bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface chemistry of metastable oxygen. I. Production and loss of the 4–5 eV states
Authors: Sharpless R. L. , Jusinski L. E. , Slanger T. G.
Published: July 2002

In an attempt to isolate the important parameters governing the production of the 4–5 eV states of O2 generated from O-atom recombination on metallic surfaces, we have investigated the effects of substrate composition and temperature on the intensity of the UV glows that are generated. Of the 22 metals investigated, glow-producing activity is localized in the VIIIb group on the Periodic Table, led by Ni and including Pt, Co, Pd, Fe, and also Au. One parameter that appears to be directly or indirectly linked to this effect is the electronic heat capacity. The six active metals exhibit unique temperature signatures, with Fe, Ni, and Co being distinctly different from Pd, Pt, and Au, measured over the 220–470 K range. The spectra of the UV glows, consisting of the A–X, A′–a, and c–X transitions of O2, differ only in intensity between the metals, not in state or vibrational level distributions. This suggests that the observed spectra are mediated by gas-phase interactions. The data indicate that at high temperatures, surface quenching of the A, A′, and c states is rapid, and that under low-pressure conditions, this effect can be observed even for excited molecules generated in the gas phase. Studies with nickel alloys indicate that the percentage of Ni in a sample is the overriding determinant of the glow intensity, regardless of the other components. Although existing data on energy accommodation coefficients lead to the conclusion that most of the energy in O2 molecules leaving surfaces should be in vibrational modes of the ground state, we have not been able to substantiate this idea.
Molecular theory of curvature elasticity in surfactant films
Authors: Szleifer Igal , Kramer Diego , Ben-Shaul Avinoam , Gelbart William M. , Safran S. A.
Published: July 2002

We develop a microscopic-level formulation for the curvature elasticity of monolayer and bilayer systems of typical surfactant molecules. It is argued that both the bending and saddle-splay force constants k and k̄ are determined primarily by the conformational entropy of the flexible hydrocarbon chain rather than by the electrostatic interactions associated with hydrophilic head groups. A priori estimates of the chain contributions are made for the first time, without the use of any adjustable parameters. Both k and k̄ are shown to be calculable wholly from the conformational statistics describing the planar film. In particular, these constants are expressed in terms of the derivatives and moments of the lateral pressure profile characterizing chain packing in the unbent layers. By considering the dependence of the curvature elasticity on chain length, area per molecule, and composition in mixed films, we are able to account for the order-of-magnitude variations in k observed in a variety of different surfactant systems. The replacement of long chain molecules by short ones is shown to be especially efficient in lowering the bending energy from 10’s of kBT to kBT. The effect of ‘‘free’’ vs ‘‘blocked’’ exchange are also presented and contrasted with the case of fixed area-per-molecule bending deformation. Finally, monolayer vs bilayer results are compared and the calculated signs and magnitudes of k and k̄ are discussed in the context of planar bilayer stability.
Laser-induced fluorescence measurements of drift-velocity distributions for Ba+ in Ar: Moment analysis and a direct measure of skewness
Authors: Penn Stephen M. , Beijers Johannes P. M. , Dressler Rainer A. , Bierbaum Veronica M. , Leone Stephen R.
Published: July 2002

Velocity distributions for Ba+ ions drifting in argon under the influence of an electric field are measured using single-frequency, laser-induced fluorescence techniques. A moment analysis of these data yields values for the ion mobilities, temperatures, and, for the first time, skewness parameters of the ion velocity distributions. Doppler profiles of the drifting ions are obtained with the laser beam propagating both parallel and perpendicular to the electric field direction as a function of E/N up to 148 Td. The velocity-component distributions perpendicular to the electric field direction are nearly Maxwellian at all E/N values studied, but the parallel distributions become noticeably skewed toward high velocities at the largest electric field strengths. The reduced mobilities for Ba+ in Ar, obtained from the first moments of the parallel velocity-component distributions, increase from 1.80±0.16 cm2 V−1 s−1 at zero field to 2.38±0.11 cm2 V−1 s−1 at 148 Td. The second central moments, which describe the widths of the distributions, increase monotonically with increasing E/N values for both the parallel and perpendicular distributions. Temperatures derived from the second central moments agree well with those calculated from a parametrized version of the three-temperature theory [M. Waldman and E. A. Mason, Chem. Phys. 58, 121 (1981)]. The third central moments of the parallel velocity-component distributions characterize the asymmetry of the distributions. Skewness parameters derived from the third central moments increase with electric field strength over the range of E/N studied.
Computer simulations of domain growth under steady shear flow
Authors: Ohta Takao , Nozaki Hanae , Doi Masao
Published: July 2002

Computer simulations of domain growth under shear flow in binary fluids are performed by means of the cell dynamic approach in two dimensions. The hydrodynamic interaction is entirely ignored in the present simulations. The time-evolution of both a single circular domain and randomly distributed domains undergoing phase separation is investigated by changing the shear rate S. We observe deformation and burst of the single domain and tearing and recombination of the interconnected domains. For the random domains with the volume fractions φ=0.5 and φ=0.3, the shear viscosity Δη and the normal stress ΔN1 are obtained as a function of the shear strain γ. These results are compared with our theoretical consideration. The anisotropic scattering function is also obtained.
Theoretical study of the spectroscopy of NO+
Authors: Partridge Harry , Langhoff Stephen R. , Bauschlicher Charles W.
Published: July 2002

The spectroscopy of the NO+ molecule has been studied using multireference configuration-interaction calculations. The peak at 21.7 eV in the photoelectron spectrum of NO is assigned to the (2) 3Π state (c 3Π) and the vibrational progression centered at 23.1 eV is assigned to the diabatic (3) 1Π state (B 1Π). Several additional bound states with binding energies in excess of 1 eV are characterized in the present work. Radiative lifetimes are presented for many of the states.
Theoretical characterization of tetrahedral N4
Authors: Lee Timothy J. , Rice Julia E.
Published: July 2002

The tetrahedral (Td) form of the N4 molecule has been investigated via ab initio quantum mechanical methods. The lowest triplet state of N4 in the tetrahedral region of the potential energy surface, and the barrier separating TdN4 from two N2 molecules have also been examined. Both the transition state and 3A″ state have Cs symmetry. The singlet state of tetrahedral N4 is predicted to lie 13 kcal/mol below the 3A″ state. The energy difference between TdN4 and two N2 molecules is accurately determined to be 186 kcal/mol while the barrier separating these species is found to be about 61 kcal/mol. The possibility that TdN4 may be used as a high energy density fuel is investigated and discussed. In addition, a highly accurate determination of the equilibrium structure, harmonic vibrational frequencies, and infrared intensities of TdN4 has been performed using large atomic natural orbital (ANO) basis sets in conjunction with the coupled-cluster single and double (CCSD) excitation level of theory and the CCSD(T) extension. The largest ANO basis sets used for N4 contain f-type functions. Analogous studies of the N2 molecule are presented in order to judge the reliability of the theoretical predictions for the experimentally unknown TdN4.
Dynamics of Frenkel excitons in disordered molecular aggregates
Authors: Fidder Henk , Terpstra Jacob , Wiersma Douwe A.
Published: July 2002

This article reports on the optical dynamics in aggregates of pseudoisocyanine-bromide and iodide. For PIC-Br in an ethylene glycol/water glass, the results of resonance light scattering (RLS), time-resolved emission, and photon echo decay measurements are discussed. Band structure calculations based on a linear-chain model for the J aggregate have also been performed. The results show that the J band can be described as a disordered Frenkel exciton band in which superradiant states exist that extend over about 100 molecules. Numerical simulation studies of the J band, based on Anderson’s Hamiltonian with uncorrelated diagonal site energies, show that the ratio κ of the disorder parameter D over the nearest-neighbor coupling parameter J12 is about 0.11. Using the frequency dependence of the ratio between the yields of vibrational fluorescence and Raman scattering as a probe, the dephasing process and derived parameters for the bath correlation function at three different temperatures have also been examined. It is shown that at all temperatures the exciton dephasing process occurs in the fast modulation limit. For PIC-I in a Langmuir–Blodgett film the optical excitation can be described also in terms of a band transition but the disorder is much larger than in a glass. For this system, a low-temperature fluorescence lifetime of about 10 ps is measured, which suggests that the excitation is much more delocalized than in the case of self-assembled aggregates in a glass. Resonance Rayleigh scattering experiments on PIC in a bilayer show that the exciton-dephasing rate increases dramatically at energies above the renormalized band edge.
Complete active space self-consistent field potential energy surfaces, dipole moment functions, and spectroscopic properties of O3, CF2, NO−2, and NF+2
Authors: Peterson Kirk A. , Mayrhofer Rudolph C. , Sibert Edwin L. , Woods R. Claude
Published: July 2002

Three-dimensional potential energy and dipole moment surfaces have been calculated for the 24 electron triatomics O3, CF2, NO−2, and NF+2 using complete active space self-consistent field wave functions (CASSCF) and a basis set of 87 (99 for NO−2) contracted Gaussian-type orbitals (cGTOs). The analytical potential energy functions (PEFs) have been used in perturbation and variational calculations of anharmonic spectroscopic constants and vibrational energy levels. The results for O3 and CF2 are compared to the available experimental data, and predictions of the rotational and vibrational spectra of NO−2 and NF+2 have been made by comparison to these species. The equilibrium geometries of NO−2 and NF+2 are predicted to be re=1.260 Å, θe=116.5° and re=1.242 Å, θe=108.0°, respectively. The fundamental vibrational frequencies (ν1,ν2,ν3) and absolute band intensities (at 300 K) of these two ions are predicted to be 1286 cm−1/31 cm−2 atm−1, 782 cm−1/13 cm−2 atm−1, 1232 cm−1/2900 cm−2 atm−1 (NO−2) and 1272 cm−1/181 cm−2 atm−1, 711 cm−1/25 cm−2 atm−1, 1210 cm−1/1230 cm−2 atm−1 (NF+2). Band intensities for several low-lying vibrational transitions have been calculated for all four species from the CASSCF dipole moment functions, and comparisons to experiment have been made for O3 and CF2.
The reduced multiplication scheme of the Rys quadrature and new recurrence relations for auxiliary function based two-electron integral evaluation
Authors: Lindh R. , Ryu U. , Liu B.
Published: July 2002

The reduced multiplication scheme of the Rys quadrature is presented. The method is based on new ways in which the Rys quadrature can be developed if it is implemented together with the transfer equation applied to the contracted integrals. In parallel to the new scheme of the Rys quadrature improvements are suggested to the auxiliary function based algorithms. The two new methods have very favorable theoretical floating point operation (FLOP) counts as compared to other methods. It is noted that the only significant difference in performance of the two new methods is due to the vectorizability of the presented algorithms. In order to exhibit this, both methods were implemented in the integral program seward. Timings are presented for comparisons with other implemenations. Finally, it is demonstrated how the transfer equation in connection with the use of spherical harmonic Gaussians offers a very attractive path to compute the two-electron integrals of such basis functions. It is demonstrated both theoretically and with actual performance that the use of spherical harmonic Gaussians offers a clear advantage over the traditional evaluation of the two-electron integrals in the Cartesian Gaussian basis.
Reactive dynamics in a multispecies lattice-gas automaton
Authors: Kapral Raymond , Lawniczak Anna , Masiar Paul
Published: July 2002

A multispecies reactive lattice-gas automaton model is constructed and used to study chemical oscillations and pattern formation processes in a spatially distributed two-dimensional medium. Both steady state and oscillatory dynamics are explored. Nonequilibrium spatial structures are also investigated. The automaton simulations show the formation of rings of chemical excitation, spiral waves, and Turing patterns. Since the automaton model treats the dynamics at a mesoscopic level, fluctuations are included and nonequilibrium spatial structures can be investigated at a deeper level than reaction–diffusion equation descriptions.
Theoretical study of the bond dissociation energies of methanol
Authors: Bauschlicher Charles W. , Langhoff Stephen R. , Walch Stephen P.
Published: July 2002

We present a theoretical study of the bond dissociation energies (D0) for H2O and CH3OH. The C–H and O–H bond energies are computed accurately with the modified coupled-pair functional (MCPF) method using a large basis set. For these bonds, an accuracy of ±2 kcal/mol is achieved, which is consistent with the C–H and C–C single bond energies of other molecules. The C–O bond is much more difficult to compute accurately because it requires higher levels of correlation treatment and more extensive one-particle basis sets.
Nonlinear and non-Markovian fluctuation–dissipation processes: A Fokker–Planck treatment
Authors: Bianucci Marco , Grigolini Paolo
Published: July 2002

This paper is devoted to the derivation of the Fokker–Planck equation in a case where the external potential acting on the system and the coupling between system and bath are not harmonic. This problem has already the subject of many preceding investigations, which left open, however, the problem of deriving the Fokker–Planck equation with no assumption but the Born approximation. Within the present treatment the problem of the derivation of the Fokker–Planck equation is solved with no limitation on the bath time scale.
Photogeneration of ions via delocalized charge transfer states. I. Xe2H+ and Xe2D+ in solid Xe
Authors: Kunttu H. , Seetula J. , Räsänen M. , Apkarian V. A.
Published: July 2002

Delocalized charge-transfer excitations in solid xenon multiply doped with atomic halogens (I, Br, Cl) and hydrogen are demonstrated to lead to charge separation by trapping of the positive charge. As evidence of such a concept we present the first vibrational spectra of Xe2H+ and Xe2D+, which are believed to be vibrationally bound ions.
Laser vaporization generation of B 14NH, B 15NH, B 14ND, B 16O, and B 17O: Electron-spin-resonance investigation in neon matrices under ultracold trapping conditions
Authors: Knight Lon B. , Herlong John O. , Kirk Thomas J. , Arrington C. A.
Published: July 2002

Reactive laser vaporization conditions have been employed to generate various isotopic combinations of the new radical species, BNH, for neon matrix electron-spin-resonance (ESR) study. The BNH radical was found to have a linear geometry with an X 2Σ electronic ground state, in agreement with ab initio theoretical calculations, which were conducted as part of this experimental investigation. ESR results for the nuclear hyperfine interactions (A tensor) indicate that the electronic structure of BNH is quite similar to that of the isoelectronic BO radical. Results of a detailed reinvestigation of the 11,10B 16,17O radicals trapped in neon matrices are reported which now show excellent agreement with earlier gas-phase microwave spectroscopic measurements of the dipolar boron hyperfine interaction. The 17O A tensor for BO has been fully resolved for the first time into its isotropic and anisotropic components. A new ultracold neon trapping procedure is described which was used to produce matrix samples of randomly oriented and nonrotating radicals in order to measure the full extent of the dipolar nuclear hyperfine interaction. The neon matrix magnetic parameters for 11B 14NH are g∥=2.0020(5) and g⊥=2.0010(5), A∥=1089(1) and A⊥=1013(1) MHz for 11B, A∥=49(1) and A⊥=37(1) MHz for 14N, and A∥=29(1) and A⊥=23(1) MHz for H.
Molecular dynamics and Monte Carlo simulations in the grand canonical ensemble: Local versus global control
Authors: Papadopoulou Aphrodite , Becker Ezra D. , Lupkowski Mark , van Swol Frank
Published: July 2002

In inhomogeneous systems the diffusion paths available to particles are often restricted by the topology of the system itself. In search of a realistic scheme to simulate such inhomogeneous systems, we developed a new method, of local control of the chemical potential. The local control method, which limits creation and destruction of particles to a control volume, can be successfully implemented in both Monte Carlo (MC) and molecular dynamics (MD) simulations of the grand canonical ensemble (GC). We present applications of local GCMC calculations of the one-dimensional lattice gas and hard rods confined by hard walls, as well as local GCMC and GCMD of a three-dimensional bulk Lennard-Jones fluid system. The simulation results compare successfully with analytical solutions and equation of state approximations.
Factors affecting lifetimes and resolution of Rydberg states observed in zero-electron-kinetic-energy spectroscopy
Authors: Chupka William A.
Published: July 2002

It is shown that under the usual conditions of zero-electron-kinetic-energy, pulsed field ionization (ZEKE–PFI) spectroscopy the lifetimes of very high-lying Rydberg states are increased by at least approximately the factor n (in addition to the expected factor of n3), the principal quantum number, due to strong l mixing by the Stark effect. Additional factors may increase lifetimes by still another factor of approximately n. Pulsed field ionization under typical conditions is shown as likely to be predominantly diabatic and the effect on resolution is assessed. Factors affecting rotational line intensities are also discussed.
A general relation between the nucleation work and the size of the nucleus in multicomponent nucleation
Authors: Oxtoby David W. , Kashchiev Dimo
Published: July 2002

We prove a general theorem that relates the variation of the work of formation of the critical nucleus with chemical potential and the size and composition of the critical nucleus. Applications are made to multicomponent nucleation and to both isothermal and nonisothermal phase transformations. We show that the excess number of molecules and the excess entropy of the critical nucleus are thus accessible to experimental determination with the help of data for the dependence of the nucleation rate on supersaturation. The results derived do not rely on classical nucleation approximations and thus apply down to the smallest nuclei of a few molecules only.
Dissociation dynamics of H2O(D2O) following photoexcitation at the Lyman-α wavelength (121.6 nm)
Authors: Mordaunt D. H. , Ashfold M. N. R. , Dixon R. N.
Published: July 2002

The technique of H(D) atom photofragment translation spectroscopy has been used to investigate the collision free photodissociation of jet cooled H2O(D2O) molecules following excitation to their B̃(1A1) excited state at 121.6 nm. The resolution of the total kinetic energy release spectrum obtained with this technique, allows assignment of the eigenvalues for the individual rotational quantum states and an estimation of the respective quantum state population distributions for the nascent OH(X 2Π) and OH(A 2Σ+) photofragments (and their deuterated analogs). This provides us the first experimental observations of high angular momentum states of OD(X). Analysis of the quantum state population distribution show both the ground (X 2Π) and electronically excited (A 2Σ+) OH(OD) fragments to be formed with little vibrational excitation but with highly inverted rotational distributions. Spectral simulation enables estimation of relative branching ratios for these two dissociation channels, and for the three-body fragmentation yielding ground state atoms. The observed energy disposal has been rationalized by considering the motion of a wavepacket launched on the B̃ state surface at a geometry corresponding to the ground state equilibrium configuration. Electronically excited OH(OD) fragments result from that fraction of the photoexcited molecules that dissociate on the B̃ state surface; their rotational excitation results from the marked angular anisotropy of the B̃ state surface. Ground state OH(OD) fragments can arise as a result of radiationless transfer to the lower Ã(1B1) or X̃(1A1) surfaces. The wavepacket calculations show that B̃■X̃ transfer via the conical intersection linking these two surfaces leads to the most highly rotationally excited OH(OD) fragments. These calculations also show that the contribution made by B̃■Ã radiationless transfer to the overall rotational distribution in the ground state OH(OD) fragments scales with the amount of a-axis rotational excitation in the photoexcited molecules: The detailed form of the OH(OD) product state population distribution is thus predicted to be temperature dependent.
Molecular dynamics of conformational substates for a simplified protein model
Authors: Grubmüller Helmut , Tavan Paul
Published: July 2002

Extended molecular dynamics simulations covering a total of 0.232 μs have been carried out on a simplified protein model. Despite its simplified structure, that model exhibits properties similar to those of more realistic protein models. In particular, the model was found to undergo transitions between conformational substates at a time scale of several hundred picoseconds. The computed trajectories turned out to be sufficiently long as to permit a statistical analysis of that conformational dynamics. To check whether effective descriptions neglecting memory effects can reproduce the observed conformational dynamics, two stochastic models were studied. A one-dimensional Langevin effective potential model derived by elimination of subpicosecond dynamical processes could not describe the observed conformational transition rates. In contrast, a simple Markov model describing the transitions between but neglecting dynamical processes within conformational substates reproduced the observed distribution of first passage times. These findings suggest, that protein dynamics generally does not exhibit memory effects at time scales above a few hundred picoseconds, but confirms the existence of memory effects at a picosecond time scale.
Benchmark calculations with correlated molecular wave functions. VI. Second row A2 and first row/second row AB diatomic molecules
Authors: Woon David E. , Dunning Thom H.
Published: July 2002

Benchmark calculations employing the correlation consistent basis sets of Dunning and co-workers are reported for the following diatomic species: Al2, Si2, P2, S2, Cl2, SiS, PS, PN, PO, and SO. Internally contracted multireference configuration interaction (CMRCI) calculations (correlating valence electrons only) have been performed for each species. For Cl2, P2, and PN, calculations have also been carried out using Mo/ller–Plesset perturbation theory (MP2, MP3, MP4) and the singles and doubles coupled-cluster method with and without perturbative triples [CCSD, CCSD(T)]. Spectroscopic constants and dissociation energies are reported for the ground state of each species. In addition, the low-lying excited states of Al2 and Si2 have been investigated. Estimated complete basis set (CBS) limits for the dissociation energies, De, and other spectroscopic constants are obtained from simple exponential extrapolations of the computed quantities. At the CBS limit the root-mean-square (rms) error in De for the CMRCI calculations, the intrinsic error, on the ten species considered here is 3.9 kcal/mol; for re the rms intrinsic error is 0.009 Å, and for ωe it is 5.1 cm−1.
A multireference configuration interaction study of the hyperfine structure of the molecules CCO, CNN, and NCN in their triplet ground states
Authors: Suter H. U. , Huang M.-B. , Engels B.
Published: July 2002

The hyperfine structures of the isoelectronic molecules CCO, CNN, and NCN in their triplet ground states (X 3Σ−) are investigated by means of ab initio methods. The infrared frequencies and geometries are determined and compared with experiment. Configuration selected multireference configuration interaction calculations in combination with perturbation theory to correct the wave function (MRD-CI/BK) employing extended atomic orbital (AO) basis sets yielded very accurate hyperfine properties. The theoretical values for CCO are in excellent agreement with the experimental values determined by Smith and Weltner [J. Chem. Phys. 62, 4592 (1975)]. For CNN, the first assignment of Smith and Weltner for the two nitrogen atoms has to be changed. A qualitative discussion of the electronic structure discloses no simple relation between the structure of the singly occupied orbitals and the measured hyperfine coupling constants. Vibrational effects were found to be of little importance.
Multiconfigurational self-consistent field calculations of nuclear magnetic resonance indirect spin–spin coupling constants
Authors: Barszczewicz Andrzej , Helgaker Trygve , Jaszuński Michael/ , Jo/rgensen Poul , Ruud Kenneth
Published: July 2002

Ab initio calculations of all the nuclear magnetic resonance (NMR) indirect nuclear spin–spin coupling constants in the HN3 molecule and in four isomers of CH2N2 are described. For each molecule, SCF and two multiconfiguration self-consistent field (MCSCF) wave functions that take into account valence shell correlation effects are applied. All mechanisms contributing to the coupling constants are included. While the SCF results are meaningless, the agreement of the correlated values with those known from experiment is satisfactory. Our most accurate calculations are carried out with the wave functions previously used successfully to compute the NMR shielding constants. All parameters determining the NMR spectra of these molecules have thus been obtained at uniform accuracy from the same reference wave function.
Electronic transitions in α-oligothiophene thin films. Comparison of ultraviolet/visible absorption spectroscopy and high resolution electron energy loss spectroscopy investigations
Authors: Oeter D. , Egelhaaf H.-J. , Ziegler Ch. , Oelkrug D. , Göpel W.
Published: July 2002

Vapor deposited thin films of a series of α-oligothiophenes are investigated comparatively with polarized ultraviolet/visible absorption spectroscopy (UV/VIS) and by high resolution electron energy loss spectroscopy (HREELS) in specular reflection geometry. The complementary selection rules of these methods allow an assignment of the observed absorption and loss bands according to a Hückel molecular orbital model. By plotting the transition energies of corresponding bands of different members of the homologous series vs the reciprocal of the number of rings, the development of the one-dimensional ‘‘π-band-structure’’ with an increasing number of rings could be followed. The extrapolation to infinite chain length leads to the electronic properties of an ideal (defect free) polythiophene. Furthermore, characteristic differences were observed in the results obtained from the two methods. The orientation of the molecules in thin films is only detectable with UV/VIS spectroscopy. It is most pronounced for α-quinquethiophene. On the other hand, HREELS gives information about the position of optical parity forbidden electronic transitions.
Solution to linearized Poisson–Boltzmann equation with mixed boundary condition
Authors: Hsu Jyh-Ping , Tseng Ming-Tsan
Published: July 2002

The Poisson–Boltzmann equation governing the electrostatic potential distribution for a charged surface immersed in an electrolyte solution is discussed. The conventional analysis on a constant potential/charge density problem is extended to a mixed boundary condition problem in which a linear combination of potential and its derivative is specified at the charged surface. We show that the difficulty of solving this type of problem can be circumvented by adopting a finite dimension, least square approximation method coupled by a random selection algorithm. The solution procedure provides an efficient and accurate way for the estimation of the unknown coefficients in the general solution of a linearized Poisson–Boltzmann equation. It is essentially independent of the geometry of a charged surface and the type of boundary condition specified, as long as the general solution can be expressed as a linear combination of harmonic functions.
The frequency dependence of nonlinear optical processes
Authors: Bishop David M. , De Kee D. W.
Published: July 2002

Explicit formulas are derived for the sum rules for the frequency-dependent hyperpolarizability-diagonal-components. These are the counterparts to the Cauchy moments for the dynamic polarizabilities. The formulas allow for the frequency dependence of any nonlinear optical process to be expressed as a single general expansion up to terms which are of fourth power in the optical frequencies, Xnα,α,...,α(−ωσ;ω1,...,ωn)=Xn α,α,...,α(0)+AW2+BW22+B′W4, where ωσ=∑iωi, W2=ω2σ+ω21+...ω2n, and W4=ω4σ+ω41+...ω4n (in conventional notation X1=α, X2=β, X3=γ, etc.). The advantages of determining the frequency dependence of all NLO processes, for a given species, in a single calculation are stressed. We focus mainly on the sum rules (A, B, and B′) for X3 and X5. These are applicable to both atoms and molecules (with the exception of X5 for noncentrosymmetric molecules) and we evaluate them, using near-exact wave functions, for H and He. It is apparent that B′ is generally smaller than B and this accounts for the reasonable success of the Shelton–Bishop dispersion formula which is often used to fit experimentally-derived dynamic hyperpolarizabilities.
Molecular dynamics simulation of trivalent europium in aqueous solution: A study on the hydration shell structure
Authors: Chaussedent S. , Monteil A.
Published: July 2002

As a prelude to a spectroscopic investigation of doped wet gels, we have made molecular dynamics simulations of a very dilute EuCl3 solution. The symmetry properties of the first hydration shell of Eu3+ have been determined. The 8 water molecules of this shell are roughly arranged according to the square antiprism D4d pseudo-symmetry. Though the arrangement of oxygens is little distorted from regular positions, the orientation of the water molecules themselves is found to be not so well organized at room temperature.
Rovibrationally averaged nuclear magnetic shielding tensors calculated at the coupled-cluster level
Authors: Sundholm Dage , Gauss Jürgen , Schäfer Ansgar
Published: July 2002

Nuclear magnetic shielding tensor functions for H2, HF, N2, CO, and F2 are calculated at the coupled-cluster singles and doubles level augmented by a perturbative correction for triple excitations [CCSD(T)]. The shielding constants for the lowest rovibrational states of these diatomics are obtained by solving the rovibrational Schrödinger equation with the finite-element method. For H, C, and F, absolute scales for the nuclear magnetic shielding constants have been obtained by combining computed diamagnetic shieldings with paramagnetic contributions deduced from measured spin-rotation constants and calculated rovibrational corrections. Since the experimental spin-rotation constants for N2 and CO are inaccurate, shielding scales for N and O based on coupled cluster calculations are probably the most accurate available.
Multiple dumbbell structures of vanadium–C60 clusters
Authors: Nakajima Atsushi , Nagao Satoshi , Takeda Hiroaki , Kurikawa Tsuyoshi , Kaya Koji
Published: July 2002

Vanadium (V)–C60 mixed clusters were produced by two laser vaporization methods. In the mass spectra of cationic Vn(C60)m+, the abundant clusters were produced at the composition of (n,m)=(1,1), (1,2), (2,3), (3–4,4), and (5,5). This pattern is explicable in terms of a multiple dumbbell structure; V atom and C60 are alternatively piled up. The ionization energy and the reactivity of the dumbbell clusters toward O2 and CO show V1(C60)2 takes a structure of V1(η6−C60)2.
Determination of iodine adlayer structures on Au(111) by scanning tunneling microscopy
Authors: Huang Lin , Zeppenfeld Peter , Horch Sebastian , Comsa George
Published: July 2002

Chemisorbed iodine adlayers on Au(111) films on quartz were studied using scanning tunneling microscopy (STM) at room temperature in air. The iodine was adsorbed by successive deposition of droplets of a dilute solution of iodine in methanol. As a function of coverage, various adlayer structures were obtained. By changing the tunneling parameters, either the iodine adlayer or the Au(111) substrate can be imaged with atomic resolution. In this way, the adlattice properties such as periodicity, orientation and the local absolute coverage have been characterized with high accuracy. In the low coverage range (θ<0.33), due to the high mobility of iodine atoms, only the unreconstructed Au(111) substrate lattice could be imaged. At θ∼0.33, a (3×3)R30° structure is evident. With increasing coverage, a (p×3) structure of higher iodine packing density is observed, which can be described as a uniaxially compressed (striped) phase. Finally, near monolayer saturation coverage the iodine atoms form a hexagonal moiré-like pattern with long-range height modulation. The results are compared with previous measurements on this system under UHV and electrolyte conditions.
Broadband dipolar recoupling in the nuclear magnetic resonance of rotating solids: A compensated C7 pulse sequence
Authors: Hohwy M. , Jakobsen H. J. , Edén M. , Levitt M. H. , Nielsen N. C.
Published: July 2002

We introduce an improved variant of the C7 pulse-sequence for efficient recoupling of spin-1/2 pair dipolar interactions in magic-angle spinning solid-state NMR spectroscopy. The tolerance of C7 toward isotropic as well as anisotropic chemical shift offsets and rf inhomogeneity is improved considerably by replacing the original basic element Cφ44̄=(2π)φ(2π)φ+π with the cyclically permuted element Cφ14̄3=(π/2)φ(2π)φ+π(3π/2)φ. The improved performance of this permutationally offset stabilized variant of C7 is analyzed by average Hamiltonian theory to fifth order, numerical simulations, and demonstrated by experiments on powder samples of doubly 13C-labeled barium oxalate hemihydrate and diammonium fumarate.
Removal of pressure and free energy artifacts in charged periodic systems via net charge corrections to the Ewald potential
Authors: Bogusz Stephen , Cheatham Thomas E. , Brooks Bernard R.
Published: July 2002

A uniform neutralizing background plasma is employed by default in standard Ewald calculations for net-charged systems. We show here that this plasma leads to serious artifacts in both system energy and pressure, which lead to unrealistic behavior. These artifacts are especially critical to simulations where either net charge or volume is allowed to change. To correct these problems we institute a net-charge correction term that consists of subtracting off the Ewald sum for a single particle with charge equal to the net charge of the full system and an optional Born or Poisson–Boltzmann term. This correction decreases pressure artifacts by three orders of magnitude and allows rapid energy convergence as the cell size increases. The correction term is general, in that it applies to systems of any shape or net charge. The net-charge correction was tested for systems in all three phases of matter: gas, solid, and liquid, and found to be markedly superior to standard Ewald in all three cases. In the gas phase, isolated molecular energies are quickly achieved as the cell size increases. In the solid phase the vaporization energy of the NaCl crystal is reproduced using free energy perturbation techniques where a single atom is removed. In the liquid phase, the solvation free energy of Na+ was investigated. It is demonstrated that both the net-charge correction and an energy term twice the size of a Born term is required to obtain optimal free energies of ionic hydration.
On the observation of propagating sound modes at high momentum transfer in viscous liquids and glasses
Authors: Ribeiro M. C. C. , Wilson M. , Madden P. A.
Published: July 2002

The dynamic structure factors, S(Q,ω), at a range of scattering vectors (Q) in the range 0.2<Q<3.0 Å−1 are calculated in a molecular dynamics (MD) simulation of the viscous, network-forming liquid, ZnCl2. At intermediate Q (0.2<Q<0.5 Å−1), Brillouin features, with shifts and widths proportional to Q and Q2, respectively, are observed. These features persist above the frequency of the “boson peak,” in apparent discord with some suggestions for its origin. At high values of Q, close to the first peak of the static structure factor, a Q-dependent feature is seen in the inelastic part of S(Q,ω). This is analogous to a recent experimental observation on the similar, network-forming system B2O3 which was assigned to a “sound mode” contribution. An instantaneous normal mode (INM) analysis was applied to understand the relationship between these observations and the nature of the underlying modes of the liquid. The INM analysis accurately reproduces the observed (in MD) behavior of S(Q,ω) but does not support the existence of well-defined, plane-wave-like sound modes in the fluid in the relevant régimes of Q and ω. Alternative explanations of the origin of the Brillouin and S(Q)-dependent features are proposed on the basis of the INM results, and results related to the origin of the boson peak in the density of states are presented.
Electrorheology of miscible blended liquid crystalline polymer: A dielectric property approach
Authors: Kawai A. , Ide Y. , Inoue A. , Ikazaki F.
Published: July 2002

A miscible blended liquid crystalline polymer prepared by Asahi Chemical Industry Co. Ltd. was used to study the effect of temperature on electrorheology. The electrorheological (ER) effect was measured using a rotational viscometer at temperatures between 20 and 60 °C. The polymer showed no yield shear stress under an external electric field, and its ER effect increased with decreasing temperature. We explain the electrorheology of the miscible blended liquid crystalline polymer using our ER mechanism, based on Block’s model. In our mechanism, for an ER fluid to have an appreciable ER effect, its relaxation frequency must be between 100 and 105 Hz and the difference in the dielectric constant must be large below and above the relaxation frequency. The relaxation frequency of the miscible blended liquid crystalline polymer increased with increasing temperature, and the difference in its dielectric constant below and above the relaxation frequency increased with decreasing temperature. These dielectric properties are explained by the Debye theory and account for the electrorheology of the polymer.
Supercooled liquids and the glass transition: Temperature as the control variable
Authors: Ferrer Maria Luisa , Lawrence Christopher , Demirjian Berj G. , Kivelson Daniel , Alba-Simionesco Christiane , Tarjus Gilles
Published: July 2002

It has long been appreciated that both temperature and density play roles in determining the extremely super-Arrhenius, low-temperature behavior of the viscosity and long α-relaxation times that characterize fragile supercooled liquids. But what has not been generally appreciated, and what we believe we have established (by focusing on a model-free analysis in terms of temperature and density, rather than upon temperature and pressure) is that over the range of densities and temperatures spanned by the experiments carried out at 1 atm pressure, temperature is the dominant control variable. This information is essential input to the formulation of a theory or model of the long-time dynamics of low-temperature fragile liquids, and it suggests a focus on activated dynamics rather than on free volume. This work indicates that, except possibly at very high densities (very high pressures), the glass transition is not a result of congestion due to a lack of free volume.
Molecular relativistic calculations of the electric field gradients at the nuclei in the hydrogen halides
Authors: Visscher Lucas , Enevoldsen Thomas , Saue Trond , Oddershede Jens
Published: July 2002

Electric field gradients at the position of the nuclei in the hydrogen halides are calculated using four-component relativistic methods. Benchmark values at the Dirac–Hartree–Fock level of theory are obtained by using large uncontracted basis sets. Electron correlation corrections are obtained by means of finite field MP2, CCSD, and CCSD(T) calculations in smaller basis sets. The importance of spin–orbit coupling and the so-called picture change effect are discussed.
Comparison of a combined quantum mechanics/interatomic potential function approach with its periodic quantum-mechanical limit: Proton siting and ammonia adsorption in zeolite chabazite
Authors: Brändle Martin , Sauer Joachim , Dovesi Roberto , Harrison Nicholas M.
Published: July 2002

Comparison is made between a combined quantum mechanics/interatomic potential function approach (QM-Pot) and its fully quantum-mechanical limit, ab initio calculation applying periodic boundary conditions. The Hartree–Fock (HF) method is combined with ab initio-parametrized ion pair shell model potential functions. The CRYSTAL code is employed for the periodic Hartree–Fock calculations. The same double-/valence triple-zeta polarization basis sets are used in both the approaches. The proton siting and ammonia adsorption in a high-silica acidic zeolite catalyst, H-chabazite (Si/Al=11, space group P1, unit cell H–AlO2[SiO2]11) are examined. The combined QM-Pot relative stabilities and reaction energies deviate from the periodic full QM results by 4–9 kJ/mol only, which demonstrates the power of our combined approach. This conclusion is also supported by comparison of the electrostatic potential inside the zeolite pore, calculated from the periodic wave function and by the QM-Pot approach. Framework oxygen O1 is found to be the preferred proton site and on interaction with NH3 the proton is predicted to move to NH3 yielding NH4+. The NH4+ surface species is coordinated to two framework oxygen atoms. It is by 30–35 kJ/mol more stable than the neutral adsorption complex of NH3. Evidence is produced that the failure of previous periodic HF calculations to predict a stable NH4+ ion is due to the limitations of the minimum basis set used.
Constrained Boltzmann-Gibbs measures and effective potential for glasses in hypernetted chain approximation and numerical simulations
Authors: Cardenas Miguel , Franz Silvio , Parisi Giorgio
Published: July 2002

By means of an effective potential associated with a constrained equilibrium measure and apt to study frozen systems, we investigate glassy freezing in simple liquids in the hypernetted chain (HNC) approximation. Unlike other classical approximations of liquid theory, freezing is naturally embedded in the HNC approximation. We give a detailed description of the freezing transition that is analogous to the one given in a large class of mean-field long range spin glass. We compare our findings with Monte Carlo simulations of the same system and conclude that many of the qualitative features of the transition are captured by the approximated theory.
What can short-pulse pump-probe spectroscopy tell us about Franck-Condon dynamics?
Authors: Shen Yu-Chen , Cina Jeffrey A.
Published: July 2002

We examine the signal from pump-probe spectroscopy of a model system—nonrotating I2—at short time delays and compare signals calculated without approximation (a full quantum calculation), with a semiclassical Franck-Condon approximation, and with a classical simulation of the nuclear wave packet. In order to assess the complications of simulation and interpretation when the probe window lies in the spectroscopically and dynamically important Franck-Condon region, we concentrate on a case where pump and probe resonances are at the same internuclear distance. We find that the common practice of ignoring the pump-truncation effects of pulse overlap leads to an overestimate of the signal at short times. Moreover, both classical simulations and semiclassical Franck-Condon treatments can deviate significantly in form from the actual signal even with proper treatment of pulse overlap. The sources of these deviations can be seen in the evolution of the excited-state nuclear distributions calculated classically and under the semiclassical Franck-Condon approximation. Specifically, the differences in evolution of the classical and full quantum excited-state nuclear distributions are due to differing initial momentum distributions. We introduce an efficient method for calculating the pump-probe signal that takes advantage of the brevity of ultrashort pulses and can include pulse characteristics such as chirp. This short-pulse expansion method aids in the proper treatment of pulse-overlap and nonzero pulse duration and promises to simplify the incorporation of relaxation processes.
Dynamically induced loss and its implications on temperature scans of relaxation processes
Authors: Snyder Chad R. , Mopsik Frederick I.
Published: July 2002

It is demonstrated that a constant frequency measurement of a purely real property that varies due to a change in some state variable (e.g., pressure, volume, or temperature) with time will show a dynamically induced loss that is not present when the variable is held constant. This conclusion is demonstrated through both generalized arguments and by examining some specific functional forms of time evolution equations, including one that resembles a glass transition. Our results show that techniques such as dielectric thermal analysis, dynamic mechanical thermal analysis, and modulated differential scanning calorimeter, which perform measurements of complex quantities while scanning in temperature, may have some serious problems. Therefore, results obtained from these techniques should be examined carefully before using them to prove or disprove theoretical model predictions, especially in the neighborhood of a phase transition.
Rovibrationally averaged magnetizability, rotational g factor, and indirect spin–spin coupling of the hydrogen fluoride molecule
Authors: Åstrand Per-Olof , Ruud Kenneth , Mikkelsen Kurt V. , Helgaker Trygve
Published: July 2002

The magnetizability tensor, the rotational g factor, and the indirect nuclear spin–spin coupling constant of the hydrogen fluoride molecule have been calculated using large multiconfigurational self-consistent field wave functions and large basis sets. For a critical comparison with experiment, rovibrational corrections have also been calculated. For the magnetizability tensor and the spin–spin coupling constant, we present results with higher precision than available experimental data; for the rotational g factor, our results are in good agreement with experiment.