bluuebunny/crossref_metadata_2025 huggingface dataset:
A photoion–photoelectron coincidence study of Kr and Xe dimers
Authors: Norwood K. , Luo G. , Ng C. Y.
Published: July 2002

The photoion–photoelectron coincidence (PIPECO) spectra for Kr+2 and Xe+2 in the wavelength regions of 825–970 and 900–1030 Å, respectively, have been measured at different nozzle temperatures and stagnation pressures (P0). The ionization energies (IE) for Kr2 and Xe2 to Kr+2[I(1/2)u] and Xe+2[I(1/2)u] determined by the PIPECO spectra are in excellent agreement with the results of previous photoionization experiments. The PIPECO measurements for Kr+2 and Xe+2 also provide lower limits for the IEs of Kr2 and Xe2 to Kr+2[II(1/2)u] and Xe+2[II(1/2)u]. The PIPECO spectra for Kr+2 and Xe+2 display strong autoionization structures similar to those resolved in the corresponding photoionization efficiency spectra, indicating that a significant fraction of autoionizing electrons are slow electrons with near zero kinetic energies. The extreme weakness of the Kr+2[II(1/2)u] and

Xe+2[II(1/2)u] PIPECO bands observed at low P0 support the conclusion that excited Kr+2[II(1/2)u] and Xe+2[II(1/2)u] ions are dissociative with dissociation lifetimes shorter than 68 and 85 μs, respectively. These values are in accord with the calculated radiative lifetimes for the II(1/2)u→I(1/2)g transitions in Kr+2 and Xe+2. The intensities for the II(1/2)u PIPECO bands relative to the I(1/2)u and I(3/2)g PIPECO bands for Kr+2 and Xe+2 are found to increase as P0 is increased, an observation attributed to the formation of Kr+2 and Xe+2 from fragmentation of excited Kr+n and Xe+n ions produced in the photoionization of Krn and Xen, n≥3. The fragmentation of excited Kr+n and Xe+n , n≥3, ions have the effect of lowering the appearance energies for the Kr+2[II(1/2)u] and Xe+2[II(1/2)u] PIPECO bands.
Mixed metal–silicon clusters formed by chemical reaction in a supersonic molecular beam: Implications for reactions at the metal/silicon interface
Authors: Beck Steven M.
Published: July 2002

We report observation of a reaction between a metal atom and silicon in a supersonic jet to form metal atom–silicon clusters. Using the technique of laser vaporization supersonic expansion with metal carbonyl seeded carrier gas, clusters of the form MSin have been detected by ArF and F2 laser photoionization time-of-flight mass spectrometry. Three group-VIB transition metals and copper have been investigated. The dominant product cluster peaks observed in the mass spectra obtained for all three group VIB metals corresponds to identical but remarkable cluster stoichiometries. The dominant product peaks have formulas given by MSin where n=16. Copper results are different than the other three metals, indicating the importance of the metal valence electronic structure to the chemistry. The metal–semiconductor clusters are relatively more stable towards photofragmentation than the bare silicon cluster of the same size. The observation of these new species may be relevant to reactions which occur at the interface between a silicon wafer and deposited metals.
Statistical fluctuations of decay rates
Authors: Zimmermann Th. , Köppel H. , Cederbaum L. S.
Published: July 2002

We introduce a general Hamiltonian describing two coupled subsystems, each having a finite zero-order decay probability. With use of simple statistical assumptions on the underlying states, we derive new probability distributions of individual decay rates. We analyze the cases of weak and intermediate to strong coupling, respectively. The resulting distributions often resemble a suitable χ2 distribution, but do not belong to that class of functions. An interpretation of decay rates in terms of a χ2 model thus may lead to wrong conclusions. As a concrete realization, we study a Hamiltonian describing the non-Born–Oppenheimer coupling of two electronic states via the nuclear motion. The model is applied to the calculation of absorption-type spectra of NO2 and C2H+4. We investigate statistical properties of energy levels, line intensities, and decay rates. For NO2, we find from all statistics a completely irregular behavior, consistent with random matrix predictions and demonstrating the strong mixing of zero-order states due to the nonadiabatic coupling. For C2H+4, all statistics exhibit characteristic deviations from the irregular limit that can be given a consistent interpretation.
Dynamical bottleneck in the reaction H+O2→OH+O at high collision energies
Authors: Kleinermanns K. , Linnebach E. , Pohl M.
Published: July 2002

The nascent rotational and λ-doublet state distributions of OH produced in the reaction H+O2→OH(N,v,f )+O were probed by fast atom–laser induced fluorescence experiments at average collision energies E=100, 183, 200, 220, and 243 kJ/mol. With increasing E, the rotational product distributions become increasingly nonstatistical with a narrow peak at high rotational states, in good agreement with quasiclassical trajectory calculations on an ab initio potential energy surface. The calculations show the narrow product rotational peak to be due to an increasing specifity of the initial H–O2 configurations leading to reactions. At high E the impact parameters and initial polar angles are confined to a rather small range allowing reaction only for an optimal approach. The OH λ-doublet distributions show preference for the Π(A′) component probed by R lines at all collision energies investigated. This suggests a planar reaction path and little importance of out of plane rotation of the HO2 complex at the high collision energies of the experiment. The experimental λ-doublet distributions are quantitatively compared with the degree of in-plane scattering obtained from the trajectory calculations.
Solvent effects on proton-transfer reactions
Authors: Cukier R. I. , Morillo M.
Published: July 2002

We present a theory of solvent effects on the rate of intramolecular proton-transfer (IPT) reactions. The proton tunnels between two vibrational levels of a double minimum potential. The proton’s coupling to the solvent is modeled with an oscillator bath, appropriate to reactions where a charge interacts with many solvent molecules. The rate is evaluated by use of the Golden Rule; the perturbation is the level splitting. The IPT rate constant has several limiting expressions, one of which has an activated form. The activation energy is related to the medium reorganization energy, and provides a mechanism to slow the IPT reaction. Since reorganization energies are small in nonpolar and large in polar solvents, the rate is expected to be smaller in the latter class of solvents. Isotopic substitution is predicted to only affect the prefactor of the rate expression. Another regime is obtained for smaller reorganization energies where the solvent dynamics, as described by a dielectric relaxation time, are important. Comparison is made with recent experimental studies of IPT in solution.
Electron densities in momentum and position spaces. I. Mapping relation and density functional aspect
Authors: Kryachko Eugene S. , Koga Toshikatsu
Published: July 2002

Based on the theory of point transformations, a possible exact relationship between the atomic electron densities in momentum and position spaces is studied through the inverse mapping from the electron density to the N-electron wave function with an appropriate generator or reference N-electron wave function. The results are used to construct the ground-state energy density functional of atoms in momentum space.
Optical and near-infrared spectroscopy of neutral indium phosphide clusters
Authors: Kolenbrander K. D. , Mandich M. L.
Published: July 2002

Spectra are obtained for a wide distribution of sizes and stoichiometries of indium phosphide clusters using photodissociation techniques. These spectra are presented at two different cluster temperatures throughout the spectral range of 0.65 eV (1900 nm) to 2.0 eV (640 nm) for indium phosphide clusters containing from 5 to 14 atoms. Though the spectral behaviors of the clusters are found to be qualitatively similar, significant quantitative differences exist. A Rice–Ramsberger–Kassel–Marcus (RRKM) dissociation model employing significant fitting constraints is found to accurately describe the observed dissociation behaviors at both temperatures. Dissociation energies ranging from 2.0 to 2.4 eV are determined for the In5Py–In9Py clusters using the RRKM model. Experimental evidence suggests that the dissociation process involves loss of a small phosphorus containing moiety such as P1–2 or InP1–2. Results of the RRKM fitting process also clearly indicate that most of these InxPy clusters have nearly uniform absorption cross sections from 0.65 to 2.0 eV. This range extends well below the band gap of bulk InP. This is the first clear evidence of the presence of electronic states in closed shell semiconductor clusters at energies well below the band gap of the bulk material. Select clusters show significant deviations from pure RRKM behavior. Their behavior is consistent with the presence of spectral structure in their absorption cross sections in the energy range of 0.65–2.0 eV.
A spectroscopic characterization of the ã 3Π−, A 3Π+, and E 3∑+ states of the MgAr van der Waals molecule
Authors: Bennett Robert R. , McCaffrey John G. , Breckenridge W. H.
Published: July 2002

The metastable ã(3Π0−) and A(3Π0+) states of the Mg(3s3p)Ar molecule were synthesized in a free-jet expansion of metastable Mg(3PJ) atoms, produced by the laser vaporization of a magnesium rod, in pure Ar. The ã(3Π0−), A(3Π0+), and E(3∑+) states were characterized spectroscopically by laser-induced fluorescence. The 3Π(v=0) states were found to be very near the Hund’s case (a) limit, with only 0.31 cm−1 separating the 3Π0+ and 3Π0− levels. The internuclear separations, re, were found to be 3.63 Å and the dissociation energies 316 and 296 cm−1, respectively. The E(3∑+) state is quite deeply bound (1128 cm−1) with a much smaller re, 2.83 Å, and it is postulated that the Ar atom has substantially penetrated the outer lobe of the 4s Rydberg orbital of the Mg(3s4s 3S1) atomic state to which this state correlates. Despite large populations of both Mg(3P1) and Mg(3P2), neither the 3Π1 nor 3Π2 states of MgAr were observed, apparently because the molecular 3Π1,2 states are collisionally quenched to the lower-energy 3Π0 state much faster than the corresponding atomic 3P1,2 multiplets are deactivated to the 3P0 level.
Entropy production in a chemical system involving an autocatalytic reaction in an isothermal, continuous stirred tank reactor
Authors: Yoshida Nobuo
Published: July 2002

The rate of entropy production due to chemical reaction is calculated for various combinations of parameter values in the cubic autocatalator model in an isothermal, continuous stirred tank reactor (CSTR) proposed by Gray and Scott and by Escher and Ross. Values of the entropy production averaged over periods of limit cycle oscillations are compared with those in coexistent unstable stationary states. It is found that in ranges of the residence time over which there are limit cycles, the entropy production in coexisting stationary states increases as the residence time is shortened, i.e., as the system is removed farther from thermodynamic equilibrium. The average entropy production over a limit cycle is less than that in the corresponding stationary state over wide ranges of parameter values, but not necessarily for the whole oscillatory region. More specifically, the former inequality always prevails in ranges where the entropy production of stationary states is larger, i.e., the residence time is shorter, but in some cases the inequality is reversed in ranges of lower magnitudes of the entropy production.
Rotationally inelastic gas–surface scattering: HCl from Au(111)
Authors: Lykke Keith R. , Kay Bruce D.
Published: July 2002

A quantum-resolved molecular beam–surface scattering study of HCl scattered from Au(111) is described. The HCl is detected in a quantum-resolved manner via (2+1) resonant enhanced multiphoton ionization (REMPI). Greater than 85% of the incident HCl molecules are in a single-quantum state (v=0, J=0) with a narrow velocity distribution (Δυ/υ<0.10). The scattered HCl is strongly peaked about the specular angle, and both its final velocity and rotational distributions are indicative of direct inelastic scattering. The scattered rotational distributions exhibit features characteristic of rotational rainbows and have a mean rotational energy that displays a bilinear dependence upon the incident normal kinetic energy and surface temperature. The final velocity distributions are largely insensitive to the rotational level and indicate that the energy loss to phonons is small (<20%). Analysis of the scattered data indicates an orientation-averaged attractive well depth of ∼5 kcal/mol for the HCl–Au(111) interaction.
Photodissociation and metastable decay of solvated cluster ions
Authors: Syage Jack A.
Published: July 2002

The stepwise solvation of dissociative ions was studied using a new technique in which a time-of-flight mass spectrometer is operated as a tunable low mass rejection filter. Two studies reflecting different modes of operation are reported. (1) Electronically excited aniline cation C6H5NH+2 was produced by picosecond photoionization and the rate of metastable dissociation to C5H6++HNC was investigated as a function stepwise solvation by NH3. The addition of just one or two NH3 solvent molecules was found to significantly reduce the rate of dissociation relative to the bare ion at comparable ion energies. (2) The resonant photodissociation of (CH3I)+n cluster ions, formed by electron impact (EI) ionization, was investigated in a crossed electron-laser-molecular beam configuration. No evidence for C–I dissociation, which is prominent in the bare ion, was observed. Instead van der Waals dissociation occurred by one- and two-photon mechanisms that varied depending on cluster size.
A new water potential including polarization: Application to gas-phase, liquid, and crystal properties of water
Authors: Cieplak Piotr , Kollman Peter , Lybrand Terry
Published: July 2002

We present a new set of potential function parameters for simulations involving water. This function has been used to calculate the energy and structure of water dimer, the second virial coefficient of gaseous water, the energy and density of liquid water, and the structural parameters and energy of sublimation of ice Ih and VII. Most of the results are in good agreement with experiments, but the second peak in the O–O radial distribution function of the liquid and the density of ice are less well represented. Nonetheless, the calculated H-bond energy distribution in the liquid has the properties found in other simulations which reproduce the first minimum and second peak in the radial distribution function, suggesting that the H bonding network found in the function is reasonable.
Lattice model of self-assembly
Authors: Dawson K. A. , Kurtović Z.
Published: July 2002

A microscopic lattice model of two-component mixtures of water and amphiphile is introduced. The model is defined on a simple cubic lattice, the vertices of which are occupied by Ising and Heisenberg degrees of freedom which model water- and amphiphile-filled cubes. The orientations of the lipid molecules are described by the directions of the Heisenberg spin–vectors. Microscopic interactions between two water-filled cubes, two lipid-filled cubes, or a water- and a lipid-filled cube are related to the coupling constants of the model. In this paper we discuss the zero-temperature states of the model and, for some parts of the parameter space, these are explicitly constructed from local octahedral clusters of the cubic lattice. We then introduce a mean-field theory that preserves the full symmetry of the Hamiltonian, and thereby construct the global phase diagrams for different choices of the coupling parameters. In the conclusions to our paper we comment on other theoretical methods of solution being applied to the present Hamiltonian.
On the infrared spectroscopy of SiF4 and SF6 in Ar clusters: Location of the solute
Authors: Gu X. J. , Levandier D. J. , Zhang B. , Scoles G. , Zhuang D.
Published: July 2002

We have measured the infrared photodissociation spectra of argon clusters containing SiF4, as a function of the cluster size n (for n≤ 103) using molecular beam laser spectroscopy. The clusters were produced by both the conventional seeded expansion of a dilute mixture and by a ‘‘pickup’’ method where, upon colliding with it, the chromophore sticks to the surface of a cluster made in a neat solvent expansion. Furthermore, the spectra of small SF6/Arn clusters (n≤50) have been remeasured with the improved resolution resulting from the use of two line and tunable isotopic CO2 lasers. These data, together with previously published data on SF6/Ar, indicate a remarkably different behavior for these two solute–solvent combinations. The preferred ‘‘site’’ for SiF4 is at the surface of Ar clusters of all sizes, regardless of how the molecule is introduced to the cluster, while appreciable amounts of SF6 reside at the surface only when the cluster is large and the impurity is deposited onto the cluster surface. The behavior of SiF4 and SF6, together with the analogous behavior of other polyatomic chromophores, the IR spectra of which have been measured and reported previously [D. J. Levandier, M. Mengel, R. Pursel, J. McCombie, and G. Scoles, Z. Phys. D 10, 337 (1988); D. J. Levandier, S. Goyal, J. McCombie, B. Pate, and G. Scoles, J. Chem. Soc. Faraday Trans. 86, 2361 (1990)], can be rationalized in terms of molecular dynamics simulations of similar systems which are presented in the paper by Perera and Amar [L. Perera and F. G. Amar, J. Chem. Phys. 93, 4884 (1990)]. The combination of the theoretical and experimental results confirm the usefulness of infrared photodissociation spectroscopy for the study of the structure of clusters and suggest that assuming any particular location for an impurity in a cluster in the absence of experimental evidence or, at least, a dynamics calculation, can easily lead to wrong conclusions.
Hylleraas six-term wave function: Correction
Authors: Koga Toshikatsu
Published: July 2002

The Hylleraas six-term wave function, Ψ6=exp(−ζs)(1+c1u+c2t2+c3s+c4s2 +c5u2), is known as a typical function which shows the importance of the electron correlation for two-electron atoms. We report that the wave function associates an energy much lower than the value given in the literature. In the case of helium atom, for example, we have obtained −2.903 329 354 au, while the literature states −2.903 24 au. The accurate parameters are given for atoms from H− through Ne8+.
A theoretical analysis of nuclear magnetic resonance experiments on proton transfer in benzoic acid crystals
Authors: Cukier R. I. , Morillo M.
Published: July 2002

A theory of proton transfer reactions oriented toward transfer in solids is presented. This theory is used to interpret the experimental data of Nagaoka et al. [S. Nagaoka, T. Terao, F. Imashiro, A. Saika, and N. Hirota, J. Chem. Phys. 79, 4694 (1983)], who measured proton transfer rates in dicarboxylic acids by the use of NMR T1 measurements. The experiments, carried out over a large range of temperature, imply that the proton transfer rate is activated at high temperature and falls to a temperature-independent rate at low temperature. It is assumed that the proton motion occurs by tunneling between equivalent sites and is modulated by coupling to the acoustic phonons of the solid. The rate of transition is evaluated by the ‘‘golden rule,’’ the perturbation being the splitting between the localized states. It is shown that for the potential surfaces of interest here, another pair of localized states exist in the potential well and, for the higher temperatures of the experiment, excitation to this higher tunneling pair dominates the observed activation energy. Since the acoustic coupling model leads to a very small reorganization energy for the transfer, the medium’s nuclear degrees of freedom are tunneling even at high temperature. Therefore, the rate of tunneling between either pair of levels is found to be only very weakly temperature dependent.
Photoionization studies of transition metal clusters: Ionization potentials for Fen and Con
Authors: Yang Shihe , Knickelbein Mark B.
Published: July 2002

The photoionization efficiency (PIE) spectra for cold iron clusters (Fe6–Fe90) and cobalt clusters (Co3, Co7–Co92) have been measured using a tunable ultraviolet laser combined with time-of-flight mass spectrometry. Vertical ionization potentials (IPs) assigned from the observed photoionization thresholds decrease rapidly but nonmonotonically for both Fen and Con up to n≅20, beyond which IPs evolve more slowly and smoothly. The measured IPs for both iron and cobalt clusters are lower than those predicted by the conducting spherical droplet model. The evolution of IP with increasing cluster radius R is observed to occur more gradually than the R−1dependence predicted by this model. This suggests that the Fermi energy for clusters in this size range decreases with increasing cluster size. The shapes of the PIE spectra near threshold are observed to continuously evolve with increasing cluster size, with the smallest clusters displaying photoionization efficiencies which rapidly increase with energy above threshold, and larger clusters displaying spectra with more gradual slopes. It is proposed that this trend reflects the evolution in the density and distribution of low-lying electronic states which occurs with increasing cluster size.
Adiabatic ionization potential of Al2 and vibrational spectrum of the X 2Σ+g state of Al+2
Authors: Harrington Joel E. , Weisshaar James C.
Published: July 2002

We have measured the adiabatic ionization potential of the aluminum dimer, AIP(Al2)=5.989±0.002 eV (±2σ uncertainty), using threshold photoelectron spectroscopy with resonant two photon ionization (ω1 fixed and ω2 scanned). We obtain the Al+2 , X 2Σ+g vibrational constants ωe =178±8 cm−1 and ωexe =2±2 cm−1 (±2σ uncertainty), the first spectroscopic data for a cationic Al cluster. These results are in excellent agreement with ab initio calculations.
Near-threshold photoionization of nickel clusters: Ionization potentials for Ni3 to Ni90
Authors: Knickelbein Mark B. , Yang Shihe , Riley Stephen J.
Published: July 2002

The threshold photoionization efficiency (PIE) curves for nickel clusters in the size range Ni3 to Ni90 have been measured by laser photoionization with detection by time-of-flight mass spectrometry. Both warm (≤298 K) and cold (≤77 K) clusters have been studied. The PIE curves for 298 K clusters display thermal tails, while these tails are smaller for cold clusters. Cluster ionization potentials (I.P.s) have been determined by two methods: the Watanabe procedure and linear extrapolation of the PIE curves. Dramatic dependence of I.P. on cluster size is found for clusters smaller than 11 atoms, while the I.P.s of larger clusters decrease relatively smoothly and nearly monotonically from 5.84 eV for Ni11 to 5.56 eV for Ni90. The I.P.s for clusters larger than Ni40 show the linear dependence on reciprocal radius (R−1) predicted by the conducting spherical drop model of small particle I.P.s, but do not fit the model quantitatively unless the limiting (R−1→ 0) work function is reduced by 0.46 eV from the bulk polycrystalline value. The differences between the thermal tails of the room temperature and 77 K PIE curves diminish with increasing cluster size, suggesting a reduced difference between neutral and ionic structures for larger clusters. In general, there is poor agreement between our experimental results and theoretically calculated I.P.s for small nickel clusters, with the exception of the recently reported tight-binding theory results of Pastor et al. [Chem. Phys. Lett. 148, 459 (1988)].
Dynamics of diffusion in small cluster systems
Authors: Beck Thomas L. , Marchioro T. L.
Published: July 2002

Classical dynamical studies are presented of melting and diffusive behavior in the Ar13 cluster. A comparison is made between the standard Einstein form for the diffusion constant (and the equivalent Green–Kubo form) and a recently developed theory for dynamics and diffusion in liquids based on the dynamical Hessian matrix. Comparisons are also made to similar calculations on melting in bulk argon systems. In the molecular dynamics calculations, it is found that the onset of liquidlike behavior occurs rather suddenly across the transition region. Well-defined Fick’s Law diffusive behavior occurs in liquidlike clusters over time scales long relative to the characteristic vibrational period. In studies involving the dynamical matrix, the distributions and numbers of positive and negative eigenvalues of the Hessian matrix are monitored as a function of energy and temperature, and the Einstein frequency of the cluster is computed.
Direct determination of long-range inverse-power potential coefficients from spectroscopic data
Authors: Davies Mark R. , Shelley John C. , Le Roy Robert J.
Published: July 2002

This paper presents and tests a method for determining the set of inverse-power coefficients defining the long-range potential tail of a given molecular state by least-squares fitting directly to the energies and inertial rotational constants (Bv ’s) of vibrational levels lying very near dissociation. This approach is superior to the widely used procedure of determining potential points by some inversion procedure and fitting them to the inverse-power form, in that the fit is directly sensitive to the uncertainties in the experimental quantities, and that the parameter uncertainties due to interparameter correlation are greatly reduced. Moreover, the new method gives direct estimates of the number, energies, and other properties of unobserved levels lying above those being fitted. Improved results, including potential parameters up to C10, are presented for the B(3Π+0u) state of I2.
Microscopic solvation of acetone to 9,9′-bianthryl studied in a free jet: Polar excited state formation
Authors: Honma Kenji , Arita Koji , Yamasaki Katsuyoshi , Kajimoto Okitsugu
Published: July 2002

Dynamics of the electronically excited state of 9,9′-bianthryl (BA)–acetone complexes was studied in a free jet. The BA–acetone complexes were produced by supersonic expansion of BA/acetone/He mixtures. The laser induced fluorescence spectrum of the BA–acetone complexes showed several bands which were ascribed to the complexes containing different number of acetone molecules. The number of acetone molecules attaching to BA was determined by mass-selected resonance-enhanced multiphoton ionization (REMPI) spectra for each band. The Stokes shift and lifetime were measured for each band, i.e., for the BA–acetone complex with a specific number of ‘‘solvent’’ molecules. These measurements revealed that there are two kinds of BA–acetone complexes; one giving the broad laser-induced fluorescence (LIF) spectrum and largely redshifted fluorescence, and the other yielding the structured LIF and the sharp fluorescence spectrum similar to that of bare BA. The Stokes shift and the lifetime of the former complex increased with increasing number of solvated acetone molecules, whereas those of the latter complex are insensitive to the degree of solvation. These findings can be explained in terms of the importance of both the ‘‘symmetry breaking’’ and the ‘‘polar microscopic solvation’’ in forming the so-called twisted intramolecular charge–transfer state.
Critical dynamics of polymer blends
Authors: Stepanek P. , Lodge T. P. , Kedrowski C. , Bates F. S.
Published: July 2002

The critical behavior of six polyisoprene/poly(ethylene–propylene) binary blends has been examined, primarily by dynamic light scattering. Specifically, the decay rate (Γ) of fluctuations in composition has been measured as a function of scattering vector (q) and temperature, above the stability limit. The results are interpreted in the context of mode-coupling corrections to the dynamics. Theoretically predicted crossovers between mean-field and nonclassical regimes, and between mode-coupled and mode-decoupled dynamics, are clearly seen. The mean-field to nonclassical crossover, or Ginzburg criterion, is localized, and the mode-coupled to mode-decoupled transition in the dynamics occurs well into the static mean-field regime. In addition, geometric crossovers between Γ∼q2 (‘‘diffusive’’) and Γ∼q3 or Γ∼q4 (‘‘critical nondiffusive’’) regimes are in quantitative agreement with theory. The applicability of the Kawasaki–Stokes relationship between Γ and the dynamic correlation length is also demonstrated.
Observation and analysis of the CN–Ne B–X transition
Authors: Lin Yaomin , Heaven Michael C.
Published: July 2002

Spectra for the CN–Ne B–X transition are reported. Rotational analyses provided an average Ne to CN distance of 3.79±0.07 Å for both the B and X states. The barrier to internal rotation in the excited state was found to be 17.2±1.0 cm−1.
Self-diffusion in n-alkane fluid models
Authors: Padilla P. , Toxvaerd S.
Published: July 2002

Molecular dynamic simulations of fluid n-pentane and n-decane have been performed in order to analyze the self-diffusion. An isotropic united-atom (UA) model as well as anisotropic united-atom (AUA) models have been used to represent the molecular interactions. Self-diffusion coefficients have been calculated. The sensitivity of the self-diffusion coefficient to the shape of the intermolecular potential as well as the torsion potential has been analyzed. The simulation results for the diffusion coefficient are in excellent agreement with experimental data when the molecular interaction is represented by an anisotropic United-Atom model and the internal rotation is governed by a torsion potential proposed in this work.
Studies of electron collisions with polyatomic molecules using distributed-memory parallel computers
Authors: Winstead C. , Hipes P. G. , Lima M. A. P. , McKoy V.
Published: July 2002

Elastic electron scattering cross sections from 5–30 eV are reported for the molecules C2H4, C2H6, C3H8, Si2H6, and GeH4, obtained using an implementation of the Schwinger multichannel method for distributed-memory parallel computer architectures. These results, obtained within the static-exchange approximation, are in generally good agreement with the available experimental data. These calculations demonstrate the potential of highly parallel computation in the study of collisions between low-energy electrons and polyatomic gases. The computational methodology discussed is also directly applicable to the calculation of elastic cross sections at higher levels of approximation (target polarization) and of electronic excitation cross sections.
On the equation of state of charged colloidal systems
Authors: Groot Robert D.
Published: July 2002

The mean spherical approximation (MSA) is applied to the ‘‘primitive model’’ of electrolytes to calculate the osmotic pressure of a charged colloidal system. For a two-component system of large particles (1 μm diameter) and counterions, the MSA predicts a correction to the ideal gas law which is proportional to ρ3/4. This result should be compared to the Debye–Hückel correction, which is proportional to ρ3/2. As a result of this power (3)/(4) the MSA equation of state describes phase separation in combination with a very low spinodal density. To study the importance of the nonlinear effects, a correction based on the Poisson–Boltzmann equation is proposed. The resulting equation of state qualitatively shows the same behavior as the MSA does, however, the correction to the ideal gas law is reduced by a factor of four. In the three-component system (colloid particles, counterions, and coions) the parameter range where phase separation occurs is limited to not only low dielectric permittivity, but also to low salt concentrations.
Measurements of kinetic energy release following the unimolecular and collision-induced dissociation of argon cluster ions, Ar+n, for n in the range 2–60
Authors: Woodward C. A. , Stace A. J.
Published: July 2002

A double-focusing mass spectrometer in conjunction with a cluster beam source has been used to measure the average kinetic energy released following the unimolecular and collision-induced fragmentation (CID) of argon cluster ions. Measurements on unimolecular decay have been made for clusters in the range Ar+5–Ar+60, and for the CID studies the range was Ar+2–Ar+30. Within the observation time window, the kinetic energy release results for the loss of a single argon atom via unimolecular decay are consistent with internal energy being partitioned statistically. Three separate CID routes are identified: (i) loss of one Ar atom; (ii) rapid (<10−7 s) loss of two Ar atoms within the confines of a collision cell; (iii) sequential loss of two Ar atoms on a time scale >10−7 s. It is proposed that the CID of small cluster ions proceeds via electronic excitation; but that as the clusters increase in size (n>4) vibrational excitation predominates. A simple spectator model of collisional excitation accounts for the experimental CID results in cluster ions beyond Ar+15.
Integral equation theory for associating liquids: Weakly associating 2–2 electrolytes
Authors: Kalyuzhnyi Yu. V. , Holovko M. F. , Haymet A. D. J.
Published: July 2002

Formation of ion pairs in a 2–2 aqueous electrolyte is studied using a generalization of Wertheim’s formalism, designed to treat explicitly association between molecules. The ions in the model electrolyte interact via a continuous potential energy (unlike previous studies), which combines the long-range Coulomb interaction with a soft repulsive potential at short distances. Both the derivation of new equations, including approximately the formation of trimers, and numerical solution of the resultant equations are presented here. The predictions of the theory are in excellent agreement with new molecular dynamics (MD) simulations of the same electrolyte. Explicit predictions are made for the fraction of monomers, dimers, and trimers in the electrolyte, over the full range of concentrations of interest for association, from 0.001 to 0.2 M.
Icosahedral structure in hydrogenated cobalt and nickel clusters
Authors: Klots T. D. , Winter B. J. , Parks E. K. , Riley S. J.
Published: July 2002

Reactions with ammonia and with water are used to probe the geometrical structures of cobalt and nickel clusters that are saturated with hydrogen. Ammonia saturation experiments allow the determination of the number of primary NH3 binding sites on cluster surfaces, and this number shows a repeated minimization to values of 12 for many cluster sizes in the 50- to 200-atom size region. These sizes correspond to closed shells and subshells of icosahedra, suggesting that the ammoniated clusters have metal frameworks with icosahedral structure. The equilibrium reaction of the hydrogenated clusters with a single water molecule shows a pattern of local maxima in the cluster–water binding energy, with the maxima in most cases coming at clusters having one metal atom more than those showing minima in ammonia binding. This correlation suggests that nonammoniated clusters likewise have icosahedral structure, and is consistent with the nature of the metal–water interaction. Some of the larger clusters do not show clear evidence for icosahedral structure at room temperature, although they begin to do so at elevated temperature. Annealing experiments suggest that many of these clusters are icosahedral in their most stable configuration at room temperature, although the 147-atom nickel cluster is not. In general, hydrogenation enhances the icosahedral features in the ammonia and water binding patterns compared to those seen for bare clusters, and extends the cluster size region over which icosahedral structure is evident.
Colloid stability: The forces between charged surfaces in an electrolyte
Authors: Valleau J. P. , Ivkov R. , Torrie G. M.
Published: July 2002

Monte Carlo methods are presented for the evaluation of the various components of the force between parallel charged surfaces due to the presence between them of an electrolyte, represented by the restricted primitive model. This is a significant problem because some recent theoretical results, and computations on even simpler models, have put in question the adequacy of traditional Derjaguin–Landau–Verwey–Overbeek (DLVO) theory in offering a basic understanding of colloid stability. The methods are applied principally to 2:2 electrolytes at a variety of surface charge densities. We find, in contrast to DLVO theory, that the force between the surfaces resulting from the electrolyte is almost everywhere attractive; in detail these forces turn out to have a remarkably rich behavior resulting from the interaction of their various contributions. The results bolster the perception that we need to take a new look at some basics of colloid science.
The dynamics of hydrogens in double well potentials: The transition of the jump rate from the low temperature quantum-mechanical to the high temperature activated regime
Authors: Heuer Andreas , Haeberlen Ulrich
Published: July 2002

The temperature dependence of the deuteron spin lattice relaxation time T1 in four carboxylic acids, which were deuterated in the carboxyl groups, is presented. These data allow determination of the rate Γ of the hydrogen transfer between the two minima of the double well potential in which the hydrogens move along the hydrogen bonds. The temperature dependence of Γ has clearly distinct low temperature quantum-mechanical and high temperature thermally activated regimes. Evidence is presented that the transfer of hydrogens in dimers of carboxylic acids is strongly influenced by a high-frequency mode which is strongly coupled to the hydrogens. This supports the conclusions recently put forward by the ETH group [J. Chem. Phys. 93, 1502 (1990)]. A model based on the spin–Boson–Hamiltonian with a bath consisting of acoustic phonons plus a unique high frequency mode explains fully the jump rate in the quantum mechanical regime of all four compounds studied. The way in which the unique bath mode influences the hydrogen transfer is clarified.
Laser induced photodissociation of s-tetrazine probed by photoelectron spectroscopy with synchrotron radiation
Authors: Nahon Laurent , Morin Paul , Larzilliere Michel , Nenner Irene
Published: July 2002

We have combined a cw visible laser with synchrotron radiation, respectively, to photodissociate the s-tetrazine molecule and to probe, with an electron analyzer, the vibrational energy of the nascent fragments. No fragments other than N2 and HCN were detected on the time scale of our experiment. We find that 5.4%±0.5% of the nitrogen fragment departs with one quantum of vibration and at least 26% of each HCN fragment is vibrationally excited, mainly in the bending mode (n=1 to 6) and probably to a very small extent in the C–N stretching mode (n=1). These data are fully consistent with a pure three body fragmentation mechanism governed by the geometry of the transition state as calculated by Scheiner et al., J. Am. Chem. Soc. 108, 8160 (1986).
Quantum chemistry by random walk: Higher accuracy for H+3
Authors: Anderson James B.
Published: July 2002

An improved quantum Monte Carlo calculation for the ground state of the molecular ion H+3 gives higher accuracies than previously attained. The nonrelativistic electronic energy for the equilateral triangle configuration of side length 1.6500 bohrs is found to be −1.343 835±0.000 001 hartrees.
Ne++C60: Collision energy and impact parameter dependence for endohedral complex formation, fragmentation, and charge transfer
Authors: Wan Zhimin , Christian James F. , Anderson Scott L.
Published: July 2002

We report measurements of C60−2nNe+ and C+60−2n formation in collisions of Ne+ with C60 over a range from 2–212 eV (CM). Activation energies and crude angular information are given for each channel. Endohedral complex formation and fragmentation both require low impact parameter collisions, while charge transfer can occur with larger impact parameters as well.
Cross section and spin polarization for photoionization of the HI molecule: Rotationally resolved theoretical results in the spin–orbit autoionization region
Authors: Büchner M. , Raşeev G. , Cherepkov N. A.
Published: July 2002

The theoretical calculations of rotationally resolved total and differential cross sections and spin polarization parameters for the photoionization of HI molecule are presented. These calculations are based on the theoretical expressions derived by Raşeev and Cherepkov [Phys. Rev. A 42, 3948 (1990)]. The transition moments used in the calculation are taken from an ab initio calculation without rotation. Molecular rotation is included in the framework of a simplified model taking into account Hund’s case (c)–(e) frame transformation. The results of ab initio calculations of cross section and electron spin polarization parameters for state-to-state transition are obtained for the first time. Averaged over the rotational levels of the initial state and summed over the rotational levels of the final state of the ion, these calculations are compared with the recent measurements of the total cross section and integrated spin polarization parameters performed in Bielefeld.
State-resolved studies of the laser-induced desorption of NO from Si(111) 7×7: Low coverage results
Authors: Richter Lee J. , Buntin Steven A. , King David S. , Cavanagh Richard R.
Published: July 2002

The results of a quantum-state-resolved study of the laser-induced desorption (LID) of NO from Si(111) 7×7 at a surface temperature of 100 K are reported. All aspects of the LID are found to be sensitive to the initial coverage. The coverage dependence indicates that there are two desorption mechanisms, one operative at low coverages that is quenched with increasing NO exposure, and one operative at high coverage. This report characterizes the low coverage channel. Most of the energy in the desorbed NO occurs as vibration and translation, with the rotations substantially cooler. The desorption is selective for production of the ground spin–orbit state. The energy partitioning shows strikingly little change as the desorption-laser wavelength was varied from 1907 to 355 nm. This, coupled with a quantitative study of the yield over the same photon energy range and selective coadsorption experiments, establishes that the desorption is specifically due to an interaction involving photogenerated holes in the rest-atom localized, intrinsic surface state of the 7×7 reconstructed surface. It is suggested that the surface state hole drives the desorption by neutralization of a NO−δ adsorbate.
Structure of phosphorus clusters using simulated annealing. II. P9, P1, P11, anions P2−4, P2−10, P3−11, and cations P+n to n=11
Authors: Jones R. O. , Seifert G.
Published: July 2002

Previous calculations on the structures of isomers of phosphorus clusters P2 to P8 [density functional calculations with simulated annealing (SA), J. Chem. Phys. 92, 6710 (1990)] have been extended to P9, P10, and P11. The tendency of phosphorus to form tubular structures is already evident in clusters of this size. We have also performed calculations on additional isomers of the hexamer P6, positive ions up to P+11, and the P2−4, P2−10, and P3−11 anions. Ionization energies agree well with available measurements [P1 to P4] and show trends that can be correlated with the structures and the measured relative abundances of the ions. The calculated structure of P3−11 agrees very well with x-ray diffraction data, and we observe isomerization in P9 resulting from charge transfer. A simplified SA scheme using a linear combination of atomic orbitals (LCAO) has been used to aid the analysis of the results.
Quasiclassical trajectory calculations of the thermal rate coefficients for the reactions H(D)+O2→OH(D)+O and O+OH(D)→O2+H(D) as a function of temperature
Authors: Varandas A. J. C. , Brandão J. , Pastrana M. R.
Published: July 2002

Thermal rate coefficients are calculated for the reaction, (1), H+O2→OH+O and its reverse, (−1), O+OH→O2+H, using the quasiclassical trajectory method and the most recently reported double many-body expansion (DMBE IV) potential energy surface for the ground electronic state of the hydroperoxyl radical. The full range of temperatures for which experimental data is available in the literature has been covered, namely, 1000≤T≤3000 K for reaction (1) and 150≤T≤3000 K for reaction (−1). The equilibrium constant has also been evaluated. In addition, calculations of the isotopic effect on the thermal rate coefficient for the deuterated reactions D+O2→OD+O and O+OD→O2+D, and equilibrium constant, are reported. The theoretical results are shown to be in good agreement with the most recent and accurate experimental measurements.
Investigations on the production kinetics of ionic alkali–rare gas excimers
Authors: Mantel M. , Schumann M. , Giez A. , Langhoff H. , Hammer W. , Petkau K.
Published: July 2002

Ionic rare gas alkali excimers Rg+A were produced by exciting Rg/A mixtures in a buffer gas atmosphere by means of an ion or electron beam. As a representative candidate the formation of Kr+K decaying at 135 nm was investigated. The rate constant k(He) for the main reaction Kr++K→Kr+K was determined varying the buffer gas density [He] between 1017 and 1020 cm−3. Values between k(He)=1×10−11 and 5×10−10 cm3 s−1 were observed. At high buffer gas densities the quantum yield ηq for the emission of the 135 nm radiation depends on the ratio [K]/[Kr]. At the optimum value, [K]/[Kr]=0.06, an absolute yield of ηq=1.5% was obtained. Up to a deposited energy of 3 mJ/cm3 the quantum yield is independent on the excitation density. For neon as buffer gas the rate constants k(Ne) are smaller by about a factor of 3, while the efficiency is only slightly less than for He. Kr+K is split into five fine structure states. The quenching of these states by krypton and buffer gas causes the low quantum yield.
Excited-state proton transfer in 1-naphthol/ammonia clusters
Authors: Hineman M. F. , Brucker G. A. , Kelley D. F. , Bernstein E. R.
Published: July 2002

Excited-state proton transfer dynamics are reported for the 1-naphthol(NH3)n cluster system for n=3 and 4. Picosecond time- and mass-resolved pump (S1←S0)–probe (I←S1) experiments demonstrate the following results: (1) excited-state proton transfer occurs for n=3 and 4 clusters only; (2) for n=5 clusters the proton is transferred in the ground state and for n=2 clusters no proton transfer can be observed; (3) the proton transfer time in the n=3 cluster at the 000 transition is ca. 60 ps; (4) this time is reduced to ca. 40 ps and ca. 10 ps for 800 and 1400 cm−1 of vibrational energy in S1, respectively; (5) for the n=4 clusters these times are approximately 70, 70, and 30 ps, for 0, 800, and 1400 cm−1 of vibrational energy in S1, respectively; (6) both n=3 and 4 clusters exhibit a second low-amplitude decay component, which is about an order of magnitude slower than the initial decay; and (7) 1-naphthol-d1(ND3)n clusters have a greatly reduced rate constant for the excited-state proton transfer dynamics. These observations are well fit and explained by a simple statistical/barrier penetration model involving proton tunneling and the effect of van der Waals vibrations on the height and width of the barrier to proton transfer.
Ab initio studies of small sodium–sodium halide clusters, NanCln and NanCln−1 (n≤4)
Authors: Weis Patrick , Ochsenfeld Christian , Ahlrichs Reinhart , Kappes Manfred M.
Published: July 2002

The geometries, energetics, and some first and second order molecular properties (dipole moments, force constants, polarizabilities) of small stoichiometric (NaCl)n and nonstoichiometric (NanCln−1), n≤4 clusters were studied at the SCF and various correlated levels.