bluuebunny/crossref_metadata_2025 huggingface dataset:
The librational ground state of monodeuterated methane adsorbed on graphite
Authors: Inaba A. , Skarbek J. , Lu J. R. , Thomas R. K. , Carlile C. J. , Sivia D. S.
Published: July 2002

High resolution inelastic neutron scattering spectra of CH4, CH4/CD4 mixtures, and CH3D adsorbed on exfoliated graphite at 4.5 K have been obtained. The resolution used was approximately five times higher than previously used and has revealed extra detail in the spectra. For CH4 it is shown that the parameter, H, related to rotation about the twofold axes is not exactly zero. It has an unexpected small positive value. For a 1:2 CH4/CD4 mixture the tunneling spectra of the CH4 show that the mixture must be ordered at 4.5 K and that the barriers to rotation of the CH4 are increased slightly when the surrounding CH4 molecules are replaced by the heavier CD4. For CH3D the higher resolution spectra make it possible to make an unambiguous assignment of the tunneling spectra, which is different from that previously proposed. The assignment indicates that there are terms in the orientational potential higher than the threefold Fourier components previously assumed, but that these probably do not affect the derived heights of the barriers to rotation of methane on graphite.
Inner filter effect: Phase relationships between oscillatory fluorescence and oscillatory Ru(II) concentration in the Belousov–Zhabotinsky reaction
Authors: Solli Gunn Marit , Ruoff Peter
Published: July 2002

The oscillatory fluorescence of the Ru-ion-catalyzed Belousov–Zhabotinsky reaction has been studied and analyzed in relation to the fluorimetric behavior of the isolated Ru(bpy)2+3 ion. Here we show how different forms of fluorescence oscillations, observed at various excitation wavelengths and Ru(bpy)2+3 concentrations are due to an inner filter effect of the isolated Ru(bpy)2+3 ion. Three types of phase relationships, i.e., in-phase, opposite-phase, and complex phase relationships, between oscillatory fluorescence and the oscillatory Ru(bpy)2+3 concentration can be predicted and have experimentally been observed.
Partitioned energetics for propene internal rotation
Authors: Kundu Tapanendu , Goodman Lionel , Leszczynski Jerzy
Published: July 2002

Energy partitioning in terms of symmetry components shows that σ-bonding changes contribute importantly to the energetics of the (∼700 cm−1) methyl torsional barrier in propene. This σ-conjugation effect, larger in propene than in acetaldehyde, has its primary origin in the partial unmaking of the bond between the methyl carbon and adjacent carbon atoms. The role of strain in generating torsional barriers is discussed. Rigid rotation leaves both molecules in a strained metastable conformation with the barrier primarily originating from π contributions to a repulsive nuclear virial. But overall energetics and hence barrier heights in the relaxed molecule are controlled by σ interactions. Extension to the propene+2 π cation provides insight into competition between σ and π interactions forming the barrier. In neutral propene dominant π-electron repulsion decreases the barrier, in the cation dominant σ-electron repulsion increases the barrier {predicted [MP2/6-311G(3df,2p)] height, 1241 cm−1}.
Photoelectron spectroscopy of C4H4−: Ab initio calculations and dynamics of the 1,2-hydrogen shift in vinylvinylidene
Authors: Gunion Robert F. , Köppel Horst , Leach Gary W. , Lineberger W. C.
Published: July 2002

The ultraviolet photoelectron spectrum of HXC=C− (X=C2H3) is reported along with ab initio calculations. The adiabatic electron affinity of the X 1A′ state is measured to be 0.914(15) eV for C4H4 and 0.909(15) eV for C4D4. The term energy of the C4H4 ã3A′ state is 1.923(15) eV and the b 3A″ term energy is 2.035(30) eV. Geometries and frequencies of several stationary points on the C4H4 and C4D4 neutral and anion surfaces at the configuration interaction with singles and doubles level of theory are reported, as well as an intrinsic reaction coordinate calculation at the restricted Hartree Fock level on the C4H4 singlet surface. Calculations and experiment are combined to estimate the lifetime of singlet vinylvinylidene for rearrangement to vinylacetylene to be 20–200 fs, corresponding to lifetime broadening of 35–3 meV.
High resolution proton energy loss spectroscopy of the high overtone vibrations in CF4
Authors: Maring W. , Toennies J. P. , Wang R. G. , Levene H. B.
Published: July 2002

Proton energy loss spectroscopy (PELS) has been used to investigate the very high overtone states of the ν3 mode in CF4. With a new scattering apparatus, which is described here in detail, it has been possible to directly measure the energy levels of the dominantly excited ν3 mode up to its 14th overtone with an energy resolution of δE=10–15 meV, corresponding to 80–120 cm−1. The data are interpreted using the Hecht vibrational Hamiltonian which predicts the tensor splitting of the ν3 mode energy levels into sublevel bands. By adjusting the values of the anharmonicity parameters X33, G33, and T33, previously determined by IR spectroscopy and introducing a second-order anharmonicity parameter X333, it was possible to fit the ν3 mode energy levels to the measured energy loss distributions. A model based on the earlier theoretical work of Levene and Perry [H. B. Levene and D. Perry, J. Chem. Phys. 84, 4385 (1986)] for the proton–CF4 scattering process is extended to allow the simulation of complete spectra including contributions from combinations with the ν1 and ν4 mode for comparison with the experimental energy loss spectra. The influence of all relevant parameters on the simulated data is analyzed in detail.
Quantum delocalization and cluster melting
Authors: Chakravarty Charusita
Published: July 2002

Quantum effects on the cluster solid–liquid transition (CSLT) are studied by using the Fourier path integral Monte Carlo technique to simulate quantum Lennard-Jones clusters. Quantum delocalization effects are shown to induce a CSLT transition analogous to the thermal cluster melting transition. The CSLT should therefore be represented in terms of a two-dimensional phase diagram with the reduced temperature and the de Boer parameter indexing the relative magnitudes of thermal and quantum effects respectively.
Photoinduced electron transfer and geminate recombination in liquids: Analytical theory and Monte Carlo simulations
Authors: Swallen S. F. , Fayer M. D.
Published: July 2002

Photoinduced electron transfer and geminate recombination in liquid solution are addressed with analytical theory and Monte Carlo simulations. The time-dependent probabilities of the donor being excited and of an ion pair existing are obtained for a system of a donor and many acceptors undergoing diffusive motion. Multiparticle simulations are modeled as a Markov chain and are shown to agree with the analytical formalism presented previously. The calculations are performed using both a simple exponential form of the distance dependence of the transfer rate and using the more general Marcus distance-dependent transfer rate. For a static donor, in the absence of acceptor–acceptor excluded volume, theory and simulations provide identical results, confirming the accuracy of the analytical method. For the calculation of properties of real systems in which both the donor and acceptors diffuse, to make the mathematics tractable, the donor is held static and each acceptor is given a Fick diffusion constant equal to the sum of the diffusion constants of the donor and acceptor, D=Dd+Da. The validity of this approximation is examined in the absence of acceptor–acceptor excluded volume and found to work extremely well under all conditions. It is also examined with acceptor–acceptor excluded volume. In this case, the static donor approximation is found to work generally well up to moderately high acceptor concentrations, <5% packing fraction. However, the results suggest that at even higher packing fractions, the static donor approximation loses its validity.
Electron spin resonance and theoretical studies of the PO2 and AsO2 radicals in neon matrices at 4 K: Laser vaporization and x-irradiation radical generation techniques
Authors: Knight Lon B. , Jones Glenn C. , King Gina M. , Babb Robert M. , McKinley Allan J.
Published: July 2002

The utilization of high energy generation techniques for trapping ion radicals and other reactive intermediates in neon matrices at 4 K is discussed. Electron spin resonance (ESR) results for several small radicals are presented to demonstrate the application of x irradiation and reactive laser vaporization for neon matrix isolation studies. Detailed ESR studies of the PO2 and AsO2 radicals, along with ab initio theoretical computations of their nuclear hyperfine interactions, are presented. No previous ESR observations have been reported for these radicals, although PO2 has been studied under high resolution in the gas phase utilizing far-infrared laser magnetic resonance and microwave spectroscopies.
Computation of high-energy vibrational eigenstates: Application to C6H5D
Authors: Wyatt Robert E.
Published: July 2002

In this study, a two loop iteration scheme, similar to one developed recently [Phys. Rev. E 51, 3643 (1995)], is applied to the computation of high energy vibrational eigenstates in 21-mode planar C6H5D. The computational method is based upon the use of a spectral filter to extract a small number of eigenpairs (near the test input energy E) from the interior of the dense energy spectrum. In the outer iteration loop, a very effective filter, the Green function G(E)=(E1−H)−1, is used to drive the Lanczos recursion algorithm through a small number of steps (frequently <10). The result is a small tridiagonal representation of the Green function. The Lanczos algorithm converges quickly because the desired eigenvalues, those near the test energy, are mapped to the extreme edges of the spectrum of the filter. In order to apply the Green function to the current Lanczos vector, a matrix partitioning technique is combined with a perturbation–iteration method in the inner iteration loop. The Green function–Lanczos algorithm, GFLA, was then used to compute eigenstates for 21-mode planar C6H5D near the energy of the v=3 CD overtone (about 6700 cm−1). These computations were done using an active space with the dimension 20 000. The resulting eigenfunctions were then subjected to several types of analysis, including basis state and vibrational mode distributions. It is shown that the energetic distribution of basis functions in the eigenvectors exhibits multifractal scaling (finer features built upon coarser features).
Rotationally resolved photoelectron spectroscopy of the [a 1Δ]3dπ 2Φ Rydberg state of the SH radical
Authors: Milan J. B. , Buma W. J. , de Lange C. A. , Wang Kwanghsi , McKoy V.
Published: July 2002

Results of calculated and measured photoelectron spectra via the [a 1Δ]3dπ 2Φ state of SH are reported. These spectra show a strong asymmetry away from classical expectations and only transitions up to ΔN=±2 are observed, in contrast to ΔN=±4 expected for photoionization of a 3dπ orbital. Agreement between the calculated and measured spectra is excellent.
A molecular dynamics study of electronic absorption line broadening in high-pressure nonpolar gases
Authors: Kalbfleisch T. , Fan R. , Roebber J. , Moore P. , Jacobsen E. , Ziegler L. D.
Published: July 2002

The observed electronic line broadening of the X→B (6s Rydberg) absorption spectrum of CH3I vapor in moderate to high pressures (55–140 atm, reduced densities ∼0.08–0.14) of Ar and CH4 is analyzed via molecular dynamics simulations. Good fits to the absorption line shapes are found in this pressure/density range for a given set of ground and excited state solute–solvent potential parameters in the static limit, i.e., the absorption linewidths and shapes are dominated by inhomogeneous broadening on the time scale of the decay of the dipole correlation function. The pressure dependence of these absorption line shape changes is explained in terms of the shape of the solute–solvent ground-excited state difference potential. Consistent with the static limit description at these moderate to high pressures, the corresponding transition energy correlation function, a quantity of central importance in stochastic and Brownian oscillator line shape theories, decays on a much longer time scale than the inverse absorption widths. At moderate to high pressures, simulations find relatively long-lived solvent clusters surrounding the CH3I solute. The slow decay of the energy correlation functions, and hence the validity of the static approximation, is attributed to these cluster dynamical time scales. At bath pressures lower than observed here, MD simulations reveal that the static limit is no longer valid and satellite bands, due to an underdamped solvent response, are found. The evolution of a Gaussian optical line shape at higher densities is discussed with respect to the difference potential shape, the number density, and the central limit theorem.
Analytical solutions of the proper integral equations for interaction site fluids: Molecules composed of hard-sphere interaction sites
Authors: Lue Leo , Blankschtein Daniel
Published: July 2002

Interaction site models are used quite extensively to describe molecular fluids. However, theories for these fluids are not as well developed or tested as compared to those for simple fluids. With this in mind, it appears useful to develop analytical expressions for the thermodynamic properties of fluids whose molecules are composed of hard-sphere interactions sites, since these systems can serve as a convenient reference state for perturbation theories for molecular fluids. In an effort to achieve this goal and advance our understanding of molecular fluids, in this paper, we present an analysis of the Chandler–Silbey–Ladanyi (CSL) equations, a diagrammatically proper set of integral equations for interaction site fluids, with the specific aim of solving them analytically. First, we rewrite the CSL equations to explicitly account for the presence of equivalent sites. We find that the mathematical structure of the resulting CSL equations remains the same as that of the original CSL equations, subject to slight modifications in some of the matrices which appear in these equations. Subsequently, we apply the Wiener–Hopf factorization technique to the CSL equations with the Percus–Yevick (PY) closure for a general fluid composed of hard-sphere interaction sites. We then analytically solve these equations for symmetric n-atomic tangent hard-sphere molecules (n≤4: spheres, diatomics, triangles, and tetrahedrals), which results in analytical expressions for the equation of state and other thermodynamic properties of the fluid. Finally, we compare the predictions of the analytical equation of state with those of other theories as well as with those of Monte Carlo simulations of these systems. The CSL equations with the PY closure are found to provide fair predictions for the equation of state of the fluids under investigation. More specifically, the CSL–PY equations tend to perform better for smaller molecules and at lower densities.
Transport properties of nonconventional lead cuprate glass
Authors: Hazra S. , Ghosh A.
Published: July 2002

The dc electrical transport properties of the nonconventional lead cuprate glasses of compositions (CuO)x(PbO)100−x for x=15–50 mol % are reported in the temperature range 150–500 K. The experimental results have been analyzed in the light of existing theoretical models. It has been observed that the high-temperature conductivity data are consistent with Mott’s nearest-neighbor hopping model, while at low temperatures Mott’s variable-range-hopping model fits the data very well. The polaron hopping models of Schnakenberg and Emin can predict the conductivity data in the entire temperature range of measurement. The percolation model of Triberis and Friedman is not consistent with the experimental results.
The classical statistical theory of three-atom reactions governed by short-range forces: Energy transfers and recoil energy distribution
Authors: Bonnet L. , Rayez J. C.
Published: July 2002

When the nascent products of a three-atom reaction governed by chemical forces separate, energy transfers may occur between vibrational, rotational, and translational motions. In the first part of the paper, we show from quasiclassical trajectory calculations on a model potential energy surface that (a) the vibrational energy is adiabatic on average as usually assumed in statistical theories, (b) rotational-translational energy transfer mainly favors translational motion (as was initially suggested by Marcus), but that (c) this transfer is inefficient when the product atom is sufficiently light with respect to the other two. A qualitative analysis of these findings is proposed based on arguments differing from those of Marcus, and Quack and Troe. In the second part of the paper, we extend the classical statistical formalism proposed recently by ourselves, initially limited to reactions governed by long-range forces, to the present more general case of reactions involving tight transition states and for which energy transfers are inefficient. In such a case, energy distributions at the exit transition state and in the products are the same. We focus our developments on the recoil energy distribution. Agreement between our theoretical result and the quasiclassical trajectory approach is shown to be very satisfactory.
A helium atom scattering study of water adsorption on the NaCl(100) single crystal surface
Authors: Bruch L. W. , Glebov A. , Toennies J. P. , Weiss H.
Published: July 2002

Structural and dynamic properties of H2O adsorbed on an in situ cleaved NaCl(100) crystal surface were studied by He atom scattering in the range of temperatures and partial pressures of 80–160 K and 5×10−7–5×10−10 mbar, respectively. A well-ordered 2D H2O layer was formed during adsorption as well as by heating of 3D ice. The isosteric heat of adsorption in the monolayer regime was found to be in the range 58–63 kJ/mol. The structure of the 2D condensed phase was investigated for both modes of preparation and in both cases a (1×1) diffraction pattern was found. Time-of-flight spectra of the monolayer H2O/NaCl were measured in the 〈100〉 direction and phonon dispersion curves were determined. The observed dispersion-free vibrational mode, with ℏω=5.5 meV, was identified as a frustrated translation of the water molecule parallel to the surface, via calculations for physisorption models of the H2O/NaCl interaction. Other salient features of the adsorption, the large heat of adsorption, and the magnitude of the frequency of molecular vibration perpendicular to the substrate, are reproduced in the calculations.
The liquid–liquid interface of simple liquids
Authors: Stecki J. , Toxvaerd S.
Published: July 2002

Molecular-dynamics simulations of liquid–liquid interface at low temperatures between two simple liquids reveal that the interface is a sheet of vacuum between liquid slabs. Density profiles, order parameters, the pressure dependence of the gap, and the dynamics of liquid slabs, are reported. The Fowler approximation for interfacial tension γ between immiscible liquids is formulated. Contributions to γ=γaa+γab+γbb are also reported.
Magnetic circular dichroism of CD3I in the vacuum ultraviolet
Authors: Felps W. S. , Scott J. D. , McGlynn S. P.
Published: July 2002

The magnetic circular dichroism (MCD) of CD3I is reported in the 48 700–60 100 cm−1 region. This region embraces the 5p → 6s manifold, much of the 5p →6 p manifold, and perhaps some members of the 5p → 5d Rydberg manifold of states. The results in the 5p →6 s manifold are confirmatory of the same work in CH3I [Scott et al., J. Chem. Phys. 68, 4678 (1978)]; some questions occur concerning the conferral of absorption intensity on the highly forbidden 5p →6 s[1] state of these molecules. The MCD of part of the 5p →6 p manifold of CD3I is also reported. Five possible 5 p→6 p origins have been securely identified and a sixth has been tentatively assigned. Apart from one of these, that at 58 930 cm−1, the other five origins assigned here match none of those found by MPI spectroscopy [Dobber, Buma, and de Lange, J. Chem. Phys. 99, 836 (1993)], suggesting a degree of complementarity of both data sets. Relative magnetic moments for all six observed states in the 5p → 6p region have been determined: the 58 930 cm−1 origin has been assigned as a 1Σ+ state, possibly an interloper from the 5d manifold, and the 59 630 cm−1 origin has been assigned as a 1Π1 state. The 5p → 6p manifold of CH3I is also discussed and assignments, which are largely vested in correlations of energies, absorption intensities, and MCD specifics with the corresponding excitations of CD3I, are given.
The interaction representation and nonadiabatic corrections to adiabatic evolution operators
Authors: Schwartz Steven D.
Published: July 2002

This paper presents a new approach to operator resummation corrections to adiabatic evolution operators. It is shown that an infinite order correction produces an operator that is equivalent to a propagator in the interaction representation. For a problem in which the adiabatic approximation assumes that certain degrees of freedom are held fixed, the interaction representation correction is just the interaction propagator of the coupling for these degrees of freedom. This formulation allows simple physical interpretation and simple mathematical evaluation of the full correction. No power series or cumulant methods are needed. Application to double well splitting when coupled to a bath oscillator shows the approach to be highly accurate.
Percolative diffusion of CO during CO oxidation on Pt(100)
Authors: Tammaro M. , Evans J. W.
Published: July 2002

During CO-oxidation on Pt(100), CO diffuses in a ‘‘disordered environment’’ produced by a complex pattern of reconstructed and unreconstructed regions of the substrate. Macroscopic diffusion of CO is effectively only possible on percolating 1×1-regions of the substrate. We treat the spatio-temporal behavior observed in this reaction system accounting in the simple way for the percolative nature of CO-diffusion. This is done via incorporation into the reaction-diffusion equations of a suitable chemical diffusion coefficient, exploiting ideas from the theory of transport in disordered media. We use these equations to analyze the propagation of reactive, O-rich pulses into a CO-covered 1×1-background.
Microscopic model with temperature-dependent interactions for the free molecule and for the trigonal phase of benzil
Authors: Zieliński P. , More M. , Cochon E. , Lefebvre J.
Published: July 2002

The molecule of benzil (diphenylethanedione, C14H10O2) has been approximated by a system of rigid segments to model the lowest-frequency part of its vibrational spectrum. The interactions of internal degrees of freedom have been described with the use of phenomenological force constants. The structure of the trigonal (P3121) phase has then been modelled by means of a temperature-dependent atom–atom potential based on thermal motions of atoms. The potential gives the correct account of the softening of an E-symmetry, zone-center mode which underlies the phase transition to the low-temperature monoclinic phase (P21). The low-frequency modes at the zone center, supposed until now to be difference overtones, have been shown to result from a coupling between internal and external degrees of freedom. A low-frequency soft mode at the point M of the zone border has been found, which explains the behavior of observed peaks in diffuse x-ray scattering experiments. The values and the temperature evolution of the effective elastic constants calculated within the model are in a very good agreement with the results of ultrasonic and Brillouin scattering data. The model has been shown insufficient in the description of dielectric and piezoelectric properties of benzil.
The planar reaction OH+H2→H2O+H: A quasiclassical trajectory study
Authors: Palma Juliana , Echave Julián
Published: July 2002

We report a quasiclassical trajectory (QCT) study of the reaction OH+H2→H2O+H performed on the same model as that used in a recent exact quantum treatment. The reaction is constrained to occur on a plane, with the OH bond frozen. We used Clary’s modification to the Schatz–Elgersma potential energy surface. Initial state selected reaction probabilities and cross sections were calculated at several energies between 0.31 and 0.65 eV. The effect of rotational excitation of H2 and OH on reaction probabilities was studied and QCT results were compared with exact quantum results for the same model. We found very good quantitative agreement between quantum and QCT results. Furthermore, QCT calculations confirm the qualitative effects predicted by the quantum treatment; exciting the rotation of H2 decreases reactivity while the rotation of OH has almost no effect. Similarities and differences between the present QCT results with 2D and 3D quantum and classical results by other authors are discussed.
Quantum molecular dynamics studies of H2 transport in water
Authors: Mei H. S. , Coker D. F.
Published: July 2002

The transport of H2 in liquid water is studied using adiabatic, nonadiabatic, and classical molecular dynamics methods in an attempt to understand the influence of transitions between translational states of the H2 molecule driven by solvent fluctuations. Quantum autocorrelation functions of the H2 center-of-mass velocity are computed in various dynamical limits. We find that there are strong nonadiabatic couplings between the instantaneous adiabatic translational states of H2 in water which result in rapid decorrelation of the H2 center-of-mass velocity for the time evolving translational mixed state. Transitions to excited translational states reduce the effects of caging dynamics in the velocity autocorrelation function dramatically. Classical and adiabatic descriptions of the dynamics predict that caging is much more important than we find nonadiabatically. Diffusion constants and frequency spectra are compared for the different limits and with experiment.
High resolution Fourier transform infrared spectroscopy of the ν6 and ν10 bands of jet-cooled Fe(CO)5
Authors: Asselin P. , Soulard P. , Tarrago G. , Lacome N. , Manceron L.
Published: July 2002

Rovibrational FTIR spectra of Ni(CO)4 and Fe(CO)5 have been recorded in supersonic argon expansions at resolutions up to 0.004 cm−1. Rotational temperatures as low as 4 K for the ν6 band of Fe(CO)5 and 6 K for the ν5 band of Ni(CO)4 have been determined from the populations of the rotational levels, and an efficient vibrational cooling has been evidenced by the large reduction of hot band absorptions. An analytical model including the instrumental line shapes and the intensity distribution is proposed to enable simulations of overlapping rovibrational transitions of symmetric top molecules. The spectroscopic study of the ν6 and ν10 stretching bands of Fe(CO)5 yielded the following parameters (3σ uncertainties in parenthesis): ν0=2038.106 97(91) cm−1, B6=0.026 797(12) cm−1, and B0=0.026 826(10) cm−1 for the 610 transition, ν0=2015.5513(12) cm−1, ΔA10=−0.000 086(5) cm−1, B10=0.026 786(4) cm−1, and ξ10=−0.049(3) for the 1010 transition based on the literature value of A0.
Rotational diffusion and internal motions of circular DNA. II. Depolarized photon correlation spectroscopy
Authors: Chirico Giuseppe , Baldini Giancarlo
Published: July 2002

The conformation and slow internal motions of a DNA plasmid molecule pUC18 (2687 base pairs) are studied by means of depolarized photon correlation spectroscopy. The autocorrelation functions are measured at different scattering angles between 6.7° and 90° and analyzed as heterodyne autocorrelation functions. One obtains both a relaxation time of ≂140 μs, which is interpreted as the tumbling rotational diffusion coefficient of the plasmid, and some faster component with relaxation time around 15 μs. The value of the rotational diffusion relaxation time is in good agreement with the results of Monte Carlo simulations, while the faster decay has a nonsingle exponential decay behavior. The results of measurements made in the depolarized scattering configuration are compared to those obtained from polarized configuration and critically discussed.
Quantum dynamics simulations of nonadiabatic processes in many-atom systems: Photoexcited Ba(Ar)10 and Ba(Ar)20 clusters
Authors: Jungwirth Pavel , Gerber R. Benny
Published: July 2002

Quantum simulations are reported for the dynamics following the photoexcitation Ba(1S)→Ba(1P) in Ba(Ar)10 and Ba(Ar)20 clusters. The evolution in time is studied in a framework that treats quantum-mechanically all the coupled degrees of freedom. The focus is on the role of nonadiabatic transitions between the three adiabatic surfaces corresponding to the P states of the Ba atom. The time scales of electronic relaxation and of electronic depolarization (orbital reorientation) are computed, and the competition between adiabatic and nonadiabatic effects is assessed. The calculations are carried out by a new scheme that extends the recent classically based separable potential method. Semiclassical surface-hopping simulations are used to produce effective single-mode potentials on which nuclear ‘‘orbitals’’ are then generated. The full wave packet is constructed from the electronic states involved, and from these nuclear wave functions. Among the main results we find that nonadiabatic transitions become appreciable around 1 ps after photoexcitation, and they are stronger in the smaller cluster. Comparing Tully’s semiclassical method with the quantum simulations, good qualitative agreement is found. Quantitatively, the semiclassical predictions for the electronic states branching rations deviate from the quantum results roughly by a factor of 2 after 1 ps. In the smaller cluster direct dissociation of the Ba atom dominates over energy redistribution within the cluster, the opposite being true for the large system. This example demonstrates the feasibility of quantum simulations of nonadiabatic processes in large systems with the new method.
Lifetime measurements of selectively excited rovibrational levels of the V 1B2 state of CS2
Authors: Weyh Th. , Demtröder W.
Published: July 2002

The radiative lifetimes of selectively excited rovibrational levels of the V 1B2 state of the CS2 molecule have been measured under collision-free conditions in a cold molecular beam using a pulse-modulated frequency-doubled single-mode cw dye laser. Pure exponential fluorescence decays were found for all excited levels. The lifetimes, scattering considerably in the range of 2–13 μs, give evidence of interactions of the V 1B2 state with background states. A deperturbation analysis performed for rotational levels involved in local two-level interactions yields detailed quantitative information on the V 1B2 state and on the background levels. It is shown that these levels represent rovibrational levels of the B2(3A2) state and highly excited rovibrational levels of the X 1Σg+ electronic ground state. The coupling mechanisms are: (1) the spin–orbit interaction between the V 1B2 state and the B2(3A2) state where the latter is coupled to the X 1Σg+ ground state by spin–orbit/vibronic and spin–orbit/Coriolis interaction and (2) the Coriolis coupling of the V 1B2 state to the X 1Σg+ ground state. Further sources of perturbations are the spin-uncoupling in the 3A2 state and the Renner–Teller coupling of the V 1B2 state to the 1A2 state. From the measured radiative lifetimes of the background states information on the electronic structure of the CS2 molecule is gained.
Real space analysis of the nonlocal optical response of PPV oligomers
Authors: Wagersreiter Thomas , Mukamel Shaul
Published: July 2002

We investigate the linear optical polarizabilities of poly(p-phenylene vinylene) oligomers using the time dependent Hartree Fock procedure. Our analysis is based on the computation of a nonlocal response function, which describes the effects of an interaction with the electromagnetic field at one carbon atom on the distribution of charges at other atoms. This provides physical insight into intramolecular charge dynamics and the anisotropic tensorial properties of the linear polarizabilities. The results are analyzed using the coupled electronic oscillator (CEO) representation, which allows the assignment of absorption peaks to distinct transitions between Hartree Fock orbitals. The size dependence of the linear absorption shows that at 10 repeat units the optical gap has almost converged to its infinite size value, and the oscillator strength of the lowest absorption line scales linearly with size. This implies that the sizes studied exceed the exciton coherence size and the resulting spectra should mimic the infinite size limit.
Molecular dynamics simulations of compressible ions
Authors: Wilson M. , Madden P. A. , Pyper N. C. , Harding J. H.
Published: July 2002

A representation of the short-range repulsion energy in an ionic system is described which allows for the fact that an ion may be compressed by its neighbours. The total energy of the system is expressed in a pairwise additive form, but the interionic interactions have a many-body character. The form of this representation and the parameters required to represent MgO and CaO are obtained from recent ab-initio electronic structure calculations. The fact that the representation is transferable between crystals with different coordination number is demonstrated by direct comparison with ab-initio results on the different crystal types. Comparison with experimental results on the equation of state of different isomorphs and on the location of the pressure of the transition between them confirms the accuracy of the ab-initio results and of the potential derived from them in representing perfect crystal properties. A computationally efficient molecular dynamics (MD) scheme may be derived for this representation. The additional degrees of freedom which represent the varying ionic radii are constrained to their adiabatic values in the course of the simulation by an adaptation of Car and Parrinello’s method. The MD scheme is used to examine whether an ab-initio parameterized potential model which allows for the spherical compression of an oxide ion by its neighbours and for dipole polarization effects is a sufficiently good representation of the interactions in MgO to allow an accurate calculation of the phonon dispersion curves.
Two-photon dissociation/ionization beyond the adiabatic approximation
Authors: Vardi Amichay , Shapiro Moshe
Published: July 2002

An accurate theory of two-photon dissociation with strong laser pulses, which goes beyond the adiabatic approximation, is developed. Criteria for adiabatic behavior in two photon dissociation (enabling via adiabatic passage the complete population transfer to the continuum), are derived. We obtain the minimal pulse duration needed to ensure adiabaticity as a function of the field intensities and detuning. In addition, we develop a simple, rapidly converging, iterative scheme, built on the adiabatic approximation, for the exact solution of the two-photon dissociation problem. Each iteration step requires a computational effort that scales as N2 (N being the system dimension) for full matrices, or N log N for sparse matrices. The iterative scheme is tested by comparing it to the Runge–Kutta–Merson direct integration algorithm. It is found to work well even when the adiabatic approximation fails completely.
An investigation of hydrogen transfer in water clusters
Authors: Tozer David J. , Lee Chengteh , Fitzgerald George
Published: July 2002

The potential energy surface of the (H2O)5 water cluster is examined using Kohn–Sham density functional theory, Hartree–Fock theory and second-order Mo/ller–Plesset theory. Two distinct minima on the energy surface may be interconverted through the transfer of two hydrogen atoms, representing a possible mechanism for ionic dissociation in water clusters. Our calculations suggest a concerted mechanism where the two hydrogen atoms move simultaneously through a late transition state.
Molecular quadrupole moments, second moments, and diamagnetic susceptibilities evaluated using the generalized gradient approximation in the framework of Gaussian density functional method
Authors: de Luca Giorgio , Russo Nino , Sicilia Emilia , Toscano Marirosa
Published: July 2002

A series of monoelectronic properties, i.e., molecular dipole and quadrupole moments, diamagnetic susceptibility and second moments of a number of organic and inorganic systems (CO2, OCS, CS2, C2H2, HCN, SO2, CH3CN, C2H6, C6H5F, C5H5N, C4H4N2, and C2H2N4) have been determined by using the linear combination of Gaussian-type orbitals-density functional method employing both local spin density (LSD) and nonlocal spin density (NLSD) approximations and triple zeta quality basis sets. The possible influence of an increase of radial grid points on the calculated properties has been also examined. Results show a general good agreement between all calculated monoelectronic properties and the available experimental counterparts even at local level and with a fine grid employing 32 radial grid points. In particular for the considered molecules the average error, at nonlocal level, with respect to the experiment is about 0.4×10−26 e.s.u. cm2 for quadrupole moments, 2.0×10−16 cm2 for 〈r2〉 and 5.6×10−6 ergs/G2 mol for diamagnetic susceptibility, that is in the range of the experimental error.
Correlation effects and vibronic coupling features in the interaction of H− ions with N2 molecules
Authors: Gianturco F. A. , Kumar Sanjay , Schneider F.
Published: July 2002

This paper discusses the calculation of the potential energy surfaces (PES) for the electronic singlet states of C2v and C∞v symmetries of a nitrogen molecule interacting with the atomic hydrogen negative ion. The behavior of such surfaces is analyzed as a function of relative orientations and also of the molecular internal coordinate. The PES’s have been obtained using an ab initio, multireference configuration interaction method (MRDCI) and the effects of correlation forces and of basis set size are analyzed in order to understand the role of electron transfer (ET) processes which are likely to take place during closer collisions between partners and which are suggested to be responsible for the vibronic coupling effects which occur during low-energy scattering. The general features of the orientational anisotropy of this interaction, of its dependence on the molecular coordinate, and of the strength of its coupling with the impinging negative ion are also analyzed and discussed.
Two-electron transfer reactions in polar solvents
Authors: Zusman Leonid D. , Beratan David N.
Published: July 2002

Chemical, biological, and electrode based electron transfer (ET) processes involve multielectron events. However, an adequate framework in which to describe these complex reactions does not yet exist. A theory for two-electron transfer reactions in Debye solvents is developed. The theory is formulated by generalizing Zusman’s model of ET reactions [L. D. Zusman, Chem. Phys. 49, 295 (1980)] to those involving three parabolic wells: One for the doubly reduced donor, one for the singly reduced donor/singly reduced acceptor, and one for the doubly reduced acceptor. The ET processes are described in terms of diffusional motion along a one-dimensional reaction coordinate with tunneling transitions at the intersection points of the parabolas. Two competing mechanisms of two-electron transfer arise. One is a process with two sequential single electron steps D=A→D−A−→DA=. The other involves ET in one concerted two-electron step (D=A→DA=). The general rate expressions for two-electron transfer are obtained. When the stepwise mechanism dominates, the free energy of activation is predicted to depend upon the driving forces of the two sequential steps but is independent of the overall driving force of the reaction. When concerted two-electron transfer dominates, the Marcus relation is obtained with a reorganization energy associated with the shift of two electrons. The dynamical solvent effect in two-electron ET processes is predicted to be unusual. Two distinct regimes exist, each with essentially linear 1/τL dependence (with τL the solvent longitudinal relaxation time): one for slow solvents and one for fast solvents. A combination of solvent and free energy studies could be used to elucidate the mechanism of multielectron processes in chemical and biological systems.
The notion of error in Langevin dynamics. I. Linear analysis
Authors: Mishra Bimal , Schlick Tamar
Published: July 2002

The notion of error in practical molecular and Langevin dynamics simulations of large biomolecules is far from understood because of the relatively large value of the timestep used, the short simulation length, and the low-order methods employed. We begin to examine this issue with respect to equilibrium and dynamic time-correlation functions by analyzing the behavior of selected implicit and explicit finite-difference algorithms for the Langevin equation. We derive: local stability criteria for these integrators; analytical expressions for the averages of the potential, kinetic, and total energy; and various limiting cases (e.g., timestep and damping constant approaching zero), for a system of coupled harmonic oscillators. These results are then compared to the corresponding exact solutions for the continuous problem, and their implications to molecular dynamics simulations are discussed. New concepts of practical and theoretical importance are introduced: scheme-dependent perturbative damping and perturbative frequency functions. Interesting differences in the asymptotic behavior among the algorithms become apparent through this analysis, and two symplectic algorithms, ‘‘LIM2’’ (implicit) and ‘‘BBK’’ (explicit), appear most promising on theoretical grounds. One result of theoretical interest is that for the Langevin/implicit-Euler algorithm (‘‘LI’’) there exist timesteps for which there is neither numerical damping nor shift in frequency for a harmonic oscillator. However, this idea is not practical for more complex systems because these special timesteps can account only for one frequency of the system, and a large damping constant is required. We therefore devise a more practical, delay-function approach to remove the artificial damping and frequency perturbation from LI. Indeed, a simple MD implementation for a system of coupled harmonic oscillators demonstrates very satisfactory results in comparison with the velocity-Verlet scheme. We also define a probability measure to estimate individual trajectory error. This framework might be useful in practice for estimating rare events, such as barrier crossing. To illustrate, this concept is applied to a transition-rate calculation, and transmission coefficients for the five schemes are derived.
A general and efficient filter-diagonalization method without time propagation
Authors: Chen Rongqing , Guo Hua
Published: July 2002

A general and efficient filter-diagonalization method is proposed to solve the quantum mechanical eigenvalue/eigenstate problem in a pre-specified energy range. This method is in similar spirit to the original filter-diagonalization approach, but it eliminates the time propagation by expanding the filter operator directly in the energy space in terms of Chebychev polynomials. Our approach is more efficient than the existing methods because neither time propagation nor time-to-energy transformation is needed. It also allows the choice of the most efficient filter operator for the system of interest. This method is tested for a one-dimensional Morse oscillator and for the two-dimensional Henon–Heiles system. The results show excellent convergence and accuracy.
STM images of guanine on graphite surface and the role of tip–sample interaction
Authors: Wang X. W. , Tao N. J. , Cunha F.
Published: July 2002

A combined experimental and theoretical study is conducted on guanine molecules adsorbed on graphite substrate. The main effort is to understand the scanning tunneling microscopic images and the effect of scanning tip force on guanine molecules and their images. The force on guanine molecule is determined from the tip-induced deformation in the substrate and is found to be a few nano-Newtons per guanine molecule. A jellium substrate is used to model the graphite substrate in ab initio calculations of electronic density of states. Both the equilibrium height of guanine and tip force on guanine are determined using pseudopotential density functional calculations of system’s total energy. The calculated height of guanine molecule above the substrate is in good agreement with measurements. Electronic states are calculated and it is shown that features in surface density of states are mostly due to admixture with the guanine LUMO and the admixture increases with increasing scanning tip force.
Diffusion in zeolites via cage-to-cage kinetics: Modeling benzene diffusion in Na-Y
Authors: Auerbach Scott M. , Metiu Horia I.
Published: July 2002

We have performed kinetic Monte Carlo simulations of benzene diffusion in Na-Y (Si:Al=2.0) over the temperature range 200–500 K. For hopping on a tetrahedral lattice, we derive the analytical formula for D in terms of hopping lengths and times, yielding the simple-cubic relation D=1/6ka2, even though the lattice is very different from simple cubic. We have calculated the distribution of cage residence times for benzene in Na-Y, finding single exponential decay controlled by the SII→W rate coefficient, even though several processes contribute to intercage motion. Exact agreement between mean square displacement slopes and 1/6ka2 is obtained only when using kinetic intercage hopping lengths, which are found to be in excess of the static length by as much as 0.54 Å. Constructing diffusion coefficients from intercage lengths and times provides overwhelming computational speedups over calculating mean square displacements.
Susceptibilities of liquid–vapor interface of simple liquids
Authors: Stecki J. , Toxvaerd S.
Published: July 2002

Quantities related to two-point correlation function in the inhomogeneous interfacial zone of a planar liquid–vapor interface are determined from molecular dynamics simulations for a Lennard-Jones liquid at equilibrium with its vapor at the reduced temperature kT/ε=1.0. Susceptibilities and density fluctuations in planes parallel to the interface are reported. Stabilization of the liquid slab surrounded by vapor, by an external potential, is also discussed. Qualitative agreement was found for the contribution predicted from capillary-wave theory—in contrast to the case of liquid–liquid interface at low temperature, where fully quantitative agreement was obtained.
Photoelectron spectroscopy of silicon–fluorine binary cluster anions (SinF−m)
Authors: Kawamata Hiroshi , Negishi Yuichi , Kishi Reiko , Iwata Suehiro , Nakajima Atsushi , Kaya Koji
Published: July 2002

Electronic properties of silicon–fluorine cluster anions (SinF−m; n=1–11, m=1–3) were investigated by photoelectron spectroscopy using a magnetic-bottle type electron spectrometer. The binary cluster anions were generated by a laser vaporization of a silicon rod in an He carrier gas mixed with a small amount of SiF4 or F2 gas. The highly abundant clusters are SiF−m (m=3 and 5) and SinF− (n=6, 7, and 10) in their mass spectra. In the photoelectron spectra of SiF−m (m=1–5), the clusters having odd m have higher electron affinity (EA) than those having even m, indicating that the even/odd alternation in EA is attributed to their electronic structures of a closed/open valence shell. Comparison between photoelectron spectra of SinF− and Si−n (n=4–11) gives the insight that the doped F atom can remove one electron from the corresponding Si−n cluster without any serious rearrangement of Sin framework, because only the first peak of Si−n, corresponding singly occupied molecular orbital (SOMO), disappears and other successive spectral features are unchanged with the F atom doping. In some clusters, furthermore, the vibrational structures could be resolved to determine a vibrational frequency and to presume the geometry with ab initio molecular orbital calculations.