bluuebunny/crossref_metadata_2025 huggingface dataset:
Rovibronic spectrum of the N3 radical in the X 2Πg state
Authors: Chambaud Gilberte , Rosmus Pavel
Published: July 2002

Based on ab initio electronic structure calculations, the three-dimensional potential energy functions for the electronic ground state X 2Πg of N3 have been generated and used in beyond Born–Oppenheimer calculations of the rovibronic energy levels by a variational approach accounting for anharmonicity, rotation–vibration, electronic angular momenta, and electron spin coupling effects. The vibronic levels (J=P) for energies up to 4300 cm−1 are given. The few experimentally known vibronic energy differences have been reproduced with an accuracy of better than 10 cm−1. For several vibronic levels also the rovibronic levels are reported. The electron–nuclear motion and anharmonic coupling effects have been analyzed. The strength of the Born–Oppenheimer breakdown depends strongly on the rovibronic symmetry. Like in the isoelectronic CO+2 ion, the anharmonic coupling in the vibronic states occurs mainly within the blocks of Fermi polyads for (2ν1+ν2) =constant. In low lying states the anharmonic coupling effects are weaker in N3 than in CO+2. The presence of the Fermi polyad blocks of interacting states strongly alleviates the characterization of the excited rovibronic states in the N3 radical.
Analysis of the intermolecular potential of Ar–CH4: An a
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Authors: Szczȩśniak M. M. , Chal/asiński G. , Cybulski S. M.
Published: July 2002

The perturbation theory of intermolecular forces in conjunction with the supermolecular Mo/ller–Plesset treatment is applied in the analysis of the potential energy surface of Ar–CH4. The anisotropy of the Ar–CH4 potential energy surface is chiefly due to exchange repulsion. In the equilibrium structure, Ar approaches the face of CH4 tetrahedron thus avoiding contacts with C–H bonds. The equilibrium Ar–C separation was found equal to 7.5 a0 and the De energy to 113 cm−1. We estimate that our De may be too small by up to 25% with respect to the accurate value. The properties of Ar–CH4 are also compared with other Ar-molecule systems, such as Ar–NH3, Ar–H2O, and Ar–HCl. We find that the equilibrium structures of weak proton donors bound to Ar (CH4, NH3) are determined by the exchange repulsion, while those of efficient proton donors, such as HCl (and to a lesser extent H2O), result from the strong polarization of Ar in the field of a molecule.
Reactive collision dynamics of Na*(4 2P)+H2 and HD: Experiment and theory
Authors: Bililign Solomon , Kleiber P. D. , Kearney W. R. , Sando K. M.
Published: July 2002

We have used a ‘‘half-collision’’ pump–probe technique to measure the far wing absorption profiles of the NaH2 collision complex leading to the nonreactive formation of Na* and to four distinct final rotational states of the reaction product NaH(v″=1, J″=3, 4, 11, and 13). We have observed reaction on both the attractive potential energy surfaces and over a barrier on the repulsive surface. We have observed the effect of the Na* reagent electronic orbital alignment on the NaH final product rotational state distribution. Specifically, absorption to the repulsive surface leads preferentially to low-rotational product states, while absorption to the attractive surfaces leads preferentially to high-rotational product states of NaH. Isotopic substitution experiments give evidence of a kinematic isotope effect on the product rotational state distribution for reactive trajectories on the repulsive surface. We have developed a simple model using a quantum mechanical line shape calculation to estimate the NaH2 absorption probability as a function of wavelength. We then make simple phenomenological dynamical arguments to predict final state branching. There is an overall qualitative agreement between the experimental results and theoretical model predictions.
The Laplacian of the intracule and extracule densities and their relationship to the shell structure of atoms
Authors: Sarasola C. , Dominguez L. , Aguado M. , Ugalde J. M.
Published: July 2002

Numerical evidence of the monotonicity of both the intracule density I(r) and the extracule density E(R) in atoms are presented. However, it is found that their associated Laplacian fields ∇2E(R) and ∇2I(r) show a rich structure. Roots and extrema of these functions have been compared with the corresponding values of the Laplacian of the charge density ∇2ρ(r). Radii corresponding to minima in ∇2I(r) are found to match closely their corresponding values in ∇2ρ(r). Linear relationships between the inverses of the roots of ∇2E(R) and ∇2I(r) and the nuclear charge Z are found. Calculations demonstrate that atomic shell structure is well resolved by the odd numbered zeros of ∇2E(R) and by the radii corresponding to minima in ∇2I(r).
Wave packet dynamics of resonance decay: An iterative equation approach with application to HCO→H+CO
Authors: Gray Stephen K.
Published: July 2002

Three-dimensional solutions to the time-dependent Schrödinger equation, corresponding to the decay of resonances in the HCO radical, are studied. Theoretical tools to facilitate such studies are discussed. A novel propagation scheme, based on iterative equation equivalents to the Schrödinger equation, is developed. The HCO calculations are performed by representing the wave function with a grid and basis set expansion, and by using a Lanczos procedure to evaluate operator actions. Resonance energies and decay widths are obtained by analyzing correlation functions over relatively short time periods with Prony’s method of spectral analysis. Product distributions are also estimated, and results are compared with previous time-independent scattering results. Inspection of the time-evolving probability density yields a picture of the decay mechanism, and points to the origins of structure in the product distributions.
Photoreaction dynamics of CO oxidation on Pt(111)
Authors: Ukraintsev V. A. , Harrison I.
Published: July 2002

The dynamics of the surface aligned photoreaction between submonolayers of O2 and CO coadsorbed on a Pt(111) surface at 25 K and laser irradiated at 308 nm are explored by measurement of angle-resolved translational energy distributions for the CO2 photoproduct.
Optical measurements of methyl group tunneling in molecular crystals: Temperature dependence of the nuclear spin conversion rate
Authors: Hartmann Claudius , Joyeux Marc , Trommsdorff H. Peter , Vial Jean-Claude , von Borczyskowski Christian
Published: July 2002

The tunneling methyl groups in dimethyl-s-tetrazine (DMST) doped single crystals of durene were investigated by high resolution optical spectroscopy using spectral hole burning. The experiments probe the level structure as well as the relaxation dynamics of the tunneling methyl groups in different electronic states of DMST. The tunneling splitting differs by 1.24 GHz in the ground and the first excited singlet states of DMST. In the ground electronic state, relaxation (spin conversion) between the spin 3/2 (A) and 1/2 (E) tunneling levels was measured between 1.5 and 12 K. The spin conversion time is larger than 100 h at 1.5 K and decreases with Arrhenius-type behavior above 3.5 K. The activation energy of 20 cm−1 also is observed as a phonon sideband in emission, and is, in agreement with theoretical predictions, tentatively assigned to a librational mode of the methyl group.
Vibrational energy transfer on hydrogen-terminated vicinal Si(111) surfaces: Interadsorbate energy flow
Authors: Morin M. , Jakob P. , Levinos N. J. , Chabal Y. J. , Harris A. L.
Published: July 2002

We report measurements of excited-state lifetimes for Si–H stretching vibrational modes of steps and terraces on chemically prepared, hydrogen-terminated vicinal Si(111) surfaces using picosecond pump–probe surface spectroscopy. The steps present on these vicinal surfaces are shown to play an important role in the vibrational energy relaxation pathways. Three types of vicinal Si(111) surfaces are studied, all having monohydride-terminated terraces but differing in step termination or in step density. Two surfaces are cut along the 〈1̄1̄2〉 direction, 9° and 5° from the (111) plane, respectively. Both of these surfaces have dihydride-terminated steps. A third surface is cut 9° from the (111) plane along the 〈112̄〉 direction and has monohydride-terminated steps. Two normal modes of the dihydride-terminated steps show vibrational energy relaxation times of ∼100 ps [≤80 ps and 130(20) ps, uncertainty in parentheses], while the monohydride-terminated steps relax 10 times more slowly with an 1100(120) ps lifetime. On the dihydride-stepped 9° surface the lifetime of the terrace mode is shortened to 420(40) ps from the flat surface lifetime of 950(100) ps, while on the monohydride-stepped surface the terrace mode lifetime is 820(80) ps. The results are explained by energy transfer between the terrace and the step Si–H modes. The different dynamics on the monohydride- and dihydride-stepped surfaces arise because the short-lifetime dihydride steps act as energy drains, while the long-lifetime monohydride steps do not. Dipole–dipole coupling between the Si–H stretching modes can account for the interadsorbate vibrational energy transfer observed.
The relative intensity contributions of axial and equatorial CH bonds in the local mode overtone spectra of cyclohexane
Authors: Kjaergaard Henrik G. , Henry Bryan R.
Published: July 2002

Vapor phase room temperature overtone spectra of cyclohexane are measured with conventional near infrared spectroscopy for the lower overtones, ΔvCH=2–4, and with intracavity dye laser photoacoustic spectroscopy for the higher overtones, ΔvCH=5–7. The relative intensity of the axial to equatorial peak is explained in terms of the local mode model of harmonically coupled anharmonic oscillators, where all modes but the two CH-stretching modes are neglected. The dipole moment function is expanded in the two CH-stretching coordinates, where the expansion coefficients are determined from ab initio molecular orbital calculations. This simple calculation, which contains no adjustable parameters, can account very well for the observed relative intensity of axial and equatorial pure local mode peaks in the overtone spectra from ΔvCH=3–7. We also investigate different ways of obtaining the dipole moment derivatives, and find that a fourth order series expansion around the equilibrium geometry is a good approximation when calculating the relevant matrix elements.
Symmetry restrictions in diatom/diatom reactions. I. Group theoretical analysis
Authors: Gellene Gregory I.
Published: July 2002

New permutation inversion (PI) symmetry groups are presented which can be used to symmetry classify the nuclear-rotational–vibrational–electronic–translational wave function (Ψnrvet) of an A2/A2, A2/AB, A2/B2, or AB/AB collision system, where the diatomic electronic states can be of any spatial symmetry or spin multiplicity. A symmetry correlation scheme based on these PI groups is developed which connects symmetry distinct nuclear motion states of Ψnrvet at infinite diatom/diatom separation with distinct electronic symmetries of the system in the diatom/diatom interaction region. Consideration of the effect of isotopic substitution on the correlation indicates that a kinetic isotope effect which is based on symmetry rather than mass considerations can occur in reactions where (1) the molecules are identical, but in different electronic states, or (2) at least one of the reactants is in a degenerate electronic state. Application of the correlation scheme to the kinetics of the H+2/H2 proton/atom transfer reaction (and its isotopic analogs) indicates that the experimentally observed unusual isotope effects can be understood as a consequence of this new symmetry based kinetic isotope effect. The possibility of observing this effect in the reactions of more complex molecules is discussed briefly.
Sequential reactions of SiH+2 with silane: A theoretical study
Authors: Raghavachari Krishnan
Published: July 2002

The reaction mechanism for the interaction of SiH+2 with SiH4 has been studied by means of accurate molecular orbital techniques including polarized basis sets, effects of electron correlation, and zero-point corrections. The reaction leads to the formation of the disilene cation (H2Si–SiH+2) without any overall energy barrier. In addition, the formation of the hydride transfer product (SiH+3) is calculated to be exothermic. Further reactions of the stable ion H2Si–SiH+2 with silane require significant endothermic energy barriers. The calculated results are in good agreement with the experimental observations of Reents and Mandich.
Structural, thermodynamic, and electrical properties of polar fluids and ionic solutions on a hypersphere: Theoretical aspects
Authors: Caillol J. M.
Published: July 2002

We generalize previous work [J. Chem. Phys. 94, 597 (1991)] on an alternative to the Ewald method for the numerical simulations of Coulomb fluids. This new method consists in using as a simulation cell the three-dimensional surface of a four-dimensional sphere, or hypersphere. Here, we consider the case of polar fluids and electrolyte solutions. We derive all the formal expressions which are needed for numerical simulations of such systems. It includes a derivation of the multipolar interactions on a hypersphere, the expansion of the pair-correlation functions on rotational invariants, the expression of the static dielectric constant of a polar liquid, the expressions of the frequency-dependent conductivity and dielectric constant of an ionic solution, and the derivation of the Stillinger–Lovett sum rules for conductive systems.
Transient diode laser absorption spectroscopy of the ν2 fundamental of trans-HOCO and DOCO
Authors: Sears Trevor J. , Fawzy Wafaa M. , Johnson Philip M.
Published: July 2002

We report the observation and assignment of the ν2 fundamental vibration in the HOCO and DOCO radicals. The radical was made by the photolysis of acetic acid or acetic acid-d at 193 nm in a flow system. The observed spectra indicate that the excited vibrational level is perturbed in both HOCO and DOCO. In HOCO, the rotational levels in ν2=1 have an irregular dependence on the Ka quantum number, probably caused by anharmonic interactions with combinations of lower frequency in plane vibrations. An a-type Coriolis interaction involving perturbation by a level containing one quantum of the out-of-plane torsional vibration cannot be rigorously excluded however. In DOCO, there are also N-dependent perturbations in the excited state. Only parallel transitions were assigned in the ν2 fundamental of both isotopomers. The K=0 band origin for HOCO is at 1852.567 cm−1 which compares with 1843.7 cm−1 reported previously for the radical trapped in a low-temperature argon matrix. This absorption spectrum will be useful in future studies of chemical reactions involving the radical.
Effects of solvation by a single atom on photodissociation: Classical and quantum/classical studies of HCl photolysis in Ar⋅⋅⋅HCl
Authors: García-Vela A. , Gerber R. B. , Valentini J. J.
Published: July 2002

The photolysis of HCl in the cluster Ar⋅⋅⋅HCl is studied theoretically with the objective of elaborating on the effect of a single ‘‘solvent’’ atom on the dynamics of chemical bond breaking. The focus is on observable properties, such as the velocity distribution and the angular distribution of the H atom product, on how these properties reflect the ‘‘solvent’’ effect, and on the physical mechanisms involved. The main results obtained are the following. (1) There is a high probability for at least a single ‘‘hard’’ collision between the H photofragment and the Ar atom before the H atom leaves the cluster. Multiple collisions between the H and the heavy atoms also occur with significant probability. (2) The final kinetic-energy distribution of the H atom shows a long pronounced tail due to energy transfer in the collisions with the heavy atoms. (3) There are pronounced peaks in the angular distribution of the H atom due to the single and multiple collision events. (4) Comparison of photolysis of Ar⋅⋅⋅HCl with that of Ar⋅⋅⋅DCl shows a large isotope effect, again due to collisions within the cluster during its fragmentation. The results were mostly obtained from classical trajectory calculations, but also in part from calculations using a hybrid quantum/classical method in which the H atom is treated by quantum wave packets while the heavy atoms are described classically. The quantitative results show some quantum effects, but for most purposes the classical description is sustained. Implications of the results for experimental studies of molecular photodissociation in clusters are discussed.
CO2 laser assisted vibrational overtone spectroscopy
Authors: Settle R. D. F. , Rizzo T. R.
Published: July 2002

We report the implementation of a new spectroscopic scheme for measuring vibrational overtone spectra of molecules in collisionless environments. This approach is based upon selective infrared multiphoton dissociation (IRMPD) of molecules that have undergone a light atom stretch vibrational overtone transition. Overtone excitation is ultimately monitored by laser induced fluorescence (LIF) detection of the dissociation products. Application of this technique to measure the 5←0 OH stretch transition methanol at 50 m Torr produces an excitation spectrum which is virtually identical to a photoacoustic spectrum recorded at 40 Torr, indicating that the former faithfully represents the vibrational overtone absorption. When applied to methanol cooled in a supersonic expansion, this method results in considerable simplification of the 5νOH vibrational overtone spectrum and reveals the presence of a strong Fermi resonance which divides the 5νOH oscillator strength equally between two vibrational levels. This new spectroscopic technique should permit the application of supersonic expansion and double resonance techniques to vibrational overtone spectroscopy of a wide variety of molecules.
Cation vibrational spectroscopy of trans and gauche n-propylbenzene rotational isomers. Two-color threshold photoelectron study and ab initio calculations
Authors: Takahashi M. , Kimura K.
Published: July 2002

In this paper, a full account of an earlier report [Takahashi, Okuyama, and Kimura, J. Mol. Struct. 249, 47 (1991)] on the cation vibrational spectra of trans and gauche n-propylbenzene, which were obtained by means of a two-color (1+1′) multiphoton ionization threshold photoelectron technique is presented. The trans and gauche cations were separately produced by (1+1′) multiphoton ionization resonant through the S1 vibronic levels of n-propylbenzene in supersonic free jets in which both the trans and gauche isomers exist. From the observed threshold photoelectron spectra, adiabatic ionization energies are determined as Ia(trans)=70 278 cm−1 (8.7134 eV) and Ia(gauche)=70 420 cm−1 (8.7311 eV) with an accuracy of ±8 cm−1 (±1 meV). Furthermore, four benzene ring modes (6b+, 1+, 12+, and 18a+) as well as several low-frequency torsional and bending modes have been identified which are sensitive to the relative conformations of the n-propyl group with respect to the benzene ring [(trans)+: 82, 212, and 300 cm−1; (gauche)+: 46, 73, 207, and 252 cm−1]. The present vibrational assignments of the trans and gauche cations are based on a normal mode analysis performed for the trans cation by ab initio calculations. In the present work, this technique is demonstrated to be quite powerful for distinguishing the vibrational spectra of different cation isomers.
Femtosecond transient stimulated emission pumping studies of ozone visible photodissociation
Authors: Chen Y. , Hunziker L. , Ludowise P. , Morgen M.
Published: July 2002

We report a new variant of the femtosecond transition-state spectroscopy technique−transient stimulated emission pumping (TSEP). The initial application of TSEP to studies of ozone visible photodissociation indicates that the molecule is trapped in the Franck–Condon region for ∼90 fs. The wave packet then spreads along the reaction coordinate.
A theoretical study of hydrogen diffraction following photodissociation of adsorbed molecules
Authors: Kosloff Ronnie , Zeiri Yehuda
Published: July 2002

A new probe of surface structure is presented which is based on the photodissociation of hydrogen from an adsorbate molecule. The event creates an atomic hydrogen fragment, positioned between the adsorbate layer and the solid surface. Due to its light mass, the hydrogen dynamics is quantum mechanical in nature. A useful image is of the hydrogenic wave function behaving like a liquid able to fill all cracks. The coherent character of the hydrogenic wave function is crucial in the ability of the photodissociation experiment to act as a probe. A series of case studies has been carried out whose aim is to reveal the relation between the structure of the surface and the asymptotic energy resolved angular distribution of the hydrogen fragment. The dynamics of the hydrogen atom motion was modeled by the time dependent Schrödinger equation. The cases studied include the dissociation of a single HBr adsorbate on flat and corrugated surfaces. A broad specular peak was observed, in addition to diffraction peaks which can be correlated with the corrugation. Moreover, selective adsorption peaks, which can be correlated with the attractive part of the surface potential, have been identified. Systems in which the hydrogenic wave function scatters from several adsorbates were also investigated. It was found that the scattering is dominated by the trapping of the wave function by unstable periodic orbits. The quantization rules of these periodic orbits have been identified, creating a link between the structure of the adsorbates and the asymptotic angular distributions.
I2− photofragmentation/recombination dynamics in size-selected I2−(CO2)n cluster ions: Observation of coherent I...I− vibrational motion
Authors: Papanikolas John M. , Vorsa Vasil , Nadal María E. , Campagnola Paul J. , Gord James R. , Lineberger W. C.
Published: July 2002

We have employed picosecond pump–probe techniques in conjunction with a tandem time-of-flight mass spectrometer to investigate the caging dynamics of photodissociated I2− solvated with a specific number of CO2 molecules. In this paper, we report the observation of a recurrence at ≊2 ps in the I2− absorption recovery, a feature which is attributed to coherent I...I− nuclear motion following I2− photoexcitation.
Friction kernels for the relative dynamics of ion pairs in water
Authors: Rey R. , Guàrdia E. , Padró J. A.
Published: July 2002

The dynamic properties of Cl−–Cl−, Cl−–Na+, and Na+–Na+ ion pairs in water are investigated. It is assumed that the interionic separations obey the generalized Langevin equation. The random forces on the ion pairs are calculated from constrained molecular-dynamics simulations at three interionic distances for each ion pair. The Gaussianity of the random forces is checked. The time-dependent friction kernels for the relative motions are determined from the random forces by using the fluctuation–dissipation theorem. The dependence of the friction kernels on both the solute species and the intersolute separation is explored. Special attention is paid to the relationship of the friction kernels with the characteristics of the configurations of the solvent molecules in the neighborhood of the ions. The coupling between the intramolecular motions of water and the relative ionic motions is discussed. The association–dissociation processes for the three ion pairs are analyzed within the framework of the Kramers and Grote–Hynes theories for activated barrier crossing kinetics.
Vibrationally enhanced dissociative scattering of NO+(Etrans, v=0–6) on GaAs(110)
Authors: Martin J. S. , Greeley J. N. , Morris J. R. , Jacobs D. C.
Published: July 2002

A hyperthermal beam of state-selected NO+ X Σ+(v=0–6) ions is scattered on a clean, well-characterized GaAs(110) surface. The threshold for the appearance of scattered O− ions occurs at a collision energy of 25 eV. Vibrational energy proves to be an order of magnitude more effective than translational energy in enhancing the yield in this channel.
Accurate proton affinities: Ab initio proton binding energies for N2, CO, CO2, and CH4
Authors: Komornicki Andrew , Dixon David A.
Published: July 2002

A set of large-scale ab initio molecular orbital calculations on the title molecules and their protonated forms has been performed. The aim of the present study has been to help establish very accurate absolute proton affinities for each of these molecules. For each molecule a series of calculations was performed using increasingly larger atomic natural orbital (ANO) one-particle spaces. The energetics of protonation were then evaluated using four methods. These include self-consistent-field (SCF), second-order perturbation theory (MP2), the singles and doubles coupled-cluster (CCSD) ansatz, and the CCSD(T) method, which includes a perturbational estimate of connected triple excitations. At each of these levels of theory the incompleteness of the one and N-particle spaces was ascertained by an evaluation of the basis set superposition error (BSSE) for the protonation reaction. We believe that the final proton affinities all attain chemical accuracy in that they contain <1 kcal/mol error. Our results are in extremely good agreement and consistent with the recently revised absolute proton affinity scale.
Nonlinear dielectric constant of nonpolar fluids
Authors: Hinsen K. , Bratz A. , Felderhof B. U.
Published: July 2002

We study the third-order electrical susceptibility of a fluid of nonpolar spherical molecules with linear polarizability and cubic hyperpolarizability. In the electrostatic dipole approximation, we derive an expression for the third-order susceptibility, which allows its evaluation from a property of the linear system. To second order in the linear polarizability, the correction to a mean field approximation, obtained by Buckingham, may be related to the correction to the Clausius–Mossotti formula for the linear dielectric constant. We present numerical results, obtained by computer simulation, for a system of hard spheres.
The role of nonbond and charge flux in hydrogen bond interactions. The effect on structural changes and spectral shifts in water dimer
Authors: Dinur U. , Hagler A. T.
Published: July 2002

An analysis of Hessian matrix elements in molecular systems is presented. It is shown that elements that couple nonbonded atoms lead to an exact pairwise decomposition of intermolecular potential energy surfaces. The analysis further shows that potential energy surfaces of dimers in general contain terms that couple inter and intramolecular coordinates. Such terms arise from the ‘‘flux’’ of the internal charge distribution as the monomers distort in response to each other. These fluxes may be represented analytically by allowing atomic parameters such as point charges and van der Waals parameters to be coordinate dependent. Three hydrogen bonded dimers are studied in some detail with special focus on the short-range nonbond repulsions. It is shown that the pairwise hydrogen bond interactions in water–water, water–formaldehyde, and water–formamide are very similar, and that the nonbond flux is effective mainly along the O–H bond of the donor water molecule. The flux and the functional form of the nonbonded interaction in the water dimer are parametrized. Intramolecular structural changes and IR shifts and splittings in this system are shown to be strongly dependent on charge and nonbond flux terms. It is suggested that such flux terms provide a general mechanism for controlling structural and spectral changes in dimers.
On the magnetic field dependence of the excess electron Hall mobility observed in liquid argon and liquid xenon
Authors: Muñoz Raúl C. , Pennington Tad
Published: July 2002

A comparison is made between the magnetic field dependence of the excess electron Hall mobility recently reported in liquid Ar and liquid Xe, to the predictions based on solutions of Boltzmann transport equation (BTE) within the relaxation time approximation. The results show that, for the scattering mechanisms occurring in semiconductors, BTE predicts a sublinear magnetic field dependence of the Hall field which is weaker than what is experimentally observed.
Stress relaxation in a polymer melt of freely-rotating chains
Authors: Gao J. , Weiner J. H.
Published: July 2002

It has been generally assumed that the nonbonded interactions in a dense polymer melt make only an isotropic contribution to the stress. Recent computer simulations of stress relaxation in systems of idealized chain models have shown that this is not the case. In fact, the nonbonded interactions are primarily responsible for the anisotropic stress during a stress relaxation process. To check that this result is not due to the idealized character of the chain models, simulations are performed for a melt employing a more realistic model, namely that representing a freely-rotating chain. The parameters employed correspond to the freely-rotating chains (FRC) model melt studied in extensive simulations by Takeuchi and Roe. We find that the primary role of the nonbonded interactions remains unchanged. Simulations are performed at several temperature levels at constant pressure. It is found that the temperature–time equivalence principle applies to the simulated shear relaxation modulus. The required shift factor aT satisfies the WLF (Williams–Landel–Ferry) equation with fairly reasonable parameter values.
The B3Li molecule’s electronic and geometrical structure
Authors: Earl Edward , Hernandez Ramon , Simons Jack
Published: July 2002

The geometries, electronic energies, dipole moments, and local harmonic vibrational frequencies of the singlet ground and low-energy singlet valence excited states of B3Li have been investigated using (i) correlation-consistent double-zeta plus polarization basis sets, (ii) small valence-space complete active space multiconfiguration self-consistent field electronic wave functions, and (iii) analytical energy derivative techniques combined with automated potential energy surface ‘‘walking’’ algorithms. The lowest-energy structure of B3Li is found to have C3v symmetry and to consist of a Li atom that sits above the center of an equilateral B3 moiety; the dipole moment of this species is 6.7 D and its dissociation energy (not zero point corrected) to ground state B3+Li is 60 kcal/mol. The B3H analog of this C3v structure is not a geometrically stable species for reasons that are analyzed here. A second stable structure (with dipole moment of 8.5 D and dissociation energy of 47 kcal/mol) is planar and has the Li atom σ bonded to one of the B atoms in the triangular B3 group. The B3H analog of this σ-bonded species is stable and, in fact, is the lowest-energy structure of B3H. Finally, in contrast to findings for B3H, no bridge-bonded structure was found to be stable for B3Li. Both the σ-bonded and C3v structures of B3Li are shown to possess significant charge–transfer character near their equilibrium geometries, but to dissociate to ground state neutral B3+Li fragments.
Theoretical determination of the geometric and electronic structures of oligorylenes and poli(peri-naphthalene)
Authors: Viruela-Martín R. , Viruela-Martín P. M. , Ortí E.
Published: July 2002

We present a theoretical investigation of the electronic structure of oligorylenes (from perylene to heptarylene, including also the naphthalene molecule) and their corresponding polymer poly(peri-naphthalene) (PPN) using the nonempirical valence effective (VEH) method. The geometry of the unit cell used to generate the polymer is extrapolated from the PM3-optimized molecular geometries of the longest oligorylenes. That geometry shows some bond alternation along the perimeter carbon chains and a bond length of ≊1.46 Å is calculated for the peri bonds connecting the naphthalene units. The VEH one-electron energy level distributions calculated for oligorylenes are used to interpret the experimental trends reported for the first ionization potentials, redox potentials, and lowest energy optical transitions. An excellent agreement is found between theoretical estimates and experimental values. The VEH band structure calculated for an isolated chain of PPN is interpreted in terms of the molecular orbitals of naphthalene. The ionization potential, electron affinity, and bandwidths obtained for PPN suggest a large capacity to form conducting p- or n-type materials. The small band gap of 0.56 eV predicted for PPN from VEH band structure calculations is in good agreement with theoretical and experimental estimates calculated by extrapolating the data reported for the oligomers.
Minimum energy conformation of ortho-xylene in its ground and first excited electronic states
Authors: Disselkamp R. , Bernstein E. R. , Seeman Jeffrey I. , Secor Henry V.
Published: July 2002

The geometry of ortho-xylene is studied through supersonic jet cooling and one color mass resolved excitation spectroscopy. By examining the S1←S0 transition origin region of various (d0–d2) methyl deuterated o-xylene species, the conformation of the two methyl groups in S0 and S1 can be determined. A comparison between the predicted and experimentally observed number and intensity of origin features for the various partially deuterated o-xylenes shows that two highly symmetric methyl group structures are possible, each structure having C2V point group symmetry—the antiplanar conformation (A1), in which τ1(C2–C1–Cα–Hα)=180°; τ2(C1–C2–Cα′–Hα′)=180°; and the syn, planar conformation (A2) in which τ1=τ2=0°. The experimentally determined structures are consistent with ab initio calculations and microwave studies which predict the anticonformation to be most stable. Spectroscopic data, in conjunction with further ab initio calculations, are used to investigate the ground and excited state potential energy surfaces. In addition, a repulsive, intramolecular, nonbonded interaction between the two methyl groups is identified through an empirical potential energy calculation to be the most important interaction defining the lowest energy structure.
Kinetics in excited He/CO gas mixtures
Authors: Wan B. N. , Langhoff H.
Published: July 2002

Gas mixtures of He and CO were excited by electron beams. The transient species CO+(X,A,B), CO(A,C,a,b), C2(X,d), C*(3s) and O*(3p5P) were identified by fluorescence and absorption measurements. The time dependence and absolute population yield for most of these species were determined. Based on these results a reaction scheme is proposed which reproduces the experimental data quantitatively. Optical gain, previously observed by Waller et al. [Appl. Phys. Lett. 27, 323 (1975)] on the CO+(B)→CO+(X) transition was not reproduced.
The singlet–triplet energy separations of GeCl2, GeBr2, GeI2, GeHCl, GeHBr, and GeHI
Authors: Benavides-Garcia M. , Balasubramanian K.
Published: July 2002

The singlet–triplet energy separations of GeCl2, GeBr2, GeI2, GeHCl, GeHBr, and GeHI were obtained using the complete-active-space multiconfiguration self-consistent-field (CAS–MCSCF) method followed by multireference configuration interaction (MRCI). We also studied the effect of spin–orbit coupling on the heavier GeI2 using the relativistic CI (RCI) method. In addition to the 1A1 ground state we found 3B1 and 1B1 excited states for GeCl2, GeBr2, and GeI2. The X 1A1–A 3B1 calculated energy splittings are 60.3 kcal/mol for GeCl2, 55.5 kcal/mol for GeBr2, and 42.4 kcal/mol for GeI2. The ground-state properties for GeCl2 are Re=2.191 Å, Θe=100.5°, and μe=2.956 D. The corresponding value for GeBr2 are Re=2.373 Å, Θe=101.8°, and μe=2.394 D, while our computed constants for GeI2 are Re=2.574 Å, Θe=102.8°, and μe=1.422 D. For GeHCl, GeHBr, and GeHI molecules we computed the properties of the 1A′ ground state and 3A″ and 1A″ excited states. The X 1A′–A 3A″ energy splittings are computed as 39.9 kcal/mol for GeHCl, 37.9 kcal/mol for GeHBr, and 34.8 kcal/mol for GeHI. The spectroscopic properties for 1A′ state are Re(Ge–H)=1.589 Å, Re(Ge–Cl)=2.201 Å, Θe=94.3°, and μe=2.553 D for GeHCl, Re(Ge–H)=1.572 Å, Re(Ge–Br)=2.388 Å, Θe=93.9°, and μe=2.341 D for GeHBr, and Re(Ge–H)=1.575 Å, Re(Ge–I)=2.580 Å, Θe=93.5°, and μe=1.947 D for GeHI.
A search for the wetting transition in 2,6-dimethyl pyridine/water mixtures in contact with glass near the lower critical point
Authors: Kreuser H. , Woermann D.
Published: July 2002

The pendant drop method and the capillary rise method are used to measure the interfacial tension σ of 2,6-dimethyl pyridine(2,6-lutidine)/water mixtures near the lower critical point. It is found that at temperatures 0.2 K≤(T−Tc)≤1.5 K the temperature dependence of σ is represented by a power law of the form σ=σ0 tμ (t=(T−Tc)/Tc) with μ=(1.25±0.01) and σ0=(32.4±0.6) mN m−1. The value of μ is consistent with the theoretically expected universal value of μ=1.26. At large temperature differences σ increases slower than expected by the power law. The values of the amplitude ratios Rσ,ξ+[=σ0(ξ0+)2/(kBTc)] and Rσ,A+[=(Av,0/kB)2/3(kBTc/σ0)] calculated from the experimental data [correlation length of local concentration fluctuations, ξ0+=(0.230±0.009) nm] and Av,0 (heat capacity per unit volume at constant pressure; Av,0=1.81 J cm−3 K−1) are consistent with accepted values. This indicates that the interfacial tension measurements are free from large systematic errors. The value of a2 (a, capillary constant) determined with the pendant drop method as well as with the capillary rise method agree with each other within the uncertainty of the measurements in a temperature range (T−Tc)≤7 K. The capillary rise data is analyzed by assuming that the contact angle θ of the lower water rich phase at the capillary wall is zero (θ=0). The data follows the same curve as that obtained with the pendant drop method in a temperature range (T−Tc)≤25 K. From these findings it is concluded that the water rich (lower) phase wets the wall of the glass capillary completely at least up to temperature differences (T−Tc)≤7 K. A wetting transition cannot be detected in that temperature range.
Molecular theory of transition energy correlations for pairs of chromophores in liquids or glasses
Authors: Sevian H. M. , Skinner J. L.
Published: July 2002

The absorption spectrum of an optical transition of a dilute solute in a glassy or liquid solvent is usually inhomogeneously broadened. In a concentrated solution, the question arises as to whether or not the transition energy distributions of nearby solutes are correlated. Such correlation has important implications for coherent or incoherent transport and optical dephasing experiments. We present a molecular theory of this correlation. For a simple model of Lennard-Jones solutes in a Lennard-Jones liquid solvent, we compare our theory to Monte Carlo simulations, finding reasonable agreement. For a model with longer range solute–solvent interactions, where the excited state solute is ionized, the theory predicts very significant correlation effects. This suggests that for more realistic models with dipolar interactions, significant correlation effects will also be present.
Extreme ultraviolet laser spectroscopy on CO in the 91–100 nm range
Authors: Levelt Pieternel F. , Ubachs Wim , Hogervorst Wim
Published: July 2002

Transitions from the ground state X 1Σ+ v=0 to eleven highly-excited states of CO were investigated using extreme ultraviolet laser spectroscopy. Excited states were detected using a two-photon ionization scheme. Improved values of band origins could be deduced from an absolute calibration of the extreme ultraviolet laser radiation against the I2 standard in the visible wavelength region. Accurate predissociation rates followed from linewidth analyses of individual rotational lines.
Photochemical decomposition of AsH3 on GaAs(100)
Authors: Zhu X.-Y. , Wolf M. , White J. M.
Published: July 2002

Molecular AsH3, adsorbed on Ga-rich GaAs(100) at 115 K, dissociates readily upon uv irradiation with 193, 248, and 351 nm excimer laser light. In the initial photodissociation step one As–H bond cleaves, leaving all the AsH2, and a large fraction of the H, adsorbed to As. The AsH2 further photodissociates to give As–H and Ga–H. The final steps, photochemical removal of hydrogen from Ga–H and As–H, lead to As deposition. The photodissociation cross section decreases sharply with the extent of photolysis. The wavelength dependence, compared to the gas-phase absorption cross section, extends to much lower photon energies and indicates that substrate-mediated excitation dominates the observed chemistry. There are strong isotope effects in all the cross sections; these are related to mass-dependent substrate-mediated quenching of the excited states. Implications for photon-assisted organometallic chemical vapor deposition are discussed.
Surface forces in athermal polymer solutions
Authors: Woodward C. E.
Published: July 2002

Forces between planar surfaces immersed in a polymer solution are compared with those of the pure solvent in order to establish the effect of adding polymer. We use an off-lattice model consisting of hard-sphere polymers in a hard sphere (athermal solvent). The model is analyzed using a recently introduced density-functional theory. Due to the typically high density of liquids, the forces in both solution and solvent are oscillatory, leading to a more complicated picture than the classical description of polymer depletion forces. Rather than a well-defined attractive depletion regime in the surface forces, the effect of polymers is better described as a damping of the oscillatory force in the pure solvent. It is only when one goes to low (solvent and polymer) density that an attractive depletion regime can be identified.
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o second- and fourth-order Mo/ller–Plesset study on structure, stabilization energy, and stretching vibration of benzene⋅⋅⋅X (X=He,Ne,Ar,Kr,Xe) van der Waals molecules
Authors: Hobza P. , Bludský Ota , Selzle H. L. , Schlag E. W.
Published: July 2002

The C6v structure of benzene⋅⋅⋅X (X=He, Ne, Ar, Kr, Xe) complexes was investigated with second-order Mo/ller–Plesset (MP2) theory; for the benzene⋅⋅⋅He the whole potential-energy surface (PES) was also studied. The stabilization energy of the benzene⋅⋅⋅He was also determined at the fourth-order Mo/ller–Plesset (MP4) level; the respective MP4 stabilization energy is almost identical with MP2 stabilization energy which is due to the compensation of MP3 and MP4 contributions. The ab initio MP2 intermolecular distances agree nicely for all the complexes studied with the experimental value. While the stabilization energy of benzene⋅⋅⋅He and benzene⋅⋅⋅Ne (67 cm−1; 99 cm−1) is considerably smaller than that of benzene⋅⋅⋅Ar (429 cm−1), the intersystem distance differs less (3.32 Å, 3.50 Å, 3.53 Å). The stabilization energies and intersystem distances for benzene⋅⋅⋅Kr and benzene⋅⋅⋅Xe are 485 and 601 cm−1 and 3.71 and 3.89 Å, respectively. The PES of benzene⋅⋅⋅He differs from that of benzene⋅⋅⋅Ar and can be characterized as more isotropic. The harmonic and anharmonic stretching frequencies were determined for all the complexes; anharmonicity plays a crucial role only for the benzene⋅⋅⋅He complex. Theoretical and experimental stretching frequencies for benzene⋅⋅⋅Ar and benzene⋅⋅⋅Kr agree very well.
Rotational state dependence of decay dynamics in the superexcited 7f Rydberg state (υ=1) of NO
Authors: Fujii Asuka , Morita Norio
Published: July 2002

The decay dynamics of the 7f Rydberg state (υ=1) of NO has been investigated with laser multiphoton excitation methods; not only NO+ ions generated by autoionization, but also fragment nitrogen atoms produced by predissociation have been directly probed. The fragment atoms have been found to populate both the 2p3 2DJ and 2p3 4S3/2 states. Population yield in the 4S state shows strong dependence on each 7f rotational level of NO, and this dependence is remarkably correlated with autoionization yield, while no such significant dependence exists for the 2D state. From the fact that only odd ℒ levels generate the 4S state, dissociative states causing this predissociation process have been identified as Σ+ states. Through detailed analysis, it has been shown that the predissociation rate of this channel is much larger than the autoionization rate, and that the N(2D)-generating predissociation is also mainly caused by dissociative Σ+ states. Moreover, it has been found that a major part of the total decay rate of each rotational level is strongly correlated with magnitude of its fσ character. From these facts, it has been concluded that the decay process in the 7f state is mainly governed by predissociation due to direct coupling with dissociative Σ+ valence states, which have been identified as A′ 2Σ+ and I 2Σ+ for the N(4S)- and N(2D)-generating predissociation channels, respectively.
Monte Carlo simulation of the collapse-coil transition in homopolymers
Authors: Szleifer Igal , O’Toole Eamonn M. , Panagiotopoulos Athanassios Z.
Published: July 2002

We have performed Monte Carlo simulations of the transition of polymers from an expanded to a collapsed state on a cubic lattice using a variation of the scanning technique of Meirovitch. The technique allows the study of highly compact polymer conformations that could not be studied with previously available methods. Our results indicate that the universal scaling function α3(1−T/θ)n1/2 vs (1−T/θ)n1/2 has a pronounced maximum before leveling off in the fully collapsed regime. Available experimental data for this function only cover the range up to and including the maximum. Good agreement is obtained between our calculations and experimental data for the polystyrene/cyclohexane system.
Segmental motion in poly(styrene-b-1,4-isoprene) block copolymers in the disordered state
Authors: Rizos A. K. , Fytas G. , Roovers J. E. L.
Published: July 2002

The segmental dynamics associated with two primary relaxation processes in three poly(styrene-b-1,4-isoprene) diblock copolymers with different proximity to the order–disorder transition (ODT) were investigated by photon correlation spectroscopy. Utilizing the selectivity of the depolarized light scattering, the presence of two distinct orientational processes were unambiguously revealed by the same experimental technique near but above ODT. These primary α-relaxation modes result from the composition field configuration (‘‘pattern’’) in disordered block copolymers with correlation length ξc larger than the cooperative size ξα of the α relaxation near the glass transition Tg. The existence of a slowly fluctuating composition pattern can lead to the observed excess broadening of the segmental distribution of relaxation function L(log τ) beyond that of bulk homopolymers. For the slow α process, assigned to segmental orientation within microregions rich in polystyrene, a new finding is the temperature dependence of the width of the relaxation time distribution function. The relaxational characteristics of the two processes are discussed in the framework of free volume concepts and composition fluctuations within the cooperative volume (∼ξα3).
On the scaling of the critical temperature with the degree of polymerization in symmetric polymer blends
Authors: Yethiraj Arun , Schweizer Kenneth S.
Published: July 2002

The scaling of the critical temperature (Tc) with the degree of polymerization (N) in symmetric polymer blends is investigated via a new molecular closure to the polymer reference interaction site model integral equation theory. The theory predicts Tc∼N, which is qualitatively consistent with classical mean field ideas.
Many-body perturbation theory of frequency-dependent polarizabilities and van der Waals coefficients: Application to H2O–H2O and Ar–NH3
Authors: Wormer Paul E. S. , Hettema Hinne
Published: July 2002

Correlation contributions to the multipole moments and frequency dependent polarizabilities of molecules are described within the framework of time-dependent coupled Hartree–Fock and many-body perturbation theory. Computationally feasible expressions are given for the ‘‘true’’ correlation contributions to the multipole moments and frequency dependent polarizabilities. The polarizabilities of argon, ammonia and water and the van der Waals induction and dispersion coefficients of H2O–H2O and Ar–NH3 are presented.