bluuebunny/crossref_metadata_2025 huggingface dataset:
A deperturbation analysis of the B 3Σu−(v′=0–6) and th B″ 3Πu(v′=2–12) states of S2
Authors: Green M. E. , Western C. M.
Published: July 2002

Laser-induced fluorescence spectra of 32S2 have been obtained, covering v′=0–6 of the B 3Σ−u–X 3Σ−g transition and v′=2–12 of the B″ 3Πu–X 3Σ−g transition, using static cell and supersonic free jet techniques. The spectra include transitions to the previously unseen B″ 3Π2,u components. Analysis of the many perturbations between the two upper electronic states has been carried out using a Hamiltonian matrix including all the B and B″ states simultaneously rather than deperturbing individual pairs of vibronic states. This takes into account longer range interactions and gives deperturbed molecular constants that vary smoothly with vibrational state. Our model for the B″ v′=2–12 and B v′=0–6 levels covering J up to 100 can fit all 3320 observed transitions with an average error of 0.064 cm−1. The widely ranging fluorescence lifetimes of the B″ state vibronic levels provide independent information about the state mixing and confirm the model, since the observed B″–X fluorescence arises almost entirely via this mixing. However, lifetime measurements of the newly observed Ω=2 components of the B″ state showed little variation in lifetimes, about an average of 4.2±0.4 μs. This indicates a small intrinsic B″–X transition strength as the Ω=2 components are essentially not mixed with the B state. A model for the perturbation parameters is developed, based on Franck–Condon factors between the two states. The magnitude of the perturbations and transition moments are discussed in relation to the electronic configurations of the B and B″ states.
Numerical methods with a high order of accuracy applied in the quantum system
Authors: Zhu Wusheng , Zhao Xinsheng , Tang Youqi
Published: July 2002

Two kinds of numerical methods with a high order of accuracy are developed in this paper. In the general classical Hamiltonian system, it was claimed that no explicit n-step symplectic difference method with the nth order of accuracy can be achieved if n is larger than 4. We show that there is no such constraint in the quantum system. We also exploit to investigate the high order Newton–Cotes differential methods in the quantum system. For the first time, we work out the generalized derivation of explicit symplectic difference methods with any finite order of accuracy in the quantum system. We point out that different coefficients in the same multistep symplectic method will lead to quite different results. The choices of coefficients and order of accuracy for the best efficiency in multistep symplectic methods and Newton–Cotes differential methods are studied. The connections between explicit symplectic difference structure, Newton–Cotes differential schemes, and other methods are presented. Numerical tests on the model system have also been carried out. The comparison shows that the explicit symplectic difference methods and the Newton–Cotes differential methods are both accurate and efficient.
Inert gas scattering from molten metals: Probing the stiffness and roughness of the surfaces of atomic liquids
Authors: Ronk Warren R. , Kowalski Daniel V. , Manning Michelle , Nathanson Gilbert M.
Published: July 2002

The surface rigidity and corrugation of the liquid metals indium, gallium, and bismuth are investigated by scattering neon, argon, and xenon from each liquid. The inert gas fractional energy transfers and trapping–desorption fractions are much lower for these atomic liquids than for molecular fluids, reflecting the high mass densities and high surface tensions of the liquid metals. The peak energy transfer at a 55° specular angle can be predicted by using hard sphere collision mechanics and by estimating the mass ratio μ=mgas/msurf by (0.28±0.02 Å−1) (ρgas/ρliq), where ρgas is the mass per cross-sectional area of the gas atom and ρliq is the bulk metal density. The specular scattering intensity decreases as the liquid’s temperature increases at a fractional rate approximately equal to −1/(2Tliq). The intensity appears to scale nearly inversely with the amplitude of the surface atom fluctuations.
Photofragment imaging of methane
Authors: Heck Albert J. R. , Zare Richard N. , Chandler David W.
Published: July 2002

The photolysis of methane is studied using photofragment imaging techniques. Our study reveals that the photolysis of methane proceeds via many different pathways. The photofragment imaging technique is used to resolve and characterize these various pathways and provides therefore unique insight into the dynamical processes that govern this photodissociation. The formation of H-atom photofragments following absorption of a Lyman-α photon, and H2 photofragments following absorption of two ultraviolet photons (λ=210–230 nm) are studied. The measured H-atom photofragment images reveal that a channel that produces fast H atoms concomitant with methyl fragments is dominant in the Lyman-α photolysis of methane. This channel leads to an anisotropic recoil of the fragments. A secondary channel is observed leading to the formation of somewhat slower H atoms, but an unique identification of this second channel is not possible from the data. At least part of these slower H atoms are formed via a channel that produces H atoms concomitant with CH and H2 photofragments. The recoil of these slower H atoms appears to be isotropic. The measured, state-resolved H2(v,J), photofragment images reveal that two channels lead to H2 photofragments from the two-photon photolysis of methane: a channel that leads to H2 products concomitant with methylene fragments; and a channel that leads to H2 products concomitant with CH and H fragments.

H2(v,J) rotational and vibrational distributions are measured for each of these two channels separately. The H2 products formed via the H2+CH2 channel are rotationally and vibrationally highly excited, whereas those formed via the H2+CH+H channel are rotationally and vibrationally cooler. Rotational distributions of H2 formed via the H2+CH+H channel are well reproduced by Boltzmann distributions. Results on D2 elimination following two-photon photolysis of CD4 are in general similar and in qualitative agreement with the results on CH4.
Multidimensional semiclassical tunneling between asymmetric wells via two channels
Authors: Guo Yin , Qin Yue , Sorescu Dan C. , Thompson Donald L.
Published: July 2002

We have developed a semiclassical method, based on the models proposed by Miller and co-workers, for calculating tunneling effects in asymmetric double-well systems. The procedure can be easily implemented within standard classical trajectory simulations and thus allows for explicit treatment of the full-dimensional dynamics. We have applied the method to HSiOH cis–trans isomerization.
The quadrature discretization method (QDM) in the solution of the Schrödinger equation with nonclassical basis functions
Authors: Shizgal Bernie D. , Chen Heli
Published: July 2002

A discretization method referred to as the Quadrature Discretization Method (QDM) is introduced for the solution of the Schrödinger equation. The method has been used previously for the solution of Fokker–Planck equations. The Fokker–Planck equation can be transformed to a Schrödinger equation with a potential of the form that occurs in supersymmetric quantum mechanics. For this class of potentials, the ground state wave function is known. The QDM is based on the discretization of the wave function on a grid of points that coincide with the points of a quadrature. The quadrature is based on a set of nonclassical polynomials orthogonal with respect to a weight function determined by the potential function in the Schrödinger equation. For the Fokker–Planck operator, the weight function that provides rapid convergence of the eigenvalues are the steady distributions at infinite time, that is, the ground state wave functions. In the present paper, the weight functions used in an analogous solution of the Schrödinger equation are related to the ground state wave functions if known, or some approximate form. Calculations are carried out for a model systems, the Morse potential, and for the vibrational levels of O2 and Ar–Xe with realistic pair potentials. For O2, the wave functions are used to calculate the vibrationally inelastic transition amplitudes for a Morse potential and compared with exact analytic results. The eigenvalues of a two-dimensional Schrödinger equation with the Henon–Heiles potential are also calculated. The rate of convergence of the eigenvalues and the eigenfunctions of the Schrödinger equation is very rapid with this approach.
An exactly solvable model for the collapsed state of Gaussian polymer chains
Authors: Washington Gary E.
Published: July 2002

The influence of intermolecular and intramolecular interactions on the end-to-end distance is shown to be proportional to N−1/4, where N is the contour length of the polymer chain, for a single chain with intramolecular interactions. The same scaling behavior is obtained as a consequence of intermolecular interactions between two polymer chains. The apparent collapsed state chain size is reduced below that of experiment due to the absence of excluded volume. The onset of the collapsed state occurs above a threshold contour length at constant temperature where intramolecular interactions dominate the space of available conformations.
Wertheim cluster development of free energy functionals for general nearest-neighbor interactions in D=1
Authors: Brannock G. R. , Percus J. K.
Published: July 2002

We develop a particle cluster technique for representing the equilibrium thermodynamics of an inhomogeneous multispecies classical fluid in one dimension with nearest-neighbor interactions. The corresponding free energy functionals take on the general Wertheim association form and make contact with the Widom insertion argument. Application is made to adhesive rod mixtures, for which first-order thermodynamic perturbation theory is seen to be exact, and to more general interactions, such as the square well, in which activity is no longer a local functional of the density and explicit relations must be generated by recursion. For a fluid on a loop, auxiliary two-point feedback functions appear, which correct the simply connected form.
The frequency dependence of hyperpolarizabilities for noncentrosymmetric molecules
Authors: Bishop David M. , De Kee D. W.
Published: July 2002

Formulas are given which describe the quadratic and quartic frequency dependence of hyperpolarizabilities for noncentrosymmetric molecules.
Nuclear magnetic resonance polarization and coherence echoes in static and rotating solids
Authors: Tomaselli M. , Hediger S. , Suter D. , Ernst R. R.
Published: July 2002

The mechanisms of defocusing and refocusing of spin order in extended dipolar coupled nuclear spin systems are investigated by experiments on static and on rotating solids. It is demonstrated that polarization or coherence echoes are possible also under magic-angle sample spinning. The dipolar interactions, averaged by the spinning, are recovered by rotor-synchronized multiple-pulse sequences. By a simple modification of the pulse sequences, it is possible to reverse the sign of the effective dipolar Hamiltonian and to induce the refocusing of polarization or coherence. The creation of multiple-spin order in the course of polarization evolution or free precession is monitored by a modified echo experiment. Experimental results for a polycrystalline sample of calcium formate are presented.
Ground and excited states of the complex of CO with water: A diffusion Monte Carlo study
Authors: Sandler P. , Buch V. , Sadlej J.
Published: July 2002

We present an analysis of the complex of water with CO which includes (a) a new potential energy surface obtained by fitting ab initio points, followed by adjustment against experimental rotational, spin–spin and quadrupole coupling constants; (b) diffusion Monte Carlo (DMC) studies of the ground vibrational state, and of three excited vibrational states, at J=0. A new approach is suggested for DMC calculation of intermolecular vibrational frequencies.
The (2n+1)th-order off-resonant spectroscopy from the (n+1)th-order anharmonicities of molecular vibrational modes in the condensed phase
Authors: Okumura K. , Tanimura Y.
Published: July 2002

Assuming that the polarizability is a linear function of the nuclear coordinate, i.e., α(q)=α0+α1q, we obtain analytical expressions of the (2n+1)th-order signals and show that the leading order of the signals (n>1) is proportional to gn+1, where gn+1 is the coefficient of the anharmonic potential V(q)=g3q3/3!+g4q4/4!+⋅⋅⋅. In other words, detection of the (2n+1)th-order signal implies the direct observation of the (n+1)th-order anharmonicity within the approximation. Based on this fact we discuss a possibility to detect the (n+1)th-order anharmonicity directly from the (2n+1)th-order experiment. Calculations are made by using novel Feynman rules for the nonequilibrium multitime correlation functions relevant to the higher-order off-resonant spectroscopy. The rules have been developed by the authors and are presented compactly in this paper. With the help of a conventional double-sided Feynman diagram, we draw physical pictures of higher-order off-resonant optical processes. Representative calculations for CHCl3 of the fifth-, seventh-, and ninth-order optical processes are presented and discussed.
Measurement of the hydrodynamic corrections to the Brownian motion of two colloidal spheres
Authors: Crocker John C.
Published: July 2002

The hydrodynamic coupling between two isolated 0.97 μm diameter polystyrene spheres is measured by reconstructing their Brownian motion using digital video microscopy. Blinking optical tweezers are used to facilitate data collection by positioning the spheres in the microscope’s focal plane and in close proximity to one another. The observed separation dependence of the spheres’ relative and center of mass diffusion coefficients agree with that predicted by low Reynolds number hydrodynamics and the Stokes-Einstein relation.
The equilibrium structure of silene H2C=SiH2 from millimeter wave spectra and from ab initio calculations
Authors: Bailleux Stephane , Bogey Marcel , Demaison Jean , Bürger Hans , Senzlober Michael , Breidung Jürgen , Thiel Walter , Fajgar Radek , Pola Josef
Published: July 2002

Silene, H2CSiH2, has been efficiently produced by pyrolysis of 5,6-bis(trifluoromethyl)- 2-silabicyclo[2.2.2]octa-5,7-diene (SBO). Seven isotopomers have been observed by millimeter- and submillimeter-wave spectroscopy. From the different sets of experimental molecular parameters and from ab initio calculations of the rovibrational interaction parameters, the equilibrium structure has been obtained by a least squares analysis of the rotational constants. The results are: re(Si=C)=1.7039(18) Å, re(C–H)=1.0819(12) Å, re(Si–H)=1.4671(9) Å, ∠HCSi=122.00(4)°, and ∠HSiC=122.39(3)°. This experimental structure is in excellent agreement with the equilibrium geometry calculated at the CCSD(T) level of theory with a cc-pV(Q,T)Z basis set. This is the first experimental determination without any constraint of the Si=C double bond length in the parent compound of the silaalkene family. A lifetime of 30 ms has been observed for this molecule in the gas phase at low pressure.
Ab initio relativistic effective potentials with spin-orbit operators. VII. Am through element 118
Authors: Nash Clinton S. , Bursten Bruce E. , Ermler Walter C.
Published: July 2002

Ab initio averaged relativistic effective core potentials (AREP) and spin-orbit (SO) operators are reported for the elements Am through element 118. Two sets have been calculated for certain elements to provide AREPs with varying core/valence space definition, thereby permitting the treatment of core/valence correlation interactions. The AREPs and SO operators are tabulated as expansions in Gaussian-type functions (GTF). GTF valence basis sets are derived for the lowest energy state of each atom. The reliability of the AREPs and SO operators is gauged by comparing calculated atomic orbital eigenvalues and SO splitting energies with all-electron relativistic values.
Large scale simulation of macromolecules in solution: Combining the periodic fast multipole method with multiple time step integrators
Authors: Figueirido Francisco , Levy Ronald M. , Zhou Ruhong , Berne B. J.
Published: July 2002

Large scale simulations of macromolecules in solution that do not suffer from artifacts arising from force truncations are becoming feasible. New force evaluation algorithms such as the fast multipole method (FMM) and multiple time scale integration methods such as the reversible reference system propagator algorithm (r-RESPA) have been combined and used to perform fast and stable simulations of large macromolecular systems. A consistent treatment of the long-range forces in simulations with periodic boundary conditions requires the use of a periodic form of the Coulomb potential. In this article, the FMM is extended to periodic systems, and combined with RESPA, yielding a new algorithm that is successfully applied to the simulation of large biomolecules in solution. If the interactions at different stages are separated smoothly, good energy conservation is obtained even for time steps as large as 12 fs on a system of over 40 000 atoms, and a CPU speedup of more than a factor of 20 is achieved compared to the standard Verlet integrator with Ewald sum for the Coulombic interaction. As compared with the recently developed particle-mesh Ewald (PME) method, the periodic r-RESPA/FMM has a break-even point at about 20 000 atoms; for larger systems, r-RESPA/FMM is expected to be more efficient.
Vibrations and soliton dynamics of positively charged polyacetylene chains
Authors: Champagne B. , Deumens E. , Öhrn Y.
Published: July 2002

Ab initio molecular dynamics simulation is performed on a small polyacetylene chain with a positive soliton defect. The dynamics is initialized by an external electric field. The collective motion of the carbon and hydrogen atoms are compared to some low frequency vibrational modes of positively charged polyacetylene chains of varying lengths having the characteristics of the soliton displacement. The soliton effective mass estimated using a variety of schemes is found to be about 10 electron masses. The static linear polarizability of singly charged polyacetylene chains of varying lengths is computed and compared with that of undoped chains. The electronic contributions to the polarizability are computed at the level of the coupled Hartree–Fock or the random phase approximation, and the vibrational contributions are estimated by invoking the double harmonic oscillator approximation. The soliton defect causes some enhancement of the electronic term, which covers 10–15 carbon–carbon double bonds, and it generates a large increase of the vibrational part, which in the absence of counter ions show no sign of leveling off even for the longest chains considered.
Determination of methyl radical concentrations in a methane/air flame by infrared cavity ringdown laser absorption spectroscopy
Authors: Scherer J. J. , Aniolek K. W. , Cernansky N. P. , Rakestraw D. J.
Published: July 2002

Infrared cavity ringdown laser absorption spectroscopy (IR-CRLAS) is employed to determine absolute methyl radical concentrations in a 37.5 Torr laminar methane/air flame. IR-CRLAS rovibrational absorption spectra of the ν3 fundamental band system near 3200 cm−1 are combined with N2-CARS temperature measurements to obtain methyl radical concentrations as a function of height above the burner surface. These data are compared with flame chemistry simulations under both stoichiometric and rich flame conditions. Issues regarding the applicability of IR-CRLAS for combustion studies are discussed, including the uncertainties present for the specific case of methyl radical. These IR-CRLAS measurements indicate the ability to monitor reactants, intermediates, and products within a narrow spectral window, and, to our knowledge, constitute the first infrared detection of a polyatomic radical in a flame.
Six-dimensional quantum dynamics of dissociative chemisorption of H2 on Cu(100)
Authors: Kroes G. J. , Baerends E. J. , Mowrey R. C.
Published: July 2002

The dissociative chemisorption of H2 on Cu(100) has been studied using a six-dimensional wave packet method. All molecular degrees of freedom are treated quantum mechanically, with no dynamical approximations. The potential-energy surface used is an analytical fit to calculations employing density functional theory (DFT), using the generalized gradient approximation (GGA) and a slab representation of the metal surface. The dependence of the reaction probability on normal incidence energy has been obtained for both (v=0, j=0) and (v=1, j=0) H2. Comparison to experiment suggests that, on average, the DFT method overestimates the barriers to dissociation by ∼0.15 eV for H2+Cu(100). In broad agreement with experiments on H2+Cu(111) and H2+Cu(110) the calculations show large vibrational inelasticity. A prediction of the survival probability of (v=1, j=0) H2 as a function of incidence energy is also presented. The measurement of this quantity is now feasible and should provide another sensitive test of the DFT potential-energy surface.
High resolution spectroscopy of Ne and Ar containing noble gas clusters
Authors: Xu Yunjie , Jäger Wolfgang
Published: July 2002

Detailed studies of the first rotational spectra of mixed noble gas trimers and tetramers containing Ne and Ar atoms are described. Transitions of various isotopomers of Ne2Ar, NeAr2, Ne3Ar, Ne2Ar2, and NeAr3 were measured. Accurate values for rotational constants and centrifugal distortion constants were obtained from the spectral analyses. The spectroscopic parameters are consistent with isosceles triangular geometries of the trimer systems, and with distorted tetrahedral arrangements for the tetramers. Further evidence for the geometries was found in the effects of spin statistics in the spectra of isotopomers with C2v symmetry and of those with C3v symmetry. The simplicity of these fundamental systems makes them well suited for the determination and characterization of many-body nonadditive interactions. The signal strengths observed in these first high resolution spectroscopic studies of noble gas tetramers indicate the possibility of detecting the spectra of larger noble gas clusters.
Surface viscoelastic relaxation on aqueous solutions of tetrahydrofuran
Authors: Sharpe D. , Earnshaw J. C.
Published: July 2002

Surface light scattering was used to study thermally excited capillary waves on aqueous solutions of tetrahydrofuran. The data indicate the presence of various surface viscoelastic relaxation processes in this system. In particular, the surface dilatational elastic modulus shows clear evidence of a relaxation with time constant ∼5 μs, which is here associated with reorientation of the adsorbed tetrahydrofuran molecules. The time constant of this process increases significantly for surface films which are older than 5 h; this change is associated with other changes in the surface viscoelasticity, including the rate of increase of the surface pressure. The dilatational surface viscosity deduced from the surface light scattering data is negative, as has been found in other recent studies of soluble surfactants. These negative surface viscosities are interpreted as effective properties only, reflecting a reduction in the dilatational wave damping due to some process or processes not included in the theory used in the data analysis. The transverse shear surface viscoelastic modulus exhibits a complex frequency dependence, more than one relaxation process of different time constant being involved. Possible molecular mechanisms are discussed.
Patterns in hydrogen bonding via electrostatic potential topography
Authors: Gadre Shridhar R. , Bhadane Pravin K.
Published: July 2002

Topography of molecular electrostatic potential has been explored for understanding the primitive binding patterns in hydrogen bonded complexes. A linear relationship has been found between the distance of an atom from the minimum in the electrostatic potential of B molecule and the experimental hydrogen bond length for B⋯HF-type complexes.
Production of maximally aligned O(1D) atoms from two-step photodissociation of molecular oxygen
Authors: Eppink A. T. J. B. , Parker D. H. , Janssen M. H. M. , Buijsse B. , van der Zande W. J.
Published: July 2002

Two-step photodissociation of O2 with a total excitation energy of 7.7 eV yields O(1D) atoms with their angular momenta aligned perpendicular to the fragment recoil axis. Correlation rules linking the bound molecule with the separated atoms predict such strong alignment only for an adiabatic process.
An approximate formula for the intermolecular Pauli repulsion between closed shell molecules. II. Application to the effective fragment potential method
Authors: Jensen Jan H. , Gordon Mark S.
Published: July 2002

The accuracy and efficiency of an approximate formula for the intermolecular Pauli repulsion between closed shell molecules, derived earlier [Mol. Phys. 89, 1313 (1996)], is demonstrated for dimers of H2O, CH3OH, CH2Cl2, CH3CN, (CH3)2CO, and (CH3)2SO. The energy derivative with respect to a Cartesian coordinate and rigid rotation about the center-of-mass (torques) are presented. The Pauli repulsion energy term is then combined with the Coulomb and classical induction energy terms of the effective fragment potential method [J. Chem. Phys. 105, 1968, 11081 (1996)] to give a general intermolecular interaction potential. This potential is applied to water and methanol clusters.
Dissociative electron attachment in nanoscale ice films: Thickness and charge trapping effects
Authors: Simpson W. C. , Orlando T. M. , Parenteau L. , Nagesha K. , Sanche L.
Published: July 2002

The yield and kinetic energy (KE) distributions of D− ions produced via dissociative electron attachment (DEA) resonances in nanoscale D2O ice films are collected as a function of film thickness. The B12, A12, and B22 DEA resonances shift to higher energies and their D− ion yields first increase and then decrease as the D2O films thicken. The D− KE distributions also shift to higher energy with increasing film thickness. We interpret the changes in the DEA yield and the D− KE distributions in terms of modifications in the electronic and geometric structure of the surface of the film as it thickens. A small amount of charge build-up occurs following prolonged electron beam exposure at certain energies, which primarily affects the D− KE distributions. Charge trapping measurements indicate that an enhancement in the trapping cross section occurs at energies near zero and between 6 and 10 eV.
Linear and nonlinear optical properties of platinum-ethynyl
Authors: McKay T. J. , Bolger J. A. , Staromlynska J. , Davy J. R.
Published: July 2002

We report an experimental investigation of absorption, fluorescence, and nonlinear absorption in a platinum-ethynyl complex. We have observed strong singlet absorption at 355 nm, and a weaker triplet absorption at 520 nm. Photoluminescence measurements show a fluorescence band at 400 nm and a phosphorescence band at 520 nm. Pump–probe nonlinear absorption measurements confirmed that S0→T1 followed by excited triplet-state absorption corresponds to the dominant excitation path in the region between 513 and 532 nm.
Molecular dynamics simulation with the charge response kernel: Diffusion dynamics of pyrazine and pyrazinyl radical in methanol
Authors: Morita Akihiro , Kato Shigeki
Published: July 2002

The present study involves two themes. The first is to incorporate the charge response kernel (∂Qa/∂Vb) into the molecular dynamics simulation, where Qa denotes the partial charge at the site a and Vb the electrostatic potential at the site b. The response kernel was ab initio calculated in our previous study [A. Morita and S. Kato, J. Am. Chem. Soc. 119, 4021 (1997)], and it provides a useful way to describe the polarization effect in solution, with several advantages discussed in Sec. I. The second theme is to elucidate the anomalously slow diffusion of some aromatic radicals using the above procedure. It demonstrated that the difference in the polarization effect of pyrazinyl radical (product of hydrogen abstraction) and pyrazine (parent) manifests itself in the diffusion coefficients. The analysis of the frictional force revealed the different dynamics of diffusion.
A statistical rate theory study of interface concentration during crystal growth or dissolution
Authors: Dejmek M. , Ward C. A.
Published: July 2002

The concentration at the interface of a growing or dissolving crystal has been previously found experimentally to be different from the equilibrium value; however, a method for predicting this concentration has not been available. We report an investigation that uses statistical rate theory to obtain the expression for the rate of molecular transport across the interface of a growing or dissolving crystal. This expression is in terms of the concentration at the interface, and serves as the boundary condition for the convective diffusion equation. The solution of this system of equations contains two constants that appear in the expression for the equilibrium exchange rate. To evaluate the validity of the system of equations obtained, KAlum dissolution in a rotating disk apparatus was examined using data that had been previously reported. In the series of experiments, saturated, aqueous solutions of KAlum were prepared, and the temperature changed to form undersaturated solutions. KAlum{111} crystals were then dissolved isothermally at a series of different temperatures and rotation rates, and their steady state rates of dissolution measured. Using the measured dissolution rates at one temperature, the values of the two constants were determined, and the system of equations was then used to predict the rate of dissolution at the other four temperatures. The predictions were found to be in agreement with the measured dissolution rates at the other temperatures. The predicted interfacial concentration was less than the equilibrium value at all dissolution rates and the difference increased with increasing dissolution rates.
Optical control over two-dimensional lattice vibrational trajectories in crystalline quartz
Authors: Wefers Marc M. , Kawashima Hitoshi , Nelson Keith A.
Published: July 2002

Control over the trajectory of two-dimensional vibrational motion is demonstrated for a degenerate E-symmetry optic phonon mode in crystalline quartz. Pairs of ultrashort optical pulses of different polarizations are used to impart momentum along orthogonal directions of the degenerate mode through impulsive stimulated Raman scattering. Variation of the temporal separation of the two excitation pulses permits various linear, elliptical, or circular trajectories to be selected. More complex two-dimensional trajectories are also possible if arbitrarily shaped excitation waveforms are used.
Characterization of polymer networks using the dipolar correlation effect on the stimulated echo and field-cycling nuclear-magnetic resonance relaxometry
Authors: Fischer Elmar , Grinberg Farida , Kimmich Rainer , Hafner Siegfried
Published: July 2002

Chain dynamics in a series of styrene-butadiene rubbers (SBR) was studied with the aid of the dipolar correlation effect (DCE) and field-cycling NMR relaxometry (FCR). The typical time scales of the two techniques are t>10−4 s and t<10−3 s, respectively, and therefore complementary. The crosslink density of the polymer networks was varied in a wide range. In order to prevent sinusoidal undulations of the stimulated-echo attenuation curves due to spin exchange between groups with different chemical-shift offsets, the DCE of the samples was examined using a modified radio frequency pulse sequence with additional π pulses inserted in the free-evolution intervals. Residual dipolar couplings can thus be probed in samples where chemical-shift and dipolar interactions are of the same order. The dipolar correlations probed with the DCE in SBR networks turned out to exist on a time scale exceeding 300 ms. The short-time fluctuations (probed by FCR) and the long-time dynamics (probed by DCE) can be approached by power-law dipolar correlation functions with exponents −0.78±0.02 and −1.5±0.1, respectively. The crossover time is in the order of 1 ms. In contrast to FCR, the DCE data strongly depend on the crosslink density but not on the temperature in a range from 30 to 80 °C. On this basis determinations of the crosslink density may be possible as an alternative to the usual mechanical torsion modulus measurements.
Dissipative tunneling control by elliptically polarized fields
Authors: Hartmann Ludwig , Grifoni Milena , Hänggi Peter
Published: July 2002

The tunneling dynamics of a dissipative two-level system that is strongly driven by elliptically polarized electric fields is investigated. The dissipative dynamics is governed within the noninteracting-blip-approximation for the stochastic forces by a generalized master equation (GME). With the focus being on viscous friction, we compare exact numerical solutions of the GME with analytical approximations to both the transient and the asymptotic, long-time periodic dynamics. Novel phenomena are identified: These are a selective control on localization (or, as well, on delocalization) of the tunneling dynamics, or the inversion of an initially induced localization by a static bias via multiphoton-assisted tunneling. These effects can be selectively tuned as a function of the eccentricity parameter p=Ey/Ex of corresponding field amplitudes. In particular, the case of a circularly polarized driving field with p=±1 yields a dramatic enhancement of the relaxation rate at resonances, when an integer multiple of the angular driving frequency matches the asymmetry energy induced by a static bias.
Frequency and wave-vector dependent dielectric function of water: Collective modes and relaxation spectra
Authors: Bopp Philippe A. , Kornyshev Alexei A. , Sutmann Godehard
Published: July 2002

The longitudinal frequency and wave-vector dependent complex dielectric response function χ(k,ω)=1−1/ε(k,ω) is calculated in a broad range of k values by means of molecular dynamics computer simulation for a central force model of water. Its imaginary part, i.e., Im{ε(k,ω)}/|ε(k,ω)|2, shows two main contributions in the region of small k values: Debye-like orientational relaxation in the lower frequency part of the spectrum and a damped librational resonance at the high frequency wing. The Debye relaxation time does not follow a de Gennes-like pattern: τ(k) goes through a maximum at k≈k*≈1.7 Å−1, while the static polar structure factor S(k) peaks at k≈3 Å−1. The resonance frequency ω(k) and the decay decrement γ(k) show a dispersion law, indicative of a decaying optical-like mode, the libron. With an approximate normal mode approach, we analyze the origin of this mode on a molecular level which shows that it is due to a damped propagation of molecular orientational vibrations through the network of hydrogen bonds. At high k the decay, due to dissipation of collective into single particle motions, dominates. The static dielectric function is calculated on the basis of the response function spectra via the Kramers–Kronig relation. In the small k region ε(k) decreases from the macroscopic value ε≈80 to a value ≈15, i.e. it exhibits a Lorentzian-type behavior. This behavior is shown to be determined by higher order multipole correlation functions. In the intermediate and high k range, our results on ε(k) and χ(k) are in excellent agreement with data extracted from experimental partial pair correlation functions: ε(k) exhibits two divergence points on the k axis with a range of negative values in between where a maximum in χ(k) is found with χmax(k)≫1, indicative of overscreening. Consequences of quantum corrections to χ(k) with respect to a purely classical calculation are discussed and consequences are shown for the interaction energy between hydrated ions.
Ab initio study of the electronic spectrum of C2H2+: Investigation of structure of spectra involving low-lying doublet electronic states
Authors: Perić Miljenko , Ostojić Bojana , Engels Bernd
Published: July 2002

Ab initio calculated potential energy surfaces for low-lying doublet electronic states of C2H2+ are employed to investigate the structure of spectra involving these species. Particular attention is paid to the X 2Πu, A 2Ag, and B 2Σu+ states arising by loss of an electron from one of the three uppermost molecular orbitals populated in the ground electronic state of the neutral molecule.
Microscopic derivation of the Hubbard–Onsager–Zwanzig expression of limiting ionic conductivity
Authors: Bagchi Biman
Published: July 2002

An outstanding problem in the theory of ionic conductivity is a derivation of the well-known Hubbard–Onsager–Zwanzig expression for the dielectric friction on an ion from first principles. In fact, microscopic theories have repeatedly failed to reproduce the Hubbard–Onsager–Zwanzig expression under any limiting conditions. We show in this article that the existing molecular theories and the continuum model treatments calculate two entirely different contributions to the friction on the ion. While the former calculates the contribution from the solvent density mode alone, the latter includes only the contribution from the current mode. Thus the existing molecular theories can never be reduced to the Hubbard–Onsager (H–O) theory. In addition, we show that the existing molecular theories become inconsistent for larger ions where the H–O theory is accurate. The reverse is true for smaller ions. An expression is derived here which is valid at both the limits and for all ion sizes and its consequences discussed.
Heat capacity anomaly near the critical point of aniline-cyclohexane
Authors: Rebillot Paul F. , Jacobs D. T.
Published: July 2002

The heat capacity of the liquid–liquid mixture aniline-cyclohexane has been measured for the first time near its upper critical consolute point using an adiabatic calorimeter. Two data runs provide heat capacity data that are fitted by equations with background terms and a critical term. The critical exponent α was determined to be 0.104±0.011, consistent with theoretical predictions. When α was fixed at its theoretical value of 0.11 to determine the critical amplitudes A+ and A−, our value for the amplitude ratio A+/A−=0.59±0.03 was consistent with most experimental determinations in liquid–liquid mixtures, but was slightly larger than either theoretical predictions or recent experimental values in liquid–vapor systems. The two-scale-factor universality ratio X, now consistent among experiments and theories with a value between 0.019 and 0.020, is consistent in this system using one published value for the correlation length, but not with another.
Model studies of nonadiabatic dynamics
Authors: Kohen Daniela , Stillinger Frank H. , Tully John C.
Published: July 2002

Mixed quantum-classical methods are applied to an increasingly challenging series of model problems, and their accuracy is examined. The models involve one light and one heavy degree of freedom, and exhibit substantial nonadiabatic behavior. In all of the models the coupling between the light and heavy particles is linear (harmonic). In addition, different external potentials are applied to the heavy particle only. The energies of the light particle quantum states, as a function of the position of the heavy particle, define a sequence of ground and excited Born–Oppenheimer potential energy curves. Because the light particle experiences a purely harmonic potential, the potential energy curves are parallel and equally spaced for all of the models. In addition, the nonadiabatic couplings among potential energy curves persist for all times due to the nonvanishing linear coupling between light and heavy particles. The model problems were used to test two strategies for carrying out mixed quantum-classical dynamics in systems involving nonadiabatic transitions: mean field and surface hopping. The model calculations reported here suggest that, in cases where linear couplings dominate, the mean field mixed quantum-classical method displays useful accuracy and is robust to the introduction of anharmonic heavy-particle interactions. The model calculations also reveal special situations in which the surface hopping approximation is inappropriate.
Magnetic-field effects on the recombination fluorescence of anthracene cation and perfluorocarbon anions
Authors: Grigoryants V. M. , McGrane S. D. , Lipsky S.
Published: July 2002

The recombination fluorescence from anthracene that has been photoionized at 188 nm in isooctane solutions containing low concentrations of either perfluorocyclobutane (C4F8) or perfluorobenzene (C6F6) has been examined as a function of magnetic-field strength. Level crossing resonances are found at magnetic-field positions predicted by the theory of a hyperfine driven spin evolution on the perfluoroanion (i.e., 0, 452, 757, and 903 G for C4F8 and at 0, 411, and 822 G for C6F6). The widths of the zero-field resonances have been studied as a function of perfluorocarbon concentration. For perfluorocyclobutane, the peak to peak widths of the derivative spectra vary from 6.8 to 13 G over a concentration range from 0.005 to 0.21 M. This variation is quantitatively predicted using the analytic solution to the time-dependent diffusion equation in a Coulomb field to determine how the distribution function for thermalized electron–cation distances evolves into its concentration dependent counterpart for anion–cation separation distances. For perfluorobenzene, the widths are observed to vary over a wider range from 3.0 to 38.5 G for concentrations from 0.0044 to 0.13 M. The additional concentration broadening in this case is consistent with previous estimates of the spin-dephasing rate constant for C6F6–C6F6− charge exchange.
Elementary structural transitions in the amorphous Lennard-Jones solid using multidimensional transition-state theory
Authors: Kopsias Nikolaos P. , Theodorou Doros N.
Published: July 2002

In this work we construct molecular configurations of an amorphous Lennard-Jones solid, which, for given values of the temperature and stress/pressure, constitute local minima of the free energy under the quasi-harmonic approximation (QHA). From the volumetric behavior of these configurations for various values of the temperature and/or pressure we conclude that the QHA is very reasonable for our system and we calculate the values of the isothermal compressibility and of the elastic constants. We then identify representative paths in configuration space leading from one free energy minimum to an adjacent one at fixed temperature and pressure. For each of these transitions or elementary structural relaxation events we determine the corresponding rate constant using the principles of multidimensional transition-state theory. The distribution of free energy barriers for a system of 198 particles is found to be strongly asymmetric and extremely broad, spanning more than 100 kBT at T=0.083ε/kB. The corresponding distribution of activation entropies is narrow, spanning ca. 10kB. There is a strong positive correlation between the volume change and the free energy change accompanying each of these elementary transitions, as would be expected from observed volume relaxation phenomena in glasses.
The liquid–liquid phase transition in ionic solutions: Coexistence curves of tetra-n-butylammonium pricrate in alkyl alcohols
Authors: Kleemeier M. , Wiegand S. , Schröer W. , Weingärtner H.
Published: July 2002

We report coexistence curves of the liquid–liquid phase transition in ionic solutions. The phase diagrams of tetra-n-butylammonium pricrate solutions in a series of alkanols (2-propanol, 1-decanol, 1-dodecanol, 1-tridecanol and 1-tetradecanol) are determined either by measuring the refractive index in the two phase region in one sample of near-critical composition as a function of the temperature, or by direct observation of the composition dependent phase separation temperatures. With the exception of the 2-propanol system, the critical points are in accordance with the predictions by the restricted primitive model. The coexistence curves are analyzed in terms of different composition variables, of which the volume fraction seems to be the most appropriate one. For the volume fraction, deviations from asymptotic Ising behavior are observed which are equally well described by a critical exponent slightly different from the Ising value or by Wegner corrections. Although the deviations are quite small, they show a systematic increase with decreasing dielectric constant of the solvent, thus suggesting an approach to the mean-field case. The significance of this finding is, however, weakened by the fact that the corrections to scaling are also affected by the choice of the composition variable. For all investigated systems, the diameter of the coexistence curve shows a pronounced nonanalytic temperature dependence.
Field-cycling NMR relaxometry of water in synthetic saponites: Lévy walks on finite planar surfaces
Authors: Zavada Tatiana , Kimmich Rainer , Grandjean Jean , Kobelkov Anton
Published: July 2002

Aqueous suspensions of synthetic saponite clays have been studied using field-cycling NMR relaxometry. The saponite platelets form planar surfaces on which water molecules are adsorbed. As the strong-adsorption limit applies, “bulk mediated surface diffusion” as originally suggested by Bychuk and O’Shaughnessy is expected to occur. That is, within the so-called retention time, adsorbate molecules tend to perform Lévy walks along the surfaces. A corresponding formalism (the “pancake model”) is derived and compared with the experimental data. The low-frequency spin-lattice relaxation dispersion can perfectly be described on this basis. The platelet diameter can be estimated from the peculiar adsorbate dynamics on the surfaces in accordance with literature data.
Locating all transition states and studying the reaction pathways of potential energy surfaces
Authors: Westerberg K. M. , Floudas C. A.
Published: July 2002

We propose a new method for calculating all stationary states, including saddle points of all orders, of a potential energy surface based on the αBB deterministic branch and bound global optimization algorithm. This method is based on rigorous optimization methods and offers a theoretical guarantee of enclosing all solutions to the equation ∇V=0. We apply this method to Murrel–Sorbie analytic potential energy surfaces of HCN, HSiN, HBO, and CS2, and to the Empirical Conformational Energy Program for Peptides (ECEPP/3) potential energy surfaces of alanine, alanine dipeptide, and tetra-alanine. For alanine, alanine dipeptide, and tetra-alanine, we proceed to analyze the topography of the potential energy surface by calculating reaction pathways, transition rate matrices, time-evolution of occupation probabilities, and rate disconnectivity graphs.