bluuebunny/crossref_metadata_2025 huggingface dataset:
The production of O(3P) in the 157 nm photodissociation of CO2
Authors: Zhu Yi-Fei , Gordon Robert J.
Published: July 2002

The branching ratio was measured for the production of O(3P) in the photodissociation of CO2 at 157 nm. A gas mixture consisting of CO2, H2, and Ar was irradiated with an F2 excimer laser, while the relative concentration of O(3P) was monitored continuously using atomic resonance fluorescence. The O(1D) product was removed by either reacting with H2 or by being quenched by CO2. At a high H2/CO2 ratio a residual O(3P) signal persisted which was due to the nascent photofragments of CO2. A Stern–Volmer analysis indicated that the fraction of O(3P) produced is 5.9%. Control experiments using O2 and N2O as precursor molecules confirmed this interpretation of the data. A mechanism is proposed based on curve crossing from the 1B2 to the 3B2 potential energy surfaces of CO2. Since the 1B2 state is bent, a substantial fraction of the absorbed energy is initially in bending motion, resulting in a long-lived chaotic trajectory which has many opportunities to cross over to the triplet surface. A similar model proposed by Tully to explain the quenching of O(1D) by CO is in qualitative agreement with our data.
Nonadiabatic coupling between the E
F+G
K+H 1Σ+g, I 1Πg, and J 1Δg states of the hydrogen molecule. Calculation of rovibronic structures in H2, HD, and D2
Authors: Quadrelli P. , Dressler K. , Wolniewicz L.
Published: July 2002

The Born–Oppenheimer potential curves of the first two excited 1Σ+g states of H2, EF and GK, exhibit double minima which arise from avoided crossings between the electronic energy curves of the doubly excited 1σ2u configuration and of the 1Σ+g Rydberg states of the 1σg2sσg and 1σg3dσg configurations. The vibrational structures of the adiabatic electronic states are mutually coupled via strongly R-dependent off-diagonal electronic matrix elements of the d/dR and d2/dR2 operators. Furthermore the rotational structures are characterized by strong angular coupling within the complex of 3d-singlet-gerade states G 1Σ+g, I 1Πg, and J 1Δg.

We have evaluated the nonadiabatic coupling functions involving the first three excited 1Σ+g states EF, GK, and H, and the I 1Πg and J 1Δg states, and have calculated their nonadiabatic rovibronic structures for the J=0–5 levels of H2, HD, and D2 up to the H(1s)+H(2s,2p) dissociation limit. The simultaneous radial and angular couplings within these five electronic states have been treated using ab initio potential curves, adiabatic corrections, and nonadiabatic coupling functions. The coupled equations have been transformed into a diabatic electronic basis and solved numerically, and the resulting eigenfunctions have been transformed back into the adiabatic basis. Energy eigenvalues, nonadiabatic energy shifts, B values, Landé g factors, band transition moments, Einstein coefficients, and radiative lifetimes have been evaluated and compared with spectroscopic results where possible. The discrepancies between observed and calculated rovibronic term values depend systematically on electronic state, vibrational energy, and isotopic mass, and their magnitudes indicate that the absolute energy errors of the ab initio potential curves amount to less than 1 cm−1 near the minima of the EF 1Σ+g and I 1Πg states, approximately 2 cm−1 in the J 1Δg state, and 5 cm−1 in the H 1Σ+g state. The nonadiabatic ab initio results have already aided spectroscopic identifications of new energy levels and they indicate that several old assignments must be discarded.
Location of melting point at 300 K of nitrogen by Monte Carlo simulation
Authors: Meijer Evert Jan , Frenkel Daan , LeSar Richard A. , Ladd Anthony J. C.
Published: July 2002

We present an accurate new method to compute absolute free energies of molecular solids by computer simulations. As a first application, we computed the thermodynamic phase transition between the fluid phase and the orientational disordered solid β phase of nitrogen at 300 K, using a well tested pair potential. The computed coexistence pressure and the volume change coincides within the error margins with the experimental values. The coexistence volume differed by 2% from the experimental value. To our knowledge these results constitutes the first numerical calculation of the thermodynamic stability for a model of a realistic molecular solid.
Raman spectroscopic study of ion–ion interaction and its temperature dependence in a poly(propylene-oxide)-based NaCF3SO3 –polymer electrolyte
Authors: Kakihana M. , Schantz S. , Torell L. M.
Published: July 2002

Raman scattering measurements have been carried out on poly(propylene oxide) complexed with NaCF3SO3 salt of concentration O:M=30:1 (where O:M is the PO:Na ratio) over a temperature range of 186–360 K in order to study ion–ion associations of the dopant salt and their temperature dependence. Splitting of the symmetric stretching mode of the CF3SO−3 anion into a double band was observed and attributed to the existence of different environments of the anions. A two-component band analysis led to the identification of coexisting dissociated free ions and ion pairs, suggested to be in contact. Below the glass transition temperature, Tg, the intensity of the mode corresponding to the free ions was more or less constant with temperature; the amount of free ions in the glassy state was found to be about 84% of the total salt concentration. Above Tg the amount of dissociated free ions decreased rapidly with temperature in an Arrhenius-type behavior. The resulting reduction of the number of charge carriers has little influence on the conductivity, which is reported to dramatically increase with increasing temperature. It is concluded that the major factor determining the temperature dependence of the conductivity is the mobility rather than the number of charge carriers.
Collision-induced intramolecular energy transfer and dissociation of acetone molecular ion
Authors: Qian Kuangnan , Shukla Anil , Futrell Jean
Published: July 2002

Two crossed-beam instruments with different geometry and with low pressure and differentially pumped ion sources were used to investigate collision-induced dissociation (CID) of acetone molecular ions activated by He and Ar collisions over the kinetic energy range 0.45–10 eV (center of mass). Long-lived electronically excited state(s) which back-scatter superelastically with conversion of 2.2±0.3 eV of internal energy into translational energy dominate the low collision energy (Erel <2 eV) CID of the acetone ion. This energy release matches the X←A electronic excitation energy difference, suggesting a very efficient E→T energy transfer mechanism for this system. The location of the intensity maximum for this mechanism on the relative velocity vector (fully rebound peak) indicates that small impact parameter ‘‘head-on’’ collisions trigger this energy transfer process. A curve-crossing mechanism is suggested to rationalize these results. At collision energies of 6 eV and higher the superelastic scattering mechanism is no longer observed. Translationally endothermic forward (but nonzero angle) scattering takes over as the predominant CID mechanism. The dynamics are consistent with the hypothesis that the endothermic CID channel proceeds on the electronically excited surface and involves the efficient T→E excitation of ground state acetone ions.
H+H2(0,0)→H2(v′, j′)+H integral cross sections on the double many body expansion potential energy surface
Authors: Manolopoulos David E. , Wyatt Robert E.
Published: July 2002

The log derivative version of the Kohn variational principle is used to calculate integral cross sections for H+H2(0,0)→H2(v′, j′)+H, in the energy range Etot =0.95−1.35 eV, on the double many body expansion (DMBE) potential energy surface. The resulting DMBE cross sections are qualitatively very similar to, but somewhat larger than, the Liu–Siegbahn–Truhlar–Horowitz cross sections reported by Zhang and Miller. In particular, they show none of the interesting resonant structure seen in the recent experiments of Nieh and Valentini. While this does not prove that these experiments are wrong, it does at least indicate the extent to which errors in the best available H3 potential energy surfaces are likely to affect the calculated dynamical results.
Optimal phase modulation in stored wave form inverse Fourier transform excitation for Fourier transform mass spectrometry. I. Basic algorithm
Authors: Guan Shenheng , McIver Robert T.
Published: July 2002

A new signal processing method has been proposed for generating optimal stored wave form inverse Fourier transform (SWIFT) excitation signals used in Fourier transform mass spectrometry (FTMS or FT-ICR). The excitation wave forms with desired flat excitation power can be obtained by using the data processing steps which include: (1) smoothing of the specified magnitude spectrum, (2) generation of the optimal phase function, and (3) inverse Fourier transformation. In contrast to previously used procedures, no time domain wave form apodization is necessary. The optimal phase functions can be expressed as an integration of the specified power spectral profiles. This allows one not only to calculate optimal phase functions in discrete data format, but also to obtain an analytical expression (in simple magnitude spectral cases) that is for theoretical studies. A comparison is made of the frequency sweeping or ‘‘chirp’’ excitation and stored wave form inverse Fourier transform (SWIFT) excitation. This shows that chirp excitation and SWIFT excitation with a square magnitude spectrum and a quadratic phase are counterparts of the Fourier transformation. Therefore, the results of theoretical work on chirp excitation can be used for the analysis of the time domain excitation wave forms in the SWIFT technique.
Molecular dynamics calculation of the equation of state of alkanes
Authors: Toxvaerd So/ren
Published: July 2002

Molecular dynamics calculations of the pressure in fluids of propane, pentane, and decane show that the pressure in the traditional united-atom model which treats a methylene group as one isotropical interaction unit, scales incorrectly with respect to temperature and density. The shortcoming can, however, easily be overcome by introducing realistic anisotropic intermolecular potential functions. The anisotropy is achieved by taking the geometrical mean position of the valence electrons as the origin for intermolecular interactions of the methylene subunits.
Wave vector dependent static dielectric properties of associated liquids: Methanol
Authors: Fonseca Teresa , Ladanyi Branka M.
Published: July 2002

The static dielectric properties of liquid methanol and their wave vector dependence have been investigated using a molecular dynamics (MD) computer simulation and the reference hypernetted chain (RHNC) integral equation formalism. In the MD simulation we have used the site–site interaction model of methanol proposed by Haughney et al. [M. Haughney, M. Ferrario, and I.R. McDonald, J. Phys. Chem. 91, 4934 (1987)] and we have adopted a simplified, spherical core, model of methanol in the RHNC calculation. We find that the static dielectric constant is equal to 24 and the Kirkwood factor is 1.9 for the MD model. Although these values are not in good agreement with the corresponding experimental ones (32 and 2.94, respectively), we are able to show that the size of our system is large enough such that its static dielectric properties are consistent with those of a real fluid. Both MD and RHNC results for the wave vector dependence of the static dielectric tensor show that the longitudinal component of the dielectric permittivity is negative for almost the entire range of wave vectors studied. We argue that there is nothing unusual about this seemingly curious behavior, and that it should in fact be common to most polar liquids. The consequences of this behavior for the evaluation of free energies of solvation are outlined.
The spectroscopy and dynamics of π hydrogen-bonded complexes: Benzene–HCl/DCl and toluene–HCl/DCl
Authors: Gotch Albert J. , Zwier Timothy S.
Published: July 2002

The benzene–HCl/DCl and toluene–HCl/DCl complexes have been studied using both fluorescence and multiphoton ionization detection. These complexes are prototypical of π hydrogen-bonded complexes involved in the chemically important process of electrophilic aromatic attack. Laser-induced fluorescence (LIF) etalon scans of the 610 rotational band contour are used to determine the S1 state geometry of the benzene–HCl as one in which HCl is on the sixfold axis with a center-of-mass separation of 3.64±0.03 Å. The lack of significant van der Waals’ intensity points to the complex having a hydrogen-bonded geometry similar to that found in the ground state. Dispersed fluorescence scans are used to put crude bounds on the S0 and S1 binding energies of the benzene–HCl complex of 1.8≤D″0 ≤ 3.8 kcal/mol and 1.5≤D′0≤3.5 kcal/mol. The fluorescence lifetimes of bound levels of the complexes are factors of 7–12 times shorter than the corresponding levels of the free molecules. In contrast, the C6H6–CH3Cl complex, which has a similar geometry and binding energy, has a fluorescence lifetime nearly as long as C6H6. We argue that the differences observed are consequences of the hydrogen bonding interactions present in benzene–HCl and toluene–HCl. Two-color multiphoton ionization experiments in which the delay between the S0–S1 laser and the ionization lasers is continuously scanned give evidence that the hydrogen bonding interactions lead to enhanced intersystem crossing to the triplet state. One-color RE2PI scans show that fragmentation of the [benzene–HCl]+ and [toluene–HCl]+ ionic complexes proceeds nearly quantitatively (≥98%) from the hydrogen-bonded S1 state. This fragmentation occurs by virtue of the repulsive geometry formed for the ionic complex in Franck–Condon excitation from the S1 state.
Mode specific internal and direct rotational predissociation in HeHF, HeDF, and HeHCl: van der Waals complexes in the weak binding limit
Authors: Lovejoy Christopher M. , Nesbitt David J.
Published: July 2002

The near-infrared vibration–rotation spectra of the weakly bound complexes HeHF, HeDF, and HeHCl are observed in a slit supersonic expansion. The spectra correspond to simultaneous excitation of vibration and internal rotation of the H(D)X subunit within the complex. The HeHF and HeDF P/R branch transitions show J-dependent excess linewidths, which are attributed to rapid predissociation of the excited states from intramolecular rotation–translation energy transfer. The corresponding P/R branch transitions in HeHCl are not observed despite good S/N on the Q branch, suggesting even more rapid predissociation for the upper state of this complex. The Q branch transitions for all three complexes abruptly terminate at low J, yielding lower limits to the number of bound rotational states and good estimates of the dissociation energies D0=7.1±0.1 cm−1 for HeHF and HeDF, and 10.1±1.2 cm−1 for HeHCl. In addition to isotropic intermolecular potentials, the HeHF/HeDF data yield considerable information on the potential anisotropy in the region sampled by the bound and quasibound states. The information so obtained is complementary to results from scattering studies and provides sensitive tests for refining trial potential energy surfaces.
Wave functions with terms linear in the interelectronic coordinates to take care of the correlation cusp. III. Second-order Mo/ller–Plesset (MP2-R12) calculations on molecules of first row atoms
Authors: Klopper Wim , Kutzelnigg Werner
Published: July 2002

The MP2-R12 method (Mo/ller–Plesset second-order perturbation theory with terms linear in the interelectronic coordinate r12) in the approximations A and B as outlined in paper I of this series is applied to the ground states of the molecules H2, LiH, HF, H2O, NH3, CH4, Be2, N2, F2, C2H2, and CuH in their experimental equilibrium geometry, and to the van der Waals interaction between two He atoms. In all cases MP2 correlation energies are obtained that are supposed to differ by at most a few percent from the basis set limit. For CH4 the dependence of the energy on the symmetric stretching coordinate is studied, which together with other information leads to a recommended bond length of 1.086 Å for the CH bond length. For He2 and F2 the canonical and localized descriptions are compared. The latter is superior for the K-shell contributions, otherwise there is a little difference. For He2 in the localized representation rather good results for the dispersion interaction are obtained. The potential curve of Be2 is significantly improved in MP2-R12 as compared to conventional MP2. The examples C2H2 and CuH show that the method is not limited to very small systems.
Hyperpolarizability of H2O
Authors: Maroulis George
Published: July 2002

A complete description of the electric dipole moment (μ), the dipole polarizability (α), the first dipole (β), and the second dipole (γ) hyperpolarizability tensors is reported for the ground state of the water molecule at its equilibrium geometry. Self-consistent-field (SCF) and complete fourth-order many-body perturbation theory (MP4) values of the independent components are calculated via a finite-field method from the perturbed energies of the molecule in the presence of a homogeneous electric field. The dependence of the calculated values on the basis set is studied at both the SCF and the MP4 levels. Electron correlation has a strong effect on the hyperpolarizability. Our best SCF values are calculated with a large (13s10p6d2f/9s6p2d)[9s7p6d2f/6s5p2d] basis set comprising 136 contracted Gaussian-type functions and are 0.7789 ea0 for the dipole moment and 8.531 e2a20E−1h, −10.86 e3a30E−2h, and 979 e4a40E−3h for the mean dipole polarizability and first and second dipole hyperpolarizabilities, respectively. The electron correlation correction to these properties is estimated at −0.055±0.005 ea0, 1.11±0.14 e2a20E−1h, −7.1±1.3 e3a30E−2h, and 749±113 e4a40E−3h. Agreement with experiment is very good for the dipole moment and mean dipole polarizability. As regards the hyperpolarizability, satisfactory agreement with the frequency-dependent values of Ward and Miller may also be deduced, but further experimental and theoretical work on the dispersion of the hyperpolarizability is needed for an effective rapprochement of theory and experiment.
On the geometry of transient relaxation
Authors: Roussel Marc R. , Fraser Simon J.
Published: July 2002

Coupled chemical reactions are often described by (stiff) systems of ordinary differential equations (ODEs) with widely separated relaxation times. In the phase space Γ of species concentration variables, relaxation can be represented as a cascade through a nested hierarchy of smooth hypersurfaces (inertial manifolds) {Σ}: If d is the number of independent concentration variables, then Γ≡Γd⊇Σd−1⊇Σd−2⋅⋅⋅. The last three sets in this hierarchy have special chemical importance: Σ0 is the stagnation point of the ODEs, i.e., chemical equilibrium; ℳ(≡Σ1) is the linelike slow manifold describing the dynamical steady state in closed systems; Σ(≡Σ2) is the two-dimensional surface containing the slowest transient flow that reaches ℳ. Thus ℳ and Σ are the structures underlying most steady-state and transient kinetics experiments. The ODEs describe the velocity field in Γ, which may be used to define functional equations for ℳ, Σ, and other members in the hierarchy {Σ}. These functional equations can be solved to give explicit formulas for ℳ, Σ, etc. In a model three-step mechanism, ℳ is described by a vector functional equation involving ordinary derivatives, whereas Σ is described by a scalar functional equation involving partial derivatives. We show how Σ may be found by iterative solution of this functional equation if decay is monotonic, and comment on the complications introduced by oscillatory transients. The functional equation approach may be generally applied in higher dimensional systems.
Quantum mechanics of small Ne, Ar, Kr, and Xe clusters
Authors: Leitner David M. , Doll J. D. , Whitnell Robert M.
Published: July 2002

We compute energy levels and wave functions of Ne, Ar, Kr, and Xe trimers, modeled by pairwise Lennard-Jones potentials, using the discrete variable representation (DVR) and the successive diagonalization-truncation method. For the Ne and Ar trimers, we find that almost all of the energy levels lie above the energy required classically to achieve a collinear configuration. For the Kr and Xe trimers, we are able to determine a number of energy levels both below the classical transition energy as well as above it. Energy level statistics for these heavier clusters reveal behavior that correlates well with classical chaotic behavior that has previously been observed above the transition energy. The eigenfunctions of these clusters show a wide variety of behavior ranging from very regular behavior for low lying eigenstates to a combination of regular and irregular behavior at energies above the transition energy. These results, along with quantum Monte Carlo calculations of the ground states for a variety of small clusters, lead to the construction of a local mode model for the eigenstates of these clusters. This simple model reproduces quite well many of the calculated energy levels. From the findings here, a simple picture emerges for quantum mechanical clusters of Ne, Ar, Kr, and Xe that correlates very strongly to classical models of these systems.
Temperature dependence of the rate constants for the H+Br2 and D+Br2 reactions
Authors: Wada Youichi , Takayanagi Toshiyuki , Umemoto Hironobu , Tsunashima Shigeru , Sato Shin
Published: July 2002

The rate constants for the reactions of H+Br2 and D+Br2 were measured by employing a pulse radiolysis–resonance absorption technique. The rate constants could be expressed by the following Arrhenius equations between 214 and 295 K: k(H+Br2) =6.7×10−10 exp(−680/T), k(D+Br2)=6.0×10−10 exp(−720/T), in units of cm3 s−1. Sudden transition state theoretical calculations were performed on the basis of modified LEPS surfaces. The calculated results were compared with the experimental ones.
Local structure of polyethylene melts
Authors: Honnell Kevin G. , McCoy John D. , Curro John G. , Schweizer Kenneth S. , Narten Alfred H. , Habenschuss Anton
Published: July 2002

Polymer-RISM (Reference-interaction-site-model) theory is used to examine the local structure of a dense polyethylene melt near the freezing point. Predictions for the static structure factor are found to be in near quantitative agreement with new x-ray diffraction data obtained at 430 K and 1 atm.
Excited state intermolecular proton transfer in isolated clusters: 1-naphthol/ammonia and water
Authors: Kim Seong K. , Li Shijian , Bernstein Elliot R.
Published: July 2002

The excited singlet state intermolecular proton transfer reaction in jet-cooled clusters of 1-naphthol/ammonia and water is investigated employing mass resolved excitation, threshold photoionization, and emission spectroscopy. The complete data set indicates that no proton transfer occurs for 1-naphthol(NH3)1,2 and (H2O)n, n=1,...,20 clusters. Proton transfer occurs for (at least) one configuration of the 1-naphthol(NH3)3 cluster, as well as all 1-naphthol(NH3)n, n≥4, clusters. The (at least) two configurations of 1-naphthol(NH3)3 clusters are distinguished by threshold photoionization studies. The 1-naphthol(NH3)3 cluster for which proton transfer is indicated has a threshold photoionization energy ∼2000 cm−1 below the other 1-naphthol(NH3)3 cluster configurations. These results are employed to explain the previous discrepancy between static spectroscopic experiments and picosecond time resolved dynamic experiments concerning proton transfer in the 1-naphthol(NH3)3 cluster. Calculations of cluster geometry in 1-naphthol/ammonia and water cluster systems suggest some qualitative explanations of these observations.
Mass resolved excitation spectroscopy of radicals: Benzyl and phenylnitrene
Authors: Im H. S. , Bernstein E. R.
Published: July 2002

Two-color mass resolved excitation spectra are obtained for the benzyl and ‘‘phenylnitrene’’ radicals. The spectrum previously assigned to the benzyl radical is generated by a species with mass 91 amu. This finding is consistent with the proposed C6H5ĊH2 benzyl radical structure. The origin of this 1 2A2←1 2A1 transition lies at 21 997 cm−1. The ionization energy for the benzyl radical is ∼7.236 eV. The phenylnitrene radical spectrum is associated with a species of 90 amu. The phenylnitrene designation is not appropriate for this mass number: the suggested cyanocyclopentadienyl radical species (C5H4CN) is consistent with this mass. This radical has an ionization threshold of ∼9.05 eV. Lifetimes are reported for a number of the vibronic states of both radicals.
Photoelectron spectroscopy of IO−
Authors: Gilles Mary K. , Polak Mark L. , Lineberger W. C.
Published: July 2002

The 351 nm (hν=3.531 eV) photoelectron spectrum IO− shows transitions to two spin–orbit states of neutral IO. The electron affinity of IO is found to be 2.378(5) eV. Vibrational frequencies for the neutral (2Π1/2) and anion (1Σ+) states are reported. The spin–orbit splitting of the IO ground state is determined to be −2091(40) cm−1. Franck–Condon analysis of the IO vibrational progressions were used along with the previously determined 2Π3/2 molecular constants to obtain the equilibrium bond lengths of the anion (1Σ+) and neutral (2Π1/2) states.
A new approach to the surface plasma resonance of small metal particles
Authors: Xu Muliang , Dignam Michael J.
Published: July 2002

A differential equation for the polarizability of a small, continuum, spherically symmetric medium is developed in the local-dielectric-response approximation. The dynamic polarizability of a small metal sphere is then calculated through this equation using the Drude dielectric expression with the spatial dispersion of the free electron density given by the results of self-consistent density functional calculations on jellium. This approach is used to examine the size dependence of the plasmon resonance absorption of small metal particles. It is able to account for the observed broadening and peak frequency shifts, both ‘‘red’’ and ‘‘blue,’’ as well as the additional absorption feature seen for small particles. To the extent that comparison with experimental data is possible, good agreement is found.
Spin polarization and ferromagnetic coupling in metallocenium charge transfer complexes
Authors: Kollmar Christian , Kahn Olivier
Published: July 2002

Spin polarization effects in metallocenium complexes across the transition metal series Cr, Mn, Fe, Co, Ni are discussed in a qualitative fashion using a restricted open-shell Hartree–Fock approach. The spin polarization is taken into account by a limited configuration interaction (CI) with singly excited configurations. In Fe(C5Me5)+2, Mn(C5Me5)+2, and Cr(C5Me5)+2, the unpaired electrons are localized in molecular orbitals with predominant metal 3d character. The polarization of the metal–cyclopentadiene π bonds then leads to a negative spin density on the cyclopentadiene (Cp) rings. The amount of negative spin density on Cp increases with increasing spin on the metal (S=1/2, S=1, and S=3/2 for ferrocenium, manganocenium, and chromocenium, respectively). The Cp spin density of nickelocenium on the other hand is positive because there is a direct delocalization of spin density to the Cp rings due to the considerable Cp contributions to the e*1g molecular orbitals in which the unpaired electron of nickelocenium is located. Correspondingly, the charge transfer complexes of these compounds with the acceptors tetracyanoethylene (TCNE) or tetracyanoquinodimethane (TCNQ) in an alternating arrangement ...D+A−D+A−... are expected to exhibit ferromagnetic donor–acceptor coupling for Fe(C5Me5)+2, Mn(C5Me5)+2, and Cr(C5Me5)+2 and antiferromagnetic coupling for Ni(C5Me5)+2 according to a model suggested previously by the authors and based on an idea of McConnell. This is in agreement with the experimental results. The observed nuclear magnetic resonance (NMR) shifts of some metallocenes in relation to these findings are discussed.
Structural properties and medium-range order in calcium-metasilicate (CaSiO3) glass: A molecular dynamics study
Authors: Abramo Maria C. , Caccamo C. , Pizzimenti G.
Published: July 2002

Structural properties and medium-range order (MRO) in calcium metasilicate (CaSiO3) glass are investigated at different concentrations of calcium oxide through the molecular dynamics technique. Calculations are based on the Born–Mayer–Huggins potential, and show that the experimental Ca–Ca partial structure factor, which documents the existence in the glass of Ca MRO, can qualitatively be reproduced within such a model. The characteristics of Ca MRO are then examined in the context of the overall structure of the system. At equimolar concentrations of CaO and SiO2, the simulation evidentiates the formation in the glass of clusters of CaO6 octahedra in which Ca ions are roughly lying on a plane, in a configuration that closely resembles the one of crystalline CaSiO3. On the other hand, the ‘network’ structure and MRO of pure SiO2 glass appear sensibly affected by the presence of Ca ions, with considerable loose of connectivity between SiO4 tetrahedral units, and reduction of the first sharp diffraction peak in the neutron structure factor. The q-space investigation of charge–charge correlations signals the presence of medium-range charge ordering in the calcium vs oxygen ion distribution, at a wave vector equal to that associated to Ca MRO. The meaning and implications of this result are discussed.
Liquid chlorine in shear and elongational flows: A nonequilibrium molecular dynamics study
Authors: Hounkonnou M. N. , Pierleoni C. , Ryckaert J.-P.
Published: July 2002

A diatomic fluid (chlorine) is studied by the nonequilibrium molecular dynamics (NEMD) technique in various isochoric homogeneous flows at a unique state point. Rheological and orientational properties are investigated in planar Couette flow and also, for the first time for a molecular system, in various elongational flows. Moreover, the phenomenological coefficients (zero strain rate viscosity η0, Maxwell constant for molecular orientations) describing the linear regime in strain rate are probed through appropriate time correlation functions measured along an equilibrium trajectory. Large statistics are required given the presence in all relevant time autocorrelation functions of an exponential tail with a characteristic time close to the single molecule reorientation time at equilibrium τr (≊103 integration steps). At low strain rates, rheological and reorientational properties of molecular fluids are difficult to measure with accuracy by NEMD, not only because of the usual signal-to-noise ratio problems, but also as a result of the relatively slow reorientational dynamics which follows the imposition of an homogeneous flow. All flows lead to a somewhat similar shear thinning behavior for the generalized viscosity in terms of the second scalar invariant of the strain rate tensor, a quantity which provides a measure, valid for all flows, of the deformation rate based on the heat dissipation. The effective shear viscosity almost decreases by a factor of 2 with respect to η0 at a reduced shear rate of τrγ̇≊1. Orientational coupling with flow is studied in detail both in linear (Newtonian) and nonlinear regimes. A strong analogy is observed between nonlinear behavior of stress and orientation tensors under flow.
Phase transition of a poly(acrylic acid) gel induced by polymer complexation
Authors: Yu Xiaohong , Tanaka Ayako , Tanaka Kazunori , Tanaka Toyoichi
Published: July 2002

Swelling equilibrium of poly(acrylic acid) (PAA) gels in aqueous poly(ethylene glycol) (PEG) solutions has been studied. First order volume transition was observed as the concentration of PEG was varied. The pH of the solution and the molecular weight of PEG are the key factors in determining the phase behavior of the PAA gel in this system. The transition concentration of PEG varies many orders of magnitude for different molecular weights. PEG with molecular weight 600 does not induce a transition. Reentrant phase transition was observed for the molecular weight of 18 500. Namely, at low PEG concentrations, the PAA gel is swollen; as the concentration is increased the gel collapses, but swells again as PEG is further added to the solution. PEG and PAA are known to form a polycomplex through hydrogen bonds. As the hydrogen bonds are formed, the hydrophilic side of polymers are hidden and the polymers become effectively hydrophobic, inducing the collapse. A simple theoretical model is presented to qualitatively explain the phenomena.
Newton–Raphson optimization of the many-body nonadiabatic wave function expressed in terms of explicitly correlated Gaussian functions
Authors: Kozlowski Pawel M. , Adamowicz Ludwik
Published: July 2002

A nonadiabatic many-body wave function is represented in terms of explicitly correlated Gaussian-type basis functions. Motions of all particles (nuclei and electrons) are treated equally and particles are distinguished via permutational symmetry. The nonadiabatic wave function is determined in a variational calculation with the use of the method proposed recently [P. M. Kozlowski and L. Adamowicz, J. Chem. Phys. 95, 6681 (1991)]. In this approach no direct separation of the center-of-mass motion from the internal motion is required. The theory of analytical first and second derivatives of the variational functional with respect to the Gaussian exponents and its computational implementation in conjunction with the Newton–Raphson optimization technique is described. Finally, some numerical examples are shown.
Dimensional scaling for quasistationary states
Authors: Kais S. , Herschbach D. R.
Published: July 2002

Complex energy eigenvalues which specify the location and width of quasibound or resonant states are computed to good approximation by a simple dimensional scaling method. As applied to bound states, the method involves minimizing an effective potential function in appropriately scaled coordinates to obtain exact energies in the D→∞ limit, then computing approximate results for D=3 by a perturbation expansion in 1/D about this limit. For resonant states, the same procedure is used, with the radial coordinate now allowed to be complex. Five examples are treated: the repulsive exponential potential (e−r); a squelched harmonic oscillator (r2e−r); the inverted Kratzer potential (r−1 repulsion plus r−2 attraction); the Lennard-Jones potential (r−12 repulsion, r−6 attraction); and quasibound states for the rotational spectrum of the hydrogen molecule (X 1∑g+, v=0, J=0 to 50). Comparisons with numerical integrations and other methods show that the much simpler dimensional scaling method, carried to second-order (terms in 1/D2), yields good results over an extremely wide range of the ratio of level widths to spacings. Other methods have not yet evaluated the very broad H2 rotational resonances reported here (J≳39), which lie far above the centrifugal barrier.
(n,3s) Rydberg spectra of diethyl ether, diisopropyl ether, and methyl vinyl ether: Analysis of the torsional motion
Authors: Shang Q. Y. , Moreno P. O. , Disselkamp R. , Bernstein E. R.
Published: July 2002

Two-photon resonance mass resolved excitation spectra are obtained for diethyl ether-h10, diethyl ether-d10, diisopropyl either, and methyl vinyl ether cooled in a supersonic jet expansion. The spectra are assigned as due to 2p3s←(2p)2 Rydberg excitations: significant progressions in a low energy vibrational mode are observed for both diethyl and diisopropyl ethers, but not for methyl vinyl ether. The transition energies for the vibronic progressions are modeled by a double well potential function of polynomial form truncated at the quartic term. The transition intensities for the progressions are calculated based on a Franck–Condon analysis of the excited state potential surface and a harmonic ground state surface. Calculations identify the progression forming mode as the antigeared torsion of the C–O–C–C dihedral angles. Excitation of diethyl ether from its ground electronic state to its 2p3s Rydberg state leads to changes in the dihedral angles (τ1, τ2) from ±180° to ±157°. Similar electronic excitation of diisopropyl ether yields changes in (τ1,τ2) from ±157° in the ground state to ±146° in the Rydberg state. The torsional displacement active in the 2p3s←(2p)2 oxygen Rydberg transition is suggested to arise from the interaction between the diffuse 3s electron and the β-methyl groups on both diethyl and diisopropyl ethers.
Evolution with temperature of the middle phase from micellar to bicontinuous microemulsion
Authors: Matsen M. W. , Schick M. , Sullivan D. E.
Published: July 2002

We study the evolution of a middle phase microemulsion, from micellar solution to bicontinuous microemulsion to inverse micellar solution, as temperature is increased in a lattice model of a ternary mixture of oil, water, and nonionic amphiphile. Our model, which is studied within the Bethe approximation, includes orientational degrees of freedom of water as well as of amphiphile. It yields a two-phase to three-phase to two-phase progression with changing temperature, i.e., a triple line bounded by lower and upper critical endpoints. The cmc surfaces of normal and inverted micelles are calculated as a function of temperature, and we find that at both critical endpoints, the middle phase emerges from a critical micellar phase. Osmotic pressures are calculated to display the manner in which they reflect the cmcs. The connectivities of oil and water regions are obtained so that the region in which the middle phase is bicontinuous can be delineated. We find that when the temperature is increased from the lower critical endpoint, the micelle concentration falls sharply as the middle phase becomes bicontinuous; when the temperature is decreased from the upper critical endpoint, the much smaller population of inverse micelles decreases less sharply as the system becomes bicontinuous. The correlation functions of the middle phase are calculated, and we determine the region in which they decay nonmonotonically at large distances. Comparison shows that while the regions in which the middle phase displays its structure to connectivity and to scattering experiments do overlap, they do not coincide. Most of our results compare well to experiments on ternary mixtures which contain a weak, nonionic amphiphile.
Many-electron explicitly correlated Gaussian functions. I. General theory and test results
Authors: Cencek Wojciech , Rychlewski Jacek
Published: July 2002

The Gaussian functions containing correlation factors of the type exp(−βrij2), employed so far in variational calculations of two-electron atoms and molecules are generalized for many-electron systems. Explicit formulas for necessary one-, two-, three-, and four-electron integrals over s-type correlated Gaussians are given. Preliminary computations for the H3 and LiH molecules yield significantly lower energy values than all previously published variational results.
Chemisorption on stepped metal surfaces: CO/vicinal Ni(100)
Authors: Sinniah K. , Dorsett H. E. , Reutt-Robey J. E.
Published: July 2002

The chemisorption properties of carbon monoxide on two vicinal Ni(100) surfaces have been studied with surface infrared reflection–absorption spectroscopy and low energy electron diffraction. For coverages ≤0.50 monolayer, equilibrium adlayers are formed in which CO populates atop sites on the low-index (100) terrace, as well as twofold bridging sites along both the highly-kinked and close-packed step edges of the Ni[(100)-1.4°(01̄0)] and Ni[(100)-9°(01̄1̄)] surfaces investigated. Low energy electron diffraction (LEED) measurements confirm that all three long-range structures established on the (100) surface—c(2×2) at 0.50 ML, hexagonal at 0.61 ML, and compressed-hexagonal at 0.69 ML—are also formed on the Ni [(100)-1.4°(01̄0)] surface. On the Ni [(100)-9°(01̄1̄)] surface, however, only the ordered c(2×2) structure appears. A simple Arrhenius analysis of the relative population of step and terrace sites estimates a small binding energy preference for populating step sites. This weak preference is of comparable magnitude to the CO–CO interactions that produce long range structures. To evaluate quantitatively the binding energy difference between adsorption at step and terrace sites, step adsorption isotherms are measured as a function of total coverage at select temperatures over the 90–300 K window. The isotherms are modeled with simple Monte Carlo simulations of adsorption on stepped surfaces, which include a 1.0 kcal/mol binding energy preference for step sites. The data and simulations indicate that the primary role played by the steps in the chemisorption of CO is to serve as nucleation centers for island growth.
Inclusion of nonequilibrium continuum solvation effects in variational transition state theory
Authors: Truhlar Donald G. , Schenter Gregory K. , Garrett Bruce C.
Published: July 2002

We define a general set of effective solvent coordinates for reactions in solution that should be especially well suited for polar solvents. No specific functional form is assumed for the variation of charges with motions along or orthogonal to the reaction coordinate, and as a consequence the formalism allows the treatment of arbitrary reaction types using electronic structure calculations for the solute charge distribution. A critical assumption of the theory is that the nonequilibrium polarization field may be represented in terms of the adiabatic polarization fields that arise for linear geometric displacements. The results are combined with the Cartesian-coordinate canonical variational transition state theory presented previously to provide a general method for including both equilibrium and nonequilibrium solvation effects of a nondissipative, continuum-dielectric solvent in practical calculations of rate constants.
Pressure shifts and electron scattering lengths in atomic and molecular gases
Authors: Asaf U. , Rupnik K. , Reisfeld G. , McGlynn S. P.
Published: July 2002

Photoabsorption or photoionization spectra of CH3I are discussed as a function of perturber pressure for 11 different binary gas mixtures consisting of CH3I and each one of 11 different gaseous perturbers. Five of the perturbers were rare gases and six were nondipolar molecules. The energy shifts of CH3I Rydberg states become independent of n, the principal quantum number, for n≥10. The energy shifts for n≥10 vary in a linear fashion with perturber number density. The electron scattering lengths for the perturbers are extracted from the shifts using Fermi theory in which the polarization term is that of Alekseev and Sobel’man. These scattering lengths are compared with those from swarm and time-of-flight experiments. It is found that the uncorrected shift scattering lengths correspond to the zero energy or near-zero energy scattering lengths obtained from extrapolated swarm and time-of-flight data. It is found that plots of scattering length vs polarizability α (ᾱ for molecules) define two linearities, one for the rare gases and one for molecules, CO2 being an exception to the latter linearity (presumably because of its large quadrupole moment). For a given polarizability, it is also found that molecules exhibit a larger scattering length than the rare gases. These results are discussed and consequences for scattering cross sections are elaborated.
Molecular dynamics and semiclassical electronic spectra of naphthalene⋅Arn clusters (n≤4)
Authors: Troxler Thomas , Leutwyler Samuel
Published: July 2002

Molecular dynamics simulations were performed for van der Waals clusters naphthalene⋅Arn, n=1 to 4. For all isomers and conformers of these clusters, dynamical quantities such as velocity autocorrelation functions, vibrational power spectra, and semiclassical electronic absorption spectra were calculated over a wide energy range, and averaged over a canonical distribution at temperatures in the range T=5 to 30 K. Electronic absorption spectra were calculated for the origin bands according to the semiclassical method [L. E. Fried and S. Mukamel, J. Chem. Phys. 96, 116 (1992)] and are compared with the corresponding experimental naphthalene⋅Arn R2PI spectra [T. Troxler and S. Leutwyler, J. Chem. Phys. 95, 4010 (1991)]. The appearance of distinct absorption bands due to specific isomers for a given cluster size, as observed experimentally, is well reproduced by the simulations. Comparison of calculated electronic shifts for different isomers allows clear assignments in the experimental spectra. Increasing the simulation temperature to T=15–25 K is accompanied by band broadening and the appearance of sidebands towards the blue. Especially strong sidebands appear for naph⋅Ar2 and all clusters containing the Ar2 subunit, due to large-amplitude surface rotation/translation of the argon dimer on the naphthalene surface, in agreement with experiment. For clusters containing the n=3 and n=4 subunits the spectral broadening is smaller. For the n=4 (4+0)-isomer, the calculated band shape increases less than for the other n=4 isomers, mainly due to the motional narrowing effect of cluster fluxionality. Above 25 K, isomerization between different possible topological structures also occurs by side-crossing motion of one or several argon atoms.
New effective strategy of generating Gaussian-type geminal basis sets for correlation energy calculations
Authors: Bukowski Robert , Jeziorski Bogumil/ , Szalewicz Krzysztof
Published: July 2002

Gaussian-type geminal basis sets for correlation energy calculations are usually obtained by optimization of the second-order energy functional. In the present work we propose a new strategy of generating the initial sets of geminals for these optimizations. Taking advantage of the information about the analytical structure of the second-order pair functions, this strategy produces reasonable starting values of the nonlinear geminal parameters, which lead to accurate second-order energies after only a short, crude reoptimization.
Calculation of shapes of dipolar domains in insoluble monolayers: Analysis of shape stability and transitions
Authors: Mayer M. A. , Vanderlick T. K.
Published: July 2002

Dispersed domains in two-phase systems often exhibit complex and intriguing morphologies. For many of these systems, it is possible to predict the shape of such a domain through an evaluation of the free energy. In this paper, we present a numerical technique to calculate domain shapes through a variational approach. This is a significant extension to previous work which has primarily involved examining free energies of specific classes of shapes. The solution diagram is presented in the neighborhood of the first four bifurcations from a circle and the affect of domain size is examined. We find the only stable domain shapes are circular or bilobed. More importantly, we find that the transition between these two shapes is discontinuous, contrary to the findings of previous shape studies. Examining the free energy functional in the neighborhood of stable solutions, we find that, in general, domain shape calculations are best applied to small nonelongated domains. Finally, we present graphical differentiation, a numerical technique we developed to enable solution of the problem.