bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory and simulation of cohesive diffusion in nanopores: Transport in subcritical and supercritical regimes
Authors: Saravanan Chandra , Auerbach Scott M.
Published: July 2002

We have studied a lattice model of self-diffusion in nanopores, to explore how loading, temperature, and adsorbate coupling influence benzene self-diffusion in Na–X and Na–Y zeolites. We propose a simple method for determining how adsorbate–adsorbate interactions modify activation energies of site-to-site jumps. We apply a mean-field approximation that describes transport semiquantitatively for a wide variety of system parameters, simplifying kinetic Monte Carlo simulations. We also derive an analytical diffusion theory that provides semiquantitative apparent activation energies, and qualitatively reasonable loading dependencies. We have found that supercritical systems exhibit three characteristic loading dependencies of diffusion, depending upon the degree of degeneracy of lattice sites. Subcritical diffusion systems are dominated by cluster formation, exhibiting intriguing loading dependencies with broad regions of constant diffusivity. Our model for benzene in Na–X is in excellent qualitative agreement with pulsed field gradient nuclear magnetic resonance (NMR) diffusivities, and in qualitative disagreement with tracer zero-length column (TZLC) data. We suggest that high-temperature TZLC experiments should be performed, to test whether the coverage of maximum diffusivity decreases with increasing temperature.
Nonequilibrium distribution function theory of diffusion-influenced reversible energy-transfer reactions
Authors: Sung Jaeyoung , Chi Junhoa , Lee Sangyoub
Published: July 2002

Recently, we proposed a new systematic approach to evaluate the many-particle effects on the diffusion-influenced reactions. The method gives an improved result over that obtained by using the superposition approximation. In the present paper, we apply the method to treat the kinetics of reversible energy-transfer reactions of the type A*+B⇄A+B*. Until now, most theories were inapplicable when the lifetime of A* is shorter than that of B*, and a notable exception was the integral encounter theory (IET) of Burshtein et al. The present theory can be applied irrespective of the relative magnitude of the lifetimes of donor and acceptor molecules, and becomes exact for the irreversible target model. In addition, it is applicable to the system with higher reactant densities than IET; the result of IET is recovered as a limiting form in the present theory.
New means of spatially manipulating molecules with light
Authors: Seideman Tamar
Published: July 2002

We develop a general framework for describing the interaction of molecules with a nonperturbative intensity gradient and note several limits, including the two-level Hamiltonian of atom trapping and the quasistatic Hamiltonian of molecular optics. The theory is used to propose a new molecular optics scheme which complements previous work in the area of spatial manipulation of molecules in intense fields by introducing the possibility of reflecting molecules with light. Repulsive interaction obtains in the limit of small level spacings as compared to the field frequency, attained by exciting the molecules to high Rydberg states. Some of the potential applications of the repulsive interaction are illustrated and others are noted. We address the problem of molecular alignment in intense laser fields and describe the possibility of simultaneously focussing the center-of-mass motion of molecules and aligning their figure axis after exit from the laser focus, under field-free conditions.
Intramolecular vibrational redistribution and fragmentation dynamics of I2 ⋯ Nen (n=2–6) clusters
Authors: Fernandez Alberti S. , Halberstadt N. , Beswick J. A. , Bastida A. , Zúñiga J. , Requena A.
Published: July 2002

Intramolecular vibrational energy redistribution and fragmentation dynamics in I2(B,v=22) ⋯ Nen (n=2–6) and I2(B,v=21) ⋯ Nen (n=2–5) clusters is studied by hybrid quantum/classical techniques and the results are compared with experiments. A vibrational version of the molecular dynamics with quantum transitions (MDQT) treatment is used in which the vibrational degree of freedom of I2 is treated quantum mechanically while all the other degrees of freedom are treated classically. The potential energy surface is represented as a sum of pairwise interactions with parameters taken from the literature. The calculated product state distributions are in very good agreement with the experiments. Fragmentation lifetimes were also calculated and agree reasonably well with those measured in time-dependent experiments. Fragmentation proceeds via sequential ejection of Ne monomers through three different mechanisms: (i) sequential intramolecular vibrational redistribution plus vibrational predissociation (in which the I2 molecule loses more than one quantum of vibration); (ii) direct vibrational predissociation (in which the I2 molecule loses only one quantum of vibration); (iii) evaporation (in which the I2 molecule remains in the same vibrational state).
Electron dynamics in gold and gold–silver alloy nanoparticles: The influence of a nonequilibrium electron distribution and the size dependence of the electron–phonon relaxation
Authors: Link S. , Burda C. , Wang Z. L. , El-Sayed M. A.
Published: July 2002

Electron dynamics in gold nanoparticles with an average diameter between 9 and 48 nm have been studied by femtosecond transient absorption spectroscopy. Following the plasmon bleach recovery after low power excitation indicates that a non-Fermi electron distribution thermalizes by electron–electron relaxation on a time scale of 500 fs to a Fermi distribution. This effect is only observed at low excitation power and when the electron distribution is perturbed by mixing with the intraband transitions within the conduction band (i.e., when the excitation wavelength is 630 or 800 nm). However, exciting the interband transitions at 400 nm does not allow following the early electron thermalization process. Electron thermalization with the lattice of the nanoparticle by electron–phonon interactions occurs within 1.7 ps under these conditions, independent of the excitation wavelength. In agreement with the experiments, simulations of the optical response arising from thermalized and nonthermalized electron distributions show that a non-Fermi electron distribution leads to a less intense bleach of the plasmon absorption. Furthermore, the difference between the response from the two electron distributions is greater for small temperature changes of the electron gas (low excitation powers). No size dependence of the electron thermalization dynamics is observed for gold nanoparticles with diameters between 9 and 48 nm. High-resolution transmission electron microscopy (HRTEM) reveals that these gold nanoparticles possess defect structures. The effect of this on the electron–phonon relaxation processes is discussed. 18 nm gold–silver alloy nanoparticles with a gold mole fraction of 0.8 are compared to 15 nm gold nanoparticles. While mixing silver leads to a blue-shift of the plasmon absorption in the ground-state absorption spectrum, no difference is observed in the femtosecond dynamics of the system.
Unitary group based open-shell coupled cluster theory: Application to van der Waals interactions of high-spin systems
Authors: Jankowski Piotr , Jeziorski Bogumil
Published: July 2002

The performance of the unitary group approach (UGA) based coupled cluster singles and doubles (CCSD) method in application to van der Waals interactions involving high-spin open-shell systems is examined. The tested approach is fully spin-adapted in the sense that any intermediate quantity appearing in the formulation of the theory is free from spin contamination contributions. Interaction energies are computed within the supermolecular approach and corrected for the basis set superposition error. Several methods of solving UGA CCSD equations are used with the emphasis on iterative processes based on the Hamiltonian partitionings employed in the spin-restricted many-body perturbation theories. Test calculations are performed for the ground states of HeLi, H2Li, and for the excited a 3Σu+ state of Li2. The UGA CCSD interaction energies are compared with those computed using the spin-unrestricted and valence universal coupled cluster methods, spin-restricted and spin-unrestricted many-body perturbation expansions, and the full configuration interaction method. The obtained results show that the UGA CCSD method performs very well in applications to weakly bound open-shell systems, giving results as good or better than other open-shell coupled cluster approaches.
Structures and energetics of Nen–HN2+ clusters
Authors: Meuwly Markus
Published: July 2002

The structures of Nen–HN2+ for (n⩽17) and vibrational red shifts for (n⩽6) clusters are investigated. The potential energy surfaces are based on a rigid monomer ab initio interaction potential which has been modified using a vibrational adiabatic correction. In order to reproduce mid-infrared experimental results on the Ne–HN2+ dimer, the potentials describing the interaction between Ne–HN2+ (v1=0) and Ne–HN2+ (v1=1) are subjected to a simple energy scaling. The final potential energy surfaces reproduce all experimental observables satisfactorily. The dimer potentials are used in the calculation of minimum energy structures assuming pairwise additivity of the interactions. In larger clusters the neon atoms form solvation rings around the ionic core. Because the differences in total energies do not reproduce the observed vibrational red shifts, diffusion Monte Carlo calculations are performed to assess ground-state energies of the clusters. For n⩽3, the calculated and observed red shifts are comparable. However, for n>3, calculated values systematically overestimate the red shifts. The discrepancy is mainly attributed to the neglect of many-body interactions and the marginally defined shape of the dimer interaction potential for larger angles.
Extension of path integral quantum transition state theory to the case of nonadiabatic activated dynamics
Authors: Schwieters Charles D. , Voth Gregory A.
Published: July 2002

In this paper, a methodology is introduced by which one may compute rates of nonadiabatic processes for arbitrary potential forms. This method augments the extant path-integral transition state theory approach in which information about the inherently dynamical rate process is obtained from a static analysis of a free energy surface. The present resulting formula reduces to previous single adiabatic surface results when the adiabatic surface are well separated. Numerical examples show that the method well approximates the exact results in the nonadiabatic limit and over a large range of temperatures for quadratic and for nonquadratic potentials. Corroborating these results, analysis of the rate formula for a single-oscillator spin-boson Hamiltonian in the nonadiabatic classical-limit reveals close agreement with the known exact result.
On the strongly bound B 3Π state of the CAr van der Waals complex: Bonding and predissociation
Authors: Sohlberg Karl , Yarkony David R.
Published: July 2002

For the open shell van der Waals molecule, CAr, the potential energy curves (PECs) for the B 3Π and 1 5Σ− states, the B 3Π–1 5Σ− spin–orbit coupling and the B 3Π fine structure splitting are determined using multireference configuration interaction wave functions as large as ∼8 million configuration state functions. The B 3Π state is strongly bound, with De=5100 cm−1. Re(B 3Π)=3.7a0 and is considerably shorter than Re(X 3Σ−)=6.07 a0. The PEC for the repulsive 1 5Σ− state crosses that of the B 3Π state at Rx(1 5Σ−,B 3Π)=3.31a0 leading to spin–orbit induced predissociation. The B 3Π–1 5Σ− spin–orbit coupling is the result of valence-Ryberg mixing in the B 3Π state and is considerably enhanced by the heavy atom effect. The heavy atom effect is also reflected in a marked decrease in the fine structure splitting of the B 3Π state with increasing vibrational level. The implications of these results for using CAr(B 3Π) in laser induced fluorescence detection of CAr(X 3Σ−,v) are discussed.
Water↔ice transformation in micron-size droplets in emulsions
Authors: Tombari E. , Ferrari C. , Salvetti G. , Johari G. P.
Published: July 2002

To investigate the crystallization and melting behaviors of micron-size droplets in a water-emulsion, the heat capacity, Cp, has been studied by adiabatic calorimetry over the 233–273 K range. Water droplets in the emulsions began to crystallize at ca. 243 K, but the crystallization rate was slow. This was caused partly by the relatively slow coalescence of crystallized and uncrystallized droplets at that temperature, in a highly nonNewtonian viscous media of the emulsion. Crystallization occurred rapidly on heating and remained incomplete even when it occurred at 260 K. Thus a substantial amount of water droplets coexisted with ice droplets in the emulsion. The onset of crystallization shifted to lower T and the number of water droplets in the emulsion at a given T decreased as the droplets grew on thermally cycling the emulsion. Cp of emulsion increased progressively more rapidly as 273 K was approached, which is attributable to premelting of the ice droplets beginning at 260 K. Analysis of the Cp data showed that interaction between the droplets and the surfactant in the emulsion changes on crystallization. This is attributable to the decrease in the entropy at the surfactant–water interface.
Critical properties of mixtures of alkanes from perturbation theory
Authors: MacDowell L. G. , Vega C. , López-Rodríguez A.
Published: July 2002

The critical properties of binary mixtures of linear and branched alkanes have been evaluated using perturbation theory. An accurate equation of state for the reference system is combined with a mean field treatment of the perturbation term, along with a reasonable potential model. No adjustable parameters were used either for the pure compounds or for the binary mixtures. The aim of this work is to show which features of the critical properties of alkane mixtures can be obtained from a fully molecular based theory. It is shown that the main trends of the critical properties of alkane binary mixtures can be described qualitatively by the theory.
Interaction of gaseous D atoms with alkyl halides adsorbed on Pt(111), H/Pt(111), and C/Pt(111) surfaces: Hot-atom and Eley–Rideal reactions. II. Ethyl iodide
Authors: Wehner S. , Küppers J.
Published: July 2002

The kinetics of reactions of D atoms with ethyl iodide adsorbed on Pt(111), H covered Pt(111), and monolayer C covered Pt(111) surfaces were studied using direct product detection. Ethane, C2H5D, was observed as gaseous product formed through ethyl abstraction from the halide. On C/Pt(111) substrates the reaction kinetics are compatible with the operation of Eley–Rideal mechanisms. At annealed halide submonolayers on C/Pt(111) ethane forms in one step; at three-dimensional halide islands a two-step reaction was observed, probably with a C2H5 radical as intermediate. The measured kinetics could be very well reproduced by a sequence of two Eley–Rideal reaction steps with cross-sections in line with expectations. On Pt(111) and H/Pt(111) substrates the abstraction kinetics of ethyl from the adsorbed halide is controlled by hot-atom mechanisms. Coadsorbed H leads to simultaneous abstraction of H from the surface towards HD and H2, as well as to a gaseous C2H6 product. The ethyl abstraction cross-sections are in the range 0.5 to 1 Å2, in line with direct processes. The results provide further evidence that the substrate, nonmetallic or metallic, is instrumental in controlling the operating Eley–Rideal or hot-atom mechanisms of atom–adsorbate reactions.
Interaction of gaseous D atoms with alkyl halides adsorbed on Pt(111), H/Pt(111), and C/Pt(111) surfaces: Hot-atom and Eley–Rideal reactions. III. Isopropyl iodide
Authors: Wehner S. , Küppers J.
Published: July 2002

The kinetics of reactions of D atoms with isopropyl iodide adsorbed on Pt(111), H covered Pt(111), and monolayer C covered Pt(111) surfaces were studied using direct product detection. Propane, C3H7D, was observed as gaseous product formed through propyl abstraction from the halide. At low temperatures, below the propane desorption temperature, and submonolayer coverages the reaction products remain adsorbed on the surface. At higher temperatures (coverages) they desorb. With C/Pt(111) as substrate, the reaction kinetics are compatible with the operation of an Eley–Rideal mechanism. On Pt(111) and H/Pt(111) substrates the abstraction kinetics of propyl from the halide is controlled by hot-atom mechanisms. Coadsorbed H leads to simultaneous abstraction of H from the surface towards HD and H2, as well as to a gaseous C3H8 product. Propyl abstraction cross-sections are in the range 0.5 to 1.5 Å2, in line with direct processes. The results confirm the role of a metallic or nonmetallic substrate on the mechanisms of atom–adsorbate reactions.
Chemisorption of acrylonitrile on the Cu(100) surface: A local density functional study
Authors: Crispin X. , Bureau C. , Geskin V. M. , Lazzaroni R. , Salaneck W. R. , Brédas J. L.
Published: July 2002

The possibility of chemically grafting polyacrylonitrile onto transition metal electrodes via electropolymerization leads to promising applications in the fields of corrosion protection or metal surface functionalization. The initial step of the electrografting mechanism is the adsorption of the acrylonitrile monomer on the metal surface from solution. Here, we investigate theoretically this adsorption process on the copper (100) surface; Density Functional Theory is used in the Local Spin Density approximation to describe the electronic and structural properties of acrylonitrile adsorbed on copper clusters. The chemisorption of acrylonitrile on the copper surface is confirmed experimentally via X-Ray Photoelectron Spectroscopy. The thermodynamic characteristics of the adsorption process are also studied via statistical mechanics. Finally, determining the influence of the copper cluster size on the adsorption of acrylonitrile allows to extrapolate the properties of the acrylonitrile/Cu(100) surface from those of acrylonitrile/copper clusters.
Ab initio study of the dipole-bound anion (H2O…HCl)−
Authors: Skurski Piotr , Gutowski Maciej
Published: July 2002

The (H2O…HCl)− anion has been studied at the coupled cluster level of theory with single, double, and noniterative, triple excitations whereas lowest energy structures have been determined at the second-order Mo/ller–Plesset level. The vertical electron detachment energy and the adiabatic electron affinity were found to be 475 and 447 cm−1, respectively, and they agree very well with the maximum of the dominant peak in the photoelectron spectrum of (H2O…HCl)− at 436 cm−1, recorded by Bowen and collaborators. Our results indicate that electron correlation contributions to the electron binding energy are important and represent ∼70% of its total value. Both for the neutral and the anion, the two equivalent Cs symmetry minima are separated by a C2v transition state and the energy barrier amounts to only 77 cm−1. Thus the equilibrium structures averaged over vibrations are effectively planar (C2v). The Franck–Condon (FC) factors, calculated in harmonic approximation, indicate that the neutral complex formed in photoelectron spectroscopy (PES) experiments may be vibrationally excited in both soft intermolecular and stiff intramolecular modes. The theoretical photoelectron spectrum based on the calculated FC factors is compared to the experimental PES spectrum of Bowen et al.
A coupled-cluster study of the molecular structure, vibrational frequencies, and energetics of COBr+ and BrCO+ cations
Authors: Flowers Bradley A. , Francisco Joseph S.
Published: July 2002

The structural and energetic properties of COBr+ and BrCO+ are examined using the singles and doubles coupled-cluster method that includes a perturbational estimation of the effects of connected triples excitation (coupled-cluster single double triple CCSD (T)). Of the two isomers, the lowest energy isomer is the BrCO+ species. It is stable to dissociation into CO++Br and CO+Br+ by 140.9 and 111.6 kcal mol−1, respectively. Moreover, it is 74.6 kcal mol−1 below the COBr+ isomer. Harmonic vibrational frequencies, as well as charge distributions and dipole moments for COBr+ and BrCO+, are presented.
Simulation of an asymmetric electrolyte with charge asymmetry 60:1 using hard-sphere and soft-sphere models
Authors: Lobaskin Vladimir , Linse Per
Published: July 2002

Thermodynamic and structural properties of an asymmetric electrolyte containing macroions with 60 elementary charges and monovalent counterions in aqueous solution at different concentrations have been studied by means of Monte Carlo (MC) simulations and molecular dynamics (MD) employing two different short-range potentials. The long-range Coulombic interactions were handled by using Ewald summation and the MC simulations were accelerated by a cluster-move technique, which was found to be two orders of magnitude more efficient for this system than the standard MC method. An effective repulsion was found to operate between the macroions at all concentrations. The electrostatic screening of the macroion repulsion by the counterions was stronger in the hard-sphere model as compared to a soft-sphere model. The origin of this difference arises primarily from the deeper macroion–ion potential in the former model. The results of the hard-sphere model have been compared with different more approximate theories such as the cell model solved by MC simulations, the cell model solved by the Poisson–Boltzmann (PB) equation, and the Derjaguin–Landau–Vervey–Overbeek (DLVO) theory. We have found that the cell model solved by MC simulations and combined with a charge renormalization approach is superior to the other simplified approaches and its predictions are in excellent agreement with the exact simulation results.
Complex L2 calculation of the variation of resonance widths of HOCl with total angular momentum
Authors: Skokov Sergei , Bowman Joel M.
Published: July 2002

Complex L2 calculations of the variation of the resonance width of HOCl(6νOH)→Cl+OH with total angular momentum, J, are reported, using a recently developed, accurate ab initio potential energy surface [S. Skokov, J. M. Bowman, and K. A. Peterson, J. Chem. Phys. 109, 2662 (1998)]. The calculations are carried out using the adiabatic rotation approximation for the overall rotation and a truncation/recoupling method for the vibrational states. An ab initio calculation of the J and K dependence of the intensity of the absorption spectrum of the Q branch in the neighborhood of the 2νOH→6νOH transition is presented, and compared to results of recent experiments of Rizzo and co-workers. The variation of the resonance width of the 6,0,0 and the 3,8,0 states with J and K is presented, and comparisons with recent double-resonance experiments of the Rizzo and Sinha groups for the 6,0,0 state show encouraging qualitative agreement. The fluctuations of the dissociation rate with J is shown to be due to rotation-induced coupling of the 6νOH state to a dense set of highly excited OCl stretch states. A simple model describing the coupling of 6νOH with background states, using a coupling constant of 0.05 cm−1 is shown to give a qualitatively correct picture of the fluctuation of the resonance width with J. Finally, the energies of many nonoverlapping resonances, some of which are assigned, for J=18 and K=0 are presented and compared to Rice–Ramsperger–Kassel–Marcus (RRKM) theory. It is found that due to slow, rate limiting, intramolecular vibrational relaxation the RRKM overestimates the average dissociation rate by an order of magnitude.
Ultrafast predissociation and coherent phenomena in CS2 excited by femtosecond laser pulses at 194–207 nm
Authors: Farmanara P. , Stert V. , Radloff W.
Published: July 2002

The ultrafast predissociation dynamics of CS2 molecules excited to the B21(1Σu+) electronic state by femtosecond laser pulses with 6.0–6.4 eV photon energy has been studied in pump–probe experiments. The analysis of the time-dependent ion signals has revealed lifetimes decreasing from 620 fs down to 180 fs for tuning the excitation wavelength from 207 nm to 194 nm. A nearly constant plateau at about 200 nm in the energy dependence of the lifetime reflects the barrier of the transition from the bent to a quasilinear geometry of the excited CS2 molecule. If two vibrational bands of the B21(1Σu+) state were excited simultaneously by the femtosecond laser pulses we observed quantum beats with a modulation frequency corresponding to the energy difference of the two modes. Thus, the coherent excitation process is directly visualized despite the ultrafast decay of the excited molecular states due to predissociation.
Effect of twisting on the behavior of a double-stranded polymer chain: A Monte Carlo simulation
Authors: Velichko Yu. S. , Yoshikawa K. , Khokhlov A. R.
Published: July 2002

DNA and some proteins have a multistranded chain structure and exhibit characteristic tertiary structures due to twisting or anisotropic rigidity. We studied the effect of chain twisting on the behavior of a circular double-stranded polymer chain using a Monte Carlo simulation with bond fluctuation model. We found that there are two regimes for the chain conformation, depending on its twisting. In the first regime, with weak twisting, a circular chain behaves like a coiled linear chain. In the second regime, the chain shows scaling dependencies characterized by supercoiled structures.
General formulation of the vibrational kinetic energy operator in internal bond-angle coordinates
Authors: Frederick John H. , Woywod Clemens
Published: July 2002

A general formulation of the vibrational kinetic energy operator expressed in internal bond-angle coordinates is presented. This formulation is based on Podolsky’s expression for the covariant form of the Laplace–Beltrami operator. When a valid set of internal bond-angle coordinates is employed, it is possible to adapt a systematic approach to solve for the Jacobian determinant governing the coordinate transformation from Cartesian coordinates. In the general case of an arbitrary N-atom system, this Jacobian always factorizes to a simple form. This allows one to evaluate all the terms that contribute to V̂′, the effective potential that arises from transforming the kinetic energy operator to internal coordinates. We discuss restrictions on the choice of internal vibrational coordinates that may be included in a valid set. We then provide tabular information from which the vibrational kinetic energy operator for any molecular system can be constructed directly with no matrix inversion or chain rule manipulation required.
Theoretical and spectroscopic studies of gap-states at ultrathin silicon oxide/silicon interfaces
Authors: Kubota Tomohiro , Asano Akira , Nishioka Yasushiro , Kobayashi Hikaru
Published: July 2002

The energy distribution of interface states in the Si forbidden gap at ultrathin thermal oxide/Si(111) interfaces is obtained from x-ray photoelectron spectroscopy measurements under bias. All the observed interface state spectra have peaked structure, indicating that they are due to Si dangling bonds. For thermal oxide layers formed at 350 °C, only one interface state peak is present near the midgap. The interface state peak has ∼0.1 eV width, showing that the effective correlation energy of the Si dangling bond interface state is less than ∼0.1 eV. For oxide layers produced above 550 °C, on the other hand, two peaks are observed, one above and the other below the midgap. It is found using a density functional theory method by employing clusters containing 27 bulk-like Si atoms (interior atoms, without H passivation) that an isolated Si dangling bond, with which no atoms in the oxide layer interact, has an energy level near the midgap. It is also found from the calculations that weak interaction of the Si dangling bond with a Si atom having an unpaired electron lowers the Si dangling bond energy below the midgap, while the interaction with an oxygen or Si atom having lone-pair electrons elevates it above the midgap. When the oxide layers are formed at low temperatures, the atomic density of the oxide layer is low, leading to a long distance between a Si dangling bond and the atom in the oxide layer, thus resulting in the isolated Si dangling bond interface state near the midgap. The higher the formation temperature of the oxide layer, the higher the atomic density, resulting in a shorter distance between a Si dangling bond and the interacting atom in the oxide layer. The interface state peaks are shifted from the midgap due to the weak interaction.
Semiclassical theory of multidimensional tunneling and the hopping method
Authors: Mil’nikov G. V. , Varandas A. J. C.
Published: July 2002

The paper presents a new semiclassical theory of multidimensional tunneling and its application to the decay problem. A simple semiclassical expression for the decay rate constant is derived in terms of complex valued family of classical trajectories in the decay valley while the algebraic form of the family in the tunneling region is found by means of classical canonical perturbation theory. This provides a tool to analytically continue the classical trajectories into the decay valley, with a numerical illustration of such a continuation being done for a model 2D decay rate problem. The calculated results for the rate constant are found to be in good agreement with the exact ones for the high levels where the suggested perturbative treatment is applicable. It is also shown that the formulated theory can be directly compared with a previously proposed hopping method which gives a way to examine the accuracy of the latter without exact quantum calculations.
Molecular simulation of static hyper-Rayleigh scattering: A calculation of the depolarization ratio and the local fields for liquid nitrobenzene
Authors: Janssen R. H. C. , Theodorou D. N. , Raptis S. , Papadopoulos M. G.
Published: July 2002

Molecular dynamics (MD) simulation is used to assess the hyper-Rayleigh scattering (HRS) depolarization ratio of liquid nitrobenzene subject to vertically polarized light. In contrast to previous theoretical work, we have quantified both incoherent and coherent scattering arising from positional and orientational inhomogeneities in the molecular distribution. Although coherent scattering is shown to be much less important than in the case of Rayleigh scattering, it can not be neglected. Therefore, our analysis supports the current practice of working with dilute solutions (for which coherent contributions to HRS are truly negligible) to extract the first molecular hyperpolarizability from HRS measurements. In cases where experiments with pure liquids can not be circumvented, our analysis may be used to separate coherent and incoherent signals. Our work, which uses as input static “gas-phase” (hyper)polarizabilities obtained from ab initio calculations, also provides information on the orientations and magnitudes of the local electric fields experienced by the individual molecules in the liquid. For nitrobenzene it is found that the local fields are largely determined by specific dipolar alignment between neighboring pairs of molecules, with consequences on the HRS signal.
Comparison between two methods for mapping fluctuations in a simulation cell onto a macrovolume
Authors: Reiss Howard , Bowles Richard K.
Published: July 2002

This paper compares two methods for extending the results of simulations of physical clusters (of interest to nucleation theory) to the macrosystem in which the simulation cell is supposed to be immersed. A particular cluster model is studied in which one of the defining parameters is the volume of a “container” to which the cluster molecules are confined. The two mapping processes are conveniently and respectively referred to as the “tiling method” and the “macromethod.” Although the tiling method has been the method of choice in simulations, it is shown to be only approximate, because of a redundant counting of molecular configurations, and it is also shown that the tiling method can be derived if redundancy is ignored. On the other hand, the macromethod is derived when redundancy is properly accounted for. In certain limit situations, the tiling method converges on the macromethod but it still represents an approximation. The issue dealing with redundancy is subtle. We emphasize the fact that, short of a full, direct molecular dynamics simulation, all theories of nucleation require the use of a model. We also present a simple cluster center of mass argument that verifies the “macromethod.” The mapping problem could arise in connection with localized fluctuations beyond those that represent physical clusters.
On the origin of dynamic heterogeneities in glass-forming liquids
Authors: Qian J. , Hentschke R. , Heuer A.
Published: July 2002

We employ classical Molecular Dynamics simulations to study the cooperative dynamics of two low molecular weight glass-formers, propylene carbonate and salol. The length scales of dynamic heterogeneities are estimated. After appropriate scaling both glass-formers display a similar temperature dependence of this length scale. Local structural properties like density, regularity, and potential energy are correlated with different local dynamical observables. We find that the dynamic heterogeneities are most strongly related to the local potential energy. To obtain an optimum correlation the local dynamics has to be characterized by the residence time.
Spectroscopy of O3 trapped in rare gas matrices. I. Theoretical model for low-lying vibrational levels
Authors: Dahoo P. R. , Lakhlifi A. , Chabbi H.
Published: July 2002

A theoretical model is elaborated which allows methods used in gas phase to be applied to calculate the vibrational energies and transition moments for low-lying levels of O3 trapped in rare gas matrices. The model used in a previous work allowed only one mode to be handled at a time. With the new approach, an overall treatment of low-lying levels is achieved. The trapping site, a single or double substitutional one is distorted to minimize the free energy of the molecule–matrix system. The molecule is considered to be submitted to the net electric field present in the site as the result of the distortion and polarization of the matrix atoms. New harmonic and anharmonic constants that lead to matrix dependent calculated energy levels and transition moments can then be determined. Besides confirmation of two trapping sites, a single (S1) and a double (S2) substitutional site in a distorted face-centered-cubic (fcc) lattice structure, two other S1 sites in argon and krypton in a distorted hexagonal-closed-packed (hcp) lattice structure are shown to be possible. A fit within experimental uncertainty is reached between observed and calculated frequencies for fundamental bands v1, v2, and v3.
A multilayer theory for interfacial properties of systems containing hydrogen bonding molecules. II. A simple, yet exact form for segment potentials arising from association interactions
Authors: Suresh S. J. , Naik V. M.
Published: July 2002

We present, in this note, a simple, highly computationally efficient, and yet exact set of expressions to accurately account for H-bond interactions among molecules in any multilayer theory of interfaces. We also demonstrate that these expressions, derived using the bond-counting approach, are entirely consistent with concepts inherent in the reaction equilibrium approach proposed by F. Z. Dolezalek [Phys. Chem. 64, 727 (1908)].
Diffusion Monte Carlo simulations of the dipole-bound state of the water dimer anion
Authors: Clary David C. , Benoit David M.
Published: July 2002

It is shown how the diffusion quantum Monte Carlo method for solving the Schrödinger equation can be adapted to calculate the lowest energy state of a dipole-bound cluster anion. Specifically, the method can be used to calculate the rotationally adiabatic potential curve that arises due to the interaction of an electron with a dipolar molecular cluster. The theory allows the motion of each monomer molecule to be coupled explicitly with the angular motion of the electron. Application is made to the anion of the water dimer. The method gives a clear picture of the reorientation of the monomer water molecules that produces an overall dipole moment for the water dimer that is favorable for forming a dipole-bound state with the electron.
Intermolecular vibrations of jet-cooled (2-pyridone)2: A model for the uracil dimer
Authors: Müller Andreas , Talbot Francis , Leutwyler Samuel
Published: July 2002

The 2-pyridone dimer, (2PY)2, which is linked by two antiparallel N–H⋅⋅⋅O hydrogen bonds, is a model for hydrogen bonded nucleotide base pairs, e.g., the uracil dimer. Mass- and isomer-selected S2←S0 vibronic spectra of supersonically cooled (2PY)2 were measured by laser two-color resonant two-photon ionization and UV/UV-holeburning techniques. The latter allows the identification of the spectrum of the 2-pyridone⋅2-hydroxypyridine mixed dimer, present at ≈5% relative concentration. S2→S0 fluorescence emission spectra show dominantly the hydrogen bond shearing vibration ν4″ at 98.5 cm−1 and the stretching vibration ν6″ at 163.5 cm−1. The hydrogen bond stretching vibration force constant was determined to be 75.4 N/m, or 37.7 N/m per hydrogen bond, a very high value. The ν2″ (au) torsional and the ν3″ (bu) slanting vibrations were also identified. Ground state structures, rotational constants, harmonic intermolecular and intramolecular vibrational frequencies, interaction, and dissociation energies were calculated using Hartree–Fock and density functional (B3LYP) methods. The B3LYP/6-311++(2d,2p) results are in excellent agreement with all experimental observations.
Integral equation theory for polyelectrolyte solutions containing counterions and coions
Authors: Harnau Ludger , Reineker Peter
Published: July 2002

Integral equations for equilibrium correlation functions of a three-component polyelectrolyte solution (polyions, counterions, coions) are solved numerically. Various pair correlation functions and structure factors are investigated. It is shown that added salt screens the Coulomb interaction between the negatively charged polyions. The comparison of the calculated polyion–polyion partial structure factor with experimental results of light scattering experiments on tobacco mosaic virus in a solution with added salt exhibits good agreement. Moreover, the counterion condensation on rodlike polyions is studied by means of pair correlation functions.
Topology versus temperature: Thermal behavior of H+(H2O)8 and H+(H2O)16
Authors: Singer Sherwin J. , McDonald Shannon , Ojamäe Lars
Published: July 2002

Monte Carlo simulations based on the OSS2 potential indicate the structure of the small protonated water clusters, H+(H2O)8 and H+(H2O)16, is far from what could be expected for the proton solvated in bulk water. Near room temperature we find H+(H2O)n, n=8,16 clusters have a treelike topology with chains of waters emanating from a central H3O+ moiety. Only at lower temperatures do cycles and cages of water appear. These findings bear upon experiments in a variety of disciplines.
Molecular dichotomy within an intense high-frequency laser field
Authors: Nguyen Nam A. , Nguyen-Dang T.-T.
Published: July 2002

It is shown that, under an intense high-frequency laser field, electronic distributions in molecules exhibit a dichotomy effect just as previously found in atoms. The generalization of the formal demonstration of the dichotomy effect as given in M. Gavrila and J. Shertzer, Phys. Rev. A 41, 477 (1990) to many-electron, polyatomic molecules is considered and the validity of the α0−2/3 scaling law of the Floquet eigenvalues, with respect to the field intensity parameter α0 of the HFFT, is discussed. To test the molecular dichotomy effect, numerical calculations are performed using a quantum chemical package (Gaussian 94), modified appropriately to incorporate the cycle-averaged displacements of the nuclear–electron Coulomb potential as found in the HFFT hamiltonian. Results of calculations on the two-electron H2 molecule are presented with an emphasis placed on the character of the total and orbital charge distributions and on trends to be observed in the electronic correlation at high intensities.
Photofragmentation of ammonia at 193.3 nm: Bimodal rotational distributions and vibrational excitation of NH2(Ã)
Authors: Loomis Richard A. , Reid Jonathan P. , Leone Stephen R.
Published: July 2002

Time-resolved Fourier transform infrared emission spectroscopy is used to measure the nascent rovibrational distribution of low-lying electronically excited NH2(Ã 2A1) produced in the 193.3 nm photolysis of room-temperature and jet-cooled ammonia. Emission is observed predominantly from NH2(Ã) states with rotational motion about the a-axis and without bending excitation, υ2′=0. A bimodal N′=Ka′ rotational state population distribution is observed with up to Ka′=7 in υ2′=0 and with maxima at Ka′=5 and Ka′=1. We suggest that the bimodal rotational distribution may result from the competition between planar and bent geometries during dissociation. Weaker emission from NH2(Ã) with bending excitation, υ2′=1 and 2, is detected; the υ2′=1, N′=Ka′ rotational state population distribution spans from Ka′=0 to the energetic limit of Ka′=4. The vibrational energy partitioning for the formation of NH2(Ã,υ2′=0):NH2(Ã,υ2′=1) is 3:1 and 2:1 in the room-temperature and jet-cooled conditions, respectively. An upper limit of the NH2(Ã,υ2′=2) population is ∼10% of the total NH2(Ã) photofragments. Emission from rotational states with N′>Ka′ (molecules with rotational excitation about the b/c-axes) is also observed. Under jet-cooled conditions the NH2(Ã) b/c-axes rotational temperature of ∼120 K is higher than that expected from the rotationally cold parent species and is attributed to a mapping of the zero-point bending motion in the ν4 H–N–H scissors bending coordinate of the NH3(Ã) predissociative state onto the NH2(Ã,υ2′,N′,Ka′)+H photofragments.
Signatures of non-Markovian relaxation in two-level atoms due to environmental perturbations
Authors: Vemuri Gautam , Kemple Marvin D.
Published: July 2002

The effect of environmental perturbations on a system of two-level atoms is to alter the coupling between the atoms and their surroundings, which manifests itself as a change in the relaxation rates of the atoms. One of the ways in which environmental fluctuations can affect an atom is to modulate its transition frequency in a stochastic manner. We study the response of such two-level atoms to irradiation by a monochromatic laser, and report the results of numerical experiments on the spectrum of scattered light. It is observed that this spectrum is a sensitive indicator of the parameters that describe the stochastic modulation. In addition, the spectra exhibit substantially different features depending on whether the stochastic process follows a diffusion or a jump mechanism. The numerical results are based on a Monte Carlo procedure, which can be applied to stochastic processes in a wide variety of biological, chemical, and physical processes.
Effects of asymptotic interactions on the photodissociation of HCl
Authors: Lee Sungyul , Jung Kyung-Hoon
Published: July 2002

Theoretical analysis is presented on the influence of the interactions among the asymptotically degenerate states correlating with Cl (2P) on the photodissociation of HCl. The spin–orbit branching ratios of Cl(2Pj, j=1/2, 3/2) are computed for low photon energy (50 000–110 000 cm−1). We predict that the branching ratios are very different from the high energy recoil limit in this energy regime and may even be inverted, due to the large influence of the interactions in the recoupling region. The angular distributions of Cl(2Pj, j=1/2, 3/2) and the alignment parameter of Cl(2P3/2) are also treated. We show that these vector properties can be significantly different from those resulting from perpendicular A 1Π–X 1Σ+ transition, indicating the role of the ground X 1Σ+ state as the dissociation channel interacting with the A 1Π state.
Spin dynamics study in doped polyaniline by continuous wave and pulsed electron paramagnetic resonance
Authors: Magon C. J. , de Souza R. R. , Costa-Filho A. J. , Vidoto E. A. , Faria R. M. , Nascimento O. R.
Published: July 2002

Continuous-wave (cw) and pulsed electron paramagnetic resonance (pulsed EPR) techniques were used to investigate spin dynamics in moderately and slightly doped poly(o-methoxyaniline), a polyaniline derivative. EPR line shapes, measured by standard cw EPR, and relaxation rates, measured by electron spin–echo methods, were studied as a function of temperature, in the ranges 6–300 K (cw) and 6–100 K (pulsed). Experimental results were explained by a model which considered two types of spins: isolated polarons in an insulating matrix, and delocalized ones in conductive islands (clusters) formed by dopant aggregates. Experimental data agreed with the assumption that EPR spectra are generated by isolated polarons coupled to the clusters by dipole–dipole interactions. Furthermore, some experimental evidence suggests that spin dynamic within the clusters, which play an essential role in the interpretation of the EPR results, is determined by exchange interactions. Consequences of the proposed model are discussed.
LDA and GGA calculations of alkali metal adsorption at the (001) surface of MgO
Authors: Snyder James A. , Jaffe John E. , Gutowski Maciej , Lin Zijing , Hess Anthony C.
Published: July 2002

The adsorption geometry, binding energy and electronic structure of alkali metal overlayers on the MgO (001) surface have been studied by means of density functional theory, using Gaussian-type orbitals to expand the wave functions and electronic charge density. A two-dimensionally periodic slab of MgO with alkali metal adsorbed at one surface was used to model the semi-infinite system. Li, Na, and K were considered at both half- and quarter-monolayer coverage. Results were compared for the local density approximation and for two different forms of the generalized gradient approximation. In all cases Li was found to interact with the surface approximately twice as strongly as Na and three times as strongly as K. The epitaxial binding energies were, however, always less than or close to the bulk cohesive energies of the respective alkali metals, suggesting an instability of the adsorbed film toward the formation of two- or three-dimensional islands, in agreement with experiment. Spin polarized and unpolarized calculations were compared to detect metal–insulator transitions in the alkali overlayer. Only Li adsorbed at 1:4 coverage was found to have lower energy in a spin polarized (hence nonmetallic) state.
Recent nuclear magnetic resonance experiments on polymer melts: Comments
Authors: Herman Michael F.
Published: July 2002

The role of the relative motions of protons on different chain segments in the relaxation of the dipolar coupling measured in nuclear magnetic resonance experiments is considered. The relaxation of the coupling for a single intersegmental proton–proton pair is evaluated numerically, assuming that the decay of this coupling is dominated by the translational motion of the monomers. It is found that the relaxation of this coupling scales as gr(t)−5/2, where gr(t) is the mean squared relative displacement of two monomers in the melt. This scaling applies for times at which gr(t) is greater than the square of the initial separation of the two coupled protons. The effect of the short range equilibrium structure of the melt on the intersegmental relaxation is qualitatively considered by separating out, from the summation over all interactions, the interactions between the protons corresponding to the first intersegmental peak in the hydrogen–hydrogen radial distribution function. This analysis indicates that the short range melt structure results in a more rapid decay of the intersegmental interactions at moderately short times, than would be predicted if the short range structure is ignored.