bluuebunny/crossref_metadata_2025 huggingface dataset:
Radiative decay lifetimes of CH−2
Authors: Okumura M. , Yeh L. I. , Normand D. , van den Biesen J. J. H. , Bustamente S. W. , Lee Y. T. , Lee Timothy J. , Handy Nicholas C. , Schaefer Henry F.
Published: July 2002

Recently the presence and radiative decay of vibrationally excited CH−2, generated in a hot cathode discharge of methane, was established by measuring the time dependent photodetachment from excited states of CH−2 as it radiatively relaxed in a high vacuum ion trap. The time dependence of the photodetachment was found to be consistent with an electron affinity of 5250 cm−1 (0.65 eV) for ground state X̃ 3B1 methylene. The radiative decay lifetimes of the first three excited bending vibrations of CH−2 were also tentatively assigned. Here, we report a more refined analysis of the experimental data along with theoretical ab initio determinations of the radiative decay lifetimes of the first four excited bending vibrational levels of CH−2. There is some discrepancy between the ab initio values (431, 207, 118, and 68 ms for the v2=1, 2, 3, and 4 levels respectively) and the experimental values (525, 70, and 14 ms for v2=1, 2, and 3 respectively) for v2=2 and 3. Possible reasons for this discrepancy are discussed but none of the alternatives are entirely satisfactory.
The infrared spectrum of the acetylene radical cation C2H+2. A theoretical study using SCF, MCSCF, and CI methods
Authors: Lee Timothy J. , Rice Julia E. , Schaefer Henry F.
Published: July 2002

Within the harmonic approximation, the vibrational frequencies and infrared intensities of the C2H+ 2 electronic ground state have been predicted using ab initio molecular quantum mechanics. A standard double zeta plus polarization (DZ+P) basis set of contracted Gaussian functions was employed. Self-consistent-field (SCF), multiconfiguration (MC) SCF, and configuration interaction (CI) methods were used.
ESR study of optically enhanced phase transition in (BEDT–TTF)3Ta2F11
Authors: Acrivos J. V. , Hughes H. P. , Parkin S. S. P.
Published: July 2002

We have identified an optically enhanced magnetic phase transition in the newly synthesized organic molecular charge transfer salt, (BEDT–TTF)3Ta2F11 (BEDT–TTF 3/4 bis-ethylenedithiolotetrathiafulvalene) by ESR absorption measurements in the X band microwave region. At room temperature, only a doublet state ESR absorption is observed, but below 30 K several triplet ESR absorptions appear. The orientation dependence of the ESR absorption under illumination at energies near the band gaps in the material (640 nm, T=12 to 5 K, H0<0.34 T) indicates that there are rapid spin exchange processes with times τe<10−8 s near 7 to 5 K along certain crystallographic directions with a temperature dependence suggesting spin-lattice relaxation times which proceed via Van Vleck ‘‘direct processes.’’ This, to our knowledge, is the first case where the magnetic properties of a charge transfer salt are altered by the interaction with photons of energy equal to the band gaps in a low dimensional solid, providing a new, interesting way to investigate these materials.
Binary diffusion coefficients for the systems He4–He3, He4–D2, He4–H2, He3–D2, He3–H2, and H2–D2 from 273 to 320 K: Tests of several potential functions
Authors: Dunlop Peter J. , Bignell C. M. , Taylor W. L. , Meyer B. A.
Published: July 2002

Diffusion coefficients are reported for the six binary systems which can be formed from the components He4, He3, D2, and H2. The results are used to test the assumption that the potential function for a given system is independent of the isotopic nature of each component.
Ultraviolet absorption spectroscopy of dissociating molecules: Effects of cluster formation on the photodissociation of CH3I
Authors: Donaldson D. J. , Vaida Veronica , Naaman Ron
Published: July 2002

The ultraviolet absorption spectra of jet-cooled CH3I, (CH3I)2, and (CH3I)n, n>2 are measured in the valence A state and the Rydberg B state. A significant blue shift of the valence state is observed upon cluster formation. The magnitude of this shift yields a dimer bond strength of at least 500 cm−1 which implies extensive dimer formation at room temperature and moderate pressures. The methyl iodide B-state spectrum consists of a single progression in the ν2 mode. Dimerization retards the predissociation and therefore increases the excited state lifetime; consequently many new modes are observed which in the monomer are coupled to the dissociation. The implications of this result to the photodissociation dynamics are discussed. The Rydberg spectra of (CH3I)n, n>2 show asymmetric line shapes which may be due to interferences between the B state and an underlying continuum. The nature of these interferences and the possible indentity of this continuum are discussed.
The analytic gradient for the coupled pair functional method: Formula and application for HCl, H2CO, and the dimer H2CO⋅⋅⋅HCl
Authors: Rice Julia E. , Lee Timothy J. , Handy Nicholas C.
Published: July 2002

A general analytic gradient expression is formulated for the coupled pair functional (CPF) electron correlation procedure. This method is implemented by modification of our existing configuration interaction (CI) gradient. It is demonstrated that since the CPF energy is not invariant to a unitary transformation of the occupied orbitals, additional self-consistent-field (SCF) redundant orbital rotation contributions must be included to evaluate the CPF energy gradient. The new procedure is used to rigorously determine the equilibrium structure, harmonic frequencies and dipole moment of the H2CO⋅⋅⋅HCl hydrogen bonded complex and its constituent monomers. These results are compared with the predictions of SCF, second order perturbation theory (MP2), singles and doubles configuration interaction (CISD) and the experimental values. The CPF method in conjunction with a large basis set is shown to obtain highly accurate structures and molecular properties for the H2CO and HCl molecules. The H–Cl frequency of the complex is the most intense band in the infrared spectrum. The harmonic frequency shift of the H–Cl stretch is predicted to be 151 cm−1 with the double-zeta plus polarization basis for the gas phase dimer. Since it is well known that the structure of a dimer is dependent on the properties of the monomers and because the CPF triple zeta plus double polarization (TZ2P) level of theory obtains the most accurate results for the monomers, it is concluded that the CPF TZ2P method gives the most reliable prediction of the equilibrium structure of the complex. The hydrogen bond is predicted to have a length, rO⋅⋅⋅H of 1.952 Å and to be nonlinear, ∠O⋅⋅⋅H–Cl=163.2°. The angular orientation of the two monomers in the complex, described by ∠C=O⋅⋅⋅H, is determined to be 109.0° at equilibrium. Based upon the CPF TZ2P structure, the harmonic frequency shift of the H–Cl stretch is believed to be greater than 151 cm−1.
Interaction between plates in a polymer melt
Authors: ten Brinke Gerrit , Ausserre Dominique , Hadziioannou Georges
Published: July 2002

The effect of confining a polymer melt between two parallel plates has been investigated using Monte Carlo simulations. As soon as the distance between the plates becomes comparable to twice the radius of gyration of the polymer molecules in the bulk, the shape of all molecules is affected. Reducing the separation even more reduces the actual number of deformed molecules while the remaining molecules become on the average more deformed. The deformation of the molecules is due to those sections which are very near one of the two plates. The excess number of bonds parallel to the plates with respect to the bulk isotropic contribution is nearly independent of the distance between the plates. The correlation length is estimated to be of the order of the lattice spacing. All observations are in agreement with the predictions of de Gennes that no long-range force between plates in a melt should exist.
The structure and reactivity of chemisorbed aromatics: Spectroscopic studies of benzene on Mo(110)
Authors: Liu A. C. , Friend C. M.
Published: July 2002

The reactions of benzene on Mo(110) have been investigated under ultrahigh vacuum conditions using temperature programmed reaction spectroscopy (TPRS), x-ray photoelectron spectroscopy (XPS), and near-edge x-ray absorption fine structure (NEXAFS) measurements. Benzene undergoes competing decomposition and desorption processes during temperature programmed reaction; saturation exposures of C6H6 made at a crystal temperature of 120 K yield a small amount of benzene desorption at 360 K as well as substantial H2 evolution at 400 and 530 K. Additionally, a trio of molecular benzene desorptions attributed to weakly bound and multilayer states is observed below 200 K. Isotopic exchange experiments demonstrate that the 360 K molecular desorption arises from intact chemisorbed benzene. For surface temperatures up to 300 K, carbon 1s photoelectron spectra show only a single photoemission peak at a binding energy of 284.0 eV, attributed to molecular benzene. Above 300 K, increasing amounts of atomic carbon are observed at a binding energy of 282.8 eV, indicating that the kinetics for C–C and C–H bond activation in this system are similar. Decomposition to surface C and gaseous H2 is complete by 700 K. At 200 K, near-edge x-ray absorption fine structure measurements confirm the presence of molecular benzene in an orientation with the C6 plane parallel to the surface. After annealing to 450 K, the NEXAFS is complicated by the admixture of absorption structures from a surface intermediate and atomic carbon. Comparison of this NEXAFS with that for surface benzyne (C6H4) produced by annealing benzenethiol (C6H5SH) adsorbed on Mo(110) to 600 K suggests that the intermediate formed from benzene is C6H4 as well.
The photoelectron spectrum of HCl and DCl studied with ultraviolet excitation, high resolution x-ray excitation, and synchrotron radiation excitation: Isotope effects on line profiles
Authors: Svensson S. , Karlsson L. , Baltzer P. , Wannberg B. , Gelius U. , Adam M. Y.
Published: July 2002

The HCl and DCl molecules have been studied with monochromatized x-ray, ultraviolet, and synchrotron radiation excited photoelectron spectroscopy. Isotope effects are detected in the outer and inner valence bands using all the different excitation sources. These effects are used to describe the nature of the potential curves for the 5σ−1 state and various inner valence correlation states. The 5σ−1 state is shown to predissociate at 18.0 eV into Cl+(3P)+H(2S). The potential curves for the two outermost states in the 4σ−1 region are approximately determined. The outermost state is shown to be repulsive, whereas the second state, that corresponds mostly to the 4σ−1 single hole state, is found to be bound. One weak structure that must be associated with the 2∏ manifold of states is observed at 28.6 eV binding energy. Three structures at binding energies larger than 40 eV are reported for the first time.
Breakdown of the Onsager theory of geminate ion recombination
Authors: Tachiya M.
Published: July 2002

We investigate how the dynamics of geminate recombination between a positive ion and an electron depends upon the mean free path of the electron for scattering by the solvent. We calculate the escape probability of geminate recombination as a function of the electron mean free path, by use of the Monte Carlo method we previously developed. When the mean free path is negligibly short compared with the Onsager length, the calculated escape probability agrees with the Onsager result. However, it increases with increasing mean free path and becomes much higher than the Onsager result, for long mean free paths. We also study how the effect of an applied electric field on the escape probability is influenced by the length of the electron mean free path. Our theory quantitatively explains recent experimental data on free ion yields in liquids such as methane, argon, and tetramethylsilane, which sustain high electron mobilities or equivalently long electron mean free paths.
On the molecular theory of aqueous electrolyte solutions. II. Structural and thermodynamic properties of different models at infinite dilution
Authors: Kusalik P. G. , Patey G. N.
Published: July 2002

This paper describes a comparative theoretical study of the infinite dilution properties of solutions of univalent ions in different water-like solvents. The theoretical results are obtained by solving the reference hypernetted-chain (RHNC) approximation as described in the first article of this series [J. Chem. Phys. 88, 7715 (1988)]. It is shown that the inclusion of the octupole moment of the water molecule in the model solvent leads to strong preferential solvation of negative ions, and that this has important effects upon certain thermodynamic properties and upon the ion–solvent and ion–ion structure. Questions concerning the definition and physical interpretation of individual ion partial molar volumes are raised and considered in detail. It is argued that in general the individual ion results obtained experimentally may not accurately reflect the true microscopic ion–solvent structure.
Locating stationary paths in functional integrals: An optimization method utilizing the stationary phase Monte Carlo sampling function
Authors: Beck Thomas L. , Doll J. D. , Freeman David L.
Published: July 2002

A method is presented for determining the stationary phase points for multidimensional path integrals employed in the calculation of finite-temperature quantum time correlation functions. The method can be used to locate stationary paths at any physical time; in the case that t≫βℏ, the stationary points are the classical paths linking two points in configuration space. Both steepest descent and simulated annealing procedures are utilized to search for extrema in the action functional. Only the first derivatives of the action functional are required. Examples are presented first of the harmonic oscillator for which the analytical solution is known, and then for anharmonic systems, where multiple stationary phase points exist. Suggestions for Monte Carlo sampling strategies utilizing the stationary points are made. The existence of many and closely spaced stationary paths as well as caustics presents no special problems. The method is applicable to a range of problems involving functional integration, where optimal paths linking two end points are desired.
Photochemical hole burning for 1,3,5,7-octatetraene in n-hexane
Authors: Adamson George , Gradl Gerhard , Kohler Bryan E.
Published: July 2002

It is now well established that the unsubstituted linear polyene 1,3,5,7-octatetraene efficiently undergoes cis–trans photoisomerization even when substituted in an n-alkane matrix cooled to liquid-helium temperatures. The fact that this photochemical reaction takes place under these conditions opens the possibility of using the techniques of photochemical hole burning to uncover details of the microscopic mechanism of this isomerization. In this paper we report the demonstration of photochemical hole burning for all-trans-octatetraene in n-hexane, show that the hole width for the zero-phonon component of the 0–0 band in the limit of zero temperature and zero hole depth is within experimental error equal to the value predicted from the measured fluorescence lifetime, analyze the dependence of hole width on temperature, and show that the relative quantum yield for hole burning increases by approximately a factor of 35 as vibrational energy in the excited state is increased beyond a threshold of approximately 950 cm−1. This threshold agrees well with the previously determined barrier to trans, cis isomerization.
Hydrogen bond cooperativity in simulated water: Time dependence analysis of pair interactions
Authors: Sciortino F. , Fornili S. L.
Published: July 2002

Hydrogen bonding in water systems is investigated by introducing a new method to analyze the time dependence of pair-interaction data obtained from a molecular dynamics simulation of 216 ST2 [F. H. Stillinger and A. Rahman, J. Chem. Phys. 60, 1545 (1974)] water molecules at 280 K. This approach avoids the use of cutoff values and yields a more realistic bond population, whose distributions of geometric and energetic properties are reported as a function of the bond lifetimes. For the fraction of long-lived bonds, correlation among bond stability, molecular mobility, and local structure is elecited. Percolation analysis of HB network evidences cooperativity in the spatial distribution of bonds, which does not originate from proton polarizability of HB and/or from many-body terms of the interaction potentials since a rigid water model and pair potentials are used. These features can play a role in the anomalous properties of liquid water.
Electronic structure and ultraviolet absorption spectrum of polyimide
Authors: LaFemina John P. , Arjavalingam G. , Hougham G.
Published: July 2002

The ultraviolet absorption spectra for polyimide and several polyimide model compounds have been measured and the spectroscopically parametrized CNDO/S3 model is used to provide a detailed quantitative description of the spectra. Analysis of the optical absorption spectrum yields a value of 30° for the phenylimide torsional angle in these systems. In addition, the ultraviolet absorptions are found to be intramolecular charge transfer transitions.
Spinodal decomposition of a symmetric critical mixture of deuterated and protonated polymer
Authors: Bates Frank S. , Wiltzius Pierre
Published: July 2002

A nearly symmetric critical mixture (φc=0.486) of perdeuterated and protonated 1,4-polybutadiene exhibiting an upper critical solution temperature Tc=61.5±1.5 °C has been quenched from the homogeneous state (≂75 °C) to various temperatures between 25 and 57.5 °C. Light scattering measurements document the subsequent spinodal decomposition process which we describe based on a four-stage model: early, intermediate, transition, and final. The early stage is accounted for by the Cahn theory, yielding initial correlation lengths and effective diffusion coefficients in quantitative agreement with mean-field predictions. Nonlinear effects mark the beginning of the intermediate stage, which exhibits a simple power-law growth of heterogeneity length Lm(t)∼tneff, but with a temperature dependent exponent neff. As the composition fluctuation amplitude approaches the equilibrium values, the spinodal decomposition process enters the transition stage, characterized by a decreasing interfacial thickness and an increasing Lm(t). Once the interfacial profile equilibrates, a crossover to the final stage occurs. Subsequent growth of L(t) leaves the morphology unaffected as evidenced by a universal structure factor. These findings are discussed in the context of the current theory and are compared with prior studies involving polymer–polymer and simple liquid mixtures.
Simulation of aggregating colloids in shear flow. II
Authors: Chen D. , Doi M.
Published: July 2002

Three-dimensional computer simulation is carried out for the aggregation process of colloids using the sticky sphere model proposed in the previous paper. Time evolution of the structure of aggregates and macroscopic viscosity is studied when a shear flow is started in a suspension of the sticky spheres. In the transient state, the viscosity is found to increase in sigmoidal manner in agreement with real experiments. This phenomena is shown to be caused by the cooperative aggregation. In the steady state, it is found that (i) as the volume fraction increases, the structure transforms from compact cluster to loose network; (ii) the infinite network appears above the volume fraction about 0.1. and this value is rather insensitive to the shear rate; (iii) below and above the percolation transition, the steady state viscosity depends on the shear rate γ̇ in the same power law.
Temperature evolution of single particle correlation functions of liquid water
Authors: Frattini R. , Ricci M. A. , Ruocco G. , Sampoli M.
Published: July 2002

Molecular dynamics simulations of liquid water at various temperatures is reported. The autocorrelation functions of linear and angular velocities and of the first and second rank rotation matrices are studied and compared with previous data obtained using different potential models. Comparisons with the results of molecular dynamics simulations of heavy water and hydrogen sulfide are also shown.
Electronic structure determination and dynamical properties of iron (II)-guanosine-5′-monophosphate complex via mossbauer and magnetic susceptibility measurements
Authors: Alabart J. R. , Moreno V. , Labarta A. , Tejada J. , Molins E.
Published: July 2002

The electronic structure of the ferrous ion in the Fe(II)-5′-GMP complex has been determined by means of least-squares fittings of the temperature dependence of both the quadrupole splitting and the magnetic susceptibility experimental data, using a crystal-field approximation. The resulting low lying energy levels in order of increasing energy were 5B2, 1A1, 5E, and 3E, but after applying the rhombic perturbation, the rhombic-split 5E term became the ground state due to a quite large rhombic distortion D=−2236 cm−1. The set of crystal-field parameters obtained from the fit has been correlated with the molecular structure of the Fe(II)-5′-GMP derivative. The great value of the ε2 parameter is indicative of a strong N7–Fe interaction while the values of the ε3 parameter and the rhombic distortion are in agreement with a large distortion in the octahedron plane. Finally, the Debye temperature of the solid has been derived from the thermal variation of the Lamb–Mössbauer factor and has been used to calculate the second-order Doppler shift of the ferrous cation which, together with the assumption of a linear temperature dependence for the intrinsic isomer shift, reproduces the isomer shift experimental data.
Electronic structure and chemical bonding of the first row transition metal dichlorides, MnCl2, NiCl2, and ZnCl2: A high resolution photoelectron spectroscopic study
Authors: Wang Lai-Sheng , Niu B. , Lee Y. T. , Shirley D. A.
Published: July 2002

High resolution He i (584 Å) photoelectron spectra of ZnCl2, MnCl2, and NiCl2 were measured using a high temperature supersonic molecular beam source. In ZnCl2, vibrational structures were resolved, and spectroscopic constants were derived for the observed molecular ion states. A single ν1 vibrational progression was observed for the C 2Σ+g state of ZnCl+2. A Franck–Condon factor calculation allowed us to obtain a Zn–Cl equilibrium bond length increase of 0.095(5) Å and a ν1 vibrational frequency of 290(8) cm−1. For the open-shell molecules, MnCl2 and NiCl2, no vibrational structure could be resolved because of their very low bending frequencies. Transitions from the ligand orbital and metal d orbital ionizations were clearly resolved, with those of the d orbitals having considerably narrower band widths. Even though many final states are expected for ionization of the open-shell d orbitals, only a few states were observed. This was explained in MnCl2 by the one-electron spin selection rule: Sf=Si ±1/2. Besides the spin selection rule, a propensity toward high spin was proposed to account for the spectrum of NiCl2. From the metal d orbital and ligand orbital splittings, the degree of covalent bonding was inferred to be in the order of: MnCl2>NiCl2>ZnCl2.
First observation of alkyne radical anions by electron spin resonance spectroscopy: Hexyne/n-hexane mixed crystals
Authors: Matsuura Kaoru , Muto Hachizo
Published: July 2002

The radical anions of alkynes have been first observed by electron spin resonance spectroscopy following alkene anions previously studied. Hexyne radical anions were formed in 1-, 2-, or 3-hexyne/n–hexane mixed crystals irradiated at 4.2 or 77 K. The characters of the anions were as follows; (a) the α-proton hyperfine coupling is very large (∼4.5 mT for the 1-hexyne anion), (b) the β-proton couplings are very small (∼1.0 mT for C–Hβ proton with the conformational angle of 0°), and (c) the radicals show a negative g shift (2.0014). From these observations, it was found that the anions have a nonlinear(bent) molecule structure in the anticonfiguration (trans C–C≡C–C) with the bend angle ∼60°, and that the unpaired electron orbital is approximately composed of the anticombination of the sp2 hybrid orbitals of the C≡C carbon atoms. A discussion based on complete neglect of differential overlap (CNDO) molecular orbital (MO) calculations was given for the observed negative g shift, which was shown to be characteristic of the alkyne anions which have a high-lying unpaired electron orbital and an antibonding 2p–2p π carbon orbital just above it on the upper energy side.
Monte Carlo simulation of microstructural transitions in surfactant systems
Authors: Larson R. G.
Published: July 2002

In a lattice model of mixtures of idealized surfactant, oil, and water molecules, the microseparation of hydrophobic components (oil and surfactant tails) from hydrophilic ones (water and surfactant heads) is simulated by a Monte Carlo technique. In water, symmetric surfactants, i.e., with heads as long as the tails, achieve lamellar or hexagonal-cylindrical order as the temperature is reduced; the lamellae and cylinders form at surfactant concentrations that are similar to the concentrations at which symmetric block copolymers mixed with homopolymers have been found to form these structures. The lamellae containing tails can have many holes; as the temperature is reduced the holes attain hexagonal order within each layer. At low concentrations in water, symmetric surfactants form spherical micelles; the size distribution of these is computed, as well as the critical micelle concentration. When the surfactant tail is larger than the head, the micelles are cigar shaped or cylindrical. Cylindrical micelles can intersect each other to form a bicontinuous phase. Other disordered bicontinuous phases, including a symmetric spongelike phase, are observed, and the validity of film theories for these phases is examined.
Collisional energy transfer in highly excited molecules: Calculations of the dependence on temperature and internal, rotational, and translational energy
Authors: Clarke David L. , Oref I. , Gilbert Robert G. , Lim Kieran F.
Published: July 2002

Classical trajectory calculations of the rate of collisional energy transfer between a bath gas and a highly excited polyatomic method, and the average energy transferred per collision, as functions of the bath gas translational energy and temperature, are reported. The method used is that of Lim and Gilbert [J. Phys. Chem. 94, 72 (1990)], which requires only about 500 trajectories for convergence, and generates extensive data on the collisional energy transfer between Xe and azulene, as a function of temperature, initial relative translational energy (E′T), and azulene initial internal energy (E′). The observed behavior can be explained qualitatively in terms of the Xe interacting in a chattering collision with a few substrate atoms, with the collision duration being much too brief to permit ergodicity but with a general tendency to transfer energy from hotter to colder modes (both internal and translational). At thermal energies, trajectory and experimental data show that the root-mean-squared energy transfer per collision, 〈ΔE2〉1/2, is relatively less dependent on E′ than the mean energy transfer 〈ΔE〉.

The calculated temperature dependence is weak: 〈ΔE2〉1/2∝T0.3, corresponding to 〈ΔEdown〉∝T0.23. Values for the calculated average rotational energy transferred per collision (data currently only available from trajectories, and required for falloff calculations for radical–radical and ion-molecule reactions) are of the order of kBT, and similar to those for the internal energy; there is extensive collision-induced internal-rotational energy transfer. The biased random walk ‘‘model B,’’ as discussed in text, is found to be in accord with much of the trajectory data and with experiment. This suggests that energy transfer is through pseudorandom multiple interactions between the bath gas and a few reactant atoms; the ‘‘kick’’ given by the force at the turning point of each atom–atom encounter governs the amount of energy transferred. Moreover, a highly simplified version of this model explains why average energies transferred per collision for simple bath gases have the order-of-magnitude values seen experimentally, an explanation which has not been provided hitherto.
Vibrational overtone spectroscopy of the 4νOH+νOH′ combination level of HOOH v
i
a sequential local mode–local mode excitation
Authors: Luo X. , Fleming P. R. , Rizzo T. R.
Published: July 2002

Sequential pumping of the local OH stretch vibrations in hydrogen peroxide using infrared-optical double resonance permits spectroscopic access to the 4νOH+νOH′ combination level. Analysis of the rotationally resolved vibrational overtone spectra generated by this technique determines approximate rotational constants for this level and a value of 17 051.8±3.4 cm−1 for the O–O bond dissociation energy. The linewidths of individual zeroth-order rotational transitions increase sharply with increasing K and change from smooth Lorentzian profiles to clumps of individual lines. The K dependence of the clump widths suggests that an a-axis Coriolis interaction is the primary coupling mechanism between the zeroth-order bright state and dark bath states. As a function of increasing J, each clump coalesces into a smooth Lorentzian profile. We interpret this J dependence in terms of a model that includes rotationally induced vibrational coupling among zeroth-order dark states.
Molecular symmetry in methods for electron correlation
Authors: Hollauer Eduardo , Dupuis Michel
Published: July 2002

This paper is concerned with a procedure to obtain molecular integrals from symmetry unique integrals over atomic orbitals, a step found in many methods dealing with electron correlation in the context of the linear combination of atomic orbitals (LCAO) approach. This ‘‘symmetrization’’ procedure incorporated in the four-index transformation is based on symmetry coupling of the molecular orbitals to create charge distributions followed by an inverse transformation. The method involves the use of Clebsch–Gordan coefficients and works for all point groups, including those with complex representations. It is currently implemented in the hondo8 computer program. The relationship between this approach and other previously proposed approaches is shown. A formal expression of Pitzer’s equal contribution theorem is provided. Timing data related to the computer implementation of the proposed procedure is analyzed.
Two-photon absorption and second hyperpolarizability measurements in diphenylbutadiene by degenerate four-wave mixing
Authors: Sutherland R. L. , Rea E. , Natarajan L. V. , Pottenger T. , Fleitz P. A.
Published: July 2002

We have investigated two-photon absorption (TPA) in diphenylbutadiene at 532 nm using degenerate four-wave mixing (DFWM) of nanosecond laser pulses. We present a theory which describes the development of double-peaked phase conjugate pulses produced in DFWM due to TPA-induced gratings superimposed on the usual electronic third-order processes. Our analysis suggests a novel technique for measuring both the real and imaginary parts of the third-order susceptibility of a molecular solution, hence leading to a determination of the TPA cross section σ2 and second hyperpolarizability γ of the solute molecules. Applying this experimental technique to diphenylbutadiene in chloroform, we obtain σ2=(40±8)×10−50 cm−4 s/photon–molecule and ‖γ‖=(420±80)×10−36 esu for this diphenyl polyene. We show that these values are in agreement with related measurements in diphenylbutadiene, and that they are consistent with a two-photon resonance near 39 500 cm−1. The DFWM measurement technique we describe is very sensitive and should be applicable for measuring the two-photon spectra of a variety of molecules.
Observation of ArHF(3000) and its combination modes by laser-induced fluorescence
Authors: Chang Huan-C. , Klemperer William
Published: July 2002

Laser-induced fluorescence is used to obtain the second overtone spectrum of ArHF. The method exploits intracavity circulating power of a Ti–sapphire ring laser to pump the weakly bound complex generated in a supersonic slit jet from v=0 to v=3. Fundamental (Δv=−1) emission is monitored using an infrared PbS detector. Intense fluorescence allows recording of the rotationally resolved sub-Doppler spectra of (3000)←(0000), (3100)←(0000), and (3110)←(0000) transitions. We determine vibrational band origins of ν0=11 339.034 cm−1, 11 412.438 cm−1, 11 422.378 cm−1 and rotational constants of B=0.103 30 cm−1, 0.102 76 cm−1, 0.101 18 cm−1 for the (3000), (3100), and (3110) bands, respectively. Both the band origins and the rotational constants indicate that the weak Ar–HF van der Waals bond is strengthened as the HF stretch is vibrationally excited to higher states. All the observations are in near perfect accord with extrapolations of related constants in the HF stretching states of v=0–2.
Alternative expressions for the Fermi hole curvature
Authors: Dobson John F.
Published: July 2002

The Fermi hole curvature C(r,s) is defined as the Laplacian of the parallel-spin pair distribution, evaluated at zero separation r′=r for a pair of Fermions in a many-Fermion system. It has been used by a number of authors to discuss electron localization, properties of the exchange and correlation hole, and exchange and correlation energies of inhomogeneous electron gases. Here, the discussion of this quantity is extended in two directions. First, for the special case of a single-determinant many-electron state, it is shown that a previously derived macroscopic expression for C can be generalized in a simple fashion to apply to current-carrying states. Second, it is shown that a recently given interpretation of C(r,s), in terms of relative kinetic energy of pairs, is valid for a general many-Fermion state and is not limited to the single-determinant case investigated previously.
The [FHCl]− molecular anion: Structural aspects, global surface, and vibrational eigenspectrum
Authors: Klepeis Neil E. , East Allan L. L. , Császár Attila G. , Allen Wesley D. , Lee Timothy J. , Schwenke David W.
Published: July 2002

The [FHCl]− molecular anion has been investigated in detail by means of state-of-the-art ab initio electronic structure methods, including restricted Hartree–Fock (RHF), Mo/ller–Plesset perturbation theory (MP2–MP4), and coupled-cluster and Brueckner methods incorporating various degrees of excitation [CCSD, CCSD(T), BD, BD(T), and BD(TQ)]. The one-particle Gaussian basis sets ranged in quality from F[6s4p2d], Cl[10s7p2d], and H[4s2p] to F[18s13p6d4f], Cl[20s14p7d5f], and H[8s3p2d1f]. The first phase of the investigation focused on the prediction of thermochemical, spectroscopic, and bonding properties of [FHCl]− and the chemical interpretation thereof.

The final proposals for the geometric structure and binding energy of the complex are re(H–F)=0.963±0.003 Å, Re(H–Cl)=1.925±0.015 Å, and D0(HF+Cl−)=21.8±0.4 kcal mol−1. A Morokuma decomposition of the ion-molecule bonding gave the following electrostatic (ES), polarization (PL), exchange repulsion (EX), dispersion (DISP), and charge-transfer plus higher-order mixing (CT+MIX) components of the vibrationless complexation energy: −27.3 (ES), −5.2 (PL), +18.3 (EX), −4.5 (DISP), and −5.0 (CT+MIX) kcal mol−1. The second phase of the work involved the construction of a CCSD global surface from 208 and 228 energy points for linear and bent conformations, respectively, these being fit to rms errors of only 3.9 and 9.3 cm−1, respectively, below 8000 cm−1. The surface was represented by a flexible analytic form which reproduces the quartic force field at equilibrium, exhibits the proper asymptotic properties, and is generally applicable to ion-molecule systems. The final phase of the study entailed the determination of converged J=0 and J=1 variational eigenstates of the [FHCl]− surface to near the HF+Cl− dissociation threshold by employing Jacobi coordinates and vibrational configuration interaction expansions in terms of natural modals.

The fundamental vibrational frequencies given by the analysis were ν1=247, ν2=876, and ν3=2884 cm−1. The complete vibrational eigenspectrum was then analyzed in terms of several contemporary dynamical issues, including vibrational adiabaticity, anharmonic resonances, densities of high-lying states, and signatures of quantum ergodicity.
Time-resolved small-angle neutron scattering study of spinodal decomposition in deuterated and protonated polybutadiene blends. I. Effect of initial thermal fluctuations
Authors: Jinnai Hiroshi , Hasegawa Hirokazu , Hashimoto Takeji , Han Charles C.
Published: July 2002

Time-resolved small-angle neutron scattering (SANS) experiments have been performed on the self-assembling process of a binary mixture of deuterated polybutadiene and protonated polybutadiene at the critical composition. This mixture has an upper critical solution temperature type of phase diagram with the spinodal temperature at 99.2 °C. Specimens held in the single-phase state at an initial temperature (Ti) were quenched to a point inside the spinodal phase boundary at a final temperature (Tf) to induce phase separation via spinodal decomposition (SD). In order to examine the effect that thermal concentration fluctuations have on SD, three different initial temperatures, Ti=102.3 °C, 123.9 °C, and 171.6 °C, were chosen while Tf was fixed at −7.5 °C. The time-dependent SANS structure factor, S(q,t;Tf), showed clear scattering peaks corresponding to the early and intermediate stages of SD. The time changes in the wave number qm(t;Tf) and the intensity Sm(t;Tf) at the peak of S(q,t;Tf) followed different paths depending on the initial temperature. This fact evidences a definite effect of thermal concentration fluctuations on SD (i.e., a significant ‘‘memory’’ effect). A critical test of the linearized Cahn–Hilliard–Cook theory led to the conclusion that this theory can describe satisfactorily the early stage SD in the deep-quench region.
Open-shell restricted Hartree–Fock perturbation theory: Some considerations and comparisons
Authors: Lee Timothy J. , Rendell Alistair P. , Dyall Kenneth G. , Jayatilaka Dylan
Published: July 2002

A comparative study is presented of the various recently developed open-shell perturbation theories that are based on a restricted Hartree–Fock reference wave function. Included in this study are issues concerning spin contamination, implementational considerations, and numerical comparisons at the second-order of perturbation theory for equilibrium geometries, vibrational frequencies, and singlet–triplet energy differences. Based on all of these considerations, it is concluded that the z-averaged perturbation theory (ZAPT) method is to be preferred over the other recently devised spin–orbital perturbation theories, while the spin-free OPT2 method possesses some advantages and disadvantages relative to the ZAPT method. In particular, it is shown that OPT2 energies are not invariant to rotations among singly-occupied degenerate molecular orbitals.
Sub-Doppler spectroscopy of thioformaldehyde: Excited state perturbations and evidence for rotation-induced vibrational mixing in the ground state
Authors: Clouthier Dennis J. , Huang G. , Adam A. G. , Merer A. J.
Published: July 2002

High-resolution intracavity dye laser spectroscopy has been used to obtain sub-Doppler spectra of transitions to 350 rotational levels in the 410 band of the Ã 1A2–X̃ 1A1 electronic transition of thioformaldehyde. Ground state combination differences from the sub-Doppler spectra, combined with microwave and infrared data, have been used to improve the ground state rotational and centrifugal distortion constants of H2CS. The upper state shows a remarkable number of perturbations. The largest of these are caused by nearby triplet levels, with matrix elements of 0.05–0.15 cm−1. A particularly clear singlet–triplet avoided crossing in Ka′ = 7 has been shown to be caused by interaction with the F1 component of the 3162 vibrational level of the ã 3A2 state. At least 53% of the S1 levels show evidence of very small perturbations by high rovibronic levels of the ground state. The number of such perturbations is small at low J, but increases rapidly beyond J=5 such that 40%–80% of the observed S1 levels of any given J are perturbed by ground state levels. Model calculations show that the density and J dependence of the number of perturbed levels can be explained if there is extensive rotation-induced mixing of the vibrational levels in the ground state.
Excitation transfer from Kr(5s′,3P) and Kr(5s,3P2) atoms to 12CO and 13CO
Authors: Sadeghi N. , Colomb I. , Stoyanova J. , Setser D. W. , Zhong D.
Published: July 2002

Emission spectra have been used to characterize the excitation-transfer reactions from Kr(5s′,3P0) and Kr(5s,3P2) metastable atoms to 12CO and 13CO at 300 K. The most important products from the Kr(3P0) reactions are 12CO and 13CO(b 3Σ+,v′=0 and 1) and 12CO(a′ 3Σ+,v′=34 and 35) and 13CO(a′ 3Σ+,v′=35 and 36). The rotational distributions of the CO(a′ 3Σ+) and CO(b 3Σ+,v′=1) levels are cold, but the CO(b 3Σ+,v′=0) distribution is rotationally excited. The populations in the 12CO(a′,v′=34 and 35) levels are transferred to CO(b,v′=0) by collisions with He and the rate constants are 0.4–1.0×10−10 cm3 s−1. Emission spectra from the Kr(3P2) reaction identified 12CO(a′,v′=23–26) and 13CO(a′,v′=24–27) and CO(d 3Δ,v′=20 and 21) for both 12CO and 13CO as important products; the CO(d 3Δ,v′=20 and 21) states previously were identified by Tsuji and co-workers. The vacuum ultraviolet spectra from the Kr(3P2) reaction with 12CO and 13CO showed that CO(A 1Π) is a primary product and that it also is formed from CO(d 3Δ) and CO(a′ 3Σ+) by collisions with He and Ar. The Kr(3P2)+CO reaction also generates some unassigned CO triplet state emission. The propensity for formation of 3Σ+ states rather than the e 3Σ− state of CO is discussed. An improved transition dipole function for the CO(b 3Σ+–a 3Π) transition is presented in the Appendix.
Methods for molecular dynamics with nonadiabatic transitions
Authors: Coker D. F. , Xiao L.
Published: July 2002

We show how the dynamically nonlocal formulation of classical nuclear motion in the presence of quantal electronic transitions presented many years ago by P. Pechukas [Phys. Rev. 181, 166 (1969); 181, 174 (1969)] can be localized in time using time dependent perturbation theory to give an impulsive force which acts when trajectories hop between electronic surfaces. The action of this impulsive force is completely equivalent to adjusting the nuclear velocities in the direction of the nonadiabatic coupling vector so as to conserve energy, a procedure which is widely used in surface hopping trajectory methods [J. C. Tully, J. Chem. Phys. 93, 1061 (1990)]. This is the first time the precise connection between these two formulations of the nonadiabatic dynamics problem has been considered. We also demonstrate that the stationary phase approximation to the reduced propagator at the heart of Pechukas’ theory is not unitary due to its neglect of nonstationary paths. As such mixed quantum-classical evolution schemes based on this approximation are not norm conserving and in general must fail to give the correct branching between different competing electronic states. Tully’s phase coherent, fewest switches branching algorithm is guaranteed to conserve the norm. The branching between different alternatives predicted by this approach, however, may be inaccurate, due to use of the approximate local dynamics. We explore the relative merits of these different approximations using Tully’s 1D two state example scattering problems for which numerically exact results are easily obtained.
A coupled cluster study of the structures, spectroscopic properties, and isomerization path of NCS− and CNS−
Authors: Pak Youngshang , Woods R. Claude , Peterson Kirk A.
Published: July 2002

Three-dimensional near-equilibrium potential energy surfaces and dipole moment functions have been calculated for the X 1Σ+ ground states of NCS− and CNS−, using the coupled cluster method with single and double substitutions augmented by a perturbative estimate of triple excitations [CCSD(T)] with a set of 154 contracted Gaussian-type orbitals. The corresponding equilibrium bond lengths at their linear geometries are re(NC)=1.1788 Å and re(CS)=1.6737 Å for NCS−, and re(CN)=1.1805 Å and re(NS)=1.6874 Å for CNS−. The predicted equilibrium rotational constants Be of NCS− and CNS− are 5918.2 and 6282.7 MHz, respectively. The former agrees very well with the known experimental value (5919.0 MHz). Full three-dimensional variational calculations have also been carried out using the CCSD(T) potential energy and dipole moment functions to determine the rovibrational energy levels and dipole moment matrix elements for both NCS− and CNS−. The corresponding fundamental band origins (cm−1) ν1, ν2, and ν3 and their absolute intensities (km/mol) at the CCSD(T) level are 2060.9/306.1, 451.5/2.2, and 707.5/12.8, respectively, for NCS− and 2011.4/6.6, 343.7/2.3, and 624.9/0.2 for CNS−. The calculated ν1 (CN stretching) value for NCS− is in very good agreement with the experimental result, 2065.9 cm−1. The calculated dipole moments of NCS− and CNS− in their ground vibrational states are 1.427 and 1.347 D, respectively. The transition state geometry (saddle point) for the isomerization of NCS−→CNS− is predicted at the CCSD(T) level to be r(NC)=1.2044 Å, R(CS)=1.9411 Å and θ(∠NCS)=86.8°. Its calculated energy is 62.6 and 26.5 kcal/mol above the minima of NCS− and CNS−, respectively, including zero-point energy corrections. The structure of the NCS radical was also optimized at the same level of theory, yielding ion to neutral bond length shifts in excellent agreement with those derived from recent photoelectron spectroscopy experiments.
The heat of formation of HNO
Authors: Lee Timothy J. , Dateo Christopher E.
Published: July 2002

The heat of formation of HNO is determined from ab initio calculations to very high accuracy. Two independent approaches have been used to verify the reliability. The singles and doubles coupled-cluster approach that includes a perturbational estimate of the effects of connected triple excitations, denoted CCSD(T), has been used in conjunction with very large one-particle basis sets that include up to g functions. Correlation of the core electrons has also been investigated. The two approaches agree to within 0.4 kcal/mol, and our best estimate for ΔHf,0o (ΔHf,298o) is 26.7±0.8 kcal/mol (26.0±0.8 kcal/mol). This value is 2.2 kcal/mol larger than the currently accepted experimental value, indicating an error in the experimental determination.
Overtone spectroscopy in nitrous oxide
Authors: Campargue Alain , Permogorov Dmitri , Bach Mohamed , Abbouti Temsamani Mohammed , Vander Auwera Jean , Herman Michel , Fujii Masaaki
Published: July 2002

The near infrared and visible absorption spectrum of nitrous oxide 14N216O has been recorded by Fourier transform absorption spectroscopy, between 6500 and 11 000 cm−1, and by Intracavity Laser Absorption Spectroscopy, between 11 700 and 15 000 cm−1. Nineteen new bands are observed and, altogether, 34 cold and 10 hot bands are rotationally analyzed. The related upper term values, vibrational assignments, and principal rotational constants, as well as the relative band intensities are quantitatively discussed in terms of the formation of vibrational clusters, on the basis of the effective Hamiltonian developed by J. L. Teffo, V. I. Perevalov and O. M. Lyulin [J. Mol. Spectrosc. 168, 390 (1994)].