bluuebunny/crossref_metadata_2025 huggingface dataset:
Singlet–triplet surface crossings and low-temperature rate enhancement for O(3P)+H2→OH+H
Authors: Donaldson D. J. , Wright James S.
Published: February 2003

Low-temperature curvature in Arrhenius plots of H-atom transfer reactions is usually attributed to tunneling. This paper investigates the feasibility of an alternative mechanism involving nonadiabatic transitions to enhance the low-T rate of O(3P)+H2→OH+H. 3A″ and 1A′ potential surfaces are calculated for the configuration OHH over a range of bond angles. The ab initio surface calculations use two basis sets: double-zeta + polarization (DZP), and DZP augmented by bond functions and Rydberg orbitals (BF), followed by extensive configuration interaction. The CI data points are fitted by a rotated Morse curve-cubic spline functional form. The surface crossing is analyzed in detail. The singlet surface crosses the triplet surface near 120° at an energy which is slightly above (2 kcal/mol) the energy of the lowest triplet barrier (13.2 kcal/mol for the linear geometry). The implications for low-temperature kinetics are discussed.
Raman spectroscopic study of EtNH3X (X=Cl,Br) and several deuterated analogs
Authors: Hagemann H. , Bill H.
Published: February 2003

A room temperature Raman study has been performed on the following compounds: EtNH3X (X=Cl,Br), CD3CH2NH3Cl, CD3CH2ND3Cl, CH3CD2NH3Cl, CH3CD2ND3Cl, and EtND3Cl. The complete vibrational spectra are discussed in terms of external and internal modes. The external modes of the halogens have been identified on the basis of the isotopic substitutions. The internal vibrations are divided into three frequency ranges. Several modes are clearly identified through the observed isotopic shifts, while other modes are found to interact strongly. The Fermi resonances of the carbon–hydrogen stretching modes with the carbon–hydrogen deformational modes are analyzed.
Time-resolved two-color photoacoustic and multiphoton ionization spectroscopy of aniline
Authors: Moll D. J. , Parker G. R. , Kuppermann Aron
Published: February 2003

The multiphoton ionization and photoacoustic signals from aniline have been detected as a function of time delay between two laser pulses of different wavelength. The first pulse excited the S0 → S1 0–0 transition. The second pulse either excited S1 to a higher singlet state, or a triplet state produced by intersystem crossing to a higher triplet state, depending on the time delay between pulses. Both ionization and dissociation processes are observed. By varying the conditions of excitation it appears that a given amount of energy can be selectively channeled almost exclusively into either ionization or dissociation pathways. The results are explained using a simple Franck–Condon factor model.
Differential scattering (CIDS) of circularly polarized light by dense particles
Authors: Bustamante Carlos , Maestre Marcos F. , Keller David , Tinoco Ignacio
Published: February 2003

The circular intensity of differential scattering (CIDS) for structures composed of an arbitrary collection of point polarizable groups has been obtained in analytical form using the second Born approximation, both for oriented and rotationally averaged structures. It is found that: (i) A CIDS signal is present in the forward direction parallel to the light beam. This forward CIDS does not appear in CIDS patterns calculated using the first Born approximation alone; (ii) the magnitude and shape of CIDS patterns for oriented periodic structures depend only on the properties of one unit cell of the structures and are independent of the overall size; (iii) CIDS can exist for scatterers made up of a collection of spherically symmetric point polarizable groups. In the first Born approximation anisotropic polarizable groups are required for the CIDS to be different from zero. In the second Born approximation the anisotropy necessary for CIDS is generated by the interaction between the scattering groups; (iv) the CIDS is largest when the chiral dimensions of the scatterer are of the order of the wavelength of light.
On the absence of I(2P1/2) as a product of the reactions of F atoms with HI, I2, and ICN
Authors: Das P. , Venkitachalam T. , Bersohn Richard
Published: February 2003

It is shown by two photon laser induced fluorescence that virtually no I(2P1/2) atoms are products of the reactions of F atoms with HI, I2, and ICN. A simple electrostatic model is used to prove that the potential surface beginning at F*(2P1/2)+RI and ending at RF+I*(2P1/2) must have an intermediate maximum. Moreover, this surface must cross a surface leading to ground state I atoms at two points. It is concluded that the propensity to form I rather than I* atoms is not general but depends on specific properties of the potential surfaces.
A measurement of the vibrational band strength for the ν3 band of the HO2 radical
Authors: Zahniser Mark S. , Stanton Alan C.
Published: February 2003

The HO2 radicals generated in a discharge-flow system have been observed with tunable diode laser absorption in the P branch of the ν3 vibrational band at 1080 cm−1. The observed line positions agree within (2 to 5)×10−3 cm−1 with those calculated from the molecular constants for the ν3 band obtained from a previous study using laser magnetic resonance spectroscopy. The band strength has been determined by observing line center absorptions when HO2 is produced in the reaction F+H2O2→HO2+HF (k1) with a measured concentration of atomic fluorine and excess hydrogen peroxide. F-atom concentrations are measured by diode laser absorption of the spin-orbit transition at 404 cm−1. The analysis accounts for HO2 losses due to the reactions of HO2+HO2→H2O2+O2 (k3) and F+HO2→HF+O2 (k4). The experimental data are consistent with a value of k3=(1.6±0.3)×10−12 cm3 s−1 and a ratio k4/k1=1.0±0.4. The line strength for the 615←716 F1 transition is 2.9×10−21 cm2 molecule−1 cm−1 which corresponds to a ν3 band strength of 35±9 cm−2 (STP atm)−1. This value is a factor of 1.6 to 6 lower than previous ab initio calculations.
Time dependent reactivity of species embedded in micelles: Lifetime distribution of photoproduced N,N,N′,N′-tetramethylbenzidine cation radicals in sodium dodecyl sulfate micelles
Authors: Plonka Andrzej , Kevan L.
Published: February 2003

Photoproduced N,N,N′,N′-tetramethylbenzidine cation radicals in sodium dodecyl sulfate micelles decay in a pseudo-first-order reaction with an apparent rate constant k(t) depending on time according to k(t)=Btα−1 , where B and α(0<α≤1) are constants. At a given temperature the numerical value of B decreases with the storage time between preparation of the micellar solution and photoionization achieving a constant value for a storage time of a few days. The numerical value of α is essentially independent of the storage time and determines the distribution of cation radical lifetimes for a given value of B.
Potential energy curves for the A 1Σ+u and C 1Σ+g states of He2 obtained by combining scattering, spectroscopy, and a
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Authors: Jordan R. M. , Siska P. E.
Published: February 2003

Crossed beam scattering experiments on He*(21S)+He at energies of 0.95, 1.50, and 2.23 kcal/mol have been combined with spectroscopic and ab initio theoretical data to yield reasonably unique potential energy curves for the A 1Σ+u and C 1Σ+g excited states of He2 over a range of interatomic distances from 0.7 to 7 Å. In this energy range the scattering is particularly sensitive to the long-range barrier in the A state; the resulting barrier height is 1.158±0.02 kcal/mol and the position of the maximum 3.132±0.02 Å. At longer range the potentials are determined by a semiclassical inversion method described earlier [J. Chem. Phys. 69, 4634 (1978)] and the classical theory that underlies this method is used to interpret the complicated scattering patterns at higher energy.
Spectroscopy and intramolecular relaxation of methyl salicylate in its first excited singlet state
Authors: Kuper Jerry W. , Perry David S.
Published: February 2003

High resolution fluorescence excitation experiments are reported for the blue emitting rotamer of methyl salicylate in its first excited singlet state. These experiments employ moderate expansions of methyl salicylate seeded in argon ( P0D=5–8 Torr cm) to achieve rotational and vibrational cooling in a pulsed supersonic jet. The rotational contour of the electronic origin at 30 055.3 cm−1 is shown to be consistent with a geometrically distorted π−π* excited state, partially polarized along the A axis and with a rotational temperature of 5–7 K. A noticeable broadening of the spectral features beyond the rotational contour begins at 500 cm−1 above the origin and then increases rapidly above 900 cm−1 reaching a width of 12 cm−1 near 1200 cm−1. The constancy of fluorescence decay lifetimes in this region indicate that intramolecular vibrational relaxation in the S1 manifold is the broadening mechanism.
Large molecule relaxation: Spectroscopy, structure, and vibrational energy redistribution in naphthazarin
Authors: Rentzepis P. M. , Bondybey V. E.
Published: February 2003

Analysis of IR and laser induced fluorescence spectra of naphthazarin in rare gas matrices shows clearly that its vibrational potential is characterized by well localized, deep, C2v potential minima. Fluorescence quantum yield in the deuterated molecule is nearly an order of magnitude higher than the H2 and HD isotopic species. Vibrational relaxation in the upper state is remarkably slow and intense unrelaxed emission from a large number of excited vibrational levels is observed. Studies of the unrelaxed emission provide insight into the vibrational relaxation process. Vibrational energy redistribution clearly proceeds via specific pathways and not in a random, statistical manner. The relaxation rates and pathways exhibit only a mild dependence upon temperature and upon the matrix medium.
Cooperative two-photon absorption. II
Authors: Andrews David L. , Harlow Michael J.
Published: February 2003

In this paper we extend our earlier work on cooperative two-photon absorption to the case of one- and three-photon allowed transitions using the principles of quantum electrodynamics. After deriving a general rate equation we consider two specific molecular systems: (i) van der Waals molecules in which the two centers have a fixed mutual orientation; and (ii) fluid samples with random molecular orientation. Calculations are shown to involve a new type of rotational average which gives rise to two-photon circular dichroism in case (i). The dependence of the rate upon molecular separation is discussed in detail, as is the possibility of resonance enhancement if suitable intermediate energy levels are present. Finally we draw attention to an effect specific to this mode of cooperative absorption and speculate on the possibility of direct observation of the effects of retardation.
The microwave spectrum and properties of the propyne–HF complex
Authors: Shea J. A. , Bumgarner R. E. , Henderson Giles
Published: February 2003

The microwave spectrum of the weakly bound propyne–HF/DF complex was recorded in the region between 6 and 16 GHz. The J=0–1 and J=1–2 a-dipole lines, each with a torsional counterpart due to internal rotation of the methyl top, were observed. The spectrum was characteristic of a distorted T-shaped asymmetric top exhibiting torsional splitting caused by a low barrier to internal rotation of the methyl top relative to the propyne–HF frame. Deuterium substitution of HF confirms that the acidic hydrogen of HF is located between the F atom and the propyne triple bond. The spectroscopic constants given below are consistent with the fluorine atom’s being displaced toward the methyl group from a line perpendicular to and bisecting the propyne triple bond, suggesting a weak hydrogen bond interaction between fluorine and the methyl protons. UFRULE2 Propyne–HF Propyne–DF UFRULE1 A(MHz) 8722(9)  8644(12) B(MHz) 3919(3)  3886(6)  C(MHz) 2753(1)  2728(2)  Daa(kHz) ⋅ ⋅ ⋅   −23(6)  χaa(kHz) ⋅ ⋅ ⋅  163(6)  V3(cm−1)  100(50)  100(50) UFRULE2
Raman intensities of ethane and deuterated derivatives
Authors: Martín J. , Montero S.
Published: February 2003

Experimental gas phase Raman scattering differential cross sections and depolarization ratios of ethane and its deuterated derivatives are reported. A consistent interpretation of these data is accomplished in the frame of the bond polarizability model. The bond polarizability derivatives ∂αCH/∂rCH here obtained are, within the estimated uncertainties, the same as for other hydrocarbon molecules not only for the mean polarizability but for the anisotropy as well. On the other hand, the C–C bond properties are proved to be essentially the same in ethane and in diamond. In ethane isotopic species with synclinal and antiperiplanar conformers the experimental intensities are not always in the ratio 2:1. A satisfactory explanation of this apparent anomaly is achieved on the basis of the calculated Raman tensors of homologous normal modes which are quite different in some cases.
Kinetic study of the VUV xenon emissions using selective multiphoton excitation
Authors: Salamero Y. , Birot A. , Brunet H. , Galy J. , Millet P.
Published: February 2003

Selective excitation of xenon by multiphoton absorption of a dye laser beam gives, in the VUV, the emissions characteristic of this gas, i.e., the resonance line at 147 nm, the first continuum at 150 nm and the second continuum at 173 nm. The kinetic analysis of these emissions was carried out, using a correlation method, between 10−2 and 600 Torr. At low pressures the temporal variation of the density of the resonant 3P1 states allowed the resonance line trapping phenomenon to be demonstrated and the natural lifetime of these states to be determined (τn =4.3 ns). At high pressures the metastable 3P2 states are involved in the emission of the second continuum. The excitation spectra plotted for the blue satellite showed the creation of states in the 4f configuration. This phenomenon indicates that multiphoton ionization is not responsible for the creation of the metastable states. These states are created from a crossing of two molecular levels, one of which is bound (0+u) and responsible for the first continuum and the other of which is dissociative (2u) and correlated to the 3P2 state.
Infrared photodissociation of the hindered internal rotors Ne⋅C2H4 and Ar⋅C2H4
Authors: Western Colin M. , Casassa Michael P. , Janda Kenneth C.
Published: February 2003

Infrared photodissociation spectra of the van der Waals molecules Ne⋅C2H4 and Ar⋅C2H4 are reported. Spectra obtained near the ν7 frequency of free ethylene exhibit the sharpest and most complex structure yet observed for an ethylene-containing van der Waals molecule. Calculated spectra based on a hindered-internal rotor model are in satisfactory agreement with those observed. The barrier heights for hindered rotation of C2H4 about the C3 ♯ C axis are 12.5 and 30 cm−1 for Ne⋅C2H4 and Ar⋅C2H4, respectively. The natural linewidths, 0.5 cm−1 for Ne⋅C2H4 and 3.0 cm−1 for Ar⋅C2H4, are compared to those of other ethylene-containing clusters and it is concluded that the widths reflect vibrational predissociation, constrained by conservation of angular momentum.
Coordination of Cu(II) in Cu(NO3)2 aqueous solutions
Authors: Licheri G. , Musinu A. , Paschina G. , Piccaluga G. , Pinna G. , Sedda A. F.
Published: February 2003

Two concentrated copper (II) nitrate solutions have been examined by x-ray diffraction. The inspection of the radial distribution curves and the direct fitting of the structure functions suggested that a distorted arrangement of six oxygen atoms exists around the Cu2+ ion, probably in a tetragonally distorted, elongated octahedron. The best agreement with experimental data was achieved through a model in which extensive inner-sphere complex formation between Cu2+ and NO−3 ions takes place.
Three-dimensional quasiclassical trajectory study of the reaction He+H+2 →HeH++H on an accurate a
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Authors: Joseph Tomi , Sathyamurthy N.
Published: February 2003

State-selected integral cross sections computed via the three-dimensional quasiclassical trajectory method using an accurate ab initio potential-energy surface are compared with the classic photoionization experimental results for different vibrational states and relative translational energies of the reactants for the reaction He+H+2 →HeH++H. The agreement between theory and experiment is nearly quantitative.
Comparisons of classical and quantum dynamics for initially localized states
Authors: Davis M. J. , Heller E. J.
Published: February 2003

We compare the dynamics of quantum wave packets with the dynamics of classical trajectory ensembles. The wave packets are Gaussian with expectation values of position and momenta which centers them in phase space. The classical trajectory ensembles are generated directly from the quantum wave packets via the Wigner transform. Quantum and classical dynamics are then compared using several quantum measures and the analogous classical ones derived from the Wigner equivalent formalism. Comparisons are made for several model potentials and it is found that there is generally excellent classical–quantum correspondence except for certain specific cases of tunneling and interference. In general, this correspondence is also very good in regions of phase space where there is classical chaos.
Molecular properties from MCSCF-SCEP wave functions. II. Calculation of electronic transition moments
Authors: Werner Hans-Joachim
Published: February 2003

A method for calculating first order reduced transition density matrices between general singles-doubles multireference-CI wave functions (MCSCF-SCEP) is presented. The internal coupling coefficients are given in terms of simple overlap and first order transition density matrix elements between a set of N−1 and N−2 electron core functions, which need not necessarily to be orthogonal. As characteristic for the SCEP technique no external coupling coefficients appear in the formalism, and the transition density matrix is obtained in terms of matrix operations involving coefficient matrices and vectors. Some test calculations for the A–X transitions of the OH and CN radicals are presented. The stability of calculated transition moments with respect to the number of reference configurations and the size of the basis sets is investigated. Furthermore, the singles-doubles CI results are compared to those obtained from first order and internal CI wave functions corresponding to the same reference wave function.
Multireference-CI calculations of radiative transition probabilities in C−2
Authors: Rosmus Pavel , Werner Hans-Joachim
Published: February 2003

Electronic transition moment functions for the dipole allowed transitions among the X 2∑+g, A 2Πu, and B 2∑+u states of C−2 have been calculated from highly correlated MCSCF-SCEP wave functions. Vibrational band transition probabilities for all bound–bound transitions have been evaluated, and radiative lifetimes of bound vibrational levels are obtained. For the B 2∑+u, v′=0 state, the calculated lifetime of 76.5 ns is in excellent agreement with recent experimental values. Strong emission from the A 2Πu state in the infrared spectral region is predicted.
Eigenvalues of the Schrödinger equation for a periodic potential with nonperiodic boundary conditions: A uniform semiclassical analysis
Authors: Connor J. N. L. , Uzer T. , Marcus R. A. , Smith A. D.
Published: February 2003

A uniform semiclassical expression for the eigenvalues of a one dimensional periodic Schrödinger equation with nonperiodic boundary conditions has been derived. The potential energy function can have any number of symmetric or asymmetric barriers and wells. The treatment is uniform in that the classical turning points can come close together, coalesce, and move into the complex plane as the energy passes through a barrier maximum. A detailed application is made to Mathieu functions of integer order; the equations themselves include the case of fractional order. Approximate semiclassical expressions are derived for the widths of the energy bands and the energy gaps of the periodic Mathieu equation when these quantities are small. The semiclassical results give a physical interpretation to formulas present in the mathematical literature and to the decrease in the splitting of a sequence of avoided crossings with increasing quantum numbers in coupled oscillator systems. Numerical calculations are reported to illustrate the high accuracy of the semiclassical formulas.
Electronic structure and bonding in the Pd2 molecule by all electron a
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Authors: Shim Irene , Gingerich Karl A.
Published: February 2003

In this study, we present results of all electron ab initio Hartree–Fock (HF) and configuration interaction (CI) calculations of the low-lying electronic states of the Pd2 molecule. The basis set used is of double zeta quality in general, but the 4d orbital has been represented by a triple zeta function. Analysis of basis set superposition errors reveals that the interaction between two Pd atoms in their 1S(4d)10 ground term is nonbonding. Interaction between two Pd atoms in the (4d)9(5s)1 configuration results in formation of a chemical bond almost entirely due to the 5sσg molecular orbital. The six lowest lying states 1Σ+g , 1Γg, 1Σ−u , 3Σ−g , 3Γ+u , and 3Γu are very close in energy, and they all originate from a configuration with a hole in the 4dδ subshell of each of the two atoms. Spin-orbit coupling causes a considerable mixing of the low-lying electronic states. Taking spin-orbit coupling into account reveals the ground state as 0+g with two other states, 5u and 0−u , very close to the ground state. Spectroscopic constants have been derived. Our mass spectrometric measurements combined with our theoretical data yielded the dissociation energy D°0 of Pd2 as 99±15 kJ mol−1. The corresponding heat of formation is 653±15 kJ mol−1.
The equation of state of polytetrafluoroethylene to 80 GPa
Authors: Morris C. E. , Fritz J. N. , McQueen R. G.
Published: February 2003

Hugoniot data for polytetrafluoroethylene (PTFE) have been reported. These data can be fitted by two linear us–up Hugoniots which indicate the existence of a phase change. As usual for polymers, a linear low-pressure us–up fit does not extrapolate to the zero-pressure bulk sound-speed. The phase transition occurs at ∼34 GPa and a compression of 0.396. To investigate this high-pressure phase transition, shock-recovery experiments were performed. The recovery system used is described. The recovered samples indicate the high-pressure phase transition is a dissociation reaction into carbon and gaseous fluorocarbons. Release isentropes at various pressures have been measured using the ASM probe. From these measurements, the variation of the Grüneisen parameter was calculated for states both on and off the Hugoniot. The Grüneisen parameter on the Hugoniot monotonically increased from its initial value until the phase transition where it dropped slightly and then continued to increase with further increase in pressure. The Grüneisen parameter has been theoretically modeled in the solid phase and the model helps to explain some of these unusual experimental results.
Connectivity of hydrogen bonds in liquid water
Authors: Blumberg Robin L. , Stanley H. Eugene , Geiger Alfons , Mausbach Peter
Published: February 2003

We report on a molecular dynamics (MD) study of the connectivity of hydrogen bond networks in liquid water, focusing primarily on the microscopic distribution functions giving the weight fraction of molecules belonging to a ‘‘net’’ of M molecules (M=1,2,3,...). The MD data compare favorably—using no adjustable parameters—with predictions of random bond percolation theory. We also study the connectivity of those molecules with four intact hydrogen bonds, and compare the corresponding distribution functions with correlated-site percolation theory. Our analysis supports the proposal that when looking at the bond connectivity, water appears as a macroscopic space-filling network—as expected from continuum models of water. When looking at the correlated site percolation problem defined by the four-bonded molecules, water appears as a myriad of tiny ramified low-density patches, somewhat reminiscent of mixture theories and cluster models. In Appendix A, we find a strong correlation between the number of molecules within a sphere of radius rc around a given molecule and the total interaction energy of that molecule with its neighbors residing in that sphere; for most choices of rc, the energy becomes less negative when more molecules are in the sphere, in contrast to the behavior of a normal fluid. This result supports the finding of Geiger and Stanley that regions of high bond connectivity are correlated with regions of low density. In Appendix B we describe in detail how we adapt conventional percolation theory to the calculation of cluster size distribution functions for hydrogen bond networks in water.
A SIMS and classical dynamics study of the chemisorption of CO on Ni{7 9 11}
Authors: Foley K. E. , Winograd N. , Garrison B. J. , Harrison D. E.
Published: February 2003

The dependence of secondary ion intensities on azimuthal angle of incidence of the primary ion beam in SIMS experiments on a stepped Ni{7 9 11} surface with adsorbed CO has been examined in detail. The surface was examined after various exposures to CO and at temperatures of 231 and 300 K. The angular anisotropies for the Ni+, Ni+2, Ni+3, Ni2CO+, and especially the NiCO+ species were found to be quite sensitive to surface structure changes suggested using other surface science techniques. Ion yield ratios as a function of exposure of CO to the surface, however, were found to be nearly insensitive to these structural changes. Of particular significance was the presence of a sharp feature in the NiCO+ ion yield at an Ar+ ion angle of incidence of φ=110° with the crystal temperature at 231 K and after a CO exposure adequate to populate the step edge sites. At higher exposures or temperatures, this feature was washed out when presumably mostly terrace sites are occupied. Using these ion yield versus azimuthal angle curves, it is also apparent that the saturation coverage structures at 231 and 300 K are different. By introducing 10−7 Torr of CO into the chamber at 300 K the ion yield curves are identical to those for the saturation coverage at 231 K, indicating similar surface structures. Classical dynamics calculations aimed at modeling the structure corresponding to a CO exposure of 0.6 L at 231 K support the idea that the CO molecules reside in twofold bridge sites along the bottom of the step edge.
Microscopic theory of membrane transport. II. Darcy–Brinkman flow
Authors: Vertenstein Mariana , Ronis David
Published: February 2003

Viscous flow through a permeable membrane is examined using linear response theory. An equation similar to the Darcy–Brinkman equation, with a correlation function expression for the permeability, is obtained. Only membrane force time correlation functions are needed to compute the permeability to leading order. The corrections arising from surface structure are examined and pressure and two-dimensional shear flow are presented as examples.
Charge renormalization, osmotic pressure, and bulk modulus of colloidal crystals: Theory
Authors: Alexander S. , Chaikin P. M. , Grant P. , Morales G. J. , Pincus P. , Hone D.
Published: February 2003

The interactions between charged colloidal particles with sufficient strength to cause crystallization are shown to be describable in terms of the usual Debye–Huckel approximation, but with a renormalized charge. The effective charge in general is smaller than the actual charge. We calculate the relationship between the actual charge and the renormalized charge by solving the Boltzmann–Poisson equation numerically in a spherical Wigner–Seitz cell. We then relate the numerical solutions and the effective charge to the osmotic pressure and the bulk modulus of the crystal. Our calculations also reveal that the renormalization of the added electrolyte concentration is negligible, so that the effective charge computations are useful even in the presence of salts.
On the calculation of the self-diffusion coefficient of interacting Brownian particles
Authors: Lekkerkerker H. N. W. , Dhont J. K. G.
Published: February 2003

We consider two ways to calculate the self-diffusion coefficient of interacting Brownian particles. The first approach is based on the calculation of the mean square displacement of a Brownian particle starting from the Smoluchowski equation. In the second approach the self-diffusion coefficient is obtained as the product of the thermodynamic driving force and the mobility. The advantages and limitations of the two methods are discussed.
Inner-sphere reorganization in optical electron transfer
Authors: Delahay P. , Dziedzic A.
Published: February 2003

Free energies for photoelectron emission by aqueous solutions of hexaquo cations (V2+, Cr2+, Fe2+), metal complexes [Fe(CN)4−6, Co(NH3)2+6], and inorganic anions (OH−, Cl−, Br−, I−) are calculated from theory and compared with experimental threshold energies. Good agreement is obtained within the estimated error (±0.2 eV) on emission free energies. The free energy for outer-sphere reorganization is calculated from a continuous medium model. The inner-sphere reorganization energy is obtained from a bond-stretching model for hexaquo cations and metal complexes. A new method is developed for the calculation of the inner-sphere reorganization energies of univalent anions from their free energies of hydration. Reorganization free energies for electron transfer reactions [V2+/3+, Cr2+/3+, Mn2+/3, Fe2+/3+, Fe(CN)4−/3−6 ] calculated from experimental threshold energies and computed outer-sphere reorganization free energies yield activation free energies in agreement with the values obtained from kinetic measurements. Improvements are discussed for the determination of threshold energies by extrapolation.
Thermotropic ionic liquid crystals. IV. Structural effects in sodium methylpentanoates
Authors: Plesko S. , Phillips M. L. , Cassell R. , Jonas J.
Published: February 2003

Thermotropic ionic liquid crystals sodium 4-methylpentanoate and sodium 3-methylpentanoate have been studied by polarization microscopy and NMR. The quadrupole coupling constant e2qQ/h of 23Na is obtained for the mesophase and the 23Na spin-lattice relaxation time T1 is reported as a function of temperature in both the mesophase and the isotropic melt. A model is presented for the arrangement of the polar groups in the mesophase of all short chain Na carboxylates—the smectic A bilayer consists of R–COO−Na+ ion pairs with interdigitated polar groups. The observed dependence of the defect size and e2qQ/h of 23Na upon the molecular structure is interpreted in terms of this model. A qualitative discussion of possible relaxation mechanisms contributing to the T1 of 23Na is presented.
NMR study of molecular motions in cyclohexanol, a glass-forming rotor crystal
Authors: Kuhns Philip L. , Conradi Mark S.
Published: February 2003

The motions in the rotor and glassy crystal phases of solid cyclohexanol are studied with proton NMR from the melt down to 5 K. Particular attention is paid to the variation of the linewidth with temperature and to the temperature and frequency dependences of T1. We find there are two distinct motions that cause minima in T1 as a function of temperature. These two motions are also responsible for the two dielectric loss peaks observed previously. From the proton line narrowing and C13 high-resolution solid state spectra the high temperature α motion is identified as overall molecular rotation. The low temperature β motion is identified as a uniaxial internal rotation of the cyclohexyl ring about the CO bond, with the COH group remaining stationary. This explains both the strong spin relaxation and the weak dielectric relaxation associated with the β motion. Both motions have distributions of correlation times, as seen from the shallow T1 minima and the weak temperature and frequency dependences of T1. From 100 K down to 5 K the temperature dependence continues to be weak and the frequency dependence remains less than ω20. These surprising results indicate that some components of the motion remain faster than NMR frequencies even at 5 K. The behavior of cyclohexanol is compared to that of other disordered solids.
Collisional energy transfer in the low-pressure-limit unimolecular dissociation of HO2
Authors: Brown Nancy J. , Miller James A.
Published: February 2003

Classical trajectory calculations are used to investigate the energy transfer properties of HO2–He collisions under conditions where HO2 is initially excited to energies near dissociation. The emphasis in this investigation is on determining the dependence of vibrational energy transfer characteristics on heat bath temperature, total molecular energy, and total molecular angular momentum. Vibrational energy transfer is a function of all three variables. Energy transfer averages, correlation coefficients, and energy transfer cross sections are used to determine the energy transfer mechanism. Evidence is found for all types of energy transfer, but the specific mechanism for a particular ensemble is highly dependent on the initial variables. At fixed vibrational energy and heat bath temperature, the magnitude of the average vibrational energy transferred per collision ‖〈ΔE′〉‖ increases with increasing molecular angular momentum. At fixed initial molecular angular momentum and heat bath temperature, −〈ΔE′〉 increases as the total energy in the molecule increases, except for very low values of initial angular momentum. Increasing the heat bath temperature for fixed values of the other initial variables decreases the magnitude of 〈ΔE′〉. The relative importance of weak and strong collisions in governing the energy transfer characteristics is discussed. In particular, the probability density function for vibrational energy transfer is not well represented by a simple ‘‘exponential down’’ model. A double exponential function is required to represent the long tail of the distribution adequately. Total vibrational energy transer cross sections determined from vibrational energy transfer histograms are weak functions of total energy, total angular momentum, and heat bath temperature. They are systematically higher than the corresponding Lennard-Jones cross sections.
One variable map prediction of Belousov–Zhabotinskii mixed mode oscillations
Authors: Rinzel John , Schwartz Ira B.
Published: February 2003

The structure of the nonlinear dynamics of the Belousov–Zhabotinskii reaction in a continuously stirred tank reactor is studied via a new one-dimensional discrete dynamical system (map). The map, obtained from an Oregonator differential equation model, is defined on all domains of interest and for a wide range of flow rates. It predicts the qualitative behavior of complex oscillation patterns observed for the differential equations as a function of flow rate. These patterns correspond to the multiple peak oscillations observed experimentally by Hudson et al. The derived map exhibits the tangent bifurcation structure associated with intermittency. However, for this model, that intermittency does not appear to be chaotic over a detectable parameter range.
Collisional redistribution and fine-structure transitions in Na–Ar optical collisions
Authors: Kulander Kenneth C. , Rebentrost Frank
Published: February 2003

Cross sections for optical collisions between Na and Ar are calculated for a range of detunings and a single collision energy. Results are obtained from a fully quantum mechanical coupled channels calculation including the ground and three excited molecular states. The effects of fine-structure transitions between the excited states during dissociation of the molecule following absorption determine the relative intensities of the two Na fluorescence lines. The calculated intensity ratios are compared to measured results. A quasiclassical model also is presented and is shown to produce total absorption line shapes in excellent agreement with the quantum mechanical results.
Electronic properties of polypyrrole: An a
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o Hartree–Fock study
Authors: André J. M. , Vercauteren D. P. , Street G. B. , Brédas J. L.
Published: February 2003

We present band structure calculations on neutral polypyrrole. The calculations are performed at the Hartree–Fock ab initio level with the use of a program specifically adapted for polymer applications. The density of states computed from the band structure compares very well with the UPS data on neutral polypyrrole.
Geometric approach to the semiclassical bound-state energies of quantum-mechanical models
Authors: Fernández F. M. , Arteca Gustavo A. , Castro E. A.
Published: February 2003

A simple geometrical analysis reveals a close relation between variational methods and the WKB approximation. The procedure leads to quite accurate, analytical expressions for the discrete spectrum of quantum-mechanical models. Numerical results for the anharmonic oscillator are shown.
On the nature of the ligand core hole states of transition metal carbonyls
Authors: Ford Peter C. , Hillier Ian H.
Published: February 2003

Configuration interaction calculations of the O1s and C1s core hole states of Cr(CO)6 and C3O2 are described. The increased shielding of the primary core hole states of these molecules compared to those of CO leads to the shift in core ionization energy being determined by final state effects. The intense satellite band in the O1s and C1s spectrum of Cr(CO)6 arises from M(3d)→L(π*) charge transfer transitions involving all six CO ligands.
Nonrelativistic Compton scattering in Furry’s picture: Beyond the sudden impulse approximation by means of the complex coordinate method
Authors: Froelich P. , Weyrich Wolf
Published: February 2003

Basic electrodynamical and collision theory presumptions related to Compton scattering are critically reconsidered (with particular emphasis on identifying the proper transition matrix element effectively controlling the process, and on framing it within time dependent perturbation theory and scattering theory) in order to indicate the main features and the validity of the proposed method. The on-energy-shell transition matrix element governing the cross-sectional formula is obtained from Furry’s two-potential formalism, which makes possible a treatment of the primary electron–photon interaction to first order while incorporating the effects of the final state Coulombic interaction to all orders. Furry’s procedure, in addition to defining clearly which initial and final states should enter the transition matrix element, also brings additional insight into the nature of the so-called ‘‘sudden impulse approximation.’’ A treatment is proposed in which the decisive transition matrix element is obtained without explicit calculation of the distorted continuum solutions implied by Furry’s procedure, but is instead extracted from the dispersion relations of the ‘‘beat-frequency’’ dependent generalized polarizability by means of the L2 treatment based on the complex-coordinate method. The practical advantage of the method will be its ability to incorporate correlation between the ejected electron and the electrons remaining in the ion.
Systematic GVB study of harmonic vibrational frequencies and dipole moment derivatives: The vinyl radical C2H3 and other simple molecules
Authors: Dupuis Michel , Wendoloski J. J.
Published: February 2003

Structure, harmonic vibrational frequencies, and dipole moment derivatives of methane, ethylene, and acetylene have been obtained from generalized valence bond (GVB) wave function calculations. The results are compared to Hartree–Fock (HF) data, and other correlated wave function data. It is found that the GVB method consistently overemphasizes left–right electron correlation effects, and predicts bond lengths ∼0.01–0.02 Å longer than experiment. However, the calculated harmonic frequencies are within 4.2% of the observed fundamentals and within 3.5% of the experimental harmonic frequencies. Dipole moment derivatives are in semi-quantitative agreement with experiment. The method is used to predict the IR spectrum of the vinyl radical, including a very intense out of plane bending mode with frequency near 1000 cm−1.
A Monte Carlo method for quantum Boltzmann statistical mechanics using Fourier representations of path integrals
Authors: Freeman David L. , Doll Jimmie D.
Published: February 2003

By expanding Feynman path integrals in a Fourier series a practical Monte Carlo method is developed to calculate the thermodynamic properties of interacting systems obeying quantum Boltzmann statistical mechanics. Working expressions are developed to calculate internal energies, heat capacities, and quantum corrections to free energies. The method is applied to the harmonic oscillator, a double-well potential, and clusters of Lennard-Jones atoms parametrized to mimic the behavior of argon. The expansion of the path integrals in a Fourier series is found to be rapidly convergent and the computational effort for quantum calculations is found to be within an order of magnitude of the corresponding classical calculations. Unlike other related methods no special techniques are required to handle systems with strong short-range repulsive forces.
Isotropy group description of a phase transition in NaNO3
Authors: Stornetta W. Scott , Hatch Dorian M.
Published: February 2003

Predictions of the possible post-transition symmetries of an R3̄c zone center continuous phase transition are made using recent extensions of the Landau theory. The Landau theory is summarized and direct group theoretical methods are applied to the R3̄c phase. General techniques for solving systems of nonlinear polynomial equations are reviewed and these techniques are applied to the minimization of the free energy F for the aforementioned possible symmetries. The results of the direct methods and the minimization procedures are shown to be compatible. It is shown that the transition to R3c in NaNO3 is consistent with the above results as well as with the observed atomic displacements.
Resonance Raman scattering study of azulene. I. Experiment and theoretical analysis via transform techniques
Authors: Brafman O. , Chan C. K. , Khodadoost B. , Page J. B. , Walker C. T.
Published: February 2003

Room temperature resonance Raman (RR) profiles for the 1400, 1260, 900 and 2×825 cm−1 lines of azulene in CS2 and for the 825 cm−1 line in methanol are reported. These profiles display pronounced multimode interference effects within the measured spectral range (570–720 nm). Using previously developed transform techniques, we analyze our RR data by calculating profile line shapes directly from our measured optical absorption data. The calculated and measured profile line shapes are in good agreement for all of the modes studied. These results are consistent with the model assumptions upon which the transform analysis rests. To further test our model assumptions, we extract ratios of the RR Stokes loss parameters from the scale factors of our profile line shape fits. RR Stokes loss parameters estimated from these ratios are found to be in good correlation with published 2K emission intensity ratios. However, their correlation with published 4K absorption intensity ratios indicates the presence of some intensity borrowing and hence implies deviations from our model assumptions. Nevertheless, the good line shape agreement for all of the modes studied, together with the lack of strong intensity borrowing, indicate that large deviations from our model assumptions are unlikely. The present study serves as a useful prelude for simultaneous modeling of the optical absorption spectra, the RR profile line shapes, and the RR relative intensities. Such model studies will be reported in a future publication.
Sub-Doppler resolution infrared spectroscopy of the acetylene dimer: A direct measurement of the predissociation lifetime
Authors: Miller R. E. , Vohralik P. F. , Watts R. O.
Published: February 2003

Rotationally resolved, infrared predissociation spectra have been obtained for (C2H2)2 using a molecular beam apparatus in conjunction with a single mode F-center laser operating near 3300 cm−1. Several well separated bands are seen in the low resolution spectra, possibly indicating that more than one stable dimer structure exists. The high resolution spectra indicate that the widths of the individual rovibrational transitions, and hence predissociation lifetimes, depend on the band excited. The lifetimes obtained from the widths vary from 80×10−9 to 1.6×10−9 s.
The electron spectrum of UF6 recorded in the gas phase
Authors: Mårtensson N. , Malmquist P.-Å. , Svensson S. , Johansson B.
Published: February 2003

Gas phase core and valence electron spectra from UF6, excited by AlKα monochromatized x rays, in the binding energy range 0–1000 eV are presented. It is shown that the AlKα excited valence electron spectrum can be used to reassign the highest occupied molecular orbital (HOMO) in UF6. Many-body effects on the core levels are discussed and core level lifetimes are determined. The shift between solid phase and gas phase electron binding energies for core lines is used to discuss the U5 f population in UF6.
On the efficiency and accuracy of quantum and classical methods of calculating diatomic spectra
Authors: Tellinghuisen Joel
Published: February 2003

Several methods of calculating low-resolution diatomic spectra under thermal equilibrium conditions are examined with respect to accuracy and computational efficiency. When the vibrational interval in the initial state is ∼kT/2 or less, the quasistatic classical method is reliable to better than 1% of peak intensity. In quantum computations, discrete transitions can often be modeled as bound–free for convenience. Some CPU time bench marks are given for the various calculations.
Molecular dynamic simulation of quadrupole relaxation of atomic ions in aqueous solution
Authors: Engström Sven , Jönsson Bo , Impey Roger W.
Published: February 2003

Molecular dynamics (MD) simulations were performed in order to investigate the molecular origin of the nuclear quadrupole relaxation mechanism for Li+, Na+, and Cl− ions in dilute aqueous solution. Different boundary conditions were investigated in the simulations, but neither the boundary conditions nor the system size have any significant effect on the different time correlation functions (tcf:s) calculated. It is found that the field gradient tcf, determining the NMR relaxation rate for these ions, is almost completely due to the water molecules in the first shell. The calculated field gradient tcf:s show a complex time-dependence not describable with a mono-exponential decay. The very rapid decay shown by the field gradient tcf for all three ions can partly be attributed to a correlated water motion in the first hydration shell. The relaxation rates obtained from the simulations are in excellent agreement with experimental data for Na+ and Cl−, while the Li+ relaxation rate is less well produced.
Line broadening in rigid and nonrigid clusters and molecular electronic spectra. The spectral density formalism
Authors: Islampour Reza , Mukamel Shaul
Published: February 2003

The spectral density formalism is used to solve for intramolecular line broadening in simple model systems corresponding to rigid (solid-like) and nonrigid (droplet-like) clusters of varying sizes. Both quantum mechanical and classical techniques are used to calculate the spectral density J(ω), which subsequently yields the absorption spectrum. Our calculations demonstrate how the averaging of molecular information takes place in the broadened line shapes. This occurs in two steps: The molecular eigenstates are grouped to form the spectral density J(ω), and the line shape in turn averages the information contained in J(ω). The semiclassical approximation for J(ω) is excellent for rigid clusters of any size. For nonrigid clusters it holds only when the cluster size is large enough. The present approach may be used also towards the calculation of molecular electronic spectra and overtone line shapes.