bluuebunny/crossref_metadata_2025 huggingface dataset:
Computer simulation study of the conformational statistics of long chain hydrocarbons in cylindrical potentials
Authors: Yamamoto Takashi
Published: July 2002

The Monte Carlo method is applied to the study of disordered conformations of polymethylene chains confined in cylindrical mean-field potentials. It is assumed that the molecule, which is composed of 30 united atoms (methylene groups), has fixed bond length and bond angle, and makes quasicontinuous bond rotations. Various statistical properties of the molecule, such as dihedral angle distributions, dihedral angle pair correlations, transverse fluctuations, etc., are calculated vs strength of the mean-field potential. The dihedral angle distributions calculated exhibit the marked reduction of the gauche peaks with increasing potential; it implies the increasing inaccuracy of the usual rotational isomeric model. The dihedral angle pair correlations reveal novel characteristics of the dihedral angle fluctuation: the fluctuation has approximate period of four bonds with marked tendency for the next nearest bonds to counter-rotate. The characteristics are more conspicuous under weaker potential constraint. There are large transverse fluctuations of the chain, the average linear form of the chain being still maintained. These characteristic dihedral angle fluctuation and the transverse deviation of the chain are found to be well understood by a small scale kink model.
Dynamic bond percolation theory for diffusion of interacting particles: Tracer diffusion in a binary mixture lattice gas
Authors: Granek Rony , Nitzan Abraham
Published: July 2002

Dynamic percolation theory is used to obtain the tracer diffusion coefficient in binary mixtures of ‘‘noninteracting’’ lattice gas (with only the blocking interactions, i.e., double occupancy of a lattice site is forbidden) within the effective medium approximation (EMA). Our approach is based on regarding the background particles as a changing random environment. The result is expressed in terms of two fluctuation time parameters which we attempt to determine self-consistently. We compare two possible choices for these parameters which are consistent with our former results for the single component system. The resulting tracer diffusion coefficient for both choices compares well with numerical simulations whenever single bond EMA is expected to be reliable. Comparison is also made with the theoretical results of Sato and Kikuchi [Phys. Rev. B 28, 648 (1983)] and discrepancies between both theories are discussed.
Predissociation linewidths of the (3,0)–(11,0) Schumann–Runge absorption bands of 18O2 and 16O 18O in the wavelength region 180–196 nm
Authors: Chiu S. S.-L. , Cheung A. S.-C. , Yoshino K. , Esmond J. R. , Freeman D. E. , Parkinson W. H.
Published: July 2002

Predissociation linewidths of the (3,0)–(11,0) Schumman–Runge bands of 18O2 and 16O 18O in the wavelength region 180–196 nm have been obtained from the published measurements of the absolute absorption cross sections of Yoshino et al. [Planet. Space Sci. 36, 1201 (1988); 37, 419 (1989)] and spectroscopic constants of these molecules of Cheung et al. [J. Mol. Spectrosc. 131, 96 (1988); 134, 362 (1989)]. The linewidths are determined as parameters in the nonlinear least-squares fitting of calculated to measured cross sections. Predissociation maxima are found at upper vibrational levels with v′=4, 7, and 10 for 18O2 and for 16O 18O. Our predissociation linewidths are mostly greater than previous experimental values for both isotopic molecules.
Time-dependent treatment of scattering. II. Novel integral equation approach to quantum wave packets
Authors: Sharafeddin Omar A. , Judson Richard S. , Kouri Donald J. , Hoffman David K.
Published: July 2002

The time-dependent form of the Lippmann–Schwinger integral equation is used as the basis for a novel wave-packet propagation scheme. The method has the advantage over a previous integral equation treatment in that it does not require extensive matrix inversions involving the potential. This feature will be important when applications are made to systems where in some degrees of freedom the potential is expressed in a basis expansion. As was the case for the previous treatment, noniterated and iterated versions of the equations are given; the iterated equations, which are much simpler in the present new scheme than in the old, eliminate a matrix inversion that is required for solving the earlier noniterated equations. In the present noniterated equations, the matrix to be inverted is a function of the kinetic energy operator and thus is diagonal in a Bessel function basis set (or a sine basis set, if the centrifugal potential operator is incorporated into an effective potential). Transition amplitudes for various orbital angular momentum quantum numbers can be obtained from: (1) Fourier transform of the amplitude density from the time to the energy domain, and (2) direct analysis of the scattered wave packet. The approach is illustrated by an application to a standard potential scattering model problem.
Invariant surfaces and phase space flux in three-dimensional surface diffusion
Authors: Gillilan Richard E.
Published: July 2002

Investigation of the classical three-dimensional motion of a helium atom on a (001) xenon surface uncovered new invariant manifolds that are in close accord with recent theoretical developments in nonlinear dynamics. The relationship between these manifolds, phase space flux, and recrossing in transition-state theory is explored. General theory and computational methods are also discussed.
Phase diagram in a lattice model of surfactant solutions
Authors: Ciach A.
Published: July 2002

Phase diagrams and correlation function of three-component mixtures containing amphiphilic particles are calculated from a model introduced previously within mean-field approximation. The phase diagram depends on interparticle interactions, and agrees qualitatively with experiment already in the simplest one-dimensional case. The surface tension between oil- and water-rich phases at coexistence with the lamellar phase is very low for all temperatures. A form of water–water structure factor in microemulsion agrees qualitatively with neutron-scattering experiments.
Localization versus band crossing transitions in a multiband model of spatially disordered materials
Authors: Winn Martyn D. , Logan David E.
Published: July 2002

A theory is developed to describe the electronic densities of states and localization characteristics of a multiband tight-binding model for a system characterized by quenched liquid-like disorder. For the case of an sp3 system, and with parameters appropriate to fluid mercury at an independent electron level of description, the role of localization is found to be important: localized Fermi-level states persist up to densities considerably higher than that at which the s–p band gap closes.
Nonequilibrium kinetics of the reaction H+Br2 in xenon within a Lorentz gas model
Authors: Stiller W. , Schmidt R. , Popielawski J. , Cukrowski A. S.
Published: July 2002

The reaction H+Br2→products in the carrier gas xenon is studied in the framework of the Lorentz gas model. The nonequilibrium velocity distribution function fH of the light component H is calculated from the Lorentz–Fokker–Planck equation. This permits the determination of the nonequilibrium temperature TH and the nonequilibrium rate coefficient k of this reaction. These kinetic quantities are numerically calculated and compared with various other approaches for solutions of the appropriate Boltzmann equation (Kostin method, Chapman–Enskog and Shizgal perturbation method, Keizer approximation).
Femtosecond laser study of energy disposal in the solution phase isomerization of stilbene
Authors: Sension R. J. , Repinec S. T. , Hochstrasser R. M.
Published: July 2002

Femtosecond laser studies of cis-stilbene isomerization show the absorption of the hot trans product decaying in 20 ps. An initial internal temperature of 725±100 K is estimated which decays with a ca. 14 ps time constant. This temperature accounts for less than one-half the photon energy implying significant energy loss by solvent friction.
Coil to rod transitions in Monte Carlo simulations of a short polyelectrolyte. III. Contact as a new type of bond
Authors: Brender Chava
Published: July 2002

In previous articles we introduced a new thermal effect which causes a fully charged chain to unfold under cooling. In this article we develop a new method to monitor the progress of the unfolding process. A new order parameter is suggested and an attempt is made to define a new kind of what we call statistical–physical bond.
Intramolecular vibrational redistribution in alkylbenzenes. II. Spectroscopy and dynamics
Authors: Gruner Daniel , Brumer Paul
Published: July 2002

Dispersed fluorescence spectra from 00, 6b1, and 121 excitation of ethylbenzene, pentylbenzene, and hexylbenzene are computed within the normal mode approximation and compared with experiment. The harmonic model is shown to be capable of describing the appearance of the ‘‘new shifted spectrum,’’ originally identified as a ‘‘relaxed spectrum,’’ which becomes increasingly dominant with increasing chain length. Thus, this feature cannot be a consequence of intramolecular vibrational redistribution (IVR). By contrast, the appearance of substantial spectral broadening can only be accounted for through the introduction of anharmonic coupling. An assessment of standard assumptions in the analysis of IVR in ‘‘ring+tail’’ molecules is also provided. Results show that some crucial assumptions, e.g., that electronic transitions are not accompanied by changes in chain excitation, are unjustified.
Core–valence correlation effects using approximate operators
Authors: Pettersson Lars G. M. , Åkeby Hans
Published: July 2002

The effects of core and core–valence correlation on the properties of several Cu containing compounds (Cu atom, CuH, CuF, and Cu2) are investigated using both model operator techniques and CI including core–core and core–valence correlation. Good agreement is found for all properties studied. The effects of core-correlation and different aspects of the model operator method are analyzed in detail.
On the formation of the isomeric cluster ions (CO)+n
Authors: Hiraoka Kenzo , Mori Toshiharu , Yamabe Shinichi
Published: July 2002

The kinetics and equilibria of clustering reactions (CO)+n−1+CO=(CO)+n (n=3–18) were studied, using a pulsed electron-beam high-pressure mass spectrometer. It was found that there are two isomers for the tetramer (CO)+4. The energy barrier for the isomerization reaction was measured to be 6.1 kcal/mol. Anomalous van’t Hoff plots for the clustering reactions were obtained with n=8–10, probably due to the existence of several isomeric cluster ions. The structures of the cluster ions (CO)+n with n=2–5 were examined by ab initio MO calculations. Two core ions of n=3, σ and π radical cations, are found to give different clustering patterns.
Relaxation dynamics in molecular alloys. I. Annealed (C2F6)1−x(CClF3)x mixtures
Authors: Böhmer R. , Loidl A.
Published: July 2002

Solid solutions of (C2F6)1−x(CClF3)x were investigated using dielectric spectroscopy. In part I of this work results on thermally equilibrated (C2F6)1−x(CClF3)x alloys are presented. The molar polarizability of liquid and plastic C2F6 was determined. The phase diagram of (C2F6)1−x(CClF3)x was studied in detail. It exhibits two eutectic points and a large miscibility gap. In a high-temperature orientationally disordered phase mixed crystals can be grown up to CClF3 concentrations x≊0.5; in the low-temperature ordered state solid solutions are stable for x≤0.25. The dielectric loss spectra indicate that in (C2F6)1−x(CClF3)x different relaxation channels are available for the CClF3 dipoles.
Determination of the singlet/triplet branching ratio in the photodissociation of ketene
Authors: Kim Sang Kyu , Choi Young S. , Pibel Charles D. , Zheng Qi-Ke , Moore C. Bradley
Published: July 2002

The rotational distributions of CO products from the dissociation of ketene at photolysis energies 10 cm−1 below, 56, 110, 200, 325, 425, 1107, 1435, 1720, and 2500 cm−1 above the singlet threshold (30 116.2 cm−1 ), are measured in a supersonic free jet of ketene. The CO(v″=0) rotational distributions at 56, 110, 200, 325, and 425 cm−1 are bimodal. The peaks at low J′s, which are due to CO from the singlet channel, show that the product rotational distribution of CO product from ketene dissociation on the singlet surface is well described by phase space theory (PST). For CO(v″=0) rotational distributions at higher excess energies (1107, 1435, 1720, and 2500 cm−1 ), the singlet and triplet contributions are not clearly resolved, and the singlet/triplet branching ratios are estimated by assuming that PST accurately predicts the CO rotational distribution from the singlet channel and that the distribution from the triplet channel changes little from that at 10 cm−1 below the singlet threshold. The singlet yield shows a rapid increase in the low excess energy region (0–300 cm−1 ), and a slower increase above. The singlet and triplet rate constants are derived from the directly measured total rate constants using the singlet yields. The triplet rate constant increases monotonically with increasing photolysis energy through the singlet threshold region. The singlet rate constant is accurately established in the threshold region and found to increase much less rapidly than predicted by phase space theory. At 2500 cm−1 excess energy, the CO(v″=1) rotational distribution is obtained, and the ratio of CO(v″=1) to CO(v″=0) products for the singlet channel is measured to be 0.045±0.017. This ratio is close to the variational Rice–Ramsberger–Kassel–Marcus (RRKM) calculation 0.038, and the separate statistical ensembles (SSE) prediction 0.041, but much greater than the PST prediction, 0.016.
Photodissociation dynamics of Mo(CO)6 at 266 and 355 nm: CO photofragment kinetic-energy and internal-state distributions
Authors: Buntin Steven A. , Cavanagh Richard R. , Richter Lee J. , King David S.
Published: July 2002

The internal-state and kinetic-energy distributions of the CO photofragments from the 266 and 355 nm photolysis of Mo(CO)6 have been measured under collision-free conditions using vacuum-ultraviolet laser-induced fluorescence. The rotational-state distributions for CO(v″=0) and (v″=1) are well represented by Boltzmann distributions with effective rotational ‘‘temperatures’’ of Tr(v″=0)=950±70 K and Tr(v″=1)=935±85 K for 266 nm and Tr(v″=0)=750±70 K and Tr(v″=1)=1150±250 K for 355 nm photolysis. The CO(v″=1/v″=0) vibrational-state ratios for 266 and 355 nm photolysis are 0.19±0.03 and 0.09±0.02, respectively. The Doppler-broadened CO photofragment line shapes indicate that the translational energy distributions are isotropic and Maxwellian. There is no photolysis-laser wavelength or internal-state dependence to the extracted translational ‘‘temperatures.’’ The observed energy partitioning and kinetic-energy distributions are inconsistent with an impulsive ejection of a single CO ligand. CO photofragment line shapes for 266 nm photolysis are not consistent with a mechanism involving the repulsive ejection of the first CO ligand, followed by the statistical decomposition of the Mo(CO)5 fragment. While phase-space theories do not predict quantitatively the energy disposal, the photodissociation mechanism appears to be dominated by statistical considerations. The results also suggest that the photodissociation of Mo(CO)6 at 266 and 355 nm involves a common initial ‘‘state’’ and that similar exit channel effects are operative.
Direct absorption spectra of jet-cooled benzene in 130–260 nm
Authors: Hiraya Atsunari , Shobatake Kosuke
Published: July 2002

The direct absorption spectrum of benzene in a free jet has been measured in the 130–260 nm region (S1, S2, and S3 states, Rydberg series, and the first ionization limit) using synchrotron radiation as a light source. The absolute molar extinction coefficients (ε) of benzene in jets have been determined by scaling measured free-jet values to the known value in the vapor phase for a broadband at 200.1 nm in the S2 state. The vibrational temperature for ν16 mode was estimated to be 185 K. The maximum value of ε of the S1 absorption system was found to be 1400 ℓ mol−1 cm−1 (spectral bandwidth=0.065 nm). A shoulder observed at 205.45 nm in the S2 absorption system is assigned to the S2 origin, induced by pseudo-Jahn–Teller distortion.
Extended hydrates interaction model: Hydrate formation and the energetics of binary homogeneous nucleation
Authors: Kulmala Markku , Lazaridis Mihalis , Laaksonen Ari , Vesala Timo
Published: July 2002

The classical hydrates interaction model presented by Jaecker-Voirol et al. is extended into systems where the gas-phase number concentrations of acid and water molecules are of the same order of magnitude. Besides the sulfuric acid–water system, the hydrogen iodide–water and the nitric acid–water systems are considered. The distribution Nh,k of hydrates containing h water and k acid molecules has been calculated as a function of relative humidity and relative acidity. An extended formula for the Gibbs free energy of droplet formation is derived. The fraction of free molecules to the total number of molecules (free molecules+hydrates) is solved numerically and therefore the equilibrium constants of hydrate formation are not needed. Hydrate formation often has a significant effect on energetics of nucleation in the acid–water systems and the extended hydrates interaction model represents a definite improvement over the older hydrates interaction model.
Do variational formulations for inhomogeneous density functions lead to unique solutions?
Authors: Lovett Ronald , Stillinger Frank H.
Published: July 2002

In principle, the equilibrium density in an inhomogeneous system is that density field which extremalizes the free energy and all the system’s equilibrium properties can be deduced from this. A simple, but qualitatively realistic model free energy is presented which shows that approximate free energy functionals can easily possess a large number of extremalizing solutions. The usual interpretation when multiple solutions are found is that the correct solution is the one associated with the lowest value of the free energy. This rule is not very reassuring when, as the model exhibits for some range of parameter values, a continuum of solutions can be found. A more careful analysis of the variational problem shows that a variational formulation only provides a complete characterization of an equilibrium system when the variational problem possesses a unique solution. A multiplicity of solutions actually corresponds to the existence of a multiplicity of Hamiltonians which could give rise to the postulated free energy functional. There is no variational basis for comparing different Hamiltonians, however, and hence choosing from among a multiplicity of solutions on the basis of the value of the free energy is an additional extrathermodynamic rule.
Optothermal-detected microwave-sideband CO2-laser spectroscopy of Ar–NH3
Authors: Fraser G. T. , Pine A. S. , Kreiner W. A.
Published: July 2002

A microwave-sideband CO2-laser optothermal spectrometer with a resolution better than 1 MHz has been used to record the infrared spectrum of Ar–NH3 in the vicinity of the aR(0,0) line of the ν2 vibration of free NH3. A Π←∑ type band is observed, giving a positive l-type doubling constant q, of 90.9 MHz for the upper state. The positive q indicates that the j=1, k=0, ∑ state is above the j=1, k=0, Π state in the v2=1 excited state, where j and k specify the correlation of the internal-rotor state of the Ar–NH3 complex to the NH3 monomer rotational state j,k. The ν2 vibrationally excited complex is found to predissociate in less than the 0.9 ms transit time between the bolometer detector and laser-excitation region. A lower limit to the upper-state lifetime can be obtained from the observed linewidths, which range from 1.5 to 3 MHz (FWHM). The present results agree with and extend the previous free-jet diode-laser absorption measurements on this band.
Magnetic properties of simple alkylene–aromatic polymers
Authors: Tanaka Kazuyoshi , Yoshizawa Kazunari , Takata Akira , Yamabe Tokio , Yamauchi Jun
Published: July 2002

This paper describes the results of the magnetization and the electron-spin resonance (ESR) measurements of the simplest members of alkylene–aromatic polymers (AAPs). In contrast with the previous claim of the observation of the ferromagnetism in AAPs, a distribution of the spin alignment giving 1<S<3/2 on the average has been concluded from the magnetization measurement. The spin concentration certainly coming from carbon π spins has turned out to be relatively small (∼1017–1018 spins⋅g−1 ). The ESR observation has revealed a forbidden Δms=±2 transition characteristic of a triplet or higher multiplet state of radical pairs in agreement with the observation of the local spin alignment. The relationship between the spin state and the chemical structure of AAPs has also been examined with the use of the 13C NMR spectroscopy. It is concluded that in the present material there is a ferromagnetic coupling between a few of spins such as triphenylmethyl radicals forming spin clusters of triplet or higher spin multiplet state diluted in the whole bulk.
Molecular dynamics study of a model isomerization reaction at the liquid–vapor interface of a Lennard-Jones fluid
Authors: Benjamin Ilan
Published: July 2002

The dynamics of a model isomerization reaction at the liquid–vapor interface of a Lennard-Jones fluid is studied using molecular dynamics simulation and is compared to the same reaction in bulk liquid. The reaction system consists of two atoms moving in a double well potential between a compact and an extended state. The potential of mean force and the dynamical friction are evaluated for the reaction in the bulk and at the interface, and are used to estimate the effects on the rate of the reaction. The transition from the bulk to the interface reduces both the friction along the reaction coordinate and the activation barrier for the transition from the compact to the extended state. The rate of the reaction at the interface can be larger than the rate in the bulk by a factor of ≊2, depending on the system’s potential parameters. These effects on the rate of the reaction can be understood in terms of the density variation in going from the bulk to the interface.
Coadsorption of water and chlorine on Ag(110): Evidence for adsorbate-induced hydrophilicity
Authors: Kizhakevariam N. , Stuve E. M. , Döhl-Oelze R.
Published: July 2002

Coadsorption of H2O and Cl was studied on a Ag(110) surface under conditions of ultrahigh vacuum with thermal desorption spectroscopy, low energy electron diffraction, and electron stimulated desorption ion angular distribution. The experiments were conducted over the temperature range of 100–650 K for water coverages ranging from zero to several multilayers and chlorine coverages θCl of 0–0.75 monolayers (ML). Water adsorption is stabilized by chlorine; the thermal desorption peak for water interacting with chlorine, called the α2 state, shifts to higher temperature by 25–40 K from the α1 state for desorption from the clean surface. A c(2×2) bilayer for H2O forms for coadsorption with less than 0.25 ML of Cl. The surface solvation number (SSN), defined as the number of stabilized water molecules per chlorine atom, varies from 13 to about 4 as θCl increases from 0 to 0.25. The unusually large SSN and the c(2×2) structure is evidence that Cl(a) promotes water adsorption to the metal surface itself in an effect called adsorbate-induced hydrophilicity. Coadsorption with higher chlorine coverages produces a p(4×3) structure for 0.25<θCl<0.4 and a c(4×4) structure for 0.4<θCl<0.5. Chlorine interacts directly with water in these structures in the form of surface solvation seen in previous studies of water coadsorption. The p(4×3) and c(4×4) patterns are evidence that coadsorbed water alters the distribution of chlorine on the surface. These results are interpreted in terms of the balance of forces among the two adsorbed species and the surface.
Hydration of carbon dioxide: The structure of H2O–H2O–CO2 from microwave spectroscopy
Authors: Peterson K. I. , Suenram R. D. , Lovas F. J.
Published: July 2002

The structure of the gas-phase trimeric complex H2O–H2O–CO2 is determined through an analysis of the rotational spectra of ten isotopically substituted species. These spectra were measured in the region between 7.5 and 18 GHz using a pulsed-molecular-beam Fourier-transform microwave spectrometer. The nondeuterated species display two sets of transitions separated by ∼1 MHz. The splittings of the perdeuterated form are smaller and three partially deuterated forms have no splittings. The rotational constants for the lower frequency set of transitions of the normal species are A=6163.571(4) MHz, B=2226.157(2) MHz, C=1638.972(1) MHz, δJ=0.000 83(3) MHz, ΔJ=0.002 98(4) MHz, ΔJK=−0.0005(2) MHz. The differences in the rotational constants between the upper and lower states are ΔA=498 kHz, ΔB=520 kHz, and ΔC=−133 kHz. The dipole moments are μa=1.571(5) D and μb=0.761(4) D with μc=0 D. The dipole moments and the intertial defect of −0.620 uÅ2 both indicate an essentially planar complex. The structure is found to be cyclical with the dimer-type bond lengths within the trimer being approximately the same as those found in the free heterodimers. One water molecule is oxygen bound to the carbon atom of the CO2 and is also hydrogen bonded to the oxygen of the second water molecule. The second water molecule is in turn hydrogen bonded to one of the oxygens of the CO2 molecule. The observed splittings are most likely due to a hydrogen-exchanging internal rotation of this second water molecule.
The effect of bending vibrations on product rotations in the fully state-resolved photodissociation of the Ã state of water
Authors: Schinke R. , Vander Wal R. L. , Scott J. L. , Crim F. F.
Published: July 2002

Theoretical calculations and experimental measurements determine the influence of initial bending vibration on the rotational states of the OH product following the photodissociation of water through its Ã state. Excitation from an initial state with no bending excitation reaches only a rather isotropic region of the excited state potential energy surface, but excitation from states with excited bending vibrations reaches more anisotropic regions. Close-coupling calculations that include the anisotropy of the Ã-state potential show that the torque exerted by the potential reduces the amount of rotational excitation below that predicted in the absence of anisotropy of the Ã-state potential. The experimental measurements for initial bending states having no, one, and two quanta of bending excitation, along with three or four quanta of stretching excitation, agree with the theoretical predictions. The measured product rotation increases with increasing bending excitation, and the observed rotational excitation is less than the limit for an isotropic potential.
Photoemission studies of Ru3(CO)12 adsorption and formation of Cu–Ru bimetallics on Cu(111)
Authors: Sham T. K. , Liu Z.-F. , Tan K. H.
Published: July 2002

Photoemission spectra of Ru3(CO)12 adsorbed on Cu(111) and its decomposition products induced by electron irradiation and annealing are reported. It is found that electron bombardment and subsequent annealing of Ru3(CO)12/Cu(111) lead to the formation of Cu–Ru bimetallic aggregates which exhibit similar features as those reported previously on polycrystalline Cu, that is that Ru–Cu aggregates are formed of which Ru atoms are at the core and the surface is wetted with Cu and there exist distinct interface states. The mechanism for the formation of the bimetallic Cu–Ru aggregates on Cu(111) is almost certainly one in which two-dimensional Ru aggregation occurs on the surface upon initial annealing (∼225 °C) followed by three-dimensional aggregation and wetting of the Ru cluster surface by Cu after further annealing at higher temperatures (≳450 °C). The interaction between Ru surface intermediates and Cu(111) before Cu wetting occurs is also examined with photoemission and SW–Xα calculations.
A Fourier transform infrared reflection absorption spectroscopy study of chemisorbed PF3 on Ni(111): Coverage and temperature dependence
Authors: Guo Xingcai , Yates John T. , Agrawal Vijay K. , Trenary Michael
Published: July 2002

The P–F symmetric stretch fundamental mode of PF3 chemisorbed on the Ni(111) surface has been measured with Fourier transform infrared reflection absorption spectroscopy. Strong adsorbate–adsorbate interactions lead to a net increase in frequency from 858 cm−1 at the lowest coverage to 916 cm−1 at the highest coverage. At 273 K the band shape is symmetric with a FWHM of 6 cm−1 at both low and high coverages indicating a uniform environment for PF3 molecules in both coverage limits. At intermediate coverages the band is broad and asymmetric with unresolved components indicating a wide distribution of intermolecular distances on the surface. The low coverage frequency is 40 cm−1 lower than in the Ni(PF3)4 complex indicating interactions between the F atoms of PF3 and nearest-neighbor Ni atoms on the Ni(111) surface. This interaction is reduced as higher lying torsional states of chemisorbed PF3 become thermally populated, leading to a strong temperature dependence of the P–F stretch band shape at low coverages.
Dielectric and conductivity relaxation in poly(propylene glycol)–lithium triflate complexes
Authors: Fu Y. , Pathmanathan K. , Stevens J. R.
Published: July 2002

The electric modulus representation is used to display conductivity and dielectric relaxation occurring in poly(propylene glycol)-4000 (PPG) complexed with LiCF3SO3, as the ether oxygen to lithium (O:Li) ratio is varied from 300:1 to 12:1. The frequency range covered is 10 to 107 Hz. Results are obtained for temperatures above the glass transition temperature appropriate for each PPG–LiCF3SO3 complex. For O:M=300:1, the conductivity peak and the α and α′ relaxation are clearly resolved. As the concentration is increased, there is a coupling between the structural and the conductivity relaxations; the various peaks begin to overlap. The coupling is greatest for an O:Li ratio in the range 30:1 to 12:1, where the highest conductivity is measured. Below room temperature, the 30:1 complex has the highest conductivity, above room temperature the 12:1 complex has the highest. At any particular temperature there is a concentration above which the conductivity drops. This drop is due to reduced ion mobility resulting from a dramatic increase in viscosity to values above ∼100 poise.
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o multiple reference double-excitation configuration-interaction ground and excited state potential curves for nitromethane decomposition
Authors: Roszak S. , Kaufman Joyce J.
Published: July 2002

Reliable ab initio quantum chemical calculations for the ground and electronically excited potential energy surfaces for C–NO2 bond dissociation pathways in addition to the bond dissociation energy from the ground electronic state are crucial to evaluating energetic materials. The excited electronic state potential energy surfaces are important especially for laser induced decomposition. The curve crossings and avoided crossings are important since they determine the electronic states of the resulting fragments which influences their subsequent reactivities. We have carried out large basis set multiple reference double-excitation configuration-interaction (MRD-CI) calculations for the lowest several singlet and triplet potential curves for the H3C – NO2 decomposition of nitromethane. The proper ground state decomposition should lead to two radicals. The self-consistent-field (SCF) treatment (for the ground electronic state) was found to be satisfactory around the equilibrium structure (except for the ever-present ππ* transition of the electrons in the NO2 group) while failing completely to predict the proper dissociation limit. The analysis of the MRD-CI wave function allows distinguishing of two regions which can be interpreted as interaction within bonded molecule and interaction between species of already dissociated molecule.
Diffusion disallowed crystal growth. II. A parabolic model
Authors: Löwen Hartmut , Schofield Sarah A. , Oxtoby David W.
Published: July 2002

The growth of a crystal into an undercooled liquid is studied within a phase field model. In contrast to the usual Landau–Ginzburg approach, the free energy as a function of the order parameter is taken to be a pair of intersecting parabolas. This model is completely solvable and shows a transition from heat-diffusion allowed to disallowed steady-state crystal growth. Analytical expressions for the interface velocity and for the order parameter and temperature profiles are obtained and extensively discussed. In comparison with the Landau–Ginzburg model, most qualitative and quantitative features are the same. However, the solution of the present model is more general, avoiding perturbation theory, and gives a clearcut picture of the underlying transition.
Exceptional solid-state properties of organic 2:1 donor–acceptor metals with integral charge transfer
Authors: Böhm Michael C.
Published: July 2002

The material properties of low-dimensional 2:1 donor (D)–acceptor (A) salts with integral mutual charge transfer (CT) are exceptional in the class of the synthetic metals. The electronic charge fluctuations in the majority component of D+2A− and A−2D+ compounds are of extremal character in the presence of strong electronic correlations. In the minority component the fluctuations are extensively suppressed. The corresponding solids are one-chain conductors. The charge fluctuations are investigated by a simple analytic model formulated on the basis of the local approach for the many-particle problem and the bond-orbital approximation for the independent-particle wave function. The Fermi surfaces of the D+2A− and A+2D− CT salts are effectively flat also for smaller anisotropy ratios. Structural phase transitions of the Peierls type leading to one-dimensional as well as two- and/or three-dimensional ordering are analyzed in a tight-binding framework. There exists a large range, where the one-dimensional dimerization is suppressed, but where n-dimensional (n=2, 3) ordering can still exist. Fermi-surface nesting in materials with smaller anisotropy ratios allows for the conservation of metastable solid-state configurations, i.e., configurations where the electron–lattice interaction is enhanced, but where a metal–insulator transition is suppressed. For the quasi-one-dimensional metals with D+2A− and A−2D+ stoichiometry it is shown that electronic correlations lead to a remarkable enhancement of the metal–insulator transition temperature TP. Experimental data available for the respective 2:1 CT salts can be rationalized by simple analytic models. A possible violation of particle–hole symmetry between D+2A− and A−2D+ compounds in their ability to stabilize a low-temperature superconducting ground state is suggested.
Multiphoton ionization of SiH3 and SiD3 radicals. II. Three-photon resonance-enhanced spectra observed between 450 and 610 nm
Authors: Johnson Russell D. , Hudgens Jeffrey W.
Published: July 2002

The electronic spectra of silyl radicals, SiH3 and SiD3, were observed between 450 and 610 nm (49 200-65 200 cm−1 ) by resonance enhanced multiphoton ionization (REMPI) spectroscopy. The spectra were produced through a 3+1 REMPI mechanism. Spectra of four new planar Rydberg states were observed and assigned. In SiH3 the observed states and spectroscopic constants are D̃ 2A′1(3d)■ T0=49 787(30), ω2=810(31) cm−1; Ĩ′ (4d): T0=56 253(30), ω2=814(25) cm−1; J̃′ (4d): T0=57 726(30), ω2=835(26) cm−1; and L̃ (5d): T0=59 615(30) cm−1, ω2=839(26) cm−1. In SiD3 the observed states and spectroscopic constants are D̃ 2A1(3d): T0=49 685(30), ω2=600(28) cm−1; Ĩ′ (4d):T0=56 205(30), ω2=600(17) cm−1; and J̃′ (4d):T0=57 840(30), ω2=603(20) cm−1. The differences between 2+1 and 3+1 REMPI spectra are explained by a Δl rule, where l is the electronic orbital angular momentum. The H̃, K̃, and Ñ states observed in the 2+1 REMPI spectrum of SiD3 spectrum and reported in J. Chem. Phys. 91, 3340 (1989) are reassigned as the H̃ 2E′ (4p), K̃ 2E′ (4f), and Ñ 2E′ (5f).
Thermochromism in polyalkylthiophenes: Molecular aspects from vibrational spectroscopy
Authors: Zerbi G. , Chierichetti B. , Ingänas O.
Published: July 2002

It is known that polyalkylthiophenes show reversible thermochromism within a well-defined temperature range. The vibrational infrared and Raman spectra are used as structural probes for understanding the structures of polyhexyl and polyoctyl thiophenes at room temperature and their evolution with temperature during the thermochromic process. The seemingly sample IR and Raman spectra of these materials are explained in terms of the theory of the effective conjugation coordinate which also accounts for the observed ‘‘dispersion’’ of the Raman spectrum with exciting wavelength or from solid to solution states in terms of changes of effective conjugation length. A detailed description of the structure of the system is reached. At room T the sample consists mainly of two phases: (i) an ordered phase with the alkyl side chains in the transplanar structure and the main chain in a quasicoplanar or coplanar conformation and (ii) a disordered phase with the alkyl residue fully conformationally coiled and the main chain conformationally twisted with the torsional angle of ∼ 30°. Upon heating, the relative concentration of the disordered phase increases. The temperature dependence of the side chain and the main chain conformations are similar, thus showing that the coiling of the side chain drives the twisting of the main chain. The thermochromism is thus accounted for.
Vibrational frequencies in the Ã 3E state of methylnitrene
Authors: Ferrante Robert F.
Published: July 2002

New observations have enabled the identification of another excited state vibration in the Ã 3E –X̃  3A2 ultraviolet absorption spectrum of matrix-isolated methylnitrene. Two progressions in the C–N stretching frequency ν3 were detected in both CH3N and CD3N, with average intervals of 710 and 695 cm−1, respectively. For each isotopomer, corresponding lines in the pair of progressions are offset by frequencies appropriate for a methyl vibration. This provides the first clear evidence for excitation of the CH3 symmetric deformation (umbrella mode) ν2 in the upper state of the transition, with vibrational frequencies of 1166 cm−1 for CH3N and 1044 cm−1 for CD3N. These observations confirm earlier speculation on the absorption spectrum of matrix-isolated CH3N, and correlate with unidentified lines in the gas phase emission spectrum. The use of a new precursor, methyl isocyanate, in the generation of this radical is also of interest.
Photodissociation of ammonia at 193.3 nm: Rovibrational state distribution of the NH2(Ã 2A1) fragment
Authors: Woodbridge Eric L. , Ashfold Michael N. R. , Leone Stephen R.
Published: July 2002

The rovibrational state distribution of the nascent NH2(Ã 2A1) fragments generated by 193.3 nm photodissociation of a room temperature sample of NH3 is determined through an analysis of a major portion (6000–13 000 cm−1) of the NH2(Ã 2A1→X̃ 2B1) near infrared emission spectrum obtained by time-resolved Fourier transform infrared emission spectroscopy. The NH2(Ã) fragments are observed to be formed predominantly in their zero-point vibrational level, with substantial rotational excitation about their a-inertial axis up to the limit of the available energy, ∼3150 cm−1, but with little excitation about the other axes. The pattern of this energy disposal is discussed within the framework of existing knowledge regarding the form of the NH3 Ã state potential energy surface on which the dissociation occurs. The essential features are entirely consistent with a direct carry over, into the fragment, of the out-of-plane bending vibrational motion introduced in the parent molecule by the photoexcitation process.
Effects of blending on local chain dynamics and glass transition: Polystyrene/poly(vinyl methyl ether) blends as studied by high-resolution solid-state 13C nuclear magnetic resonance spectroscopy
Authors: Takegoshi K. , Hikichi Kunio
Published: July 2002

To evaluate effects of blending on molecular motion of the individual polymer components in a compatible polymer blend, polystyrene/poly(vinyl methyl ether)(PS/PVME), the temperature dependence of 13C nuclear magnetic resonance linewidth for CH carbon resonance of PVME was studied under conditions of magic-angle spinning and proton dipolar decoupling. The observed temperature dependence was satisfactorily explained by assuming the following effects of molecular motion: (1) averaging the dispersion of isotropic chemical shifts in the glassy state and (2) the interference between local anisotropic motion and high-power proton decoupling. The activation parameters for motion of PVME of the blends were determined, and the compositional dependence is discussed. It is concluded that microscopically homogeneous mixing is achieved for PS/PVME blends.
Homogeneous nucleation in gases: A three-dimensional Fokker–Planck equation for evaporation from clusters
Authors: Nowakowski B. , Ruckenstein E.
Published: July 2002

A theory of homogeneous nucleation in gases is developed, based on independently obtained expressions for the rates of evaporation from and condensation on the clusters. The rate of evaporation was calculated using a diffusion equation in the energy space, derived from the three-dimensional Fokker–Planck equation averaged with respect to the distribution of the angular variables. The obtained rates of nucleation are compared with those obtained previously by the authors on the basis of a diffusion equation in the energy space and a unidimensional Fokker–Planck equation, as well as with the predictions of the classical theory. The present approach predicts rates of nucleation which are closer to the classical theory than those provided by the former approaches. Nevertheless, the difference between the present approach and the classical result is significant for small clusters, i.e., for high supersaturations.
Vibrational relaxation of N+2(X 2Σ+g, ν=1) in collisions with He. A theoretical study
Authors: Zenevich V. A. , Freysinger W. , Pogrebnya S. K. , Lindinger W. , Dmitrieva I. K. , Porshnev P. I. , Tosi P.
Published: July 2002

Vibrational relaxation of N+2(X 2Σ+g, ν=1) in collisions with He is studied by means of both the quasiclassical trajectory moment method and a semiclasssical analytical approach. To carry out trajectory calculations the potential-energy surface of N+2–He is modeled using ab initio data. Rate constants k10(T) calculated by the moment method reproduce the plateau-like behavior in the Landau–Teller plot of the experimental values at the near thermal energy limit. Simpler analytical models allowing for the long-range interaction are also used to estimate k10(T). The presented theoretical results are in good qualitative agreement with experimental data.
Picosecond measurements of the vibrationally resolved proton-transfer rate of the jet-cooled 1-azacarbazole dimer
Authors: Fuke Kiyokazu , Tsukamoto Keizo , Misaizu Fuminori , Kaya Koji
Published: July 2002

Proton-transfer reaction (PTR) of the jet-cooled 1-azacarbazole (1-AC) dimer has been studied by directly measuring, in real time, the fluorescence intensity from the precursor dimer and from the tautomer formed in the excited-state reaction process after exciting various intermolecular vibrational levels of the dimer. For the vibrational levels of the 1-AC dimer examined here, PTR is found to be predominant in the overall decay process. The rate of PTR is strongly dependent on the vibrational mode and is enhanced pronouncedly by excitation of the intermolecular symmetric-stretching vibration in the hydrogen bond.
The diffusion tensor for a flowing dilute solution of Hookean dumbbells: Anisotropy and flow rate dependence
Authors: Prakash J. Ravi , Mashelkar R. A.
Published: July 2002

For a dilute solution of Hookean dumbbells, the consistent averaging and Gaussian approximations for the hydrodynamic interaction have recently been shown to lead to an anisotropic translational diffusion tensor, which is shear rate dependent in steady shear flow. These properties of the diffusion tensor arise because of the nature of the average friction experienced by the dumbbell as it translates through the solvent. In this paper, the relationship between the average friction on the dumbbell and the spatial distribution of mass about the center of the dumbbell is examined. It is found that the anisotropic and flow rate dependent distribution of mass caused by the solvent flow field is responsible for the average friction having the same properties.
Microscopic rate coefficients in reactions with flexible transition states: Analysis of the transitional-mode sum of states
Authors: Smith Sean C.
Published: July 2002

Expressions are derived for the energy- and angular-momentum-resolved transitional-mode sum of states, WTM(E,J), for flexible transition states in unimolecular, recombination, or bimolecular collision–complex-forming reactions. The expressions are derived classically by evaluation of the phase-space volume integral. The phase-space integral is so arranged that the total available momentum-space volume, Φ(E,J,q), for a frozen configuration q is first evaluated. Accurate analytic expressions for Φ(E,J,q) are obtained for all relevant pairwise combinations of atom, linear, spherical-top, symmetric-top, and asymmetric-top fragments in flexible transition states. The analytic expressions for Φ(E,J,q) indicate clearly the conditions under which the common method of imposing angular momentum conservation, which assumes that J≊L (L being the orbital angular momentum), will fail. WTM(E,J) is then obtained by integration of Φ(E,J,q) over configuration space. Exact evaluation of the integral over configuration space involves at most a five-dimensional numerical integral. Accurate analytical expressions for WTM(E,J) are derived for model potentials which admit sufficient flexibility for the fitting of more-accurate potentials. These expressions enable the calculation of accurate microscopic rate coefficients k(E,J) by microcanonical variational Rice–Ramsperger–Kassel–Marcus (RRKM) theory at little more computational expense than a standard RRKM calculation.
Determination of the bipolar moments in the photodissociation of BrCN
Authors: Wannenmacher Elizabeth A. J. , Lin Hua , Fink William H. , Paul Albert J. , Jackson William M.
Published: July 2002

The photodissociation vector correlations, also referred to as the bipolar moments, are studied following the photodissociation of BrCN in the spectral region from 206 to 260 nm. From the analysis, it is shown that there are at least two potential energy surfaces involved in the photodissociation of BrCN; one that produces CN+B(2P3/2), and another that yields CN+(2P1/2). There is also clear evidence that the fragment translational recoil velocity v is predominantly parallel to the parent transition dipole moment μ, while the fragment rotational angular momentum J is primarily perpendicular to μ. The values of the bipolar moments are lower than the limiting values for μ∥v⊥J and appear to be reduced by the dynamics of the photodissociation process. The primary factor causing the loss of correlation between μ and J appears to be the bent exit channel which leads to a mixed parallel and perpendicular transition. The loss of correlation between μ and v is evidence confirming the existence of out of plane forces being present in the photolysis. These out of plane forces were previously invoked to account for unequal populations in the F1 and F2 spin–rotation components of the CN product.