectron angular distributions were measured for the series of four olefins: ethylene, isobutylene (2-methyl-propene), trimethylethylene (2-methyl-2-butene), and tetramethylethylene (2,3-dimethyl-2-butene). From these, the asymmetry parameter β as a function of photoelectron energy was obtained for each of these molecules. The following important effects in the behavior of β are observed: (a) In the π orbital ionization regions of the spectrum of each molecule, β increases with increasing electron energy across the vibrational envelopes. (b) With increasing methyl substitution (and at a fixed photoelectron enegy) β for this band decreases. (c) In the region of the spectra of each of the methyl-substituted ethylenes involving several 2pσ bands, this energy dependence of β behaves as if they constituted a single band, in spite of the widely differing orbital symmetries. (d) Over most of the 2pσ region of each molecule, β decreases with increasing photoelectron energy, except for the high ionization potential end of this region, where β increases instead. We attribute effects (b) and (d) to σ–π orbital mixing.
Quantum chemical calculations of bonded and nonbonded interactions in saturated hydrocarbons, amines, alcohols, and ethers
Authors: Chung-Phillips Alice , Kao James W. H.
Published: February 2003

The energy-partitioning technique developed by us recently has been applied to the binding energies calculated from a modified CNDO method for selected compounds in the saturated hydrocarbon, amine, alcohol, and ether series. The binding energy is partitioned into energy terms that are representative of atom–pair interactions. Calculated results are analyzed in light of existing theoretical and empirical evidence and useful trends are deduced based on the relative magnitudes of these terms. Specifically, the values for the bonded interactions compare exceedingly well with the empirical bond energy values; but those for the nonbonded interactions are drastically reduced owing to the approximation of zero differential overlap empolyed in the CNDO model. Nonetheless, the knowledge of the attractive and repulsive nature revealed by these MO calculations for the various nonbonded interactions is important to the understanding of conformational properties of molecules.
Optically detected magnetic resonance of the μ-oxo bridged porphyrin dimer
Authors: Leenstra Willem R. , Gouterman Martin , Kwiram Alvin L.
Published: February 2003

The scandium μ-oxo dimer of octaethyl porphyrin has been investigated by optically detected magnetic resonance. By comparing the zero-field splitting parameters of the monomer and the dimer using the strong coupling exciton model it is concluded that the two porphyrin rings are essentially coplanar in the dimer. (AIP)
Absorption lines in the ArF and KrF laser spectra
Authors: Shimauchi Midori , Miura Tokiwa , Karasawa Shiro
Published: February 2003

The ten-meter vacuum spectrograph was employed to study absorption spectra of ArF and KrF excimer laser. A diffuse-looking line was formed near 1931 Å for Ar F and a rotational analysis was performed. (AIP)
Rydberg states of H3
Authors: Martin Richard L.
Published: February 2003

Herzberg6 has recently observed very highly excited states of H3 in the emission from a hollow cathode discharge through H2. The emission bands were assigned to Rydberg transitions. Calculations are reported to interpret these transitions. (AIP)
Correlation function modeling via the third-order memory function: Application to ethane
Authors: Wilde Richard E.
Published: February 2003

In order to understand the vibrational correlation function of the ν3 band of liquid C2H6, it has been necessary to include third-order memory functions in the modeling procedure. The analysis indicates that the vibrational dephasing process may be a complex one involving both the spinning motion around the threefold symmetry axis and the tumbling motion of the axis itself. Information about the molecular dynamics in the temperature range 93–168 K has been obtained out to 7 ps.
Potential energy curves for NO+
Authors: Albritton D. L. , Schmeltekopf A. L. , Zare R. N.
Published: February 2003

In recent years, high-resolution photoelectron spectroscopy and ab initio calculations have considerably revised and enlarged the understanding of the electronic structure of the NO+ ion. Based on these results, new potential energy curves for the electronic states of NO+ below 24 eV have been constructed and are given here. RKR potentials were calculated from new molecular constants for the eight experimentally well-defined states. For the other states, the main features of the potentials were estimated from theoretical calculations and from isoelectronic analogies. A brief review is given of the information on which each potential energy curve is based.
On the product ratios of the reactions of Ar+ with NO and NO+ (a3Σ+) with Ar at 300 K
Authors: Dotan I. , Fehsenfeld F. C. , Albritton D. L.
Published: February 2003

The reaction of Ar+ with NO to yield the possible ground-state NO+(X1Σ+) and metastable-state NO+(a 3Σ+) product ions has been studied near 300 K in a flow-drift tube using helium and argon buffer gases separately. The ’’apparent’’ rate constant obtained in the argon buffer is an order of magnitude smaller than the rate constant obtained in the helium buffer. The difference in the rate constants is due to the ’’back’’ reaction of the NO+(a 3Σ+) product ions with the argon buffer-gas atoms to yield the possible product ions NO+(X 1Σ+) and Ar+. The ratio of the rate constants places precise limits on the product ratios in both cases; namely, the Ar+ + NO reaction yields greater than 85% NO+(a 3Σ+) ions and the ratios NO+(a 3Σ+) + Ar reaction yields greater than 85% Ar+ ions. The data also show that the rate constant for the quenching of NO+(a 3Σ+) ions by NO is (8.0±4.0) ×10−10 cm3/s at 300 K.
Investigation of the dielectric constants and the diamagnetic anisotropies of cyanobiphenyls (CB), cyanophenylcyclohexanes (PCH), and cyanocyclohexylcyclohexanes (CCH) in the nematic phase
Authors: Schad Hp. , Baur G. , Meier G.
Published: February 2003

We have measured the dielectric constants of cyanobiphenyls (CB), cyanophenylcyclohexanes (PCH), and cyanocyclohexylcyclohexanes (CCH) using a capacitance bridge with a dielectric cell having a guard ring. The ratio Δχ/Δε was also measured for these materials using a new technique which is applicable when Δχ is small or even negative. As a surprising result we have found that this ratio is almost independent of temperature. The temperature dependence of the diamagnetic anisotropies and the dielectric constants of these compounds are given and the results are discussed in terms of the molecular properties.
Sub-nanosecond time resolved transient photoconductivity in anthracene
Authors: Schein L. B. , Anderson R. W. , Enck R. C. , McGhie A. R.
Published: February 2003

Observations are reported of current, time resolved to 800 psec, in a measurement of transient photoconductivity in anthracene. These measurements indicate that the transient currents associated with photogeneration, polaron formation, and hot carrier motion are over within 800 psec of the light illumination. These measurements also have revealed a new transient photoconductivity effect, a current peak whose duration equals the duration of the laser illumination. Possible origins of the current peak are considered.
Alkali halide photofragment spectra. I. Alkali iodide bond energies and excited state symmetries at 266 nm
Authors: Su Tzu-Min R. , Riley Stephen J.
Published: February 2003

Photofragment spectroscopy studies of the five alkali iodides have been done at 266 nm. For the four heavier iodides exclusive production of excited I2P1/2 atoms is found; for LiI, both ground and excited I atoms are produced, with ground state atoms favored two to one. From the measured alkali atom recoil time-of-flight distributions the following bond dissociation energies (D00) in kcal mol−1 are determined: LiI 81.6±1.0; NaI 71.8±0.5; KI 76.2±0.5; RbI 75.3±0.5; CsI 79.7±0.5. No evidence of dimer photodissociation is found. From recoil angular distributions the percentage parallel character of the transitions at 266 nm is found to be 60±4, 54±2, 51±2, and 51±2 for NaI,, KI, RbI, and CsI, respectively. The transition in LiI leading to ground state I atoms is 29%±3% parallel. Implications of these and other results for the nature of the first and second excited states of the alkali iodides are discussed.
Production of NO+(a 3Σ+) ions in the reaction of N+ ions with O2 at 300 K
Authors: Albritton D. L. , Viggiano A. A. , Dotan I. , Fehsenfeld F. C.
Published: February 2003

It is well known that NO+ ions constitute about 45% of the product ions of the ionospherically important reaction of N+ ions with O2. However, NO+ ions in both the ground X 1Σ+ state and the metastable a 3Σ+ state are energetically possible. The present study uses a selected-ion flow tube to determine the percentage of NO+(a 3Σ+) product ions. It is known from separate studies that NO+(a 3Σ+) ions react rapidly with N2 to form N2+; therefore, the addition of N2 and the occurrence of N2+ ions provide a monitor for the possible NO+(a 3Σ+) product ions from the N+ reaction with O2. It is found that NO+(a 3Σ+) production accounts for less than 2% of the total rate constant at 300 K.
Molecular dynamics study of a polymer chain in solution
Authors: Rapaport D. C.
Published: February 2003

The behavior of a polymer chain in solution has been simulated by means of molecular dynamics techniques. The polymer itself is represented by a freely linked chain of hard spheres and the solvent by a hard sphere fluid. Both the equilibrium configurational properties of chains of various lengths and their time-dependent autocorrelation functions are studied. The equilibrium behavior agrees with the results of previous work. The decay of the autocorrelation functions is exponential over a considerable range and the relaxation times, thus defined, appear to scale with the chain length. The effect of the solvent density is also examined.
On the quantum mechanical implications of classical ergodicity
Authors: Stratt Richard M. , Handy Nicholas C. , Miller William H.
Published: February 2003

The concept of quantum ergodicity is examined empirically through a quantitative study of the eigenfunctions of both the Barbanis and Casati–Ford potentials. By considering the behavior of the nodes of the wave functions and by employing a combination of natural orbital and spectral analyses, many of the higher eigenstates can apparently be classified as ergodic. These particular states seem to correspond quite well not only to classical ergodic trajectories but also to (currently) unquantizable semiclassical wave functions. The relationship between this quantal ergodicity and its classical equivalent is discussed.
Vibrational analysis of (SCN)2 and the transient (SCN)2−
Authors: Jensen N. H. , Wilbrandt R. , Pagsberg P. , Hester R. E. , Ernstbrunner E.
Published: February 2003

The vibrational spectra of thiocyanogen and the transient radical anion (SCN)2− are interpreted in detail through molecular orbital and normal coordinate calculations. The results support the assignment of (SCN)2− to the anion of thiocyanogen and indicate a substantial weakening of the S–S and C≡N bonds in going from the parent molecule to its radical anion.
Relaxation of HCN (001) using stimulated electronic Raman scattering in Cs vapor as a tunable infrared source
Authors: Petersen Alan B. , Smith lan W. M.
Published: February 2003

Rate constants are reported for the relaxation of HCN (001) by HCN, Ar, N2, CO, and CO2. The possible mechanisms of energy transfer are discussed, especially the extent to which HCN behaves like a hydrogen halide. These are the first measurements in which HCN(001) has been prepared by direct optical pumping. A novel source of coherent, tunable, infrared radiation was employed. The output of a pulsed dye laser was downshifted using stimulated electronic Raman scattering in cesium vapor. The potential of this method for other experiments requiring selective vibrational excitation is considered briefly.
Luminescence of barium molybdate (BaMoO4)
Authors: ’t Lam R. U. E. , Blasse G.
Published: February 2003

Luminescence spectra are reported for BaMoO4. Thee emission occurs only for samples precipitated from acid solutions. Samples show at liquid helium temperature a green emission. (AIP)
Oxygen chemisorption on a stepped Ru (∼001) crystal
Authors: Parrott S. L. , Praline G. , Koel B. E. , White J. M. , Taylor T. N.
Published: February 2003

The 300 K chemisorption of oxygen on a stepped Ru (∼001) single crystal [17(001) × (100)] gives 2×2 complete half-order, 1×2 complete or incomplete half-order, and 1×1 integral-order LEED patterns depending on oxygen exposure and sample annealing. The corresponding relative AES oxygen signals are 0.25, 0.50, and 1.0, respectively. Under certain surface conditions, the presence of the steps is seen to break the threefold degenerate growth habit of 1×2 domains. The 1×1 pattern forms at 300 K only after lengthy exposure at relatively high pressure (10−5 Torr) and is stable to 600 K. Prolonged exposure gives rise to relative AES oxygen signals greater than unity suggesting penetration into the subsurface region.
Sulfur oxide: Low-lying bound molecular electronic states of SO
Authors: Swope William C. , Lee Yuan-Pern , Schaefer Henry F.
Published: February 2003

We present here the results of self-consistent field (SCF) and configuration interaction (CI) theoretical studies of seven low-lying electronic states of sulfur oxide. The basis set was of double zeta quality augmented with polarization functions. The CI space for each electronic state consisted of all configurations constructed from single and double substitutions of electrons from the valence orbitals of the Hartree–Fock reference occupation. Spectroscopic constants as well as dipole moments for each electronic state were predicted both at the SCF and CI levels of theory. Of particular significance is the prediction of excitation energies and properties for three low-lying states for which experimental information is either unavailable or only very recently available. These states are the c1Σ− state (Te=28 100±300 cm−1), the A′ 3Δ state (29 200±300 cm−1) and the A″ 3Σ+ state (30 200±300 cm−1).
Time-of-flight measurements of the rotational excitation of para- and ortho-H2 by collisions with Li+ ions. I. Angular dependent transition probabilities for the j=0→2, 2→0, 2→4, and 1→3 transitions at Ec.m.=0.6 eV
Authors: Faubel Manfred , Toennies J. Peter
Published: February 2003

Time-of-flight spectra for the scattering of Li+ ions from para-H2 in the j=0 and j=2 rotational states and from ortho-H2 in the j=1 state have been measured for center of mass scattering angles between 17° and 50° at a center of mass energy of 0.6 eV. In all the spectra two or more maxima were observed. For 100 °K para-H2 they could be attributed to the j=0→0 and j=0→2 transitions. For para-H2 at 300 °K additional maxima due to the j=2→0 and the j=2→4 transitions were observed. By using normal H2 and subtracting off the known contributions from para-H2 the transition probability for the j=1→3 transition was also obtained. The angular resolution was sufficient to resolve the ’’fast’’ undulations as well as the rainbow maximum for both the j=0→0 and j=0→2 transitions. The results are compared directly with differential cross sections calculated by Schaefer and Lester [J. Chem. Phys. 62, 1913 (1975)] using a close coupling procedure with the inclusion of a number of closed channels for the best ab initio CI-type potential hypersurface. The comparison reveals a noticeable discrepancy in the angular location of the rainbow. This difference suggests that the ab initio potential well is too shallow in the experimentally probed region by about 17% to 35%.
The noncrossing rule and degeneracy in Hubbard models: Cyclobutadiene and benzene
Authors: Bondeson S. R. , Soos Z. G.
Published: February 2003

The unexpected crossing of energy levels with the same symmetry, as a function of an on-site correlation U, in the Hubbard model for benzene is shown to reflect the degeneracies of the site Hamiltonian, ◊0. Diagrammatic valence bond (VB) methods greatly simplify the symmetry analysis and allow a complete treatment of configuration interactions (CI) for both on-site and extended interactions. The single forbidden crossing in the 1A subspace of cyclobutadiene disappears for arbitrarily small nearest-neighbor interactions. The many forbidden crossings in the 1A subspace of benzene also become avoided crossings when nearest-neighbor intersite interactions are included. Since the degeneracies of ◊0 in benzene are only partially lifted, CI among nondegenerate states is a sufficient, but not a necessary condition for the noncrossing rule.
Does diamond melt?
Authors: Grover R.
Published: February 2003

Recent dynamic compression data for carbon are discussed which suggests that metallic carbon has unexpected thermodynamics properties, both in having a lower density and energy of formation and a higher melting temperature. On the basis of these properties the diamond phase of carbon is now predicted to transform to a solid phase of metallic carbon at all pressures above the graphite triple point.
Critical exponent of the heat of transport of water– isobutyric acid mixtures
Authors: Rowley Richard L. , Horne Frederick H.
Published: February 2003

The Dufour effect (diffusion thermoeffect) has been used to obtain the critical exponent of the heat of transport in binary mixtures of isobutyric acid and water near the liquid–liquid critical point. As the critical temperature is approached experimentally, the heat of transport vanishes with a critical exponent of about 2/3. The i critical exponent for the Onsager coefficient Ω01 is zero. This value is consistent with the semidefinite positive entropy production r requirement and is identical to the best literature value for the critical exponent of the reciprocal Onsager coefficient Ω10. In order to explain the observed critical behavior of the heat of transport a new molecular interpretation based on the random thermal motion model for diffusion is suggested.
Statistical behavior and the detailed dynamics of collinear F+H2 trajectories
Authors: Duff James W. , Brumer Paul
Published: February 2003

Collinear F+H2 collisions are discussed. The time evolution of the quantity D(t), the distance between pairs of initially adjacent trajectories, seems to identiy the statistical component in collinear F+H2 ccollisions.
Effective collision cross sections governing the effect of an external magnetic field on the thermal conductivity of a pure gas of linear molecules
Authors: Liu Wing-Ki , McCourt F. R.
Published: February 2003

Using Liouville-space algebra, general expressions are derived for the independent effective collision cross sections governing the thermal conductivity Senftleben–Beenakker effect in a pure gas of linear molecules. These cross sections are presented in terms of S-matrix elements in the total-J representation since it is particularly useful for the dynamical approximation procedures in current use.
Bulk viscosity of dense simple fluid mixtures
Authors: Karkheck John , Stell G.
Published: February 2003

The contribution to the bulk viscosity of fluid mixtures arising from the deviation of the two-particle distribution function from its local-equilibrium form can be significant in comparison with that arising from the local-equilibrium part of this function when analyzed in the framework of Enskog theory. Quantitative results are given for hard spheres.
ENDOR of F+ centers in SrO
Authors: Abraham M. M. , Chen Y. , Peters W. C. , Rubio O. J. , Unruh W. P.
Published: February 2003

Electron–nuclear double resonance (ENDOR) measurements are reported for the F+ center in neutron-irradiated SrO single crystals.
The supersonic expansion of pure copper vapor
Authors: Preuss D. R. , Pace S. A. , Gole J. L.
Published: February 2003

A high temperature source (2800 K) has been developed to produce a free jet expansion of copper vapor (Tsource=2500 K, Psource∼100 Torr). The supersonic beam formed under these conditions was characterized through analysis of the laser induced excitation spectrum for the B 1Σu+←X 1Σg+ band system of Cu2. Excitation was accomplished with an argon ion pumped stilbene cw dye laser. The Cu2 formed in the expansion was characterized by a vibrational temperature, Tvib=950±100 K and a rotational temperature, Trot=800±50 K. A new band system attributed to diatomic or triatomic copper has been observed.
Matrix studies of the ? 2A’2→? 2E transition of the s
y
m-C6Cl3F3+ radical cation
Authors: Bondybey V. E.
Published: February 2003

The laser induced fluorescence spectra of the sym-C6Cl3F3+ radical cation were observed and analyzed. The cations are generated in rare gas matrices by VUV photoionization of the parent halobenzene. They fluoresce in solid Ne and Ar with near unity quantum efficiency and exhibit a well resolved vibronic structure. A radiative lifetime of 29±5 nsec is found for the emission in Ne host. Both the totally symmetric a1 and degenerate e′ modes are active in the spectrum. Analysis of the ground state vibrational structure yields the Jahn–Teller distortion parameters for several e′ vibration modes.
Time resolved measurement of CF2 formation in the infrared multiphoton dissociation of CF2HCl
Authors: Duperrex R. , van den Bergh H.
Published: February 2003

The infrared multiphoton dissociation of CF2HCl is investigated by monitoring the appearance of the CF2 product in real time during and after the laser pulse. The difluorocarbene radicals are detected quantitatively by their optical absorption in the ? 1B1←? 1A1 band system around 250 nm. The fragmentation is studied under collision-free conditions as well as in the presence of up to 500 Torr Ar.
The electronic structure of the formyl radical HCO
Authors: Adams George F. , Bent Gary D. , Purvis George D. , Bartlett Rodney J.
Published: February 2003

Many-body perturbation theory (MBPT) and couple-cluster doubles (CCD) calculation are reported for the formyl radical and for points on the potential energy surface corresponding to decomposition of the radical to hydrogen plus carbon monoxide. The predicted equilibrium structure (rCH=2.1 b, rco=2.245 b, and ϑ=124°) and dissociation energy (De=16.6 kcal mol−1) are in excellent agreement with experimental data. An analysis of the saddle-point region of the hypersurface provides a structure for the activated complex (rCH=3.35 b, rco=2.15 b, ϑ=115°) and predicts the critical energy (E0=18.5 kcal mol−1). Comparison of MBPT and CCD results for the dissociation energy and barrier height shows that equivalent results are obtained. A RRKM prediction of the decomposition rate coefficient for HCO→H+CO is also given.
Vibrational analysis by EPR spectroscopy: The ethyl radical
Authors: Griller D. , Marriott P. R. , Preston K. F.
Published: February 2003

The isotropic 13Cα and proton hyperfine interactions of the ethyl radical have been measured in the temperature range 112–313 K in the liquid solvents propane and 2,2,4-trimethylpentane. The data are entirely consistent with a value of 500 cm−1 for the out-of-plane bending frequency and with the assumption of a harmonic potential well for the vibration. Calculations show that the accuracy of vibrational analysis by EPR spectroscopy is limited by the quality of the experimental data rather than by the assumption of a harmonic potential function. The 13Cα hyperfine splittings for simple alkyls in their ’’planar’’ configuration are calculated to be 37.1 G [⋅CH(CH3)2]; 34.8 G(⋅CH2CH3); 33.1 G (⋅CD3) and 31.9 G (⋅CH3). These results show that hyperfine splittings which are uncorrected for vibrational contributions are not a reliable guide to spin densities.
Infrared multiphoton generation of radicals: A new technique for obtaining absolute rate constants. Application to reactions of CF3
Authors: Rossi Michel J. , Barker John R. , Golden David M.
Published: February 2003

The infrared multiphoton dissociation of CF3I is used as a convenient source of CF3 radicals under very low pressure conditions. Measurement of thermal absolute rate constants at 298 K for the reactions CF3+Br2→CF3Br+Br, CF3+ClNO→CF3Cl+NO, CF3+O3→CF3O+O2, and CF3+NO2→CF3O+NO yield (7.8±1.3) ×108, (3.5±0.5) ×108, (5.6±0.8) ×108, and (1.6±0.3) ×109 M−1 s−1, respectively. This new technique shows great promise for production of other free radicals of interest and measurement of thermal absolute rate constants over a wide temperature range.