bluuebunny/crossref_metadata_2025 huggingface dataset:
Resonance Raman detected triplet state magnetic resonance
Authors: Clarke Richard H. , Graham Daniel J. , Hanlon Eugene B. , Mitra Pradip
Published: February 2003

The zero field triplet state spin transitions of chlorophyll b have been measured by monitoring the microwave induced intensity changes of bands in the resonance Raman spectrum. The magnitude of the effect is found to be comparable to that observed in fluorescence detected ODMR.
Electron affinities of the alkali dimers: Na2, K2, and Rb2
Authors: Partridge Harry , Dixon David A. , Walch Stephen P. , Bauschlicher Charles W. , Gole James L.
Published: February 2003

Ab initio calculations on the ground states of the alkali dimers, Na2, K2, and Rb2, and their anions are reported. The calculations employ large Gaussian basis sets and account for nearly all of the valence correlation energy. The calculated atomic electron affinities are within 0.02 eV of experiment and the calculated adiabatic electron affinities for Na2, K2, and Rb2 are, respectively, 0.470, 0.512, and 0.513 eV.
An ab initio study of core–valence correlation
Authors: Partridge Harry , Bauschlicher Charles W. , Walch Stephen P. , Liu B.
Published: February 2003

Including core–valence correlation in ab initio CI calculations results in a contraction of the atomic valence shell, particularly for the first two columns of the periodic table. In addition, including core–valence correlation improves the calculated atomic ionization potentials and the atomic energy separations. For the alkali dimers, Na2, K2, and Rb2, our calculated Re’s are in excellent agreement with experiment when core–valence is included. We find for the alkali dimers that the valence De’s are accurate.
Treatment of resonances with the dilated electron propagator: The 2P shape resonance in e-Mg scattering
Authors: Mishra Manoj , Kurtz Henry A. , Goscinski Osvaldo , Öhrn Yngve
Published: February 2003

The dilated electron propagator technique based on an underlying bivariational SCF and a second order self-energy is applied to study the 2P shape resonance in e-Mg scattering. Basis set effects are studied and comparisons with the results from an alternative construction of the second order dilated electron propagator are made.
Maximum-entropy inference and momentum density approach
Authors: Koga Toshikatsu , Morita Mutsuo
Published: February 2003

Previously, we have proposed a rigorous method to obtain the total energy E of a system of interacting atoms based on the electron momentum density and its redistribution during the interaction process. In this paper, we consider an inverse problem of constructing the momentum density distribution from the knowledge of the total energy E. Using the method of the maximum-entropy inference in information theory, analytical expressions are derived for the radial momentum density as a function of E. The results are applied to the interactions in the H2 system and the reliability of the inference is examined. In spite of its simplicity, the inferential momentum distribution is shown to be reliable semiquantitatively.
Kinetic Ising model for polymer dynamics: Applications to dielectric relaxation and dynamic depolarized light scattering
Authors: Skinner James L.
Published: February 2003

We introduce a new one-dimensional kinetic Ising model to study the cooperative dynamics of linear chain molecules. Correlation functions for the model are calculated approximately. We discuss the connection between these correlation functions and certain relaxation functions that can be measured by dielectric and dynamic depolarized light scattering experiments. These relaxiation functions are extremely well represented by the Williams–Watts empirical function φ(t)=exp[−(t/τ)β] with β between 1/2 and 3/4. This is in reasonable agreement with both dielectric and light scattering experiments on amorphous polymers. Our results lead to a microscopic interpretation of this ‘‘universal’’ nonexponential decay function.
Mobilities and interaction potentials for Br−–Ar, Br−–Kr, and Br−–Xe
Authors: Lamm D. R. , Chelf R. D. , Twist J. R. , Holleman F. B. , Thackston M. G. , Eisele F. L. , Pope W. M. , Gatland I. R. , McDaniel E. W.
Published: February 2003

Mobilities of Br− ions in Ar, Kr, and Xe gases at 300 K have been measured in a drift tube mass spectrometer over a wide range of E/N (the ratio of the electric field to the neutral gas number density). For any ion–neutral gas interaction potential, implementation of the Viehland–Mason kinetic theory (accurate for arbitrary E/N) yields theoretical mobility data. This theory is employed in conjunction with an iterative-inversion scheme which uses experimental mobility data to modify an initial ‘‘guess’’ 4,6,12 potential to produce a ‘‘directly determined’’ potential which accurately reproduces the experimental mobilities for a given ion–neutral atom pair. These potentials are tabulated for Br−–Ar, Br−–Kr, and Br−–Xe and their salient features are compared to previous ‘‘directly derived’’ potentials of other ions in these gases.
Mobilities of various mass-identified positive ions in helium, neon, and argon
Authors: Böhringer H. , Durup-Ferguson M. , Fahey D. W.
Published: February 2003

The mobilities of several mass-identified positive ions in helium, neon, and argon buffer gases were measured in a flow drift tube apparatus as a function of E/N at 297 K. The systems studied were N2O+, NO2+, and SO2+ in He, H2O+, O2+, and N2O+ in Ne, and H2O+, H3O+, N2O+, NO2+, and SO2+ in Ar. The values obtained were compared to previous measurements and Langevin mobilities and were used to estimate the well depth of the ion buffer interaction potential.
Transit time conception for ion diffusion through membrane channels
Authors: Schröder H.
Published: February 2003

The diffusion path of an ion is considered as a one-dimensional random walk between binding sites along the channel, where the potential energy profile may consist of an arbitrary sequence of different barriers and binding sites. It is shown that the conception of discrete transit times leads to a very simple calculation of transport quantities such as diffusion constant and stationary transmembrane current. The diffusion constant obtained here is the discrete counterpart of the diffusion constant obtained from the exact solution of the Smoluchowski equation. The resulting current–voltage characteristics indicate that the assumption of a discrete random walk represents a fairly good approximation. Furthermore, the first passage time problem for the present model is discussed and solved for the homogeneous case with the aid of a difference equation for successive first passage times.
Kinks and polarons in polyacetylene
Authors: Sano Tatsuji , I’Haya Y. J.
Published: February 2003

Various defect states in a finite trans-polyacetylene chain are studied by a geometry optimization method with a tight-binding Hamiltonian of Longuet-Higgins and Salem. It is shown that the defect obtained forms a φ-four kink or two combined kinks such as a polaron, being independent of chain size whenever the chains are longer than the defect width. The defect formation energy is local in the vicinity of the defect. The creation energies of the kink and polaron at rest have been determined by means of extrapolation to infinite chain size. The results obtained are in good agreement with those estimated from the analytical solutions in the continuum models.
The one-dimensional n → 0 vector model and the helix–coil transition
Authors: ten Brinke Gerrit , Karasz Frank E.
Published: February 2003

The one-dimensional n → 0 vector model is shown to be closely related to a model describing helix–coil transitions in polypeptides. This makes available a vast amount of theory developed in this area and allows a simple derivation of the self-avoiding walk length distribution corresponding to the one-dimensional n → 0 vector model. The effect of the peculiar feature of this model, the extra factor 1/2 for single site walks, is shown to be minor. The interpretation of the one-dimensional n → 0 vector model as a model for helix–coil transitions commits little error and the corresponding ‘‘helix sequence’’ length distribution is in a first approximation identical to the helix sequence length distribution of existing models for the helix–coil transition. The equivalence between the n → 0 vector model and an equilibrium polymerization model found by Wheeler et al., implies a close connection between the self-avoiding walk length distribution and the molecular weight distribution. Our results confirm the mean-field predictions that the molecular weights of the polymer species are distributed exponentially. The solution of the one-dimensional n → 0 vector model shows a phase transition in the limit of zero magnetic field.
A classic example of an Evans hole caused by intermolecular interaction in solid 2ND3⋅D2O at 15 to 145 K
Authors: Bertie John E. , Devlin J. Paul
Published: February 2003

The infrared spectra of 2ND3⋅D2O are reported at temperatures between 15 and 140°K. A sharp dip at 2333cm−1 is found to be an Evans hole2−5. The cause of this dip is discussed. (AIP)
The structure of dimethylamine
Authors: McKean D. C.
Published: February 2003

An r0 structure is determined for dimethylamine from earlier microwave data. With the constraint δr0(CH–CD)=δr0(NH–ND), CH bond lengths of 1.0958, 1.0981, and 1.1060 A° are obtained, in excellent agreement with earlier predictions from ‘‘isolated’’ CH stretching frequencies. The new value of r0NH of 1.011 A° is significantly shorter than the earlier rs value of 1.022. The angular asymmetry of the methyl group is greater than previously supposed.
Experimental results of homogeneous nucleation of supersaturated vapors
Authors: Garnier J. P. , Mirabel P. , Rabeony H.
Published: February 2003

An r0 structure is determined for dimethylamine from earlier microwave data. With the constraint δr0(CH–CD)=δr0(NH–ND), CH bond lengths of 1.0958, 1.0981, 1.1060 A≡ are obtained, in excellent agreement with earlier predictions from ‘‘isolated’’ CH stretching frequencies. The new value of r0NH of 1.011 A≡ is significantly shorter than the earlier rs value of 1.022. The angular asymmetry of the methyl group is greater than previously supposed.
The rotational Zeeman effect in ArHBr: An interpretation of the rotational Zeeman parameters in rare gas hydrogen halide complexes
Authors: Read W. G. , Campbell E. J.
Published: February 2003

The rotational Zeeman effect in the ArHBr van der Waals molecule has been studied using the method of pulsed Fourier transform microwave spectroscopy carried out in Fabry–Perot cavity located in the bore of a superconducting magnetic solenoid. The rotational magnetic dipole moment, magnetic susceptibility anisotropy, and bromine bulk shielding constant were determined. The values for ArH79 Br are: g⊥=0.000 20(5), χ∥–χ⊥ =−0.13(9)×10−9 MHz/G2, and σBr=4000(2000) ppm. The equations describing these molecular Zeeman parameters in terms of the properties of the constituent subunits of the complex are derived from the full dynamic molecular Hamiltonian. The rotational g value is a sensitive measure of the degree of Coriolis coupling in the complex. The molecular quadrupole moment of ArHBr is calculated.
Infrared laser multiple-photon dissociation of CDCl3 in a molecular beam
Authors: Herman Irving P. , Magnotta Frank , Buss Richard J. , Lee Y. T.
Published: February 2003

Pulsed CO2 laser multiple-photon dissociation of CDCl3 was examined in a molecular beam using 11 μ radiation. The only observed dissociation channel was hydrogen chloride elimination (≳99.1%), with no evidence of simple chlorine atom cleavage (<0.9%). Implications for isotope separation are discussed.
Collisional quenching of A 2Σ+ OH at elevated temperatures
Authors: Fairchild Paul W. , Smith Gregory P. , Crosley David R.
Published: February 2003

Thermally averaged cross sections σQ for collisional quenching of the A 2Σ+ state of the OH molecule have been measured near 1100 K. The OH was produced and detected in a laser pyrolysis/laser fluorescence experiment, in which a mixture of SF6, H2O2, and the collision partner M was heated by a pulsed CO2 laser. Thermal decomposition of the peroxide produces OH which is then excited by a tunable laser; the real-time decay of the fluorescence signal at different pressures of M furnishes σQ. These σQ at elevated temperatures are generally less than the room temperature values. This result, the size of σQ, and its variation with M suggest the importance of attractive forces in the collisional quenching. The experimental results have been compared with a theoretical calculation of σQ based on multipole attractive forces with a repulsive centrifugal barrier. Good correlation is obtained for eight of the 11 quenchers studied; the experimental values of σQ for N2 and SF6 are conspicuously low.
Picosecond transient Raman spectroscopy: The photoisomerization of trans-stilbene
Authors: Gustafson T. L. , Roberts D. M. , Chernoff D. A.
Published: February 2003

We measured picosecond transient resonance Raman spectra of t-stilbene in hexane using a high repetition rate, amplified, synchronously pumped dye laser. We assign four vibrational frequencies to the olefinic moiety. Two of these frequencies, 1238 and 1565 cm−1, have broad, asymmetric bands. We attribute this band shape to a distribution of excited state molecular conformers having different single and double bond strengths.
Photoabsorption and photoionization of HD
Authors: Dehmer P. M. , Chupka W. A.
Published: February 2003

Relative photoabsorption and photoionization cross sections have been measured for HD at a temperature of 78 K in the wavelength region from 735 to 805 Å. The present wavelength resolution of 0.016 Å represents an improvement of more than two orders of magnitude over that of previous photoionization studies of this molecule. Bands of the 3pπ D 1Πu ← X 1Σ+g system are observed to v′=17, and ionization efficiencies are reported for a number of Rydberg states of low principal quantum number. As in the case of H2, the ionization efficiency is close to unity for Rydberg states that can autoionize with Δv=−1, but drops to zero for states that can autoionize only with a large change in vibrational quantum number and that are significantly predissociated (such as the 3pπ D 1Πu state). The breakdown of (g,u) symmetry in HD and the resulting effects on the absorption spectrum and on the decay paths of the Rydberg states are discussed.
The photophysics and photochemistry of weakly bound dimers of s-tetrazine
Authors: Young Linda , Haynam Christopher A. , Levy Donald H.
Published: February 2003

Energy redistribution within the excited states of the planar and T-shaped dimers of s-tetrazine was monitored by dispersed emission following selective vibronic excitation. When the dimer contained excess vibrational energy above the electronic origin, three processes were observed: (1) energy transfer from the initially populated ν6a vibration (in-plane stretch) to low frequency dimer vibrations involving motion of one monomer subunit with respect to the other; (2) electronic energy transfer from the high energy ring (∥ polarized) to the low energy ring (⊥ polarized) in the T-shaped dimer; and (3) photodissociation of the T-shaped dimer into monomer subunits. Simultaneously observed fluorescence from the initially populated state allows us to estimate the rates at which these processes occur. The observation of photodissociation (τ=38 ps) of the T-shaped dimer with an input of 1595 cm−1 of excess vibrational energy yields an estimate for the binding energy in the ground state Eb<1552 cm−1.
Absorption and circular dichroism spectra of dimeric systems. Extended dimer model
Authors: Zgierski M. Z. , Pawlikowski M.
Published: February 2003

An extended dimer model is formulated. It differs from the simple dimer model in allowing for mixing of monomer electronic states by intermolecular interaction. The exact and approximate mathematical treatments of the model are discussed in detail. The model is applied to studies of absorption and circular dichroism spectra of dimers. It is shown that the transfer of intensity and rotatory power due to interstate mixing leads to readily observable changes in the vibronic structure of the corresponding spectra. The model is used to reinterpret the observed CD spectra of two different 1,1′-bianthryls.
One-dimensional Brownian motion near an absorbing boundary: Solution to the steady state Fokker–Planck equation
Authors: Mayya Y. S. , Sahni D. C.
Published: February 2003

The problem of steady, one-dimensional Brownian motion under prescribed flux in the presence of an infinite plane absorbing boundary is discussed from the point of view of the eigenfunction expansion solution to the relevant Fokker–Planck equation. The expansion coefficients are determined by reducing it to a problem of the initial value type, using a parametrized Maxwellian for the velocity distribution of incoming particles only at the boundary. The number density function expressed in the form of an infinite series of exponentials is cast into a rapidly convergent form at the boundary layer. While some of the physical quantities obtained by this method are in essential agreement with those obtained by certain previous investigators, the following new results are to be noted: (i) the fractional power law variation of the number density in the vicinity of the boundary; and (ii) the complete vanishing of the space dependent diffusion coefficient at the boundary. The results are further discussed.
Shear rate dependence of non-Newtonian viscosity of fluidsa)
Authors: Eu Byung Chan
Published: February 2003

A pair of analytic formulas for the shear viscosity of simple fluids are derived from kinetic equations as functions of shear rate (γ), temperature, and density. One of the formulas is of the form η=η0 ln[τχ+(1+τ2χ2)1/2]/(τχ), where χ=√2γ and τ is a function of Newtonian shear viscosity η0, temperature, and density. The shear viscosities calculated with the formulas compare well with the molecular dynamics shear viscosities of argon.
Observations upon the dynamic structure factor of interacting spherical polyelectrolytes
Authors: Phillies George D. J.
Published: February 2003

Experimental studies are reported on the light-scattering spectrum of 0.038 μ carboxylate-modified polystyrene latex spheres in deionized water. In concentrated suspensions, the long-time decay of the two-particle dynamic structure factor is given by a power law (t−α) form, not by a slow exponential. Nonlinear least squares fits found α≂1.2–1.5, smaller values of α corresponding to higher sphere concentrations. The reduction of the mutual diffusion coefficient of 0.15 μ spheres in mixtures of 0.15 and 0.038 μ spheres was examined, and is interpreted in terms of the dynamic friction modification to the drag coefficients. Multiple scattering artifacts were shown to be absent with homodyne coincidence spectroscopy.
Langevin analysis of the diffusion model for surface chemical reactions
Authors: Freeman David L. , Doll Jimmie D.
Published: February 2003

An analysis is presented of the magnitude of some of the potential sources of error in a recently developed diffusion model of surface chemical reactions. Using single absorber Langevin simulations, comparisons are made between the diffusion equation model and the Fokker–Planck equation for the rates of diffusion controlled surface chemical reactions. The diffusion equation is found to predict rates in good agreement with the Fokker–Planck equation for physical values of the diffusion constant. For unphysically large diffusion constants, the rates predicted by the diffusion equation are found to be in error. By employing multiple absorber Langevin simulations errors in the single absorber approximation used in the diffusion model of surface reactions are examined. The single absorber model is found to be accurate for weakly bound adsorbates. For strongly bound adsorbates rate expressions derived from a two-dimensional model are found to be appropriate. The relative rates of Eley–Rideal and Langmuir–Hinshelwood mechanisms are also studied by multiple absorber Langevin simulations. The ratios of the Eley–Rideal to the Langmuir–Hinshelwood rate is found to be in good agreement with the predictions of the diffusion equation model for physically reasonable diffusion constants. The time dependent solution to the diffusion equation considered in a previous publication is given in an appendix.
Vapor pressures, vaporization reactions, and thermodynamics of vaporization of VS(s)
Authors: Edwards Jimmie G. , Pelino Mario , Starzynski John S.
Published: February 2003

Two studies of the vaporization of VS(s) were conducted in the temperature range 1770–2069 K by the simultaneous Knudsen-torsion-effusion method. The vapor pressure of VS(s) was best represented by the equation log(P/Pa)=−(2.161±0.057)×104 K/T+11.02±0.29. Limiting thermodynamic results were obtained on the basis of the assumption that the partial pressure of V(g) was the vapor pressure of V(s). The ΔH°(298 K) of sublimation of VS(s) to give VS(g) was 551.5±2.1 kJ/mol, of dissociation of VS(g) to give gaseous atoms was 484.5±3.3 kJ/mol, and of formation of VS(s) was −243±5 kJ/mol. The second result agrees exactly with that of a mass spectrometric study in the literature, and indicates that the temperature range of the present study is near the temperature beyond which VS(s) does not vaporize congruently.
Effects of pressure on conformer equilibria in liquid n-hexane
Authors: Wong Patrick T. T. , Mantsch Henry H. , Snyder Robert G.
Published: February 2003

From a study of the dependence of the polarized Raman spectra of liquid n-hexane (30 °C) on pressure (0.26–7.38 kbar), we have confirmed the findings of Schoen et al. [Nature 270, 412 (1977)] that pressure is effective in perturbing the distribution of conformers. However, our results on the behavior of the gauche forms differ from these authors. We find that increased pressure decreases the concentration of the ttt conformer and increases the concentrations of both the single- and double-gauche forms. Surprisingly, the pressure dependence of the single- and double-gauche conformers is nearly the same. Thus while the volume change for ttt→gtt is found to be about −1.1±0.0 cm3/mol, that for the single-gauche to double-gauche conversion is about 0.0±0.0 cm3/mol. These results appear to rule out a simple additivity model for the n-alkanes in which CC bonds respond independently to pressure. Our measurements also suggest that the concentration of the all-trans conformer in liquid n-hexane is significantly higher than the value obtained from the rotational-isomeric-state model with Eg and Egg′ equal to the widely used values of 500 and 3000 cal/mol.
Evaluation of dielectric permittivity by dc transient current
Authors: Yagihara S. , Nozaki R. , Takeishi S. , Mashimo S.
Published: February 2003

Inverse Laplace transformation of the Havriliak–Negami (HN) equation of complex dielectric permittivity has been obtained analytically both in short time and long time regions. A convenient procedure for determining the relaxation parameters in the HN equation from the dc transient current has been presented. Even if the current measurement is not carried out over the whole time range but only in a restricted time range, this procedure gives satisfactory values to the relaxation parameters.
The effect of surface diffusion on surface reaction rates
Authors: Cukier R. I.
Published: February 2003

We present a simple model to explore the role of surface diffusion on the rate of reaction between a surface diffusing species and a set of fixed randomly distributed reactive sites. A Langmuir adsorption/desorption term is used to describe the influence of the bulk phase on the surface. Reaction is modeled by a fast, with respect to surface diffusion, absorption on the perimeters of the reactive sites. A general effective medium theory is formulated for this model and solved approximately for the rate kf and diffusion coefficient D, which characterize the transport of the reactive density on a scale large compared with intersite dimensions. We find that surface diffusion can be an important contributing factor to the rate of reaction. In particular, when surface diffusion is rapid relative to the Langmuir adsorption/desorption rate, the reaction rate increases strongly with increasing reactive site surface concentration relative to its value for low surface site concentration.
A one-dimensional model for phonon-induced desorption
Authors: Efrima Shlomo , Jedrzejek Czeslaw , Freed Karl F. , Hood Eric , Metiu H.
Published: February 2003

We derive the kinetic equations for a one-dimensional model of phonon-induced desorption. We start with a generalized master equation and discuss approximations that reduce it to a master equation of the Pauli type. This describes desorption as a stochastic process in which the energy of the chemisorptive bond has random variations caused by the thermal fluctuations of the lattice atoms. Desorption occurs when the bond energy exceeds a certain value, placing the adsorbed particle in a continuum state of the surface-particle potential. The calculation of the rates of energy transfer includes multiphonon processes and allows transitions between all the bound levels of the adsorbed particle. Numerical calculations are presented in a subsequent paper.
Effects from bead size and hydrodynamic interactions on the translational and rotational coefficients of macromolecular bead models
Authors: García de la Torre José , Rodes Vicente
Published: February 2003

In this work, we analyze a deficiency of the currently used form of the Kirkwood–Riseman (KR) theory for macromolecular bead models in regard to the finite size of the beads and illustrate the importance of a more complete description of hydrodynamic interactions. We consider several exact or very accurate theories for two-bead models and compare their results with different KR-based methods. One of them is the substitution technique in which the beads are substituted by a cubic array of smaller spheres. Its disadvantage is an increase in computer requirements. A novel result of our work is a very simple correction for the KR rotational coefficients intended to correct, in part, the failure originated by size effects. The comparison of the various approaches has been made for two specific cases, the rigid dumbbell and prolate ellipsoids, for which rigorous results are available. We show that the performance of the substitution technique is excellent, and that our correction improves in many cases the KR results for rotation.
X-ray total external reflectance–diffraction as a probe of the structure of the liquid–vapor interface
Authors: Mohanty Udayan , Rice Stuart A.
Published: February 2003

For potential use in the study of the structure of the liquid–vapor interface of a liquid metal, we analyze the scattering of a collimated beam of x rays which strike the plane surface of a liquid metal at an angle below the range for total external reflection. Our analysis of the diffraction pattern is based on the Von Laue model of a condensed phase scatterer and the distorted wave approximation. The x-ray total reflectance–diffraction is expressed in terms of the structure factor of the inhomogeneous transition zone between the liquid and vapor of a metal.
Temperature dependence of associative detachment reactions
Authors: Viggiano A. A. , Paulson John F.
Published: February 2003

The temperature dependences of the rate coefficients for the associative detachment reactions O−+NO, S−+CO, and S−+O2 have been measured. All rate coefficients varied as T−0.74±0.1. In addition, the rate coefficients and branching ratios for the reactions O−+C2H2 and O−+C2H4 have also been studied as a function of temperature. Both reactions were found to have a large associative detachment channel over the entire temperature range 140–494 K.
Calculation of molecular polarizabilities using a semiclassical Slater-type orbital-point dipole interaction (STOPDI) model
Authors: Birge Robert R. , Schick G. Alan , Bocian David F.
Published: February 2003

The point dipole interaction model for molecular polarizability proposed by Applequist, Carl, and Fung [J. Am. Chem. Soc. 94, 2952 (1972)] is modified by replacing the point dipole interaction tensor with a descaled distributed charge interaction tensor. Our procedure is based on the descaled tensor algorithm proposed by Thole [Chem. Phys. 59, 341 (1981)] and uses a Slater-type orbital (STO) function to represent the charge distribution. The resulting STOPDI formalism calculates mean molecular polarizabilities and the components of the molecular polarizabilities with errors comparable to experimental uncertainty. Furthermore, these procedures require only one optimized parameter per atom, the average atomic polarizability. The formalism is invariant to coordinate transformations and avoids the discontinuities and/or false resonances that are characteristic of previous classical and semiclassical formalisms. The STOPDI algorithm requires less parameterization and computation time than the anisotropic atom point dipole interaction (AAPDI) model of Birge [J. Chem. Phys. 72, 5312 (1980)] and is more reliable for the calculation of polarizability derivatives and Raman cross sections. We demonstrate, however, that none of the above formalisms are reliable for calculating absolute Raman cross sections for normal modes involving significant bond stretching components. This is an inherent limitation of any formalism which does not explicitly account for electron density redistribution accompanying changes in the internuclear distances of covalently bonded atoms.
A theoretical study of the radiative lifetime of the CH A 2Δ state
Authors: Larsson Mats , Siegbahn Per E. M.
Published: February 2003

The complete active space (CASSCF) method has been used to generate wave functions for calculations of the A 2Δ–X 2Π transition moment. This transition provides an interesting test case for different ab initio methods since the experimental data for the A state radiative lifetime should be very accurate. The two most accurate recent experiments agree to within 1% of each other, with a value of 536 ns. The best earlier theoretical value is 397 ns. Our calculations show an extremely slow convergence with basis sets and active spaces. A calculation with a basis set of more than quadruple zeta quality with two d and one f function on carbon and three p and one d function on hydrogen gave an error of 20% (446 ns compared to 536 ns) for the lifetime, even when a large active space of 5σ,3π, and 1δ orbital was used. Our best calculated value 525 ns finally agreed very well with the experimental value, and is consequently larger compared to the earlier theoretical value by more than 30%.
Basis set effects in Hartree–Fock studies on aromatic molecules: Hartree–Fock calculations of properties in benzene and hexafluorobenzene
Authors: Almlöf Jan , Faegri Knut
Published: February 2003

A number of selected properties have been evaluated for the benzene and hexafluorbenzene molecules, using more than 20 different Gaussian basis sets. The effects of polarization functions have been studied and scaling of the orbital exponents in energy-optimized basis sets is commented on. The effects of extending the primitive basis sets are also discussed and the particular problems associated with LCAO studies on polar molecules are investigated.
The use of modified virtual orbitals in perturbative polarization propagator calculations
Authors: Oddershede Jens , Sabin John R.
Published: February 2003

The use of modified virtual orbital potentials in perturbative polarization propagator approaches has been investigated. We find that the random-phase approximation (RPA), which is identical to a first order polarization propagator approach, is invariant to modifications in the virtual orbital potential, provided that we include all possible excitations within a given basis set. Even in a moderately truncated excitation space we find only a small change in RPA. However, the second order polarization propagator approximation is not invariant to unitary transformations of the virtual orbitals. Certain denominator shifts in the correlation coefficients and in the two-particle, two-hole terms will reduce this dependence on the virtual orbital potential. Numerical applications to Be and CO show that we may have simple VN−1 potentials which do not give a convergent scheme. This happens when the virtual orbitall energies are too close to the orbital energies of the occupied orbitals.
Quantum beating of vibrational factor group components in molecular solidsa)
Authors: Velsko S. , Trout J. , Hochstrasser R. M.
Published: February 2003

We report the first observation of coherent Raman induced quantum beats from exciton states in molecular crystals. The paper describes the relevant crystal optics and phasematching conditions needed to ensure coherent excitation of more than one Raman transition in an anisotropic solid. The study of the 991 cm−1 mode of C6H6 at 1.6 K has allowed the identification of two nearby (0.6 cm−1) factor group components and their lifetimes. The larger damping of the weaker Ag→B2g transition suggests that intraband scattering is operative.
NaH: Studies of the X 1Σ+ state near dissociation and of the radiative lifetimes and collision cross sections of the A 1Σ+ state
Authors: Nedelec O. , Giroud M.
Published: February 2003

With a dye laser pumped by an excimer laser, the NaH A 1Σ+ state has been excited up to v′=21. Its decay to the X 1Σ+ ground state has been observed up to v″=19, a vibrational level close to the dissociation limit, allowing us to obtain a precise value of the X 1Σ+ dissociation energy: De=15 785±20 cm−1. For v″≳15, the vibrational spacings and the rotational constants decrease very rapidly, as also observed recently in CsH, due to a sudden change of configuration mixing. The A 1Σ+ radiative lifetimes do not vary appreciably from v′=3 to v′=18, τ=27±2 ns, but increase to 35±2 ns at v′=21. The quenching and rotational transfer collision cross sections of the A 1Σ+ state with H2 have nearly the same values for a given v′. They increase from v′=4 to v′=21 as do the calculated electric dipole moments and the geometric extent of the levels.
A test of an effective pair potential for liquid water
Authors: Townsend Michael , Rice Stuart A. , Morse Michael D.
Published: February 2003

Matsuoka, Clemeuti and Yoshimine1, potential is tested on the structure of ice VIII. The structure of ice VIII as obtaiend by other models and experimental data is compared with that of the calculated one. (AIP)
High energy elastic and inelastic electron scattering by the Ne and Ar atoms: Electron correlation effects
Authors: Duguet A. , Bennani A. Lahmam , Rouault M.
Published: February 2003

Elastic and inelastic differential cross sections (DCS) of high energy electrons (25 keV) scattered by neon and argon have been separately measured by totally independent methods. The effects of electronic correlations on the DCS and on the radial distribution functions D(R) and P(R) are deduced from experiment by comparison with theoretical Hartree–Fock values. An estimate of the correlation energy is also given. In the case of neon, the differences, with respect to Hartree–Fock, of the various contributions to atomic potential energy Δ Vne, Δ VeeCoul, and ΔVeeexch are calculated from experiment and compared to theoretical results.
Second order rate constants for intramolecular conversions: Application to gas-phase NMR relaxation times
Authors: Bauer S. H. , Lazaar K. I.
Published: February 2003

The usually quoted expression for the second order rate constant, for a unimolecular reaction at the low pressure limit, is valid only for strictly irreversible processes. Its application to isomerization reactions (which are to some extent reversible) is demonstrably in error; corrected expressions have been published. Attention is directed to intramolecular conversions over low barriers, for which the inappropriateness of the unidirectional expression becomes obvious. For such isomerizations we propose a model which incorporates only operationally observable states, so that an essential conceptual ambiguity is avoided. Use of this model is illustrated for the syn⇄anti conversions of methyl nitrite, derived from a gas phase NMR coalescence curve (Mc:Tc). The present data suggest that during isomerization the alkyl nitrites may not be completely ergodic on a time scale of 10−9 s. A regional phase-space model is proposed which has the appropriate formalism to account for this behavior.
MRD-CI potential surfaces using balanced basis sets. I. First-row diatomic hydrides
Authors: Wright James S. , Williams Richard J.
Published: February 2003

Multireference configuration-interaction (MRD-CI) methods are used to calculate binding energies, potential curves, and vibrational energy levels for the diatomic hydrides H2, BH, CH, NH, OH, and FH. The curves are calculated out to the dissociation limit. It is shown how the use of bond functions (located midway between the nuclei) play an essential role in augmenting the standard double-zeta plus polarization basis set. This provides a relatively small but balanced basis set which describes equally well both atomic and molecular regions. The potential curves are accurately fitted by generalized Morse functions over a range of distance which spans 90% of the well depth. Computed binding energies are within 0.1 eV of experimental values, except for the experimentally uncertain NH, which is within 0.2 eV of an estimated value. Potential curves and vibrational levels for H2, OH, and FH agree well with spectroscopic (and RKR) values. Vibrational levels and potential curves for the other hydrides are compared to theoretical values since few experimental data are known, with less good but still reasonable agreement. The selection/extrapolation and multireference features of the MRD-CI method provide good energy continuity at modest computational expense over the whole range of internuclear distances.
Vibration-rotation coupling in anisotropic environments. II. Quadrupolar couplings of methanes in liquid crystals
Authors: Snijders J. G. , de Lange C. A. , Burnell E. E.
Published: February 2003

It is shown that deuterium quadrupolar couplings observed when the deuterated methanes are dissolved in liquid crystals can be understood on the basis of the same vibration-rotation coupling mechanism which can explain the measured dipolar couplings. Two quantitites are obtained from the analysis: (i) the electric field gradient at the site of the deuterium nucleus when the deuterated methane possesses its equilibrium geometry; and (ii) the derivative of this field gradient with respect to the F2 stretch symmetry mode, taken at the equilibrium geometry. In order to obtain consistency, the presence of an external electric field gradient in the liquid–crystal solvent is required and its magnitude is estimated.
OH(X 2Π) state distribution from HNO3 and H2O2 photodissociation at 193 nm
Authors: Jacobs Axel , Kleinermanns Karl , Kuge Helmut , Wolfrum Jürgen
Published: February 2003

The photodissociation of HNO3 and H2O2 at 193 nm is reported. The rotational vibrational and fine structure state distributions of OH radicals obtained in the photodissociation are described. (AIP)
Negative thermal expansion due to the hindered rotation of the ammonium ion or the methyl group in solids with low barriers to rotation and the anisotropic thermal expansion in solids having polyatomic groups
Authors: Smith David
Published: February 2003

The thermal expansion due to the hindered rotation of the ammonium ion or the methyl group is negative at liquid helium temperatures, if the tunneling frequencies of these polyatomic groups are greater than 0.3 cm−1. Since the thermal expansion due to the hindered rotation of polyatomic groups with large tunneling frequencies is much greater than that due to the lattice vibrations at very low temperatures, the observed thermal expansion should also be negative. For barriers to rotation less than 500 cal/mol, the tunneling frequencies of NH+4 and CH3 are large enough to cause a negative thermal expansion. The thermal expansion due to the hindered rotation of NH+4 has been computed for NH+4 in crystalline fields of trigonal, tetrahedral, and octahedral symmetry. Some noncubic ammonium compounds with relatively high barriers to rotation have been found to have a negative thermal expansion along one of their crystallographic axes. Expressions for the contribution to the thermal expansion from the hindered rotation of polyatomic groups are derived for tetragonal, hexagonal, and orthorhombic crystals. From these expressions, the probable source of the negative thermal expansion in one of the crystallographic directions in compounds such as NH4ReO4 and NH4ClO4 is deduced.
Infrared dephasing and librational motion in liquid methyl iodide
Authors: Rothschild Walter G. , Soussen-Jacob Janine , Bessière Josette , Vincent-Geisse Josette
Published: February 2003

A theoretical model of vibrational dephasing with motional narrowing by the librational motions of the molecules in their liquid cages is proposed and tested on the infrared ν6 degenerate fundamental of liquid methyl iodide and perdeutero methyl iodide over a temperature range of −190 to + 40 °C. The theory, which does not require artificial separation of vibrational and orientational coordinates, is particularly applicable to associated and sterically hindered liquids.
Lattice dynamics of solid I2 under high pressure
Authors: Kobashi Koji , Etters R. D.
Published: February 2003

A calculation of the lattice dynamics of solid I2 is made on the basis of a semiphenomenological potential with terms describing the intramolecular interaction, orientation-dependent charge transfer interactions, and nonbonding interactions. In addition, two terms describing a coupling between intramolecular vibrations of different molecules and a vibrational coupling between charge transferred bonds are included. At normal pressure, calculated vibrational frequencies are in good agreement with Raman and neutron scattering data. Agreement with experiment at high pressures requires that some potential parameters be volume dependent, and the implication of this result to the pressure induced change in the electronic states of solid I2 is discussed. The merging of the intramolecular Ag and B3g Raman bands upon compression is explained quantitatively. The pressure dependence of several lattice vibrational mode frequencies are predicted but the observed softening of the librational Ag mode is not reproduced. The crystal energy is also calculated and compared with experiment.
Gravity-induced density and concentration profiles in binary mixtures near gas–liquid critical lines
Authors: Chang R. F. , Sengers J. M. H. Levelt , Doiron T. , Jones J.
Published: February 2003

We have calculated gravity-induced density and concentration gradients using scaled equations of state fashioned after that of Leung and Griffiths for binary mixtures near gas–liquid critical lines. The mixtures considered here are those of helium-3 and helium-4 and of carbon dioxide and ethane. Our calculations show that the density profiles for both mixtures in any proportion of the components are similar to those of pure fluids. The concentration gradients in the helium mixture have the same appearance as the density gradients. In the carbon dioxide–ethane system, however, the form of the concentration profile varies greatly, depending on the overall composition. Moreover, the temperature at which a mixture separates into two phases is slightly different from that expected for the mixture in the absence of gravity. We have also examined the case where a mixture is subjected to a large gravitational field such as can be generated in a centrifuge and found that, although the density gradient in all the mixtures is like that in pure fluids, the concentration gradients in the mixtures of carbon dioxide and ethane have complex features related to the presence of critical azeotropy.
Extensions of the molecular dynamics simulation method. II. Isothermal systems
Authors: Haile J. M. , Gupta S.
Published: February 2003

In this paper we systematically develop two general classes of equations of motion for performing molecular dynamics simulations at constant temperature. One member of each class is found to be identical to one of the two commonly used methods for performing such simulations. Of those two, one, namely the momentum scaling procedure, is proven to have no effect on the static properties of the simulated system. In addition, we have tested the two methods via equilibrium molecular dynamics on the Lennard-Jones fluid and find the resulting velocity autocorrelation functions and self-diffusion coefficients to be in good agreement with those determined by the standard NVE molecular dynamics method.
Charge carrier photogeneration in iodoform. Applicability of the Onsager model
Authors: Nešpurek S. , Samoć A. , Samoć M. , Sworakowski J.
Published: February 2003

The photogeneration of electrons and holes in thin layers of iodoform has been investigated as a function of the electric field by the method of photoinduced discharge current. The experimental results can be described by the Onsager theory. The fraction of absorbed photons which produce thermalized electron-hole pairs is close to unity. The pair separation distance is ∼3 nm. Equal values of the primary rates of production of electrons and holes support the idea of an intrinsic photogeneration mechanism.
Ground state energy and structure of physisorbed monolayers of linear molecules
Authors: Bruch L. W.
Published: February 2003

Model calculations of the ground state configuration and energy of monolayers of CO2 and of N2 adsorbed on the basal plane surface of graphite are reported. The interaction models consist of atom–atom and site–site potentials and point quadrupole moments, as in models for the three-dimensional bulk solids, supplemented by effects of static and dynamic screening of multipole fields by surface charges on the graphite. There is a good quantitative agreement with estimated zero temperature heats of adsorption for N2/Gr and the calculated ground state configuration is the √3 R 30° registered monolayer solid. For CO2/Gr, the calculated low coverage isosteric heat differs by 15% from the experimental value, but the calculated lateral energy of the monolayer solid is consistent with available experimental information. Within the uncertainty caused by this discrepancy, the calculations are consistent with a suggestion that CO2/Gr is a nonwetting system at low temperatures. The ground state configuration of monolayer CO2/Gr is calculated to be an incommensurate solid of number density within 5% of the number density of the √3 R 30° solid.
Infrared studies of molecular ions. I. The ν3 band of NH4+
Authors: Crofton Mark W. , Oka Takeshi
Published: February 2003

Infrared spectrum of the ν3 fundamental vibration-rotation band of the ammonium ion NH4+ has been observed. The ac discharge method recently developed by Saykally and his colleagues was used with a difference frequency laser system as the radiation source. Some 150 lines have been observed and assigned to P,q, and R branches up to J=10. An analysis of the low J transition gave the following molecular constants. ν0−2(Bζ)′ =3342.573 (21) cm−1, B′−(Bζ)′=5.523 53(70) cm−1, B′−B0=−0.049 997(19) cm−1, Zt=−7.59(80)×10−3 cm−1, Ft=−1.6(11)×10−4 cm−1, and Ds=7.2(32)×1p−5 cm−1. A calculated Coriolis constant was used to obtain the ν0 and b2d0 values as ν0=3343.130 (56) cm and b0=5.852 (31) cm−1. These results confirm the expected tetrahedral structure of this molecular ion and give an estimated equilibrium bond length of 1.026(5) Å. (AIP)