bluuebunny/crossref_metadata_2025 huggingface dataset:
Interaction-induced infrared spectra of H2 dissolved in water and in carbon tetrachloride
Authors: Alvarez Jorge , Fernández Prini Roberto , Japas María L.
Published: July 2002

Interaction-induced infrared (IR) absorption bands for H2 dissolved in liquid water at room temperature are reported for the first time. The Q1(1) absorption band present in aqueous solution is compared to those observed for the same gas dissolved in CCl4 and in mixtures of CCl4 with acetonitrile. The analysis of the observed bands together with available information about the effect of electric fields on the integrated absorption of this normally forbidden band give an estimated maximum value for the local electric field on the solute molecule due to the solvent dipole moment. The results suggest that the electric field-induced dipole on H2 dissolved in water is very small compared to that induced in low density H2–H2O gaseous mixtures.
Dynamical aspects of anisotropic correlations in supercooled liquids
Authors: Mountain Raymond D. , Thirumalai D.
Published: July 2002

The dynamics of anisotropic correlation functions for the supercooled liquid states of soft-sphere mixtures is examined using molecular dynamics. Two measures of the local anisotropy are considered. These dynamic functions, which are representations of local cooperative motions, exhibit considerable slowing down long before the glass transition temperature is reached. It is clear that the decay of these functions over the time interval considered here cannot be fit by a single exponential function. The numerical evidence suggests that the data are consistent with the stretched exponential function exp[−(ωrt)β], where the exponent β is dependent on the degree of supercooling and on the specific correlation function. Estimates for the decay rate, ωr, provide a quantitative measure of the slowing down in the supercooled liquid of the decay of local, anisotropic correlations. The temperature dependence of the decay rate ωr for both measures is found to follow a Vogel–Fulcher equation. On the other hand, the dependence of the single particle self-diffusion coefficients on temperature seem to follow the Arrhenius equation. By analogy with the behavior observed in supercooled molecular liquids, it is suggested that these correlation functions provide a measure of α relaxation processes in these binary alloys.
Comment on the paper ‘‘Further contributions to the energy levels of a perturbed anharmonic oscillator: Application to adiabatic corrections’’
Authors: Ogilvie J. F.
Published: July 2002

An error in the theoretical treatment by Hadinger et al., using the experimental data on HCl of Coxon and Ogilvie, is pointed out.
Optical nonlinearity of a proposed quantum-confined semiconductor colloid
Authors: Horan P. , Blau W.
Published: July 2002

Optical saturation and degenerate four-wave mixing were observed in a bismuth tri-iodide (BiI3) colloid, which has been reported to exhibit quantum confinement effects. A third-order optical nonlinearity ‖χ(3)eff‖=1.4×10−10 esu was measured at 364 nm with ∼500 ps pulses. The temporal decay of the nonlinear mixing signal is slow (>6 ns) and shows acoustic oscillations. All results were found to be consistent with the formation of tri-iodide ions I−3 only, and the effects could be explained by photochemical dissociation. The four-wave mixing signal is thermal in origin.
Geometrical structures and vibrational frequencies of the energetically low-lying isomers of SiC3
Authors: Alberts Ian L. , Grev Roger S. , Schaefer Henry F.
Published: July 2002

The ground state of the SiC3 molecule is found to be a closed-shell cyclic C2v symmetry structure which can be described as a four-membered ring with a transannular (cross ring) carbon–carbon bond, r(C–C)=1.469 Å. Theoretical studies with a triple-zeta plus double-polarization function (TZ2P) basis set in conjunction with the configuration-interaction technique at the TZ2P self-consistent-field optimized geometries predict this rhomboidal structure to be 4.1 kcal/mol more stable than the linear triplet Si–C–C–C isomer. A second closed-shell rhomboidal C2v symmetry structure with carbon–silicon transannular bonding, r(Si–C)=1.880 Å, was located and characterized as a local minimum lying 4.3 kcal/mol above the ground-state rhomboidal structure at this level of theory. Higher-level theoretical methods, including contributions from triple excitations, with larger basis sets will be required to obtain a more definitive set of relative energies.
Construction of chromophore orbitals
Authors: Petke J. D.
Published: July 2002

A method is presented in which the delocalized orbitals of a large molecule containing one or more distinct chromophores are transformed to give orbitals which possess spatial properties similar to those of small isolated molecules which resemble each chromophore. The purpose of the method is to provide orbitals which, when employed in configuration interaction or other orbital-based calculations, permits excited state wave functions to be characterized in terms of local chromophore and/or charge-transfer excitations. The method is applicable to any set of orbitals given as a basis set expansion, obtained from either ab initio or zero differential overlap procedures. The effectiveness of the method is demonstrated with applications to a glycine dipeptide.
Trajectory study of Si4 formation and decay and of exchange and abstraction reactions in Si+Si3 collisions
Authors: Kay Ronald D. , Raff Lionel M. , Thompson Donald L.
Published: July 2002

The formation and subsequent decay of Si4 complexes as well as the direct exchange and abstraction processes in Si+Si3 collisions have been studied using quasiclassical trajectories on a new global Si4 potential energy surface fitted to available experimental and ab initio data, and on Bolding and Andersen’s (BA) recently formulated silicon potential for arbitrary cluster sizes. Cross sections for Si4 formation, σf(Et), were computed as a function of initial relative translational energy Et over the range 0.01 to 4.0 eV, with the Si3 internal energy described by the Boltzmann distribution at 800 K. The cross section was found to peak sharply near Et=0, as expected, and to fall off linearly at high energy. An analytical expression for kf(T), the thermal rate constant for Si4 formation, was found by averaging σf(Et) over the Maxwell–Boltzmann distribution for Et.

The analytical values of kf(T) lie between 6×1014 and 8×1014 cm3/mol s for the range 800–1500 K, and are in excellent accord with trajectory calculations of kf at 800 and 1200 K. Unimolecular dissociation rate constants for Si4, kd, were calculated as a function of Et over the 0.4 to 4.0 eV. The values of kd are well described by the RRK expression, with a value of 4.67 for the effective number of vibrational modes. Averaging the dissociation rate constant over the Maxwell–Boltzmann distribution yields an average Si4 lifetime of 413 ps at 800 K, which is not long enough for a stabilizing collision to occur at pressures characteristic of low-pressure CVD experiments. The direct exchange reaction is found to be unimportant for Et less than 1 eV, since for lower relative energies essentially all reactions proceed indirectly via Si4 complex formation. Direct atomic abstraction is energetically forbidden, on average, for Et less than 0.9 eV, and is unlikely for Et less than 2 eV. At higher energies, the end-atom exchange and abstraction channels, which are statistically favored over the apex-atom channels, are dynamically favored as well.

When exchange or abstraction proceeds indirectly via an Si4 intermediate, the distinction between apex-atom, end-atom, and no-reaction channels is lost. Both the direct and indirect pathways leave a large fraction of the energy and angular momentum in the reaction products. Cross sections for Si4 formation on the BA surface are smaller than those on the global Si4 surface due to the cutoff function in the BA two-body potential terms; Si4 dissociation rates for total energies between 1.3 and 2 eV above threshold agree to within a factor of 2.3 or better with corresponding values for the Si4 surface.
Generalized random sequential adsorption
Authors: Tarjus G. , Schaaf P. , Talbot J.
Published: July 2002

Adsorption of hard spherical particles onto a flat uniform surface is analyzed by using generalized random sequential adsorption (RSA) models. These models are defined by releasing the condition of immobility present in the usual RSA rules to allow for desorption or surface diffusion. Contrary to the simple RSA case, generalized RSA processes are no longer irreversible and the system formed by the adsorbed particles on the surface may reach an equilibrium state. We show by using a distribution function approach that the kinetics of such processes can be described by means of an exact infinite hierarchy of equations reminiscent of the Kirkwood–Salsburg hierarchy for systems at equilibrium. We illustrate the way in which the systems produced by adsorption/desorption and by adsorption/diffusion evolve between the two limits represented by ‘‘simple RSA’’ and ‘‘equilibrium’’ by considering approximate solutions in terms of truncated density expansions.
Relaxation of intermediate wave-vector density fluctuations in dense binary liquids
Authors: Vijayadamodar G. V. , Bagchi Biman
Published: July 2002

Binary liquids are known to be better glass formers than one component liquids. To understand this, we carried out a theoretical study of relaxation of density fluctuations in a dense liquid of binary hard spheres. This study is based on a coupled, modified, Smoluchowski equations where effects of interparticle interactions are included through two particle direct correlation functions of the binary mixture. Such a modified Smoluchowski equation description is reliable at intermediated values of wave vector k, but not at small k, so we have limited our study only to intermediate k. We find that the eigenvalues of the coupled equations show rich structure that depends on the composition of the binary liquid and also on the size ratio of the two constituents. At high density, the larger of the two eigenvalues becomes positive at intermediate wave vectors, indicating an instability of density fluctuation at that wave vector. As the density is further increased, the range of k over which the eigenvalue is positive, also increases. Although nonlinearities in density fluctuations and coupling with other hydrodynamic modes (neglected in this calculation) must be considered to gain an understanding of glass transition, the present results do indicate that the relaxation of density fluctuations in a binary liquid mixture is considerably different from that in a one component liquid.
On the topology of loop-erased self-avoiding random walks
Authors: Windwer S.
Published: July 2002

Loop-erased self-avoiding random walks were generated on a 5-choice cubic lattice. Knot formation in these walks were determined by counting the number of crossings in each walk. The results were fitted to a power law.
Convective–diffusive macrotransport processes in the presence of an inhomogeneous volumetric chemical reaction: Perturbation expressions for the effective reaction and transport coefficients in terms of comparable nonreactive properties
Authors: Pagitsas Michael , Brenner Howard
Published: July 2002

A systematic perturbation solution scheme is developed for calculating the laterally averaged effective reaction velocity constant K* for a chemically reactive solute undergoing a laterally inhomogeneous chemical reaction within a laterally bounded (but longitudinally unbounded) domain through which the solute is transported by convection and diffusion. This is accomplished by use of a perturbation operator technique which is used to obtain a time-dependent effective reaction velocity constant via a systematic perturbation expansion about the nonreactive, purely convective–diffusive Taylor dispersion state. The generally nonequilibrium initial spatial solute distribution requires use of a ‘‘fictitious’’ macroscale initial condition in the first-order macroscale linear decay law governing the total amount of solute present in the system at any time. This fictitious value differs from the true value of the total amount of solute initially present. Comparison is made between the present formalism and that developed by Wilemski-Fixman and others in the analysis of partially diffusion-controlled reactions of tracer corpuscles and polymers diffusing within laterally unbounded domains (from which convection is absent). An example drawn from this field confirms the viability of the scheme. Perturbative expressions are also derived for the mean tracer velocity vector U* and Taylor dispersion dyadic D* about this mean. The asymptotic expression obtained for the convective contribution to D* explicitly manifests the effect of the inhomogeneous chemical reaction upon this effective transport coefficient.
Icosahedral clustering in a supercooled liquid and glass
Authors: Kondo Toshiharu , Tsumuraya K.
Published: July 2002

We study the icosahedral clustering process in a supercooled single-component metal liquid by use of Voronoi polyhedron analysis. The number of the cluster increases with decreasing temperature from just below the melting temperature. The life of the cluster increases with decreasing temperature from slightly above the glass transition temperature Tg. The atoms with 0.56 fraction in the system become the central atoms of the cluster at least one time during the cooling process, although the total fraction of the clusters is 0.04 at temperatures far below the Tg. After atoms become the central atoms for the first time which we call fresh atoms, the atoms become central atoms of the other types of clusters, and after then revive as the central atoms of the icosahedral cluster. The life is longer for the revived atoms than for the fresh atoms in the low temperature region. The longer life is due to the energetic stability of the coordinate atoms around the revived atoms. We present atomistic transition processes from the cluster to the other types of clusters.
Photodissociation dynamics of carbon suboxide at 193 and 248 nm
Authors: Anderson D. J. , Rosenfeld R. N.
Published: July 2002

The nascent CO vibrational, rotational, and translational energies, following the UV photolysis of carbon suboxide at 193 and 248 nm, were determined by time-resolved tunable diode laser infrared absorption spectroscopy. A statistical model for energy disposal fits the experimentally observed vibrational distributions and average translational energies at both photolysis wavelengths. The model fails to account for the experimental rotational distributions that are much ‘‘colder’’ than the model predictions. An impulsive model for dissociation could not account for the observed energy partitioning at both photolysis wavelengths. These results suggest that at 193 nm the primary photoproducts are CO(X1Σ+) and C2O(ã1Δ), and that at 248 nm the primary photoproducts are CO(X1Σ+) and C2O(X̃3Σ−).
Molecular structure of first-row transition metal dihalides from combined electron diffraction and vibrational spectroscopic analysis
Authors: Hargittai Magdolna , Subbotina Natalya Yu. , Kolonits Maria , Gershikov Alexander G.
Published: July 2002

The average and equilibrium molecular geometries and vibrational characteristics of MnCl2, FeCl2, CoCl2, NiCl2, MnBr2, FeBr2, CoBr2, and NiBr2 have been determined by a combined analysis of gas-phase electron diffraction and vibrational spectroscopic data. The nozzle temperatures ranged between 960 and 1100 K in the electron diffraction experiments, paralleled with mass spectrometric control of the vapor composition. Four approximations have been examined for the molecular Hamiltonian in the joint analysis. The overall utility of combined analysis has been demonstrated. The dynamic behavior of the first-row transition metal dihalides is best described by the semirigid model. The equilibrium bond distance is best approximated by the cubic anharmonic potential. The cubic force constants and the bond Morse anharmonicity parameter can be determined even from electron diffraction data alone provided that experimental information is collected to large enough scattering angles. The present analysis has confirmed the linearity of equilibrium configuration for all dihalides studied. The thermal average and equilibrium bond distances obtained are as follows, rg (re) with estimated total errors not exceeding ±0.007 Å, MnCl2 2.202 (2.184), FeCl2 2.151 (2.128), CoCl2 2.113 (2.090), NiCl2 2.076 (2.056), MnBr2 2.344 (2.328), FeBr2 2.294 (2.272), CoBr2 2.241 (2.223), and NiBr2 2.201 (2.177) Å. Appreciable amounts of dimeric species, with four-membered ring structures, were detected in the vapors of FeCl2, CoCl2, and FeBr2 under the electron diffraction experimental conditions. The largest amount, 11%, occurred for iron dibromide and a reliable geometry was determined for the Fe2Br4 molecule. The bridging metal–halogen bonds are about 0.2 Å longer than the terminal bonds, and the monomer bonds, in all three systems. The applicability of the joint diffraction/spectroscopic analysis is severely reduced in the presence of dimeric species.
Dynamic magnetic resonance line shapes in a symmetric threefold potential
Authors: Zamir S. , Poupko R. , Luz Z. , Alexander S.
Published: July 2002

A method for calculation of dynamic nuclear magnetic resonance (NMR) line shapes of molecules undergoing reorientation diffusion about a single rotation axis in a symmetric threefold potential is developed using the general theory of Freed. The method is used to calculate deuterium NMR spectra of molecules undergoing reorientational diffusion in a model potential of the form V(φ)=−V0 cos 3φ, as a function of the diffusion constant DR, and the potential strength V0. It is found that for V0 smaller than kT the resulting line shapes are very similar to those obtained in a potential-free diffusion. When V0>kT the uneven distribution of the molecules and the hindering effect of the potential barrier have significant effects on the line shape. In this region two distinct motional effects of the diffusion process are observed: (i) At low DR values molecular diffusion within the potential wells results in averaging of the local distribution and consequently to line narrowing. (ii) At sufficiently high DR values diffusion between the potential wells results in lifetime broadening effects similar to those observed in jump processes. Relations between the diffusion constants and the discrete jump rates are discussed.
The onset of intramolecular vibrational energy redistribution and its intermediate case: The ν1 and 2ν1 molecular beam, optothermal spectra of trifluoropropyne
Authors: Pate B. H. , Lehmann K. K. , Scoles G.
Published: July 2002

Using the optothermal method for molecular beam, infrared spectroscopy, we have measured both the fundamental and first overtone of the acetylenic C–H stretch in CF3CCH. In the fundamental we observe a spectrum which shows only few perturbations. The majority of lines can be successfully fit to a model assuming an anharmonic coupling of the C–H stretch to a single, near-resonant background state with a coupling matrix element of 0.006 cm−1. We have observed other perturbations in this spectrum, including a state coupled by a weak perpendicular Coriolis interaction. All observed couplings are very weak and local in nature. In the overtone, where the density of background states increases by a factor of 100, we observe a spectrum characteristic of a system in the intermediate case of IVR (intramolecular vibrational energy redistribution). Analysis of the R(0) and P(1) transitions provides a homogeneous IVR lifetime of about 2 ns, which is long compared to lifetimes generally quoted for overtone vibrational relaxation. The root-mean-square coupling matrix element in the overtone is about 0.0008 cm−1. The higher J transitions in the overtone suggest that Coriolis interactions are present in the spectrum. The interpretation of these spectroscopic results in the context of IVR is discussed.
Local scaling analysis of state-to-state rotational energy-transfer rates in N2 from direct measurements
Authors: Bonamy L. , Thuet J. M. , Bonamy J. , Robert D.
Published: July 2002

An analysis of recent rotational energy state-to-state rates data in N2 is achieved by using an exact local equation valid within the energy corrected sudden (ECS) frame. This equation connects the rates belonging to each group of five cross-shaped elements characterized by the rotational quantum numbers {(J,J′), (J,J′+2), (J, J′−2), (J−2,J′), (J+2,J′)}. Such an analysis is free of any assumption on the dependence of the basis rates on the rotational energy. It allows one to perform a study of the local validity of the scaling law and a more direct estimation of the scaling length. Previous ECS studies, only based on the inversion–fitting procedure of line-broadening coefficients, are compared with the present results. It is shown that, in spite of good agreement of these last phenomenological approaches with stimulated Raman profiles for compressed N2 gas, they may be unadapted to describe other physical properties. This point is clearly illustrated by the behavior of the rotational angular momentum correlation function. The origin of such a discrepancy is evidenced and a convenient modification of the adiabaticity correcting factor is introduced. Finally, excellent consistency with available experimental data is obtained.
An analysis of the methyl rotation dynamics in the S (X̃ 1A1) and T1 (ã 3A2) states of thioacetone, (CH3)2CS and (CD3)2CS from pyrolysis jet spectra
Authors: Moule D. C. , Smeyers Y. G. , Senent M. L. , Clouthier D. J. , Karolczak J. , Judge R. H.
Published: July 2002

Jet-cooled, laser-induced phosphorescence excitation spectra (LIP) of thioacetone (CH3)2CS/(CD3)2CS have been recorded over the region 16 800–18 500 cm−1 using the pyrolysis jet spectroscopic technique. The responsible electronic transition, T1←S0, ã 3A″←X̃ 1A1, results from an n→π* electron promotion and gives rise to a pattern of vibronic bands that were attributed to activity of the methyl torsion and the sulphur out-of-plane wagging modes. The intensities of the torsional and wagging progressions in the excitation spectra were interpreted in terms of a C2v–Cs molecular distortion of the triplet molecule from its singlet ground state equilibrium structure. A complete unrestricted Hartree–Fock (UHF) ab initio molecular orbital (MO) structural optimization of the T1 state predicted that the sulphur was displaced by 27.36° from the molecular plane and the methyl groups were rotated by 10.93° in clockwise–counterclockwise directions. Restricted Hartree–Fock (RHF) calculations were used to generate the V(θ1,θ2) potential surface governing methyl rotation for the S0 state. This was incorporated into a two-dimensional Hamiltonian, symmetrized for the G36 point group and solved variationally for the torsional frequencies. The calculated frequencies of 159.97/118.94 for the ν17(b1) mode of S0 (CH3)2CS/(CD3)2CS were found to agree with the experimental values, 153.2/114.7 cm−1.
Electron tunneling pathways in proteins: A method to compute tunneling matrix elements in very large systems
Authors: Onuchic José Nelson , de Andrade Paulo C. P. , Beratan David N.
Published: July 2002

A tight-binding Hamiltonian and Dyson’s equation method are described that allow the computation of the tunneling matrix elements between electron donor and acceptor sites in a protein. The method is exact and computationally tractable. The Green’s function matrix elements of the bridge are computed using a strategy that builds up the bridge one orbital at a time, allowing inclusion of all orbitals on proposed tunneling pathways and elsewhere. The tunneling matrix element is determined directly from the bridge Green’s function. A simple representation of a helical protein segment is used to illustrate the method and its ability to include contributions from high-order backscattering and multiple pathway interference in the donor–acceptor coupling.
Generalized Flory equations of state for square-well chains
Authors: Yethiraj Arun , Hall Carol K.
Published: July 2002

The hard chain Dickman–Hall generalized Flory (GF) and Honnell–Hall generalized Flory-dimer (GF-D) equations of state are extended to square-well chain fluids. The molecules are modeled as a pearl necklace of freely jointed spheres that interact via site–site square-well intermolecular potentials. Equations of state for square-well monomers and square-well dimers (required in the GF-D theory) are obtained from integral equations with a mean spherical approximation (MSA) closure. The theories are compared to Monte Carlo simulation data for the pressure of square-well 4-mers, 8-mers, and 16-mers. The GF-D theory is in excellent agreement with the simulation data; the GF theory overestimates the pressure in all cases. A closed-form equation of state for square-well chains is obtained by employing equations of state for square-well monomers and for square-well dimers using second order perturbation theory. The resulting equation is very accurate when compared to simulations, but not as accurate as when the monomer and dimer equations of state are obtained via the MSA. The effect of intramolecular attractions on the compressibility factor is investigated via Monte Carlo simulation and the GF-D theory. It is found that, for the chain lengths studied, intramolecular attractions have a very small effect on the pressure of the system.
Predissociation lifetimes of OH A 2Σ+ (v′=3) obtained from optical–optical double-resonance linewidth measurements
Authors: Gray Jeffrey A. , Farrow Roger L.
Published: July 2002

The excitation of rapidly predissociating v′=3 levels in the A 2Σ+ state of OH has recently been suggested as a means of avoiding collisional quenching corrections when performing laser-induced fluorescence (LIF) measurements in flames. We report measurements of predissociation rates of several rotational levels in v′=3, obtained from linewidths in optical–optical double-resonance (OODR) spectra. The magnitude of the observed predissociation rates places an upper limit on the pressures at which OH LIF can be considered ‘‘quench-free.’’ In addition, analysis of spectral line shapes suggests that rotational transfer can significantly affect LIF quantum yields at pressures of 1 atm and higher. We conclude that variations of the predissociation rate with rotational level in v′=3 must be included in determinations of rotational temperatures from LIF.
Diagram techniques for solving Schwinger–Dyson equations: Electron transfer pathways in biological molecules
Authors: Magarshak Y. , Malinsky J. , Joran A. D.
Published: July 2002

A graph method is developed to solve Schwinger–Dyson equations for electron transfer reactions in biological molecules. Feynman diagrams provide a convenient technique for the calculation of self-energy. Multiple pathway mechanisms of electron transfer can be examined by splitting of the graphical representation into clusters in case of rate-limiting steps. The approximation of weak hopping greatly simplifies the problem of calculating Green functions, which powerfully express a number of characteristics of the process of electron transfer such as the spectral density of states and the correlational function. Rules of graph transformations are derived, and applied to calculate Green matrix elements corresponding to a single hydrogen bond-coupled path in polypeptides, and to the case of the through-backbone pathway. The relation between cluster graphs and Feynman diagrams in locator representation is discussed. Formulas up to the second-order perturbations for linear structure of the cluster graph are given. Calculations of the electron transfer rate dependence on donor–acceptor distance are presented. It is shown that taking into account the second-order perturbation makes the dependence of the logarithm of the electron transfer rate on donor–acceptor distance nonlinear. This effect is especially significant for large distances.
Statics and dynamics of the two-spin–facilitated kinetic Ising model
Authors: Reiter Johannes
Published: July 2002

The two-spin–facilitated kinetic Ising model, or Fredrickson–Andersen model, in two dimensions is investigated, where down-spins are energetically favored over up-spins due to a suitably oriented magnetic field and the only interaction between spins is a dynamic constraint which allows any spin to flip only if it has at least two nearest up-spin neighbors. From Monte Carlo simulations it is known that the model exhibits glasslike behavior where the autocorrelation time of a spin as a function of temperature is well described by the Adam–Gibbs expression. Due to the dynamic constraint, the state space is divided into nonconnected partitions the most important of which is the high temperature partition which includes the all-spins-up state. A proof given earlier by Fredrickson and Andersen is completed, which states that for infinitly large lattices and for a temperature larger than zero, the probability that a randomly picked state is part of the high temperature partition approaches unity. It is also shown that the system is ergodic, i.e., that no glass transition may occur at a temperature larger than zero. These results hold also for the model in higher dimensions than two and for the three-spin–facilitated kinetic Ising model in three dimensions. It is suggested that the model equilibrates by cooperative diffusion of a critical droplet of up-spins.
Low-energy excitations in amorphous polymers
Authors: Inoue K. , Kanaya T. , Ikeda S. , Kaji K. , Shibata K. , Misawa M. , Kiyanagi Y.
Published: July 2002

In order to investigate low-energy excitations below 10 meV, inelastic neutron-scattering measurements have been carried out on various organic amorphous polymers at 10 and 50 K and three inorganic glasses at 150 and 295 K. It was found that a broad excitation peak is observed for all amorphous materials in the ω range of 1.5–4.0 meV irrespective of different chemical structures. On the other hand, highly crystalline polyethylene with a degree of crystallinity 0.96 shows no such broad peak, indicating that the low-energy excitation is characteristic of amorphous materials. We have employed an asymmetric double-well potential as a common origin for the low-energy excitation in amorphous materials and analyzed the results of amorphous polyisobutylene to confirm validity of this model and alternatively to determine parameters of the potential. Analysis of the temperature dependence of the inelastic-scattering intensity of the low-energy excitation leads to a concept of phonon-assisted tunneling in the asymmetric double-well potential.
The infrared spectrum of clustered carbon dioxide: An interpretation via bulk optical constants
Authors: Barnes J. A. , Gough T. E. , Stoer M.
Published: July 2002

The diffusive trapping technique, in which a condensable vapor is injected into a cold buffer gas, has been used to produce clusters of carbon dioxide and nitrous oxide containing approximately 109 molecules. The infrared spectrum of these clusters was recorded using a Fourier-transform spectrometer. Major features of the carbon dioxide spectrum were reproduced by solving Maxwell’s equations for light interacting with a sphere having a complex dielectric response equal to that of solid carbon dioxide. The calculation also allowed for an estimation of the cluster size. Additional features of the spectrum suggest the presence of edges and corners on the clusters.
Line mixing in the Q branches of the ν1+ν2 band of nitrous oxide and of the (1110)I←(0220) band of carbon dioxide
Authors: Margottin-Maclou M. , Henry A. , Valentin A.
Published: July 2002

Line mixing effects have been observed in absorption spectra recorded, at room temperature, with a Fourier spectrometer. They concern the Q branches of the ν1+ν2 band of N2O, centered at 1880.2 cm−1 and of the (1110)I←(0220) band of CO2, centered at 740.8 cm−1. For the N2O spectra, the gas was pure and its pressure has been raised up to more than 6 atm. The CO2 spectra were registered with a total gas pressure below 1 atm and the gas was either pure or mixed with argon or nitrogen. Comparison is made between the experimental results and computations taking into account the off-diagonal elements of the relaxation matrix.
Molecular dynamics monitored by cross-correlated cross relaxation of spins quantized along orthogonal axes
Authors: Brüschweiler Rafael , Ernst Richard R.
Published: July 2002

A new approach is proposed for the analysis of molecular dynamics on a wide range of time scales by the measurement of specific cross-correlated nuclear magnetic resonance cross-relaxation pathways of spins quantized along orthogonal spin-lock axes. Cross-correlated time modulation of (i) chemical shifts and J couplings, and of (ii) dipolar coupling and chemical shielding anisotropy, respectively, are determined using a sensitive technique, called ortho-ROESY (orthogonal rotating frames Overhauser effect spectroscopy). The two processes (i) and (ii) can be separated experimentally by their distinct dependence on the spin-lock radio frequency amplitude. The relaxation rate constant depends on the power spectral density sampled at a frequency close to zero. This renders the experiment attractive for investigations of slowly tumbling (bio)molecules.
Photoinduced dissociative electron attachment of CH3Br on Pt(111): The role of the local work function
Authors: Ukraintsev V. A. , Long T. J. , Gowl T. , Harrison I.
Published: July 2002

The photoinduced dissociative electron attachment of CH3Br on Pt(111) has been examined by measuring CH3 photofragment translational energy distributions from CH3Br adsorbed on Xe multilayers deposited upon CH3Br covered Pt(111). Subvacuum level, photoexcited substrate electrons produced by 308 nm laser irradiation were found to propagate through the Xe layer where attachment to CH3Br led to fragmentation. A simple model for surface dissociative electron attachment, DEA, was found to quantitatively predict the observed fragmentation dynamics for the CH3Br/Xe/CH3Br/Pt(111) system. The role of the local work function in surface DEA processes was explored by examining the dependence of the photofragment yield upon the thickness of the intermediate Xe layer and the fractional coverage of the Pt(111) bound first layer of CH3Br. Three dimensional variation of the local work function above the substrate was required to account for the observed DEA dynamics.
Quadratic response of a chemical reaction to external oscillations
Authors: Astumian R. Dean , Robertson Baldwin , Li Ru Sheng , Ross John
Published: July 2002

We develop a second-order response theory to investigate the effects of external periodic perturbations on a chemical reaction at a stable steady state in an open reactor. We apply the theory to the quadratic Schlögl model, a single-variable nonlinear reaction. In the presence of oscillating reactant or product concentrations or oscillating rate coefficients, the average intermediate concentration, the fluxes, and the dissipation are each a Lorentzian function of frequency with midpoint at the inverse relaxation time of the system. Thus even very short relaxation times can be determined by measuring average rates as a function of frequency of the perturbation. The amplitude of the Lorentzian depends on the chemical mechanism of the reaction and is proportional to the square of the amplitude of the applied perturbation. We also show that energy from the perturbation can be used to drive the reaction in a direction opposite of that predicted by the Gibb’s free energy difference of reactants and products, even under circumstances where the overall affinity is independent of the perturbation.
Optical–optical double resonance spectroscopy of Cl2: Analysis of the 1g(3P1)–A 3Π(1u) system
Authors: Ishiwata Takashi , Si Jin-hai , Obi Kinichi
Published: July 2002

The 1g (3P1)–A 3Π(1u) transition is analyzed by optical–optical double resonance using the 1g (3P1)–A 3Π(1u)–X 1Σ+g photoexcitation sequence. The analysis of the 409 transitions in the v′=0–14 and J′=2–51 range of 35Cl2 isotope species yields a set of molecular parameters for the 1g(3P1) state in a Dunham-type expansion: Y00=59 295.651(5), Y10=256.6412(30), Y20=−1.205 36(48), Y30=3.724(22)×10−3, Y01=0.114 313 8(60),Y11=−7.507(15) ×10−4, Y21=1.96(13)×10−6, and Y02=−8.10(22)×10−8 with the Ω-type doubling constant, qv=B(f)v−B(e)v =[3.176(31)−0.0660(50)×(v+1/2)]×10−4 (all in cm−1 and σ in parentheses). An empirical model is given to interpret the spin–orbit couplings between the nascent six ion-pair states of g-type symmetry correlating to Cl−(1S)+Cl+(3P).
Velocity autocorrelation functions of particles and clusters in liquids. A possible criterion for correlation length of incipient glass formation
Authors: Variyar Jayasankar E. , Kivelson Daniel , Lynden-Bell R. M.
Published: July 2002

We have carried out molecular-dynamics simulations over a range of densities in two and three dimensions for particles that interact through soft repulsive potentials. We have also carried out calculations of the corresponding systems in which all particles except a tagged particle and its neighbors within a certain distance are frozen. Velocity autocorrelation functions for a single particle, for clusters containing the particle, and for the velocity of the particle relative to an embedding cluster were obtained. The single-particle velocity autocorrelation function can be resolved into correlation functions describing the local rattling in a cage or a cluster, the motion of the cluster itself, and a small cross-correlation term; the function for the single particle is sensitive to the structure of the fluid over a much shorter time scale than are those of clusters, and the shape of the single-particle velocity autocorrelation function comes primarily from rattling motion within a cage. We show that the velocity autocorrelation functions of clusters are probably better probes than that for the single particle for investigating incipient glass formation since they can be used to establish a correlation length which increases when a liquid is cooled. The dynamics of clusters at a given state point depend upon their sizes, and the nature of their motions changes qualitatively from ‘‘rattling’’ for small to ‘‘diffusional’’ for large clusters, the ‘‘critical’’ size at which the change occurs increasing with decreasing temperature. A simple model for this cluster behavior is presented.
Triplet Rydberg states of the imidogen radical characterized via two-photon resonance-enhanced multiphoton ionization spectroscopy
Authors: Clement Simon G. , Ashfold Michael N. R. , Western Colin M. , Johnson Russell D. , Hudgens Jeffrey W.
Published: July 2002

Five new triplet excited states of the ND radical (three in the case of NH) in the wave-number range 85 000–91 000 cm−1 have been identified through analysis of the two-photon resonance enhancements they provide to the wavelength-resolved multiphoton ionization spectrum of X 3Σ− state NH(ND) radicals. The lowest energy of these, the B 3Π state, is found to be a ‘‘regular’’ Rydberg state which, on the basis of its observed quantum defect and its deduced rotational and spin–orbit coupling constant, is surmised to be the 3Π state derived from a 3pσ electron built on the 2Π ground-state ion core. Perturbations are evident in the B 3Π–X 3Σ− origin bands of both NH and ND. In the case of ND the perturbing state provides its own resonance enhancements, the analysis of which enables its definitive identification as the C 3Σ− state. The very small spin–orbit splitting found for the D 3Π state is taken to indicate that (at least in the Franck–Condon region) its wave function is dominated by the configuration involving one 3pπ Rydberg electron and a 4Σ− ion core. To still higher wave number we identify two more 3Σ− excited states, the upper of which (the F 3Σ− state) has a very small rotational constant which we take to imply that it has substantial valence character. Further indications that the F 3Σ− (and C 3Σ−) states possess significant valence character is provided by the observation that both parent and daughter (N+) ions contribute to the overall ion yield when the multiphoton ionization proceeds via these two states. Daughter-ion formation is considered to occur via an overall four-photon excitation process in which the coherent two-photon excitation to the 3Σ− state of interest is followed by a one-photon excitation to a ‘‘superexcited’’ state of the neutral. This is then presumed to absorb a further photon to yield the observed N+ ions and/or to predissociate, yielding highly excited N* atoms which then undergo a direct one-photon ionization.
Total N-channel control in the weak field domain
Authors: Shapiro Moshe , Brumer Paul
Published: July 2002

The existence of a complete solution to the problem of controlling molecular photodissociation using weak laser fields is explored from the perspective of the theory of coherent radiative control. Specifically, the usual two-path coherent radiative control theory is extended to the N-path case and the magnitudes and phases of a series of light frequencies which generate a particular distribution of product photofragments are obtained. Results show that complete control over the distribution of product yields is possible if (a) the continuum absorption spectrum is composed of a series of sufficiently narrow resonances, and (b) that the photodissociation process is nonfactorizeable, i.e., it cannot be broken up to the excitation of a ‘‘bright’’ state and dissociation to a set of ‘‘dark’’ states.
Laser-induced interaction of ammonia with GaAs(100). I. Dissociation and nitridation
Authors: Zhu X.-Y. , Wolf M. , Huett T. , White J. M.
Published: July 2002

UV laser irradiation of ammonia adsorbed on GaAs(100) leads to molecular desorption and dissociation. A nitride passivation layer can be formed on the GaAs surface at 100 K by simultaneous exposure to ammonia and uv photons in a UHV environment. The nitride layer consists of a mixture of Ga and As nitrides. While the dominating GaN surface species is thermally stable, AsN desorbs below 800 K. Surface NH2 is identified as an intermediate. The implication of this study for selective area passivation and GaN growth is discussed.
Atomic level simulations on a million particles: The cell multipole method for Coulomb and London nonbond interactions
Authors: Ding Hong-Qiang , Karasawa Naoki , Goddard William A.
Published: July 2002

The N2 computations implicit in the Coulomb and other long range interactions remain the critical bottleneck in atomic-level simulations of the structure and dynamics of large systems. We report here the cell multipole method which scales linearly with N and requires only modest memory. To demonstrate the feasibility of this approach, we report systematic calculations on realistic polymer systems with up to 1.2 million atoms on a laboratory workstation. The method becomes faster than the exact method for systems of 300 atoms, and for a 1.2 million-atom polymer, it is 2377 times faster. The method treats a class of interactions of the form qiqj/rijp, which includes Coulomb (p=1), London dispersion (p=6), or shielded Coulomb (p=2) interactions. This method is well suited for highly parallel and vector computers.
What is a typical dipolar coupling constant in a solid?
Authors: Lacelle Serge , Tremblay Luc
Published: July 2002

Some statistical properties of the distributions of dipolar coupling constants and their products are discussed in the context of nuclear magnetic resonance (NMR) of solids. Transformations of the distributions (absolute values and their logarithms) reveal new statistical properties. The ‘‘relative fluctuations’’ of the absolute value distributions, i.e., (second absolute central moment)1/2/(first absolute moment) are shown to scale as N+1/2, where N is the number of spins. The ratio (first absolute moment)/(most probable value) is shown to diverge as N+3/2. Products of the absolute values of dipolar coupling constants follow lognormal distributions in large spin networks. The relative fluctuations of these products also scale with N+1/2. Numerical studies of spin systems, with 10≤N≤105, in one-, two-, and three-dimensional crystalline lattices support several of these results. Some implications for multiple quantum NMR spectroscopy are presented.
Dynamic scattering function of a semidilute suspension of hard spheres
Authors: Cichocki B. , Felderhof B. U.
Published: July 2002

We study the time-dependent intermediate scattering function of a semidilute colloidal suspension of hard spheres with neglect of hydrodynamic interactions. The dynamics of the suspension is assumed to be governed by the generalized Smoluchowski equation. The Laplace transform of the scattering function involves the static structure factor and an irreducible memory kernel. The latter is approximated by the first term of its expansion in powers of volume fraction. The time-dependent scattering function is found by integration over the spectrum of relaxation rates.
Attractive double layer forces from hard-core correlations
Authors: Bolhuis P. G. , Åkesson T. , Jönsson Bo
Published: July 2002

The osmotic pressure in an electric double layer is studied by means of Monte Carlo simulations. The net osmotic pressure is calculated as the difference between the internal and external pressures calculated at the same chemical potential. This can be obtained with sufficient accuracy using the Widom insertion particle technique in the canonical ensemble. The same technique can also be applied in the calculation of different pressure contributions. We find that the net osmotic pressure is always repulsive in a system with monovalent co- and counterions, which is in agreement with the Poisson–Boltzmann equation as well as with previous simulation studies. In asymmetric salts, when the coion is multivalent we find that the external pressure exceeds the internal pressure resulting in a net attractive interaction. This attraction appears at high particle density or volume fraction in the double layer, for example, at short separations, high salt concentration, and/or high surface charge density. The origin of the attraction seems mainly to be the hard-core interactions, which the ions experience in a dense double layer, which is manifested in a high chemical potential. The corresponding bulk solution at this chemical potential has a very high salt concentration and hence also a high osmotic pressure.
Computation of the energy V of an excess electron in dense helium and argon
Authors: Boltjes B. , de Graaf C. , de Leeuw S. W.
Published: July 2002

In this article we present a method and results for the computation of the energy of the ground state of an excess electron in undisturbed helium and argon configurations at various densities. To reproduce experimental data of the excess electron–argon system, the interactions between the induced dipoles of the argon atoms have been taken into account. These interactions were included through an effective screening function. The effective screening function used was a simple approximation to the exact result obtained by computing the electric fields at the particles self-consistently. The latter screening function is similar to a screening function calculated from the radial distribution function via a self-consistent integral equation as will be shown.