bluuebunny/crossref_metadata_2025 huggingface dataset:
Tight-binding simulations of argon cation clusters
Authors: Morales Guillermo A. , Faulkner Jessica , Hall Randall W.
Published: July 2002

A simple, semiempirical model was used to study the ground and excited state properties of argon cation clusters at 60 K. The model is a tight-binding Hamiltonian whose parameters are determined from atomic and diatomic properties. Monte Carlo simulations were used to calculate the average properties of these clusters. The photoabsorption spectrum was in good agreement with previous calculations and experiments. The splitting of the photoabsorption spectrum for clusters with greater than 14 atoms was investigated. The two excited states corresponding to the splitting arise from a 3-atom ion core, perturbed by a 4th atom, with solvation from the remaining atoms. The perturbation of the 3-atom ion core by the 4th atom is of the form ψion core±ψ4. The splitting can be decomposed into a contribution solely from the 4 atom wave functions (75% of the splitting) and to additional solvation stabilization of the low energy excited state over the high energy excited state (25%).
Lifetimes and transition dipole moment functions of NaK low lying singlet states: Empirical and ab initio approach
Authors: Tamanis M. , Auzinsh M. , Klincare I. , Nikolayeva O. , Ferber R. , Zaitsevskii A. , Pazyuk E. A. , Stolyarov A. V.
Published: July 2002

The paper presents experimental D 1∏ state lifetime τv′J′ data and develops empirical and ab initio approaches concerning D 1∏ and B 1∏ lifetimes, as well as D 1∏–X 1∑+, B 1∏–X 1∑+ and D 1∏–A 1∑+ transition dipole moment functions μ(R) of the NaK molecule. Experimental D 1∏(v′,J′) state τv′J′ values for v′ varying from 1 to 22 have been obtained from experimentally measured electric radio frequency-optical double resonance (rf-ODR) signal contours. The rf-ODR signals have been produced by D 1∏←X 1∑+ laser induced optical transition and rf field (1–900 MHz) induced e–f transition within the D 1∏(v′,J′) level. The possibility to determine empirical absolute μ(R) function in a wide R range from experimental τv′J′ dependence on v′ and J′ has been demonstrated; such an approach has been applied to obtain μ(R) for the B 1∏–X 1∑+ transition on which relative intensity data are absent. The empirical D 1∏–X 1∑+μ(R) function has been considerably improved by simultaneous fitting of relative intensity and lifetime data implicitly accounting for the J′ dependence of measured lifetime values. The finite-field technique combined with the many-body multipartitioning perturbation theory was used for ab initio all-electron transition moment calculations. This approach appeared to be adequate to compute reliable μ(R) functions due to a proper description of core-valence correlations. As a result, excellent agreement between ab initio and empirical B 1∏–X 1∑+ and D 1∏–X 1∑+ transition dipole moment functions has been achieved.
Model for the electrostatic response of the copper–water interface
Authors: Walbran S. , Mazzolo A. , Halley J. W. , Price David Lee
Published: July 2002

We present a model for the electrostatic response of the copper–water interface, which takes into account the electronic, Stern layer, and double layer contributions. The electronic and Stern layer contributions are not separated, but are modeled together using a direct dynamics model of the copper–water interface which we introduced earlier. The boundary condition for the double layer contribution is chosen to be consistent with electric fields calculated explicitly from the molecular dynamics simulation. We find results which are consistent with some experimental results for the capacitance of the low-index surfaces of copper. A remarkable feature of the results is that the calculated macroscopic electric field is much more effectively screened near the interface than it is in classical molecular dynamics models.
Alternating current conductivity and electrical conductivity relaxation in an intercalated polymer electrolyte
Authors: Jeevanandam P. , Vasudevan S.
Published: July 2002

Sodium cations solvated by polyethylene oxide have been confined by intercalation in the interlamellar space of an insulating layered solid, CdPS3. The frequency-dependent conductivity and the associated conductivity relaxation in the intercalated polymer electrolyte have been investigated. The temperature-dependent conductivity displays a distinct change in behavior at T∼225 K. Below 225 K the dc conductivity, σdc, shows an Arrhenius temperature dependence, while for T>225 K σdc rises sharply and follows the Vogel–Tamann–Fulcher relation, characteristic of ionic motion coupled to polymer relaxation modes. An analysis of the frequency-dependent conductivity using the electrical modulus representation showed that the electrical conductivity relaxation may be described in terms of the stretched exponential correlation function in the time domain, φ(t)=exp(−t/τσ)β. The absence of any anionic contribution to the electrical response makes these intercalated polymer electrolyte systems attractive for fundamental studies on ionic motion in disordered polymeric media.
On the electronic absorption spectrum of Cr4+ in Rb2CrF6
Authors: Pascual José Luis
Published: July 2002

In this note, we report results of ab initio model potential embedded cluster calculations of the d-d spectrum of Cr4+ in Rb2CrF6. Our calculations support the experimental assignment, recently questioned theoretically; however, a metal–ligand distance considerably larger than the one used before has to be used to obtain accurate results for the spectrum, suggesting that the actual Cr–F distance in the Rb2CrF6 lattice has been experimentally underestimated.
On the Einstein–Stern model of rotational heat capacities
Authors: Dahl Jens Peder
Published: July 2002

The Einstein–Stern model for the rotational contribution to the heat capacity of a diatomic gas has recently been resuscitated. In this communication, we show that the apparent success of the model is illusory, because it is based on what has turned out to be an unfortunate comparison with experiment. We also take exception to the possibility of assigning any meaning to the rotational zero-point energy introduced by the model.
The dependence of the rate constant for isomerization on the competition between intramolecular vibrational relaxation and energy transfer to the bath: A stochastic model
Authors: Gershinsky Gidon , Berne B. J.
Published: July 2002

A new stochastic model is introduced to emulate intramolecular vibrational relaxation in molecules undergoing isomerization. In this model the Hamiltonian flow is punctuated at random times by an exchange of vibrational kinetic energies. When applied to the study of the rate of barrier crossing, this model reproduces many of the salient features of rate processes in real molecules. For example when the mean time τ between kinetic energy exchange events is short compared to the crossing time of a well the model gives the Rice–Ramsperger–Kassel–Marcus (RRKM) rate constant. When τ is longer than the crossing time the rate constant is smaller than the RRKM value. In this paper the stochastic kinetic energy exchange model (SKEEM) combined with the Bhatnagar–Gross–Krook model for impulsive collisions with a bath is used to explore the dependence of the rate constant on collision rate. This model clarifies why at low collision rates the rate constant for barrier crossing reflects the full dimensionality of the molecule, whereas at higher collision rates the rate constant seems to reflect a lower dimensionality.
Gas phase spectroscopy of alkali carbides: The pure rotational spectrum of KC (X 4Σ−)
Authors: Xin J. , Ziurys L. M.
Published: July 2002

The pure rotational spectrum of the KC radical in its X 4Σ− ground state has been recorded using millimeter/submillimeter direct absorption spectroscopy. This study is the first gas phase observation of potassium carbide, and of any alkali metal carbide species. The molecule was produced under d.c. discharge conditions by the reaction of potassium vapor and CH4; the vapor was generated in a Broida-type oven. Eleven rotational transitions were measured for KC in the frequency range 344–515 GHz; fine structure was resolved in every transition, which consisted of a quartet pattern. The data were analyzed using nonlinear least-squares methods in a Hund’s case (b) basis, and rotational and fine structure parameters were accurately determined. The third-order contribution to the spin–rotation term was not found necessary for the data fit, although it has been suggested for states of quartet multiplicity and higher. The spin–spin interactions in KC appear to have a significant direct dipolar component. The bond length and electron configuration in KC have also been established, which suggest some covalent character to its bonding.
Isomorphism of 2-methylnaphthalene and 2-halonaphthalenes as a revealer of a special interaction between methyl and halogen
Authors: Calvet T. , Cuevas-Diarte M. A. , Haget Y. , Mondieig D. , Kok I. C. , Verdonk M. L. , Van Miltenburg J. C. , Oonk H. A. J.
Published: July 2002

The systems 2-methylnaphthalene+2-chloronaphthalene and 2-methylnaphthalene+2-bromonaphthalene belong to the exceptional group of binary systems where the formation of mixed crystals goes together with a solid–liquid phase diagram with a maximum. For these systems a thermodynamic analysis is presented which is based on new phase diagram and thermochemical data. The excess Gibbs energies, excess enthalpies, and excess entropies of the mixed crystalline state all are negative. These properties correspond to a net attraction between methyl and substituted halogen. Additional evidence of such an attraction is given by the outcome of a statistical search on intermolecular contacts, in the crystalline state of pure substances, among methyl+halogen, halogen+halogen, and methyl+methyl.
Microinhomogeneities of glasses of the system PbO–SiO2
Authors: Golubkov V. V. , Bogdanov V. N. , Pakhnin A. Ya. , Solovyev V. A. , Zhivaeva E. V. , Kabanov V. O. , Yanush O. V. , Nemilov S. V. , Kisliuk A. , Soltwisch M. , Quitmann D.
Published: July 2002

Small angle x-ray scattering (SAXS) and Rayleigh–Mandelstam–Brillouin (RMB) light scattering as well as ultrasonic sound velocities have been studied in glasses of the system PbO–SiO2 which has an unusually wide range of glass forming ability. The results of scattering are compared with calculations based on the concept of frozen-in equilibrium thermal fluctuations as the origin of static microinhomogeneities (MIH) in glasses. MIH of compositions seem to be the main source of scattering, and the calculations are found to be in qualitative agreement with both SAXS and RMB measurements. Glasses with PbO content above 40 mol.% are more homogeneous than ideal solutions of PbO and SiO2 whereas MIH in glasses with smaller PbO content are comparable with those expected for ideal solutions of PbO and SiO2. In the latter range SAXS measurements indicate the existence of medium-range order with correlation length of 5–7 Å.
Many chain correlated dynamics in polymer fluids
Authors: Guenza Marina
Published: July 2002

Starting from the formally exact Liouville equation, we derive a generalized Langevin equation for the simultaneous correlated motion of several molecules in dense melts. Our calculations are based upon phase space kinetic theory and Mori–Zwanzig projection operator techniques. In the overdamped regime, a set of nonlinear coupled equations is found containing cross-interaction contributions to the frequency matrix and to the memory functions. Specializing the problem to the correlated dynamics of polymer melts enables an analytical solution for the nonlinear cross contribution of the frequency matrix. The memory functions are calculated using a generalization of the Schweizer’s single molecule mode-coupling approach. A self-consistent procedure allows a numerical solution of the diffusive dynamics of the chains on the scale of the intermolecular interactions. At long time, depending on the strength of the intermolecular mean-force potential, two different scenarios take place. For weak intermolecular interactions the short-time correlated diffusive dynamics crosses over to the uncorrelated single chain dynamics. For stronger intermolecular interactions, when the time scale of relaxation of the many chain domains exceeds the time scales investigated, collective diffusion dominates the dynamical behavior.
Mean field lattice model for adsorption isotherms in zeolite NaA
Authors: Ayappa K. G. , Kamala C. R. , Abinandanan T. A.
Published: July 2002

Using a lattice model for adsorption in microporous materials, pure component adsorption isotherms are obtained within a mean field approximation for methane at 300 K and xenon at 300 and 360 K in zeolite NaA. It is argued that the increased repulsive adsorbate–adsorbate interactions at high coverages must play an important role in determining the adsorption behavior. Therefore, this feature is incorporated through a “coverage-dependent interaction” model, which introduces a free, adjustable parameter. Another important feature, the site volume reduction, has been treated in two ways: a van der Waal model and a 1D hard-rod theory [van Tassel et al., AIChE J. 40, 925 (1994)]; we have also generalized the latter to include all possible adsorbate overlap scenarios. In particular, the 1D hard-rod model, with our coverage-dependent interaction model, is shown to be in best quantitative agreement with the previous grand canonical Monte Carlo isotherms. The expressions for the isosteric heats of adsorption indicate that attractive and repulsive adsorbate–adsorbate interactions increase and decrease the heats of adsorption, respectively. It is concluded that within the mean field approximation, our simple model for repulsive interactions and the 1D hard-rod model for site volume reduction are able to capture most of the important features of adsorption in confined regions.
Green bands of the CsHg molecule
Authors: Polly R. , Dinev S. , Windholz L. , Milošević S. , Hess B. A.
Published: July 2002

We report the first observation of laser-induced fluorescence of the CsHg molecule. Cs/Hg vapor mixtures contained in a heat-pipe oven were excited by several different argon ion laser lines. Following these excitations, we observed three distinct peaks at λ=492.8, λ=498.3, and λ=521.5 nm in addition to the well-known fluorescence of the Cs2 molecule. Based on an analysis using ab initio potential energy curves of Cs2 and CsHg, we identified the origin of these three distinct peaks as bound–free transitions of CsHg (λ=492.8: IV3/2→X1/2, λ=498.3: VII1/2→X1/2, and λ=521.5 nm: VI1/2→X1/2). Furthermore, we could show that two different production channels are responsible for the formation of the CsHg molecules in the excited states. The excitation of the Cs/Hg vapor mixtures with wavelengths λexc=476–514 nm of the argon ion laser results in the production of the CsHg molecules via a photoassociation process, whereas the CsHg molecules are produced in a three-body collision following an excitation with λexc=457 nm. These results are confirmed by subsequent quantum-mechanical simulations of the observed laser-induced fluorescence spectra of CsHg.
Nascent rovibrational distribution of NO+(A 1Π) produced from charge-transfer reaction of He2+ with NO at thermal energy
Authors: Tsuji Masaharu , Nishimura Yukio , Ishimi Hiroaki , Hisano Masahiro , Oota Hidefumi
Published: July 2002

The NO+(A 1Π-X 1Σ+) emission resulting from the He2+/NO charge-transfer reaction at thermal energy has been observed in a He flowing afterglow. The vibrational and rotational distributions of NO+(A) were determined from a spectral simulation. The average vibrational and rotational energies deposited into NO+(A) were determined to be 0.22±0.02 and 0.10±0.1 eV, respectively. The vibrational population of NO+(A) decreases rapidly for v′=0–2 and becomes flat for v′=3,4, indicating that the vibrational distribution is bimodal. The bimodal vibrational distribution was explained as due to either two different entrance channels or two different dynamics (Demkov or Landau–Zener type). The rotational distributions were expressed by single Boltzmann temperatures of 1170±100 K.
Electronic relaxation and vibrational predissociation of benzene-acetylene van der Waals complexes
Authors: Shelley M. Y. , Dai H.-L. , Troxler T.
Published: July 2002

Fluorescence excitation spectra and dispersed fluorescence spectra of van der Waals complexes formed between benzene and acetylene, B⋅An with n=1,2, were measured in the vicinity of the benzene S1←S0 60110n transitions. Based on spectral shifts, fluorescence lifetimes and an acetylene concentration study, the complex exhibiting the largest blue shift (+136 cm−1) is assigned to the B⋅A dimer complex. Two other complex peaks, blue shifted by +125 and +116 cm−1, respectively, are assigned to two different B⋅A2 isomers. Vibrational predissociation is observed for all the clusters above various 611n levels in the excited state. Upper bounds of binding energies of both the electronic ground and excited states are deduced. Furthermore, analysis of the dispersed fluorescence spectra from product benzene after predissociation shows that extensive vibrational energy redistribution occurs prior to predissociation, leading to a complete loss of the ν1 mode excitation. A lengthening of the fluorescence lifetimes of nondissociating B⋅A2 levels is observed. Suppression of rotation-induced intramolecular relaxation caused by complexation is suggested for understanding this phenomenon.
Multidimensional solvation dynamical effects on quantum yields in model triad systems
Authors: Okada A. , Bandyopadhyay T.
Published: July 2002

The dynamical effects on the quantum yield for the electronic state, D+MA− (the final state) representing the efficiency of the system as an energy conversion system in a model triad system made out of an electron donor, D, an acceptor, A and a medium, M are investigated. The effective quantum yield for the final state is introduced, which is calculated only from the rates in a long time limit, and is applicable even when a memory effect from nuclear dynamics on the rates is considerable. Applying the effective quantum yield formalism, and taking into account multidimensional solvation dynamics by the theory previously developed by the authors, the yield for the final state, is calculated. Considerable dynamical effects are found in some specific situations. For example, the larger yield for the final state is obtained for the larger solvation time scale when the transfer from the initial state, D*MA to the final state occurs through the intermediate state, D+M−A prior to nuclear thermalization in the intermediate state; whereas the similar transfer but with nuclear thermalization in the intermediate state is almost impossible.
The photodissociation of carbonyl cyanide CO(CN)2 at 193 nm studied by photofragment translational energy spectroscopy
Authors: Scheld Heiner A. , Furlan Alan , Huber J. Robert
Published: July 2002

The photodissociation of carbonyl cyanide CO(CN)2 at 193 nm was investigated by photofragment translational energy spectroscopy. For all the fragments created (CO, CN, OCCN, NCCN), the kinetic energy distributions were measured and two decay channels identified. The radical decay, CO(CN)2+hν→OCCN+CN, dominates with a yield of 94%±2% and shows the available energy mainly (82%) channeled into the internal degrees of freedom of the fragments. A fraction of 18%±6% of the nascent OCCN radicals has sufficient energy to spontaneously decay to CO+CN involving a barrier ⩽160 kJ/mol. With a yield of 6%±2% the molecular decay produces the fragments CO+NCCN. These fragments acquire a high available energy owing to the formation of the new C–C bond in NCCN. An average fraction of 70% is partitioned into internal fragment energy. Even the fastest fragments are still internally hot, indicating that with the high barrier expected, a substantial exit channel interaction is operative. The isotropic recoil distribution found for the products CN, OCCN, and NCCN further suggests that both the radical and the molecular decay are, on the time scale of a parent rotation, slow and probably indirect.
Photodissociation of gas phase I3− using femtosecond photoelectron spectroscopy
Authors: Zanni Martin T. , Greenblatt B. Jefferys , Davis Alison V. , Neumark Daniel M.
Published: July 2002

The photodissociation dynamics of gas phase I3− following 390 nm excitation are studied using femtosecond photoelectron spectroscopy. Both I− and I2− photofragments are observed; the I2− exhibits coherent oscillations with a period of 550 fs corresponding to ∼0.70 eV of vibrational excitation. The oscillations dephase by 4 ps and rephase at 45 and 90.5 ps on the anharmonic I2− potential. The gas phase frequency of ground state I3− is determined from oscillations in the photoelectron spectrum induced by resonance impulsive stimulated Raman scattering. The dynamics of this reaction are modeled using one- and two-dimensional wave packet simulations from which we attribute the formation of I− to three-body dissociation along the symmetric stretching coordinate of the excited anion potential. The photodissociation dynamics of gas phase I3− differ considerably from those observed previously in solution both in terms of the I2− vibrational distribution and the production of I−.
Construction of an accurate potential energy surface by interpolation for quantum dynamics studies of a three-body system
Authors: Lin Shi Ying , Park Seung C. , Kim Myung Soo
Published: July 2002

A method to construct an accurate potential energy surface (PES) by interpolation for a three-body reaction which is suitable for quantum dynamics studies is presented using Cl+H2→HCl+H as an example. Use of the exponential coordinates led to a significant improvement. Dynamics results, both classical and quantal, on the LEPS and LEPS-interpolated PESs were nearly indistinguishable. An accurate analytic PES can be constructed with the ab initio results also, as manifested with the PES contours.
Condensation pressures in small pores: An analytical model based on density functional theory
Authors: Nilson Robert H. , Griffiths Stewart K.
Published: July 2002

Integral methods are used to derive an analytical expression describing fluid condensation pressures in slit pores bounded by parallel plane walls. To obtain this result, the governing equations of density functional theory (DFT) are integrated across the pore width assuming that fluid densities within adsorbed layers are spatially uniform. The thickness, density, and free energy of these layers are expressed as composite functions constructed from asymptotic limits applicable to small and large pores. By equating the total free energy of the adsorbed layers to that of a liquid-full pore, we arrive at a closed-form expression for the condensation pressure in terms of the pore size, surface tension, and Lennard-Jones parameters of the adsorbent and adsorbate molecules. The resulting equation reduces to the Kelvin equation in the large-pore limit. It further reproduces the condensation pressures computed by means of the full DFT equations for all pore sizes in which phase transitions are abrupt. Finally, in the limit of extremely small pores, for which phase transitions may be smooth and continuous, this simple analytical expression provides a good approximation to the apparent condensation pressure indicated by the steepest portion of the adsorption isotherm computed via DFT.
A new and more direct test of Hubbard relations from molecular mass distribution influence on linear liquid dynamics
Authors: Calero S. , Lago S. , Garzón B.
Published: July 2002

Molecular dynamics simulations varying the molecular mass distribution in linear molecules are performed to search for a direct test of Hubbard relations for liquid state dynamics. Results are obtained for series of models with different moments of inertia and the same multipoles and molecular length. The intermolecular potential is composed of different contributions including a nonpolar term. This potential allows for a steady variation of moment of inertia. Correlation times are directly checked with Hubbard relations and a nearly quantitative agreement found. Nonpreviously reported relationships between single correlation times and transport coefficients versus molecular moment of inertia are shown.
Different description levels of chemical wave front and propagation speed selection
Authors: Lemarchand A. , Nowakowski B.
Published: July 2002

The results of stochastic descriptions and microscopic simulations of a chemical wave front propagating into an unstable stationary state are compared to investigate the problem of speed selection. Three different effects, the space discretization, the mean number of particles in a cell, and the deviation from equilibrium molecular velocity distribution, are shown to induce corrections to the propagation speed predicted by a macroscopic continuous description. The speed correction due to the perturbation of the particle velocity distribution in the presence of a fast reaction is isolated from the other effects.
The molecular geometry, harmonic force field, bonding characteristics, and nuclear shielding parameters of OPCl, as determined from high resolution microwave spectra
Authors: Brupbacher-Gatehouse Bethany , Brupbacher Thomas
Published: July 2002

The reactive gas phase species OPCl has been prepared in an electric discharge, and its microwave spectrum has been measured for the first time; both quadrupole and spin–rotation hyperfine splittings were resolved. The determined spectroscopic constants have been used to derive experimental values for many properties of this molecule: the centrifugal distortion constants and inertial defects have been combined with all available infrared data to determine a harmonic force field; the centrifugal distortion corrected rotational constants have been used to yield the first direct experimental determination of the OPCl geometric parameters; the Cl quadrupole coupling tensor has been diagonalized and these values have been used to obtain an approximate value for the ionic character of the P–Cl bond; and the spin–rotation coupling constants have been used to derive nuclear shielding parameters for both the P and Cl nuclei. In all cases, comparisons with the corresponding parameters determined for other phosphenous halides (OPX), phosphenous hydride (OPH), the analogous second row compounds (ONX and ONH), and other related species have been made. Some of these parameters have also been calculated using theoretical techniques; these comparisons are given where appropriate.
Two novel approaches to the Kramers rate problem in the spatial diffusion regime
Authors: Drozdov A. N.
Published: July 2002

At present, there are two general theoretical approaches to calculating the rate of thermally activated escape of a Brownian particle over a barrier out of a metastable well in the spatial diffusion regime. A direct approach involves techniques entirely based on the underlying Fokker–Planck equation, such as the Kramers flux over population method, the mean first passage time formalism, and the eigenmode expansion. An alternative consists of replacing the original one-dimensional stochastic dynamics by an infinite dimensional Hamiltonian system. The rate is then calculated using reactive flux methods. Both approaches are rather efficient when treating bistable potentials with high parabolic barriers. However, complications arise if the barrier is not parabolic. In such a case, large deviations of theoretical predictions from exact numerical rates are observed in the intermediate friction region. The latter holds true even though the barrier is infinitely high, to say nothing of low barriers for which the problem of finite barrier height corrections remains effectively unresolved. Based on the expansion of the Fokker–Planck equation in reciprocal powers of the friction coefficient, two novel methods for calculating analytically the rate of escape over an arbitrarily shaped barrier are presented. These are a continued fraction expansion method and a self-similar renormalization technique developed recently for summation of divergent field-theoretical series, respectively. In this way, two different rate expressions are constructed that agree in the limiting case of high friction with the rate following from the corresponding Smoluchowski equation and reduce to the transition state theory rate at zero damping. Comparison with a known rate expression for a purely parabolic barrier and from numerical simulations for bistable potentials with cusped and smooth barriers of different heights show excellent agreement between the present theories and exact numerical results. As long as the escape dynamics is dominated by spatial diffusion across the barrier top, the maximal relative errors attained with the continued fraction method and the self-similar renormalization technique are less than 3% and 7%, respectively. This is in drastic contrast to known rate formulas derived by other means, whose relative errors are larger by factors and even by orders of magnitude.
Dissociation of ICl−(CO2)n by its impact on silicon surface: The role of core ion
Authors: Koizumi Shin’ichi , Yasumatsu Hisato , Terasaki Akira , Kondow Tamotsu
Published: July 2002

Collisional dissociation induced by impact of a cluster anion, ICl−(CO2)n (n=0–20), onto a silicon surface was studied by measuring the branching fractions of the ICl− dissociation and the translational energies of the product anions as functions of n and the collision energy (per ICl− of ICl−(CO2)n in an apparatus based on a tandem time-of-flight mass spectrometer. It was found that the branching fraction of the ICl− dissociation did not change with n at the collision energies of 30–70 eV. Molecular dynamics simulation showed that the impinging core ion, ICl−, tends to orient with the molecular axis of the core ion being at the angle of 55° with respect to the surface normal, whereas in the X2−(CO2)n (X=Br, I) collision, the molecular axis of the X2− core ion being in parallel to the surface plane. This finding together with prominent wedge effect in the X2−(CO2)n collision leads us to conclude that the CO2 molecules do not work as a “wedge” in the ICl−(CO2)n collision because of the unfavorable orientation of the incoming cluster anion. No discernible cage effect could also be related to the orientation. The translational energies of the product anions were interpreted in terms of energy redistribution of the collision energy among the degrees of freedom of the cluster anion and the surface atoms involved in the collision. It was also found that the I and Cl product ions reach quasiequilibrium with the surface by measuring these translational energy distributions.
Phase behavior of an asymmetric vector lattice model for oil-water-amphiphile mixtures
Authors: Pretti Marco , Buzano Carla
Published: July 2002

A vector model describing oil-water-amphiphile mixtures, originally proposed by Ciach, Ho/ye and Stell, is investigated on the simple cubic lattice. Our previous approach, involving an exact integration over the amphiphile orientational degrees of freedom, is reformulated to consider the more general case of asymmetric amphiphiles. The resulting spin-1 effective Hamiltonian, containing many-site temperature-dependent interactions, is then treated in a local mean field approximation and the phase behavior at finite temperature is extensively analyzed. In order to stress asymmetry effects, a detailed comparison with the symmetric case is performed.
Spectral diffusion in low temperature polymers: Deviation from logarithmic temporal hole broadening
Authors: Osad’ko I. S. , Yershova L. B.
Published: July 2002

A deviation from the logarithmic temporal behavior of the inverse dephasing time 1/T2 found recently in polymethylmethacrylate (PMMA) at low temperature in time scale ranging from nanoseconds to milliseconds and that of spectral hole width found in time scale ranging from minutes to weeks is discussed. It is shown that existing stochastic spectral diffusion theories cannot explain all these experimental data. These data can be described by the formula 1/T2(τ)=B[A(1−τ−1/4) +ln τ+a(τ1/2−1)] which is derived within the scope of a dynamical approach to low temperature spectral diffusion in polymers and glasses. The formula is based on an assumption that two-level systems (TLS) in PMMA can be divided into three groups: with a charge (e), with dipole (d), and with quadrupole (q) moments.
Wave packet dynamics of pulsed laser catalysis in two dimensions
Authors: Vardi Amichay , Shapiro Moshe
Published: July 2002

We apply the time-dependent theory of laser catalysis to the collinear exchange reaction A+BC→AB+C. We have performed detailed calculations of pulsed laser catalysis on two-dimensional potential surfaces resembling the ground and first excited electronic potentials of the collinear H+H2 reaction, at various incident energies and laser central frequencies. As in the one-dimensional case, the resulting reactive lineshapes are Fano-type curves, with tunneling enhancement on the blue side of the radiatively broadened line and suppression on the red side. Transmission coefficients range from zero to unity, depending upon the laser detuning. The points of complete transmission and complete reflection are explained by constructive and destructive interference between nonradiative tunneling and the optical path.
Spin polarization of zero kinetic energy electrons from HBr
Authors: Drescher M. , Irrgang R. , Spieweck M. , Heinzmann U. , Cherepkov N. A. , Lefebvre-Brion H.
Published: July 2002

The range of observables in zero kinetic energy (ZEKE) electron spectroscopy of molecules, previously restricted to the total electron intensity, was extended by measuring the integral spin polarization of ZEKE electrons at the HBr Ω+=3/2 thresholds after single-photon excitation with narrow-band circularly polarized light. A comparison with calculated values from multichannel quantum defect theory underlines the importance of autoionization for the decay dynamics.
Electronic transfer dynamics for bridged binuclear mixed-valence compounds: Density functional theory study on electronic structure in the ground state for the Creutz–Taube ion under asymmetric distortions
Authors: Chen Zhida , Bian Jiang , Zhang Lei , Li Shuzhou
Published: July 2002

In order to explore the dynamic problem of the electronic structure in the ground state for the Creutz–Taube ion, the calculations on the electronic potential surfaces along the totally symmetric vibrational difference coordinate of the Ru–N stretch and the net charge distribution on the Ru atoms are carried out by using the density functional theory in the two schemes with or without the Ru–N(pyz) vibration. It is shown that the vibration associated with the totally symmetric vibrational difference coordinate Q− can be regarded as a harmonic oscillation with 123.16 eV Å−2 of the force constant, and 430 cm−1 of the fundamental frequency is obtained on the basis of the five-body vibrator model. From the electronic structure calculated in the ground state it is concluded that the asymmetric net distribution on the Ru atoms in the Creutz–Taube ion occurs in the vibration associated with the totally symmetric vibrational difference coordinate, and the charge transfer between the Ru atoms and the ligands is responsible for the asymmetric charge distribution. The total symmetric vibrations of Ru–N for the related monomer [(NH3)5Ru(pyz)]2+/3+ are further examined. It is found that the reduction of Ru(III) to Ru(II) corresponds to 0.05 Å of the difference in Ru–N distance between the two Ru-subunits under the asymmetric distortion and to 0.012 eV of the potential energy over the zero point energy, which is comparable with kT. The Born–Oppenheimer approximation and the neglected coupling effect between the electronic and nuclear motion in the present paper are briefly discussed.
Conductance of symmetric electrolyte solutions: Formulation of the dressed-ion transport theory (DITT)
Authors: Varela L. M. , Perez-Rodriguez M. , Garcia M. , Sarmiento F. , Mosquera V.
Published: July 2002

The concentration dependence of the electrical conductance of several symmetrical electrolytes is studied using the dressed-ion transport theory (DITT) formalism. This theoretical scheme is constructed from the Fuoss–Onsager theory using the equilibrium dressed-ion theory (DIT) distribution functions for electrolyte and colloid media. The conductance equation is constructed from previous results for the relaxation of the ionic atmosphere [Varela et al., J. Chem. Phys. 110, 4483 (1999)], now completed with the calculation of the concentration-dependent electrophoretic velocity of ions on the basis of a DIT in a hydrodynamic point of view. Onsager’s limiting law for the electrophoretic velocity based on the Debye–Hückel (DH) equilibrium distribution function is reformulated in terms of the DIT renormalized quantities, effective charges and effective screening length. The electrophoretic correction to the velocity calculated in the framework of this DITT is shown to exhibit a nontrivial dependence on the renormalized dielectric constant of the medium (ε*). The concentration dependence of both the deviations of the renormalized dielectric constant from the classical limit and the electrophoretic correction to the mobility is analyzed using the modified-mean spherical approximation (MMSA) value of the DIT linear response function α̂(k). The behavior of ε* and therefore of the electrophoretic correction to the ionic (or colloidal) mobilities is studied for both the random-phase approximation (RPA) and the MMSA and they are related to ionic association through the dimensionless coupling parameter, ζ, made up from the ratio between the Bjerrum length and the mean ionic radius. The expression for the conductance of the electrolyte solutions obtained in this new theoretical approach is cast in the form of a generalized Onsager’s limiting law with a concentration-dependent slope which is predicted in terms of the formation of DIT renormalized quasiparticles in the solution. Conductance predictions of several theoretical expressions for the renormalized magnitudes of electrolyte solutions are compared to direct experimental data of KCl, and are in very good agreement with the theoretical predictions of the mean-field DITT, which confirms the convenience of understanding transport processes in terms of effective dynamical entities with renormalized ionic charges interacting by means of a renormalized screened potential.
Inelastic scattering of molecules from a liquid polymer surface
Authors: Los J. , Gleeson M. A. , Koppers W. R. , Weeding T. L. , Kleyn A. W.
Published: July 2002

The (dissociative) scattering of CF+ and CF2+ ions from a perfluoropolyether is analyzed on the basis of the conservation laws of energy and momentum. The ion–surface collisions are treated as binary collisions between the primary ions and a representative group of the perfluoropolyether molecule. The energy partition between the two particles is analyzed on the basis of a statistical distribution between harmonic oscillators. The energy transfer between the individual atoms is treated as inelastic rovibrational excitation. This method not only allows us to draw some general conclusions concerning the dynamics of this type of molecule–surface collision, but also to estimate the dissociation probability of the scattered molecules as a function of the scattering angle and the energy loss.
The strong field photoelectron spectroscopy of acetylene: Evidence for short-lived 4p gerade states via electric field-induced resonance-enhanced multiphoton ionization
Authors: Moore Noel P. , Levis Robert J.
Published: July 2002

We report the intense field photoelectron spectra of acetylene, excited using 780 nm, 135 fs radiation for intensities ranging from 5.7 to 8.6×1013 W cm−2. Three features observed at 1.59, 1.95, and 2.13 eV are repeated at higher energies throughout the spectra as above threshold ionization features. A method for analysis of the spectra and assignment of peaks is presented, based on intensity dependent shifting of intermediate states and ponderomotive shifts of the ionization potential. We present evidence for the observation of previously unobserved gerade states in the region of 9.81–10.35 eV.
Tumbling and spinning diffusions of acetonitrile in water and organic solvents
Authors: Wakai Chihiro , Saito Hirokazu , Matubayasi Nobuyuki , Nakahara Masaru
Published: July 2002

The spin–lattice relaxation times T1 for H2 and N14 of acetonitrile-d3(CD3CN) are measured in pure liquid, n-hexane, carbon tetrachloride, acetone, methanol, water, and heavy water as functions of temperature and concentration. From the T1 values, the rotational diffusion coefficients for the tumbling and spinning motions are separately obtained. The tumbling motion is strongly dependent on the solvent and temperature and its diffusion coefficient is qualitatively explained by a simple dielectric friction model. The spinning motion is, on the other hand, weakly dependent on the solvent and temperature. The observed large anisotropy ratio (∼9) of the spinning to the tumbling modes represents the anisotropy of the solvation shell and is explained by neither the free rotor model nor the hydrodynamic continuum model. The tumbling motion is more strongly hindered than the spinning motion due to the dipolar interaction between the solute and solvent.
Tunneling splittings in water trimer by projector Monte Carlo
Authors: Blume D. , Whaley K. B.
Published: July 2002

We demonstrate the Monte Carlo calculation of multidimensional tunnel splittings in the water trimer without the use of explicit nodal approximations. The calculations are made for the three-dimensional torsional model of the nonbonded hydrogen atom motions. We employ the projection operator imaginary time spectral evolution (POITSE) method, which requires specification of a set of projector operators. The permutation symmetry leading to exchange tunneling is incorporated into these projectors directly. We analyze the optimal choice of the real space vibrational coordinates with a comparative study of local vs nonlocal symmetry-adapted nuclear coordinates. Extraction of the lowest four sets of tunneling states yields good agreement with available discrete variable representation calculations.
Reaction of gaseous oxygen with adsorbed carbon on Pt{110}(1×2)
Authors: Walker A. V. , King D. A.
Published: July 2002

The C oxidation reaction on Pt{110}(1×2) has been investigated using molecular beam techniques. The reaction products are CO and CO2. As the surface temperature is increased from 550 to 750 K, the proportion of CO2 produced decreases from ∼50% to <1%. When oxygen first impinges on the surface, the formation rate of CO is observed to rise immediately (⩽0.1 s), and then rise more slowly to a maximum before decreasing sharply as the C adlayer is depleted. The production of CO2 is initiated after a measurable delay. Angle-resolved product distribution measurements demonstrate that CO desorbs in a sharp lobe centered at an angle of 32° to the surface normal, fitted to cosn(θ−32), where n=50±5. The C oxidation reaction site is identified with the (111) microfacets. It is concluded that two processes are operative, a Langmuir–Hinshelwood mechanism and a reaction in which CO is impulsively desorbed.
Phase-space derivation of propensity rules for energy transfer processes between Born–Oppenheimer surfaces
Authors: Segev B. , Heller E. J.
Published: July 2002

We consider a simple method for calculating weak Franck–Condon factors. The Wigner transform of the wave function on an initial Born–Oppenheimer state is calculated for the donor potential surface and projected onto the acceptor energy surface energy shell. The integrated projection yields an approximation for the relevant Franck–Condon factors, while phase-space integrand shows where in phase space the leakage occurs between the donor and acceptor states. This in turn determines the initial conditions on the acceptor surface for subsequent IVR and energy flow. Propensity rules are obtained by recognizing phase-space points of closest approach of the initial-state Wigner function and the final-state energy surface. The example of two coupled harmonic oscillators is explicitly solved to demonstrate the power of this phase-space approach.
Photophysics of O2 excited by tunable laser radiation around 193 nm
Authors: Bakker B. L. G. , Parker D. H.
Published: July 2002

The photodissociation/ionization dynamics of O2 around 193 nm have been studied using a narrowband tunable ArF excimer laser and the velocity map imaging technique. Angular and kinetic energy distributions of the product O+ ions and O(3P2) atoms are recorded and analyzed. The production of O(3P2) atoms is resonance enhanced on the one-photon level by the B 3Σu−(v=4) state, which is part of the B 3Σu−←X 3Σg− Schumann–Runge bands. Angular distribution measurements for individual rotational levels of the B state yield values for the anisotropy parameter, βSR, which are in good agreement with the values predicted by independent measurements of predissociation lifetimes from spectral linewidths. An average value of β=0.48 is found for the underlying Herzberg continuum at 193 nm. O2+ production is enhanced on the two-photon level via members of the nsσg(n−1)dπg1Σg+ Rydberg series terminating at higher vibrational levels of the ion. The high Rydberg states autoionize into the O2+ X 2Πg ground state or absorb one more photon and then autoionize into the A 2Πu and b 4Σu− states of O2+, which subsequently fluoresce. Production of O+ from one- and two-photon dissociation of the O2+ formed after two-photon absorption is also observed and characterized.
Experimental and theoretical study of the electronic spectrum of the BAr2 complex: Transition to the excited valence B(2s2p2 2D) state
Authors: Krumrine Jennifer R. , Alexander Millard H. , Yang Xin , Dagdigian Paul J.
Published: July 2002

The 2s2p2 2D←2s22p 2P valence transition in the BAr2 cluster is investigated in a collaborative experimental and theoretical study. Laser fluorescence excitation spectra of a supersonic expansion of B atoms entrained in Ar at high source backing pressures display several features not assignable to the BAr complex. Resonance fluorescence is not observed, but instead emission from the lower 3s state. Size-selected fluorescence depletion spectra show that these features in the excitation spectrum are primarily due to the BAr2 complex. This electronic transition within BAr2 is modeled theoretically, similarly to our earlier study of the 3s←2p transition [M. H. Alexander et al., J. Chem. Phys. 106, 6320 (1997)]. The excited potential energy surfaces of the fivefold degenerate B(2s2p2 2D) state within the ternary complex are computed in a pairwise–additive model employing diatomic BAr potential energy curves which reproduce our previous experimental observations on the electronic states emanating from the B(2D)+Ar asymptote. The simulated absorption spectrum reproduces reasonably well the observed fluorescence depletion spectrum. The theoretical model lends insight into the energetics of the approach of B to multiple Ar atoms, and how the orientation of B p-orbitals governs the stability of the complex.