bluuebunny/crossref_metadata_2025 huggingface dataset:
Complete basis set correlation energies. III. The total correlation energy of the neon atom
Authors: Petersson G. A. , Yee Arnold K. , Bennett Andrew
Published: July 2002

Within the framework of many-body perturbation theory, the total correlation energy can be partitioned into: intraorbital pair energies, eii; interorbital pair energies, αβeij and ααeij; double-excitation pair-coupling terms eij,kl(D); and higher-excitation pair-coupling terms, eij,kl(S,T,Q,...). The asymptotic convergence of pair natural orbital expansions for each of these terms is determined for the model problem of n infinitely separated helium-like ions with infinite nuclear charge. For example, the asymptotic form of the basis set truncation error in an αβ-interorbital pair energy is LimitNij→∞Δαβeij =αβfij (𝒥μ=1Nij Cμij)2 ((−225/ 4608)) (Nij+δij)−1 , where Nij is the number of pair natural orbitals and Cμij is the coefficient of pair natural orbital configuration  μij. Numerical studies of the neon atom verify that this model behavior applies to real many-electron systems. The pair-coupling terms beyond third-order contribute less than 1% of the total correlation energy in a variety of atoms and molecules and can therefore be neglected. As a practical test of the use of the asymptotic forms to extrapolate the remaining terms, a double zeta plus polarization set of pair natural orbitals was used. Extrapolation of each of the neon pair energies to the value for a complete basis set yields an independent electron pair approximation equal to −0.4233 hartree, which is 108.6% of the experimental correlation energy (−0.3896±0.001 hartree). Including the third-order MP-MBPT pair-coupling terms and extrapolating to a complete basis set gives a total correlation energy equal to −0.3904 hartree, which is 100.2±0.2% of the experimental value. A similar

calculation on H2O gave equally good results (calc. −0.3706; expt. −0.370±0.003 hartree) indicating that this DZ+P CBS method is applicable to polyatomic potential energy surfaces.
Lifetime-vibrational interference effects in the ultra-soft x-ray emission spectrum of CO
Authors: Flores-Riveros Antonio , Correia Nestor , Ågren Hans , Pettersson Lennart , Bäckström Mats , Nordgren Joseph
Published: July 2002

The effect of lifetime-vibrational interference in the carbon and oxygen ultra-soft x-ray emission (USX) spectra of CO is theoretically analyzed. Computed spectral functions are compared with high-resolution experimental recordings. A time dependent Franck–Condon formalism is employed in the theoretical treatment of the interference. The electronic states involved in the analysis are determined by means of large scale MCSCF and CI calculations and the nuclear wave functions are solved for numerically. Certain features of the spectra are found to be sensitively dependent on the interference and can hence be used as a monitor for the core state lifetime.
A grand canonical Monte Carlo simulation study of polyelectrolyte solutions
Authors: Vlachy V. , Haymet A. D. J.
Published: July 2002

We present the results of grand canonical Monte Carlo simulations on a model of ‘‘rod-like’’ polyelectrolyte solutions. The model is approximate, but well studied by a variety of techniques. The parameters in the model are chosen to mimic DNA and polystyrenesulfonate solutions. We find that the Poisson–Boltzmann equation retains its semiquantitative utility in this model if 1:1 electrolyte is present, and that integral equation theories are measurably even more accurate. By comparison with experimental results, we argue that there are limitations in the present model. Finally, a simulation of polyions surrounded by divalent mobile ions provides additional evidence for charge inversion under certain thermodynamic conditions. This feature is not predicted by the Poisson–Boltzmann equation.
The reaction of O(3P) with NO clusters
Authors: Nieman J. , Naaman R.
Published: July 2002

The reaction of O(3P) with NO clusters, NO/Ar, NO/He, and NO/CO2 have been studied in a crossed-beam experiment. Both angular distribution and velocity of the chemiluminescent NO2 product were measured. An isotropic distribution was found for (NO)2, while NOAr yielded forward peaking of the scattered products. These results were rationalized based on the lifetime of the collision complex. The upper limits for the NO–NO and the NO–Ar bonds were found to be 6.3±1 and 1.4±0.3 kJ/mol, respectively. The NO ⋅ CO2 complexes did not react to produce electronically excited NO2.
An approximate variational method for improved thermodynamics of molecular fluids
Authors: Shaw M. S. , Johnson J. D. , Ramshaw J. D.
Published: July 2002

For a certain class of thermodynamic perturbation theories, a generalization of the Gibbs–Bogoliubov inequality holds through second order of perturbation theory and for a subset of terms the inequality is true to infinite order. Using this approximate variational principle, a perturbation theory is chosen for which the Helmholtz free energy of the reference system is minimized under the constraint that the first order term is identically zero. We apply these ideas to the determination of effective spherical potentials that accurately reproduce the thermodynamics of nonspherical molecular potentials. For a diatomic-Lennard-Jones (DLJ) potential with l/σ=0.793, the resulting spherical reference potential is identical to the median average over angles for the repulsive part of the potential, but differs in the attractive well. The variational effective spherical potential leads to more accurate thermodynamics than the median, however, particularly in the triple point region.
Collision induced dissociation of diatomic molecules on surfaces: A charge transfer mechanism
Authors: Gadzuk J. W. , Holloway S.
Published: July 2002

A theory is presented which accounts for one of the possible mechanisms responsible for dissociative scattering of diatomic molecules from surfaces. If on the incident trajectory of the molecule, a surface-to-molecule electron transfer occurs and on the outgoing trajectory, the reverse, then the temporary negative molecular ion formed for the time duration between electron hops will displace in its intramolecular vibrational coordinate. The molecule will emerge as a vibrationally excited neutral, with some of the excited states lying within the dissociative continuum. A model is described for this process in which the center-of-mass translational motion is handled classically and the intramolecular motion via wave packet dynamics. The theory is energy and probability conserving and microscopically reversible. Dissociation probabilities calculated as a function of incident energy and system parameters are discussed in the light of experimental measurements for the system of I2 dissociatively scattered from MgO surfaces.
Analysis of the kinetic energy functional in density functional theory
Authors: Yang Weitao , Harriman John E.
Published: July 2002

The density matrix that leads to a minimum kinetic energy for a given density is considered as a convex superposition of pure states. It is shown that the conditions of stationarity of the kinetic energy and collapse to the given density require that each of the pure state wave functions involved be a single determinant in the same eigenspace of a particular, n-electron Hamiltonian and that all of the orbitals are eigenfunctions of the same effective one-electron Hamiltonian. The potential function arises originally as a Lagrange multiplier associated with the density constraint. In some cases it can (at least in principle) be determined. The role of electron–electron interactions and possible treatment of excited states are considered.
Hydrostatic pressure effects on TO–LO splitting and softening of infrared active phonons in α-quartz
Authors: Wong P. T. T. , Baudais F. L. , Moffatt D. J.
Published: July 2002

Infrared spectra of crystalline α-quartz have been measured as a function of pressure up to 50.3 kbar. Pressure induced large frequency shifts, increase in TO–LO splittings and ‘‘softening’’ in various phonon bands have been observed. The pressure dependences of the mode Grüneisen parameters for selected phonons have been calculated. The results are discussed in terms of the pressure induced changes in crystal structure and Szigeti effective charge.
ESR study of the formation of complex defects involving molecular oxygen by electron–hole reactions in x-irradiated KClO3 and KBrO3
Authors: Byberg J. R.
Published: July 2002

X irradiation of solid KXO3, X=Cl, Br, is shown to produce complex defects of composition [XO−, O2] and [XO, O2]. [XO−, O2], which is produced also by UV irradiation of KXO3, is formed during and after x irradiation in a reaction between electrons, self-trapped as XO2−3, and holes, self-trapped as XO3. [XO,O2] is shown to arise in the oxidation of [XO−,O2] by reaction with XO3. In KClO3, the complex [ClO2,O2] is formed also, possibly in a reaction between ClO3 and [ClO−2,O2] .
Total differential scattering cross sections for hydrogen scattered from nitrogen and hydrogen fluoride
Authors: Miller R. E. , Vohralik P. F. , Watts R. O.
Published: July 2002

In-plane total differential scattering cross sections for H2 scattered from N2 and HF are reported. The data are analyzed in terms of spherical and anisotropic potentials using single channel and IOS methods for calculating the differential cross sections. In the case of H2/HF the diffraction oscillations are highly damped at small angles, suggesting that the anisotropy is large.
Deuterium isotope effects in binary critical mixtures
Authors: Schön W. , Wiechers R. , Woermann D.
Published: July 2002

The shape of the liquid–liquid coexistence curve of five binary liquid mixtures containing methanol (methanol, methanol-d1, methanol-d4) and cyclohexane (cyclohexane, cyclohexane-d12) is studied using a differential refractometer. It is found that all systems have approximately the same critical composition (mole fraction) and approximately the same shape of the coexistence curve in the vicinity of the critical point. However, the critical temperature changes in a characteristic manner. Replacement of hydrogen atoms involved in hydrogen bonding by deuterium leads to an increase of the critical temperature of about 1.2 K for each replaced hydrogen atom. Replacement of hydrogen atoms not involved in hydrogen bonding leads to a decrease of the critical temperature of about 0.3 K for each replaced hydrogen atom. These are average values referring to the systems methanol/cyclohexane, phenol-d6/D2O, deutero-isobutyric acid (COOH)/H2O, and deutero-isobutyric acid (COOD)/D2O. Substitution of H2O by D2O in the four component system benzene, ethanol, water, ammonium sulfate of near tricritical composition increases a characteristic temperature by about 1 K.
Fluids in contact with a hard surface: Universality of the bridge functions for the density profile
Authors: Rosenfeld Y. , Blum L.
Published: July 2002

The modified hypernetted chain theory (MHNC) for density profiles of fluids in contact with a hard, smooth surface is found to possess a remarkably simple and interesting property: The accuracy of the MHNC and the universality of the bridge functions for the density profiles can be tested without resort to any detailed solution of the integral equations. It is given by the degree of universality of the bulk bridge parameter (e.g., η for the Percus–Yevick hard-sphere bridge function) when expressed in terms of the second partial derivative of the free energy with respect to the density F. The function η (F) as obtained from MHNC calculations for bulk simple fluids is found to be remarkably independent of the pair potential. As an example for the accuracy of the resulting method, the one-component plasma near a hard wall is discussed.
Properties of a generalized diffusion equation with a memory
Authors: Jäckle Josef , Frisch Harry L.
Published: July 2002

A third-order partial differential equation for the concentration or chemical potential of a penetrant diffusing in a relaxing medium with a memory described by a simple Maxwell–Debye relaxation function is shown to be equivalent to generalized diffusion equations with memory terms. We establish some properties of solutions of this equation for both suddenly or adiabatically prepared initial conditions. We extend our general treatment to account for osmotic pressure effects in mechanical equilibrium which can arise due to swelling of the solvent medium in which the transport occurs.
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o study on the c
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s energetics of polyacetylene
Authors: Teramae Hiroyuki
Published: July 2002

The cis–trans energetics of three polyacetylene isomers are studied by means of ab initio crystal orbital calculations. Systematic convergence data for optimized geometry, total energy, Koopman’s ionization potential, band gap, and population on hydrogen are given for both the minimal STO-3G and the split valence 4-31G basis sets with respect to the number of neighboring cells. Using a modified symmetric cutoff criterion, the relative energies of the all-trans, cis–transoid, and trans–cisoid isomers are calculated to be 0.0, 1.8, and 1.9 kcal/mol per C2H2 units at the STO-3G level, and 0.0, 2.1, and 3.6 kcal/mol per C2H2 units at the 4-31G level. The STO-3G results agree well with those of two previous works, whereas the 4-31G calculations show a relatively accurate energy difference between cis–transoid and trans–cisoid polyacetylene. The modified symmetric cutoff procedure is proven to be suitable for calculating the energy difference between polyacetylene isomers.
Supersonic molecular jet studies of the pyrazine and pyrimidine dimers
Authors: Wanna J. , Menapace J. A. , Bernstein E. R.
Published: July 2002

Mass selected optical spectra for the first excited singlet nπ* states of the pyrazine and pyrimidine dimers are presented. The species are created in a pulsed supersonic jet expansion. The spectra are analyzed based on ionization energy, vibronic structure, and relative energy with respect to the isolated monomer (cluster spectroscopic shift). Calculations of binding energy and geometry for these dimers are carried out employing a Lennard-Jones (6-12-1) and hydrogen bonding (10-12-1) potential. In the case of pyrazine, calculations and experiments agree that both parallel planar hydrogen bonded and perpendicular dimers are present in the expansion. The calculations also predict a parallel stacked and 90° rotated pyrazine dimer which is not observed. This latter species most likely forms an excimer in the excited state with a short lifetime and a highly red shifted broad spectrum. In the case of pyrimidine, calculations yield four planar hydrogen bonded species and a parallel stacked and displaced species. The spectra for the pyrimidine dimer are consistent with these configurations, in agreement with the calculations. No perpendicular configuration is calculated for the pyrimidine dimer and no spectroscopic features require postulating the existence of such a configuration. To explore further the agreement between calculated and experimental results for aromatic dimers, calculations are also presented for the tetrazine dimer. Three calculated geometries are obtained for the tetrazine dimer: a parallel stacked and 90° rotated species, a planar hydrogen bonded species, and a perpendicular species. Experimental spectra and calculations are in basic agreement for all dimers studied and, in general, support one another.
Late stage spinodal decomposition in polymer mixtures
Authors: Onuki Akira
Published: July 2002

Late stages are examined in the spinodal decomposition process of polymer blends. The coarsening due to surface tension is much delayed in the entangled case even on the dimensionless time scale. Before its inception the hydrodynamic interaction is strongly suppressed and the coarsening should be analogous to that of the kinetic Ising model.
Resonance effects in the 5σ photoionization of CO
Authors: Smith Maile E. , Lynch Diane L. , McKoy Vincent
Published: July 2002

We report vibrational branching ratios and vibrationally resolved photoelectron angular distributions for resonant photoionization of the 5σ level of CO leading to X 2Σ+CO+. These results were obtained using frozen-core Hartree–Fock photoelectron continuum orbitals. These studies provide a clear quantitative picture of the role of the kσ shape resonance in the observed non-Franck–Condon behavior and useful insight into the interplay between this shape resonance and a valence-like autoionizing resonance around 23 eV.
Excitation energies in Be: A comparison of multiconfigurational linear response and full configuration interaction calculations
Authors: Graham Richard L. , Yeager Danny L. , Olsen Jeppe , Jo/rgensen Poul , Harrison Robert , Zarrabian Sohrab , Bartlett Rodney
Published: July 2002

Using a 〈9s9p5d〉 contracted GTO basis we have calculated low-lying excitation energies of singlet and triplet symmetry for the Be atom using Δfull CI, ΔCI(1s) with double occupancy in the 1s orbital, multiconfiguration linear response (MCLR), and ΔMCSCF approaches. The Δfull CI results agree very closely with the experimental excitation energies except for higher excitations where obvious basis set defects occur. The MCLR calculations shows that with an adequately chosen MCSCF reference state the MCLR calculation is capable of mimicking the Δfull CI results. The MCLR results are closer to the Δfull CI results than the ΔCI(1s). The ΔMCSCF excitation energies show that this approach can only be used with extreme care to determine excitation energies.
Adsorbate ordering processes and infrared spectroscopy: An FT-IRAS study of N2 chemisorbed on the Ni(110) surface
Authors: Brubaker Mary E. , Trenary Michael
Published: July 2002

A newly constructed apparatus for Fourier transform-infrared reflection absorption spectroscopy (FT-IRAS) studies has been used in a detailed study of the N–N stretching band of N2 weakly chemisorbed on the Ni (110) surface. The high resolution and the high signal-to-noise ratios of the spectra allow observation of subtle changes of the band shape which accompany changes in the overlayer structure. The results are discussed with respect to a detailed two-dimensional phase diagram recently proposed for the N2/Ni (110) system. The IR data clearly reveal that for low coverages nonequilibrium adsorption occurs at 81 K, while equilibrium is attained at 125 K in agreement with the phase diagram. The present results indicate that the incommensurate overlayer structure which forms at the highest coverages is characterized by an intense sharp IR band at 2194 cm−1, a weak shoulder at 2204 cm−1, and a still weaker satellite peak at 2220 cm−1. The IR results for the incommensurate N2 overlayer are compared with models originally proposed to reconcile LEED and vibrational studies of incommensurate CO overlayers.
Hyperpolarizability dispersion measured for CO2
Authors: Shelton D. P.
Published: July 2002

The frequency dependence of the second hyperpolarizability (γ) of CO2 has been measured in the visible (488<λ<693 nm) by means of gas phase electric-field-induced second-harmonic generation (ESHG). Comparison of these results with the results for several other third-order nonlinear optical processes suggests that there are significant vibrational contributions to γCO2.
A method of calculating electron correlations for large molecules involving C, N, and H atoms
Authors: Oleś Andrzej M. , Pfirsch Frank , Fulde Peter , Böhm Michael C.
Published: July 2002

A way of calculating the electron correlation energy for the ground state of large organic molecules is presented. It is demonstrated that various contributions to the correlation energy may be described by simple analytic expressions. In the case of interatomic correlations, they depend only on the bond length, its kind (e.g., σ or π bond) and the atoms involved in the bonding. Intraatomic correlations for a given atom are shown to be determined by its total charge and the fraction of p electrons. The method is developed by starting from semiempirical (self-consistent field) calculations and building correlations into it. It is straightforward and simple to apply. Moreover, it provides considerable physical insight into the phenomenon of electron correlations. A number of tests of its accuracy are presented by considering small molecules for which a comparison with other calculations can be made. An overall satisfactory agreement is found.
Excess free energy in solutions of isotopic isomers. I. Monatomic species. II. Polyatomic species
Authors: Singh Rajiv R. , Van Hook W. Alexander
Published: July 2002

The excess free energy Aex for solutions of isotopes is discussed. We consider the volume dependence of the free energy and integrate over the concentration weighted molar volume isotope effect, but under the constraint of constant g(r*). The result is significantly different than results from integration along the isothermal equilibrium path employed by previous authors. Both internal and external modes can make important contributions to Aex. This is exemplified by treating solutions of 36Ar/40Ar, C6H6/C6D6, H216O/H218O, and H2O/HDO.
Energy dependence in coupled channel models for electronic structures in atoms
Authors: Ginter Dorothy S. , Ginter Marshall L.
Published: July 2002

Data for Si i, Al i, and Ca i are used to determine parametric representations of eigenquantum defect theory which reproduce level energies and transition oscillator strengths to the order of the average experimental uncertainty. The results of these calculations demonstrate an insensitivity to the placement of energy dependence within the parameter sets of coupled channel models. Thus, while energy dependence is essential to precise representation of the level structures of coupled, closed channels, a unique determination of where energy dependence resides is not (1) necessary for quantitative correlation of experimental observables and (2) possible from existing closed-channel data.
Fourier transform infrared spectroscopy of a single aerosol particle
Authors: Grader Gideon Sageev , Arnold Stephen , Flagan Richard C. , Seinfeld John H.
Published: July 2002

A method is developed for obtaining the molecular composition of a single suspended microparticle by Fourier transform infrared spectroscopy. The particle is held in an electrodynamic balance and irradiated simultaneously by the infrared output from a Michelson interferometer and the visible light from a dye laser. The laser is tuned to an edge of an optical resonance of the particle while the IR beam is chopped. Through evaporation and condensation the chopped IR beam causes a size modulation of the droplet, which in turn induces a fluctuation in the laser light scattered from the particle. The scattered light is detected at 90° with a photomultiplier, and the amplitude of the light fluctuation is measured with a lock-in amplifier. The lock-in signal is then inverted by a discrete fast Fourier transform routine (FFT), to yield the particle absorption spectrum. Spectra of (NH4)2SO4 droplets at different solute concentrations are presented. The data shown include the first infrared spectrum of a highly supersaturated solution.
Full CI benchmark calculations on CH3
Authors: Bauschlicher Charles W. , Taylor Peter R.
Published: July 2002

Full CI calculations have been performed on the CH3 radical. The full CI results are compared to those obtained using CASSCF/multireference CI and coupled-pair functional methods, both at the equilibrium CH distance and at geometries with the three CH bonds extended. In general, the performance of the approximate methods is similar to that observed in calculations on other molecules in which one or two bonds were stretched.
Interionic potentials in alkali halides
Authors: Patil S. H.
Published: July 2002

Analytic expressions are obtained for the Coulomb and exchange energies in alkali halides, incorporating the correct asymptotic behavior of electron densities. We have also introduced suitable damping factors for the polarizability and van der Waals terms. This allows us to obtain accurate values for the potentials which are found to be conformal.
A study of the infrared emission from the O I (3d 3D0) and (4s 5S0) states produced by electron impact excitation of O2
Authors: Erdman P. W. , Zipf E. C.
Published: July 2002

We have measured the absolute emission cross section value for the O i(3p 3P–3d 3D0, λ=11 287 Å) and the (3p 5P–4s 5S0, λ=11 299 Å) multiplets excited by electron impact on O2. The former cross section has a value of 3.05×10−19 cm2±15% at 100 eV, while the latter’s value is 4.14×10−19 cm2±15%. The quintet emission at λ=11 299 Å is the only deexcitation channel for the 4s 5S0 state, and, therefore, represents the total excitation cross section for that state. However, the λ=11 287 Å branch from the O i(3d 3D0) state only constitutes ∼25% of the total 3d 3D0 cross section, with the major branch from this state being the transition to the ground state observed at λ=1027 Å. The branching ratio obtained in this study is in good agreement with the theoretical calculation of Pradhan and Saraph (ω̃11 287=0.28), but our absolute σ(λ=11 287 Å) value is substantially smaller than the result reported recently by Schulman et al.
Optimized orbitals for the description of electron correlations
Authors: Dieterich Klaus , Fulde Peter
Published: July 2002

A method is described for the construction of (local) orbitals which are optimized for a correlation energy calculation. A distinction can be made between orbitals for intrabond and interbond correlations. The correlation energy is calculated with the optimized orbitals by applying the local ansatz. It is demonstrated for CH4 that the results agree with those of a CEPA. The number of optimized orbitals is considerably less than the number of pseudonatural orbitals (PNOs) which is required to attain the same accuracy. A physical interpretation of the orbitals is given and their transferability is briefly discussed.
CARS spectroscopy in supersonic jets of ammonia monomers and clusters
Authors: Barth Hans Dieter , Huisken Friedrich
Published: July 2002

Supersonic molecular beams of ammonia seeded with various rare gases are investigated in the expansion region using one-dimensional coherent anti-Stokes Raman scattering (CARS). A Raman spectrum of the ν3 vibration of the NH3 monomer with resolved rotational structure and inversion splitting is reported. Spectroscopy of this vibration is ideally suited to determine the rotational temperature in the beam. In the spectral region between 3100 and 3500 cm−1, broad structures are observed which can be attributed to clusters. These structures arise at strong stagnation conditions and increase when going from pure NH3 expansions to NH3/He and NH3/Ne mixtures. Analysis of the employed beams by mass spectrometry suggests that large clusters containing ∼5–50 NH3 molecules are responsible for the observed features. A narrow-peaked structure, red shifted by 3.5 cm−1 from the ν1 Q branch, is attributed to small clusters or NH3 molecules residing on the outside of the cluster whose H atoms are free and not bound to the complex.
Circular dichroism in photoelectron angular distributions from two-color (1+1) REMPI of NO
Authors: Appling J. R. , White M. G. , Dubs R. L. , Dixit S. N. , McKoy V.
Published: July 2002

A detailed experimental and theoretical study of dichroic effects in photoelectron angular distributions is reported for (1+1), two-color REMPI of NO via the A 2Σ+, v=0 state. Optically aligned A state rotational levels are probed through ionization by circularly polarized light. Resultant photoelectron angular distributions exhibit significant left–right asymmetry, the phase and magnitude of which are shown to be related to the curvature of the excited state MJ distribution. Theoretical calculations involving a full ab initio treatment of the ionization dynamics result in circularly dichroic angular distribution (CDAD) parameters in good agreement with those derived experimentally. Additional effects including hyperfine depolarization and coherence are also discussed in relation to the observed CDAD data.
Comparison of experiment and theory for the resonance Raman spectrum of I2 in solution. III. Perfluorohexane and chloroform
Authors: Sension Roseanne J. , Kobayashi Takamichi , Strauss Herbert L.
Published: July 2002

The absolute Raman excitation profiles (REP) of I2 in perfluorohexane and chloroform have been determined. The experimental results are compared to calculations and to our previous results in n-hexane. Although there is good overall agreement between experiment and theory, certain discrepancies turn up in all the solvents. In particular, the experimental cross section is higher than that calculated in the high frequency wing of the REP. This discrepancy cannot be due to charge transfer states since it exists in all the solvents. The excited state linewidth, Γ has been determined to be about 15 cm−1 in all three solvents.
Methyl reorientation in methylphenanthrenes. I. Solid state proton spin–lattice relaxation in the 3-methyl, 9-methyl, and 3,9-dimethyl systems
Authors: Conn K. G. , Beckmann P. A. , Mallory C. W. , Mallory F. B.
Published: July 2002

We have investigated the dynamics of methyl group reorientation in solid methyl-substituted phenanthrenes. The temperature dependence of the proton spin–lattice relaxation rates has been measured in polycrystalline 3-methylphenanthrene (3-MP), 9-methylphenanthrene (9-MP), and 3,9-dimethylphenanthrene (3,9-DMP) at Larmor frequencies of 8.50, 22.5, and 53.0 MHz. The data are interpreted using a Davidson–Cole spectral density which implies either that the correlation functions for intramolecular reorientation are nonexponential or that there is a distribution of exponential correlation times. Comparing the fitted parameters that characterize the relaxation data for the three molecules shows that the individual contributions to the relaxation rate from the 3- and 9-methyls in 3,9-DMP can be separated and that the parameters specifying each are similar to the equivalent group in the two single methylphenanthrenes. The 9-methyl group is characterized by effective activation energies of 10.6±0.6 and 12.5±0.9 kJ/mol in 9-MP and 3,9-DMP, respectively, whereas the 3-methyl group is characterized by effective activation energies of 5.2±0.8 and 5±1 kJ/mol in 3-MP and 3,9-DMP, respectively. The agreement between the fitted and calculated values of the spin–lattice interaction strength, assuming only intramethyl proton dipole–dipole interactions need be considered, is excellent. A comparison between experimentally determined correlation times and those calculated from a variety of very simple dynamical models is given, and the results suggest, as have several previous studies, that at high temperatures where tunneling plays no role, methyl reorientation is a simple, thermally activated, hopping process. We have also analyzed many published data in methyl-substituted phenanthrenes, anthracenes, and naphthalenes (14 molecules) in the same way as we did for the phenanthrene data presented here, and a consistent picture for the dynamics of methyl reorientation emerges.
Infrared spectroscopy of NH+: An analysis of the perturbation between the X 2Π and a 4Σ− states
Authors: Kawaguchi K. , Amano T.
Published: July 2002

The v=1–0 vibration–rotation transitions in the X 2Π and a 4Σ− states as well as those between the two electronic states were observed with a difference frequency laser as a radiation source. The two electronic states (X 2Π and a 4Σ−) lie close together and interact each other strongly through the spin–orbit coupling. A merged least-squares fit was carried out with the present infrared transition wave numbers, some of the previous optical term values, and the recent far-infrared rotational transition frequencies to determine the spectroscopic parameters precisely. The equilibrium internuclear distance was obtained to be 1.0692±0.0002 and 1.0924±0.0001 Å for the X and a states, respectively. The Λ-type doubling transition frequencies were calculated for several of the lowest J states with the molecular constants obtained and the hyperfine coupling constants determined from the far-infrared transitions.
The Laplacian of the charge density and its relationship to the shell structure of atoms and ions
Authors: Sagar Robin P. , Ku Andrew C. T. , Smith Vedene H. , Simas Alfredo M.
Published: July 2002

The Laplacian of the spherically averaged charge density ∇2ρ̄(r) has been computed from nonrelativistic SCF wave functions for the neutral atoms from hydrogen to uranium, and the singly positive ions, from helium to barium and lutetium to radium, in order to examine the shell structure. ∇2ρ̄(r) exhibits a number of extremal points and zeros with the absolute value of the function becoming smaller at each successive extremal point. The zeros, in particular the odd numbered zeros, are shown to exhibit good correlation with the Bohr theory of an atom while the extremal points correlate to a lesser extent. At most five shells are seen in the studied atomic cases based on the fact that the odd numbered zeros are the topological feature of ∇2ρ̄(r) most indicative of a shell.
Molecular hardness and softness, local hardness and softness, hardness and softness kernels, and relations among these quantities
Authors: Berkowitz Max , Parr Robert G.
Published: July 2002

Hardness and softness kernels η(r,r′) and s(r,r′) are defined for the ground state of an atomic or molecular electronic system, and the previously defined local hardness and softness η(r) and s(r) and global hardness and softness η and S are obtained from them. The physical meaning of s(r), as a charge capacitance, is discussed (following Huheey and Politzer), and two alternative ‘‘hardness’’ indices are identified and briefly discussed.
Rotational spectra and structures of small clusters containing the HCN dimer: (HCN)2–Ar, a T-shaped trimer
Authors: Ruoff R. S. , Emilsson T. I. , Klots T. D. , Chuang C. , Gutowsky H. S.
Published: July 2002

Microwave rotational spectra have been observed for the four 14N/15N isotopic species of an (HCN)2–Ar trimer with the pulsed nozzle Fourier transform method using the Flygare Mark II spectrometer. Thirteen J→J′ asymmetric top transitions were measured in the 2.5 to 10 GHz region for the parent 14N,14N trimer and nine for each of the other three species. The 14N nuclear quadrupole hyperfine structure was analyzed for the transitions and the interaction constants and line centers determined. The line centers were fitted to obtain ground vibronic state rotational and quartic centrifugal distortion constants. For the 14N/14N trimer these are (in MHz) for A″, B″, and C″: 2013.5993(10), 1759.2756(3), 932.3709(2); and for τ1, τ2, τaaaa, τbbbb, and τcccc: −0.3017(1), −0.0660(1), −0.1700(9), −0.013 71(1), and −0.0088(1). The inertial defect is 3.7881 amu Å2. The zero-point vibrationally averaged geometry is planar and T shaped with the structure of the linear HCN dimer remarkably unperturbed by presence of the Ar atom. The substitution N⋅⋅⋅N distance in the cluster is 4.453 Å compared with 4.461 Å in the free dimer. The argon is attracted by the π electrons of the two –C≡N groups, being equidistant from them. The Ar⋅⋅⋅N(1) and Ar⋅⋅⋅N(2) distances are 3.671 and 4.316 Å, respectively.
Vibration-induced first-order Stark effect in tetrahedral molecules: SiH4 and SiF4
Authors: Frye Joan M. , Magerl Gottfried , Kreiner Welf A. , Oka Takeshi
Published: July 2002

The vibration-induced permanent dipole moments in SiH4 and in SiF4 have been determined by measuring the first-order Stark effect on the infrared vibration–rotation lines in the triply degenerate ν4 and ν3 fundamental bands, respectively. Microwave modulation sidebands on CO2 laser lines were generated with sufficient power and spectral purity to be used as the frequency-tunable infrared source for the sub-Doppler saturation Lamb dip measurements. Theory is also presented for calculating the dipole moment matrix elements in the excited vibrational state. From the observed Stark splittings, the vibration-induced permanent dipole moment P has been determined to be 1.46(5)×10−2 D for SiH4 in the v4=1 state, and 4.21(12)×10−2 D for SiF4 in the v3=1 state. In addition, Watson’s coefficient for the rotation-induced permanent dipole moment θxyz has been determined to be 2.97(19)×10−5 D in SiH4 (v4=1).
Integral equation for the Smith–Nezbeda model of associated fluids
Authors: Wertheim M. S.
Published: July 2002

Our recent reformulation of statistical thermodynamics for fluids of molecules interacting by site–site bonding forces is extended to the Smith–Nezbeda (SN) model of associated fluids, where sites bond to the hard core of another molecule. The formal theory of the SN model with one molecular site is similar to the case of site–site bonding with one site, with many formal expressions identical. The difference in physical behavior is attributable to the quite different graph content of the functions that enter. The reformulated theory contains two densities, a number density ρ and a density ρ0 of molecules with site unbonded. An integral equation of Percus–Yevick type is derived and transformed using factorization methods. Accurate numerical solutions are obtained and pressures, internal energies, and concentrations of molecules with site unbonded, bonded, and bonded partaking in a double bond are calculated. No gas–liquid phase transition is found. This is explained by insufficient clustering due to a preference for double bond formation, which is strongest at low ρ and temperature T. The degree of consistency of the virial and compressibility equations of state improves with decreasing T and becomes extraordinarily high over an extended density range at low T. Agreement with the twelve Monte Carlo simulation states of SN is excellent for the internal energy and the pair distribution function near contact. For the pressure the agreement is not quite as good, with no clear trend visible in the discrepancy between integral equation and simulation.
Influence of the fractal geometry of trajectories on the rate of diffusion-controlled bulk ion recombination
Authors: López-Quintela M. A. , Buján-Núñez M. C. , Pérez-Moure J. C.
Published: July 2002

Analysis of the influence of the differential fractal dimension of reagent trajectories on the rate of diffusion-controlled bulk ion recombination provides explanations of both Tachiya’s recent Monte Carlo results on the dependence of the rate constant on the mean free path and Nakamura’s results on electron-ion recombination in liquid methane.
Experimental and theoretical studies of the radiative properties of the N2+2 D 1Σ+u state
Authors: Olsson B. J. , Kindvall G. , Larsson M.
Published: July 2002

The fluorescence lifetime of the v′=0 level of the N2+2 D 1 ∑+u electronic state has been measured, with the high frequency deflection technique, for the first time. The result, τ=6.0±0.5 ns, is in good agreement with a recent photoion–photon of fluorescence coincidence measurement which yields a lifetime, averaged over an unspecified number of vibrational levels, of 8±3 ns. The electronic transition moment function for the N2+2 D 1 ∑+u–X 1 ∑+g transition was calculated with nonorthogonal complete active space SCF wave functions. Potential energy curves for the X 1 ∑+g and D 1 ∑+u states were calculated with a multireference contracted CI treatment and these potential curves were used, together with the electronic transition moment function, to obtain vibrationally averaged transition moments and hence a theoretical radiative lifetime. The calculated lifetime, 8.0 ns, is in good agreement with the experimentally determined value indicating that the decay of D 1 ∑+u, v′=0 occurs primarily via fluorescence to the X 1 ∑+g state.
Microscopic observation of ordered colloids in sedimentation equilibrium and importance of Debye-screening length. VII. Colloidal silica spheres (specific gravity=1.58)
Authors: Okubo Tsuneo
Published: July 2002

A metallurgical microscope is used to directly observe the ordered structures of colloidal silica spheres (340 nm in diameter, specific gravity=1.58) in sedimentation equilibrium and in aqueous suspensions. The two-dimensional distance distribution function is obtained for the crystal-like structures. The center-to-center interparticle distances (2Dexp) in the hexagonal ordering increase with time after the sample is set into the cell, and reach equilibrium values after ∼10 days, which shows that complete deionization is essential to get reliable data for 2Dexp. 2Dexp continues to decrease as the initial concentration of the spheres increases and from these data Young’s modulus for the ordered lattices is estimated to be 0.30 Pa. The way 2Dexp is influenced by the concentration and ionic strength shows that electrostatic interparticle repulsion and the elongated Debye-screening length around the spheres are both essential for the ordering.
Ensemble dephasing in vibrationally excited jet-cooled tetrazine and its complexes with Ar, Kr, and Xe
Authors: Weber Peter M. , Rice Stuart A.
Published: July 2002

A time delayed, fully resonant, version of coherent anti-Stokes Raman spectroscopy (CARS) has been used to study intramolecular and intermolecular dephasing in vibrationally excited jet-cooled tetrazine and its van der Waals complexes with argon, krypton, and xenon. When there is 1200 to 2200 cm−1 of vibrational energy in the ground electronic state manifold, the dephasing of all species is dominated by the macroscopic interference associated with the heterogeneous structure of the Q-branch transitions. The observed macroscopic dephasing times are about 300 ps to 2 ns, which implies that the lower limit for the intramolecular dephasing times falls in the same range. The influence of excited state resonances on the dephasing characteristics of these molecules is qualitatively accounted for by formulas derived from the wave packet theory of Tannor, Rice, and Weber.
Computational studies of SiH2+SiH2 recombination reaction dynamics on a global potential surface fitted to a
b i
n
i
t
i
o and experimental data
Authors: Agrawal Paras M. , Thompson Donald L. , Raff Lionel M.
Published: July 2002

The recombination dynamics for the SiH2+SiH2→H2Si=SiH2 reaction are studied by quasiclassical trajectory methods using a global potential-energy surface fitted to the available experimental data and the results of various ab initio calculations. The potential surface is written as the sum of 18 many-body terms whose functional forms are motivated by chemical and physical considerations. The surface contains 41 parameters which are fitted to calculated geometries, fundamental vibrational frequencies, and energies for H2Si=SiH2, H2Si=SiH, H2Si=Si, HSi=Si, Si2, H2, and SiH2, and to various calculated and/or measured reaction barrier heights and activation energies. In general, the equilibrium bond lengths and angles given by the global surface are in agreement with ab

i n i t i o results to within 0.03 Å and 0.5°, respectively. The calculated exothermicities for various reactions involving silicon and hydrogen atoms are in excellent agreement with previous MP4 calculations and with experimental data. The average absolute error is 1.90 kcal/mol. The average absolute deviation of the predicted fundamental vibrational frequencies for H2Si=SiH2, H2Si=SiH, H2Si=Si, and SiH2 from the results reported by Ho et al. is 52.9 cm−1. The calculated barrier height for molecular hydrogen elimination from SiH2 is 34.27 kcal/mol with a backreaction barrier of 0.63 kcal/mol. The barrier for 1,2 elimination of H2 from H2Si=SiH2 is 115.3 kcal/mol with a backreaction barrier of 30.7 kcal/mol. The formation cross sections for H2Si=SiH2 decrease with both relative translational energy and internal SiH2 energy with translational energy being the more effective in reducing the cross sections. Thermally averaged formation cross sections vary from 66.3 Å2 at 300 K to 28.7 Å2 at 1500 K. The corresponding thermal rate coefficients lie in the range 2–4×1014 cm3/mol s over this temperature range and exhibit a maximum at an intermediate temperature. The trajectory details indicate that the reaction exothermicity is primarily partitioned into the Si–Si stretch and the H–Si–H bending modes upon formation of Si2H4. Energy transfer from the Si–Si stretch to the Si–H stretching modes is a relatively slow process occurring on a time scale of 10−12 s, which is about three to four times that previously computed for other polyatomic systems. Transfer from the Si–Si stretch to the H–Si–H bending modes is a faster process.
Multifrequency electron spin-echo envelope modulation: The determination of nitro group 14N hyperfine and quadrupole interactions of DPPH in frozen solution
Authors: Flanagan Heather L. , Gerfen Gary J. , Singel David J.
Published: July 2002

Two- and three-pulse electron spin-echo modulations from 14N in the nitro groups of DPPH in toluene at 77 K have been measured at electron spin excitation frequencies of 4.45, 4.94, 5.47, 6.68, and 9.10 GHz. Analysis of the frequencies, amplitudes, and linewidths of the modulation components, together with their variation with excitation frequency, establishes that the 14N hyperfine interactions are approximately isotropic and enables the evaluation of the isotropic coupling. Distinctive spectral trends apparent in multifrequency ESEEM are employed to identify the cancellation of hyperfine and Zeeman interactions; modulation components accordingly assigned as pure quadrupole frequencies afford accurate determination of quadrupole coupling constants. Multifrequency ESEEM is shown to be a vital tool of general applicability in 14N ESEEM spectroscopy.
Infrared double-resonance measurements in CD3H: Vibrational energy transfer between the ν3 and ν6 Coriolis-coupled modes and between A and E symmetry species
Authors: Menard-Bourcin F. , Doyennette L.
Published: July 2002

IR pulse–probe transient absorption measurements using two CO2 lasers were performed to investigate the collisional energy transfers in CD3H. The results indicate that the vibrational energy transfer between the Coriolis-coupled ν3 and ν6 modes is very fast (≂107 s−1 Torr−1) approaching the rate of gas kinetic collisions, and occurs only between rovibrational states of the same symmetry type. Besides, evidence of a V–V transfer between the A and E species of CD3H is given, and the corresponding measured rate constant is (2.3±0.2)×105 s−1 Torr−1. It is the first time, to our knowledge, that such a V–V transfer between two different symmetry species is directly observed. The V–T, R relaxation rate constant was also determined and found to be (1.16±0.35)×103 s−1 Torr−1, in agreement with previous studies.
Observation of vibronic quantum beat in dye-doped polymers using femtosecond accumulated photon echo
Authors: Saikan S. , Nakabayashi T. , Kanematsu Y. , Imaoka A.
Published: July 2002

Fourier-transform photon echo spectroscopy has been applied to the vibronic absorption band in dye molecule. The quantum beat between the vibronic lines has been observed in cresyl-violet and tetraphenyl-porphine doped in polymer films. The vibronic lines associated with the quantum beat have been identified with comparing the Fourier-transformed spectrum of the echo signal and the persistent hole-burning spectrum. The observed steep temperature dependence of the vibronic coherence has been discussed in comparison with the mild temperature dependence of vibrational coherence (sublevel coherence between vibronic states).