bluuebunny/crossref_metadata_2025 huggingface dataset:
Impact of global interactions on patterns in a simple system
Authors: Middya Usuf , Luss Dan
Published: July 2002

Numerical simulation by Pearson revealed a surprising variety of spatiotemporal patterns in a simple two-dimensional reaction–diffusion system. The mutual (or global) interaction between the overall reaction rate in a thin slab and the reactants concentrations in the surrounding vessel may have a strong impact on the dynamics of the system. It may lead to formation of patterns even when diffusion–reaction interactions cannot create patterns. Changes in this global interaction, which can be accomplished by changes in the reactant flow rate, may change the qualitative shape of the pattern and cause it to appear or disappear. Patterns of qualitatively different nature may be obtained for different initial conditions.
Fast ion beam laser spectroscopy of N2O+: Observation and interpretation of the Fermi resonance in the 100 2Π vibronic level of the ground state
Authors: Chafik el Idrissi M. , Larzilliere M. , Carré M.
Published: July 2002

We present the analysis of the 100 Ã 2Σ–100 X̃ 2Π3/2 band of N2O+ observed by fast ion beam laser spectroscopy. The obtained spectrum allowed us to interpret the Fermi interaction effects in the X̃ 2Π ground state. The two Fermi parameters W1 and W2 were determined for the first time. The obtained values are in good agreement with those calculated theoretically and those obtained for other isoelectronic linear triatomic molecules.
Microscopic theory of molecular crystal surface polarization, piezoelectricity, and pyroelectricity
Authors: Munn R. W.
Published: July 2002

Nonpolar crystals must exhibit a surface polarization induced by the loss of bulk symmetry at the surface. For crystals of nonpolar molecules, the surface polarization is normally induced by incomplete cancellation of the electric fields of the molecular quadrupole moments; for crystals of polar molecules, it is produced by incomplete cancellation of the electric fields of the molecular dipole moments. Algebraic treatments are developed for both cases, including all local-field effects via planewise dipole sums. Surface piezoelectricity and pyroelectricity follow from the strain and temperature dependences of the surface polarization. Numerical calculations are presented in an accompanying paper.
Calculation of surface quadrupole field, polarization, and pyroelectricity for anthracene and pyrene crystals
Authors: Gill A. J. , Munn R. W.
Published: July 2002

Surface quadrupole tensor sums have been calculated for unrelaxed (001), (010), and (100) surfaces of anthracene and pyrene, with the molecules treated as points. The surface has most effect on sums relating molecules in different surface layers. With the molecular quadrupole moments, the quadrupole sums give the surface electric fields, which reach 5 GV m−1 for anthracene and 15 GV m−1 for pyrene. The fields approach the bulk value by about the fourth molecular layer below the surface. With the molecular polarizability and surface dipole tensor sums, the electric fields give the surface polarizations, which reach 80 mC m−2 for anthracene and 340 mC m−2 for pyrene. With the crystal thermal expansion for anthracene, the polarizations give the surface secondary pyroelectricity, which is found to be of the order of 10 μC m−2 K−1 for all the surfaces. Treating the molecules as point submolecules at the center of each aromatic ring tends to reduce the anisotropy of the results somewhat.
Guided ion beam measurement of the product branching ratios for the ion-molecule reaction N++O2 as a function of collision energy
Authors: Guettler Robert D. , Jones Glenn C. , Posey Lynmarie A. , Kirchner Nicholas J. , Keller Beat A. , Zare Richard N.
Published: July 2002

A quadrupole-octopole-quadrupole mass spectrometer has been constructed for comparing ion-molecule reaction product intensities as both the internal excitation and the kinetic energy of the reactant ion are varied. Such comparisons require an ion beam with a known kinetic energy distribution and, most importantly, they require product intensity measurements made without significant bias in detection of the different product channels. To assess the characteristics of our instrument, we have studied the ion-molecule reaction N++O2 that is known to yield three different product channels: N+O+2, NO++O, and NO+O+. Ion beam trajectory simulations combined with experimental measurements show that the spread in the kinetic energy of the reagent ions has a fixed value in the range of 0.6 to 1.1 eV full width at half maximum in the center of mass (c.m.). Relative cross sections for the three different product channels are reported as a function of c.m. collision energy. A comparison of the observed product branching ratios with those determined previously by other workers shows that no serious product discrimination occurs over the collision energy range of 1.5 to 10.0 eV c.m. Discrepancies in the product branching ratios below 1.5 eV c.m. are believed to be caused by the overall collision energy uncertainty that results from both the ion beam kinetic energy spread and the thermal motion of the O2 reactant.
Hard square lattice gas
Authors: Baram Asher , Fixman Marshall
Published: July 2002

It is shown that the fluid branch of the hard square lattice gas terminates at a finite activity zf. Estimates of zf indicate that it is identical to the termination activity of the solid branch zs, found by Baxter, Enting, and Tsang [J. Stat. Phys. 22, 465 (1980)] to be at zs=3.7962(1), resulting in a second order phase transition with zf=zs=zc. The radius of convergence of the virial series p(ρ) is found to be smaller than the value of the critical density ρc. For a finite lattice a complex Yang–Lee zero of the grand partition function converges to zc like the reciprocal width of the lattice.
Photodecomposition of Mo(CO)6/Si(111) 7×7: CO state-resolved evidence for excited state relaxation and quenching
Authors: Chu Pamela M. , Buntin Steven A. , Richter Lee J. , Cavanagh Richard R.
Published: July 2002

State-resolved detection techniques have been used to characterize the ultraviolet photodecomposition dynamics of Mo(CO)6 on Si(111) 7×7 at 100 K. Details of the excitation/fragmentation mechanism including adsorbate energy transfer were examined by measuring the cross sections and the internal and translational energies of the photoejected CO from submonolayer through multilayer coverage regimes. The CO energy distributions are found to be independent of Mo(CO)6 coverage, and can be characterized by two components with markedly different mean energies. In contrast to the coverage independence of the measured energy disposal, the cross section was found to decrease by a factor of 3 from multilayer coverages to submonolayer coverages.
A comprehensive experimental study of the dynamical interaction of He atoms with Cu(001) surface phonons
Authors: Hofmann F. , Toennies J. P. , Manson J. R.
Published: July 2002

We present new measurements of inelastic He atom scattering from the surface phonons of Cu(001) as a function of crystal temperature, incident energy, and parallel momentum transfer. A careful subtraction of the multiphonon intensity and other background contributions from the time-of-flight intensities reveals three distinct surface-localized vibrational modes which are ascribed to the Rayleigh phonon, the longitudinal bulk resonance, and a further acoustic bulk resonance at higher energy transfers. The longitudinal resonance couples very strongly to the scattering He atoms and, for a wide range of incident conditions, gives peaks which are more intense than those due to the Rayleigh mode. The energy and momentum dependence of these peak intensities are analyzed with the aid of a simple distorted wave Born approximation, and the different coupling parameters for the two modes are determined and compared with other available data. The incoherent diffuse elastic peak is shown to decrease as a function of parallel momentum transfer according to the theory of Fraunhofer scattering from a random array of point defects. The multiphonon background is shown to be in agreement with a quick scattering approximation.
Frequency effect on the charge screening in electrolyte solutions. Charge fluctuation and spatial oscillations studied by Brownian dynamics simulation
Authors: Vallejo Diego F. , Grigera J. Raúl
Published: July 2002

A series of Brownian dynamics simulations of electrolyte solutions were performed with the purpose of studying the effects of charge fluctuation and dynamics on charge screening. The interaction potential between ions was selected as having a Coulombic and a repulsive component with the proper parameters to simulate a NaCl-like solution. The electric field generated by the ions including the charge screening, and the effective electrical forces were computed for regular solutions and for cases in which the magnitude of some charges fluctuate or have fast oscillations. The results show that, in agreement with the theory, for fast fluctuating charges there is no charge screening. It was also observed that in the cases for which the charge is moving fast enough the results are equivalent to the fluctuating charge situation.
Theoretical study of Ca(4s5p 1P)→Ca(4s5p 3P) energy transfer in collisions with He. Initial and final state alignment
Authors: Duhoo Thierry , Pouilly Brigitte
Published: July 2002

In this paper we present the results of close-coupling calculations of the cross sections for Ca(4s5p 1P)→Ca(4s5p 3P) energy transfer in collisions with He, based on new potential energy curves. Particular attention is devoted to the simulation of the recent experiment of Smith and co-workers [J. Chem. Phys. 96, 8212 (1992)], in which, for the 1P→3P2 transfer both initial and final alignment are controlled with respect to the initial relative velocity vector of the two partners Vrel. The calculated polarization ratios (σ⊥/σ∥), defined as the ratio of the cross sections for the 1P→3P transfer summed over final levels for initial alignment of the 5p orbital perpendicular and parallel to Vrel are in good agreement with the experimental results. The theoretical cross sections for the 1P→3P2 transfer determined in the so-called collision frame (σj1m1m1→j2m2m2), where the axis of quantization is taken along Vrel are in good agreement with the experiment in the case of initial perpendicular excitation, but show quantitative and qualitative disagreement in the case of initial parallel excitation. We find that relative populations into the final m2 levels depend strongly on the initial orientation of the 5p orbital with respect to the initial collision plane. However, we show, that in the case where the 5p orbital lies in the initial collision plane, the relative populations in the final m2 components of the 3P2 state depend exclusively on interactions among the exit channels.
Irreversible adsorption of macromolecules at a liquid–solid interface: Theoretical studies of the effects of conformational change
Authors: Van Tassel Paul R. , Viot Pascal , Tarjus Gilles , Talbot Julian
Published: July 2002

The effects of particle conformational changes on the kinetics and saturation coverage of irreversible macromolecular adsorption at liquid–solid interfaces are investigated by computer simulation of a modified random sequential adsorption model. In this model, macromolecules (modeled as disks of diameter σα) adsorb onto a surface at a rate ka. Once adsorbed, the particles spread symmetrically and discretely to a larger diameter σβ at a rate ks. Adsorption or spreading events which result in the overlap of particles on the surface are not allowed. We investigate the effects of changes in spreading magnitude Σ (=σβ/σα) and relative spreading rate Ks (=ks/ka). We observe that the saturation coverage of spread particles decreases while that of unspread particles increases with spreading magnitude. This dependence is most pronounced for small spreading: the derivative of the surface coverage of both spread and unspread particles with respect to Σ diverges logarithmically when Σ→1. An increase in the rate of spreading increases the saturation coverage of spread particles while decreasing that of unspread particles. The dependence of the coverage on spreading rate is weaker than its dependence on spreading magnitude: a four order of magnitude change in Ks results in a factor of 2 change in the partial coverages. The coverage of unspread particles may become nonmonotonic in time for certain values of Σ and Ks. The total density of particles on the surface decreases and the average particle size increases with Ks, in accordance with recent protein adsorption experiments.
Quantitative determination of the activity of defect sites on a single-crystalline surface: C–H bond activation of carbon-13 labeled ethane on Ir(111)
Authors: Johnson Dale F. , Weinberg W. Henry
Published: July 2002

The C–H bond activation of carbon-13 labeled ethane 1,2-di-13C–C2H6 at defect sites on an Ir(111) single-crystalline surface, cut and polished to 0.70°±0.15° of the (111) direction, has been determined quantitatively. These results have been obtained from a kinetic model accounting for diffusion from sites on the less reactive terraces to step edges on the surface, combined with the appropriate trapping-mediated description of the overall rate processes operative at the defects. The activation energy for reaction at the defect sites was found to be 4500±1500 cal/mol, which is ∼6000 cal/mol less than the reaction barrier at the terrace sites. The preexponential factor of the reaction rate coefficient at the defect sites was found to lie between 5×1011 and 1012 s−1, which is five to ten times greater than the preexponential factor of the reaction rate coefficient at the close-packed (111) terrace sites that dominate the surface. The net effect is that at room temperature, the defect activity is approximately four and one-half orders of magnitude greater than that of the close-packed terrace sites.
Electronic energy transfer kinetics of Xe+(2P1/2) at very low temperatures
Authors: Latimer Darin R. , Smith Mark A.
Published: July 2002

Nonadiabatic Xe+(2P1/2) electronic fine structure relaxation by collision with several gases has been investigated using a monitor method at temperatures near 4 K. Of the several collision partners, only CH4 and N2O exhibited measurable spin–orbit relaxation rates of 6.1×10−10 and 1.2×10−10 cm3 s−1, respectively. Both CH4 and N2O possess parallel exothermic charge transfer and electronic quenching channels which may suggest a sequential two electron charge transfer channel facilitated electronic relaxation mechanism.
Hyperfine and Zeeman quantum beats in the B 2Π1/2 state of NO
Authors: Baker Jacob , Bramble Simon K. , Hamilton Peter A.
Published: July 2002

Quantum beat signals have been observed in v=0, 1, 2, and 3 of the B 2Π1/2 state of NO for the first time and are used to carry out Doppler free spectroscopy in these levels. Beat signals are observed at zero magnetic field in all four vibrational levels between different hyperfine components for J=3/2 and J=5/2, and are analyzed to derive accurate hyperfine constants and quadrupole coupling parameters. Although from their magnetic tuning rates the beats between the different hyperfine levels can be assigned to particular F′↔F separations, the energy ordering of the F levels and the parity assignment remain to be determined. Independently of this exact assignment a small variation in the hyperfine parameters with vibrational state is observed which is irregular in v=3 indicating the presence of an interaction with another electronic state. When a small magnetic field is applied much more intense beat signals are observed which occur between the Zeeman split components of a single hyperfine level. The B 2Π1/2 state is nominally nonmagnetic in the Hund’s case (a) limit and so the effective g values vary rapidly with J due to spin–orbit uncoupling. The relative intensities of these beat signals and their variation with both magnetic field and vibrational level is accurately predicted using the known spectroscopic constants and a fitted value of gr.
Effect of fluid flow on the fluctuations at the surface of an elastic medium
Authors: Kumaran V.
Published: July 2002

The effect of a linear shear flow of a Newtonian fluid in the region 0<z<∞ on the fluctuations at the surface of an elastic medium of thickness H in the region −H<z<0 is analyzed in the regime Re≫1 and Λ∼1, where Re=ργH2/η is the Reynolds number and Λ=(ργ2H2/E)1/2 is the ratio of the inertial stresses in the fluid and the elastic stresses in the solid. Here ρ and η are the fluid density and viscosity, E is the coefficient of elasticity of the solid, and γ is the mean strain rate in the fluid. A linear analysis is used to determine the effect of the flow on the fluctuations in the surface displacement, and an asymptotic expansion in the small parameter ε=(Λ/Re) is employed. The dynamics in the bulk of the fluid is inviscid in the leading approximation, and the leading order growth rate is imaginary because energy is conserved in the absence of viscous dissipation. There are multiple frequencies of oscillation, all of which satisfy the equations of motion. An increase in the fluid velocity increases the frequency of the downstream traveling waves, and decreases the frequency of the upstream traveling waves. The structure factor for the surface modes of the upstream traveling waves increases with an increase in the fluid velocity because the kinetic energy of the fluctuations decreases due to the lower frequency.

An opposite effect is observed for the downstream traveling waves; in addition, it is observed that the structure factor has a double-peaked structure and reaches zero at an intermediate value at sufficiently high velocities. This is due to a divergence in the ratio of the tangential and normal displacements, and a consequent divergence in the energy required for the normal fluctuations at the surface. There is an O(ε1/2) correction to the growth rate due to the presence of a viscous boundary layer of thickness Hε1/2 in the fluid at the interface. The O(ε1/2) calculation shows that the real part of the growth rate is negative for all values of Λ and wave number k, except along certain lines in the Λ−k parameter space where the real part of the growth rate is zero, because the amplitude of the boundary layer velocity becomes zero along these lines. The real part of the O(ε) correction to the growth rate at these points is negative, indicating the presence of a small stabilizing effect due to the dissipation in the bulk of the fluid and the elastic medium.
Geometrical representation of coherence transfer selection by pulsed field gradients in high-resolution nuclear magnetic resonance
Authors: Mitschang Lorenz , Ponstingl Hannes , Grindrod David , Oschkinat Hartmut
Published: July 2002

A formalism for the calculation of suitable sequences of pulsed field gradients for signal selection in high-resolution NMR spectroscopy is presented. It is based on a geometrical interpretation of coherence transfer pathway selection by pulsed field gradients. The formalism allows the calculation of the suppression rates for undesired pathways and the determination of the most efficient sequence of pulsed field gradients. As an example, sequences for multiplicity filtered 13C/1H correlation experiments are calculated and analyzed.
U
q
(4)⊇O
q(4) description of diatomic molecules: Vibrational spectra
Authors: Gupta Raj K. , Cooper Ian L.
Published: July 2002

Quantum-deformed algebra (q-DA) of the O(4) dynamical symmetry of the U(4) group is studied in respect of the vibrational spectra of diatomic molecules. It is shown that, as in the case of the U(6) group for nuclei, the q-deformation parameter in Uq(4)⊇Oq(4) must also be complex for any meaningful gain in going from the classical to the q-deformed algebra. Application is made to the ground electronic state (X 1Σ+g) of the H2 molecule.
Computer simulations of the electron spin resonance spectra of steroid and fatty acid nitroxide probes in bilayer systems
Authors: Eviatar Hadass , van der Heide Uulke A. , Levine Yehudi K.
Published: July 2002

Monte Carlo dynamics (MCD) techniques are used to simulate the orientational behavior and rotational motion of probe molecules in lipid bilayers. The trajectories of molecular orientations generated from the simulations are then used to calculate the order parameters and the orientational time correlation functions. The behavior of the time correlation functions is compared with the predictions of the rotational diffusion (RDM) and the compound motion (CM) models. The MCD trajectories are also used to produce electron-spin resonance (ESR) spectra, employing a recently developed time-domain algorithm. Two questions which have been the subject of debate in the literature are addressed. The first question concerns the discrepancy between the ability of motional models to describe ESR spectra and fluorescence depolarization measurements on rigid molecules in vesicles—while the RDM does an excellent job of fitting the former, the latter require the CM to describe them properly. It is argued that the key to resolving this lies in the fact that the ESR line shapes are sensitive to the tumbling motions of the long molecular axes as well as to rotational motions about them, while fluorescence anisotropy is blind for the latter. The rotation about the long molecular axis introduces a fast decay into the correlation functions in a way independent of the tumbling motion of the axis. The second question concerns the fidelity of reporting by fatty acid spin probes in lipid bilayers. It is shown that the motion of the bulky hydrophillic doxyl group does not, in fact, reflect the motion of the chains about it and consequently these spin probes cannot be considered good reporters for these applications.
Ab initio study of the dication carbon trimer C32+
Authors: Hogreve H.
Published: July 2002

Motivated by the recent experimental observations of multiply positively charged fullerene ions Cnk+, n=56, 60, 70, k=1–7, the doubly charged trimer C32+ is investigated by multireference configuration interaction computations. Studying the lowest potential energy surfaces of various electronic states, several local minima are found that entail not only metastability of the system, but the involved large and wide barriers render C32+ almost stable against dissociative tunneling. Our computations predict a linear configuration as the most favorable one for C32+, with a 1Σg+ ground state and an energy slightly below the minima found for the lowest triplet state 3Σu+ or occurring in C2v geometry (state 1A1). Stability and additional properties of C32+ are discussed and compared with those of other trimer dications. Moreover, this study includes accurate data for low-lying states of the dimer ions C2+ and C22+ that are of relevance for the dissociation channels of C32+.
Orbital alignment cross sections by stimulated emission probing: The state-to-state Ca Rydberg process Ca(4s17d 1D2)+Xe→Ca(4s18p 1P1)+Xe
Authors: Spain Eileen M. , Dalberth Mark J. , Kleiber Paul D. , Leone Stephen R. , Op de Beek Stefan S. , Driessen Jan P. J.
Published: July 2002

The initial state alignment effect vs relative velocity is measured for a state-to-state Ca Rydberg collisional energy transfer process. The stimulated emission detection method is used to determine the alignment effect for the n,l-changing transition: Ca(4s17d 1D2)+Xe→Ca(4s18p 1P1) +Xe+ΔE=−1.7 cm−1. The rate of electronic energy transfer in this state-changing collision is observed to vary with the direction of the Rydberg electron charge cloud relative to the collision axis. Both the expected cos(4β) and cos(2β) dependencies are observed. The alignment data are analyzed to obtain the relative cross sections for the individual Ca(1D2) magnetic sublevels. The values of the m-sublevel cross sections σ0:σ‖1‖:σ‖2‖ are 1.13±0.02:1.11±0.02:0.83±0.02. Qualitative interpretations of the relative cross sections in terms of both molecular (van der Waals) Born–Oppenheimer potentials and the impulse approximation are presented.
Use of relativistic effective core potentials in calculating the electronic spectrum of the antimony dimer
Authors: Das Kalyan K. , Liebermann Heinz-Peter , Hirsch Gerhard , Buenker Robert J.
Published: July 2002

A relativistic configuration interaction study including the spin–orbit coupling has been carried out for a large number of electronic states for the antimony dimer and comparisons with analogous results for the isovalent Bi2 molecule have been made. Bond lengths re, vibrational frequencies ωe, excitation energies Te, and dissociation energies De have been computed for all bound states up to 40 000 cm−1, as well as transition probabilities and radiative lifetimes based on electric–dipole matrix elements. The calculations confirm that the A1u state derives mainly from the π→π* 3Δu λ–s state and not from the lower-lying 3Σ+u. The lifetime of this state (0.3 ms) is computed to be 50 times shorter than for the B0+u state, suggesting that emissions from either of these states will be quite difficult to observe in gas-phase studies. By contrast the lifetime of the D0+u state is relatively short (35.6 ns), consistent with the fact that the D–X band system is quite intense. No suitable candidate is indicated for the K–X transition observed by Sontag and Weber, however, and thus it is suggested that the corresponding spectrum is a part of the D–X system.
The electronic spectroscopy of the Ba+–Ar complex: Potential surface and dissociation energies
Authors: Panov Sergey I. , Williamson James M. , Miller Terry A.
Published: July 2002

Ba+–Ar open-shell ionic complexes were produced in a pulsed free-jet expansion. The dispersed emission and both the low and high resolution A 2Π–X 2Σ+ excitation spectra of the Ba+–Ar complex are reported. The data obtained were used to construct potentials for the ground and excited states. A simple quantum mechanical model was introduced in order to simulate the experimentally measured potentials. The model potential is used to estimate the dissociation energy of the ground 2Σ+ state. This value, when combined with the spectral red shift, allows the dissociation energies of the two components of the excited 2Π state to be determined. The same electrostatic interaction model also explains the observed angular momentum coupling scheme as well as the much stronger binding in the excited 2Π state.
Study of N2O2 by photoelectron spectroscopy of N2O2−
Authors: Arnold Don W. , Neumark Daniel M.
Published: July 2002

Photoelectron spectra of the N2O−2 anion, collected at hν=4.657 and 5.822 eV, are presented. The spectra originate from the C2v isomer of the anion. Vibrationally resolved progressions corresponding to transitions to several electronic states of the previously unobserved N2O2 molecule are observed. All of the observed transitions lie above the dissociation asymptotes for N2+O2, NO+NO, and O+N2O, and several lie above the N+NO2 and N2+O+O asymptotes. Ab initio calculations have been carried out for the anion ground state and several singlet and triplet states of neutral N2O2. By comparing the observed spectra with Franck–Condon simulations based on these calculations, the lowest bands observed in our spectra were assigned to transitions to the 3A2 and 3A1 states (C2v symmetry) of N2O2. These spectra thus represent the first experimental characterization of metastable, high energy forms of N2O2. Both the N2O−2 and the N2O2 species are considered in terms of their roles as reactive intermediates in the O−+N2O and N+NO2 chemical reactions.
A theoretical investigation of the interactions between thiophene and vanadium, chromium, copper, and gold
Authors: Elfeninat F. , Fredriksson C. , Sacher E. , Selmani A.
Published: July 2002

We have employed quantum chemical methods, at the local spin density approximation level, to study the interaction between an organic semiconductor, polythiophene, and potential metals for hole injecting contacts in devices: vanadium, chromium, copper, and gold. The results show that there is a strong interaction between vanadium and the thiophene ring, mainly due to covalent bond formation between the metal and the S and Cα atoms of the thiophene. Vanadium is therefore predicted to provide good conditions for chemisorption and mechanical stability at the polymer/contact interface. A similar, but considerably weaker, covalent interaction is found between chromium and all the conjugated atoms of the thiophene molecule. For both these metals, the interactions cause the thiophene ring to lose its aromaticity and planarity which, as a consequence, would interrupt the π-electron system in a polymer and impair charge transport along the chains. In the case of copper, the metal is found to react only with the sulfur atom of the thiophene and to a very small extent. For gold, the results indicate that there are no significant chemical interactions. Our results are in good qualitative agreement with XPS measurements performed during metallization of thin films of poly(3-hexylthiophene). The calculations confirm the general trend of reactivities for this series of metals: V≳Cr≳Cu≳Au=0.
Collision complex model of molecules scattering from corrugated surfaces
Authors: Andersson Mats B. , Pettersson Jan B. C.
Published: July 2002

A collision complex that gives almost quantitative agreement with a large set of data for inelastic scattering of atoms and molecules from surfaces is presented. In the model, a scattering molecule and a small part of the surface form a collision complex, that decomposes in a unimolecular fashion after statistical redistribution of energy. Both molecular translation and rotation are included in the model, and the surface is represented by a small number of harmonic oscillators. The surface is considered as locally flat at the place of impact, and surface corrugation is represented by a Gaussian distribution of local normal directions. Analytical solutions of simple integrals clearly illustrate the functional dependence on the principal parameters: translational energy, scattering angle, surface temperature, the relative size of the surface directly interacting with a scattering molecule, and the active degrees of freedom. Angular distributions for atoms, diatomic and polyatomic molecules scattering from metals, graphite and liquid surfaces are shown to be in good agreement with experimental results at thermal translational energies, and at least up to 0.5 eV. The model provides a simple and useful way to interpret and inter-relate experimental results, and makes it possible to evaluate the total information content in experimental data.
Formation and metastable decomposition of unprotonated ammonia cluster ions upon femtosecond ionization
Authors: Buzza S. A. , Wei S. , Purnell J. , Castleman A. W.
Published: July 2002

The formation and metastable dissociation mechanism of unprotonated ammonia cluster ions, (NH3)+n, produced by multiphoton ionization (MPI) at 624 nm and a nominal pulse width of 350 fs, are investigated through a reflectron time-of-flight (TOF) mass spectrometric technique. Detection of the unprotonated ions after femtosecond and nanosecond multiphoton ionization under various intensity conditions is explained. The role of the energy of the ionizing photons, and the observation of these ions after femtosecond MPI is examined. The formation of the unprotonated series is found to be a function of intensity in the case of ionization on the nanosecond time scale, but not so for the femtosecond time domain. The results can be explained in terms of ionization mechanisms and ionizing pulse durations. The findings of the present study suggest that the unprotonated ions are trapped behind the barrier to intracluster proton transfer and/or concomitant NH2 loss. The studies of metastable decomposition also reveal that the unprotonated ammonia cluster ions dissociate in the field-free region of the TOF by losing an NH2 radical rather than via the evaporative loss of NH3 as occurs for protonated clusters. Additionally, isotopic investigations of the unimolecular decay reveal a strong dependence on the conditions of cluster formation. The cluster formation condition dependence of the unimolecular decay is further investigated by altering formation temperatures and observing the consequences reflected by changes in the spontaneous metastable decay rate constant. This is a unique example of a cluster system whose metastable dissociation does not obey an evaporative ensemble model.
Scattering aligned NO+ on Ag(111): The effect of internuclear-axis direction on NO− and O− product formation
Authors: Greeley J. N. , Martin J. S. , Morris J. R. , Jacobs D. C.
Published: July 2002

Hyperthermal energy reactions of NO+ with Ag(111) were studied as a function of collision energy and the incident diatom’s internuclear-axis direction. Laser photoselection via (1+1′) resonance enhanced multiphoton ionization (REMPI) prepared an aligned distribution of NO+(X 1Σ+, Etrans, v=0, J) prior to the molecules impact with the surface. The emergence of scattered O− products was enhanced when NO+ approached the surface with an ‘‘end-on’’ rather than a ‘‘side-on’’ orientation. Moreover, the magnitude of the reaction’s alignment preference increased with decreasing collision energy. The appearance threshold for O− was approximately 20 eV. Classical trajectory calculations demonstrated that these results are consistent with a collision-induced dissociation mechanism mediated by both vibrational and rotational excitation upon impact. Reorientation of the internuclear axis during the collision drastically affects the simulated dissociation dynamics. No alignment preference for scattered NO− emergence could be discerned from the experiments. As a complement, the angular momentum mechanics are provided to predict the internuclear-axis distribution of ions created via two-photon REMPI.
Spin dynamics in oriented lithium phthalocyanine thin films investigated by pulsed electron spin resonance
Authors: Wachtel H. , André J.-J. , Bietsch W. , von Schütz J. U.
Published: July 2002

Thin films of monolithium phthalocyanine (PcLi) were prepared by vacuum deposition on cleaved mica. The highly anisotropic spin diffusion in these films was studied by electron spin resonance (ESR). Using averaging techniques, we succeeded in the detection of films as thin as 100 nm using pulsed ESR and 2 nm with cw ESR. The electronic relaxation is shown to depend on the substrate temperatures Ts during film deposition which determine the morphology of the films. We find relaxation rates of films which are faster by a factor of 20 (T1e−1) to 30 (T2e−1) than those of single crystals. At ambient temperature, a low-dimensional spin exchange mechanism is proposed for the single crystal. At low temperatures, in single crystals the electronic relaxation rates T1e−1 and T2e−1 hint at a reduced exchange with increased dimensionality d≳1. This holds for the films at all temperatures and is explained by different crystal structures of single crystals and films. A generally applicable relaxation model is developed which describes the transition from one-dimensional to three-dimensional spin propagation.
Electronic structures of new π-conjugated cyclic polymers with quinoid structures
Authors: Hong Sung Y. , Kwon Si J. , Kim Shi C.
Published: July 2002

Geometrical and electronic structures of new π-conjugated five-membered ring polymers were theoretically investigated. These polymers are analogous to heterocyclic polymers, but adopt as bridging groups ≳CH2, ≳CF2, ≳SiH2, ≳SiF2, ≳C=CH2, ≳C=O and ≳C=S moieties instead of heteroatoms. The ground-state geometries of the polymers were predicted to be quinoid from semiempirical band calculations with AM1 Hamiltonian. The electronic properties of these systems were obtained using the modified extended Hückel method. The calculated band gaps (Eg) were analyzed in terms of geometrical relaxations and electronic effect of the bridging groups using the equation of Eg=ΔEδr+ΔE1–4+ΔEel. The effect of bond-length alternation (ΔEδr) amounts to 1.1–1.4 eV for the aromatic forms and 1.8–1.9 eV for the quinoid forms of the polymers. The interactions (ΔE1–4) between C1 and C4 atoms of the cis-PA type backbone tend to decrease the band gaps of the aromatic forms and to increase the gaps of the quinoid forms as much as 0.2–0.5 eV, depending on the size of a bridging atom. It is found that the electronic effect (ΔEel) of these bridging groups is quite small compared to that found in heterocyclic polymers such as polythiophene, polypyrrole, and polyfuran. ΔEel of ≳CF2, ≳SiH2, and ≳SiF2 bridging groups are negligible and that of the other groups amounts to 0.3–1.0 eV. Therefore, the band gaps of these systems almost correspond to the ΔEδr values which arise from the bond-length alternations, except the case of the polymers with ≳C=O and ≳C=S bridging groups whose π* orbitals strongly interact with the π system of the polymeric backbone.
Monte Carlo studies of heat capacity anomalies in two-dimensional nanoclusters
Authors: Rivera Yanira , Weber Desiree C. , López Gustavo E.
Published: July 2002

The characterization of heat capacity anomalies in a two-dimensional nanocluster is presented. Calculations of the heat capacity as a function of temperature show the presence of two coexistence regions. The first region corresponds to a solid/liquid coexistence and the second to a liquid/vapor equilibrium. These two transitions observed are analogous to melting and evaporation transitions observed in various two-dimensional bulk systems. The standard Metropolis Monte Carlo algorithm fails in the identification of these transitions due to quasiergodicity problems. The implementation of the classical J-walking algorithm reduces this problem, and allows the precise identification of these two transitions.
Nonisothermal dissolution and crystallization of ellipsoidal particles in liquid solutions
Authors: Liu Bo-Tau , Hsu Jyh-Ping
Published: July 2002

The kinetic behavior of both crystallization and dissolution of solid particles in liquid solutions is investigated. The effect of heat on the phenomena under consideration is taken into account. Although the present analysis is based on ellipsoidal particles, it can be extended to particles of various geometries by choosing an appropriate transformation. We show that the rate of dissolution increases with the decrease in particle size, and the greater the curvature of a surface, the faster the rate. On the other hand, the rate of growth of a crystal decreases with the increase of its size. For both dissolution and crystallization, the greater the curvature of a surface, the faster the rate of change in the particle size. In the analysis of the effect of the heat of dissolution/crystallization, we conclude that the rate of variation in the size of a particle for dissolution follows the order exothermic≳isothermal≳endothermic. No general rule is found for crystallization. The experimental data for the growth of ferrite allotriomorphs in Fe–C alloys reported in the literature is analyzed to justify the applicability of the present model.
A density functional study of M–C2H4 complexes (M=Li, Na, K): Singularity of the Li atom
Authors: Alikhani M. E. , Hannachi Y. , Manceron L. , Bouteiller Y.
Published: July 2002

Quantum chemical calculations on the Li–C2H4 complex have been performed with coupled-cluster and density functional methods. For both methods the electronic ground state of the complex is calculated to be 2B2, with a C2v symmetry equilibrium structure, and the calculated binding energy is quite small (around 2 kcal/mol), and therefore very much basis set dependent. The vibrational spectrum has been calculated at the harmonic approximation, including 13C/12C, 7Li/6Li, and H/D isotopic substitutions. The agreement between experimental and calculated infrared frequencies is correct, except for the low frequency symmetric Li–C stretching mode. These calculations also allow to propose an assignment for the observed C–H/C–D stretching modes. The observed blue-shift of the symmetric CH2 bending mode as well as the red-shift of the antisymmetric CH2 bending, CD2 bending, and C–C stretching modes with respect to the free ethylene have been confirmed by the density functional calculations. The Na...C2H4 complex has been found to be unstable in its 2B2 electronic state. The study of the 2A1 electronic state for both Na...C2H4 and K...C2H4 complexes show that they are at most very weak van der Waals complexes. This result confirms the conclusions of matrix isolation experiments.
Photoelectron–neutral–neutral coincidence studies of dissociative photodetachment
Authors: Hanold K. A. , Sherwood C. R. , Continetti R. E.
Published: July 2002

Photoelectron–neutral–neutral coincidence spectra have been measured for the dissociative photodetachment of O4− (O4−+hν→O2+O2+e−) at 523 and 349 nm. The neutral photofragment translational energy spectrum, the photoelectron spectrum, and the correlations of the translational energy and photoelectron energy are presented here. The correlation spectra reveal phenomena that are not discernable in either one-dimensional measurement. Features are observed which indicate that non-Franck–Condon processes play a role in the dissociative photodetachment of O4− at 349 nm.
Molecular dynamics and simple transition-state theory predictions of rates of atomic diffusion in rare-gas matrices
Authors: Guo Yin , Thompson Donald L.
Published: July 2002

The results of molecular dynamics calculations of thermal rates of diffusion of C and H atoms in Ar matrices are presented. The computed results are compared with those computed by a simple transition-state theory (STST). The results of the STST, which involve trivial computations, are in good agreement with the results of the classical molecular dynamics calculations, which require large amounts of computer time. These results show that the STST is adequate for predicting thermal rates of diffusion. The applicability of STST to diffusion in matrices provides an easy method for predicting rates since it greatly reduces the amount of information needed about the potential and effort in computation compared with methods such as Monte Carlo variational transition-state theory and molecular dynamics calculations. Calculations using STST were also carried out for O- and H-atom diffusion in xenon. The computed rates are smaller than the experimental values, indicating that the experimentally observed processes are likely occurring in matrices with imperfections, as has been suggested previously by others.
A classical trajectory study of Ar+Ar2 collisions: Phase space structures in three degrees of freedom
Authors: Atkins Keith M. , Hutson Jeremy M.
Published: July 2002

A classical trajectory study of Ar + Ar2 collisions is described. The system provides a model chaotic scattering system in more than two degrees of freedom. The initial conditions that give rise to ejection of each of the three Ar atoms, and the corresponding collision mechanisms, are investigated. There are some large regions of the initial condition space in which the identity of the atom ejected does not change, and other regions in which it changes rapidly. Attention is focused on long-lived trajectories, which lie at the boundaries between different product identities. The long-lived trajectories are associated with sequences of periodic orbits. The different stability possibilities for periodic orbits in three degrees of freedom are discussed, and a sequence of periodic orbits responsible for dividing the initial condition space is identified. These periodic orbits are born at ‘‘avoided bifurcations,’’ at which a saddle-center bifurcation occurs close to a parent periodic orbit. The generalization to systems with more than three degrees of freedom is discussed.
δ -target optimal control of molecular dynamics: Application to a rotating diatomic molecule
Authors: Dubov Valery , Rabitz Herschel
Published: July 2002

The δ-target method proposed recently [Chem. Phys. Lett. 235, 309 (1995)] was shown to significantly simplify the task of optimal control of molecular localization. The present paper is intended to study the application of the δ-target technique to a practically interesting case of a diatomic molecule including rotation. Illustrative numerical examples are presented for the molecules NO and Na2. The δ-target method is shown to provide good control when the rotational temperature is not too high. The optimal control solution is very robust and insensitive to small changes in the potential curves and transition dipole moment functions. It is shown that the field obtained within the linearized δ-target optimal control technique can be successfully used in the nonlinear regime. The possibility of extending the δ-target technique is considered for treating strong field nonlinear optimal control tasks. The computational advantages of the method are discussed.
High specific enthalpies from the photochemically induced isomerization: BOH↔HBO
Authors: Gole J. L. , Michels H. H.
Published: July 2002

The photochemical interconversion of BOH to the considerably more stable HBO isomer is evaluated. The BOH and HBO isomers, while differing considerably in energy, possess a significant barrier to interconversion on the ground 1A′ potential energy surface. However, by accessing the low-lying double minimum a 3A′ state, we define an interconversion pathway that facilitates the interconversion process, allowing a substantial energy release. Here, we calculate the potential surfaces for the X 1A′ and a 3A′ states of HBO–BOH and present limited evaluations for the b 3A″ and A 1A′ states. We outline means to distinguish the BOH and HBO isomers, following the progress of the isomerization process using optical, infrared, and mass spectrometric techniques. The energetics of this geometric isomerization process and the potential utilization of BOH as a storable high energy material are discussed.
Entanglement between a polymeric ring and a rod
Authors: Koniaris Kleanthes , Muthukumar M.
Published: July 2002

Ring polymers are represented on a computer with the bead–stick model. If an infinitely thin spike is inserted at some (minimum) distance r from the center of mass of a Gaussian ring of contour length N, the probability P(r,N,w) of the ring winding a nonzero number of times w about the spike is nearly given by A exp[−(r/Rg)2/2σ2], where A and σ depend on N and w, and Rg is the radius of gyration of the polymer. We also find that the asymptotic results of related theoretical calculations cannot be realized in systems of practical size.
On the equation of state for thermal polymer solutions and melts
Authors: Wolfgardt M. , Baschnagel J. , Binder K.
Published: July 2002

We measure the chemical potential in a polymer melt with a modification of Widom’s insertion method. Our method is based on partial insertion of a testchain by controlling the interaction between the testchain and the melt by means of a control parameter. The chemical potential can be obtained from a single run in a multicanonical-like simulation over a wide range of temperatures and densities. From the chemical potential the osmotic pressure and the entropy are calculated. We study a variant of the bond fluctuation model of a polymer melt where an energy is gained when the bond vector is taken from the set (±3,0,0) lattice spacings (or permutations thereof). From extensive previous studies it is known that the model exhibits a glass-like freezing at low temperatures. We show that the configurational entropy decreases strongly when one cools the system through this transition, but it remains distinctly nonzero in the glass phase.