bluuebunny/crossref_metadata_2025 huggingface dataset:
Selective collision-induced desorption: Measurement of the π-bonded C2H4 binding energy on Pt{111} precovered with atomic oxygen
Authors: Velic D. , Levis Robert J.
Published: July 2002

Collision-induced desorption (CID) is used to selectively probe the well depth of one particular adsorbate–surface potential energy surface in the presence of multiple adsorbates. Ethylene adsorbed at saturation coverage on Pt{111} precovered with atomic oxygen represents a system with three adsorbates: oxygen and two different adsorption forms of ethylene. Both the π-bonded ethylene (π-C2H4) and di-σ-bonded ethylene (di-σ-C2H4) species are formed at coverages of 0.25 ML preadsorbed atomic oxygen and 0.15 ML ethylene on Pt{111} at 100 K. Deconvolution of Al Kα x-ray photoelectron (XP) spectra suggests that C(1s) XP binding energy is 283.1 eV for π-C2H4 and is 283.7 eV for di-σ-C2H4. The C(1s) XP spectra together with the CID result reveal that the coverage ratio of π-C2H4 and di-σ-C2H4 at saturation at 100 K is ∼1:1. The selectivity of CID is demonstrated by desorbing only π-C2H4 using a neutral Xe atomic beam with translational energies ranging from 2.1 to 4.1 eV. In this translational energy range, di-σ-C2H4 remains intact on the O/Pt{111} surface. The threshold energy for CID of π-C2H4 was determined by extrapolation to the Xe energy where the CID cross section is equal to zero. The threshold of 2.4±0.1 eV can be related to the π-C2H4–Pt binding energy using classical collision mechanics. An upper limit for the π-C2H4 binding energy on O/Pt{111} is calculated to be 0.95±0.04 eV from the CID threshold measurement, using a Pt effective mass of 1.5 Pt atom.
Exact analytic solution for the correlation time of a Brownian particle in a double-well potential from the Langevin equation
Authors: Kalmykov Yu. P. , Coffey W. T. , Waldron J. T.
Published: July 2002

The correlation time of the positional autocorrelation function is calculated exactly for one-dimensional translational Brownian motion of a particle in a 2–4 double-well potential in the noninertial limit. The calculations are carried out using the method of direct conversion (by averaging) of the Langevin equation for a nonlinear stochastic system to a set of differential–recurrence relations. These, in the present problem, reduce on taking the Laplace transform, to a three-term recurrence relation. Thus the correlation time Tc of the positional autocorrelation function may be formally expressed as a sum of products of infinite continued fractions which may be represented in series form as a sum of two term products of Whittaker’s parabolic cylinder functions. The sum of this series may be expressed as an integral using the integral representation of the parabolic cylinder functions and subsequently the Taylor expansion of the error function, thus yielding the exact solution for Tc. This solution is in numerical agreement with that obtained by Perico et al. [J. Chem. Phys. 98, 564 (1993)] using the first passage time approach while previous asymptotic results obtained by solving the underlying Smoluchowski equation are recovered in the limit of high barrier heights. A simple empirical formula which provides a close approximation to the exact solution for all barrier heights is also given.
Geometry of hard ellipsoidal fluids and their virial coefficients
Authors: Singh G. S. , Kumar B.
Published: July 2002

This article is primarily addressed to the geometry of hard ellipsoidal molecules which is essentially required in the study of the fluid structure, thermodynamics and transport properties of the model hard-biaxial fluids or liquid crystals. A methodology has systematically been followed in obtaining the known as well as hitherto unknown results for a biaxial system. The computationally convenient and neat expressions have been achieved for the surface and volume elements of a biaxial molecule. New expressions have been obtained for the surface area S and the mean radius of curvature R of a rigid ellipsoid in terms of the elliptic integrals of the first and second kinds. In the limit when the two axes become equal, the expressions obtained for the ellipsoidal molecules reduce to those of the spheroidal systems. Exact expression has been obtained for the second virial coefficient of the ellipsoidal fluid and furthermore the significance of our analytical results for such a fluid are enunciated in the context of the study of the higher virial coefficients and equation of state.
Realistic molecular distortions and strong substrate buckling induced by the chemisorption of benzene on Ni{111}
Authors: Held G. , Bessent M. P. , Titmuss S. , King D. A.
Published: July 2002

A detailed structural determination of the Ni{111}–(√7×√7)R19.1°–C6H6 overlayer has been performed using automated fully dynamical low energy electron diffraction I–V analysis. In the most likely geometry (RP = 0.26, total energy range: 1552 eV) benzene adsorbs with its center 1.91 Å above an hcp site and with its C–C bonds oriented parallel to the close-packed rows of the substrate. The molecular radius is found to be slightly expanded relative to the gas phase (1.48 Å and 1.50 Å vs 1.40 Å) and no significant vertical buckling can be seen (< 0.04±0.05 Å). The topmost Ni layer is strongly buckled (0.14 Å) with the height of the Ni atoms decreasing with increasing lateral distance from the molecule (+0.08, 0.00, and –0.06 Å with respect to the clean surface). A second similarly low RP factor minimum for adsorption on a bridge site was dismissed because of significantly higher R1 and R2 values and the extreme molecular distortions of this geometry. The resulting structure is fully consistent with recent UPS and PED studies.
Dynamical properties and transport coefficients of Kihara linear fluids
Authors: MacDowell L. G. , Garzón B. , Calero S. , Lago S.
Published: July 2002

Transport properties of spherical and linear molecules modeled by the Kihara potential are studied by molecular dynamics simulations. Diffusion coefficients, shear viscosities, and thermal conductivities are calculated for a wide range of the fluid region and for several elongations. The corresponding individual and collective correlation functions are discussed along with angular velocity and reorientational correlation functions. Relaxation times and simple models relevant to orientational motion are also studied. The results obtained are discussed in a corresponding states framework, using previous Gibbs ensemble Monte Carlo data for the liquid–vapor equilibria of the models. In this way, the role of elongation can be studied. It is found that in most of the liquid region, the diffusion coefficient is weakly dependent on elongation. On the other hand, both viscosity and thermal conductivity are found to decrease with elongation. The dependence of transport coefficients on density and temperature is also discussed. On testing the Stokes–Einstein relation, it was observed that, unlike previous findings for hard spheres, stick boundary conditions perform just as good as slip boundary conditions for the Lennard-Jones fluid and the low-elongated Kihara fluid.
Fourier transform infrared observation of the vibrational spectrum of the linear SiCCH radical in Ar at 10 K
Authors: Han Dongsheng , Rittby C. M. L. , Graham W. R. M.
Published: July 2002

A Fourier transform infrared study has resulted in the first observation of a vibrational spectrum of the linear SiCCH radical produced by trapping the products of the vacuum ultraviolet photolysis of a mixture of SiH4 silane, and C2H2 acetylene, in an Ar matrix at 10 K. Two fundamentals have been observed, the ν2(σ), carbon–carbon stretching mode at 1989.8 cm−1 and the ν3(σ), silicon–carbon stretching mode at 636.0 cm−1. The frequency of the 1989.8 cm−1 band and the results of extensive measurements of D and 13C isotopic shifts are in good agreement with the predictions of new CCSD(T) (coupled cluster with single, double, and triple excitations) ab initio calculations for the ν2 mode, while the assignment of the 636.0 cm−1 band is considered somewhat less certain.
Excess heat capacities due to the low-energy excitations of molecular glasses: An approach using the soft potential model
Authors: Lindqvist Anna , Yamamuro Osamu , Tsukushi Itaru , Matsuo Takasuke
Published: July 2002

We have investigated the low-temperature heat capacities arising from the low-energy excitations in several molecular glasses. The heat capacities of glassy toluene, ethylbenzene, and 3-methylpentane were measured with an adiabatic calorimeter. The first two samples were doped with 10 mol % of benzene to prevent crystallization. The low-temperature heat capacities of the molecular glasses measured in this study and those measured previously were all reproduced well by the sum of a normal part represented by the Debye function and an excess one represented by the soft potential model. The peak energy of G(ω)/ω2 [G(ω): density of states, ω: energy] was found to be proportional to M−1/2 (M: molecular mass) for the hydrocarbon molecules but not for the alcohol molecules. Intermolecular hydrogen bonds in the alcohols may be responsible for the different behavior.
Light scattering investigations in a ternary liquid mixture: Quest for a critical inflection point
Authors: Jaffar Ali B. M. , Kumar A.
Published: July 2002

We identify the existence of both a double critical point (DCP) and a critical double point (CDP) in a ternary liquid mixture consisting of ethanol plus water plus potassium carbonate. This system has a potential to yield a critical inflection point (CIP), a point at which a DCP and its associated CDP merge. We present measurements of osmotic compressibility (χT) near to and away from the DCP and the CDP. The approach to DCP is signalled by the variation of loop size (ΔT) from 35 to 2 K. The convexity of the coexistence surface was probed along different thermodynamic paths. The data for χT are quantified better by the mean-field (or classical) behavior (i.e., critical exponent, γ=1) than by 3D Ising behavior (γ=1.24). The universal value of γ (=1) is recovered for any distance to DCP or CDP by invoking a field variable that is third order in temperature (tULU) in lieu of the usual field variable (i.e., reduced temperature, t). There is a lack of symmetry between the approaches to the upper and to the lower convex paths near to and away from DCP. One of the unusual features of our findings is a rather large limiting value of the correlation length amplitude (ξ0∼0.3–0.8 nm) for the upper convex paths. We rationalize our observations in terms of the geometric picture and the phenomenological theories of phase transitions. The fortuitous similarity between this solvophobic system and the truly ionic fluids is highlighted.
Characterization of foldable protein models: Thermodynamics, folding kinetics and force field
Authors: Hao Ming-Hong , Scheraga Harold A.
Published: July 2002

We propose three analytical models that describe the characteristics of proteins that can be folded into unique native structures. Model I is characterized by a mean-field single-residue energy which favors the native state and has a large energy gap between the native and non-native states; Model II involves mean-field cooperative interactions among the residues in the native states, and Model III is characterized by the mean-field single-residue energy at a low degree of folding and by the cooperative interactions among native residues at a high degree of folding. The thermodynamics of all three protein models exhibit two-state transition behavior, in which the non-native state is dominated by large entropy while the native state is determined by low energy. The folding kinetics of the models are studied by means of the master equation method. While the kinetics of folding of all three models are driven by the energetic biases of individual residues which favor the native state, the different interaction modes lead to different folding rates. It is found that the models with long-range cooperativity (i.e., Models II and III) fold several orders of magnitude faster than the model with only localized interactions (Model I). The intramolecular interactions that are responsible for the different properties of these models are examined, and the ways that these models may be used for developing the force fields for realistic proteins are discussed.
Reactions of N and NO on Pt(335)
Authors: Wang Hong , Tobin R. G. , DiMaggio Craig L. , Fisher Galen B. , Lambert David K.
Published: July 2002

As part of a study of species important in automotive exhaust chemistry, the reactivity of atomic N and NO on Pt(335) at low temperature has been studied. The atomic N was produced by dissociating adsorbed NO with a 76 eV electron beam. Cross sections for electron-stimulated desorption and dissociation are estimated for NO on terrace and step sites. Terrace NO is at least five times more likely to desorb than to dissociate. Step NO has a lower desorption cross section than terrace NO, but probably a higher dissociation cross section. Temperature-programmed desorption was used to monitor desorption, dissociation, and the formation of N2 and N2O from adsorbed N and NO. Five distinct desorption states of N2 formed by NO dissociation are identified. The dominant N2 peak (435 K) comes from electron-dissociated step NO; its desorption temperature is higher than the N2 peaks from electron-dissociated terrace NO. Coadsorbed N and NO react to form N2O even below 100 K, with an activation barrier of ∼6 kcal/mol. Only terrace NO participates in this reaction; step NO does not react to form N2O. This site dependence resembles that for CO oxidation on Pt(112) and Pt(335) and can be rationalized with simple steric considerations. All of the forms of atomic N participate in N2O formation, but that formed by the dissociation of step NO exhibits the lowest reaction temperature. Hence, the same N atoms that only recombine to form N2 at 435 K, react with NO to form N2O at 100 K. We found no evidence for an NO reaction with N atoms to form N2 and adsorbed O, or for NO formation from the recombination of adsorbed N and adsorbed O2.
Fine structure relaxation of aluminum by atomic argon between 30 and 300 K: An experimental and theoretical study
Authors: Le Picard S. D. , Bussery-Honvault B. , Rebrion-Rowe C. , Honvault P. , Canosa A. , Launay J. M. , Rowe B. R.
Published: July 2002

This paper reports the first measurements at low temperatures (44–137 K) of rate coefficients for collision induced spin-orbit transitions, using a swarm experiment consisting of a uniform supersonic flow coupled with pulsed laser photolysis and laser-induced fluorescence techniques. It has been stated that both excitation and relaxation rates have a positive temperature dependence. The excitation (respectively relaxation) rate constant is 0.15×10−12 cm3 s−1 (respectively 1.63×10−12) at 53 K and 2.76×10−12 cm3 s−1 (respectively 4.48×10−12) at 137 K. A theoretical modelization of the Al–Ar system has been developed in the 30–300 K range, that reproduces the temperature dependence.
Generalized molecular mechanics including quantum electronic structure variation of polar solvents. II. A molecular dynamics simulation study of water
Authors: Bursulaya Badry D. , Jeon Jonggu , Zichi Dominic A. , Kim Hyung J.
Published: July 2002

By employing the truncated adiabatic basis set (TAB) description developed in the preceding article [B. D. Bursulaya and H. J. Kim, J. Chem. Phys. 108, 3277 (1998), preceding paper], solvent water under an ambient condition is studied via a molecular dynamics (MD) computer simulation method. The evolving charge distribution of each water molecule is described by the mixing of the TAB functions, which fluctuates with its local environment. The parametrization of these basis functions is couched in terms of the complete active space self-consistent field (CASSCF) ab initio calculations in vacuum. By using an interaction site representation for the diagonal and overlap charge distributions of the basis functions, electronic polarizability both in and out of the water molecular plane is accounted for. The ground-state charge distribution for the entire solvent system is determined at the self-consistent field (SCF) level with a numerical iteration method. Two different models, TAB/10 and TAB/10D, are studied. The average water dipole moment in liquid is found to be 2.58 D for the former and 2.65 D for the latter, while it is 1.85 D in vacuum for both models. The solution-phase electronic polarizability distributions, characterized by a narrow but finite width, show that nonlinear hyperpolarizability makes a non-negligible contribution to instantaneous electronic response of water even though its average response mainly falls in a linear regime. It is found that the TAB water predictions for structural, dynamic, spectroscopic, dielectric, and transport properties are in good agreement with corresponding experimental results.
The equilibrium structure and fundamental vibrational frequencies of dioxirane
Authors: Stanton John F. , Lopreore Courtney L. , Gauss Jürgen
Published: July 2002

Complete sets of quadratic and cubic force constants calculated for four isotopomers of dioxirane (CH2OO) are used to estimate vibration-rotation interaction contributions to observed values of rotational constants (B″), thereby yielding empirical estimates of the corresponding equilibrium values (Be). At the highest levels of theory, least-squares refinements of atomic coordinates to both the empirical Be values and the associated isotope shifts yield consistent sets of structural parameters. Recommended values are re(CO)=1.3846±0.0005 Å; re(OO)=1.5133±0.0005 Å; re(CH)=1.0853±0.0015 Å and θe(HCH)=117.03±0.20°. Semidiagonal quartic force constants (in the normal coordinate representation) also calculated for CH2OO are used to estimate anharmonic contributions to the fundamental vibrational frequencies. Arguments based on the latter set of results support those made in a previous theoretical study and clearly show that two infrared features assigned to dioxirane in a matrix-isolation experiment must be due to a different carrier.
The thermodynamic properties of the gaseous dimer of CdI2
Authors: Kuncewicz-Kupczyk Wioletta , Kapała Jan , Roszak Szczepan , Miller Mirosław
Published: July 2002

The vaporization of CdI2(s) was investigated in the temperature range between 534 and 613 K by the Knudsen effusion mass spectrometry. The Cd2I4(g)-dimer content in the equilibrium vapor of CdI2(s) was determined for the first time. The enthalpies of sublimation and dissociation of Cd2I4(g) were evaluated according to the third-law method using the experimental p(CdI2)/p(Cd2I4) ratio. Molecular parameters and the thermodynamic functions of Cd2I4(g) were determined theoretically. Enthalpy of sublimation, 2 CdI2(s)=Cd2I4(g), was obtained as: ΔsubH0(298.15 K)=222.5±6.2 kJ mol−1, and enthalpy of dissociation, Cd2I4(g)=2 CdI2(g), was determined as ΔdH0(298.15 K)=70.7±7.0 kJ mol−1. The standard enthalpy of formation of Cd2I4(g) was obtained as ΔfH0(298.15 K)=−188.1±6.3 kJ mol−1.
Kinetics of a homopolymer collapse: Beyond the Rouse–Zimm scaling
Authors: Klushin L. I.
Published: July 2002

We present a phenomenological theory describing the self-similar coarsening stage in the collapse of a long flexible homopolymer chain in a dilute solution upon a sudden quench. The approach is based on the “necklace” picture of the collapsing chain being composed of clusters separated by strands as demonstrated by computer simulations. The model represents a special class in the cluster growth problem where aggregation is driven by tension in the connecting strands. The mean cluster size in the free-draining limit is predicted to grow with time as s(t)∼t6/7(1+ν) where ν is the exponent characterizing the initial conformation of the coil. The characteristic collapse time scales as τc∼N1+ν∼N1.6 in agreement with the Langevin dynamics simulations. Incorporation of hydrodynamic effects leads to τc∼N1/3+ν∼N0.93. For a realistic experimental situation the theory presented thus predicts a much faster collapse than suggested by self-consistent field calculations.
Virial expansion of a non-additive hard-sphere mixture
Authors: Saija F. , Fiumara G. , Giaquinta P. V.
Published: July 2002

We have computed the fourth virial coefficient of a binary mixture composed of hard spheres with equal contact diameters for like particles but unequal collision diameter for particles belonging to unlike species. The independent irreducible cluster integrals which contribute to the fourth virial coefficient were evaluated numerically using the Monte Carlo method. The coexistence line relative to the fluid–fluid phase transition, calculated through the truncated virial expansion for small positive values of the non-additivity parameter, is found to be in good agreement with the Gibbs ensemble Monte Carlo results.
A comparison of quasi-harmonic lattice dynamics and Monte Carlo simulation of polymeric crystals using orthorhombic polyethylene
Authors: Rutledge G. C. , Lacks D. J. , Martoňák R. , Binder K.
Published: July 2002

The temperature dependence of lattice parameters, elastic constants and other physical properties of crystalline polyethylene at zero pressure in the orthorhombic phase is discussed. Two complementary approaches, self-consistent quasi-harmonic lattice dynamics and Monte Carlo simulation, both of which are predicated on the use of empirical force fields to describe the interatomic potentials, are critically compared. Both techniques are studied in their classical and quantum mechanical versions, to assess the accuracy and limitations of each method. Particular attention is paid to the classical approximation, the onset of anharmonicities in dynamical behavior which are not captured by the quasi-harmonic approximation, and finite size effects. It is shown that quantum effects are important throughout the range of temperatures 0⩽T⩽300 K. At temperatures below about 23 of the melting temperature (i.e., 250 K for polyethylene) the two approaches yield consistent results in both classical and quantum mechanical cases for a given empirical force field, provided that finite size effects are avoided. Above 300 K, anharmonic effects become quite pronounced. The combined treatment of these effects in the framework of path integral Monte Carlo (PIMC) pushes the limits of current computational feasibility, due to simulation sizes required. Guidelines are offered for choosing between classical simulations, quasi-harmonic methods, and full path integral Monte Carlo simulation.
Transition probabilities in the A 2Σ+−X 2Πi electronic system of OH
Authors: Luque Jorge , Crosley David R.
Published: July 2002

A complete set of experimental vibrational band transition probabilities, including recent measurements on v′=2 and 3, allows us to calculate an empirical electronic transition moment, Re(r), for the A 2Σ+−X 2Πi system of OH, over the range of internuclear distance 0.70–1.70 Å. A comparison with an ab initio moment confirms that theoretical shape and validates its quantitative results, as the calculated Re(r) is within experimental uncertainty of the present result. The experimentally derived Re(r) is used along with Rydberg–Klein–Rees (RKR) wave functions to calculate transition probabilities for a wide range of vibrational and rotational levels. The study is extended to the isotope OD.
Accurate simulation of highly asymmetric electrolytes with charge asymmetry 20:1 and 20:2
Authors: Lobaskin Vladimir , Linse Per
Published: July 2002

Thermodynamic and structural properties of asymmetric electrolytes in aqueous solution have been studied by means of molecular dynamic and Monte Carlo simulations employing two different short-range potentials and the Ewald summation technique for handling the long-range Coulombic interactions. The macroion carried 20 elementary charges and both monovalent and divalent counterions have been considered. Previous accurate results for the 20:1 soft-sphere system, obtained from integral equations and simulations, have been reexamined in view of the present results. Data from extensive simulations covering a wide concentration range of the 20:1 and 20:2 soft-sphere systems have been obtained. The valency of the counterions did not affect the structure of the system in a qualitative manner. In particular, an effective repulsive potential was operating between the macroions even in the 20:2 system at all concentrations. On the contrary, data from simulations of the 20:1 and 20:2 hard-sphere systems indicated a much stronger dependence on the counterion valency. In the case of divalent counterions, the electrostatic repulsion between the macroions was nearly completely screened, but no attractive component arising from the electrostatic interaction could be discerned. Hence, the type of the short-range part of the macroion-counterion potential plays a decisive effect on the long-range structure of the system. Finally, we also discussed the optimization of the Ewald summation and the choice of dielectric boundary conditions applied to asymmetric electrolytes.
The second virial coefficient of hard alkane models
Authors: MacDowell L. G. , Vega C.
Published: July 2002

The second virial coefficient for hard models of alkanes and other flexible molecules is determined numerically using a new algorithm which increases the speed of the calculations by a few orders of magnitude. For alkanes with few carbon atoms, linear and branched chains were considered and the effect of branching was analyzed. For linear hard alkanes, the second virial coefficient was computed for chains with up to 600 carbon atoms and the scaling behavior was studied. The effect of changes in the chemical structure of a flexible molecule (i.e., bond length, bond angle) was also studied. A fast and efficient empirical methodology to estimate the second virial coefficient of hard chains is given. This methodology uses ideas of convex body geometry. It is shown that the proposed methodology yields very good estimates of the second virial coefficient for chains with up to 100 monomer units, but it predicts incorrectly the scaling law. The virial coefficients provided in this work can be useful in the search of an equation of state for hard alkanes, since it is likely that a good equation of state should provide good estimates of the second virial coefficient.
Collision-induced absorption by H2 pairs in the second overtone band at 298 and 77.5 K: Comparison between experimental and theoretical results
Authors: Brodbeck C. , Bouanich J.-P. , Nguyen-van-Thanh  , Fu Y. , Borysow A.
Published: July 2002

The collision-induced spectra of hydrogen in the region of the second overtone at 0.8 μm have been recorded at temperatures of 298 and 77.5 K and for gas densities ranging from 100 to 800 amagats. The spectral profile defined by the absorption coefficient per squared density varies significantly with the density, so that the binary absorption coefficient has been determined by extrapolations to zero density of the measured profiles. Our extrapolated measurements and our recent ab initio quantum calculation are in relatively good agreement with one another. Taking into account the very weak absorption of the second overtone band, the agreement is, however, not as good as it has become (our) standard for strong bands.
A molecular Ornstein–Zernike study of popular models for water and methanol
Authors: Richardi J. , Millot C. , Fries P. H.
Published: July 2002

Some effective models of water (TIP3P, SPC, SPC/E, TIP4P) and methanol (OPLS, H1) are studied with the help of the molecular Ornstein–Zernike (MOZ) theory using the hypernetted chain (HNC) approximation. The quality of the results obtained within the HNC approximation is discussed by comparison with values from molecular dynamics (MD) simulations. The MOZ-HNC theory yields internal excess energies and dielectric constants which are about 20% smaller than the simulation results found in the literature. The relative trends of the properties observed by simulation for the different interaction models are correctly predicted. In order to calculate the rotational invariant coefficients which define the liquid structure, new MD simulations were carried out. The rotational invariant coefficients derived from the simulation and from the MOZ theory strongly differ. In particular, the center–center distribution functions show that the theory is not able to reproduce the tetrahedral structure of water. In this solvent a comparison of the O–H distribution function indicates that the MOZ theory underestimates the H bonding. The use of a spherically symmetric bridge function in the reference HNC approximation does not lead to an improvement of the MOZ results. The observed defects of the MOZ-HNC approach contrast with the good agreement found for aprotic solvents and are presumably due to the association by H bonds in water and methanol.
Infrared spectrum of CCH+ in solid argon and neon
Authors: Andrews Lester , Kushto Gary P. , Zhou Mingfei , Willson Stephen P. , Souter Philip F.
Published: July 2002

Laser-ablation of over ten different transition, lanthanide, and actinide metals with concurrent codeposition of acetylene/argon samples at 7 K produced metal independent absorptions for CCH, CCH−, C4H, and C4H2, in agreement with previous matrix isolation work, and a sharp new 1820.4 cm−1 band. Isotopic substitution showed this band to be due to a largely C–C stretching mode of a species with one H and two inequivalent carbon atoms. The same species were observed in solid neon samples at 4 K, and the neon matrix counterpart of the new band was found at 1832.2 cm−1. When CO2 was added to serve as an electron trap, the yield of CCH− at 1772.8 cm−1 decreased and the 1832.2 cm−1 band increased relative to CCH at 1837.9, 1835.0 cm−1. Quantum chemical calculations at the coupled-cluster and density functional levels predict the C–C stretching mode of CCH+ between this mode for CCH and CCH− and support assignment of this new infrared absorption to the CCH+ cation in solid argon and neon.
Photolysis experiments on SiC mixed clusters: From silicon carbide clusters to silicon-doped fullerenes
Authors: Pellarin M. , Ray C. , Lermé J. , Vialle J. L. , Broyer M. , Blase X. , Kéghélian P. , Mélinon P. , Perez A.
Published: July 2002

Silicon carbon binary clusters are generated in a laser vaporization source from SixC1−x mixed targets (x=0 to 50%). We have first analyzed stoichiometric (SiC)n (n⩽40) clusters grown from a silicon carbide target (x=50%). Both high fluence photoionization of (SiC)n neutral clusters and photofragmentation of size-selected (SiC)n+ natural positive ions show that silicon-doped fullerenes emerge as stable photoproducts through the laser induced annealing of these clusters. They are detected as stable species as soon as a sufficient amount of silicon is eliminated through unimolecular processes involving the sequential losses of Si2C and Si3C neutral molecules in the earliest evaporation steps. This result is in favor of an efficient substitution of silicon atoms (about 12) into stable “cagelike” carbon networks. We will also show that an efficient doping of carbon fullerenes with silicon atoms can be obtained in carbon-rich mixed clusters directly grown as positive ions from nonstoichiometric targets (x<25%). Mass abundance spectroscopy gives a clear signature of cagelike structures where silicon atoms are substituted for carbon ones. The results on the favored stability of even-numbered C2n−qSiq+ clusters with q=0, 1, 2 are presented here in the size range: 2n=32–80. More largely doped species (q⩾3) cannot be evidenced in abundance mass spectroscopy because of unavoidable mass coincidences. A careful analysis of the photofragmentation behavior of selected sizes relative to the laser fluence nevertheless succeeds in indicating the contribution to the photofragmentation spectra of largely doped heterofullerenes C2n−qSiq+ (q=7 at least) that mainly dissociate by the loss of small even-numbered mixed molecules such as Si2,Si3C,… . Both approaches are consistent with the surprising capability of substituting a large number of silicon atoms into fullerenes without destabilizing their cage structure too much. In this respect, a value close to 12 seems to be an upper limit.
Controlling the self-assembly and dynamic response of star polymers by selective telechelic functionalization
Authors: Vlassopoulos Dimitris , Pakula Tadeusz , Fytas George , Pitsikalis Marinos , Hadjichristidis Nikos
Published: July 2002

We report on the structure and dynamics of model mono-, di-, and tri-ω-functionalized three-arm star polybutadiene melts. By using x-ray scattering and dynamic rheological measurements, we find that functionalization of the arm ends can lead to distinctly different supramolecular structures and material behavior. The monofunctionalized samples behave like multiarm nonionic star-like dendrimers, whereas the difunctional stars resemble a transient network consisting of highly branched structures with a very broad relaxation spectrum. On the other hand, the trifunctional stars seem to develop an unusually regular structure of dominant intramolecularly aggregated functional groups leading to collapsed star conformations, resembling soft spheres, and a well-defined terminal relaxation. These results suggest that by tailoring the telechelic functionalization of regular star polymers, a route to design and obtain a wealth of controlled supramolecular structures exhibiting a rich and variable dynamics could open.
Embedded atom model calculations of the diffusion coefficient of Ni impurity in liquid Al
Authors: Alemany M. M. G. , Rey C. , Gallego L. J.
Published: July 2002

Using the embedded atom model potential for solid Ni–Al systems proposed by Voter and Chen, we performed molecular dynamics simulations to compute the diffusion coefficient of Ni impurity in liquid Al. Our results are in excellent agreement with available experimental data.
Microwave and millimeter wave study of Ortho-N2 states of CO–N2
Authors: Xu Yunjie , Jäger Wolfgang , Surin L. A. , Pak I. , Panfilov L. A. , Winnewisser G.
Published: July 2002

Microwave and millimeter wave transitions of the CO–N2 complex were investigated using three different instruments, namely, a pulsed molecular beam Fourier transform microwave spectrometer in the frequency region from 4 to 26 GHz, a microwave-millimeter wave double resonance spectrometer in the frequency regions from 8 to 18 GHz for the microwave and 107–118 GHz for the millimeter wave range, and an OROTRON spectrometer in the frequency range from 107 to 132 GHz. Both a- and b-type transitions associated with the ground-state K=0 levels and the lower K=1 levels of the ortho-N2 states, and with rotational quantum number J up to 19, were measured and analyzed. Nuclear quadrupole hyperfine splittings due to the presence of two equivalent N14 nuclei were resolved and analyzed to give additional information about the angular anisotropy of the interaction potential. The nuclear quadrupole coupling constants obtained are χaa=0.196 41(52) MHz for K=0 levels, and χaa=−1.0391(17) MHz, χbb=0.0633(17) MHz for the lower K=1 levels, respectively. The drastic difference between these two sets of coupling constants suggests that the orientation and motion of the N2 subunit are very different in these two states, and that the complex cannot be adequately described by a semirigid rotor model. In addition, measurements of the rotational spectra of the two new isotopomers containing C13O16 and C13O18 subunits provide further important information about the CO–N2 interaction potential.
Network domain structure in phase-separating polymer solutions
Authors: Liu Hong , Bhattacharya Aniket , Chakrabarti Amitabha
Published: July 2002

Results from molecular dynamics simulations of phase separation in a quenched polymer solution are presented for both two- and three-dimensional systems. Simulations clearly demonstrate the existence of a networklike pattern formation at an early to intermediate time regime for sufficiently dense polymer solutions. This network structure, however, relaxes at late times as the minimization of the interface energy wins over the elastic stress of the network. A crossover of domain growth exponent from a smaller value at intermediate times to the Lifshitz–Slyozov value of 1/3 at late times is seen in three-dimensional (3D) simulations of network breakup. Our results for the late-time growth exponent and dynamical scaling of the structure factors strongly indicate that the late-time growth kinetics of quenched polymer solutions belong to the same universality class of small molecular mixtures.
Studies of local structure of Cm3+ in borosilicate glass using laser and x-ray spectroscopic methods and computational modeling
Authors: Liu G. K. , Zhorin V. V. , Antonio M. R. , Li S. T. , Williams C. W. , Soderholm L.
Published: July 2002

The local environment of Cm3+ in a borosilicate glass has been probed by a combination of laser spectroscopy, structural modeling, and extended x-ray absorption fine structure (EXAFS) spectroscopy. The Stark splitting for the Cm f–f state transitions is significantly larger than the inhomogeneous line broadening that results from the disordered environment. As a result, the Cm optical spectrum can be fit using an effective operator Hamiltonian to obtain a set of crystal-field parameters. The fitting procedure, which requires the use of a descent-in-symmetry approach, provides a set of parameters for a best fit within tetragonal symmetry. These parameters are then linked to the local environment of Cm through exchange-charge modeling (ECM) of crystal field interactions. Cm in our borosilicate glass is best modeled with six oxygen ions with approximately tetragonal symmetry, and at an average distance of 2.31 (3) Å. The results of crystal-field modeling are supported by EXAFS results.
Friction and mobility for colloidal spheres in Stokes flow near a boundary: The multipole method and applications
Authors: Cichocki B. , Jones R. B. , Kutteh Ramzi , Wajnryb E.
Published: July 2002

We obtain the many-body hydrodynamic friction and mobility matrices describing the motion in a fluid of N hard-spheres with stick boundary conditions in the presence of a planar hard wall or free surface using (1) a multipole expansion of the hydrodynamic force densities induced on the spheres and (2) an image representation to account for the fluid boundary. The coupled multipole equations may be truncated at any order to give positive definite approximations to the exact friction and mobility matrices. An extension of the Bossis–Brady lubrication correction to the friction matrix is also discussed and included. The resulting method for computing the mobility matrix may be used for the Stokesian or Brownian dynamics simulation of N spheres subject to interparticle and external forces and imposed shear flow. We illustrate the method by performing Stokesian dynamics simulation of particles near a hard wall. The simulations exhibit the rapid convergence of the multipole truncation scheme including lubrication corrections.
Femtosecond fluorescence upconversion studies of barrierless bond twisting of auramine in solution
Authors: van der Meer M. J. , Zhang H. , Glasbeek M.
Published: July 2002

Femtosecond fluorescence upconversion studies have been performed for auramine (a diphenylmethane dye), dissolved in ethanol, as a function of temperature. It is found that the (sub)picosecond decay components in the fluorescence slow down as the temperature is lowered from 293 K to 173 K. From the observation of a residual fluorescence, with a viscosity-dependent lifetime of about 30 ps (or longer at higher viscosity), and transient absorption results it is concluded that the two-state sink function model [B. Bagchi, G. R. Fleming, and D. W. Oxtoby, J. Chem. Phys. 78, 7375 (1983)] does not apply in the case of auramine. Comparison of the auramine fluorescence kinetics in ethanol and decanol shows that diffusional twisting and not solvation is the main cause for the (sub)picosecond excited state relaxation. To explain the experimental results, adiabatic coupling between a locally excited emissive state (F) and a nonemissive excited state (D) is considered. Torsional diffusion motions of the phenyl groups in the auramine molecule are held responsible for the population relaxation along the adiabatic potential of the mixed state, S1 (comprised of the F and D states). Simulation of the excited state dynamics is feasible assuming a barrierless-shaped potential energy for S1 and applying the Smoluchowski diffusion equation. The temporal behavior of the auramine band emission was simulated for the temperature range 293 K >T>173 K, with the temperature, T, and the viscosity coefficient, η, being the only variable parameters. The simulated temporal behavior of the emission in the investigated temperature range is compatible with that obtained experimentally. The rotational diffusion coefficient for the auramine phenyl groups as extracted from the simulations is found to follow the Einstein–Stokes relation. From the numerical calculations the effective radius of the twisting phenyl groups is determined as 1.0 Å which compares well with the actual value of 1.2 Å.
Low-energy electron-energy-loss spectroscopy of condensed acetone: Electronic transitions and resonance-enhanced vibrational excitations
Authors: Lepage M. , Michaud M. , Sanche L.
Published: July 2002

We report electron-energy-loss spectroscopy, within the incident electron energy range 1 to 19 eV, of solid films of acetone condensed at 18 K. The strong Rydberg progressions, which usually dominate the spectra in the gas phase, are found to completely disappear in the solid phase. In the absence of these transitions, the remaining broad bands centered at 4.3, 4.5, 6.2, 8.7, and 9.8 eV energy loss can be assigned to the 1 3A2(n→π*), 1 1A2(n→π*), 1 3A1(π→π*), 1 3B1(σ→π*), and 2 3A2(σ→π*) valence electronic transition of acetone, respectively. A broad feature ranging from 11 to 16 eV and having a maximum around 13.8 eV is ascribed to several overlapping autoionizing excited states. From a comparison with infrared and Raman spectra, the energy-loss peaks observed below 1 eV are found to be due to excitation of the fundamental, overtone, and combination vibrational modes of the molecule. Their incident energy dependence is showing broad vibrational enhancement maxima at 4, 7, and 9 eV, which are attributed to the formation of single-particle or shape resonances of 2B1, 2A1, and 2A2 (or 2B2) symmetries, respectively.
Light scattering study of the liquid–glass transition of meta-toluidine
Authors: Aouadi A. , Dreyfus C. , Massot M. , Pick R. M. , Berger T. , Steffen W. , Patkowski A. , Alba-Simionesco C.
Published: July 2002

An experimental study of the glass transition of meta-toluidine combining several light scattering techniques was performed. The structural relaxation time is measured in depolarized geometry from the glass transition temperature up to well above the melting point and found to vary over 13 time decades. An analysis by means of the idealized Mode Coupling Theory shows that, as found in other aromatic liquids, experimental results obtained in depolarized light scattering can be described by this theory above Tc in a two-decade frequency range. The polarized Brillouin doublet, measured in the backscattering geometry between 176 K and 300 K, is also analyzed. None of the sets of parameters we obtained in fitting those spectra could fulfil all the requirements of this Mode Coupling Theory.
Configurational and vibrational entropies and molecular relaxation in supercooled water
Authors: Johari G. P.
Published: July 2002

In order to investigate whether the anomalous decrease in the net entropy of water on supercooling indicates a structural change, its entropy and relaxation time data have been examined by equating the Adam-Gibbs expression with the Vogel-Fulcher-Tamman equation. This gave values of the minimum size of the cooperatively rearranging region as 4.7 molecules at 150 K, and the temperature-invariant energy as 7.42 kJ mol−1. On the premise that a liquid’s configurational entropy, Sconf, differs from its excess entropy over the ordered crystal state, Sconf of water has been estimated over the 150–273 K range by using the available value of its excess entropy at ∼150 K. Water’s Sconf at 273 K is found to be less than half of its entropy of fusion and to further decrease continuously on supercooling. This puts into question the conjecture that water structurally transforms near 228 K, as deduced by (wrongly) assuming that water’s configurational entropy is equal to its excess entropy. The analysis also indicates that the vibrational entropy of supercooled water, Svib, becomes less than the calorimetric entropy of hexagonal ice at T<193 K, which is seen as a reflection of the relatively tighter and strained intermolecular H-bonding in water than in hexagonal ice. This is supported by the known higher frequency of translational modes in water than in hexagonal ice. The ratio of Sconf to Svib for water at 273 K is 0.19, which is comparable with the corresponding ratio determined here for other supercooled liquids.
Macroscopic pressure–volume–temperature properties versus free-volume characteristics of isotropic pressure-densified amorphous polymer glasses
Authors: Schmidt Marcus , Olsson Mattias , Maurer Frans H. J.
Published: July 2002

We made a series of isotropic pressure-densified (0–200 MPa) amorphous homopolymer [atactic polystyrene (a-PS)] and copolymer [poly(styrene-co-acrylonitrile) (SAN) and poly(styreneco-maleic anhydride) (SMA)] glasses and studied their macroscopic pressure–volume–temperature (PVT) properties vs their free-volume characteristics from the Simha–Somcynsky equation-of-state (EOS) theory and from positron annihilation lifetime spectroscopy (PALS). The glass densities lie in the range of 1.0403–1.0535 g/cm3 (PS), 1.0573–1.0759 g/cm3 (SAN), and 1.0989–1.1196 g/cm3 (SMA). With increasing formation pressure, all pressure-densified glasses exhibit decreasing volume and free-volume characteristics such that the changes in specific volume (1.26%–1.85%) are <ortho-positronium (o-Ps) lifetime τ3 (5.5%–9.1%) <free-volume hole size V(τ3) (10.3%–17.1%) <free-volume fraction h (25.1%–30.5%). We find, furthermore, that the o-Ps formation probability I3 is independent of formation pressure. Likewise, the glasses’ thermal expansivity α0 remains constant. There is a one-to-one correlation of τ3 and of I3 when calculated from POSITRONFIT and the maximum entropy lifetime (MELT) program. The full width at half maximum of free-volume distributions determined from PALS data and MELT does not change with formation pressure within established limits of uncertainty. Neither h vs V(τ3) nor h vs bulk modulus K, calculated from the Tait EOS of the glasses, show unique relations that are common to all the studied glasses. On the other hand, K vs V(τ3) gives a universal curve for all pressure-densified glasses from this study and from our previous study on PMMA: K=8.190–4.479×10−2V(τ3) (r2=0.92). Moreover, three more polymers from the literature are well described by this curve. The bulk moduli increase by up to 11.5% (PMMA), 7.6% (PS), 11.2% (SAN), and 10.2% (SMA) and they follow the order PS<SMA<SAN<PMMA.
Collective diffusion in a two-dimensional Brownian fluid
Authors: Acuña-Campa Heriberto , Medina-Noyola Magdaleno
Published: July 2002

The collective dynamics of a simple model colloidal suspension, namely, a two-dimensional Brownian Yukawa fluid, is studied by Brownian dynamics simulations without hydrodynamic interactions. The results for the intermediate scattering function F(k,t) are employed to assess the quantitative and qualitative accuracy of the single-exponential memory function (SEXP) approximation. We find that, contrary to self-diffusion dynamics, the additional, accesorial approximation of neglecting the three-body correlations in the third moment condition, leads to unphysical predictions of the SEXP approximation. We also show that these spurious features are readily removed by including the contribution of the three-particle distribution function to the third moment of F(k,t), even in a simple approximate manner implemented in this work.