bluuebunny/crossref_metadata_2025 huggingface dataset:
Weak-stripe magnetic domain evolution with an in-plane field in epitaxial FePd thin films: Model versus experimental results
Authors: Marty A. , Samson Y. , Gilles B. , Belakhovsky M. , Dudzik E. , Dürr H. , Dhesi S. S. , van der Laan G. , Goedkoop J. B.
Published: July 2002

The magnetic domain structure in thin ferromagnetic films with perpendicular anisotropy is investigated. The effect of an in-plane magnetic field on the presence of domains and the magnetization profile is analyzed within the framework of a one-dimensional analytical model. The general results are reported in a state diagram as a function of the two relevant dimensionless parameters: the quality factor and the ratio of the film thickness to the exchange length. The experimental results, including hysteresis measurements, magnetic force microscopy, and x-ray resonant magnetic scattering obtained on a generic FePd thin layer, are reported and compared to the model.
Dynamics of nonadiabatic reactions (theory). II. Product vibrational excitation
Authors: Mayne H. R. , Polanyi J. C.
Published: July 2002

The same model potentials as were used in part I (preceding paper) have been employed in a study of the effect of nonadiabatic exchange reaction on product vibrational energy distributions. The exit-valley seam at which hopping from a lower (L) to an upper (U) potential energy surface (pes) occurred could be altered from early (E) to late (L), and from inner (I) to outer (O). The splitting 2ε, between the pes could be made small or large. The computations involved trajectory surface hopping for the mass-combination light plus heavy light (L+HL), in three dimensions (3D). The effects on P(v′) of alterations in reagent energy, and changes in the pes, could be understood in terms of the local motion, part way through the reactive encounter in the configurations at which the seam was encountered. The seam was effective as a ‘‘filter’’ favoring reaction into low v′U′, , following the hop to the upper pes, particularly if ε was large, since the seam allowed trajectories with higher speed along the exit valley to pass onto the U surface and these correlated with low vU. Surface hopping strongly favored low v′U if the seam location was of the ‘‘inner’’ type (I), and favored high vU if the seam was an ‘‘outer’’ one (O), since the reaction intermediate leading to the upper surface was a compressed one in the former case, and an extended one in the latter.
Bounds to two- and three-body long-range interaction coefficients for S-state atoms
Authors: Standard J. M. , Certain P. R.
Published: July 2002

New upper and lower bounds to the van der Waals C6, C8, and C10 coefficients for hydrogen, noble gas, alkali, and alkaline earth atoms are determined by using Padé approximants to bound the dynamic multipole polarizabilities. Also, the nonadditive, three-body coefficients involving dipole, quadrupole, and octupole interactions are bounded.
Lifetimes of single rotational states in the ‘‘channel three’’ region of C6H6
Authors: Schubert U. , Riedle E. , Neusser H. J. , Schlag E. W.
Published: July 2002

Lifetimes of single rotational states in the 14112 vibrational state of benzene, C6H6, at 3412 cm−1 excess energy were measured. The lifetimes depend strongly on the rotational quantum number J and we find them to agree with our recent homogeneous linewidth measurements of the same states. We present a coupling scheme including the coupling of the excited narrow light zero order state with a broadened dark background state. The broadening is due to a fast electronic nonradiative relaxation process typical for a state containing quanta of out-of-plane modes. It is demonstrated that the observed J dependence is due to perpendicular Coriolis coupling in the weak coupling limit. The IVR process resulting from this coupling explains the onset of ‘‘channel three’’ in a natural way without any further mystery states in the model.
VUV-optical oscillator strength distributions of C2H6O and C3H8O isomers
Authors: Koizumi Hitoshi , Hironaka Katsuyuki , Shinsaka Kyoji , Arai Shin , Nakazawa Hidenobu , Kimura Atsushi , Hatano Yoshihiko , Ito Yoshiro , Zhang Yunwu , Yagishita Akira , Ito Kenji , Tanaka Kenichiro
Published: July 2002

The photoabsorption cross sections of C2H6O isomers (ethyl alcohol and dimethyl ether) and C3H8O isomers (n-propyl alcohol, isopropyl alcohol, and ethyl methyl ether) have been measured in the wavelength region from 30 nm to the ionization potential of each molecule (about 120–130 nm) using an ionization chamber. The obtained photoabsorption cross sections show a maximum value at about 70–80 nm for each molecule. In the wavelength region shorter than about 50 nm, the cross sections are almost the same among isomers and equal to the sum of the cross sections for the constituent atoms. In the longer wavelength region, however, the cross sections show different peaks and shoulders depending on its molecular structure. Especially in the cases of dimethyl ether and ethyl methyl ether vibrational structures of the cross section are observed in the wavelength region from 70 to 100 nm.
Dynamics of the activated dissociative chemisorption of CH4 and implication for the pressure gap in catalysis: A molecular beam–high resolution electron energy loss study
Authors: Lee M. B. , Yang Q. Y. , Ceyer S. T.
Published: July 2002

The dynamics of the activated dissociative chemisorption of CH4 on Ni(111) are studied by molecular beam techniques coupled with high-resolution electron energy loss spectroscopy. The probability of the dissociative chemisorption of CH4 increases exponentially with the normal component of the incident molecule’s translational energy and with vibrational excitation. The dissociative chemisorption probability of CD4 exhibits the same trends with a large kinetic isotope effect. High-resolution electron energy loss spectroscopy identifies the nascent products of the dissociative chemisorption event as an adsorbed methyl radical and a hydrogen atom. These results, which have shown that there is a barrier to the dissociative chemisorption, are interpreted in terms of a deformation model for the role of translational and vibrational energy in promoting dissociative chemisorption. The barrier likely arises largely from the energy required to deform the molecule sufficiently to allow a strong attractive interaction between the carbon and the Ni surface atoms. Tunneling is suggested as the final process in the C–H bond cleavage. The presence of this barrier to dissociative chemisorption presents a plausible explanation for the pressure gap in heterogeneous catalysis.
On the growth of two dimensional crystals. I. Fluctuations and the relation of step free energies to morphology
Authors: Sadler David M.
Published: July 2002

Some fundamental aspects of crystallization of two dimensional (2D) crystals are examined, including an analysis of the fluctuations in step positions, computer simulation results for the step density and growth rates, and analysis for predicting crystal shapes. A discussion is given of whether nucleation can occur on the 1D surface of a 2D crystal. During the course of the paper it is shown that a more appropriate description, for any infinite 1D surface, is of a step density related to the equilibrium value and a growth rate which is proportional to the density and speed of these steps. The speed depends essentially linearly on the net thermodynamic driving force for crystallization. (This description is not appropriate for a 2D surface since the equilibrium step density can then be zero.) Nevertheless, one can define a criterion, depending both on the density of steps and on the extent of the fluctuations in step positions, for a regime of growth with some similarities to nucleation (1D nucleation). Kinetic (nucleation) theories for polymer crystallization, using a rate equation approach, have implicitly assumed the conditions for 1D nucleation to hold. Even when the conditions do not hold, the results of this rate equation approach are in reasonable agreement with the simulation results (though a rather curious discrepancy of a factor of 21/2 occurs since the rate equation approach omits a growth mediated approach to equilibrium). The agreement in general trend is shown to be the fortuitous consequence, in the rate equation approach, of neglecting both the enhancement of step annihilations caused by fluctuations and the creation of pairs of steps at cavities. If the latter effect is neglected but not the former, anomalous results can be obtained which violate thermodynamic principles. Growth rate results from simulation are obtained as a function of the obliquity of the growth face with respect to the principal directions in the lattice. These growth rate anisotropy data are used to predict the shapes of crystal outlines. Comparisons between these results and

experimental observations on shapes of polymer crystals are made, and a preliminary account is given of how the step free energy per length of step decreases with increasing temperature. In general, the total step free energies tend to decrease as the crystal thickness increases for polyethylene.
Self-diffusion in fluids in microporous solids
Authors: Vanderlick T. K. , Davis H. T.
Published: July 2002

Enskog’s kinetic theory of tracer diffusion has been generalized recently to strongly inhomogeneous fluids and is here applied to self-diffusion in fluids in slit pores, i.e., fluids confined between parallel, flat, solid walls. In pores narrower than ten molecular diameters, the diffusivity deviates significantly from its value in bulk phase at the same temperature and chemical potential. Because of the confining pore walls, the fluid tends to form layers. The pore diffusivity oscillates as a function of pore width, a local minimum occurring when the packing of a given number of fluid layers is favored and a local maximum in the region of transition between these favored packing widths. The theory predicts fluid density distributions in good agreement with computer simulations of a similar fluid. The predicted diffusivities are in good qualitative agreement with computer simulations, although quantitatively the predicted oscillations are sharper than those observed in the computer simulations. This work represents the first application of Enskog’s kinetic theory of diffusion in strongly inhomogeneous fluids.
Tracer diffusion in concentrated colloidal dispersions. II. Non-Gaussian effects
Authors: van Megen W. , Underwood S. M.
Published: July 2002

The incoherent dynamic structure factor for concentrated dispersions of interacting, monodisperse, hard, spherical, colloidal particles has been measured by dynamic light scattering and displays weak but systematic departures from Gaussian behavior. At particle concentrations just below the crystallization transition, these non-Gaussian effects are comparable with those measured and calculated for simple atomic liquids.
Spectroscopic identification of surface phenyl thiolate and benzyne on Mo(110)
Authors: Roberts Jeffrey T. , Friend C. M.
Published: July 2002

Chemisorbed phenyl thiolate (C6H5S) and surface benzyne (C6H4) formed during the temperature programmed reaction of benzenethiol on Mo(110) have been characterized using x-ray photoelectron and high resolution electron energy loss spectroscopies. Electron energy loss spectroscopy demonstrates that at all exposures the S–H bond of benzenethiol breaks upon adsorption at 120 K, while x-ray photoelectron spectroscopy confirms that the C–S bond is intact. The intermediate formed upon adsorption is assigned as a chemisorbed phenyl thiolate. At high exposures, approximately 40% of the phenyl thiolate reacts by way of hydrogenolysis at 350 K to form gaseous benzene and atomic sulfur, while 60% undergoes dehydrogenation at ≊370 K to form surface benzyne and atomic sulfur. Electron energy loss spectroscopy reveals that surface benzyne is aromatic in nature, and x-ray photoelectron spectroscopy confirms that the C–S bond is no longer intact. Surface benzyne is unusually stable on the sulfided Mo(110), decomposing at 680 K to gaseous dihydrogen and atomic carbon, with an activation energy of 42 kcal/mol in the limit of high coverage. At low coverages, the phenyl thiolate decomposes on Mo(110) to surface carbon, surface sulfur, and gaseous dihydrogen, with decomposition complete below 600 K. At low coverages, no formation of gaseous benzene or surface benzyne occurs. The coverage dependence of the reaction kinetics of the phenyl thiolate on Mo(110) are attributed, in part, to a change in the structure of the adsorbed thiolate at high coverages.
Fluorescence polarization and intramolecular dynamics in S1 of pyrazine, pyrimidine, and s-triazine vapors
Authors: Ohta Nobuhiro , Sekiguchi Osamu , Baba Hiroaki
Published: July 2002

The excitation polarization (EP) spectra of fluorescence of pyrazine, pyrimidine, and s-triazine in the vapor phase have been obtained at room temperature along the rotational contour of the absorption bands belonging to the S0→S1 transition. The decay curves of the parallel and perpendicularly polarized fluorescence of these molecules have also been observed. At low pressures, the degree of polarization of both fast and slowly decaying components of fluorescence emitted from the initially prepared vibronic level (IPL) exhibits a remarkable variation along the rotational contour in every case, whereas the broad fluorescence of pyrimidine and s-triazine emitted from the levels to which nonradiative transition occurs from the IPL is not polarized with excitation to any rotational level. The EP spectra of the IPL fluorescence are well reproduced theoretically by considering the resonance fluorescence from fully resolved rotational levels in S1 and by employing a symmetric top approximation. The EP spectra of the broad fluorescence are also reproduced fairly well by assuming that K scrambling occurs within the singlet manifold following the intramolecular vibrational redistribution from the IPL and that the broad fluorescence is emitted from such K scrambled levels. On the basis of these results, polarization characteristics of fluorescence and the role of molecular rotation in the intramolecular dynamics of these molecules are discussed.
Laser-induced-fluorescence spectroscopy of jet-cooled C3
Authors: Rohlfing Eric A.
Published: July 2002

We present a spectroscopic study of jet-cooled C3 in which laser-induced-fluorescence (LIF) excitation spectra and dispersed-fluorescence (DF) spectra are taken for a new, vibronically induced band system (1Πg and/or 1Δu)−X̃ 1Σ+g in the ultraviolet. In addition, DF spectra are taken for the well-known cometary band system, Ã 1Πu–X̃ 1Σ+g. The DF spectra of the new system are very rich and display strong fluorescence bands in stretch–bend progressions that reach as high as 17 000 cm−1 in the 1Σ+g ground state. The data from the DF spectra of both systems is used to assign vibrational term energies to 144 ground-state levels that cover the range: 0≤v1≤8, 0≤v2≤37, and 0≤v3≤4. The observed level structure clearly demonstrates the highly anharmonic nature of the potential energy surface of C3 and the unusual internal dynamics that occurs upon it. These include pronounced barriers to linearity in the v3=2 and v3=4 states and an increase in molecular rigidity as the symmetric stretch is excited.
The Onsager theory of the isotropic–nematic liquid crystal transition: Incorporation of the higher virial coefficients
Authors: Tjipto-Margo Broto , Evans Glenn T.
Published: July 2002

In the Onsager theory for the phase transition from the isotropic fluid to the nematic liquid crystal phase, the Helmholtz free energy of a fluid of hard convex bodies (HCBs) is expressed as the sum of an entropy of a mixing-like term and an energy-like term (from the interaction of the HCBs). Whereas the Onsager theory expresses the interaction term in a virial expansion and determines the consequences of B2 alone, here we extend that treatment to incorporate B3 (with its attendant dependence on the mutual orientation of three HCBs). For HCBs (and specifically for D∞h ellipsoids) with large aspect ratios (5:1 or greater), the incorporation of B2 and B3 suffices to predict the variation of the order parameter 〈P2[cos(θ)]〉 with density in accord with the Monte Carlo (MC) results of Allen and Wilson. As the aspect ratio decreases (from 5:1) to more spherical molecules (say 3:1), virial coefficients of higher order than B3 contribute to the interaction term and their effect is represented in part by the y-expansion (or resummation) theory proposed by Barboy and Gelbart. In this y-expansion–third virial-Onsager theory, the predicted transition densities are in accord with the MC values of Frenkel and Mulder for prolate ellipsoids. Neither the y expansion nor the direct B2 and B3 theories find the phase diagram (i.e., transition density and order parameter regarded as a function of aspect ratio) to be symmetric for prolate and oblate ellipsoids. The dependence of B3 on the mutual orientation of the ellipsoids is also discussed and previous work is also addressed.
The microphase separation transition in symmetric diblock copolymer melts: A Monte Carlo study
Authors: Fried H. , Binder K.
Published: July 2002

We use Monte Carlo methods to study a lattice model of a dense melt of symmetric diblock copolymers. The calculations are performed in three dimensions using a slithering snake algorithm. In this work we present results for temperatures at and above the microphase separation transition (MST). The location of the MST is found through a study of the relaxation time associated with lamellar compositional fluctuations. A range of copolymer chain lengths are studied, and from our results, approximate scaling relationships are established. A collective length scale characterizing the compositional fluctuations is found which results from a nontrivial compositional mesostructure. At infinite temperature (athermal conditions), the copolymers exhibit the expected Gaussian conformational behavior. We find, however, that at finite temperatures, the copolymers exhibit non-Gaussian behavior indicative of a stretched or elongated state. The magnitude of this stretching, and hence of the deviation from Gaussian behavior, is found to increase as the temperature decreases towards the MST.
Group theory and nuclear spin statistics of weakly-bound (H2O)n, (NH3)n, (CH4)n, and NH+4(NH3)n
Authors: Balasubramanian K.
Published: July 2002

We show that the irreducible representations, character tables, and nuclear spin statistical weights of tunneling levels of weakly bound van der Waals polymers such as (H2O)n, (NH3)n, (CH4)n, and NH+4(NH3)n can be obtained readily using the cycle-type matrix algebra and plethysm substitutions. The permutation-inversion (PI) groups of (H2O)n, (NH3)n, and (CH4)n are expressible as wreath product groups. A tree-pruning method is developed to generate the PI groups of NH+4(NH3)n. The tunneling levels and the nuclear spin statistical weight tables of (H2O)n (n up to 8), (NH3)n (n up to 4), and (CH4)n (n up to 3) are obtained for the first time using the wreath product algebra although, e.g., the PI group of (H2O)8 considered in this study contains 20 643 840 operations.
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o calculations on HC+5, a cation of interest to astrochemistry
Authors: Botschwina Peter
Published: July 2002

HC+5, a cation of astrochemical interest, has been studied by the coupled electron pair approximation (CEPA) using a basis set of 111 contracted Gaussian-type orbitals (cGTOs). It has a large equilibrium dipole moment of −2.88 D with the positive end at the proton site and is thus a suitable candidate for future investigation by radioastronomy or microwave spectroscopy. Most promising for a study in the infrared region of the spectrum is the Fermi resonance doublet ν2/ν4+ν5 with band origins calculated at 2199 and 2174 cm−1. Using larger basis sets of 146 and 160 cGTOs, the equilibrium proton affinity of C5 is calculated to be 887.7 and 886.6 kJ mol−1, respectively. For the proton affinity at 298 K, a value of 860±5 kJ mol−1 is recommended.
On the correlation between like ion pairs in water
Authors: Dang Liem X. , Pettitt B. Montgomery , Rossky Peter J.
Published: July 2002

The consequences of the subtle balance of intermolecular forces for like charge interionic correlations in solution are considered. The potential of mean force (PMF) for Cl−–Cl− and Na+–Na+ ion pairs using simple point charge (SPC and SPC/E) water models are examined in detail, and it is demonstrated that the results depend qualitatively on the details of the model. Features in the ion–ion cavity potential are shown to be responsible for much of the observed sensitivity with respect to parameters.
Vacuum ultraviolet photoionization mass spectrometric study of C6
Authors: Yoo R. K. , Ruscic B. , Berkowitz J.
Published: July 2002

Gaseous C60 has been studied by photoionization mass spectrometry between the ionization threshold and 40.8 eV. An adiabatic threshold of 7.57±0.01 eV is observed, which may be slightly low due to hot bands. The energy derivative of the photoion yield curve is in rough agreement with the He i photoelectron spectrum on the positions of some peaks, but others are weak or absent. The discrepancy is not attributed to autoionization, but rather to selection rules governing the ejection of low energy electrons into high angular momentum waves. C++60 is observed at higher energies, and becomes ∼0.6 as intense as C+60 at 40.8 eV. The photoion yield curve of C++60, approximately linear well above threshold, appears to exhibit curvature near threshold, thwarting an attempt to make a distinction between two alternative values of the second ionization potential. Fragmentation to form C+58 is only observed at the highest energy, 40.8 eV. The unimolecular decay is modelled by quasiequilibrium theory. In this model, the kinetic shift is of the order of 30 eV, and the minimum energy for dissociation into C+58+C2 seems to be 6.0–6.5 eV.
Nascent product population distribution in the reaction 16O(1D2)+H218O→16OH+18OH
Authors: Cleveland Cheryl B. , Wiesenfeld John R.
Published: July 2002

Nascent rotational and vibrational population distributions for the reaction 16O(1D2)+H218O→16OH+18OH have been determined using laser induced fluorescence detection of both OH species. Distributions were corrected for quenching by H2O. The rotational distribution for ν″=0 of 16OH is significantly hotter (19 500 K) than that obtained previously (2600 and 4600 K). In addition, rotational population in ν″=0 of both OH species out to the thermochemical limit (9930 cm−1 ) cannot be explained by a model in which the OH rotational angular momenta of both fragments must be equal. Further evidence against that model is provided by the preferential population of Π(A′)Λ sublevels, with ν″=0 and 1 having Λ population ratios of 1.5 and 1.2, respectively. The current data are better explained by an HOOH collision complex formed by insertion of 16O(1D2) into the H218O molecule. The complex probably has a relatively short lifetime, however, because the 16OH fragment is more rotationally and vibrationally excited than the 18OH fragment.
Monte Carlo simulation of folding transitions of simple model proteins using a chain growth algorithm
Authors: O’Toole Eamonn M. , Panagiotopoulos Athanassios Z.
Published: July 2002

We have performed Monte Carlo simulations of folding transitions of model proteins on a simple cubic lattice with the Rosenbluth and Rosenbluth (1955) chain growth algorithm combined with Boltzmann weighting and multilink additions. We use a model proposed by Dill (1985) that represents the protein as a connected sequence of hydrophobic and hydrophilic beads on the lattice, with nearest-neighbor interactions between the constituent beads. The algorithm allows the calculation of equilibrium folding curves for arbitrary sequences of chain lengths of at least 48 in three dimensions. This is significantly longer than the maximum chain length for which equilibrium curves can be determined with the Metropolis (1953) based algorithms used in previous work. The thermal denaturation curve for the model is found to vary greatly with the sequence used, in some special cases approximating closely the sharpness observed for real proteins. These special sequences appear to be simple, periodic arrangements of beads. However, it seems that the unique or almost unique lowest energy states that exist for certain sequences of moderate length approaching the length of the simplest real proteins cannot be recovered in feasible simulation times with our particular model/algorithm combination.
Structure and dynamics of polymer colloid suspensions from dynamic light scattering and Brownian dynamics simulation
Authors: Härtl W. , Versmold H. , Wittig U. , Linse P.
Published: July 2002

A dilute suspension of ordered polymer colloid particles was investigated by static and dynamic light scattering. The obtained results are compared with a Brownian dynamic (BD) simulation of a model system consisting of hard spheres interacting with an exponentially screened Coulomb potential. The long time properties of the self- and collective correlation functions are investigated in detail. The obtained mean squared displacement is best described by a stretched exponential model and the collective long time diffusion from the BD simulation can only be approximately described by a kinetic theory based on a hard sphere system.
Excited state vibrational dynamics of 4-ethylaniline (X)1 clusters (X=Ar, N2, and CH4)
Authors: Hineman Max F. , Bernstein Elliot R. , Kelley David F.
Published: July 2002

Intracluster vibrational redistribution (IVR) and vibrational predissociation (VP) dynamics of 4-ethylaniline (Ar)1, (N2)1, and (CH4)1 clusters have been studied by time-correlated single photon counting, mass-resolved excitation spectroscopy, and dispersed emission spectroscopy. The 4-ethylaniline molecule has a low frequency ethyl group torsion vibrational mode, which is similar in energy to the van der Waals modes of the clusters. This mode, because of its low energy (∼35 cm−1), plays a role in the vibrational dynamics of the clusters studied. The cluster dissociation rates and product state distributions can be modeled by a serial IVR/VP mechanism for which the VP step is treated by the Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The resulting agreement between the calculated and experimental rates and product state intensities indicates that a statistical distribution of energy among all low frequency modes exists for 4-ethylaniline/polyatomic solvent clusters in which kIVR≫kVP. For 4-ethylaniline (Ar)1 clusters kVP≳kIVR and a statistical distribution of energy among the chromophore and van der Waals modes is not achieved. The central determining factor for the vibrational dynamics of these clusters is overall density of low energy modes.
The thermodynamic stability of clathrate hydrate. II. Simultaneous occupation of larger and smaller cages
Authors: Tanaka Hideki , Kiyohara Kenji
Published: July 2002

The thermodynamic stability of a clathrate hydrate encaging methane or xenon has been investigated by examining the free energy of formation. The total free energy is divided into several contributions: the interaction between water and guest molecules, the entropic contribution arising from the combinations of cage occupancy, and also the free energy arising from the guest molecular motions inside cages. Our method is based on the generalized van der Waals and Platteeuw theory used for the study of the hydrate encaging propane and is free from some of the fundamental assumptions in the original theory. This enables us to evaluate separately the factors which contribute significantly to the thermodynamic stability of the hydrate, and to address a question as to what extent the original theory is applicable to the prediction of the phase diagrams. While the potential energy curve of the guest molecule with surrounding water molecules in a smaller cage has a single minimum and the molecular motion can be accurately approximated to a collection of harmonic oscillators strongly coupled with the host molecules, the guest molecular motion in a larger cage does not couple with the host. To show evidence that the fixed lattice approximation is sufficient to describe molecular motions inside the larger cage, two kinds of molecular dynamics simulations have been carried out.

In one simulation, both host water and guest molecules move according to the classical equations of motion. In another simulation only guest molecules are allowed to move, interacting with fixed host molecules. We perform two kinds of analyses on those motions. In the first analysis, the velocity autocorrelation functions are calculated from molecular dynamics simulations at several temperatures and the power spectra are obtained by the Fourier transform of the correlation functions. In the second, a so-called normal mode analysis is performed and the densities of state for intermolecular vibrations are obtained. The densities of state (corresponding to 0 K) are compared with the power spectra. It is revealed that the fixed lattice approximation can be applied when describing the molecular motions of methane and xenon in larger cages. The free energy for the accommodation of an extremely large CF4 or small argon guest is also examined.
Characterization of the first excited 1Π1 and the ground X 1Σ+ states of MgXe. I. Analysis of the 1Π1←X 1Σ+ bound–bound transitions
Authors: McCaffrey John G. , Funk David J. , Breckenridge W. H.
Published: July 2002

Laser induced fluorescence (LIF) excitation spectra recorded for the vibrational bands in the Mg(3s3p 1P1)⋅Xe(1Π1)←Mg(3s3s 1S0)⋅Xe (X 1Σ+) system have been analyzed, yielding absolute vibrational assignments and values of ωexe=1.585±0.02 and ωe=97.5±1.0 cm−1 for the 1Π1 state of 24Mg132Xe. From a Birge–Sponer extrapolation, the well depth of this state is estimated to be 1500 cm−1. Simulations of rotationally structured spectra of three of the most intense vibrational bands are consistent with R″e=4.56±0.12 Å for the X 1Σ+ state. From Morse function extrapolation of the excited state rotational constants from the simulations, and Franck–Condon intensity simulations of the 1Π1←X 1Σ+ vibrational progressions, R′e for the 1Π1 state is estimated to be 3.07±0.10 Å. The 1Π1 state of MgXe fluoresces strongly. The corresponding 1Π1 states of ZnXe and CdXe do not fluoresce, but ‘‘action’’ spectra from the production (via predissociation) of metal atom 3PJ states are observed. Possible reasons for these differences are discussed in terms of spin–orbit induced predissociation. It is concluded that predissociation of the MgXe(1Π1) state is not observed because the crossing between the repulsive 3∑+1 and the attractive 1Π1 potential curves does not occur until energies higher than those accessible experimentally. Possible reasons for the behavior of the diatomic MgXe(1Π1) state vs that of Mg(3s3p 1P1) isolated in solid Xe, where production of Mg(3s3p 3PJ) states competes with Mg(3s3p 1P1) fluorescence, are also discussed. Finally, the attractive ‘‘bonding’’ interactions in the MgXe(1Π1) state are analyzed in terms of electrostatic interactions and compared with those for other Π-type states of metal/rare-gas van der Waals diatomic molecules.
Phase transitions and chemical transformations of benzene up to 550 °C and 30 GPa
Authors: Cansell François , Fabre Denise , Petitet Jean-Pierre
Published: July 2002

The phase diagram of benzene has been investigated up to 550 °C and 30 GPa in a diamond anvil cell by Raman scattering. A new triple point at 400 °C and 5 GPa and a new phase of solid benzene, called phase IV, have been located at high temperature. The boundaries have been determined between the following solid phases: (i) benzene II and benzene III; (ii) benzene III and benzene III′. The irreversible chemical transformation line of benzene has been determined and can be divided in three parts leading to three different recovered compounds. The P–T diagram obtained by our experimental results is discussed in comparison with published works.
The effect of finite film thickness on the surface segregation in symmetric binary polymer mixtures
Authors: Hariharan Arvind , Kumar Sanat K. , Rafailovich Miriam H. , Sokolov Jonathan , Zheng X. , Duong Daihung , Schwarz Steven A. , Russell Thomas P.
Published: July 2002

We report neutron reflectivity and dynamic secondary ion mass spectroscopy measurements of surface segregation from symmetric, isotopic polystyrene blends, spin coated onto oxide covered silicon wafers, as a function of film thickness. The results of this analysis show that the segments of the deuterated polymer always partition to both the air and the substrate interfaces. Furthermore, the surface segregation is affected significantly if the film thicknesses are reduced below ∼four times the correlation length in the systems, and the segregation to both surfaces decreases with decreasing thickness. These results are in good agreement with the predictions of a mean-field lattice model which incorporates composition and chain length independent values of the surface energy parameter χs at each surface.
The equilibrium geometry and spectroscopic constants of cyanogen calculated by the single, double, and perturbative triple excitation coupled-cluster method
Authors: Botschwina Peter
Published: July 2002

The equilibrium geometry and several spectroscopic constants of cyanogen (NCCN) have been calculated by coupled-cluster theory with single and double excitations using a large basis set of 220 contracted Gaussian-type orbitals and correlating all electrons. The results are in excellent agreement with experimental data. An improved equilibrium geometry is R1e(CN)=1.1579(2) Å and R2e(CC)=1.3839(2) Å.
Interchannel coupling and ground state correlation effects in the photoionization of CO
Authors: Rescigno T. N. , Lengsfield B. H. , Orel A. E.
Published: July 2002

We describe a general procedure for applying the complex Kohn variational method to the calculation of molecular photoionization cross sections and asymmetry parameters. In this initial application of the method, we examine the effects of interchannel coupling and ground state correlation on the X 2Σ+(5σ−1), A 2Π(1π−1), and B 2Σ+(4σ−1) partial photoionization cross sections and asymmetry parameters for the CO molecule. We find that the dominant effect of interchannel coupling is to remove a spurious π→π* resonance feature from the continuum that appears at the frozen-core Hartree–Fock level. We also find that it appears to be important to combine the effects of final channel coupling with a correlated initial target state to achieve quantitatively correct cross sections.
Macromolecular electrostatic energy within the nonlinear Poisson–Boltzmann equation
Authors: Zhou Huan-Xiang
Published: July 2002

A fundamental problem in macromolecular electrostatics is the calculation of the electrostatic energy of a macromolecule solvated in an electrolyte solution, i.e., the work required to charge up the macromolecule in the presence of the electrolytic ions. Through numerical calculations with the nonlinear Poisson–Boltzmann (PB) equation, Sharp and Honig [J. Phys. Chem. 94, 7684 (1990)] observed that this energy can be obtained with equal accuracy from the charging integral and from their energy–density integral. Here we give an elementary analytical proof of the exact equivalence of the two different formulations of the energy. In order to calculate the macromolecular electrostatic energy, a boundary element method [Biophys. J. 65, 954 (1993)] previously developed for the linearized PB equation is modified to solve the nonlinear PB equation. Illustrative calculations show that for globular proteins under physiological ionic strengths, the electrostatic energy calculated from the linearized PB equation differs very little from that calculated from the nonlinear equation.
A systematic comparison of molecular properties obtained using Hartree–Fock, a hybrid Hartree–Fock density-functional-theory, and coupled-cluster methods
Authors: Oliphant Nevin , Bartlett Rodney J.
Published: July 2002

We present results of a systematic study of the theoretical determination of equilibrium geometries, harmonic frequencies, total atomization energies, and dipole moments using Hartree–Fock, a hybrid Hartree–Fock density-functional-theory, and coupled-cluster methods in conjunction with a triple zeta basis set for a large set of molecules. This allows a direct comparison of the three theoretical methods applied to a range of chemical systems. The average errors (‖experimental value-theoretical value‖) for the Hartree–Fock, hybrid Hartree–Fock density-functional-theory, and coupled-cluster methods, respectively, are bond length (Å) 0.022, 0.005, 0.005; bond angle (degrees) 2.7, 1.7, 1.9; harmonic frequencies (cm−1) 144, 40, 30; atomization energies (kcal/mol) 81.9, 3.6, 11.5; and dipole moments (debye) 0.29, 0.14, 0.10. This clearly demonstrates that the relatively inexpensive hybrid Hartree–Fock density-functional-theory method yields results which represent a reliable, significant improvement over those obtained with the Hartree–Fock method. The results obtained using the hybrid Hartree–Fock density-functional-theory method are, in fact, quite comparable with the corresponding results obtained using the high level, ab initio coupled-cluster method. For certain difficult open shell examples, the hybrid Hartree–Fock density-functional theory using a spin restricted open shell Hartree–Fock density is much improved over the corresponding hybrid Hartree–Fock density-functional-theory results obtained using a spin unrestricted Hartree–Fock density.
Aqueous solvation dynamics with a quantum mechanical Solute: Computer simulation studies of the photoexcited hydrated electron
Authors: Schwartz Benjamin J. , Rossky Peter J.
Published: July 2002

We have used molecular dynamics simulation to explore aqueous solvation dynamics with a realistic quantum mechanical solute, the hydrated electron. The simulations take full account of the quantum charge distribution of the solute coupled to the dielectric and mechanical response of the solvent, providing a molecular-level description of the response of the quantum eigenstates following photoexcitation. The solvent response function is found to be characterized by a 25 fs Gaussian inertial component (40%) and a 250 fs exponential decay (60%). Despite the high sensitivity of the electronic eigenstates to solvent fluctuations and the enormous fractional Stokes’ shift following photoexcitation, the solvent response is found to fall within the linear regime. The relaxation of the quantum energy gap due to solvation is shown to play a direct role in the nonradiative decay dynamics of the excited state electron, as well as in the differing relaxation physics observed between electron photoinjection and transient hole-burning (photoexcitation) experiments. A microscopic examination of the solvation response finds that low frequency translational motions of the solvent play an important role in both the inertial and diffusive portions of the relaxation. Much of the local change in solvation structure is associated with a significant change in size and shape of the electron upon excitation. These results are compared in detail both to previous studies of aqueous solvation dynamics and to ultrafast transient spectroscopic work on the hydrated electron.
New method for determining the chemical potential for condensed matter at high density
Authors: Powles J. G. , Holtz B. , Evans W. A. B.
Published: July 2002

We apply a space-dependent external potential to the fluid such as to produce both a high density, virtually homogeneous, region and also a low density, not necessarily homogeneous, region. We easily measure the chemical potential in the low density region by Widom’s fictitious- particle-insertion method by computer simulation. By constancy of the chemical potential the value is the same in the high density region, which is difficult to measure directly by this or any other method. The method has been tested for Lennard-Jones fluids, for which independent values of chemical potential at high density are known. The method is quite general and can be used for molecular liquids, polymer melts, mixtures, etc.
Chemical association in simple models of molecular and ionic fluids. IV. New approximation for the cavity function and an application to the theory of weak electrolytes
Authors: Zhou Yaoqi , Stell George
Published: July 2002

A multiplicative interpolation scheme (MIS) for the cavity function of associating hard-core particles is proposed and applied to the case of our shielded sticky-point model in which the particles associate into fused hard-sphere diatomic molecules (hard dumbbells). The results indicate that the MIS is a significant improvement over the simple interpolation scheme (SIS) of our earlier paper when the bonding length is much less than the hard-core diameter. The MIS is also applied to the case of a weak electrolyte modelled by shielded sticky-point particles that are charged, which associate into hard dipolar dumbbells. Using a near-exact hard-sphere cavity function along with the mean spherical approximation (MSA) for the electrostatic part of the activity coefficient, we find that the resulting version of the MIS is already in reasonable agreement with the experimental data for monochloroacetic acid, despite the equal-diameter restriction of the simple version of the model that we are using.
A time-dependent approach to flux calculation in molecular photofragmentation: Vibrational predissociation of HF–DF
Authors: Zhang Dong H. , Wu Qian , Zhang John Z. H.
Published: July 2002

We present in this paper a time-dependent approach to the calculation of photofragmentation dynamics using the flux formulation. The method is essentially a time-dependent version of the flux formulation for photodissociation calculation recently pursued by Manolopoulos and Alexander. In the present approach, the partial decay width of photofragmentation is obtained by calculating the flux at a given surface using a time-dependent method. This particular time-dependent approach for photofragmentation has two principal advantages. First, it is superior in computational scaling: CPU time ∝Nα(α<2) vs N3 in standard time-independent propagation method. Second, it is quite straightforward to handle the photofragmentation process involving rearrangement with the application of optical potentials. In addition, no bound state projection is necessary using the time-dependent flux method, which is required using the time-dependent golden rule method. This time-dependent method is applied to the calculation of decay width for vibrational predissociation of hydrogen-bonded HFDF, and the computed lifetime are compared with the recent experimental measurement of Farrell and Nesbitt. We also present the results of the full dimensional (6D) calculation of bound state energies for the HFDF complex. The exact dissociation energies are calculated to be 1057.33 cm−1 for (HF)2, 1166.6 cm−1 for (DF)2, 1142.7 cm−1 for HF–DF, and 1078.4 cm−1 for DF–HF. All theoretical calculations have used the SQSBDE potential energy surface due to Quack and Suhm.
Electron impact ionization of the SO2 molecule
Authors: Basner R. , Schmidt M. , Deutsch H. , Tarnovsky V. , Levin A. , Becker K.
Published: July 2002

We report measurements of the absolute cross sections for the electron-impact ionization of SO2 from threshold to 200 eV. Absolute cross sections for the formation of the SO+2 parent ions and of the SO+, S+, O+, and O+2 fragment ions were obtained independently in two different laboratories using two different experimental techniques with uncertainties ranging from ±18% to ±25%. The level of agreement between the absolute cross sections (at 70 eV) obtained by the two techniques ranges from about 10% for SO+2 and SO+ to 20% for (S++O+2) and O+, which in all cases is well within the combined error margins of the two measurements. The high resolution capability of the mass spectrometer employed in one experiment enabled the separation of the S+ and O+2 fragment ions, which are separated by only 0.017 76 atomic mass units (amu), for the first time. The single positive ion formation is the dominant process for all observed product ions. The total single SO2 ionization cross section obtained by the two techniques agreed to within 8%. A comparison of the experimentally determined total SO2 single ionization cross sections with calculated cross sections based on a modified additivity rule revealed agreement to within 20%.
Immiscibility of large and small symmetric diblock copolymers
Authors: Matsen M. W.
Published: July 2002

We examine binary mixtures of large and small symmetric AB diblock copolymers using self-consistent field theory. In accord with experiment, we find that when the ratio of their polymerization indexes exceeds about 1:5, they become immiscible producing a coexistence between long- and short-period lamellar phases. As in experiment, the short-period phase contains few large diblocks, but the long-period one contains a substantial volume fraction of small diblocks.
Rotational spectra of the mixed rare gas dimers Ne–Kr and Ar–Kr
Authors: Xu Yunjie , Jäger Wolfgang , Djauhari Joyce , Gerry Michael C. L.
Published: July 2002

Pure rotational spectra of several isotopomeric species of the rare gas dimers Ne–Kr and Ar–Kr have been measured using a pulsed jet cavity microwave Fourier transform spectrometer. Equilibrium internuclear distances have been evaluated by taking advantage of the isotopic data, for both these dimers and three Xe-containing dimers, whose spectra were reported earlier [Jäger et al., J. Chem. Phys. 99, 919 (1993)]. The dipole moments have been estimated using the ‘‘π/2-pulse’’ excitation condition. 83Kr nuclear quadrupole hyperfine structure has been observed in some rotational transitions of 20Ne–83Kr and of Ar–83Kr, and the corresponding quadrupole coupling constants have been derived.
The quasi-Gaussian entropy theory: Free energy calculations based on the potential energy distribution function
Authors: Amadei A. , Apol M. E. F. , Di Nola A. , Berendsen H. J. C.
Published: July 2002

A new theory is presented for calculating the Helmholtz free energy based on the potential energy distribution function. The usual expressions of free energy, internal energy and entropy involving the partition function are rephrased in terms of the potential energy distribution function, which must be a near Gaussian one, according to the central limit theorem. We obtained expressions for the free energy and entropy with respect to the ideal gas, in terms of the potential energy moments. These can be linked to the average potential energy and its derivatives in temperature. Using thermodynamical relationships we also produce a general differential equation for the free energy as a function of temperature at fixed volume. In this paper we investigate possible exact and approximated solutions. The method was tested on a theoretical model for a solid (classical harmonic solid) and some experimental liquids. The harmonic solid has an energy distribution, which can be derived exactly from the theory. Experimental free energies of water and methanol could be reproduced very well over a temperature range of more than 300 K. For water, where the appropriate experimental data were available, also the energy and heat capacity could be reproduced very well.