bluuebunny/crossref_metadata_2025 huggingface dataset:
Raman scattering by high-density dispersions. I. Formulation of theory and application to a pair of coated particles
Authors: Xu Muliang , Dignam Michael J.
Published: July 2002

The methods developed in our earlier papers, dealing with Raman scattering from small isolated, spherical particles, uniformly coated with radially uniaxial molecules, are extended to include scattering driven by applied multipolar fields. Such fields are generated by the action of the incident electromagnetic radiation on extraneous material, such as other particles belonging to the same high-density dispersion. In this paper we treat inelastic scattering associated with the Raman dipole matrix elements of the adsorbed molecules ignoring other contributions. Analytical expressions are developed for calculating the primary Raman–Stokes moments given the amplitudes of the applied multipolar fields. As a test of the practicability of the procedure, a full calculation for parallel polarization has been performed for two identical contacting, CO-coated Ag particles oriented with their line of centers parallel to the direction of polarization of the incident radiation. For this case, the effect of interparticle coupling is to broaden and intensify the excitation spectrum, extending the region of intense Raman scattering to much lower frequencies. Intensity enhancements approaching 106 are calculated for frequencies as low as 80% of the single-particle surface-plasmon resonance frequency.
Intramolecular excited-state proton transfer in jet-cooled 2-substituted 3-hydroxychromones and their water clusters
Authors: Ito Akitoshi , Fujiwara Yoshihisa , Itoh Michiya
Published: July 2002

Intramolecular excited-state proton transfer (ESPT) of 3-hydroxychromone (3-HC), 3-hydroxyflavone (3-HF) and 2-(2-naphthyl)-3-hydroxychromone (2-NHC), and of their water clusters, was investigated in a supersonic expansion. The visible tautomer fluorescence excitation spectra of these compounds exhibit considerably well-resolved vibrational structures, while no significant uv fluorescence excitation spectrum due to the normal form was observed. The upper limit of the rate constant of the tautomer formation was estimated to be 1.77×1012 s−1 for 3-HC, 6.5×1011 s−1 for 3-HF, and 1.54×1011 s−1 for 2-NHC in supersonic expansion by simulation of the linewidth of the respective origin bands in the visible fluorescence excitation spectra. The order of rate constants of the tautomer formations in the supersonic free jet are consistent with that of the ESPT of these compounds in nonpolar solution. The fluorescence excitation and dispersed fluorescence spectra demonstrate 1:1 and 1:2 water complex formations of 3-HF and 2-NHC and no ESPT in these water clusters. However, no evidence of water complex formation was obtained in jet-cooled 3-HC.
A theoretical investigation on the molecular and electronic structure of the SFn compounds n=1–5 and their singly charged negative ions
Authors: Ziegler Tom , Gutsev G. L.
Published: July 2002

The electronic and molecular structures of binary sulphur fluorides SFn (n=1,5) as well as their singly charged anions have been studied by a computational method based on density functional theory. Ground state conformations as well as geometries for all excited states below 4 eV are presented for SFn and SF−n. It was found that all anions have more than one state which is stable with respect to ligand dissociation and electron detachment. Calculations have been carried out on the first S–F ligand dissociation energy D(SFn−1−F), as well as the adiabatic electron affinity EAad. The D(SFn−1−F) and EAad values made it possible to estimate decomposition energies for SF−n with respect to different fragmentation channels. The topmost dissociation channel of SF−n corresponds to the SFn−1+F− decay, followed by the SFn−2+F−2 dissociation. The electronic structure of binary sulphur fluorides is largely determined by the four 3sS and 3pS sulphur orbitals. The sulphur based orbitals are either nonbonding or antibonding with respect to the fluorine atoms and make up the highest occupied and lowest unoccupied orbitals in all systems. The shapes of the sulphur fluorides and their anions have been rationalized in terms of these four orbitals.
Path-integral study of magnetic response: Excitonic and biexcitonic diamagnetism in semiconductor quantum dots
Authors: Pollock E. L. , Runge Karl J.
Published: July 2002

In the path-integral formulation of quantum mechanics, the response of a system of charges to a static magnetic field is determined by the flux enclosed by the particle paths. In particular, the diamagnetic susceptibility is proportional to the mean-squared path area normal to the field. This relation is derived, discussed, and illustrated by computing the diamagnetic susceptibility for excitons and biexcitons in a spherical sample of semiconductor material (quantum dot model).
Chemisorption of CO on NbC(111) at 80 K: Angle-resolved photoemission study
Authors: Edamoto Kazuyuki , Shiobara Eishi , Anazawa Toshihisa , Hatta Masanori , Miyazaki Eizo , Kato Hiroo , Otani Shigeki
Published: July 2002

Angle-resolved photoemission spectroscopy with synchrotron radiation has been used to study the adsorption of CO on a NbC(111) surface at 80 K. CO is adsorbed molecularly on the NbC(111) surface. Binding energies of the CO valence levels are in good agreement with previously published results for metal–CO systems. The adsorbed CO forms a (1×1) overlayer. The two-dimensional band dispersions of CO valence levels in the (1×1) phase are measured in the Γ̄M̄ direction. The considerable dispersions (∼0.35 eV for both 4σ and 5σ+1π levels) are observed as in the case for the periodic CO layer on the metal surfaces. A shape resonance of the 4σ level of the adsorbed CO is observed at ∼32 eV, which is nearly the same energy as that for the gas phase CO. The 4σ cross section at the resonance energy is measured as a function of incidence angles and the result is consistent with a molecular orientation normal to the surface.
Photodissociation of S4N4. I. 248 nm
Authors: Henshaw Thomas L. , Ongstad Andrew P. , Lawconnell Robert I.
Published: July 2002

S4N4 was photolyzed by the KrF excimer laser source at 248 nm. The excited state photofragments produced are the NS(B 2Π)r and NS(H 2Π)1/2, NS(G 2Σ−) and NS(I 2Σ+) states. Single band progression fluorescence out of v′=0 for NS(H)1/2 and v′=2 for NS(G,I) are observed. Franck–Condon factors have been calculated for NS(H) up to v′=3. The electronic transition moment for the NS(H–X) transition was found to decrease slightly with increasing r̄. The radiative lifetime of the 0,5 band of NS(H)1/2 is determined to be 87±11 ns. The collisional quenching constants at 335 K for various species including N2 (kq=3.4±0.7×10−10 cm3 s−1 ), CF4 (kq=1.8±0.4×10−10 cm3 s−1 ), SF6 (kq=3.9±0.7×10−10 cm3 s−1 ) and He (kq=5.6±2.2×10−11 cm3 s−1 ) are reported. A semiempirical calculation using a PM3 Hamiltonian was used to calculate the heats of formation of various (SN)x species. A mechanism is proposed to account for the presence of these excited states based on laser fluence, excited state time histories, spectral composition and calculated heats of formation. NS(B) is postulated to arise directly from an intermediate photolysis product which is assumed to be acyclic S3N3. The NS(H 2Π)1/2 subband, NS(G 2Σ−) and NS(I 2Σ+) states are thought to be produced from a resonant interaction with the KrF line at 248 nm and vibrationally excited ground state NS. Using the I2(D′2g−A2u) emission as an actinometer, the overall efficiency on the conversion of absorbed photons by S4N4 into NS(B 2Π) is 2.6±0.7%.
A global constraint and the inverse problem for ternary phase diagrams
Authors: Teubner Max
Published: July 2002

A global constraint is derived for ternary phase diagrams at constant pressure and temperature. This constraint rules out certain diagrams with rotating tie lines. The inverse problem for phase diagrams is stated. It consists in proving, for a given phase diagram which obeys the known constraints, that a free energy function f  exists which generates just this phase diagram.
Computational studies of submicron probing of polymer surfaces. I
Authors: Sumpter Bobby G. , Getino Coral , Noid Donald W.
Published: July 2002

The atomistic details of the interaction of an atomic force microscopic (AFM) probe with a polymer surface are examined by using the molecular-dynamics method. It is found that the perturbation of the AFM probe can produce a deformation of the local structure of the polymer surface. The dynamics study reveals how the structural changes evolve during the surface probing experiment, and whether the deformations result in permanent or reversible structural damage upon removing the probe. The effects of probe features (radius of curvature) and load force on the surface deformation(s) and image resolution are investigated in both a constant-force and constant-height AFM mode. Load forces between 10−8 and 10−11 N were determined to be an optimum working range for nondestructive AFM probing of polymer surfaces, and consequently for well-resolved surface image production.
Gas–gas equilibria of aqueous fluid binary mixtures
Authors: Scalise O. H.
Published: July 2002

In a previous work [Chem. Phys. Lett. 100, 378 (1983)], by the application of thermodynamic perturbation theory to the polarizable hard-spheres dipole model, we found that on the P–T diagram the critical curve for the Ar–H2O mixture exhibits a negative slope. The same behavior on the critical curve has been found in a recent experimental reinvestigation of this mixture. Here it is investigated whether the theoretical approach can also predict the gas–gas equilibria of type I of the aqueous fluid binary mixtures. Good agreement with the experimental data is found for He–H2O and H2–H2O binary mixtures.
Mapping of parent transition-state wave functions into product rotations: An experimental and theoretical investigation of the photodissociation of FNO
Authors: Ogai Amy , Brandon James , Reisler Hanna , Suter Hans Ulrich , Huber J. Robert , von Dirke Michael , Schinke Reinhard
Published: July 2002

We study experimentally and theoretically reflection-type structures in the rotational distributions of NO following the photodissociation of FNO via excitation of the S1 state. Exciting quasibound states with zero quanta of bending vibration in the FNO(S1) state yields Gaussian-type rotational distributions, while excitation of states with one bending quantum leads to bimodal distributions. In the latter case, the ratio of the two intensity maxima depends on the number of NO stretching quanta in the S1 state. The accompanying calculations employing a three-dimensional ab initio potential energy surface for the S1 state of FNO are performed in the time-dependent wave packet approach. They reproduce the main features of the experimental distributions, especially the bimodality. The analysis of two-dimensional calculations for a frozen NO bond distance shows that the final rotational state distributions can be explained as the result of a dynamical mapping of the stationary wave function on the transition line onto the fragment rotational quantum number axis. Here the transition line is defined as the line which separates the inner part of the FNO(S1) potential energy surface from the strongly repulsive F+NO product channel.
Real-time path-integral simulation of vibrational transition probabilities of small molecules in clusters: Theory and application to Br2 in Ar
Authors: Amini Abolfazl M. , Herman Michael F.
Published: July 2002

A discretized real-time Feynman path integral is applied to a system containing a diatomic in a small cluster of solvent molecules. The system considered consists of Br2 in Ar. An adiabatic separation of variables is assumed. The solvent as well as the Br2 center of mass are gathered into a vector called bath or solvent coordinates. The forced oscillator approximation is used to analytically obtain the vibrational contribution to the transition amplitude. The discretized real-time propagators (bath dependent only, since the vibrational part is carried out analytically) are highly oscillatory and, therefore, not suitable for Monte Carlo calculations. The coarse-graining technique introduced by Filinov and developed by Freeman and Doll, and Miller is employed to make the integrals more suitable for Monte Carlo calculations. The computations are carried out for five different times. For each time, we study the convergence of the technique for a range of Gaussian widths used as conditioning functions. We also examine the convergence as a function of the number of points in the discretized description of the solvent path.
Optimized calculations of reaction paths and reaction-path functions for chemical reactions
Authors: Melissas Vasilios S. , Truhlar Donald G. , Garrett Bruce C.
Published: July 2002

In this paper we optimize several algorithms for the computation of reaction rates based on information calculated along minimum energy reaction paths and we evaluate the efficiencies of the optimized algorithms. The investigations are based on the calculation of chemical reaction rate constants using variational transition state theory and multidimensional semiclassical transmission coefficients including reaction path curvature. Several methods are evaluated and compared by a systematic set of applications to test cases involving the hydrogen-atom transfer reactions CH3+H2→CH4+H and OH+H2→H2O+H. For each method we present general recommendations for all algorithmic choices other than gradient step size so that future calculations may be carried out reasonably efficiently by varying only one parameter. In the process of these optimizations we have found that the accuracy of the Euler stabilization method can be significantly increased by choosing the auxiliary parameters differently than in previous work; the optimized algorithm is called ES1*. Our final recommendations for future work are (i) when the Hessian/gradient computational cost ratio is low (≲3): the Page–McIver algorithm with the Hessian recalculated at every step, with a cubic starting step, and with curvature calculated from the derivative of the gradient, and (ii) when the Hessian/gradient computational cost ratio is moderate or large: the ES1* algorithm with a Hessian step size three times larger than the gradient step size, with a quadratic starting step, and with curvature calculated from the derivative of the gradient.
Molecular-beam study of the collisional intramolecular coupling of N2(B 3Πg) with the N2(A 3Σ+u) and N2(W 3Δu) states
Authors: Bachmann R. , Li X. , Ottinger Ch. , Vilesov A. F.
Published: July 2002

Collision-induced intramolecular energy transfer between N2 triplet states has been investigated for the first time under single-collision conditions. A beam containing N2 molecules in the long-lived N2(A 3Σ+u) and N2(W 3Δu) states interacted with target particles (H2, N2, NO, and all rare gases) either in a collision cell or in a secondary, pulsed molecular beam. From the collision region N2(B 3Πg) emission was observed with a linear dependence on the target-gas density. It is due to collision-induced intramolecular energy transfer (‘‘collisional coupling’’) N2(W→B) and N2(A→B). These two contributions were differentiated by means of data taken at different distances from the beam source, using the known radiative rate decay of N2(W). The spectra (40 Å full width at half maximum) show clearly the importance of energy resonance between reactant and product vibrational levels, with an exponentially decreasing dependence of the cross section on the energy mismatch. Relative cross sections were obtained for all collision partners and the B-state vibrational levels v′=3–10. For the W→B collisional coupling, absolute cross sections were derived by relating the intensity of the collision-induced luminescence to that of the collisionless beam afterglow, which is also due to the N2(W) species and was studied previously.
Modeling spatiotemporal behavior of the NO+CO reaction on Pt
Authors: Evans J. W. , Madden H. H. , Imbihl R.
Published: July 2002

Various features of NO+CO reaction kinetics on Pt(100) surfaces, including temporal oscillations, are well described by a three-variable model incorporating only the CO, NO, and O coverages. Here we analyze the corresponding reaction–diffusion equations demonstrating the existence of chemical waves where an ‘‘oscillating phase’’ displaces an unreactive NO/CO phase leaving a spatially periodic structure in its wake; pulses excited via inhomogeneities from an unreactive NO/CO background; and Turing structures for sufficiently unequal NO and CO diffusion rates.
Comment on: Recent calculations of liquid-state band structures
Authors: Winn M. D. , Logan D. E.
Published: July 2002

The mean spherical approximation for the DoS calculation of a liquid is modified. (AIP)
Triple-resonance spectroscopy of the higher excited states of NO2. IV. Trends in the mode dependence of vibrational autoionization via asymmetric stretch versus symmetric stretch and bend
Authors: Bryant Gregg P. , Jiang Yanan , Grant Edward R.
Published: July 2002

Multiresonant stepwise excitation of NO2 isolates series of individual rovibronic states converging to vibrationally excited levels of NO+2. Resonances detected by third-photon photoionization of two-photon photoselected intermediate states show characteristic patterns of intensity and linewidth that convey information on the dynamics of vibrational autoionization for relaxation in specific normal modes of the linear NO+2 core. Earlier studies characterized vibrational-to-Rydberg-electronic energy transfer from symmetric stretching (100) and bending (010) excited states [J. Chem. Phys. 93, 2308, 7731 (1990)]. In the present work, a definitive assignment of double-resonant rotational structure confirms two-color selection of 3pσ 2∑+u gateway states that are core excited by one quantum of asymmetric stretch. Ionization-detected optical-absorption scans from the double-resonantly selected N′=2 level of the 3pσ (001) state yield spectra of s, d, and g Rydberg series of vibrationally autoionizing resonances converging to the (001) ionization threshold. Transitions assigned to s and d series range in principal quantum number from 7 to more than 30. As with the spectrum of states built on the (100) core, transitions assigned to g states form a series of sharp resonances that disappears at relatively low n.

Regions of the (001) autoionizing spectrum are perturbed by discrete–discrete interactions with underlying series converging to lower thresholds, and comparatively low intensities are found for whole series of states with higher Rydberg orbital angular momentum. Nevertheless, resonant linewidths for (001) can be compared with (100) and (010) for the most penetrating series (ns), and a clear trend manifests itself over complete autoionizing intervals for all three vibrational states of the core. Symmetric stretching states, with the largest dynamic volume change over a cycle of vibration, autoionize fastest. Asymmetric stretch, with a smaller volume change, but comparable N–O bond displacements, is coupled less strongly. Bending, which supports a large dynamic dipole, but little change in the volume of the core charge distribution, displays the slowest relaxation to the NO+2 (000) continuum. These trends in vibrational state dependence are in qualitative accord with a simple long-range picture that relates the strength of Rydberg-continuum coupling, induced by vibrational motion in the core, to the size of the associated dynamic monopole.
The reaction of sulfur atoms with carbon disulfide: Potential energy surface features
Authors: Froese Robert D. J. , Goddard John D.
Published: July 2002

The lowest singlet and triplet potential energy surfaces of the reaction, S+CS2→CS3→CS+S2, were investigated by the 6-31G* ab initio self-consistent-field (SCF) method with the inclusion of electron correlation by Mo/ller–Plesset perturbation theory. The triplet reactants and products [S(3P)+CS2(1Σ+g),S2(3Σ−g)+CS(1Σ+)] are predicted to be more stable than their singlet counterparts [S(1D)+CS2(1Σ+g),S2(1Δg)+CS(1Σ+)] in agreement with experiment. However, the CS3 complex is more stable in its singlet as opposed to triplet state, leading to interesting surface crossings in the intermediate regions of the surfaces. The triplet surface contains shallow wells corresponding to cis- and trans-CS3 chain isomers. A low-lying singlet C2v ring structure, carbon trisulfide, was connected to a chain molecule, carbon disulfide S-sulfide, by a relatively low-lying transition state. Another transition state was found with a modest barrier, which joined the C2v ring structure to another relatively low-lying minimum, a symmetric D3h structure. Relative to the singlet ring compound, the singlet chain isomer and the D3h structure are 14.3 and 4.1 kcal/mol higher in energy, and the triplet cis and trans minima 31.2 and 34.1 kcal/mol higher in energy. At the unrestricted Hartree–Fock level (UHF or UMP2), transition states join the chain structure to both reactants and products on the singlet surface. From the experimental evidence and the predictions of this work, an intersystem crossing from the triplet potential energy surface to the singlet one is expected in the region of those transition states leading to the products, CS+S2.
Master equation for nonlinear dissipative systems
Authors: Gangopadhyay Gautam , Ray Deb Shankar
Published: July 2002

Based on the system-reservoir theory we have established a generalization of the master equation for a class of nonlinear oscillators within the Born–Markov approximation. We have made a detailed study of the two model systems, e.g., the Morse oscillator and the quartic oscillator as immediate application. Significant interplay of dissipation and nonlinearity has been demonstrated even in the restricted case of weak damping and low finite temperatures.
A new perspective on local scaling analysis
Authors: Eno Larry
Published: July 2002

In a recent paper, an energy-sudden-based local scaling analysis has been introduced. We show that this local analysis can be incorporated into a much more general scaling framework−a framework which encompasses the ‘‘regular’’ energy sudden scaling relations. We also make some suggestions with regard to utilizing the general framework in extending present scaling theoretic analysis.
Mechanism of quasiresonant vibration–rotation energy transfer in atom–diatom encounters
Authors: Miklavc Adolf , Marković Nikola , Nyman Gunnar , Harb Vili , Nordholm Sture
Published: July 2002

We present an analysis based on the quantized version of the effective mass theory and on classical trajectory calculations which leads to an interpretation of all the characteristic features of the striking new phenomenon named rotation-induced quasiresonant vibration–rotation energy transfer and observed recently in Li*2(nvi,Ji) + X → Li*2(nvf,Jf)+ X collision (X=He, Ne, Ar, Xe). The ‘‘ordinary branch’’ of the effective mass theory reproduces the broad Jf distribution in the nonresonant internal energy transfer found experimentally at lower Ji values (Ji = 8 and 22). The ‘‘extraordinary branch’’ of the same theory gives the narrow, sharply peaked quasiresonant Jf distributions as observed at Ji values 42 and higher. The conclusions derived from the effective mass theory and classical trajectory calculations enable us to elucidate the celebrated experimental correlations between the most probable changes in the rotational and vibrational quantum numbers, as well as the ‘‘locking mechanism’’ by which these correlations prevail over large intervals of the initial angular momentum Ji. On the same basis we explain why the total vibrationally inelastic cross section is greatly enhanced by increasing Ji and why the shape of the final Jf distribution is identical (within the experimental uncertainty) for Ne, Ar, and Xe, but not for the He.
Geometry, interaction range, and annealing
Authors: Chang Xiao-yan , Berry R. Stephen
Published: July 2002

A method introduced by Stillinger for finding global minima of potential surfaces and for achieving the optimization objectives of simulated annealing is shown to require caution in some of its applications. In particular, a range parameter is varied to eliminate secondary minima on the potential surface or hypersurface of the objective function of interest. We show that in some cases, using minimizations on the simplified surface to reveal minima on the more complex surface require scrutiny of the sequences of low-energy minima, as functions of the range parameter whose variation controls the complexity of the surface.
Use of approximate scattering theories as interpolation guides
Authors: Cross R. J.
Published: July 2002

A simple method is given for using a fast but not necessarily accurate scattering approximation to interpolate the results of an exact calculation. The goal is to minimize the number of points where the expensive, exact calculation must be done. The approximate theory is used to remove the rapidly varying parts of the exact S-matrix to obtain a slowly varying correction matrix which can then be interpolated over a large interval in energy or angular momentum. After the interpolation, the rapidly varying approximate parts are put back in to give an accurate S-matrix. The method preserves the phase of the S-matrix. It is demonstrated for the cases of symmetric molecular charge exchange and for rotationally inelastic scattering.
Collisional dynamics of the BrF B 3Π(0+) state. II. Vibrational energy transfer
Authors: Perram Glen P. , Melton David W. , Thompson Tim L. , Roh Won B.
Published: July 2002

Pulsed, laser induced fluorescence techniques have been used to study vibrational energy transfer within the B 3Π(0+) state of bromine monofluoride. Rate coefficients for state-to-state vibrational transfer were determined from the observed BrF(B–X) emission in the presence of He, Ne, Ar, Kr, Xe, and CF4. Vibrational states v′=2, 5, and 6 were probed and time-resolved emission observed from states v′=0–6. The vibrational relaxation is well described by the Montroll–Shuler model for harmonic oscillators. The fundamental rate coefficient for vibrational transfer from v′=1 to v′=0 ranged from 2.2×10−13 to 3.9×10−12 cm3/molecule s for the rare gases and was 4.1±1.0×10−12 cm3/molecule s for CF4. The scaling of the vibrational transfer rates with vibrational quantum number is consistent with the predictions of the Landau–Teller theory. Quantum-resolved vibrational transfer rates for BrF, IF, and BrCl are compared to the predictions of the Schwartz, Slawsky, and Herzfeld (SSH) theory.
Determining potential-energy surfaces from spectra: An iterative approach
Authors: McCoy Anne B. , Sibert Edwin L.
Published: July 2002

A general method for iteratively fitting the coefficients of a Taylor-series expansion of the potential-energy surface for a polyatomic molecule to the observed transition frequencies and rotational constants is presented. This approach utilizes the efficiency of fourth-order Van Vleck perturbation theory for calculating these properties, scaled to the results of converged variational calculations. Three fits to the transition frequencies and rotational constants obtained from absorption and stimulated emission pumping spectra of HCN are presented. Comparison of these potentials sheds light on the uniqueness of fit potential surfaces, given this set of observables. We fit the vibrational energies for 71 states with a mean absolute deviation of 0.69 cm−1.
Mean number of distinct sites visited by correlated walks. II. Disordered lattices
Authors: Argyrakis Panos , Kehr Klaus W.
Published: July 2002

The mean number of distinct sites Sn visited by correlated walk of particles in two-dimensional lattices with randomly blocked sites is studied by numerical simulations, for various values of the forward probability pf and of the concentration of open sites p. At the percolation threshold, the behavior of Sn changes from a power law for smaller values of pf to more complicated behavior at larger pf. For complete forward correlation, pf=1, trapping of the particles in effective traps is observed at all values of p≳pc. The probability distribution P(S) of the number of distinct sites visited until trapping is also examined.
A reference hypernetted-chain equation for soft potentials. Atomic and molecular Lennard-Jones systems
Authors: Lombardero M. , Martín C. , Lomba E.
Published: July 2002

A recently proposed reference hypernetted-chain equation based on a semiphenomenological bridge function due to Verlet is reformulated for atomic and molecular Lennard-Jones liquids. In this new approximation we treat the size of the reference-system hard particles as a function of density and temperature through a functional relationship presented herein. The theory yields excellent results for the structure and thermodynamics of atomic Lennard-Jones liquids over a wide range of temperature and density and is quite satisfactory for Lennard-Jones homonuclear diatomics.
Dynamic light scattering from a semiflexible polymer at very low concentrations
Authors: Jamil T. , Russo P. S.
Published: July 2002

Dynamic light scattering measurements on semiflexible poly(γ-benzyl-α,L-glutamate) have been performed at very low concentrations, near the limit for reliable multiangle detection, in N,N-dimethylformamide. By several methods of data analysis, the mutual diffusion coefficient Dm is initially level or slightly decreasing with polymer concentration. It then increases substantially, as expected in a good solvent. Static light scattering measurements offer no thermodynamic explanation for the initially level or decreasing behavior, which is apparently a manifestation of the decrease in lateral mobility as the rods become entangled in semidilute solutions. The coefficient C describing the dependence of the first cumulant of the electric field autocorrelation function upon squared scattering vector decreases with concentration, even in the most dilute regime measured. The reliability of persistence lengths estimated for this polymer by dynamic light scattering is considered.
Stirring sense discriminates between stationary and oscillatory states
Authors: Hauser Marcus J. B. , Lebender David , Schneider Friedemann W.
Published: July 2002

The bifurcation behavior of the minimal bromate system (BrO3−/Br−/Ce3+/H2SO4) in a flow reactor shows a wider oscillatory region if the symmetric stirrer rotates in one direction than if the stirring direction is reversed. Thus, at appropriate flow rates, a reversal in the direction of stirring causes a transition between an oscillatory and a steady state. This phenomenon can be explained by a change in the mixing order of the feedstreams.
Reversible multiple time scale molecular dynamics
Authors: Tuckerman M. , Berne B. J. , Martyna G. J.
Published: July 2002

The Trotter factorization of the Liouville propagator is used to generate new reversible molecular dynamics integrators. This strategy is applied to derive reversible reference system propagator algorithms (RESPA) that greatly accelerate simulations of systems with a separation of time scales or with long range forces. The new algorithms have all of the advantages of previous RESPA integrators but are reversible, and more stable than those methods. These methods are applied to a set of paradigmatic systems and are shown to be superior to earlier methods. It is shown how the new RESPA methods are related to predictor–corrector integrators. Finally, we show how these methods can be used to accelerate the integration of the equations of motion of systems with Nosé thermostats.
Analytical theory of solution of attracting molecules of different size derived from hypernetted-chain equation for radial distribution function
Authors: Nishimura H. , Hirao M.
Published: July 2002

The Flory–Huggins formula of mixing entropy of different size molecules originally proved for polymer solution is generally proved for attracting spherical molecules different in size. The Scatchard formula of the excess energy known for application in the solubility parameter is also proved theoretically. The proof is based on analytical solutions of the radial distribution functions for mixture and does not rely on the lattice model. The hypernetted-chain (HNC) equation is perturbed taking hard-sphere system as reference and the attractive force as perturbation. The obtained equation for the perturbed radial distribution function for mixture is solved analytically under simplifying assumptions, which enabled to give the equation of state in analytical form. In order to give the excess Gibbs free energy in explicit form, the vapor pressure of liquid mixture is obtained by applying a phase equilibrium condition corresponding to the Maxwell’s equal-area rule to mixture. The excess entropy and enthalpy are derived directly.
Semistochastic approach to many electron systems
Authors: Grossjean M. K. , Grossjean M. F. , Schulten K. , Tavan P.
Published: July 2002

A Pariser–Parr–Pople (PPP) Hamiltonian of an 8π electron system of the molecule octatetraene, represented in a configuration-interaction basis (CI basis), is analyzed with respect to the statistical properties of its matrix elements. Based on this analysis we develop an effective Hamiltonian, which represents virtual excitations by a Gaussian orthogonal ensemble (GOE). We also examine numerical approaches which replace the original Hamiltonian by a semistochastically generated CI matrix. In that CI matrix, the matrix elements of high energy excitations are choosen randomly according to distributions reflecting the statistics of the original CI matrix.
The vibrational line shape of diatomic adsorbates on metal clusters
Authors: Blaisten-Barojas Estela , Gadzuk J. W.
Published: July 2002

A decrease of at least an order of magnitude in the vibrational relaxation time T1 has been measured for CO bonded to Rh and Co clusters when the size of the cluster increases from 5 to 35 Å. We propose that this effect is mainly due to the coupling of the molecular vibration ω0 with the electron-hole excitations in the cluster. This is described via a model Hamiltonian. The finite size of the clusters give rise to a discrete electronic spectrum, and hence to a discrete pair excitation spectrum. This effect is measured in terms of D, the mean spacing between nearest-neighbor levels in the conduction band of the cluster. We find that: (1) the proposed mechanism starts to contribute to T1 only when D<ℏω0; (2) T1 is at least several hundred ps for clusters less than 15 Å in size; (3) there is a sharp decrease of T1 to about 10 ps as the cluster size increases from 15 to 40 Å; (4) T1 decreases smoothly towards the bulk value for larger clusters.
Comment on the classical theory of the rate of isomerization
Authors: Zhao Meishan , Rice Stuart A.
Published: July 2002

The classical theory of the rate of unimolecular isomerization developed by Gray and Rice is extended in two ways. First, an improved state dependent approximation to the system separatrix is introduced. Second, a systematic approximation for the rate of intramolecular energy transfer in one isomer is introduced. These new approximations considerably improve the accuracy of the prediction of the rate of isomerization. Indeed, using the analysis described in this paper, the calculated rates of isomerization as a function of energy for the model system studied by Gray and Rice are in excellent agreement with the exact rates obtained from trajectory calculations.
Bound-state effects on the classical Boltzmann equation
Authors: Curtiss C. F.
Published: July 2002

In some derivations of the Boltzmann equation, the equation which describes the time evolution of the distribution function in the phase space of the atoms of a low-density gas, it has been assumed that all of the trajectories in the phase space of the pairs of atoms are such that transforming back sufficiently far in time leads to separated atoms. In the present development, it is shown that if this is not true the ‘‘bound-state’’ region of the phase space of pairs of atoms leads to an additional term in the linearized Boltzmann equation. This additional term will lead to differences in the low-density limiting values of the transport coefficients from those which have been obtained from the classic equation of Boltzmann.
Effect of temperature on the formation of a bound-state of positronium with nitrobenzene and on the related hyperfine interaction parameters
Authors: Billard I. , Abbé J. Ch. , Duplâtre G.
Published: July 2002

The strong magnetic field effect on the chemical bound-state formation of positronium with nitrobenzene in n-hexane previously studied at 294 K has been further examined as a function of temperature, from 264 to 328 K. The model proposed before to explain the data, implying the definition of triplet and singlet positronium bound-state, appears to be valid at all temperatures studied. The variation with temperature of the parameters descriptive of the bound-state are derived; the hyperfine splitting is constant at 1.06×10−5 eV, while the contact density parameter, ηc, shows an abrupt change from 0.43 to 1.56 between 286 and 292 K. These experiments give new insight on the structure of the bound-state formed, which may be considered as a charge-transfer complex.
Multiquantum vibrational energy transfer into surface Rayleigh, bulk shear, and pressure waves by atom–solid-surface collisions: A discrete-continuum hybrid treatment with applications to He–Pt(111)
Authors: Vilallonga Eduardo , Rabitz Herschel
Published: July 2002

A discrete-continuum hybrid treatment is developed for energy transfer into solid-surface vibrations by atomic collisions. Surface vibrations are described in terms of the displacement field of a three-dimensional elastic continuum with a stress-free boundary. The displacement field is evaluated discretely at the surface lattice sites and it is quantized by the standard methods for harmonic vibrations. This hybrid approach can extend classical Debye models to incorporate surface corrugation, lattice structure, and the Bose–Einstein statistics of phonons. The treatment is illustrated on He scattering from Pt(111) at superthermal collision energies, e.g., E=0.5 eV, to probe the repulsive cores of the gas–surface potential. Accordingly, the projectile motion is approximated by classical trajectories, whereas all vibrational modes are treated quantum mechanically. The differential (in final angles and transferred energy) scattered intensity is obtained from time-correlation functions of the semiclassical transition operator, which incorporate numerous vibrational states as well as surface temperature. A computational procedure is described for efficiently calculating multiquantum transitions of very high order using fast Fourier transforms. Scattered intensities are calculated for the He–Pt(111) system over a wide range of angles (0°–75°) and surface temperatures (0–600 K). The distributions of transferred energies are analyzed in terms of the continuum vibrational modes, which include surface Rayleigh, shear-horizontal (SH), and coupled pressure/shear-vertical (PSV) waves. The mode-specific distributions are found to vary in distinct ways as one changes collisional angles. On average, surface Rayleigh waves absorb approximately one-half of the transferred energy and the remainder is shared in comparable amounts by bulk SH and PSV waves.
Ab initio study of the geometry, stretching, vibrations, and assignment of the observed frequencies of the ground state C6H (hexatriynyl) radical
Authors: Liu Ruifeng , Zhou Xuefeng , Pulay Peter
Published: July 2002

Unrestricted natural orbital–complete active space (UNO–CAS) calculation on the ground state C6H radical confirmed the matrix IR frequency at 1953.4 cm−1 in the vacuum ultraviolet photolysis of acetylene and butadiene is the ω2(C≡C stretching) mode. The calculation also predicts another strong C≡C stretching IR mode, ω4, at about 1770 cm−1.
Quadratic response functions for a multiconfigurational self-consistent field wave function
Authors: Hettema Hinne , Jensen Hans Jo/rgen Aa. , Jo/rgensen Poul , Olsen Jeppe
Published: July 2002

We describe an efficient implementation of the quadratic response function for a multiconfiguration self-consistent field reference wave function. The quadratic response function determines the hyperpolarizability and its residues determine the two-photon transition matrix elements and the transition matrix elements between excited states. We report sample calculations for the hyperpolarizability of Ne and for the two-photon transition matrix elements of Ne and H2.
Defect chain motions in the low temperature phase of multiple mixtures of n-alkanes by means of nuclear magnetic resonance spin–lattice relaxation time measurements
Authors: Basson I. , Reynhardt E. C.
Published: July 2002

Nuclear magnetic resonance spin–lattice relaxation time measurements in the laboratory and rotating frames, supported by differential scanning calorimetry and x-ray diffraction measurements, are reported for a series of multiple mixtures consisting of Gaussian chain length distributions of n-alkanes. The results revealed three types of molecular motions, viz., threefold reorientations of methyl groups, gauche defect motions of the chain ends of the long chains and twofold screw motions of the short chains in these mixtures. The main factor influencing the latter two motions is the width of the chain length distribution.
Energy transfer in condensed media. III. Trimers
Authors: Cortés Emilio , Lindenberg Katja
Published: July 2002

We consider the relaxation and equilibration of an excitation in a trimer coupled to a phonon bath. Energy transfer within the trimer occurs via resonance interactions and the coupling between the trimer and the phonon bath occurs via the modulation of the monomer energies by the phonons. The problem is dealt with fully quantum mechanically. We focus on the effects of certain forms of bond disorder, in particular, on the effects of two different resonance interactions in a trimer of otherwise identical monomers. We find that ‘‘bond disorder’’ even unaccompanied by ‘‘site disorder’’ leads to spontaneous energy localization. This result would not be predicted by semiclassical master equation models or even by most stochastic Liouville equation models−it requires the proper (finite temperature) quantum analysis not only of the trimer system but also of the heat bath. Indeed, a nonuniform distribution of energy among the monomers is a necessary consequence of the proper thermal distribution of populations among the eigenstates of the trimer. We also discuss the relaxation of the trimer when the highest energy level of the trimer is initially excited. We find that the decay of the highest level and the population of the lowest level proceed monotonically, but the intermediate energy level in general has a ‘‘population overshoot’’ before achieving equilibrium. Correspondingly, the populations of the various monomers go to equilibrium in a nonmonotonic way.
Robust optimal control theory for selective vibrational excitation in molecules: A worst case analysis
Authors: Beumee Johan G. B. , Rabitz Herschel
Published: July 2002

Recent research has demonstrated that optimal electromagnetic fields capable of producing selective vibrational excitation in molecules can be designed employing linear quadratic control methods using a cost functional that balances the energy distribution in the molecule, the fluence of the optical field, and a final cost to insure the desired excitation. Practical computations of molecular control theory for large molecules especially with anharmonic potentials become difficult to obtain due to the increased dimensionality and the accompanying uncertainty in the Hamiltonian. In this paper we reduce the complexity of the problem by treating a portion of the molecule including the target and optical dipoles in full detail, while the remainder of the molecule is modeled as an external disturbance of bounded energy. The optimal control field now minimizes the cost functional which is simultaneously maximized with respect to the energy constrained external disturbance to assure robustness. This optimal design process is commensurate with taking the most pessimistic view of the disturbance. This conservative view was born out in the numerical calculations such that practical laboratory studies should reach results much improved over the worst case design. As an illustration we investigate disturbances of varying energy content for a truncated 20 atom molecular chain where the uncontrolled remainder of the chain is the source of the system disturbance. The sensitivity of the system with respect to the disturbance was found to be strongly dependent on the distance of the disturbance to the target bond and the dipole arrangement. In addition, in the range of physically reasonable disturbance energy the optimal field could be accurately predicted from an asymptotic expansion involving only the reference undisturbed case. Although the present paper takes advantage of linear system techniques, the same robust optimal control procedure can be generalized to nonlinear systems by a variety of means.
Photodissociation dynamics of S4N4 at 222 and 248 nm
Authors: Ongstad Andrew P. , Lawconnell Robert I. , Henshaw Thomas L.
Published: July 2002

Emission from several electronically excited states of NS is observed when the energetic molecule S4N4 is photolyzed with radiation from an excimer laser. Photolysis at 248 nm generates fluorescence from the B 2Π1/2,3/2, H 2Π1/2, G 2Σ−, and I 2Σ+ states of NS. NS(B 2Π1/2,3/2) and NS(C 2Σ+) fluorescence is observed when the photolysis wavelength is changed to 222 nm. The NS(H) and NS(C) spectra are postulated to arise from a resonant interaction between the KrF and KrCl excimer photons, respectively, and vibrationally hot ground state NS. LIF excitation scans on the NS X 2Π1/2,3/2 → B 2Π1/2,3/2 system confirm the production of rotationally and vibrationally excited NS(X) up to v″=4. A mechanism, based on the experimental data (i.e., spectral composition, laser fluence studies, excited state time histories), calculated heats of formation, and Gaussian molecular orbital calculations, is proposed to account for the observed emissions. For photolysis at 248 nm it is hypothesized that a two photon absorption promotes the ground singlet state of S4N4 to an upper repulsive singlet state, which rapidly dissociates (τ≪30 ns), producing an acyclic S3N3 fragment and vibrationally excited monomeric NS(X). The photofragments can interact further with the excimer radiation to produce NS(B) and NS(H), respectively. A similar mechanism is proposed to account for the presence of the NS(B) and NS(C) excited states for the 222 nm photolysis.
Vibrational autoionization in H2: Vibrational branching ratios and photoelectron angular distributions near the v+=3 threshold
Authors: Dehmer J. L. , Dehmer P. M. , West J. B. , Hayes M. A. , Siggel M. R. F. , Parr A. C.
Published: July 2002

Vibrational branching ratios and photoelectron angular distributions are reported for photoionization of normal (the equilibrium ortho/para mixture) hydrogen in the region just above the v+=3 ionization limit. This region contains a number of vibrationally autoionizing Rydberg states, and the goal of the work was to observe experimentally the characteristic behavior associated with the decay of these states that was predicted by Raoult and Jungen [J. Chem. Phys. 74, 3388 (1981)] on the basis of an MQDT (multichannel quantum defect theory) calculation on para-H2 (J″=0). We have indeed observed that vibrational autoionization strongly favors the ionization channel representing the minimum change in the vibrational state of the ion core, as predicted. We also observed a sharp reduction in the photoelectron asymmetry of the v+=3 (and, to a lesser extent, the v+=2) ionization channel for resonant photoionization. Hence, qualitative agreement with theory is observed; however, a quantitative comparison now requires an extended calculation that includes the additional rotational levels that were populated under the present experimental conditions.