bluuebunny/crossref_metadata_2025 huggingface dataset:
Intermolecular potential and rovibrational levels of Ar–HF from symmetry-adapted perturbation theory
Authors: Lotrich Victor F. , Williams Hayes L. , Szalewicz Krzysztof , Jeziorski Bogumil , Moszynski Robert , Wormer Paul E. S. , van der Avoird Ad
Published: July 2002

A two-dimensional intermolecular potential energy surface for Ar–HF has been calculated using the many-body symmetry-adapted perturbation theory (SAPT). The H–F distance was kept constant at its equilibrium value. The interaction energies have been computed using an spdfg-symmetry basis optimized for intermolecular interactions. In addition, the dispersion and induction energies have been calculated in a few progressively larger basis sets to determine the basis set convergence and validity of the asymptotic scaling of those components. Converged results for the dispersion energy have been obtained by using a large basis set containing spdfgh-symmetry orbitals. The ab initio SAPT potential agrees well with the empirical H6(4,3,2) potential of Hutson [J. Chem. Phys. 96, 6752 (1992)], including a reasonably similar account of the anisotropy. It predicts an absolute minimum of −207.4 cm−1 for the linear Ar–HF geometry at an intermolecular separation of 6.53 bohr and a secondary minimum of −111.0 cm−1 for the linear Ar–FH geometry at an intermolecular separation of 6.36 bohr. The corresponding values for the H6(4,3,2) potential are −211.1 cm−1 at an intermolecular separation of 6.50 bohr and −108.8 cm−1 at an intermolecular separation of 6.38 bohr. Despite this agreement in the overall potentials, the individual components describing different physical effects are quite different in the SAPT and H6(4,3,2) potentials. The SAPT potential has been used to generate rovibrational levels of the complex which were compared to the levels predicted by H6(4,3,2) at the equilibrium separation. The agreement is excellent for stretch-type states (to within 1 cm−1), while states corresponding to bending vibrations agree to a few cm−1. The latter discrepancies are consistent with the differences in anisotropies of the two potentials.
Canonical flexible transition state theory revisited
Authors: Robertson Struan H. , Wagner Albert F. , Wardlaw David M.
Published: July 2002

A simple formula for the canonical flexible transition state theory expression for the thermal reaction rate constant is derived that is exact in the limit of the reaction path being well approximated by the distance between the centers of mass of the reactants. This formula evaluates classically the contribution to the rate constant from transitional degrees of freedom (those that evolve from free rotations in the limit of infinite separation of the reactants). As a result of this treatment, the formula contains the product of two factors: one that exclusively depends on the collision kinematics and one that exclusively depends on the potential energy surface that controls the transitional degrees of freedom. This second factor smoothly varies, in the classical limit, from harmonic oscillator to hindered rotor to free rotor partition functions as the potential energy surface varies from quadratic to sinusoidal to a constant in its dependence on the relative orientation angles of the fragments. An application to the recombination of CH3+H essentially demonstrates exact agreement with a previous flexible transition state theory study in which all integrals are carried out numerically. The simple formulas presented in this paper allow the classical inclusion of large amplitude motion of arbitrary complexity in the determination of the canonical rate constant for reactions whose reaction path is dominated by the distance between the centers of mass of the reactants.
Relativistic corrections to molecular dynamic dipole polarizabilities
Authors: Kirpekar Sheela , Oddershede Jens , Jensen Hans Jo/rgen Aagaard
Published: July 2002

Using response function methods we report calculations of the dynamic isotropic polarizability of SnH4 and PbH4 and of the relativistic corrections to it in the random phase approximation and at the correlated multiconfigurational linear response level of approximation. All relativistic corrections obtained from the use of the Darwin and mass-velocity operators to first order are included at both levels of approximation. We find that correlation and relativistic contributions are not even approximately additive for the two molecules. The importance of the relativistic corrections is smallest in the correlated calculations, as has also been observed for other properties. For SnH4 the correlation contribution and the pure relativistic correction are of the same order of magnitude, whereas for PbH4 the relativistic correction becomes more important than the correlation contribution. We report estimated Cauchy moments, obtained from fitting the dispersion of the calculated corrections as a function of ω2. The frequency dependence of the nonrelativistic polarizability is most pronounced at the correlated level, mainly due to lower excitation energies in the multiconfigurational calculations than those in the random phase approximation.
A theory for time correlation functions in liquids
Authors: Cao Jianshu , Voth Gregory A.
Published: July 2002

A theory for time correlation functions in liquids is developed based on the optimized quadratic approximation for liquid state potential energy functions. The latter approximation leads to the rigorous mathematical definition of inherent structures in liquids and their vibrational fluctuations, in turn leading to the concept of inherent normal modes in the liquid state. These normal modes are called ‘‘optimized normal modes.’’ Unlike normal modes based on instantaneous liquid state configurations, the optimized normal modes are stable, having real-valued frequencies, and each inherent liquid state structure has a different set of modes associated with it. By including a single phenomenological decay function which captures the average transition rate between the different sets of normal modes, velocity time correlation functions and dynamical friction kernels for solute bonds can be predicted in good agreement with direct molecular dynamics simulation results.
Thermal diffusivity and structural relaxation in a supercooled liquid by a holographic grating technique
Authors: Köhler W. , Fytas G. , Steffen W. , Reinhardt L.
Published: July 2002

The decay of a spatial transient temperature grating created within the molecular glass former phenolphthalein-dimethylether (PDE) relates to the hydrodynamic heat diffusion and the dynamics of local density fluctuations. The characteristics of the latter process agree well with the structural α relaxation observed by photon correlation and dielectric spectroscopy near and above the glass transition temperature Tg. When the α-relaxation time approaches the experimental timescale set-in by the thermal diffusivity the specific heat undergoes a dispersion. In either side of this dynamic regime, heat diffusion and α relaxation are independent modes. ‘‘Two fluid’’ models of the vitreous state conform to the experimental observations.
Multidimensional tunneling in terms of complex classical mechanics: Wave functions, energy splittings, and decay rates in nonintegrable systems
Authors: Takada Shoji
Published: July 2002

A semiclassical theory of multidimensional tunneling is formulated to calculate the tunneling wave function, energy splitting in a double well and decay rate constant from a metastable state. First, the tunneling wave function is calculated by analytic continuation of a quantized torus prepared in analytic forms using either the Chapman–Garrett–Miller method or the Birkhoff–Gustavson normal form method. For a weakly nonintegrable system, tunnelings are confirmed to be classified into two qualitatively different domains; pure tunneling in the I region and mixed tunneling in the C region. Semiclassical wave functions agree with quantum mechanical ones within a few percent both in classically allowed and tunneling regions. Breakdown of this simple picture is exemplified for cases of relatively strong couplings. Second, expressions of the tunneling energy splitting in a double well potential and the decay rate from a metastable state are derived. The wave function near a well formulated above is connected with the semiclassical Green’s function in the deep tunneling region. The latter is expressed by complex trajectories which start from the complex quantized torus. A preliminary numerical comparison with the quantum mechanical value is also done for the energy splitting.
Spatially nonlocal fluctuation theory of rapid chemical reactions
Authors: Molski Andrzej , Keizer Joel
Published: July 2002

Using ideas from statistical nonequilibrium thermodynamics we develop a spatially nonlocal theory of the influence of diffusion on rapid chemical reactions. The new approach generalizes the hydrodynamic (local) theory by including elementary events that influence both diffusion and reaction on molecular length scales and eliminates problems in the local theory that occur at short times. Examples of the nonlocal approach are given for several reaction schemes, including a simple reversible reaction A+B■C, and single species reactions of the form A+A→products. For low densities, the present theory is equivalent to the usual Smoluchowski method. In this regime, an intermediate version of the theory, including only nonlocal effects due to reaction, is compared to the full nonlocal theory and to the earlier hydrodynamic level theory and is shown to produce results for the time dependence of the radial distribution function that are similar at low density to the full theory. The application of the new approach at higher densities and to lower dimensions is described briefly.
An electron spin resonance investigation of the 12C11B12C, 12C11B13C, and 13C11B13C radicals in neon, argon, and krypton matrices: Comparison with ab initio calculations
Authors: Knight Lon B. , Cobranchi Scott , Earl Edward , McKinley Allan J.
Published: July 2002

The 11 electron CBC radical has been generated by the pulsed laser vaporization of elemental carbon–boron mixtures and trapped in neon, argon and krypton matrices for detailed electron spin resonance (ESR) studies. Extensive comparisons of the experimental nuclear hyperfine A tenors for carbon and boron were made with a variety of ab initio computational results that involved different levels of theory and basis sets. These new ESR results agree with recent vibrational studies of CBC that show it to have a nonlinear symmetric geometry with a 2A1 electronic ground state. These ESR results provide a description of the singly occupied molecular orbital. The spin density resides primarily on boron in 2s and 2pz orbitals, however a complete resolution of the 13C hyperfine structure does show that approximately 20% resides on the carbon atoms.
Statistical properties of resonance states in a double Morse potential well
Authors: Dallwig S. , Weese I. , Weiss Th. , Schlier Ch.
Published: July 2002

The resonances of a collinear ABA molecule with a double Morse potential well are computed by means of complex scaling. For each symmetry, this system has about 40 resonances in 10 channels. The extraction of the resonance parameters from the raw data has been automatized, allowing the computation of all resonances for more than 40 values of the kinetic coupling parameter δ=mA/(mA+mB). Mean and standard deviation of the width Γ as function of δ show an interesting structure, which corresponds to the behavior of the classical unimolecular decay rates determined from trajectory calculations. Unexpectedly, we noticed that Fermi’s golden rule for Γ breaks down already for values of δ near 0.1.
Quantum scattering calculations on the NH3+OH→NH2+H2O reaction
Authors: Nyman Gunnar
Published: July 2002

Quantum scattering calculations on the NH3+OH→NH2+H2O reaction have been performed at energies up to 0.8 eV. The rotating bond approximation is used, treating NH2 as a pseudoatom. The OH rotation and a reactive N–H stretch of NH3 are treated explicitly as well as the bending motion and one OH local stretch vibration of H2O. A reduced dimensionality potential energy surface is developed. It has accurate reactant and product rovibrational energy levels for the modes explicitly treated in the scattering calculations and incorporates the zero point energy of the other modes. Quantized transition states gating the flux are found and mode selectivity is observed. Reactants in their ground rovibrational states produce mainly ground state H2O and vibrationally excited NH3 produces mainly vibrationally excited H2O. Rate constants are obtained using an adiabatic approach to account for all degrees of freedom not explicitly treated in the scattering calculations. Tunneling makes a dominant contribution to the rate constants, which are in reasonable agreement with previous theoretical and experimental work.
The tunneling contributions to optical coherence in femtosecond pump–probe spectroscopy of a three level system
Authors: Chachisvilis Mirianas , Sundström Villy
Published: July 2002

We present theory and observations to show that the proper description of the interaction of femtosecond light pulses resonant with both transitions of a three level system must consider additional excitation pathways which do not include population creation on the intermediate electronic level. This leads to the extra tunneling contributions to the pump–probe signal around zero delay time when the light pulse is centered close to the isosbestic point, i.e., the wavelength where the absorption from the lower and higher transitions are equal. We use the density matrix approach in the perturbative limit to simulate the pump–probe signal under various conditions. In the conventional calculation of transient anisotropy as measured by pump–probe spectroscopy, the assumption is made that the system is excited by the pump pulse only. However, this is not the case at zero delay time when pump and probe pulses overlap. The mutual coherence of pump and probe pulses derived from the same laser source makes it necessary to include nonsequential contributions to the signal. We show that some of these terms can have distinct anisotropic properties and lead to noticeable deviations of the zero delay anisotropy from the long time value. Effects of the pulse wavelength and the relative orientation of the transition dipole moments on the transient anisotropy are examined in detail. Application is made to our earlier femtosecond pump–probe experiments on the light harvesting pigment proteins of Rhodobacter sphaeroides, whereby we present a more elaborate explanation of the short lived induced absorption signal at negative delay times.
Ultrafast quantum dynamics and resonance Raman spectroscopy of photoexcited I2(B) in large argon and xenon clusters
Authors: Jungwirth Pavel , Fredj E. , Gerber R. Benny
Published: July 2002

The early quantum dynamics following the B(3Π0u+)←X photoexcitation of I2 in large rare gas clusters is studied and the resonance Raman spectrum of these systems is calculated by a novel time-dependent quantum mechanical simulation approach. The method used is the classically based separable potential (CSP) approximation, in which classical molecular dynamics simulations are used in a first step to determine an effective time-dependent separable potential for each mode, then followed by quantum wavepacket calculations using these potentials. In the simulations for I2(Ar)n and I2(Xe)n, with n=17, 47, all the modes are treated quantum mechanically. The Raman overtone intensities are computed from the multidimensional time-dependent wavepacket for each system, and the results are compared with experimental data on I2 in Ar matrices and in liquid Xe. The main findings include: (i) Due to wavepacket dephasing effects the Raman spectra are determined well before the iodine atoms hit the rare gas ‘‘wall’’ at about 80 fs after photoexcitation. (ii) No recurrencies are found in the correlation functions for I2(Ar)n. A very weak recurrence event is found for I2(Xe)n. (iii) The simulations for I2(Ar)17 (first solvation layer) and for I2(Ar)47 (second solvation shell) show differences corresponding to moderate cluster size effects on the Raman spectra. (iv) It is estimated that coupling to the B″(1Π1u) state or to the a(1g) state have a small effect on the Raman intensities. (v) For I2(Ar)47, the results are in very good quantitative agreement with I2/Ar matrix experiments. The I2(Xe)n results are in qualitative agreement with experiments on I2 in liquid Xe. The reported calculations represent a first modeling of resonance Raman spectra by quantum dynamical simulations that include all degrees of freedom in large systems, and they demonstrate the power of the CSP method in this respect.
DNA in a liquid-crystalline environment: Tight bends, rings, supercoils
Authors: Odijk Theo
Published: July 2002

The entropy of tightly bent DNA is investigated in a variety of problems: closure probabilities, hairpin formation, nicked coils, plectonemic supercoiling, all in states with liquid-crystalline order. A new semiclassical method is presented for deriving the Green function of a tightly curved wormlike chain. Precise estimates for the entropy arising from undulations are given for tightly bent DNA in weak, intermediate, and strong nematic fields. A formal statistical mechanical analysis is outlined for hairpins and supercoils. The elongation of closed DNA without twist is computed in strong nematic fields. A scaling theory is given for a liquid crystal of untwisted DNA rings in which nematic order and ring elongation are self-consistently coupled. The elongation of plectonemic supercoils is evaluated for weak and strong nematic fields. The pitch of a cholesteric phase of plectonemic or loose supercoils is shown to be directly related to their writhe.
A transition from shear-induced order to shear-induced disorder in block copolymers
Authors: Balsara N. P. , Dai H. J.
Published: July 2002

The effect of steady shear flow on a weakly ordered block copolymer material was examined by in situ small angle neutron scattering. We demonstrate this material undergoes a transition from shear-induced ordering to shear-induced disordering at a fixed temperature above the quiescent disordering temperature. Current theories, which assume that the primary effect of shear flow on block copolymers is to suppress disordered concentration fluctuations, can only predict shear-induced ordering. Our results indicate that the coupling between flow and microstructure is not unique and that it can change qualitatively with shear rate.
The calculation of electrostatic potentials for periodic charge distributions
Authors: Johnson Elijah
Published: July 2002

A method for obtaining the Coulomb energy of a lattice of charges is presented. The method is used here to find the Madelung constant of a number of crystals. The method is simpler than the Ewald summation method. It is based on the use of a basis set of lattices. Any lattice may be represented by a linear combination of these basis lattices.
Hydrodynamic- and viscosity-radius expansion factors of polymer chains with excluded volume: Monte Carlo methods
Authors: Yoshizaki Takenao , Yamakawa Hiromi
Published: July 2002

In order to examine the effect of fluctuating (nonpreaveraged) hydrodynamic interaction on the hydrodynamic-radius expansion factor αH and also the viscosity-radius one αη, a Monte Carlo simulation study is made on the basis of the polymethylenelike rotational isomeric state chain in the Zimm rigid-body ensemble approximation. All the results obtained are consistent with the recent finding (by Yamakawa’s group) that the experimental values of αH are appreciably smaller than the values predicted by any available theory and rather agree with the theoretical and experimental values of αη, indicating that the effect is significant on αH but is negligibly small on αη if any. The present results along with the recent experimental ones also confirm the validity of the quasi-two-parameter scheme for αH and αη as well as for the gyration-radius expansion factor αS; i.e., these expansion factors are functions only of the scaled excluded-volume parameter z̃ defined in the Yamakawa–Stockmayer–Shimada theory of the excluded-volume effect in the Kratky–Porod wormlike chain and the helical wormlike chain.
Highly vibrationally excited 12C2H2 in the X̃ 1∑+g state: Complementarity of absorption and dispersed fluorescence spectra
Authors: Temsamani Mohammed Abbouti , Herman Michel , Solina Stephani A. B. , O’Brien Jonathan P. , Field Robert W.
Published: July 2002

Direct absorption and dispersed fluorescence (DF) spectra sample the 12C2H2 X̃ 1Σ+g potential energy surface via profoundly different zero-order bright states. Despite the complementary nature of the data sets, a polyad model based on the approximate quantum numbers, Ns=v1+v2+v3, Nr=5v1+3v2+5v3+v4+v5, l=l4+l5, accounts for the energies (±0.35 cm−1) and relative intensities for all transitions into pure bending levels (Ns=0) at Evib≤12 000 cm−1 and Nr≤15, l=0 and 2. The parameters that define this model are obtained by fitting 41 pure bending levels, of which 9 are J=0 energies derived from components of the [Ns=0, Nr=8, 10, 12, l=0, g] polyads observed in the DF spectrum. This polyad model provides a basis for extrapolating a description of otherwise indescribably complex spectra and dynamics to the Evib≊16 000 cm−1 region of the barrier to acetylene ↔ vinylidene isomerization, and could therefore provide a basis for detecting the pattern-breaking signature of the onset of isomerization.
Effects of external electric field on semidilute polymer solutions probed by dynamic light scattering
Authors: Wang C. H. , Huang Q. R.
Published: July 2002

Dynamic light scattering studies of polystyrene (PS) in tetrahydrofuran (THF) in 1,4-dioxane (DO) and in THF/DO mixed solvent, subject to an external dc field, have been carried out. In THF the rate constant for the decay of the intensity autocorrelation function (IACF) is found to increase with increasing the external applied electric field; in DO, the electric field is found to have only a negligible effect on the IACF; in the mixed solvent it exhibits an intermediate effect, depending on the THF content. A hydrodynamic theory is proposed to account for the observed electric field induced effect in THF.
Hopping reactions of charged particles
Authors: Burshtein A. I. , Fedorenko S. G.
Published: July 2002

The hopping theory of transfer reactions is essentially generalized to account for the electrostatic interaction between reactants. An application is made for a solvated electron that may be trapped by ions in the course of a random walk in liquid solutions. The hopping reaction constant differs essentially from the diffusional constant but exhibits a similar dependence on the Onsager radius: reactions with anions are strongly suppressed while reactions with cations are significantly accelerated.
Constant pressure simulations of lattice gas models
Authors: Pendzig Peter , Dieterich Wolfgang , Nitzan Abraham
Published: July 2002

A method for constant pressure Monte Carlo simulations in lattice gas models is described. The simulation box is placed between two hard walls with fluctuating distance, and periodic boundary conditions are applied in the perpendicular directions. Continuous volume fluctuations in the bounded direction are made possible by introducing a generalized volume, which interpolates between the discrete values that correspond to the given lattice. This is achieved by using a surface potential variable which makes the lattice surface layer next to the hard wall gradually less accessible to particle occupation. The method is applied to the equation of state of noninteracting lattice gas models, where exact results are available for comparison, and also to less trivial models of interacting point-particles and athermal lattice chains, for which the quasichemical approximation (QCA) provides reliable results to compare with. For the chain simulations the method can be used in conjunction with the configuration biased Monte Carlo procedure in order to enhance its performance. However, since the Monte Carlo moves can be chosen to fit any desired kinetic model, our method can be used not only to generate constant pressure equilibrium ensembles, but also in the context of dynamic Monte Carlo studies. The center of mass diffusion in dense systems of athermal chains is investigated as an example. In all our static applications the method performs very well in comparison with exact or with QCA results, provided that the system size is large enough in the bounded direction. For small systems finite size effects are observed and analyzed, suggesting potential applications in the study of confined systems.
Laser-induced fluorescence spectrum of the FCO radical
Authors: Williams Bradley A. , Fleming James W.
Published: July 2002

Laser-induced fluorescence spectra of FCO are reported between 29 000 cm−1 and 32 800 cm−1. FCO was prepared by three separate procedures: photolysis of CF2O and C2F2O2, and photolysis of F2 in the presence of CO. The observation of the same spectral features from all three production schemes confirms the assignment of FCO as the spectral carrier. Although the LIF spectrum lies in the same wavelength region as the UV absorption spectrum, the two spectra do not have the same appearance and so represent different upper states. The LIF spectrum is assigned as a transition to the A″ Renner–Teller component of a linear 2Π state predicted by ab initio calculations. The spectrum shows a progression of bands at approximately 430 cm−1 intervals, in good agreement with the predicted spacing of bending levels in the 2Π state. The lower frequency stretching mode ν1 occurs at approximately 960 cm−1. The vibronic bands are strongly degraded to the red, consistent with the calculated geometry of the linear state but not the bent A state to which the absorption spectrum in this region is assigned. Resolved emission from the longest wavelength prominent bandhead at 29 872 cm−1 shows progressions in the C–O stretch and bend modes of the ground state. The fluorescence lifetime of this band extrapolated to zero pressure is 95±4 ns, while that of the band at 31 136 cm−1 is 63±5 ns.
Analysis of Rayleigh light scattering spectra of pivalic acid in plastic and liquid phase
Authors: Kubicki J. , Wróż T. , Naskręcki R.
Published: July 2002

The shape and integral intensity of the depolarized component of Rayleigh light scattering on molecules of pivalic acid has been analyzed in the temperature range covering partly plastic and liquid phase. The occurrence of fast reorientations of pivalic acid monomers, earlier searched for by other authors, has been confirmed. This kind of motion was observed only in the liquid phase. The motion in the plastic phase is, as earlier established, a slow reorientation which involves breaking up of the double hydrogen bonds joining two adjacent molecules of pivalic acid and formation of a new bond with one of the twelve closest neighbours. The decay constant of the pure interaction-induced processes in the liquid and plastic phase as well as the squares of the effective anisotropy of the optical polarizability and anisotropy due to the pure interaction-induced processes were determined. A comparative analysis of integral intensity of the depolarized components at the transition from the plastic to the liquid phase, was performed for pivalic acid and succinonitrile.
Rotor-vibrator couplings in partially deuterated toluenes
Authors: Siebrand Willem , Zgierski Marek Z. , Zerbetto Francesco , Wójcik Marek J. , Boczar Marek , Chakraborty Tapas , Kofron William G. , Lim Edward C.
Published: July 2002

Fluorescence excitation spectra covering the origin–band region of the two partially methyl-deuterated toluenes (α-deuterotoluene, C6H5CH2D and α-dideuterotoluene, C6H5CHD2) are reported. The spectra, observed in a cold supersonic beam, show transitions between low methyl–rotor levels, and are analyzed in terms of vibrationally adiabatic torsional potentials with two-, four-, and sixfold periodicity. To interpret the resulting potential parameters, ab initio vibrational force field calculations are carried out at the four stationary structures of each isotopomer. The calculations yield accurate values for the adiabatic torsional barriers and qualitatively correct results for the vibrationally adiabatic potentials. Specifically, they show that the twofold potential term dominates in the partially methyl-deuterated isotopomers and that this component has the same magnitude but opposite sign in the mono- and dideuterocompounds. The potential parameters are interpreted in terms of nonbonded interactions between a CH bond of the methyl group and the phenyl ring; specifically, they represent the difference between the interactions of a methyl-CH and a methyl-CD bond with the ortho-CH bonds of the phenyl ring. Modeling of this interaction as a function of H–H separation in terms of a simple analytical expression shows that the repulsive potential is very soft and that individual H–H interactions are of the same order of magnitude as vibrational frequencies.
Simulations of hydrocarbon adsorption and subsequent water penetration on an aluminum oxide surface
Authors: de Sainte Claire P. , Hass K. C. , Schneider W. F. , Hase W. L.
Published: July 2002

Static and dynamic equilibrium properties of butane octane, and dodecane films adsorbed on α-Al2O3(0001) at a variety of coverages and temperatures, and the subsequent penetration of such films by 30 molecule water clusters are examined using classical molecular dynamics. Model potential functions are constructed from existing alkane united atom and “simple point charge” model water parameters, experimental alkane desorption energies and other available theoretical information. The adsorbed films exhibit a distinct layering parallel to the surface, and a pronounced densification, reduction in gauche defects and orientational ordering within the innermost layer. Strong surface corrugation allows molecules to rotate relatively freely about their long axes at intermediate temperatures and assists them in orienting their zig-zag planes perpendicular to the surface at lower temperatures. Only butane molecules show any tendency to tilt their long axes out of the first layer toward the second. (H2O)30 clusters are attracted toward the alumina surface and easily penetrate most of the adsorbed alkane films, either by displacing alkane molecules to more distant layers or causing them to pack more closely within existing layers. The molecules in the clusters tend to remain connected during penetration. Kinetic barriers to penetration become increasingly significant for higher alkane coverages, lower temperatures, and longer chains.
Eigenstates for internal rotors with angle dependent moment of inertia
Authors: Mellor W. E. , Kalotas T. M. , Lee A. R.
Published: July 2002

A new calculational method is outlined for the eigenstates of internal rotors characterized by angle-dependent moments of inertia. The procedure is applied to the 1,3-butadiene molecule giving good agreement with transitions calculated by standard techniques. Independent checks are included to justify the numerical accuracy of the outlined procedure.
Electron–electron coalescence and counterbalance densities for atoms in Hartree–Fock theory
Authors: Koga Toshikatsu , Matsuyama Hisashi
Published: July 2002

The electron–electron coalescence I(0) and counterbalance E(0) densities are probability densities of finding any two electrons, respectively, at the same position and at the reflection points in the three-dimensional space. For a single Slater determinant wave function, these electron-pair properties are shown to be exactly expressible in terms of the spin-traced one-electron density function ρ(r) and its orbital components ρi(r): I(0)=(1/4){〈ρ〉−ΔI} and E(0)=2{〈ρ〉−ΔE}, where 〈ρ〉 is the average electron density, and ΔI and ΔE are linear combinations of overlaps between two orbital densities, that depend on the electronic configuration and the LS multiplet state of the atom under consideration. For the atoms He through Ne in their experimental ground state, the explicit forms of ΔI and ΔE are derived, and the electron–electron coalescence and counterbalance densities obtained from the numerical Hartree–Fock calculations are discussed.
The photophysical behavior of 3-chloro-7-methoxy-4-methylcoumarin related to the energy separation of the two lowest-lying singlet excited states
Authors: Seixas de Melo J. , Becker Ralph S. , Elisei Fausto , Maçanita A. L.
Published: July 2002

The photophysical behavior of 3-chloro-7-methoxy-4-methylcoumarin (ClMMC) was studied as a function of the solvent and temperature. The radiative lifetime τF0=1/kF is essentially solvent independent and its value (≈4.6 ns) is totally commensurate with the fluorescence originating from a 1(π,π*) state as the lowest excited state. From the fluorescence data obtained in 24 solvents plus nine solvent mixtures, and the triplet formation quantum yields for three representative solvents, it was found that the internal conversion rate constant (kIC) dictates the photophysical behavior of ClMMC and changes of two orders of magnitude occur from nonpolar to polar solvents. From the temperature dependence (20 to −100 °C) of the fluorescence lifetimes in five solvents it was found that a change of the internal conversion rate constant of the same order of magnitude occured as above. The rate constants and the activation energies for the radiationless processes were determined. The results show that the reason for the dramatic variation of kIC is the fact that when the S2(n,π*) state is close lying to S1(π,π*), there is a decrease of the activation energy of the internal conversion process (S1∼∼→S0). Increasing solvent polarity increases the energy gap between these states, and decreases the magnitude of the effect. Decreasing the temperature to sufficiently low values, disables the deactivation channel. The interpretation of the foregoing results can be satisfied by either a model involving a fast equilibrium between two close lying S2 and S1 states or in terms of the so-called “proximity effect.”
Assessment for the mean value total dressing method: Comparison with coupled cluster including triples methods for BF, NO+, CN+, C2, BeO, NH3, CH2, H2O, BH, HF, SiH2, Li2, LiNa, LiBe+, NeH+, and O3
Authors: Garcı́a-Cuesta I. , Sánchez-Marı́n J. , Sánchez de Merás A. , Amor N. Ben
Published: July 2002

Limited previous experience with the mean value total dressing (MVTD) method had shown that MVTD energies for closed shell systems are generally better than CCSD(T) ones compared to FCI. The method, previously published as total dressing 2′(td-2′), is based on the single reference intermediate Hamiltonian theory. It is not a CC method but deals in a great part with the same physical effects that CC methods that incorporate amplitudes of triples such as CCSDT or its CCSDT-1n approaches. A number of test calculations comparing to diverse CC methods, as well as FCI and experiment when available, have been performed. The tests concern equilibrium energies in NH3 and CH2, equilibrium energies and distances in some diatomics (BF, NO+, CN+, C2, BeO), different bond breaking situations (H2O, BH, HF, SiH2) and spectroscopic properties of different bonding conditions (Li2, LiNa, LiBe+, NeH+, and O3). The results are in general closer to the full CCSDT ones in the equilibrium regions and close to CCSDT-1 along most dissociation curves. A few exceptions to this rule are analyzed with the help of an approach to MVTD that does not take into account the effects of linked quadriexcitations. Such analysis suggests the interest of improving the treatment of effects of linked triples in the MVTD model. The separate contributions of linked and unlinked triples and quadruples are also analyzed for some of the above diatomics representing different behaviors of bond breaking. The interest of such analysis is illustrated in the NeH+ molecule. The MVTD results show, in general, a high quality, provided that the nature of the correlation problem does not become largely multiconfigurational, as occurs in multiple bond dissociation or in the asymmetric stretching of ozone.
A three-dimensional reduction of the Ornstein–Zernicke equation for molecular liquids
Authors: Cortis Christian M. , Rossky Peter J. , Friesner Richard A.
Published: July 2002

The derivation of a three-dimensional integral equation for solute molecule-solvent site correlation functions is presented. The equation is obtained by averaging the Ornstein–Zernicke equation for molecular liquids over orientations of the solvent molecule consistent with one site of the solvent remaining at a fixed distance from a solute-based origin. The approach is similar to that adopted in the reduction leading to the reference interaction site model (RISM) equations but retains full three-dimensional information regarding the structure of the reference solute molecule. The proposed equation can be solved using three-dimensional HNC-like closures, of which three different forms are discussed. A formulation which allows the introduction of long range interactions through a renormalization of the equation is also presented. Applications to various molecular liquids indicate that the proposed theory provides pair correlation functions that are in better agreement with molecular dynamics simulations than those obtained using the extended RISM formulation. Furthermore, qualitative errors in the correlation functions, frequently seen in results from RISM calculations are completely eliminated through geometrical averaging of the Mayer function in the 3D HNC closure. Prospects for the development of a novel mean field theory of solvation are also discussed.
Sequential solvation of HCl in argon: High resolution infrared spectroscopy of ArnHCl (n=1,2,3)
Authors: Anderson David T. , Davis Scott , Nesbitt David J.
Published: July 2002

High-resolution near-infrared spectra of the vHCl=1←0 fundamental stretch in Ar2HCl and Ar3HCl have been characterized using a slit-jet infrared spectrometer. Analysis of the jet-cooled, rotationally resolved spectra (i) permits unambiguous identification of the cluster size, (ii) provides vibrationally averaged geometries in the vHCl=1 excited state, and (iii) allows the vibrational shift of the HCl chromophore to be measured as a function of the number of Ar atoms in the complex. The equilibrium structures of ArnHCl (n=1–3) clusters calculated using accurate Ar–Ar and Ar–HCl pair potentials are consistent with the vibrationally averaged structures inferred spectroscopically. The vibrational red-shifts for ArnHCl (n=1–3) reflect a near-linear dependence on the number of Ar atoms, which is qualitatively reproduced by simple classical calculations on vHCl=0 and 1 pairwise additive potential surfaces. Theoretical predictions of the ArnHCl red-shifts in a fcc lattice indicate good agreement with experimental matrix results. However, to achieve this asymptotic limit requires up to n≈54 Ar atoms; this underscores a clear sensitivity to non-nearest neighbor Ar–HCl interactions significantly outside the first solvation shell. Finally, for smaller ArnHCl clusters with only one solvation shell (n=12), the potentials predict an energetic preference for HCl in surface vs interior sites. Analysis indicates that this effect is predominantly due to Ar/HCl size mismatch, which destabilizes the nearest neighbor Ar shell for HCl solvated in the center of the cluster.
Three component spin echo generation by radiation damping
Authors: Augustine M. P. , Hahn E. L.
Published: July 2002

Exploration of the theory for spin–cavity coupling in inhomogeneously broadened spin ensembles shows that all three components of magnetization can be refocused. The conventional spin echo generated by two components of magnetization is a special case of this three component refocusing in the limit of negligible radiation damping. We demonstrate this effect by an analytical theorem, numerical simulation, and experiment.
Fourier transform infrared spectroscopy of UF6 clustering in a supersonic Laval nozzle: Cluster configurations in supercooled and near-equilibrium states
Authors: Tanimura Shinobu , Okada Yoshiki , Takeuchi Kazuo
Published: July 2002

The Fourier transform infrared spectra of UF6 clusters seeded in Ar were measured in a continuous supersonic Laval nozzle flow at various pressures. The spectra of the clusters showed completely different features when the total pressure P(total) was changed from 3.0 to 6.0 Torr at a fixed mole fraction of UF6, X(UF6)=0.08. From measurements of the temperature and UF6 monomer density in the nozzle flow, it was found that the clusters grew in a supercooled state, i.e., at a supersaturation ratio S≫1, when P(total)=3.0 Torr. In contrast, UF6 clusters grew in a near-equilibrium state, i.e., at S≈1, when P(total)=6.0 Torr. A comparison between the measured and calculated spectra revealed that the distribution of the potential energy of the cluster configurations was similar to an inverted population at P(total)=3.0 Torr, and similar to the Boltzmann distribution at P(total)=6.0 Torr. The fact that the distribution of the potential energy of the cluster configurations was similar to an inverted population in the supercooled state at P(total)=3.0 Torr was explained qualitatively on the basis of the RRK theory. The spectra measured at X(UF6)=0.04 support our explanation.
Distributions of rovibrational states of secondary product NO X 2Π from photodissociation of nitric acid at 193 nm
Authors: Leu Gen-Hou , Chen I-Chia
Published: July 2002

The distributions of vibrational, rotational, spin–orbit and lambda–doublet states of nascent NO are obtained using the A 2Σ–X 2Π transition via the technique of laser-induced fluorescence. NO is produced mainly from secondary dissociation of fragment NO2 from nitric acid after photolysis at 193 nm. According to the measured distribution the partition of energy in NO X 2Π is calculated to be ∼840 cm−1 in vibration and ∼1410 cm−1 in rotation. The spin–orbit state Π21/2 of NO is populated about twice of that in Π23/2 for the vibrational levels υ=0, 1, and 2. The measured populations of vibrational states υ=0, 1, and 2 are 0.75, 0.19, and 0.07, respectively; these populations agree with results calculated with the statistical model, prior theory. The distributions of rotational states of NO X 2Π for these three vibrational levels have single maxima of Gaussian shape. The experimental data indicate that although NO2, produced photochemically from nitric acid, was proposed to be in an electronically excited state, the state distributions of NO from unstable NO2 agree with those from NO2 excited with monochromatic light in the UV-vis range after averaging for a broad distribution of internal energy. In the UV-vis range of excitation the NO fragment dissociates from the ground electronic surface of NO2. Hence, this unknown electronic state of NO2 is expected to be coupled to the ground electronic surface then leading to dissociation.
Ab initio studies on the structures, vertical electron detachment energies, and stabilities of CnP− clusters
Authors: Zhan Chang-Guo , Iwata Suehiro
Published: July 2002

In this work, the structures, vertical electron detachment energies and fragmentation energies of CnP− clusters (n=1 to 7) have been investigated by carrying out the geometry optimizations with MP2 method using 6-31G(d) augmented with a set of diffuse sp functions followed by the energy calculations at higher levels. It can be concluded from our calculated results that for the ground states the linear structures are stable only for C2P− and C3P−. Other CnP− (n=4 to 7) clusters are slightly bent, and the bent CnP− with odd n is very floppy for bending motion. The contributions of the bending of the structures of CnP− with odd n to the stability can be ignored, and therefore these clusters could appear pseudolinear in experiment. The calculated vertical electron detachment and fragmentation energies of CnP− (n=1 to 7) in their ground states reveal that the CnP− clusters with odd n are more stable than those with even n, which is consistent with the observed odd-even alternation in the TOF signal intensities. It has been also found that the odd-even alternation in fragmentation energies of CnP− is weaker than that of CnN−. Therefore, the stability difference for CnP− between the odd n and even n is smaller than that for CnN−. This conclusion is also coincident with the experimental observation reported.
The nature of metal-oxide chemical bond: Electronic structure of PdMgO and PdOMg molecules
Authors: López Núria , Illas Francesc
Published: July 2002

The molecular and electronic structure of the complexes formed from interaction of Pd and MgO have been determined from ab initio self-consistent field and complete active space self-consistent field wave functions. The electronic structure of PdMgO and PdOMg appears to be rather involved because of the energy proximity between the ground 4d10 and excited 4d9 5s1 electronic configurations of Pd atom and the near degeneracy between the HOMO and LUMO of MgO. The only stable structures of PdMgO and PdOMg were linear and correspond to the lowest Σ+1, 3Σ+, 1Π, 3Π, 1Δ, and Δ3 electronic states. The dissociation energies with respect to the isolated fragments have been calculated through multireference configuration interaction expansions. Several methods of analysis of the wave functions indicate that the bonding in the O- and Mg-bonded complexes is quite different while, for a given complex, the nature of the chemical bond in the different electronic states appear to be rather similar.
A modified bubble model for positronium atoms in liquids
Authors: Mukherjee Tapas , Ganguly Bichitra Nandi , Dutta-Roy Binayak
Published: July 2002

Positronium self-trapping in liquids is frequently described through the bubble model in order to account for the observed decay characteristics, namely, the enhanced pick-off lifetime and the details of the angular correlation of the decay gammas. This model is improved upon in the present paper by modifying the idealised picture of a sharp bubble radius and thereby a formalism is developed which is not only more realistic but also provides elegant analytic solutions.
Energy transfer in the ground state of OH: Measurements of OH(υ=8,10,11) removal
Authors: Dyer Mark J. , Knutsen Karen , Copeland Richard A.
Published: July 2002

OH collisional removal rate constants for υ=8, 10, and 11 with O2, N2, CO2, N2O, and He are measured. Vibrationally excited OH molecules in υ=5, 7, and 8 are prepared from the reaction of atomic hydrogen with ozone, and selectively excited to υ=8, 10, and 11, respectively, with a pulsed infrared laser via direct overtone excitation. Subsequent collisional deactivation of these laser-populated levels is probed as a function of collider gas partial pressure using a time-delayed, pulsed ultraviolet laser resonant with OH(B 2Σ+−X 2Π) transitions. Fluorescence from OH(B 2Σ+−A 2Σ+) is detected. These results complete a series of removal rate constant measurements for OH(X 2Π) up to υ=12. Resonance enhancement effects in vibration-to-vibration energy transfer for CO2 and N2O factor significantly in the vibrational-level-dependent rate constants.
The nonradiative decay of the allyl radical excited B 2A1 state studied by picosecond time-resolved photoelectron spectroscopy
Authors: Schultz Thomas , Fischer Ingo
Published: July 2002

The nonradiative decay rates of the vibrational ground state and the υ7 fundamental in the electronically excited B 2A1 state of the allyl radical, C3H5, are studied by picosecond time-resolved pump-probe photoelectron spectroscopy. The states decay by internal conversion with a time constant of 20 and 16 ps, respectively.