bluuebunny/crossref_metadata_2025 huggingface dataset:
Simple modeling of line-mixing effects in IR bands. II. Nonlinear molecules applications to O3 and CHClF2
Authors: Hartmann J. M. , Nguyen-Van-Thanh  , Brodbeck C. , Benidar A. , LeDoucen R. , Regalia L. , Barbe A.
Published: July 2002

A simple semiempirical approach is developed in order to model the shape of infrared absorption bands. It is based on use of the strong collision model and of a classical representation of rotational levels. The absorption coefficient then has a simple analytical expression whose wavenumber and pressure dependencies are computed by using eleven parameters which depend on the considered vibrational transition, the temperature, and the nature of the perturber only. These quantities, which are band-averaged values of the detailed spectroscopic and collisional parameters of the molecular system, can be deduced from direct fits of measured spectra. The model thus requires no previous knowledge of the characteristics of the molecules and is thus applicable to complex systems; in particular it seems a promising approach for very dense molecular spectra for which only absorption cross sections are now available. Tests are presented in the case of O3 and CHClF2 bands perturbed by N2 at room temperature for which new measurements have been made. They demonstrate the accuracy of our semiempirical approach in predicting the spectral shape in a wide range of density provided that effective parameters are used.
Mid-infrared spectra of He–HN+2 and He2–HN+2
Authors: Meuwly M. , Nizkorodov S. A. , Maier J. P. , Bieske E. J.
Published: July 2002

Mid-infrared vibrational spectra of He–HN+2 and He2–HN+2 have been recorded by monitoring their photofragmentation in a tandem mass spectrometer. For He–HN+2 three rotationally resolved bands are seen: the fundamental ν1 transition (N–H stretch) at 3158.419±0.009 cm−1, the ν1+νb combination band (N–H stretch plus intermolecular bend) at 3254.671±0.050 cm−1, and the ν1+νs combination band (N–H stretch plus intermolecular stretch) at 3321.466±0.050 cm−1. The spectroscopic data facilitate the development of approximate one-dimensional radial intermolecular potentials relevant to the collinear bonding of He to HN+2 in its (000) and (100) vibrational states. These consist of a short range potential derived from an RKR inversion of the spectroscopic data, together with a long range polarization potential generated by considering the interaction between the He atom and a set of multipoles distributed on the HN+2 nuclei. The following estimates for binding energies are obtained: D0″=378 cm−1 [He+HN+2(000)], and D0′=431 cm−1 [He+HN+2(100)]. While the ν1 band of He2–HN+2 is not rotationally resolved, the fact that it is barely shifted from the corresponding band of He–HN+2 suggests that the trimer possesses a structure in which one of the He atoms occupies a linear proton-bound position forming a He–HN+2 core, to which a second less strongly bound He is attached.
A unitary group based open-shell coupled cluster study of vibrational frequencies in ground and excited states of first row diatomics
Authors: Li Xiangzhu , Paldus Josef
Published: July 2002

The performance of recently introduced coupled cluster (CC) method exploiting the unitary group approach (UGA) to many-electron systems, truncated at the first order interacting space level [UGA-CCSD(is)] and using the 6-31G* basis set, in computations of equilibrium bond lengths and harmonic vibrational frequencies, is examined for a series of open-shell (OS) states of the first row diatomics and hydrides. Altogether, 48 distinct electronic states are considered for 9 diatomic hydrides (BeH, BH, CH, CH+, NH, NH+, OH, OH+ and FH) and 18 diatomics (BeF, BN, BO, C2, C2+, C2−, CN, CO, CO+, CF, N2+, NO, NO−, NF, O2, O2+, OF and F2+), involving both high and low spin cases. Very good agreement with the available experimental data is found in all cases, except when the experimental values are marked as ‘‘uncertain’’ or where only the ΔG(1/2) values of harmonic frequencies are available. For the so-called ‘‘difficult’’ systems, namely NO(X 2Π), O2(X 3Σg−), O2+(X 2Πg), OF (X 2Π) and F2+ (X 2Πg), the geometries and vibrational frequencies are also calculated using the TZ2P [5s4p2d] basis sets, and the results are compared with both the experiment and existing perturbation theory and CC results. All results indicate that UGA CCSD(is) represents a versatile, reliable and computationally affordable method that can handle a great variety of OS states, including OS singlets.
Multi-rank nuclear magnetic resonance studies of half-integer quadrupolar nuclei in solids by three-dimensional dynamic-angle correlation spectroscopy
Authors: Medek Ales , Sachleben Joseph R. , Beverwyk Peter , Frydman Lucio
Published: July 2002

The present work introduces a new three-dimensional nuclear magnetic resonance (3D NMR) experiment for the analysis of half-integer quadrupolar nuclei in solids. The method is based on the multi-rank expansion of the high-field NMR Hamiltonian governing the central transition of these spins in terms of irreducible spherical tensor elements. This approach leads to a temporal spin evolution given by an isotropic term characteristic of each chemical site, as well as by second- and fourth-rank anisotropies depending on the principal values and relative orientations of the shielding and quadrupolar interactions. A method for extracting the 3D spectral distribution correlating these three frequency components is presented, based on the acquisition of dynamic-angle spinning NMR signals collected as a function of different initial and final spinning axes. Computational and instrumental details involved in the acquisition of these 3D dynamic-angle correlation spectroscopy (DACSY) data are discussed, and applications of the DACSY methodology to the analysis of different rubidium salts are illustrated. The new type of chemical information that this experiment can provide and its relation to other NMR techniques that have been recently developed for the analysis of quadrupolar nuclei in solids are also discussed.
Fluorescence-dip infrared spectroscopy of the tropolone-H2O complex
Authors: Frost Rex K. , Hagemeister Fredrick C. , Arrington Caleb A. , Schleppenbach David , Zwier Timothy S. , Jordan Kenneth D.
Published: July 2002

Fluorescence dip infrared spectroscopy (FDIRS) is used to probe the effect of a solvent water molecule on intramolecular H-atom tunneling in tropolone. As with the bare molecule discussed in paper I, the FDIR spectrum of the tropolone-H2O complex is recorded in the O–H and C–H stretch regions. Three OH stretch fundamentals are observed in the spectrum, and can be assigned nominally to a free OH stretch of the water molecule (3724 cm−1), a hydrogen bonded OH stretch of water (3506 cm−1), and the OH stretch of tropolone (∼3150 cm−1). The breadth and complexity of the bands is highly mode specific. The free OH stretch transition is sharp (1.8 cm−1 FWHM) and has weak combination bands built on it at +73 and +1600 cm−1. The former is assigned to a combination band with the in-plane bending mode of the tropolone-H2O hydrogen bond, while the latter is the free OH/intramolecular water bend combination band. The water hydrogen-bonded OH fundamental is also a sharp transition which, after correction for the decreased infrared power at its frequency, is clearly the strongest transition in the spectrum. It is flanked by three close-lying satellite bands 13, 23, and 34 cm−1 above it, and also supports a weak combination band at +69 cm−1 due to the in-plane intermolecular bending mode. The tropolone OH absorption is in the same frequency region as in the bare molecule, but broadened to over 100 cm−1 in TrOH–H2O. Distinct substructure in the band is present, with spacings reminiscent of those in the water H-bonded OH stretch region. Ab initio calculations on tropolone-H2O are carried out at both the MP2 and Becke3LYP levels of theory. Two isomers with similar binding energies and vibrational frequencies are identified. In one isomer (isomer I), the water molecule serves as a hydrogen-bonded bridge between the tropolone OH and keto groups. In the other (isomer II), the water molecule is exterior to the tropolone and hydrogen bonded to the keto oxygen. The experimental evidence does not conclusively distinguish between these two possibilities, though the exterior structure seems somewhat more in keeping with the data as a whole.
Hydration of Ni2+ and Cl− in a concentrated nickel chloride solution at 100 °C and 300 °C
Authors: de Jong P. H. K. , Neilson G. W. , Bellissent-Funel M.-C.
Published: July 2002

The first order difference method of neutron diffraction and isotopic substitution has been applied to the Ni2+ and Cl− ions in a 2 m nickel chloride heavy water solution at 100 °C and 1000 bar, and at 300 °C and 1000 bar. The results show that both the Ni2+ and the Cl− hydration shell become progressively weakened with increasing temperature. The hydration number of Ni2+ drops from ∼6 under ambient conditions to ∼5.4 at 100 °C and to ∼4.5 at 300 °C. The hydration number of Cl− at 100 °C (6.9) is similar to the value under ambient conditions (6.4) and reduces to ∼4.9 at 300 °C.
Accurate measurement of thermophoretic effect in microgravity
Authors: Toda Akira , Ohi Yosuke , Dobashi Ritsu , Hirano Toshisuke , Sakuraya Takashi
Published: July 2002

The behavior of particles under thermophoretic effect was examined experimentally by getting rid of the effect of natural convection utilizing microgravity environment. The microgravity environment was realized by using a drop tower facility. In this system, measurement fields with temperature gradient were established between two metal plates. In order to provide a steep temperature gradient within a limited experimental duration, the preheated metal plate was forced to approach the cold plate quickly after the microgravity environment was established. Temperature distribution and the behavior of individual particles were observed simultaneously. The fields in an almost steady state with monotonic temperature gradient were obtained. In these fields, no other forces than thermophoretic force which might induce the particles movement were confirmed to exist. The fields ideal for the measurement of the thermophoretic effect were obtained in our experiments. The measured thermophoretic velocities were found to be higher than the corresponding one estimated from previously proposed theoretical equations; coefficients included in which were determined on the basis of experimental data obtained using a less accurate method.
The initial vibrational level distribution and relaxation of HCN[X̃ 1Σ+(v1,0,v3)] in the CN(X 2Σ+)+CH4→HCN+CH3 reaction system
Authors: Bethardy G. A. , Northrup F. J. , Macdonald R. Glen
Published: July 2002

The reaction of the cyano radical (CN) with methane was studied by time-resolved infrared absorption spectroscopy by monitoring individual rovibrational states of the HCN and CH3 products. The initial vibrational level distribution of the bendless vibrational levels of HCN(v1,0,v3) was determined by plotting the time dependence of the fractional population of a vibrational level and extrapolating these curves to the origin of time. About 20% of the HCN products were observed to be initially produced in the HCN(v1,0,v3) vibrational levels, with v1 and v3=0,1,2. The CN radical was created by laser photolysis of three different precursors. Each photolyte provided a different initial vibrational level distribution of CN; however, similar initial HCN(v1,0,v3) vibrational level distributions were obtained independent of the CN radical precursor. This may indicate that the CN radical does not act as a spectator bond during the course of a reactive encounter for this system. The time dependence of the CH3 (00000) ground state was also followed using time-resolved infrared absorption spectroscopy. Preliminary data indicates that a large fraction, if not all, the CH3 radicals are produced in their ground state in the title reaction.
The rotator phases of neat and hydrated 1-alcohols
Authors: Sirota E. B. , Wu X. Z.
Published: July 2002

We report an x-ray scattering study of the rotator phases in both dry and hydrated 1-alcohols CH3–(CH2)n−1–OH(12≤n≤26). Hydration was accomplished by using both fixed composition and controlled relative humidity. A hexagonal phase and two tilted phases are reported in the temperature versus chain length and temperature versus humidity phase diagrams. Hydration tends to drive the system from one tilted rotator state to the other, and then to the untilted hexagonal phase. The hexagonal phase shows a six-layer periodicity illustrating the crossover between trilayer alkane behavior and bilayer surfactant behavior. Hydration and chain-length mixing increase the temperature range of the rotator phase. The rotator phase will take in about 1 H2O per 2 alcohols which is twice the saturated water content of the liquid phase. This is shown to explain the unusual rise in the melting temperature with hydration.
Nuclear magnetic resonance spin–spin coupling constants from density functional theory: Problems and results
Authors: Malkina Olga L. , Salahub Dennis R. , Malkin Vladimir G.
Published: July 2002

Our recently developed method for the calculation of indirect nuclear spin–spin coupling constants is studied in more detail. For the couplings between nuclei other than N, O, and F (which have lone pairs) the method yields very reliable results. The results for 1J(Si–H) couplings are presented and their dependence on the basis set quality is analyzed. Also, 2J(H–H) and 1J(X–H) couplings (X=C, Si, Ge, Sn) in XH4 molecules are presented and the relativistic effects on 1J(X–H) are discussed. The limitations of the method, which is based on density functional theory, are connected with the inability of the present LDA and GGA exchange-correlation functionals to describe properly the spin-perturbations (through the Fermi-contact mechanism) on atoms to the right of the periodic table (containing lone pairs). However, the deviations from experiment of the calculated couplings for such nuclei are systematic, at least for one-bond couplings, and therefore these calculated couplings should still be useful for NMR structure determinations.
Determination of quadrupolar and chemical shielding tensors using solid-state two-dimensional NMR spectroscopy
Authors: Shore J. S. , Wang S. H. , Taylor R. E. , Bell A. T. , Pines A.
Published: July 2002

The quadrupolar and chemical shift tensors, as well as the relative orientation of the two principle axis systems, are accurately determined using a two-dimensional nuclear magnetic resonance technique. Good agreement between experimental and simulated two-dimensional spectra is obtained for a series of rubidium and sodium compounds at multiple magnetic field strengths. Extension of this technique to correlate the quadrupolar and dipolar interactions, as well as the incorporation of a purely isotropic dimension resulting in a three-dimensional experiment is also discussed.
Caging phenomena in reactions: Femtosecond observation of coherent, collisional confinement
Authors: Wan C. , Gupta M. , Baskin J. S. , Kim Z. H. , Zewail A. H.
Published: July 2002

We report striking observations of coherent caging of iodine, above the B state dissociation threshold, by single collisions with rare gas atoms at room-temperature. Despite the random nature of the solute–solvent interaction, the caged population retains coherence of the initially prepared unbound wave packet. We discuss some new concepts regarding dynamical coherent caging and the one-atom cage effect.
Reactant-product decoupling method for state-to-state reactive scattering: A case study for 3D H+H2 exchange reaction (J=0)
Authors: Zhu Wei , Peng Tong , Zhang John Z. H.
Published: July 2002

In this paper, we present theoretical and computational details of implementing the recently developed reactant-product decoupling (RPD) method (J. Chem. Phys. 105, 6072 (1996)) for state-to-state quantum reactive scattering calculations of the prototypical H + H2 reaction in three dimensions. The main purpose of this paper is to explore important features of the RPD scheme for use as a general and efficient computational approach to study state-to-state quantum dynamics for polyatomic reactions by using 3D H + H2 as an example. Specific computational techniques and numerical details are explicitly provided for efficient application of this method in the time-dependent (TD) implementation. Using the RPD method, the calculated state-to-state reaction probabilities for the 3D H + H2 reaction are in excellent agreement with those from the time-independent variational calculations, and the computational cost of the RPD method is significantly lower than other existing TD methods for state-to-state dynamics calculations.
Two-dimensional transport and wall effects in the thermal diffusion cloud chamber. II. Stability of operation
Authors: Bertelsmann Anne , Heist Richard H.
Published: July 2002

In this paper, the second of a series of two presenting a detailed description of thermal diffusion cloud chamber operation, we address the operational stability of the vapor–gas mixture in a diffusion cloud chamber with respect to density profile extrema and the accompanying possibility of buoyancy-driven convective flow disturbances. We examine conditions for stable operation (no convective flow disturbances) in the central portion of the cloud chamber, as well as conditions necessary for stable operation in the vicinity of the cloud chamber wall. We find that the total density profile in the central portion of the cloud chamber can pass through a density minimum even though the density at the upper plate surface is less than the density at the lower plate surface. This local density profile inversion can result in unstable (convective) behavior that propagates through the cloud chamber. Furthermore, we find that local extrema in the total density profile near the chamber wall can lead to subtle, convective flows that are difficult to detect yet can exert a profound influence on nucleation in the central portion of the cloud chamber. We have developed a simple method to estimate the limiting total pressure in a cloud chamber that will support stable operation. From results of our investigations based upon this method, it appears that the thermal diffusion cloud chamber is best suited for experiments at higher temperatures where the accessible total pressure range is largest. Finally, we find that results of our investigation into the effects of total pressure and kind of background gas on nucleation in diffusion cloud chambers involving the low molecular weight alcohols and hydrogen and helium background gases cannot be explained on the basis of these kind of density disturbances occurring within the diffusion cloud chamber. Also, for (relatively) low vapor pressure materials, such as 1-pentanol or other high molecular weight alcohols and alkanes stability limitations may preclude nucleation measurements at low temperatures using a diffusion cloud chamber altogether.
Raman spectra of ruthenium dimers
Authors: Wang Huaiming , Liu Yifei , Haouari Hanae , Craig R. , Lombardi J. R. , Lindsay D. M.
Published: July 2002

The absorption (scattering depletion) spectrum and resonance Raman spectrum for Ru2 in an argon matrix prepared by the mass-selected ion deposition technique have been obtained. The absorption spectrum in the visible region shows a single broad transition centered at 470 nm. Resonance Raman spectra, obtained by excitation into this band with dye laser radiation, display a single sharp progression with lines at 343.8(26) cm−1, 681.6(32) cm−1, 1017.5(26) cm−1, 1350.2(25)cm−1,and 1678.9(8) cm−1. These data give ωe=347.1±0.9 cm−1 with ωexe=1.85±1.5 cm−1, leading to a spectroscopic dissociation energy of 2.0±0.2 eV. Sidebands in the Raman spectra are indicative of a weak coupling (Huang–Rhys factor, S∼0.1–0.7) between the dimer and the phonons of the argon host.
Statics and dynamics of dense copolymer melts: A Monte Carlo simulation study
Authors: Hoffmann Alexander , Sommer Jens-Uwe , Blumen Alexander
Published: July 2002

Using the bond-fluctuation-algorithm we perform dynamic Monte Carlo simulations on dense symmetric copolymer melts. We cover structures from homogeneous melts to strongly separated regimes. We confirm the scaling laws found in previous simulations for the static properties of the chains and extend these laws into the strong segregation region. The structure factor and various other static properties of the single chain scale with εN. We observe that in the homogeneous phase even for short chains the A- and B-blocks contract due to the prevailing comonomer repulsion. The center-of-mass diffusion of single chains as well as the relaxation of the end-to-end vector exhibit a crossover between two regimes above and below the order-disorder transition. We find that in both regimes the dynamics do not scale with εN. In the strong segregation limit the diffusion coefficient turns out to be lower than the expected value of 2D0/3. The end-to-end-vector displays two relaxation processes even well above the order-disorder transition. Besides a fast mode we find a slow mode, more than a decade apart. The simulations show that composition fluctuations above the order-disorder transition have a profound influence on the dynamics.
Molecular dynamics simulation of a liquid crystalline mixture
Authors: Sandström Dick , Komolkin Andrei V. , Maliniak Arnold
Published: July 2002

We present results from a molecular dynamics simulation of benzene dissolved in the mesogen 4-n-pentyl-4-cyanobiphenyl (5CB). The computer simulation is based on a realistic atom-atom potential and is performed in the nematic phase. Singlet orientational distribution functions are reconstructed from order parameters employing several methods, and the estimated distributions are compared with those obtained directly from the trajectory. Transport properties have been studied by calculating translational diffusion coefficients in directions both parallel and perpendicular to the liquid crystalline director. The simulated diffusion coefficients were found to be of the same order of magnitude as those measured in experiments. Second rank orientational time correlation functions are used to investigate molecular reorientations and significant deviations from the small step rotational diffusion model are established. Molecular structure and internal dynamics of 5CB have been examined by correlating the time dependence of dihedral angles with effective torsional potentials.
Ghost levels and near-variational forms of the discrete variable representation: Application to H2O
Authors: Wei Hua
Published: July 2002

The quadrature error in the discrete variable representation (DVR) corresponds to first-order perturbation corrections of the potential matrix elements, and is remarkably large. It causes an unphysical (ghost) level at Eghost≈(3/4)Emax in a one-dimensional problem. In a multidimensional problem the ghost components of the wave functions create large errors in levels and intensities of spectral transitions. Estimation of the quadrature error for a general formula of numerical integral is presented. The convergence direction of the DVR levels is explained based on the error analysis. Near-variational forms of the DVR are presented, which remove the ghost levels, improve convergence of the levels and intensities and are easily combined with the Lanczos method to reach high computational efficiency. As an application, vibrational band intensities and levels of H2O were calculated by using the best potential energy surface available and two dipole surfaces.
Compensatory effects in the establishment of spontaneous polarization in ferroelectric liquid crystals
Authors: Cherkaoui Mohammed Zoubair , Nicoud Jean-François , Galerne Yves , Guillon Daniel
Published: July 2002

Electro-optical properties of optically active mesogenic n-alkyl sulfinates are reported. Two homologous series have been considered, and the properties have been analyzed as a function of the length of aliphatic terminal chains. It is shown that the anisotropic distribution of the transverse dipole moments of the molecules, f, decreases strongly (by about 50%) with increasing chain length for a given tilt angle; this behavior is attributed to large orientational fluctuations. It is shown also that f increases slightly with increasing chain length at a given distance from the transition temperature between the C* and A phases; this is attributed to a better defined building of different smectic sublayers. These two antagonistic effects, together with the dilution effect, act simultaneously in order to keep the spontaneous polarization practically independent of the aliphatic chain length, whereas the tilt angle of the molecules within the smectic C* layers is quite dependent of aliphatic chain length, at a given distance from the transition temperature.
Thin films of block copolymer
Authors: Matsen M. W.
Published: July 2002

We develop a numerical method for examining complex morphologies in thin films of block copolymer using self-consistent field theory. Applying the method to confined films of symmetric diblock copolymer, we evaluate the stability of parallel, perpendicular, and mixed lamellar phases. In general, lamellar domains formed by the diblocks are oriented parallel to the film by surface fields. However, their orientation can flip to perpendicular when the natural period of the lamellae is incommensurate with the film thickness. Experiments and Monte Carlo simulations have indicated that mixed lamellar phases may also occur, but for symmetric diblocks, we find these phases to be slightly unstable relative to perpendicular lamellae. Nevertheless, just a small asymmetry in the molecule stabilizes a mixed lamellar phase. Although our work focuses on confined films, we do discuss the behavior that results when films are unconfined.
Long-range interaction of the 39K(4s)+39K(4p) asymptote by photoassociative spectroscopy. I. The g− pure long-range state and the long-range potential constants
Authors: Wang H. , Gould P. L. , Stwalley W. C.
Published: July 2002

This paper reports on a comprehensive study of the long-range interaction of the K39(4s)+39K(4p) asymptotic system. We present a detailed discussion of the R-dependent angular momentum couplings and correlation between the Hund’s case (a) and case (c) molecular states. Analytical expressions for the 16 adiabatic Hund’s case (c) long-range potential curves are derived including the higher order dispersion forces and the effects of retardation. Experimentally, six Hund’s case (c) long-range molecular states (0u+, 1g, and 0g− dissociating to the 4 2S1/2+4 2P3/2 asymptote and 0u+, 1g, and 0g− to the 4 2S1/2+4 2P1/2 limit) are observed with rovibrational resolution by photoassociative spectroscopy of ultracold K39 atoms in a high density magneto-optical trap (MOT). Among the six observed long-range states, the upper 0g− “pure long-range” state has negligible short-range chemical exchange contributions and the measured molecular binding energies (v=0–26) are used to precisely determine the long-range potential constants of the 4s+4p interaction. We determine: C3Π=8.436(14) a.u., C3Σ=16.872(28) a.u., C6Π=6272(94) a.u., and C6Σ=9365(141) a.u.. Molecular constants for the three special pure long-range states, the 0g− and 1u (dissociating to the 4 2P3/2 limit and with potential minimum) and the 1u (dissociating to the 4 2P1/2 and with potential maximum), are reported. The internal consistency of the theoretical model used in this work is confirmed by the excellent agreement between the long-range potential curve of the 1g state obtained in present work (from the 0g− state) and the long-range portion of the RKR potential curve of the 1 1Πg state previously determined by conventional molecular spectroscopy. The radiative lifetime of the K 4p state derived from the dipole–dipole interaction constant C3Π is also in excellent agreement with a recent fast-beam measurement.
A quasiclassical trajectory study of H+CO2: Angular and translational distributions, and OH angular momentum alignment
Authors: Bradley Kimberly S. , Schatz George C.
Published: July 2002

We present a quasiclassical trajectory study of the H+CO2 reaction dynamics, with emphasis on product angular and translational distributions, and OH angular momentum alignment. A new potential surface has been developed for this study, based on modifications of a previously developed full dimensional empirical HCO2 potential surface. The modifications include correcting errors that cause the HO⋅⋅⋅CO dissociation barrier to be too loose, and adjusting the depth of the HCO2 minimum and the heights of several barriers, in order to bring them into agreement with their best estimates determined from ab initio calculations. We compare cross sections, energy partitioning, and mechanistic information calculated using the unmodified and modified surfaces with experimental results. Results from the modified surface improve the comparison with experiment for the product OH energy partitioning, but the product CO internal excitation is still high. The translational distributions have the same shape as measured distributions, and the average translational excitation matches some experiments but is lower than others. The angular distribution calculated at 2.6 eV on the modified surface is in good agreement with experimental results, showing both forward and backward scattered peaks, with a slight preference for backward scattering. By studying the average lifetime of the HOCO collision complex, we find that the lifetime is comparable to the rotational period so that there is considerable forward scattering (half rotation) and backward scattering (full rotation). The OH product angular momentum alignment indicates no preference for polarization of the OH rotational angular momentum vector. This result–an essentially isotropic distribution–agrees within the experimental uncertainty for measurements of OH Π(A′) polarization dependent differential cross sections and center-of-mass frame alignment parameters, but not with OH Π(A″)
Slow manifold structure and the emergence of mixed-mode oscillations
Authors: Goryachev Andrei , Strizhak Peter , Kapral Raymond
Published: July 2002

A detailed study of the slow manifold of a model exhibiting mixed-mode oscillations is presented. A scenario for the emergence of mixed-mode states which does not involve phase locking on a 2-torus is constructed. We show that mixed-modes correspond to the periodic orbits embedded in the horseshoe-type strange set and demonstrate how transformations of this set determine the transitions of mixed-mode states into each other.
A Green’s-function perturbation theory for nonuniform semiflexible polymers: Phases and their transitions near attracting surfaces
Authors: Kuznetsov Dmitri V. , Sung Wokyung
Published: July 2002

The chain stiffness is an important characteristic that affects the properties of many biological and synthetic polymers. We develop a perturbation theory in the framework of a Green’s-function approach to treat the chain stiffness effects systematically. This scheme incorporates the standard flexible-chain Edwards equation in the zeroth order. When applied to the chains on the surface, the theory predicts novel features in adsorption–desorption transitions coupled with types of isotropic to nematic ordering transitions, giving a rigorous quantitative basis for our previous scaling theory results. We analyze the phase diagrams, and various quantities that detail the chain distribution including orientation-order-parameter distribution of the adsorbed semiflexible polymers.
Energetics and geometries of carbon nanoconic tips
Authors: Han Jie , Jaffe Richard
Published: July 2002

The density functional theory (DFT), semiempirical molecular orbital calculations, and π-orbital axis vector (POAV) analysis are used to study the energetics and geometries of carbon nanoconic tips. The tip models are formed by introducing two to five pentagons into a graphene network corresponding to experimentally observed cone angles of 19 to 84°. The calculations show a pronounced energetic preference of (1,1) pentagons over (2,0) pentagons in tip structures, where one pentagon is at hexagonal coordinate (1,1) or (2,0) relative to the other at (0,0). The DFT energies of tips containing (1,1) pentagon pairs are lower than those with (2,0) pairs by 18.0–55.9 kcal/mol. This is attributed to less in-plane and out-of-plane strain being induced by (1,1) pentagon pairs than (2,0) pentagon pairs. The bond lengths, bond angles, torsion angles, and POAV angles are presented and discussed. The isolated pentagon rule is also tested for tip molecules containing two pentagons. The lower tip energy is obtained by increasing (a,a) pentagon separation or by decreasing (b,0) pentagon separation with b>1.
Dangers of counterpoise corrected hypersurfaces. Advantages of basis set superposition improvement
Authors: Liedl Klaus R.
Published: July 2002

The convergence properties of counterpoise corrected hypersurfaces towards the limit of infinite basis set size within a specific method are discussed. In the case of concerted hydrogen exchange of the cyclic hydrogen fluoride trimer counterpoise correction is shown to be highly counterproductive for the convergence speed towards the basis set limit. A best estimate for the MP2 basis set limit of −15.2 kcal/mol for the stabilization energy and of 18.7 kcal/mol for the concerted hydrogen exchange barrier of the cyclic hydrogen fluoride trimer is deduced by calculations using basis sets up to aug-cc-pV5Z. Both results are very well predicted in the uncorrected case already with the smaller basis sets in the MP2/aug-cc-pVxZ series, whereas counterpoise correction completely fails to improve predictions mainly due to a wrong correction of the dynamic electron correlation contribution. Only with very large basis sets it is possible to obtain counterpoise corrected results of comparable quality.
Mass density of polystyrene thin films measured by twin neutron reflectivity
Authors: Wallace W. E. , Beck Tan N. C. , Wu W. L. , Satija S.
Published: July 2002

Neutron reflectivity measurements on polystyrene thin films (6.5–79.0 nm thick) supported on silicon substrates indicate that the mass density is near the bulk value regardless of film thickness. To account for possible inaccuracies arising from sample misalignment, reflectivity measurements were made both from the free-surface and silicon-substrate sides of the thin film, a method termed twin reflectivity. For films spin coated on the hydrogen-terminated silicon surface the relative uncertainty in the density measurement was on the order of 1%, but for films spin coated onto the silicon native-oxide surface the analysis was more difficult because of subtleties in data fitting due to the oxide layer. Nevertheless, within the limits of greater uncertainty, these films also showed no systematic change in density with thickness.
Phase lag near the resonances in (ω1,ω3) coherent control
Authors: Lee Sungyul
Published: July 2002

Molecular phases are computed near the resonances in the (ω1,ω3) coherent control scheme for excitations to predissociating final states. We show that the phase factors for the one- and three-photon transition amplitudes are identical, giving a vanishing molecular phase, when the intermediate manifolds consist only of discrete states and when the final continuum states do not interact. The phase lag between the dissociation products identically vanishes across the resonance in this case. On the other hand, when there is contribution of the intermediate continuum states, the imaginary part of the three-photon transition operator can give different phase factors for the one- and three-photon transition amplitudes, yielding very important effects of the molecular phase in (ω1,ω3) coherent control. It is also shown that couplings between the final continuum states can give different molecular phase for different product channels in the absence of the effects of the intermediate continuum states, exhibiting the phase lag of the products near resonances. We demonstrate that the phase lag may show minimum near the top of the resonance, as was observed in Gordon and co-workers’ recent observations [Zhu et al., Science 270, 77 (1995)] on the phase lag in HI coherent control experiments, indicating that the observed phase lag is due to the effects of the molecular phase.
Monte-Carlo simulations of polymer crystallization in dilute solution
Authors: Chen C.-M. , Higgs Paul G.
Published: July 2002

Polymer crystallization in dilute solution is studied by three-dimensional Monte-Carlo simulations using the bond fluctuation model. We study monodisperse chains of moderate length, intended to model recent experiments on monodisperse alkanes with length of a few hundred carbon atoms, and we also investigate chain folding of very long polymers. For monodisperse flexible chains we observe both extended-chain and once-folded-chain crystals. The simulations illustrate the range of defects and irregularities which we expect to find in polymer crystals. The roughness of the top and bottom surfaces of the lamellae is measured. Chain ends can be seen as cilia emerging from the surfaces. Folds are found to occur with approximately equal frequency on top and bottom surfaces. Although most chain folds are aligned perpendicular to the growth direction, a significant number of chains folding parallel to the growth direction are found as defects. The simulation includes a chain stiffness parameter which has an important effect on chain folding kinetics. When chains are semi-flexible the crystals formed are extremely irregular with many defects including holes and blocks of extended chains within the folded chain lamellae. For very long chains we show that the lamellar thickness is determined by the folding kinetics. The thickness diverges as the temperature approaches the infinite chain melting point T∞. For T→T∞, the thickness is close to the theoretical minimum thickness, which indicates the dominant importance of the entropic barrier in crystallization.
UV-induced desorption of CH3X (X=I and Br)/TiO2(110)
Authors: Kim Seong Han , Stair Peter C. , Weitz Eric
Published: July 2002

The UV photodesorption of methyl halides from TiO2(110) has been investigated using quadrupole mass spectrometry. For low fluence irradiation (<800 μJ/cm2) and ⩽1 ML coverages of methyl iodide, the photodesorption yield decreases exponentially with photon dose, indicating a nonthermal, single-photon process. The wavelength and coverage dependence of the photodesorption yield and the effect of spacer layers support the conclusion that photodesorption of CH3X (X=I and Br) is initiated by substrate excitation, i.e., photogenerated electrons in the conduction band of TiO2. CH3I and CH3Br desorb from TiO2(110) on 257 nm irradiation, while CH3Cl does not photodesorb in the absence of a photoactive coadsorbate, such as CH3I. This indicates that desorption results from a resonant interaction of subvacuum-level electrons with an adsorbate electronic state. Momentum transfer from energetic photofragments to adsorbates can account for only a minor fraction of the total photodesorption yield for coverages of 1 ML or less. For high fluence irradiation (⩾7 mJ/cm2), the nonlinearity of the desorption yield, the calculated transient surface temperature rise, and the desorption of an inert adsorbate (CH3OH at ⩾12 mJ/cm2) indicate that a laser-induced thermal desorption mechanism is dominant.
Collision dynamics of large water clusters
Authors: Svanberg Marcus , Ming Liu , Marković Nikola , Pettersson Jan B. C.
Published: July 2002

Classical trajectory calculations of (H2O)n+(H2O)n collisions are carried out for n=125 and n=1000. We investigate energy redistribution and fragmentation behavior for relative collision velocities up to 3000 ms−1, impact parameters up to 4 nm, and initial cluster temperatures of 160 and 300 K. Three main scattering channels are identified; coalescence, stretching separation, and shattering collisions. For small impact parameters, low collision velocities produce coalesced clusters while high velocities yield shattering behavior. Large impact parameters combined with high velocities result in stretching separation collisions. A decreased internal temperature influences the dynamics by increasing the stability of the collision complex. The results for (H2O)125 and (H2O)1000 are comparable, although the smaller size allows individual molecules to have a larger influence on the overall behavior. We find good agreement between the cluster simulations and experimental data for water drops in the micrometer range concerning the transition between coalescence and stretching separation, which shows that the clusters in some respects resemble “macroscopic” objects.
Normal mode dynamics in simple liquids
Authors: Krämer N. , Buchner M. , Dorfmüller Th.
Published: July 2002

A dynamical generalization of the instantaneous normal mode (INM) theory of liquid state dynamics is presented. Due to anharmonicities the eigenvalues and eigenvectors of the Hessian matrix change with time. Therefore, regular INM theory gives a description of molecular dynamics valid only for short times. Starting out from the classical equations of motion the velocity correlation function is expressed in terms of a series of propagation matrices. These are calculated by diagonalizing the Hessian matrix at configurations equidistant in time along a short piece of trajectory. Correlation functions calculated by this normal mode propagation (NMP) for a representative selection of atomic systems agree quantitatively with results from molecular-dynamics simulation.
Product representation of potential energy surfaces. II
Authors: Jäckle A. , Meyer H.-D.
Published: July 2002

An efficient method was recently introduced [J. Chem. Phys. 102, 5605 (1995); 104, 7974 (1996)] to represent multidimensional potential energy surfaces as a linear combination of products of one-dimensional functions, so-called natural potentials. Weight functions were shown to be easily implemented in the product representation scheme as long as they are separable, i.e., defined as a product of one-dimensional weight functions. Here the constraint imposed by the special product form of the separable weights is removed. Nonseparable weights are emulated by dividing the potential energy surface in arbitrary regions of minor and major physical relevance and by utilizing a so-called relevant region iteration procedure. Maintaining the advantageous computational scaling properties of the product representation scheme, this relevant region iteration procedure allows the stepwise improvement of the surface representation in the regions of major relevance. The quality of the product representation in the regions of minor relevance remains nevertheless acceptable. As a consequence, the number of potential expansion coefficients can be reduced substantially. The product representation of potential energy surfaces is especially well suited to be employed within the framework of the multiconfiguration time-dependent Hartree (MCTDH) approximation. To check the performance of the proposed method the Liu–Siegbahn–Truhlar–Horowitz (LSTH) surface is represented in Jacobian coordinates, and initial-state selected reaction probabilities for the H+H2(ν=j=0)→H2+H exchange reaction are computed.
Spectroscopy and dynamics of the H2–CN van der Waals complex
Authors: Chen Yaling , Heaven Michael C.
Published: July 2002

Spectra for the H2–CN and D2–CN complexes have been observed in association with the monomer B 2∑+–X 2∑+ and A 2∏–X 2∑+ transitions. Ground state dissociation energies of D0″=38±1 cm−1 (H2–CN) and D0″=42±1 cm−1 (D2–CN) were determined. Electronically nonadiabatic predissociations of H2/D2–CN(A) complexes were characterized using double resonance techniques.
Structure formation during the crystallization induction period of a short chain-molecule system: A molecular dynamics study
Authors: Takeuchi Hisao
Published: July 2002

A molecular dynamics simulation has been performed for a short chain-molecule system to study structure formation during the crystallization induction period. A model simulating n-alkane having 20 methylene units was used for the short-chain molecule. A model system containing either 250 or 2000 chains was quenched from a high temperature, and the structure formation was examined at constant temperature and pressure. The model system containing 250 chains eventually forms an ordered phase after the induction period, during which macroscopic quantities, such as volume, remain almost constant. In spite of these small changes in the macroscopic quantities, development of local parallel order is significant during the induction period. We also found that the development of local parallel order causes density fluctuations which appear as a weak small-angle peak in the structure factor. A characteristic length scale corresponding to the density fluctuations becomes larger as time elapses in the induction period. These features are qualitatively in accord with recent experimental findings on a different polymer system [M. Imai et al., Phys. Rev. B 52, 12 696 (1995)].
Effects of tetramethylurea on the tertiary structure of lysozyme in water
Authors: Castelletto Valeria , Arêas Elizabeth P. G. , Arêas José A. G. , Craievich Aldo F.
Published: July 2002

We studied by small angle x-ray scattering (SAXS) the tetramethylurea (TMU) effects on the tertiary structure of lysozyme in water. Samples containing TMU with mass fractions, w, ranging from 0 to 0.7 were analyzed. The SAXS results clearly demonstrate a structural transition from a spheroidal molecule (0⩽w⩽0.6) to a highly anisometric molecule for w=0.7. Additional SAXS results for samples with w=0.7, from which the TMU was removed, showed that this transition is reversible. The variation of the maximum dimension of the molecule with w fits an equation corresponding to a “two-state” model. At w=0.6 (critical fraction) a mixture of folded (spheroidal) and unfolded (anisometric) molecules coexists in the solution.
Phase behavior of ionic solutions: Comparison of the primitive and explicit solvent models
Authors: Shelley J. C. , Patey G. N.
Published: July 2002

Grand canonical Monte Carlo calculations are used to investigate the demixing transition in model ionic solutions where the solvent is explicitly included. Charged hard sphere ions in hard sphere, dipolar hard sphere and quadrupolar hard sphere solvents are considered and the results are compared with the primitive (continuum solvent) model. For all solvents considered, it is found that the demixing transition is in the same general region of the phase diagram and is roughly described by liquid-vapor equilibrium in the primitive model. However, details such as the precise location of the critical point and the width of the unstable region depend upon the exact nature of the solvent.