bluuebunny/crossref_metadata_2025 huggingface dataset:
On the tunnel electron transport in metal/Langmuir–Blodgett film/metal systems
Authors: Barraud André , Millie Philippe , Yakimenko Irina
Published: July 2002

This paper presents a new method for the calculation of the tunnel current through a metal/multilayer of conjugated molecules/metal sandwich structure under low and moderate electric fields. The electron transfer from one electrode to the other through the cascaded molecules is treated step by step by considering hopping integrals between neighboring molecules. A rather simple computer calculation gives access to the tunnel current. A second approach is also given which consists in approximating the cascaded tunnel mechanism by a homogeneous rectangular insulating barrier, the height of which can be calculated simply from the basic parameters of the system. These two calculation methods have been applied to a practical system: Langmuir–Blodgett films made of a two-dimensional porphyrin polymer. The lowest unoccupied molecular orbital (LUMO) level in tetrapyridino–porphyrin bromide, which is the monomeric unit of the polymer, has been calculated and found at 2.8 eV below vacuum level. From this value and literature values for the other parameters, the tunnel current densities vs voltage have been calculated for various film thicknesses (2–5 monomolecular layers). The two calculation methods give quite close values for the tunnel current densities. The tetrapyridinoporphyrin polymeric Langmuir–Blodgett film is suggested to check the model experimentally, because once fabricated, the rigid polymeric film cannot undergo molecular reorganisation or evaporation and hence should contain few pinholes.
Mode-coupling theory for self-diffusion in polymer blends and blend solutions
Authors: Tang Hai , Schweizer Kenneth S.
Published: July 2002

The microscopic polymer mode-coupling theory is generalized to treat diffusion in unentangled and entangled homogeneous polymer blends. Concentration fluctuations are shown to result in additional frictional resistance to chain translation, due to physical clustering and nonrandom mixing effects. The modification of the pure component effective friction coefficient depends sensitively on polymer degree of polymerization, temperature, blend composition, solution density, and nonuniversal local structure. Stronger effects are predicted for entangled systems near a liquid–liquid phase boundary. Model calculations and analytic results are presented for self-diffusion in structurally symmetric binary mixtures. Our predictions are qualitatively consistent with existing experimental measurements. Based on the theory, new experiments are suggested for blends in which the influence of concentration fluctuations on measured diffusion constants is maximized.
Polymers in periodic and aperiodic potentials: Localization effects
Authors: Sommer Jens-Uwe , Blumen Alexander
Published: July 2002

We investigate the behavior of polymer chains embedded in a lamellar matrix by considering both a regular periodic environment and the effect of disturbances. By using the Green’s function formalism and an attractive Kronig–Penney model, we obtain analytically exact results. For the case of a regular lamellar matrix of period ξ, a long polymer chain is characterized by an effective segment length leff, in analogy to the effective mass of electrons in solids. For potential wells deep enough there appears a gap of forbidden states which separates the low-lying, adsorption band from the higher lying, desorption band. Due to the ground-state dominance, for polymers only the lowest lying states are of physical relevance. Isolated defects of the periodic structure may localize the polymer, in the sense that infinitely long chains are confined inside a region of finite extent L around the defect. For a single defect we find L=1/(εΔξ), where ε is the strength of the periodic potential and Δξ is the deviation from the periodicity. This is also valid for finite chains when their number of segments exceeds the cross-over value NL=2L2/l2eff.
Unimolecular decomposition of NO3: The NO+O2 threshold regime
Authors: Mikhaylichenko K. , Riehn C. , Valachovic L. , Sanov A. , Wittig C.
Published: July 2002

The unimolecular decomposition of expansion-cooled NO3 has been investigated in the threshold regime of the NO+O2 channel. Photoexcitation in the region 16 780–17 090 cm−1 (596–585 nm) prepares ensembles of molecular eigenstates, each of which is a mixture of the B 2E′ bright state and lower electronic states. The X 2A2′ ground state is believed to be the probable terminus of 2E′ radiationless decay, though participation of A 2E″ is also possible. For these photon energies, unimolecular decomposition occurs exclusively via the NO+O2 channel, and NO yield spectra and state distributions have been obtained. The yield spectra are independent of the rotational state monitored, as expected for a large reverse barrier. The state distributions are insensitive to the photolysis photon energy and can be rationalized in terms of dynamical bias. The NO yield goes to zero rapidly above the O+NO2 threshold (17 090±20 cm−1). Because of tunneling, the NO+O2 channel does not have a precise threshold; the value 16 780 cm−1 is the smallest photon energy that yielded signals under the present conditions. Very small decomposition rates were obtained via time-domain measurements in which reactive quenching of long-lived NO3 fluorescence was observed. The rates varied from 1×104 at 16 780 cm−1 to 6×107 s−1 at 16 880 cm−1, and their collision free nature was confirmed experimentally. These data were fitted by using a one-dimensional tunneling model for motion along the reaction coordinate combined with the threshold Rice–Ramsperger–Kassel–Marcus (RRKM) rate. The top of the NO+O2 barrier is estimated to lie at 16 900±15 cm−1. Translational energy measurements of specific NO (X 2ΠΩ,v,J) levels showed that O2 is highly excited, with a population inversion extending to energies above the a 1Δg threshold, in agreement with previous work. It is possible that the main O2 product is X 3∑g−, though some participation of a 1Δg cannot be ruled out. Within the experimental uncertainty, b 1∑+g is not produced.
Combining ab initio computations, neural networks, and diffusion Monte Carlo: An efficient method to treat weakly bound molecules
Authors: Brown David F. R. , Gibbs Mark N. , Clary David C.
Published: July 2002

We describe a new method to calculate the vibrational ground state properties of weakly bound molecular systems and apply it to (HF)2 and HF–HCl. A Bayesian Inference neural network is used to fit an analytic function to a set of ab initio data points, which may then be employed by the quantum diffusion Monte Carlo method to produce ground state vibrational wave functions and properties. The method is general and relatively simple to implement and will be attractive for calculations on systems for which no analytic potential energy surface exists.
Numerical quantum propagation with time-dependent Hamiltonian
Authors: Zhu Wusheng , Zhao Xinsheng
Published: July 2002

A numerical method was proposed to propagate the quantum system with a time-dependent Hamiltonian. The propagator is composed of the time-independent-Hamiltonian-type multisplit forms, which becomes possible by realizing that the evolution operator has a close link with the time-shift operator. The present algorithm can achieve a high order of accuracy through the recurrence of multisplit forms with predetermined coefficients, and it has the advantage that only the Hamiltonian itself at different time instants is involved, while other currently available schemes need to perform either a time integration or a time differential of the Hamiltonian. This essential difference has made our proposal highly competitive. A one-dimensional model system is considered to verify the accuracy and efficiency of the numerical scheme.
Fluctuations effects in diblock copolymer fluids: Comparison of theories and experiment
Authors: Guenza Marina , Schweizer Kenneth S.
Published: July 2002

The analytic Polymer Reference Interaction Site Model (PRISM) theory of structurally and interaction symmetric Gaussian diblock copolymer fluids is reformulated, extended, and applied to make predictions for experimentally observable equilibrium properties of the disordered state. These include the temperature, degree of polymerization, copolymer composition, and polymer density or concentration dependences of the peak scattering intensity, effective chi-parameter, and heat capacity. The location of the order-disorder transition is empirically estimated based on the disordered, strongly fluctuating state scattering function. Detailed numerical applications of PRISM theory demonstrates it provides an excellent and consistent description of the data. An in depth comparison of the mathematical structure and predictions of PRISM theory with the highly coarse-grained, incompressible Brazovski–Leibler–Fredrickson–Helfand (BLFH) fluctuation corrected field theory is also carried out. Under some conditions (nearly symmetric composition, high melt densities, moderate temperatures) there are striking mathematical similarities between the predictions of the physically very different theories, although quantitative differences always persist. However, for strongly asymmetric copolymer compositions, short chains, compressible copolymer solutions, and low temperatures many qualitative differences emerge. The possibility of multiple, self-consistent fluctuation feedback mechanisms within the most general PRISM approach are identified, their qualitative features discussed, and contrasted with alternative versions of the fluctuation-corrected incompressible field theories due to BLFH and Stepanow. The predictions of PRISM and BLFH theory for the composition, copolymer density, temperature, and molecular weight dependence of the effective chi-parameter are presented, contrasted, and qualitatively compared with recent experiments.
Symplectic splitting methods for rigid body molecular dynamics
Authors: Dullweber Andreas , Leimkuhler Benedict , McLachlan Robert
Published: July 2002

Rigid body molecular models possess symplectic structure and time-reversal symmetry. Standard numerical integration methods destroy both properties, introducing nonphysical dynamical behavior such as numerically induced dissipative states and drift in the energy during long term simulations. This article describes the construction, implementation, and practical application of fast explicit symplectic-reversible integrators for multiple rigid body molecular simulations. These methods use a reduction to Euler equations for the free rigid body, together with a symplectic splitting technique. In every time step, the orientational dynamics of each rigid body is integrated by a sequence of planar rotations. Besides preserving the symplectic and reversible structures of the flow, this scheme accurately conserves the total angular momentum of a system of interacting rigid bodies. Excellent energy conservation can be obtained relative to traditional methods, especially in long-time simulations. The method is implemented in a research code, ORIENT, and compared with a quaternion/extrapolation scheme for the TIP4P model of water. Our experiments show that the symplectic-reversible scheme is far superior to the more traditional quaternion method.
Delay-induced chaos in catalytic surface reactions
Authors: Khrustova N. , Mikhailov A. S. , Imbihl R.
Published: July 2002

Deterministic chaos related to a sequence of period-doubling bifurcations (the Feigenbaum transition) has been observed in the NO+CO and NO+H2 reactions on Pt(100). On a microscopic scale, these reactions are accompanied by the formation of 1×1 adsorbate islands due to the properties of the 1×1↔hex phase transition of Pt(100). A simple skeleton model is constructed that describes the behavior of a population of reacting islands which are globally coupled together via the gas phase. Investigations of this model show that the experimentally observed chaotic behavior can result from delays in the response of the reacting islands to partial pressure variations in the gas phase.
Normal, single-file, and dual-mode diffusion of binary adsorbate mixtures in AlPO4-5
Authors: Sholl David S. , Fichthorn Kristen A.
Published: July 2002

We have used molecular simulations to examine the diffusion of Ne, Ar, Kr, Xe, CH4, CF4, CCl4, SF6, SnCl4, and SnBr4 in the molecular sieve AlPO4-5, both as single species and as coadsorbed mixtures. Single adsorbed species exhibit a transition from normal to single-file diffusion as a function of increasing adsorbate size. In addition to normal and single-file diffusion, coadsorbed mixtures exhibit a qualitatively new diffusion mode in which one species performs normal diffusion while the other undergoes single-file diffusion. We discuss the prospects for experimental verification of this phenomenon, and present measurements of the loading dependence of diffusion rates during the dual-mode diffusion of Ne/CF4 mixtures.
Three-body distribution functions in hard sphere fluids. Comparison of excluded-volume-anisotropy model predictions and Monte Carlo simulation
Authors: Ben-Amotz Dor , Stamatopoulou Argyroula , Yoon B. J.
Published: July 2002

Hard sphere three-body distribution functions predicted by the recently developed Excluded-Volume-Anisotropy (EVA) model are compared with Monte Carlo computer simulation measurements. Two types of simulations, both based on the Widom insertion method, are performed as a function of solvent density (0.1⩽ρσ3⩽0.8), solute structure (linear, triangular, and bent 3-bead chain), and solute–solvent sphere diameter ratio (0⩽σ/σS⩽3). Comparisons of these results with those of previous studies illustrate the accuracy of the EVA model in predicting multi-body distribution functions near contact separations (and inside of contact), where the Kirkwood-Superposition-Approximation is least accurate.
The electronic spectra of jet-cooled HGeCl and HGeBr
Authors: Harper Warren W. , Clouthier Dennis J.
Published: July 2002

Laser-induced fluorescence spectra of jet-cooled HGeCl, DGeCl, HGeBr, and DGeBr have been obtained using the pulsed discharge technique. The germylenes were produced by an electric discharge through argon seeded with the appropriate monohalogermane (H3GeX or D3GeX). All three excited state vibrational frequencies have been obtained for each of the four molecules. Analysis of the rotational structure of the 000 bands of the Ã 1A″–X̃ 1A′ band system allowed the determination of r0 structures in the ground and excited states, with the bond angles constrained to previous ab initio values. The results for HGeCl are: r0″(Ge–Cl)=2.171(2) Å, r0″(H–Ge)=1.592(1) Å, θ0″(HGeCl)=94.3°, r0′(Ge–Cl)=2.146(15) Å, r0′(H–Ge)=1.613(2) Å, θ0′(HGeCl)=114.5°; and for HGeBr: r0″(Ge–Br)=2.329(12) Å, r0″(H–Ge)=1.598(6) Å, θ0″(HGeBr)=93.9°, r0′(Ge–Br)=2.308(1) Å, r0′(H–Ge)=1.615(1) Å, and θ0′(HGeBr)=116.3°. The radiative lifetimes of the Ã 1A″ states of HGeCl and HGeBr have been measured to be 548±19 ns and 736±24 ns, respectively.
Quantum dynamics with real wave packets, including application to three-dimensional (J=0)D+H2→HD+H reactive scattering
Authors: Gray Stephen K. , Balint-Kurti Gabriel G.
Published: July 2002

We show how to extract S matrix elements for reactive scattering from just the real part of an evolving wave packet. A three-term recursion scheme allows the real part of a wave packet to be propagated without reference to its imaginary part, so S matrix elements can be calculated efficiently. Our approach can be applied not only to the usual time-dependent Schrödinger equation, but to a modified form with the Hamiltonian operator Ĥ replaced by f(Ĥ), where f is chosen for convenience. One particular choice for f, a cos−1 mapping, yields the Chebyshev iteration that has proved to be useful in several other recent studies. We show how reactive scattering can be studied by following time-dependent wave packets generated by this mapping. These ideas are illustrated through calculation of collinear H+H2→H2+H and three-dimensional (J=0)D+H2→HD+D reactive scattering probabilities on the Liu–Siegbahn–Truhlar–Horowitz (LSTH) potential energy surface.
Ab initio study of gas-phase sulphuric acid hydrates containing 1 to 3 water molecules
Authors: Arstila Hanna , Laasonen Kari , Laaksonen Ari
Published: July 2002

Sulphuric acid has a tendency to form hydrates, small clusters containing a few water molecules, in the gas phase. Hydrate formation has a stabilising effect on the vapour as the pressure of sulphuric acid drops (relative to unhydrated vapor), decreasing the nucleation rate. In classical nucleation theories the hydration energies and the hydrate distribution are predicted assuming that hydrates can be described as liquid droplets having thermodynamic properties of bulk liquid. To obtain a better understanding of the structures and formation energies of the smallest clusters, we have performed ab initio density functional calculations of the mono-,di-, and trihydrates. The hydrogen bonds between the molecules are found to be strong. The more water molecules the hydrate contains, the clearer ring-like structure is formed. Comparison to classical values for the hydration enthalpies confirms that the properties of bulk liquid do not describe the properties of the smallest clusters too well. The energy barrier for proton transfer reaction H2SO4⋅H2O →HSO4−⋅H3O+ for mono- and dihydrate is high, and protonisation is unlikely to occur, but in trihydrate the protonisation has almost occurred and the barrier is very low. We also studied the singly protonised monohydrate, and found that while sulphuric acid forms H bonds with the OH parts, the hydrogen sulphate ion tends to bind with the O (S=O) part, and the second proton stays tightly in the ion.
Catalytic oxidation of carbon monoxide by platinum cluster anions
Authors: Shi Yang , Ervin Kent M.
Published: July 2002

Catalysis of the oxidation reaction CO+12O2→CO2 by metal clusters is examined by gas phase ion chemistry and mass spectrometry techniques. Small platinum cluster anions, Ptn− (n=3–6), are effective catalysts for the reaction under thermal conditions. The proposed reaction mechanism involves atomic oxygen chemisorbed on the clusters.
X-ray natural circular dichroism in a uniaxial gyrotropic single crystal of LiIO3
Authors: Goulon José , Goulon-Ginet Chantal , Rogalev Andrei , Gotte Vincent , Malgrange Cécile , Brouder Christian , Natoli Calogero R.
Published: July 2002

We produce the first experimental evidence of x-ray natural circular dichroism (XNCD) in a uniaxial gyrotropic crystal of α-LiIO3 that is known to crystallize with space group P63 and to exhibit a very strong nonlinear susceptibility. For the sake of simplicity, the optical axis was set colinear to the direction of the exciting x-ray beam in order to get rid of undesirable birefringence and linear dichroism effects. The nicely structured XNCD spectra recorded at the LI, LII, and LIII edges of iodine are assigned to the electric dipole–electric quadrupole (E1.E2) interference terms which do not vanish in gyrotropic crystals but contribute to a second order polarizability. Our interpretation is consistent with a band structure calculation and is fully supported by ab initio multiple scattering simulations. The signatures recorded at the LII and LIII edges are similar and have the same sign. This is in contrast to x-ray magnetic circular dichroism (XMCD) spectra which usually exhibit opposite signs at the LII and LIII edges: while the exchange and spin-orbit interactions are the driving terms in XMCD, this is not the case in XNCD. The XNCD signal detected at the LI edge is found to be relatively more intense and has the opposite sign. These results leave very little space for a contribution of the electric dipole–magnetic dipole (E1.M1) interference terms which are usually dominant at optical wavelengths but should be barely detectable in the x-ray range.
The spectroscopy of high Rydberg states of ammonia
Authors: Langford Stephen R. , Orr-Ewing Andrew J. , Morgan Ross A. , Western Colin M. , Ashfold Michael N. R. , Rijkenberg Arjan , Scheper Connie R. , Buma Wybren Jan , de Lange Cornelis A.
Published: July 2002

This paper extends our knowledge of the higher excited states of the ammonia molecule by presenting detailed measurements of the 2+1 resonance enhanced multiphoton ionization (REMPI) spectrum of both NH3 and ND3 obtained following excitation in the wavelength range 298–242 nm, i.e., at energies up to the first ionization energy. Complementary analyses of the wavelength resolved REMPI spectrum and the accompanying REMPI-photoelectron spectra leads to the identification of ten new Rydberg origins of NH3 (four for ND3) with principal quantum numbers n⩽8 and, in most cases, of the accompanying out-of-plane bending vibrational progression. Symmetry assignments for the various newly identified excited states are offered, based on band contour simulation and/or quantum defect considerations. Dominant amongst these are the Ẽ″ 1A2″ (5sa1′←1a2″) state: ν0=74 118(2) cm−1 [NH3], ν0=74 258(2) cm−1 [ND3], the F̃″ 1E″ (5pe′←1a2″) state: ν0=76 220(50) cm−1 [NH3], ν0=76 240(50) cm−1 [ND3], the F̃′ 1A1′ (5pa2″←1a2″) state: ν0=76 674(1) cm−1 [NH3], ν0=76 770(5) cm−1 [ND3], and the G̃′ 1A1′ (6pa2″←1a2″) state: ν0=78 494(1) cm−1 [NH3]. The present work serves to reinforce the previously noted dominance of np←1a2″ Rydberg excitations in the 2+1 REMPI spectrum of ammonia. In addition, the adiabatic ionization energy of ND3 is estimated to be 82 280±40 cm−1 based on the assumption that analogous Rydberg states of NH3 and ND3 will have very similar quantum defects.
The role of electron delocalization in the ionization of C6 hydrocarbons using intense 780 nm laser pulses of femtosecond duration
Authors: DeWitt Merrick J. , Levis Robert J.
Published: July 2002

The photoionization/dissociation mass spectra are reported for the series of molecules benzene (C6H6), 1,3,5-hexatriene (C6H8), cyclohexane (C6H12), and n-hexane (C6H14) as a function of laser power intensity from 1 to 3.8×1013 W cm−2 using a pulse duration of 170 fs and wavelength 780 nm. The ionization orders are localized around 8.3 for benzene, 1,3,5-hexatriene, and cyclohexane and the relative ionization probabilities are measured to be 1,79, and 0.15, respectively. No ion current was observed for n-hexane. The dissociation yield is observed to increase exponentially as a function of the number of atoms in the molecule with cyclohexane undergoing the most dissociation and benzene undergoing essentially no dissociation. These observations are interpreted in light of a field ionization model that incorporates both the ionization potential and the electronic and nuclear structure of the molecule.
Theoretical treatment of predissociation of the (4pσ) 1,3Πu rovibrational levels in the spectrum of the oxygen molecule
Authors: Li Yan , Hirsch Gerhard , Buenker Robert J.
Published: July 2002

Ab initio configuration interaction calculations are carried out for eight low-lying Πu1,3 and four Δu1 states of the O2 molecule. Three different types of couplings are considered: nonadiabatic, spin–orbit, and rotational, and the complex scaling method has been employed to determine rovibrational energies and wave functions. The calculations correctly predict a large number of trends in the measured level locations, predissociation linewidths, rotational constants, and line intensities for the (4pσ) 1,3Πu states. It is found that the v=1 level has the lowest frequency and strongest absorption intensity among the four observed vibrational levels of the (4pσ) 3Πu state, while the v=2 has the broadest predissociation linewidth and v=0, the smallest rotational constant, all in agreement with experimental findings. The linewidth of the v=0 level of the Πu1 state is computed to be 0.01 cm−1, consistent with the measured value of less than 0.1 cm−1. The calculations indicate that the v=0 level of the (4pσ) 1Πu state borrows intensity from the allowed (4pσ) 3Πu–X 3Σg− through spin–orbit interaction and that the v=4 level of the (4pσ) 3Π1u component, which has not yet been identified experimentally, is strongly perturbed by the v=3 level of the (4pσ) 1Πu state. The L-uncoupling-type interaction between Πu1 and Δu1 states is also studied, and the explanation for the missing lines of the v=0, J⩾2 rovibrational levels of the (4pσ) 1Πu states is sought based on the calculated results.
On absolute calibration with xenon of laser diagnostic methods based on two-photon absorption
Authors: Goehlich A. , Kawetzki T. , Döbele H. F.
Published: July 2002

A novel calibration method for the determination of absolute species densities by laser spectroscopy with two-photon absorption is presented. The method is based on a comparative measurement with a noble gas which has a two-photon resonance spectrally close to the transition investigated. Application of this scheme for the calibration of LIF measurements to determine atomic oxygen, generated in a capacitively coupled rf reactor, and for calibration of a RIS measurement of atomic oxygen and carbon generated by sputtering is demonstrated. The relevant excitation cross section of the involved 7p[3/2]2 xenon state, which is used for calibration of atomic oxygen densities, is determined.
Optimal control of multisurface molecular systems
Authors: Cheng Jixin , Shen Zhenwen , Yan YiJing
Published: July 2002

We report a theoretical framework for the study of the optimal control of multisurface molecular systems via a set of nondegenerate excitation fields. The resulting control equations in the strong response regime are presented in terms of both the Liouville-space density matrix dynamics and the Hilbert-space wave function evolution. We further derive a pair of eigenequations for the optimal pump-pump fields in the pure-state control of three-surface molecular systems in the weak response regime. The globally optimal pair of pump-pump fields in this case are identified. Application to the control of a rovibronic level on the final excited surface reveals a symmetry relation within the optimal pair of pump-pump fields in the weak response regime. For numerical demonstrations, we consider the control of the I2 molecular system involving the initial ground X, the intermediate B, and the final E surface. The target is chosen as an outgoing vibrational wave packet in the bound region of the final E electronic state. The optimal control fields in both the strong and weak response regimes are calculated and further parameterized to fit simple experimentally realizable laser pulses.
Calculation of the aqueous solvation free energy of the proton
Authors: Tawa G. J. , Topol I. A. , Burt S. K. , Caldwell R. A. , Rashin A. A.
Published: July 2002

The value of the proton hydration free energy, ΔGhyd(H+), has been quoted in the literature to be from −252.6 to −262.5 kcal/mol. In this article, we present a theoretical model for calculating the hydration free energy of ions in aqueous solvent and use this model to calculate the proton hydration free energy, ΔGhyd(H+), in an effort to resolve the uncertainty concerning its exact value. In the model we define ΔGhyd(H+) as the free energy change associated with the following process: ΔG[H+(gas)+H2nOn(aq)→H+(H2nOn)(aq)], where the solvent is represented by a neutral n-water cluster embedded in a dielectric continuum and the solvated proton is represented by a protonated n-water cluster also in the continuum. All solvated species are treated as quantum mechanical solutes coupled to a dielectric continuum using a self consistent reaction field cycle. We investigated the behavior of ΔGhyd(H+) as the number of explicit waters of hydration is increased from n=1 to n=6. As n increases from 1 to 3, the hydration free energy decreases dramatically. However, for n=4–6 the hydration free energy maintains a relatively constant value of −262.23 kcal/mol. These results indicate that the first hydration shell of the proton is composed of at least four water molecules. The constant value of the hydration free energy for n⩾4 strongly suggests that the proton hydration free energy is at the far lower end of the range of values that have been proposed in the literature.
Free energy from constrained molecular dynamics
Authors: Sprik Michiel , Ciccotti Giovanni
Published: July 2002

Activated processes can be studied in the molecular dynamics (MD) approach by imposing a mechanical constraint on the corresponding reaction coordinate and by performing a kind of thermodynamic integration. The blue-moon ensemble method provides us with the correct algorithm for computing the potential of mean force and the transmission coefficient. Here we show a procedure for obtaining the mean force directly from the average force of constraint and a geometric correction term which is easy to compute in MD simulations. Previous work on the same problem will be also discussed.
Diffraction and rotational transitions in the scattering of D2 from Cu(001) at energies up to 250 meV
Authors: Bertino Massimo F. , Graham Andrew P. , Rusin Lev Y. , Toennies J. Peter
Published: July 2002

Absolute diffraction probabilities for the scattering of D2 from a clean Cu(001) surface along the [100] azimuth have been measured at incident kinetic energies between 20 and 250 meV. The measured attenuation of the diffraction intensities with surface temperature corresponds to a surface Debye temperature of ΘD=341 K. The high-resolution angular distributions show clear evidence of rotationally inelastic diffraction (RID) peaks. The RID probability increases with incident energy and represents as much as 30% of the elastic diffraction probability at energies above Ei=200 meV. An Eikonal approximation analysis gives a value h=0.075 Å for the surface corrugation which is independent of incident energy. The rotational transition probabilities correspond to an effective value of δ=0.3 for the molecular eccentricity. The experimental results indicate that diffraction of D2 from Cu(001) can be accounted for by a hard-wall collision mechanism over the whole range of investigated energies.
Gel friction: A model based on surface repulsion and adsorption
Authors: Gong Jianping , Osada Yoshihito
Published: July 2002

A model describing the frictional force produced when a polymer gel is sliding on a solid surface has been proposed from the viewpoint of solvated polymer repulsion and adsorption theory at a solid surface. General relations for the frictional force f expressed as functions of the normal loading P, sliding velocity ν, the polymer volume fraction φ, or the elastic modulus E of the gel, etc., have been derived by applying scaling relations to the model. For the repulsive case, f is ascribed to the viscous flow of solvent at the interface and f is theoretically demonstrated to be proportional to the sliding velocity ν and the normal pressure P when the pressure is smaller than the elastic modulus of the gel. For the attractive case, in addition to the hydrodynamic friction, the force to detach the adsorbing chain from the substrate appears as friction. When ν is not very large, f∝ν. At an intermediate velocity, f has a velocity dependence less than linear, depending on the strength of adsorption. At a higher sliding velocity, f∝ν again since the hydrodynamic friction becomes predominant at this stage. The theoretical results coincide well with some experimental observations that confirm the essential feature of the model.
Ab initio molecular orbital and density functional characterization of the potential energy surface of the N2O+Br reaction
Authors: Lesar Antonija , Hodošček Milan
Published: July 2002

The lowest doublet (2A′) potential energy surface for the reaction N2O+Br→N2+OBr was investigated using ab initio and nonlocal density functional theory calculations. Geometries, energies and vibrational frequencies for stationary points were evaluated at HF/6-31G(d), MP2/6-31G(d) and B3LYP/6-31G(d) levels of theory. All levels of calculation give a similar geometry for the transition state, but the MP2 barrier is narrower. Intrinsic reaction coordinate (IRC) calculations starting from the transition state and proceeding toward the two local minima confirmed that IRC trajectories reached the reactant and product regions, respectively. Calculations of kinetic isotope effects were also performed. They are influenced by the theoretical level and the B3LYP method gives results in best agreement with experimental data. The HF method predicts the relative values of both the primary and secondary nitrogen kinetic isotope effects less accurately. At the MP2 level of calculations only the oxygen kinetic isotope effect is reproduced satisfactorily. Finally, the best value for the activation energy is again provided by the B3LYP method.
Gas-phase reactions of oxide and superoxide radical anions with C6F6
Authors: Knighton W. B. , Miller T. M. , Viggiano A. A. , Morris R. A. , Van Doren J. M.
Published: July 2002

The reactions of O− and O2− with C6F6 and C6F6− with O2 were studied using the selected ion flow tube (SIFT) technique. The reaction rate constants and branching fractions were measured. The reactions of O− and O2− with C6F6 occur with near unit efficiency while that of C6F6− with O2 is about 2% efficient. The reaction of O− produces three ionic products, F−, C5F4−, and C6F5O−. The reaction of O2− with C6F6 produces five ionic decomposition products, C4F4−, F−, C4F2−, C5F4O−, C5F5O−, and the electron transfer product, C6F6−. The reaction of C6F6− with O2 produces products similar to those observed for the reaction O2− with C6F6.
Segmental motion of entangled random coil polymers studied by pulsed gradient spin echo nuclear magnetic resonance
Authors: Komlosh M. E. , Callaghan P. T.
Published: July 2002

Pulsed gradient spin echo nuclear magnetic resonance (NMR) is used to investigate polymer mean-squared segmental displacements in semidilute solutions of high molar mass polystyrene in deuterio-toluene. Nine molar masses from 1 to 20 million daltons are studied at a fixed concentration of 5% w/v, and a range of concentrations from 5% to 20% at fixed molar mass of 3 million daltons. The distance and time scales accessed are 20 to 1000 nm and 10 to 3000 ms, respectively. Evidence for intrachain spin diffusion is found and its effect corrected for. The time dependence of mean-squared segmental displacements is fitted to the predictions of the Doi–Edwards model and tube disengagement times and tube diameters obtained.
Systematic study of electron localization in saltrich Mx(MX)1−x melts (M=Na, K, Cs; X=Cl, I) by optical absorption spectroscopy
Authors: von Blanckenhagen B. , Nattland D. , Bala Kiran , Freyland W.
Published: July 2002

We report optical absorption spectra of Kx(KI)1−x, Csx(CsI)1−x, and Csx(CsCl)1−x solutions at temperatures around 800 °C and in the saltrich concentration range up to metal mole fractions of xM⩽0.04 for energies 0.5⩽ℏω⩽5 eV. Employing a high-temperature electrochemical cell, we were able to change and determine the alkalimetal activity and the alkalimetal mole fraction in situ simultaneously with the optical absorption spectra. The high quality of the absorption spectra allows to distinguish spectral contributions to localized electronic states and mobile electrons. Together with previously measured spectra of Nax(NaI)1−x melts a systematic investigation of the alkali iodide melts reveals differences in the nature of the strongly localized electronic states with varying cation. Interpretation of the spectroscopic results with the aid of a chemical defect model shows that the formation of localized dimeric electron states (bipolarons) is pronounced in melts of smaller cations. The analysis of the optical contribution due to mobile electrons has been performed with a simple Drude model for nearly free electrons and the results are in good agreement with independent measurements of electronic transport properties.
First-passage-time-distribution approach to gated trapping problems
Authors: Sheu Wen-Shyan
Published: July 2002

Two-state gated trapping systems with a static trap are solved by a first-passage-time-distribution (FPTD) approach. The intrinsic microscopic rate for either state need not be zero, an important condition toward developing a theory for gated trapping problems of multiple or continuous states. A one-state trap with a slower intrinsic rate is shown to be physically equivalent to a partially open trap with a faster intrinsic rate. This result is applied to explicitly express the two-state gated FPTD in terms of their corresponding ungated FPTD. Consequently, the gated trapping rates of the systems are calculated and discussed. An approach to experimentally determine gating mechanisms is also proposed.
Divergent behavior of Mo/ller–Plesset perturbation theory for molecular electric dipole and quadrupole moments
Authors: Halkier Asger , Larsen Helena , Olsen Jeppe , Jo/rgensen Poul
Published: July 2002

The molecular electric dipole and quadrupole moments of HF have been calculated in Mo/ller–Plesset perturbation theory through 20th order. From comparison with full configuration interaction results, the Mo/ller–Plesset series is found to diverge for both properties. Within the Mo/ller–Plesset series, only the second-order correction should be calculated. For higher accuracy, one should use coupled-cluster methods instead of proceeding to higher-order perturbation theory.
Nuclear magnetic shieldings in solution: Gauge invariant atomic orbital calculation using the polarizable continuum model
Authors: Cammi Roberto , Mennucci Benedetta , Tomasi Jacopo
Published: July 2002

We present the gauge invariant atomic orbital (GIAO) calculation of nuclear magnetic shieldings for solvated molecules described within the polarizable continuum model (PCM). The performance of the PCM-GIAO approach is tested in a benchmark calculation of isotropic C13, N15, and O17 shielding constants for CH3CN and CH3NO2 in vacuo and in water, both at the Hartree–Fock and density functional levels of theory. Various aspects of the calculation of solvent effects on these properties, such as the dependence on the basis set, the electron correlation, and the size of the molecular cavity embedding the solute, are taken into account and discussed. An interpretation of the gas-to-solution shielding variations in terms of a combined action of the solvent reaction field and the shielding polarizabilities is also given.
Reversal in the dielectric relaxation time during polymerization: Thermal energy compensation on macromolecular growth
Authors: Ferrari C. , Tombari E. , Salvetti G. , Johari G. P.
Published: July 2002

The increase in the dielectric relaxation time τ0 and in the number of covalent bonds n formed during polymerization of an equimolar mixture of cyclohexylamine and a diepoxide have been studies in real time, in two experiments: (i) under isothermal conditions when the dielectric spectrum monotonically shifts to the low-frequency side and the configuration entropy decreases and (ii) on heating the polymerizing liquid at a fixed rate when the spectrum initially shifts to the low-frequency side, reverses direction at a certain temperature, and shifts towards the high-frequency side. This reversal in τ0 occurs when an increase in thermal energy begins to dominate the effects of increase in n. Consequences of the two competitive effects on the measured dielectric behavior have been discussed and relations between the change in configurational entropy, and τ0 of the ultimately formed linear chain polymer, are discussed. It is shown mathematically that for a fixed τ0 and a configurational entropy, there are an infinite number of combinations of temperature and n, some of which may be explored by altering the temperature–time profile during the macromolecular growth.
The B 3Σu−←b 1Σg+ transition of molecular oxygen
Authors: Lewis B. R. , Gibson S. T. , Slanger T. G. , Huestis D. L.
Published: July 2002

The origin of the intensity of the B 3Σu−←b 1Σg+ transition of molecular oxygen, first observed recently by Eppink et al. [J. Chem. Phys. 108, 1305 (1998)], is discussed. It is shown that the B←b transition borrows its intensity principally from the dipole-allowed B←X transition, through spin-orbit mixing between the X 3Σg− and b 1Σg+ states. Estimated continuum photoabsorption cross sections and discrete oscillator strengths for the B←b system are presented.
Temperature modulation effects on a material’s properties: Thermodynamics and dielectric relaxation during polymerization
Authors: Johari G. P. , Ferrari C. , Tombari E. , Salvetti G.
Published: July 2002

Because of the nonlinear change in the properties of a material with change in temperature, sinusoidal temperature modulation will alter the magnitude of a measured property and the rate of an irreversible transformation. These effects are mathematically described and elaborated. From real-time dielectric spectroscopy and calorimetric measurements performed on a model liquid undergoing stepwise network structure polymerization until its vitrification, it is shown that the effects of sinusoidal temperature modulation are substantial, and need to be considered in the analysis of the data. Features that appear in the plots against time vanish in plots against the extent of polymerization, an intrinsic property. Fourier transform of the heat flow in response to a sinusoidal temperature modulation may help determine the magnitude of this effect in the form of second or higher order harmonic terms. This in turn may be used to calculate the temperature and time dependence of the nonlinear effects. These fundamental aspects will be useful for the current efforts in incorporating sinusoidal temperature modulation in calorimetry.
State to state Ne–CO rotationally inelastic scattering
Authors: Antonova Stiliana , Lin Ao , Tsakotellis Antonis P. , McBane George C.
Published: July 2002

Measurements of state-to-state integral cross sections for rotational excitation of CO by collisions with Ne are reported. The measurements were performed in crossed molecular beams with resonance enhanced multiphoton detection at collision energies of 711 and 797 cm−1. The cross sections display strong interference structure, with a propensity for odd Δj below Δj=10. Predictions of the ab initio potential surface of Moszynski et al. [J. Phys. Chem. A 101, 4690 (1997)] and the new ab initio surface of McBane and Cybulski [J. Chem. Phys. 110, 11734 (1999), preceding paper] are compared to the data. The new surface agrees more closely with the observed interference structure, although significant disagreements remain.
Polymers simulated with an improved “rebound selection”
Authors: Alexandrowicz Z.
Published: July 2002

An improved “rebound selection” simulation constructs a polymer chain in biased excluded volume avoiding steps. A construction that in midcourse accumulates a low bias weight tends to be arrested by a selection. Conversely a high weight construction sprouts reserve bifurcation and if subsequently it is arrested, it falls back and rebounds from such bifurcations. The simulation is applied to a condensation transition of thermal 2d lattice chains, and to a dilute–semidilute transition of athermal chains. Semidilute 2d chains do not mix, 3d ones mix as blobs.
Electron trapping and hydrogen atoms in oxide glasses
Authors: Shkrob Ilya A. , Tadjikov Boris M. , Chemerisov Sergey D. , Trifunac Alexander D.
Published: July 2002

Trapped hydrogen atoms generated in 3 MeV β-radiolysis of B2O3:OH glass below 140 K were studied using electron paramagnetic resonance (EPR). Two types of trapped H atoms were present in this glass; one was an interstitial atom located in a void between several BOB fragments, another was the atom trapped in a cage between two B3O6 (boroxol) rings connected by hydrogen bonds. The geometry of the trapping site was determined using electron spin echo envelope modulation (ESEEM) spectroscopy. Time-resolved pulsed EPR was used to observe mobile H atoms at 300–500 K. The lifetimes (10–100 μs) of the H atoms were controlled by ∼1018 cm−3 of metastable spin centers. The H atoms migrated with diffusion constant of 1.5×107 cm2/s (activation energy of 0.13–0.16 eV), mean residence time at the site of 4–5 ns, and mean jump length of 0.56 nm (at 300 K). This site-to-site migration causes rapid spin relaxation due to modulation of magnetic interactions, such as dipole–dipole interaction of the unpaired electron of the H atom with B10 and B11 nuclei. Though there was no observed H/D kinetic isotope effect on the decay/diffusion of the hydrogen atoms, there was a significant isotope effect on their radiolytic yield (α≈1.5–1.6). This effect is comparable to the one observed in SiO2:OH and aqueous acid glasses. This similarity suggests that in the room-temperature “wet” SiO2 and B2O3 glasses, mobile H atoms are generated via electron trapping at the proton(s) associated with threefold coordinated oxygen (–OH2+ and/or >OH+ centers). Semiempirical MNDO simulations were used to estimate energetics of such electron trapping reactions.
Molecular simulation of the temperature- and density-dependence of ionic hydration in aqueous SrCl2 solutions using rigid and flexible water models
Authors: Driesner Thomas , Cummings Peter T.
Published: July 2002

Molecular dynamics simulations of aqueous SrCl2 solutions have been performed with two flexible water models [the Bopp–Jancsó–Heinzinger (BJH) and modified Toukan–Rahman simple point charge model (SPC-mTR)] as well as the rigid simple point charge (SPC) model. Recent extended x-ray absorption fine structure spectroscopy (EXAFS) studies of Sr2+ hydration reported a decrease of the average distance between Sr2+ and water molecules in the first hydration shell with increasing temperature. The available Sr2+–water potential for rigid SPC water and its variants is not able to reproduce this hydration shell contraction. Adding intramolecular flexibility in the form of the SPC-mTR potential only slightly improves the performance of the SPC model, while the BJH model performs significantly better. All models predict an expansion of the first hydration shell of the Cl− ion with increasing temperature. The degree of expansion is density and concentration dependent. Large shifts of the position of the first minimum in the gClO(r) make the comparison of Cl− coordination numbers at different temperatures and densities difficult. We demonstrate that although the coordination number as determined from nearest neighbor hydrogen atoms (as preferred by neutron diffraction experimentalists) appears to decrease with increasing temperature, it is in fact increasing when the coordination number is properly defined as the number of nearest neighbor water molecules. When identical definitions for the hydration shells are used, the results for Cl− are in good agreement with the available experimental data. Hence, care has to be taken when discussing trends in hydration “strength” with temperature and density.
Polymerization-induced propulsion of a two-dimensional vesicle
Authors: Baumgaertner A. , Sambeth R.
Published: July 2002

The stochastic motion of a two-dimensional self-avoiding ring (“vesicle”) can be rectified and propelled by an enclosed end-polymerizing chain. The rectification mechanism is based on polymerization processes which induce an asymmetric “recoil” relaxation of the positions of vesicle and chain. The rectification process has been studied using Monte Carlo methods and a Langevin approach.
Detection of “ended” NO recoil in the 355 nm NO2 photodissociation mechanism
Authors: Nestorov Vilen K. , Cline Joseph I.
Published: July 2002

Circularly polarized 1+1′ REMPI with ion imaging detection was used to probe μ–v–j angular correlations of the NO (X 2Π1/2,3/2, v=0) photoproduct from linearly polarized 355 nm photodissociation of NO2 in a molecular beam. Rotational angular momentum orientation was measured for the NO product, with the sense and magnitude of the orientation depending upon the direction of the NO recoil velocity with respect to the polarization vector of the photolysis light. The rotational orientation is shown to originate from the directionality of the torque responsible for NO rotational motion. The experimentally observed sense of rotation in the plane of the NO2 parent molecule is consistent with an effective recoil impulse force applied to the N end of the NO product. A preliminary measure of the bipolar moment associated with this orientation for the j=21.5 and j=36.5 rotational levels gives a value of approximately β02(21)=0.2.