bluuebunny/crossref_metadata_2025 huggingface dataset:
Sensitivity analysis of distribution functions of liquid water
Authors: Zhu Sheng-Bai , Wong Chung F.
Published: July 2002

We have carried out sensitivity analysis studies to identify the most important potential parameters of the SPC and the TIP3P flexible water models in determining three distribution functions of liquid water—the site–site radial distribution functions, the distribution function of the interaction energy of a water molecule with its surrounding [Pu(ub)], and the distribution function of the local electric field at the oxygen of a water molecule projected along the permanent dipole moment vector of the water molecule [Pe(E⋅μ0)]. The site–site radial distribution functions of each water model are most sensitive to the equilibrium O–H bond length (rOH) and the Lennard-Jones radius (σ) of the model. In addition to these two parameters, the cut-off radius (Rcut) in the reaction field geometry and the atomic partial charges are also important in affecting Pu(ub) of each model, especially the wings of the distribution. The oxygen charge affects the low energy wing of Pu(ub) more than the high energy wing, whereas the hydrogen charge mainly affects the high energy wing and has little effect on the low energy wing of the distribution. As for Pe(E⋅μ0), which provides an indirect check of the validity of the effective charge approximation in accounting for molecular polarizability, the leading factors are rOH, σ, and Rcut. These parameters affect the distribution wings of Pe(E⋅μ0) the most. The key advantage of the sensitivity analysis technique is that it provides a systematic and economical means for studying the role of each parameter of a water model in affecting the properties of the model.
Photodepletion and dynamics of oxygen atoms in xenon matrices
Authors: Ryan E. Todd , Weitz Eric
Published: July 2002

The photodepletion and reaction dynamics of O atoms isolated in xenon matrices have been studied. Oxygen atoms are generated by 193 nm photolysis of a N2O dopant and monitored via the laser-induced fluorescence of XeO exciplexes produced by the 248 nm excitation of Xe/O pairs. Dissociative relaxation of XeO exciplexes generates energetic O atoms which may travel significant distances in the matrix. This photoinduced mobility can lead to O atom loss. The O atom loss process is found to be bimolecular from 15 to 42 K at all fluences studied and is ascribed to an O+O recombination process. Higher temperature data (42 K) may exhibit behavior characteristic of a transition from second order to first order kinetics. A kinetic scheme is derived which can be used to model the data and infer a microscopic description of the O atom dynamics.
Nonadiabatic transitions in liquid phase reactions. Net electron polarization in radical pair recombination and triplet-radical quenching
Authors: Shushin A. I.
Published: July 2002

The nonadiabatic transitions leading to the net chemically induced dynamic electron polarization (NCIDEP) in radical–radical (RR) recombination and triplet–radical (TR) quenching are analyzed in detail. The theory treating the transitions induced by both the fluctuating and the nonfluctuating interactions in a unified manner is developed. The discussed nonadiabatic mechanism of NCIDEP generation can be considered as an extension of the well-known ST−-mechanism. Simple analytical formulas for NCIDEP are derived which are applicable in different regions of parameters of the model suggested. It is shown that the fluctuating (anisotropic) hyperfine interaction (hfi) can make significant contribution to NCIDEP RR-recombination which is comparable with the contribution of nonfluctuating (isotropic) hfi. The fluctuating zero-field splitting interaction dominates the generation of NCIDEP in TR-quenching. Contribution of the fluctuating interaction appears to strongly depend on the correlation time of fluctuations. Theory predicts essentially different dependencies of the contributions of fluctuating and nonfluctuating interactions on the viscosity of a solution. The difference can easily be distinguished experimentally.
Rotational constants for the C̃ 2A2 state of NO2
Authors: Crawford T. Daniel , Schaefer Henry F.
Published: July 2002

The coupled-cluster method including all single and double excitations (CCSD) and CCSD including perturbatively applied triple excitations [CCSD(T)] have been used with large basis sets to study the rotational constants and associated geometrical parameters of the C̃ 2A2 state of NO2. The results compare favorably to earlier theoretical work but remain in conflict with recent optical–optical double resonance spectroscopic results.
A multireference configuration interaction study of the low-lying electronic states of ClO+2 and the X 1A1 state of ClO−2
Authors: Peterson Kirk A. , Werner Hans-Joachim
Published: July 2002

Near-equilibrium two-dimensional (C2v symmetry) potential energy functions of the first seven electronic states of ClO2+ and the ground X 1A1 electronic state of ClO2− have been calculated using internally contracted multireference configuration interaction (CMRCI) wave functions. In the case of ClO2+, five electronic states (3B2, 3A2, 1A2, 3B1, and 1B1) are predicted to lie within 3 eV of the X 1A1 ground state, while a 1B2 state has a calculated Te of just over 5 eV. In the X state, the equilibrium geometry is calculated by CMRCI to be re=1.423 Å and θe=120.8°. Harmonic vibrational frequencies of 1012 (ω1), 511 (ω2), and 1283 cm−1 (ω3) were derived from the computed potential energy function, and ω1 and ω2 were found to be in good agreement with the values estimated from the photoelectron spectrum of ClO2. For the X 1A1 state of ClO2−, calculated spectroscopic constants include re=1.573 Å, θe=113.9°, ω1=789 cm−1, ω2=378 cm−1, and ω3=848 cm−1. Electric dipole moment functions have also been derived for both species in their electronic ground states. The calculated equilibrium dipole moments for ClO2+ and ClO2− 1.46 and 2.55 D, respectively, make them good candidates for observation by pure rotational spectroscopy. Within the double harmonic approximation, fundamental infrared intensities have also been derived, and these results predict a relatively intense infrared vibrational spectrum in the ν3 mode of ClO2−.
Charge transfer excitations in the photoelectron spectrum of Cl−NH3: Experiment and calculation
Authors: Markovich Gil , Cheshnovsky Ori , Kaldor Uzi
Published: July 2002

A charge transfer (CT) excited state has been identified in the photoelectron spectrum of Cl−NH3. A narrow band at 4.0 eV binding energy (BE) is assigned to ionization of Cl−, and a broad band peaking at 6.45±0.15 eV BE corresponds to a CT state, Cl−NH+3. The assignments are confirmed by ab initio calculations, yielding binding energies of 3.88 and 6.36 eV.
Fluctuations in state-selected unimolecular decomposition: Double-resonance infrared visible photofragment yield spectroscopy of NO2
Authors: Reid S. A. , Brandon J. T. , Hunter M. , Reisler H.
Published: July 2002

Fluctuations in the NO(2Π) state distributions from the state-selected simple bond fission unimolecular reaction of NO2 are probed using double-resonance photofragment yield (PHOFRY) spectroscopy. These spectra are sensitive probes of the correlation of selected NO rovibronic levels with specific resonances in the excited molecule.
A molecular theory of the line shape: Inhomogeneous and homogeneous electronic spectra of dilute chromophores in nonpolar fluids
Authors: Saven J. G. , Skinner J. L.
Published: July 2002

Kubo’s stochastic theory of the spectral line shape provides an elegant phenomenological description of inhomogeneous and homogeneous broadening and the transition between the two. This theory has been used profitably in the analysis of many experiments. In this paper we attempt to provide a microscopic foundation for the Kubo model by developing a completely molecular theory of the line shape. For definiteness we focus on the optical line shape of dilute chromophores in nonpolar fluids. Many of the features of the Kubo theory are found in the molecular theory; indeed, the molecular theory produces microscopic expressions involving the solvent structure and dynamics for Kubo’s phenomenological parameters, and provides some justification for the Gaussian assumption in the stochastic theory. On the other hand, the molecular theory produces a transition frequency time-correlation function that is distinctly nonexponential, in contrast to the exponential assumption of the Kubo theory, and it is found that this nonexponentiality is necessary for the accurate description of line shapes in the regime intermediate between inhomogeneous and homogeneous broadening. For a model of Lennard-Jones particles the molecular theory is compared with molecular dynamics computer simulations.
Electronic structure of semiconductor nanoclusters: A time dependent theoretical approach
Authors: Hill Nicola A. , Whaley K. Birgitta
Published: July 2002

We present a time dependent theoretical approach to calculating the electronic properties of semiconductor nanoclusters. The technique can be applied to ground and excited electronic states without using the effective mass approximation or perturbative expansions. The effects of surface properties on the electronic structure can be calculated at an atomic level. We illustrate the method with calculations of ground state densities of states for CdS and CdSe crystallites between 19 and 33 Å in diameter. The size-dependence of the bandwidths and band gap is studied, and the influence of surface states and surface polarity is discussed. The calculated shift in the valence band edge with cluster size is compared with experimental results from valence-band photoemission. Good agreement is obtained.
Electron solvation in methane and ethane
Authors: Liu Zhihua , Berne Bruce J.
Published: July 2002

The solvation of excess electrons in fluid methane and ethane is studied by path integral Monte Carlo computer simulation and by the fast-Fourier-transform–Lanczos diagonalization method using a newly developed electron-alkane pseudopotential. Many-body polarization interactions between solvent molecules are treated using a mean field approximation in the simulation. In methane, it is found that the electron is in an extended state throughout the whole fluid density range studied. In ethane, it is found that the solvated electron gradually becomes localized or ‘‘self-trapped,’’ with cavity formation occurring at a fluid density where experimentally the electron is found to have a very low mobility and the threshold value for electron photoconduction rises above zero. The electronic ground state energies in the unperturbed solvent and in the electron-equilibrated solvent were compared. At the same density these electronic energies are very close to each other in methane and in the low density ethane fluids, but at higher ethane densities, where cavity formation takes place, the ground state energy in the electron solvated fluid is lower than that in the unperturbed fluid.
C2 from laser produced carbon plasma
Authors: Thareja Raj K. , Abhilasha
Published: July 2002

We report the appearance of molecular C2 Swan bands and high pressure band (ν′=6) in the presence of helium gas in photoablated carbon. The vibrational temperature estimated using the Swan bands in the pressure range of 10−2 to 100 Torr of helium gas is presented.
Detection and reactions of the HOCO radical in gas phase
Authors: Miyoshi Akira , Matsui Hiroyuki , Washida Nobuaki
Published: July 2002

The HOCO radical has been successfully detected by a photoionization mass spectrometry in gas phase. The HOCO radical produced by the reaction of Cl+HCOOH (with small excess energy; 13.0 kcal mol−1) was found to be stable and its lifetime was confirmed to be very long (≳10 ms) at room temperature. On the other hand, the HOCO radical formed by the 193 nm photolysis of C2H3COOH (with large excess energy; 55.8 kcal mol−1) exhibited fast, nonexponential, and total pressure-dependent decay. This behavior was well interpreted by a competition of dissociation and relaxation of ‘‘hot’’ radicals. Further, the decay of ‘‘hot’’ radicals showed an apparent isotope effect (DOCO/HOCO). This fact together with an energetic consideration suggests that the observed decay is dominated by a tunneling dissociation to H+CO2, which implies the importance of the tunneling in the OH+CO reaction. The vibrational relaxation rate of the HOCO radical was roughly evaluated from the pressure dependence of the decay profile of ‘‘hot’’ HOCO radicals. The rate of reaction of HOCO radical with molecular oxygen has also been determined, and appeared pressure-independent.
First-order radial distribution functions based on the mean spherical approximation for square-well, Lennard-Jones, and Kihara fluids
Authors: Tang Yiping , Lu Benjamin C.-Y.
Published: July 2002

Following the assumption of the mean spherical approximation, analytical expressions of the first-order radial distribution function are obtained for the square-well, Lennard-Jones, and Kihara potentials with a hard core through a complex-plane analysis. The developed expression yields good agreement with the available computer simulation data for the radial distribution function of the Lennard-Jones fluid.
Surface scattering of NO from Ag[111]: A statistical description of rotational energy distributions
Authors: Pettersson Jan B. C.
Published: July 2002

A statistical model is applied to inelastic scattering of NO molecules from the Ag[111] surface. Calculated final rotational energy distributions are found to be in good agreement with experimental distributions including pronounced ‘‘rotational rainbows’’ [Phys. Rev. Lett. 47, 1169 (1981)]. The model has previously been applied to NO scattering from graphite at lower collision energies [J. Chem. Phys. 93, 845 (1990)]. In the model, a scattering molecule and a small part of the surface form a collision complex which decomposes in a unimolecular fashion. The molecule is treated as a rigid rotor, and the simulated part of the surface as a few harmonic oscillators. The calculations indicate that the experimental results to a first approximation are statistical, and that no detailed dynamics have to be taken into consideration to explain them. The shape of the rotational energy distributions is due to conservation of the angular momentum component in the surface normal direction, introduced since the surface is treated as flat. Rotational rainbows are thus reproduced without introducing any detailed information about the molecule–surface interaction potential. The number of surface oscillators used in the model is varied, and in general one to four oscillators best reproduce the experimental results. The calculations indicate that the angular acceptance of the laser-induced fluorescence experiments is of large importance for the obtained final rotational energy distributions. An analytical solution to the statistical problem is derived for the case of fixed initial energy terms, and it is shown to describe well the experimental distributions here discussed.
Dynamical structure of water in aqueous solutions of D-glucose and D-galactose by low-frequency Raman scattering
Authors: Wang Yan , Tominaga Yasunori
Published: July 2002

Low-frequency depolarized Raman spectra of aqueous solutions of D-glucose and D-galactose have been investigated in the frequency region from −250 cm−1 to 250 cm−1 at 30.0 °C as a function of concentration up to 0.04 molar ratio. The dynamical structure of water in aqueous solution is analyzed by using the reduced Raman spectrum χ″(ν̄), which corresponds to the imaginary part of the dynamical susceptibility. The reduced spectrum is fitted with the superposition of one Cole–Cole type relaxation mode and two damped harmonic oscillator modes by a nonlinear least-squares fitting. The effects of D-glucose and D-galactose on the dynamical structure of water in aqueous solution are similar. The relaxation time of hydrogen bond among water molecules becomes slower with increasing sugar concentration. The characteristic frequencies of stretching-like and bending-like vibrations among water molecules do not change in both D-glucose and D-galactose aqueous solutions.
A discharge flow-photoionization mass spectrometric study of HOBr(X 1A′): Photoion yield spectrum, ionization energy, and thermochemistry
Authors: Monks P. S. , Stief L. J. , Krauss M. , Kuo S. C. , Klemm R. B.
Published: July 2002

The photoion yield spectrum of HOBr was measured over the wavelength range λ=108–121 nm by using a discharge flow-photoionization mass spectrometer apparatus coupled to a synchrotron radiation source. HOBr was generated by the reaction of OH with molecular bromine. A value of (10.62±0.04) eV was obtained for the adiabatic ionization energy (I.E.) of HOBr from photoion thresholds, corresponding to the HOBr+(2A″)←HOBr(1A′) transition. The structure observed in the spectrum is discussed in terms of the available states for HOBr+, which have been determined using multiconfiguration-self-consistent field calculations. A new value for ΔH0f 298(HOBr) of −9 kcal mol−1 is derived from I.E.(HOBr) and estimates of ΔHf(HOBr+).
Vacuum ultraviolet laser/time-of-flight mass spectroscopy: Ion-pair spectra of 79Br35Cl
Authors: Dimov S. S. , Lipson R. H. , Turgeon T. , Vanstone J. A. , Wang P. , Yang D. S.
Published: July 2002

A newly built vacuum ultraviolet (VUV) laser/time-of-flight mass spectrometer is described, which has been used to record the first vibrationally and rotationally resolved spectra of the E0+ ion-pair state ←X0+ ground state transition of jet-cooled bromine monochloride BrCl, near 145 nm. The mass resolution of the instrument (m/Δm≊1000) was sufficient to obtain single isotopomer data. Preliminary results for 79Br35Cl are reported. Like other interhalogens, BrCl vibronic band intensities vary widely in the VUV, presumably as a result of excited state perturbations. This scenario has been corroborated by the observation of an irregularly spaced vibrational progression, and the irregular behavior of the excited state rotational constants as a function of v′.
Induced infrared absorption of HD physisorbed on NaCl films: A second adsorption site
Authors: Dai David J. , Ewing George E.
Published: July 2002

In addition to our previously reported study of physisorption of hydrogen to Na+ sites on the (100) faces of NaCl crystallites [Dai and Ewing, J. Chem. Phys. 98, 5050 (1993)], there is a second site (the Cl− ion) available for adsorption. Here, we report the induced infrared absorption of HD adsorbed on this second site. The infrared transition of these adsorbed HD molecules is blue shifted by 10 cm−1 from the absorption of the first adsorption site molecules. Isotherms of HD on the second site yield an isosteric heat of adsorption of −1.7±0.5 kJ/mol, or approximately half that for the first site.
Rotational tunneling of orientationally coupled methyl groups: Molecular field approximation versus numerically exact solution
Authors: Timann Martin , Voll Günter , Häusler Wolfgang
Published: July 2002

The orientational coupling of methyl groups leads to a split rotational tunneling line. We study the dependence of this splitting on the type and strength of the interaction. Two types of pair potentials are considered (M1) cos 3φi cos 3φj and (M2) cos 3(φi+φj). The tunneling spectra are calculated numerically exact for N=2 and N=3 rotors by using a sparse matrix method (SMM). For model M1 we also probe the molecular field approximation (MFA). For both, N=2 and N=3, not even qualitative agreement is found between MFA and SMM. But a trend can be seen towards MFA-like behavior; with increasing N, the dependence of the splitting on the coupling strength qualitatively resembles the MFA prediction. However, by our SMM results, we are led to the conjecture, that for moderate coordination numbers a considerable splitting should occur within a much broader range of coupling strengths than predicted by MFA.
Measurement of the V–T energy transfer rates of highly excited 2A1 NO2
Authors: Mazely Troy L. , Friedl Randall R. , Sander Stanley P.
Published: July 2002

Production of electronic ground state NO2 (2A1) from 248 nm photolysis of HNO3 was detected by laser induced fluorescence (LIF). A growth in the LIF signal was observed following the photolysis and has been interpreted as the relaxation of NO2 through the higher vibrational levels of the X(2A1) state; an energy region where the probe laser photodissociates the NO2 instead of inducing fluorescence. The rate coefficients for NO2 relaxation through these high vibrational levels were determined by fits of time resolved LIF signal to a stepladder kinetic model. The results of the kinetic analysis suggest that the observed relaxation begins at the 2B2 threshold near 9500 cm−1 and extends downward through approximately 5 vibrational levels of the ground electronic surface. The derived quenching rate coefficients (in units of 10−12 cm3 molecule−1 s−1) are 0.51±0.05, 1.0±0.1, 1.4±0.2, 2.6±0.6, and 8.7±1.1 for Ar, He, N2, O2, and CO2 collision partners, respectively. The discrepancies between these coefficients and previous literature values are rationalized in terms of a dependence of the vibrational relaxation rate on total internal energy.
Dynamic Monte Carlo simulations of freezing and melting at the 100 and 111 surfaces of the simple cubic phase in the face-centered-cubic lattice gas
Authors: Moss Richard , Harrowell Peter
Published: July 2002

Dynamic Monte Carlo simulations are used to study the structure and dynamics of the 100 and the 111 crystal–liquid interfaces of the simple cubic phase in the face-centered-cubic lattice gas. At equilibrium the two surfaces are found to be distinguished only by the presence of strong surface-induced structural fluctuations at the close packed 100 surface. Away from equilibrium we find (i) the surfaces exhibit identical velocity vs temperature curves; (ii) the interfacial velocity is a linear function of temperature (except at small supercoolings); (iii) the slope of velocity vs temperature in these linear regions is, for melting, double that seen in freezing; and (iv) over a range of small supercoolings (less than 0.5% of the melting temperature) the interfacial velocity vanishes. The 111 surface is found to exhibit microfacets of 100 orientation over the entire temperature range studied. The asymmetry between freezing and melting dynamics and the origin of the stationary surface at small supercoolings are discussed in terms of the role of symmetry breaking fluctuations on the dynamics of interface motion.
Molecular beam stimulated emission pumping spectroscopy of propynal
Authors: Rogaski C. A. , Wodtke A. M.
Published: July 2002

Stimulated emission pumping (SEP) experiments were performed on propynal (H–C≡C–CHO). The SEP spectra probed the overtones of the pure C=O stretch (ν4) and a combination of the C=O stretch with the C≡C–C bend (ν9). Term energies for J≤8, K≤2 of the vibrations 42, 43, 44, 45, 4391 as well as two unidentified perturbing vibrational states were obtained. The SEP spectra show simple rotational structure that is assignable by means of a near prolate asymmetric top model. Rotational constants were obtained for all the vibrational states measured. Spectroscopic perturbations are observed for ν4=2,4,5 as either anomalous rotational constants, anomalous energy level shifts, or as extra spectral transitions. Complex state mixing is not observed at the current spectroscopic resolution (0.04 cm−1) even though vibrational state densities as high as 400 states per cm−1 were probed. The propynal SEP results are compared to other SEP experiments on aldehyde containing molecules and inferences are made.
Transport properties and interaction potential for Na+ ions in Xe gas
Authors: Tan T. L. , Ong P. P. , Hogan M. J.
Published: July 2002

The ratio, DT/K, of the transverse diffusion coefficient to mobility for Na+ ions drifting in Xe gas has been experimentally measured. Inevitable longitudinal end effects present in the drift tube were duly corrected for. Results at ∼303 K are reported at electric field to neutral gas density ratios, E/N, ranging from 5 to 440 Td. Furthermore, elaborate calculations of the reduced mobility, K0, the ratio, DL/K, of the longitudinal diffusion coefficient to mobility, and DT/K were calculated with a Monte Carlo simulation (MCS) method which used the interaction potential of Koutselos, Mason, and Viehland [J. Chem. Phys. 93, 7125 (1990)] as input. The accuracy of this potential has been evaluated by comparing the results with the experimental data and those calculated using the generalized Einstein relations technique. Agreement within 3% between the experimental values of DT/K and those from MCS calculations for the whole range of E/N suggests that the interaction potential represents fairly well the true potential. The error of the experimental measurements of DT/K is estimated to be better than 3.5% and that of the MCS calculations is better than 1% for K0 and 2.5% for DT,L/K.
Lattice theory of solvation and dissociation in macromolecular fluids. I. Mean field approximation
Authors: Olender Roberto , Nitzan Abraham
Published: July 2002

This paper analyzes solvation and association in macromolecular solvents using mean-field theory applied to several lattice–gas models. The thermodynamic functions of solvation and the equilibrium constant for association is calculated in terms of the temperature, pressure, and solvent molecular size. Applications to ion association in polymer electrolytes are discussed and the advantages and flaws of the model are critically analyzed. Exact results for a simple one dimensional model are also obtained and discussed.
Quantum dynamics and microcanonical rate theory
Authors: Schofield Sarah A. , Wolynes Peter G.
Published: July 2002

We present a model calculation for the determination of quantum microcanonical average reaction rates for isolated molecules. The model system consists of two coupled electronic states, reactant and product, each with many nuclear motion eigenstates. The reaction rate is evaluated via a resummed perturbation expansion in the electronic coupling. From this we obtain an electronic adiabaticity criterion which depends on the probability to remain at or return to the transition region. We evaluate this survival probability using a global random matrix model for the nuclear Hamiltonian. This leads to an equation for the reaction rate which interpolates between the golden rule electronic crossing rate and the electronically adiabatic rate. We discuss the classical limit and make connection with thermal rate theory.
Molecular dynamics and mean spherical approximation results for symmetric nonadditive hard core Yukawa mixtures
Authors: Rey C. , González L. E. , Gallego L. J. , González D. J.
Published: July 2002

The thermodynamic properties of some symmetric hard core Yukawa mixtures with positively and negatively nonadditive diameters are studied by molecular dynamics (MD) simulation, and the results compared with the predictions of the mean spherical approximation (MSA). MSA results for the compressibility factors are obtained by taking as reference the semiempirical equation of state proposed by Gazzillo and Pastore for symmetric nonadditive hard sphere fluids. Our calculations show that the ‘‘exact’’ computer simulation data are, in general, closely approximated by the MSA energy route to the thermodynamic properties. For the largest positive nonadditivity considered in this paper, some discrepancies appear between the compressibility factors obtained by MD and the MSA, but it is shown that the origin of the discrepancies lies in the hard core part of the compressibility factor rather than in the MSA theory of liquids.
Comment on ‘‘Temperature-enhanced electron detachment from C6F−6 negative ions’’ [J. Chem. Phys. 98, 7875 (1993)]
Authors: Chen E. C. M. , Chen E. S. D. , Wentworth W. E.
Published: July 2002

The results of three recent, independent measurements of the electron affinity of C6F6 are discussed. These utilize an electron capture detector (ECD), an atmospheric pressure ionization mass spectrometer (APIMS), and a photoelectron spectrometer (PES). Seven independent measurements of the temperature dependence of the ECD response of C6F6 give an average value of 0.86±0.02 eV for the electron affinity of C6F6 in agreement with an earlier published value. The APIMS and PES studies have been recently published and support the ECD value. On this basis, it is suggested that the value of the electron affinity of about 0.5 eV presented in the subject article refers to an excited electronic state of the anion of C6F6. Morse potential energy curves are calculated for four negative ions states of C6F6 using experimental data.
The potential energy curves of the X 2Πg, a 4Πu, A 2Πu, b 4Σ−g, B 2Σ−g, 2Πu, and c 4Σ−u states of O+2 obtained using the multiconfigurational spin tensor electron propagator method
Authors: Yeager Danny L. , Nichols Jeffrey A. , Golab Joseph T.
Published: July 2002

With electron propagator methods, electronic ionization and attachment energies are obtained directly. The multiconfigurational spin tensor electron propagator method (MCSTEP) is explicitly designed for systems with open shell and/or nondynamical correlation in the initial state. We apply MCSTEP to O2 at several internuclear separations and obtain and report the MCSTEP potential energy curves and the spectroscopic constants for the X 2Πg, a 4Πu, A 2Πu, b 4Σ−g, B 2Σ−g, 2Πu, and c 4Σ−u states of O+2.
Phenomenological microscopic treatment of optical activity in molecular crystals
Authors: Munn R. W.
Published: July 2002

Expressions are derived for the optical activity of a molecular crystal showing the contributions arising from molecular chirality and lattice chirality separately and in combination. The molecular chirality, expressed via a generalized field-gradient polarizability, modifies the effective molecular polarizability in the crystal environment. This modification implies different Davydov splittings in the optical spectra of crystals of enantiomeric molecules that adopt the same chiral structure.
Molecular dynamics simulations of the depolarized light scattering spectra of liquid OCS in comparison with experiment and simulations of CO2 and CS2
Authors: Stassen H. , Dorfmüller Th. , Ladanyi B. M.
Published: July 2002

Molecular dynamics computer simulation was used to study depolarized light scattering in liquid OCS at temperatures of 170, 243, and 298 K. Applying the first-order dipole-induced dipole model for the interaction-induced polarizabilities, the time correlation functions and spectral line shapes corresponding to the depolarized Rayleigh and nonvibrational contributions to the symmetric stretch (ν1) mode Raman spectra were calculated. The results were compared with experimental findings and simulations on the liquids CO2 and CS2. We find that the OCS simulations reflect the main spectral features in satisfactory agreement with experiment and that an intermediate position concerning all the studied particularities is assigned to liquid OCS between CO2 and CS2. For these three liquids the difference correlation functions between Rayleigh and Raman light scattering were computed and discussed in terms of their component many-body contributions.
Ab initio calculations on Ar–NO+: Structure and vibrational frequencies
Authors: Wright Timothy G. , Špirko Vladimír , Hobza Pavel
Published: July 2002

The Ar–NO+ ionic complex is studied using ab initio calculations. The complex geometry and harmonic vibrational frequencies are calculated using second order Mo/ller–Plesset perturbation theory (MP2) calculations, employing a variety of basis sets. The calculated intermolecular bond length supports the experimental value, whereas the calculated Ar–N–O bond angle suggests a possible reinterpretation of the experimental result. The vibrational frequencies are then recalculated using an anharmonic approach and the fundamentals are found to be in much better agreement with the experimental values [obtained from zero-kinetic-energy (ZEKE) spectroscopy] than are the harmonic values. However, the calculations suggest that the potential energy surface of this complex cation is very anharmonic, and that the experimental assignment of the vibrational features in the ZEKE spectrum may have to be revised. The interaction energy of the complex is calculated, both with and without the full counterpoise (CP) correction; the CP-corrected values are in much closer agreement with experiment than are the uncorrected values. The final value of the stabilization energy, taking into account the MP4 correction is ca. 950 cm−1, in excellent agreement with the (re-evaluated) experimental value of 920±20 cm−1.
Poling dynamics and relaxation of polar order in guest/host polymers by second-harmonic generation
Authors: Guan H. W. , Wang C. H. , Gu S. H.
Published: July 2002

A detailed study of the decay of the second-harmonic generation (SHG) signals of several guest–host nonlinear optical polymer systems has been carried out. The decay of the SHG signal is found to be affected by surface and trapped space charges. A fast component is observed at a low poling field and is found to be mainly due to surface charges which orient the chromophores near the surface. A second component, associated with macroscopic polarization, is induced at high field. The intensity of the second component, which decays considerably slower than the fast component, rapidly increases when the poling field is greater than a threshold voltage. Above the temperature of glass transition Tg, a single exponential function gives a good fit to the shape of the second component of the SHG intensity decay curve. The time constant of the slow component is found to increase with successive poling-decay cycles and reaches a steady-state value after several cycles have been performed. This lengthening effect is found both above and below Tg. Furthermore, the relaxation time of the slow component is also found to depend on the poling field strength: when the host is polymethylmethacrylate, it is found that the relaxation time increases with increasing poling field strength; on the other hand, it decreases with the poling field when polysulfone is used as the host. The temperature dependence of the steady-state relaxation time is not Arrhenius; the Vogel–Fulcher–Tammann equation gives a satisfactory fit to the temperature dependence data.
Temperature of large clusters
Authors: Näher Ulrich , Hansen Klavs
Published: July 2002

Free clusters that cool by evaporation will have a defined distribution of internal energies in spite of lack of contact with a heat bath. This distribution has previously been considered for small clusters. Here we present results valid for large clusters. A simple Gaussian shape is predicted in this regime and we calculate the mean and variance of the distribution. The limit of validity of previous and present work is discussed. The analytical results are compared to numerical simulations.
Simplification of the transition state concept in reactive island theory: Application to the HCN⇄CNH isomerization
Authors: De Leon N. , Ling S.
Published: July 2002

The reactive island (RI) theory of chemical reaction rates is modified so that a precise knowledge of the transition state is not required. This revised form of the theory is applied to a realistic two (J=0) and three degree of freedom model (J≠0) of the isomerization HCN⇄CNH. The detailed reaction dynamics is examined and understood in terms of the cylindrical manifolds embedded in the molecular phase space. Of special interest we find a T-shaped conformer in addition to the HCN and CNH moieties at J=0. Rates of unimolecular decay are examined by numerical simulation and theory. Good agreement is found between theory and simulation except for a case where a substantial amount of phase space is consumed by regular motion. In all cases studied, the theory is a significant improvement over the Rice–Ramsperger–Kassel–Marcus (RRKM) theory, which is up to three orders of magnitude too large.
Dissociation of Kr+2, N2Ar+, (CO)+2, CH+5, and C2H+5 ions drifting in He
Authors: Glosík J. , Skalský V. , Praxmarer C. , Smith D. , Freysinger W. , Lindinger W.
Published: July 2002

Rate coefficients for the dissociation of Kr+2, N2Ar+, (CO)+2, CH+5, and C2H+5 ions in collisions with He have been obtained in a drift tube experiment as dependent on the mean relative kinetic energy in the center-of-mass frame Ec in the range from 0.1 to 0.5 eV. The Arrhenius activation energies obtained for dissociation are 1.15, 0.91, 1.1–1.3, 1.1–1.4, and 1.3–1.7 eV for Kr+2, N2Ar+, (CO)+2, CH+5, and C2H+5 ions, respectively. For the thermal energy region, the rate coefficients for the same processes have been calculated from three-body association rate coefficients reported in the literature, equilibrium constants, and known thermodynamic quantities. The values of the dissociation rate coefficients obtained cover a range of 15 to 50 orders of magnitude. The present data indicate that the internal temperature Ti of the drifting ions is in close equilibrium with Ec, i.e., Ec=3/2.kBTi.
Free electron laser irradiation of deuterated ethane isolated in a xenon matrix: A new step in molecular isomerization studies
Authors: Roubin P. , Berset J. M. , Cardonatto S. , Dubost H. , Lefèvre J. , Ortéga J. M.
Published: July 2002

The selective excitation of different vibrational modes of the CH2D–CH2D molecule isolated in a xenon matrix at 7 K has been performed using the free electron laser source, CLIO, in order to test the selectivity of the intramolecular couplings involved in the isomerization induced by the infrared irradiation.
Excess volume of oligomer blends: A numerical study
Authors: Schulz Michael , Reineker Peter
Published: July 2002

The thermodynamic equation of state for oligomer systems of different densities and flexibilities was studied by numerical Monte Carlo simulations. The values for the thermodynamic limit (V→∞) are obtained by a finite size scaling from the numerical values calculated for the small simulation volume. The present numerical results give a good explanation of the thermodynamic behavior of oligomer mixtures as a function of the composition ratio φ.
Torsional time correlation function for one-dimensional systems with barrier crossing: Periodic potential
Authors: Perico Angelo , Pratolongo Roberto , Freed Karl F. , Szabo Attila
Published: July 2002

The one-variable Smoluchowski equation is used to study various systematic approximation schemes to the torsional angle time correlation function for a system with a multibarrier periodic potential. The memory function is evaluated as a function of the barrier height using both the Mori continued fraction expansion and a related but more efficient matrix expansion method. An exact integral relation for the correlation time is derived and is compared with the approximations.
Quantum chemical study of lithium–C60 clusters
Authors: Zimmermann U. , Burkhardt A. , Malinowski N. , Näher U. , Martin T. P.
Published: July 2002

Semiempirical quantum chemical calculations at the modified-neglect-of-diatomic-overlap self-consistent-field level are performed for differently charged clusters of composition LixC60 with x=0...14. The ground state energies of various isomers are calculated to find the most stable configuration for each cluster stoichiometry. The energies required to remove a Li-atom from these configurations are calculated to determine abundance spectra of distributions of heated LixC60 clusters. These spectra show intensity anomalies at x=6+n (where n=0...+2 is the cluster charge), interpreted to be of electronic origin, and at x=12, interpreted to be of geometric origin. Identical anomalies are observed in experimentally obtained mass spectra of LixC60 clusters.
The interaction of ions in an ionic medium
Authors: Fisher Michael E. , Levin Yan , Li Xiaojun
Published: July 2002

The energy of interaction, W12(R), of two ions at separation R in an ionic medium is considered using Debye–Hückel (DH) theory for spheres of diameter a0. The ions are treated as spheres of dielectric constant D′ which may differ from that of the medium, say D; they have radii b and carry equal or opposite point charges. The electrostatic potential φ(r) in the ionic medium satisfies the DH equation ∇2φ=κ2φ. An exact, closed-form expression is obtained for W12(R). In the limit of zero ionic strength (κ2→0) previous asymptotic results for R→∞, indicating Φ4/R4 +Φ6/R6+Φ7/R7+... corrections to Coulomb’s law, are reproduced; by contrast for κb≳0 the leading interaction term when R→∞ varies, as expected, like e−κR/DR but the first correction decays as κ2e−2κR/DR2, i.e., with a squared screening factor, and can be stated for two ions differing in charge, radii, etc. The same calculations show that, within DH theory, modeling the ions with D′≠D has no effects on the predicted thermodynamics.
The role of excitons in argon cluster ion decay
Authors: de Vries J. , Kamke B. , Steger H. , Weisser B. , Honka M. , Kamke W.
Published: July 2002

Argon clusters show metastable decay with ejection of several monomers when they are photoionized at energies where, in addition to ionization, excitonic states can be populated. When photoions are detected in coincidence with threshold photoelectrons (TPEPICO) resonancelike peak shapes in the signal as a function of photon energy for both the metastable and the spontaneous fragmentation channel are observed. Contributions of different excitonic states to the signal can be resolved and their dependence on cluster size is studied. Several possible excitation processes are being discussed.
Adsorption, desorption, and decomposition of HCl and HBr on Ge(100): Competitive pairing and near-first-order desorption kinetics
Authors: D’Evelyn Mark P. , Yang Yuemei L. , Cohen Stephen M.
Published: July 2002

We have investigated the surface chemistry of coadsorbed hydrogen and halogen atoms on Ge(100), produced by dissociative chemisorption of HCl and HBr, by temperature-programmed desorption. The initial sticking probability S0 for HCl decreases from 0.6 at a substrate temperature of 270 K to 0.05 at 400 K, indicative of a precursor state to adsorption. For HBr S0 is constant at 0.7 over the same temperature range. A fraction f of adsorbed hydrogen atoms desorb associatively as H2 near 570 K, while the remaining (1−f) H atoms recombine with adsorbed halogen atoms and desorb as the hydrogen halide (HX) near 580–590 K. The activation energies for desorption of H2, HCl, and HBr are all approximately 40 kcal/mol. For both HCl and HBr f is 0.7 at low initial coverage and decreases slightly to 0.6 at saturation. The fraction f of adsorbed halogen atoms left on the surface following the competitive desorption of H2 and HX desorb as the dihalides GeCl2 and GeBr2 near 675 and 710 K, respectively. Desorption of H2, HCl, and HBr occurs with near-first-order kinetics, similar to the behavior of hydrogen adsorbed alone, which we attribute to preferential pairing induced by the π bond on unoccupied Ge dimers. We introduce and solve a generalized doubly occupied dimer model incorporating competitive pairing of H+H, H+X, and X+X on Ge dimers to explain the near-first-order kinetics. The model quantitatively accounts for both the desorption kinetics and the relative yields of H2 and HX with pairing energies of ≊3 kcal/mol. Implications of the present results for surface thermochemistry, chemical vapor deposition, and atomic layer epitaxy of Ge and Si(100)2×1 surfaces are discussed.
Molecular evaporation and condensation of liquid n-alkane films
Authors: Xia Ting Kang , Landman Uzi
Published: July 2002

Energetic, structural, and dynamical properties of solid-to-liquid and liquid-to-vapor interfaces and molecular evaporation and condensation processes from high-temperature liquid n-C6H14 and n-C16H34 films were investigated with molecular dynamics simulations. For hexadecane all evaporation events occurred via monomers while for hexane, evaporation of monomers as well as dimers were observed. For both alkane liquids the molecular evaporation mechanism is found to be sequential in nature, starting with an end segment of a molecule leaving the surface and subsequently the evaporation of the molecule occurs via sequential ‘‘dragging’’ of the rest of the molecule. The condensation coefficients of vapor molecules onto the liquid surface are estimated as ∼0.9 for hexane and ∼1 for hexadecane. Evaporation is accompanied by significant molecular conformational changes. In hot liquid n-C16H34 the trans (t) and gauch (g+ and g−) dihedral conformations are distributed as (t,g+,g−)=(66,17,17) while in the vapor the distribution is almost uniform with a large decrease in the fraction of trans conformations, i.e., (33,31,36). On the other hand, for the shorter alkane hot liquid (t,g+,g−)=(72,14,14) while in the vapor the fraction of trans conformations is increased, i.e., (83,10,7). These results are discussed in light of theoretical treatments of evaporation processes.
Analytical model for the microscopic nonaffine deformation of polymer networks
Authors: Glatting G. , Winkler R. G. , Reineker P.
Published: July 2002

A general model for the description of the stress-strain behavior of polymer networks is introduced. We assume that the network consists of a gas of phantom chains. Its deformation is accounted for by macroscopic constraints, which allow nonaffine behavior on the microscopic scale. The advantage of the model lies in the fact that the number of assumptions is smaller than in other models and—at the same time—the main experimental features are reproduced. Comparison to mechanical deformation experiments shows good agreement with the model.
Stark energy levels of symmetric top dipoles: Analytical expressions for arbitrary field strengths
Authors: Maergoiz A. I. , Troe J. , Weiss Ch.
Published: July 2002

Analytical expressions are presented which interpolate between weak field and strong field expansions of rotational energy levels of polar symmetric top molecules in homogeneous electrical fields. Besides Stark spectroscopy, the derived formulas can be used as well for calculating adiabatic channel potential curves in capture processes of species whose interaction is characterized by an anisotropy of the cos θ type.
Detailed rate coefficients and the enthalpy change of the equilibrium reaction OH+C6H6↔MHOC6H6 over the temperature range 345–385 K
Authors: Lin Shih-Chieh , Kuo Tsu-Chi , Lee Yuan-Pern
Published: July 2002

The reaction between OH and C6H6 in He has been studied over the pressure range 250–500 Torr and the temperature range 345–385 K by means of the laser-photolysis/laser-induced-fluorescence technique. Analysis of the temporal profile of [OH] yielded the detailed rate coefficients and the equilibrium constant for the reaction OH+C6H6+M↔HOC6H6+M. The temperature dependence of the rate coefficients for the forward, the reverse, and the adduct-loss reactions have been determined to be kf=(2.0±0.1)×10−14 exp[(1420±250)/T] cm3 molecule−1 s−1, kr=(3.0±0.2)×1012 exp[−(8240±320)/T] s−1, and ka=(1.7±0.3)×106 exp[−(3500±360)/T] s−1, respectively. The temperature dependence of the equilibrium constant led to the enthalpy of reaction ΔH(365±20 K)=−(20.0±1.2) kcal mol−1 and the entropy of reaction ΔS(365±20 K)=−(33.6±2.5) cal K−1 mol−1, greater than previously reported values; after correction for temperature dependence, ΔH0(298 K)=−(19.9±1.2) kcal mol−1 and ΔS0(298 K)=−(33.4±2.6) cal K−1 mol−1.
A Raman investigation of lead haloborate glasses
Authors: Pan Zhengda , Henderson Don O. , Morgan Steven H.
Published: July 2002

Glasses of the (65−x)PbO⋅xPbX2⋅35B2O3 system (where X=F, Cl, Br, and I) have been made and investigated by Raman measurements. Three low frequency bands with high Raman intensities have been observed at about 40, 105, and 135 cm−1, respectively. It is found that the intensities of the depolarized low frequency Raman contour peaked at 40 cm−1 are exceptionally high and strongly dependent on the halogen type and content. We attribute this nonsymmetric low-frequency Raman contour to the collective modes of the BO3–PbO4 bridged local structures and propose a model to interpret the experimental data. Our results suggest that the liberation of these local structures is related to the lead as well as the halogen. Cl, Br, and I enter the glass as nonbridging anions and weaken the cohesion of the glass network in the increased order of Cl, Br, and I.