bluuebunny/crossref_metadata_2025 huggingface dataset:
The graphical spin algebra method applied to U(2n) generators
Authors: Lucht Michael W. , Gould Mark D.
Published: July 2002

An efficient method for the evaluation of the matrix elements of U(2n) spin-dependent generators in a fully spin adapted Gelfand–Tsetlin basis is given. This is done by evaluating the matrix elements of the U(2n) generators in a Yamanouchi–Kotani basis whose orbital part is equivalent, up to phase factors, to the Gelfand–Tsetlin basis. This allows the expression for the matrix elements to be separated into products of creation and annihilation operators, which are evaluated using Wick’s theorem, and products of SU(2) Clebsch–Gordan coefficients, whose spin graphs are factorized into easily evaluated segment diagrams. The matrix elements of a single U(2n) generator reduce to a sum of products of segment values. These values are given in formula form involving 3-j and 6-j symbols and in table form, where the formulas have been evaluated for all the nonvanishing segments.
Lattice cluster theory for phase behavior of rectangular mesogens. II. Nearest-neighbor interactions, phase diagrams, and competitive nematic orderings
Authors: Li Wenshuo , Freed Karl F.
Published: July 2002

The lattice cluster theory is extended to study all the possible nematic orderings of rectangular mesogens, namely, rodlike, discotic, and biaxial nematics for oblong mesogens, and rodlike and discotic nematics for square mesogens. The theory describes both the anisotropic hard core excluded volume and the anisotropic attractive van der Waals interactions on an equal footing. The partition function is expanded in terms of cluster contributions from the packing induced correlations and van der Waals interactions, with a Flory-type approximation as the zeroth-order treatment. The correlation contributions to the free energies for the isotropic liquid and the various homogeneous liquid crystalline phases are evaluated to second order in mesogen density and to first order in interaction energy. This theory therefore describes the anisotropic character of both the attractive and repulsive portions of the mesogen–mesogen interactions. Models with greater dimensionality provide greater orientational freedom and are shown to produce qualitatively different phase diagrams. As expected, the phase diagram is a strong function of the mesogen dimensions, and the placement of the biaxial phase with respect to the other phases is strongly dependent on mesogen size and the degrees of orientational freedom, in general accord with the nonuniversal behavior exhibited by experiments and computer simulations.
Site selective electron paramagnetic resonance study of photoexcited chromium doped forsterite
Authors: Regev Ayelet , Freed Jack H.
Published: July 2002

Time resolved electron paramagnetic resonance (EPR) measurements on a photoexcited chromium doped forsterite (Cr:Fo) single crystal are reported. The spectral changes with time, magnetic field, crystal orientation, microwave power and, in particular, photoexciting wavelength, provide a selective picture of the various chromium dopants and the absorption–relaxation cycle associated with each optical excitation. Both Cr+4 ions lodged at tetrahedral (Td) and octahedral (Oh) sites as well as Cr+3 ions are detected. In particular, the laser-EPR technique enabled us to monitor the spin dynamics associated with the lasing center (Cr+4/Td) in the time regime of 200 ns–100 ms following a selective photoexcitation of the crystal between 532 and 1064 nm. The transient EPR signals associated with the lasing Cr+4/Td ions, exhibit a noticeable dependence on even small changes (∼0.5 nm) in the exciting wavelengths that correspond to the visible 3A2→3T1 and the near infrared 3A2→3T2 transitions. The transient magnetization associated with each absorption–relaxation cycle is quantitatively analyzed in terms of site selectivity due to the narrow band (i.e., low intensity) microwave detection following a narrow band optical excitation. Given this observed selectivity, it is suggested that laser-EPR may be employed to study intersite interactions and site structure versus optical function relationships in forsterite as well as other solids doped with transition metal ions.
Dynamic studies of degenerate four-wave-mixing in an azobenzene-doped polymer film with an optical pump
Authors: Wang Yougui , Zhao Jiang , Si Jinhai , Ye Peixian , Fu Xingfa , Qiu Ling , Shen Yuquan
Published: July 2002

Degenerate four-wave-mixing was performed in azobenzene-doped polymethyl methacrylate film under a low power continuous-wave laser at 632.8 nm with a pulsed pump laser of 532 nm. Different temporal profiles of the degenerate four-wave-mixing signal were observed under different pump energies, which originated from the relaxation processes between the energy levels of azobenzene during the photoisomerization, especially the transferring processes between the triplet states of the two isomers, trans and cis.
The largest angle generalization of the rotating bond order potential: The H+H2 and N+N2 reactions
Authors: Garcia E. , Laganà A.
Published: July 2002

A generalization of the rotating bond order model potential is discussed. The new functional representation is used for fitting the ab initio potential energy values of the H+H2 system and the main features of a LEPS potential energy surface for the N+N2 system. For H+H2 we analyze the accuracy obtained when fitting the ab initio potential energy values using different functional representations. For N+N2 we examine the deviation of some dynamic and kinetic properties calculated on the new model potential from those obtained from the original LEPS.
Stabilization of reactants in a weakly bound complex: OH–H2 and OH–D2
Authors: Loomis Richard A. , Lester Marsha I.
Published: July 2002

Binary complexes of OH X 2Π and H2/D2 have been stabilized in a shallow well below the activation barrier to the hydrogen abstraction reaction and identified by laser-induced fluorescence in the OH A 2Σ+–X 2Π 1–0 spectral region. The characteristics of the OH (X 2Π, A 2Σ+)+H2/D2 potentials are in accord with ab initio predictions.
Ab initio all-electron Dirac–Fock–Breit calculations for UF6
Authors: Malli Gulzari L. , Styszynski Jacek
Published: July 2002

Ab initio all-electron Dirac–Fock, Dirac–Fock–Breit, and Hartree–Fock calculations are reported for UF6, assuming the experimental octahedral geometry. The spin–orbit (S–O) splitting is calculated for all the ground state levels of UF6 and the calculated S–O splittings for the 4d, 5d, and 4f levels are in excellent agreement with those reported experimentally by Martensson et al. [J. Chem. Phys. 80, 5456 (1984)]. The magnetic part of the Breit interaction for UF6 is calculated via perturbation theory as 42 hartrees (1145 eV). Our Hartree–Fock, Dirac–Fock, and Dirac–Fock–Breit wave functions predict UF6 to be bound with dissociation energy of 13.71, 23.53, and 23.27 eV, respectively. Relativistic effects lead to about 70% increment in the predicted dissociation energy of UF6. Our calculations show that the relativistic effects are so large for UF6 that it is imperative to treat them using Dirac’s fully relativistic equation.
Laser fluorescence excitation spectroscopy of BNe electronic states correlating with the excited valence B(2s2p2 2D) atomic asymptote
Authors: Yang Xin , Hwang Eunsook , Dagdigian Paul J.
Published: July 2002

The laser fluorescence excitation spectrum of the BNe van der Waals complex, in the vicinity of the B atom 2s2p2 2D←2s22p 2P transition at 208.9 nm, is reported. A total of six partially resolved molecular bands, as well as a broad, unstructured feature to the blue of these bands, have been observed. Three BNe electronic states, denoted as C 2Δ, D 2Π, and E 2Σ+, correlate with the B(2s2p2 2D)+Ne atomic asymptote, and the observed bands are assigned as (v′,0) progressions of the C 2Δ−X 2Π1/2 and D 2Π–X 2Π1/2 band systems. Rotational analysis of the C–X bands has been carried out, and spectroscopic constants characterizing the upper and lower states determined. The onset of the continuous excitation is assigned as the energy to reach the B(2s2p2 2D)+Ne atomic asymptote. Identification of this threshold has allowed the determination of dissociation energies of the X, C, and D states. The observation of banded features in this wavelength range contrasts sharply with the continuous free←bound excitation in the B 2Σ+–X 2Π1/2 transition, because of the purely repulsive B(2s23s 2S)–Ne interaction [X. Yang, E. Hwang, P. J. Dagdigian, M. Yang, and M. H. Alexander, J. Chem. Phys. 103, 2779 (1995)]. The differences in the binding energies of the BNe electronic states are discussed in terms of their expected electronic structures.
Fermi resonance and mode specificity in the vibrational autoionization of NO2
Authors: Matsui Hiroshi , Grant Edward R.
Published: July 2002

Ionization-detected absorption spectra of autoionizing Rydberg series converging to the (100), (010), (0200), and (0220) vibrational states of NO+2 have been recorded using three-color triple resonant excitation. Resonances produced by relaxation of the core, excited in one quantum of the bending vibration, (010), are found to be much sharper than those associated with decay by relaxation in symmetric stretch, (100), consistent with earlier findings on mode specificity of vibrational autoionization in this system. In contrast, series excited in the symmetric overtone of the bend, (0200), are found to exhibit dynamics that resemble much more those of series built on one quantum of symmetric stretch, indicating that the bending specificity that characterizes (010) is lost in the overtone. This loss of mode specificity with increasing vibrational energy is ascribed to moderate (100)–(0200) Fermi resonance in the NO+2 core, exemplifying how even small vibrational coupling can affect mode-specific pathways in polyatomic molecules.
A novel method for optimizing quantum Monte Carlo wave functions
Authors: Huang Hongxin , Cao Zexing
Published: July 2002

We propose an algorithm for optimizing quantum Monte Carlo wave functions. An improved steepest-descent technique is used, with step size automatically adjustable to obtain a procedure that converges superlinearly. We also propose a novel trial function, which has both correct electron–electron cusp conditions and correct electron–nucleus cusp conditions. To test the optimization procedure and the optimized trial function, the ground states for CH4 and H2O molecules were investigated using variational Monte Carlo (VMC) and fixed-node quantum Monte Carlo (FNQMC) calculations. For CH4 and H2O, the VMC recovered 73.3% and 57.9% of the correlation energy, respectively, and the FNQMC recovered 99.3% and 92.8%, respectively. The optimization procedure is three to five times faster than conventional usual steepest-descent procedures. The trial functions optimized are more accurate than prior trial functions of similar complexity.
Nonadiabatic effects in a method that combines classical and quantum mechanics
Authors: Antoniou Dimitri , Schwartz Steven D.
Published: July 2002

We have included nonadiabatic effects in the calculation of the dynamical evolution of a system where a quantum particle in a double well is coupled to a classical oscillator. By performing an exponential resummation of the evolution operator we have included ‘‘polarization’’ effects (similar to the self-energy corrections for an electron that moves in a polarizable medium) which lead to a renormalization of the energy of the quantum particle.
D2 dissociative adsorption on and associative desorption from Si(100): Dynamic consequences of an ab initio potential energy surface
Authors: Luntz A. C. , Kratzer P.
Published: July 2002

Dynamical calculations are reported for D2 dissociative chemisorption on and associative desorption from a Si(100) surface. These calculations use the dynamically relevant effective potential which is based on an ab initio potential energy surface for the ‘‘pre-paired’’ species. Three coordinates are included dynamically; the distance to the surface, the D–D bond length and a Si phonon coordinate. Other coordinates (multidimensionality) have been included via a static approximation. Both an asymmetric and symmetric reaction paths are considered. While energetics favors the asymmetric path, phase space favors the symmetric one. Under the conditions of many experiments, either could dominate. The calculations show quite weak dynamic coupling to the Si lattice for both paths, i.e., weak surface temperature dependences to dissociation and small energy loss to the lattice upon desorption. These calculations do not support previous suggestions that either a strong coupling to the lattice or ‘‘entropic’’ effects can reconcile the apparent violation of detailed balance obtained by comparing experimental dissociation to desorption barriers. In fact, the results reported here do not agree with several experimental findings. We discuss several possibilities for this disagreement, including experimental artifact, limitations in the dynamical model and even the possibility that electronically adiabatic dynamics involving the ‘‘pre-paired’’ species is not relevant to experiments on real systems.
Multidimensional nuclear magnetic resonance in complex liquids analyzed by a simple model for non-Markovian molecular reorientation
Authors: Sillescu Hans
Published: July 2002

A simple model of non-Markovian molecular reorientation is applied to multidimensional NMR experiments in supercooled liquids and polymers close to the glass transition. The process of exchange between components of a correlation time distribution is related with the notions ‘‘exponential/nonexponential, homogeneous/heterogeneous, Markovian/non-Markovian, and ergodic/nonergodic’’ often used in relation with molecular dynamics in viscous systems. By working out a detailed example of two-state exchange the difference between 2D and reduced 4D-spectra with respect to the mechanism and dynamics of molecular reorientation in complex liquids is elucidated.
Quantum chaos in collinear (He, H2+) collisions
Authors: Mahapatra Susanta , Ramaswamy Ramakrishna , Sathyamurthy N.
Published: July 2002

The quasibound spectrum of the transition state in collinear (He, H2+) collisions is obtained from time-dependent wave packet calculations. Examination of short- and long-range correlations in the eigenvalue spectra through a study of the nearest neighbor spacing distribution, P(s), and the spectral rigidity, Δ3(L), reveals signatures of quantum chaotic behavior. Analysis in the time domain is carried out by computing the survival probability 〈〈P(t)〉〉 averaged over initial states and Hamiltonian. All these indicators show intermediate behavior between regular and chaotic. A quantitative comparison of 〈〈P(t)〉〉 with the results of random matrix theory provides an estimate of the fraction of phase space exhibiting chaotic behavior, in reasonable agreement with the classical dynamics. We also analyse the dynamical evolution of coherent Gaussian wave packets located initially in different regions of phase space and compute the survival probability, power spectrum and the volume of phase space over which the wave packet spreads and illustrate the different behaviors.
The concentration dependent cooperative friction coefficient of dilute polymer solutions at the theta point
Authors: Cherayil Binny J. , Freed Karl F.
Published: July 2002

We combine multiple scattering and renormalization group methods to calculate the leading order dimensionless virial coefficient ks for the friction coefficient of dilute polymer solutions under conditions where the osmotic second virial coefficient vanishes (i.e., at the theta point Tθ). Our calculations are formulated in terms of coupled kinetic equations for the polymer and solvent, in which the polymers are modeled as continuous chains whose configurations evolve under the action of random forces in the velocity field of the solvent. To lowest order in ε=4−d, we find that ks=1.06. This result compares satisfactorily with existing experimental estimates of ks, which are in the range 0.7–0.8. It is also in good agreement with other theoretical results on chains and suspensions at Tθ. Our calculated ks is also found to be identical to the leading order virial coefficient of the tracer friction coefficient at the theta point. We discuss possible reasons for the difficulties encountered when attempting to evaluate ks by extrapolating prior renormalization group calculations from semidilute concentrations to the infinitely dilute limit.
Fluids in micropores. V. Effects of thermal motion in the walls of a slit-micropore
Authors: Diestler D. J. , Schoen M.
Published: July 2002

Previous articles in this series have concerned the prototypal slit-pore with rigid walls, in which a Lennard-Jones (12,6) monatomic film is constrained between two plane-parallel walls comprising like atoms fixed in the face-centered-cubic (fcc) (100) configuration. The behavior of molecularly thin films in the rigid-wall prototype is governed by the template effect, whereby solid films can form epitaxially when the walls are properly aligned in the lateral directions. In this article the influence of thermal motion of the wall atoms on the template effect is investigated. The walls are treated as Einstein solids, the atoms moving independently in harmonic potentials centered on rigidly fixed equilibrium positions in the fcc (100) configuration. The force constant fc is a measure of the stiffness of the walls, the rigid-wall limit being fc=∞. Formal thermodynamic and statistical mechanical analyses of the system are carried out. The results of grand canonical ensemble Monte Carlo simulations indicate that for values of fc characteristic of a soft (e.g., noble-gas) crystal dynamic coupling between wall and film has a substantial influence on such equilibrium properties as normal stress (load) and interfacial tensions. In general, the softer the walls (i.e., the smaller the value of fc), the weaker the template effect and hence the softer and more disordered the confined film.
Quasioscillations in the structure of a liquid mixture under spinodal decomposition
Authors: Bunkin N. F. , Lobeyev A. V.
Published: July 2002

Using a very thin cuvette with a stratified mixture of 2,4,6-trimethylpyrydine+water, having a lower critical point, we observed a quasiperiodic in time transformation of the liquid structure occurring at spinodal decomposition.
Estimation of fundamental frequencies of perhalogenated ethylenes in terms of molecular fragment contributions
Authors: Haase Dietrich , Krumrey Christine
Published: July 2002

A model, in which a molecule is subdivided into a number of subunits, is used to calculate approximately fundamental frequencies of perhalogenated ethylenes in terms of contributions of subunits and clusters of subunits. Different levels of approximation are defined by a subunit additivity scheme, a linear subunit pair and subunit triple scheme as well as by a nonlinear coupled subunit pair scheme. Algebraic dependencies of the approximate formulas for distinct ethylene derivatives are used as criteria for testing the different schemes. The method is applied to the fundamental frequencies of normal vibrations which, with respect to symmetry, correlate uniquely in the series of substituted ethylenes considered, otherwise the method is applied to corresponding sums of frequencies. For the fundamental frequencies of the 27 ethylene derivatives C2FnClmBr4−n−m, n+m≤4, completely known from a normal coordinate analysis, the main results are: (a) The out-of-plane frequencies correlate uniquely in these molecules, three of the in-plane frequencies and two sums formed from the remaining ones correlate as well. (b) The simple additivity scheme with three parameters for each set of correlating frequencies yields a sufficiently exact fit of the out-of-plane frequencies; their mean absolute deviation is about 5 cm−1. (c) The linear subunit pair approximation with twelve parameters for each set is sufficiently exact for all the correlating frequencies and sums of frequencies, respectively; their mean absolute deviation is about 1 cm−1.
Ab initio study of van der Waals interaction of CO2 with Ar
Authors: Marshall Peter J. , Szczȩśniak M. M. , Sadlej Joanna , Chal/asiński Grzegorz , ter Horst Marc A. , Jameson Cynthia J.
Published: July 2002

The ab initio potential energy surface of the ArCO2 cluster is calculated using the supermolecular Mo/ller–Plesset perturbation theory (S-MPPT) and dissected into its fundamental components; electrostatic, exchange, induction, and dispersion energies. The surface contains a single minimum for the perpendicular approach of Ar toward the C atom which has a well depth of ∼210 cm−1 at R=6.5 a0. This value is obtained using an extended basis set supplied with the bond functions and the fourth order supermolecular Mo/ller–Plesset calculations, and is expected to be accurate to within ±5%. The areas of the surface corresponding to the collinear approach of Ar to CO2 contain an extended plateau. The saddle point in this region for R=9.0 a0 is stabilized by 117 cm−1. The analytical pair potential for Ar–CO2 obtained by fitting to the individual interaction components is provided. The three-body effects in the related cluster, Ar2CO2, are examined for two configurations of the Ar2CO2 cluster. The overall nonadditivity is dominated by the three-body dispersion effect; however, the exchange nonadditivity is the most anisotropic. The sources of this anisotropy are discussed.
Experimental measurements of fast adsorption kinetics of H2 on vicinal Si(100) and (111) surfaces
Authors: Hansen D. A. , Halbach M. R. , Seebauer E. G.
Published: July 2002

The Si(100) and (111) surfaces have generally been classified as relatively inert toward molecular hydrogen adsorption. In the present work, we show using surface second harmonic generation that the sticking probability increases dramatically for slightly misoriented substrates, which are of interest in many integrated circuit applications. At temperatures lower than 30–50 K below where desorption becomes significant, defect sites serve as adsorption conduits from which hydrogen atoms diffuse onto the rest of the surface. The adsorption rate therefore has the activation energy of surface diffusion, found to be 0.78 eV on Si(100) and 1.50 eV on Si(111). The small energy barrier and pre-exponential factor (∼10−11 cm2/s) for diffusion on (100) seem to result from the need to break Si–Si dimer-pair bonds prior to hopping of an H atom. At higher temperatures, the adsorption mechanism changes, and H2 sticks to both surfaces with a fairly large probability near 0.1.
Phase diagram of the two-dimensional Coulomb gas: A thermodynamic scaling Monte Carlo study
Authors: Orkoulas Gerassimos , Panagiotopoulos Athanassios Z.
Published: July 2002

In this work, we report results for the phase diagram of the two-dimensional Coulomb system. We use a recently proposed simulation method, thermodynamic scaling Monte Carlo, to determine the Kosterlitz–Thouless insulator/conductor transition line and the first-order vapor–liquid coexistence curve. The Kosterlitz–Thouless line terminates at the first-order coexistence curve close to the gas–liquid critical point. Our estimate of the vapor–liquid critical point is, T*≊0.056, ρ*≊0.21, in only modest agreement with previous theoretical estimates and semiquantitative numerical results.
The low-lying electronic states of CoF
Authors: Ram R. S. , Bernath P. F. , Davis S. P.
Published: July 2002

The emission spectrum of CoF has been investigated in the 820 nm–3.5 μm spectral region using a Fourier transform spectrometer. The bands were excited in a carbon tube furnace by the reaction of cobalt metal vapor and CF4 at a temperature of about 2300 °C. The bands observed in the 3000–9000 cm−1 region have been classified into three new transitions. The bands with 0-0 R-heads at 3458 cm−1, 3759 cm−1, and 4012 cm−1 have been assigned as the 3Δ1–3Φ2, 3Δ2–3Φ3, and 3Δ3–3Φ4 subbands of the C 3Δ–X 3Φi electronic transition. To higher wave numbers, two bands with R-heads at 8396 cm−1 and 8565 cm−1 have been assigned as the 3Δ2–3Φ3 and 3Δ3–3Φ4 subbands of the D 3Δ–X 3Φi transition. In addition, the bands with R-heads at 6339 cm−1 and 6542 cm−1 have been assigned as the 0-0 3Φ4–3Δ3 and 3Φ3–3Δ2 subbands of the G 3Φ–C 3Δ transition. The G 3Φ–X 3Φ transition has been reported previously as the [10.3]3Φ–X 3Φ transition. The rotational analysis of many bands of these transitions has been obtained and the molecular constants for the two new low-lying excited states have been extracted. Six new band involving the high vibrational levels of ground state (up to v=6) have been identified in the 3Φ4–3Φ4 subband of the G 3Φ–X 3Φ transition. The rotational analysis of these bands provides improved constants for the ground state. We have noticed, as have previous workers, the strong correspondence that exists between the states of transition metal monofluorides and monohydrides. In addition, all of the low-lying states of CoF and CoH are related to the low-lying terms of the Co+ atom. We discuss these correlations between the energy levels of CoF, CoH, and Co+.
Phase transitions and critical dynamics in (C18H37NH3)2SnCl6
Authors: Lee K. W. , Lee C. H. , Lee C. E. , Kang J. K.
Published: July 2002

Phase transitions in the layer structured (C18H37NH3)2SnCl6 were studied using 1H NMR. The spin-lattice relaxation rate reflects the critical slowing down around the order–disorder phase transition temperature, and is compatible with the three-dimensional Ising model. A critical slowing down is also observed at the conformational transition in the second moment measurement, presumably for the first time.
Molecular dynamics simulation study of the dynamics of fluids in thin films
Authors: Winkler Roland G. , Schmid Rolf H. , Gerstmair Anja , Reineker Peter
Published: July 2002

The structure and dynamics of fluids in thin films are investigated by molecular dynamics simulations. Particularly the influence of surface attraction or repulsion on the structure of hexadecane melts (C16H34) is investigated. We find that for a strongly attractive surface, well ordered, crystalline like monolayers are the most stable configuration. In addition, the dynamics perpendicular to solid surfaces of the hexadecane molecules as well as of a simple Lennard-Jones fluid is investigated. For the Lennard-Jones fluid, the numerical results are compared with analytical calculations based on the diffusion equation, which shows that the numerical results can very well be described by the solution of the diffusion equation for reflecting surfaces. The diffusion coefficient is practically independent of the position within the film, although the fluid is inhomogeneous perpendicular to the surfaces. However, we observe a slight influence of the finite size of the fluid particles on their dynamics in the layer adjacent to a surface. In contrast, the dynamics of the centers of mass of hexadecane molecules perpendicular to repulsive surfaces is severely slowed down due to their extended and anisotropic nature and cannot be described by a single particle diffusion equation.
A phenomenological model for the vibrational dependence of hydrogen interchange tunneling in HF dimer
Authors: Chang H.-C. , Klemperer W.
Published: July 2002

We present a phenomenological model to account for the observations of the hydrogen interchange tunneling at N=v1+v2=1–3 of the free (v1) and the bound (v2) HF stretches of (HF)2. Good agreement is obtained between calculations and observations at the levels (v1,v2) of v1−v2=±1 for both N=1 and 3, suggesting that the observed hydrogen tunneling splittings in these levels are direct rather than the results of many extraneous perturbations. The model also predicts well the ratios of the rates of vibrational predissociation at these states, in addition to the splittings. We attribute the unusually large vibrational dependence of the splittings upon valence bond excitation to the result of interbond coupling between the two HF local oscillators. Significant differences in the vibrational energy of the level (1,2) as well as the tunneling splitting at (2,0) between observation and prediction are, however, noted.
Photochemistry of phosgene in the solid phase: State-resolved dissociation dynamics
Authors: Xin Q.-S. , Zhu X.-Y.
Published: July 2002

The translational, rotational, and vibrational state distributions of CO (g) resulting from the single photon photodissociation of Cl2CO in the condensed phase at ∼90 K have been determined by time-of-flight (TOF) distribution measurement and resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The TOF distribution of CO (g) is bimodal. Internal state characterization of the slow channel reveals a completely thermalized origin, with a rotational temperature of Trot=88±5 K, which is equal to the translational temperature as well as the substrate temperature. We believe these slow CO molecules originate from photodissociation below the topmost surface of the molecular film and achieve thermal equilibrium with the substrate before escaping into the gas phase. Internal state characterization of the fast channel shows, on the other hand, an energetic origin: at hν=5.0 eV, the rotational distribution, with an overall flux-weighted mean rotational energy of 〈Erot〉=0.12±0.01 eV, is non-Boltzmann and can be approximated by a bimodal distribution with rotational temperatures of 210±40 K at low J″(s) and 2200±300 K at high J″(s); the relative vibrational population is Nν=1/Nν=0=0.33±0.05. Both rotational and translational distributions of fast CO show positive correlation with photon energy. These CO molecules must be promptly ejected into the gas phase, carrying nascent energetic information from the photodissociation reaction on the surface of the molecular film. For electronic excitation events that result in photodissociation, 74% of the excess excitation energy is distributed in the translational and internal motions of products (CO and Cl); only 26% of the available energy is converted to motions of surrounding molecules.
Ab initio study of the O2(X 3Σ−g)+He(1S) van der Waals cluster
Authors: Cybulski Sl/awomir M. , Burcl Rudolf , Szczȩśniak Mal/gorzata M. , Chal/asiński Grzegorz
Published: July 2002

Potential energy surface for the He(1S)+O2(X 3Σ−g) interaction is calculated using the supermolecular unrestricted Mo/ller-Plesset perturbation theory approach and is analyzed via the perturbation theory of intermolecular forces. The latter has been generalized to provide a decomposition of the interaction energies into electrostatic, exchange, induction, and dispersion constituents for monomers described by unrestricted Hartree–Fock determinants. The global minimum occurs for the T-shaped geometry, around 6.0a0. Our UMP4 estimate of the well depth of the global minimum is De=27.7 cm−1. This value is expected to be accurate to within a few percent. The potential energy surface reveals also a local minimum for the collinear geometry at about 7.0a0. The well depth for the secondary minimum is estimated at De=25.5 cm−1(UMP4) and is expected to be accurate within a few percent. The minima are separated by a barrier of 7.5 cm−1. The energy partitioning reveals that the origin of interaction in this complex is typical for van der Waals clusters involving He. The global minimum is determined by the minimum in the exchange repulsion in the direction perpendicular to the O–O bond. The secondary linear minimum results from the maximum dispersion attraction and enhanced by a slight flattening of the electron density near the ends of the interoxygen axis.
Effective Hamiltonian for near-degenerate states in direct relativistic perturbation theory. I. Formalism
Authors: Rutkowski A. , Schwarz W. H. E.
Published: July 2002

Direct perturbation theory (DPT) for relativistic effects is generalized to the case of a set of near-degenerate strongly interacting states. This situation, where the standard approach breaks down, is quite common in atoms and especially in molecules. We introduce a new partitioning of the Dirac equation and apply the Mo/ller–Bloch approach. An effective Schrödinger-like equation within a nonrelativistic model space of near-degenerate states is derived. The effective Hamiltonian and metric operators are expressed with the help of a Mo/ller wave operator Ω, which generates the complete four-component Dirac wave function from the nonrelativistic Schrödinger wave function in the finite model space. The corresponding Bloch equation can be solved numerically in a basis set to infinite order by iteration. Also explicit formulas are derived for different orders of Heff and Seff. They can be used to determine the relativistic energies to different orders either directly by diagonalization, or by a perturbation approach.
Potentials of mean force for biomolecular simulations: Theory and test on alanine dipeptide
Authors: Pellegrini Matteo , Gro/nbech-Jensen Niels , Doniach Sebastian
Published: July 2002

We describe a technique for generating potentials of mean force (PMF) between solutes in an aqueous solution. We first generate solute–solvent correlation functions (CF) using Monte Carlo (MC) simulations in which we place a single atom solute in a periodic boundary box containing a few hundred water molecules. We then make use of the Kirkwood superposition approximation, where the 3-body correlation function is approximated as the product of 2-body CFs, to describe the mean water density around two solutes. Computing the force generated on the solutes by this average water density allows us to compute potentials of mean force between the two solutes. For charged solutes an additional approximation involving dielectric screening is made, by setting the dielectric constant of water to ε=80. These potentials account, in an approximate manner, for the average effect of water on the atoms. Following the work of Pettitt and Karplus [Chem. Phys. Lett. 121, 194 (1985)], we approximate the n-body potential of mean force as a sum of the pairwise potentials of mean force. This allows us to run simulations of biomolecules without introducing explicit water, hence gaining several orders of magnitude in efficiency with respect to standard molecular dynamics techniques. We demonstrate the validity of this technique by first comparing the PMFs for methane–methane and sodium–chloride generated with this procedure, with those calculated with a standard Monte Carlo simulation with explicit water. We then compare the results of the free energy profiles between the equilibria of alanine dipeptide generated by the two methods.
Surface phase behavior in binary polymer mixtures. II. Surface enrichment from polyolefin blends
Authors: Scheffold Frank , Budkowski Andrzej , Steiner Ullrich , Eiser Erika , Klein Jacob , Fetters Lewis J.
Published: July 2002

Using nuclear reaction analysis, we have measured the enrichment by one of the components at the surface of a binary mixture of random olefinic copolymers, with components of monomer structure E1−x1EEx1 and E1−x2EEx2. Here E and EE are the linear ethylene and branched ethylethylene groups (C4H8) and [C2H3(C2H5)], respectively, and x represents the fraction of the EE group randomly distributed on the chains. We examined 12 different couples covering a range x=0.38–0.97. The mixtures, whose thermodynamic behavior was established in our earlier paper, were cast in the form of films on both a silicon and on a gold-covered silicon surface, and were investigated in the one-phase region of the binodal in the vicinity of the critical temperature. We find that it is always the more flexible component—the one with a shorter statistical step length, corresponding to the higher ethylethylene fraction (higher x)—that is enriched at the polymer/air surface. Within our resolution neither component is enriched at the polymer/solid interface. These results show clearly that enthalpic rather than entropic factors dominate the surface potential driving the surface enrichment. For two of the mixtures we determined the excess of the surface-preferred species as a function of mixture composition along an isotherm in the one-phase region of the binodal. A consistent description of our data in terms of a mean-field model is provided by including in the surface potential a term in the mixture composition gradient at the polymer surface.
Novel technique for real-time monitoring of electron attachment to laser-excited molecules
Authors: Pinnaduwage Lal A. , Datskos Panos G.
Published: July 2002

We report a new experimental technique that is capable of monitoring electron attachment to laser-excited molecules in real time; the time resolution is limited only by the time constant of the detection circuit and was ∼100 ps for the experiments reported here. This technique provides information on the lifetime of the excited states responsible for electron attachment, and also allows determination of electron attachment cross sections involved. Results on dissociative electron attachment to ArF-excimer-laser-irradiated NO are reported: Electron attachment occurred to the A 2Σ+(ν=3) state populated via the absorption of a single photon, and to highly excited states populated via two-photon absorption; the cross section for low-energy electron attachment to the A 2Σ+(ν=3) state was ∼3 orders of magnitude larger compared to that for the A 2Σ+(ν=0). Decay of the electrons over the ∼200 ns lifetime of the A 2Σ+(ν=3) state was directly monitored. Negative-ion formation that occurred via the A 2Σ+(ν=3) state was suppressed in the presence of CO2 due to collisional quenching of that state by CO2, and the reduction in the A 2Σ+(ν=3) state lifetime with increasing CO2 pressure was also observed.
A self-consistent-field approach to surfaces of compressible polymer blends
Authors: Schmid F.
Published: July 2002

A self-consistent-field theory for compressible polymer mixtures is developed by introducing elements of classical density functional theory into the framework of the Helfand theory. It is then applied to study free surfaces of binary (A,B) polymer blends. Density profiles in the one- and two-phase region are calculated as well as chain end distributions and chain orientations of the minority and the majority component. In the ideally symmetric mixture, in which the individual properties of polymers A and B are the same and both have the same surface energy, polymers of the minority component segregate to the surface, where they are exposed to less polymers of the majority component. This effect can only be captured correctly, if one accounts for the fact that the monomer-monomer interaction has finite range. As a consequence, the Flory–Huggins-parameter varies in space and depends on the concentration profiles and their derivatives. The surface segregation calculated with such an ansatz, without any fit parameter, is in reasonable quantitative agreement with data from recent Monte Carlo simulations.
Electron transfer reactions in a non-Debye medium with frequency-dependent friction
Authors: Tang Jau
Published: July 2002

A generalized Zusman equation and its formal solution for electron transfer reactions in a non-Debye medium with frequency-dependent friction are presented. The derivation is based on the spin-boson model, representing a two-level system for a donor and an acceptor coupled to a non-Debye polar solvent bath. An analytical expression for the electron transfer rate constant will be derived using the Green’s function method. Because of the time retardation in such a non-Markovian process, the initial electron-transfer reaction is influenced more by the higher frequency components in the solvent relaxation, whereas the long-time behavior is influenced more by the lower frequency components. Electron transfer processes in such a medium are therefore often nonexponential.
Dynamic scattering from a homopolymer and a diblock copolymer chain in dilute and many-chain solutions
Authors: Molina Luis A. , Freire Juan J.
Published: July 2002

The intermediate dynamic light scattering function has been computed for a single A–B diblock copolymer chain in dilute and many-chain solutions, assuming a nonrefractive solvent (i.e., setting opposite contrast factors for the A and B units). Theoretical calculations, based in the Pecora scheme for the Rouse model, have been compared with the results obtained with a dynamic Monte Carlo algorithm for chains with nonoverlapping units placed in a cubic lattice. These numerical functions are also compared with the prediction of a single exponential described by the first cumulant method, which is consistent with the application of the linear response theory. The homopolymer case is also discussed.
Transient absorption spectra of vibrationally excited OH/OD groups in mordenite zeolites: Effect of Xe adsorption
Authors: Kashitani M. , Fujino T. , Fukuyama K. , Kubota J. , Kondo J. N. , Wada A. , Domen K. , Hirose C. , Wakabayashi F. , Ishida M. , Goto F. , Kano S. S.
Published: July 2002

Transient spectra following the v=1←0 excitation of the OH stretching mode of Bro/nsted acidic hydroxyl groups in normal and deuterated mordenite zeolites have been measured by a two-color picosecond pump–probe technique. When the OH and OD stretching bands were excited by resonant IR pulses, transient bleaching of the fundamental band (v=1←0) and the transient hot band (v=2←1) absorption signal were observed. For the OH and OD groups free of adsorbates, the spectral widths of the pump-induced signals were broadened by ∼4 cm−1 due to pure dephasing from the convoluted width of the pump and probe IR pulses. When the OH and OD groups were adsorbed by a Xe atom, the frequencies of the ν(OH) and ν(OD) bands shifted to lower frequency and at the same time the widths of both the transient bleaching and the transient hot band absorption were much broader than those of the isolated hydroxyl groups by the factor of more than three. Actually, the widths of the transient signals were about the same as those of the linear absorption signals. An adiabatic approximation has been employed to interpret the observed broadening/shift of the IR bands by making use of the results of DFT (density functional theory) calculations performed on a model cluster consisting of a Xe atom and an H3Si–(OH)–AlH3 substrate. The result agreed well with the observed spectral features.
Model calculations of Langmuir monolayers: Pressure effects on tilting behavior of idealized amphiphiles
Authors: Swanson D. R. , Hardy R. J. , Eckhardt C. J.
Published: July 2002

Model amphiphiles consisting of lines of Lennard-Jones (LJ) centers are investigated to determine the effect of pressure and molecular geometry on ground-state tilting behavior. Both the amphiphile length and the intramolecular distance between LJ centers is varied. The results give guidelines for understanding and controlling tilting behavior in monolayer films.
Relativistic effects on electric properties of many-electron systems in spin-averaged Douglas–Kroll and Pauli approximations
Authors: Kellö Vladimir , Sadlej Andrzej J. , Hess Bernd A.
Published: July 2002

Relativistic effects and electron correlation effects on the dipole moments of the coinage metal hydrides are investigated and compared employing one-component (scalar) relativistic approximations based on the mass–velocity and Darwin operator and, alternatively, the Douglas–Kroll-transformed spin-averaged no-pair Hamiltonian. The former of the two operators is found to perform quite accurately for CuH and AgH. For AuH the limits of the Pauli approximation seem to be reached, as can be inferred from a comparison with the values obtained within the spin-averaged Douglas–Kroll no-pair formalism. The coupled cluster calculations in the Douglas–Kroll no-pair approximation for relativistic effects establish the dipole moment values of the coinage metal hydrides as equal to 1.05 a.u. for CuH, 1.14 a.u. for AgH and 0.52 for AuH. The corresponding non-relativistic results are 1.14 a.u., 1.36 a.u., and 1.22 a.u., respectively. Some formal problems arising in applications of the Douglas–Kroll no-pair approximation are discussed. It is shown that the Hellmann–Feynman theorem leads to a rather complicated form of the first-order energy change due to external perturbation. The usual expectation value formula is, however, valid through terms proportional to 1/c4 and can be used in most applications. The invariance property with respect to a shift in the external potential is addressed for the Douglas–Kroll no-pair approximation in a finite basis set.
Rotationally resolved infrared absorption spectrum of N+4
Authors: Ruchti T. , Speck T. , Connelly J. P. , Bieske E. J. , Linnartz H. , Maier J. P.
Published: July 2002

The rotationally resolved infrared band of the antisymmetric stretching vibration (ν3) of N+4 has been recorded by tunable diode laser spectroscopy. A continuous supersonic expansion of pure nitrogen through a slit nozzle and electron impact ionization was employed. Forty-four P and R branch transitions with J up to 25 are observed. The band origin is at ν0=2234.5084(4) cm−1 and the rotational constants are determined to be B0=0.112 05(3) cm−1 and B1=0.111 76(3) cm−1. The infrared spectrum shows that N+4 has a linear ground state structure.
Liquid–liquid equilibrium for fluids confined within random porous materials
Authors: Gordon Peter A. , Glandt Eduardo D.
Published: July 2002

Using the Gibbs Ensemble Monte Carlo (GEMC) simulation technique, we examine the liquid–liquid phase behavior for a symmetric binary LJ fluid confined in a disordered solid matrix. Increasing the matrix packing fraction and adsorption strength, and lowering the ratio of adsorbent to adsorbate particle size reduces the magnitude of the miscibility gap. We also employ Voronoi tessellations to analyze the distribution of cavity sizes in mono- and polydisperse matrices to help explain the observed phase behavior.
H2 dissociation on metal surfaces: Six-dimensional approximate quantum calculations
Authors: Munn Nicholas S. , Clary David C.
Published: July 2002

We examine the dissociative adsorption of hydrogen on metal surfaces by employing a reduced dimensionality quantum scattering method. Using hyperspherical coordinates, we show how the six-dimensional (6D) H2/metal surface dissociation problem may be reduced to two dimensions by treating explicitly the bonds breaking and forming, while adding zero-point energy corrections to the effective potential for all other degrees of freedom. We present calculations of reaction probabilities for dissociation of H2 on Ni(100) in three dimensions and on Cu(111) in four dimensions using this approach, and show their substantial agreement with those obtained using far more expensive exact methods. Five-dimensional and 6D results for H2/Cu(111) are also presented, and compared to the lower-dimensional results.
Nuclear quadrupole coupling operator for a molecule with an internal rotor
Authors: Duan Yun-Bo , Sun Zhen-Dong , Takagi Kojiro
Published: July 2002

The sequential contact transformation technique is applied to the nuclear quadrupole coupling operator for a molecule containing a threefold symmetric internal rotor in order to be consistent with the transformed vibration–torsion–rotation Hamiltonian to be used in the analysis of the spectra of the molecule. Various higher-order effects on the nuclear quadrupole coupling energies, such as the centrifugal distortion, the anharmonic corrections and so on, are formulated for a molecule containing one quadrupolar nucleus. The detailed discussion is presented of vibrational–torsional–rotational dependence of these higher corrections to the quadrupole hyperfine energies.