bluuebunny/crossref_metadata_2025 huggingface dataset:
An elastic hinge model for dynamics of stiff chains. I. Viscoelastic properties
Authors: Roitman Daniel B. , Zimm Bruno H.
Published: February 2003

The behavior of linear macromolecules with some flexibility is considered. A ‘‘trumbell’’ model is used which consists of three beads connected by two rigid rods and a hinge with a restoring force proportional to the angle of bending at the middle bead. This model, which has four curvilinear nonorthogonal configurational coordinates, seems to be the most complex that can be solved completely at present. The relaxation spectrum λi is found for different values of the relative bending force constant Z=ε/kT by numerical integration of Kirkwood’s diffusion equation using Hassager’s collocation method. The viscoelastic behavior is calculated by extending Hassager’s procedure. The results show two main contributions: (i) a slow relaxation λ1 which is nearly pure rotation and which tends to the rigid-rod limit as Z increases, (ii) a fast mixed bend-rotation relaxation λ2 proportional to Z. A pure-bending mode of relaxation also exists, but it does not contribute to the viscoelastic behavior of the model.
An elastic hinge model for dynamics of stiff chains. II. Transient electro-optical properties
Authors: Roitman Daniel B. , Zimm Bruno H.
Published: February 2003

The transient Kerr effect of the ‘‘trumbell’’ model with a restoring force at the hinge is considered assuming induced-dipole orientation. The results show that two main contributions dominate the relaxation behavior of the model: (i) a slow relaxation which is nearly pure end-over-end rotation, (ii) a fast mixed-bend rotation proportional to the relative force constant Z of the spring. These relaxation modes are the same ones that dominate the viscoelastic behavior of the model. As the stiffness of the spring increases, the amplitude of the faster contribution becomes small, and the decay curves nearly fit a single exponential if Z>1. This behavior, however, does not mean that substantial bending is not present. The effect of bending is apparent since for all values of Z the end-over-end relaxation rate is faster than the relaxation rate of a rigid rod. The results of the trumbell model are used to test an approximation of averaging over an ensemble of rigid bodies for the rotational relaxation of semiflexible chains. It is found that the exact results are close to, but not identical with, those of the rigid-body ensemble.
Energy transfer in the collision of an atom with a cold lattice
Authors: Riley Merle E. , Diestler Dennis J.
Published: February 2003

We report numerically exact classical calculations on energy transfer and capture of an atom incident on an initially cold lattice in a collinear configuration. The lattice is harmonic and the adatom–surface interaction is a Morse potential. The system is completely specified in terms of only three physical parameters: the adatom-to-lattice-atom mass ratio, the ratio of the Morse interaction force constant to the lattice force constant, and the ratio of the incident energy to the Morse well depth (lattice–adatom binding energy). These exact calculations form a basis for comparing and testing several approximate dynamical models of gas–surface interactions.
The phase behavior of aqueous dipalmitoyl phosphatidylcholine bilayers at very high pressures
Authors: Wong P. T. T. , Mantsch H. H.
Published: February 2003

Raman spectra of aqueous dispersions of dipalmitoyl phosphatidylcholine (DPPC) were obtained at very high pressures, up to 32 kbar. A new structural phase transition was detected at 15 kbar, from the known GIV to a so far unknown GV gel phase. The acyl chains in the GV phase of DPPC are fully extended and their reorientational fluctuations are completely damped. More than two correlation field components for both the CH2 bending mode and the asymmetric C–C stretching mode are observed. According to the spectroscopic data and the results of a selection rules analysis, an interchain packing is suggested for the GV gel phase of DPPC which resembles that in a parallel orthorhombic cell with four nonequivalent chains.
Distribution functions for very short n-alkane chains
Authors: Mark J. E. , Curro J. G.
Published: February 2003

Monte Carlo simulations based on several rotational isomeric state models were used to generate distribution functions for the end-to-end separation of very short n-alkane chains. The unusual multimodal distributions obtained were found to be due to the reduction in the number of spatial configurations inherent in the rotational isomeric state approximation.
Excitonic and solitonic states in one-dimensional exciton–phonon systems
Authors: Venzl Gerd , Fischer Sighart F.
Published: February 2003

A one-dimensional model for Frenkel excitons interacting with acoustical phonons via a linear diagonal coupling is studied using two distinct variational trial functions which lead to delocalized (excitonic) and localized (solitonic) states. Above a certain exciton–phonon coupling strength, which depends on the exciton band width and the Debye frequency of the phonons, the soliton ground state energy lies below the continuum of dressed excitons. For exciton bandwidths not much larger than the Debye frequency this occurs in a regime where the continuum approximation for the soliton is invalid.
Brownian dynamics simulations: Statistical error of correlation functions
Authors: Lee Sangyoub , Karplus M.
Published: February 2003

Exact expressions for the statistical errors arising from the replacement of the ensemble average by a finite time average in diffusive Brownian dynamics simulations are derived for the displacement correlation function and the orientational correlation function. The results are illustrated by analyzing Brownian dynamics simulations of some oligomeric molecules. These include flexible and rigid dimers and models for the α helix. Flexibility is found to have a significant effect on rotational diffusion.
A mean field calculation of the librational properties of δ phase O2 monolayers on graphite
Authors: Etters R. D. , Kobashi K.
Published: February 2003

A mean field approximation is used to calculate the librational properties of δ-phase molecular oxygen monolayers on graphite at temperatures 0≤T≤20 K. The frequencies of the two libron modes are found to occur at 35.07 and 60.28 cm−1. The libron specific heat and the rms orientational fluctuations of the O2 molecules about equilibrium are also determined. It is found that the contribution of zero-point orientational fluctuations to librational behavior is large. The molecules exhibit well defined small angle librational motion at low temperatures and there is no evidence of an orientational order–disorder phase transition prior to melting at 25.6 K.
Calculation of optical line shapes for generalized multilevel vibronic systems
Authors: Lagos Roberto E. , Friesner Richard A.
Published: February 2003

An approximate method for calculating the one-exciton Green’s functions (and hence the linear response optical line shapes) of a multilevel excited electronic manifold is presented. Vibronic coupling to low and high frequency modes are treated by appropriate perturbative techniques; the resulting numerical algorithms are rapidly convergent and easily evaluated. For two excited levels and single vibrational mode, the method reduces (at lowest order) to previous approximations which have been extensively compared with basis set results.
A coupled cluster approach with triple excitations
Authors: Lee Yoon S. , Kucharski Stanislaw A. , Bartlett Rodney J.
Published: February 2003

The coupled-cluster model for electron correlation is generalized to include the effects of connected triple excitation contributions. The detailed equations for triple excitation amplitudes are presented, and a simplified version implemented that retains the dominant terms. The model presented, CCSDT-1, provides the energy correct through fourth order and the wave function through second order. The CCSDT-1 model is illustrated by comparing with full CI results for HF, BH, and H2O, the latter at several geometries.
Theoretical studies of the photexcited state Cu atom reactions. II. The Cu*+H2→CuH+H photochemical reaction
Authors: García-Prieto J. , Ruiz M. E. , Poulain E. , Ozin G. A. , Novaro O.
Published: February 2003

Pseudopotential SCF-LCAO-MO including extensive variational and perturbative CI calculations have been done for the side-on Cu(2S, 2P, 2D)+H2(1Σ+g )→CuH(1Σ+)+H(2S) photochemical reaction. Nine low-lying states that result from these interactions have been considered. From these nine those that correspond to the 2B2 and 2A1 representations of the C2v symmetry are analyzed in full detail; first because the HCuH intermediate has a 2B2 electronic ground state and secondly because a 2A1 state correlates with the ground states of the reactants and products (Cu+H2 and CuH+H). We analyze the possible structures of the CuH2 intermediate molecule and discuss the reasons why such species is not observed under the matrix isolation experimental conditions of the photoinduced chemical reaction.
Electronic structure and bonding in the RhC molecule by all-electron a
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o HF–Cl calculations and mass spectrometric measurements
Authors: Shim Irene , Gingerich K. A.
Published: February 2003

In the present study we present all-electron ab initio Hartree–Fock (HF) and configuration interaction (CI) calculations of the 2Σ+ ground state as well as of 16 excited states of the RhC molecule. The calculated spectroscopic constants of the lowest lying states are in good agreement with the experimental data. The chemical bond in the electronic ground state is mainly due to interaction of the 4d orbitals of Rh with the 2s and 2p orbitals of C. The bond is a triple bond composed of two π bonds and one σ bond. The 5s electron of Rh hardly participates in the bond formation. It is located in a singly occupied, nonbonding orbital. The chemical bond in RhC is polar with a charge transfer from Rh to C giving rise to a dipole moment of 2.82 D at the experimental equilibrium distance. Mass spectrometric equilibrium measurements over the temperature range 1970–2806 K have resulted in the selected dissociation D°0 = 576.0±3.8 kJ mol−1 for RhC(g).
Compact effective potentials and efficient shared-exponent basis sets for the first- and second-row atoms
Authors: Stevens Walter J. , Basch Harold , Krauss Morris
Published: February 2003

Compact effective potentials, which replace the atomic core electrons in molecular calculations, are presented for atoms in the first and second rows of the periodic table. The angular-dependent components of these potentials are represented by compact one- and two-term Gaussian expansions obtained directly from the appropriate eigenvalue equation. Energy-optimized Gaussian basis set expansions of the atomic pseudo-orbitals, which have a common set of exponents (shared exponents) for the s and p orbitals, are also presented. The potentials and basis sets have been used to calculate the equilibrium structures and spectroscopic properties of several molecules. The results compare extremely favorably with corresponding all-electron calculations.
Structural characterization of soft-core and hard-core glasses by Delaunay tessellation
Authors: Hiwatari Y. , Saito T. , Ueda A.
Published: February 2003

Methods of the Delaunay and Voronoi tessellation are applied to investigate significant structures and structural differences among liquids, glasses, and crystals of the soft-core (inverse 12th power potential) and hard-core models for which molecular dynamics (MD) techniques are used. With particle configurations obtained by MD simulations the Delaunay tessellation for the three dimensional systems defines uniquely a network of tetrahedra. Density dependent properties of the irregularity distribution which measures the magnitude of distortion of each Delaunay tetrahedron (DT) from regular shape (equal edge lengths) and of the length of edges of DT are examined in detail for both models. Clear differences between the hard-core and soft-core systems are observed. Short range correlation of DT is calculated on the pair distribution function of center-to-center distances of the circumspheres of DT, which turns out to be useful to distinguish the short range order of glasses from that of crystals or liquids. It is also shown that there are no clear differences in the distribution of various types of Voronoi polyhedron (VP) between liquids and glasses. However, among various types of polygons (i-edged faces) of VP, the contribution from i=5 increases rather rapidly with increasing density around the glass transition point.
A model of interacting clusters for microscopic nucleation theory
Authors: Kobraei Hamid R.
Published: February 2003

A model is suggested for the microscopic theory of homogeneous nucleation. This model, which will be referred to as the interacting monomers and clusters, is composed of different concentrations of clusters of different sizes. In this model, in addition to the internal structure for each cluster which is similar to the atomistic model, the monomer–monomer interaction and monomer–cluster interaction are taken into account. The calculation of the configuration integral is made possible by extending the method of correlation function to apply to the interacting monomers and clusters model. The formation energy of a cluster and the equilibrium concentration of clusters, which are necessary for obtaining the nucleation rate, have been calculated. These new expressions for the equilibrium concentration of clusters and the formation energy of a cluster reduce to the same form used in the atomistic model, when there is no interaction between monomers and clusters. Additionally, this model reduces to a system of an imperfect gas when there are only monomers in the system. The formation energy of a cluster in the interacting monomers and clusters and in the atomistic model for the Lennard-Jones potential has been studied.
X-ray diffraction study of aqueous solutions of NiSO4 and MnSO4
Authors: Licheri G. , Paschina G. , Piccaluga G. , Pinna G.
Published: February 2003

Two aqueous solutions of NiSO4 and MnSO4 2M were examined by x-ray diffraction. The best agreement with experimental data was achieved using a model in which two hydration shells around the divalent cations and one hydration shell around the sulphate ion were considered, together with some amount of inner sphere ion pairing. The criteria for the choice of the best model are also discussed.
Combined translation-rotation jumps in solid carbon dioxide
Authors: Liu Shang-Bin , Doverspike Montee A. , Conradi Mark S.
Published: February 2003

Combination translation-rotation jumps in solid CO2 have been measured using 13C NMR techniques previously applied to α-CO. In the Pa3 structure of CO2, a molecule which jumps to a neighboring (presumably vacant) site will also reorient, due to the orientationally ordered structure. The rates of translation and rotation have been measured independently by using low and high field NMR. The two rates agree, indicating that one combined motion occurs, as expected. The jump rate obeys the thermal activation expression with activation energy E/k=6600 K and frequency prefactor ω0=2×1017 s−1. The activation energy in CO2 agrees with that found previously for the same motion in N2O and α-CO, after scaling by the latent heats of sublimation. All three molecular solids belong to the family of solids composed of small, linear molecules with the Pa3 crystal structure. Unusually high frequency prefactors are seen in all three solids. The shift anisotropy of 13CO2 has been measured as 325 ppm.
Inelastic collisions of OH (2Π) with H2: Comparison between theory and experiment including rotational, fine structure, and Λ-doublet transitions
Authors: Schinke R. , Andresen P.
Published: February 2003

Detailed cross section calculations for inelastic collisions of OH (2Π) in the ground state j=3/2, Ω̄=3/2 with H2 are presented using an ab initio potential energy surface without adjustable parameters. The OH molecular wave function is described within the intermediate coupling case. The results are compared with recent experiments. The agreement is satisfactory for the final rotational state distributions within both the Ω̄=3/2 and the Ω̄=1/2 ladder. Also the relative magnitude of Ω̄=1/2 and 3/2 cross sections is in good accord with experiment and thus indicates that the difference potential VA′-VA″ is realistically described by the ab initio calculation. The dynamical calculations yield prefential excitation of one Λ-doublet state. The extent of this preference increases with j and is larger for Ω̄=3/2 in qualitative but not quantitative agreement with experiment. Possible interpretations in terms of the potential energy surfaces are briefly discussed.
Cage effect in the dissociation of van der Waals complexes RgI2 (Rg=Ar, Kr, Xe): A quasiclassical trajectory study
Authors: NoorBatcha I. , Raff L. M. , Thompson Donald L.
Published: February 2003

The dissociation dynamics of the RgI2 (Rg=Ar,Kr,Xe) van der Waals complexes have been studied using three-dimensional quasiclassical trajectories. Specifically, the unimolecular dissociation of RgI2(B 3π) with initial I2 vibrational excitation above the Rg+I+I dissociation limit of the B 3π state was studied. In addition to complete dissociation to atoms, iodine ‘‘recombination’’ was observed to be a major reaction channel. This result is interpreted as a cage-like effect due to the inert gas atom, which also carries away a large fraction of the energy when the complex dissociates to form I2. The reaction mechanism leading to the formation of molecular products has been found to involve both direct and long-lived, complex trajectories. Dissociation of the complex RgI2 is favored by near collinear orientations. The decomposition kinetics of the complex are found to be complex and non-RRKM in character. A four-step reaction mechanism involving an explicit intramolecular energy transfer step is proposed to explain the calculated time dependence of the product concentrations. The calculated product vibrational distributions are in qualitative agreement with the experimental results.
State selected ion–molecule reactions by a TESICO technique. IX. Vibrational state dependence of the cross sections in the reaction C2H+2(ν2)+D2(H2)
Authors: Honma K. , Kato Tatsuhisa , Tanaka Kenichiro , Koyano Inosuke
Published: February 2003

Vibrational-state selected reaction cross sections have been measured for four product channels of the reaction C2H+2+D2, by use of the threshold electron–secondary ion coincidence (TESICO) technique. The ν2 vibrational states of the C2H+2 ion were selected up to v=2 and the collision energies were changed from 0.2 to 3.7 eV. At low collision energies close to 0.2 eV, considerable enhancement of the cross sections for the channels producing C2H2D+ and C2HD+2 was observed when the vibrational quantum number was increased successively. As the collision energy was increased, the extent of this enhancement diminished gradually and the cross sections finally became almost independent of v at about 2 eV. The cross section for the H/D exchange channel, on the other hand, was found to decrease with increasing vibrational quantum number at low collision energies. However, the v=0 and v=1 cross sections of this channel, especially the former, decayed very quickly as the collision energy was increased, reaching essentially zero at ∼2 eV, while the cross section for v=2 remained substantial even at this and higher collision energies. These results have been discussed in terms of two concurrent reaction mechanisms, i.e., a complex mechanism predominating at low collision energies and a direct one predominating at high collision energies. The ν2 state selected cross sections for the reaction C2H+2+H2 →C2H+3+H have also been studied in a similar collision energy range for comparison.
Maximum in the translational energy dependence of the cross section for the Na+CH3I→NaI+CH3 reaction
Authors: Rabanos V. Saez , Verdasco E. , Herrero V. J. , Ureña A. Gonzalez
Published: February 2003

Relative values of the total reaction cross section for the crossed molecular beam reaction Na+CH3I→NaI+CH3 have been measured over the collision energy ET range of 0.11–0.40 eV. The measured excitation function is of a mechanistically bimodal type showing a maximum at ET ∼0.14 eV. This functionality is compared to those of closely related reactions and is discussed in the light of different dynamical models.
An improved quasiclassical trajectory method for state to state reactive scattering cross sections and rate constants
Authors: Ashton C. J. , Muckerman J. T. , Schubert F. E.
Published: February 2003

A systematic scheme is developed for the incorporation into quasiclassical trajectory (QCT) methodology of recent advances in the understanding of vibrationally adiabatic barriers in collinear atom + diatom reactions. The resulting hybrid QCT method centers on a definite set of rules for optimally combining the results of forward and reverse trajectory calculations. It is argued, and demonstrated by practical examples, that the hybrid method will give a more consistently reliable account of the threshold behavior of collinear reaction cross sections than the conventional QCT method. Extension of the method to the three dimensional F+H2 reaction gives similarly encouraging results, both for state to state reaction cross sections and for rate constants.
Rotational tunneling in CH4 at temperatures below 1 K studied by inelastic neutron scattering
Authors: Heidemann A. , Lushington K. J. , Morrison J. A. , Neumaier K. , Press W.
Published: February 2003

The temperature dependence of the tunneling states of CH4 II doped with 0.25% O2 has been studied by high resolution inelastic neutron scattering in the temperature range 0.04–5 K. The results are in good agreement with a model based on inhomogeneous broadening except that the linewidth remains finite at very low temperatures. For the first time and through slow cooling, methane has been entirely converted into the A state. With rapid cooling, however, about 4% of the molecules remain in the E state.
Thermal electron attachment to van der Waals molecules containing O2
Authors: Huo Winifred M. , Fessenden Richard W. , Bauschlicher Charles W.
Published: February 2003

Calculations on O2⋅N2 and O−2⋅N2 have been carried out to explain the large enhancement in the attachment rate of thermal electrons found in van der Waals molecules containing O2. Two geometries, T-shape and linear, are used. SCF wave functions are used to represent both the neutral molecule and the ion. The incoming electron is approximated by a plane wave. The width is determined using a shielded polarization potential. The effect of additional vibrational structures of the van der Waals molecule on the attachment process is investigated by studying the O2–N2 stretching mode using Lennard-Jones potentials. Symmetry breaking, which allows the molecule to attach a p wave electron, is shown to play a primary role. The lowering of resonance energy, due to a deeper Lennard-Jones potential of O−2⋅N2 in comparison with O2⋅N2, furthers the enhancement. The calculated attachment rate is comparable to that determined by Shimamori and Fessenden, but differs from the recent values obtained by Toriumi and Hatano, who used a different set of reactions to interpret their data.
Rotational energy transfer in HF(v=2): Energy corrected sudden approximation scaling relations applied to double resonance measurements
Authors: Copeland R. A. , Crim F. F.
Published: February 2003

Applying scaling relations based on the energy corrected sudden (ECS) approximation to a large set of laser double resonance measurements of HF self-relaxation allows the time evolution of individual rotational states from J′=2 to 6 to be described by only a few fitting parameters and permits extraction of state-to-state energy transfer rate constants. Obtaining a qualitatively correct description of the data requires inclusion of a significant (∼25%) multiple rotational quantum pathway (‖ΔJ‖≥2). The ECS approximation based scaling relation seems to provide a better description of the rotational energy transfer for high rotational levels (J′=5 and 6) than do relations based solely on energy defect. Comparing the double resonance results to those of linewidth analysis shows differences that may arise from the sensitivity of the measurements to different aspects of the energy transfer dynamics. Extrapolating the rate constants obtained by fitting the double resonance data on low rotational levels (J′≤6) yields good agreement with recent independent measurements on J′=11 to 14 in HF(v=0).
Quantum Monte Carlo for molecules: Green’s function and nodal release
Authors: Ceperley D. M. , Alder B. J.
Published: February 2003

A random walk algorithm is presented which exactly calculates the properties of a many-electron system. For that purpose both the Green’s function Monte Carlo method and nodal relaxation have been employed and both are described in detail. The scheme is applied to several small molecules, (H3, LiH, Li2, H20) and with modest computational effort and simple importance functions, ground state energies are obtained which agree with experimental energies within statistical error bars. The small energy decrease due to nodal release is accurately evaluated by a difference method.
The lowest rotational transition of several isotopic forms of KrD+
Authors: Warner Hugh E. , Conner William T. , Woods R. Claude
Published: February 2003

The J=0–1 absorption line of KrD+ has been observed for all six stable krypton isotopes in a dc glow discharge in Kr–D2 mixtures. For 83KrD+ the quadrupole coupling constant [eQq=549.1(5) MHz] and nuclear magnetic coupling constant [CN=−0.022(2) MHz] were determined. By combining the new microwave data for KrD+ with very precise published Dunham constants for KrH+ from Johns’ high resolution infrared spectral data, the mass independent Dunham parameters U01, ΔH01, and ΔKr01 were deduced. Precise Doppler shift measurements indicated a small drift velocity in the magnetic field enhanced, abnormal discharge in the direction opposite to that naively expected from the electrode polarity.
Complete determination of 14N hyperfine and quadrupole interactions in the metastable triplet state of free-base porphin via electron spin echo envelope modulation
Authors: Singel David J. , van der Poel W. A. J. A. , Schmidt J. , van der Waals J. H. , de Beer R.
Published: February 2003

Through the use of the electron spin echo envelope modulation method, we have accomplished the complete determination of the 14N hyperfine and quadrupole interaction tensors in the lowest photo-excited triplet state of free-base porphin in an n-octane single crystal at ∼1.4 K. The results give insight into the molecular structure of the porphin in the excited state, the relationship between the nitrogen spin density and observed hyperfine couplings, and the effect of hydrogen bonding interactions on the quadrupole coupling constants and principal axes.
Nonperturbative theory of temperature-dependent optical dephasing in crystals. I. Acoustic or optical phonons
Authors: Hsu D. , Skinner J. L.
Published: February 2003

We continue the discussion of the nonperturbative theory of zero-phonon impurity lines in crystals formulated by Osad’ko and more recently by us. In that paper, a diagrammatic analysis was undertaken which led to analytic results for the thermal width and shift of the zero-phonon line. Here we evaluate these results for two model phonon densities of states: the Debye model for acoustic phonons, and a sharply peaked density of states appropriate for optical phonons. For Debye phonons we find that at low temperatures the standard (perturbative) theory of optical dephasing is qualitatively correct, but at high temperatures it becomes very inaccurate. Moreover, if one of the excited state phonon frequencies tends to zero (a soft mode), then perturbation theory breaks down completely. For optical phonons our results give an Arrhenius temperature dependence for the linewidth and place an upper bound on the value of the preexponential. In paper II of this series we will discuss dephasing by pseudo-local phonons, and in paper III we will compare our results to absorption, photon echo, and hole burning experiments.
Laser induced fluorescence study of the HeBr2 van der Waals complex
Authors: van de Burgt Lambertus J. , Nicolai Jean-Philippe , Heaven M.
Published: February 2003

The HeBr2 van der Waals complex has been observed in a free jet expansion. The complex was detected by laser excitation of the bands associated with the Br2(B−X) system. High resolution (0.05 cm−1) spectra have been recorded for the 11-0 to 38-0 bands. The rotational structure has been resolved for several bands and the analysis is found to be consistent with a rigid T-shaped geometry. The distance from the Br2 bond center to the He atom is 3.8 A and 3.7 A for the excited and ground states, respectively. Vibrational predissociation of the He–Br2 bond has been observed via homogeneous broadening of the rotational lines. Predissociation rates, derived from the line shape data, showed a strong dependence on the Br2 vibrational excitation. Rates varied from 1010 s−1 for v′=11, up to 5×1011 s−1 for v′=38. The data can be adequately described by a simple ‘‘energy gap’’ model for the predissociation probability.
Adiabatic long-range rotationally inelastic collisions in Li*2(A 1Σ)–Xe: Experiment and theory
Authors: Smith Neil
Published: February 2003

A recent study of the velocity dependence of rotationally inelastic (RI) collision cross sections in Li*2 –rare gas systems has shown the Δ≡jf−ji=±2 cross sections in Li*2–Xe to be exceptional in their large size and rising velocity dependence. A simple semiclassical time dependent perturbation treatment using the long-range R−6 dispersion potential will be shown to provide an excellent description of the magnitude and velocity dependence of these cross sections. The analysis explicitly accounts for the molecular rotation during the collision, which is shown to cause the substantial adiabatic reduction of these ‖Δ‖=2 cross sections at lower velocity. The observation that almost all other RI collision cross sections in Li*2–rare gas systems decrease with larger velocity suggests that long-range collisional contributions and rotationally adiabatic effects are not of general importance for these collisions. New experimental as well as theoretical investigations into vib-rotationally inelastic collisions in Li*2–Xe suggest that vibrational inelasticity may have been previously mistaken for rotational adiabaticity when accounting for the general decrease in RI cross sections at larger ji.
Isolated C–H stretching vibrations of n-alkanes: Assignments and relation to structure
Authors: Snyder Robert G. , Aljibury A. L. , Strauss Herbert L. , Casal H. L. , Gough K. M. , Murphy W. F.
Published: February 2003

The conformational/configurational dependence of the frequencies of the deuterium-isolated C–H stretching modes of the gas-phase alkanes C1, C2, n-C3, n-C4(t and g), n-C5(tt and gt), cyclo-C6, iso-C4, and neo-C5 are reported. Most of the isolated C–H frequencies were obtained from Raman spectra of specifically and randomly protonated deuterohydrocarbons. An extraordinarily precise correlation is found between the observed isolated C–H frequencies and the corresponding ab initio calculated C–H bond lengths. In the case of the n-alkanes, the observed C–H frequencies tend to fall in clusters that are regularly spaced with an average separation of about 14.5±1 cm−1. The clustering occurs because the isolated C–H stretching frequencies are determined by the structure of the n-alkane in the immediate vicinity of the C–H bond. The relation between frequency and local structure can be expressed in a simple way and used to predict the effect of conformational change.
The role of intersystem collisional transfer of excitation in the determination of N2 vibronic level populations. Application to B′ 3Σ−u–B 3πg band intensity measurements
Authors: Benesch William , Fraedrich Douglas
Published: February 2003

This work constitutes a two-part study of molecular nitrogen in laboratory electric discharges. In the first part, an investigation is undertaken of the phenomenon of the intersystem collisional transfer (ICT) of excitation (recently advanced as the process which gives rise to the reddening of type B aurora) in which ICT is established as a dominant mechanism controlling the distribution of vibronic level populations under discharge conditions. In particular, it is determined that there occur many pairs of vibronic levels in adjacent electronic states in which the pair partners attain essentially identical populations through the agency of collisional coupling. In the second part, the knowledge of completely shared populations is combined with the results of a series of measurements of emission band intensities in N2 to obtain transition probabilities and transition moments for the infrared afterglow system B′ 3Σ−u –B 3πg. A tabulation of transition probabilities and vibronic level lifetimes for this system is presented.
Systematic investigation of the Cooper minimum for the hydrogen halides
Authors: Carlson Thomas A. , Fahlman Anders , Krause M. O. , Whitley Timothy A. , Grimm Frederick A.
Published: February 2003

Angle-resolved photoelectron spectroscopy has been carried out on the two outermost molecular orbitals of HBr using synchrotron radiation from a photon energy of 14 to 110 eV. Both partial cross sections σ and angular distribution parameters β have been determined experimentally. For comparison, calculations were also carried out on σ and β using the multiple scattering Xα method. Both the experimental and calculated results are discussed in terms of the Cooper minimum. Comparison is made with earlier results on HCl and HI and with results on the rare gases, which are isoelectronic with the hydrogen halides.
Laser induced fluorescence of CH3S and CD3S radicals
Authors: Suzuki Makoto , Inoue Gen , Akimoto Hajime
Published: February 2003

Laser-induced fluorescence of the CH3S andCD3S Ã 2A1−X̃ 2E transitions was studied. The 000 bandhead of the 2A1–2E3/2 transition lies at 26 531 cm−1 (376.82 nm) for CH3S and 26 574 cm−1 (376.20 nm) for CD3S. Prominent progressions of the C–S stretching mode (ν3) both in the excitation and dispersed fluorescence spectra are characterized by a doublet structure corresponding to a spin-orbit splitting in the ground state. The band shapes of the transitions 2A1–2E1/2 and 2A1–3E3/2 were found to be quite different from each other, which was interpreted as the difference in the geometry of the spin-orbit sublevels. From the doublet spacings, a spin-orbit coupling constant (A) was obtained; −280±20 cm−1 in CH3S and −260±20 cm−1 in CD3S, in a good agreement with the previous results of laser electron photodetachment studies. The CH3 umbrella mode vibration (ν2) is also active in the vibronic transition. Fundamental vibrational frequencies determined were 1316±4 (ν″2), 740±4 (ν″3), and 403±1 cm−1 (ν′3) for CH3S, and 837±1 (ν2), 667±1 (ν″3) and 395±1 cm−1 (ν3) for CD3S. Fluorescence lifetimes of single vibronic levels were measured to give 0.31 μs for the 000 band of CH3S. Predissociation occurs at v′3=2 and 3 for CH3S and CD3S, respectively.
Factor group splitting and dispersion on internal vibrational modes: A qualitative discussion
Authors: Durman Ray , Jayasooriya Upali A. , Kettle Sidney F. A. , Mahasuverachai Sudjit , Mortimer Richard , Norrby Lars-Johan
Published: February 2003

Factor group splittings, of common occurrence in the infrared and Raman spectra of crystals, and dispersion, usually encountered in discussions of lattice modes, are shown to be closely related phenomena. Their relationship is elucidated by a model comprising a linear array of four vibrating units. A correlation between this and a similar two vibrator model indicates a method by which, in favorable cases, dispersion and mode change contributions to factor group splittings may be separately measured by conventional vibrational spectroscopies.
The three-photon spectrum of the 1B2u←1A1g transition in benzene: Analysis of vibronic and rotational structure
Authors: Grubb S. G. , Otis Charles E. , Haber K. S. , Albrecht A. C.
Published: February 2003

A vibrational and rotational analysis of the three-photon fluorescence excitation spectrum of the 1B2u←1A1g transition in gas-phase benzene is presented. The group theoretically allowed origin and any associated vibrational structure are absent. In addition, there is no evidence for any rotational structure based on a rank-three transition tensor. Coherent loss mechanisms are carefully considered and then rejected as possible interferences in the three-photon fluorescence spectrum. Polarization measurements and observed vibronic structure indicate a dominant rank one (vibronic) contribution to the three-photon cross section. The lack of three-photon allowed structure in this 1B2u←1A1g transition of benzene is consistent with the pseudoparity selection rules of alternant hydrocarbon theory, yet such strict adherence to them would seem surprising. The appearance of the e2g mode ν8 in the three-photon spectrum marks the first experimental determination of its frequency in the 1B2u state.
Dynamics of helical worm-like chains. IV. Dielectric relaxation
Authors: Yoshizaki Takenao , Yamakawa Hiromi
Published: February 2003

Dielectric relaxation of both flexible and stiff chain polymers in dilute solution is studied on the basis of the discrete helical worm-like chain such that an electric dipole moment is attached rigidly or with a rotational degree of freedom to each of the subbodies composing the chain. The complex dielectric constant is formulated with the dipole correlation function, which may be expressed in terms of the L=1 time-correlation functions derived previously. Then, dielectrically active branches of the eigenvalue spectrum are identified for a given type of dipoles, and a mode analysis of them is made in order to inquire into the interaction between global and local modes. The decay behavior of the dipole correlation function is also examined numerically for various chains. A comparison of theory with experiment is made with respect to the dispersion and loss, and also the dielectric correlation time τD as determined from the loss peak. The theory cannot always well explain the asymmetry of the loss curve, especially for flexible chains without side chain motions. For flexible chains, however, the calculated values of τD agree rather well with the observed values, although the former are somewhat smaller than the latter. Furthermore, it is found that there is strong correlation between the static stiffness parameter λ−1 and the dynamic stiffness as defined as the ratio of τD to that of the isolated subbody (monomer unit). For stiff chains, an interpolation formula for τD is constructed, and it is shown to explain well the molecular weight dependence of τD. Finally, a possible direction toward improvement of the theory is suggested.
Dynamics of helical worm-like chains. V. Nuclear magnetic relaxation
Authors: Yamakawa Hiromi , Fujii Motoharu
Published: February 2003

Nuclear magnetic relaxation of both flexible and stiff chain polymers in dilute solution is studied on the basis of the discrete helical worm-like chain. For most cases, only the dipolar interaction is considered by attaching a spin–spin (internuclear) vector rigidly or with a rotational degree of freedom to each of the subbodies composing the chain. For some cases of stiff chains, both the dipolar interaction and the anisotropic chemical shift are considered by affixing also a shielding tensor. The spectral densities are formulated by the use of the L=2 time-correlation functions derived previously. Then, the behavior of the five branches of the eigenvalue spectrum and the amplitudes is examined, and a mode analysis of each branch is also made. With these results, the behavior of the spectral densities is examined with a comparison with those from other theories. A comparison of theory with experiment is made with respect to the spin-lattice relaxation time T1, the spin–spin relaxation time T2, and the nuclear Overhauser enhancement NOE for various chains. For flexible chains, the agreement between theory and experiment is as good for T1 as for the dielectric correlation time τD but not for T2 and NOE, especially for T2, except for the extreme narrowing case. However, there is correlation between the static stiffness parameter λ−1 and the dynamic stiffness as defined as the ratio of a properly defined magnetic correlation time τM to that of the isolated subbody. For stiff chains, the agreement is rather good. Specifically, the molecular weight dependences of T1 and T2 can be explained semiquantitatively, and a reasonable estimate of the torsional constant of, e.g., DNA can be obtained. Finally, the defect of the theory and its improvement are discussed.
Determination of electrolyte friction from measurements of the tracer diffusion coefficients, mutual diffusion coefficients, and electrophoretic mobilities of charged spheres
Authors: Gorti Sridhar , Plank Lindsay , Ware B. R.
Published: February 2003

The technique of fluorescence recovery after photobleaching has been used to measure the tracer diffusion coefficient of fluorescein-labeled charged polystyrene spheres in dilute solutions as a function of solution ionic strength. As the ratio of the particle radius (a) to the Debye–Hückel screening length (κ−1) was varied from 13 to 0.6, a 20% reduction in tracer diffusion coefficient was observed. The mutual diffusion coefficient, measured by quasielastic light scattering, increased as κa was reduced, demonstrating the dominant effect of thermodynamic factors on this parameter. The tracer data have been compared with theoretical predictions of other workers describing the influence of small-ion interactions with a charge sphere on its translational friction. The theoretical results of Schurr, and Booth, involve an explicit dependence on the electrokinetic charge of the sphere and yield estimates of this parameter (by least squares) which are significantly smaller than those obtained from laser Doppler electrophoretic light scattering results for the electrophoretic mobility of the labeled spheres. Approximations in the theoretical analyses are expected to account for these discrepancies.
A correlation between slightly endothermic reactivity and ion–molecule association reaction rates
Authors: Ferguson E. E. , Smith D. , Adams N. G.
Published: February 2003

Primary binary reaction processes which are slightly endothermic can create relatively long-lived complexes in ion–neutral molecule collisions. The endothermic process absorbs translational energy, inhibiting separation to reactants, and in addition may facilitate access to an overall reaction potential well. The endothermicity must be less than the available collision energy, i.e., the electrostatic attractive energy and possibly some chemical interaction energy. An enhanced complex lifetime can result in enhanced rate constants for three-body ion–neutral association, radiative association, vibrational relaxation or complicated binary reactions. Application is made to the three-body association of hydrocarbon ions with H2, a selective H/D isotope effect in three-body association, the three-body association of Si+ with O2, the efficient vibrational relaxation of O+2 by CH4 and the fast low temperature reaction of O+2 with CH4 to produce protonated formic acid which involves the making and breaking of four chemical bonds.
Mechanisms and rate constants for the vibrational relaxation of HD(v=4, 5, and 6) in collisions with HD, 4He, and D2
Authors: Rohlfing Eric A. , Rabitz Herschel , Gelfand Jack , Miles Richard B.
Published: February 2003

The relaxation of highly vibrationally excited HD with several collision partners is examined using direct overtone excitation and time-resolved photoacoustic detection of the subsequent change in translational energy of the gas. Analysis of such data for HD(v=4, 5 and 6) self-relaxation yields vibration-translation (V-T) and vibration–vibration (V–V) rate constants for collisions between HD(v=1 to 6) and HD(v=0). The V-T rate constant becomes larger than the V–V rate constant above v=4. Similarly, relaxation data for the HD(v=4 and 5)–4He systems is analyzed and the HD–4He V-T rate constants at v=4 and v=5 are presented. The differences in the quantum number scaling of the V-T rates for the HD–HD and HD–4He systems are discussed. Finally, the time-resolved photoacoustic data for HD(v=4) relaxing in collisions with D2 shows the effect of the rapid, near-resonant V–V energy transfer between HD in the v=1 through v=4 levels and D2 in v=0.
Monte Carlo transition-state theory: XH4→XH3+H (X=C, Si)
Authors: Viswanathan R. , Raff L. M. , Thompson Donald L.
Published: February 2003

A classical variational phase-space procedure for the computation of microcanonical dissociation rate coefficients is presented and applied to simple bond rupture in the CH4 and SiH4 systems. The method replaces the sum and density of quantum states that appear in the RRKM formalism with equivalent multidimensional phase-space integrals which are evaluated using a Metropolis Monte Carlo sampling procedure. The computed microcanonical rate coefficients for bond rupture in CH4 and SiH4 are in good agreement with previously reported trajectory results obtained on the same potential-energy surfaces, provided the dividing surface separating reactants and activated complex is reoptimized at each reactant internal energy. The procedure permits the entire k(E) vs E curve to be obtained in a single calculation. Consequently, the total computational time required is less than 5% of that required for the corresponding trajectory calculations.
Laser measurements of the effects of vibrational energy on the reactions of CN
Authors: Li Xuechu , Sayah Nahid , Jackson William M.
Published: February 2003

Pulsed laser photolysis of C2N2 at 193 nm has been used as a source of CN radicals in both the v″=0 and v″=1 levels. Individual rovibronic levels of these radicals were measured as a function of time with a tunable dye laser. From these measurements the rate constants for the reaction of each of these vibrational level with H2, O2, CO, CO2, N2, HCN, C2N2, and CH4 have been determined. Some enhancement in the rate constant with vibrational energy which could not be ascribed to quenching was observed for O2, CH4, and H2. Only vibrational quenching was observed for HCN, N2, CO2, CO, and C2N2. In the CO case the vibrational quenching rate appears to be significantly enhanced by complex formation during the quenching process.
Electronegativities of the elements from density functional theory. I. The Hartree–Fock–Slater theory
Authors: Manoli S. , Whitehead M. A.
Published: February 2003

Mathematical expressions for the electronegativities of the elements are derived exactly using the Hartree–Fock–Slater equations and Mulliken’s definition of electronegativity. Relaxation effects are neglected. These equations are the same ones used by Bartolotti, Gadre, and Parr. They are generalized for the spin-polarized case and calculations are done for the elements helium to krypton. Results show that the spin-polarized ionization potentials are closer to experiment than the non-spin-polarized ones and that the trend in the electronegativities obtained from spin-polarized calculations is the same as the one obtained by Bartolotti et al. using a non-spin-polarized program.