bluuebunny/crossref_metadata_2025 huggingface dataset:
Absorption cross sections, saturated vapor pressures, sublimation energies, and evaporation energies of some organic laser dye vapors
Authors: Schmidt J. , Penzkofer A.
Published: July 2002

A new technique of transmission measurement of overheated dye vapors is applied to determine absolute absorption cross-section spectra of three active dyes for vapor phase dye lasers. The investigated compounds are 1,4-di[2-(5-phenyloxazolyl)]-benzene (POPOP), 1,4-di[2-(4-methyl-5-phenyl-oxazolyl)]-benzene (dimethyl-POPOP), and 2,5-diphenylfuran (PPF). The vapor absorption spectra are compared with liquid solution spectra in order to obtain information on the dye–solvent interaction. The saturated vapor densities are determined by transmission measurements after knowing the absolute absorption cross-section spectra. The latent heats of sublimation, evaporation, and melting are derived by analyzing the dependences of the saturated vapor densities on the vapor temperature.
Singular value analysis and reconstruction of photon correlation data equidistant in time
Authors: Finsy R. , de Groen P. , Deriemaeker L. , van Laethem M.
Published: July 2002

The application of the algorithm of Zeiger and McEwen to the analysis of noisy photon correlation data is investigated. For the particular case where the data are sampled at equidistant time intervals a complete solution is given allowing reliable reconstruction of the spectrum of exponential decay rates without any a priori knowledge. A particular attractive feature of the method is that the singular value analysis of the Hankel matrix of autocorrelation functions offers a practical criterion for the decomposition of the data into a signal and a noise part. Some tests of the method are illustrated with experiments on monodisperse latices, gold sols, and binary mixtures of monodisperse latices. In the latter case comparable and even better results are obtained in significantly shorter computing time when compared to an analysis with Contin and the maximum entropy method. Since the present method does not require any a priori parameter setting, it is also complementary to these methods.
Tunneling of two interacting particles: Transition between separate and cooperative tunneling
Authors: Dakhnovskii Yuri I. , Semenov Mikhail B.
Published: July 2002

Tunneling of two particles interacting with each other is considered. The semiclassical (instant on) action is calculated taking into account the interaction with an oscillator bath. It is shown that for repulsive interaction between particles, there are two regions of tunneling depending on the value of the interaction constant α. For values larger than a critical value αc, simultaneous transition occurs. For a fixed interaction constant, there is a critical temperature at which a change of tunneling regime takes place (from single particle to cooperative tunneling). The same analysis has also been applied to a model including the interaction with phonons. The transition to the cooperative tunneling regime of two particles moving in opposite direction should be observable in optical absorption spectra.
A neutron diffraction and molecular dynamics investigation of the environment of Dy3+ ions in a fluoroberyllate glass
Authors: Clare Alexis G. , Etherington George , Wright Adrian C. , Weber Marvin J. , Brawer Steven A. , Kingman Donald D. , Sinclair Roger N.
Published: July 2002

A combined neutron diffraction and molecular dynamics study is reported of the Dy3+ ion environment in vitreous NaF–DyF3–BeF2, using a special version of the isotopic substitution procedure known as the null technique. To investigate the effects of both NaF and DyF3 on the basic beryllium fluoride glass network, complementary measurements and simulations have been performed for vitreous NaF–BeF2, with the same NaF:BeF2 ratio as the DyF3 containing glass, and also for pure vitreous BeF2. The neutron diffraction data indicate that for glasses simulated using pair potentials the BeF4 tetrahedra are much more distorted than those in the real materials and the mean Be–F–Be angle is too high. Both of these deficiencies in the simulations are attributed to the use of simple ionic potentials with no bond angle restoring forces. Experimentally, the distribution of Dy–F first neighbor distances is found to be narrow, having a root mean square deviation of 0.110±0.003 Å about the mean value of 2.290±0.003 Å. The Dy(F) coordination number is 7.3±0.2 which is slightly less than that in crystalline DyF3.
The water flip barrier in the Li+HCOO−⋅H2O crystal from a
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o and molecular mechanics calculations
Authors: Hermansson Kersti
Published: July 2002

A combination of ab initio calculations and energy minimization using empirical potentials is shown to be a viable approach to the study of the 180° flip rotational barrier for the water molecule in LiHCOO⋅H2O. For an isolated crystal fragment consisting of an H2O molecule tetrahedrally surrounded by Li+, HCOO−, and two H2O molecules, the ab initio SCF barrier at the DZP level is 165 kJ/mol, as compared to the NMR-determined value of 66±1 kJ/mol for the activation energy. Only when the Madelung potential and relaxation of the crystalline surroundings are taken into account do experiment and theory agree. A second-order Mo/ller–Plesset treatment shows electron correlation to have only a small effect (∼+10 kJ/mol) on the calculated barrier, as has allowance for relaxation of the internal water geometry (∼−5 kJ/mol).
Variational method for two-electron atoms
Authors: Boumerzoug M. S. , Miletic J.
Published: July 2002

The ground-state energies for helium-like atoms are computed by the Ritz variational method. The trial wave function is chosen to be of the form ψ=[ln(r12+e)+C(r1−r2)2] ×exp[−S(r1+r2)], where r1 and r2 are the distances of the electrons from the nucleus, r12 the distance between the electrons, e the natural base of logarithms, and the parameters C and S are determined by minimizing the energy of the atom. The numerical results are presented and compared with those of other authors.
Local mode of motions in amorphous solid. II. Configurational contributions in poly(vinyl chloride)
Authors: Naoki Motosuke
Published: July 2002

The dielectric dispersions of the β process are analyzed with respect to the configurational contributions for the densified glasses of poly(vinyl chloride). The vitrification pressure intensifies the intermolecular potential energy and affects the energy disparity induced by the electric field, but it only slightly affects the energy barrier for the β process. The relaxation magnitude varies with volume in accordance with the configurational change in the glasses. About 40% of the relaxation magnitude is the configurational contribution.
The effect of aggregation on colloid catalysts: A fractal model
Authors: Melrose J. R.
Published: July 2002

Theoretical models for the effect of aggregation on the catalytic properties of metal colloids are developed within the framework of the electrochemical theory of the catalytic mechanism and recently proposed models for the extremes of cluster aggregation. In the case of fast aggregation of the colloid catalyst, the catalytic rate is, in general, predicted to decrease as a power law of time. In the case of slow aggregation catalytic rates are predicted to have an exponential decrease with time. Polydispersity could be important in the case of slow aggregations. A recent experiment on aggregating colloidal silver is discussed.
Atomic and molecular diamagnetic susceptibilities from Compton scattering data
Authors: Gadre Shridhar R. , Pathak Rajeev K.
Published: July 2002

A procedure is proposed for the extraction of molar Larmor diamagnetic susceptibilities χL=−N0e2〈r2〉/(6mc2) (N0 is the Avogadro number) for closed-shell atomic and molecular systems, starting exclusively from their electron momentum densities derived from experimental or theoretical Compton profile data. This procedure is essentially founded on a transcription of the coordinate-space nonlocal-density approximation (NLDA) of Gadre and Chakravorty [J. Chem. Phys. 86, 2224 (1987)] to momentum space. Numerical tests of this scheme to a variety of closed-shell systems yield decent estimates of their 〈r2〉 values which compare well with their experimental or theoretical counterparts.
Nonadiabatic a
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o calculations of eigenfunctions and energies for the 3s,3d-triplet complex of molecular hydrogen
Authors: Bak Keld Lars , Linderberg Jan
Published: July 2002

Using the coupled equations for diatomic molecules, nonadiabatic ab initio calculations of the rovibronic eigenfunctions and energies for the 3s,3d-triplet complex of molecular hydrogen have been performed for the first time. Results for ν=0 to ν=4 and J=0 to J=4 are presented and compared to corresponding experimental results. For ν=0 to ν=2 and J=2 the results are also compared to adiabatic results. Strong nonadiabatic effects are observed and although the agreement with experimental results is only semiquantitative, it is stated that both rotational and vibrational nonadiabatic couplings have a crucial influence on both the energy spectrum and the eigenfunctions.
On the calculation of diffusive reaction rates using Brownian dynamics simulations
Authors: Zhou Huan-Xiang
Published: July 2002

An analytical derivation is presented of the expression of Northrup et al. [J. Chem. Phys. 80, 1517 (1984)] for extracting diffusive reaction rates from Brownian dynamics simulations. By working with the Smoluchowski equation governing the distribution function of the reactants, we obtain two identities that directly lead to this expression. This derivation provides a rigorous theoretical basis for this important expression.
Penning ionization electron spectroscopy of iron phthalocyanine thin films. Study of electronic structure from local electron distribution of molecular orbitals
Authors: Ozaki Hiroyuki , Harada Yoshiya
Published: July 2002

Penning ionization electron spectra (PIES) and ultraviolet photoelectron spectra (UPS) were measured for two kinds of ultrathin films of iron phthalocyanine with different orientation of surface molecules. The local electron distribution of molecular orbitals (MO) at the different parts of the molecule was reflected in the PIES. From this the character of MOs responsible for each band in the PIES and UPS was revealed. The so-called iron 3d bands were assigned to 3d⊥ (dxz , dyz, dz2)-like MOs. The IP values for π MOs were found to be less than ∼12 eV in the solid phase. It was concluded that the type of MOs probed by PIES can be selected if the orientation of molecules is controlled appropriately, which will be of great help to elucidate the electronic structure of complex molecules.
The microwave spectrum of the AlO radical
Authors: Yamada Chikashi , Cohen Edward A. , Fujitake Masaharu , Hirota Eizi
Published: July 2002

The rotational spectrum of the aluminum monoxide radical in the ground electronic state X 2Σ+ was observed in the millimeter- and submillimeter-wave region. Aluminum monoxide radicals were generated directly in a flow reactor absorption cell by the reaction of N2O with aluminum vaporized from a high-temperature crucible. The observed spectrum was analyzed to yield molecular constants including the rotational constant, centrifugal distortion constant, spin–rotation interaction constant, magnetic hyperfine interaction constants, and nuclear quadrupole coupling constant.
Spectroscopic study of the second 3Πg state of H2 : A bound triplet state perturbed by a doubly excited state
Authors: Lembo L. J. , Bjerre N. , Huestis D. L. , Helm H.
Published: July 2002

A double-resonance technique is employed to measure the positions of energy levels of the second 3Πg state of H2. Vibrational levels of this state with v>3 lie above the ionization threshold of H+2, and are observed both to autoionize and dissociate. This state, 23Πg, arises from the homogeneous interaction between the singly excited 1sσg3dπg and 1sσg4dπg diabatic Rydberg states with the core-excited 2pσu2pπu state. The energy-level spacings are in good agreement with a simple model calculation for the interacting potentials. We also include a listing of autoionizing levels lying in the same energy range.
Evidence for coupled rotational and phonon quantum excitation in the scattering of a nearly monoenergetic HD beam from the Ni(001) surface
Authors: Berndt R. , Toennies J. P. , Wöll Ch.
Published: July 2002

Angular distributions and time-of-flight spectra of a nearly monoenergetic (Δv/v≂2% FWHM) He seeded HD beam (ki=6.80 Å−1, Ei=32 meV) have been measured after scattering from a Ni(001) single-crystal surface along the 〈100〉 azimuth. The most intense peaks in the angular distributions are attributed to diffraction with rotational excitation while the rotationally elastic diffraction peaks are an order of magnitude smaller. All the time-of-flight distributions reveal intense peaks at ΔE=−11.06 meV corresponding to the HD rotational transition ji=0→jf=1. The rotational energy loss observed at angles between the diffraction peaks is attributed to incoherent inelastic scattering at defects. The other inelastic peaks in the time-of-flight spectra can be assigned to phonons which are created or annihilated in the rotationally inelastic collisions. The measured phonon dispersion curves suggest a predominant interaction with the transverse bulk edge and not with the Rayleigh mode. These results provide the first direct evidence for a coupling of rotational transitions to the phonons of a metal surface. The pronounced rotational inelasticity at step edges does not support the widely held view that nonactivated chemisorption occurs at step edges.
Raman matrix isolation spectroscopy of dihydrogen. II. Aggregation processes in argon
Authors: Alikhani M. E. , Manceron L. , Perchard J. P.
Published: July 2002

The vibrational spectra of H2, D2, and HD trapped in solid argon were examined as a function of concentration and sample annealing. Before annealing, increasing the hydrogen concentration only leads to an asymmetrical broadening and a small blue shift of the Q( J) lines assigned to monomeric species. These observations are believed due to formation of low stoichiometry aggregates. After annealing above 35 K, new features—10 cm−1 blue shifted with respect to monomer and small aggregate lines—appear with properties comparable to those reported for solid dihydrogen at 2 K. Force field calculations on aggregates involving two to six molecules (including ortho/para and isotopic mixtures) allow a semiquantitative interpretation of the experimental data, and suggest an aggregation degree larger than six. Finally it is concluded that a possible interpretation of all the data is the formation of microcrystals upon annealing, tied to the low solubility and the high mobility of dihydrogen in solid argon.
A model of the DNA transient orientation overshoot during gel electrophoresis
Authors: Lim Hwa Aun , Slater Gary W. , Noolandi Jaan
Published: July 2002

Linear dichroism and electric birefringence measurements show that when an electric field is applied to a DNA molecule at equilibrium in an agarose gel, the isotropic molecular conformation quickly orients in the field direction, reaching first a maximum ‘‘overshoot’’ orientation before it relaxes towards a somewhat less oriented but still anisotropic steady-state conformation. We present here a simple analytical model of this overshoot effect together with numerical results from a computer simulation of gel electrophoresis. The predicted dependence of the overshoot time and orientation upon field intensity and molecular size are in good agreement with experimental results. The dynamics of the overshoot involves U-shape conformations that disappear only after the internal elastic forces completely dominate the electric forces. It is also predicted that a different overshoot regime takes place for low electric fields and small molecular sizes, and that a primary and a secondary overshoot may appear for very large molecules.
Computer simulation results for the dielectric properties of a highly polar fluid
Authors: Kusalik P. G.
Published: July 2002

Despite the fact that periodic boundary conditions are employed quite frequently in computer simulations of polar liquids, relatively little is known about how the specific numerical implementation may affect the properties of interest, particularly its dielectric properties. In this article we report molecular dynamics results for the dielectric, structural and thermodynamic properties of a highly polar fluid composed of dipolar soft spheres. The long-range dipole–dipole interactions are evaluated using the Ewald summation technique and we consider in some detail appropriate choices of the parameters. The dependence of the results obtained upon the sample size and boundary conditions applied are examined in a systematic fashion with systems ranging in size from 4 to 1372 particles. In general we find simulations performed in conducting boundary conditions (i.e., εRF=∞) to be somewhat problematic. Calculations which employ boundary conditions that attempt to emulate more closely a true infinite system (i.e., using εRF≊ε) appear to be superior in most respects.
Monte Carlo study of fluid–plastic crystal coexistence in hard dumbbells
Authors: Singer Sherwin J. , Mumaugh Ruth
Published: July 2002

The fluid-cubic plastic crystal coexistence curve for the hard dumbbell model is determined from Monte Carlo simulations. A transition to a stable plastic crystal phase is found for reduced bond lengths L/σ<0.4. A metastable plastic crystal is observed at slightly longer bond lengths. Orientational correlations in both the fully ordered and plastic crystal phase are examined. This is the first calculation of the complete coexistence curve between two phases as a function of particle anisotropy. This information permits detailed comparison with several recent density functional theory predictions for the same transition.
Excess electron transport in water
Authors: Barnett R. N. , Landman Uzi , Nitzan Abraham
Published: July 2002

The properties of excess hydrated electrons in liquid water, at room temperature, are studied via coupled quantum-classical simulations. In these simulations, the system evolves dynamically on the adiabatic potential energy surface with the electron maintained in the ground state throughout the process. The diffusion constant of the hydrated electron under field-free conditions is found to be the same as that obtained, via the Nernst–Townsend–Einstein relation, from the electron mobility simulated for a system under an electric field of 3.2×106 V/cm, acting on the electron. For larger electric fields, the electron mobility is found to be field dependent. The mode of migration of the excess electron is polaronic in nature and the influence of the intramolecular degrees of freedom of the water molecules on the hydrated electron transport properties is investigated. It is shown that the electron diffusion constant obtained in simulations under field-free conditions with rigid-water molecules [D0e=(3.7±0.7)×10−5 cm2/s] is larger than that obtained from simulations where a flexible-water model potential is employed D0e=(1.9±0.4)×10−5] cm2/s] and smaller than the experimental estimated value obtained from conductivity measurements (4.9×10−5 cm2/s). The difference between the diffusion constants calculated for the two models is correlated with a marked enhancement of the probability of reversal of the direction of motion of the migrating electron in flexible water. The self-diffusion constant of water using the rigid-molecules model [Ds=(3.6±0.4)×10−5 cm2/s] is also larger than that found for the flexible-water molecule model Ds=(2.3±0.2)×10−5] cm2/s], with the latter in agreement with the experimental value (Ds=2.3×10−5 cm2/s). Structural and dynamical aspects of hydrated electron transport are discussed.
Strong short-range attractions in a fluid
Authors: Hemmer P. C. , Stell G.
Published: July 2002

We study the thermodynamics of a one-dimensional fluid with particles interacting via a hard-core pair potential with a deep narrow attractive well plus an infinitely weak and long-ranged attractive tail. The attractive well has a striking effect on the liquid–gas phase boundaries of the pressure–temperature–volume surface that describes the equation of state of the model. This is brought out most clearly by considering the limiting behavior associated with the well width going to zero and the well depth becoming infinite in such a way that the contribution of the well to the second virial coefficient is finite and nonzero at some fixed temperature. Our analysis is primarily classical, but we note the changes to be expected in the quantum description. In particular, we find that when one takes the above limit appropriate to the classical second virial coefficient, no effect at all from the well will be felt when the system is described quantum mechanically.
Rotational spectra, structure, and intramolecular force field of the Hg–OCS van der Waals complex
Authors: Iida Minoru , Ohshima Yasuhiro , Endo Yasuki
Published: July 2002

Pure rotational spectra of the mercury–carbonyl sulfide complex have been observed by a pulsed-nozzle Fourier-transform microwave spectrometer, which has been recently constructed. The structure of the complex is found to be T-shaped, which is the same as the rare-gas–carbonyl sulfide complexes analogous to this system. The mercury–carbon distance and the Hg–C–O angle have been determined to be 3.7075(2) Å and 86.98(1) deg, respectively, for 202Hg–OCS from the rotational constants. Harmonic force constants of the van der Waals modes have been derived by the centrifugal distortion constants. The electric quadrupole coupling constants for 201Hg–OCS have also been determined and the values indicate small perturbation on the electric charge distribution of the mercury atom by complex formation.
Electron spin resonance and optical studies on the radiolysis of carbon tetrachloride. II. Structure and reaction of CCl⋅−4 radical anion in tetramethylsilane low-temperature solids
Authors: Muto Hachizo , Nunome Keichi
Published: July 2002

An electron spin resonance (ESR) and optical study of carbon tetrachloride radical anion has been made to provide for a better understanding of the radiolysis of CCl4, following CCl⋅+4 cation previously studied. It was found that the anion was metastably trapped in tetramethylsilane (TMS) matrices γ irradiated at 4 or 77 K. The g tensor and the hyperfine coupling tensors of all atoms of the radical were determined from ESR spectral simulation by using 12 CCl4 and the 13C enriched compound: g∥=2.004–5, g1=2.015,(A∥,A⊥) =(24.3,18.3) mT for 13C , (0.9, 0.2) mT for one 35Cl atom, and (A1,A2=A3)=(1.98,0.45) mT for the other three equivalent 35Cl atoms. From these parameters and a consideration on the g anisotropy combined with the optical data, the anion was found to have a predissociating molecular structure (CCl3⋅⋅⋅Cl) ̇− with C3v symmetry, where the unpaired electron occupies A*1γ antibonding orbital. The carbon atom has a large spin density and near sp3 hybridization: ρp=0.62, ρs=0.18, ρp/ρs=3.4, and three Cl atoms and the other Cl atom have the spin densities ρp=0.10 and ρp=0.05, respectively. The species had two optical absorptions at λmax=265 and 370 nm which were assigned to the Eγ–A*1γ and A1γ–A*1γ electronic transitions, respectively. The anion converted to CCl ⋅3 radical by warming to ∼150 K in the TMS matrix. The present results have given unequivocal ESR and optical spectroscopic evidence and support for the assignment of the 370 nm band reported in the radiolyses of organic solutions containing CCl4.
Melting temperature of colloidal crystals of polystyrene spheres
Authors: Okubo Tsuneo
Published: July 2002

The melting temperature (Tm) of colloidal crystals of polystyrene spheres has been measured by the reflection spectroscopy as a function of sphere concentration. Seven kinds of spheres their diameters ranging from 85 to 212 nm, and their monodispersity indices (standard deviation of spheres divided by the mean diameter) between 0.014 and 0.26, are used after further purification and deionization processes. A phase diagram, including liquid-like and crystal-like (body-centered-cubic and face-centered-cubic lattices) structures, is obtained as a function of Tm against sphere concentration. The data of Tm are analyzed successfully with the theory of Williams, Crandall, and Wojtowicz, and the heat and entropy of melting are estimated. Tm values are consistent with the theory of Robbins and co-workers which treats the repulsive Yukawa potential between colloidal spheres.
Energetics of adsorption of neutral and charged molecules at the air/water interface by second harmonic generation: Hydrophobic and solvation effects
Authors: Castro Alonso , Bhattacharyya Kankan , Eisenthal Kenneth B.
Published: July 2002

Surface second harmonic generation has been used to study the adsorption of alkyl phenols and anilines and their respective ions to the air/water interface. The free energies of adsorption of the neutral molecules were obtained using a simple Langmuir model. The quantitative effects of the opposing hydrophobic and solvations forces on molecular adsorption were investigated by varying the chain length of alkylphenolate and alkylanilinium ions. An alkyl chain of at least five carbons was found to be necessary to balance the bulk ionic solvation and thereby bring the ion interface population to a level above the water background. The adsorption of the ions can be described quantitatively by using a modified Langmuir model that includes the electrostatic free energy of the electric double layer as given by the Gouy–Chapmann theory. From the excellent fit of the second harmonic data to this treatment, the chemical and electrostatic free energies of adsorption, interface electric potential, and interface pH were obtained.
Nuclear magnetic shielding of nitrogen in ammonia
Authors: Jameson Cynthia J. , de Dios Angel C. , Jameson A. Keith
Published: July 2002

The nitrogen shielding surface in ammonia is calculated using the localized orbital-local origin (LORG) method of Hansen and Bouman, in terms of the symmetry coordinates for the molecule. With respect to the inversion coordinate, the N shielding surface has a shape similar to the potential surface. Rovibrational averaging of the N shielding in NH3 and ND3 molecules is carried out using numerical wave functions which are solutions to the inversion potential which best fits the spectra of all isotopomers. The other coordinates are vibrationally averaged in the usual way, assuming small amplitude motions. The calculated temperature dependence of the N shielding due to inversion is in the opposite sense to that observed for a large number of molecules, and is nearly canceling the contributions from all the other coordinates. The temperature dependence of the nitrogen shielding in ammonia has been measured in the range 300–400 K in samples with densities in a hundredfold range (0.37–33 amagat). When the temperature-dependent intermolecular effects are separated out, the remaining temperature dependence is small and is consistent with the calculations. The inversion contribution to the deuterium-induced isotope shift is of opposite sign to the contributions from all other coordinates. The agreement with the experimental isotope shift in the liquid phase is satisfactory.
Hard-sphere radial distribution functions for face-centered cubic and hexagonal close-packed phases: Representation and use in a solid-state perturbation theory
Authors: Choi Yumi , Ree Taikyue , Ree Francis H.
Published: July 2002

The hard-sphere radial distribution functions, gHS(r/d,η), for the face-centered cubic and hexagonal close-packed phases have been computed by the Monte Carlo method at nine values of the packing fraction, η[=(π/6)ρd3], ranging from 4% below the melting density to 99% of the close-packed density. The Monte Carlo data are used to improve available analytic expressions for gHS(r/d,η). By utilizing the new gHS(r/d,η) in the Henderson and Grundke method [J. Chem. Phys. 63, 601 (1975)], we next derive an expression for yHS(r/d,η) [=gHS(r/d)exp{βVHS(r)}] inside the hard-sphere diameter, d. These expressions are employed in a solid-state perturbation theory [J. Chem. Phys. 84, 4547 (1986)] to compute solid-state and melting properties of the Lennard-Jones and inverse-power potentials. Results are in close agreement with Monte Carlo and lattice-dynamics calculations performed in this and previous work. The new gHS(r/d,η) shows a reasonable thermodynamic consistency as required by the Ornstein–Zernike relation. As an application, we have constructed a high-pressure phase diagram for a truncated Lennard-Jones potential. From this study, we conclude that the new gHS(r/d,η) is an improvement over available expressions and that it is useful for solid-state calculations.
Analytical ionization cross sections for atomic collisions
Authors: Kunc J. A. , Soon W. H.
Published: July 2002

General analytical expressions for cross sections for direct ionization in atom–atom collisions are evaluated using the classical impulse approximation. The approach is also applied to ion–atom and molecule–molecule interactions. The overall accuracy of the obtained cross sections in a broad range of energy is better, when compared with existing measurements for many collision systems, than accuracy of other analytical predictions available in literature.
High-resolution zero kinetic energy electron spectroscopy of ammonia
Authors: Habenicht Wieland , Reiser Georg , Müller-Dethlefs Klaus
Published: July 2002

Rotationally resolved ‘‘zero kinetic energy (ZEKE)’’ electron spectra of ammonia obtained in a 2+1 photon two-color pump–probe experiment are reported. The rovibronic states with J′=3, K′=1 (ortho-NH3) and J′=3, K′=2 (para-NH3) in the B̃ (1E″) v2=2 state are used as intermediate resonances. Rotational energy levels of the NH+3 ion in two vibrational states of the X̃+ (2A″2) electronic ground state, v+2 =1 and 2, are observed in the ZEKE spectra. The rotational constants are: B+=10.19±0.03 cm−1 and C+=5.30±0.04 cm−1 for v+2 =1, and B+ =9.77±0.04 cm−1 and C+ =5.39±0.05 cm−1 for v+2 =2. The adiabatic ionization energies are determined as 83 062.5±1 cm−1 for X̃+ v2=1 and 84 002.9±1 cm−1 for X̃+ v2=2, with respect to the lowest (unoccupied) v2=0+, J″=0, K″=0, Γevr=A1 rotational state of the X̃ (1A′1) electronic ground state of NH3. The observed propensity rules for the change in K quantum number ion←neutral in the rotational ZEKE transitions are explained from ‘‘near symmetry’’conservation rules in electronically allowed transitions.
Melting temperature of colloidal crystals of monodisperse silica spheres
Authors: Okubo Tsuneo
Published: July 2002

Melting temperature (Tm) of colloidal crystals of monodisperse silica spheres has been measured by the reflection spectroscopy as a function of sphere concentration. Spheres of their diameters of 110 and 178 nm from electron microscopy, and their monodispersity indices (standard deviation of sphere diameter divided by the mean diameter) of 0.041 and 0.05, respectively, are used after purification and deionization processes. Transformation from the body-centered cubic lattice to the face-centered cubic lattice subphases are observed as the suspension temperature rises. A phase diagram including liquid-like and crystal-like structures is obtained in the presence of ion-exchange resins or sodium chloride. The Tm values observed are analyzed with the theory of Crandall et al. successfully. The Tm values are also consistent with the theory of Robbins et al., which treats the repulsive Yukawa potential between colloidal spheres.
The first-order Jahn–Teller distortion and the symmetry of the electron density in the BH+3 radical
Authors: Wang Jian , Boyd Russell J.
Published: July 2002

Self-consistent field (SCF) and complete active space multiconfiguration (CAS-MCSCF) calculations with extended basis sets are used to show that the electron–nuclear attraction energy is the dominant contributor to the total energy lowering associated with the Jahn–Teller distortion of BH+3. Furthermore the Jahn–Teller distortion raises the interelectronic and internuclear repulsion energies. These observations are consistent with a contraction of the molecular framework and electron cloud. In the context of the relationship between the electron distribution and the Jahn–Teller distortion direction, it is observed that the electron density associated with the canonical highest occupied molecular orbital (HOMO) of the D3h constrained BH3 radical has C2v symmetry, which corresponds to the preferred point group of the Jahn–Teller distortion in BH+3. The effect of electron correlation on the electron density distribution is shown to be inconsequential for the prediction of the direction of the Jahn–Teller distortion.
Simulated quenching to the zero-temperature limit of the grand-canonical ensemble
Authors: Phillpot S. R. , Rickman J. M.
Published: July 2002

A formalism for obtaining the zero-temperature structure of monocomponent solids in the grand-canonical ensemble is developed. The new methodology, grand-canonical simulated quenching, is validated by simulating systems initially containing a vacancy and an interstitial. As a first application, the reconstruction of a high-angle twist grain boundary in fcc Cu is investigated.
The molecular dynamics around the glass transition and in the glassy state of molecular organic systems: A 2H−nuclear magnetic resonance study
Authors: Schnauss W. , Fujara F. , Sillescu H.
Published: July 2002

2H-nuclear magnetic resonance(NMR)-spin–lattice relaxation experiments have been performed for studying the crossover from viscous (α process) to secondary (β processes) dynamics in the van der Waals liquid orthoterphenyl and the H-bridged network glycerol. The essential and general features, observed in both systems, are the following: (a) a dominating α process in the liquid and viscous regime; (b) a change from exponential to nonexponential spin–lattice relaxation as the temperature is lowered below a characteristic temperature above Tg; (c) the existence of a slow (>10−9 s) secondary reorientational process in the highly viscous regime; and (d) the existence of a fast (∼10−12 s) local process in the glassy state. Whereas the slower process is shown to be the one known from dielectric studies, we attribute the fast mode to a β process found in quasielastic neutron scattering.
Photoelectron spectroscopy of small antimony cluster anions: Sb−, Sb2−, Sb3−, and Sb4−
Authors: Polak Mark L. , Gerber Gustav , Ho Joe , Lineberger W. C.
Published: July 2002

We report the 351 nm photoelectron spectra of Sb−, Sb2−, Sb3−, and Sb4−. The electron affinity of atomic Sb is measured to be 1.046(5) eV. The Sb2− photoelectron spectrum displays rich vibrational and electronic structure. Low-lying electronically excited states are observed for both the anion and the neutral. Several features in both the 351 and 364 nm photoelectron spectra of Sb2− cannot be explained as Franck–Condon processes, indicating that we are accessing autodetaching resonances of the negative ion at these wavelengths. The adiabatic electron affinity of Sb2 is determined to be 1.282(8) eV. For the photoelectron spectra of Sb3− and Sb4−, the observed electronic structure is explained in terms of recently reported ab initio calculations. The adiabatic electron affinity of Sb3 is estimated to be 1.85(3) eV, and an upper bound on the electron affinity of Sb4 is reported, EA(Sb4)≤1.00(10) eV. The vertical detachment energies of Sb3− and Sb4− to the neutral ground states are determined to be 1.90(2) and 1.57(5) eV, respectively. We report photoelectron angular distributions for all the observed spectra, and find that the autodetaching resonance causes unusual angular distributions for Sb2− photodetachment. Finally, electron affinity trends for group V atoms, dimers, and small clusters are discussed in light of the present study.
Quasiclassical state to state reaction cross sections for D+H2(v=0, j=0)→HD(v′,j′)+H. Formation and characteristics of short-lived collision complexes
Authors: Aoiz F. J. , Herrero V. J. , Sáez Rábanos V.
Published: July 2002

State resolved total and differential reaction cross sections, as well as reaction probabilities, have been calculated by the quasiclassical trajectory (QCT) method for the D+H2(v=0, j=0)→HD(v′,j′)+H reaction on the Liu–Siegbahn–Truhlar–Horowitz potential energy surface in the collision energy range 0.30–1.25 eV. Thus a detailed comparison with existing fully converged quantum mechanical (QM) calculations has been performed. The general agreement between both sets of results is good with some differences. QCT integral reaction cross sections for the production of HD(v′=0) are lower than the corresponding QM ones by 10%–15% for collision energies higher than 0.6 eV, and the energy dependence of the QCT reaction probability with a total angular momentum J equal to zero shows no structure when summed over all j′ states (contrary to the QM case). The differential cross sections for the lowest j′ values show, when represented as a function of energy, a ‘‘ridge’’ feature similar to the one found in exact QM calculations and attributed to a broad resonance. The analysis of the trajectories leading to low j′ shows progressively longer collision times as the scattering angle decreases. The longest-lived trajectories, related to the formation of short-lived complexes, cause local maxima in the differential cross section at the lowest scattering angles for each energy. These local maxima are the origin of the ridge. The lifetime of the classical short-lived complexes is estimated to be 15–35 fs, clearly larger than the lifetimes obtained from the width of the ridge feature, when interpreted as a quantum mechanical resonance.
Direct observation of surface reactions by scanning tunneling microscopy: Ethylene→ethylidyne→carbon particles→graphite on Pt(111)
Authors: Land T. A. , Michely T. , Behm R. J. , Hemminger J. C. , Comsa G.
Published: July 2002

We have used variable temperature, ultrahigh vacuum scanning tunneling microscopy (STM), in both static and time-dependent experiments, to study the chemistry of the ethylene/Pt(111) system. Images of ethylene which exhibit long-range order have been obtained at a sample temperature of 160 K. The conversion of ethylene to ethylidyne has been observed directly in STM images. This conversion reaction is observed to occur in a ‘‘patchy’’ manner across the surface at saturation coverage. As the reaction proceeds, well-defined islands of unreacted ethylene continue to be clearly observed. Further dehydrogenation of the ethylidyne formed from ethylene leads to carbon containing particles dispersed randomly across the sample. After annealing the ethylidyne covered sample to 500 K, the surface is uniformly covered with carbon containing particles which exhibit a bimodal distribution of heights (one and two atomic layers) consisting of an average of ten and twenty carbon atoms, respectively. Further annealing to 700 K results in the formation of larger particles which appear to be a single atomic layer high and 10–15 Å in diameter in the STM images. These particles contain an average of thirty-four carbon atoms. Pt steps do not show any preferential reactivity for these reactions. Annealing the carbon particle covered surface to higher temperatures results in the formation of monolayer thick graphite islands, which eventually accumulate at the Pt steps.
Monte Carlo simulation for a symmetrical electrolyte next to a charged spherical colloid particle
Authors: Degrève Léo , Lozada-Cassou Marcelo , Sánchez Enrique , González-Tovar Enrique
Published: July 2002

Results from Monte Carlo (MC) simulations for a restricted primitive model symmetrical electrolyte next to an isolated spherical macroion are reported. Calculations were made for various 1:1 and 2:2 electrolyte concentrations and macroion’s radii and charges. The MC results are compared to those from the hypernetted chain/mean spherical approximation (HNC/MSA) integral equation and the fifth version of the modified Poisson–Boltzmann (MPB5) and Poisson–Boltzmann (PB) differential equations. An overall good agreement of the HNC/MSA and MPB5 results with the MC data is found. On the other hand, the widely used PB theory is found to have important quantitative and qualitative disagreements with the MC results.
Simulations of the NO+NH3 and NO+H2 reactions on Pt(100): Steady state and oscillatory kinetics
Authors: Lombardo S. J. , Fink T. , Imbihl R.
Published: July 2002

A model for the reactions of NO+NH3 and NO+H2 has been developed for simulating the reduction of NO on Pt(100) in the 10−6 mbar pressure range for temperatures between 300 and 700 K. The model consists of seven ordinary differential equations for describing the coverage changes of six adsorbed species as well as an equation for describing the 1×1⇄hex phase transformation. Simulations of the N2 and H2O reaction rates for both reaction mixtures reproduced the hysteresis effects and the existence range for kinetic oscillations, which were found in the experiments. In addition, the occurrence of the so-called ‘‘surface explosion’’ in both reaction systems is well described by the model. In contrast to the NO+CO reaction on Pt(100), where oscillations may also take place on a pure 1×1 substrate, the 1×1⇄hex phase transition occurs during oscillations for the NO+NH3 and NO+H2 reactions. The transitions between different adsorbate/substrate phases during one oscillatory cycle which are predicted by the model are in agreement with experimental observations made by photoemission electron microscopy (PEEM) for the NO+NH3 reaction. Using values for the constants which were taken from experiments, the model provided quantitative predictions of the absolute reaction rates as well as the relative rates of the competing reaction channels, e.g., N2 and NH3 production in the case of the NO+H2 reaction. The similar dynamical behavior observed in the NO+H2 and NO+NH3 reactions on Pt(100) is attributed to the insensitivity of NO reduction to the source of the hydrogen atoms.
A molecular beam study of the CO-induced surface phase transition on Pt{100}
Authors: Hopkinson Andrew , Guo Xing-Cai , Bradley Josephine M. , King David A.
Published: July 2002

The CO-induced hex→(1×1) surface phase transition on Pt{100} is driven by the higher adsorption heat of CO on the (1×1) phase than on the hex phase and occurs by the sequential steps of CO adsorption on the hex phase and growth of islands of CO trapped on the (1×1) phase. We have investigated the dynamics of CO chemisorption on both the rotated hex (hex-R) and (1×1) surfaces by measuring the sticking probability as a function of surface temperature, beam energy, and beam incidence angle using the King and Wells beam reflectivity method. For both clean surfaces, the adsorption is nonactivated and there is no evidence for adsorption via an intrinsic precursor. However, at finite CO coverages, there is an additional adsorption pathway involving an extrinsic physisorbed precursor. At surface temperatures above 350 K, the net sticking probability on the initial hex-R surface at finite CO coverages is less than below 350 K due to CO desorption, primarily from the hex-R phase, competing with island growth. We have developed a technique to measure the local CO coverage on the hex-R phase during the CO-induced hex-R→(1×1) phase transformation. The island growth rate has a power law dependence on the local CO coverage on the hex-R phase with an apparent reaction order of 4.5±0.4. These kinetics manifest themselves as a strongly flux-dependent net sticking probability. The absence of a decrease in the local CO coverage on the hex-R phase after the onset of island growth is unexpected considering the nucleation-and-growth process, and two possible microscopic mechanisms for the surface phase transition are proposed.
A condensed counterion theory for polarization of polyelectrolyte solutions in high fields
Authors: Manning Gerald S.
Published: July 2002

Counterions bound to a polyion are polarized by an applied electric field. A polyelectrolyte theory for this effect is constructed by placing the Mandel polarization model within the framework of counterion condensation theory. A central feature of the Mandel model, retained here, is equilibration with the field of the local concentration of condensed counterions along the length of the polyion. Briefly, the bound counterions are assumed to polarize but not to conduct. In qualitative agreement with observations, the theoretical induced dipole moment saturates; condensed counterions respond to increases of the field strength by piling up at one end of the polyion until further polarization is not possible. The value of the saturated dipole when polyelectrolyte effects are considered is the same as in the Mandel theory, but the onset of saturation is pushed to much higher field strengths. Massive quantitative discrepancies between theory and high-field data point to reasons for dipole saturation other than the postulates of the model. It seems likely that condensed counterions respond to increases in field strength above a relatively low threshold by carrying a current, not by further increases in polarization.
Ab initio potential energy surface for H–H2
Authors: Partridge Harry , Bauschlicher Charles W. , Stallcop James R. , Levin Eugene
Published: July 2002

Ab initio calculations employing large basis sets are performed to determine an accurate potential energy surface for H–H2 interactions for a broad range of separation distances. At large distances, the spherically averaged potential determined from the calculated energies agrees well with the corresponding results determined from dispersion coefficients; the van der Waals well depth is predicted to be 75±3μEh. Large basis sets have also been applied to reexamine the accuracy of theoretical repulsive potential energy surfaces (25–70 kcal/mol above the H–H2 asymptote) at small interatomic separations; the Boothroyd, Keogh, Martin, and Peterson (BKMP) potential energy surface is found to agree with results of the present calculations to within the expected uncertainty (±1 kcal/mol) of the fit. Multipolar expansions of the computed H–H2 potential energy surface are reported for four internuclear separation distances (1.2, 1.401, 1.449, and 1.7a0) of the hydrogen molecule. The differential elastic scattering cross section calculated from the present results is compared with the measurements from a crossed beam experiment.
Path integral formulation of retardation effects in nonlinear optics
Authors: Chernyak Vladimir , Mukamel Shaul
Published: July 2002

The signatures of retardation in nonlinear optical susceptibilities are studied by starting with the multipolar Hamiltonian and using path integral techniques to develop a perturbative scheme for incorporating the retarded interaction with the electromagnetic field. The present approach accounts for cooperative radiative decay and polariton transport which show up in resonant spectroscopies of nanostructures. These effects, which require a quantum electrodynamical description of the field, are missed by conventional nonretarded theories. Application is made to the enhanced spontaneous emission rate of biexcitons, which may show up in the nonlinear reflection off molecular superlattices.