bluuebunny/crossref_metadata_2025 huggingface dataset:
Photofragmentation spectroscopy of Al+(C2H4)
Authors: Chen J. , Wong T.-H. , Kleiber P. D. , Yang K.-H.
Published: July 2002

We have studied the structure and dissociation dynamics of the weakly bound bimolecular complex Al+(C2H4) by photodissociation spectroscopy in the 216–320 nm spectral region. Experimental studies are supported by ab initio electronic structure calculations of the ground and low-lying excited states of the complex. Al+ is the dominant photofragment observed throughout the absorption profile. C2H4+ charge transfer product is also observed for shorter photolysis wavelengths, λ<252 nm. The Al+–C2H4 bond dissociation energy is measured as D0=0.37±0.15 eV. Three molecular absorption bands are observed and assigned to the transitions (2 1A1,1 1B1,1 1B2←1 1A1) in C2v equilibrium complex geometry. The excited states are of predominantly charge-transfer character correlating with the product channel Al(3s23p)+(C2H4)+. The 2 1A1 and 1 1B2←1 1A1 absorption bands appear broad and structureless. This observation is consistent with ab initio results that suggest a pathway for rapid nonadiabatic dissociation through a 1 1B2–1 1A1 surface crossing facilitated by a stretch in the C–C bond of ethylene. In contrast the 1 1B1←1 1A1 molecular band shows significant vibrational structure. Spectroscopic analysis yields a band origin (000=40 042 cm−1) and corresponding vibrational mode frequencies for the 1 1B1 excited state. The observed modes have been assigned to the intermolecular Al–C2H4 stretch of a1 symmetry (ν2=230 cm−1), the Al–C2H4 out-of-plane wag with b1-symmetry (ν3=328 cm−1), and two intramolecular ethylene modes of a1 symmetry at 1264 and 1521 cm−1. The assignment for these higher frequency ethylene modes is not conclusive.
A combined quantum mechanics and molecular dynamics study of small Jahn–Teller distorted hydrocarbons: Another difficult test for density-functional theory
Authors: Wetmore Stacey D. , Boyd Russell J. , Eriksson Leif A. , Laaksonen Aatto
Published: July 2002

Temperature, vibrational, and matrix effects on the geometry and hyperfine coupling constants of the methane and ethane radical cations are investigated with a combined quantum mechanics and molecular dynamics technique. Density-functional theory (the B3LYP functional) is implemented as the quantum mechanical method. Results obtained for the methane cation are discouraging. The hyperfine coupling constants (HFCCs) obtained from the simulations are in poor agreement with experimental results. These deficiencies are ascribed to the inadequacy of density-functional theory to describe the potential energy surface in this radical. Results obtained for the ethane radical cation with the identical method are more promising. The HFCCs obtained from the simulations are in better agreement with experimental results obtained at 4 K than those obtained from static, gas-phase calculations, indicating vibrational effects are important for this radical even at low temperatures. Temperature effects on the HFCCs in the ethane radical cation observed experimentally are also well reproduced by the simulations.
Femtosecond pump-probe study of molecular vibronic structures and dynamics of a cyanine dye in solution
Authors: Yang T.-S. , Chang M.-S. , Chang R. , Hayashi M. , Lin S. H. , Vöhringer P. , Dietz W. , Scherer N. F.
Published: July 2002

Time-resolved pump–probe spectra of 1,1′,3,3,3′,3′-hexamethyl-4,4′,5,5′-dibenzo-2,2′indotricarbocyanine (HDITC), a cyanine dye, in ethylene glycol are obtained using 11 fs and 90 fs duration pulses and analyzed in order to study its potential energy surfaces and vibrational dynamics. Ten oscillatory frequencies ranging from 30 cm−1 to 1400 cm−1 are observed in the 11 fs duration wavelength-resolved pump–probe measurements. They are assigned as fundamental vibrational frequencies of HDITC. The relative displacements of the equilibrium position between electronic excited and ground states along the resolved ten vibrational modes are determined through the wavelength dependence of the oscillatory amplitude. After considering the contributions of the ten vibrational modes, it is found that most of the Stokes shift and the early fast decays of the pump–probe signals are due to relaxation along the low frequency overdamped modes of the chromophore. The overdamped modes are characterized by the 90 fs pump–probe signals with the excitation at the red edge of the absorption band.
Adsorption of water on NaCl(001). I. Intermolecular potentials and low temperature structures
Authors: Engkvist Ola , Stone Anthony J.
Published: July 2002

Water adsorption on the NaCl(001) surface has been extensively studied both theoretically and experimentally during recent years. Here we investigate it using intermolecular potentials derived from intermolecular perturbation theory (IMPT). The water–water interactions are described by the recently developed ASP-W4 potential. For the water–NaCl surface, repulsion parameters were developed using IMPT, and C6 dispersion coefficients were calculated using coupled Hartree–Fock perturbation theory. The binding energy between a single water molecule and the NaCl surface is found to be 40 kJ mol−1. A stable tetramer can form on the surface, similar to the water tetramer in the gas phase. At a coverage of one water molecule per NaCl unit, there are several different water monolayer structures with approximately the same energy. Some have all the water oxygens located close to Na+ ions, but others have some water molecules located above the Cl− ions. The latter are farther from the surface, and are hydrogen bonded to water molecules sited above the Na+ ions. Structures were also located with 1.5 water molecules per NaCl unit. Their binding energy per water molecule was similar to the structures with one water molecule per NaCl unit. Experimentally water monolayer structures with (1×1) and (c4×2) unit cells have been suggested. Our results support the conclusion that owing to the small energy differences between different monolayer structures, several different types of monolayer structure can coexist on the surface at low temperatures.
A new method for simulating the late stages of island coarsening in thin film growth: The role of island diffusion and evaporation
Authors: Mattsson Thomas R. , Mills Greg , Metiu Horia
Published: July 2002

We have developed a method for simulating the evolution of an ensemble of one-atom-high islands from deposition and nucleation to coarsening. Using this method we have studied three regimes of coarsening; coarsening due to island coalescence, coarsening driven by evaporation, and the case in which both mechanisms act simultaneously. The parameters have been chosen to mimic coarsening of Ag on Ag(001); they are not meant to reproduce the experimental results for Ag quantitatively, but to provide simulations relevant to metal-on-metal homoepitaxy. We find that the scaling laws proposed by the mean-field theory for the time dependence of the number of islands and the island size distribution function work well in the limiting case when coarsening is dominated by island diffusion and coalescence. In the opposite limit, when coarsening is dominated by evaporation, the scaling predicted for the island size works well, but the island size distribution predicted by the mean-field theory is narrower than the one found in simulations. In the case when island migration and evaporation are both important, the evolution of the number of islands shows a crossover; at early times it scales as if coarsening takes place by island coalescence, and at later times it scales as if coarsening is dominated by evaporation. Regardless of the coarsening mechanism, most islands disappear by coalescence.
Lattice vibrations and thermodynamic stability of polymerized C60 deduced from heat capacities
Authors: Inaba Akira , Matsuo Takasuke , Fransson Åke , Sundqvist Bertil
Published: July 2002

Heat capacities of 1D and 2D pressure polymerized C60 as well as the thermally depolymerized C60 have been measured at temperatures between 4 and 350 K and the results analyzed for the lattice vibrations and the thermodynamic stability. It was found from the low-temperature heat capacity (T<100 K) that on polymerization (1) the lattice vibrations, both translational and rotational, stiffen substantially, (2) an anisotropic nature emerges in response to the lower dimensionality, and (3) an anharmonicity still remains as in pristine C60. The normal C60 has an excess entropy of 67.8 and 99.4 J K−1 mol−1 at 300 K relative to the 1D and 2D polymerized C60, respectively. The thermodynamic stability is considered with two possible phase diagrams.
Ensemble dependence of confined hard-rod fluids
Authors: Kim Soon-Chul
Published: July 2002

The finite-size effects and packing constraints on the density profile of a hard-rod fluid in both open (grand canonical) and closed (canonical) walls have been investigated. For a finite system, the grand canonical density profile shows very different density behavior compared with the canonical density profile. At low packings, the convergence of series is shown to converge very quickly, even if only a few particles are confined in hard walls. However, the significant differences at high packings arise between the canonical and the grand canonical density profiles. The convergence is much slower in the region where the peak develops.
High resolution Raman spectroscopy from vibrationally excited states populated by a stimulated Raman process. Transitions from v2=1 in C212H2 and C213H2
Authors: Bermejo D. , Martı́nez R. Z. , Di Lonardo G. , Fusina L.
Published: July 2002

The Raman spectra of the Q branches of the ν1+ν2−ν2 band of C212H2 and of the ν1+ν2−ν2 and 2ν2+2ν5−ν2 bands of C213H2 were recorded at high resolution in the gaseous phase. To obtain the spectra a two-step pump/probe technique was used. In the first step, molecules were pumped from the ground to the v2=1 excited state by means of a double pulsed stimulated Raman process. Then, spectra corresponding to the transitions from this state to higher ones are recorded by means of a high resolution quasicontinuous stimulated Raman process. The analysis of the experimental data, taking into account the fourth-order anharmonic interaction coupling the excited states for the C13 isotopomer, provided unperturbed values for the rotational and distortion constants of the excited states and for the band origins. Accurate values for the x120 anharmonicity constant have been calculated directly from the energies of the v1=v2=1 states for the first time.
Picosecond studies of the intramolecular relaxation processes in isolated C60 and C70 molecules
Authors: Sassara A. , Zerza G. , Chergui M. , Ciulin V. , Ganière J.-D. , Deveaud B.
Published: July 2002

Time-resolved fluorescence spectra for C60 molecules embedded in Ne and Ar matrices and C70 in Ne matrices were recorded using a synchroscan streak camera. In the case of C60, the decay times of the S1 state are identical for all fluorescence bands confirming that it is a state of mixed T1g, T2g, and Gg characters. Its lifetime is determined by the intersystem crossing rate to the triplet manifold which increases in going from Ar to Ne matrices. This is attributed to a change of the dominant symmetry character of S1 in the two matrices. A transient, short-lived (∼170 ps in Ne, ∼70 ps in Ar), S3 fluorescence of dominant Gg symmetry is also observed, which feeds the S2/S1 population (of dominant T2g and T1g symmetry). Using the Energy Gap Law, a change of the S3→S2/S1 decay rate from Ne to Ar matrices is attributed to the different number of phonons needed to bridge the gap between these states and/or to an environment-dependent change of the coupling matrix element. In C70, the fluorescence from both S1 and S2 are populated at the same rate despite the 165 cm−1 energy gap between them. This is explained in terms of an intersystem crossing from the triplet state manifold. Finally, S2 decays predominantly to S1, while the latter decays to the triplet states by reversible intersystem crossing. These conclusions are confirmed by a simple kinetic model.
Modeling of low-temperature depolymerization of poly (methyl methacrylate) promoted by ion beam
Authors: Raudino A. , Fragalà M. E. , Compagnini G. , Puglisi O.
Published: July 2002

The role played by the ion beam in the depolymerization of poly(methyl methacrylate) (PMMA) is the creation of initiators for the unzipping reaction at relatively low temperatures [lower than the ceiling temperature of PMMA (TC=220 °C)]. Monomer (MMA) evolution out of the polymeric film is thus governed by the kinetics of the chemical reaction and also by the diffusion of monomer inside the polymeric film. In this work this phenomenon has been described by using simple models in which both one- and two-dimension monomer diffusion is considered. Exact analytical solutions of the models are provided. The theory fit well the experimental data and allows for the determination of important parameters such as the kinetics constant of the depolymerization and the monomer diffusion coefficient, D, inside the polymer film. Thus, the pulsed ion beam technique, here modeled, is useful not only for extracting D values of slow diffusants in polymers, but even for measuring kinetics constants of depolymerization at a temperature well below that needed for the formation of suitable thermal initiators.
Suppressing the geometric phase effect: Closely spaced seams of the conical intersection in Na3(2 2E′)
Authors: Yarkony David R.
Published: July 2002

It is shown that for the Exε Jahn–Teller problem, the circulation, the line integral along a closed loop, of the actual, or an approximation to the, derivative coupling can be used to determine the number of closely spaced conical intersections in that closed loop. This approach has advantages over the use of the geometric phase theorem, which can only count conical intersections mod 2. The circulation is used to establish the locus of the seam of the conical intersection of the 2 2E′ state of Na3. It is found that in addition to the D3h seam of the conical intersection, three symmetry equivalent seams with C2v symmetry exist in close proximity to the D3h seam. The three C2v seams intersect the D3h seam. The net geometric phase effect is largely suppressed and this ostensibly Jahn–Teller pair of electronic states is more like a Renner–Teller pair.
Highly predissociative levels of the D 2Π state of CH studied with the two-color resonant four-wave mixing technique
Authors: Li Xinghua , Lee Yuan-Pern
Published: July 2002

We investigated highly predissociative transitions D 2Π–B 2Σ−(0-0) of CH in an oxyacetylene flame with two-color resonant four-wave mixing, in which two grating beams are in resonance with the D–B transition and the pump (and signal) beam is resonant with a selected B–X transition. A total of 86 predissociative lines in six branches are observed, corresponding to excitation of the D state up to N′=16; in previous experiments lines up to only N′=6 were observed. Observed rovibronic transitions of the D–B(0-0) band are fitted to known Hamiltonians to yield improved spectral parameters of the D 2Π (v=0) state. The parameters for centrifugal distortion (D,H) and Λ doubling (p,q,qD) of the D 2Π (v=0) state are deduced for the first time. The predissociative line width is about 2 cm−1 for the rotational level with N′=1 and it gradually increases with N′; for N′=15 the width is about 7 cm−1. Predissociation mechanisms of the D 2Π state are discussed. A barrier height ∼5600 cm−1 for the D state is estimated based on observed predissociative lifetimes.
Characterization of selective adsorption resonances for helium scattering from a highly corrugated surface using quantum wave packet dynamics
Authors: Subramanian Vijaya , Skodje Rex T.
Published: July 2002

We have studied the characteristics of selective adsorption resonances (SAR) for helium scattering from the highly corrugated Xe/Ag(111) surface. A time-dependent quantum mechanical procedure developed previously for the study of resonances in gas-phase molecular collisions was employed. Converged resonance positions, lifetimes, partial widths, and resonance wave functions were obtained for states lying below 20 meV. The zero-order separable description of SAR was found to fail badly for the assignment of the states as well as for their energies and lifetimes, as expected. However, it was found that the separable model improves noticeably for SAR’s with higher energy and for those states localized farther from the surface. More sophisticated separable models have been developed which can improve the agreement with exact results to some degree. The dynamical characteristics of SAR states are analyzed in detail and the role of symmetry is discussed. Energies and lifetimes along the bands of SAR were also generated.
Excitation energy transfer between molecular thin layers of poly(phenylene vinylene) and dye labeled poly(allylamine) in layer-by-layer self-assembled films
Authors: Richter Bernd , Kirstein Stefan
Published: July 2002

Steady state Förster energy transfer is investigated in multilayer structures of self-assembled films prepared via the layer-by-layer deposition of oppositely charged polyelectrolytes. The samples consist of a single layer of the conjugated polymer poly(p- phenylene vinylene) (PPV) as a donor, several layers of poly(styrene sulfonate) (PSS), and poly(allylamine hydrochloride) (PAH) as a transparent spacer, and a single layer of dye labeled PAH as an acceptor. The dyes are rhodamine B and fluorescein, whereas the molar ratio with respect to the PAH monomers is less than 0.2% for both systems. The steady state fluorescence intensity of the PPV is measured for a series of samples with different spacer thicknesses. The spacer thicknesses are evaluated from x-ray reflectivity measurements. At large thickness of the spacer the donor intensity I versus distance d follows the 1/(1+(d0/d)4) law as theoretically predicted for the case of ideal two-dimensional layers. At short distances a different behavior is observed which is explained first, by the low lateral density of the acceptor molecules and second, by a distribution of the dyes normal to the layer plane. Good agreement between the experimental data and computer simulations is obtained under the assumption of a Gaussian distribution of both donor and acceptor molecules, in the direction normal to the film plane with a width of 25–30 Å. However, a mismatch is observed between the Förster radius derived from the spectroscopic properties and that obtained from the layer-to-layer energy transfer. This is discussed under the consideration of a nonperfect layer structure and the photophysics of the PPV.
Numerical simulations of Coulomb systems: A comparison between hyperspherical and periodic boundary conditions
Authors: Caillol J. M.
Published: July 2002

Numerical simulations of Coulomb systems can be performed in various geometries, for instance in a cube with periodic boundary conditions (C3) or on the surface of a hypersphere (S3). We make a detailed comparison between electrostatics in these two geometries with a special emphasis on the problem of properly defining the zero of energy of a system of charges. This analysis enables the derivation of the correct configurational energies of important models of Coulombic fluids or plasmas in C3 and S3 in a unified way. The cases of the one component plasma and the restricted primitive model of electrolytes are considered in detail.
Ab initio potential energy surfaces of He-CO2 and Ne-CO2 van der Waals complexes
Authors: Negri F. , Ancilotto F. , Mistura G. , Toigo F.
Published: July 2002

We have calculated ab initio the ground-state potential energy surfaces of Ne-CO2 and He-CO2 van der Waals complexes by using the fourth-order Moller-Plesset perturbation theory with a large basis set containing bond functions. The rigid molecule approximation has been used for CO2. Both complexes have the lowest energy at the T-shaped configuration. In addition, a shallow minimum is found for the linear configuration, with a small barrier connecting the two minima on the potential energy surface, whose height is larger for the lighter noble-gas atom. The implications of these potentials for the wetting behavior of adsorbed films are discussed.
Scattering matrix determination by asymptotic analysis of complex scaled resonance wave functions: Model Cl+H2 nonadiabatic dynamics
Authors: Friedman Ronald S. , Ryaboy Victor M. , Moiseyev Nimrod
Published: July 2002

It has previously been shown that partial widths of resonance states can be calculated by the asymptotic analysis of the complex scaled resonance wave function [U. Peskin, N. Moiseyev, and R. Lefebvre, J. Chem. Phys. 92, 2902 (1990)] and by the complex coordinate scattering theory [N. Moiseyev and U. Peskin, Phys. Rev. A 42, 255 (1990)]. Here we use these methods for the first time to calculate complex partial width amplitudes. The complex amplitudes are independent of the complex scaling parameters and are used for calculating the resonance contribution to the scattering matrix (the S matrix) in the case of Cl+H2 scattering described by two coupled one-dimensional potential energy curves. The background contribution to the S matrix was calculated by the use of one ClH2 potential energy curve only. The sum of the resonance and the background contributions provides accurate complex S matrix elements and transition probabilities, even at the resonance energy for which total reflection is obtained due to the interference between the two contributions.
Bro/nsted acid sites in gmelinite
Authors: Benco Ľubomı́r , Demuth Thomas , Hafner Jürgen , Hutschka François
Published: July 2002

The structural properties of purely siliceous and Al-substituted protonated gmelinite, a zeolite with a medium-sized unit cell, are investigated by means of first-principles local-density functional calculations. For acid sites introduced into an Al-free structure, optimized geometries are compared with experimental data. The substitution of a Si–O fragment by Al–O–H induces a pronounced local deformation of the structure. Four symmetrically distinct O sites (O1–O4) are classified, according to the pattern of the deformation, into two categories showing rather localized (sites 2 and 3), and mostly delocalized (sites 1 and 4) distortions. Relative stabilities of protonated structures are shown to depend on the initial geometry of the site. Larger Si–O–Si angles lead to a higher stability of the acid site. Two approaches, a static and a dynamical one, are used to derive OH stretching frequencies. Both of them prove a rather complex relation between the infrared (IR) frequency, the acidity, and the local environment of the Bro/nsted acid site. The lowest OH stretching frequency is assigned to the O2 site as distinguished in the dynamical calculations. The shift occurs due to attractive interactions of H to the framework oxygen atoms.
The (NO)2 dimer and its ions: Is the solution near?
Authors: Urban Bernhard , Strobel Andreas , Bondybey Vladimir E.
Published: July 2002

In this manuscript the available experimental and theoretical information on (NO)2 and its ions is summarized and reviewed, and high resolution two photon, one color photoelectron spectra of the dimer are presented. The new spectra yield a wealth of information about the dimer cation, including possibly all six of its vibrational frequencies as well as several anharmonic intermode constants. The most consistent interpretation of the contradictory data available in the literature and of the present results is to assign the gas phase photoelectron spectrum to cis–cis ionization, and the matrix spectra to the trans (NO)2+ ionic ground state. The similarity between the observed matrix frequencies with the gas phase values reflects the insensitivity of the NO stretching fundamentals of the dimer cation to conformation, a point of view supported by the results of theoretical calculations. Despite the problems of theory in computing properties of the (NO)2 species, the experimental structures and molecular constants appear to be in an acceptable agreement with the most recent computational results.
Glass-like behavior in supercritical fluids: The effect of critical slowing down on solute dynamics
Authors: Goodyear Grant , Tucker Susan C.
Published: July 2002

Compressible supercritical fluids exhibit extended density inhomogeneities which, due to their collective many-particle nature, fluctuate only very slowly, a phenomenon known as “critical slowing down.” Our computations demonstrate that these slowly fluctuating inhomogeneities may remain effectively frozen on the time scale of fast dynamic processes such as solute vibrational relaxation. Furthermore, we find that a given solute molecule may experience any of a broad distribution of effectively static local solvent environments and that each solute’s dynamical behavior will be controlled by its own local solvent environment. Consequently, an ensemble of solute molecules will relax with a correspondingly broad distribution of lifetimes. Although such inhomogeneous broadening effects are expected in amorphous solids and glasses, this is the first confirmation of their presence in supercritical fluids.
Dynamics of sulfur adlayer transformations at metal/electrolyte interfaces
Authors: Andreasen G. , Vericat C. , Vela M. E. , Salvarezza R. C.
Published: July 2002

The dynamics of adsorbed sulfur on Au(111) at the metal/electrolyte interface has been followed by scanning tunneling microscopy. The increase in the negative surface charge density induces the transformation from rectangular S8 to a √3×√3 R30° lattice. This process involves desorption of sulfur atoms as HS− ions, formation of rectangular tetramer intermediates, and displacement of atoms from the intermediate to nearest hollow sites. A further increase in the negative charge density produces desorption of the √3×√3 R30° lattice from terraces. Sulfur atoms remain strongly bonded to step edges. Experimental data provide an estimation of the excess of binding energy close to step edges. Correlation between S and thiol adlayer desorption data has been found.
A diffusion equation for Brownian motion with arbitrary frictional coefficient: Application to the turnover problem
Authors: Battezzati Michele
Published: July 2002

After a brief re-exposition of the procedure devised by the author in order to reobtain a diffusion equation from the equations of the motion of a mechanical system driven by a random force, this method is applied to derive a third-order diffusion equation for an anharmonic oscillator undergoing Brownian motion. This equation is exact to first-order in the parameter of anharmonicity, and is valid for arbitrary values of the frictional coefficient. The confrontation of this equation with a similar equation obtained previously by asymptotic expansion in inverse powers of the frictional coefficient, shows that although the two equations are different, nevertheless they reduce to the same equation (within the limits of validity of each approximation scheme) when they are both reduced to second order. An asymptotic formula for the mean first-passage time (MFPT) for escaping over a barrier is then proved in the low-temperature limit, which is related to an eigenvalue of the diffusion operator, and to the solution of an integral equation with Smoluchowski boundary conditions. This equation yields the correct behavior of the eigenvalue in both limits of high and extremely low friction, with interpolation between the two limits, while in the oscillatory regime yields a complex eigenvalue, whose imaginary part can be interpreted as a stochastic resonance frequency between the anharmonic well and its mirror image beyond the barrier. It is shown how the Kramers’ result for moderate or strong friction fits in with the present theory, and what is the origin of the discrepancies.
Reaction dynamics on a thermally fluctuating potential
Authors: Berezhkovskii Alexander M. , Szabo Attila , Weiss George H. , Zhou Huan-Xiang
Published: July 2002

This paper analyzes the kinetics of escape of a particle over a barrier fluctuating between two states, the fluctuations being produced by thermal noise. By this we mean that the jump rates for transitions between the two states are position-dependent, satisfying detailed balance at any point along the reaction coordinate. The fast-fluctuation limit can be analyzed in terms of the potential of mean force, and for high barriers the survival probability is found to be a single exponential. In the slow-fluctuation regime the survival probability is a linear combination of two exponentials. In the case of a linear potential the slow-fluctuation solution describes the kinetics, as obtained from simulations, quite well over the entire range of the jump rates between the two states. Our analysis suggests that this is true for more general forms of the potential. Further, for a thermally fluctuating potential the mean lifetime is shown to decrease monotonically as the jump rate increases. This is in contrast to the turnover behavior, or resonant activation, which can occur when fluctuations are produced by nonthermal noise. An extension of our approach to systems with thermal fluctuations between more than two states is discussed.
Optical excitations and coupling constants in FeO42− and CrO44− complexes in oxides: Density functional study
Authors: Wissing K. , Barriuso M. T. , Aramburu J. A. , Moreno M.
Published: July 2002

The experimental crystal field (CF) and charge transfer (CT) spectra due to Fe6+-doped K2XO4 (X=S, Se, Cr) and Cr4+ in oxides have been investigated by density functional (DF) calculations on FeO42− and CrO44− complexes at different values of the metal–ligand distance, R. To this aim the coupling constants, Aα and Aε, with the A1 and the Jahn–Teller E modes have also been calculated. The present results explain reasonably the transition energies and coupling constants observed for Fe6+ and Cr4+ in oxides and account for the following experimental features: (i) The different nature of the first CF and CT excitations observed on passing from Fe6+ to Cr4+ in oxides. (ii) Why for Fe6+ in oxides the dominant coupling is with the E mode for the CF states while with the A1 mode for CT states. (iii) Why on going from Cr4+ to Fe6+ in oxides the coupling constant Aα of CT states decreases while the opposite is found for CF ones. The role played by the covalency on these questions is explained on simple grounds, stressing that it drastically decreases the Aα value for CT states. This fact together with the value να=820 cm−1 accounts for the low value of the Huang–Rhys factor Sα=2.5 in the assigned 1t1↓→4t2↓ CT excitation of K2SeO4:Fe6+. As a salient feature the R dependence of 10Dq (which determines Aα for the T23 CF state) is found to be greatly sensitive to the small admixture (≈0.5%) of 2s(O) orbitals in the antibonding 2e level. The electronic relaxation decreases the energy of the first CT transition of FeO42− by 19 000 cm−1, the charge distribution being however the same as that of the ground state. The important role played by the trigonal distortion for reducing the Jahn–Teller effect in the first excited state of Cr4+ in oxides is analyzed through a phenomenological model. Though no further neighbors of the host lattice are included, the calculated equilibrium distances for FeO42− and CrO44− are only 3% and 8% higher than experimental values found for Cr4+ to Fe6+ in oxides. Moreover the calculated value of the να frequency at the right equilibrium distance essentially coincides with experimental findings indicating that force constants between oxygen and further neighbors likely play a minor role. In comparison with recent ab initio plus configuration interaction results on FeO42− the DF scheme offers a simpler description of ground and CT states.
Rupture luminescence from natural fibers
Authors: Li W. , Haneman D.
Published: July 2002

Fibers of cotton and wool, and samples of paper, have been ruptured in tension in vacuum and in air, and give detectable luminescence in the visible range. All have a common emission peak at around 2.0 eV, which is ascribed to the deexcitation of states excited by the rupture of organic chain molecule bonds. Rubber bands give stronger emission in air, but no emission in vacuum, suggesting the material breaks only at weak interchain bonds. Mohair, cat, and horse hair also give emission in air. The phenomena reveal effects that would occur widely in nature.
Langmuir monolayers as disordered solids: Disorder and elastic fluctuations in mesophases
Authors: Luty Tadeusz , Eckhardt Craig J. , Lefebvre Jacques
Published: July 2002

Ordering in Langmuir monolayer mesophases is examined using an approach based on the elastic theory of crystals. Molecular tails are modeled as “defects” grafted onto a two-dimensional elastic medium and are characterized by elastic dipoles. It is assumed that disorder in the parent, LS, phase is due to competition between local (within a domain) and global (hexagonal arrangement of domains) structure. By treating the LS phase as a mixture of rectangular and/or oblique domains (rectangular and/or oblique defects within the two-dimensional elastic medium), density fluctuations due to elastic interactions between domains are analyzed. The correlation function for the elastic dipoles is calculated and the elastic interactions’ renormalization of the elastic properties of mesophases is analyzed. Results are shown to be compatible with very recent experiments on microscopic and macroscopic elasticity of the monolayers as well as those on positional disorder in LS and S phases. Kinetic aspects of the elastic response are considered, as is the contribution of the elastic domains’ reorientations to x-ray diffuse scattering.
The effects of bead–bead repulsion on the spacial and time correlation functions of model polymer solutions: Mesoscale simulations
Authors: Xiao C. , Heyes D. M.
Published: July 2002

Brownian dynamics simulations have been carried out for model polymer chains in a good solvent over a wide concentration range. The polymers were treated as beads linked by finitely extensible nonlinear elastic (FENE) springs and the repulsion between any two unlinked beads was modeled by a pair potential with a Gaussian analytic form, βu(r)=A exp(−r2/σ2), where β=1/kT, A and σ are characteristic energy and distance scales, respectively. The effects of the bead–bead repulsion on the structure and time-correlation functions of the chains in the polymer solution were studied as a function of polymer concentration. Three concentration regimes are distinguished, a dilute region where intrachain bead–bead repulsions dominate, a concentrated region where interchain bead–bead repulsions dominate, and a highly concentrated region where the net repulsion on any bead tends to zero owing to substantial cancellation of the effects from nearest neighbors. The pair radial distribution function, the relaxation time for the rotation of the coil, the mean square displacement of the middle-bead and that of the center of mass of the chain, the infinity frequency elastic modulus, and the viscosity of the system are examined in all the density regions. Our results show that the excluded volume repulsion strongly affects the behavior of the system in the concentrated region and that the structural features return to the Rouse-limit behavior at high density more rapidly than the dynamical properties.
A patching model for surface tension of spherical droplet and Tolman length. II
Authors: Bykov T. V. , Zeng X. C.
Published: July 2002

In the framework of density functional theory (DFT), two patching models for the density profile of spherical liquid droplet are developed. The patching is based on analytical expressions of the asymptote of the density profiles. The first model leads to analytic expressions of the Tolman length and the effective rigidity constant, from which the temperature dependence of the Tolman length and effective rigidity constant can be determined. The second model is developed particularly for small spherical droplets, from which the dependence of chemical potential and the surface tension of the droplet on the radius are obtained. The results are compared with numerical DFT calculations.
Application of the forward–backward initial value representation to molecular energy transfer
Authors: Skinner David E. , Miller William H.
Published: July 2002

It is shown how the forward–backward (FB) approximation to the semiclassical initial value representation (IVR) can be used to calculate the probability (or cross section) for molecular energy transfer. Specifically, the probability P(ΔEA) for a molecule A to gain (or lose) an amount of internal energy ΔEA by collision with a bath molecule B is given by the Fourier transform of a time correlation function C(t), which is in turn given by a single phase space average over the initial conditions of classical trajectories of the A+B collision system. Application to energy transfer of H2 by collision with He is carried out to demonstrate that the FB-IVR provides a good description of quantum effects in P(ΔEA).
Comment on “Surface diffusion near the points corresponding to continuous phase transitions” [J. Chem. Phys. 109, 3197 (1998)]
Authors: Vattulainen I. , Ying S. C. , Ala-Nissila T. , Merikoski J.
Published: July 2002

It is well known that unlike static equilibrium properties, kinetic quantities in Monte Carlo simulations are very sensitive to the details of the algorithm used for the microscopic transition rates. This is particularly true near the critical region where fluctuations are pronounced. We demonstrate that when diffusion of oxygen adatoms near the order–disorder transition of a lattice-gas model of the O/W(110) model system is studied, the transition rates must be chosen carefully. In particular, we show that the choice by Uebing and Zhdanov [J. Chem. Phys. 109, 3197 (1998)] is inappropriate for the study of critical effects in diffusion.
Vibrorotational Raman and infrared spectra of polar diatomic molecules in inert solutions. I. Spectral theory
Authors: Padilla A. , Pérez J. , Calvo Hernández A.
Published: July 2002

A unified non-Markovian theory for the vibrorotational Raman and infrared spectra of polar diatomic molecules diluted in nonpolar fluids is presented. From this theory, the physical basis of the spectra can be interpreted in terms of a few molecular properties of the isolated diatomic and of the time autocorrelation functions determining the collective effects of the solvent molecules on the vibrorotational dynamic of the diatomic. The spectrum is obtained as a diagonal part, constituted by an additive superposition of lines accounting for the integrated intensity, and an (exact) nondiagonal part accounting for the redistribution of intensity due to interbranch and intrabranch mixing effects. This theory generalizes previous theoretical frames based on a secular contribution modified by an (approximate) interference term. Also it allows the comparative analysis of the Raman and infrared spectra, and gives a clear and consistent interpretation of the theoretical lines building up the spectra.
A combined reaction class approach with integrated molecular orbital+molecular orbital (IMOMO) methodology: A practical tool for kinetic modeling
Authors: Truong Thanh N. , Maity Dilip K. , Truong Thanh-Thai T.
Published: July 2002

We present a new practical computational methodology for predicting thermal rate constants of reactions involving large molecules or a large number of elementary reactions in the same class. This methodology combines the integrated molecular orbital+molecular orbital (IMOMO) approach with our recently proposed reaction class models for tunneling. With the new methodology, we show that it is possible to significantly reduce the computational cost by several orders of magnitude while compromising the accuracy in the predicted rate constants by less than 40% over a wide range of temperatures. Another important result is that the computational cost increases only slightly as the system size increases.
A diffusion Monte Carlo accurate interaction potential between H and PsH
Authors: Mella Massimo , Morosi Gabriele , Bressanini Dario
Published: July 2002

The interaction potential between H and PsH, and between H and H− have been calculated using the diffusion Monte Carlo method. As far as the H–H− system is concerned, our results appear to be the most accurate published so far. The potential functions show the H–PsH system to be bound with respect to the H2+Ps dissociation channel for nuclear distances larger than 3.5 bohr, while the H2−−e+ threshold lies 0.25 hartree above for all the distances studied. Moreover, the H–PsH potential curve presents a minimum located at 5.2 bohr that can be attributed to the van der Waals interaction between the two fragments. The relevancy of these results for the low energy scattering of Ps on H2 and for the exchange reaction of H with Ps is discussed.
Radial moments of the electron density: Gas phase results and the effects of solvation
Authors: Worsnop S. Kent , Boyd Russell J. , Elorza Jose M. , Sarasola Cecilia , Ugalde Jesus M.
Published: July 2002

The radial moments of the electron density, 〈rn〉, have been calculated for N2 and H2O at the quadratic configuration interaction (with singles and doubles) levels of theory by use of a variety of basis sets. An optimal basis set was chosen from the first set of calculations and then used with several conventional ab initio and density functional methods to compute the moments for a representative set of molecules: N2, CO, CH4, NH3, H2O, HF, SiH4, PH3, H2S, HCl, CH3OH, and CH3SH. The effects of solvation on the radial moments were also studied using the Onsager model and an extension of the Tomasi polarized continuum model.
Molecular beam study of the chemiluminescent reaction of manganese and ozone
Authors: Green K. M. , Kampf R. P. , Parson J. M.
Published: July 2002

The electronically chemiluminescent reaction Mn+O3→MnO*+O2 was investigated using a beam-gas configuration. Light from the MnO A 6∑+–X 6∑+ transition was collected by a charge coupled device (CCD) array detector with resolutions of 0.5 and 0.1 nm. The spectrum at lower resolution (500–655 nm) encompassed the Δv=−3 to +2 sequences, while that at higher resolution (555.5–583.5 nm) encompassed only the Δv=0 sequence. These two spectra were separately fitted with a nonlinear least-squares program to obtain vibrational and rotational distributions of the nascent MnO*. The limited vibrational-state coverage of the higher-resolution spectrum made it unrealiable for determining the vibrational state distribution, and it was useful only for characterizing the rotational distribution when v′=0. The best-fit vibrational excitation is somewhat less than for the Prior model, but the rotational excitation is considerably greater. A consideration of the electronic structure of reactants and products indicates that principal changes occurring in the chemiluminescent reaction are σ–electron donation from the sdz2 hybridized Mn orbital to the O3 lowest unoccupied molecular orbital (LUMO) (2b1) and π–electron backdonation from the O–O 4b2 orbital to the Mn 3dπ orbital. Correlation of the orbitals involved indicates that direct access is allowed to the MnO A 6∑+(10σ*18σ1) state. This mechanism favors Mn approach perpendicular to the O3 plane and suggests that the product’s rotational excitation may originate in O2–OMn repulsion arising from removal of electron density from the slightly bonding 4b2 orbital of O3. However, some rotational excitation could also be attributed to conservation of angular momentum arising from a sizable reactive impact parameter. The lack of significant vibrational excitation is a consequence of the short-range nature of the partial charge transfer in this reaction channel.
State-resolved dissociation dynamics of triplet acetaldehyde near the dissociation threshold to form CH3+HCO
Authors: Huang Cheng-Liang , Chien Volume , Chen I-Chia , Ni Chi-Kung , Kung A. H.
Published: July 2002

We studied the state-resolved dynamics of S1 acetaldehyde to product channels with quantum-beat spectroscopy. Two bands near the threshold of dissociation to radical products CH3+HCO in a supersonic jet, displaying most quantum-beat features, are recorded with resolution 0.025 cm−1. Evaluated on the basis of a simple asymmetric rotor, the origins of these two bands 1402−1501 and another denoted # are 31 275.045(1) and 31 523.263(1) cm−1; effective rotational constants of excited state are A=5.7883(1), 5.0408(3), B=0.33269(2), 0.32320(2) and C=0.31026(2), 0.32091(2) cm−1, respectively; large A value results from lack of consideration of torsional motion. For these two vibrational levels most rotational states (about 70 percent) display quantum-beat features attributed to coherently excited singlet–triplet eigenstates. The linewidth in transformed spectra for level #, ∼125 cm−1 below the dissociation threshold, increases with increasing total angular momentum J whereas level 142−151 that is 375 cm−1 below shows a small linewidth independent of J. This is because correlation of the triplet state with dissociation to form radical products results in a decreased lifetime of the triplet state in the tunneling region. A systematic dependence on rotational quantum number implies Coriolis-induced vibrational coupling of triplet states to dissociating continuum on the exit side of the dissociation barrier.
Distribution function approach to irreversible adsorption of interacting colloidal particles
Authors: Faraudo Jordi , Bafaluy Javier
Published: July 2002

A statistical-mechanical description of the irreversible adsorption of interacting colloidal particles is developed. Our approach describes in a consistent way the interaction of particles from the bulk with adsorbed particles during the transport process towards the adsorbing surface. The macroscopic physical quantities corresponding to the actual process are expressed as averages over simpler auxiliary processes which proceed in the presence of a fixed number n of adsorbed particles. The adsorption rate verifies a generalized Langmuir equation, in which the kinetic resistance (the inverse of the kinetic coefficient) is expressed as the sum of a diffusional resistance and a resistance due to interaction with adsorbed particles during the transport process (blocking effect). Contrary to previous approaches, the blocking effect is not due to geometrical exclusion, instead it measures how the transport from the bulk is affected by the adsorbed particles. From the general expressions obtained, we have derived coverage expansions for the adsorption rate and the surface correlation function. The theory is applied to the case of colloidal particles interacting through DLVO potentials. This form of the kinetic coefficient is shown to be in agreement with recent experimental results, in which RSA fails.