bluuebunny/crossref_metadata_2025 huggingface dataset:
Monte Carlo simulation studies of diffusion coefficients and mobilities for Rb+–N2 with anisotropic model potential and comparison with experimental measurements
Authors: Li M. M. , Ong P. P.
Published: July 2002

Monte Carlo simulation calculations were made of the mobility, and the transverse and longitudinal diffusion coefficients of Rb+ swarms drifting in nitrogen gas using an anisotropic model potential, which is constructed by extending the Tang–Toennies model to the alkali ion–diatomic molecule system. The potential parameters have been obtained from the combining rule and some published data. Scattering data for the collision of Rb+ with N2 molecule, which are indispensable to the simulation procedure, are obtained by infinite order sudden approximation. Detailed comparison between the simulated results and recent measurements of transverse diffusion as well as previously published mobility and longitudinal diffusion data suggests that the present interaction potential may represent reasonably well the true interaction in the long and intermediate ranges. In addition, it also was found that previous experimental results of longitudinal diffusion and reduced mobility in the high E/N region were possibly too high due to systematic errors.
Modified configurational bias Monte Carlo method for simulation of polymer systems
Authors: Vendruscolo Michele
Published: July 2002

We present an extension of the configurational bias Monte Carlo method for the efficient simulation of off–lattice polymer systems. Elementary moves consist in regrowing internal segments of a polymer chain. We show that the method satisfies the detailed balance condition. We apply it to three well known simple models used in protein studies, namely homopolymers, random heteropolymers and random copolymers.
Effect of close contacts on the radius of gyration of a polymer
Authors: Zwanzig Robert
Published: July 2002

The mean squared radius of gyration S2 of a short Rouse model polymer composed of N (⩽50) monomers is determined as a function of the number M (⩽25) of imposed contacts. The contacts are modeled by extra harmonic potentials between the specified monomers. A Green’s function method is used to follow the effects of sequential addition of contacts. Averaging over random sequences of contacts leads to the estimate 〈S2〉M=〈S2〉0/(1+0.43M).
Adsorption on hydrophobic porous glass near the liquid/liquid coexistence curve of a binary liquid mixture
Authors: Grüll H. , Woermann D.
Published: July 2002

A differential refractometric method is used to study the temperature and composition dependence of the adsorptive properties of porous glass in 2-butoxyethanol (abbreviated C4E1)/water mixtures. The surface of the adsorbent carries hydrophobic -SiO-C8H17 as well as hydrophilic -SiOH groups. The experiments are carried out close to the liquid/liquid coexistence curve in the vicinity of the lower critical point of the system. Depending on the temperature and composition of the mixtures either C4E1 or water is preferentially adsorbed. There exists a line along which the net adsorption vanishes. For compositions x>xc (x, mole fraction of C4E1; xc, critical composition) the amount of substance of adsorbed water ñ ws(Tp) per unit gram of adsorbent at temperature Tp increases with increasing values of the temperature difference (Tp−Tc) in the range 0 K<(Tp−Tc)<4 K. Tp is the temperature of phase separation of the bulk mixture in the presence of the adsorbent. Beginning at temperatures (Tp−Tc)>4 K the adsorbed amount of substance ñ ws(Tp) decreases. At (Tp−Tc)>8 K it reaches a value which is smaller by a factor of about 10 compared with its maximum value. This phenomenon is reminiscent of a wetting transition.
Helium dimer potential from symmetry-adapted perturbation theory calculations using large Gaussian geminal and orbital basis sets
Authors: Korona Tatiana , Williams Hayes L. , Bukowski Robert , Jeziorski Bogumil , Szalewicz Krzysztof
Published: July 2002

The symmetry-adapted perturbation theory (SAPT) has been employed to calculate an accurate potential energy curve for the helium dimer. For major components of the interaction energy, saturated values have been obtained using extended Gaussian-type geminal bases. Some other, less significant components were computed using a large orbital basis and the standard set of SAPT codes. The remaining small fraction of the interaction energy has been obtained using a nonstandard SAPT program specific for two-electron monomers and the supermolecular full configuration interaction (FCI) calculations in a moderately large orbital basis. Accuracy of the interaction energy components has been carefully examined. The most accurate to date values of the electrostatic, exchange, induction, and dispersion energies are reported for distances from 3.0 to 7.0 bohr. After adding the retardation correction predicted by the Casimir theory, our new potential has been shown [A. R. Janzen and R. A. Aziz (submitted)] to recover the known bulk and scattering data for helium more accurately than other existing ab initio and empirical potentials. However, the calculated dissociation energy of 1.713 mK and the bond length of 45.8 Å differ somewhat from the values inferred recently from a transmission experiment using nanoscale sieves.
Electrostriction effects on electron transfer reactions in solution. I. Adiabatic regime
Authors: Jeon Jonggu , Kim Hyung J.
Published: July 2002

The outer-sphere unimolecular electron transfer reactions, DA⇌D+A−, in polar solvents are studied theoretically. By employing a recent theory of fluctuating cavities [H. J. Kim, J. Chem. Phys. 105, 6818, 6833 (1996)], we construct a nonequilibrium free energy formulation that accounts for both the solvation and electrostriction aspects of the changing solute dipole moment during the reaction. With a two-state diabatic description for the solute electronic structure, we obtain a two-dimensional free energy surface in terms of the cavity radius a and a solvent coordinate that gauges the nonequilibrium solvent orientational polarization Por. The equilibrium a values for the reactant and product states are found to differ significantly, indicating that the cavity reorganization plays an important role in the reaction free energetics and dynamics. The reaction mechanism and rate constant are analyzed via the solution-phase reaction path, which is found to vary markedly with the relative time scales of the Por and a motions. To be specific, the charge transfer proceeds in a concerted fashion with active participation of both Por and a when their characteristic times are comparable. By contrast, a stepwise mechanism results when there is a large mismatch between the Por and a time scales. In the slow cavity limit where a is much slower than Por, the reorganization through the cavity size adjustment precedes the barrier crossing that occurs exclusively along the Por coordinate. In the opposite fast cavity regime where Por is mainly responsible for activation and deactivation, it is found that the saddle point on the free energy surface is no longer rate-determining due to the nonlinear residual entropic effects. A brief comparison with other reaction systems involving charge shift in solution is also made.
Photoionization of C2F4/O2 complexes and C2F4 homoclusters
Authors: Cheng B.-M. , Preses Jack M. , Grover J. R.
Published: July 2002

Single-photon ionization of the clusters C2F4⋅O2, (C2F4)2O2, (C2F4)2, and (C2F4)3 is reported for energies up to 28 eV. The ionization potentials found are IP(C2F4⋅O2)=9.879±0.011 eV, IP([C2F4]2O2)=9.776±0.016 eV, IP([C2F4]2)=9.826±0.017 eV, and IP([C2F4]3)=9.838±0.022 eV. In addition, we measured IP(C2F4)=10.117±0.004 eV, in excellent agreement with prior work. The appearance potential of C2F4+ from C2F4⋅O2 was found to be AP=10.253±0.012 eV, which, when combined with appropriate values from above gives the cluster dissociation energies D0(C2F4⋅O2)=0.136±0.013 eV (3.14±0.29 kcal mol−1), and D0([C2F4⋅O2]+)=0.374±0.016 eV (8.62±0.38 kcal mol−1). The former value helps to understand the remarkable solubility of oxygen in fluorocarbons. No evidence was seen for photoionization-induced reactions of O2 with C2F4. A surprisingly intense Rydberg progression converging to the excited state of C2F4+ near 16.0 eV can be identified in the parent C2F4, and persists strongly in all four of the clusters studied. The lack of oxygen-containing reaction products and the persistence of the Rydberg state convey information about the structure of C2F4⋅O2.
Laser photofragmentation and collision-induced reactions of SiF22+ and SiF32+
Authors: Lee Yin-Yu , Leone Stephen R. , Champkin Philip , Kaltsoyannis Nikolas , Price Stephen D.
Published: July 2002

Laser photofragmentation processes of the doubly charged ions SiF22+ and SiF32+ are studied at 1064, 532, and 355 nm. The observed photoproducts are compared with those of the reaction pathways induced by the collisions of these dications with the rare gases. Photodissociation, via absorption of a single photon occurs readily for both dications, and the photoion yields are dominated by the products of neutral-loss pathways: SiF2+ from SiF22+ and SiF22+ from SiF32+. A minor contribution from photoinduced charge separation is also observed. For SiF22+, a neutral-loss pathway forming the atomic dication Si2+ is also detected at higher photon energies. The excitation energy required for this fragmentation process is determined to be 2.2±0.2 eV. Collisions of SiF22+ and SiF32+ with He and Ne are also dominated by neutral-loss reactions. With the heavier rare gases, charge transfer is exothermic and dominates the product ion yield, although neutral-loss reactions still occur. Interpretation of the charge transfer results using Landau–Zener theory suggests that two electronic states of SiF22+, lying at 31.0±0.5 eV and 35.5±0.5 eV above the ground state of the neutral molecule, are present in the dication beam. Ab initio calculations reveal that SiF32+ adopts an equilibrium geometry of C2v symmetry with one elongated bond. Hence, electron capture by SiF32+ results in the formation of SiF3+ in an energetically unfavorable conformation. By taking account of these geometry differences in the Landau–Zener algorithm, a satisfactory rationalization of the ion yields from the charge transfer reactions of SiF32+ is achieved.
Theoretical study of the potential energy surfaces and dynamics of CaNC/CaCN
Authors: Nanbu Shinkoh , Minamino Satoshi , Aoyagi Mutsumi
Published: July 2002

Potential energy surfaces for the ground and two low-lying electronically excited states of CaNC/CaCN, are calculated using the ab initio molecular orbital (MO) configuration interaction (CI) method. The absorption and emission spectra of the system are computed by performing time-dependent quantum dynamical calculations on these surfaces. The most stable geometries for the two lowest lying 12Σ+ and 12Π electronic states correspond to the calcium isocyanide (CaNC) structure. These two states are characterized by ionic bonding and the potential energy curves along the bending coordinate are relatively isotropic. The result of our wave packet dynamics shows that the characteristics of the experimental spectra observed by the laser-induced fluorescence spectroscopy can be explained by the Renner–Teller splitting.
Transverse diffusion measurements and comparisons with Monte Carlo simulation and Gram–Charlier calculations for Cs+ ions drifting in He gas
Authors: Li M. M. , Ong P. P. , Zhou Z. L.
Published: July 2002

The ratio of the transverse diffusion coefficient to the mobility, DT/K, has been experimentally determined for Cs+ ion drifting in He gas at E/N (electric field to neutral gas number density ratio) values ranging from 5 to 140 Td. Results at 303 K are reported with an overall accuracy of ±4%. The DT/K results were effectively corrected for longitudinal end effects existing in the drift tube. In addition, Monte Carlo simulation (MCS) calculations were made for the mobility and transport coefficients using the Koutselos, Mason, and Viehland (KMV) interaction potential as input. Comparisons of MCS results and available kinetic theory’s Gram–Charlier results with the present and previous experimental data give an indication of the validity and accuracy of the present potential. The theoretical results did not reproduce the experimental curve very well, suggesting that the proposed potential for the Cs+–He pair should be modified. Further comparisons of the transverse and longitudinal diffusion coefficients indicate that the Gram–Charlier approach of kinetic theory is quite accurate even for the large ion-neutral mass ratio.
Photoinduced charge transfer in conducting polymer C60 composites
Authors: Bruening Justin , Friedman Barry
Published: July 2002

We propose a simple model for photoinduced charge transfer in conducting polymer C60 composites. Our model consists of coupled Su–Schrieffer–Heeger models for a single polymer chain and a C60 molecule. We have included the coupling of the molecular vibrations to the π electrons but have neglected explicit electron–electron interaction. Using adiabatic and nonadiabatic molecular dynamics, we find photoinduced electron transfer takes place in 200–300 fs. This is consistent with an experimental upper bound of 300 fs. Nonadiabatic effects tend to average out pronounced oscillatory behavior in the adiabatic calculations.
On the ab initio evaluation of Hubbard parameters. II. The κ-(BEDT-TTF)2Cu[N(CN)2]Br crystal
Authors: Fortunelli Alessandro , Painelli Anna
Published: July 2002

A previously proposed approach for the ab initio evaluation of Hubbard parameters is applied to BEDT-TTF dimers. The dimers are positioned according to four geometries taken as the first neighbors from the experimental data on the κ-(BEDT-TTF)2Cu[N(CN)2]Br crystal. RHF-SCF, CAS-SCF and frozen-orbital calculations using the 6-31G** basis set are performed with different values of the total charge, allowing us to derive all the relevant parameters. It is found that the electronic structure of the BEDT-TTF planes is adequately described by the standard Extended Hubbard Model, with the off-diagonal electron-electron interaction terms (X and W) of negligible size. The derived parameters are in good agreement with available experimental data. Comparison with previous theoretical estimates shows that the t values compare well with those obtained from Extended Hückel Theory (whereas the minimal basis set estimates are completely unreliable). On the other hand, the Uaeff values exhibit an appreciable dependence on the chemical environment.
Ultraviolet laser desorption of indole
Authors: Elam Jeffrey W. , Levy Donald H.
Published: July 2002

Ultraviolet laser desorption from a thick, 120 K indole film was studied. Using a 2660 Å, 10 ns desorption laser at 75 mJ/cm2, 2.0 monolayers of indole are removed per shot. Indole0 is the only neutral species desorbed and it has an internal temperature Ti=210 K and a translational temperature Tt=3400 K. The velocity distribution is non-Boltzmann and the angular distribution is bimodal and forward peaked with major component proportional to cos7(θ). No evidence of “jetlike” structure in the desorbed plume is found: Different regions of the plume are at the same internal temperature and both internally hot and cold molecules have identical angle velocity distributions. While existing collisional models cannot account for the details of these distributions, they suggest that 2–7 collisions per molecule occur following desorption producing minor vibrational cooling (<10%). Laser desorbed indole+ is observed at a concentration of ∼10−5 that of indole0 with an angle velocity distribution similar to that of indole0. We show that indole+ results from resonant two photon ionization of indole0 by the desorption laser and that desorption laser heating of the plume occurs at 2660 Å.
Field-cycling nuclear magnetic resonance relaxometry of molecular dynamics at biological interfaces in eye lenses: The Lévy walk mechanism
Authors: Bodurka Jerzy , Seitter R.-O. , Kimmich R. , Gutsze A.
Published: July 2002

We have studied the water dynamics in whole rabbit lens and fragments using H1-field-cycling nuclear magnetic resonance relaxometry. We have measured the proton spin–lattice relaxation time T1 as a function of the Larmor frequency. The data can be interpreted well using the reorientation mediated by translational displacement model in combination with the mechanism of bulk-mediated surface diffusion, where individual water molecules perform Lévy walks with a Cauchy diffusion propagator. This gives evidence of anomalous water surface diffusion on proteins in the rabbit lens. We assume that the lens protein surface topology can be modeled by a polyhedral structure of randomly oriented faces with individual correlation lengths s0. For the whole lens we obtained the most frequent s0 value of 2.5 nm whereas for fragments from the rabbit cortex and the nucleus we obtained values of 3 and 0.3 nm, respectively. The correlation length values obtained for the lens can be attributed to the short-range order of the lens proteins necessary for maintaining lens transparency.
Point-ion versus density functional calculations of electric field gradients in ordered GaInP2
Authors: Wei Su-Huai , Zunger Alex
Published: July 2002

We investigate whether the electric field gradient (EFG) at an atomic site in the unit cell of a periodic solid can be modeled via the electrostatic field gradient set up by atomic point charges outside that site. To test this approach we contrast the EFG predicted by such point-ion models for long-range ordered GaInP2 alloys with the results obtained from self-consistent all-electron calculations in the local density approximation (LDA). We first tested our LDA approach for ZnAl2O4, for which experimental data exist, finding the quadrupole coupling constant Qcc(27Al)=3.94 MHz, compared with the measured value of |Q|=3.68 MHz. Applying next the LDA approach to perfectly ordered GaInP2 (for which experimental data do not exist), we find the LDA quadrupole coupling constant Qcc=−4.83, −2.84, and 13.08 MHz for Ga69, Ga71, and In115, respectively. We further find that more than 95% of these EFGs originate from the anisotropic electron charge distribution within a small sphere of radius ∼0.2 Å about the respective atomic site. Hence, the point-ion model significantly underestimates the magnitude of the EFG (and in some cases also gives an incorrect sign). The point-ion model also fails in reproducing the relative trends in the EFG as the crystal structure changes. We conclude that the point-ion model is not a viable alternative to calculate EFG in periodic covalent solids.
Equilibrium states of rigid bodies with multiple interaction sites: Application to protein helices
Authors: Erman B. , Bahar I. , Jernigan R. L.
Published: July 2002

Equilibrium configurations of rigid building blocks with multiple embedded interaction sites are investigated, as a coarse-grained approach for conformational sampling of protein structures with known secondary structure. First, hypothetical structures of asymmetric shapes, and pairs of rods composed of multiple interaction sites are considered. The rods are either disconnected or joined by a flexible loop. The sites are assumed to interact with a classical 6-12 Lennard-Jones potential. Subsequently, the investigation is extended to the study of two disconnected α helices composed of homogeneous interaction sites and to the ROP monomer, a small protein consisting of two heterogeneous α helices connected by a loop. Residue-specific long-range and short-range potentials extracted from a protein database are used. A Monte Carlo procedure combined with an energy minimization algorithm, originally developed by Li and Scheraga [Proc. Natl. Acad. Sci. USA 84, 6611 (1987)] is used to generate a set of low energy conformations over the full conformational space. Results show that: (i) The potential of mean force between two rods as a whole exhibits an inverse linear dependence on the separation between rods despite the individual sites interacting via a 6-12 Lennard-Jones potential. (ii) As the length of the rods (or helices) increases, they tend to align parallel to one other. (iii) This tendency to become parallel is enhanced when the density of interaction sites is higher. (iv) The angle between the principal axes of the rods is found to scale as n−5/3 with the number n of sites. (v) The native conformation of the ROP monomer, including the detailed rotational states of the virtual bonds located in the loop connecting the α helices is correctly predicted. This lends support to the adoption of such a coarse-grained model and its parameters for future simulations.
Fragmentation analysis of size selected sodium clusters
Authors: Hertel Ingolf V. , Schulz Claus Peter , Goerke Alexander , Palm Hartmut , Leipelt Gregor
Published: July 2002

The fragmentation processes of small sodium clusters induced by photoionization are investigated for a size selected neutral cluster beam. We use the collision induced momentum transfer in a crossed beam experiment to achieve the separation of the masses. Photoions produced with photon energies up to 4.66 eV are mass analyzed in a time-of-flight mass spectrometer. A considerable amount of fragmentation is observed leading exclusively to population of ionic channels with an even number of electrons. The measured fragmentation probabilities for cluster sizes up to n=8 are compared to a statistical unimolecular decay model. This gives a satisfactory agreement also for the photon energy dependence of the fragmentation probabilities if one assumes a significant contribution from cluster isomers.
Time and frequency resolved spectra of high molecular Rydberg states by dynamical computations
Authors: Remacle F. , Levine R. D.
Published: July 2002

The absorption spectrum of bound Rydberg states which can be detected by a delayed, pulsed field ionization is computed. The spectrum, measured for various delay times, provides information on the short and the longer time dynamics of high molecular Rydberg states. A quantitative dynamical theory, based on an effective Hamiltonian formalism is applied, illustrating the role of the Rydberg electron–core coupling and of an external electrical field in the delay-time dependent spectra. The sharpening of the spectra for longer delay times is reproduced by the dynamical computations. It is found that the overall intensity, as a function of the delay time before detection, is well described by a double exponential decay where the short lifetime is primarily a manifestation of the direct autoionization to the continuum, while the long lifetime is due to interseries coupling. Both lifetimes increase with the principal quantum number of the Rydberg states. The notion of trapped “reservoir states” is illustrated by the computational results, with special reference to a kinetic model analysis. The role of the initially optically accessed state(s) and of the depth of detection, in particular with regard to the intensity, is demonstrated. The effect of varying the strength of an external dc field in the time interval prior to the detection is illustrated by the dynamical computations, with respect to both the decay kinetics and the intensity of the spectrum.
Measurement of the Stark effect in the Ω=0+−X 1Σ+ transition of YbO
Authors: Steimle Timothy C. , Goodridge Damian M. , Linton Colan
Published: July 2002

The effect of applying an electric field on the MJ levels of the R(1) feature (υ=17 255.684 cm−1) in the AΩ=0+−X 1Σ+ band system of 174YbO has been examined using the technique of high resolution laser induced fluorescence spectroscopy. The results were used to obtain values for the permanent electric dipole moments, μ, in the X 1Σ+ and AΩ=0+ states of 5.89±0.02 and 5.93±0.04 D, respectively.
Comment on “A self-consistent field study of the wetting transition in binary polymer blends” [J. Chem. Phys. 106, 1257 (1997)]
Authors: Pereira Gerald G.
Published: July 2002

We consider the order of wetting transitions in polymer mixtures with particular reference to the recent self-consistent mean field calculation of Genzer and Composto [J. Genzer and R. J. Composto, J. Chem. Phys. 106, 1257 (1997)]. We discuss anomalies between these results and the simulation results of Pereira and Wang, and suggest a possible reason for the discrepancies.
The anharmonicity of motion in the liquid state and its consequences
Authors: Hlaváček B. , Křesálek V. , Souček J.
Published: July 2002

The Brownian motion in connection to anharmonic oscillations of particles is taken as a basic property associated with a liquid state. The Tg, Tm transitions are discussed in this context. It is shown that consideration of anharmonicity leads to the strong fluctuations in amplitudes and densities, as well as to the coexistence of double values of amplitudes and strong vibrational pulses randomly distributed in a liquid matrix. The analyses of Tg, Tm, or TB transitions by use of the anharmonic oscillator approach allow us to use similar mathernatical tools which are so successfully used in the Voltera–Prigogine type of thermodynamics. In spite of the fact that the anharmonicity effect has already been investigated before, the full impact of “S” shaped amplitude hystereses on the partition function is worked out in more detail in the present paper.
The near infrared spectrum of solid silane
Authors: Nucara A. , Calvani P. , Lupi S. , Roy P.
Published: July 2002

The infrared absorption spectrum of solid SiH4 has been measured between 4000 and 10500 cm−1 in both phases I and II. Eighteen new overtone and combination bands have been observed and assigned. Some of them correspond to sets of propagating vibrons, others to localized overtones with strong anharmonicities. The observations of the overtones (n−1)ν1+ν3 (n=2,3,4,5) are in excellent agreement with a local mode approach, usually applied to the vibrations of isolated molecules. Its use provides a determination of both the harmonic and anharmonic parameters of the SiH4 stretching in the solid phase.
Fluctuation of the pair-correlation function
Authors: Baranyai András , Tóth Gergely
Published: July 2002

Using a simple thermodynamic argument we derive an expression providing the r-dependent fluctuations of the pair-correlation function, g(r), of a liquid. The variance of g(r) at r is a function only of its own value at this point. Fluctuations calculated from Monte Carlo simulations fit well to this formula.
The microwave spectrum of the NHD radical in the ground electronic state, A′′2
Authors: Kobayashi Kaori , Ozeki Hiroyuki , Saito Shuji , Tonooka Mizuho , Yamamoto Satoshi
Published: July 2002

The microwave spectrum of the monodeuterated species of NH2, NHD(X̃ 2A′′), was observed in the millimeter- and submillimeter-wave region. The NHD radical was produced by a dc-glow discharge through a NH3 and D2 mixture at around 220 K. Ten rotational transitions with both b- and a-type selection rules were observed in the 177–517 GHz region. The most important rotational transitions for astronomical observations, 101–000 and 110–101, were included and their detailed fine and hyperfine components were measured with high precision. The hyperfine interaction of the nitrogen, hydrogen, and deuterium nuclei, which are similar in magnitude, caused complicated hyperfine patterns and made their assignment relatively difficult. A least-squares analysis of the measured frequencies resulted in the determination of 40 molecular constants including nuclear spin–rotation coupling constants of N, H, D and, furthermore, nonaxial components of magnetic dipolar terms of H and D. The necessity of a higher K-dependent term of the dipole–dipole interaction constant for the hydrogen nucleus relative to the principal axis system reflects the effective geometrical change of the rotational levels.
Diffusion-driven front instabilities in the chlorite–tetrathionate reaction
Authors: Horváth Dezső , Tóth Ágota
Published: July 2002

An extensive study on the instabilities of planar fronts leading to the formation of cellular structures has been carried out in the acid-catalyzed chlorite–tetrathionate reaction. A simple two-variable model based on the empirical rate law of the reaction is developed to describe the observed pattern formation. The calculated onset of instability and the size of the patterns in the cellular fronts are in good agreement with experimental observations.
The fast cooling/heating rate effects in devitrification of glasses. I. Number of nucleated particles
Authors: Shneidman V. A. , Uhlmann D. R.
Published: July 2002

We consider nucleation of crystalline phase in a glass-forming melt which is quenched at some arbitrary rate, S, and then reheated at some other (typically smaller) rate, H. In conventional (steady-state) approaches it is assumed that the nucleation rate is a function of temperature only, so that the number of nucleated crystallites is proportional to 1/S+1/H. We demonstrate, however, that in general the nucleation rate depends on the quench/heating rate and that there exists an S- and H-dependent temperature region which effectively does not contribute to nucleation, so that the aforementioned scaling does not hold. An expression for the non-steady-state nucleation rate is derived analytically. In certain cases, the number of nucleated crystallites can be reduced by orders of magnitude compared to the steady-state predictions. The results are tested against numerically exact data obtained from the Turnbull–Fisher nucleation model.
Vibrational frequencies of the 2p 2A2″ and 3d 2E″ states of the triatomic deuterium molecule
Authors: Müller U. , Braun M. , Reichle R. , Salzgeber R. F.
Published: July 2002

We investigated the vibrational energies in the 2p 2A2″ and 3d 2E″ states of the triatomic deuterium molecule D3. The experiments were performed using a fast neutral beam photoionization spectrometer recently developed at Freiburg. A depletion type optical double-resonance scheme using two pulsed dye lasers was applied. The measured vibrational frequencies of the 2p 2A2″ state of D3 are compared to those of H3 and to theoretical values calculated from an ab initio potential energy surface. The data give insight into the importance of the coupling between the valence electron and the ion core.
Isotope exchange reaction between 235UF5 nanoparticles and 238UF6 gas
Authors: Kuga Yoshikazu , Takeuchi Kazuo
Published: July 2002

The equilibrium in the isotope exchange reaction between 235U-enriched UF5 nanoparticles and natural UF6 gas was experimentally investigated. From the relationship between the isotopic fraction of UF6 gas at equilibrium and the specific surface area of the UF5 nanoparticles, it was experimentally demonstrated for the first time that all UF5 molecules on the outermost layer of the particles participated in the reaction until the isotopic fraction of UF5 on the outermost layer became equal to that of the UF6 gas. A fast rate process during the early period of the reaction and a subsequent slow rate process were quantitatively observed. These facts contradict the previously reported assumption not supported by experimental evidence that the UF5 molecules in the underlying layer of the particle participate in the reaction. A new model based on our experimental findings was proposed. Our model suggests that there are two kinds of molecules on the outermost layer with different reactivities and all UF5 molecules on the outermost layer contribute to the reaction. The rate equations were derived from the proposed model and analytically solved. The time dependency of the isotopic fraction of the UF6 gas and UF5 particles was expressed by three kinetic parameters such as the rate constant of the high reactivity molecule, ks, the rate constant of the low reactivity molecule, ks′, and the fraction of high reactivity molecules on the outermost layer, ξ. Our model reproduced well all the experimental data determined in the present study with ks=5.5×10−20 cm3/(s molecules), ks′=4.5×10−22 cm3/(s molecules), and ξ=0.078.
The barrier height for decomposition of HN2
Authors: Gu Jiande , Xie Yaoming , Schaefer Henry F.
Published: July 2002

The barrier height and exothermicity for the HN2→H+N2 reaction are predicted by high level ab initio quantum mechanical methods. The classical barrier is predicted to be 10.0±1.0 kcal/mol, and the reaction exothermicity is predicted to be 3.8±0.5 kcal/mol. The importance of these parameters to the thermal De–NOx process is discussed. The apparent conflict between the theoretical potential energy surface for HN2 and recent kinetic modeling studies is seen to persist.
Collision induced absorption in mercury–rare-gas collisions
Authors: Kurosawa T. , Ohmori K. , Chiba H. , Okunishi M. , Ueda K. , Sato Y. , Devdariani A. Z. , Nikitin E. E.
Published: July 2002

The far-wing excitation and probe technique is applied to observe quasimolecular absorption bands on the blue side of the Hg 6 1S0–6 3P2 atomic line for the Hg–Ar, Hg–Kr, and Hg–Xe mixtures. It is found that the excitation of this band is followed predominantly by a rapid elastic half-collision scattering on the excited state potential yielding the nascent product state Hg(3P2). This gives direct evidence of the assignment of the absorption to the c 31→X 10 collision induced dipole transition of the Hg–rare-gas quasimolecules. A chance of nonadiabatic transition from the c state is negligibly small compared to the elastic scattering. Analytical procedures are presented to deduce the c–X transition dipole moment from the relevant potential energy curves by making use of the Hund’s coupling schemes of the molecular electronic states. The c–X transition dipole moment is estimated as a function of the internuclear distance and is incorporated into the analysis of the observed band profiles. The potential energy curves of the c, A 30+, and B 31 states are also estimated from the analysis for Hg–Ar and Hg–Kr. The Hg–Xe system shows a small undulation in the c→X band suggesting that it involves a satellite structure due to an extremum of the c–X difference potential.
A polymer threading a membrane: Model system for a molecular pump
Authors: Carl Wilfried
Published: July 2002

A one-dimensional model of a polymer threading a membrane has been examined with Monte Carlo methods. If the membrane is separating two regions with different interactions with the polymer segments, the chain is pulled to the energetically favored region. However, in the presence of temporary adducts that alter the effective interaction, the chain is able to oscillate between the two regions. This may be viewed as a simple model for transport through membranes and constitutes a concentration-gradient driven polymeric pump.
Experimental and theoretical study of ring substituent induced effects on the structural and optical properties of poly(p-pyridylvinylene- phenylenevinelyne)s
Authors: Fahlman M. , Gebler D. D. , Piskun N. , Swager T. M. , Epstein A. J.
Published: July 2002

Optical absorption and photoluminescence spectroscopy have been carried out on a new class of (phenylene) ring substituted p-pyridylvinylenephenylenevinelyne polymers used as active materials in light emitting diodes. The effects of the ring substitutions on the optical absorption and photoluminescence energies are qualitatively explained through the use of semiempirical quantum chemical modeling of the ring torsion angles. Reduced aggregation through the use of “strap” substituents on the phenylene rings also is discussed.
Multicomponent nucleation and droplet growth in natural gas
Authors: Luijten C. C. M. , van Hooy R. G. P. , Janssen J. W. F. , van Dongen M. E. H.
Published: July 2002

The first quantitative experimental results are presented on homogeneous nucleation and droplet growth in a multicomponent gas-vapor mixture. Using the pulse-expansion wave tube technique, we investigated the condensation behavior of natural gas consisting of over 30 components. Data were obtained in the pressure range between 6 and 24 bar and at temperatures ranging from 221 to 237 K. The observed droplet growth rates are quantitatively explained using a multicomponent model for diffusion controlled growth. The nucleation rate data are for the moment mainly presented as a challenge to theoreticians, although some qualitative arguments are presented that could be helpful in the interpretation. The data appear to agree at least qualitatively with theoretical values (according to the revised binary classical nucleation theory) for a mixture of n-octane and methane, a model mixture which also shows the same macroscopic phase behavior as natural gas.
Transition energies of lanthanum, actinium, and eka-actinium (element 121)
Authors: Eliav Ephraim , Shmulyian Sergei , Kaldor Uzi , Ishikawa Yasuyuki
Published: July 2002

The relativistic coupled cluster method is applied to the transition energies (ionization potentials, excitation energies, electron affinities) of lanthanum, actinium, and eka-actinium (element 121) in several ionization states. Good agreement with available experimental data is obtained. Large relativistic effects are observed for all three atoms, with transition energies changing by up to 2.5 eV even for the relatively light La. The first ionization potential of La is reduced from 7.135 to 5.582 eV (the experimental value is 5.577 eV). In Ac, the 7s shell is occupied before 6d, yielding 7s and 7s2 ground states for Ac2+ and Ac+, respectively, instead of the 5d and 5d2 states for the La ions. The ground state of E121 is 8s28p, to be compared with the ns2(n−1)d configurations of the lighter group-3 elements. The Breit effect on transition energies is significant (0.06–0.1 eV) for states including f orbitals and small (below 0.02 eV) for others. The contribution of virtual i orbitals (l=6) shows similar behavior. The electron affinities are predicted at 0.33 eV for La, 0.35 eV for Ac, and 0.57 eV for E121.
Submillimeter-wave spectrum of the AsH2 radical in the B12 ground electronic state
Authors: Fujiwara Hideo , Kobayashi Kaori , Ozeki Hiroyuki , Saito Shuji
Published: July 2002

The pure rotational spectrum of the AsH2 radical in its B12 ground electronic state was observed for the first time by microwave spectroscopy. The AsH2 radical was generated in a free-space cell by dc-glow discharge of a mixture of H2 and O2 gases over arsenic powder. Fifty-five fine and hyperfine components of six rotational transitions were measured in the frequency region of 304–374 GHz, and were analyzed by least-squares methods. Molecular constants, including the rotational constants, the centrifugal distortion constants, the spin-rotation coupling constant incorporating the centrifugal distortion term, and the hyperfine coupling constants associated with the arsenic and hydrogen nuclei, were precisely determined. The bonding in AsH2 was discussed on the basis of the hyperfine coupling constants, first determined in the present study.
High resolution Raman study of phonon and vibron bandwidths in isotopically pure and natural benzene crystal
Authors: Pinan J. P. , Ouillon R. , Ranson P. , Becucci M. , Califano S.
Published: July 2002

The frequency and the bandwidth of the 12 Raman active lattice modes and of more than 45 internal vibrons of natural and C12 isotopically pure benzene have been measured under high resolution with the aid of a tandem interferometer–spectrometer system or alternatively of a two meter focal length grating Sopra spectrometer. For five lattice phonons and for several vibrons the measurements were extended to a sufficient temperature range to study the relaxation processes in terms of anharmonic phonon–phonon and phonon–vibron coupling processes. The effect of the isotopic impurities on the phonon lifetime has been shown to be very important for some vibrons, in particular, for those resonant with the impurity level. The bandwidth evolution with temperature has been interpreted in terms of down- and up- three- and four-phonon as well as of dephasing processes.
Time-dependent and time-independent approaches to study effects of degenerate electronic states
Authors: Baer Michael , Yahalom Asher , Englman Robert
Published: July 2002

Two types of phases are discussed in this article: (1) The topological phase as introduced by Berry [Proc. R. Soc. London, Ser. A 392, 45(1984)] and Aharonov and Anandan [Phys. Rev. Lett. 58, 1593 (1987)] and (2) the Longuet–Higgins phase [Proc. R. Soc. London, Ser. A 344, 147 (1975)]. The two types of phases have a common origin, namely the multivaluedness of the electronic adiabatic basis, a phenomenon associated with the existence of a degeneracy in configuration space. It will be shown, by studying an electronic model Hamiltonian that arises from a two-state approximation to the Mathieu equation, that the two phases differ from each other substantially, coinciding only in the adiabatic limit upon completion of a cycle.
Time correlation functions in low-dimensional conservative chaotic systems: A memory function approach
Authors: Munakata Toyonori , Hussien Hamdan A. Z. , Nakamura Yasuyuki
Published: July 2002

Time correlation functions are studied for some conservative chaotic systems with both discrete and continuous time dynamics. For low-dimensional systems a memory function formalism on a microcanonical ensemble turns out to be able to yield useful information on the time correlation function and its energy dependence.
Solid-liquid equilibrium of dipolar heteronuclear hard dumbbells in a generalized van der Waals theory: Application to methyl chloride
Authors: Gay Shawn C. , Beale Paul D. , Rainwater James C.
Published: July 2002

The solid-liquid equilibrium of hard dumbbells with embedded point dipoles is calculated using a generalized van der Waals theory to account for long range attractive forces. Molecular parameters are chosen to model a methyl chloride molecule. The solid free energy is calculated using the cell theory of Lennard-Jones and Devonshire with the dipolar contribution estimated by static lattice sums. Thermodynamic perturbation theory is used to add dipolar effects to a hard dumbbell fluid equation of state. The resulting phase equilibria show that the dipole does have a significant effect in determining the stable solid structure on freezing. In particular, the dipole moment stabilizes a non-close-packed orthorhombic structure, similar to the known solid structure of methyl chloride. An increase in the ratio of triple point temperature to critical point temperature is also observed as the dipole moment is increased, as is a decrease in the density change on freezing. At high pressure and temperature a solid-solid-liquid triple point is found, above which the system freezes into the close-packed structure.
Adhesion promotion at a homopolymer–solid interface using random heteropolymers
Authors: Simmons Edward Read , Chakraborty Arup K.
Published: July 2002

We investigate the potential uses for random heteropolymers (RHPs) as adhesion promoters between a homopolymer melt and a solid surface. We consider homopolymers of monomer (segment) type A which are naturally repelled from a solid surface. To this system we add RHPs with both A and B (attractive to the surface) type monomers to promote adhesion between the two incompatible substrates. We employ Monte Carlo simulations to investigate the effects of variations in the sequence statistics of the RHPs, amount of promoter added, and strength of the segment–segment and segment–surface interaction parameters. Clearly, the parameter space in such a system is quite large, but we are able to describe, in a qualitative manner, the optimal parameters for adhesion promotion. The optimal set of parameters yield interfacial conformational statistics for the RHPs which have a relatively high adsorbed fraction and also long loops extending away from the surface that promote entanglements with the bulk homopolymer melt. In addition, we present qualitative evidence that the concentration of RHP segments per surface site plays an important role in determining the mechanism of failure (cohesive versus adhesive) at such an interface. Our results also provide the necessary input for future simulations in which the system may be strained to the limit of fracture.
Perturbed hyperfine doubling in the A 2Π1/2 and [18.6]0.5 states of YbF
Authors: Sauer B. E. , Cahn S. B. , Kozlov M. G. , Redgrave G. D. , Hinds E. A.
Published: July 2002

We report a spectroscopic hyperfine structure study of the YbF174 molecule in the excited states A 2Π1/2(v=0), A 2Π1/2(v=1), and [18.6]0.5(v=0). The fluorine hyperfine interaction constant A∥ in each state is, respectively, −1.6±2 MHz, 48.8±3 MHz, and 63.3±3 MHz. The corresponding values of A⊥ are 4.6±1 MHz, −24.7±1 MHz, and −31.9±1 MHz. The very small hyperfine interaction characteristic of A 2Π1/2(v=0) is not seen in A 2Π1/2(v=1) because this state is strongly mixed with [18.6]0.5(v=0). Analysis of our results shows that the lower-lying of the two mixed states is predominantly of [18.6]0.5(v=0) character, not A 2Π1/2(v=1) as had previously been thought. The deperturbed hyperfine constants for [18.6]0.5 are A∥=114±5 MHz and A⊥=−61±2 MHz, comparable with those in the X state and much stronger than in the unperturbed A 2Π1/2 state. This supports the view that the state [18.6]0.5 is of 4f136s2 character.
Homogeneous nucleation of argon in an unsteady hypersonic flow field
Authors: Zahoransky R. A. , Höschele J. , Steinwandel J.
Published: July 2002

The vapor nucleation of argon was investigated in shock tube expansions starting from room temperature. The cooling from the superheated initial state to nucleation onset was attained in unsteady flow fields at high Mach numbers M=5. Thermodynamic states for homogeneous nucleation were observed between 30 and 180 Torr at corresponding temperatures in the range of 50–70 K. The high starting temperatures do not affect the nucleation onset states compared to previous experiments on the subject, where the expansions started from much lower temperatures. The slight differences in the onset states to previous experiments employing cryogenic shock and Ludwieg tubes may be attributed to the higher cooling rates and to nonisentropic expansion effects.
X-ray and neutron scattering studies of liquid formic acid DCOOD at various temperatures and under pressure
Authors: Nasr Salah , Bellissent-Funel Marie-Claire , Cortès Robert
Published: July 2002

A structural investigation of fully deuterated liquid formic acid was performed by neutron scattering at pressure up to 3 kbar. The molecular pair correlation function was also deduced from x-ray study of DCOOD at ambient pressure and at 294 K. The results could be explained in terms of an open-chain structure with only two H bonds per molecule. The mean O⋯O distance is about 2.72 Å. The effect of both temperature and pressure on the hydrogen bond network is examined.
Electric double layer interaction of ionizable surfaces: Charge regulation for arbitrary potentials
Authors: Behrens Sven Holger , Borkovec Michal
Published: July 2002

Electrostatic interactions between charged surfaces across an electrolyte solution are commonly described by boundary conditions of constant charge or constant surface potential. These two extremes are in general not appropriate for an equilibrium description of materials with ionizable surface groups, but do provide an upper and lower bound for the interaction energy. We propose a quantitative criterion which permits to evaluate the degree of charge regulation for surfaces with arbitrary electrostatic potential, as considered in the Poisson–Boltzmann theory. Our approach represents a generalization of the linearized regulation model proposed earlier for the framework of Debye–Hückel theory only. Like in the case of low surface potentials, the regulation behavior is generally determined by the competition between the capacities of the diffuse and the compact part of the electric double layer. Our results suggest a new way of using the limiting conditions of constant charge and constant potential to approximate the interaction free energy of charge regulating surfaces.