bluuebunny/crossref_metadata_2025 huggingface dataset:
Low energy vibrational relaxation of the diatomic molecule I2 (B) by collision with He
Authors: Villarreal P. , Delgado-Barrio G. , Mareca P.
Published: February 2003

A quantum mechanical study of the collision I2(B 3Π0 , n = 1, j = 0)+He→I2(B 3Π0 , n = 0, j′)+He is reported here. The initial relative kinetic energies are lower than 0.7 cm−1. The I2(B) diatom is approximated by a harmonic oscillator, while the atom–diatom interaction is described by two atom–atom Morse type potentials. The close-coupling equations for nuclear motion, within the body-fixed frame, have been solved numerically. The partial vibrational relaxation cross sections, corresponding to the J = 0,1,2 values of the total angular momentum, have been obtained as a function of energy. When several rotational channels in the n = 1 vibrational level are introduced in the calculation, resonances do appear with lifetimes of the order of several ps. These resonances enhance the efficiency of the collision. Their positions, at very low energies, cause the dependence of the mean cross section with increasing temperature to be a fast decreasing function, in agreement with experimental results.
Decay of the first triplet Ne2 excimer in the presence of O2
Authors: Manzanares E. R. , Firestone R. F.
Published: February 2003

Measurements of the rate of decay of the first triplet state of the Ne2 molecule in Ne/O2 mixtures have been performed by fast spectrophotometric monitoring of this species at 814 nm. The rate constant for electronic quenching of vibrationally relaxed Ne2 (3Σu+) by O2 has been found to be equal to 3.67 (±0.14)10−10 cm3 molecule−1 s−1 at 298 K. A new upper limit to the value of the spontaneous radiative decay constant is equal to 1.28(±0.02) 105 s−1. Dependence of the excimer yield on O2 concentraion appears to indicate that electronic three-body quenching of the atomic precursor(s) predominates markedly over vibrational relaxation of Ne2(3Σu+, v). The excimer yield is, accordingly, extremely sensitive to the presence of impurities at high neon pressures.
Fluorescence yields from photodissociation of NO2 at 1060–1620 Å
Authors: Lee L. C. , Chiang C. C.
Published: February 2003

The photoabsorption and fluorescence cross sections of NO2 were measured using synchrotron radiation in the 1060–1620 Å region. The fluorescence starts to appear at 1620 Å, and its cross section increases towards shorter excitation wavelengths. The fluorescence excitation spectra show vibrational structure of Rydberg states similar to the absorption spectrum. The quantum yields for the production of fluorescence in the 1900–8000 Å region increase with decreasing photon wavelengths, and reach a plateau of ∼10% in the 1190–1240 Å region. At photon wavelengths shorter than 1190 Å, the quantum yields increase to about 18% and show structure. Correlations of the absorption and fluorescence spectra with electronic states, as well as the photodissociation processes, are discussed.
Light scattering from dilute solutions of flexible molecules: A molecular theory of the solute–solvent interactions
Authors: Ladanyi Branka M.
Published: February 2003

A molecular theory of anisotropic light scattering from dilute solutions of flexible molecules is presented. The polarizability of a given flexible molecule is evaluated using the interacting atom model. The local field around a flexible molecule is represented in terms of dipole–induced dipole interactions between solvent molecules and atoms or sites on the flexible molecule. The mean squared effective anisotropic polarizabilities of flexible molecules are given as perturbation expansions in these interactions. Two approximations to these polarizabilities are proposed. In the first, the external ideal dipole model, the local field is evaluated using a single site on the flexible molecule; in the second, the dielectric cavity theory, the solvent density around the flexible molecule is assumed to be uniform. These approximations are applied to anisotropic light scattering from n-butane in solution. A dramatic decrease in the effective mean squared anisotropic polarizability of n-butane is found as the solvent optical dielectric constant is increased. It is shown that the same effect is produced by simply decreasing the atomic polarizabilities of an isolated n-butane molecule. The implications of the n-butane results to light scattering from dilute solutions of other flexible molecules, as well as other applications of the present theory are discussed.
Triply differential photoelectron studies of non-Franck–Condon behavior in the photoionization of acetylene
Authors: Parr A. C. , Ederer D. L. , West J. B. , Holland D. M. P. , Dehmer J. L.
Published: February 2003

Vibrational branching ratios and photoelectron angular distributions for alternative vibrational levels of C2H2+X 2Πu have been measured in the range 13 eV⩽hν⩽25 eV using synchrotron radiation. Below hν∼16 eV, these data exhibit strong non-Franck–Condon effects, namely, wavelength-dependent vibrational branching ratios and vibrational-state-dependent photoelectron asymmetry parameters. Moreover, enhanced excitation of bending modes of the ion is observed below hν∼16 eV, in addition to the C–C stretch mode, which is the only mode readily observed in photoelectron spectra of C2H2 at shorter wavelengths, e.g., at the He I (21.2 eV) resonance line. The non-Franck–Condon behavior is attributed to resonant photoionization processes, whose identification is discussed in the framework of several recent theoretical and experimental studies on acetylene and related molecules.
Depolarized Rayleigh spectroscopy of small alkanes with picosecond relaxation times
Authors: Patterson G. D. , Carroll P. J.
Published: February 2003

Depolarized Rayleigh scattering is studied as a function of temperature in a group of small alkanes under conditions where the observed relaxation time is in the picosecond time range. The molecules studied include the n-alkanes from pentane to nonane, 2-methyl butane, 2,3-dimethyl butane, 2,4-dimethyl pentane, 2,2,4,4-tetramethyl pentane, and 3,3-diethyl pentane. In the n-alkanes, 2-methyl butane, and 2,4-dimethyl pentane, the spectrum is dominated by the molecular optical anisotropy and the second rank reorientational relaxation time is determined. In the other systems, the depolarized Rayleigh spectrum is determined by the relaxation of intermolecular optical anisotropy due to dipole-induced dipole interactions between pairs of density fluctuations. The observed linewidth can be accurately predicted using the theory of Madden [Mol. Phys. 36, 365 (1978)]. The present results are intended to serve as an accurate data base for the rapidly improving computer calculations of the dynamics of small alkanes.
Magnetic field effects on the luminescence of quasi-one-dimensional crystals Mx[Pt(CN)4]⋅yH2O
Authors: Hidvegi I. , von Ammon W. , Gliemann G.
Published: February 2003

An exceptionally large influence of magnetic fields on the E⊥c-polarized emission of Mx[Pt(CN)4]⋅y H2O (M = Mg2+, K+, Li+, Ba2+, Ca2+, Na+) single crystals is reported. When the magnetic field is raised from 0 to 6 T, the emission is blueshifted by 270 cm−1, the intensity increases by a factor 13 and the lifetime is shortened by 3 orders of magnitude. The results are explained within the D′4h-symmetry of a single [Pt(CN)4]2− complex, assuming that the lowest emitting state is an A′1u.
Quasiclassical trajectory calculation of the state-specified differential cross sections and opacity functions for F+H2 → HF(v′)+H
Authors: Blais Normand C. , Truhlar Donald G.
Published: February 2003

We report well-converged quasiclassical trajectory calculations of the state-specified differential cross sections dσ/dΩ(v′) and opacity functions P(b, v′) for F+H2 → FH(v′)+H, where the final vibrational quantum numbers v′ are assigned by the histogram method and P(b, v′) is the probability of reaction into a specified v′ level at initial impact parameter b. The differential cross sections are calculated by the Legendre moment method using stratified sampling. At a relative translational energy of 0.233 eV, all dσ/dΩ(v′) are backwards peaked, and P(b,v′ = 2) and P(b,v′ = 3) both peak at b = 0. This provides confirmatory evidence that the sideways scattering and high-b maximum in P(b, v′) that have been observed in previous calculations by Redmon and Wyatt and by Jellinek, Baer, and Kouri are indeed quantal effects.
On a j
z-preserving propensity in molecular collisions. II. Close-coupling study of state-to-state differential cross sections
Authors: Khare V. , Kouri D. J. , Hoffman D. K.
Published: February 2003

In this paper, we present and analyze accurate close-coupling results for polarization transition differential cross sections for Δj≠0 atom–diatom collisions. The systems considered are He+CO and He+HCl. Cross sections for polarization transitions defined with several choices of quantization axes are presented and discussed. We find that (a) the jz-preserving cross sections are heavily favored when a generalized apse quantization scheme is used; (b) the other quantization schemes can be factorized for transitions involving lower rotor j states in terms of the dominant jz-preserving apse quantized amplitudes; (c) this factorization implies limits on the amount of information contained in polarization measurements based on nonapse quantization axes; (d) the factorization (and, hence, its limitations) does not apply to the highest accessible rotor j states; and (e) the propensity for preserving jz when quantized along a generalized apse has important implications for the well-known CS decoupling approximation and for other possible decoupling schemes.
Group theory of coupled oscillators: Normal modes as symmetry breaking
Authors: Kellman Michael E.
Published: February 2003

The unitary symmetry U(N) of the N-dimensional harmonic oscillator is used in a re-examination of the couping of a pair of single-particle local oscillators to give normal modes. The coupling p1p2/M is expressed in terms of SU(2) operators. It is shown that in degenerate perturbation theory, diagonalization of the Hamiltonian leads to exact ’’pseudospin’’ quantum numbers S and Sx to label normal modes and describe selection rules. The pseudospin quantum numbers are therefore exact constants of the motion for the coupled system at this level of approximation. It is found that the pseudospin labels are still rather good beyond the zero order approximation, even for large values of the coupling. They thus give new approximate constants of the motion for the true normal modes solutions of the problem. Finally, a class of Hamiltonians is investigated for which SU(2) symmetry labels are exact quantum numbers for all values of the coupling strength.
A convergent perturbative solution to the attractive corrugated hard wall model of atom–surface scattering
Authors: Wolfe-Brannon Karen , Weare John H.
Published: February 2003

The attractive corrugated wall model of atom–surface scattering is solved by an expansion method. The approach is similar to a Born expansion; however, projection operators must be introduced in order to obtain a convergent series near resonances. Diffraction intensities are calculated for He–LiF(001) and He–NaF(001) scattering. Good agreement with experimental intensities is obtained when the series is taken to convergence. In some cases (He–NaF) this requires calculation to as high as 12th order in the expansion parameter. At this order as many as 169 channels are included in the calculation. Preliminary estimates for the values for v01 and v11 for the He–NaF system have been found by comparison of theoretical and experimental intensities.
Effective viscosity of dilute polymer solutions near confining boundaries
Authors: Aubert James H. , Tirrell Matthew
Published: February 2003

The problem of a bead–spring model macromolecule in the presence of impenetrable interfaces is studied. Equilibrium partition coefficients are calculated for various pore geometries and are compared to data on partitioning in porous chromatographic columns. Impenetrable, purely repulsive interfaces strongly affect viscosity, resulting in apparent slip at the interfaces. The bead–spring contribution to the stress in the presence of walls is calculated at low shear rates and compared to recent data on dilute Xanthan polysaccharide solutions. The analysis compares well with data on solutions of low polymer concentration at low shear rates.
Selective vibrational excitation of formaldehyde X̃ 1A1 by stimulated emission pumping
Authors: Reisner D. E. , Vaccaro P. H. , Kittrell C. , Field R. W. , Kinsey J. L. , Dai H.-L.
Published: February 2003

Stimulated emission pumping (SEP), an optical–optical double resonance scheme whereby highly excited rotation-vibration levels of the electronic ground state may be selecetively populated, is applied to formaldehyde. This method is free of important limitations of other schemes such as IR multiphoton, high overtone, and stimuated Raman pumping. We report fully resolved, assigned, and sub-Doppler (laser linewidth limited to 0.04 cm−1) SEP spectra in the H2CO Ã 1A′′–X̃ 1A1 441 and 230401 bands. More than 0.02% of the total thermal population in the irradiation volume was excited into a single rotation-vibration level at least 4600 cm−1 above the vibrationless level in the electronic ground state.
On the attachment of hydrogen in HxMoO3
Authors: Dickens P. G. , Jarman R. H. , Slade R. C. T. , Wright C. J.
Published: February 2003

The attachment of H in H1.6MoO3 via–OH2 groups, which has been deduced from neutron scattering and NMR linewidth data, is shown to be compatible with the observed thermodynamic and electronic properties of the compound.(AIP)
Multiphoton ionization of ammonia: Mass analysis and photoelectron spectra
Authors: Glownia James H. , Riley Stephen J. , Colson Steven D. , Miller John C. , Compton Robert N.
Published: February 2003

Mass and photoelectron spectra following the resonantly enhanced four-photon ionization of ammonia have been recorded. The mass spectral data show that extensive fragmentation occurs at a distinct wavelength threshold. This threshold energetically corresponds to the minimum number of photons required to reach the first excited state of the ion. The branching ratio for the various fragments (H+, H+2, N+, NH+, NH+2, NH+3) is independent of the neutral resonantly excited vibronic state, and the ratios differ from those recorded by conventional ionization techniques. Photoelectron energy measurements show that low (∼0 eV) kinetic energy electrons are produced following ionization of all but one of the resonantly excited Rydberg states investigated. The origin of these zero energy electrons is thought to be due to vibrational autoionization. Internal conversion may give rise to the neutral vibronic levels responsible for the autoionization. For one excited state, higher electron energies are recorded corresponding to direct ionization. This mechanism leaves the positive ion in a single vibrational quantum state which is the same vibrational state as that of the initially excited neutral-ammonia with cross sections governed by the Franck–Condon principle. The results suggest that the fragmentation occurs entirely within the ionic manifold.
Application of an extended RISM equation to dipolar and quadrupolar fluids
Authors: Hirata Fumio , Pettitt B. Montgomery , Rossky Peter J.
Published: February 2003

A generalization of the RISM integral equation for site–site pair correlation functions previously proposed by us is discussed and applied to model liquids composed of strongly polar diatomic molecules. The nonuniform molecular charge distribution is represented by the introduction of charged interaction sites. The generalization consists of applying closure conditions analogous to those which are known to be reasonable for the description of atomic ionic fluids, and the corresponding renormalization of the contributions arising from long range forces. We discuss both the symmetry properties of the pair correlation functions in special cases and the dielectric properties implied by theory. Applications are presented for three two-site models which differ substantially in the degree of asymmetry of the non-Coulombic potential between the two sites, and for three three-site models for Br2. The two sites models are compared to computer simulation results, and those for Br2 to experimental results. The analysis shows that the integral equation is well balanced in that in every case the qualitative features of the liquids structure which are introduced by polarity are well represented, even in cases where the site–site potentials are individually much larger than kBT. In cases where the molecular shape and polar forces are in competition, the results are of comparable accuracy to the corresponding theory for nonpolar systems. In the extreme case where changes in orientational structure can occur without interfering with packing requirements, the results appear quantitatively less reliable.
Schrödinger fluid dynamics of many-electron systems in a time-dependent density-functional framework
Authors: Deb B. M. , Ghosh S. K.
Published: February 2003

For an N-electron system, a connection is explored between density-functional theory and quantum fluid dynamics, through a dynamical extension of the former. First, we prove the Hohenberg–Kohn theorem for a time-dependent harmonic perturbation under conditions which guarantee the existence of the corresponding steady (or quasiperiodic) states of the system. The corresponding one-particle time-dependent Schrödinger equation is then variationally derived starting from a fluid-dynamical Lagrangian density. The subsequent fluid-dynamical interpretation preserves the ’’particle’’ description of the system in the sense that the N-electron fluid has N components each of which is an independent-particle Schrödinger fluid characterized by a density function ρj and an irrotational velocity field uj, j = 1,⋅⋅⋅,N. However, the mean velocity u of the fluid is not irrotational, in general. The force densities and the stress tensor occurring in the Navier–Stokes equation are physically interpreted. The present work is another step towards the interpretation of physicochemical phenomena in three-dimensional space.
A new look at correlation energy in atomic and molecular systems. II. The application of the Green’s function Monte Carlo method to LiH
Authors: Moskowitz Jules W. , Schmidt K. E. , Lee Michael A. , Kalos M. H.
Published: February 2003

The potential energy surface of the LiH molecule is calculated using the Green’s function Monte Carlo method. The calculated correlation energy is 0.078±0.001 hartree and the binding energy is 2.56 eV. These results are within 6% and 2% of the experimental values, respectively. The Green’s function Monte Carlo method is discussed in some detail with particular emphasis on problems of chemical interest.
The solution of driven equations by R-matrix propagation methods
Authors: Schneider B. I. , Taylor H. S.
Published: February 2003

A method for the solution of the inhomogeneous (driven) Schrödinger equation is presented which is a generalization of the R-matrix propagation technique developed by Light and Walker. The method is numerically stable and allows the direct calculation of transition matrix elements for photodissociation (or photoionization) from the asymptotic form of the wave function. Thus, no additional integrals need to be computed. The method makes direct contact with the work of Freed et al. and Heller and demonstrates that straightforward modifications of standard collision theory can be used for the half-collision problem.
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o studies of methyllithium clusters
Authors: Herzig L. , Howell J. M. , Sapse A. M. , Singman E. , Snyder G.
Published: February 2003

A b initio self-consistent field calculations with STO-3, 4-31, and 6-31G basis sets are performed on methyllithium monomer dimer, trimer, and pentamer. The calculations provide data on binding energies and optimum geometries of various structures as well as indicate the formation of a lithium bond (between tetramers) similar to hydrogen bonds. Several tetrameric structures were investigated including one of D2d symmetry (resembling the ethyllithium tetramer) which is of possible importance in lithium exchange reactions.
Binary collision dynamics and numerical evaluation of dilute gas transport properties for potentials with multiple extrema
Authors: Rainwater James C. , Holland Paul M. , Biolsi Louis
Published: February 2003

Prediction of gaseous transport properties requires calculation of Chapman–Enskog collision integrals which depend on all possible binary collision trajectories. The interparticle potential is required as input, and for a variety of applications involving monatomic gases the Hulburt–Hirschfelder potential is useful since it is determined entirely from spectroscopic information and can accomodate the long-range maxima and minima found in many systems. Hulburt–Hirschfelder potentials are classified into five distinct types according to their qualitative binary collision dynamics, which in general can be quite complex and can exhibit ’’double orbiting’’, i.e., a pair of orbiting impact parameters for a single energy of collision. The collision integral program of O’Hara and Smith has been revised extensively to accomodate all physical cases of the Hulburt—Hirschfelder potential, and the required numerical methods are described and justified. The revised program substantially extends the range of potentials for which collision integrals can be calculated.
Phase equilibria in polydisperse fluids
Authors: Gualtieri J. A. , Kincaid J. M. , Morrison G.
Published: February 2003

We present a new approach for solving phase equilibria problems in multicomponent systems together with several applications. A mathematical framework is developed that provides a method for generalizing the thermodynamics of a finite-component system to that of a system with an infinite number of components—a polydisperse system. Two new functions, the mole fraction distribution function and the mole fraction density function, play a key role in our method. The phase equilibria conditions are written in terms of these functions and are formally solved. We illustrate the utility of our approach by solving, for a polydisperse generalization of the van der Waals model, three phase-equilibria problems: (1) the fractionation of a polydisperse impurity dissolved in a solvent; (2) the shift of the critical temperature and density due to the presence of a polydisperse impurity; (3) the calculation of the cloud-point surface and critical point of a completely polydisperse system.
Raman spectra of ices II and IX above 35 K at atmospheric pressure: Translational and rotational vibrations
Authors: Bertie John E. , Francis Bernard F.
Published: February 2003

The Raman scattering by the translational and rotational vibrations of normal and deuterated ice II and ice IX at atmospheric pressure and 35–100 K is reported. The infrared absorption by deuterated ice II and ice IX at atmospheric pressure and 4.3 K is also reported. Imperfect, but useful, polarized Raman spectra of single crystals of ice II have indicated the symmetries of the vibrations. The translational vibrations have been reproduced remarkably well by normal coordinate and intensity calculations based on very simple force fields and models of the dipole moment and polarizability changes. No features could be assigned to longitudinal optic modes. Attempts to calculate the rotational and translational vibrations from the one force field were made for five force fields. No force field reproduced the observed separation of translational and rotational displacements in the normal vibrations for the actual structure. This experimental fact, that the vibrations are translational or rotational but not mixed, will provide a severe test for potential functions for the interaction between water molecules. A correction is given to the sample temperature cited in a previous publication about the OH and OD stretching vibrations of these ices [J. E. Bertie and B. F. Francis, J. Chem. Phys. 72, 2213 (1980)].
The chemiluminescent reactions Ba+N2O and Ba+O3 in solid argon
Authors: Long S. Randolph , Lee Yuan-Pern , Krogh Ole D. , Pimentel George C.
Published: February 2003

Visible chemiluminescence in the spectral region 5500–7300 Å due to the Ba+N2O and Ba+O3 reactions is observed in argon matrices at temperatures below 15 K. Emission is recorded both as thermoluminescence and during deposition. Two molecular progressions of BaO, with ν00=17 035 and 17 195 cm−1, and ω″e=657 and 651 cm−1, are found and are tentatively attributed to emission in the ã 3Σ+→X̃ 1Σ+ transition for BaO* in different matrix sites, though the Ã 1Σ+→X̃ 1Σ+ transition could also be involved. The presumed excitation mechanism is the reactive population of one or both of the singlet states Ã Σ+1 and/or Ã′ 1Π followed by rapid intersystem crossing to the ã 3Σ+ state. This is the first observation of a chemiluminescent atom transfer reaction in low temperature matrices. Emission due to Ba atoms in the (6s6p1P) →(6s2 1S) transition is also observed, apparently excited by energy transfer from BaO*, most probably from the Ã′ 1Π state.
Energy partitioning in the VUV photodissociation of ClNO as a function of excitation energy
Authors: Lahmani F. , Lardeux C. , Solgadi D.
Published: February 2003

The fluorescence of NO Rydberg states resulting from the VUV photodissociation of ClNO with synchrotron radiation between 120 and 165 nm has been studied. The excitation spectra for the production of NO A 2Σ+ v′ = 0,1,2, C 2Π v′ = 0, D 2Σ+ v′ = 0 have been measured and show by comparison with the ClNO absorption spectrum a state-to-state correlation. The internal energy content (vibrational and rotational) of the NO A 2Σ+ fragment has been determined as a function of excitation energy. The rotational level distribution of NO(A) is not of Boltzmann type but follows a Gaussian-like distribution as shown from the detailed analysis of the fluorescence spectrum obtained at 147 nm. The excess energy dependence of the rotational and vibrational energy disposal deviates strongly from statistical predictions and can be explained on the basis of a simple quasidiatomic impulsive model. The time decays of NO(A) have been recorded and the rate coefficients for quenching by ClNO was found to be larger for NO(A) resulting from ClNO photolysis than for the thermalized species. This effect is attributed to the translational excitation of the separating NO fragment.
Theory of electron transfer reactions. I. An approximate Franck–Condon, exponential distorted wave scaling theory
Authors: Sears Stephen B. , DePristo Andrew E.
Published: February 2003

A simple quantum number scaling relationship is developed for electron transfer reactions. The theory, which is restricted to a single electronic state in the reactants and products, utilizes the Franck–Condon assumption in conjunction with the exponential distorted wave approximation for the dynamics. We show that the entire set of unknown state-to-state cross sections can be generated from one cross section plus two additional parameters. As an application of this result, we deconvolute the final state summed cross sections to recover the individual state-to-state values with fair to good accuracy.
Mean square torques from far infrared spectra
Authors: Evans Myron W.
Published: February 2003

The moment analysis of far infrared spectra is used to estimate mean square torques for some dipolar solutes in decalin solution. These are compared with some model dependent values of Reid and Evans.
Fluorescence polarization as a probe of molecular autoionization
Authors: Poliakoff E. D. , Dehmer J. L. , Parr A. C. , Leroi G. E.
Published: February 2003

It is shown that measurements of fluorescence polarization from excited-state molecular photoions directly probe the symmetry signatures and synamical properties of autoionization resonances. To demonstrate this relationship, the excitation wavelength dependence of CO2+A2Πu→X2πg fluorescence polarization was measured following 1Πu photoionization of CO2. The resulting fluorescence polarization spectrum showed extensive autoionization structure that is reproduced well by a simple model which averages resonant and nonresonant ionization contributions to obtain a resultant polarization spectrum. Implications regarding CO2 spectral assignments are discussed. Moreover, the measurement appears to be a generally useful complement to existing probes of molecular autoionization. (AIP)
Electronic structure of polypyrrole and oligomers of pyrrole
Authors: Ford W. K. , Duke C. B. , Salaneck W. R.
Published: February 2003

The ultraviolet photoemission and optical absorption spectra of pyrrole and polypyrrole are analyzed using the spectroscopically parametrized CNDO/S3 model. Calculated charge densities, bond orders, and orbital eigenvalues for pyrrole are compared with the extensive literature on this molecule. The densities of valence states are computed for pyrrole, oligomers of pyrrole, and an infinite planar polypyrrole chain. They provide a good description of ultraviolet valence electron photoemission spectra measured for gas-phase pyrrole, condensed thin films of pyrrole, and thin-film polypyrrole. The photogenerated radical cation states in polypyrrole seem to extend over four or more pyrrole units for the higher binding energy σ-electron states but are localized on fewer units for the lowest binding energy π-electron states. The relationship between the density of valence states and the energy band structure of polypyrrole is established. The energies of the first few dipole allowed optical absorption transitions are calculated for pyrrole and its oligomers via a limited configuration interaction analysis. Examination of the trends in these transition-state energies as a function of the size of the oligomer suggests that the lowest-energy optically excited states in polypyrrole are excitons which extend over at least four pyrrole units. Comparison of the model predictions for ring and linear-chain molecular conformations indicates that both are compatible with existing spectroscopic measurements.
A comparative study of the convergence characteristics of modified virtual orbitals within an orbitally ordered CI expansion
Authors: Cooper Ian L. , Pounder Christopher N. M.
Published: February 2003

A variety of unitary transformations on the canonical set of virtual orbitals yield sets of modified virtual orbitals which display qualitatively similar convergence characteristics within a limited CI expansion in which the orbitals are ordered via their interaction with the occupied SCF orbitals. The Hartree–Fock exchange operator, with neglect of those contributions arising from the occupied core orbitals, is found on energetic grounds to be the most effective operator for mixing the canonical virtual orbitals among themselves. In addition to providing considerable energy benefits, such modified virtual orbitals possess one-electron expectation values which are similar to those of natural orbitals, derived from an extended CI calculation.
The critical region in the Percus–Yevick approximation. A numerical study for a Lennard-Jones potential
Authors: Brey J. J. , Santos A. , Romero F.
Published: February 2003

The Percus–Yevick equation is numerically solved by using the compressibility equation of state, and its behavior near the critical point is analyzed. We have used a Lennard-Jones potential without truncation, assuming an asymptotic form for the direct correlation function. The results show that the equation of state in the critical region has the same structure as the one obtained from the van der Waals equation. Then, the classical values of the critical exponents are obtained. Furthermore, the critical isotherm, the coexistence curve, and the spinodal curve are symmetrical around the critical point. Thus, the nonclassical behavior of the Percus–Yevick equation seems to be restricted to a certain class of interaction potentials.
Adsorption of nitrogen molecules on graphite for 31<T<52 K: The fluid-to-commensurate-solid transition, multilayer condensation, and the pressure–temperature phase diagram
Authors: Diehl R. D. , Fain S. C.
Published: February 2003

LEED intensity isotherms for 45<T<52 K are used to characterize the fluid-to-commensurate-solid transition. The results are consistent with a first-order transition and the incipient triple point model. Isotherms near 32 K show clear second- and third-layer steps. The equilibrium vapor pressure vs inverse temperature phase diagram is constructed from data between 31 and 100 K. Some effects of heterogeneity on these phase transitions are discussed.
Monte Carlo simulations of liquid tetrahydrofuran including pseudorotation
Authors: Chandrasekhar Jayaraman , Jorgensen William L.
Published: February 2003

Monte Carlo statistical mechanics simulations have been carried out for liquid tetrahydrofuran (THF) with and without pseudorotation at 1 atm and 25 °C. The intermolecular potential functions consisted of Lennard-Jones and Coulomb terms in the TIPS format reported previously for ethers. Pseudorotation of the ring was described using the generalized coordinates defined by Cremer and Pople, viz., the puckering amplitude and the phase angle of the ring. The corresponding intramolecular potential function was derived from molecular mechanics (MM2) calculations. Compared to the gas phase, the rings tend to be more flat and the population of the C2 twist geometry is slightly higher in liquid THF. However, pseudorotation has negligible effect on the calculated intermolecular structure and thermodynamic properties. The computed density, heat of vaporization, and heat capacity are in good agreement with experiment. The results are also compared with those from previous simulations of acyclic ethers. The present study provides the foundation for investigations of the solvating ability of THF.
Study of rotational distribution of N2 in the Ar free jet using two-parameter power gap model
Authors: Koura Katsuhisa
Published: February 2003

The rotational distribution of N2 in the Ar free jet is studied using the rotational transition cross section of the two-parameter power gap model, which is reasonably fitted to the theoretical cross sections of the classical trajectories and quantum closed-coupling calculations for the N2–Ar intermolecular potential of Kistemaker and de Vries (KDV) or Pattengill, LaBudde, Bernstein, and Curtiss (PLBC). The rotational distribution yj, pair temperature T01, and collision number Zr along the jet axis are obtained by solving the master equation for the stagnation temperature T0=300 K and the high density levels P0d=250 and 1000 Torr mm, where the velocity distribution is taken as the local Maxwell distribution. The results are rather sensitive to the intermolecular potential: yj shows a non-Boltzmann distribution with overpopulation of the higher rotational levels and this degree of nonequilibrium for the PLBC potential is more pronounced than for KDV; T01 for PLBC is higher than for KDV; Zr shows little dependence on P0d; and Zr for PLBC (1.4≳Zr≳0.4) is larger than for KDV (0.9≳Zr≳0.2). It is also indicated that the results for the single-parameter power gap model used previously are quantitatively inadequate.
Temperature and size dependence of intermolecular energy transfer probability in a nonreacting system
Authors: Oref I.
Published: February 2003

The temperature and size dependence of collisional energy transfer probability in an equilibrium system is investigated. It is shown numerically and analytically, that for a classical system it is given by the expression P(ΔE, T)={2[π(s−1)]1/2RT}−1 exp[−ΔE2/4(s−1) (RT)2], where s is the number of degrees of freedom and ΔE is the energy transferred up and down during the collision. The average energy transferred in the up collision, 〈ΔEU〉 equals the down collision 〈ΔED〉 and is given by 〈ΔEU〉 =[(s−1)/π]1/2RT. It increases with the number of degrees of freedom and temperature.
Drastic spectroscopic perturbations due to dynamics of linearly inactive (sequence forming) and linearly active (progression forming) Jahn–Teller modes
Authors: Scharf Benjamin , Vitenberg Rachel , Katz Benjamin , Band Yehuda B.
Published: February 2003

We predict drastic changes of splittings in the v–v sequence of a linearly J–T inactive mode along a progression of a linearly J–T active mode. This phenomenon results from the mutual dynamics of the two modes. Preliminary experimental evidence is in accord with these predictions. Previously observed variations in unique splittings due to vibrational perturbations along J–T progressions resulting from asymmetric deuterations can also be explained using the theoretical analysis developed here.
Temperature dependence of the Raman active phonons in TCNQ crystals
Authors: Carlone Cosmo , Cyr Camil , Jandl Serge , Hota N. K. , Zauhar J.
Published: February 2003

The frequency dependence of 26 Raman active phonons in TCNQ crystals has been investigated as a function of temperature. Although all phonon frequencies decrease upon increasing the temperature, those at 152 and 160 cm−1 do the opposite. In general, the phonons in the middle frequency region are less sensitive to temperature than those at low and high frequency. We attribute this to mixing of the internal and external modes. By combining the pressure data with the temperature data, we have estimated the intrinsic and extrinsic contribution to the frequency changes. Volume dilatation effects are dominant for frequencies up to 400 cm−1, but phonon–phonon interactions are also present. The half-width of the phonons was measured, both at room temperature and liquid nitrogen temperature. It goes through a maximum for the phonons in the 300 cm−1 region and a minimum for those in the 1000 cm−1 region. The CN groups appear to be loosely attached to the quinone ring.
Observation of electron beam induced chemistry by tunneling spectroscopy
Authors: Kroeker R. M. , Pacansky J.
Published: February 2003

The technology of tunneling spectroscopy has been developed to allow the observation of in situ electron beam induced chemistry at liquid helium temperatures. In order to demonstrate the technique, we report the irradiation of a monolayer of cyanoacetic acid by a 10 keV electron beam. The formation of lead cyanide upon irradiation at 10 K is observed and is compared with the formation of nickel cyanide from cyanoacetic acid in tunneling junctions.
Beam maser investigation of inelastic scattering of NH3. II. Cross sections for rotational transitions induced by polar gases
Authors: Klaassen D. B. M. , ter Meulen J. J. , Dymanus A.
Published: February 2003

A molecular beam maser is used in a double-resonance scheme to measure cross sections for rotational transitions between levels of different inversion doublets of NH3 induced by collisions with NH3, CH3F, and CF3H. Theoretical values, calculated with a modification of Anderson’s theory, are in good agreement with present experimental results and steady-state double-resonance experiments of Oka [J. Chem. Phys. 48, 4919 (1968)].
Kramers’ theory of chemical kinetics: Curvilinear reaction coordinates
Authors: Larson R. S. , Kostin M. D.
Published: February 2003

Kramers’s theory of chemical kinetics is modified in order to analyze reacting systems in which the path of lowest energy between the reactants and the products is a curve in two dimensions. The potential surface is allowed to be either symmetric or asymmetric with respect to the barrier, and approximate expressions for the rate constant are obtained via both the Smoluchowski and Fokker–Planck equations. Accurate numerical results are computed for a special case and are seen to be in good agreement with the predictions of the theory. The analytical results indicate that the effect of the curved reaction path is to increase the rate constant over its straight-path value when the friction constant β is arbitrarily large. A competing effect arises when the frictional forces are reduced, however, and for sufficiently small β a decrease in the rate constant is noted. Nevertheless, the analyses tend to justify the commonly used notion of a reaction coordinate.
Model calculation of the effect of hydration on the energy band structure of a nucleotide base stack
Authors: Otto P. , Ladik J. , Corongiu G. , Suhai S. , Förner W.
Published: February 2003

The energy band structure of the nucleotide base stacks poly C, poly T, poly A, and poly G have been calculated by the ab initio SCF LCAO crystal orbital method. For poly C, model calculations have been performed to investigate the effect of water molecules on its electronic structure. The presence of the water molecules, whose positions have been determined recently by a Monte Carlo simulation technique at T=300 K, causes significant band shifts and together with positive ions could substantially influence the conductive properties of native DNA.
Generalized mean spherical approximation for a mixture of hard ions and hard dipoles against a charged hard wall
Authors: Vericat Fernando , Blum Lesser , Henderson Douglas
Published: February 2003

A mixture of charged and dipolar (nonpolarizable) hard spheres near a uniformly charged hard wall of unit dielectric constant is considered. This is a simple model of an electrode–electrolyte interface. The Ornstein–Zernike equation for this system is completely solved for a generalized closure in which the bulk electrolyte and wall-electrolyte direct correlation functions are, for distances larger than the repulsive cores, written as a sum of some parametrized terms plus the corresponding pair potentials. Once the closure parameters are evaluated, the density, charge, polarization, and potential profiles are easily obtained. Comparison is made with the quadratic hypernetted chain and mean spherical approximations for the bulk correlation functions and with the mean spherical approximation for the charge, polarization, and potential profiles.
The use of NMR linewidths to study b-axis distributions in poled and unpoled PVDF
Authors: Douglass D. C. , McBrierty V. J. , Wang T. T.
Published: February 2003

Reorientation of molecular dipoles in a strong electric field is an important contributor to the generation of piezo- and pyroelectricity in polymers. An understanding of the mechanisms involved and a knowledge of the extent to which dipoles are reoriented is clearly a desirable goal. Strong piezoelectric properties can be conferred on polyvinylidene flouride (PVDF) in the crystallographic β form for which the molecular dipoles are colinear with the crystallographic b axis. The β polymorph is usually generated through mechanical deformation by uniaxial drawing and/or rolling. The resulting material is spatially highly anisotropic at the molecular level. Anisotropy in the observed NMR linewidth of deformed polymers, monitored as a function of the angle between the sample draw or machine direction and the magnetic field H0 has been a source of useful information on the spatial distribution of molecules. Here, molecular distributions, specifically distributions of b axes about the poling direction, have been studied in poled and unpoled PVDF by observing linewidth anisotropy both as a function of draw direction alignment and poling axis alignment with respect to H0. The tendency for b axes to orient towards the direction of the applied electric field in the poling process is evident but ultimate alignment is by no means achieved. Our results are in broad agreement with earlier x-ray studies on comparable materials and it would appear, in addition, that there is still significant b-axis alignment, at large angles to the poling direction in the two poled samples investigated by NMR. The agreement achieved between theory and experiment justifies the initial assumption of (nπ/3)-fold rather than (nπ)-fold dipole rotation. Our study clearly demonstrates the unusual sensitivity of the NMR linewidth to b-axis distribution in β-form PVDF and provides measures of the means 〈cos 2δ〉 and 〈cos 4δ〉, where δ is the angle between poling and b axes.
Oxygen doping of zinc phthalocyanine thin films
Authors: Twarowski Allen
Published: February 2003

It is well established that the electrical properties of phthlocyanine solids change drastically as these solids absorb oxygen. However, the actual doping process has not been previously investigated. Here, capacitance measurements of (Al‖zinc phthalocyanine‖Nesatron) cells are reported which show the formation of a Schottky barrier at the Al‖zinc phthalocyanine interface as a function of distance from the interface and as a function of exposure time of the cell to oxygen. A careful examination of aged zinc phthalocyanine Schottky cells shows an inhomogeneous space charge density close to the aluminum contact. This inhomogeneity could lead to erroneous determinations of the barrier potential.
Global reaction: Diffusion coupling and reciprocity in linear asymmetric kinetic networks
Authors: Peusner Leonardo
Published: February 2003

The kinetics of a simple diffusion-chemical reaction system are formulated using circuit theory and network thermodynamics. It is shown that the Curie principle forbidding the local coupling of a scalar (chemical reaction) to a vector (diffusion) in isotropic systems is consistent, however, with global coupling in asymmetric processes. Equations derived in a proper frame of reference demonstrate the compatibility of the kinetic analysis with the reciprocity of Onsager’s equations. Tellegen’s theorem is used to study the stability of transient solutions and the unicity of dynamical trajectories followed in the approach towards the stationary state.
Photoionization mass spectrometry of CH3SH, CD3SH, and CH3SD: Heats of formation of CH3S+(CH2SH+), CH2S+, CH2S, and HCS+
Authors: Kutina R. E. , Edwards A. K. , Goodman G. L. , Berkowitz J.
Published: February 2003

Photoionization yield curves have been obtained for the major ions from CH3SH. Auxiliary studies with selectively deuterated species reveal that CD3S+ has a slightly lower threshold than CD2SH+ from CD3SH; but CH2SD+ has a lower threshold than CH3S+ from CH3SD. An analysis of alternative interpretations is presented. The isotopic studies also imply that CH2S+ is more stable than HCSH+, and HCS+ is more stable than CSH+. The value ΔH°f,0(HCS+)≤259.2±0.9 kcal/mol obtained in this study is in good agreement with a bracketed proton affinity for CS, but differs significantly from a recent ab initio calculation. This result confirms the view that HCS+ can be formed in interstellar clouds by exothermic reactions. The reaction CH3S+/CH2SH+→HCS++H2 proceeds weakly at threshold, and much more strongly after overcoming an activation barrier of ∼0.9 eV.
The core-hole excitation spectrum of benzene: A symmetry-adapted CNDO/S equivalent-core study including ‘‘spin-symmetry breaking’’ configurations
Authors: Bigelow Richard W. , Freund Hans-Joachim
Published: February 2003

The core-hole excitation spectrum of benzene below 12.0 eV is calculated within a symmetry-adapted first-order configuration interaction approach based on the CNDO/S equivalent-core approximation. One-electron singlet excitation energies of the neutral molecule are shown to approximate the corresponding ‘‘singlet’’-coupled doublet energies in the delocalized core-hole species. Intensities in accord with experiment are obtained within the sudden approximation by projecting the C2ν reduced-symmetry equivalent-core solutions out of the first-order delocalized description. The total core-hole excitation intensity below 12.0 eV is found to be largely attributable to three one-electron π*←π excitations of singlet-coupled doublet character which are split into the observed number of features, with correspondingly observed strengths, by first-order configuration interaction with energetically degenerate excitations in the ‘‘triplet’’-coupled doublet manifold—‘‘spin-symmetry breaking configurations.’’ The observed core-hole excitation satellite at 10.7 eV and the asymmetry on the high binding energy side of the well-resolved satellite at 8.3 eV are interpreted in terms of two excitations of 1E2g (π*←π) neutral molecule parentage. The assignment of these higher-energy features as 1E2g(π*←π) in origin provides further support for the numerous theoretical predictions and recent conclusions based on spectroscopic investigations for states of 1E2g symmetry at approximately 8.0 and 10.0 eV in neutral benzene.