bluuebunny/crossref_metadata_2025 huggingface dataset:
Excess electrons in simple fluids. II. Numerical results for the hard sphere solvent
Authors: Nichols Albert L. , Chandler David , Singh Yashwant , Richardson Diane M.
Published: February 2003

The theory for solvated excess electrons described in an earlier paper is now applied numerically. For the simple model of an electron excluded from spherical regions of radius d surrounding each of the particles in a hard sphere fluid, our results exhibit a rich behavior. At all temperatures for which λe ≫σ, where λe is the thermal wavelength of the electron and σ is the diameter of the hard sphere fluid particles, we find a relatively narrow transition region of solvent densities below which the solvated electron is extended and above which the electron is localized. For λe =10σ and d=σ/2, localization occurs near the solvent density 0.15σ−3. Both the width and location of the transition region increase with decreasing λe (i.e., increasing temperature). When λe =6σ, the region is broad, barely discernible, and located near 0.3σ−3 when d=σ/2. For λe ≲5σ, no precipitous behavior is found as density is increased and the electron passes from extended to localized behavior. The localized electron is characterized by a condition of ground state dominance, and this behavior leads to a density dependence of the electron-solvent pair structure that is very different than the dependence found when the electron is delocalized and extended. We estimate the behavior of the friction constant for the electron and find a change in direction of its temperature dependence as one passes from the localization to delocalized case. We discuss how the location of the localization depends upon model parameters and temperature, and we provide qualitative explanations of our results in terms of structural principles associated with the fact that the delocalized electron has a distribution of spatially large fluctuations while the localized electron does not fluctuate over large distances and is characterized by primarily one length scale, its range of self-correlations. The equations central to our theory embody these principles.
Determination of intramolecular bond lengths in gas phase molecules from K shell shape resonances
Authors: Sette F. , Stöhr J. , Hitchcock A. P.
Published: February 2003

A systematic analysis of K shell excitation spectra of gas phase molecules containing B, C, N, O, and F reveals a correlation between the position of a characteristic K shell excitation feature, the σ shape resonance, and the intramolecular bond length. When referenced to the 1s ionization threshold the position of the σ shape resonance is found to vary linearly with the internuclear distance between the pair of atoms which gives rise to the scattering resonance. This linear relationship holds remarkably well within different classes of molecules characterized by the total number of electrons (sum of atomic numbers) for a given pair of atoms. The empirically derived rules are compared with the predictions of a full multiple scattering theory treatment. Comparison of empirical experimental and first-principle theoretical results allows the prediction of the average intramolecular muffin tin potential in the presence of the core hole.
Volume phase transition in a nonionic gel
Authors: Hirokawa Yoshitsugu , Tanaka Toyoichi
Published: February 2003

Nonionic N-isopropylacrylamide gel was found to undergo a discontinuous phase transition by changing a solvent composition or temperature. The observation that polymer gel with and without charge can undergo a first order volume phase transition is an evidence for the universality of the phase transition of polymer gels.
Structure factor and direct correlation function of a fluid from finite range simulation data
Authors: Foiles S. M. , Ashcroft N. W. , Reatto L.
Published: February 2003

We explore a new procedure for extending the simulation results for the radial distribution function g(r) to distances larger than the basic simulation cell. The method is based on the modified hypernetted chain equation. The bridge function in this equation is initially approximated by results for hard sphere fluids and then subsequently modified to bring the computed g(r) into agreement with the available simulation data. The method appears to be successful for densities up to about 90% of the triple-point density. Applications to the Lennard-Jones fluid are presented.
Characterization of exit-channel barriers for chemical reactions producing specific vibrational states
Authors: Steckler Rozeanne , Truhlar Donald G. , Garrett Bruce C. , Blais Normand C. , Walker Robert B.
Published: February 2003

Use of the reaction-path Hamiltonian in natural collision coordinates with WKB eigenvalues for stretching vibrations is tested for predicting state-specific vibrationally adiabatic barriers for the fully three-dimensional reaction F+D2→FD (n′=4)+D. A range of potential energy surfaces, including one specifically designed to have no delayed threshold for the highest energetically allowed product vibrational state, are considered, and both approximate quantal scattering calculations and quasiclassical trajectory calculations are performed for comparison to the adiabatic predictions. We find that the adiabatic predictions correlate well with the quantal scattering results. We also study other features of the dynamics on the surface with no delayed vibrational threshold to see how changing the surface to eliminate the delayed threshold affects other reaction attributes. We find that the approximate quantal differential cross sections for this surface show an appreciable probability for forward scattering of the products. This trend is not exhibited by the quasiclassical trajectory calculations for the same surface.
An ESR study at 4 K of the reaction between H and Ni(CO)4
Authors: Morton J. R. , Preston K. F.
Published: February 2003

A new ESR spectrum has been generated by the photolysis at 4 K of Kr matrices containing HI and Ni(CO)4. Analysis of the nuclear hyperfine structure for 1H, 2H, 61Ni, and 13C leads to its assignment to the radical HNi(CO)3 which has trigonal geometry and a 2A1 ground state in C3v symmetry.
The S1(n,π*) states of cyclopentanone and cyclobutanone in a supersonic nozzle beam
Authors: Baba Masaaki , Hanazaki Ichiro
Published: February 2003

Fluorescence excitation spectra of cyclopentanone and cyclobutanone have been observed for their (n,π*) transition in a pulsed supersonic nozzle beam using a high power tunable laser. A drastic reduction of hot bands has been attained, making it possible to discuss the vibronic assignments in more detail than the previous works. The C=O out-of-plane wagging mode was found to be active for both molecules. The ring twisting and flapping in cyclopentanone and the ring puckering in cyclobutanone were also active. The molecules are pyramidally distorted in the excited states with double minimum potentials in the out-of-plane displacement coordinates. The barrier to inversion (V) and the C=O out-of-plane angle at the potential minimum (θm) have been determined for the S1(n,π*) state; V=680±17 cm−1 and θm=34° for cyclopentanone, and V=1850±50 cm−1 and θm=42° for cyclobutanone. The puckering mode (ν20′) of cyclobutanone was also found to have a double minimum potential with V=16.9 cm−1. The rotational envelope of each vibronic band has been analyzed on the basis of a computer simulation for an asymmetric top molecule. In contrast with formaldehyde, the A-type (parallel) component, as well as the B-type, was shown to be important in these cyclic ketones. The mechanism of the vibronic intensity borrowing is discussed on the basis of the band-type considerations.
Spectroscopy of phenyltetrazine, (phenyltetrazine)2, and phenyltetrazine–Ar as studied in a supersonic free jet
Authors: Park Young D. , Levy Donald H.
Published: February 2003

The 0–0 band of the 1B1←1A1 (π*←n) electronic transition of phenyl-s-tetrazine (C6H5–C2N4H) was observed in a supersonic free jet at 17 617.8 cm−1. The high resolution rotational spectrum of the origin allowed the determination of the structure of phenyltetrazine in the gas. It is planar in both ground and excited electronic states. Weakly bound phenyltetrazine dimers were observed. Structures of two distinct dimers are proposed. Several van der Waals complexes of argon with phenyltetrazine were observed. The phenyltetrazine–Ar (tetrazine) rotational spectrum was analyzed. The distance of the argon from the center of the tetrazine ring is 3.34 Å.
Effect of pressure on proton transfer rate in aqueous solutions: A picosecond study
Authors: Huppert D. H. , Jayaraman A. , Maines R. G. , Steyert D. W. , Rentzepis P. M.
Published: February 2003

Time resolved picosecond fluorescence studies of excited aqueous 8-hydroxy-1,3,6-pyrene trisulfonate (HPTS) have been carried out at pressures up to the ice transition point of H2O and D2O. The proton transfer rates derived from these studies exhibit a large linear increase with pressure from 8×109 s−1 at 1 atm and 294 K to 2.5×1010 s−1 at the liquid→ice VI transition point at 9 kbar and 294 K. In D2O, the deuteron transfer rate also increases linearly with pressure from 2.7×109 s−1 at 1 atm to 1010 s−1 at 8 kbar. The rates between H2O and D2O were found to differ by a factor of 3, which is consistent with the isotope effect kD+/kH+=1/3 which is maintained across the pressure range studied. From these results, an activation volume ΔV‡0 of −6 cc/mol is obtained. These results which show different trends than previous NMR and proton conductance studies are discussed in terms of a microscopic model.
Dynamics of late stage phase separations in two dimensions
Authors: Marqusee J. A.
Published: February 2003

The theory of late stage phase separation in two dimensions is presented. The asymptotic distribution function for droplet sizes and associated temporal power laws are derived for the case where the condensation–evaporation mechanism for growth is dominant. The average radius is shown to grow as t1/3 and the density of droplets decays as t−2/3. The effect of competition between droplets is included and shown to increase the coarsening rate. The distribution function and growth rate broaden and increase, respectively, as the equilibrium area fraction of the minority phase is increased.
Computer simulation of site formation for Na atoms trapped in Ar and Xe solids
Authors: Balling L. C. , Wright J. J.
Published: February 2003

Existing data on Na-rare gas and rare gas–rare gas dimer potentials have been used in a computer program to simulate the formation of trapping sites for Na atoms isolated in Ar and Xe solids. The dimer potentials have also been employed to calculate the matrix perturbed energy-level structure of the Na atoms to obtain theoretical wavelengths for absorption and emission bands corresponding to the sites formed in the simulations. The results support the view that the stable blue-shifted absorption band of Na in Ar and Xe is due to a substitutional site and that the thermally unstable unshifted band results from amorphous sites evolving from two-atom vacancies. In all cases, the simulated sites exhibited a local symmetry sufficient to produce a degeneracy in the perturbed Na 3p state levels.
Electron scattering with methane molecules at 20–500 eV
Authors: Jain Ashok
Published: February 2003

We report quantum mechanical calculations of the differential, momentum transfer, and integral cross sections for electron-CH4 elastic scattering in the energy range 20–500 eV. The e−-CH4 interaction term is composed of three local spherical potentials, namely, the static, the exchange, and the polarization. The static potential is generated without additional approximations from near-Hartree–Fock methane wave functions; the exchange interaction is included in the free-electron-gas-exchange (FEGE) model; and for polarization, we employ a parameter-free ab initio potential of Jain and Thompson. All the cross sections are in good accord with recent absolute experiments.
Nonadiabatic semiclassical scattering. I. Analysis of generalized surface hopping procedures
Authors: Herman Michael F.
Published: February 2003

A formal analysis of generalized, semiclassical surface-hopping approaches to nonadiabatic scattering is undertaken. Expressions are obtained for the transition and reflection amplitudes for intersurface jumps. In problems involving two adiabatic electronic states, it is found that only the component of the heavy particle momentum which is parallel to the nonadiabatic interaction vector η=〈χel1‖∇χel2〉, is adjusted so as to conserve energy in the transition between adiabatic energy surfaces, given certain reasonable assumptions concerning the form of the wave function. This is the first time a prescription for the post-transition momentum has been presented in a less than ad hoc fashion.
The effect of pressure on the adsorption of crystal violet on oriented ZnO crystals
Authors: Clark F. T. , Drickamer H. G.
Published: February 2003

The effects of pressure up to 10 kbar on the shift of the peak locations and equilibrium characteristics of the monomeric isomers and the dimer of crystal violet (CV) dye have been established by comparing, at corresponding pressures, the photocurrent spectra of CV adsorbed to the surface of an oriented ZnO(0001) single crystal electrode with the optical absorption spectra of CV in solution. Dye concentrations range from 2×10−6 to 1.5×10−5 M in the electrolyte. The peaks of the monomeric isomers and the dimer of CV both in solution and adsorbed to the surface exhibit similar red shifts with increasing pressure, as is characteristic of π-π* excitations. In addition, the volume change of the system on isomerization and dimerization of the monomer in solution and the volume change of the system on adsorption of each species of CV to the ZnO surface are calculated. The results are interpreted to indicate the mode of adsorption on the surface.
Thermodynamic equilibrium of a crystalline sphere in a fluid
Authors: Mullins W. W.
Published: February 2003

A treatment is given of the thermodynamic equilibrium of a crystalline sphere immersed in a fluid under the assumptions that the surface free energy and surface stress are isotropic and that the sphere is in a homogeneous state of hydrostatic stress. The treatment extends previous work and includes, for the sphere/fluid system, a thermodynamic treatment of surface stress, a derivation of a Gibbsian adsorption equation, a systematic discussion of the solution of the two surface equations resulting from two independent surface variations, and expressions for the work of nucleation and nucleus (sphere) composition. The most interesting theoretical result is that, for fixed temperature and chemical potentials (uniform throughout the system), the state of matter in the sphere differs, in general, from that of bulk of the same phase. For example, the composition in the sphere is not given, in general, by the parallel tanget construction using bulk free energy curves; the necessary correction is provided. Again, an expression is rederived for the difference between a given chemical potential in the sphere and in bulk crystal, holding fixed the temperature, pressure, and composition of the crystal; the chemical potential difference is traceable to a difference in the defect concentration.
1
B
2u↔1A1g spectroscopy of jet-cooled benzene: Single vibronic level fluorescence studies
Authors: Stephenson Thomas A. , Radloff Patricia L. , Rice Stuart A.
Published: February 2003

As part of an investigation of intramolecular energy transfer in jet-cooled 1B2u benzene, an extensive study of one photon 1B2u↔1A1g spectroscopy has been carried out. Our data lead to an assignment for ν8 in the 1B2u state (1516 cm−1), to a number of new assignments involving activity in ν18 and ν4, and they show that many of the higher vibrational levels in the 1B2u manifold are mixed. The activities of components of combination levels with different vibrational angular momenta have been observed and used to resolve several controversies concerning assignments in the 1B2u↔1A1g spectrum of benzene.
Vibrational state dependence of radiationless processes in 1B2u benzene
Authors: Stephenson Thomas A. , Rice Stuart A.
Published: February 2003

An extensive examination of the vibrational state dependence of fluorescence lifetimes in supersonic jet-cooled 1B2u benzene, and the inferred vibrational state dependence of the nonradiative rate constants, are reported. The qualitative features of our results agree with those obtained from previous investigations using room temperature vapor phase samples. The spectral simplification achieved in the supersonic jet expansion has, however, allowed measurement of the fluorescence lifetimes of a number of 1B2u vibrational levels not previously studied. These data indicate that excitation of ν4, ν5, ν8, and ν10 result in enhanced nonradiative decay, which suggests that these vibrations are promoting modes. A previously unobserved sensitivity of the nonradiative rate constant to the vibrational angular momentum of the initially excited level has been demonstrated. Finally, fluorescence decays which do not have simple exponential form have been observed from vibrational levels in the vicinity of the ‘‘channel three’’ region. We discuss the significance of all of these observations in terms of the existing theory of radiationless processes and with respect to other recent experimental investigations.
The uncoupled O–H stretch in ice VII. The infrared frequency and integrated intensity up to 189 kbar
Authors: Klug D. D. , Whalley E.
Published: February 2003

The integrated absorptivity of the O–H stretching vibration of a dilute solution of HDO in D2O ice VII and the peak frequencies of the O–H stretching, bending, and rotational vibrations of HDO have been measured in diamond anvils in the range 23.6–189 kbar. The integrated absorptivity of the O–H stretching vibration increases from 55 to 153 cm μmol−1 in this range, i.e., by a factor of 2.78, and the effective charge increases from 0.61 to 0.94 electronic charges, i.e., by the factor 1.54. The effective charge of the O–H stretch as a function of pressure can be correlated within the experimental precision by a simple model in which the dipole moment of an ice crystal changes sinusoidally as the protons are moved in phase along the O––O lines. It predicts that the change of dipole moment with oxygen–proton distance, when the oxygen–proton distance is small, is 1.1 D Å−1. The theory could be tested by calculating the dipole moment of a neon atom caused by adding a small dipole moment to the nucleus. The uncoupled O–H stretching frequency changes from 3402 to 3059 cm−1 in the range 23.6–189 kbar. This change is interpreted as an largely as indirect effect of the hydrogen bonding, which appears to increase the O–H distance by 0.183 Å Mbar−1 and so reduces the effective force constant because of the anharmonicity. By combining the frequency of the uncoupled O–H stretch with the frequency of the strongest Raman band reported by others, the sum k12+4k13 of the intramolecular and first-neighbor intermolecular O–H coupling constants was obtained over a range of O––O distances.
A statistical mechanical theory of chemically reacting multiphase mixtures: Application to the detonation properties of PETN
Authors: Ree Francis H.
Published: February 2003

We present a new statistical mechanical theory of multiphase, multicomponent systems. It is based on Ross’s modification of the Mansoori–Canfield–Rasaiah–Stell hard-sphere variational theory and the improved one-fluid van der Waals mixture model. Next, the new theory and exponential-6 potentials that accurately reproduce shock wave data of major detonation-product species are used to compute the detonation properties of PETN (pentaerythritol tetranitrate). The results show satisfactory agreement with the experimental Chapman–Jouguet (CJ) data at initial densities ( ρ0) between 0.25 and 1.77 g/cm3. Small (≂9%) deviations which occur between the experimental and the theoretical CJ pressures at high ρ0 (>1.55 g/cm3) are attributed to time-dependent processes associated with formation of solid carbon which is theoretically present in detonation products. We suggest that some CJ pressure experiments might have finished too soon to experimentally observe late-time reactions involved in formation of solid carbon. However, a more definitive conclusion must await further effort to reduce experimental and theoretical uncertainties in the CJ pressure. We have also compared the theoretical CJ expansion adiabat at ρ0=1.77 g/cm3. The present theory predicts that both the adiabatic gamma −(∂ ln P/∂ ln V)s and the Grüneisen gamma exhibit peaks, along the CJ adiabat, at the density range where carbon atoms in gaseous detonation products start to condense. Comparison of our calculations with other theoretical EOS models is also discussed.
Solvation thermodynamics of nonionic solutes
Authors: Ben-Naim A. , Marcus Y.
Published: February 2003

A generalized process of solvation is defined. It is argued that the thermodynamics of this solvation process is more informative as compared with other processes suggested before. Numerical examples are presented and compared with some recently published related data.
Theory of Onsager phenomenological coefficients for isothermal linear transport processes in electrolyte solutions
Authors: Thacher Thomas S. , Lin Jeong-long , Mou C. Y.
Published: February 2003

The Onsager phenomenological coefficients Lij for linear isothermal transport processes for electrolyte solutions have been calculated from the velocity time-correlation functions. Our approach is based on the assumption that the ionic velocities are evolved by the Fokker–Planck operator. The limiting law expression for Lij has been derived. In addition, numerical calculations using the HNC pair distribution function have been performed for several chloride solutions. The results indicate that our theory can qualitatively describe the experimental values up to a concentration of about 2 M.
Kinetic balance: A partial solution to the problem of variational safety in Dirac calculations
Authors: Stanton Richard E. , Havriliak Stephen
Published: February 2003

Recent publications have suggested that the satisfaction of a principle we call kinetic balance can provide variational safety in Dirac calculations. The theoretical foundation for this proposal is examined, first in simple one-electron problems, and then (less rigorously) in SCF calculations. The conclusion is that finite basis calculations using kinetic balance are safe from catastrophic variational collapse, but that the ‘‘bounds’’ provided by these calculations can be in error by an amount of order 1/c4. The bounds are applicable to total SCF energies, but not to SCF orbital energies. The theory is illustrated by a series of one-electron calculations.
The pressure and temperature dependence of the OH+C2H2 reaction above 800 K
Authors: Smith Gregory P. , Fairchild Paul W. , Crosley David R.
Published: February 2003

The rate constant for the reaction OH+C2H2 has been measured at 900, 1100, and 1300 K. The experimental method was that of laser pyrolysis/laser fluorescence, in which a pulsed CO2 laser heats a mixture of SF6, N2, H2O2, and C2H2. The rate constant is determined from the rate of decay of laser-induced fluorescence signals in OH, formed by pyrolysis of the peroxide and consumed by reaction with the acetylene. At the two higher temperatures the rate constant is independent of pressure between 10 and 120 Torr but at 900 K it was observed to be pressure dependent over a similar range. The rate constant at 1100 K is 2.7±0.6×10−3 cm3 s−1, and rises to 5.8±0.8×10−13 cm3 s−1 at 1300 K. Calculations of the temperature and pressure dependence of the addition channel OH+C2H2+M→C2H2OH+M were made using Troe’s approach, based on flow tube data at a lower temperature. These theoretical calculations are consistent with the present results as well as previous experimental measurements, showing a decrease in the importance of the addition channel with increasing temperature and the onset of a direct route in the region of 1000 K. These considerations of combined pressure and temperature dependence of reaction rates, and changes in mechanism, must be properly taken into account in detailed combustion chemistry models which cover a large range in temperature and pressure.
Atomic and molecular correlation energies with explicitly correlated Gaussian geminals. IV. A simplified treatment of strong orthogonality in MBPT and coupled cluster calculations
Authors: Szalewicz Krzysztof , Zabolitzky John G. , Jeziorski Bogumil , Monkhorst Hendrik J.
Published: February 2003

A simplified strong orthogonality projection has been introduced for the use in the many body perturbation theory (MBPT) or coupled cluster method (CCM) calculations with explicitly correlated geminals. This approximate projection efficiently eliminates the undesired strong orthogonality violating components from the perturbative or CCM pair functions. Moreover it permits dropping the exact strong orthogonality projectors from the pair equations. This results in a dramatic simplification of atomic and molecular calculations with explicitly correlated geminals.
The effect of anharmonic vibration and centrifugal distortion on nuclear shielding in linear triatomic molecules: NNO and CO2
Authors: Jameson Cynthia J. , Osten H.-Jörg
Published: February 2003

The temperature dependence of the mean bond displacement in linear triatomic molecules NNO and CO2 have been determined using the well-established anharmonic force fields for these molecules. The results are applied to explain the temperature dependence of the nuclear shielding observed in the zero-pressure limit for the 15N and 13C nuclei in 15N15NO and 13CO2. By fitting the observed σ0(T)−σ0(300 K) for T=250–350 K, empirical values of (∂σ/∂Δr)e=−220 ppm/Å (CO2), −1030 ppm/Å (NN*O), and −5190 ppm/Å (N*NO) are obtained. These derivatives are discussed in comparison to similar molecular types. The average shielding for the vibrational states (000) and (0110) are calculated for both 15N nuclei in NNO and compared with values obtained from experimental spin-rotational constants. The empirical derivatives are also used to calculate NMR isotope shifts from the mass-dependent 〈Δr〉.
Production and characterization of supersonic carbon cluster beams
Authors: Rohlfing Eric A. , Cox D. M. , Kaldor A.
Published: February 2003

Laser vaporization of a substrate within the throat of a pulsed nozzle is used to generate a supersonic beam of carbon clusters. The neutral cluster beam is probed downstream by UV laser photoionization with time-of-flight mass analysis of the resulting photoions. Using graphite as the substrate, carbon clusters Cn for n=1–190 have been produced having a distinctly bimodal cluster size distribution: (i) Both even and odd clusters for Cn, 1≤n≤30; and (ii) only even clusters C2n, 20≤n≤90. The nature of the bimodal distribution, and the intensity alterations in the observed C+n signals are interpreted on the basis of cluster formation and stability arguments. Ionizing laser power dependences taken at several different photon energies are used to roughly bracket the carbon cluster ionization potentials, and, at high laser intensity, to observe the onset of multiphoton fragmentation. By treating the graphite rod with KOH, a greatly altered carbon cluster distribution with mixed carbon/potassium clusters of formula K2C2n is produced.
Collisions of Xe(n
f) Rydberg atoms with electron attaching targets
Authors: Zollars B. G. , Smith K. A. , Dunning F. B.
Published: February 2003

This paper reports a study of Xe (nf) destruction in collisions with 1,1,1-C2Cl3F3 and C5F8. The primary Rydberg atom destruction process is collisional ionization that results in the formation of a positive and a negative ion. Rate constants for this reaction have been measured for Xe (nf) over the range 26 ≤n≤43, and the major negative ion products identified. Cross sections for Rydberg electron attachment derived from these and earlier measurements are in good agreement with recent free electron attachment cross sections obtained using swarm techniques. This agreement further confirms that the study of Rydberg atom collisions can provide information on very-low-energy electron interactions.
Study of higher excited singlet states of zinc(II)-tetraphenylporphin
Authors: Tobita Seiji , Kaizu Youkoh , Kobayashi Hiroshi , Tanaka Ikuzo
Published: February 2003

The mechanism for the production of the S2 state fluorescence following the excitation to the S1 states of zinc (II)-tetraphenylporphin has been investigated by using two-photon absorption and optical–optical double resonance techniques. The main process populating the S2 state is found to be a stepwise two-photon absorption through the S1 state (the Q state) even by the excitation of relatively low power density (<5×1015 photons cm−2 pulse−1) of a nitrogen pumped dye laser. The absorption cross sections σ1n of the Sn ← S1 transition are determined from the kinetic analyses based on the two-photon absorption measurement. The Sn ← S1 absorption spectrum obtained is structureless and its σ1n values vary from 4.74×10−16 to 1.02×10−15 cm2 molecule−1 in the wavelength region of 490–560 nm. This observation suggests that the corresponding Sn ← S1 transition possesses a strongly allowed nature and there exists a higher excited singlet state of gerade symmetry in the 17 800–20 400 cm−1 region above the ground state. The result of MO calculation taking into account doubly excited configurations and higher singly excited configurations actually indicates the presence of a strongly two-photon allowed 1A2g state in this energy region.
UV photodissociation of C2F5Br, C2F5I, and 1,2-C2F4BrI
Authors: Krajnovich D. , Butler L. J. , Lee Yuan T.
Published: February 2003

The photodissociation reactions of C2F5Br, C2F5I, and 1,2-C2F4BrI have been studied at 248 and 193 nm using the crossed laser-molecular beam technique. Photodissociation of 1,2-C2F4BrI was also studied at 266 nm. We find that: (i) C2F5Br undergoes C–Br bond fission at 193 nm (but does not absorb at 248 nm); (ii) C2F5I undergoes C–I bond fission at 248 nm (but does not absorb at 193 nm); (iii) 1,2-C2F4BrI undergoes only C–I bond fission at 248 and 266 nm, but at 193 nm both C–I and C–Br bond fission are observed, with a C–I:C–Br fission ratio of approximately 1.7:1. Center-of-mass recoil energy and angular distributions are determined for each of these photodissociation reactions. These results combined with those of other workers, are used to test simple predictions based on molecular orbital theory. The 266 nm data for C2F4BrI provide an approximate value of 19.3±3 kcal/mol for the dissociation energy of C2F4Br to C2F4+Br and also show that all or almost all the iodine produced in the primary C–I bond fission is in the excited 2P1/2 state. The prospect of doing bond-selective photochemistry in the ultraviolet is discussed.
A spherical potential for hydrogen from solid state and scattering data
Authors: Norman M. J. , Watts R. O. , Buck U.
Published: February 2003

Calculations of the ground state energy and pressure of solid hydrogen, and of the total differential scattering cross section for D2+H2, are used to assess existing isotropic potentials for the hydrogen pair interaction. Although several of the models give consistent results in the region of the potential minimum, they differ significantly at short distances. A comparison with solid state and scattering data indicates that ab initio calculations are in error and that there is a lack of consistency between this data and second virial coefficient measurements. The results are used to recommend a potential surface that is consistent with the experimental data.
Characterization of surface complexes in enhanced Raman scattering
Authors: Roy D. , Furtak T. E.
Published: February 2003

An indicator molecule, para-nitrosodimethylanaline (p-NDMA), has been used to study the chemical nature of surface complexes involving the active site for SERS in the electrochemical environment. We present evidence for positively charged Ag atoms stabilized by coadsorbed Cl− ions as the primary sites which are produced during the oxidation reduction cycle treatment of an Ag electrode. Depending on the relative number of Cl− ions which influence the Ag site the active site demonstrates a greater or lesser electron accepting character toward p-NDMA. This character is influenced by the applied voltage and by the presence of Tl+ ions in the bulk of the solution near the surface. As in previously studied systems p-NDMA/Cl−/Ag complexes demonstrate charge transfer excitation which in this case is from the p-NDMA to the Ag site. These results further solidify the importance of complex formation in electrochemical SERS and suggest that caution should be applied when using SERS as a quantitative measure of surface coverage.
An accurate and efficient scheme for propagating the time dependent Schrödinger equation
Authors: Tal-Ezer H. , Kosloff R.
Published: February 2003

A new propagation scheme for the time dependent Schrödinger equation is based on a Chebychev polynomial expansion of the evolution operator Û=exp(−iĤt). Combined with the Fourier method for calculating the Hamiltonian operation the scheme is not only extremely accurate but is up to six times more efficient than the presently used second order differencing propagation scheme.
Infinite-order sudden calculations of pressure broadening cross sections for noble gas–oxygen binary mixtures
Authors: Corey Gregory C. , McCourt Frederick R.
Published: February 2003

A detailed computational study is presented of both the self-broadening and foreign gas broadening of the magnetic dipole and rotational Raman lines of O2. The calculations are based on a recent extension of the quantum mechanical line shape function theory to paramagnetic molecules. Quantal infinite-order sudden calculations are compared with the results of classical and semiclassical trajectory calculations and with experimental measurements where available. For the O2–He system, a number of different anisotropic potential energy surfaces have been tested and a recent surface determined from molecular beam scattering data has been found to give by far the best overall agreement with the line broadening data.
Spectral diffusion of a photochemical proton transfer system in an amorphous organic host: Quinizarin in alcohol glass
Authors: Breinl W. , Friedrich J. , Haarer D.
Published: February 2003

Experimental data describing the time evolution of photochemical holes in organic glasses are reported. The photochemical system is 1,4-dihydroxyanthraquinone (quinizarin) in ethanol/methanol glasses; its photochemistry is based on proton or deuteron transfer processes. The experiments show a logarithmic increase of the hole widths in a time domain between minutes and about 104 min. The experimental results yield a pronounced deuteration effect and little variation with temperature between 1.35 and 4.2 K. The data are interpreted in a semiquantitative way using current theories of spectral diffusion in amorphous solids. The fastest measured photochemical rates are on the order of seconds, leaving a ‘‘time independent’’ linewidth of about 0.4 cm−1 at 1.35 K.
Alignment-free sequence comparison—a review
Authors: Vinga Susana , Almeida Jonas
Published: February 2003

Motivation: Genetic recombination and, in particular, genetic shuffling are at odds with sequence comparison by alignment, which assumes conservation of contiguity between homologous segments. A variety of theoretical foundations are being used to derive alignment-free methods that overcome this limitation. The formulation of alternative metrics for dissimilarity between sequences and their algorithmic implementations are reviewed.

Results: The overwhelming majority of work on alignment-free sequence has taken place in the past two decades, with most reports published in the past 5 years. Two main categories of methods have been proposed—methods based on word (oligomer) frequency, and methods that do not require resolving the sequence with fixed word length segments. The first category is based on the statistics of word frequency, on the distances defined in a Cartesian space defined by the frequency vectors, and on the information content of frequency distribution. The second category includes the use of Kolmogorov complexity and Chaos Theory. Despite their low visibility, alignment-free metrics are in fact already widely used as pre-selection filters for alignment-based querying of large applications. Recent work is furthering their usage as a scale-independent methodology that is capable of recognizing homology when loss of contiguity is beyond the possibility of alignment.

Availability: Most of the alignment-free algorithms reviewed were implemented in MATLAB code and are available at http://bioinformatics.musc.edu/resources.html

Contact: almeidaj@musc.edu; svinga@itqb.unl.pt

* To whom correspondence should be addressed.
An information theoretic approach for analyzing temporal patterns of gene
expression
Authors: Kasturi Jyotsna , Acharya Raj , Ramanathan Murali
Published: February 2003

Motivation: Arrays allow measurements of the expression levels of thousands of mRNAs to be made simultaneously. The resulting data sets are information rich but require extensive mining to enhance their usefulness. Information theoretic methods are capable of assessing similarities and dissimilarities between data distributions and may be suited to the analysis of gene expression experiments. The purpose of this study was to investigate information theoretic data mining approaches to discover temporal patterns of gene expression from array-derived gene expression data.

Results: The Kullback–Leibler divergence, an information-theoretic distance that measures the relative dissimilarity between two data distribution profiles, was used in conjunction with an unsupervised self-organizing map algorithm. Two published, array-derived gene expression data sets were analyzed. The patterns obtained with the KL clustering method were found to be superior to those obtained with the hierarchical clustering algorithm using the Pearson correlation distance measure. The biological significance of the results was also examined.

Availability: Software code is available by request from the authors. All programs were written in ANSI C and Matlab (Mathworks Inc., Natick, MA).

Contact: jkasturi@cse.psu.eduacharya@cse.psu.edu

* To whom correspondence should be addressed.
Comparisons and validation of statistical clustering techniques for
microarray gene expression data
Authors: Datta Susmita , Datta Somnath
Published: February 2003

Motivation: With the advent of microarray chip technology, large data sets are emerging containing the simultaneous expression levels of thousands of genes at various time points during a biological process. Biologists are attempting to group genes based on the temporal pattern of their expression levels. While the use of hierarchical clustering (UPGMA) with correlation‘ distance’ has been the most common in the microarray studies, there are many more choices of clustering algorithms in pattern recognition and statistics literature. At the moment there do not seem to be any clear-cut guidelines regarding the choice of a clustering algorithm to be used for grouping genes based on their expression profiles.

Results: In this paper, we consider six clustering algorithms (of various flavors!) and evaluate their performances on a well-known publicly available microarray data set on sporulation of budding yeast and on two simulated data sets. Among other things, we formulate three reasonable validation strategies that can be used with any clustering algorithm when temporal observations or replications are present. We evaluate each of these six clustering methods with these validation measures. While the ‘best’ method is dependent on the exact validation strategy and the number of clusters to be used, overall Diana appears to be a solid performer. Interestingly, the performance of correlation-based hierarchical clustering and model-based clustering (another method that has been advocated by a number of researchers) appear to be on opposite extremes, depending on what validation measure one employs. Next it is shown that the group means produced by Diana are the closest and those produced by UPGMA are the farthest from a model profile based on a set of hand-picked genes.

Availability: S+ codes for the partial least squares based clustering are available from the authors upon request. All other clustering methods considered have S+ implementation in the library MASS. S+ codes for calculating the validation measures are available from the authors upon request. The sporulation data set is publicly available at http://cmgm.stanford.edu/pbrown/sporulation

Supplementary information: http://www.mathstat.gsu.edu/~matsnd/clustering/supp.htm

Contact: sdatta@mathstat.gsu.edu

* To whom correspondence should be addressed.
Export market information use, organizational knowledge, and firm performance
Authors: Toften Kjell , Ottar Olsen Svein
Published: February 2003

Building on prior research in organizational knowledge, learning, and memory, this paper suggests that export market knowledge may provide a deeper understanding of the relationships between export market information use and export performance. Specifically, a conceptual model is developed linking different dimensions of information use to different dimensions of organizational knowledge as well as to export performance. This is then used to generate research propositions that provide insights into how export market knowledge integrates with export market information use and affects export performance.
Affective influences on risk perceptions and risk intention
Authors: Williams Steve , Zainuba Mohamed , Jackson Robert
Published: February 2003

The effects of affect, as measured by the dispositional traits of positive affectivity (PA) and negative affectivity (NA), on risk perceptions and risk intentions were tested among 149 managers from a variety of industries and companies. A risk‐assessment instrument consisting of systematically varied business scenarios was used to measure managerial perceptions of the risk dimensions of uncertainty, gains/losses, framing and personal involvement. Findings suggest that positive affect (high PA) managers viewed risk‐related uncertainty and personal involvement more optimistically, but were not more willing to seek risk. Negative affect (high NA) decision makers perceived risk‐related gains more negatively and were also significantly more likely to avoid risk.
Situational antecedents and outcomes of organizational politics perceptions
Authors: Poon June M.L.
Published: February 2003

A model of perceptions of organizational politics was developed and tested using a sample of 208 Malaysian employees from diverse occupations and organizations. Results of a path analysis on the survey data showed that job ambiguity, scarcity of resources, and trust climate were significant predictors of perceptions of organizational politics. Perceptions of organizational politics, in turn, mediated the effects of these situational antecedents on job stress, job satisfaction, and turnover intention. Specifically, employees who perceived a high level of politics in their workplace reported higher levels of stress, lower levels of job satisfaction, and higher levels of intention to quit than did employees who perceived a low level of politics. Implications of the findings and suggestions for future research are discussed.
Canadian e‐commerce
Authors: Michalak Wieslaw , Jones Ken
Published: February 2003

Examines the importance of e‐commerce in the retail economy. The analysis is placed in the broader context of economic and social change and provides a theoretical framework for evaluating the growth of e‐commerce as a major distribution channel. By exploring the results of the recently released “Household Internet survey of Canadian households”, the paper documents the actual shopping behaviour of Internet shoppers from a variety of perspectives. The Canadian experience should provide analysis with a series of benchmarks and trends on which to evaluate the adoption of e‐commerce in other markets.
Value effects of corporate consolidation in European retailing
Authors: Dragun Dmitry , Howard Elizabeth
Published: February 2003

A significant number of studies have been performed to investigate generic effects of corporate consolidations on shareholder value. Sector‐focused scrutiny has been lacking, however. European retailing has unique features that warrant investigation of consolidation effects, yet such studies are scarce or inadequate. This article seeks to fill the gap by reporting the findings of a study that ascertains the value impact of corporate consolidations in the context of European retailing. The method used in the study is analysis of the abnormal returns realised by shareholders in consolidating transactions, supplemented by examination of value‐sales and price‐earnings multiples. Its finding that retail consolidations fail to create shareholder wealth, in the short term at least, challenges retail executives to consider other ways to deliver value.
International expansion and strategies of discount grocery retailers: the winning models
Authors: Colla Enrico
Published: February 2003

Illustrates the reasons that have determined the success or failure of the discount retailers on the international scene, through the identification and analysis of the strategic groups, competitive advantages, entry barriers to the sector and barriers to mobility between the groups. Three strategic groups of discount food retailers that have adopted different internationalisation strategies have been defined and a series of key success factors of the different strategic groups has been identified. The winners in the race to international expansion are in particular the leaders of the first group of German hard discount retailers. But important niches in several foreign markets can also enable growth of retailers in the second group, the German soft discount retailers specialised abroad, and of the third group, the French soft discount retailers diversified abroad.
Effects of aging on epoxy‐based rapid tooling materials
Authors: Ottemer Xavier , Colton Jonathan S.
Published: February 2003

This study shows the effects of aging on epoxy‐based rapid tooling materials. Two epoxy‐acrylate resins used for stereolithography and one aluminum powder‐filled epoxy used for the high‐speed machining of plastic injection molds were selected to investigate their aging behavior in different environments. Four different conditions, each characterized by a specific relative humidity, were used to perform a seven week long aging study. Temperature was kept a constant at 30°C. Both the stereolithographic resins showed a drop in mechanical properties and in their glass transition temperatures in wet environments, whereas aging time showed little influence on these properties after an initial induction phase. Moisture uptake is mainly responsible for this drop and the coefficients of water diffusion were determined in this process. On the other hand, the aluminum powder‐filled epoxy showed little humidity‐dependent drop in mechanical properties and in its glass transition temperature during aging. A simple empirical model that only takes into account the moisture uptake was developed.
Adaptive slicing with cubic patch approximation
Authors: Kumar Madhup , Roy Choudhury A.
Published: February 2003

In adaptive slicing, the number of layers is drastically reduced by using sloping layer walls. For both vertical (2.5D slices) and sloping (ruled slices) outer walls, the strategies for determining slice height generally consider a number of vertical sections along the contour of a slice. Surface deviation error is calculated at these sections and slice height subsequently determined. Instead, a method is proposed which calculates error at every part of the surface. This method approximates the outer wall between two successive contours by a series of taut cubic spline patches. It is proposed that the deviation between such a patch and the actual surface is a better and more exhaustive estimate of surface error. Results show that the predicted number of slices is slightly higher than that predicted by existing methods for sloping layer walls.