bluuebunny/crossref_metadata_2025 huggingface dataset:
State-correlated two-dimensional NMR spectroscopy: Separation of local dipolar fields of protons in nematic phase of 4′-methoxybenzylidene-4-acetoxyaniline
Authors: Naito Akira , Imanari Mamoru , Akasaka Kazuyuki
Published: July 2002

A novel two-dimensional NMR experiment, state-correlated two-dimensional NMR (SC-2D NMR) spectroscopy, was successfully performed between the nematic phase and the isotropic phase of 4′-methoxybenzylidene-4-acetoxyaniline (APAPA) by using pulsed microwave heating. In this experiment, local dipolar fields of the individual protons in the nematic phase were well separated as cross sections in the F1 dimension of the SC-2D NMR spectrum. The cross sections for the aromatic protons showed main doublet patterns with a common splitting of 9.7 kHz, giving an order parameter of 0.41 for the aromatic rings of APAPA. The cross section for the methyl protons of the methoxy group clearly showed a triplet pattern with a splitting of 10.2 kHz, while the cross section for the methyl protons of the acetoxy group showed an unresolved singlet with small shoulders. Computer simulation, based on the evolution of the density matrix operator, taking the order of the ring and the local dipolar couplings of the methyl protons into account, reproduced well the triplet pattern of the methoxy group. It also showed that the difference in the splitting between the methyl and methoxy groups originates from the difference in the orientation of the two methyl axes with respect to the molecular axis. The time dependence of spectral mixing between cross sectional patterns for individual protons of APAPA was studied as a function of the mixing time introduced before the microwave pulse, and was reproduced well for the methoxy protons by a Solomon type equation that takes into account exchange of magnetizations due to dipolar interactions in the liquid crystalline phase.
Simple relationships between statistical mechanics expectation values
Authors: Hamad Esam Z.
Published: July 2002

A general procedure for obtaining exact equations between statistical mechanics expectation values is suggested. The procedure is illustrated for quantum and classical systems in a generalized ensemble. Detailed applications are given for the Stockmayer potential in classical statistical thermodynamics. Application to site–site interactions is also discussed. The resulting equations can be used to detect inconsistencies in approximate theories. This is illustrated for the Percus–Yevick equations for hard chain molecules. For a complete description of equilibrium properties, additional information is needed to supplement the derived equations.
Monte Carlo simulation of the CO+NO reaction
Authors: Kortlüke Olaf , von Niessen Wolfgang
Published: July 2002

In this paper a simple lattice gas model for the catalytic CO+NO→CO2+1/2N2 surface reaction is studied by means of Monte Carlo simulations. In the simulations we take into consideration the diffusion of all particles and the desorption of CO. The square and triangular lattices are used to model the surface of an ideal catalyst, whereas percolation clusters, diffusion limited aggregates (DLA), the Sierpinski carpet, and the Sierpinski gasket are introduced as better models for supported catalysts. In many simulations an interval of the gas phase concentration yCO∈[y1,y2] exists, where the simulation remains in a reactive state. The kinetic phase transitions of second (y1) and first-order (y2) which appear in this reaction system are studied as a function of the system parameters. The above interval is broadened with increasing diffusion of the adsorbed particles. Desorption of CO results in a shift of y2 to yCO=1. On the Sierpinski lattices only very small reactive intervals appear, whereas on DLA and percolation clusters no reaction occurs. Estimates for the order parameter β of the second order phase transition from an active into one (out of infinitely many) absorbing states show that the model belongs to the universality class of directed percolation. The very strong influence of the lattice structure on the reaction system is dicussed in detail.
Dynamics of a suspension of spheres and rigid polymers: Effect of geometrical mismatch
Authors: Mondescu Radu P. , Muthukumar M.
Published: July 2002

An effective medium approach together with a multiple scattering formalism is considered to study the steady-state dynamics of suspensions of spheres and polymer chains without excluded volume interactions. The polymer chains are assumed to obey rigid-body dynamics (without rotation) and are taken to be so long that Gaussian statistics is applicable. We have considered the dynamics of probe objects (either a sphere or a polymer) in a solution containing spheres and polymers. The significance of the dimensionless variables appearing when solving the effective medium equations and, in particular, the role of the geometrical parameter t=Rg/a (a is the radius of any sphere and Rg the radius of gyration of a polymer chain) are discussed. The translational diffusion coefficients of the moving probe sphere DS and of the center-of-mass of the probe polymer chain DP, and the shear viscosity of the suspensions have been derived. Both the friction coefficient of the probe sphere or that of the probe polymer chain and the shear viscosity diverge as ΦPOL→0.31 in the presence of polymers in the solution. Also, when polymers are diffusing in a suspension of fixed spheres an optimum range of ΦSP that maximizes the difference in the diffusion coefficients of polymer chains characterized by distinct t values has been noticed.
Microwave spectrum, large-amplitude motions, and ab initio calculations for N2O5
Authors: Grabow J.-U. , Andrews A. M. , Fraser G. T. , Irikura K. K. , Suenram R. D. , Lovas F. J. , Lafferty W. J. , Domenech J. L.
Published: July 2002

The rotational spectrum of dinitrogen pentoxide (N2O5) has been investigated between 8 to 25 GHz at a rotational temperature of ∼2.5 K using a pulsed-molecular-beam Fourier-transform microwave spectrometer. Two weak b-dipole spectra are observed for two internal-rotor states of the molecule, with each spectrum poorly characterized by an asymmetric-rotor Hamiltonian. The observation of only b-type transitions is consistent with the earlier electron-diffraction results of McClelland et al. [J. Am. Chem. Soc. 105, 3789 (1983)] which give a C2 symmetry molecule with the b inertial axis coincident with the C2 axis. Analysis of the 14N nuclear hyperfine structure demonstrates that the two nitrogen nuclei occupy either structurally equivalent positions or are interchanging inequivalent structural positions via tunneling or internal rotation at a rate larger than ∼1 MHz. For the two internal rotor states, rotational levels with Ka+Kc even have IN=0, 2, while levels with Ka+Kc odd have IN=1, where IN is the resultant nitrogen nuclear spin. This observation establishes that the equilibrium configuration of the molecule has a twofold axis of symmetry. Guided by ab initio and dynamical calculations which show a planar configuration is energetically unfavorable, we assign the spectrum to the symmetric and antisymmetric tunneling states of a C2 symmetry N2O5 with internal rotation tunneling of the two NO2 groups via a geared rotation about their respective C2 axes. Because of the Bose–Einstein statistics of the spin-zero oxygen nuclei, which require that the rotational–vibrational–tunneling wave functions be symmetric for interchange of the O nuclei, only four of the ten vibrational-rotational-tunneling states of the molecule have nonzero statistical weights. Model dynamical calculations suggest that the internal-rotation potential is significantly more isotropic than implied by the electron-diffraction analysis.
Structure of CO monolayer adsorbed on NaCl(100) from molecular dynamics
Authors: Hoang P. N. M. , Picaud S. , Girardet C. , Meredith A. W.
Published: July 2002

Classical molecular dynamics calculations are performed between 25 and 55 K to investigate the structure and the dynamics of the CO monolayer adsorbed on NaCl(100) surface. The simulation is based on the most recent potential available for this system and the results are compared with experimental data issued from polarization infrared spectroscopy and helium atom scattering. We show that the monolayer adopts a (2×1) structure at 25 K, with two molecules per unit cell. The corresponding angular motions perpendicular and parallel to the surface are clearly librational. As temperature rises, the molecular axes remain tilted with a librational motion perpendicular to the surface, while the translational and orientational motions parallel to the surface behave much more freely, leading to an orientationally disordered structure at 55 K. These features suggest that dynamical effects in the (2×1) layer are responsible for an ordered/disordered transition observed as temperature increases, instead of the phase transition from ordered (2×1) to (1×1) structures which was previously inferred.
Global three-dimensional potential energy surfaces of H2S from the ab initio effective valence shell Hamiltonian method
Authors: Stevens Jonathan E. , Chaudhuri Rajat K. , Freed Karl F.
Published: July 2002

The correlated, size extensive ab initio effective valence shell Hamiltonian (HV) method is used to compute three-dimensional potential energy surfaces for the ground and several excited electronic states of the H2S molecule. A single calculation of the HV simultaneously generates all states of interest as well as ionization potentials. Particular emphasis is placed on the two lowest 1 1A″ excited surfaces (one valencelike and the other Rydberg-type) that are involved in recent experiments probing nonadiabatic photodissociation processes. Supplementary effective operator calculations generate three-dimensional surfaces of dipole moments and transition dipole matrix elements, but emphasis is placed on the transition dipoles relevant to the dissociation process. Comparisons to both experiment and previous calculations for this system support the ability of multireference perturbation methods to describe global potential energy surfaces for open shell systems. We discuss the implication of our calculations for interpreting and reproducing experimental observations of the dissociation dynamics.
Fractal Brownian motion and polymers in external fields
Authors: Washington Gary E.
Published: July 2002

Polymer behavior in external fields is modeled using fractal Brownian motion. The chain dimensions of polymer chains with and without excluded volume show a contour length dependence of Nv where v=h−1. Chains with excluded volume have h=3/5, chains without excluded volume, Gaussian chains have h=1/2, and for chains in the collapsed state h=1/3. This nonphysical result points to the distinction between fractals as models of the trajectory or path of a polymer that are self-similar on all length scales and real polymers that possess a lower and upper limit for self-similarity.
O2 photoabsorption in the 40 950–41 300 cm−1 region: New Herzberg bands, new absorption lines, and improved spectroscopic data
Authors: Slanger T. G. , Huestis D. L. , Cosby P. C. , Naus H. , Meijer G.
Published: July 2002

The technique of cavity ring-down (CRD) spectroscopy is particularly useful for measuring absorptions of very weak optical transitions. We have in this manner investigated the 40 950–41 300 cm−1 region in O2, where only absorption in the O2(A 3Σ+u–X 3Σ−g) 11-0 band had been previously identified. Five new bands have been discovered in this range—the A′ 3Δu–X 3Σ−g 12-0 and 13-0 bands, the c 1Σ−u–X 3Σ−g 17-0 and 18-0 bands, and the A 3Σ+u−X 3Σ−g 12-0 band. The origins of the F1 and F2 components of the latter lie only 7 cm−1 below the lowest dissociation limit, and 15 lines have been identified. No F3 levels were observed; apparently all are above the dissociation limit. The high instrumental sensitivity of the CRD technique has allowed observation of weak lines of the A–X 11-0 band, and 12 of the 13 branches have been identified and their intensities measured. A very low upper limit has been set on the intensity of the thirteenth branch, Q13. We find 107 unidentified lines in the region, the stronger ones (19) lying in the vicinity of lines of the A–X 11-0 band. The weaker ones (88) are spread throughout the spectral region, up to and even beyond the O2 dissociation limit, and probably have their origin in transitions to very weakly bound O2 states, which may have atmospheric significance. These weaker lines have intensities that are typically 1%–5% of the strong A–X 11-0 band lines.
Contact pair correlation functions and equation of state for nonadditive hard-sphere mixtures
Authors: Hamad Esam Z.
Published: July 2002

We present some exact results for mixtures of nonadditive hard spheres and use them to derive two models for the contact values of the pair correlation functions and the equation of state. The new models are general in that they are not limited to binary mixtures, equal size components, or a certain range of the nonadditivity parameter. Comparison with the available computer simulation data for equal size components show that the new models predict satisfactory the equation of state data and the contact values. The prediction of the critical point of fluid–fluid phase separation is satisfactory at small and large values of the nonadditivity parameter, and less so at intermediate values.
Photoconduction in oligo-para-phenylenevinylene films
Authors: Barth S. , Bässler H. , Wehrmeister T. , Müllen K.
Published: July 2002

Steady state photoconduction was studied in optically thin spin coated and optically thick solution grown films of an oligo-para-phenylenevinylene sandwiched between indium tin oxide (ITO) and aluminum (Al) contacts. Oligomers represent model systems for polyphenylenevinylene that do not contain structural irregularities and impurities always present in polymers. Upon irradiation through the positively biased ITO contact, dissociation of excited states via electron transfer to the ITO prevails. It leads to a field and temperature dependent photocurrent that is symbatic with the absorption spectrum. The efficiency of this process increases with excess energy of the primarily excited state. Photocurrents due to bulk photoionization are observed upon irradiating through the positively biased Al contact. They result from temperature and field assisted dissociation of geminate e–h pairs generated either by field induced break-up of molecular singlet states or by direct charge transfer transition.
Ultraslow nonequilibrium dynamics in supercooled glycerol by stimulated Brillouin gain spectroscopy
Authors: Miller Rebecca S. , MacPhail Richard A.
Published: July 2002

We have used high resolution stimulated Brillouin gain spectroscopy to monitor the nonequilibrium relaxation dynamics of supercooled glycerol near its glass transition temperature. After a temperature quench from 192.4 to 179.8 K, the Brillouin frequency shift increased over a period of 17 d from 16.877±0.005 GHz to 17.195±0.005 GHz, the equilibrium value expected for the final temperature. The relaxation of the shift appears to proceed in three distinct stages. The first and most rapid stage tracks the time dependence of the temperature. This is followed by a slower relaxation process which is well characterized by a stretched exponential function exp[−(t/τ)β]. The values determined for τ (12±2 ks) and β (0.61±0.09) are consistent with values predicted from recent dielectric and enthalpy relaxation studies of glycerol in the same temperature range, as long as allowances are made for the nonlinear nature of the nonequilibrium experiment. The third and final stage in the relaxation is approximately exponential with a time constant of 430±40 ks. We speculate that this ultraslow process may have its origin in the relaxation of mechanical strain built up in the sample during the temperature quench, or that it could result from the nonlinear nature of the experiment and the heterogeneous nature of dynamics in supercooled liquids. A comparison with other methods (dilatometry, probe fluorescence) for studying nonequilibrium relaxation in supercooled liquids shows our technique to be comparable in sensitivity. Finally, the Brillouin linewidth showed no change after an initial rapid decrease to 18±2 MHz (half width at half maximum), indicating a lack of significant spatial heterogeneity in the physical properties and/or relaxation dynamics on length scales of the order of the acoustic wavelength (0.2 μm) or longer.
Monte Carlo simulation of polymer chain collapse in an athermal solvent
Authors: Suen Jason K. C. , Escobedo Fernando A. , de Pablo Juan J.
Published: July 2002

Results of molecular simulations are presented for the entropy-driven collapse of a single chain in an athermal solvent. The mean square radius of gyration (〈Rg2〉) is calculated for a flexible chain of length N immersed in a hard-sphere medium having a specified solvent chemical potential. For a ratio of chain bead diameter to solvent diameter of 0.2 and packing fraction η=0.44, we find that 〈Rg2〉∝N2ν with ν∼0.36 for N⩾100; such exponent is consistent with that predicted by renormalization group arguments for the poor solvent regime (ν=1/3). The effects of chain length and density on 〈Rg2〉 are consistent with the predictions of the Edwards–Muthukumar theory for a chain in a quenched, random medium.
An ab initio study of CrC: A comparison of different levels of theory including density functional methods
Authors: Maclagan Robert G. A. R. , Scuseria Gustavo E.
Published: July 2002

A comparison is made of various ab initio methods with density functional models for the Σ-3, Σ-5, Σ-7 and Σ-9 states of CrC. The calculations confirm that the ground state of CrC is the Σ-3 state. The bond length in the Σ-3 state is 1.68 Å and the harmonic vibrational frequency is about 675 cm−1. The hybrid density functional (B3LYP) calculations are in good agreement with the higher level multireference configuration interaction calculations.
A collaborative theoretical and experimental study of the structure and electronic excitation spectrum of the Bar and Bar2 complexes
Authors: Alexander Millard H. , Walton Andrew R. , Yang Moonbong , Yang Xin , Hwang Eunsook , Dagdigian Paul J.
Published: July 2002

We report the investigation of the 3s←2p transition in the Bar2 cluster. In a supersonic expansion of B atoms entrained in Ar, at high beam source backing pressures we observe several features in the fluorescence excitation spectrum which cannot be assigned to the Bar diatom. Using Bar(X,B) potential energy curves which reproduce our experimental observations on this molecule and an Ar–Ar interaction potential, we employ a pairwise additive model, along with variational and diffusion Monte Carlo treatments of the nuclear motion, to determine the lowest vibrational state of the Bar2 cluster. A subsequent simulation of the fluorescence excitation spectrum reproduces nearly quantitatively the strongest feature in our experimental spectrum not assignable to Bar. Because of the barrier in the Bar(B 2Σ+) potential energy curve, the 3s←2p transition in the Bar2 is predicted to have an asymmetric profile, as is found experimentally.
Inelastic collisions of fine structure and Λ-doublet resolved rotational states of PH(A 3Π, v=0) with helium
Authors: Neitsch L. , Stuhl F. , Dagdigian Paul J. , Alexander Millard H.
Published: July 2002

A state-resolved experimental and theoretical investigation of inelastic collisions of PH(A 3Π) rotational/fine-structure levels with helium is presented. Experimental state-to-state rate constants were extracted from PH A→X fluorescence spectra recorded upon laser excitation to individual levels in the A state in the presence of helium. These rate constants have been compared with and interpreted through theoretical rate constants computed with full quantum close-coupling and coupled-states calculations of cross sections based on ab initio PH(A)–He potential energy surfaces determined by Kolczewski et al. [J. Chem. Phys. 106, 7637 (1997)]. The low-J rotational levels of PH(A) are close to the Hund’s case (a) limit, and the cross sections exhibit a number of the predicted case (a) collisional propensities. For low initial N, there is a strong propensity for fine-structure conserving transitions. With increasing initial N, fine-structure changing transitions occur with increasing probability. This tendency appears to be related to the transition of the coupling scheme away from pure case (a).
Evidence for heavy atom large amplitude motions in RG-cyclopropane van der Waals complexes (RG=Ne, Ar, Kr) from rotation-tunneling spectroscopy
Authors: Xu Yunjie , Jäger Wolfgang
Published: July 2002

Rotation-tunneling spectra of the van der Waals complex Ne-cyclopropane, and pure rotational spectra of Ar-cyclopropane and Kr-cyclopropane, were measured with a Fourier transform microwave spectrometer in the frequency range from 4–19 GHz. The observed transitions are all of a-type and are in accord with those of prolate symmetric top molecules where the rare gas atom is positioned on the C3 axis of the cyclopropane subunit above its symmetry plane. Altogether transitions of five isotopomers of the Ne complex with J values ranging from 0 to 4 and K values from 0 to 1 were measured. Three isotopomers of the Ar complex (J from 1 to 5 and K from 0 to 2) and nine isotopomers of the Kr complex (J from 1 to 8 and K from 0 to 2) were observed. Effective separations between the center-of-mass of cyclopropane and the rare gas atoms were obtained from the ground state rotational constants and were found to be 3.673, 3.802, and 3.906 Å for the Ne-, Ar-, and Kr-containing complexes, respectively. The derived centrifugal distortion constants indicate that the complexes are comparatively rigid in the radial coordinate. Transitions of two isomers were observed for complexes that contain monodeuterated cyclopropane. The intensities of transitions of the D-bonded species were considerably higher compared to those of the H-bonded isomers indicating that the angular motion of the rare gas atom is much less restricted. In the case of the parent Ne-cyclopropane complex, and its Ne22- and mono C13-isotopomers, all transitions appeared as doublets. This is attributed to a tunneling internal rotation motion of the cyclopropane unit within the complex.
The symmetrized quantum thermal flux operator
Authors: Pollak Eli
Published: July 2002

Analysis of the symmetrized thermal flux operator leads to explicit expressions for its eigenvalues and eigenfunctions. At any point in configuration space one finds two nonzero eigenvalues of opposite sign. The associated eigenfunctions are L2 integrable. The eigenfunctions and eigenvalues are expressed in terms of the thermal density matrix in the vicinity of the transition state. The positive eigenvalue of the thermal flux operator gives an upper bound to the rate and allows for a formulation of a quantum mechanical variational transition state theory. This new upper bound, though, is only a slight improvement over previous theories.
A geometrically-based mean-field theory of polydisperse hard-sphere mixtures
Authors: Bartlett P.
Published: July 2002

We present a mean-field description of a system of polydisperse hard-spheres. The theory is based on the postulate that the excess statistical properties of a general polydisperse mixture are a function of the number, the mean diameter, surface area, and volume of the constituent particles. Within this model a corresponding states relationship holds between a general polydisperse system and a suitably chosen two-component mixture. This equivalence is used to derive approximate expressions for the free energy and pressure of polydisperse crystal and fluid phases. Quantitative results are presented for the case of a Schultz distribution of diameters. These free energies are used to calculate the solid–fluid phase diagram as a function of diameter polydispersity. We find a terminal polydispersity of 8.3% above which the polydisperse fluid remains stable at all densities. In contrast with recent simulations we find no evidence for a substantial fractionation in diameters between the coexisting fluid and solid phases.
Simulation of single fiber dynamics
Authors: Skjetne Paal , Ross Russell F. , Klingenberg Daniel J.
Published: July 2002

Simulation results for the motion of flexible fibers modeled as rigid spheres connected by ball and socket joints are presented. Simulations of isolated stiff fibers reproduce such features of Jeffery orbits as orbit stability, the dependence of the dimensionless orbit period on only the fiber aspect ratio (independent of shear rate and orientation), and trajectories identical to those of prolate spheroids of the same equivalent aspect ratio. Simulations of stiff fibers “pole-vaulting” near a bounding surface qualitatively reproduce experimental observations. Fiber trajectories are very sensitive to the short-range interactions between a fiber and a bounding surface. In contrast to rigid fibers, flexible fiber orientations drift in unbounded simple shear and parabolic shear flows. The drift direction and rate depend on fiber stiffness, initial orientation, as well as the ambient flow field. A wide variety of configurational dynamics are observed, which also depend on the fiber stiffness, initial orientation, and the ambient flow field. These results agree with previous experimental observations of flexible fibers in shear flows.
A grand canonical simulation technique for dense and confined fluids with application to a Lennard-Jones fluid
Authors: Attard Phil
Published: July 2002

Grand canonical simulations, (specified chemical potential), are performed with a fixed number of particles by coupling variations in the system size to the instantaneous chemical potential determined by virtual test particle methods. This is a modified form of the pseudo- grand canonical method of Mehta and Kofke [Mol. Phys. 86, 139 (1995)]. For dense fluids the method has the advantage of allowing the chemical potential to be specified without inserting particles, and is particularly suited to confined fluids, allowing the geometry to be changed at constant chemical potential. For the test particle determination of the chemical potential a preferential cell sampling method is developed that improves the excluded volume map sampling of Dietrick et al. [J. Chem. Phys. 90, 2370 (1989)]. It is several orders of magnitude more efficient than crude Monte Carlo sampling, and 4–40 times more efficient than restricted umbrella sampling. The methods are tested on Lennard-Jones fluids, in the bulk at reduced densities up to 0.95, and confined between two planar walls.
Lyapunov exponent and the solid-fluid phase transition
Authors: Kwon Kyung-Hoon , Park Byung-Yoon
Published: July 2002

We study changes in the chaotic properties of a many-body system undergoing a solid-fluid phase transition. To do this, we compute the temperature dependence of the largest Lyapunov exponents λmax for both two- and three-dimensional periodic systems of N-particles for various densities. The particles interact through a soft-core potential. The two-dimensional system exhibits an apparent second-order phase transition as indicated by a λ-shaped peak in the specific heat. The first derivative of λmax with respect to the temperature shows a peak at the same temperature. The three-dimensional system shows jumps, in both system energy and λmax, at the same temperature, suggesting a first-order phase transition. Relaxation phenomena in the phase-transition region are analyzed by using the local time averages.
Paramagnetism of caesium titanium alum and the Jahn–Teller interaction
Authors: Tregenna-Piggott Philip L. W. , O’Brien Mary C. M. , Pilbrow John R. , Güdel Hans U. , Best Stephen P. , Noble Chris
Published: July 2002

The magnetic properties of [Ti(OH2)6]3+ within CsTi(SO4)2⋅12H2O and doped into a range of isostructural diamagnetic lattices, has been reinvestigated using electron paramagnetic resonance (EPR) and magnetic susceptibility measurements. Whereas previous attempts to rationalise the variation of the magnetic susceptibility with temperature have employed a Ag2 trigonal ground term, the data are interpreted within a framework used to describe other tervalent hexa-aqua cations whereby the trigonal field is considerable and of a sign such that the eg orbital set is lower lying, resulting in a Eg2 trigonal ground term. The magnetic susceptibility is then modeled using a novel approach based on the numerical diagonalisation of the vibronic Hamiltonian. Hence, it is shown that the singular magnetic properties of CsTi(SO4)2⋅12H2O arise from a delicate balance between spin–orbit and Jahn–Teller coupling. Particular use is made of isotopic substitution to obtain information regarding the nature of the vibronic interaction. An excellent fit to both the susceptibility data and ground state g values is provided by a model in which the Jahn–Teller interaction is dominated by coupling to phonon modes involving librational modes of sulphate and water coordinated to titanium(III). This interpretation is further supported by Raman data on the corresponding compounds. This analysis provides the first satisfactory explanation of the magnetic properties of [Ti(OH2)6]3+ in the β-alum lattice.
Translational and rotational motion in molecular liquids: A computer simulation study of Lennard–Jones ellipsoids
Authors: Ravichandran S. , Perera A. , Moreau M. , Bagchi B.
Published: July 2002

In order to understand the translational and rotational motion in dense molecular liquids, detailed molecular dynamics simulations of Lennard–Jones ellipsoids have been carried out for three different values of the aspect ratio κ. For ellipsoids with an aspect ratio equal to 2, the product of the translational diffusion coefficient (DT) and the average orientational correlation time of the ℓ-th rank harmonics (τlR), converges to a nearly constant value at high density. Surprisingly, this density independent value of DTτlR is within 5% of the hydrodynamic prediction with the slip boundary condition. This is despite the fact that both DT and τlR themselves change nearly by an order of magnitude in the density range considered, and the rotational correlation function itself is strongly nonexponential. For small aspect ratios (κ⩽1.5), the rotational correlation function remains largely Gaussian even at a very large density, while for a large aspect ratio (κ⩾3), the transition to the nematic liquid-crystalline phase precludes the hydrodynamic regime. Thus, the rotational dynamics of ellipsoids show great sensitivity to the aspect ratio. At low density, τlR goes through a minimum value, indicating the role of interactions in enhancing the rate of orientational relaxation.
A nuclear magnetic resonance study of higher-order correlation functions in supercooled ortho-terphenyl
Authors: Böhmer R. , Diezemann G. , Hinze G. , Sillescu H.
Published: July 2002

Using deuteron NMR techniques two-, effective three-, and various four-time correlation functions were recorded for supercooled ortho-terphenyl at 10–15 K above the calorimetric glass transition in order to characterize the heterogeneous nature of its primary response. The experimental results could successfully be described within various energy landscape models as well as via continuous time random walk simulations. These theoretical considerations provide a suitable basis for a definition of the term dynamic heterogeneity. We discuss the power but also some limitations of the present multidimensional NMR techniques when applied to amorphous materials.
Direct evaluation of the equilibrium distribution of physical clusters by a grand canonical Monte Carlo simulation
Authors: Kusaka I. , Wang Z.-G. , Seinfeld J. H.
Published: July 2002

A new approach to cluster simulation is developed in the context of nucleation theory. This approach is free of any arbitrariness involved in the definition of a cluster. Instead, it preferentially and automatically generates the physical clusters, defined as the density fluctuations that lead to nucleation, and determines their equilibrium distribution in a single simulation, thereby completely bypassing the computationally expensive free energy evaluation that is necessary in a conventional approach. The validity of the method is demonstrated for a single component system using a model potential for water under several values of supersaturation.
High-resolution zero-kinetic-energy pulsed field ionization photoelectron spectra of the Na(H2O) complex
Authors: Wang Kwanghsi , Rodham David A. , McKoy Vincent , Blake Geoffrey A.
Published: July 2002

Measured single-photon zero-kinetic-energy pulsed field ionization (ZEKE-PFI) photoelectron spectra of the sodium–water complex are presented and compared with the results of rotationally resolved ab initio calculations. The very nonatomiclike behavior of the photoionization of this Na(H2O) complex is essential in accounting for several significant features in these spectra. Agreement between the calculated and measured photoelectron spectra is encouraging. Furthermore, these results suggest that combined experimental and theoretical studies of ZEKE-PFI spectra can be very useful in elucidating the molecular structure and intermolecular force fields of small clusters.
A study of the CS molecule with photoelectron spectroscopy using synchrotron radiation
Authors: Dyke John M. , Gamblin Stuart D. , Haggerston Darren , Morris Alan , Stranges Stefano , West John B. , Wright Timothy G. , Wright Andrew E.
Published: July 2002

The CS molecule has been studied with constant ionic state (CIS) spectroscopy in the photon energy range 11.2–19.5 eV using radiation from a synchrotron source. The spectra were obtained by monitoring the intensities of the first three vibrational components in the first photoelectron band as a function of photon energy. The structured spectra obtained have been analyzed in terms of excitation to and autoionization from Rydberg states with A 2Π, B 2Σ+, and C 2Σ+ ionic cores. Photoelectron spectra of the first band of CS recorded at selected resonant energies showed extended vibrational structure which was used to obtain improved ionic state vibrational constants for CS+ X 2Σ+ compared to those derived from an off-resonance spectrum.
Monte Carlo simulations of the adsorption of potassium on a Cu(111) surface
Authors: Padilla-Campos Luis , Toro-Labbé Alejandro
Published: July 2002

Monte Carlo simulations of the adsorption of potassium on a Cu(111) surface at 80 K and coverages going from θ=0.13 to θ=0.26 monolayer are reported. The particles interact through an adsorbate–substrate potential previously derived and through a classical adsorbate–adsorbate Morse-type potential. Interesting results concerning the nature of the adsorbed phase have been obtained: (a) We have found that the potassium adlayer is incommensurate under 1 ML coverage (θ=0.25), but it becomes commensurate at 1 ML, forming a p(2×2) adlayer with respect to the Cu substrate. (b) A procedure to determine the distribution of potassium on different adsorption sites in terms of θ has been developed with the result that top sites are the preferred sites for adsorption, although at low and medium coverages other sites are also occupied. At 1 ML only the top sites are occupied, in good agreement with the available experimental data.
Ultrafast studies of the photodissociation of the acetone 3s Rydberg state at 195 nm: Formation and unimolecular dissociation of the acetyl radical
Authors: Owrutsky J. C. , Baronavski A. P.
Published: July 2002

Ultrafast deep UV mass-resolved photoionization spectroscopy has been used to investigate the photodissociation dynamics of the 3s Rydberg state of acetone. Single photon excitation at 193–195 nm is followed by single photon (at 260 nm) and two photon (at 390 nm) ionization and the signal is measured for both the acetone and acetyl photoions. The acetone Rydberg state lifetime determined from both single and two photon detection is surprisingly long, 4.7±0.2 ps. The higher probe energy for two photon ionization results in a lower minimum acetyl internal energy for ionization, so that part of the measured signal is due to neutral acetyl dissociation dynamics rather than only dissociative ionization of excited state acetone (which is the case for single photon ionization at 260 nm). The secondary dissociation rate of the neutral acetyl intermediate is measured, clearly establishing that photodissociation via the first Rydberg state of acetone occurs by a sequential dissociation mechanism. The acetyl dissociation occurs with a characteristic time of 3.1±0.5 ps. Based on RRKM (Rice–Ramsperger–Kassel–Marcus) calculations, this suggests an average acetyl internal energy of ∼25 kcal/mole. The long lifetime of the 3s Rydberg state suggests that the dissociation dynamics may be described in terms of a fully statistical dissociation mechanism.
An infrared spectroscopic and quasirelativistic theoretical study of the coordination and activation of dinitrogen by thorium and uranium atoms
Authors: Kushto Gary P. , Souter Philip F. , Andrews Lester
Published: July 2002

The reactions of laser-ablated thorium and uranium metals in condensing pure dinitrogen streams as well as argon/dinitrogen mixtures have provided spectroscopic evidence for the presence of several previously uncharacterized actinide metal nitrides and dinitrogen complexes. Infrared spectra of the matrix isolated product species indicate that thorium and uranium atoms have a significant bond weakening effect on dinitrogen upon initial complexation and in some cases complete dinitrogen bond cleavage is observed. In the reactions of laser-ablated thorium and uranium atoms with pure dinitrogen, the primary products are the metal mononitride (M-N), the metal dinitride (N–M–N), and in the uranium reactions, two dinuclear products U(μ-N)2U and NU(μ-N)2U. In the reactions of thorium with dinitrogen in argon, the primary products are N–Th–N, Th(μ-N)2Th, and the dinitrogen complexes Th-η1-N2 and Th-η1:η1-(N2)2. The complete ground-state electronic structure and vibrational spectrum of each product molecule has been modeled using quasirelativistic density functional calculations in order to lend support to the spectral assignments and to give insight into the electronic structure of these new species.
Nitrogen recombination dynamics at Cu(111): Rotational energy release and product angular distributions
Authors: Murphy M. J. , Skelly J. F. , Hodgson A.
Published: July 2002

Nitrogen atoms adsorbed on Cu(111) desorb thermally from an ordered Cu(100)−c(2×2)N phase in a sharp, zero order desorption feature near 700 K with an activation barrier of 143 kJ mol−1. Detailed N2 product rovibrational state distributions have been measured following recombinative desorption from a 700 K Cu(111) surface exposed to a N atom beam, with an equilibrium N coverage θN⩽10−2 ML. Although desorbing N2 is translationally and vibrationally hot, with a vibrational temperature of 5100 K and 4.2 eV of translational excitation perpendicular to the surface, rotation is excited with a temperature of just 910(±50) K for the vibrational ground state and 840(±250) K for (v=1). The energy released during recombinative desorption channels effectively into translational and vibrational motion, but not into rotational excitation. The angular distribution of recombinatively desorbed N2 is sharply peaked along the surface normal, P(θ)=cos(28±1) θ, indicating a mean energy release of 0.28 eV into translation parallel to the surface. This is inconsistent with 1D models of the translational energy release based on thermal motion parallel to the surface and a repulsive energy release directed along the surface normal. The dynamics can be described by a direct, repulsive model with a transition state at extended N2 separation, similar to the models developed for H2 dissociation on the same surface. We discuss the application of detailed balance to determine N2 sticking functions S(E,v,J) and, using a simple model for these functions, estimate a rotational efficacy of ∼0.23 for sticking of N2(v=0, J⩽24) and a vibrational efficacy of 0.7 for N2(v=1). The dynamics are compared to the models developed for H2 dissociation and the role of molecular chemisorption states and the local desorption site discussed.
Temperature dependent spectral densities and quantum activated rate theory
Authors: Antoniou Dimitri , Schwartz Steven D.
Published: July 2002

The variability with temperature of spectral densities and rates calculated with quantum activated rate theory is investigated. Classical spectral densities at two temperatures are computed via molecular dynamics for a model of proton transfer in methyl chloride. In addition, quantum dynamics is computed for spectral densities which artificially boost variability at low frequency. We find significant variation in computed spectral densities at moderate frequency. These variations, however, have little effect on overall computed quantum dynamics. In contradistinction, artificial variation in spectral densities at the lowest frequencies can generate fairly significant effects on quantum dynamics. Detailed flux correlation function calculations are presented which illustrate this phenomenon.
Thermodynamic stability of ferromagnetic liquids in the presence of nematic interactions
Authors: Oukouiss A. , Baus M.
Published: July 2002

It is shown that the region of the phase diagram where the ferromagnetic liquid phase of a Heisenberg fluid (with long-ranged exchange interactions) is thermodynamically stable (relative to the solid phases) remains confined to a fairly small temperature–density domain also in the presence of nematic interactions. The present analysis is carried out on the basis of a van der Waals theory for both translationally and/or orientationally ordered and disordered phases.
Fourier transform infrared emission spectroscopy and ab initio calculations on RuN
Authors: Ram R. S. , Liévin J. , Bernath P. F.
Published: July 2002

The emission spectrum of RuN has been observed in the near infrared using a Fourier transform spectrometer. RuN molecules were excited in a hollow cathode lamp operated with neon gas and a trace of nitrogen. Two bands with 0–0 Q heads near 7354 and 8079 cm−1 and a common lower state have been assigned as 2Π1/2–2Σ+ and 2Π3/2–2Σ+ subbands, respectively, of a C 2Π–X 2Σ+ transition. A rotational analysis of these bands has been performed and molecular constants have been extracted. The principal molecular constants for the ground X 2Σ+ state of the most abundant 102RuN isotopomer are: B0=0.552 782 9(70) cm−1, D0=5.515(13)×10−7 cm−1, γ0 =−0.044 432(22) cm−1 and r0=1.573 869(10) Å. The excited C 2Π state has the following molecular constants: T00=7714.342 60(53) cm−1, A0=725.8064(11) cm−1, B0=0.516 843 4(80) cm−1, D0=5.685(16)×10−7 cm−1, p0=5.467(36)×10−3 cm−1 and r0=1.627 670(13) Å. Ab initio calculations have been carried out on RuN to ascertain the nature of the experimentally observed states and to predict the spectroscopic properties of the low-lying electronic states. Our electronic assignment is supported by these calculations and is also consistent with the observations for the isoelectronic RhC molecule [Kaving and Scullman, J. Mol. Spectrosc. 32, 475–500 (1969)]. The valence electron configuration 1σ22σ21π41δ43σ1 is proposed for the X 2Σ+ ground state of RuN and the configurations for the excited states have been discussed. There is no previous experimental or theoretical work on RuN.
Molecular dynamics simulation on a layer-by-layer homoepitaxial growth process of SrTiO3(001)
Authors: Kubo Momoji , Oumi Yasunori , Miura Ryuji , Stirling Andras , Miyamoto Akira , Kawasaki Masashi , Yoshimoto Mamoru , Koinuma Hideomi
Published: July 2002

The effect of substrate temperature on the homoepitaxial growth process of a SrTiO3(001) surface has been investigated using our crystal growth molecular dynamics simulation code. SrO molecules were continuously deposited one by one on the SrTiO3(001) surface terminated by TiO2 atomic plane at 300 K. Two-dimensional and epitaxial growth of a SrO thin layer was observed on the SrTiO3(001) surface retaining perovskite type structure and (001) oriented configuration. However, some defects were constructed in the grown film at a low temperature of 300 K, which is in significant contrast to that at 713 K. In the latter case, a single flat and smooth SrO layer was formed without any defects, which is in good agreement with the experimental results. The self-diffusion coefficient, activation energy for surface migration, and adsorption energy of the deposited SrO molecules on the SrTiO3(001) surface were discussed. A higher migration ability of the deposited SrO molecules at high temperature was found to lead to complete layer-by-layer homoepitaxial growth.
Vibrational quenching of acetylene scattered from LiF(001): Trapping desorption versus direct scattering
Authors: Wight A. C. , Miller R. E.
Published: July 2002

A new experimental technique is reported that provides information on vibrational quenching of polyatomic molecules in collision with surfaces. A bolometer detector is used to measure angular distributions for the ground state and the survival probability of vibrationally excited states of acetylene. We find that for LiF(100) there is essentially no vibrational quenching in the direct scattering channel. Even when molecules are trapped for significant periods of time, such that their translational distributions are fully accommodated with the surface, vibrational quenching can be small. Under extreme conditions, however, where the trapping times are very long, we observe appreciable quenching of the vibrationally excited molecules.
Infrared spectroscopy and ab initio potential energy surface for Ne–C2H2 and Ne–C2HD complexes
Authors: Bemish R. J. , Oudejans L. , Miller R. E. , Moszynski Robert , Heijmen Tino G. A. , Korona Tatiana , Wormer Paul E. S. , van der Avoird Ad
Published: July 2002

The rotationally resolved spectra of Ne–C2H2 and Ne–C2HD were measured in the region of the asymmetric C–H stretch (ν3) band of the acetylene monomer. The transitions in the Ne–C2H2 spectrum are substantially broadened by vibrational predissociation, while those of Ne–C2HD are quite narrow. This difference is attributed to the fact that in the former dissociation proceeds through a “doorway” state, related to a Fermi resonance involving the bending vibrations of C2H2. In C2HD this Fermi resonance is absent. The potential energy surface (PES) for the Ne–acetylene complex has been computed using symmetry-adapted perturbation theory. This PES has been fit to an analytic form and applied in calculations of the rovibrational energy levels of Ne–C2H2 and Ne–C2HD. From these levels and calculated transition intensities we generated the near-infrared spectra of these complexes in the region of the ν3 band. These complexes may be considered as nearly free internal rotors. For Ne–C2H2 the results obtained from the ground state PES gave semiquantitative agreement with the measured spectrum. For Ne–C2HD we could assign all of the (much sharper) lines in the experimental spectrum and obtain the ν3 excited state interaction potential from a fit of the calculated spectrum to the experimental one. The ground state ab initio potential was not altered in this fit; the excellent agreement between the calculated and measured infrared spectrum for Ne–C2HD demonstrates that our Ne–acetylene potential is quite accurate.