bluuebunny/crossref_metadata_2025 huggingface dataset:
On the internal temperature in polymer glass simulations
Authors: Baschnagel J. , Dickman R.
Published: July 2002

An approximate theory, including effects of the density, for the equilibrium properties of a polymer lattice model (bond-fluctuation model) is presented and compared with Monte Carlo simulations. Effects of the density are taken into account by balancing the volume requirements of the individual bond vectors with the pressure that a melt exerts at a given density. This simple theory yields rather accurate (to within a few percent) estimates for the chain length and temperature dependence of various quantities that scan different length scales of the polymers (bond length, radius of gyration, etc.). With the inclusion of the density, one can also design an improved definition of an internal temperature of the melt, which is distinct from the external temperature of the surrounding heat bath. When the melt falls out of equilibrium during the vitrification process, this internal temperature is expected to measure the temperature that the melt actually experiences, and may thus serve to eliminate the nonequilibrium effects from the simulation data. The extent to which this expectation is borne out by a comparison with the simulation is addressed.
Stick to slip transition and adhesion of lubricated surfaces in moving contact
Authors: Reiter Günter , Demirel A. Levent , Peanasky John , Cai Lenore L. , Granick Steve
Published: July 2002

The friction of dry self-assembled monolayers, chemically attached to a solid surface and comprising a well-defined interface for sliding, is compared to the case of two solids separated by an ultrathin confined liquid. The monolayers were condensed octadecyltriethoxysilane (OTE). The liquid was squalane (C30H62), a film 2.0 nm thick confined between parallel plates of mica. The method of measurement was a surface forces apparatus, modified for oscillatory shear. The principal observations were the same in both cases: (1) Predominantly elastic behavior in the linear response state was followed by a discontinuous transition to a mostly dissipative state at larger deformations. The elastic energy stored at the transition was low, of the order of 0.1 kT per molecule. This transition was exactly repeatable in repetitive cycles of oscillation and reversible with pronounced hysteresis. (2) The dissipative stress in the sliding state was almost independent of peak sliding velocity when this was changed over several decades. Significant (although smaller) elastic stress also persisted, which decreased with increasing deflection amplitude but was almost independent of oscillation frequency. (3) The adhesive energy in the sliding state was significantly reduced from that measured at rest. This similarity of friction in the two systems, dry and wet sliding, leads us to speculate that, similar to plastic deformation of solids, sliding in the confined liquid films is the result of slippage along an interface.
Threshold for quasicontinuum absorption and reduced luminescence efficiency in CdSe nanocrystals
Authors: Hoheisel W. , Colvin V. L. , Johnson C. S. , Alivisatos A. P.
Published: July 2002

Luminescence excitation spectra are employed to study the electronic states of CdSe nanocrystals ranging in size from 9 to 26 Å radius at 77 K. These studies show that all samples have, in addition to the discrete manifold of quantum confined electronic excitations, a threshold for continuum absorption. Absorption into this continuum results in substantially reduced luminescence efficiency.
Reversible work based quantum transition state theory
Authors: Schenter Gregory K. , Mills Gregory , Jónsson Hannes
Published: July 2002

A theoretical basis is presented for reversible work evaluation of transition rates within the framework of transition state theory. The method involves computing statistical averages of forces without having to evaluate transition state partition functions or densities, and therefore eliminates the need for a harmonic reference system. The method can be applied to systems of high dimensionality which is particularly important in calculations on quantum systems, where each quantum particle may be represented by several images in a Feynman path integral chain. The relationship between this method and the fixed centroid method of Gillan and centroid density theories is established. The various methods are compared on a model quantum system consisting of an Eckart barrier coupled to a harmonic oscillator.
17O hyperfine and quadrupole interactions for water ligands in frozen solutions of high spin Mn2+
Authors: Tan Xiaoling , Bernardo Marcelino , Thomann Hans , Scholes Charles P.
Published: July 2002

The magnetic couplings of 17O in H217O coordinated to high spin Mn2+ in a frozen aqueous solution were determined using the complementary magnetic resonance techniques of pulsed and continuous wave (cw) ENDOR (electron nuclear double resonance), ESEEM (electron spin echo envelope modulation), and PFSEPR [pulse field sweep electron paramagnetic resonance (EPR)]. Several complications arise from the high electron spin multiplicity of the d5, Mn2+ ion and the high nuclear spin multiplicity, I=5/2, of the 17O nucleus. At the applied magnetic field strengths in 9 GHz EPR studies, the zero-field splitting of the S=5/2 Mn2+ ion in aqueous frozen solution is small relative to the electron spin Zeeman interaction so that the MS=±1/2,±3/2,±5/2 electron spin states all contribute to the ENDOR spectrum.

This results in a complex spectrum in which the 17O ENDOR powder pattern arising from the MS=±1/2 manifolds are separately resolved but the powder patterns from the MS=±3/2,±5/2 manifolds overlap the multiple 1H ENDOR lines arising from all six MS manifolds [X. Tan, M. Bernardo, H. Thomann, and C. P. Scholes, J. Chem. Phys. 98, 5147 (1993)]. Given this complexity, a combination of complementary spectroscopic techniques and numerical simulations are used to deconvolute the overlapping spectra and to assign the spectral lines. The ENDOR spectra provided an experimental description of H217O hyperfine couplings to high spin Mn2+ in a frozen solution. The ESEEM results are consistent with the first-order assignments of the ENDOR lines and demonstrate the feasibility of ESEEM measurements of 17O ligand hyperfine couplings to Mn2+. Simulations of the 17O ENDOR hyperfine patterns of aqueous frozen solutions of Mn2+, especially those near 20 MHz, indicated an A-tensor anisotropy of A⊥=−6.5±0.5 MHz and A∥=−9.5±0.5 MHz, consistent with couplings observed by single crystal ENDOR of H217O ligated to Mn2+ doped in [LaMg(NO2)12⋅24(H2O)]. More detailed simulations of the ENDOR pattern below 10 MHz indicated the need for quadrupole couplings consistent with those measured by single crystal ENDOR and with those determined by gas phase measurements on H17OD. Simulations of the ENDOR spectra recorded by the cw and pulsed techniques have delineated important features of the techniques which must be taken into account for a quantitative analysis of the ENDOR amplitudes. It is expected that the general ENDOR conditions employed and the theory developed will be useful in frozen solution studies of 17O involved as a ligand to Mn2+ in enzymes.
Reaction path zero-point energy from diffusion Monte Carlo calculations
Authors: Gregory Jonathon K. , Wales David J. , Clary David C.
Published: July 2002

A general diffusion quantum Monte Carlo method is described for accurately calculating the zero-point energy of the vibrations orthogonal to a reaction path in a polyatomic system. The method fully takes into account anharmonic and mode–mode coupling effects. The algorithm is applied to the OH+H2→H2O+H reaction and the results are compared with a more approximate calculation. The technique will have many useful applications to kinetic and spectroscopic problems involving polyatomic molecules.
Electron-impact ionization of the SO free radical
Authors: Tarnovsky V. , Levin A. , Deutsch H. , Becker K.
Published: July 2002

We studied the electron-impact ionization of the SO free radical using the fast-neutral-beam technique. Absolute cross sections were measured for the formation of the SO+ parent ion as well as for the S+ and O+ fragment ions from threshold to 200 eV. At 70 eV, cross section values of 3.25±0.50×10−20 m2 (SO+), 1.32±0.23×10−20 m2 (S+), and 0.45±0.08×10−20 m2 (O+) were obtained. We found little evidence of the presence of vibrationally excited SO radicals in the incident target beams and only negligible contributions to the measured ion signal from excited SO radicals in Rydberg and/or metastable states were observed. The only significant pathway contributing to the formation of the observed SO+, S+, and O+ ions is the single positive ion formation. The measured partial ionization cross sections were combined to yield the absolute total single ionization cross section of SO which was compared to calculations using a recently introduced modified additivity rule. The agreement between measured and calculated cross sections was found to be very good.
Autoionizing resonance in photoionization from the 1πu level of acetylene
Authors: Mitsuke Koichiro , Hattori Hideo
Published: July 2002

Autoionizing resonance of acetylene is studied by photoelectron spectroscopy using synchrotron radiation. Pronounced vibrational excitation in the C–H stretching mode ν1 is observed in the (1πu)−1X̃ 2Πu band of C2H+2 at a restricted photon energy range from 12.8 to 14.1 eV. It is concluded that the 3σg→3σu autoionizing transition at ∼13.3 eV gives rise to an anomalously broad maximum in the (1πu)−1 photoionization cross section curve. The strong ν1 excitation is explained as that the equilibrium C–H bond length differs from the neutral and ionic ground states to the (3σg)−1(3σu)1 resonance state. Constant ionic state spectra for the v1=3 and 4 levels of the X̃ 2Πu state measured over the same energy region show fine structures with regular spacings corresponding to the vibrational levels of the (3σg)−1(3σu)1 state.
Study of the sedimentation of a single particle toward a flat plate
Authors: Walz John Y. , Suresh Lakkapragada
Published: July 2002

The sedimentation of slightly Brownian particles toward a flat plate in water has been measured using the optical technique of total internal reflection microscopy (TIRM). The particles used were 15, 20, and 25 μm diameter polystyrene latex spheres and the plate was a polished BK-7 glass slide. The measurements were performed for dimensionless separation distances (gap width/sphere radius) ranging from 0.01 to 0.05, thus the mobility of the particles was greatly reduced by hydrodynamic coupling with the plate. The average sedimentation rate was found to agree closely with predictions made using the drag equations of Brenner [H. Brenner, Chem. Eng. Sci. 16, 242 (1961)] when the measured separation distances were adjusted by a constant offset which is believed to reflect the presence of asperities on the particle surface. In addition, the Brownian movements observed during sedimentation were found to be normally distributed with a mean of zero, also in agreement with theory. Thus by simply measuring the sedimentation rate, the dependence of both the particle’s mobility and diffusion coefficient on separation distance could be obtained. Although surface roughness on the particle limited the distance of closest approach, it did not affect the particle/surface hydrodynamics.
The kinetic theory of a simple, chemically reactive fluid: Scattering functions and relaxation processes
Authors: Qin Lihong , Dahler John S.
Published: July 2002

The purpose of this investigation is to develop a computationally tractable procedure for obtaining theoretical estimates of the correlation functions characteristic of a chemically reactive small-molecule fluid and for determining the spectral characteristics of the associated many-body modes. Generalized Enskog-type kinetic equations (appropriate both to low and high densities) are derived for fluid mixtures which include species capable of participating in bimolecular reactions such as AB+CD⇄AC+BD. The dynamics of the reactive events are treated according to the model of simple reactive spheres (SRS) and to a less restrictive, modified simple reactive sphere (MSRS) model, as well. The distribution functions of the various species are represented by bilinear expansions in terms of velocity basis functions and the associated expansion coefficients are identified with correlation functions of pairs of single-particle dynamical variables. The kinetic equations are then transformed into coupled sets of linear integrodifferential equations for these correlation functions. Finally, illustrative calculations are performed and results compared with available experimental data.
The electronic spectroscopy of jet-cooled m-difluorobenzene
Authors: Graham Philippa A. , Kable Scott H.
Published: July 2002

The S1(1B2)–S0(1A1) electronic transition of m-difluorobenzene cooled in a supersonic free jet has been investigated in detail by use of laser induced fluorescence excitation and dispersed fluorescence spectroscopy. By analysis of over 50 vibronic transitions we have assigned 16 new S1 vibrational frequencies, and have confirmed seven previous assignments. This advances the total number of known S1 fundamental vibrational frequencies to 23 out of the possible 30. The assignments of two S0 frequencies have also been somewhat modified. Two different types of symmetry-forbidden transitions were identified: transitions involving one-quantum changes in b2 modes were found; and transitions involving combinations of b1 vibrations were observed to be unusually active in the vibronic spectrum. The observation of these forbidden transitions in both the fluorescence excitation and dispersed fluorescence spectra are explored in terms of first- and second-order vibronic coupling.
Static structure factor and collective diffusion of globular proteins in concentrated aqueous solution
Authors: Fine Bernard M. , Lomakin Aleksey , Ogun Olutayo O. , Benedek George B.
Published: July 2002

We report our measurement of the time average and the temporal autocorrelation function of the intensity of light scattered by the highly monomeric globular protein, bovine γII-crystallin, in aqueous solution as a function of wave number q, protein volume fraction φ, and temperature T. The time average intensity data is used to obtain the q→0 limit of the static structure factor S(φ,T), as a function of φ and T. We show that S(φ,T) may be well characterized by modeling the proteins as interacting through the Baxter adhesive hard sphere pair interaction potential. The temporal autocorrelation function data is used to determine the collective diffusion coefficient D̃(φ,T) of the proteins as a function of φ and T. We then obtain the experimental hydrodynamic factor H̃(φ,T)≡S(φ,T)[D̃(φ,T)/D0(T)], where D0(T) is the diffusion coefficient of the individual proteins in the φ→0 limit. We find that H̃ exhibits a different φ-dependence at low (φ≤0.016) and high (φ≳0.02) protein volume fractions. In the low φ domain our data for H̃ are consistent with the theoretical result for the collective diffusion in the q→0, t→0 limit. However, for φ≳0.02 we find a deviation from single exponential decay in the autocorrelation functions, and an unexpected, large change in the slope of the H̃ vs φ relation. This crossover at such low φ suggests the existence of a heretofore unappreciated length scale in the dynamics of colloid solutions. Clearly, further theoretical insights are required to understand the origin of this crossover behavior.
Rate constants in the spatial diffusion regime for a model based on the generalized Langevin equation
Authors: Rey Rossend
Published: July 2002

Rate constants have been numerically computed for a cubic oscillator subject to a Gaussian friction kernel. The system is studied over a wide range of system–bath coupling, as measured by the static friction coefficient, and the obtained rate constants are compared with theoretical predictions as well as with previous simulations. The results are consistent with variational bounds in the spatial diffusion regime; values lower or equal to the theoretical estimates are obtained throughout. This should resolve the paradox that existed for very high friction, where values higher than the theoretical predictions has been reported and tentatively assigned to a possible nonergodicity of the model. The origin of the discrepancy between the present results and previous simulations, together with the range of validity of several theoretical approaches, are discussed.
Crossover of the concentration dependence of swelling and elastic properties for polysiloxane networks crosslinked in solution
Authors: Urayama Kenji , Kohjiya Shinzo
Published: July 2002

We have investigated the preparation concentration dependence of the polymer volume fraction and the elastic modulus of the equilibrium swollen polydimethylsiloxane networks in toluene. The networks were prepared by the end-linking method in solution with a wide range of concentration. The network samples were attempted to be as small amount of structural defects as possible. The crossover has been observed for the preparation concentration dependence of the degree of equilibrium swelling and the elastic modulus. The crossover concentrations agree with each other, and are almost identical with the boundary concentration between the semidilute and the concentrated regime for the end-to-end distance of the polymer chain. The theoretical predictions which describe the experimental results have been derived as a function of the polymer volume fraction of preparation and swollen state, considering the effect of trapped entanglements on the elasticity and the concentration regime of preparation and swollen state. The good agreements of the theoretical predictions with the experimental results suggest that the trapped entanglements contribute to the elastic modulus together with the chemical crosslinks, and the crosslinking points moves affinely relative to the global polymer network on swelling.
Extension of the law of entropy increase to nonisolated systems
Authors: Yoshida Nobuo
Published: July 2002

The behavior of the entropy of stationary states in a continuously stirred tank reactor (CSTR) operated by use of ideal gases is investigated in an attempt to gain insight into the thermodynamics of open nonlinear systems. The second law of thermodynamics predicts that for an adiabatic CSTR, the change of entropy of the reactor contents in a stationary state relative to the inflow is always nonnegative. This is an example of the law of entropy increase in an isolated system. This law as such does not in general apply to a nonadiabatic CSTR, i.e., when the reactor exchanges heat with its environment. The law, however, can still be made valid for such systems under suitable conditions provided the entropy of a stationary state is measured relative to the equilibrium state that would be attained by isolating the stationary state from its surroundings. This is illustrated on the basis of a number of specific models.
Ab initio calculations and high resolution infrared investigation on XeF4
Authors: Bürger H. , Ma S. , Breidung J. , Thiel W.
Published: July 2002

Ab initio calculations employing effective core potentials and polarized triple-zeta basis sets have been carried out for XeF4 at the following levels: self-consistent-field (SCF) theory, Mo/ller–Plesset second-order perturbation theory (MP2), and coupled cluster theory with single and double excitations and a perturbational treatment of triple excitations (CCSD(T)). Pronounced correlation effects are found, especially for the Xe–F bond length and the vibrational frequencies. The theoretical predictions for the harmonic and anharmonic force fields and the associated spectroscopic constants have guided the analysis of the experimental data. Fourier transform infrared spectra of monoisotopic 136XeF4 have been recorded between 100–1170 cm−1 with an effective resolution of 0.002–0.004 cm−1, and a rotational analysis has been performed for the ν2 band. The high resolution results provide accurate molecular parameters for the ground state and the v2=1 excited state of XeF4 and allow a precise determination of the ground state Xe–F bond length, r0=193.487(3) pm. The agreement between the experimental and the correlated theoretical results is generally quite good.
Destruction cross sections for low energy collisions of H+3 and D+3 with rare gas atoms
Authors: Peko B. L. , Champion R. L. , Wang Yicheng
Published: July 2002

Absolute total cross sections for collisional dissociation and charge transfer have been measured for collisions of H+3 and D+3 with He, Ar, and Xe for projectile energies ranging from 15 to 400 eV. The cross sections for collision-induced dissociation of H+3 into H++H2 or H+H+2 are a few Å2 and exhibit a weak target dependence. It is suggested that the charge transfer cross sections, which vary markedly with target species, are due primarily to proton transfer to the rare gas target rather than electron transfer from the target. At the lower collision energies, proton abstraction is favored over deuteron abstraction for the He and Ar targets.
Vibronic spectrum and structure of the trans-bent acetylene radical anion
Authors: Kusumori Takeshi , Matsuura Kaoru , Muto Hachizo
Published: July 2002

We report the first observation of the vibronic absorption spectrum of the trans-bent acetylene radical anion, produced in 3-methylpentane matrices by γ irradiation at 77 K. Two optical absorption bands are observed at 300–360 nm and in the region λ<420 nm. The former (strong) and the latter (weak) bands correspond respectively to electronic transitions to the Ψ2(Bu) and Ψ1(Au) excited states, specifically to the vertical transitions ‖pπy*(C≡C)〉←‖pπy(C≡C)〉 and ‖pπy*(C≡C)〉←‖πz(C≡C)〉 from the ground state ΨG(Ag) having an in-plane pseudo π* type singly occupied molecular orbital: ‖pπy*(C≡C)〉. For the 12C2H2− anion, these bands are accompanied by a single-vibrational progression of 1300–1240 (±20) and ∼1150 (±60) cm−1, respectively. These progressions are assigned to the (v←0) transitions of C≡C stretching modes and to H–C≡C–H bending vibrational modes by comparison with the results for 13C2H2− and 12C2D2− anions. The appearance of only a single-vibrational mode is reasonably well understood, from a molecular orbital calculation, as a consequence of the above electronic transitions. The frequency of the Ψ2(Bu) excited anion is much smaller than the frequency of the acetylene molecule νC≡C=1974 cm−1 and that of the ground state of the cis-bent anion moiety in the reported [Li+...C2H−2] complex 1655 cm−1. Our results indicate that the C≡C bond is greatly weakened by an addition of an excess electron to the antibonding orbital and by electronic excitation. The vibronic structure of the anion is discussed in conjunction with the results of an electron spin resonance study.
Reactions of O(3P) with alkynes: The CO and H atom channels
Authors: Xing Guoqiang , Huang Xin , Wang Xuebin , Bersohn Richard
Published: July 2002

This is the second in a series of papers on the reaction of O(3P) with alkynes in which the internal state distribution of some products are studied. The first paper dealt with acetylene whose two product channels are CO+CH2 and H+HCCO. The present paper deals with the reactions of a series of higher alkynes; however, just the CO release and the H atom release channels were studied. The CO product was rotationally and vibrationally cold in every case. We therefore infer that, except possibly for acetylene, the initial ketocarbene undergoes intersystem crossing to a singlet state and isomerizes to a substituted ketene which then dissociates through a linear C–C–O transition state. The absence of CO vibration energy implies that the energy taken from the initially formed C–O bond to facilitate a 1,2 migration is not returned. The large H atom translational energy implies that the H atom is released simultaneously with the formation of a radical of high resonance energy. Finally, the CO and H atom yields decrease in the longer alkynes, presumably because the dominant reaction channel becomes C–C bond breaking leading to radical pair formation.
The IMOMO method: Integration of different levels of molecular orbital approximations for geometry optimization of large systems: Test for n-butane conformation and S
N2 reaction: RCl+Cl−
Authors: Humbel Stéphane , Sieber Stefan , Morokuma Keiji
Published: July 2002

A new theoretical method, called IMOMO (integrated MO (molecular orbital)+MO), for integration of two different levels of MO approximation is presented. Only the active or more difficult part of a molecule is treated at a higher level of approximation and the rest of the molecule at a lower level of approximation. The integrated total energy and energy derivatives are defined from three different calculations, and the structure of transition state as well as the equilibrium structure can be optimized using the integrated energy. Any combination of any molecular orbital approximations (ab initio, density functional to semi-empirical) can be used. Test calculations in the IMOMO method have been performed and compared with normal MO calculations for the conformation energy of ethane and n-butane and the SN2 reaction of ethyl, propyl, isobutyl, and neopentyl chloride with Cl−. The results indicate that these methods have a tremendous potential for theoretical study of larger molecules, in particular for transition states.
Induced transient birefringence of a resonantly pumped molecular gas
Authors: Shen Liyang , Ho Tak-San , Shi Shenghua , Rabitz Herschel , Lin Chen , Littman M. , Weiner Andrew M.
Published: July 2002

We present a theoretical study of the induced transient birefringence of a low density homogeneous molecular gas in a resonant pump–probe experiment. The molecular coherent state induced by the resonant pump field is described by second-order perturbation theory. The induced birefringence can be detected by a delayed probe pulse propagating through the molecular medium after illumination by the pump pulse. In the case of a nonresonant probe, the birefringence is linearly proportional to the mean value of the electronic polarizability of the molecular gas. The birefringence signal is composed of distinct components due to population change and those of rotational, vibrational, and mixed vibrational–rotational origins. This is demonstrated by numerical simulations on Li2 gas. Moreover, the quantum beats contained in the birefringence, as a function of the time delay between the pump and probe pulses, is dominated by the pure rotational motion. Finally, the birefringence is sensitive to the shape of the applied pump pulse and dependent on the spectral phase of the pump pulse.
The near infrared electronic spectrum of tungsten methylidyne, WCH
Authors: Barnes M. , Gillett D. A. , Merer A. J. , Metha G. F.
Published: July 2002

The laser excitation spectrum of tungsten methylidyne, WCH, has been recorded in the 12 000–15 400 cm−1 region. A total of 14 vibronic bands of WCH and 16 bands of WCD have been observed in this region. Rotational analysis shows that the ground state is X̃3/2(2Δ), with the substitution structure r0(W≡C)=1.73665 Å and r0(C–H)=1.0765 Å. The excited vibronic levels have been assigned, on the basis of their WCH/WCD isotope shifts and their wavelength resolved fluorescence patterns, to three electronic states, Ã3/2(2Δ), B̃1/2(2Π), and C̃5/2(2Φ), at 12 090, 13 392, and 14 110 cm−1, respectively. The wavelength resolved fluorescence spectra have also established the low-lying vibrational levels of the ground state. The ground state bending fundamental lies at 660 cm−1, while the W–C stretching frequency is 1006 cm−1; the corresponding frequencies in WCD are 501 cm−1 and 953 cm−1, respectively. No evidence for the C–H stretching frequency has been found. Emission has also been observed to a low-lying electronic state, 813 cm−1 above the X̃3/2 state. The pattern of rotationally resolved emission to this state clearly indicates that it is a 4Σ1/2 state. Its bending frequency is 612 cm−1, and its W–C stretching frequency is 971 cm−1, indicating a slightly longer bond length than in the X̃3/2 state. High resolution cw laser spectra of the (0,0) bands of the two lowest excited electronic states [Ã3/2(2Δ) and B̃1/2(2Π)] reveal a small splitting of the four principal tungsten isotopes (182W, 183W, 184W, and 186W) which serves to confirm the presence of tungsten in the carrier. Hyperfine splitting associated with the 183W nucleus (I=1/2) has been observed for the (0,0) band of the Ã3/2−X̃3/2 system, allowing the electron configuration of the ground state to be elucidated.
Resonant vibrational excitation of furan by low energy electron impact
Authors: Motte-Tollet F. , Eustatiu G. , Roy D.
Published: July 2002

Our purpose was to investigate the resonant vibrational excitation of furan (C4H4O) in its electronic ground state by electron collisions. The electron energy loss measurements have been carried out at intermediate and low impact energies for a scattering angle of 30°. The spectra show peaks or groups of peaks corresponding to the excitation of the fundamental normal modes as also of the first harmonics and combinations. The energy dependence curve for the excitation of the C–H stretching vibrational modes is also reported for the impact energy range of 3–10 eV. It exhibits a broad structureless band with a maximum centered around 6 eV, which reveals an excitation mechanism through the intermediate of a shape resonance built on the electronic ground state of the neutral molecule. A comparison with benzene is made in order to help the analysis and to assign the nature as well as the symmetry of the resonance.
Development and validation of reliable quantum mechanical approaches for the study of free radicals in solution
Authors: Rega Nadia , Cossi Maurizio , Barone Vincenzo
Published: July 2002

The hyperfine parameters of a number of representative free radicals have been computed by post-Hartree–Fock and density functional approaches including averaging effects from large amplitude vibrations and solvent effects through a recent implementation of the polarizable continuum model. Our results show that fully ab initio hyperfine splittings are accurate enough to back the interpretation of experimental data and to allow an unbiased judgement of the role played by electronic, vibrational, and environmental effects in determining the observed value. The very good results obtained by a density functional approach including some Hartree–Fock exchange both for intrinsic values and for solvent shifts pave the route for the investigation of large biologically significant radicals in their natural aqueous medium.
Two-dimensional proton magnetization-exchange NMR spectroscopy in cross-linked elastomers
Authors: Demco D. E. , Hafner S. , Fülber C. , Graf R. , Spiess H. W.
Published: July 2002

Solid-state two-dimensional proton magnetization-exchange NMR is used to investigate intergroup residual dipolar couplings in a cross-linking series of poly(styrene-cobutadiene) elastomers. The magnetization-exchange process between the CH and the CH2 group in the regime of short mixing time provides valuable insight regarding molecular order. A three-spin model is employed, in which the CH and CH2 protons are considered to be coupled by residual dipolar interactions. The spin-system response reflects well-localized dipolar interactions. The time scale in which the exchange process takes place justifies these assumptions as well as the interpretation of various NMR relaxation experiments previously performed on this class of polymers. The residual intergroup dipolar couplings are measured along the average polymer-chain direction and correlated with the dynamic storage modulus. It is shown that they are sensitive to both cross-link points and nonpermanent geometric constraints of the chain motion. The dynamic order parameter along the chains is evaluated. It exhibits the remarkably high value of 〈P2〉≊0.13 for the carbon–carbon bond connecting the CH and the CH2 groups in polybutadiene in the uncross-linked copolymer melt and only slightly increases with the cross-link density.
Non-Gaussian behavior and the dynamical complexity of particle motion in a dense two-dimensional liquid
Authors: Hurley M. M. , Harrowell Peter
Published: July 2002

The single particle dynamics of a 2D liquid made up of soft disks interacting by a repulsive r−12 potential are studied using molecular dynamics simulations. We find that mean squared particle displacement 〈Δr(t)2〉 behaves diffusively, i.e., increases linearly with time, within a time interval tc very much shorter than that required for structural relaxation. The non-Gaussian parameter α(t)=〈Δr4〉/2〈Δr2〉2−1, on the other hand, exhibits a significant peak at times considerably greater than tc and a subsequent slow decay. It is argued that the only picture of diffusion consistent with these results considers particles moving in a medium characterized by fluctuating local mobilities. This picture provides an explicit connection between structural fluctuations (as characterized by the local mobility) and single particle motion. The possibility of obtaining the width and lifetime of the distribution of local relaxation times from incoherent scattering is examined.
Many-body depletion interactions among protein spheres in a semidilute polymer solution
Authors: Odijk Theo
Published: July 2002

The polymer segment distribution is shown to obey the Laplace equation for a suspension of small protein spheres and semidilute polymer. The depletion interaction is computed at all protein concentrations by introducing void distributions. Within a linear response Ansatz for the solution to the Laplace equation, the average depletion energy depends on two- and three-point void (-surface) correlation functions. It is concluded that depletion correlations of long range do not appear at high protein concentrations.
Infrared spectra of vaporized Pt and Pt+H2 in rare-gas matrices: Unique phonon effects
Authors: Li S. , Weimer H. A. , Van Zee R. J. , Weltner, Jr. W.
Published: July 2002

Laser-vaporized platinum metal, trapped in solid Ne, Ar, Kr, Xe (and Ar, Kr mixture) produced a progression of absorption bands with a spacing of ∼200 cm−1 beginning at 1942 cm−1 in Ne and shifting successively to 1579 cm−1 in Xe. In contrast with the specimen in Ne, strong patterns of similar sidebands appeared in the spectra in the heavier matrices. This matrix effect is considered in some detail in attempting to assign the trapped molecule and to explain the source and structure of the sidebands. The conclusion is that the absorption spectrum is the previously unobserved Πu3(1u)←X Σg-3(0g+) transition of Pt2 (inferred from Balasubramanian’s calculation) but with unique phonon interactions in the heavier matrices. When hydrogen was present in the matrix gases, Pt hydride bands were observed but not prominently, in contrast to Ni+H2 matrices.
Picosecond dynamics of an IR sensitive squaraine dye. Role of singlet and triplet excited states in the photosensitization of TiO2 nanoclusters
Authors: Liu Di , Kamat Prashant V. , Thomas K. George , Thomas K. J. , Das Suresh , George M. V.
Published: July 2002

The photosensitization properties of a novel squaraine dye is probed using picosecond laser flash photolysis. The dye which exists in a protonation equilibrium with a pKa of 4.9 has a strong absorption in the IR region. The protonated form of the dye is highly fluorescent and is readily quenched in colloidal TiO2 suspensions. The association constant as determined from the quenching studies is 1.25×103 M−1. The singlet excited state of the protonated form of the dye participates in the charge injection process with a heterogeneous electron transfer rate constant of 8.3×109 s−1. The unprotonated form of the dye injects electrons from both the singlet and triplet excited state with electron transfer rate constants of 1×1010 s−1 and 1.25×109 s−1, respectively.
Investigation of the linear and nonlinear optical response of edge-linked conjugated zinc porphyrin oligomers by optical spectroscopy and configuration interaction techniques
Authors: Beljonne D. , O’Keefe G. E. , Hamer P. J. , Friend R. H. , Anderson H. L. , Brédas J. L.
Published: July 2002

We present a joint experimental and theoretical investigation of the linear and nonlinear optical response of butadiyne-linked zinc porphyrin oligomers. Efficient overlap between the chromophores leads to a large red-shift of the Q absorption band together with a splitting of the B band into two components when increasing the chain length from one to two repeating porphyrin units. Ultrafast pumb-probe measurements performed on these compounds show several well-defined features, associated with singlet excited state absorption. We also find clear evidence for the formation of longer-lived triplet excitons, resulting from very efficient singlet–triplet intersystem crossing processes. For the monomers, the assignment of the observed linear and photoinduced absorption features is supported by a Configuration Interaction description of the singlet and triplet excited states. In the case of the porphyrin dimer, agreement with the experimental results can only be obtained by considering a sizable cumulenic contribution in the excited-state geometry used as input for the CI calculations. The potential of porphyrin systems for Reverse Saturable Absorption and Nonlinear Optics is emphasized.
Precrystallization structures in supersaturated lysozyme solutions studied by dynamic light scattering and scanning force microscopy
Authors: Schaper Achim , Georgalis Yannis , Umbach Patrick , Raptis Jannis , Saenger Wolfram
Published: July 2002

A comparitive study of the nanostructure evolving during aggregation of hen-egg white lysozyme in supersaturated solution was carried out by dynamic light scattering (DLS) and scanning force microscopy (SFM). Lysozyme aggregate (cluster) formation was observed in solution in the presence of NaCl, (NH4)2SO4, and NaNO3 as precipitating agents. The growth kinetics were examined by DLS and revealed fractal growth of the clusters with a fractal dimension of 1.8 obtained independently of the type of inert salt. Such behavior is typical for diffusion-limited cluster–cluster (DLCA) aggregation. Initial lysozyme cluster sizes were in the range of 12–35 nm. SFM images of individual lysozyme clusters at the liquid–solid interface were obtained in the presence of NaCl and NaNO3 under crystallization conditions, and revealed cluster sizes in agreement with those determined by DLS. Extended domains of smaller sized clusters appeared on the mica surface after subjecting supersaturated lysozyme solutions to a dialysis step. The feasibility of DLS and SFM for monitoring the nano- and mesoscopic morphology of lysozyme aggregates in supersatured solutions and at the solid–liquid interface is discussed.
Self-diffusion coefficient in dense hard sphere colloids
Authors: Moriguchi Ichirou
Published: July 2002

By means of hard sphere Brownian dynamics simulation technique, it was confirmed that the ratio of short- and long-self diffusion coefficient is not affected whether the solvent-mediated long range hydrodynamic interaction exists or not, and that the density dependence of diffusion coefficient is well represented by the theory of Tokuyama and Oppenheim.
Calculation of electronic coupling matrix elements for ground and excited state electron transfer reactions: Comparison of the generalized Mulliken–Hush and block diagonalization methods
Authors: Cave Robert J. , Newton Marshall D.
Published: July 2002

Two independent methods are presented for the nonperturbative calculation of the electronic coupling matrix element (Hab) for electron transfer reactions using ab initio electronic structure theory. The first is based on the generalized Mulliken–Hush (GMH) model, a multistate generalization of the Mulliken Hush formalism for the electronic coupling. The second is based on the block diagonalization (BD) approach of Cederbaum, Domcke, and co-workers. Detailed quantitative comparisons of the two methods are carried out based on results for (a) several states of the system Zn2OH2+ and (b) the low-lying states of the benzene–Cl atom complex and its contact ion pair. Generally good agreement between the two methods is obtained over a range of geometries. Either method can be applied at an arbitrary nuclear geometry and, as a result, may be used to test the validity of the Condon approximation. Examples of nonmonotonic behavior of the electronic coupling as a function of nuclear coordinates are observed for Zn2OH2+. Both methods also yield a natural definition of the effective distance (rDA) between donor (D) and acceptor (A) sites, in contrast to earlier approaches which required independent estimates of rDA, generally based on molecular structure data.
Adsorption of fluid vesicles
Authors: Baumgärtner A. , Bhattacharjee Somendra M.
Published: July 2002

The adhesion of fluid vesicles to a planar surface has been studied using Monte Carlo methods and scaling arguments for the random surface model. Deflated as well as inflated vesicles have been considered. Inflated vesicles, with internal pressure p>0, exhibit with increasing adhesion strength a discontinuous conformational transition from unbound sphere-like conformations to strongly adsorbed two dimensional branched conformations. Deflated vesicles with p=0 exhibit a continuous transition from three dimensional to two dimensional branched conformations where the type of transition is different from adsorption transition of branched polymers. The transition temperatures scale according to εc/kT∼pN, where N is the surface area of the vesicle.
Quasielastic neutron scattering and molecular dynamics simulation studies of the melting transition in butane and hexane monolayers adsorbed on graphite
Authors: Herwig K. W. , Wu Z. , Dai P. , Taub H. , Hansen Flemming Y.
Published: July 2002

Quasielastic neutron scattering experiments and molecular dynamics (MD) simulations have been used to investigate molecular diffusive motion near the melting transition of monolayers of flexible rod-shaped molecules. The experiments were conducted on butane and hexane monolayers adsorbed on an exfoliated graphite substrate. For butane, quasielastic scattering broader than the experimental energy resolution width of 70 μeV appears abruptly at the monolayer melting point of Tm=116 K, whereas, for the hexane monolayer, it appears 20 K below the melting transition (Tm=170 K). To facilitate comparison with experiment, quasielastic spectra calculated from the MD simulations were analyzed using the same models and fitting algorithms as for the neutron spectra. This combination of techniques gives a microscopic picture of the melting process in these two monolayers which is consistent with earlier neutron diffraction experiments. Butane melts abruptly to a liquid phase where the molecules in the trans conformation translationally diffuse while rotating about their center of mass. In the case of the hexane monolayer, the MD simulations show that the appearance of quasielastic scattering below Tm coincides with transformation of some molecules from trans to gauche conformations. Furthermore, if gauche molecules are prevented from forming in the simulation, the calculated incoherent scattering function contains no quasielastic component below Tm. Modeling of both the neutron and simulated hexane monolayer spectra below Tm favors a plastic phase in which there is nearly isotropic rotational diffusion of the gauche molecules about their center of mass, but no translational diffusion. The elastic scattering observed above Tm is consistent with the coexistence of solid monolayer clusters with a fluid phase, as predicted by the simulations. For T/Tm⩾1.3, the elastic scattering vanishes from the neutron spectra where the simulation indicates the presence of a fluid phase alone. The qualitative similarities between the observed and simulated quasielastic spectra lend support to a previously proposed “footprint reduction” mechanism of melting in monolayers of flexible, rod-shaped molecules.
Kinetic energies of Cn+ fragment ions (58⩾n⩾4) produced by electron impact on C60
Authors: Muigg D. , Denifl G. , Scheier P. , Becker K. , Märk T. D.
Published: July 2002

We used mass spectrometric techniques in conjunction with the ion deflection method to determine the kinetic energies of Cn+ fragment ions (58⩾n⩾4) produced by controlled electron impact on C60 under single collision conditions. The recorded ion beam profiles for the various fragment ions were analyzed using two independent methods. One method extracts the average kinetic energy of the fragment ion under study from the measured half-width of the ion beam profile (half-width method), whereas the second method analyzes the entire measured beam profile (profile method). For each Cn+ fragment ion, the kinetic energies obtained were interpreted assuming two possible formation pathways, (i) the fission of the excited parent C60+ ion in a single-step, two-fragment break-up, C60+→Cn++C60−n and (ii) the sequential decay of excited C60+ into Cn+ via the successive removal of Cm units (m=1, 2, or 3). Both the half-width method and the profile method yield very similar results for the kinetic energies of the Cn+ fragment ions for both formation mechanisms, viz., an average kinetic energy of roughly 0.45 eV in the case of the single-step fission and a linearly increasing average kinetic energy from 0.43 eV for the formation of C58+ to roughly 12 eV for the formation of C4+ in case of the sequential decay mechanism (in this case an essentially constant energy of also about 0.45 eV was obtained for the last fragmentation step in each case, which was confirmed by a third method using a fitting procedure). These results will be discussed in the context of the most likely fragmentation mechanism leading to the formation of the various fragment ions.
A unified model of the dynamics and spectroscopy of the g 3Σ− and E 1Σ+ states of hydrogen chloride
Authors: Strizhev Alex , Li Xiaonong , Liyanage Rohana , Gordon Robert J. , Field Robert W.
Published: July 2002

The yield, Y, the spin–orbit branching ratio, Γ, and the angular anisotropy, β2, of Cl atoms produced by predissociation of the E 1Σ+ and g 3Σ0− Rydberg states of HCl and DCl are reported as functions of the rotational angular momentum J. For the E state, Y increases with J for HCl and decreases with J for DCl, whereas Γ and β2 are independent of J for both isotopomers. For the g0 state Y increases with J, whereas Γ and β2 both decrease with J for both isotopomers. The different dynamical behavior of these two 0+-symmetry states belonging to the same zero-order electronic configuration is explained in terms of their diabatic characters.
The surface plasmon enhancement effect on adsorbed molecules at elevated temperatures
Authors: Chiang H.-P. , Leung P. T. , Tse W. S.
Published: July 2002

The surface plasmon enhancement effect on adsorbed molecules at elevated substrate temperatures is studied theoretically using surface enhanced Raman scattering (SERS) as an example. The surface structure is idealized to be a monodisperse spherical particle with its nonlocal dielectric response accounted for. The temperature effects are modeled using a temperature-dependent collision frequency in the Drude model. Numerical results show that only a small decrease in the SERS enhancement ratio occurs for temperatures up to the melting point of the substrate, even for scattering close to the surface plasmon resonance frequency of the metal. More definitive results are subjected to more realistic modeling as well as systematic experimental studies. The implication of this result to other surface photochemical processes is discussed.