bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibrational spectral diffusion and population dynamics in a glass-forming liquid: Variable bandwidth picosecond infrared spectroscopy
Authors: Tokmakoff A. , Urdahl R. S. , Zimdars D. , Francis R. S. , Kwok A. S. , Fayer M. D.
Published: July 2002

The temperature-dependent vibrational population dynamics and spectral diffusion of the CO stretching mode of tungsten hexacarbonyl in 2-methylpentane are observed from the room temperature liquid to the low temperature glass using picosecond infrared transient grating and pump–probe experiments. These experiments were performed between 10 and 300 K on the triply degenerate T1u asymmetric CO stretching mode at 1984 cm−1 using pulses with bandwidths narrower and wider than the absorption bandwidth of the transition. The rate of vibrational population relaxation (100≤T1<150 ps) is observed to decrease with increasing temperature. The orientational dynamics for this transition are observed on a faster time scale than the population relaxation. Although the liquid viscosity changes over 14 orders of magnitude, the orientational relaxation rate slows by less than one order of magnitude over the full temperature range. By comparing polarization-dependent experiments performed with both narrow and broad bandwidth transform-limited pulses, it is possible to measure temperature-dependent spectral diffusion in both the liquid and the glass. The spectral diffusion and the orientational relaxation are shown to be intimately related. It is proposed that both arise from the time evolution of the superposition of the three degenerate states created by the excitation pulse.
Optical and vibrational properties of conjugated polymeric heterostructures
Authors: Botta C. , Destri S. , Porzio W. , Tubino R.
Published: July 2002

We present an optical and vibrational analysis of two copolymers consisting of alternating thienylene and phenylene blocks linked by azomethines moieties which might represent models of a one-dimensional superlattice. Both the pristine polymers and their GaCl3 complexes are studied. Their electronic structure is investigated with absorption-emission spectra, preresonant and resonant Raman scattering. Though the backbone conjugation is not interrupted by the phenylene moieties, resonant Raman modes of the thienylene and phenylene blocks are selectively enhanced by exciting in the visible and in the UV regions, respectively. Radiative recombination occurs within the thienylenic blocks which act as efficient trapping centers for the photoexcitations. While the thienylene ring vibrations feel the presence of the potential barriers and are confined within the thienylenic blocks of the copolymers, phenylene vibrations are characteristic of a phenylenic chain segment longer than the biphenylene block. By GaCl3 complexation, copolymers with lower energy gap are obtained. A weakening of the electronic density on the thienylene blocks is proposed to account for their vibrational and electronic properties.
Liquid phases of Langmuir monolayers
Authors: Schmid F. , Schick M.
Published: July 2002

The liquid phases of Langmuir monolayers are studied in a simple continuum model in which molecules are treated as flexible chains made up of rodlike segments, with the head confined to the air-water interface. The model is studied within mean-field approximation. We show that both properties of chain flexibility and of segment rigidity, with concomitant anisotropic rodlike interactions, are needed to produce two stable liquid phases. Uniform tilt can occur in the liquid-condensed phase, but requires a mismatch between the size of the head segment and that of the others comprising the chain. The chain conformational order, and the effect of chain length on phase behavior, are also discussed.
Properties and structure of crystal-solution interfaces of normal alkane crystals: Influence of solvents
Authors: Liu Xiang-Yang
Published: July 2002

The step energy of the {110} faces of n-paraffin crystals grown from various solutions is measured in thermal roughening experiments, and the values are compared with those calculated using a self-consistent field (SCF) lattice model calculation technique. The results turn out to be in good agreement. The interfacial fluid structure of n-paraffin crystals in various solutions is further examined on a molecular scale by means of SCF calculations. It is found that the interfacial structure of the solid–fluid interface of n-alkane crystals changed substantially, depending on the molecular structure and properties of the solvent. Subsequently, a ternary system, i.e., the n-C25H52–butanol-n-hexane solution, is analyzed. It follows that when the segmental density of the solute is kept constant, mixing a small amount of one solvent with another may lead to an increase in the step energy. Finally, the solvent-dependent critical behavior of the roughening transition for n-alkane crystals grown from various solutions is also discussed.
Charge renormalization at the large-D limit for N-electron atoms and weakly bound systems
Authors: Kais S. , Bleil R.
Published: July 2002

We develop a systematic way to determine an effective nuclear charge ZRD such that the Hartree–Fock results will be significantly closer to the exact energies by utilizing the analytically known large-D limit energies. This method yields an expansion for the effective nuclear charge in powers of (1/D), which we have evaluated to the first order. This first order approximation to the desired effective nuclear charge has been applied to two-electron atoms with Z=2–20, and weakly bound systems such as H−. The errors for the two-electron atoms when compared with exact results were reduced from ∼0.2% for Z=2 to ∼0.002% for large Z. Although usual Hartree–Fock calculations for H− show this to be unstable, our results reduce the percent error of the Hartree–Fock energy from 7.6% to 1.86% and predicts the anion to be stable. For N-electron atoms (N=3–18, Z=3–28), using only the zeroth order approximation for the effective charge significantly reduces the error of Hartree–Fock calculations and recovers more than 80% of the correlation energy.
Ammonia rotations in ytterbiumhexaammine
Authors: Nöldeke C. , Press W. , Asmussen B. , Damay P. , Leclercq F. , Foukani M. , Dianoux J.
Published: July 2002

The results of inelastic neutron scattering experiments on ytterbiumhexaammine are presented. At temperatures below 35 K, the dominant dynamical process is a uniaxial rotation of the ammonia in hindering potentials of about 35 meV. At higher temperatures, another type of ammonia motion is observed, which is described as reorientations of entire Yb(NH3)x complexes (x≊6).
Spin–orbit interaction between c 3Σ+ and B 1Π states of ScF: Effects on the fine and hyperfine structures
Authors: Lebeault-Dorget M.-A. , Effantin C. , d’Incan J. , Bernard A. , Shenyavskaya E. A. , Vergès J.
Published: July 2002

The introduction of an off-diagonal spin–orbit interaction element in the Hamiltonian matrix representing the close-lying B 1Π and c 3Σ+ states of ScF allows us to interpret the anomalies previously observed, i.e., the large Λ-type doubling in B 1Π and the large spin-splitting in c 3Σ+ at v=0, both of which decrease rapidly with increasing vibration. Deperturbed molecular constants for the v=0,1,2 levels in both states, together with values of the interaction parameter, are obtained from the numerical treatment of the wave numbers of 1454 lines in the B 1Π→X 1Σ+(0-1), (1-1), (2-0), (2-2), and c 3Σ+→X 1Σ+(0-1), (1-0), (2-0) bands. The interaction between B 1Π and c 3Σ+ is greatest at v=0 and decreases rapidly with increasing v. The model of the states including hyperfine effects is then used to interpret the observed hyperfine structure which is attributed to strong interaction between the 4sσ3dσ electrons and the 45Sc nucleus of spin 7/2. The hyperfine structure of the rotational levels in B 1Π(v=0,1,2) and c 3Σ+(v=0,1,2) is thus investigated from analysis of the line profiles in the Q branches of the two systems, the only ones to exhibit measurable hyperfine broadenings at the resolution achieved by Fourier transform spectrometry of the thermal emission of ScF. Line profiles are synthesized by convolution of the apparatus function with the intensity weighted Doppler profiles attached to the multiplet components positioned at calculated energies. A value of the Fermi contact term, bF=0.032±0.008 cm−1, in c 3Σ+ is derived by matching the calculated profiles to the experimental ones. Hyperfine interactions nondiagonal in J are shown to be negligible. The present value of bF for ScF in the c 3Σ+ state agrees well with the corresponding value for ScO in its ground state.
Nonadiabatic effects in avoided crossings of electronic potentials: An application of the generator coordinate approximation
Authors: Broeckhove J. , Keutgens W. , Lathouwers L. , Van Leuven P.
Published: July 2002

The generator coordinate approximation (GCA), a recently proposed nonadiabatic theory of molecular structure, is briefly reviewed. Results of this method, applied to a series of avoided crossings of electronic potentials in diatomics with different topologies, are presented. It is found that in all cases the GCA significantly improves the adiabatic vibrational energies. Large percentages of the nonadiabatic effects are removed. As the electronic energy gap at the crossing decreases, the GCA recovers a larger part of the nonadiabatic effect, and in the crossing limit, it becomes exact.
High approximations to the transport properties of pure atomic gases
Authors: Viehland Larry A. , Janzen Alec R. , Aziz Ronald A.
Published: July 2002

It is shown how to determine the matrix elements needed to calculate the viscosity and thermal conductivity of a pure atomic gas in arbitrarily high approximation to the solution of the Boltzmann equation. For the first time, explicit expressions are given for calculating the fourth and fifth approximations. Computer programs based on these expressions have been tested for various models of the interatomic interaction potential.
Geometric phase in two Kramers doublets molecular systems
Authors: Koizumi Hiroyasu , Sugano Satoru
Published: July 2002

The geometric phase in two Kramers doublets molecular systems is considered. We obtain the general formula for the gauge potential arising from the vibronic interaction and spin–orbit interaction between two Kramers doublet electronic levels. Simple models for the Jahn–Teller and Renner–Teller problems with spin–orbit coupling are considered. It is demonstrated that the energy spectra obtained by the Born–Oppenheimer approximation with the gauge potential agree quite well with the exact energy spectra in strong vibronic cases. It is also shown that the inclusion of the scalar gauge potential is important in order to obtain accurate zero point energy. As an application, vibronic levels of the X̃ 2E′ state of Cu3 are reexamined including spin–orbit interaction.
Thermodynamic and stochastic theory of nonequilibrium systems: Fluctuation probabilities and excess work
Authors: Peng Bo , Hunt Katharine L. C. , Hunt Paul M. , Suárez Alberto , Ross John
Published: July 2002

For a nonequilibrium system described at the mesoscopic level by the master equation, we prove that the probability of fluctuations about a steady state is governed by a thermodynamic function, the ‘‘excess work.’’ The theory applies to systems with one or more nonequilibrium steady states, for reactions in a compartment that contains intermediates Xj of variable concentration, along with a reactant A and product B whose concentrations are held constant by connection of the reaction chamber to external reservoirs. We use a known relation between the stationary solution Ps(X) of the master equation and an underlying stochastic Hamiltonian H: to logarithmic accuracy, the potential that gives Ps(X) is the stochastic action S evaluated along fluctuational trajectories, obtained by solving Hamilton’s equations of motion starting at a steady state. We prove that the differential action dS equals a differential excess work dφ0, and show that dφ0 can be measured experimentally in terms of total free energy changes for the reaction compartment and the reservoirs. Thus we connect the probability of concentration fluctuations in an open reaction compartment to thermodynamic functions for the entire closed system containing the compartment. The excess work dφ0 is the difference between the total free energy change for a specified change in the quantities of A, X, Y, and B in the state of interest, and the free energy change for the same changes in species numbers, imposed on the same system in a reference state (A,X0,Y0,B).

The reference-state concentration for species Xj is derived from the momentum pj canonically conjugate to Xj along the fluctuational trajectory. For systems with linear rate laws, the reference state (A,X0,Y0,B) is the steady state, and φ0 is equivalent to the deterministic excess work φdet* introduced in our previous work. For nonlinear systems, (A,X0,Y0,B) differs from the deterministic reference state (A,X*,Y*,B) in general, and φ0≠φdet*. If the species numbers change by ±1 or 0 in each elementary step and if the overall reaction is a conversion A→X→Y→B, the reference state (A,X0,Y0,B) is the steady state of a corresponding linear system, identified in this work. In each case, dφ0 is an exact differential. Along the fluctuational trajectory away from the steady state, dφ0≳0. Along the deterministic kinetic trajectory, dφ0≤0, and φ0 is a Liapunov function. For two-variable systems linearized about a steady state, we establish a separate analytic relation between Ps(X), φdet*, and a scaled temperature T*.
S
0↔S1 transition of trans-β-methyl styrene: Vibronic structure and dynamics
Authors: Haas Y. , Kendler S. , Zingher E. , Zuckermann H. , Zilberg S.
Published: July 2002

The fluorescence excitation and emission spectra of trans-β-methyl styrene have been measured in a supersonic jet. A complete vibrational assignment of the S0 and S1 states’ frequencies is reported, assisted by ab initio quantum chemical calculations and by comparison with the IR spectrum. The fluorescence lifetime, τf, of the isolated molecule changes monotonously from 24.5 to 15 ns as the excitation energy increases from the origin band to an excess of 3000 cm−1. The fluorescence quantum yield from the zero-point energy level of S1 is about 38%, similar to the liquid solution value; The major radiationless process being intersystem crossing to a triplet level. The increasing congestion of the emission spectra as the excitation energy is increased is interpreted as due to intramolecular vibrational energy redistribution. The data are consistent with the fact that in the isolated molecule intramolecular vibrational energy redistribution is faster than intersystem crossing. Beyond an excess energy of about 3200 cm−1, a more pronounced decrease in τf is observed, indicating that the barrier to trans–cis isomerization on the S1 surface, in the isolated molecule is higher than 3200 cm−1.
Absolute integrated cross sections for some O2 Herzberg I transitions near 248–249 nm
Authors: Bao Zhen-Chuan , Yu Wa On , Barker John R.
Published: July 2002

A frequency doubled tunable dye laser system with ∼0.4 cm−1 resolution was used to measure the integrated absorption cross sections of more than 20 rotational transitions in the O2 Herzberg I (A 3Σ+u←X 3Σ−g) 8-0 and 9-0 vibrational bands near 248 nm and 249 nm. Oxygen pressures from 200 to 800 Torr and path lengths from 5 to 25 m were employed. The measured absorbances were fitted using a nonlinear least squares analysis and Beer’s Law to obtain absolute values for the individual transition integrated cross sections in good agreement with a recent spectral simulation and experimental data. By using the spectral simulation in conjunction with the present experimental results, total oscillator strengths in reasonable agreement with literature values were estimated for the 8-0 and 9-0 vibrational bands.
Potential energy surfaces of C2v and D3h ozone complexes with Li+
Authors: Alcamí M. , Cooper I. L. , Mó O. , Yáñez M.
Published: July 2002

Ab initio molecular orbital calculations have been used to study the most important features of the potential energy surfaces corresponding to Li+ association to C2v and D3h ozone. For this purpose highly correlated techniques [CASSCF, QCISD, and QCISD(T)] have been used. Our results confirm that these highly correlated techniques are unavoidable in so far as a correct description of ozone–Li+ complexes is needed. Good agreement between CASSCF and QCI results is attained for C2v ozone when the UHF function is taken as the reference function and triple excitations are considered in the QCI treatment. Results for D3h ozone are in agreement only when a proper description of the π correlation is included in the CASSCF treatment. Interactions with Li+ are stronger for open chain ozone than for the cyclic isomer. Thus on complex formation the energy gap between C2v and D3h ozone increases. There exist three structures which are predicted to lie very close in energy. The global energy minimum corresponds to an isomer in which the Li+ bridges both terminal oxygens, but other structures corresponding to the attachment of Li+ to one of the terminal oxygens and to an insertion of the Li+ in one of the O–O bonds, respectively, are about 1 kcal/mol less stable.
Conditions for invariance of molecular magnetic properties in Landau gauge transformations
Authors: Lazzeretti P. , Malagoli M. , Zanasi R. , Caputo M. C. , Ferraro M. B.
Published: July 2002

General constraints for invariance of magnetic properties in a gauge transformation are analyzed. Sum rules relative to the transformation from Coulomb to Landau gauges are examined in particular. Numerical tests for hydrogen fluoride, water, ammonia, and methane molecule have been carried out in large basis set calculations, using random-phase approximation. The conditions for invariance are severe conditions for accuracy of variational molecular wave functions.
Comment on a proposed method for finding barrier height distributions
Authors: Zwanzig Robert
Published: July 2002

A recently proposed method for finding the distribution of potential energy barrier heights in many-body systems is tested on a simple one-dimensional potential. In this test, the method does not work.
The microwave spectrum and OH internal rotation dynamics of gauche-2,2,2-trifluoroethanol
Authors: Xu Li-Hong , Fraser G. T. , Lovas F. J. , Suenram R. D. , Gillies C. W. , Warner H. E. , Gillies J. Z.
Published: July 2002

The microwave spectra of CF3CH2OH and CF3CH2OD have been investigated from 5 to 26 GHz with a pulsed-nozzle Fourier-transform microwave spectrometer and from 26 to 42 GHz with an electric resonance optothermal spectrometer. Tunneling of the OH proton between the two isoenergetic gauche conformations splits the observed transitions into two tunneling components. An effective rotation-tunneling Hamiltonian is used to fit the aan both isotopomers to better than 5 and 13 kHz for the OH and OD forms, respectively. The tunneling splittings determined from the fits for the OH and OD isotopomers are 5868.6952(16) and 208.5037(42) MHz, respectively. A structural analysis using the moments of inertia of the OH and OD isotopomers determines that the hydroxyl hydrogen is directed toward the fluorine with a F...H separation of 2.561(1) Å and a dihedral angle of φ(CCOH)=68.97(6)°. The observed tunneling splittings are fit to a double-minimum potential, giving gauche–gauche tunneling barriers of 763 and 720 cm−1 and OH torsional fundamental frequencies of 364 and 271 cm−1 for CF3CH2OH and CF3CH2OD, respectively. The uncertainties shown in parentheses throughout the paper are one standard deviation.
Multiconfigurational self-consistent reaction field theory for nonequilibrium solvation
Authors: Mikkelsen Kurt V. , Cesar Amary , Ågren Hans , Jensen Hans Jo/rgen Aa.
Published: July 2002

We present multiconfigurational self-consistent reaction field theory and implementation for solvent effects on a solute molecular system that is not in equilibrium with the outer solvent. The approach incorporates two different polarization vectors for studying the influence of the solvent. The solute, an atom, a molecule or a supermolecule, is assumed to be surrounded by a linear, homogeneous medium described by two polarization vector fields, the optical polarization vector and the inertial polarization vector fields. The optical polarization vector is always in equilibrium with the actual electronic structure whereas the inertial polarization vector is not necessarily in equilibrium with the actual electronic structure. The electronic structure of the compound is described by a correlated electronic wave function—a multiconfigurational self-consistent field (MCSCF) wave function. This wave function is fully optimized with respect to all variational parameters in the presence of the surrounding polarizable dielectric medium having two distinct polarization vectors. We develop from a compact and simple expression a direct and second-order convergent optimization procedure for the solvent states influenced by the two types of polarization vectors. The general treatment of the correlation problem through the use of complete and restricted active space methodologies makes the present multiconfigurational self-consistent reaction field approach general in that it can handle any type of state, open-shell, excited, and transition states. We demonstrate the theory by computing solvatochromatic shifts in optical/UV spectra of some small molecules and electron ionization and electron detachment energies of the benzene molecule. It is shown that the dependency of the solvent induced affinity in benzene is nonmonotonic with respect the optical dielectric constant if inertial polarization effects also are accounted for.
Slowing down of the kinetics of liquid/liquid phase separation along the binodal curve of a binary liquid mixture with a miscibility gap approaching the critical point
Authors: Steinhoff B. , Woermann D.
Published: July 2002

A homogeneous mixture of iso-butoxyethanol/water of noncritical composition is brought from a state in the thermodynamic stable region of its phase diagram in the vicinity of its lower critical point to a metastable state close to the binodal curve by a fast pressure jump. It reaches its thermodynamic equilibrium state by separating into two coexisting liquid phases of different compositions. The emerging second liquid phase is formed by nucleation and growth of droplets. After a short transition time, the early stages of the diffusion limited growth of the droplets of the emerging second liquid phase are studied by light scattering (Mie scattering). The experiments are carried out as function of the temperature difference (Tp−Tc) along the two branches of the liquid/liquid coexistence curve (Tc, critical temperature; Tp, temperature of phase separation). The mutual diffusion coefficient D extracted from the kinetics of the droplet growth decreases approaching the critical point along the two branches of the binodal curve. The values of D obtained in this way are in good agreement with those characterizing the dynamics of concentration fluctuations in each of the coexisting phases in thermodynamic equilibrium monitored by dynamic light scattering experiments.
Molecular dynamics observed 60 ps behind a solid-state shock front
Authors: Lee I-Yin Sandy , Hill Jeffrey R. , Suzuki Honoh , Dlott Dana D. , Baer Bruce J. , Chronister Eric L.
Published: July 2002

Microfabricated monolithic shock target arrays with embedded thin layers of dye-doped polymer films, termed optical nanogauges, are used to measure the velocity and pressure (Us=3.5 km/s; P=2.1 GPa) of picosecond-laser-driven shock waves in polymers. The 60 (±20) ps rise time of absorbance changes of the dye in the nanogauge appears to be limited by the transit time of the shock across the 300 nm thick gauge. The intrinsic rise time of the 2 GPa shock front in poly-methyl methacrylate must therefore be ≤60 ps. These measurements are the first to obtain picosecond resolution of molecular dynamics induced by the passage of a shock front through a solid. Good agreement was obtained between the nanosecond time scale shock-induced adsorption redshift of the dye behind the P=2 GPa shock front, and the redshift of a nanogauge, under conditions of static high pressure loading in a diamond anvil cell at P=2 GPa. Transient effects on the ≊100 ps time scale are observed in the dye spectrum, primarily on the red absorption edge where hot-band transitions are most significant. These effects are interpreted as arising from transient overheating and subsequent fast cooling of the dye molecules behind the shock front.
Photoassociative spectroscopy of long-range states of ultracold 6Li2 and 7Li2
Authors: Abraham E. R. I. , Ritchie N. W. M. , McAlexander W. I. , Hulet R. G.
Published: July 2002

We have obtained spectra of the high-lying vibrational levels of the A1Σu+ and 13Σg+states of both 6Li2 and 7Li2 via photoassociation of colliding ultracold atoms confined to a magneto-optical trap. Because of the low collision energies (∼1 mK), very high spectroscopic resolution is realized, and sensitivity is greatest for the usually difficult to access long-range states. Binding energies relative to the center of gravity of the resolved hyperfine structure are given for the A1Σu+ vibrational levels v =62–88 for 6Li2 and v=65–97 for 7Li2, and the 13Σg+ vibrational levels v=56–84 for 6Li2 and v=62–90 for 7Li2. These are extremely long-range levels with outer classical turning points ranging from 25 a0 to 220 a0, where a0 is the Bohr radius. The spectra arise mainly from s-wave collisions, and therefore exhibit little rotational structure.
Phase instability of inhomogeneous gels crosslinked within the spinodal region
Authors: Nakazawa Hatsumi , Sekimoto Ken
Published: July 2002

We consider theoretically the gels crosslinked in the early stage of spinodal decomposition. A weakly crosslinked and homogeneous gel is brought into the thermodynamically unstable state, and additional crosslinks are introduced while the spinodal decomposition takes place. The inhomogenization of gel is ‘‘recorded’’ as position dependent distribution of chain conformations. The model is based on the theory of permanent set and that of nonequilibrium thermodynamics of gels. We investigate the effect of the rate of crosslinking on the process of spinodal decomposition. We also analyze the stability of the inhomogeneous gel thus processed. Our study shows that the spinodal instability may still occur in an inhomogeneous ‘‘opaque’’ gel, and a possible experimental test is proposed.
Computation of polarizability anisotropies for molecules in dilute solution: An approach to the local field correction
Authors: Sun Huai , Mumby Stephen J.
Published: July 2002

This study shows that calculated polarizability anisotropies for molecules in solution are highly dependent on the near-molecule local-field correction, which is neglected in the Lorentz continuum local-field approximation. A direct approach that relies on a knowledge of the solute molecular structure, the liquid structure of the solvent, and density functional theory (DFT) calculation techniques, is proposed to compute the local-field correction, and consequently the apparent polarizabilities for molecules in dilute solution in nonpolar solvents. Polarizability anisotropies for a group of organic molecules in solution in carbon tetrachloride are calculated, and the results compare favorably with experimental data.
Distribution of end-to-end length of an excluded-volume chain
Authors: Valleau John P.
Published: July 2002

Monte Carlo computations employing a pivot algorithm have been used to obtain rather precise results for a linear bead-straw chain. The results are used to discriminate between several proposals for the asymptotic long-chain universal form of the distribution of the end-to-end length. It is found that a prediction based on the analysis of des Cloizeaux is far superior to other candidates examined.
Electron attachment to isolated nucleic acid bases
Authors: Desfrançois C. , Abdoul-Carime H. , Schermann J. P.
Published: July 2002

Uracil, thymine, and adenine anions were produced in charge-exchange collisions with laser-excited Rydberg atoms. Anion creation rates for uracil and thymine exhibit Rydberg electron energy dependences which are interpreted as due to the creation of both dipole-bound and conventional (valence) anions while only dipole-bound anions are observed for adenine.
X-ray reflectivity study of behenic acid Langmuir–Blodgett mono- and multilayers on SiO2 surfaces as-deposited and after thermal treatment: Influence of substrate/film interactions on molecular ordering and film topology
Authors: Asmussen Axel , Riegler Hans
Published: July 2002

Langmuir–Blodgett films of various layer numbers of behenic acid on SiO2 surfaces were investigated by x-ray reflectivity before and after thermal treatment (annealing). The thicknesses of the monolayers at the substrate/film interface indicate that the molecules are oriented normal to the interface. For samples without further treatment, at some distance from the interface, three different layer spacings are found which correspond to molecular tilt angles of 21, 28, and 36 deg. The transition region of decreasing layer spacings extends roughly over the first five layers. The films are not in equilibrium after deposition. Their molecular packing and topology can be altered irreversibly by thermal treatment (annealing) at temperatures well below the main phase transition into an isotropic phase. Whereas the molecular ordering of the first monolayer is hardly affected by the heating, all other layer spacings are uniform after the heating, with molecular tilts of 36 deg. In the case of thick multilayers the thermally induced restructuring is accompanied by an increase of the total layer number due to the reduced layer spacing and material conservation. Thin multilayers breakup into multilayer islands and monolayer-covered substrate surface (dewetting).
Quantum state-to-state reaction probabilities for the H+H2O→H2+OH reaction in six dimensions
Authors: Zhang Dong H. , Light John C.
Published: July 2002

A time-dependent wave packet method has been employed to calculate the state-to-state reaction probability for the H+H2O(0,0,0)→H2(v1,j1)+OH(v2,j2) reaction for J=0 and initial nonrotating H2O on the modified Schatz–Elgersman potential energy surface in full six dimensions (6D). Starting from a wave packet for an atom–triatom asymptotic state in atom–triatom Jacobi coordinates, we transfer the wave packet to diatom–diatom Jacobi coordinates after the wave packet moves into the interaction region. Propagation is then carried out in the diatom–diatom Jacobi coordinates until the reaction flux measured in the diatom–diatom asymptotic region is converged.
New measurements of the a3 Σ+u state of K2 and improved analysis of long-range dispersion and exchange interactions between two K atoms
Authors: Zhao Guoxing , Zemke Warren T. , Kim J. T. , Ji Bing , Wang H. , Bahns J. T. , Stwalley W. C. , Li Li , Lyyra A. M. , Amiot C.
Published: July 2002

Resolved fluorescence from the K2 43 Σ+g state to the a3 Σ+u state has been measured by the perturbation-facilitated optical–optical double resonance (PFOODR) technique. Data have been fit to an improved set of molecular constants for the a3 Σ+u state. In particular, the new Te value for this state has been determined as 4197.935±0.047 cm−1, nearly 1.8 cm−1 higher than previously reported. By combining the new results for the a3 Σ+u state and the recent results for the ground X1 Σ+g state [J. Chem. Phys. 103, 3350 (1995)], we report in this paper an improved analysis of long-range dispersion and exchange interactions between two K atoms and of the X1 Σ+g and a3 Σ+u state dissociation energies De of 4450.674±0.072 cm−1 and 252.74±0.12 cm−1, respectively.
Adsorbed state of thiophene on Si(100)-(2×1) surface studied by electron spectroscopic techniques and semiempirical methods
Authors: Jeong Hyun Dam , Lee Yoon Sup , Kim Sehun
Published: July 2002

The adsorbed state of thiophene on Si(100)-(2×1) surface at 300 K has been investigated using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and ultraviolet photoelectron spectroscopy (UPS). (2×1) LEED pattern at 300 K is sustained after the saturated exposure of thiophene, and the saturation coverage is estimated to be ∼0.6 by AES, suggesting that thiophene molecule is chemisorbed molecularly on the Si(100) surface most likely by σ bonds between C and Si atoms. UPS spectrum for the chemisorbed thiophene shows not only the π orbital shift but also the σ orbital shift. Semiempirical PM3 calculations based on the cluster model propose that the thiophene molecule adsorbs on the Si(100)-(2×1) surface by forming di-σ bonds between C atoms of thiophene and Si atoms of the surface.
Effectiveness of nonequilibrium closures for the many body forces in concentrated colloidal dispersions
Authors: Lionberger R. A. , Russel W. B.
Published: July 2002

Here we present a theory for predicting the effect of interparticle interactions on the nonequilibrium dynamics of concentrated colloidal dispersions. A configuration-space conservation equation for the pair density P2 provides a fundamental basis for calculating the nonequilibrium microstructure; however, it includes pairwise additive three-body couplings. The resulting forces depend on the three-particle distribution function, necessitating an additional equation to completely specify P2. In this paper nonequilibrium Percus–Yevick and hypernetted chain closures complete the formulation by relating these forces to the interparticle force and pair distribution function. A computational algorithm exploiting Fast Fourier Transforms solves the resulting integro-differential equations for weak perturbations from equilibrium, yielding the perturbed pair density as a function of the volume fraction φ and the interparticle potential. The advantage of a fundamental approach is that clearly defined approximations lead from the characteristics of the individual colloidal particles to the nonequilibrium structure and macroscopic properties. The calculation of all dynamic properties, both rheological stresses and diffusion coefficients, is accomplished with the same approximations. Detailed predictions of the structure provide an additional comparison with simulation and experiment lacking in theories that calculate only bulk properties. The numerical methods demonstrated here allow efficient solution of a class of models more sophisticated than previously attempted. To test the merits of nonequilibrium closures we present predictions of the low-shear viscosity and long-time self-diffusion coefficient as a function of volume fraction for various repulsive potentials without hydrodynamic interactions. Comparison with results available from computer simulations demonstrates that the closures capture the trends in the transport properties with volume fraction and interparticle potential and yield realistic predictions for the nonequilibrium structure. The hypernetted chain closure yields the best agreement with the available data for bulk properties at moderate volume fractions (φ<0.4), but significant quantitative deviations appear at φ⩾0.45.
Density functional results for isotropic and anisotropic multipole polarizabilities and C6, C7, and C8 Van der Waals dispersion coefficients for molecules
Authors: Osinga V. P. , van Gisbergen S. J. A. , Snijders J. G. , Baerends E. J.
Published: July 2002

The generalized gradient-approximated (GGA) energy functionals used in density functional theory (DFT) provide accurate results for many different properties. However, one of their weaknesses lies in the fact that Van der Waals forces are not described. In spite of this, it is possible to obtain reliable long-range potential energy surfaces within DFT. In this paper, we use time-dependent density functional response theory to obtain the Van der Waals dispersion coefficients C6, C7, and C8 (both isotropic and anisotropic). They are calculated from the multipole polarizabilities at imaginary frequencies of the two interacting molecules. Alternatively, one might use one of the recently-proposed Van der Waals energy functionals for well-separated systems, which provide fairly good approximations to our isotropic results. Results with the local density approximation (LDA), Becke–Perdew (BP) GGA and the Van Leeuwen–Baerends (LB94) exchange-correlation potentials are presented for the multipole polarizabilities and the dispersion coefficients of several rare gases, diatomics and the water molecule. The LB94 potential clearly performs best, due to its correct Coulombic asymptotic behavior, yielding results which are close to those obtained with many-body perturbation theory (MBPT). The LDA and BP results are systematically too high for the isotropic properties. This becomes progressively worse for the higher dispersion coefficients. The results for the relative anisotropies are quite satisfactory for all three potentials, however.
Transient nutation electron spin resonance spectroscopy on spin-correlated radical pairs: A theoretical analysis on hyperfine-induced nuclear modulations
Authors: Weber Stefan , Kothe Gerd , Norris James R.
Published: July 2002

The influence of anisotropic hyperfine interaction on transient nutation electron paramagnetic resonance (EPR) of light-induced spin-correlated radical pairs is studied theoretically using the density operator formalism. Analytical expressions for the time evolution of the transient EPR signal during selective microwave excitation of single transitions are derived for a model system comprised of a weakly coupled radical pair and one hyperfine-coupled nucleus with I=1/2. Zero-quantum electron coherence and single-quantum nuclear coherence are created as a result of the sudden light-induced generation of the radical pair state from a singlet-state precursor. Depending on the relative sizes of the nuclear Zeeman frequency and the secular and pseudo-secular parts of the hyperfine coupling, transitions between levels with different nuclear spin orientations are predicted to modulate the time-dependent EPR signal. These modulations are in addition to the well-known transient nutations and electron zero-quantum precessions. Our calculations provide insight into the mechanism of recent experimental observations of coherent nuclear modulations in the time-resolved EPR signals of doublets and radical pairs. Two distinct mechanisms of the modulations are presented for various microwave magnetic field strengths. The first modulation scheme arises from electron and nuclear coherences initiated by the laser excitation pulse and is “read out” by the weak microwave magnetic field. While the relative modulation depth of these oscillations with respect to the signal intensity is independent of the Rabi frequency, ω1, the frequencies of this coherence phenomenon are modulated by the effective microwave amplitude and determined by the nuclear Zeeman interaction and hyperfine coupling constants as well as the electron-electron spin exchange and dipolar interactions between the two radical pair halves. In a second mechanism the modulations are both created and detected by the microwave radiation. Here, the laser pulse merely defines the beginning of the microwave-induced coherent time evolution. This second mechanism appears the most consistent with current experimental observations.
Electronic structure of poly-oxadiazoles
Authors: Brocks G. , Tol A.
Published: July 2002

The electronic and the geometrical structures of the polymers poly-1,3,4-oxadiazole and poly-(p-phenylene-1,3,4-oxadiazole) are studied using density functional calculations. These polymers are fully conjugated, having band widths which are comparable to the more common conjugated polymers. They have however a significantly higher ionization potential, which makes them suitable hole blocking materials for organic light emitting diodes. In addition poly-1,3,4-oxadiazole is a convenient electron transport material, due to its high electron affinity. Due to the presence of the phenyl rings, the electron affinity of poly-(p-phenylene-1,3,4-oxadiazole) is much lower.
Computational study of many-dimensional quantum vibrational energy redistribution. II. Statistics of the spectrum with dynamical Implications
Authors: Schofield Sarah A. , Wyatt Robert E.
Published: July 2002

We continue a study in which we statistically analyze the dynamics of vibrational energy flow in a model system of anharmonic oscillators which are nonlinearly coupled, with a local topology. Average spectra are obtained from individual spectra of many basis states of similar energy, for different values of the magnitude of the coupling between states. The survival probabilities of the density are then determined from the average spectra. When the average fluctuation in spectral intensities is small then the density survival probability closely follows the average survival probability presented in our earlier paper for short times. For longer times, when the average survival probability shows a power law decay, this decay does not appear in the density survival probability. In addition, when spectral fluctuations are large, the two survival probabilities differ strongly.
Virial/Fisher models of molecular cluster populations
Authors: Ford I. J.
Published: July 2002

The Fisher cluster model provides a phenomenological description of the populations of molecular clusters in a vapor. However, it was not designed for use at all cluster sizes, including monomers, although this is often assumed to be the case when constructing models of droplet nucleation. In order to study the accuracy of this assumption, a set of models is constructed in which, progressively, the monomer, dimer, and trimer populations are described using empirical virial coefficients, while the Fisher model accounts for higher clusters. The models are used to study the populations at the critical temperature, which is an important reference point for model parametrisation. Remarkably, the cluster populations are little changed by altering the crossover point between the virial and Fisher expressions, and furthermore, the original Fisher model provides a reasonable description even for monomers, as desired. The model is extended in one of a number of possible ways to describe cluster populations at temperatures below the critical point with less success: the Fisher model can then no longer account for the whole size distribution.
A model for low temperature electrochemical proton transfer: Temperature and isotope effects on kinetic parameters
Authors: Kornyshev A. A. , Kuznetsov A. M. , Stimming U.
Published: July 2002

Simple framework is suggested for low temperature proton transfer reactions at electrodes. Temperature and potential dependence of the electrode current and apparent transfer coefficient, αobs, are calculated. The results are in accordance with experimental observations [M. Cappadonia, S. Krause, and U. Stimming, Electrochim. Acta, 42, 841 (1997)], including the tendency for αobs extrapolated to T=0 K to approach negative values. This behavior of αobs results in a weak and rather inverse dependence of activation energy on overpotential and a strong potential dependence of the “entropic” factor. The observed isotope effect is basically accounted for by the change of the mass of the tunneling hydrogen isotope and the corresponding changes in the isotope-dependent characteristic frequencies of the medium.
Variation of dipole–dipole interaction with rotational state: Experiment and theory
Authors: van Duijn E. J. , Nienhuis G. , Hermans L. J. F. , Kuščer I.
Published: July 2002

An experimental investigation of the rotational-state dependence of intermolecular interactions for dipole–dipole systems was performed with the aid of light-induced drift. Data are presented of the relative change in collision rate upon excitation, Δν/ν, of HF with respect to polar and nonpolar collision partners (CH3F, CH3Cl, HCl, OCS, and CH4, H2, CO2, respectively). A continuously tunable color-center laser was used to rovibrationally excite HF in the fundamental vibrational band (v=0→1). Both P- and R-branch excitations were studied, with the rotational quantum number ranging from J=0 to 6. By combining Δν/ν for all pairs of P(J) and R(J−1) transitions, the separate v and J dependencies of the collision rate ν are obtained. It is found that for HF–CH3F ν decreases by more than 40%, and for HF–CH4 by only 4%, as J increases from 0 to 6. These data show that the familiar 1/r3 dipole–dipole interaction is highly J dependent. We attribute this to the increased averaging-out of the dipole–dipole interaction as the rotational quantum number increases. A theoretical treatment based upon the first Born approximation for the total cross section of two rotating dipolar molecules is proposed. Comparison between measurements and theoretical results shows good agreement.
Solvent-induced interactions between hydrophobic and hydrophilic polyatomic sheets in water and hypothetical nonpolar water
Authors: Koga Kenichiro , Zeng X. C. , Tanaka Hideki
Published: July 2002

Hydrophobic and hydrophilic interactions are two major intermolecular forces between hydrophobic nonpolar and hydrophilic polar sites of macromolecules or materials surfaces in solvents. To further understand these two interactions at the microscopic level, an idealized polyatomic model is devised, which includes hydrophobic, hydrophilic, and partially hydrophilic polyatomic planar square molecular sheets. The hydrophobic molecular sheet is composed of the Lennard-Jones particles while the hydrophilic molecular sheet consists of positive and negative charge sites. In the framework of the extended reference interaction site model integral equation theory the solvent-induced interactions (or the potential of mean forces) between two parallel molecular sheets in water and in the hypothetical nonpolar water are investigated in a systematic fashion. Such a highly idealized model allows us to isolate and to explore the important effects of molecular size, relative intermolecular position (e.g., in- or out-of-registry configuration), and hydrophilic site distribution on the hydrophobic and hydrophilic interactions in both water and the hypothetical nonpolar water. Significant insight into these effects at the molecular level is obtained. For the hydrophobic planar molecules in water we find solvent separated hydrophobic interaction becomes less favored as sheet size increases. Moreover, the contact hydrophobic interaction between two molecular sheets in the out-of-registry configuration is always most favorable. For the latter case we find it is the van der Waals attraction, rather than the hydrophobic attraction, that dominates the total interaction. We also find that in both water and the hypothetical nonpolar water the solvent-induced interaction between two hydrophobic sheets behaves similarly. One possible explanation is that the hydrophobic hydration originating from the hydrogen bonding network in water plays an insignificant role in the solvent-induced interaction, at least in the infinitely dilute aqueous solution. For hydrophilic planar molecular sheets in water, we find water-induced hydrophilic interaction is much more substantial compared with the hydrophobic one. In many cases, the hydrophilic interaction is found directly against the intermolecular force between two parallel molecular sheets in vacuum. Finally, for the partially hydrophilic planar molecules in water, a newly discovered feature is that a disperse hydrophilic site distribution gives rise to stronger solvent-induced interaction compared with the clustered hydrophilic site distribution.