bluuebunny/crossref_metadata_2025 huggingface dataset:
Solvation dynamics in a Brownian dipole lattice: A comparison between theory and computer simulation
Authors: Zhou Huan-Xiang , Bagchi B. , Papazyan A. , Maroncelli M.
Published: July 2002

Papazyan and Maroncelli [J. Chem. Phys. 95, 9219 (1991)] recently reported computer simulations of solvation dynamics of an ion in a Brownian dipole lattice solvent. In the present article we compare these results to predictions of a number of theories of solvation dynamics in the diffusive limit. The frequency-dependent dielectric response functions needed as input to many of the theories are derived from further simulations of the lattice solvent [H. X. Zhou and B. Bagchi, J. Chem. Phys. 97, 3610 (1992)]. When properly applied, all of the currently popular molecular theories yield reasonable predictions for the time scale of the solvation response. The dynamical MSA model [P. G. Wolynes, J. Chem. Phys. 86, 5133 (1987)] and the memory function theory of Fried and Mukamel [J. Chem. Phys. 93, 932 (1990)] both provide nearly quantitative agreement with all aspects of the solvation dynamics observed in these simulations.
Ground state optical properties of charge transfer crystals close to the neutral-ionic interface: Tetrathiafulvalene-2,5-dichloro-p-benzoquinone
Authors: Girlando Alberto , Painelli Anna , Pecile Cesare , Calestani Gianluca , Rizzoli Corrado , Metzger Robert M.
Published: July 2002

The crystal structure and the optical data of the (1:1) charge-transfer (CT) complex of tetrathiafulvalene (TTF) with 2,5-dichloro-p-benzoquinone (2,5Cl2BQ) are reported, yielding a full characterization of the ground state properties. The complex crystallizes in the triclinic space group P1̄, Z=1. TTF and 2,5Cl2BQ form mixed regular stacks along the b axis. The degree of ionicity ρ is ∼0.18, and the CT integral ∼0.22 eV. A detailed analysis of the optical data yields estimates of the strength of electron–molecular vibration coupling and of intersite electron–electron interactions. The latter has also been computed in the atom-in-molecule fractional charge approximation. The TTF-2,5Cl2BQ structure and microscopic parameters at ambient conditions are found to be rather similar to those of the extensively studied complex of TTF with tetrachloro-p-benzoquinone (CA). The different behavior of the two complexes under pressure (both exhibit a neutral–ionic phase transition, but ρ changes discontinuously for TTF–CA and continuously for TTF-2,5Cl2BQ) is ascribed to the competitive role of the Madelung energy and of the CT integral.
High-resolution infrared spectroscopy of DF trimer: A cyclic ground state structure and DF stretch induced intramolecular vibrational coupling
Authors: Suhm Martin A. , Farrell John T. , Ashworth Stephen H. , Nesbitt David J.
Published: July 2002

We present high resolution, infrared laser absorption spectra of (DF)3 in a slit supersonic jet expansion. In contrast to previous structureless near IR spectra of (HF)3, the (DF)3 data reveal clear rotational structure characteristic of a cyclic, 6-membered ‘‘ring,’’ and therefore provide the first accurate experimental evidence for the equilibrium geometry of any hydrogen fluoride oligomer beyond the well-studied dimer. Furthermore, the spectra display homogeneous rotational fine structure 2–3 orders of magnitude in excess of what could be anticipated from a single vibrational band. Analysis of this fine structure elucidates a novel IVR mechanism which involves single hydrogen bond cleavage, and consequent opening of the (DF)3 ring on the 40 ps time scale.
A laser-induced fluorescence study of the BiF A O+→X1 O+ system in the region 6550 to 7400 Å: Rotational analysis and evidence for an A O+ state potential maximum
Authors: Oberlander M. D. , Parson J. M.
Published: July 2002

Eighteen previously unobserved bands belonging to the BiF A O+→X1 O+ system have been identified in the region between 6550 and 7400 Å. Nine bands representing v′=27–29 and v″=40–45 have been rotationally analyzed, and the v′=27 and v′=29 states exhibit strong homogeneous perturbations. These upper state levels are also observed to predissociate, providing evidence for an A O+ state potential maximum which has been theoretically predicted. A limiting curve of dissociation suggests a rotationless potential maximum at 32 674±822 cm−1. This energy must correspond to a potential maximum if it is to be reconciled with recent thermochemical measurements of the ground state dissociation energy, D00=3.76±0.13 eV. The new spectroscopic constants have been included in a calculation of Rydberg–Klein–Rees potential curves for both states. Franck–Condon factors calculated from these potentials are qualitatively consistent with the observed intensity distribution. Measured collision free radiative lifetimes are consistent with known A O+ state lifetimes for nonpredissociated states and are found to be shorter for predissociated states.
The frequency dependent conductivity of electrolyte solutions
Authors: Chandra A. , Wei Dongqing , Patey G. N.
Published: July 2002

The frequency dependent conductivity, σ(ω), of model electrolyte solutions is investigated employing molecular dynamics simulations. Approximate analytical theories are also derived and evaluated. It is found that σ(ω) is generally not a simple function and that its detailed frequency dependence contains much useful information about the structure and dynamics of ionic solutions. By varying molecular and state parameters such as the dipole moment and moment of inertia of the solvent, the ionic charge, the solution density, etc., we are able to obtain a physical interpretation of the behavior of σ(ω). For example, very interesting dynamical solvation effects as well as features associated with ion pairs are identified and described.
A coupled-cluster based effective Hamiltonian method for dynamic electric polarizabilities
Authors: Stanton John F. , Bartlett Rodney J.
Published: July 2002

A coupled-cluster based approach for calculating dynamic polarizabilities is described. In this procedure, the polarizability is calculated by a strategy that is formally equivalent to a sum over states corresponding to the diagonal representation of a similarity transformed Hamiltonian operator. However, the explicit evaluation of excited state wave functions and energies is avoided. The present treatment is closely related to the equation of motion coupled-cluster approximation for excited states and offers an accurate approximation to the second derivative of the energy with respect to an applied electric field; the two approaches are equivalent in the limit that the spectrum of states corresponding to the effective Hamiltonian is exact within the basis set. Terms contributing to the second derivative, but neglected in the proposed approach are shown to be insignificant for a representative set of small molecules. The method is applied to calculate the polarizability of benzene at the wavelength of the sodium D line as well as in the static field limit, using a basis set of 192 contracted Gaussian functions.
Low energy electron attachment to clusters of nitric oxide
Authors: Carman Howard S.
Published: July 2002

The attachment of low energy (9–80 meV) electrons to clusters of nitric oxide (NO) has been studied by means of Rydberg electron transfer (RET) from selected nd states of rubidium (n=15–40). The product negative ions have stoichiometry (NO)−x (x=2–60) and exhibit even/odd intensity alternations (odd≳even) which increase in magnitude with cluster size such that only odd cluster ions are observed for x≳20. The experimental data are consistent with an evaporative attachment mechanism in which the production of odd cluster ions is kinetically favored due to differences in stabilities between odd and even cluster ions. These differences in stabilities are attributed to the spin pairing of valence electrons which results in extensive dimerization of NO in condensed phases. It is postulated that, for x≳3, the excess electron becomes localized to form an (NO)−3 ion which is solvated by NO dimers. Interaction potentials governing the initial electron capture process are also discussed.
Solving Schrödinger’s equation around a desired energy: Application to silicon quantum dots
Authors: Wang Lin-Wang , Zunger Alex
Published: July 2002

We present a simple, linear-in-size method that enables calculation of the eigensolutions of a Schrödinger equation in a desired energy window. We illustrate this method by studying the near-gap electronic structure of Si quantum dots with size up to Si1315H460(≊37 Å in diameter) using a plane wave pseudopotential representation.
Characterization of the S1–S2 conical intersection in pyrazine using ab initio multiconfiguration self-consistent-field and multireference configuration-interaction methods
Authors: Woywod Clemens , Domcke Wolfgang , Sobolewski Andrzej L. , Werner Hans-Joachim
Published: July 2002

Potential-energy surfaces of the three lowest singlet states of pyrazine have been calculated as a function of ab initio determined ground-state normal coordinates, using complete-active-space self-consistent-field (CASSCF) and multireference configuration interaction (MRCI) techniques. The conical intersection of the S1 and S2 adiabatic potential-energy surfaces has been mapped out in selected subspaces spanned by the most relevant vibrational coordinates. A unitary transformation from the adiabatic to a quasidiabatic electronic representation is performed, which eliminates the rapid variations of the wave functions responsible for the singularity of the nonadiabatic coupling element. Transition-dipole-moment functions have been obtained in the adiabatic and in the diabatic representation. The leading coefficients of the Taylor expansion of the diabatic potential-energy and transition-dipole-moment surfaces in terms of ground-state normal coordinates at the reference geometry have been obtained at the CASSCF/MRCI level. Using a vibronic-coupling model Hamiltonian based on this Taylor expansion, the absorption spectrum of the interacting S1–S2 manifold has been calculated, taking account of the four spectroscopically most relevant modes.
Multiphoton studies of jet-cooled Xe2 near the Xe*5d[5/2]3 state: Characterization of the ground and excited state potential curves
Authors: Green David S. , Wallace Stephen C.
Published: July 2002

The two-photon resonant, three-photon (2+1) ionization spectra of jet-cooled mXenXe, at energies near the Xe* 5d[5/2]03 state, are reported. A new progression has been observed and is attributed to transitions from the van der Waals ground state, X 1Σ+g(0+g), through bound vibrational levels of an excited state of gerade symmetry. The analysis of some 26 closely spaced vibronic bands and isotope effects provides information on the excited and ground state potential energy curves. The vibrational quantum number of the lowest frequency band near 82 539.1 cm−1 is assigned to v′=6±1. For v′=6 this leads to molecular constants Te′ ≂ 82 514.9 cm−1, ωe′ ≂ 5.7955 cm−1, and ωexe′ ≂ 0.07491 cm−1. The upper state can be described by a Morse potential with dissociation energy De′ ≂ 112.10 ± 0.05 cm−1 and internuclear separation Re′ ≂ 5.51 ± 0.03 Å. This is consistent with assignment to a Rydberg molecular state of either the B 2Π1/2g or D 2Σ+1/2g ion core. At the Xe 1S0+Xe* 5d[5/2]03 threshold the molecular spectrum terminates and continuum absorption is evidenced by a rise and fall in the fragment ion yield. The direct determination of the dissociation limit for the excited state is used to derive the ground state dissociation energy De″≂ 196.32 ± 0.05 cm−1.
The interaction of hydrogen with a cobalt(101̄0) surface
Authors: Ernst K.-H. , Schwarz E. , Christmann K.
Published: July 2002

The adsorption of hydrogen on a cobalt(101̄0) surface was investigated in ultrahigh vacuum (UHV) between 85 and 500 K using Video-LEED, temperature-programmed thermal desorption (TPD), work function (ΔΦ) measurements, and high-resolution electron energy loss spectroscopy (HREELS). Between 90 and 200 K, hydrogen adsorbs dissociatively with high sticking coefficient (s0≥0.8) via precursor kinetics and forms, with increasing exposure, a c(2×4), a p2mg (2×1) and a (1×2) LEED structure (hydrogen coverages ΘH=0.5, 1.0, and 1.5, respectively). While the first two structures represent true ordered hydrogen phases there is strong evidence that the (1×2) phase is reconstructed, likely in a paired-row configuration. The formation of the (1×2) phase is slightly thermally activated; its decomposition produces a sharp thermal desorption maximum (α state) appearing on the low-energy side of a β-TPD signal which reflects the hydrogen desorbing from the unreconstructed surface. The activation energies for desorption from the α and β states are 62 and 80 kJ/mol, respectively. Chemisorption in the β state [(2×1) phase up to ΘH=1.0] is associated with a ΔΦ of +207 meV, while the fully developed (1×2) reconstructed phase (α state) causes a ΔΦ of approximately −122 meV resulting in an overall work function change of +85 meV at saturation. From HREELS, we determine the H adsorption site in all superstructures to be threefold with a local CS symmetry. Our results are discussed and compared with previous findings for similar metal–hydrogen interaction systems.
Implications of the volume dependent convergence of anharmonic free energy methods
Authors: Lacks Daniel J. , Rutledge Gregory C.
Published: July 2002

The free energies calculated by lattice dynamics in the quasiharmonic approximation and anharmonic perturbation theory are compared with exact results obtained by Monte Carlo simulation. The free energies obtained with the approximate methods are more accurate at smaller volumes. One implication of this result is that inclusion of interactions beyond nearest neighbor increases the accuracy of these methods due to the compression associated with the attractive longer range interactions. Also, these methods will be accurate to higher temperatures when thermal expansion is smaller, as is shown to be the case for the Morse crystal as compared to the Lennard-Jones crystal.
Anomalous surface diffusion: A numerical study
Authors: Bychuk Oleg V. , O’Shaughnessy Ben
Published: July 2002

We present a numerical study of bulk-mediated effective surface diffusion at liquid surfaces where surface-active molecules adsorb and desorb on experimental time scales. Adsorbed molecules execute Lévy walks on the interface, each step entailing desorption followed by bulk diffusion and readsorption elsewhere. Our results confirm the predicted anomalous scaling of surface displacement r at times before particles are finally lost to the bulk. Moments grow as 〈rq〉∼tζ(q), where ζ(q)=q for q<1, ζ(q)=(q+1)/2 for q≳1. We have also confirmed that the ‘‘speed’’ c which characterizes the q<1 behavior, r≊ct, is universally related to other observables: c=D/h where D and h are, respectively, the bulk diffusivity and the slope of the equilibrium adsorption isotherm.
Models and equilibrium properties of stiff molecular chains
Authors: Winkler Roland G. , Reineker Peter , Harnau Ludger
Published: July 2002

The partition functions of discrete as well as continuous stiff molecular chains are calculated using the maximum entropy principle. The chain is described by mass points, and their connectivity is taken into account by harmonic constraints (flexible segments) in addition to the bending restrictions. For comparison and as a test of the formalism the freely rotating chain as well as the Kratky–Porod wormlike chain (rigid segments) are reexamined treating the bending restrictions as constraints. It is shown that the second moments for the chain of flexible segments agree exactly with those known from the freely rotating chain for the discrete as well as the continuous chain and for all stiffnesses. Moreover, the Green’s function for the continuous chain is determined, which allows to obtain any desired two point distribution function. The influence of various bending restrictions on equilibrium properties is discussed. Furthermore, a comparison to other existing models, especially the Harris and Hearst model, is given and the validity of the various models is examined.
Femtosecond solvation dynamics determining the band shape of stimulated emission from a polar styryl dye
Authors: Bingemann Dieter , Ernsting Nikolaus P.
Published: July 2002

Spectra of transient absorption and stimulated emission are recorded for the styryl dye DASPI, after excitation at 470 nm, with experimental resolution of 100 fs. The evolution of the S1→S0 transition energy distribution is obtained for the solvents methanol and acetonitrile at several temperatures. It is described by the dependence of the mean (first moment), width, and asymmetry (second and third central moments) of the distribution on time. The observed time-dependence of the mean transition energy is simulated by appropriate models for the solvation dynamics. In both methanol and acetonitrile an ultrafast component is observed. Width and asymmetry change most rapidly and characteristically during this initial part of solvation. In the evolution of the higher moments, different relaxation contributions apparently are better distinguished than in the evolution of the first moment. For methanol at 50 °C, an oscillatory evolution is observed mainly in the higher moments which may indicate underdamped coherent solvent motion.
Rotational dielectric friction and dipole solvation: Tests of theory based on simulations of simple model solutions
Authors: Papazyan A. , Maroncelli M.
Published: July 2002

Simulations of a simplified model system are used to test analytical theories of dielectric friction and explore its connection to dipole solvation dynamics. The simulation model consists of a point dipole solute interacting with a finite collection of dipolar solvent molecules, all situated on a simple cubic lattice and undergoing rotational Brownian motion in the pure diffusion limit. An extensive set of simulations are reported in which four model properties, the solute dipole moment and charge, and the solvent polarity and relaxation time, have been systematically varied. Static and dynamic aspects of dipole solvation observed in these systems are compared to the predictions of the simple continuum and dynamical mean spherical approximation (MSA) theories. Within the linear solvation regime the MSA theory is found to yield essentially quantitative predictions for both static and dynamic solvation properties. The simple continuum model, on the other hand, provides a poor description of either the static or the dynamic behavior. Solute rotational correlation functions of various rank and the dielectric friction functions calculated from them are compared to a variety of theories of rotational dielectric friction. Since all of the analytical theories examined rely on simple continuum descriptions of dipole solvation, they all fail to yield quantitatively accurate results. However, the more sophisticated theories do generally provide useful guides for understanding the trends observed in the data. The one instance where all of the theories fail in a qualitative manner is in predicting the rotational dynamics in the slow solvent limit. Reasons for this failure are discussed and a semiempirical approach for understanding the actual behavior in this limit is presented.
Ordering of block copolymer melts in confined geometry
Authors: Brown Gregory , Chakrabarti Amitabha
Published: July 2002

Microphase separation of symmetric diblock copolymers between two rigid walls is simulated by a coarse-grained model to study the geometry of the microdomains without presupposing their basic shape. The geometry is found to be dependent on the wall separation and the domains can be either aligned horizontally or vertically. The horizontal geometry is induced by wetting at the walls and lamellas parallel to them develop quickly after a quench from the one-phase region. The vertical geometry involves domains oriented perpendicular to the walls and develops much more slowly. For identical walls, some segregation of the wetting phase to the walls is observed for the vertical geometry. This segregation does not significantly change the structure. If the vertical phase develops between walls that attract opposite phases, however, a linear concentration profile develops. In some cases the structure of this vertical phase is found to be quite different from simple, perpendicularly oriented lamellas.
Critical behavior of the Widom–Rowlinson lattice model
Authors: Dickman Ronald , Stell George
Published: July 2002

We report extensive Monte Carlo simulations of the Widom–Rowlinson lattice model in two and three dimensions. Our results yield precise values for the critical activities and densities, and clearly place the critical behavior in the Ising universality class.
Spatiotemporal concentration patterns associated with the catalytic oxidation of CO and Au covered Pt(110) surfaces
Authors: Asakura K. , Lauterbach J. , Rotermund H. H. , Ertl G.
Published: July 2002

Submonolayers of gold were evaporated onto a Pt(110) surface in order to study the influence of surface modification on the formation of spatiotemporal concentration patterns during catalytic oxidation of carbon monoxide as imaged by means of photoemission electron microscopy (PEEM). Already the presence of 0.05 monolayers of uniformly distributed Au atoms affects markedly the kinetic parameters and thereby the properties of spatiotemporal patterns. The excitable regime is reached at lower CO pressures than with bare Pt(110), the width of the oxygen waves and the wavelength of spiral patterns decrease, the velocity of front propagation is reduced, and the density of nucleation centers for waves increases. The observed effects can be rationalized on the basis of the well-established underlying mechanism in terms of modification of the sticking coefficients for oxygen and carbon monoxide and of the diffusion coefficient for adsorbed CO. Additional effects such as penetration of waves into a monostable region, refraction, etc., come into play if waves pass the border between surface areas with differing chemical properties.
Effect of the treatment of long-range forces on the dynamics of ions in aqueous solutions
Authors: Perera Lalith , Essmann Ulrich , Berkowitz Max L.
Published: July 2002

The goal of the present work is to study the dependence of the limiting ionic mobility of such anions as fluoride, chloride, and bromide in water on the way the long-range forces are treated in the computer simulations. With this in mind we have performed molecular dynamics computer simulations where the long-range electrostatic forces were treated using: (a) simple truncation procedure, (b) energy switching procedure, (c) reaction field method, and (d) Ewald summation technique. Our analysis shows that the switching procedure with the short-range switching function introduces artifacts into the simulations. These artifacts are responsible for the faster decay and oscillations in the velocity autocorrelation function of the ions and therefore for the lower value of the diffusion coefficients.
Competition between energy and phase relaxation in electronic curve crossing processes
Authors: Jean John M. , Fleming Graham R.
Published: July 2002

We present results from simulations of vibrational energy and phase relaxation and electronic curve crossing using a multilevel formulation of Redfield theory, which demonstrate the shortcomings of the optical Bloch approximation and the importance of coherence transfer processes in the relaxation dynamics of multilevel systems. Specifically, we show that for a harmonic well, energy relaxation can occur with retention of vibrational phase, and that for sufficiently strong electronic coupling, the product of an electronic curve crossing process can be formed vibrationally coherent even when no coherence is present in the initially excited state.
Rate expressions for excitation transfer. IV. Energy migration and superexchange phenomena
Authors: Scholes Gregory D. , Ghiggino Kenneth P.
Published: July 2002

General microscopic mechanisms of electronic excitation (energy) transfer (EET) in multichromophoric assemblies are investigated. Aspects of superexchange-mediated EET and energy migration (EM) and their contribution to the efficiency of donor-to-trap energy transport processes in macromolecules are discussed from a quantum mechanical viewpoint. The possibility of superexchange pathways for EM via higher excited states of the intermediate chromophores is introduced. The role of quasicoherent EM pathways, and how they are manifested in the quantum mechanical rate expression, is investigated and the significance of contributions to the rate arising through quantum mechanical interference between pathways is elucidated. The theory indicates conditions under which coherent EM pathways may significantly increase the efficiency of energy transport and trapping and the applications to natural and synthetic light-harvesting systems are outlined.
Photoionization of oriented CO molecule: Linear dichroism in the angular distribution of photoelectrons
Authors: Cherepkov N. A. , Raşeev G.
Published: July 2002

Theoretical investigation of linear dichroism in the angular distribution (LDAD) of photoelectrons obtained in a molecular photoionization process is presented. The developed theory applies to the investigation of both molecules adsorbed on a surface and gas phase molecules when the molecular orientation is fixed by coincidence between a photoelectron and a residual ion resulting from fast molecular dissociation. Several experimental geometries are proposed which are used to unravel the properties of the adsorbed or oriented molecules. The first numerical calculations of LDAD in the case of photoionization of the 4 σ-orbital of fixed-in-space CO molecules are presented. We calculate the differential cross section and the LDAD for two of the geometries of the experiment we have proposed: One allows a simple extraction of theoretical parameters from the experimental data in a kind of complete experiment with molecules; the other allows the determination of molecular orientation in space, for example, with respect to the surface if the nuclear motion of adsorbates is neglected.
Dynamics and conformational properties of polyampholytes in external electrical fields
Authors: Schiessel H. , Oshanin G. , Blumen A.
Published: July 2002

In this work we report first analytical results on the dynamics and conformational properties of polyampholytes (PAs, polymers containing positive and negative charges) in the presence of external electrical fields. In terms of the Rouse model of polymer dynamics and in the so-called weak coupling limit we obtain for PAs explicitly the mean-square displacement both of the center of mass and of individual beads, and also determine the PAs’ equilibrium end-to-end distance. For a singly charged PA we also relate the findings to a fractional differential equation.
Molecular dynamics simulation study of adsorption of polymer chains with variable degree of rigidity. I. Static properties
Authors: Kramarenko E. Yu. , Winkler R. G. , Khalatur P. G. , Khokhlov A. R. , Reineker P.
Published: July 2002

The adsorption of a single polymer chain onto a solid surface is investigated by molecular dynamics simulations. The chain is composed of mass points interacting via a truncated Lennard-Jones potential, i.e., the excluded volume interaction is taken into account, and grafted to the surface with one end. The average adsorption degree is calculated for various chain lengths (N = 16, 32, 64, 128) and adsorption energies. In addition, the scaling behavior of the adsorption degree and the radius of gyration is investigated. The adsorption degree and the average length of loops and tails are obtained for chains of various stiffnesses. In this context, we find that stiffer chains adsorb more easily. Moreover, the distribution of the mass points perpendicular to the surface as well as the orientation of the bonds with respect to the surface is discussed for various adsorption energies and stiffnesses.
Ab initio configuration interaction calculations of the potential curves and lifetimes of the low-lying electronic states of the lead dimer
Authors: Das Kalyan K. , Liebermann Heinz-Peter , Buenker Robert J. , Hirsch Gerhard
Published: July 2002

The low-energy electronic spectrum of the lead dimer is described by means of a multireference configuration interaction treatment based on a semicore relativistic effective core potential (RECP) including spin–orbit coupling. The X0+g ground state is found to be a heavy mixture of the ...σ2π2 3Σ−g, the ...σπ2π* 5Πg and ...π4 1Σ+g Λ–S states, underscoring the importance of the spin–orbit interaction in determining the electronic structure of this heavy system. The first excited state has 1g symmetry and is predominantly 3Σ−g but also with a heavy admixture of 5Πg character. The lowest-lying excited state as yet observed (A) seems to be the 2u(I) state, however, with a 0.09 Å smaller computed re value than for X0+g. The B state with an experimental Te value of 12 457 cm−1 appears to be second 0−u state which arises from an avoided crossing between the ...σπ3 3Πu and the ...σ2ππ* 1Σ−u Λ–S states. Another avoided crossing between the lowest two 0+u states is shown to produce the experimental C and F states, which possess the strongest transitions to X0+g of any of the low-lying Pb2 states. The present computed radiative lifetime for the C state is in very good agreement with the measured value of Bondybey and English (1.5 μs). The calculations also find that the F state’s lifetime is only about half as long as for the C state, whereas the experimental results give a smaller ratio of close to 0.1, indicating that nonradiative transitions may also be important for depopulating this state. Eleven other states are found to lie between the F and C states, despite their relatively small Te value difference of 4500 cm−1. Two of these are believed to have been observed in emission processes to the A state, but there is insufficient experimental data to make specific assignments in this case.
Suppression and enhancement of van der Waals interactions
Authors: Cho Minhaeng , Silbey Robert J.
Published: July 2002

The van der Waals interaction of a pair of anisotropic molecules near planar dielectric surfaces is studied by using the linear response formalism. The spatial correlation function (Green function) of the vacuum electric field in the presence of dielectric surfaces is obtained by using suitable Fresnel mode functions of the quantized electric field. In the short-distance limit, it is observed that the long-range interaction potential is significantly modified by the dielectric surfaces and strongly depends on the geometry of the two molecules near dielectric surfaces. When the two molecules are anisotropic, depending on the molecular alignments with respect to the surfaces, the van der Waals interaction is enhanced or suppressed by the existence of the surfaces. When the two molecules are in between two dielectric surfaces, the overall magnitude of the van der Waals interaction is suppressed in comparison to that in the free space because the vacuum electromagnetic field intensity is reduced by the transmissivities that are generally less than unity.
Multiplet-specific multichannel electron-correlation effects in the photoionization of NO
Authors: Stratmann R. E. , Zurales Robert W. , Lucchese Robert R.
Published: July 2002

We present partial photoionization cross sections and photoelectron asymmetry parameters for the photoionization of NO leading to the (2π)−1X 1Σ+, (5σ)−1b 3Π, (5σ)−1A 1Π, (4σ)−1c 3Π, and (4σ)−1B 1Π states of NO+. The results were obtained with multichannel multiplet-specific interaction potentials derived from correlated target states. The resulting scattering equations were solved using the Schwinger variational method. The calculations considered excitation energies in the 10–40 eV range. It was found that selective orthogonalization eliminated spurious resonances that were encountered. We found that in the channel leading to the (2π)−1X 1Σ+ state of NO+, the structure seen experimentally in the 14–17 eV region is due to two pronounced valence autoionizing states, one of which is broadened by interaction with a shape resonance. We predict the existence of a third strong σ→π* transition of 2Σ− symmetry in the photoabsorption cross section at approximately 14.8 eV photon energy which, due to symmetry restrictions, cannot decay by autoionization. In addition, our results indicate that the broad structure seen experimentally in the 20–40 eV region in the (2π)−1 channel might be due to coupling to shape resonances which occur in other ionization channels. Our predicted total cross sections and photoelectron asymmetry parameters differ from those obtained by previous theoretical approaches, all of which neglected correlation effects. The present results were found to be in good agreement with the available experimental data. We found that the existence and position of the various resonances were sensitive to the level of correlation and interchannel coupling included in the calculation. In particular, we found that the resonant enhancement in the channels leading to both the (5σ)−1b 3Π and (5σ)−1A 1Π states of NO+ was due to a single 5σ→σ* resonant state which decayed into both ionization channels.
Theoretical study of the water pentamer
Authors: Wales David J. , Walsh Tiffany R.
Published: July 2002

Geometry optimizations, rearrangement mechanisms, spectral intensities, and tunneling splittings are reported for the water pentamer. Two low energy degenerate rearrangements are identified for the chiral cyclic global minimum which are analogous to processes that lead to observable tunneling splittings in the water trimer. Fourteen different pathways are characterized by ab initio calculations employing basis sets up to double-zeta plus polarization (DZP) quality with subsequent reoptimization of the associated minima using the Becke exchange and the Lee–Yang–Parr correlation functionals (BLYP) with the same basis. All the pathways have been recomputed for a number of different empirical potentials, some of which reproduce the two lowest energy degenerate rearrangements quite well. However, none of the empirical potentials support all the higher energy ab initio minima. Qualitative estimates of the two tunneling splittings associated with the lowest energy pathways suggest that at least one might be observable experimentally; the associated splitting patterns and nuclear spin weights are also reported. The corresponding stationary points were finally reoptimized using DZP basis sets plus diffuse functions with the BLYP exchange-correlation functional.
The solvation of Cl−, Br−, and I− in acetonitrile clusters: Photoelectron spectroscopy and molecular dynamics simulations
Authors: Markovich Gil , Perera Lalith , Berkowitz Max L. , Cheshnovsky Ori
Published: July 2002

We present the photoelectron spectra of Cl−, Br−, and I− solvated in acetonitrile clusters (CH3CN)n with n=1–33, 1–40, and 1–55, respectively, taken with 7.9 eV photon energy. Anion–solvent electrostatic stabilization energies are extracted from the measured vertical electron binding energies. The leveling of stabilization energies beyond n=10–12 for the three halides signifies the completion of the first solvation layer. This is different from the behavior of anion–water clusters which probably do not fill the first solvation layer, but rather form surface solvation states. Classical molecular dynamics simulations of halide–acetonitrile clusters reproduce the measured stabilization energies and generate full solvation shells of 11–12, 12, and 12–13 solvent molecules for Cl−, Br−, and I−, respectively. Ordered shell structures with high stability were found for the clusters of Cl−, Br−, and I− with n=9, 9, and 12. This special stability is reflected in the intensity distribution of the clusters in the mass spectra. Larger anion–acetonitrile clusters have the molecules beyond the first solvation layer packed in a small droplet which is attached to the first layer. It is suggested that in general, anions solvated in large clusters of polar solvents, might be located close to their surface.
Vibrationally mediated photodissociation of isocyanic acid (HNCO): Preferential N–H bond fission by excitation of the reaction coordinate
Authors: Brown Steven S. , Metz Ricardo B. , Berghout H. Laine , Crim F. Fleming
Published: July 2002

We report the bond selected photodissociation of HNCO from a vibrationally excited state containing three quanta of N–H stretch (3ν1) and demonstrate that initial vibrational state preparation strongly influences the photochemical branching in HNCO, producing either NCO (X  2Π) from cleavage of the N–H bond or NH (a 1Δ) from cleavage of the C–N bond. Initial excitation of the N–H dissociation coordinate by excitation of the N–H stretching overtone enhances the probability for breaking the N–H bond in the electronically excited state. Compared to isoenergetic photolysis of the ground vibrational state, photodissociation of the 3ν1 state alters the NCO quantum yield by roughly a factor of 4 at the largest photolysis energy used in this work, changing the channel that breaks the N–H bond from the minor to the major dissociation pathway. In addition, the experiment measures the quantum yields for production of NCO (ΦNCO) in the one-photon dissociation at three different photolysis wavelengths and provides a correction for the influence of photodissociation from vibrationally and rotationally excited states.
Variational energy band theory for polarons: Mapping polaron structure with the Merrifield method
Authors: Zhao Yang , Brown David W. , Lindenberg Katja
Published: July 2002

In this paper we revisit from a contemporary perspective a classic problem of polaron theory following the variational approach originally taken by Merrifield. Polaron structure is represented by a variational surface giving the optimal values of the complete set of phonon amplitudes for every value of the joint exciton–phonon crystal momentum κ. Quantities such as complete ground state energy bands (all κ) and effective masses (κ=0) are obtained. The parameter space of the problem is mapped, with careful attention given to the self-trapping transition. Through this examination of the complete parameter space at all κ, it is found that the common notion of a sharp self-trapping phenomenon associated with κ=0 is a limiting aspect of a more general finite-κ phenomenon. The idea of polaron Wannier states is addressed briefly, and the properties of such states tied to characteristics of the polaron energy band. The successes and failures of the Merrifield method are assessed.
A dynamic Monte Carlo algorithm for exploration of dense conformational spaces in heteropolymers
Authors: Ramakrishnan R. , Ramachandran Bala , Pekny J. F.
Published: July 2002

This paper concerns configurational sampling methods for dense single chains for use in the study of low energy states of heteropolymers. The efficiency of current sampling techniques decreases with increasing density and breaks down completely when the volume fraction of the solvent approaches zero. Methods proposed for dense multichain systems are also ineffective for single chain conformational sampling. We propose a new elementary Monte Carlo move for dense single chains which generates new configurations by breaking and patching the chain. The effectiveness of this method is studied by testing its ability to identify global energy minima for maximally compact lattice chains. The algorithm is able to determine the ground state for a HP model protein of length 48 in around 37 000 steps. Computational results are presented for compact lattice chains of length up to 1000 on a 10×10×10 cubic lattice.
Mass-resolved two-photon spectra of Xe2 in the region of Xe*(5d). I. Vibronic analyses
Authors: Hu X. K. , Mao D. M. , Dimov S. S. , Lipson R. H.
Published: July 2002

Eight band systems of jet-cooled Xe2 between 83 763.9 and 80 149.8 cm−1 involving gerade excited states that dissociate to Xe(1S0)+Xe*5d[3/2]10, Xe(1S0)+Xe*5d[5/2]30, Xe(1S0)+Xe*5d[5/2]20, Xe(1S0)+Xe*5d[7/2]30, Xe(1S0)+Xe*5d[7/2]40, and Xe(1S0)+Xe*6p[1/2]0 have been characterized using (2+1) resonance enhanced multiphoton ionization/time-of-flight mass spectrometry, many for the first time. Excited state vibrational numbering and constants, including dissociation energies were obtained from bandhead measurements made from separate single isotopomer spectra. Bond lengths were estimated from Franck–Condon factor intensity simulations, while excited state electronic symmetries were deduced primarily from polarization measurements. In three instances, energy barriers due to double well potential energy curve formation are identified.
A hybrid semiempirical quantum mechanical and lattice-sum method for electrostatic interactions in fluid simulations
Authors: Gao Jiali , Alhambra Cristobal
Published: July 2002

A method is described to incorporate the Ewald lattice-sum method into quantum mechanical calculations in hybrid quantum and molecular mechanical (QM/MM) fluid simulations. The procedure is illustrated through standard free energy perturbation calculations in the context of Monte Carlo simulations. The free energy of hydration of chloride ion was computed using the hybrid QM/MM-Ewald method, and comparison was made to results obtained with standard spherical cutoff. The results indicate that the hybrid QM/MM-Ewald method can be effectively used to include long-range electrostatic interactions in quantum mechanical calculations of condensed media.
Methyl group tunneling—A quantitative probe of atom–atom potentials
Authors: Neumann M. , Johnson M. R.
Published: July 2002

A molecular mechanics calculation of the rotational potential experienced by methyl groups in molecular crystals is presented. Good agreement for seven out of the eight cases considered is obtained and the methyl group becomes a quantitative probe of its molecular environment. The calculation is used to gain insight into the coupling between methyl groups. For the smallest molecules, comparison between the generalized semiempirical calculation and a limited ab initio calculation on dimers of molecules suggests the simple rotational model employed is adequate, and that discrepancies between measured and calculated values are due to incorrect modeling of the atom–atom potentials.
Off-resonant fifth-order nonlinear response of water and CS2: Analysis based on normal modes
Authors: Saito Shinji , Ohmine Iwao
Published: July 2002

Off-resonant fifth-order nonlinear response functions of liquid water and liquid CS2 are analyzed based on two normal-mode schemes, quenched and instantaneous normal modes. It was found that the fifth-order response function is very sensitive to the mode mixing in polarization, arising from the quadratic term of polarization with respect to the different modes. The echo signal is drastically reduced by this off-diagonal mode mixing in polarization even without any rapid frequency modulation mechanism. The near absence of echo signal thus obtained for liquids is consistent with the recent experimental results for liquid CS2. The present calculation yields the different fifth-order signals for different polarization geometries, as experimentally shown by Tokmakoff and Fleming [J. Chem. Phys. 106, 2569 (1997)]. The mode mixing dynamics is investigated in terms of the bispectra of total potential energy and polarizability.
Perfect families of mixed crystals: The rotator I N-alkane case
Authors: Oonk H. A. J. , Mondieig D. , Haget Y. , Cuevas-Diarte M. A.
Published: July 2002

The experimental systems considered in this paper are isobaric binary mixed crystals, and their properties studied are the thermodynamic mixing properties, actually the excess enthalpy and excess entropy. More in particular, the excess behavior is examined for families of systems where the components of each of the member systems belong to a group of chemically coherent substances. The accent is on a group of 18 systems composed of n-alkanes in the range from C11H24 to C26H54, such that the components of an individual system are either neighbors or next nearest neighbors. In these systems mixed crystalline solid forms are stable in which the molecules have rotational freedom around their long axis. One of these forms is rotator I, and it is this form for which thermodynamic mixing properties have been determined. The magnitudes of the excess enthalpy and excess entropy are system-dependent and can be correlated to the geometric mismatch, i.e., the relative difference in number of carbon atoms of the molecules of the components of the system. The quotient of the two excess properties, on the other hand, is shown to be system-independent: it corresponds to a temperature which is characteristic of the family as a whole. For the rotator n-alkane family, its value is 320 K. The result obtained for the n-alkanes gives support to the treatment (of a similar nature) we applied earlier to the common-ion alkali halide systems and the p-dihalobenzenes. We propose to refer to such groups of systems of mixed crystals as a “perfect family of mixed crystals.”
The rotational spectrum and nuclear quadrupole hyperfine structure of CO2–N2O
Authors: Leung Helen O.
Published: July 2002

The microwave spectrum of CO2–N2O has been obtained in the 7–19 GHz region using a Fourier transform microwave spectrometer. The nuclear quadrupole hyperfine structure in 26 a- and b-type rotational transitions has been analyzed using the Watson S-reduced Hamiltonian with the inclusion of nuclear quadrupole interactions. The rotational constants and six centrifugal distortion constants (in MHz) are A=8843.4133(1), B=1738.777 37(6), C=1449.807 41(5), DJ=6.510(3)×10−3, DJK=−3.7405(8)×10−2, DK=2.3459(3)×10−1, d1=−1.3751(4)×10−3, d2=−8.3(1)×10−5, and HJ=−1.3(4)×10−7. The nuclear quadrupole coupling constants (in MHz) for the terminal nitrogen nucleus are χaa=−0.0966(4), χbb=−0.3111(4), and χcc=0.4077(4), and those for the central nitrogen nucleus are χaa=−0.0411(6), χbb=−0.0968(6), and χcc=0.1380(6). The spectroscopic constants are consistent with an approximately slipped parallel structure where the distance between the centers of mass of the subunits is 3.472 Å, the acute angle between the CO2 molecular axis and the intermolecular axis is 62.8°, and the acute angle between the N2O axis and the intermolecular axis is 58.1°. The experimental data cannot identify whether the terminal nitrogen or the oxygen in N2O is closest to the C in CO2. The nuclear quadrupole coupling constants show that the electric field gradients at the nitrogen nuclei are perturbed to differing extents.
Tracer-diffusion in colloidal mixtures: A mode-coupling scheme with hydrodynamic interactions
Authors: Nägele Gerhard , Dhont Jan K. G.
Published: July 2002

In this work, we develop a general theoretical scheme to study tracer-diffusion in mixtures of interacting colloidal particles where the influence of solvent-mediated hydrodynamic interactions is also considered. Based on the many-body Smoluchowski diffusion equation, we derive in a first step an exact microscopic expression of the irreducible memory function (self-friction function) associated with the self-intermediate scattering function and with the mean squared displacement of a tagged particle. By applying a mode-coupling scheme (MCS) to the irreducible memory function, we obtain explicit expressions for the tracer-diffusion quantities in terms of partial static structure factors and hydrodynamic functions. The influence of hydrodynamic interactions (HI) is accounted for using a far-field expansion of the two-body hydrodynamic diffusivity tensors. For charge-stabilized colloids, this is a good approximation due to strong electrostatic repulsion between the particles. Various applications are discussed in order to illustrate the versatility of our mode-coupling scheme.