bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron impact ionization of the SiDx (x=1–3) free radicals
Authors: Tarnovsky V. , Deutsch H. , Becker K.
Published: July 2002

We report measurements of absolute cross sections for the electron-impact ionization and dissociative ionization of the SiDx (x=1–3) free radicals from threshold to 200 eV using the fast-neutral-beam technique. The deuterated rather than the protonated target species were used in order to allow a better separation of the various product ions from a given parent in our apparatus. A common feature of all three radicals studied in this work is a dominant parent ionization cross section with essentially the same absolute value of roughly 3.7×10−16 cm2 at 70 eV. Dissociative ionization processes for all three targets are less significant with a single dissociative process dominating in each case, viz. the removal of a single D atom (SiDx+e−→SiD+x−1+D+2e−). The cross section for this dominant dissociative ionization channel also had the same maximum value of about 1.2×10−16 cm2 for all three targets. A comparison of the experimentally determined total single ionization cross sections with calculated cross sections using a modified additivity rule showed good to satisfactory agreement for all three targets in terms of the absolute values, but reveals some discrepancies in the cross section shapes.
A dynamical theory of electron transfer: Crossover from weak to strong electronic coupling
Authors: Stockburger Jürgen T. , Mak C. H.
Published: July 2002

We present a real-time path integral theory for the rate of electron transfer reactions. Using graph theoretic techniques, the dynamics is expressed in a formally exact way as a set of integral equations. With a simple approximation for the self-energy, the rate can then be computed analytically to all orders in the electronic coupling matrix element. We present results for the crossover region between weak (nonadiabatic) and strong (adiabatic) electronic coupling and show that this theory provides a rigorous justification for the salient features of the rate expected within conventional electron transfer theory. Nonetheless, we find distinct characteristics of quantum behavior even in the strongly adiabatic limit where classical rate theory is conventionally thought to be applicable. To our knowledge, this theory is the first systematic dynamical treatment of the full crossover region.
Comparative study on the dynamics, polarity, and thermal properties of the isomers of (4-acetyloxyphenyl)-(chlorophenyl)-methanone
Authors: Saiz Enrique , Alvarez Cristina , Riande Evaristo , Pinto Mauricio R. , Salom Catalina
Published: July 2002

The effect of the location of the chlorine atoms on the dynamics, polarity, and thermal properties of the isomers derived from (4-acetyloxyphenyl)-(chlorophenyl)-methanone is discussed. The changes occurring in the dipole moments along the trajectories in their respective conformational spaces are rather large (between 6.0 and 0.3 D) for (4-acetyloxyphenyl)-(2-chlorophenyl)-methanone (2CPM), slightly lower for (4-acetyloxyphenyl)-(3-chlorophenyl)-methanone (3CPM), and relatively small (between 2 and 4 D) for (4-acetyloxyphenyl)-(4-chlorophenyl)-methanone (4CPM). The values of the mean-square dipole moment of 2CPM, 3CPM and 4CPM, calculated from the evolution of their dipole moments along their respective trajectories, are 15.9, 13.1, and 11.2 D2, respectively, in very good agreement with the experimental results 15.8, 13.4, and 11.2 D2, respectively. Both melting and glass transition temperatures of the compounds are discussed in terms of their respective conformational entropies.
Copolymers in asymmetric interface potentials: A Monte Carlo study
Authors: Sommer Jens-Uwe , Peng Gongwen , Blumen Alexander
Published: July 2002

We investigate using scaling methods and Monte Carlo simulations random AB-copolymers at the interface between two good solvents. The asymmetry of the interface potential gives rise to an adsorption–desorption transition which shows for infinitely long chains a critical point. For finite chains we analyze the crossover scaling behavior near this critical point. Two new interface exponents, namely the crossover exponent φ and the interface order parameter exponent β are proposed. Simulation results obtained using the bond fluctuation model agree well with the scaling predictions. From the simulation data we obtain as best estimates φ=1.3±0.1 and β=0.35±0.04.
Interfacial diffusion controlled reactions with time varying absorbing domains
Authors: Baldo M. , Grassi A. , Lombardo G. M. , Raudino A.
Published: July 2002

A theoretical study for diffusional crossing of a tracer through a membrane, where the size of the adsorbing domain (receptor) is time dependent, is presented. A periodic array of absorbing and reflecting domains at the membrane surface was assumed. This model is described by a time-dependent diffusion equation with mixed boundary conditions, where the boundaries are themselves varying with time. Both analytical and numerical methods were employed in order to calculate the total flux of a tracer across the membrane as a function of the oscillation frequency (ω) and amplitude (ε) of the receptor site. The results show an increase in the absorption rate with frequency, reaching a plateau at very high values of ω. The space and time behavior of the diffusing tracer concentration was analyzed, evidencing dephasing effects at high ω which are common to many nonlinear systems where the dynamic response becomes time independent in the high frequency limit. This approach could be useful in studying the anomalous effect of an oscillating electric field on the membrane permeability.
Plane waves basis sets in the description of diatomic anions and valence charge density
Authors: Ireta Joel , Galván Marcelo
Published: July 2002

The performance of plane wave basis sets to describe bond lengths, and vibrational frequencies for diatomic anions is described within the context of ab initio total energy density functional pseudopotential method. Also, the behavior of the charge density as a function of the plane wave expansion size is studied for a molecule containing C, H, O, N, and S atoms. For the three properties studied, a critical size of the basis set that assures a reasonable description of them was found. These critical values are around a cutoff energy of at least 20 Ry below the cutoff energy used to design the pseudopotentials.
Effective O-17 quadrupole moments for the calibrated computation of quadrupole coupling parameters at different levels of theory
Authors: Ludwig R. , Weinhold F. , Farrar T. C.
Published: July 2002

The electric field gradient (EFG) components at the oxygen nucleus in CO, OCS, HNCO, H2CO, H2O, C2H4O, and O2 were calculated in fully optimized ab initio geometries for a wide variety of correlated and uncorrelated methods employing small and extended Gaussian basis sets. Within a specific type of theory and fixed basis level, the calculated EFG components are found to be accurately proportional to quadrupole coupling constants which are accurately known from microwave studies. We determine the characteristic proportionality constant, slightly different for each level of theory, which corresponds to a ‘‘calibrated’’ oxygen-17 quadrupole moment Q that is optimal for theoretical EFG calculations at the chosen level of theory. This generalizes a calibration method due to Huber, offering an improved procedure for determining absolute quadrupole moments (by extrapolating calibrated values to higher levels of theory) without the need for experimental structural data. More importantly, this procedure allows quadrupole coupling properties of quite reasonable accuracy to be obtained, even from very modest levels of theory (e.g., HF/3-21G), despite current uncertainties in the experimental oxygen-17 quadrupole moment.
Ab initio-discrete variable representation calculation of vibrational energy levels
Authors: Kauppi Esa
Published: July 2002

A technique to calculate vibrational energy levels of a triatomic molecule without any explicit functional form for the potential energy surface (PES) is presented. The approach uses potential optimized discrete variable representation (DVR) to calculate the vibrational energy levels while ab initio electronic structure calculations are used to evaluate the potential energy at the nuclear configurations needed in the DVR calculation. The approach is called the ab initio-discrete variable representation or ABI-DVR technique. Example calculations for the water molecule are performed. Vibrational energy levels of H2 16O are calculated up to 14 000 cm−1 above the ground vibrational state within convergence better than 1 cm−1. The potential energy is evaluated using GAUSSIAN 92 program suite. The 6-311+G** Gaussian basis set is used and the electron correlation is taken into account by second-order Möller–Plesset perturbation theory. The ABI-DVR results are compared with results of calculations in which some analytic form for the PES is used to represent the ab initio calculated potential energies and some aspects of how to construct accurate analytic PESs are discussed.
Electronic structure of pyridine-based polymers
Authors: Blatchford J. W. , Gustafson T. L. , Epstein A. J.
Published: July 2002

We present the results of semiemiprical quantum chemical calculations on oligomers of poly(p-pyridyl vinylene) (PPyV) and poly(p-pyridine) (PPy). The presence of a nitrogen heteroatom in the conjugated backbone of these polymers presents a potentially severe breaking of both spatial and charge-conjugation symmetry (CCS), and the addition of nonbonding (n) orbitals has potentially major effects on the photophysics of these systems. Geometries are optimized at the PM3 Hartree–Fock level for neutral, singly charged and doubly charged oligomers. We find that the geometric distortions associated with polaron formation are centered on the vinylene linkages in PPyV-based systems and on the interring bonds in the PPy-based systems. We discuss the electronic structure at the PM3 level applying configuration interaction between singly excited states (SCI), and we demonstrate that the lowest-lying (n→π*) states of the ideal polymer chain are well above the lowest (π→π*) states, leading to strong fluorescence in these systems. Nonplanarity, however, leads to substantial mixing of the (π→π*) and (n→π*) manifolds, thereby altering this conclusion. We calculate absorption spectra for neutral, singly charged (polaron), doubly charged (bipolaron), and triplet-state oligomers using the intermediate neglect of differential overlap/single-excitation configuration interaction (INDO/SCI) technique. For PPyV, comparison of oligomers with differing spatial symmetry allows the isolation of the effects of CCS breaking. All calculated spectra are in good agreement with experimental results and indicate that the symmetry breaking due to the nitrogen heteroatom is weak. In particular, the polaron induces a two-peak in-gap feature into the absorption spectrum and the bipolaron a single-peak feature, as is seen in the analogous all-hydrocarbon polymers.
The C6H6–(H2O)2 complex: Theoretical predictions of the structure, energetics, and tunneling dynamics
Authors: Sorenson Jon M. , Gregory Jonathon K. , Clary David C.
Published: July 2002

A detailed theoretical study of the C6H6–(H2O)2 complex is presented. We characterize the structure and energy by means of various potentials and correlated ab initio calculations. The potential surface is extremely flat but the structures obtained with the empirical potentials and ab initio optimizations agree fairly well. Transition states and corresponding reaction paths are calculated for four possible degenerate rearrangements. The splittings for these mechanisms are calculated from quantum simulations with the diffusion Monte Carlo (DMC) approach. We predict that two splittings should be observable in the spectrum. The DMC calculations also allow prediction of vibrationally averaged structures, bond energies, and rotational constants.
Asymmetric tunneling through ordered molecular layers
Authors: Benjamin Ilan , Evans Deborah , Nitzan Abraham
Published: July 2002

Electron tunneling through ordered molecular barriers is found to depend on the tunneling direction. In particular, the transmission probability of an electron incident in the normal direction on a 10 Å water layer strongly polarized in the tunneling direction is 4–5 orders of magnitude larger when it enters from the positive (hydrogen) face than from the negative (oxygen) face, in contrast to continuum dielectric model prediction. This emphasizes the importance of the discrete multidimensional nature of the barrier structure and suggests possible rectifying properties of such barriers, beyond the linear regime.
Raman spectroscopy of the ν1 N–H stretch fundamental in isocyanic acid (HNCO): State mixing probed by photoacoustic spectroscopy and by photodissociation of vibrationally excited states
Authors: Brown Steven S. , Berghout H. Laine , Crim F. Fleming
Published: July 2002

We report the first gas-phase Raman spectrum of isocyanic acid. Using stimulated Raman excitation (SRE) to prepare vibrationally excited states, we detect transitions by both photoacoustic Raman spectroscopy (PARS) and action spectroscopy. In this paper we present results on the ν1 N–H stretch fundamental, leaving the spectra of the N–C–O symmetric and antisymmetric stretch modes for a separate publication. The Raman spectrum shows extensive state mixing in the ν1 fundamental, in agreement with previous infrared work. Measurement of the effective b-axis rotational constants for different mixed vibrational states in this near-prolate symmetric top limits the number of candidates for perturbing states and shows which vibrational modes participate. Double resonance photodissociation further probes the vibrational spectroscopy of isocyanic acid. The scheme is first to prepare a vibrationally excited state by SRE, then photodissociate only the molecules prepared in the first step, and finally probe the decomposition products by laser-induced fluorescence (LIF). An action spectrum, obtained by scanning the vibrational excitation laser (Stokes) wavelength with the photolysis laser wavelength fixed and the probe laser tuned to a LIF transition in one of the photofragments, is the key to unraveling the spectroscopy. The intensity differences between PARS and action spectrum transitions reveal the vibrational state mixing and provide the Franck–Condon factors for transitions to the excited electronic state.
The ground and first excited singlet state torsional potentials of 2-phenylindole from supersonic jet fluorescence spectra
Authors: Sinclair Wayne E. , Yu Haiping , Phillips David , Hollas J. M.
Published: July 2002

The S1←S0 transition of 2-phenylindole (2PI) was studied in a supersonic jet by laser induced fluorescence. The spectra are dominated by the progression of a low frequency mode assigned to the torsional motion around the bond joining the phenyl and heterocycle moieties. The torsional potentials in the S0 and S1 states are determined from the spectra by fitting the torsional vibrational spacings to a one-dimensional potential of the form V(φ)=12ΣnVn(1−cos nφ). The equilibrium conformation of 2PI in the S0 state is nonplanar with a twist angle between two ring systems of 27° and a barrier to planarity of ∼130 cm−1. 2PI is planar in the S1 state. Franck–Condon analyses were performed to reproduce the intensity distributions in the spectra. Large Duschinsky effects among the low frequency modes are observed to accompany the large geometry change following the S1←S0 excitation.
Fluctuating Euler characteristics, topological disorder line, and passages in the lamellar phase
Authors: Hol/yst Robert , Góźdź Wojciech T.
Published: July 2002

We introduce a concept of topological disorder line for systems with ordered internal surfaces. At one side of the line the ordered structure exhibits strong topological fluctuations, accompanied by changes in the Euler characteristics. At the other side topological fluctuations are rare. In a system of oil-water-surfactant, in the lamellar phase, the crossover between two regimes is marked by the appearance of thin wormhole passages and their further proliferation. Close to the lamellar-microemulsion phase boundary thin wormhole passages merge leading to the formation of large channels between lamellas pierced with holes. The lamellar phase with many large “torus-like” passages strongly resembles the microemulsion phase. In order to illustrate these concepts we perform Monte Carlo simulations of the one scalar order parameter Landau–Ginzburg model of microemulsions. We show how the Euler characteristics can be effectively used in such simulations to identify different ordered phases and count the number of wormhole passages.
Control of long-range electron transfer in dynamically disordered molecular systems by an external periodic field
Authors: Goychuk I. A. , Petrov E. G. , May V.
Published: July 2002

The influence of a strong periodic field on the long-range electron transfer is studied theoretically within the driven spin-boson model extended by a dichotomically fluctuating tunneling coupling. Applying the noninteracting blip approximation, a set of coupled kinetic equations is derived. It describes the time-development of the electronic population difference between the donor and acceptor states averaged with respect to the stochastic process, the quantum fluctuations of the bath and over the fast oscillating field. The strength of the periodic field is taken into account in a nonperturbative manner. A detailed analysis is carried out for the case of a strong coupling of the transferred electron to a specific reaction coordinate. Analytical expressions for the effective transfer rate are obtained and their numerical analysis is presented. For the case of adiabatically controlled (gated) transfer the existence of a transfer regime is demonstrated where the strong external field does not influence the effective transfer rate, however, it can completely revert the direction of the transfer. Finally, it is shown that the periodic field can induce transitions between the quasi-adiabatic (gated) and the nonadiabatic regime of electron transfer.
Electron affinity of hydrogen, deuterium, and tritium: A nonadiabatic variational calculation using explicitly correlated Gaussian basis functions
Authors: Kinghorn Donald B. , Adamowicz Ludwik
Published: July 2002

Nonadiabatic variational calculations for the anion ground state energies, mass shifts, and electron affinities of Hydrogen, Deuterium and Tritium are reported. Electron affinities values are 6083.0994, 6086.7137, and 6087.9168 cm−1 for Hydrogen, Deuterium, and Tritium, respectively. These results were obtained using a basis of explicitly correlated Gaussians. Exact nonrelativistic energy bounds are carefully predicted: E(H−)=−0.5 274 458 811, E(D−)=−0.5 275 983 247, and E(T−)=−0.5 276 490 482 in hartree energy units. It is shown that these new bounds cannot be obtained using the infinite nuclear mass approximation plus Rydberg scaling and the usual first order mass polarization correction.
Internal dynamics contributions to the CH stretching overtone spectra of gaseous nitromethane NO2CH3
Authors: Cavagnat D. , Lespade L.
Published: July 2002

The methyl CH stretching overtone spectra of gaseous nitromethane NO2CH3 have been recorded with Fourier transform infrared conventional near infrared spectroscopy in the ΔvCH=1–4 regions and by intracavity laser photoacoustic spectroscopy in the ΔvCH=5 and 6 regions. They all exhibit a complex structure with, at ΔvCH=1 and 2, a characteristic asymmetric top vibration-rotation profile which vanishes as vibrational energy increases. In addition, in these two lower energy spectra, the perpendicular stretching vibrations exhibit a widely spaced fine structure profile resulting from a Coriolis coupling induced by the methyl internal rotation. These excited spectra have been analyzed with a theoretical model which takes into account, in the adiabatic approximation, the coupling of the anharmonic CH stretch vibrations, described by a Morse potential, with the quasi-free internal rotation of the methyl group and with isoenergetic combination states involving methyl bending modes. Till Δv=3, the theoretical treatment yields normal modes. From the third overtone, the vibrational energy is seen to be localized and thus the calculations proceed in a local mode basis from Δv=4–6. Most of the parameters of this model and their variation with the internal rotation coordinate θ are the same as those used to account for the overtone spectra of the monohydrogenated species. Fermi resonance phenomena, also modeled with θ dependent parameters, lead to only weak IVR localized at the second overtone where only the two first tiers are effective and at the third overtone where three tiers must be considered to give a good reproduction of the experimental spectral features. This simple calculation successfully describes the relative intensity and frequency of each peak within a given overtone.
Photoionization studies of manganese clusters: Ionization potentials for Mn7 to Mn64
Authors: Koretsky Geoffrey M. , Knickelbein Mark B.
Published: July 2002

The photoionization spectra of manganese clusters consisting of between seven and sixty-four atoms have been measured near threshold. As for other transition metal clusters previously investigated, the ionization potentials (IPs) decrease rapidly but nonmonotonically up to n≅20 and more slowly and smoothly beyond that. No correlation is observed between Mnn IPs and their reactivity toward molecular hydrogen, reported previously by Parks et al. [J. Chem. Phys. 104, 3531 (1996)]. In particular, the absence of any discontinuity in IP at Mn16 suggests that the onset of reactivity toward hydrogen noted at that size is not due to a sudden change in electronic structure (e.g., a nonmetal-to-metal transition), but rather to a change in cluster geometry.
Hydrogen bonding in acid–base complexes: The trans-hydroquinone-NH3 complex in its S and S1 electronic states
Authors: Humphrey Susan J. , Pratt David W.
Published: July 2002

We deduce information about the geometry of the hydrogen bond between hydroquinone and ammonia from an analysis of the fully resolved S1←S0 fluorescence excitation spectrum of the 1:1 complex in the gas phase. The complex is planar in both electronic states, with NH3 forming a nearly linear hydrogen bond to one of the hydroxy hydrogen atoms of hydroquinone. The O–H…N heavy atom separation is R=2.85 Å and the barrier to internal rotation of the NH3 group about its C3 axis is V3=35.5 cm−1 in the S0 state. Excitation of the complex to its S1 state decreases the heavy atom separation to R=2.77 Å and increases the torsional barrier to V3=58.8 cm−1. These changes are a direct consequence of the increased acidity of hydroquinone in its S1 state.
Angular distribution functions and specific local structures in liquid water
Authors: De Santis Alberto , Rocca Dario
Published: July 2002

A method of structural analysis, based on restricted averages of the angular pair correlation function, is proposed and applied to liquid water. Minimum free energy paths for a molecule moving in an average local structure are obtained. Around a central molecule, accepting and donating neighbors form hydrogen bonds having the same energy minima but differing for the availability of low free energy states. Some bounded states have specific bridging functions and connect hydrogen-bond and transition states along negative free energy pathways, thus favoring local diffusion. Bridging and transition states play the role of structural defects and include bifurcated hydrogen-bond and interstitial water geometries. Our methods and results can be relevant to understand, from a structural point of view, the anomalous behavior of stressed and confined water.
Low temperature viscosity in elongated ferrofluids
Authors: Alarcón T. , Pérez-Madrid A. , Rubı́ J. M.
Published: July 2002

We have studied the relaxation and transport properties of a ferrofluid in an elongational flow. These properties are influenced by the bistable nature of the potential energy. Bistability comes from the irrotational character of the flow together with the symmetry of the dipoles. Additionally, the presence of a constant magnetic field destroys the symmetry of the potential energy magnetizing the system. We have shown that at a moderate temperature, compared to the height of the energy barrier, the viscosity decreases with respect to the value it would have if the potential were stable. This phenomenon is known as the “negative viscosity” effect. Thermal motion induces jumps of the magnetic moment between the two stable states of the system leading to the aforementioned lowered dissipation effect.
Comparative study of wormlike polymer solutions using statistical mechanics, two-parameter theory, and blob theory
Authors: Dondos Anastasios
Published: July 2002

Using the Han equation derived from the blob theory we obtain that the chains of wormlike polymers consist of one Kuhn statistical segment at the onset of excluded volume behavior. This holds when these polymers are dissolved in any solvent. This is in accordance with the predictions of Yamakawa–Stockmayer based on the statistical mechanics theory. Combining the same equation of the blob theory with a relation derived from the two-parameter theory, and accepting that at the onset of excluded volume behavior we have one Kuhn statistical segment for the wormlike polymers, we obtain a relation from which we calculate the statistical segment length of these polymers from the K and a_ parameters of the Mark–Houwink–Sakurada equation. The obtained values are in good agreement with the values given in the literature.
Hydrodynamic surface waves in concentrated polymer solutions in the presence of surface adsorption
Authors: Wang C. H. , Huang Q. R.
Published: July 2002

A theory of light scattering from the polymer solution/air interface that includes the effect of surface adsorption is developed. The polymer solution is assumed to exhibit a linear viscoelastic response. The surface laser light scattering (SLLS) spectrum is calculated from surface hydrodynamic equations appropriate for linear viscoelastic fluids using the method of initial value problems. Numerical calculations are made to investigate the effects of surface tension and dilational modulus on the SLLS spectrum. The results show that the SLLS spectrum from polymer solutions having a high surface tension solvent displays a two-peak structure. The high-frequency peak exhibits more capillary wave character, and diminishes rapidly with decreasing surface tension or increasing dilational modulus as a result of surface adsorption. The two-peak structure is, however, found to be much less pronounced in a solution with a low surface tension solvent.
The microwave spectrum and nuclear quadrupole hyperfine structure of HCCH-N2O
Authors: Leung Helen O.
Published: July 2002

The microwave spectrum of HCCH-N2O has been collected in the 7–16 GHz region using a Fourier transform microwave spectrometer. The nuclear quadrupole hyperfine structure owing to the two N14 nuclei has been assigned in 15 rotational transitions. Using a Watson S-reduced Hamiltonian with the inclusion of nuclear quadrupole interactions to analyze the spectrum, the rotationaland centrifugal distortion constants (in MHz) are determined to be: A=9394.2683(2), B=2831.85640(8), C=2168.07804(7), DJ=1.2290(3)×10−2, DJK=5.677(4)×10−2, d1=−3.365(2)×10−3, and d2=−7.3(1)×10−4. The nuclear quadrupole coupling constants are also determined. For the terminal nitrogen nucleus, χaa=377.5(4), χbb=−773.1(5), χcc=395.6(5) kHz and for the central nitrogen nucleus, χaa=84.1(9), χbb=−246.6(7), χcc=162.5(7) kHz. The rotational constants give a 3.305-Å separation between the centers of mass of the subunits. HCCH and N2O are approximately parallel to each other, and each is approximately perpendicular to the intermolecular axis. A comparison between the nuclear quadrupole coupling constants for free N2O and HCCH-N2O shows that the electric field gradient at the central nitrogen nucleus is greatly affected by complexation. A distributed multipole calculation suggests that this distortion cannot simply be due to the presence of the charge distribution of HCCH, but is likely to result from an actual electronic redistribution of N2O upon complexation.
An embedded cluster study of dimer buckling on the Si(100) surface
Authors: Yang Cheng , Lee S. Y. , Chuan Kang H.
Published: July 2002

Both Si9H12 and Si15H16 cluster models for the Si(100) surface were studied using the Hartree–Fock molecular-orbital method and density functional theory. Our investigation shows that the ground state of the Si(100) surface consists of buckled dimers, contrary to the results of a number of previous embedded cluster calculations, but in agreement with some recent slab calculations. The relaxation constraints used in previous embedded cluster studies probably do not model the relaxation of the Si(100) surface layers adequately.
Equilibrium properties of the solvated electron in polar liquids: Finite solvent size effects
Authors: Rips Ilya , Tachiya Masanori
Published: July 2002

A microscopic theory of a solvated electron in simple polar liquid is presented. The Pekar-Fröhlich variational approach is extended to account for the finite size of the solvent molecules. The solvated electron is assumed to be completely localized within the cavity created in the solvent. The total energy of the localized electron in the solvent of polarizable hard spheres is derived within the framework of the mean-spherical approximation (MSA). The effect of the molecular size of the solvent, its polarity and polarizability on the equilibrium properties of the solvated electron, namely the radius of the cavity and the binding energy, is studied. Results for the hydrated electron are in a good agreement with the numerical simulations.
Rotational- and vibrational-state resolved HF-surface interactions investigated by surface light-induced drift
Authors: van Duijn E. J. , Nokhai R. , Hermans L. J. F. , Pankov A. Yu. , Krylov S. Yu.
Published: July 2002

Experiments using surface light-induced drift are performed to yield information on the rotational (J) and vibrational (v) state dependence of molecule–surface interactions. Data are presented for the change in accommodation coefficient for tangential momentum transfer α upon excitation of HF interacting with a polycrystalline LiF surface (on a Cu substrate) and a hydrophobic stearic-acid monolayer (on a stainless-steel substrate). We employed both P- and R-branch excitation of HF in the fundamental vibrational band (v=0→1) with J=0–4, using a continuously tunable color-center laser (λ≈2.5 μm). By combining the results for the P- and R-branch, we find that the influences of J and v upon the molecule–surface interaction can be considered independent to a good approximation. It is found that α decreases upon vibrational excitation v=0→1, whereas it increases with increasing J. The J and v dependences of α are discussed in the framework of a unified kinetic theory of molecule-surface interaction.
Extended ensemble molecular dynamics method for constant strain rate uniaxial deformation of polymer systems
Authors: Yang Liu , Srolovitz David J. , Yee Albert F.
Published: July 2002

We describe a novel molecular dynamics (MD) method to simulate the uniaxial deformation of an amorphous polymer. This method is based on a rigorously defined statistical mechanics ensemble appropriate for describing an isothermal, displacement controlled, uniaxial stress mechanical test. The total number of particles is fixed and the normal stresses in the direction normal to the applied strain are constant, i.e., an NTLxσyyσzz ensemble. By using the Lagrangian of the extended system (i.e., including additional variables corresponding to the temperature and cross-sectional area fluctuations), we derive a set of equations of motion for the atomic coordinates and the additional variables appropriate to this ensemble. In order to avoid the short MD time step appropriate for the stiff covalent bonds along the polymer chains, we introduce bond length constraints. This is achieved using a variation of the commonly used SHAKE [J. P. Ryckaert, G. Ciccotti, and H. J. C. Berendsen, J. Comp. Phys. 23, 327 (1977)] algorithm. A numerical method for integrating the equations of motion with constraints via a modification of the velocity Verlet [W. C. Swope, H. C. Andersen, P. H. Berens, and K. R. Wilson, J. Chem. Phys. 76, 637 (1982)] algorithm is presented. We apply this new algorithm to the constant strain rate deformation of an amorphous polyethylene in a model containing several distinct polymer chains. To our knowledge, this is the first time that bond length constraints were applied to a macromolecular system together with an extended ensemble in which the simulation cell shape is allowed to fluctuate.
Living poly(α-methylstyrene) near the polymerization line. V. Heat capacity as a function of temperature
Authors: Zhuang Juewen , Andrews A. Ploplis , Greer S. C.
Published: July 2002

We present here measurements of the heat capacity at constant pressure near the ceiling temperature for the anionic polymerization of an organic monomer. The measurements support the suggestion [S. J. Kennedy and J. C. Wheeler, J. Chem. Phys. 78, 953 (1983); J. C. Wheeler and P. M. Pfeuty, Phys. Rev. Lett. 71, 1653 (1993)] that the onset of equilibrium polymerization in initiated monomer solutions as the temperature is changed can be viewed as a second-order continuous phase transition. We compare our measurements on living poly(α-methylstyrene) initiated by sodium naphthalide in tetrahydrofuran to the theoretical predictions, and find qualitative agreement. We attribute quantitative differences to the dependence of the enthalpy of polymerization on the degree of polymerization and on the tacticity.
Extracting signatures of quantum chaos from the time resolved fluorescence of isolated molecules
Authors: Wilkie Joshua , Brumer Paul
Published: July 2002

We show how to extract a statistically significant temporal signature of quantum chaos from the fluorescence decay of mixed molecular states prepared by multimode partially coherent laser pulses. Specifically, the average fluorescence from appropriately prepared molecules is shown to be an incoherently averaged quantity which, aside from radiative damping factors, is linearly related to the transient part of the average survival probability which distinguishes regular from irregular vibrational dynamics. A three step procedure is developed for removing the masking effect of radiative damping, thereby allowing a direct extraction of a dynamical signature of quantum chaos. Simulated fluorescence signals with regular and irregular vibrational dynamics are analyzed to demonstrate the effectiveness of this approach and to motivate experimental studies.
Ab initio scaling of the second hyperpolarizabilities of carbon cages
Authors: Fanti Marianna , Fowler Patrick W. , Orlandi Giorgio , Zerbetto Francesco
Published: July 2002

A sum over molecular orbitals scheme in an ab initio 6-31G* basis set is used to calculate the static second hyperpolarizabilities, γ, of the eight carbon cages now available in macroscopic quantities: C60, C70, one isomer of C76, three isomers of C78, and two isomers of C84. For C60 and C70, there is good agreement between calculations and available experimental data. The range of nuclearities is extended to cover the most stable isomers of C40 and C100. It is found that γ increases as a nonlinear function of both the number of atoms and the number of partial double bonds (hexagon–hexagon edges). Two proposed scaling laws lead to very different values of γ for systems with large numbers of carbon atoms but, even for the less steeply rising function, comparison with experimental data shows that, weight for weight, fullerenes should have a larger response than polyenic systems.
Infrared laser spectroscopy of P2O
Authors: Bell Iain S. , Qian Hai-Bo , Hamilton Peter A. , Davies Paul B.
Published: July 2002

The P=O stretching fundamental, ν3, of P2O has been observed using diode laser absorption spectroscopy. The P2O species was generated in situ in a multipass cell from the reaction of O atoms with white phosphorus vapor (P4). These measurements significantly extend the preliminary observation of this species and in conjunction with the results of microwave spectroscopy enable the rotational structure of the band to be reliably assigned for the first time. The (0111)←(0110) hotband is also weakly observed in this system, with many of the transitions having resolved l-type doubling. The observed data are fitted to standard models to determine the full range of rotational parameters. Band origins of 1277.6454(3) and 1274.8993(10) cm−1 are obtained for the (001)←(000) and (0111)←(0110) bands, respectively.
Cluster translation and growth in density functional theories of homogeneous nucleation
Authors: Barrett Jonathan
Published: July 2002

Density functional theories of nucleation relate the thermodynamic properties of a supersaturated vapor to a density profile which is usually interpreted as that of a critical cluster. It is argued here that mean field density functional theory gives a profile which includes a small amount of translational motion, as well as growth, of the cluster. These fluctuations, equivalent to those of a hard sphere gas in a suitably-shaped potential well, must be factored out before the nucleation rate is obtained. Correction for the growth fluctuation has little effect and the usual Zeldovich factor is reasonably accurate. However, correction for translational motion leads to a large enhancement to the usual nucleation rate expression, by a factor approximately proportional to the ratio of liquid to vapour densities and between about 106 and 108 for nonane under usual experimental conditions.
Reaction site switching in carbon monoxide oxidation on platinum (113): A spatial distribution study of desorbing product
Authors: Yamanaka Toshiro , Moise Calin , Matsushima Tatsuo
Published: July 2002

The reaction sites involved in the CO oxidation were studied on a stepped Pt(113) [or (s)2(111)×(100)] surface through analysis of the spatial distribution of desorbing product CO2. The distribution was measured with angle-resolved thermal desorption spectroscopy. Five CO2 formation peaks are observed during heating of the coadlayer of CO and oxygen, around 370 K (P1-CO2), 300 K (P2-CO2), 270 K (P3-CO2), 235 K (P4-CO2), and 180 K (P5-CO2). The reaction sites working for these CO2 formations depend strongly on the CO and oxygen coverage. Generally, the CO2 formation on the (111) terrace is predominant at high CO coverage, whereas the contribution from the (001) step is enhanced at high oxygen coverage. A mechanism is proposed for this reaction site switching.
Two-photon spectroscopy of the low lying Rydberg states of NO. II. Application to the NO–Ar complex
Authors: Meyer Henning
Published: July 2002

The rotational structure of two-photon spectra of van der Waals complexes involving an open shell diatomic molecule is analyzed in terms of the spherical tensor components of the two-photon absorption operator of the uncomplexed diatom. The formalism is used to explain the rotational structure of several band systems of the NO–Ar complex correlating with Rydberg states of the bare NO molecule. All spectra recorded suggest an N–O bond distance typical for a Rydberg state. For the C 2Π state, the vibrationally averaged structure is T shaped. Within the effective experimental resolution no P-type doubling could be identified. For the bands correlating with the state E 2Σ, a significant deviation from the T-shaped structure is found. Once the rotational contour can be simulated, a complete analysis of the vibrational structure due to the intermolecular vibrations can be performed. For the Ar–NO E state, the bending and stretching frequencies suggest a close resemblance to the cationic complex Ar–NO+.
Low-energy electron attachment to brominated methanes
Authors: Sunagawa Takeyoshi , Shimamori Hiroshi
Published: July 2002

The rate constants as a function of the mean electron energy from thermal to about 2 eV at room temperature have been measured for electron attachment to CBr4, CHBr3, CFBr3, CF2Br2, CH2BrCl, CHBr2Cl, and CBrCl3 using the pulse-radiolysis microwave-cavity method combined with microwave heating. The electron attachment cross sections, derived from the rate constant data, all show maximum at zero energy with no noticeable peak at higher electron energies. Based on the differences observed in the absolute magnitude of the cross sections among the brominated compounds as well as those between brominated and the corresponding chlorinated methanes, a model for the dissociative attachment to brominated methanes has been presented.
Electronic structure and photoelectron spectroscopy of AlSi mixed dimer
Authors: Wang Xue-Bin , Wang Lai-Sheng
Published: July 2002

The electronic structure of the heterodimer AlSi is experimentally studied using anion photoelectron spectroscopy. Four low-lying electronic excited states are observed for AlSi. The electron affinity of AlSi is measured to be 1.32 (5) eV, which is lower than that of both Al2 and Si2. The electronic structure of AlSi is understood by comparing to that of the known Al2 and Al2− molecules. The ground state of AlSi is determined to be X 4Σ− with a vibrational frequency of 400 (50) cm−1. The four excited states are A 2Σ−, B 2Δ, C 2Π, and D 2Σ+ with excitation energies of 0.23, 0.67, 0.82, and 1.13 eV, respectively. The photoelectron spectra of Al2− are also presented and excited states of the Al2− anion are definitively observed. The electronic structure of AlSi is discussed and compared to that of the homonuclear dimers, Al2 and Si2.
The microwave spectrum of the cyanomethyl radical CH2CN(2B1)
Authors: Saito Shuji , Yamamoto Satoshi
Published: July 2002

The rotational spectrum of the cyanomethyl radical CH2CN in its B12 ground electronic state was studied by microwave spectroscopy using a source-modulated spectrometer equipped with a free-space cell. The CH2CN radical was generated in the cell by dc-glow discharge in pure CH3CN, and CD2CN in CD3CN. Fifty a-type R-branch rotational transitions were measured in the frequency region of 80 to 282 GHz, where fine structure and hyperfine structures due to the nitrogen and hydrogen nuclei were resolved for lower-N transitions, but only the fine structure for higher-N transitions. Similarly fine-structure resolved rotational transitions were measured for CD2CN in the frequency region of 190 to 286 GHz. The molecular constants of CH2CN, including the fine-structure constants and hyperfine coupling constants due to both the nuclei of N and H, were precisely determined by least-squares methods from 146 observed spectral lines, including 36 astronomically observed lines for the Ka=0components of N=1–0 and 2–1 from a dark cloud TMC-1 [Kaifu et al., Astrophys. J. Suppl. (submitted 1997)]. Those of CD2CN were similarly observed from 92 observed spectral lines. The rotational constants of both the species were used to derive molecular structural parameters of CH2CN:r0(CN)=1.1919(13) Å, r0(CC)=1.3680(12) Å, and r0(CH)=1.0894(7) Å on the assumption of 120.22° for ∠HCH. The spin densities of unpaired electrons estimated from the hyperfine coupling constants for the N and H atoms are consistent with the molecular structure obtained.
The dissociation of HNO. I. Potential energy surfaces for the X̃ 1A′, Ã 1A″, and ã 3A″ states
Authors: Mordaunt David H. , Flöthmann Heiner , Stumpf Michael , Keller Hans-Martin , Beck Christian , Schinke Reinhard , Yamashita Koichi
Published: July 2002

Three-dimensional potential energy surfaces for the X̃ 1A′, Ã 1A′′, and ã 3A′′ states of HNO have been calculated at the multireference configuration interaction (MRCI) level of ab initio theory. Energy points are calculated at 1728 molecular configurations, predominantly sampling the HNO well and the H+NO product channel regions. Energies between grid points are obtained by interpolation with a three-dimensional cubic spline. The well depths are 2.14, 1.27, and 0.38 eV for X̃ 1A′, ã 3A′′, and Ã 1A′′, respectively. Saddle points to inversion, isomerization, and dissociation are reported and their importance discussed to relevant processes. The HNO(X̃ 1A′) potential energy surface is purely attractive along its minimum energy path to ground-state products, whereas the ã 3A′′ and Ã 1A′′ states have barriers of 0.21 and 0.50 eV, respectively. Vibrational term values and rotational constants for HNO and DNO are reported for the fundamental vibrations for all three electronic states. Where comparison with experimental data is possible, the agreement is satisfactory. The three potential energy surfaces are appropriate to study the predissociation dynamics in the Ã 1A′′ state and the unimolecular dissociation in the X̃ 1A′ state.
Photoionization efficiency spectrum and ionization energy of C2H5SO
Authors: Cheng Bing-Ming , Hung Wen-Ching , Chen Wei-Chen , Yu Chin-hui , Lee Yuan-Pern
Published: July 2002

The photoionization efficiency (PIE) spectrum of C2H5SO was measured in the spectral range 110–150 nm by means of a photoionization mass spectrometer coupled to a synchrotron radiation source. C2H5SO radicals were generated by reacting O atoms with C2H5SC2H5 or C2H5SSC2H5 in a discharge flow tube. The ionization energy of C2H5SO was determined to be (8.71±0.02) eV, in agreement with theoretical calculations of 8.8 eV for ionization from C2H5SO to singlet C2H5SO+ using the G2 method. PIE spectrum also shows a sharp increase at 10.0 eV, corresponding to the ionization energy from C2H5SO to triplet C2H5SO+ predicted by theory. Theoretical calculations also predict two stable conformers with similar energies for each of C2H5SO, singlet C2H5SO+, and triplet C2H5SO+, respectively; these conformers are indistinguishable in our experiments.
Effects of a quantum-mechanically driven two-state gating mode on the diffusion-influenced bimolecular reactions
Authors: Jung Younjoon , Hyeon Changbong , Shin Seokmin , Lee Sangyoub
Published: July 2002

The reduced distribution function formalism of diffusion-influenced bimolecular reactions is generalized to incorporate a quantum-mechanical gating mode in the description. An analytical expression for the reaction rate coefficient is obtained in the Laplace transform domain for a general initial condition. For a simple reaction model, the time-dependent reaction rate coefficient and the product yield are calculated numerically for two representative initial conditions. Dependence of the rate coefficient and the product branching ratio on various reaction parameters is discussed.
Control of molecular chirality
Authors: Shao Jiushu , Hänggi Peter
Published: July 2002

We present a theoretical study of controlling molecular chirality in chemical reactions by circularly polarized fields. A theory of absolute asymmetric synthesis is formulated by standard perturbation theory. An estimation of the chiral polarization of the product is provided. We show that an unstable chiral configuration can be described as a spin one-half particle. The circularly polarized field with well adjusted parameters is applied to stabilize chiral molecules (destruction of quantum coherence in the two-level system).