bluuebunny/crossref_metadata_2025 huggingface dataset:
Computational study of transition dynamics in 55-atom clusters
Authors: Stillinger Frank H. , Stillinger Dorothea K.
Published: July 2002

Molecular dynamics computer simulation has ben employed to study structure and isomerization dynamics of intact 55-atom clusters. The interactions used were selected to represent the heavier noble gases Ar, Kr, and Xe. As an aid for interpretation of results, the molecular dynamics computation was coupled to steepest-descent mapping to locate relevant cluster inherent structures (potential energy minima). A relatively sharp melting transition has been reproducibly observed. In its low-temperature ‘‘solid state’’ the cluster predominately inhabits the basins for the Mackay icosahedral inherent structure, with occasional excursions into and out of particle–hole states (an atom promoted from filled second to empty third icosahedral shell). Most inherent structures for the liquid droplet state are amorphous, are higher in energy than those for the solid, have no obvious icosahedral ancestry, and display surface capillary excitations. Freezing can produce defective solid structures which then can be annealed to the ground-state icosahedral structure. Root-mean-square distances under mapping to minima have been evaluated vs temperature; they show behavior qualitatively similar to, but quantitatively shifted from, the bulk-phase behavior prescribed by the Lindemann melting criterion and its conjugate freezing criterion.
Chemistry of semiconductor clusters: Large silicon clusters are much less reactive towards oxygen than the bulk
Authors: Jarrold Martin F. , Ray Urmi , Creegan Kathleen M.
Published: July 2002

The chemical reactions of Si+n (n=10–65) with O2 have been investigated using selected ion drift tube techniques. The smaller clusters are etched by O2 to give Si+n−2 (and two SiO molecules) and the larger clusters chemisorb oxygen forming an SinO+2 adduct. The transition occurs between n=29 and 36 under the conditions employed. There are large variations in the reactivity of the smaller clusters: Si+13, Si+14, and Si+23 are particularly inert. The variations in reactivity are rapidly damped with increasing cluster size and for clusters with 40–65 atoms the reactivity is nearly independent of size. However, these large clusters are ∼102 times less reactive towards O2 than most bulk silicon surfaces. Studies of the temperature dependence of the reactions reveal that they proceed through a metastable precursor state which is probably molecular O2 physisorbed to the cluster surface. Variations in the size of the activation barrier for dissociative chemisorption account for the changes in reactivity with cluster size. However, the difference between the cluster and surface reactivities is not due to the size of the activation barrier, but could be accounted for by the presence of only a few reactive sites on the clusters.
Computer simulations of solvation dynamics in acetonitrile
Authors: Maroncelli Mark
Published: July 2002

Computer simulations of the solvation of monatomic ions in acetonitrile are used to investigate dynamical aspects of solvation in polar aprotic solvents. The observed dynamics depend significantly on solute charge and on which multipole moment of the solute is perturbed. In all cases, the solvation response has a two-part character. One part consists of a fast initial relaxation and attendant oscillations, both of which occur on a time scale of 0.1–0.2 ps. The initial response is well fit by a Gaussian function and accounts for ∼80% of the total relaxation. The second dynamical component occurs on a much slower, ∼1 ps time scale, and accounts for the remainder of the relaxation. The fast response results from small amplitude inertial dynamics of solvent molecules within the confines of their instantaneous environment. The slow component reflects larger amplitude motions involving the breakup and reorganization of these local environments, especially in the first solvation shell of the solute. Comparison of the observed solvation dynamics to predictions of available theories points out the inadequacy of the latter due to their neglect of inertial parts of the solvent dielectric response.
Electronic structure of small GaAs clusters
Authors: Lou L. , Wang L. , Chibante L. P. F. , Laaksonen R. T. , Nordlander P. , Smalley R. E.
Published: July 2002

The electronic structure of small GaxAsy clusters (x+y≤10) are calculated using the local density method. The calculation shows that even-numbered clusters tend to be singlets, as opposed to odd-numbered clusters which are open shell systems. This is in agreement with the experimental observations of even/odd alternations of the electron affinity and ionization potential. In the larger clusters, the atoms prefer an alternating bond arrangement; charge transfers are observed from Ga sites to As sites. This observation is also in agreement with recent chemisorption studies of ammonia on GaAs clusters. The close agreement between theoretical calculations and experimental results, together with the rich variation of electronic properties of GaAs clusters with composition makes GaAs clusters an ideal prototype system for the study of how electronic structure influences chemical reactivity.
Radiative lifetimes of van der Waals heteroclusters
Authors: Shalev Eli , Ben-Horin Narda , Jortner Joshua
Published: July 2002

In this paper we advance and apply a molecular theory of pure radiative lifetimes τr of (aromatic molecule)⋅(rare gas)n heteroclusters. The modification of τr in the heterocluster, relative to the bare molecule value, originates from intermolecular interactions between molecular multicenter transition monopoles, which are described by π electron approximation, and the rare-gas atom transition dipoles, which are specified in terms of the static atomic polarizability α. The calculated changes of τr of 9,10 dichloroanthracene⋅A1 (A=Ar, Kr, Xe) heterodimers are in good agreement with experiment; the theory accounting for the nearly linear dependence of the change of τr on α.

Calculations of the radiative lifetimes of 9,10 dichloroanthracene⋅Arn (n=1–34) heteroclusters were performed using structural information from potential optimization for small (n=1–3) heteroclusters, static structural data for small and medium-sized (n=1–8) heteroclusters, and (constant energy) finite-temperature molecular dynamics simulations for medium-sized and large (n=5–34) heteroclusters. The structural sensitivity of τr is manifested by different values of τr for distinct structural isomers, by the nonmonotonous size effects on τr with increasing n, and by pronounced decrease of τr with increasing temperature. For medium-sized clusters (n=5–18), the temperature dependence of τr originates from the enhancement of atomic motion on both sides of the aromatic microsurface and the occupation of the peripheral region, while for large clusters (n≂34) two layer–one layer isomerization processes with increasing temperature result in dramatic changes of τr. Satisfactory overall agreement between theory and experiment for 9,10 dichloroanthracene⋅Arn n=1–34 heteroclusters was accomplished.

The dominating 9,10 dichloroanthracene⋅Arn structures for n=1–18, which yield agreement between theory and experimental for τr, correspond to nearly equal distribution of the rare gases on both sides of the aromatic microsurfaces at T≤20 K, while large n=34 two-layered structures in the temperature domain 22 K≤T≤40 K account for the experimental τr result. τr serves as a useful spectroscopic probe for the interrogation of the structure and isomerization dynamics in heteroclusters.
A simple method to adjust potential energy surfaces: Application to HCO
Authors: Bowman Joel M. , Gazdy Bela
Published: July 2002

A simple method is described to adjust a potential energy surface to improve agreement with experiment. An application is made to the ab initio HCO potential energy surface.(AIP)
Does the atomic structure factor show atomic structure?
Authors: Schmider Hartmut , Sagar Robin P. , Smith Vedene H.
Published: July 2002

Numerical evidence for the monotonicity of the isotropic atomic form factor, F(k), is presented. It is shown that the structural features of F(k) can be considerably enhanced by taking its Laplacian, ∇2F(k). Roots and extrema of this function can be assigned to different atomic shells. Linear relationships between the position of the roots and extrema and the atomic number Z are found for the neutral atoms with Z≤92.
Rotational branching ratios and photoelectron angular distributions in resonance enhanced multiphoton ionization of HBr via the F 1Δ2 Rydberg state
Authors: Wang Kwanghsi , McKoy V.
Published: July 2002

Results of theoretical studies of rotational ion distributions in the X 2Π1/2 ground state of HBr+ resulting from (2+1) resonance enhanced multiphoton ionization (REMPI) via the S(2) branch of the F 1Δ2 Rydberg state are reported. These results show a strongly parity-favored ion distribution with about 80% population in the (−) component of the Λ doublet of J+ rotational levels. The 20% population in the other parity component of the Λ doublet can be seen to be due to odd partial wave contributions to the photoelectron matrix elements which arise primarily from non-atomic-like behavior of the electronic continuum. This, in turn, is due to angular momentum coupling in the photoelectron orbital brought about by the torques of the nonspherical molecular ion potential. We demonstrate that the effect of alignment on these ion distributions, although not large, is important. Photoelectron angular distributions and alignment of the J levels of the HBr+ ions are also presented. Rotational branching ratios and photoelectron angular distributions resulting from (2+1′) REMPI of HBr via several S branches of the F 1Δ2 state are also shown for near-threshold photoelectron energies.
Electronic energy transfer in anisotropic systems. III. Monte Carlo simulations of energy migration in membranes
Authors: Johansson Lennart B.-Å. , Engström Sven , Lindberg Maria
Published: July 2002

Electronic energy migration in membrane systems has been studied by means of Monte Carlo (MC) simulations according to an algorithm described previously [S. Engström, M. Lindberg, and L. B.-Å. Johansson, Chem. Phys. 89, 204 (1988)]. In the systems investigated, the interacting donor molecules are randomly localized in mono-, bi-, and multilayers and are either oriented with their transition dipoles isotropically or parallel to the layers. The mean-square displacement [〈R(t)2〉] of the excitation and experimental observables in terms of different fluorescence depolarizations were determined. All results are relevant for the ‘‘slow case,’’ which means that translational and rotational motions of the donors are much slower as compared to the rate of fluorescence. A two-particle approximation for calculating the excitation probability of the initially excited molecule [denoted by Gs(t) ] and the fluorescence depolarizations in two-dimensional systems has been published previously [J. Baumann and M. D. Fayer, J. Chem. Phys. 85, 4087 (1986)]. By using the MC algorithm, we have in this work tested this model extensively.

The excitation probability Gs(t) is found to be in excellent agreement with the MC simulations for all of the systems studied. For isotropically oriented donor molecules in multilamellar systems, the simulations show that Gs(t) is very well approximated by that of a monolayer at distances of d≥3 R0 between the layers. At distances of d≤0.5 R0, the function Gs(t) is equal to that of a three-dimensional solution. For the in-plane oriented dipoles in a multilayer system, Gs(t) is very well approximated by that for a single bilayer at d>2 R0. In general, the depolarizations obtained by the two-particle model and MC simulations differ depending on the particular orientational distribution and the experimental geometry. To obtain a physically correct behavior of the fluorescence anisotropy at long times (i.e., the limiting anisotropy) is not possible within a two-particle approach. We instead propose a phenomenological model for calculating the fluorescence anisotropy which is a function of Gs(t) obtained within the two-particle approximation. This model gives a remarkably good agreement with the MC simulations and has the correct limiting anisotropy. The MC simulations of energy migration show that 〈R(t)2〉 is about twice as large for in-plane oriented donors in mono- and bilayers, as compared to the case of isotropically oriented transition dipoles. For multilayer systems consisting of layers separated by d=R0, we find that 〈R(t)2〉 is only slightly larger for the in-plane oriented transition dipoles as compared to the isotropic orientational distribution.
Temperature dependence of the inverted regime electron transfer kinetics of betaine-30 and the role of molecular modes
Authors: Åkesson Eva , Johnson Alan E. , Levinger Nancy E. , Walker Gilbert C. , DuBruil Thomas P. , Barbara Paul F.
Published: July 2002

The inverted regime photoinduced electron transfer kinetics of betaine-30 have been investigated over a broad temperature range, revealing very little temperature dependence. For example, for betaine-30 in a polystyrene film, the electron transfer rate constant, kET, changes by less than a factor of 3 from T=293 K to T=34 K. The results are in striking contrast to predictions of contemporary electron transfer theories which employ classical nuclear modes to accept some or all of the energy of the electron transfer event. The comparison of theory and experiment for the betaines demonstrates that a full quantum mechanical theory is necessary to accurately describe the electron transfer kinetics of the betaines in environments with slow dielectric relaxation. The conclusions drawn for the betaines may also apply to other molecular examples of inverted regime electron transfer in slowly relaxing environments.
Quantitative determination of molecular structure in multilayered thin films of biaxial and lower symmetry from photon spectroscopies. I. Reflection infrared vibrational spectroscopy
Authors: Parikh Atul N. , Allara David L.
Published: July 2002

A semitheoretical formalism based on classical electromagnetic wave theory has been developed for application to the quantitative treatment of reflection spectra from multilayered anisotropic films on both metallic and nonmetallic substrates. Both internal and external reflection experiments as well as transmission can be handled. The theory is valid for all wavelengths and is appropriate, therefore, for such experiments as x-ray reflectivity, uv–visible spectroscopic ellipsometry, and infrared reflection spectroscopy. Further, the theory is applicable to multilayered film structures of variable number of layers, each with any degree of anisotropy up to and including full biaxial symmetry. The reflectivities (and transmissivities) are obtained at each frequency by solving the wave propagation equations using a rigorous 4×4 transfer matrix method developed by Yeh in which the optical functions of each medium are described in the form of second rank (3×3) tensors. In order to obtain optical tensors for materials not readily available in single crystal form, a method has been developed to evaluate tensor elements from the complex scalar optical functions (n̂) obtained from the isotropic material with the limitations that the molecular excitations are well characterized and obey photon–dipole selection rules.

This method is intended primarily for infrared vibrational spectroscopy and involves quantitative decomposition of the isotropic imaginary optical function (k) spectrum into a sum of contributions from fundamental modes, the assignment of a direction in molecular coordinates to the transition dipole matrix elements for each mode, the appropriate scaling of each k vector component in surface coordinates according to a selected surface orientation of the molecule to give a diagonal im(n̂) tensor, and the calculation of the real(n̂) spectrum tensor elements by the Kramers–Kronig transformation. Tensors for other surface orientations are generated by an appropriate rotation matrix operation. To test the viability of this approach, three sets of experimentally derived infrared spectra of oriented monolayer assemblies on quite distinctively different substrates were chosen for simulation: (1) n-alkanethiols self- assembled onto gold, (2) n-alkanoic acid salt Langmuir–Blodgett (LB) monolayers on carbon, and (3) n-alkanoic acid salt LB monolayers on silica glass. The formalism developed was used to simulate the spectral response and to derive structural features of the monolayers. Good agreement was found where comparisons with independent studies could be made and, in general, the method appears quite useful for structural studies of highly organized thin films.
Path-integral Monte Carlo studies of para-hydrogen clusters
Authors: Scharf Daphna , Klein Michael L. , Martyna Glenn J.
Published: July 2002

Path-integral Monte Carlo calculations have been used to study para-hydrogen clusters, (p-H2)N, with N=13, 19, 33, and 34. Particular attention has been given to the low temperature structures and their evolution with increasing temperature. The structures of these quantum clusters have a clear relationship to their classical counterparts. In particular, the core of the N=19, 33, and 34 clusters is bipyramidal, in constrast to that of the N=13 cluster, which is icosahedral. At T∼2 K, these clusters exhibit a large propensity for exchanging particles, even without explicitly including exchange permutations (Bose statistics). This behavior has its origin in the large zero-point motion of the very weakly bound small p-H2 clusters.
A three-component model of the electrical double layer
Authors: Tang Zixiang , Scriven L. E. , Davis H. T.
Published: July 2002

A three-component model of the electrical double layer is studied using a free energy density functional theory, which incorporates in an appropriate way the excluded volume effects and the electrostatic correlations. The new model differs from the primitive model in that the solvent is represented as a dense fluid of neutral hard spheres instead of as merely a dielectric continuum. The interfacial structure shows much richer features in the three-component electrolyte solution than either in the primitive electrolyte or in the pure hard sphere fluid. In particular we observe the coexistence of the particle layerings and the charge inversion. The electrical double layer in the three-component model is of comparatively shorter range and has a lower electrostatic potential difference across the interface than in the primitive model. The inclusion of the solvent also has large effects on the thermodynamic properties of the electrical double layer.
Diffusion in porous systems and the influence of pore morphology in pulsed gradient spin-echo nuclear magnetic resonance studies
Authors: Callaghan P. T. , Coy A. , Halpin T. P. J. , MacGowan D. , Packer K. J. , Zelaya F. O.
Published: July 2002

We report a combined experimental, theoretical, and simulation study of pulsed gradient spin-echo (PGSE) nuclear magnetic resonance (NMR) for fluid saturated porous media. A simple pore hopping theory is developed on the basis of the assumption that diffusion within pores is very much faster than diffusion between pores. For suitable periodic media, the theoretical results are found to be in good agreement with random-walk simulations. The theory for glasslike media is then used to analyze experimental PGSE NMR data for a water-saturated random loose pack of nearly monodisperse polystyrene spheres. The structural parameters extracted by this method are consistent with the known geometry of such packings. An important observation from the simulations is that the long-time effective diffusion constant is already accessed at times so short that a single spin will only have diffused across one pore width.
A vector and parallel full configuration interaction algorithm
Authors: Bendazzoli Gian Luigi , Evangelisti Stefano
Published: July 2002

A full configuration interaction (FCI) algorithm is presented and discussed. It is an integral driven formalism based on the explicit construction of tables which realize the correspondence between the FCI vector x and the vector Hx, H being the Hamiltonian matrix of the system. In this way no decomposition of the identity is needed, and in the simplest implementation only the two vectors x and Hx need to be kept on disk. The main test has been done on the cyclic polyene C18H18 in the Pariser–Parr–Pople approximation, where the size of the FCI vector can be reduced to about 73 million components. Running on a CRAY Y-MP with 4 CPU and 32 MW of core memory, we obtained an elapsed CPU time per iteration of about 300 s and a total elapsed time of 1000 s, which correspond to about 4 and 14 s per million determinants, respectively. The parallel CPU speed-up obtained by running with the 4 CPU is greater than 3, without any substantial increasing of the memory or disk requirements.
The thermal equilibrium aspects of the time dependent Hartree and the locally enhanced sampling approximations: Formal properties, a correction, and computational examples for rare gas clusters
Authors: Ulitsky Alex , Elber Ron
Published: July 2002

The thermal properties of the ‘‘trajectory bundles’’ and the LES version of the classical Hartree approximation are investigated. A systematic protocol to find an effective Lagrangian from which the trajectory bundles equations of motion can be derived is introduced. It is shown that the trajectory bundles give incorrect virials. The errors are proportional to the number of copies. A correction is proposed that is based on a binary collision model. The corrected LES approximation is tested by calculating the following properties: (i) The expectation values of the kinetic energy and the virials; (ii) the spatial correlation function; (iii) the relaxation time of excess local heat to the ‘‘bath;’’ and (iv) the diffusion time for a He atom through a Xe cluster. The collision correction of LES (cLES) significantly improves the representation of the above four properties compared to exact calculations.
Dissociative charge-transfer reactions of Ar+ with simple aliphatic hydrocarbons at thermal energy
Authors: Tsuji Masaharu , Kouno Hiroyuki , Matsumura Ken-ichi , Funatsu Tsuyoshi , Nishimura Yukio , Obase Hiroshi , Kugishima Hirofumi , Yoshida Kouichi
Published: July 2002

A flowing-afterglow apparatus coupled with a low pressure chamber has been used to measure product ion distributions and rate constants in the charge-transfer reactions of Ar+ with CH4, C2Hn(n=2,4,6), and C3Hn(n=6,8) at thermal energy. Only parent cation is formed for C2H2 due to energy restriction. Major product channels are dissociative charge transfer followed by cleavage of C–H bond(s) for CH4, C2H4, C2H6, and C3H6, while by cleavage of a C–C bond for C3H8. A comparison of the product ion distributions with the photoelectron–photoion coincidence data for CH4, C2H4, and C2H6 leads us to conclude that the mean energies of precursor (pre)dissociative states are 15.3–15.5 eV, which are 0.3–0.5 eV below the resonance states. Thus the fractions of available energy deposited into internal modes of precursor parent ions at the instant of charge transfer are estimated to be 85%–95%, indicating that most of the CT reactions occurs without significant momentum transfer. The total rate constants for CH4, C2Hn(n=4,6), and C3Hn(n=6,8) are (0.78–1.1)×10−9 cm3 s−1, corresponding to 60%–92% of the calculated values from the Langevin theory. The rate constant for C2H2, 4.2×10−10 cm3 s−1, amounts to 38% of the kcalcd value. The small kobsd/kcalcd ratio is attributed to the lack of ionic states with favorable Franck–Condon factors for ionization.
Theory of surface enhanced Raman scattering in colloids
Authors: Rojas Roberto , Claro F.
Published: July 2002

We present a theory of surface enhanced Raman scattering (SERS) that gives a response enhancement of up to ten orders of magnitude for pyridine on silver. The signal obtained depends on the laser frequency and is consistent with available experimental data. Our theory emphasizes the effect of the molecule itself on the polarization of the environment as well as the effect of high multipoles, required by the nonuniformity of the local fields. The host could be metallic or insulating. We explain the laser frequency dependence of the relative size of Raman signals for different vibrational lines and predict the presence of a doublet at each electromagnetic resonance. The effect on the Raman signal of depositing a metallic or an oxide coating over a metallic host is discussed.
Finite element analysis of DNA supercoiling
Authors: Yang Yang , Tobias Irwin , Olson Wilma K.
Published: July 2002

A DNA polymer with hundreds or thousands of base pairs is modeled as a thin elastic rod. To find the equilibrium configurations and associated elastic energies as a function of linking number difference (ΔLk), a finite element scheme based on Kirchhoff’s rod theory is newly formulated so as to be able to treat self-contact. The numerical results obtained here agree well with those already published, both analytical and numerical, but a much more detailed picture emerges of the several equilibrium states which can exist for a given ΔLk. Of particular interest is the discovery of interwound states having odd integral values of the writhing number and very small twist energy. The existence of a noncircular cross section, inhomogeneous elastic constants, intrinsic curvature, and sequence-dependent bending and twisting can all be readily incorporated into the new formalism.
Classical trajectory simulation of the cluster–atom association reaction I–Arn+I→I2+nAr. I. Capture of iodine by the I(Ar)12 cluster
Authors: Hu Xiche , Martens Craig C.
Published: July 2002

The atom–cluster association reaction I(Ar)n+I→I2+nAr (n=12) is studied theoretically as a prototypical model of the effect of microscopic solvation on reaction dynamics. Classical trajectory methods are employed to model the dynamics. This paper focuses on the initial capture of I by the I(Ar)12 cluster. Two distinct minimum energy configurations for I(Ar)12 are considered: Ar6(I)Ar6, an icosahedron with I located at the center of the cluster; and IAr12, an icosahedron with I replacing one of the vertex Ar atoms. Both the structure and the temperature dependence of the capture cross section are investigated. Capture rate constants at temperatures of 10 and 30 K are computed. Capture cross sections for Ar6(I)Ar6+I predicted by a Langevin model agree well with those computed by classical trajectory simulation, revealing that the capture process under investigation is determined by the long range interaction potential. In comparison with its gas phase counterpart I+I, Ar6(I)Ar6+I has a much larger capture cross section. One of the most important roles played by the microscopic solvation of chemical reactants in clusters is this enhancement of the cross section for the initial capture process.
The COS2 potential energy surface: Aspects of the lowest singlet and triplet potential energy surfaces for the reaction of oxygen atoms with carbon disulfide
Authors: Froese Robert D. J. , Goddard John D.
Published: July 2002

Portions of the lowest singlet and triplet potential energy surfaces for the reaction of oxygen atoms with carbon disulfide O(3P)+CS2(1Σg+)→CS(1Σ+)+SO(3Σ−), →OCS(1Σ+)+S(3P),→CO(1Σ+)+S2(3Σg−) were investigated by 6-31G* ab initio self-consistent-field methods and with the inclusion of electron correlation by Mo/ller–Plesset perturbation theory. Earlier experiments and the present ab initio results indicate that the dominant reaction pathway is the direct stripping mechanism leading to the CS and SO diatomic products. The ab initio results support the experimentally observed branching ratios as a relatively simple mechanism was located for the formation of CS and SO, while more complex paths with higher barriers lead to the OCS and S or CO and S2 products. The most stable structure on either the singlet or triplet surface is singlet dithiiranone, a three-membered CSS ring with a carbonyl group. As the predominant reaction forms CS(1Σ+)+SO(3Σ−), the triplet cis or trans carbon disulfide S-oxide molecules are probable intermediates.
A new graphical representation for dielectric data
Authors: Wei Yan-Zhen , Sridhar S.
Published: July 2002

A method of graphically representing complex dielectric data is described, which is particularly useful for situations where the Cole–Cole representation is unsuitable. The representation emphasizes the orientational dipolar contribution to the conductivity by plotting σχ″=ωεv(ε′−ε∞) vs σχ′=ωεvε″. We discuss the utility of the representation for conducting dielectrics and for studying multirelaxation processes, particularly modes with smaller amplitudes (absorption) at high frequencies. For a Debye relaxation with dc conductivity, this representation leads to a semicircle, whereas the Cole–Cole plot shows a divergence. Even in the absence of conductivity, this representation is useful particularly at high frequencies, where it enables the identification of multirelaxation processes not apparent in the Cole–Cole representation. A particularly striking example is the clear observation of two relaxation processes in dielectric data on 1-propanol. We also show that in this σχ representation, all the well-known functional forms such as Cole–Cole and Cole–Davidson, approach an asymptotic slope at high frequencies. Applications of the σχ representation to analysis of dielectric spectra taken up to 20 GHz are discussed for pure glycerol and LiCl/propanol solutions.
A small angle neutron scattering study of density fluctuations at near-critical region and a van der Waals model in a three-component microemulsion
Authors: Seto Hideki , Schwahn Dietmar , Mortensen Kell , Komura Shigehiro
Published: July 2002

A small angle neutron scattering investigation near the critical point of D2O/n-decane/AOT water-in-oil microemulsion was carried out. Although the previous results in literature indicated the Ising behavior, the meanfield behavior for the susceptibility was observed. From the analysis of the susceptibility divergence, binodal point and spinodal point for each composition were given. In order to explain these points, the Gibbs free energy of mixing with the Flory–Huggins interaction parameter is introduced in the framework of the van der Waals model.
Consistent dielectric properties of the simple point charge and extended simple point charge water models at 277 and 300 K
Authors: Smith Paul E. , van Gunsteren Wilfred F.
Published: July 2002

The dielectric constants (ε) of the simple point charge (SPC) and extended simple point charge (SPC/E) models of liquid water have been determined at 277 and 300 K using a reaction field (εRF) treatment for the long-range electrostatic interaction. Consistent dielectric constants are obtained by using statistical mechanical perturbation theory, such that the properties of the system correspond to εRF=ε, and are therefore representative of a true infinite system. The dielectric constants of SPC were determined to be 55.8 and 54.0 at 277 and 300 K, respectively, while the dielectric constants of SPC/E were 68.5 and 62.3 at 277 and 300 K, respectively.
The vibronic structure of the S↔S1 and S↔S2 transitions in simple oligomers of thiophene
Authors: Negri Fabrizia , Zgierski Marek Z.
Published: July 2002

We present a quantum-chemical study of the electronic spectra of the oligomers of thiophene. Geometries and vibrational force fields of the ground and excited electronic states are obtained by an updated version of the semiempirical quantum consistent force field/π electron method implemented to describe sulphur atoms and by ab initio Hartree–Fock and configuration interaction singles methods. The displacement parameters of totally symmetric modes are then obtained and used to model the vibrational structure of the electronic spectra. The contribution of sulphur atoms to the description of the excited state is predicted to be negligible both by ab initio and semiempirical methods which, conversely, indicate a close similarity of thiophene oligomers and polyenes. Based on the results of the simulated spectra a reassignment of some of the bands is proposed. It is shown that mode mixing upon excitation, and not large frequency changes, are responsible for the different Franck–Condon structure of the absorption and emission spectra. In addition, a vibronic coupling mechanism analogous to that active in simple polyenes is identified. It accounts for the ‘‘apparent’’ frequency increase of the most active ag mode upon excitation to the 1B state.
Production of the electronically excited NaCd excimer via resonant excitation of the metastable Cd(5p 3P1) level
Authors: Gruber D. , Domiaty U. , Windholz L. , Jäger H. , Musso M. , Allegrini M. , Fuso F. , Winkler A.
Published: July 2002

We report the first evidence for the production of the electronically excited NaCd excimer following resonant excitation of the metastable Cd(5p 3P1) state. Dependence of intensity of emission from NaCd upon buffer gas pressure, temperature, and exciting laser wavelength is discussed and possible production mechanisms are proposed.
On the assignment of Jahn–Teller effects in the ultraviolet absorption spectrum of Ag3
Authors: Wedum E. E. , Grant E. R. , Cheng P. Y. , Willey K. F. , Duncan M. A.
Published: July 2002

A Jahn–Teller linear-plus-quadratic Hamiltonian is shown to account for most of the observed band positions and intensities in the absorption and emission spectra of Ag3. Coupling parameters obtained for a simultaneous fit to absorption and emission results are k=1.93, g=0.25 for the E′ ground state and k=0.19, g=0.02 for the E″ excited state. At higher vibrational energies, simple Jahn–Teller calculations predict fewer bands than observed. Calculations including spin–orbit coupling with larger linear coupling partially quenched by the Ham effect offer a reasonable explanation for this higher energy structure. Splittings and intensity sharing at lower vibrational energies, however, demand higher-order Jahn–Teller coupling, indicating the need for more extensive calculation simultaneously incorporating quadratic Jahn–Teller effects, spin–orbit coupling, and perhaps anharmonicity.
Structure and fundamental vibrations of phenoxyl radical
Authors: Chipman Daniel M. , Liu Ruifeng , Zhou Xuefeng , Pulay Peter
Published: July 2002

The geometry and harmonic force field of the phenoxyl radical are determined by ab initio calculations of the electronic structure. All the π electrons are involved in strong nondynamical correlations, which are treated in a complete π active space wave function. Results from using multiconfigurational self-consistent-field orbitals in the active space are found to be closely approximated by use of the more economical unrestricted Hartree–Fock natural orbitals. Large polarized basis sets are also required to obtain qualitatively correct results. Properties of the carbonyl chromophore are particularly interesting. Compared to closed-shell analogs, the equilibrium geometry indicates the CO bond in phenoxyl to be nearly as short as a typical double bond, whereas the scaled quantum-mechanical force field shows it to have strength intermediate between those of typical single and double bonds. The calculated fundamental vibrational frequencies generally confirm current interpretations of the major features of the observed experimental resonance Raman spectra, while leading to reassignments of some minor features. Some limited theoretical characterization of valence excited states provides qualitative explanation of the observed intensity pattern. Calculations carried out at lower levels of theory consistently find the CO bond to be too long and weak, leading to misassignment of the observed vibrational spectrum.
Ab initio studies of the polarizabilities of retinal analogs
Authors: Toto Joseph L. , Toto Teressa Tangredi , de Melo Celso P. , Robins K. A.
Published: July 2002

Axial components of the polarizability, hyperpolarizability, and second order hyperpolarizability of retinal model molecules are calculated using the ab initio coupled-perturbed Hartree–Fock theory. Conformational rearrangements are shown to affect some components in a systematic fashion. We also find that the general trend in the hyperpolarizability behavior as a function of protonation and charge transfer is in agreement with available experimental data. The second hyperpolarizability of the protonated Schiff base is found to be highly dependent on the adopted geometry optimization. We explore a possible explanation for this as well as present a brief discussion with regard to future experimental and theoretical developments towards the construction of molecularly based nonlinear optical devices.
The Lifshitz line in binary systems: Structures in water/C4E1 mixtures
Authors: Koehler R. D. , Schubert K.-V. , Strey R. , Kaler E. W.
Published: July 2002

In a recent publication we compared phase behavior and scattering data obtained from SANS in water, n-alkane, n-alkyl polyglycol ether (CiEj) mixtures. By analyzing the scattering spectra using the Teubner–Strey formula, it is possible to determine a measure for the amphiphilic strength of each system called the amphiphilicity factor, fa. It was demonstrated that the amphiphilicity factor is constant on a variety of surfaces within the three-dimensional space of composition and temperature. For instance, the Lifshitz surface, where fa=0, depends on composition and temperature and may be a precursor for the Lα phase. Here we show that the same sequence exists in the binary system water/C4E1, where the surfaces become lines in the two-dimensional composition-temperature planes. Variations in amphiphile concentration and solution temperature allow one to reach almost the entire accessible amphiphilicity scale (∞≳fa≳−1) within this simple binary mixture. The occurrence of the Lifshitz surface at high C4E1 concentrations and its disappearance as a function of temperature give more evidence that the Lifshitz region can be treated as a precursor of the lamellar liquid crystalline phase. While earlier investigations of water/C4E1 solutions in the dilute regime support the existence of micellar aggregates, the current work provides the first definitive evidence of the presence of supramolecular aggregates and short range order in the water/C4E1 system in the amphiphile-rich region.
Differences between L3 and L2 x-ray absorption spectra of transition metal compounds
Authors: de Groot F. M. F. , Hu Z. W. , Lopez M. F. , Kaindl G. , Guillot F. , Tronc M.
Published: July 2002

The differences between L3 and L2 edges of 3d and 4d transition metal complexes and compounds in octahedral symmetry are discussed. The main origin of these differences are the multiplet effects due to the coupling of the 2p core wave function and the 3d and 4d valence wave functions. The 3d and 4d spin–orbit coupling is a second origin of difference. For 3d systems the multiplet effects dominate all other interactions and the L3 and L2 edge are completely mixed and reordered. For 4d systems the core hole spin–orbit coupling is large and the L3 and L2 are separated by about 100 eV with a ratio close to 2:1. The differences between the L3 and L2 edge originate from the weight transfer between the t2g and eg peaks due to the multiplet effect. This weight transfer is about 25% for the L3 edge and about 5% for the L2 edge, which implies that for a comparison to single-particle calculations the L2 edge is preferable to use. Partly filled 4d systems are low-spin and the occupation of the t2g states implies a decrease of the first peak. This decrease is stronger for the L2 edge, implying an increase in the L3:L2 ratio. For 4d5 systems transitions to the t2g hole are only possible at the L3 edge due to the combined effects of 4d spin–orbit coupling and the dd multiplet effects.
Shear viscosity for fluids of hard ellipsoids: A kinetic theory and molecular dynamics study
Authors: Tang Shuang , Evans Glenn T. , Mason Carl P. , Allen Michael P.
Published: July 2002

The frequency-dependent shear viscosity η(ω) and the shear-orientation coupling parameter R are calculated for fluids of hard prolate ellipsoids of revolution, using kinetic theory and molecular dynamics. Two effects which contribute to the shear viscosity are considered. First, the D∞h particle symmetry influences both the spectrum of the collision operator and the multi-Lorentzian decay of η(ω). These multiple decays are most important at low densities for elongated particles. Second, coupling between the stress tensor and the collective molecular orientation leads to a minimum in η(ω) between its zero-frequency limit η and its infinite-frequency plateau value η∞. At densities approaching the transition to a nematic phase, the zero-frequency shear viscosity is predicted to be dominated by its infinite-frequency plateau value, i.e., η/η∞∼1. Molecular dynamics simulations, extended to longer times than those previously reported, confirm the existence of the dip in η(ω) and the importance of stress-orientation coupling in determining the value of η. Discrepancies previously thought to exist between simulation and theory are attributed to this coupling between the stress and slow collective reorientation; we now obtain good agreement between simulation and theory at low and moderate densities. The kinetic theory predicts the position of the dip quite well, but underestimates its magnitude for elongated particles at medium and high densities.

The theory also predicts a weak density dependence of the shear-orientation coupling parameter R, and a decrease in this parameter with increasing elongation, whereas the simulations show a steady increase in R with both density and elongation. Moreover, the ratio η/η∞ falls below unity, rather than approaching unity, as the density increases, in several of the cases studied. The Stokes–Einstein relation and the Stokes–Einstein–Debye relation are investigated, and a consistent picture emerges of a crossover between a regime in which kinetic theory is accurate (at low density and elongation) to one in which a hydrodynamic description with slip boundary conditions applies (at high density and elongation). The change to hydrodynamic behavior is more apparent for the rotational motion than for the translational motion.
Absorption spectroscopy and photodissociation dynamics of small helium cluster ions
Authors: Haberland H. , Issendorff B. v. , Fröchtenicht R. , Toennies J. P.
Published: July 2002

The optical absorption of size-selected helium cluster ions was studied via photofragmentation spectroscopy. Absorption cross sections were measured for He+n (n=3, 4, 10, 21, and 30). A broad absorption peak was found, which for He+3 is centered at ≊5.3 eV, and which with increasing cluster size shifts slightly to the red. In addition, the kinetic energy release to the ionized and neutral photofragments was measured by a time-of-flight technique for cluster sizes between n=3 and 10. From the energy balance the total binding energy of the He+3 trimer ion was determined to be 2.6±0.15 eV. The results further indicate that a charged linear trimer acts as a core molecule for the sizes n=4–7. For n≳7, an additional isomer with a tetramer core is identified. The results are compared with recent ab initio calculations.
Photoelectron spectroscopy of AlnS1− clusters (n=1−9)
Authors: Nakajima Atsushi , Taguwa Tetsuya , Nakao Kojiro , Hoshino Kuniyoshi , Iwata Suehiro , Kaya Koji
Published: July 2002

Photoelectron spectra of AlnS−1 (1≤n≤9) cluster anions were measured at the photon energies of 3.49 and 4.66 eV, using a magnetic bottle electron spectrometer having ∼60 meV resolution. AlS− has an electron affinity of as large as 2.6 eV, attributed to large stability by six bonding electrons. A new electronic state, A′ 2Π, was found 0.4 eV above the AlS ground state, which is in agreement with theoretical calculations. AlnS−1 cluster anions are produced efficiently up to n=5, but the production of larger cluster sizes is found difficult. The mass distribution of AlnS−1 cluster anions and electron binding energy can be explained by a geometric structure obtained by theoretical calculation.
Studies of an off-lattice model for protein folding: Sequence dependence and improved sampling at finite temperature
Authors: Irbäck Anders , Potthast Frank
Published: July 2002

We study the thermodynamic behavior of a simple off-lattice model for protein folding. The model is two dimensional and has two different ‘‘amino acids.’’ Using numerical simulations of all chains containing eight or ten monomers, we examine the sequence dependence at a fixed temperature. It is shown that only a few of the chains exist in unique folded state at this temperature, and the energy level spectra of chains with different types of behavior are compared. Furthermore, we use this model as a testbed for two improved Monte Carlo algorithms. Both algorithms are based on letting some parameter of the model become a dynamical variable; one of the algorithms uses a fluctuating temperature and the other a fluctuating monomer sequence. We find that by these algorithms one gains large factors in efficiency in comparison with conventional methods.
Phase equilibria in extended simple point charge ice-water systems
Authors: Báez Luis A. , Clancy Paulette
Published: July 2002

The characteristics of the solid/liquid transition for a modified Simple Point Charge model of water have been determined using free energy calculations supported by nonequilibrium Molecular Dynamics (NEMD) simulations. We have considered the behavior of liquid water and of a variety of ice phases. Unlike real water, the stable crystalline phase at 1 bar is not hexagonal ice I, but a denser new ice phase. The melting point of this ice was found to be near 295 K. The lower-density ices, Ih and Ic, are less stable than water down to the glass transition temperature. The conclusions are supported by NEMD simulations of the behavior of the planar crystal–liquid interface for these different cases. The first report of the growth of ice from water using molecular simulation is shown here. The influence of the components of the intermolecular potential on the stability of the ice polymorphs is investigated. It is found that, for ice I to be the stable phase, the Lennard–Jones attractive part should be reduced, and the potential switching function should be used at longer distances. Properties of the supercooled liquid water are also presented.
Direct observation of the intersystem crossing in poly(3-octylthiophene)
Authors: Kraabel B. , Moses D. , Heeger A. J.
Published: July 2002

Using subpicosecond photoinduced absorption, we monitor the evolution of singlet excitons in solutions of regiorandom poly(3-octylthiophene) (P3OT), regioregular poly(3-hexylthiophene) (P3HT), and a well-defined α-oligothiophene with 12 repeat units (T12). We find a luminescence lifetime of 500 ps, and we observe the intersystem crossing from the singlet manifold to the triplet manifold, with a rate constant of k−1ISC=1.2 ns. By measuring the quantum efficiency of luminescence, we estimate an intrinsic lifetime of 2 ns for the singlet excitons, and a nonradiative decay rate of k−1nr=1.5 ns. We find no difference in the intersystem crossing time for the different thiophene derivatives, implying the intersystem crossing rate is determined primarily by the relatively large spin–orbit interaction due to the sulfur heteroatom and not by chain defects, chain ends, or effects due to the side groups. In addition, we find that for t<50 ps a fraction of the photoexcitations undergo one-dimensional diffusion limited recombination.