bluuebunny/crossref_metadata_2025 huggingface dataset:
Phase equilibria of a lattice model for an oil–water–amphiphile mixture
Authors: Mackie Allan D. , Onur Kaan , Panagiotopoulos Athanassios Z.
Published: July 2002

Monte Carlo simulation and quasichemical theory are used to study the phase behavior of a lattice model for oil–water–amphiphile ternary systems. Several short amphiphiles with varying tail and head lengths are studied. Two- and three-phase coexistence regions are observed as well as the formation of microemulsions. In contrast to previous work on this model, quantitative phase diagrams are determined for both symmetric and asymmetric amphiphiles. The removal and regrowth of whole chains by configurational bias methods is used to help equilibration and sampling. Near quantitative agreement is found between quasichemical theory and our simulations except when one of the phases self-assembles or in the vicinity of a critical point. In these areas the quasichemical theory is still qualitatively correct.
The S1–S(1B2–1A1) transition of jet-cooled toluene: Excitation and dispersed fluorescence spectra, fluorescence lifetimes, and intramolecular vibrational energy redistribution
Authors: Hickman Christopher G. , Gascooke Jason R. , Lawrance Warren D.
Published: July 2002

The fluorescence excitation spectrum of the S1–S0(1B2–1A1) transition in jet-cooled toluene has been measured up to 2000 cm−1 above the origin band. Dispersed fluorescence spectra of the major features have been recorded and used to assign the levels observed in excitation. Collisional energy transfer experiments have been used to confirm assignments for some of the lower lying S1 fundamentals that were not accessible via direct optical pumping. The number of known S1 fundamentals has been extended to 13. The dispersed fluorescence spectra reveal the onset of intramolecular vibrational energy redistribution (IVR) at low S1 vibrational energies. Fluorescence lifetimes of all of the major bands observed in the excitation spectrum have been measured. The lifetimes decrease from 86 ns for 00 to 48 ns at an S1 vibrational energy of 1900 cm−1. To alleviate the confusion that exists over the mode numbering in toluene a new scheme is proposed which obviates this problem. This system is similar to that used for other substituted aromatics and should rationalize future work.
Optical measurement of the Soret coefficient and the diffusion coefficient of liquid mixtures
Authors: Zhang K. J. , Briggs M. E. , Gammon R. W. , Sengers J. V.
Published: July 2002

A temperature gradient in a liquid mixture causes a concentration gradient through the Soret effect. We have developed an instrument to measure the Soret effect by observing the bending of a laser beam propagating horizontally through the liquid mixture subjected to a temperature gradient in the vertical direction. Our design of the liquid cell, with a long path length and controlled temperature uniformity, enables us to measure Soret coefficients with an accuracy of 1–3 %, higher than that obtained by previous investigators. In addition, by measuring the dynamic response of the beam deflection after imposition of the temperature gradient, we can also determine the mutual diffusion coefficient. We have applied the technique to mixtures of toluene and n-hexane over the temperature range 5–45 °C and to mixtures of ethanol and water at 25 °C. We have verified that the measured value of the Soret coefficient is independent of the magnitude of the temperature gradient imposed up to 14 K/cm. The Soret coefficients obtained for mixtures of toluene and n-hexane differ from the values obtained by previous investigators with a thermogravitational column method, but they are in good agreement with the results recently obtained by Köhler and Müller with a forced Rayleigh scattering method. For mixtures of ethanol and water, our Soret coefficients agree with the results obtained earlier by Kolodner et al., also with an optical beam-bending technique.
Symplectic integrators tailored to the time-dependent Schrödinger equation
Authors: Gray Stephen K. , Manolopoulos David E.
Published: July 2002

We discuss and illustrate how symplectic integrators can be tailored to solve the time-dependent Schrödinger equation, yielding a large new family of wave packet propagation methods. These methods are interesting because of their algorithmic simplicity and minimal storage requirements. A variety of such methods are obtained. Calculations and comparisons with various other methods are presented for a one-dimensional Morse oscillator and a three-dimensional unimolecular dissociation problem.
A concise redefinition of the solid spherical harmonics and its use in fast multipole methods
Authors: Pérez-Jordá José M. , Yang Weitao
Published: July 2002

Several fast algorithms for the approximation of particle–particle interactions by means of multipole expansions in spherical harmonics have appeared recently. In this letter we present a redefinition of the solid spherical harmonics that is real and gives simple expressions for the evaluation of the functions and their derivatives. Application to the recursive bisection method [J. M. Pérez-Jordá and W. Yang, Chem. Phys. Lett. 247, 484 (1995)] greatly improves its performance.
Experimental and theoretical vibrational overtone spectra of vCH=3, 4, 5, and 6 in formaldehyde (H2CO)
Authors: Luckhaus D. , Coffey M. J. , Fritz M. D. , Crim F. F.
Published: July 2002

We present the first photoacoustic absorption spectra of higher C–H stretching overtones in formaldehyde up to the visible spectral region. The analysis of the coarse rotational structure on the basis of asymmetric rotor simulations provides band centers up to the fifth overtone (vCH=6). A two dimensional normal coordinate model for the C–H stretching degrees of freedom explains the observed polyad structure of the spectrum using ab initio (MP2/6-311G**) potential energy and electric dipole functions. The observed intensity distribution reflects the increasing local mode character of higher overtone wave functions within the normal coordinate subspace.
Breaking symmetry with hydrogen bonds: Vibrational predissociation and isomerization dynamics in HF–DF and DF–HF isotopomers
Authors: Farrell John T. , Suhm Martin A. , Nesbitt David J.
Published: July 2002

High-resolution near-IR spectra of jet-cooled HF–DF and DF–HF isotopomers are presented and analyzed for fundamental excitation in the HF-stretching region (3870 cm−1–3960 cm−1) and DF-stretching region (2840 cm−1–2880 cm−1), based on direct absorption of tunable IR difference frequency radiation in a slit-jet supersonic expansion (10 K). Spectra are obtained for excitation of all four stretching modes, *HF–DF, HF–*DF, *DF–HF, and DF–*HF (* denotes the vibrationally excited subunit), which probe both the hydrogen/deuterium bond donor and acceptor moieties in the complex. Vibrational redshifts and predissociation broadening measurements are compared with full 6D quantum calculations on theoretical HF dimer potential surfaces, which exhibit trends in qualitatively good agreement with experiment. Each of the three DF-stretch-excited bands are fit to spectroscopic precision (Δν≲0.0001 cm−1) by conventional high-resolution rotational analyses, whereas each of the four corresponding HF-stretch-excited bands appear extensively perturbed (Δν≊0.01 cm−1). This H/D isotope effect is interpreted as vibrational state mixing of the HF-stretch-excited species with a dense manifold of combination band states built on DF-stretch excitation, and therefore reflects intermolecular energy flow in the complex. Such vibrational state mixing is further corroborated by observation of ‘‘dark’’ state transitions that can be tentatively assigned to *HF–DF isomer interacting with the nearly isoenergetic DF–*HF isomer. This state mixing would correspond to a vibrationally promoted ‘‘isomerization’’ over the tunneling barrier, and yield a spectroscopic measure of the difference in hydrogen bond dissociation energies [ΔD0=74.7(5) cm−1] for the HF–DF and DF–HF isotopomers.
Nonequilibrium fluctuation theory in electrochemical nucleation. I. Derivation of nucleation current equations
Authors: Tadano Atsushi , Aogaki Ryoichi
Published: July 2002

In this paper, mathematical procedures to describe the time development of nonequilibrium fluctuations in nucleation are examined. General equations depicting the time development of amplitudes of the fluctuations are firstly deduced. Secondly, using the Rayleigh theorem, the rms values of the fluctuations are obtained, being defined as the average values. Finally, some analytical equations of the nucleation current are derived for the characteristic parts of current-time transient as follows: After applying a constant potential step to the electrode, the current attains a minimum state, which gives the ratio of the average critical concentration fluctuation to the autocorrelation distance. Then, the nucleation current arising from unstable growth of the fluctuations is derived as a function of supporting electrolyte concentration, depositing metal ion concentration and overpotential. This is attributed to the fact that the instability occurs from the electrostatic interaction between electrode surface and solution particles in electric double layer. As the growth progresses, the enhancement of diffusion process yields the change of rate-controlling step to the electron transfer process, so that the observed current approaches a maximum state composed of reaction current. After passing the maximum current, the increase of concentration overpotential leads to decrease of overall double-layer overpotential. The nucleation process regains diffusion control; it is concluded that at the final stage, the diffusion limiting current flows in the same manner as the Cottrel equation.
Nonequilibrium fluctuation theory in electrochemical nucleation. III. Experimental determination of fluctuation growth rate in silver nucleation onto platinum electrode
Authors: Tadano Atsushi , Aogaki Ryoichi
Published: July 2002

Following part II, in this paper, some analytical equations concerning nucleation current were experimentally examined for silver nucleation onto platinum electrode in AgNO3+NaNO3 solution. After passing the minimum state, the nonequilibrium fluctuations being to unstably grow, being accompanied with active nucleation. Resultant current can be theoretically expressed as an exponential function of the second order of time, and its time coefficient depends on the concentrations of both depositing metal ion and supporting electrolyte and also the overpotential. These theoretical predictions were experimentally ascertained. Accordingly, it was concluded that the nucleation current is described by the quantities with regard to the fluctuations and the double layer. Furthermore, the maximum peak current appearing after nucleation growth current was investigated. As discussed before, the semilog plot of the current against the overpotential gave almost the same slope of the Tafel equation for silver deposition. Observed values of the current were almost equal to those measured by applying current steps to silver electrode. Finally, the diffusion current was expected after passing the peak current. Then, it was clarified that limiting diffusion current, indifferent to the initial value of the overpotential, flows according to the Cottrel equation.
Semiclassical-limit molecular dynamics on multiple electronic surfaces
Authors: Martens Craig C. , Fang Jian-Yun
Published: July 2002

In this paper, we present a new approach to treating many-body molecular dynamics on coupled electronic surfaces. The method is based on a semiclassical limit of the quantum Liouville equation. The formal result is a set of coupled classical-like partial differential equations for generalized distribution functions which describe both the nuclear probability densities on the coupled surfaces and the coherences between the electronic states. The Hamiltonian dynamics underlying the evolution of these distributions is augmented by nonclassical source and sink terms, which allow the flow of probability between the coupled surfaces and the corresponding formation and decay of electronic coherences. The formal results are shown analytically to reproduce the well-known Rabi and Landau–Zener results in appropriate limits. In addition, a direct numerical solution of the phase space partial differential equations is performed, and the results compared with exact quantum solutions for a model curve-crossing problem, yielding excellent agreement. Future trajectory-based implementation of the method in molecular dynamics simulations is also discussed.
Nonequilibrium fluctuation theory on pitting dissolution. III. Experimental examinations on critical fluctuation and its growth process in nickel dissolution
Authors: Asanuma Miki , Aogaki Ryoichi
Published: July 2002

Following part II, current–time transient in nickel pitting dissolution has been examined. After the passive film is broken, the resultant diffusion current decreases to the minimum value. The minimum current, as shown in part I, gives the ratio of the average critical concentration fluctuation on the completely active surface to its autocorrelation distance. Therefore, using the experimental data of the minimum current mentioned above and the average concentration fluctuation obtained in part II, the critical autocorrelation distance has been calculated. Since, in the stable region, the fluctuation is suppressed to maintain that the electrode surface is flat and stable, the autocorrelation distance tends to approach infinity. Therefore, the distance at the critical state is expected to be extraordinarily large; the value of about 1 mm has been actually obtained. This value is regarded to have some generality independent of the characteristics of the reaction because it is purely determined by the thermal motion of the solution particles. In fact, it is approximately agreeable with the value obtained for silver nucleation. Then, the fluctuations start unstable growth. As shown in part I, reflecting the fact that the instability arises from the electrostatic interaction of the solution particles in the electric double layer, the growth current substantially depends on the concentration of the supporting electrolyte and the externally applied electrode potential.
Proton-coupled electron transfer reactions in solution: Molecular dynamics with quantum transitions for model systems
Authors: Fang Jian-Yun , Hammes-Schiffer Sharon
Published: July 2002

A general minimal model for proton-coupled electron transfer (PCET) reactions in solution is presented. This model consists of three coupled degrees of freedom that represent an electron, a proton, and a solvent coordinate. Altering the parameters in this model generates a wide range of PCET dynamics. This paper focuses on three model systems corresponding to three different mechanisms: a concerted mechanism in which the proton and electron are transferred simultaneously, a sequential mechanism in which the proton is transferred prior to the electron, and a sequential mechanism in which the electron is transferred prior to the proton. The surface hopping method ‘molecular dynamics with quantum transitions’ (MDQT) is applied to these model systems. The proton and electron coordinates are treated quantum mechanically, and the solvent coordinate is treated classically. Thus the adiabatic quantum states are two-dimensional wavefunctions that depend on both the electron and the proton coordinates. The MDQT method incorporates nonadiabatic transitions between these mixed proton/electron adiabatic quantum states. The MDQT simulations presented in this paper provide insight into the fundamental physical principles and the dynamical aspects of PCET reactions. Nonadiabatic effects are shown to play an important role in determining the rates and mechanisms of PCET reactions. This represents the first application of MDQT to a system in which both a proton and an electron are treated quantum mechanically.
Dynamics of regular star polymers: Viscoelasticity and oscillatory flow birefringence
Authors: Ganazzoli Fabio
Published: July 2002

The dynamics of linear and regular star polymers in a good solvent and in the Θ state is investigated theoretically considering the viscoelastic and the oscillatory flow birefringence response under an alternating shear deformation. We show that in a good solvent the phase angles of the viscoelastic and of the birefringence response are different. The physical reason of this difference is that viscoelasticity depends on the orientation of the chain stresses with the applied deformation, whereas birefringence measurements probe the orientation of the chain bonds. Only in the phantom chain the two orientations are the same, whereas this is no more true in a good solvent because of the long-range intramolecular interactions. Moreover, the phase angles show a maximum in the case of stars at ω≅1/τ1, where ω is the applied frequency and τ1 the longest relaxation time, that is strictly related to a shoulder in the imaginary part of the complex modulus. The reasons of this feature, which is absent in linear chains, are discussed in connection with the discrete spectrum of relaxation times.
A high resolution pulsed field ionization photoelectron study of O2 using third generation undulator synchrotron radiation
Authors: Hsu C.-W. , Heimann P. , Evans M. , Stimson S. , Fenn P. T. , Ng C. Y.
Published: July 2002

We have improved a newly developed experimental scheme for high resolution pulsed field ionization photoelectron (PFI-PE) studies [Hsu et al., Rev. Sci. Instrum. (in press)] using the high resolution monochromatized multibunch undulator synchrotron source of the Chemical Dynamics Beamline at the Advanced Light Source. This improved scheme makes possible PFI-PE measurements with essentially no contamination by background electrons arising from direct photoionization and prompt autoionization processes. We present here a preliminary analysis of the rotationally resolved PFI-PE spectrum for O2 obtained at a resolution of 0.5 meV (full-width-at-half-maximum) in the photon energy range of 18.1–19.4 eV, yielding accurate ionization energies for the transitions O2+(b 4Σg−, v+=0–9, N+=1)←O2(X 3Σg−, v=0, N=1).
Time-resolved vibrationally mediated photodissociation of HNO3: Watching vibrational energy flow
Authors: Bingemann Dieter , Gorman Michael P. , King Andrew M. , Crim F. Fleming
Published: July 2002

Ultrafast excitation of an O–H stretching vibrational followed by photodissociation of the energized molecules allows direct observation of the time for intramolecular energy redistribution in isolated nitric acid. We excite the first overtone of the O–H stretch vibration in HNO3 with a 100 fs laser pulse. A second, time-delayed pulse preferentially photodissociates molecules having vibrational excitation in modes orthogonal to the O–H stretch. The photodissociation yield increases as a function of time because energy flows out of the initially excited O–H bond into other more efficiently dissociated vibrations. The single exponential time constant for this intramolecular vibrational relaxation is 12 ps, consistent with moderate coupling of the O–H stretch to states close in energy.
Variational energy band theory for polarons: Mapping polaron structure with the Toyozawa method
Authors: Zhao Yang , Brown David W. , Lindenberg Katja
Published: July 2002

In this article we revisit from a contemporary perspective a classic problem of polaron theory in one space dimension following the variational approach originally taken by Toyozawa. Polaron structure is represented by variational surfaces giving the optimal values of the complete set of exciton and phonon amplitudes for every value of the joint exciton-phonon crystal momentum κ. Through this exfoliation of the exciton-phonon correlations comprising the polaron, characteristic small polaron, large polaron, and nearly free phonon structures are identified, and the manner in which these compete and/or coexist is examined in detail. Through such examination, the parameter space of the problem is mapped, with particular attention given to problematic areas such as the highly quantum mechanical weak-coupling regime, the highly nonlinear intermediate-coupling regime, and to the self-trapping transition that may be said to mark the onset of the strong-coupling regime. Through such examination of the complete parameter space at all κ, it is found that the common notion of a self-trapping phenomenon associated with κ=0 is a limiting aspect of a more general finite-κ phenomenon. Quantities such as phonon number distributions, complete ground state energy bands, and effective masses are obtained for all κ. The inverse problem of associating localized functions with the variational energy bands is addressed, with attention given to the concept of solitons and with the explicit construction of polaron Wannier states. The successes and failures of the Toyozawa method are assessed.
A new quantum transition state theory
Authors: Pollak Eli , Liao Jie-Lou
Published: July 2002

An old challenge in rate theory is the formulation of a quantum thermodynamic theory of rates which gives accurate estimates but does not demand any real time propagation. In this paper we attempt to answer the challenge by extending an idea suggested by Voth, Chandler and Miller [J. Phys. Chem. 93, 7009 (1989)]. A new quantum expression for the rate is derived by replacing the exact time dependent dynamics with the analytically known dynamics of a parabolic barrier and utilizing the symmetrized thermal flux operator. The new rate expression is exact for a parabolic barrier, and leads by derivation rather than by ansatz to a phase space integration of a Wigner thermal flux distribution function. The semiclassical limit is similar but not identical to Miller’s semiclassical transition state theory. Numerical computations on the symmetric and asymmetric one dimensional Eckart barrier give results which are equal to or greater than the exact ones, as expected from a transition state theory. In contrast to other approaches, the present theory is a leading term in an expansion which may be used to systematically improve the results and assess their validity.
Molecular scale precursor of the liquid–liquid phase transition of water
Authors: Shiratani Eli , Sasai Masaki
Published: July 2002

Spatiotemporal fluctuations of the local structure in liquid water are studied with the molecular-dynamics simulation. At temperatures around and above the melting point, each molecule alternately goes through the structured period and the destructured period. Lifetime of each period spreads from several hundred fs to 10 ps at 0 °C at 1 atm. The order parameter to describe this structural switching fluctuations is derived by carefully filtering out the fast oscillating components from the simulation data. By analyzing the neutron-weighted pair correlation function, we find that the clusters of the structured molecules and the clusters of the destructured molecules are similar to the clusters of low-density amorphous (LDA) ice and the clusters of high-density amorphous (HDA) ice, respectively. Simulated liquid water is, therefore, a composite of the LDA-like clusters and the HDA-like clusters even at temperatures well above the melting point. The large amplitude structural fluctuation of water at around and above the melting temperature should be regarded as the molecular-scale precursor of the possible liquid–liquid phase transition in the supercooled region.
Density and index of refraction of water ice films vapor deposited at low temperatures
Authors: Westley M. S. , Baratta G. A. , Baragiola R. A.
Published: July 2002

The density of 0.5–3 μm thick vapor-deposited films of water ice were measured by combined optical interferometry and microbalance techniques during deposition on an optically flat gold substrate from a capillary array gas source. The films were of high optical quality with an index of refraction of 1.29±0.01 at 435.8 nm, a density of 0.82±0.01 g/cm3, and a porosity of 0.13±0.01. In contrast to previous studies, none of the measured properties exhibited any significant variation with growth rate or temperature over the range studied (0.6–2 nm/min, 20–140 K).
Molecular dynamics in low-spin excited states
Authors: Frank Irmgard , Hutter Jürg , Marx Dominik , Parrinello Michele
Published: July 2002

A Kohn–Sham-like formalism is introduced for the treatment of excited singlet states. Motivated by ideas of Ziegler’s sum method and of restricted open-shell Hartree–Fock theory, a self-consistent scheme is developed that allows the efficient and accurate calculation of excited state geometries. Vertical as well as adiabatic excitation energies for the n→π* transitions of several small molecules are obtained with reasonable accuracy. As is demonstrated for the cis-trans isomerization of formaldimine, our scheme is suited to perform molecular dynamics in the excited singlet state. This represents a first step towards the simulation of photochemical reactions of large systems.
Stress relaxation in a diatomic liquid
Authors: Picu R. C. , Weiner J. H.
Published: July 2002

By the use of nonequilibrium molecular dynamics we have studied the stress relaxation following imposition of a constant-volume elongation in the x1 direction on a model diatomic liquid. Three consecutive modes of relaxation of the stress difference τ=t11−12(t22+t33) are found, each governed by exponentials e−αit with α1>α2>α3. Each mode is shown to correspond to the return to isotropy of a different characteristic of the liquid structure that has been rendered anisotropic by the deformation, namely (1) r̄(θ), the mean distance from a generic atom of interacting atoms in the θ direction, with the angle θ measured from the stretch axis x1; (2) N̄(θ), the mean number density of interacting atoms in the θ direction; and (3) 〈〈P2(θb)〉〉, a measure of the orientation θb of molecular axes with respect to x1. The first two modes are identical in form to those studied previously [R. C. Picu and J. H. Weiner, J. Chem. Phys. 107, 7214 (1997)] for stress relaxation in a monatomic liquid, but their rates of decay differ because of differences in packing and in atomic mobility. During the third mode of relaxation it is found that τ=C〈〈P2(θb)〉〉, where C may be understood in terms of the intrinsic stress tensor, a tensor referred to a coordinate system fixed to the molecule [J. Gao and J. H. Weiner, J. Chem. Phys. 90, 6749 (1989)]. The relevance of these results to stress relaxation in polymer melts is discussed.
Brownian motion of spins revisited
Authors: Miyazaki Kunimasa , Seki Kazuhiko
Published: July 2002

A phenomenological theory to describe the spin relaxation is presented. We consider one classical spin surrounded by a medium which serves as a heat bath. A Langevin equation and associated Fokker-Planck equation for the spin and the fluctuating magnetic field are derived. They guarantee the thermal equilibrium state as stationary solutions. In the narrowing and high temperature limits, the equation for the spin reduces to the well-known Landau-Lifshitz equation and Bloch equation.
The contribution of particle core and surface to strain, disorder and vibrations in thiolcapped CdTe nanocrystals
Authors: Rockenberger Jörg , Tröger Larc , Rogach Andrey L. , Tischer Markus , Grundmann Marius , Eychmüller Alexander , Weller Horst
Published: July 2002

Thiolcapped CdTe nanocrystals (18 Å diameter) are investigated by extended x-ray absorption fine structure (EXAFS) measurements between 8 and 290 K at the Cd and Te K-edge. The different coordination sites of Cd in the particle core and at the surface are identified and are consistent with a CdTe particle core which is covered by a Cd–SR surface layer (R=organic rest). We are able to study individually changes of the properties of the interior and the surface of the nanoparticle with respect to bulk material. Structure and dynamics of the CdTe nanocrystals are mainly altered by the requirement of heteroepitaxial growth at the interface between the CdTe core and the Cd–SR shell. As a consequence, bond lengths and Debye temperatures of particle core and surface show a tendency to accommodate in thiolcapped CdTe nanocrystals. The trends in bond lengths variation observed in the experiment can be reproduced by calculations of the strain distribution induced by the lattice mismatch in a simplified isotropic model of a spherical CdTe nanoparticle which is encapsulated by a CdS bulklike shell. The experiment also shows a significantly enhanced static disorder both in particle core and surface. In contrast, the asymmetry of the radial pair distribution function of the Cd–S surface bonds is strongly elevated whereas the Cd–Te bonds in the interior of the particle show no enhancement with respect to bulk material. Experimental coordination numbers at the Cd and Te K-edge and the Cd/Te edge jump ratio are in good agreement with the expected values of a Cd54Te32(SR)528−-nanocrystal. This structural model is a larger homolog of a series of analogous CdS nanoparticles and consists of a CdTe tetrahedron which is partially coated by a Cd–SR surface layer.
Multiphoton transition moments and absorption cross sections in coupled cluster response theory employing variational transition moment functionals
Authors: Hättig Christof , Christiansen Ove , Jo/rgensen Poul
Published: July 2002

Based on an analysis of the first residues of coupled cluster response functions we devise variational functionals from which the transition moments for n-photon excitations can be calculated as nth derivatives. Combining these functionals with variational perturbation theory, we obtain a new approach for the derivation of multiphoton transition moments which allows us to utilize the full strengths of variational perturbation theory without the roundabout way via residues of response functions. Coupled cluster multiphoton transition moments derived by this approach are formally equivalent to the one identified from the first residues of the ground state response functions. The introduction of the variational functionals makes the mathematical structure of the transition moments more transparent and provides an interpretation of intermediates in terms of responses of excited state vectors and Lagrangian multipliers. 2n+1 and 2n+2 rules are formulated for the transition moments and build the basis for a straightforward derivation of a computational efficient formulation. The strength of the new approach is demonstrated by the derivation of three- and four-photon transition moments.
Measurement of the absolute yield of CO(a 3Π)+O products in the dissociative recombination of CO2+ ions with electrons
Authors: Skrzypkowski Miroslaw P. , Gougousi Theodosia , Johnsen Rainer , Golde Michael F.
Published: July 2002

A flowing-afterglow technique is described for measuring the absolute yield of a radiative product state from ion–electron recombination. The technique is applied to CO2++e− dissociative recombination. The measured yield of CO(a 3Π)+O(3P) is 0.29±0.10. This includes cascade from higher triplet states of CO. The vibrational distribution in CO(a 3Π,v=0–3) is approximately Boltzmann, with an effective temperature of 4200±300 K. The measured rate constant for quenching of CO(a) by CO2 is (1.0±0.2)×10−11 cm3 s−1, somewhat lower than previous measurements.
On the use of the quasi-Gaussian entropy theory in noncanonical ensembles. I. Prediction of temperature dependence of thermodynamic properties
Authors: Amadei A. , Apol M. E. F. , Berendsen H. J. C.
Published: July 2002

In previous articles we derived and tested the quasi-Gaussian entropy theory, a description of the excess Helmholtz free energy in terms of the potential energy distribution, instead of the configurational partition function. We obtained in this way the temperature dependence of thermodynamic functions in the canonical ensemble assuming a Gaussian, Gamma or Inverse Gaussian distribution. In this article we extend the theory to describe the temperature dependence of thermodynamic properties in an exact way in the isothermal-isobaric and grand canonical ensemble, using the distribution of the appropriate heat function. For both ensembles restrictions on and implications of these distributions are discussed, and the thermodynamics assuming a Gaussian or (diverging) Gamma distribution is derived. These cases have been tested for water at constant pressure, and the results for the latter case are satisfactory. Also the distribution of the heat function of some theoretical model systems is considered.
Static and dynamic polarizabilities and first hyperpolarizabilities for CH4, CF4, and CCl4
Authors: Bishop David M. , Gu Feng Long , Cybulski Sławomir M.
Published: July 2002

Complete polarizabilities and first hyperpolarizabilities (which include zero-point-vibrational averaging and other vibrational corrections) have been calculated at the self-consistent-field and Møller–Plesset second order perturbation levels of approximation for CH4, CF4, and CCl4. As well as the static values, α(−ω;ω), β(−ω;ω,0), and β(−2ω;ω,ω) have also been determined. The values of β(−2ω;ω,ω) for CCl4 are germane to a proposed standard for the determination of nonlinear optical properties in solution. For CCl4, our best estimate of β(−2ω;ω,ω) at ℏω = 0.040 a.u. is 13.6 a.u., the experimentally-derived gas value at ℏω = 0.043 a.u. is 20.7± 1.6 a.u. This latter value has been used to substantiate the proposed liquid phase value.
Rotationally resolved pulsed field ionization photoelectron study of O2+(B 2Σg−,2Σu−; v+=0–7) at 20.2–21.3 eV
Authors: Evans M. , Stimson S. , Ng C. Y. , Hsu C.-W. , Jarvis G. K.
Published: July 2002

We have obtained rotationally resolved pulsed field ionization photoelectron (PFI-PE) spectra of O2 in the energy range of 20.2–21.3 eV, covering the ionization transitions of O2+(B 2Σg−, v+=0–7, N+)←O2(X 3Σg−, v″=0, N″). Only the ΔN=−2, 0, and +2 (or O, Q, and S) rotational branches are observed in the PFI-PE bands for O2+(B 2Σg−, v+=0–7), indicating that the outgoing electron continuum channels with angular momenta l=1 and 3 dominate in the ionization transitions. This experiment allows the determination of accurate spectroscopic constants, such as ionization energy (20.29825±0.0005 eV) for the formation of O2+[B 2Σg−, v+=0, N+=1 (F2)] from O2(X 3Σg−, v″=0, N″=1), vibrational constants (ωe+=1152.91 cm−1, ωe+χe+=20.97 cm−1_, and rotational constants (Be+=1.255±0.0015 cm−1, αe+=0.0241±0.00037 cm−1_ for O2+(B 2Σg−, v+). The (nominal) effective lifetimes for high-n Rydberg states converging to O2+(B 2Σg−, v+=0–6) are measured to be ≈0.2–0.6 μs, which are significantly shorter than those of ≈1.9 μs observed for O2+(b 4Σg−, v+=0–5). The shorter (nominal) effective lifetimes for high-n Rydberg states converging to O2+(B 2Σg−, v+=0–6) are attributed to the higher kinetic energy releases (or velocities) of O++O fragments resulting from predissociation of the O2+(B 2Σg−, v+=0–6) ion cores. Rotationally resolved PFI-PE measurements also make possible the identification of the weak vibrational progression with the origin at 20.35 eV as associated with transitions to O2+(2Σu−, v+=0–7). The analysis of the rotationally resolved PFI-PE bands for O2+(2Σu−, v+=0 and 1) has yielded accurate rotational constants and IE values for these states. The rotational structures resolved in the O2+(2Σu−, v+=0 and 1) PFI-PE bands are contributed overwhelmingly by the ΔN=−3, −1, +1, and +3 (or N, P, R, and T) rotational branches, showing that the angular momenta for the outgoing photoelectron are restricted to l=0, 2, and 4. Based on simulation of the observed rotational structures, we also obtain the predissociative lifetimes for O2+(B 2Σg−, v+=0–7) and O2+(2Σu−, v+=0–1) to be in the range of 0.45–2 ps.
Kinetic-energy dependence of competitive spin-allowed and spin-forbidden reactions: V++CS2
Authors: Rue Chad , Armentrout P. B. , Kretzschmar Ilona , Schröder Detlef , Harvey Jeremy N. , Schwarz Helmut
Published: July 2002

The kinetic-energy dependence of the V++CS2 reaction is examined using guided ion-beam mass spectrometry. Several different ion sources are used to systematically vary the V+ electronic state distributions and elucidate the reactivities of both the ground and excited state V+ cation. The cross section for VS+ formation from ground state V+(5D) exhibits two endothermic features corresponding to the formation of ground state VS+(3Σ−) and excited state VS+(5Π). The thresholds for these two processes are in good agreement with theoretically determined excitation energies. The cross section for spin-forbidden formation of ground state VS+(3Σ−) exhibits an unusual variation with kinetic energy that is attributed to the energy dependence of the surface-crossing probability. From the thresholds associated with the formation of VS+ and V(CS)+, D0(V+–S)=3.72±0.09 eV and D0(V+–CS)=1.70±0.08 eV are derived. Further, circumstantial evidence for formation of a high-energy isomer of V(CS)+ is obtained.
Dynamic density functional theory of fluids
Authors: Marconi Umberto Marini Bettolo , Tarazona Pedro
Published: July 2002

We present a new time-dependent density functional approach to study the relaxational dynamics of an assembly of interacting particles subject to thermal noise. Starting from the Langevin stochastic equations of motion for the velocities of the particles we are able by means of an approximated closure to derive a self-consistent deterministic equation for the temporal evolution of the average particle density. The closure is equivalent to assuming that the equal-time two-point correlation function out of equilibrium has the same properties as its equilibrium version. The changes in time of the density depend on the functional derivatives of the grand canonical free energy functional F[ρ] of the system. In particular the static solutions of the equation for the density correspond to the exact equilibrium profiles provided one is able to determine the exact form of F[ρ]. In order to assess the validity of our approach we performed a comparison between the Langevin dynamics and the dynamic density functional method for a one-dimensional hard-rod system in three relevant cases and found remarkable agreement, with some interesting exceptions, which are discussed and explained. In addition, we consider the case where one is forced to use an approximate form of F[ρ]. Finally we compare the present method with the stochastic equation for the density proposed by other authors [Kawasaki, Kirkpatrick etc.] and discuss the role of the thermal fluctuations.
Mesoscopic model for solvent dynamics
Authors: Malevanets Anatoly , Kapral Raymond
Published: July 2002

Complex fluids such as polymers in solution or multispecies reacting systems in fluid flows often can be studied only by employing a simplified description of the solvent motions. A stochastic model utilizing a synchronous, discrete-time dynamics with continuous velocities and local multiparticle collisions is developed for this purpose. An H theorem is established for the model and the hydrodynamic equations and transport coefficients are derived. The results of simulations are presented which verify the properties of the model and demonstrate its utility as a hydrodynamics medium for the study of complex fluids.
Multistate continuum theory for multiple charge transfer reactions in solution
Authors: Soudackov Alexander , Hammes-Schiffer Sharon
Published: July 2002

In this article we present a multistate continuum theory for multiple charge transfer reactions such as proton-coupled electron transfer and multiple proton transfer reactions. The solute is described with a multistate valence bond model, the solvent is represented as a dielectric continuum, and the transferring protons are treated quantum mechanically. This theory provides adiabatic free energy surfaces that depend on a set of scalar solvent variables corresponding to the individual charge transfer reactions. Thus this theory is a multidimensional analog of standard Marcus theory for single charge transfer reactions. For processes involving significant inner-sphere (i.e., solute) reorganization, the effects of solute intramolecular vibrations can be incorporated into the adiabatic free energy surfaces. The input quantities required for this theory are gas phase valence bond matrix elements fit to standard quantum chemistry calculations and solvent reorganization energy matrix elements calculated with standard continuum electrostatic methods. The goal of this theory is to provide insight into the underlying fundamental physical principles dictating the mechanisms and rates of multiple charge transfer reactions.
Characterization of fullerenes and fullerene derivatives by small-angle neutron scattering and transmission measurements
Authors: Melnichenko Y. B. , Wignall G. D. , Compton R. N. , Bakale G.
Published: July 2002

Small-angle neutron scattering (SANS) has been shown to be an appropriate technique for the structural characterization of fullerenes in solvents with strong SANS contrast (e.g., CS2). The dimensions of C60, C70, and C84 moieties derived from neutron scattering via standard Guinier analysis are typically ∼8% higher than calculated from the atomic coordinates. However, when the SANS data are fitted to an appropriate model form factor, there is good agreement between the measured and calculated values and the results are independent of the fullerene concentration within the experimental errors. Deuterated solvents (e.g., toluene-d8) have a high scattering length density (SLD), which is close to that of C60, C70, and C84, so there is virtually no SANS contrast with the solvent and these particles are practically “invisible” in such media. Conversely, the negative scattering length of hydrogen lowers the SLD, giving strong contrast with toluene-d8 and SANS may therefore be used to study the size and shapes of buckyballs modified with H1-containing materials. These may also be studied via neutron transmission measurements and this paper describes complementary studies of cyclohexane-substituted fullerenes.
A quantum mechanical view of molecular alignment and cooling in seeded supersonic expansions
Authors: Aquilanti V. , Ascenzi D. , de Castro Vı́tores M. , Pirani F. , Cappelletti D.
Published: July 2002

Experimental investigations on the collisional alignment of the rotational angular momentum, occurring in supersonic seeded beams and in drift tubes, have recently documented a strong dependence of the observed effects on the final molecular velocity. The present investigation aims at elucidating the possible mechanisms at the molecular collision level. Quantum state-to-state differential scattering cross sections, calculated for the prototype system O2–He, for an interaction potential previously obtained in this laboratory, exhibit propensities relevant to reveal nature and selective role of the elastic and inelastic scattering events, participating in the overall mechanisms which lead to molecular alignment and cooling. The present analysis shows that the dynamics of such phenomena crucially depends on the initial and final rotational state, on the collision energy, on the involved orbital angular momentum and therefore alternative routes are possible for molecular polarization and relaxation. These routes lead to scattering into specific angular cones and therefore observations from different experiments provide complementary pieces of information which, exploiting studies of various molecular systems under diverse experimental conditions, can be correlated in a single mosaic.
Photoelectron spectroscopy of the CH3N− ion
Authors: Travers Michael J. , Cowles Daniel C. , Clifford Eileen P. , Ellison G. Barney , Engelking Paul C.
Published: July 2002

We have observed the negative ion photoelectron spectrum of the methylnitrene ion, CH3N−, and measured the electron affinity of methylnitrene, EA(CH3N)=0.022±0.009 eV. In addition to detaching the methylnitrene anion to the ground state of CH3N(X̃ 3A2), we also detect the first electronically excited state of methylnitrene, ã 1E. We measure the singlet/triplet splitting to be ΔE(ã 1E−X̃ 3A2)=1.352±0.011 eV. The photoelectron spectrum of CH3N ã 1E contains relatively sharp vibronic structure. Unlike the spectra from H2CC−, the photoelectron spectra for CH3N− show no evidence for a barrier separating the rearrangement of singlet methylnitrene to methyleneimine, [CH31N] → CH2=NH.
Nuclear spin relaxation in paramagnetic complexes of S=1: Electron spin relaxation effects
Authors: Bertini Ivano , Kowalewski Jozef , Luchinat Claudio , Nilsson Tomas , Parigi Giacomo
Published: July 2002

Electron spin relaxation for an S=1 system and its field dependence in the presence of static zero-field splitting (ZFS) has been described and incorporated in a model for nuclear spin-lattice relaxation in paramagnetic complexes in solution, proposed earlier by the group in Florence. Slow reorientation is assumed and the electron spin energy level structure (at any orientation of the molecule with respect to the laboratory frame) is described in terms of the Zeeman interaction and of the static ZFS. The electron spin relaxation is assumed to be caused by a transient ZFS modulated by the deformation of the complex described as a distortional (or pseudorotational) motion and the Redfield theory is used to derive the electron spin relaxation matrices. In the description of the electron spin relaxation we neglect any contribution from mechanisms involving modulation by reorientation, such as those of the static ZFS and the less important Zeeman interaction, as we limit ourselves to the slow-rotation limit (i.e., τR≫τS). This in general covers the behavior of proteins and macromolecules. The decomposition (DC) approximation is used, which means that the reorientational motion and electron spin dynamics are assumed to be uncorrelated. This is not a serious problem, due to the slow-rotation condition, since reorientational and distortional motions are time-scale separated. The resulting nuclear magnetic relaxation dispersion (NMRD) profiles obtained using the Florence model are calculated and compared with the calculations of the Swedish approach, which can be considered essentially exact within the given set of assumed interactions and dynamic processes. That theory is not restricted by the Redfield limit and can thus handle electron spin relaxation in the slow-motion regime, which is a consequence of not explicitly defining any electron spin relaxation times. Furthermore, the DC approximation is not invoked, and in addition, the electron spin relaxation is described by reorientationally modulated static ZFS and Zeeman interaction besides the distortionally modulated transient ZFS. The curves computed with the Florence model show a satisfactory agreement with these more accurate calculations of the Swedish approach, in particular for the axially symmetric static ZFS tensor, providing confidence in the adequacy of the electron spin relaxation model under the condition of slow rotation. The comparison is also quite instructive as far as the physical meaning of the electron spin relaxation and of its interplay with the nuclear spin system are concerned.