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Notes: Quadratic response theory for equilibrium and nonequilibrium solvation has been extended to include both singlet and triplet perturbations. The approach is tested by investigating the effect of solvent on the phosphorescence lifetime of formaldehyde. © 1999 American Institute of Physics.

Notes: Optical Kerr effect spectroscopy has been employed to study the behavior of six symmetric-top liquids (acetonitrile, acetonitrile-d3, benzene, carbon disulfide, chloroform, and methyl iodide) over a broad range of temperatures. In all of the liquids, an exponential intermolecular response is observed on a time scale of a few hundreds of femtoseconds. Comparison of the temperature dependence of the time scale of this relaxation with the viscosity and single-molecule and collective orientational times in the liquids suggests that the exponential relaxation arises from motional narrowing. © 1999 American Institute of Physics.

Notes: Dynamical effects of a solvent (environment) on an electron transfer (ET) reaction are investigated by using the Sumi–Marcus reaction–diffusion equation; this equation describes the time evolution of population distribution function of a reactant in a slow nuclear coordinate system. Assuming that viscosity of the solvent (environment) is proportional to a relaxation time scale of the slow nuclear mode, power dependence of a mean lifetime of ET on the relaxation time scale becomes the same as the one on the viscosity. Therefore, the former power dependence is investigated instead of the latter, and it is found that the power in the limit of the (infinitely) large relaxation time scale is 1−r when r〈1, and 0 when 1≤r, where r is the ratio of the reorganization energy of fast nuclear modes to the slow nuclear mode. However, this limit cannot always be reached in a realistic situation. Therefore, the present theory is extended to a large but finite relaxation time scale. The values of the power obtained by the present theory are in reasonable agreement with the ones calculated numerically by W. Nadler and R. A. Marcus [J. Chem. Phys. 86, 3906 (1987)]. Finally, a difficulty in numerical calculations is shown. An expansion of the population distribution function in some basis set of functions is common in numerical calculations. However, the use of that finite basis set of functions which is independent of the relaxation time scale leads to a value of the power that is either zero or unity in the limit of the large relaxation time scale, and as such cannot reproduce the correct asymptotic behavior of the mean lifetime. © 1999 American Institute of Physics.

Notes: Dynamical properties of the soft sticky dipole (SSD) model of water are calculated by means of molecular dynamics simulations. Since this is not a simple point model, the forces and torques arising from the SSD potential are derived here. Simulations are carried out in the microcanonical ensemble employing the Ewald method for the electrostatic interactions. Various time correlation functions and dynamical quantities associated with the translational and rotational motion of water molecules are evaluated and compared with those of two other commonly used models of liquid water, namely the transferable intermolecular potential-three points (TIP3P) and simple point charge/extended (SPC/E) models, and also with experiments. The dynamical properties of the SSD water model are found to be in good agreement with the experimental results and appear to be better than the TIP3P and SPC/E models in most cases, as has been previously shown for its thermodynamic, structural, and dielectric properties. Also, molecular dynamics simulations of the SSD model are found to run much faster than TIP3P, SPC/E, and other multisite models. © 1999 American Institute of Physics.

Notes: Time-of-flight scattering and recoiling spectrometry (TOF-SARS) was used to investigate the scattered and recoiled ion fractions from 3 keV Ar+ ion beams on LiTaO3(100) single crystals. The TOF-SARS measurements were found to be sensitive to the electrical properties of the crystal. ac impedance measurements of the electrical conductivity showed that LiTaO3 is an insulator at room temperature and that its conductivity increases by ∼103 at temperatures in the range 100–200 °C. This increase in conductivity could be monitored in TOF-SARS by measuring the current through the crystal induced by the impinging Ar+ ions as a function of temperature. The activation energy for this transition was estimated from both the impedance and scattering measurements to be ∼1 eV. Azimuthal anisotropy of the scattered Ar+ ions from Ta atoms was observed at room temperature but not at elevated temperatures. Scattered Ar+ ion fraction measurements showed that scattered Ar+ ions are enhanced by charge buildup on the LiTaO3 surface, whereas recoiled ions are not affected. The effects of surface charging phenomena on TOF-SARS could be eliminated by either heating the LiTaO3 crystal to ∼200 °C or by application of a low energy electron beam to the crystal surface. © 1999 American Institute of Physics.

Notes: We present a new time domain technique for studying molecular orientational relaxation in viscous liquids. A molecular velocity gradient (acoustic disturbance) associated with a density change induced by weak absorption of a 1.06 μm excitation pulse, causes molecular alignment through translational–rotational coupling. Using an optical heterodyne detection method, molecular orientational relaxation is monitored. An eightfold experimental cycle, analogous to phase cycles in NMR, is used to separate the DIHARD signal (density induced heterodyne amplified rotational dynamics) from optical Kerr effect (OKE) contributions and thermal lensing effects. Calculations combining the Navier–Stokes equation with translational–rotational coupling are presented that describe the nature of the method. The method is analyzed theoretically and demonstrated with experiments on supercooled salol (phenyl salicylate). DIHARD experiments on salol combined with heterodyne detected OKE experiments are used to examine long time scale orientational relaxation over a wide range of times and temperatures. While OKE experiments measure the time derivative of an orientational correlation function, it is shown that DIHARD directly measures the time dependence of an orientational correlation function. The experimental results are compared to those previously reported in the literature, which were obtained with other methods. © 1999 American Institute of Physics.

Notes: In linear response, the complex heat capacity is equal to the entropy compliance divided by temperature. The fluctuation dissipation theorem gives a relation to the correlation function (time domain) or spectral density (frequency domain) of entropy fluctuations. Another formal description of frequency dependent heat capacity at the glass transition is based on the Tool–Narayanaswamy–Moynihan approach of structural recovery. In the limit of small temperature perturbations both models are able to describe measured complex heat capacity. We have measured the response at medium temperature amplitudes, when first nonlinearities appear, and have compared the results with the behavior expected from both approaches. We have studied higher harmonics in the temperature amplitude range 0.1–15 K. At the glass transition of poly(vinyl acetate) (PVAc) we observe 1% of higher harmonics at temperature amplitudes of 1 K. Even harmonics can be described quantitatively by the temperature dependence of first harmonic (heat capacity) up to temperature amplitudes of 6 K. This is not possible for odd harmonics. The remaining small odd harmonics support, on a qualitative basis, the fluctuation approach to glass transition. © 1999 American Institute of Physics.

Notes: Despite the relevance to most aspects of crystallization, a comprehensive understanding of the kinetics of heterogeneous nucleation has not been well established yet. In this paper, a new kinetic model based on the "steady-state" approach will be put forward to describe both heterogeneous and homogeneous nucleation. As a key point in this model, the effect of foreign particles on both the nucleation barrier, the "chain reaction" process and the transport of structural units is taken into account. Ranging from low to high supersaturations, heterogeneous nucleation plays a more comprehensive role in nucleation than we expected, depending on the size of foreign particles and the interaction and structural matching between foreign particles and the nucleating phase. It follows that genuine homogeneous nucleation may only be possible at very high supersaturations, and can become easier when convection is eliminated. The results have been verified by the CaCO3 nucleation experiments. This general model allows us to describe many unexpected nucleation phenomena for the first time, and will have significant impact on the control of nucleation, understanding of crystallization in microgravity, the measurement of the crystal–fluid interfacial tension, and crystallization in general. © 1999 American Institute of Physics.

Notes: Supra-valence electron transfer from surface Cs-doped MoS2(0002) to electron acceptor adsorbates was investigated by high resolution x-ray photoelectron spectroscopy (HRXPS) in the valence band region and above the valence band maximum (VBM). Deposition of a sub-monolayer amount of Cs onto the basal plane of MoS2 introduced a new electron density of states at ca. 1.25 eV above VBM. Angle-resolved HRXPS and theoretical analysis located this electron density in the MoS2 layer. Upon the reaction with Cl2, this Cs-induced photoelectron almost completely disappeared and the Cs 3d and Cl 2p core levels indicated the formation of a surface Cs-chloride species. The Cs-covered MoS2(0002) surface also reacted with O2 to form surface peroxides and superoxides, as evidenced by two distinct binding energies of the O 1s core level peaks. However, the reaction with water proved to be more difficult: Exposure of the Cs-covered MoS2(0002) surface to H2O at 10−5 Torr did not result in electron transfer reaction, but the Cs/MoS2(0002) surface exposed to H2O at 1 Torr showed a substantial decrease in the density of states above VBM as well as formation of a surface-hydroxide, indicated by the O 1s core level position. Theoretical calculations using a full-potential linearized augmented plane wave density functional theory (FLAPW-DFT) confirm the conclusion based on experimental intensity anisotropy of the new peak: the Cs 6s electron transfers into the MoS2 substrate, forming the Cs/MoS2 electron donor–acceptor complex with Csδ+. In addition, all phenomena observed during the adsorption of electron donor–acceptor molecules are quantitatively accounted by the theory. © 1999 American Institute of Physics.

Notes: Previous pulsed-field gradient (PFG) nuclear magnetic resonance (NMR) measurements on different sorbate–zeolite systems suggest that there exist at least five different types of dependence of self-diffusivity of the sorbate, D, on the concentration, c, of the sorbate. Sorbate–zeolite systems have been modeled as a two-dimensional lattice gas and studied by carrying out Monte Carlo simulations under different conditions. Among the different factors that have been varied are the arrangement of adsorption sites, hop length, nature and strength of the sorbate-sorbate interaction, and the degree of confinement. Surprisingly, even the simple 2-D lattice gas model could yield more than one type of D vs c dependence. The present study provides insight into the possible reasons for a given type of D vs c dependence. © 1999 American Institute of Physics.

Notes: The microscopic diffusional dynamics of H and D on Pt(111) have been studied over length scales between 3 and 14 Å by quasielastic helium atom scattering. Data taken over a wide range of parallel wave vectors (0.3 Å−1≤|ΔK|≤3.1 Å−1) in the surface temperature range 140 K≤Ts≤250 K and at coverages 0.05 ML≤aitch-thetaH≤0.66 ML, provide evidence for an isotropic single jump mechanism with an activation energy of 68±5 meV and a pre-exponential factor of D0=1.1±0.5×10−3 cm2 s−1 at low coverages. The diffusion coefficient is 2 to 3 orders of magnitude higher than found in an earlier laser induced desorption experiment, whereas the dependence on coverage is similar. © 1999 American Institute of Physics.

Notes: Rheological properties and the equilibrium colloidal phase behavior of concentrated dispersions of a temperature sensitive microgel were investigated. The temperature sensitive hydrogel particles consist of poly (N-isopropylacrylamid) (PNiPAM) chemically crosslinked with N,N′ methylenbisacrylamid (BIS). With increasing temperature the microgel particles decrease in size (hydrodynamic radius 142 nm at 10 °C and 58 nm at 35 °C) and with it the effective volume fraction, which leads to dramatic changes in rheology—vanishing yield stress and decreasing viscosity and elastic properties. The relative zero-shear viscosity and the plateau modulus at different temperatures superpose to mastercurves when plotted vs the effective volume fraction. The monodisperse hydrogel particles form colloidal crystals and glasses in concentrated solution but at higher effective volume fractions as compared to model hard sphere suspensions. Comparison of the experimental freezing point with soft sphere computer simulations indicate a repulsive interaction potential of the order 1/r12. The frequency independent shear modulus exhibited a power law concentration dependence which also agrees with the soft sphere behavior. © 1999 American Institute of Physics.

Notes: We study the nematic–isotropic transition in model colloidal systems composed of platelets of various shapes using grand canonical simulations. This is of relevance for recently synthesized hard platelet systems, since the platelets in such systems are found to be not circular but irregular hexagons, and so cannot be described by the simulation data currently available. We show that the coexistence densities scale with an effective volume related to the isotropic orientation-averaged excluded volume of a pair of platelets. This excluded volume can be obtained from the perimeter of the face of the particles and so can be easily calculated for both regular and irregular particle shapes. © 1999 American Institute of Physics.

Notes: The role played by the ion beam in the depolymerization of poly(methyl methacrylate) (PMMA) is the creation of initiators for the unzipping reaction at relatively low temperatures [lower than the ceiling temperature of PMMA (TC=220 °C)]. Monomer (MMA) evolution out of the polymeric film is thus governed by the kinetics of the chemical reaction and also by the diffusion of monomer inside the polymeric film. In this work this phenomenon has been described by using simple models in which both one- and two-dimension monomer diffusion is considered. Exact analytical solutions of the models are provided. The theory fit well the experimental data and allows for the determination of important parameters such as the kinetics constant of the depolymerization and the monomer diffusion coefficient, D, inside the polymer film. Thus, the pulsed ion beam technique, here modeled, is useful not only for extracting D values of slow diffusants in polymers, but even for measuring kinetics constants of depolymerization at a temperature well below that needed for the formation of suitable thermal initiators. © 1999 American Institute of Physics.

Notes: We compare the linear viscoelastic spectra of star polymer melts, with varying functionality (4-128) and chemistry (isoprenes, butadienes), with a recent parameter-free theory of arm relaxation [S. T. Milner and T. C. B. McLeish, Macromolecules 30, 2159 (1997)]. The theory, which considers this activated process within the framework of dynamic dilution and with appropriate account of the entanglement length scaling and the higher Rouse modes, is universal as it works remarkably well for a very wide range of star functionalities and arm molecular weights. However, for hyperstars consisting of 64 or 128 arms, the viscoelastic response is characterized by the presence of a slow relaxation process in addition to the faster arm relaxation. This additional process is due to the soft ordering of these systems because of their nonuniform monomer density distribution, and exhibits a very strong functionality and molecular weight dependence. It is accounted for by a mean field approach which considers the structural relaxation of the ordered stars as an activated process involving partial disentanglement of the interpenetrating arms and a jump of the star over a distance of its size, in good qualitative agreement with the experimental findings. © 1999 American Institute of Physics.

Notes: Molecular dynamics simulations are conducted for concentrated solutions of flexible polymers. The results are contrasted with literature dielectric spectroscopy data, in an attempt to elucidate the observed phenomena from a molecular level perspective. A bead-spring model is used and systems with chain sizes up to N=150 beads at reduced densities 0.5≤ρ≤0.8 are studied. The dimensions of the chains follow a universal behavior with ρ/ρ*, where ρ* is the crossover density demarcating the onset of chain overlapping. All the chains are found to follow random-walk behavior. The global motion of the chains is investigated in terms of the dielectric loss E″. As in dielectric spectroscopy experiments, the motion of the chains induces prominent dielectric relaxation at low frequencies. The shape of E″ broadens with increasing density, and a normal-mode analysis indicates that overlapping of the chains with increasing density progressively renders the distribution of relaxation times more heterogeneous. For denser systems a second, smaller peak appears at the high frequency end of the spectrum. This secondary peak is not identified with segmental motion, since the simulated chains lack components of the segmental dipoles perpendicular to the chain contour. Entanglement effects are investigated calculating the mean squared displacement g1(t), and the results suggest that the topological constraints of entanglements render at least two different relaxation mechanisms with disparate time scales important. An attempt to explain the shape of the spectra in terms of a phenomenological separation of the motion of chains into a rotational and a stretching mode showed that stretching plays no important role in the relaxation function and the shape of E″. © 1999 American Institute of Physics.

Notes: Recently, we proposed a new systematic approach to evaluate the many-particle effects on the diffusion-influenced reactions. The method gives an improved result over that obtained by using the superposition approximation. In the present paper, we apply the method to treat the kinetics of reversible energy-transfer reactions of the type A*+B(arrow-right-and-left)A+B*. Until now, most theories were inapplicable when the lifetime of A* is shorter than that of B*, and a notable exception was the integral encounter theory (IET) of Burshtein et al. The present theory can be applied irrespective of the relative magnitude of the lifetimes of donor and acceptor molecules, and becomes exact for the irreversible target model. In addition, it is applicable to the system with higher reactant densities than IET; the result of IET is recovered as a limiting form in the present theory. © 1999 American Institute of Physics.

Notes: A methodology is developed for finding a formal solution to the entire stationary Bogoliubov, Born, Green, Kirkwood, Yvon (BBGKY) hierarchy for a classical inhomogeneous fluid in thermal equilibrium. The method is unique in that neither a closure relationship nor a restriction on the density or the temperature of the fluid is required. The decisive step in the analysis is the creation of a more general system of coupled integrodifferential equations through the introduction of a coupling parameter. Recognizing that the coupling parameter can also act as an expansion parameter, series solutions to the generalized hierarchy can be obtained through the application of regular perturbation theory. At the end of the calculation, a formal solution to the original BBGKY hierarchy can be recovered by setting the coupling parameter to unity. © 1999 American Institute of Physics.

Notes: Many systems approach equilibrium slowly along surfaces of dimension smaller than the original dimensionality. Such systems include coupled chemical kinetics and master equations. In the past the steady state approximation has been used to estimate these lower dimensional surfaces, commonly referred to as "manifolds," and thus reduce the dimensionality of the system which needs to be studied. However, the steady state approximation is often inaccurate and sometimes difficult to define unambiguously. In recent years two methods have been proposed to go beyond the steady state approximation to improve the accuracy of dimension reduction. We investigate these methods and suggest significant modifications to one of them to allow it to be used for the generation of low-dimensional manifolds in large systems. Based on the geometric investigations, two other approaches are suggested which have some advantages over these two methods for the cases studied here. All four approaches are geometric and offer advantages over methods based on the evaluation of time-dependent behavior, where phenomenological rate laws are extracted from the time-dependent behavior. © 1999 American Institute of Physics.

Notes: Two nonempirical kinetic energy density dependent approximations are introduced. First, the local τ approximation (LTA) is proposed in which the exchange energy Ex depends only on a kinetic energy density τ. This LTA scheme appears to be complementary to the local spin density (LSD) approximation in the sense that its exchange contribution to the atomization energy ΔEx=Exatoms−Exmolecule is fairly accurate for systems where LSD fails. On the other hand, in cases where LSD works well LTA results for ΔEx are worse. Secondly, the τPBE approximation to Ex is developed which combines some of the advantages of LTA and of the Perdew–Burke–Ernzerhof (PBE) exchange functional. Like the PBE exchange functional, τPBE is free of empirical parameters. Furthermore, it yields improved atomization energies compared to the PBE approximation. © 1999 American Institute of Physics.

Notes: An ab initio molecular dynamics study of the SN2 reaction Cl−+CH3Br→CH3Cl+Br− has been performed at the Becke, Lee, Yang, and Parr (BLYP) level of theory by the blue-moon method. The potential energy and the free energy profile along the reaction coordinate have been determined and compared with the available experimental and calculated data. An analysis of the structural parameters along the reaction pathway is presented. Results of impact studies are also reported. It is shown that, depending on impact velocity, recrossing of the barrier can occur. Strong polarization effects are reported. © 1999 American Institute of Physics.

Notes: Using symplectic integrator schemes, we calculate the classical trajectory of a Rydberg electron in external electric and magnetic fields. We also solve the equation of motion obtained by taking the mean values over one revolution of the electron in the undisturbed motion. The resulting secular motion is periodic. When only an electric field F is applied, as long as the modulation period in the orbital angular momentum l is longer than the revolution period, the motion agrees with the secular one and the duration for which l is much larger than its low initial value is stretched. The residence time (RT), namely, the probability of finding the electron at the distance r, is hence smaller than that at F=0. In crossed electric and magnetic fields, the secular motion predicts that an additional time stretching due to a magnetic field occurs up to the critical value of magnetic field strength, Bc=33nF (n is the principal action). In the actual simulations, the RT near the core is smaller than that at B=0 even beyond Bc, regardless of the magnitude of the non-Coulombic interaction C2/r2. Slow modulations in l are generated by transitions to secular motions that maintain high l, in addition to the fast modulation originating from the secular motion. When the magnetic field is so strong as to induce chaotic motion (∼4000 G for the energy of −5 cm−1), the RT is one order of magnitude as large as those in weak field cases around 40 G. In the intermediate region (〉 a few hundred Gauss), without a non-Coulombic interaction, the RT monotonically increases as B increases. In the presence of C2/r2, transitions from low l states to high l states occur: the RT decreases. The motions in high l states can be explained by the well-known model in which an electron bound to the core by a harmonic force moves in a magnetic field. © 1999 American Institute of Physics.

Notes: Equilibria involving the isomers AlNC and AlCN above a mixture of aluminum nitride, graphite, and gold contained in a graphite Knudsen cell were investigated with a mass spectrometer. The enthalpies of formation, ΔfH0o, and of atomization, ΔaH0o, in kJ mol−1, for AlNC and AlCN, were derived as 281.3±14 and 303.8±14, and as 1228.1±15 and 1205.6±15, respectively. © 1999 American Institute of Physics.

Notes: We reconsider the potential energy surface of the He–LiH system recently examined by Gianturco and co-workers [F. A. Gianturco et al., Chem. Phys. 215, 227 (1997)]. We compute the He–LiH interaction energy at the CCSD(T) level using large correlation consistent atomic basis sets supplemented with bond functions. To capture the severe anisotropy of the He–LiH potential, we interpolate our ab initio points in the angular direction with cubic splines, then expand the splines in terms of Legendre polynomials. The resulting smooth potential surface differs substantially from that of Gianturco et al.; in particular, our attractive He–LiH well is more than twice as deep as that of Gianturco et al., with a He–LiH binding energy of De=176.7 cm−1. © 1999 American Institute of Physics.

Notes: High quality ab initio calculations for the interaction of He with the B 3Π0u+ state of Cl2 for three r(Cl–Cl) distances, and for the He(1S)+Cl(2P) interaction are used to obtain a three-dimensional potential energy surface for the system. The surface was used to calculate HeCl2 excitation spectra, predissociation lifetimes, and product state distributions for comparison with experimental data, and yields a remarkably good agreement. The largest discrepancy is in the dependence of the lifetime on the excited state vibrational level. The calculated lifetimes are too short for the lowest measured vibrational levels. To investigate how the surface could be modified to obtain even better agreement, a microgenetic algorithm was used to adjust the potential parameters to improve the fit. The adjusted surface has a softer repulsive wall for small Cl–Cl separations which helps to lengthen the excited state lifetimes and yields better agreement with the data. Also, the shape of the well region is adjusted somewhat in the fitting process, which yields a stronger dependence of lifetime on vibrational level. © 1999 American Institute of Physics.

Notes: The study of the phenol–(NH3)3 cluster with two-color two-photon ionization shows that the main ion observed with delays between the lasers up to a few hundred nanoseconds is the (NH4)+(NH3)2 fragment, resulting from direct ionization of the (NH4)(NH3)2 product coming from the reaction: PhOH(S1)–(NH3)3→PhO•+(NH4)(NH3)2. © 1999 American Institute of Physics.

Notes: The infrared (IR) absorption spectra of the jet-cooled C6H6 and C6D6 cations, complexed with Ar, are measured throughout the 450–1500 cm−1 region via IR-laser-induced vibrational dissociation spectroscopy. The IR spectrum of the C6H6–Ar cation is dominated by a Fermi resonance between the IR active ν11 mode and two components of the combination mode of the lowest frequency modes ν6 and ν16. A stringent upper limit of 316 cm−1 is found for the value of the dissociation limit D0 of the neutral C6D6–Ar complex. © 1999 American Institute of Physics.

Notes: The empirical force fields used for protein simulations contain short-ranged terms (chemical bond structure, steric effects, van der Waals interactions) and long-ranged electrostatic contributions. It is well known that both components are important for determining the structure of a protein. We show that the dynamics around a stable equilibrium state can be described by a much simpler midrange force field made up of the chemical bond structure terms plus unspecific harmonic terms with a distance-dependent force constant. A normal mode analysis of such a model can reproduce the experimental density of states as well as a conventional molecular dynamics simulation using a standard force field with long-range electrostatic terms. This finding is consistent with a recent observation that effective Coulomb interactions are short ranged for systems with a sufficiently homogeneous charge distribution. © 1999 American Institute of Physics.

Notes: A general formalism for time-dependent linear response theory is presented within the framework of linear-combination-of-atomic-orbital crystalline orbital theory for the electronic excited states of infinite one-dimensional lattices (polymers). The formalism encompasses those of time-dependent Hartree–Fock theory (TDHF), time-dependent density functional theory (TDDFT), and configuration interaction singles theory (CIS) (as the Tamm–Dancoff approximation to TDHF) as particular cases. These single-excitation theories are implemented by using a trial-vector algorithm, such that the atomic-orbital-based two-electron integrals are recomputed as needed and the transformation of these integrals from the atomic-orbital basis to the crystalline-orbital basis is avoided. Convergence of the calculated excitation energies with respect to the number of unit cells taken into account in the lattice summations (N) and the number of wave vector sampling points (K) is studied taking the lowest singlet and triplet exciton states of all-trans polyethylene as an example. The CIS and TDHF excitation energies of polyethylene show rapid convergence with respect to K and they are substantially smaller than the corresponding Hartree–Fock fundamental band gaps. In contrast, the excitation energies obtained from TDDFT and its modification, the Tamm–Dancoff approximation to TDDFT, show slower convergence with respect to K and the excitation energies to the lowest singlet exciton states tend to collapse to the corresponding Kohn–Sham fundamental band gaps in the limit of K→∞. We consider this to be a consequence of the incomplete cancellation of the self-interaction energy in the matrix elements of the TDDFT matrix eigenvalue equation, and to be a problem inherent to the current approximate exchange–correlation potentials that decay too rapidly in the asymptotic region. © 1999 American Institute of Physics.

Notes: We present an investigation of Hermite polynomials as a basic paradigm for quantum dynamics, and make a thorough comparison with the well-known Chebyshev method. The motivation of the present study is to develop a compact and numerically efficient formulation of the spectral filter problem. In particular, we expand the time evolution operator in a Hermite series and obtain thereby an exponentially convergent propagation scheme. The basic features of the present formulation vìs a vìs Chebyshev scheme are as follows: (i) Contrary to the Chebyshev scheme Hamiltonian renormalization is not needed. However, an arbitrary time scaling may be necessary in order to avoid numerical hazards, and this time scaling also provides a leverage to accelerate the convergence of the Hermite series. We emphasize the final result is independent of the arbitrary scaling. (ii) As with the Chebyshev scheme the method is of high accuracy but not unitary by definition, and thus any deviation from unitarity may be used as a guideline for accuracy. The calculation of expansion coefficients in the present scheme is extremely simple. To contrast the convergence property of present method with that of the Chebyshev one for finite time propagation, we have introduced a time–energy scaling concept, and this has given rise to a unified picture of the overall convergence behavior. To test the efficacy of the present method, we have computed the transmission probability for a one-dimensional symmetric Eckart barrier, as a function of energy, and shown that the present method, by suitable time–energy scaling, can be very efficient for numerical simulation. Time–energy scaling analysis also suggests that it may be possible to achieve a faster convergence with the Hermite based method for finite time propagation, by a proper choice of scaling parameter. We have further extended the present formulation directed toward the spectral filter problem. In particular, we have utilized the Gaussian damping function for the purpose. The Hermite propagation scheme has allowed all the time integrals to be done fully analytically, a feature not completely shared by the Chebyshev based scheme. As a result, we have obtained a very compact and numerically efficient scheme for the spectral filters to compute the interior eigenspectra of a large rank eigensystem. The present formulation also allows us to obtain a closed form expression to estimate the error of the energies and spectral intensities. As a test, we have utilized the present spectral filter method to compute the highly excited vibrational states for the two-dimensional LiCN (J=0) system and compared with the exact diagonalization result. © 1999 American Institute of Physics.

Notes: The inversion mechanism of a T-shaped Ar3 is studied both classically and quantum mechanically. Regular states, localized in the region of the transition state for the inversion of the axial argon atom are found and are assigned by the symmetric stretch stable periodic orbits which emanate from the saddle point of the potential. These states inhibit the inversion process. States which promote the inversion are mainly irregular, but a few of them are localized and they have their nodes perpendicularly arranged along periodic orbits which originate from saddle node bifurcations. The two types of periodic orbits, inhibiting and isomerizing, are used to produce distinctly different spectra and to extract the corresponding eigenfunctions by solving the time dependent Schrödinger equation using a variable order finite difference method [J. Chem. Phys. 111, 10827 (1999), preceding paper]. © 1999 American Institute of Physics.

Notes: Optical/Stark measurements have been performed on the (0,0) bands of both the A″ 1Σ+–X 1Σ+ system (ν0(approximate)12 643 cm−1) and the A′ 1Π–X Σ+ system (ν0(approximate)13 196 cm−1) of platinum monocarbide. The PtC molecules were produced in a pulsed supersonic molecular beam source following the reaction of laser ablated platinum vapor with a mixture of a few percent of methane in argon. The newly determined permanent electric dipole moments obtained are 1.94(2)D (A″ 1Σ+) and 1.919(9)D (A′ 1Π). These results are discussed in terms of a proposed molecular orbital correlation diagram for platinum containing diatomics. The laser-induced fluorescence spectrum of the (0,0)A″ 1Σ+–X 1Σ+ transition of PtC has been re-recorded at high resolution (full width of half-maximum ∼40 MHz) and analyzed to yield rotational constants for the four most abundant isotopomers of PtC, extending the previous analysis [Appelblad, Nilsson, and Scullman, Phys. Scr. 7, 65 (1973)]. The anomalously large value (∼15 MHz) for the newly derived nuclear-spin rotation parameter, CI(195Pt), for the A″ 1Σ+ state is discussed. © 1999 American Institute of Physics.

Notes: A time-dependent quantum mechanical approach has been used to investigate the reaction He+HD+(v=0–4,j=0–3)→HeH++D; HeD++H in three dimensions for total angular momentum J=0. The vib-rotation (v,j) state-selected reaction probability (Pv,jR) is shown to increase with v over the collision energy (Etrans) range (0.95–2.25 eV) investigated for both the exchange channels, in accord with the experimental results. The isotopic branching ratio Γ=PR(HeH+)/PR(HeD+) generally remains less than unity for different v states at different Etrans in agreement with experiment. But at Etrans=1.0 eV, for v=4, Γ obtained from our calculations for j=0 of HD+ is ∼0.8, in excellent agreement with the earlier quasiclassical trajectory calculations, but a factor of 2 less than that obtained from experiment. This difference could arise from the inclusion of nonzero j states in the experimental study, as Pv,jR is found to be j dependent for both the channels. While Pv,jR (HeH+) decreases initially with increase in j from 0 to 2 and then increases when j is increased further to 3, Pv,jR (HeD+) reveals an unusual j dependence; it is larger for even j states of HD+ than for odd j. As a result, Γ is strongly dependent on j, in contrast to the marginal dependence shown by the earlier quasiclassical trajectory calculations. © 1999 American Institute of Physics.

Notes: In order to explore the dynamic problem of the electronic structure in the ground state for the Creutz–Taube ion, the calculations on the electronic potential surfaces along the totally symmetric vibrational difference coordinate of the Ru–N stretch and the net charge distribution on the Ru atoms are carried out by using the density functional theory in the two schemes with or without the Ru–N(pyz) vibration. It is shown that the vibration associated with the totally symmetric vibrational difference coordinate Q− can be regarded as a harmonic oscillation with 123.16 eV Å−2 of the force constant, and 430 cm−1 of the fundamental frequency is obtained on the basis of the five-body vibrator model. From the electronic structure calculated in the ground state it is concluded that the asymmetric net distribution on the Ru atoms in the Creutz–Taube ion occurs in the vibration associated with the totally symmetric vibrational difference coordinate, and the charge transfer between the Ru atoms and the ligands is responsible for the asymmetric charge distribution. The total symmetric vibrations of Ru–N for the related monomer [(NH3)5Ru(pyz)]2+/3+ are further examined. It is found that the reduction of Ru(III) to Ru(II) corresponds to 0.05 Å of the difference in Ru–N distance between the two Ru-subunits under the asymmetric distortion and to 0.012 eV of the potential energy over the zero point energy, which is comparable with kT. The Born–Oppenheimer approximation and the neglected coupling effect between the electronic and nuclear motion in the present paper are briefly discussed. © 1999 American Institute of Physics.

Notes: Two different definitions of phase shifts and time delays are contrasted and shown to match different experimental methods of generating delayed pulses. Phase shifts and time delays are usually defined in terms of a carrier wave in magnetic resonance, but definitions based on the envelope of a single pulse are useful in optics. It is demonstrated experimentally that a frequency domain measurement using spectral interferometry can simultaneously measure phase shifts with an accuracy of 0.1 rad (2σ) and time delays with a precision of 40 attoseconds (2σ) for 25 femtosecond optical pulses. Envelope time delays are generated by pathlength differences in an interferometer. Constant spectral phase shifts are demonstrated by diffracting pulses from a variable phase volume diffraction grating. Experimental requirements for phase-resolved spectroscopy are outlined. The theory of phase-locked pulse pair techniques is reexamined, and it is concluded that linear experiments with phase-locked pulse pairs are completely equivalent to Fourier transform absorption spectroscopy and do not measure the refractive index or real part of the susceptibility. It is shown that Fourier sine and cosine transformations of truncated time domain signals which do not match the symmetry of the complete signal can produce a false dispersive susceptibility because they are equivalent to Kramers–Kronig inversion of finite bandwidth absorption data. A procedure for shifting π/2 phase-locked transients by a quarter cycle of delay to generate a transient with a π/2 spectral phase shift is given. Equations used to calculate femtosecond nonlinear optical signals have assumed carrier wave delays. Modifications to these equations are required when envelope delays are generated by interferometer pathlength differences and modified equations are given. The modified equations yield significantly different results for phase-resolved or interferometric experiments. In particular, the modified equations are needed to calculate indirectly (interferometrically) detected frequencies and the real and imaginary parts of two-dimensional Fourier transform spectra. The role of the refractive index and real part of the frequency domain susceptibility in nonlinear experiments with phase-locked pulse pairs is explored. It is concluded that experiments such as the heterodyne detected stimulated photon echo are insensitive to nonlinear refractive index changes under some circumstances. Finally, modifications of some equations used in the theory of coherent control are needed to match theory with experimental practice. © 1999 American Institute of Physics.

Notes: To gain insight into the effects of the weakening of the electrostatic interactions on molecular dynamics when polar molecules are dissolved in a nonpolar solvent, the dielectric polarization and relaxation behaviors of iso-amylbromide and its 50 mol % solution in 2-methylpentane have been studied in detail over the frequency range, 1 mHz–1 MHz, and a temperature range approaching their liquid to glass transition. Features of the (i) α-relaxation spectrum, (ii) the Johari–Goldstein relaxation process in the liquid state at low temperatures, with an asymmetric spectral shape, and (iii) the temperature dependence of the relaxation dynamics have been determined and the effects of weakening of the electrostatic interaction on these features examined. The high-frequency wing of the loss spectrum of the α-relaxation is proportional to ω−β. The dynamics of its α-relaxation follows the Arrhenius equation initially at high temperatures and thereafter the Vogel–Fulcher–Tamman equation. Alternative equations for the change in the relaxation rate have been discussed. A decrease in the dipole–dipole interaction and reduction in the internal field in a solution with a nonpolar solvent leads to a remarkable change in the shape of the relaxation spectra at high frequencies such that the dielectric loss for the α-relaxation becomes proportional to ω−αβ, with α, β〈1. The relaxation spectra of iso-amyl bromide dissolved in 2-methylpentane follows the H–N function and therefore behaves similar to a polymer, whereas for pure iso-amyl bromide follows the Davidson–Cole behavior. © 1999 American Institute of Physics.

Notes: Stochastic resonance (SR) is studied numerically in a modified Oregonator-type model, which was proposed recently to account for the photosensitivity of the Belousov–Zhabotinsky (BZ) reaction in a flow system. When either of the two control parameters, light flux and a flow rate, is modulated by multiplicative external noise, noise induced coherent oscillations (NICO) in the absence of deterministic oscillations are observed near Hopf bifurcation point, where the external noise is added to one parameter or the other. The signal-to-noise ratio (SNR) goes through a maximum with the increment of noise intensity indicating occurrence of SR. The aspects of the two-parameter SR in this system are discussed. © 1999 American Institute of Physics.

Notes: New intermolecular potential models for benzene and cyclohexane have been developed, parameterized to the vapor–liquid coexistence properties. The models utilize the Buckingham exponential-6 potential to describe nonbonded interactions. Histograms reweighting grand canonical Monte Carlo methods were used to obtain the model parameters. A new algorithm for insertion of molecules with complex molecular architectures or stiff intramolecular constraints has been developed. The algorithm is based on the creation of a reservoir of ideal chains from which structures are selected for insertion during a simulation run. The new potential models reproduce the experimental saturated liquid densities and vapor pressures to within average absolute deviations of 0.3% and 2.2%, respectively. Critical parameters are also in good agreement with experiment. The infinite dilution behavior of these two cyclic molecules in water was studied. A combination of Widom insertion and expanded ensemble techniques were used to determine the Henry's law constant of benzene and cyclohexane in water. The results obtained have qualitatively correct temperature dependence. However, the Henry's constant of benzene in water is overestimated and that of cyclohexane is underestimated at all temperatures by approximately a factor of 3. © 1999 American Institute of Physics.

Notes: The adsorption of molecular hydrogen gas onto charged single-walled carbon nanotubes (SWNTs) is studied by grand canonical Monte Carlo (GCMC) computer simulation. The quadrupole moment and induced dipole interaction of hydrogen with "realistically" charged (0.1 e/C) nanotubes leads to an increase in adsorption relative to the uncharged tubes of ∼10%–20% for T=298 K and 15%–30% for 77 K. Long-range electrostatic interactions makes second layer (exohedral) adsorption significantly higher. Hydrogen orientation-ordering effects and adsorption anisotropy in the electrostatic field of the nanotube were observed. The geometry of nanotube arrays was optimized at fixed values of charge, temperature, and pressure. In general, negatively charged nanotubes lead to more adsorption because the quadrupole moment of hydrogen is positive. Calculated isotherms indicate that even charged nanotube arrays are not suitable sorbents for achieving the DOE target for hydrogen transportation and storage at normal temperatures, unless the charges on the nanotubes are unrealistically large. © 1999 American Institute of Physics.

Notes: We have measured rotational excitation into rotational states J=3, 4, and 5 for H2 scattered from Pd(111) as a function of surface temperature and incident translational energy. Excitation is found to occur even when the incident H2 translational energy is less than the energy level spacing between the initial and final rotational states. Thus, part of the excitation energy is coming from the surface, not from solely translational–rotational energy coupling. There is a strong surface temperature dependence to the rotational excitation that is well described by an Arrhenius-type expression. When fit to the Arrhenius equation, the apparent activation energy is less than the rotational energy level spacing and decreases as the translational energy of the incident molecules is increased. Based on inspection of the calculated H2/Pd(111) potential energy surface, we attribute this lowered activation energy to an extension of the bond length when the molecule interacts with the surface. The stretching of the molecular bond increases the moment of inertia of the molecule, which then decreases the spacing between the rotational energy levels. We suggest that the final states of molecules which do not dissociate reflect features of the potential energy surface associated with open, dissociative pathways. © 1999 American Institute of Physics.

Notes: The diffuse behavior of penetrants in simple polymer melts was investigated by molecular dynamics simulation. For the case where the polymer melt consisted of pearl-necklace chains, the diffusive behavior of the loose pearl penetrants was seen to be qualitatively different than would be expected in realistic models of polymer melts. In particular, there was little or no "non-Fickian" region; the variation of the diffusion coefficient with the penetrant diameter was what one would expect for diffusion through small molecular liquids; and, finally, the long time tail of the velocity autocorrelation displayed a "−3/2" power law form, also as in the small molecular liquid case. When the chains' backbone motion was further constrained by the introduction of a bond angle potential, the qualitative nature of the penetrant diffusion became more "polymer-like." A non-Fickian region developed; the diffusion coefficient varied more rapidly with penetrant diameter; and the velocity autocorrelation function developed a "−5/2" power law tail. © 1999 American Institute of Physics.

Notes: Ab initio configuration interaction calculations have been carried out on the ground and excited electronic states of the BeOH and MgOH molecules as well as of the cations BeOH+ and MgOH+, for linear and bent geometries. The excited states of the above molecules have not previously been calculated and experimental information exists only for the A–X system in MgOH and MgOD. The present results show that in the excited states the molecules MgOH and BeOH have similar M–O (M=Mg,Be) stretching and bending potentials. In general, the stretching potentials are rather complicated, showing a number of avoided crossings. Furthermore, most of the excited states show minima at R near Rmin of the corresponding cations BeOH+ and MgOH+, indicating Rydberg contributions to the molecular excited states. The first excited state in both BeOH and MgOH is 2 2A′, which along with 1 2A″, forms the 1 2Π state of linear geometries and which in both systems has minimum energy at a bent geometry with bond angle near 115°. In MgOH, the 2 2A′ state is the A state of the observed A–X spectra and the theoretical transition energy and the barrier to linearity are in good agreement with the corresponding experimental quantities. Analogous spectra for BeOH, not reported as yet, would be expected on the basis of the present calculations at higher energies than the MgOH spectra by 0.6 eV. The results on the molecular ground-state potentials are similar to those of previous calculations, showing a linear minimum geometry for MgOH but with a very shallow bending potential, and for BeOH a bent minimum but with only a 50 cm−1 barrier to linearity. © 1999 American Institute of Physics.

Notes: To study the effect of fluorination of the core of a phospholipid bilayer we performed two molecular dynamics computer simulations. The first simulation was performed on a regular dimyristoylphosphatidylcholine (DMPC) bilayer, the second simulation was performed on its fluorinated counterpart FDMPC. In FDMPC, bilayer hydrogen atoms belonging to the last four hydrocarbon groups in both chains of phospholipid molecules were replaced by fluorines. From our simulations we observed that as a result of fluorination the core of the bilayer represented a tightly packed structure, while the structure of the head groups and of the region in the beginning of the tails remained similar to the one observed in the ordinary bilayer. A simple model for charge distribution in the phospholipid tails was proposed which explained the change in the sign of the dipole potential due to fluorination. © 1999 American Institute of Physics.

Notes: Fluids of hard bent-core molecules have been studied using theory and computer simulation. The molecules are composed of two hard spherocylinders, with length-to-breadth ratio L/D, joined by their ends at an angle 180°−γ. For L/D=2 and γ=0,10,20°, the simulations show isotropic, nematic, smectic, and solid phases. For L/D=2 and γ=30°, only isotropic, nematic, and solid phases are in evidence, which suggests that there is a nematic-smectic-solid triple point at an angle in the range 20°〈γ〈30°. In all of the orientationally ordered fluid phases the order is purely uniaxial. For γ=10° and 20°, at the studied densities, the solid is also uniaxially ordered, whilst for γ=30° the solid layers are biaxially ordered. For L/D=2 and γ=60° and 90° we find no spontaneous orientational ordering. This is shown to be due to the interlocking of dimer pairs which precludes alignment. We find similar results for L/D=9.5 and γ=72°, where an isotropic-biaxial nematic transition is predicted by Onsager theory. Simulations in the biaxial nematic phase show it to be at least mechanically stable with respect to the isotropic phase, however. We have compared the quasi-exact simulation results in the isotropic phase with the predicted equations of state from three theories: the virial expansion containing the second and third virial coefficients; the Parsons–Lee equation of state; an application of Wertheim's theory of associating fluids in the limit of infinite attractive association energy. For all of the molecule elongations and geometries we have simulated, the Wertheim theory proved to be the most accurate. Interestingly, the isotropic equation of state is virtually independent of the dimer bond angle—a feature that is also reflected in the lack of variation with angle of the calculated second and third virial coefficients. © 1999 American Institute of Physics.

Notes: Two-wavelength measurements of isotropic scattering coefficient in concentrated, multiply scattering colloidal suspensions are used to extract S(0,φ) values, using a small wave number expansion of the interparticle structure factor, S(q,φ). Owing to the small particle size (150 nm) and near-infrared wavelengths (670–820 nm) used in this study, the approximation is reasonable and allows accurate estimation of S(0,φ). The estimated values of S(0,φ) agree well with that predicted using the Carnahan–Starling equation of state. © 1999 American Institute of Physics.

Notes: The instantaneous normal mode spectrum of a Lennard-Jones liquid undergoing shear flow is determined as a function of shear rate. Shear flow is shown to deplete the density of states at low frequencies and augment the density of states at high frequencies, for both the real and imaginary modes. Shear flow also leads to an increase in the fraction of modes with imaginary frequencies. The implications of these changes are discussed in regard to other system properties. © 1999 American Institute of Physics.

Notes: We propose a method which uses centroid molecular dynamics (CMD) [J. Cao and G. A. Voth, J. Chem. Phys. 100, 5106 (1994)] real-time data in conjunction with the imaginary-time data generated using path integral Monte Carlo simulations in a numerical analytic continuation scheme based on the maximum entropy approach. We show that significant improvement is achieved by including short-time CMD data with the imaginary-time data. In particular, for a particle bilinearly coupled to a harmonic bath, these methods lead to significant improvements over previous calculations and even allow accurate determination of transport coefficients such as the diffusion coefficient and mobility for this system. In addition we show how maximum entropy method can be used to extract accurate dynamic information from short-time CMD data, and that this approach is superior to the direct Fourier transform of long-time data for systems characterized by broad, featureless spectral distributions. © 1999 American Institute of Physics.

Notes: The automatic generation of natural internal coordinates is extended to all cases except multiply fused ring systems, i.e., "cages." This is achieved by a relatively simple code; most of the geometrical cases can now be treated by a single subroutine. To handle molecules containing cages we combine the natural internal coordinates with the delocalized coordinates of Baker, Kessi, and Delley. We have also modified the delocalized coordinates to separate modes with strongly different force constants. The new methods show very good convergence properties in geometry optimizations. Our test set of molecules covers a wide range of atoms and of different bonding situations. For such molecules good internal coordinates are particularly important because no reliable initial force constant matrix can be obtained to accelerate convergence. © 1999 American Institute of Physics.

Notes: The interaction of energetic hydrogen (25–50 eV) with fullerite is studied theoretically to determine if endohedral H@C60 is feasible. Ab initio quantum calculations are used to calculate the binding energy of various H–C60 configurations and these are used in the fitting of a classical many-body C–H potential. Molecular-dynamics simulations are carried out of the interaction of individual H atoms with a fullerite crystal at both 25 and 50 eV using this classical potential. It is shown to be feasible to implant H atoms with a good probability within the surface layer fullerene molecules, thus suggesting an experimental procedure for the production of H@C60. © 1999 American Institute of Physics.

Notes: A retarding field technique coupled with a quadrupole mass analyzer has been used to obtain the kinetic energy release distributions (KERDs) for the C2H3Br+→[C2H3]++Br dissociation as a function of internal energy. The KERDs obtained by dissociative photoionization using the He(I), Ne(I), and Ar(II) resonance lines were analyzed by the maximum entropy method and were found to be well described by introducing a single dynamical constraint, namely the relative translational momentum of the fragments. Ab initio calculations reveal the highly fluxional character of the C2H3+ ion. As the energy increases, several vibrational modes are converted in turn into large-amplitude motions. Our main result is that, upon increasing internal energy, the fraction of phase space sampled by the pair of dissociating fragments is shown to first decrease, pass through a shallow minimum around 75%, and then increase again, reaching almost 100% at high internal energies (8 eV). This behavior at high internal energies is interpreted as resulting from the conjugated effect of intramolecular vibrational redistribution (IVR) and radiationless transitions among potential energy surfaces. Our findings are consistent with the coincidence data of Miller and Baer, reanalyzed here, and with the KERD of the metastable dissociation. © 1999 American Institute of Physics.

Notes: The electronic absorption spectra of mono-hydrogenated carbon chain anions C2nH− (n=5–10) have been measured in the gas-phase and in 6 K neon matrices (n=8–12). The techniques of resonant two-color electron photodetachment in the gas-phase and absorption spectroscopy of mass-selected anions in neon matrix were used. A homologous series is observed, with band system origins shifting from 304 nm for C10H− to 590 nm for C20H−. In conjunction with ab initio calculations the band systems are attributed to a 1Σ+←X 1Σ+ transition of linear acetylenic anions. Another near lying electronic transition due to a second isomer is also apparent for C10H− up to C24H−. Comparison with tables of the known diffuse interstellar bands indicates possible matches for the origin bands of the C18H− and C20H− isomers. © 1999 American Institute of Physics.

Notes: Infrared ultraviolet double resonance (IRUVDR) experiments have been performed to investigate the rotational specificity of the vibrational–vibrational (V–V) exchange process, NO(X 2Π1/2,v=3,Ji)+NO(v=0)→NO(X 2Π1/2,v=2,Jf)+NO(v=1), for which the vibrational energy discrepancy corresponds to 55.9 cm−1. Radiation from an optical parametric oscillator was used to excite NO molecules into a specific rotational level (Ji) in the X 2Π, Ω=〈fraction SHAPE="CASE"〉12, v=3 state. Laser-induced fluorescence (LIF) spectra of the (0,2) band of the A 2Σ+–X 2Π1/2 system were then recorded at delays corresponding to a fraction of a collision. From the relative line intensities, rate coefficients were determined for transfer of the excited NO molecule from the level X 2Π1/2, v=3, Ji to different final rotational levels (Jf) in the X 2Π1/2, v=2 state. Results are reported for Ji=3.5, 4.5, 7.5, 10.5, and 15.5. The data show a significant, though not strong, propensity for J to decrease by one; i.e., for ΔJ=Jf−Ji=−1, especially for the higher Ji levels. This result is interpreted as arising from a combination of (a) the tendency to minimize the energy that has to be accommodated in the relative translation of the collision partners, and (b) the favoring of ΔJ=±1 changes when V–V intermolecular exchange occurs under the influence of dipole–dipole interactions. © 1999 American Institute of Physics.

Notes: The shapes of the ν1 and 2ν2 isotropic Raman Q-branch of CO2 perturbed by argon and helium have been measured by Stimulated Raman Spectroscopy (SRS) or coherent anti-Stokes Raman Spectroscopy (CARS) techniques. The data have been successfully analyzed with an energy corrected sudden (ECS) approximation model based on basic rates determined independently. Finally comparison of the present data with time resolved double resonance experiments allows us to discuss the physical origin of the two empirical constants which account for the shift and broadening of the branch due to vibrational effects. © 1999 American Institute of Physics.

Notes: Creep experiments were carried out on amorphous selenium (Se) at temperatures in the vicinity of the glass temperature. The recoverable compliance lacks a plateau, indicating Se chains are too short to form an entanglement network. The measured compliance function was thermorheological complex, even after subtraction of the glassy level and normalizing by the steady state compliance. The temperature dependence determined from the viscosity was in accord with previous viscosity data, although weaker than the near-Arrhenius dependence deduced from the stress relaxation of Se. Based on a comparison to other, small-molecule glass-formers, the dynamic fragility calculated from the viscosity was larger than expected from Se's thermodynamic fragility (i.e., steepness of the normalized Kauzmann curve). In contrast, although polypropylene (PP) is substantially more dynamically fragile than Se, PP is less thermodynamic fragile. Thus, when compared to either small-molecule liquids or polymers, Se exhibits a disconnect between dynamic and thermodynamic measures of fragility. © 1999 American Institute of Physics.

Notes: The thermal diffusivity of fluid oxygen in the diamond-anvil cell has been measured from 1 to 12.6 GPa and 25 to 300 °C. These constitute the first experimental measurements of thermal transport properties of simple fluids above 1 GPa. Diffusivities are found to rise sharply from a minimum at intermediate pressures and then to level off at ∼6 GPa. Thermal conductivities derived from these measurements do not vary as (square root of)T, rather the excess conductivities are approximately independent of temperature. The diffusivities of nitrogen, previously measured to 1 GPa, closely match those of oxygen when scaled as suggested by a simple, corresponding states theory. © 1999 American Institute of Physics.

Notes: A simple membrane, supporting charge densities σ1 and σ2=−σ1 on its inner and outer surfaces, is considered. In addition to the electrostatic potential, the membrane interacts with the surrounding fluid by a short range van der Waals-like potential. The fluid beyond the outer surface is a three-component restricted primitive electrolyte consisting of two cations and one anion. The membrane is impermeable to one of the cations so that the fluid in the membrane and beyond the inner surface is a two-component restricted primitive electrolyte. We use Monte Carlo simulations and density functional theory to study the density profiles of the electrolyte and the charge-electrostatic potential relationship for the membrane surfaces. Even though σ2=−σ1, the potentials on the membrane surfaces are not equal and opposite. We also study a membrane consisting of a single charged plane. For both models, the density functional results are in good agreement with the simulations. © 1999 American Institute of Physics.

Notes: We have carried out a systematic investigation of the kinetics of domain growth of a model binary mixture in contact with a fractal network similar to the fumed silica network used in recent experiments with polymer blends. This network has selective affinity for one component of the blend. We study the morphology of the growing domains and the wetting layer, the growth exponent, and dynamical scaling behavior for a critical composition of the mixture. We find that the characteristic size of domains grows as t1/3 for deep quenches, as long as the average domain size is small compared to the average "pore" radius of the unoccupied region. This suggests that the kinetics of domain growth at intermediate times is not appreciably perturbed by the presence of the network. For off-critical compositions, we study domain growth in two different situations where either the majority or the minority component of the blend wets the network. When the majority component wets the network, a network-induced nucleation is possible for sufficiently off-critical mixtures due to the reduction or elimination of the local nucleation barrier for the minority phase near the network. For this reason, minority droplets nucleate predominantly near the network before they appear in the bulk. When the minority phase wets the network, a slowdown in wetting layer growth is found to occur at late times due to a depletion of the system of its minority component. © 1999 American Institute of Physics.

Notes: High-pressure Brillouin scattering measurements have been carried out in orientationally disordered (OD) phases I and I′ of solid HBr, which yielded the first determination of acoustic velocities for all directions, the refractive index, the polarizability, the density, adiabatic elastic constants (C11, C12, and C44), bulk modulus, and the elastic anisotropy as a function of pressure up to 7 GPa and at room temperature. The I→I′ phase transition in solid HBr was confirmed on the discontinuous pressure dependence at P=2.3 GPa for average errors of the least-squares fit by the analysis of in situ Brillouin spectroscopy. These elastic properties of solid HBr are compared with recent results in the OD phase I of solid HCl. We suggest that the form of the potential energy function for intermolecular forces in solid HBr is similar to that in HCl. © 1999 American Institute of Physics.

Notes: The effect of competition between catalytic particles C on the rate of the reversible diffusion-controlled reaction A+C(r harp over l)B+C is studied in a self-consistent approximation. The C-particles are assumed static with random configuration. The A- and B-particles diffuse in a fluid or solid background medium. Frequency-dependent effective rate coefficients are calculated as a function of the volume fraction of C-particles. © 1999 American Institute of Physics.

Notes: Using nonadiabatic molecular dynamics simulations, we present evidence that the 2 ps peak in the pump–probe spectrum of I2− dissociated inside CO2 clusters is due to transitions from the ground state to the spin–orbit excited states, rather than to excited-state absorption as previously assigned. © 1999 American Institute of Physics.

Notes: Efficient extraction of frequency information from a discrete sequence of time signals can be achieved using the so-called low storage filter diagonalization approach. This is possible because the signal sequence can be considered as a correlation function associated with a quantum Hamiltonian. The eigenvalues of the Hamiltonian (i.e., the frequencies in the signal) in a pre-specified energy range are obtainable from a low-rank generalized eigenequation in a subspace spanned by the filtered states. This work presents an efficient and accurate method to construct the Hamiltonian and overlap matrices directly from correlation functions for several types of propagators. Emphasis is placed on a recurrence relationship between the Hamiltonian and overlap matrices. This method is similar to, but more efficient than, several existing methods. Numerical testing in a triatomic system (HOCl) confirms its accuracy and efficiency. © 1999 American Institute of Physics.

Notes: A correlated function expansion (CFE) is presented to systematically sample an N-dimensional dual composition-processing variable space to efficiently guide the laboratory discovery complex materials with desirable properties. The CFE breaks down the material properties in terms of the independent, pair and higher order cooperative roles of the composition-processing variables. The CFE is expected to rapidly converge in the N-dimensional space of variables to specify (1) minimally sized hierarchical libraries of materials, and (2) how to utilize the observed properties of the library members to rapidly estimate the material properties throughout the entire composition-processing variable space. As an illustration the material properties (i.e., alloy bond length and the direct optical band gap E0) over the full composition space of the multicomponent semiconductor alloys, GaxIn1−xPyAs1−y, GaxIn1−xAsySb1−y, and GaxIn1−xPySbzAs1−y−z, are expressed through the CFE in terms of existing ternary experimental data. Band gap experimental results for GaxIn1−xPyAs1−y lattice matched to InP and for GaxIn1−xAsySb1−y lattice matched to GaSb are in good agreement with the CFE estimates from ternary input data alone. The alloy GaxIn1−xPySbzAs1−y−z is found to provide more diverse opportunities to achieve desired band gaps while still maintaining the lattice matching conditions by controlling the concentration of Sb at the anion site. For even broader classes of materials the CFE is generic tool designed to guide laboratory syntheses to aid in the discovery of new materials with desired properties. © 1999 American Institute of Physics.

Notes: The mechanism of noncollisional H+ and O+ sputtering from reduced, hydrogenated and oxygenated TiO2(110) surfaces has been investigated by low-energy noble-gas ions and electron bombardment. The noncollisional sputtering is found to be initiated by formation of the O 2s core hole via the quasiresonant charge exchange. Desorption of oxygen and hydrogen occurs from core-excited oxygen and hydroxyl group, respectively, which have an antibonding character with a long lifetime. To survive efficient reneutralization, ionization of oxygen should occur after bond breakage via the intraatomic Auger decay of the O 2s hole while hydrogen can be ionized in the course of the O–H bond breakage at the surface via the interatomic Auger decay of the O 2s hole. On the basis of these findings, the interaction of oxygen and hydrogen with the reduced TiO2(110) surface is investigated using resonant ion stimulated desorption (RISD) by He+. At the reduced surface, the chemisorbed oxygen atom either fills a vacancy site of bridging oxygen atoms or chemisorbs at a fivefold-coordinated Ti4+ site as an adatom. In the RISD experiment, the oxygen adatom is detected much more efficiently than the bridging oxygen. Such oxygen adatoms are found to be unstable on the TiO2(110) surface; they diffuse into the bulk at the oxygenated surface or segregate from the bulk at the reduced surface. Hydrogen, bonded to Ti at the reduced surface, tends to be reorganized to form hydroxyl group upon oxygenation. © 1999 American Institute of Physics.

Notes: The condensation coefficient of water vapor on liquid water and the thermal accommodation coefficient of air on liquid water are poorly known despite their importance in many applications, such as cloud physics. We have developed a new technique for determining the condensation and thermal accommodation coefficients experimentally. The technique consists of simultaneously measuring the homogeneous nucleation rate of ice and the evaporation rate of liquid water droplets as a function of pressure (droplet Knudsen number). As the Knudsen number increases, surface kinetic processes limit mass and energy fluxes and, as a result, the equilibrium temperature of an evaporating droplet is a function of the condensation and thermal accommodation coefficients. The homogeneous freezing nucleation rate is used as a sensitive measure of the droplet temperature. The nucleation and evaporation rates are determined by observing the scattered light from evaporating water droplets suspended in an electrodynamic levitation system housed within a controlled environment. The observed rates are consistent with a condensation coefficient between 0.04 and 0.1, with 0.06 being most probable, and a thermal accommodation coefficient between 0.1 and 1, with 0.7 being most probable. © 1999 American Institute of Physics.

Notes: We have developed a new method for finding and representing dividing surfaces which can, for example, be used to identify "atoms" in molecules or condensed phases based on Bader's definition. Given the total electron density of the system, the dividing surface is taken to be the zero-flux surface, i.e., the surface on which the normal component of the gradient vanishes. Our method for finding this surface involves creating an "elastic sheet" represented by a swarm of fictitious particles which interact with each other so as to give a nearly uniform distribution of points on the sheet. Two kinds of forces act on the particles: (1) the component of the gradient of the density normal to the elastic sheet, and (2) an interparticle force which only acts in the local tangent plane of the sheet. Starting with a spherical surface and applying an optimization algorithm that minimizes the forces leads to convergence of the particles to the zero-flux surface. The elastic sheet tends to round off regions where the zero-flux surface has sharp cusps or points, but this appears not to be a serious problem in cases we have studied. The elastic sheet method is robust and can converge in situations where currently used methods fail. We demonstrate the method with a study of water clusters and a Si interstitial in a Si crystal. © 1999 American Institute of Physics.

Notes: We report a novel transient instability upon temperature quench in weakly ordered block copolymer microphases possessing a soft direction or directions, such as the lamellar and hexagonal cylinder (HEX) phases. We show that reequilibration of the order parameter is accompanied by transient long wavelength undulation of the layers or cylinders—with an initial wavelength that depends on the depth of the temperature quench—that eventually disappears as the structure reaches its equilibrium at the new temperature. Such undulation leads to a transient transverse broadening of the scattering peaks near the Bragg positions. We argue that this instability might be responsible for the experimentally observed unusual ordering dynamics of the HEX phase of a diblock copolymer after quenching from the disordered state. © 1999 American Institute of Physics.

Notes: Electron-pair intracule (relative motion) h¯(υ) and extracule (center-of-mass motion) d¯(P) densities in momentum space are studied for the 3P, 1D, and 1S terms of five group 14 atoms with p2 configurations, the 4S, 2D, and 2P terms of five group 15 atoms with p3 configurations, and the 3P, 1D, and 1S terms of five group 16 atoms with p4 configurations. Common to all fifteen atoms, the intracule densities show that a low energy term has a greater probability of finding a pair of electrons with a large relative momentum υ than a high energy term. The Fermi hole effect in a high spin term appears naively in momentum space, and the average relative momentum 〈υ〉 is larger in a high angular momentum term. For the terms arising from the pm electronic configurations (m=2–4), the differences in the radial extracule densities are found to be almost isomorphic with the corresponding intracule ones. In a term with a high angular momentum, the average center-of-mass momentum 〈P〉 of an electron pair is always larger, and two electrons are less likely to have opposite momenta. The major origin of these differences in the electron-pair densities lies in the valence np orbitals, and the contribution of different electron-pair motions to the relative stability of terms can be explained by a rigorous relation between the kinetic energy Tnp and the second intracule 〈υ2〉np and extracule 〈P2〉np moments of the valence np subshell. © 1999 American Institute of Physics.

Notes: Three different types of isosbestic points observed for liquid water are dealt with in this paper—temperature induced Raman, temperature induced structural, and pressure induced structural isosbestic points. These isosbestic points leave no doubt as to the precise two-state outer-neighbor mixture model description of this important substance from supercooled temperatures up to at least 40 °C. New pressure-dependent Raman experiments are suggested that will help confirm further this most simple idea that has already been shown to be quantitatively consistent with the temperature- and pressure-dependent properties of liquid water, including all its anomalies. © 1999 American Institute of Physics.

Notes: We report a new type of photofragment caging reaction that is only possible because of the strong solvent-induced perturbation of the inherent electronic structure of the chromophore. The photoexcitation of I2− at 395 nm promotes it to a dissociative state correlating with I−+I*(2P1/2), the only near-ultraviolet dissociation channel for unsolvated I2−. In I2−(CO2)n and I2−(OCS)n clusters, interaction with the solvent is observed to result in extremely fast spin-orbit relaxation. In general, we detect three reaction pathways: (1) direct dissociation of the chromophore to I−+I*(2P1/2); (2) the I2−→I−+I* dissociation, followed by spin-orbit quenching leading to I−+I(2P3/2) products; and (3) the I2−→I−+I* dissociation, followed by spin-orbit quenching and I−+I(2P3/2)→I2− recombination and vibrational relaxation. We present experimental evidence of the spin-orbit relaxation and caging and discuss possible mechanisms. The results include: the measured translational energy release in 395 nm photodissociation of unsolvated I2−, indicating that solvation-free dissociation proceeds exclusively via the I−+I* channel; ionic product distributions in the photodissociation of size-selected I2−(CO2)n and I2−(OCS)n clusters at the same wavelength, indicating the above three reaction channels; and ultrafast pump-probe measurements of absorption recovery, indicating picosecond time scales of the caging reaction. We rule out the mechanisms of spin-orbit quenching relying on I*-solvent interactions without explicitly considering the perturbed electronic structure of I2−. Instead, as described by Delaney et al. (companion paper), the spin-orbit relaxation occurs by electron transfer from I− to I*(2P1/2), giving I(2P3/2)+I−. The 0.93 eV gap between the initial and final states in this transition is bridged by differential solvation due to solvent asymmetry. Favorable comparison of our experimental results and the theoretical simulations of Delaney et al. yield confidence in the mechanism and provide understanding of the role of cluster structure in spin-orbit relaxation and recombination dynamics. © 1999 American Institute of Physics.

Notes: A two variable model, which has been proposed to describe a first-order, exothermic, irreversible reaction A→B carried out in a continuous stirred tank reactor (CSTR), is investigated when the control parameter is modulated by random and/or periodic forces. Within the bistable region where a limit cycle and a stable node coexist, stochastic resonance (SR) is observed when both random and periodic modulations are present. In the absence of periodic external signal noise induced coherent oscillations (NICO) appear when the control parameter is randomly modulated near the supercritical Hopf bifurcation point. In addition, the NICO-strength goes through a maximum with the increment of the noise intensity, characteristic for the occurrence of internal signal stochastic resonance (ISSR). © 1999 American Institute of Physics.

Notes: To clarify the mechanism of cross-interfacial molecular transport and the role of subcritical cluster population in determining the kinetics of crystal nucleation, cluster dynamics calculations based on viscosity-governed rate coefficients are confronted with experiments on crystal nucleation in six stoichiometric oxide glasses (lithium disilicate, barium disilicate, two soda-lime-silica glasses, wollastonite glass, and lithium diborate). Systematic deviations are observed in the thermal activation of the measured and predicted induction times that lead to a crossover near the glass transition. Below crossover, the viscosity based induction times are higher than the experimental ones, a relation that is reversed at higher temperatures. The differences, that may amount to orders of magnitude far from the crossover temperature, cannot be removed by taking into account the size dependence of the interfacial free energy, the depletion of the monomers, or by enforcing the proper (zero) value of the free energy of monomers. Rather, it appears that while crystal nucleation and viscosity are both diffusion related processes, they are governed by different diffusion modes. © 1999 American Institute of Physics.

Notes: Absorption signals of single terrylene molecules in n-hexadecane and naphthalene crystals were recorded at liquid-helium temperatures. The method is based upon rf Stark effect modulation in the megahertz range. The electric rf field strength was applied by means of interdigitating electrodes with 18 μm spacing. Signal-to-noise ratios better than 10 were obtained with approximately 300 ms integration time. The measured line shapes depend on the relative contributions of the linear and the quadratic Stark shift. © 1999 American Institute of Physics.

Notes: Raman intensities have been computed for a series of test molecules (N2, H2S, H2O, H2CO, CH4, C2H2, C2H4, C2H6, SiO2, NH3, CH2F2, and CH2Cl2) using Hartree–Fock, second-order Møller–Plesset perturbation theory (MP2), and density functional theory, including local, gradient-corrected, and hybrid methods (S-VWN, B-LYP and B3-LYP, and MPW1-PW91) to evaluate their relative performance. Comparisons were made with three different basis sets: 6-31G(d), Sadlej, and aug-cc-pVTZ. The quality of basis set used was found to be the most important factor in achieving quantitative results. The medium sized Sadlej basis provided excellent quantitative Raman intensities, comparable to those obtained with the much larger aug-cc-pVTZ basis set. Harmonic vibrational frequencies computed with the Sadlej basis set were in good agreement with experimental fundamentals. For the quantitative prediction of vibrational Raman spectra, the Sadlej basis set is an excellent compromise between computational cost and quality of results. © 1999 American Institute of Physics.

Notes: Recurrence relations are derived for constructing rotation matrices between complex spherical harmonics directly as polynomials of the elements of the generating 3×3 rotation matrix, bypassing the intermediary of any parameters such as Euler angles. The connection to the rotation matrices for real spherical harmonics is made explicit. The recurrence formulas furnish a simple, efficient, and numerically stable evaluation procedure for the real and complex representations of the rotation group. The advantages over the Wigner formulas are documented. The results are relevant for directing atomic orbitals as well as multipoles. © 1999 American Institute of Physics.

Notes: The resonance Raman spectrum of Ag7, mass selected out of a cation beam of sputtered silver, neutralized, and codeposited with solid Ar, is presented. By comparing the observed spectrum with one calculated using density functional theory one concludes the structure of the silver septamer to be a tricapped tetrahedron. A partial resonance Raman spectrum of Ag9 is also included. Both spectra are dominated by totally symmetric "breathing" modes at ∼165 cm−1 (as is also true for Ag5 [T. L. Haslett et al., J. Chem. Phys. 108, 3453 (1998)]). This frequency is close to the Debye frequency (156 cm−1) of solid silver, implying that the nature of the chemical bond in these silver clusters already approximates closely that which exists in bulk silver. © 1999 American Institute of Physics.

Notes: Ab initio configuration interaction (CI) calculations are performed to study the ground state of small neutral and singly charged silver bromide clusters AgnBrp(±) (n,p≤2). The results are obtained at complete active space self-consistent field and also at variational plus second order perturbational multireference CI (MRPT2) levels of approximation. We discuss more particulary the structural properties and the stability of the lowest isomers. Adiabatic and vertical ionization potentials and electron affinities have also been determined. © 1999 American Institute of Physics.

Notes: Vibrationally resolved 355 and 266 nm anion photoelectron spectra of B2N are presented. Photodetachment to two electronic states of linear B–N–B is observed and, aided by electronic structure calculations, assigned to the X˜ 1Σg+→X˜ 2Σu++e− and X˜ 1Σg+→A˜ 2Σg++e− transitions. The electron affinity of B2N is 3.098±0.005 eV and the A˜ 2Σg+ term energy T0 is 0.785±0.005 eV. Observation of excitations involving uneven quanta of the antisymmetric stretching mode (v3) indicates a breakdown of the Franck–Condon (FC) approximation and results from Herzberg–Teller vibronic coupling between the X˜ 2Σu+ and A˜ 2Σg+ states involving the v3 mode. Measurement of the angular dependence of the photodetached electrons serves as a sensitive probe for the identification of these FC forbidden transitions. A linear vibronic coupling model qualitatively reproduces the perturbed v3 potentials of the X˜ and A˜ states. Artifactual symmetry breaking along the v3 coordinate is observed in the ab initio wave functions for the neutral ground state up to the coupled-cluster level of theory, even when Brueckner orbitals are used. No evidence is found for an energetically low-lying cyclic state of B2N, which has been invoked in the assignment of the matrix infrared spectrum of B2N. However, the matrix infrared data agrees well with the peak spacing observed in the photoelectron spectra and reassigned to the linear X˜ 2Σu+ ground state. © 1999 American Institute of Physics.

Notes: We have performed quantum scattering calculations to predict pressure broadening, pressure shift, and inelastic depopulation cross-sections for the rotational transitions 11,0←10,1 and 22,0←21,1 of the H2S molecule in collision with helium atoms over a temperature range from 1 to 600 K. The calculated cross-sections are compared with experimental values obtained by millimeter wave spectroscopic techniques and the collisional cooling method. We observe good agreement between theory and experiment over the temperature region from 20 to 600 K, but increasing differences below 20 K. Possible reasons for the deviations at lower temperatures are discussed. The calculations also illustrate the contribution of elastic collisions to the pressure broadening cross-sections. © 1999 American Institute of Physics.

Notes: We have obtained rotationally resolved pulsed field ionization–photoelectron (PFI-PE) spectra of CO in the energy range of 13.98–21.92 eV, covering the ionization transitions CO+(X 2Σ+,v+=0–42,N+)←CO(X 1Σ+,v″=0,N″). The PFI-PE bands for CO+ (X 2Σ+, v+=8–22, 24, and 28–39) obtained here represent the first rotationally resolved spectroscopic data for these states. The high-resolution features observed in the PFI-PE spectra allow the identification of vibrational bands for the CO+ (X 2Σ+, v+=10, 14, 15, 17, 18, 21, 24, 25, 29–31, 33, 35–37, and 39) states, which strongly overlap with prominent vibrational bands of the CO+(A 2Π3/2,1/2,B 2Σ+) states. The simulation using the Buckingham–Orr–Sichel model has provided accurate molecular constants for CO+(X 2Σ+,v+=0–42), including ionization energies, vibrational constants (ωe+=2218.8±3.5 cm−1, ωe+xe+=16.20±0.32 cm−1, ωe+ye+=0.074±0.011 cm−1, and ωe+ze+=−0.001 83±0.000 13 cm−1), and rotational constants [Be+=1.9797±0.0051 cm−1, αe+=0.0201±0.0011 cm−1, γe+=0.000 122±0.000 067 cm−1, ze+=−(5.2±1.1)×10−6 cm−1]. Enhancement of ΔN〈0 rotational branches, attributable to field-induced rotational autoionization, was clearly discernible in PFI-PE bands for CO+ (X 2Σ+, v+=0–5, 11, and 12). Significant local enhancements due to near-resonance autoionization were observed for low v+ (〈10) PFI-PE bands of CO+(X 2Σ+), where the density of interloper Rydberg states converging to higher ionic levels is high as manifested in the photoion spectrum. The observation of a long vibrational progression in the Franck–Condon gap region, where strong autoionization states are absent, is consistent with the suggestion that high-n Rydberg states converging to highly excited vibrational levels of CO+(X 2Σ+) are partially populated via direct excitation to a repulsive neutral state. The relatively minor band intensity variation observed for high v+ PFI-PE bands is also in accord with the direct excitation model. Since ΔN=0, ±1, ±2, and ±3 rotational branches are observed in the PFI-PE spectra, we conclude that the ejected photoelectrons are restricted to angular momentum continuum states l=0–4. © 1999 American Institute of Physics.

Notes: Theoretical cross-sections are presented for Penning detachment of negatively-charged sodium clusters, receiving their detachment energy from Na*(3p0,2P). Two clusters are examined: Na7− and Na19−. Classical trajectories describe the relative motion of the colliding species. Kohn–Sham density functional theory in local approximation, with exchange, correlation, and self-interaction corrections, and a spherical jellium potential, describe the electrons involved in the transition. In the range of collision energies from 0.1 to 10 eV/amu, the cross-sections for Penning detachment are approximately 10−13 cm2. This implies that Penning detachment may be an effective means to prepare neutral clusters from size-selected negative-ion clusters in the laboratory. © 1999 American Institute of Physics.

Notes: The formation of a vibrationally excited photoproduct of nickel octaethylporphyrin (NiOEP) upon (π, π*) excitation and its subsequent vibrational energy relaxation were monitored by picosecond time-resolved resonance Raman spectroscopy. Stokes Raman bands due to the photoproduct instantaneously appeared upon the photoexcitation. Their intensities decayed with a time constant of ∼300 ps, which indicates electronic relaxation from the (d, d) excited state (B1g) to the ground state (A1g), being consistent with the results of transient absorption measurements by Holten and co-workers [D. Kim, C. Kirmaier, and D. Holten, Chem. Phys. 75, 305 (1983); J. Rodriguez and D. Holten, J. Chem. Phys. 91, 3525 (1989)]. The Raman frequencies of NiOEP in the (d, d) excited state are shifted to lower frequencies compared to those of the ground state species, and it is reasonably interpreted by the core size expansion of the macrocycle by 0.05 Å upon the electron promotion from the dz2 to the dx2−y2 orbital. Anti-Stokes ν4 intensity in the vibrationally excited (d, d) state of NiOEP appeared promptly and decayed with time constants of 11±2 and 330±40 ps. The former is ascribed to vibrational relaxation, while the latter corresponds to the electronic relaxation from the (d, d) excited state to the electronic ground state. In contrast, the rise of anti-Stokes ν7 intensity was not instantaneous, but delayed by 2.6±0.5 ps, which indicates that intramolecular vibrational energy redistribution has not been completed in subpicosecond time regime. The peak position of the ν4 band shifted by nearly 5 cm−1 between 0 and 50 ps. The time constant for the shift of the ν4 band was 9.2±1.3 ps, which was close to that for the fast component of intensity decay of anti-Stokes bands. The ν4 band became narrower and symmetric as the delay time increases. These can be ascribed to intramolecular anharmonic coupling of the ν4 mode with the low frequency modes. The intra- and intermolecular vibrational energy relaxation in the metal excited state will be discussed. © 1999 American Institute of Physics.

Notes: Molecular-dynamics simulations of the photoisomerization of cis-stilbene in supercritical argon were performed. The stilbene molecule is represented by ab initio quantum chemistry, while the solvent, the interaction with solvent, and the time evolution were described by classical mechanics. Reaction rate constants are estimated and their dependence on temperature, pressure, and viscosity are investigated. Agreement with available experimental data was obtained. Our simulations strongly suggest a minimum on the excited-state potential-energy surface at a gauche conformation which is very rapidly reached after excitation, which leads to nonequilibrium barrier transitions. Specific solvent effects were identified. Implications on the current opinion on stilbene photoisomerization are discussed. © 1999 American Institute of Physics.

Notes: We study the freezing of CCl4 in microporous activated carbon fibers (ACF), using Monte Carlo simulation and differential scanning calorimetry (DSC). Microporous activated carbon fibers are well characterized porous materials, having slit-shaped pores due to the voids formed between graphitic basal planes. They serve as highly attractive adsorbents for simple nonpolar molecules, the adsorbent–adsorbate interaction being mostly dispersive (of the van der Waals-type). Recent molecular simulation studies have predicted an upward shift in the freezing temperature (ΔTf=Tf,pore−Tf,bulk〉0) for simple fluids confined in such highly attractive carbon slit pores. Our DSC experiments verify these predictions about the increase in Tf. The results also indicate significant deviation from the prediction of ΔTf based on the Gibbs–Thomson equation (simple capillary theory). We employ a recently developed free energy method to calculate the exact freezing temperature in these confined systems using molecular simulation, in order to address the failure of the simple capillary theory. © 1999 American Institute of Physics.

Notes: We address the intramolecular vibronic interactions in the C36 tri- and tetra-anions to understand the Jahn–Teller effects and possible superconductivity in "electron-doped" C36 solids. We use the B3LYP hybrid Hartree–Fock/density-functional-theory method for our theoretical analyses. Neither the highest occupied molecular orbital (HOMO) nor the lowest unoccupied molecular orbital (LUMO) of the C36 molecule with D6h symmetry are degenerate, but the next LUMO is twofold degenerate. One can therefore expect Jahn–Teller distortions and interesting electronic properties in the C36 anions. Computed vibronic and electron–phonon coupling constants of the tetra-anion are about twice as large as those of the tri-anion. The second lowest Jahn–Teller active E2g mode of 561 cm−1 is predicted to have the largest coupling constants in both anions. We calculate superconducting transition temperature Tc from McMillan's formula using the coupling constants as well as electronic densities of states at the Fermi level and Coulomb pseudopotentials as parameters. © 1999 American Institute of Physics.

Notes: Photoionization-efficiency (PIE) spectra in the wavelength range of 108–143 nm are measured for C2H5SCl produced from reaction systems Cl/Cl2/C2H5SH and Cl/Cl2/C2H5SSC2H5 in a discharge-flow reactor coupled to a photoionization mass spectrometer employing a synchrotron as the source of radiation. According to PIE spectra of C2H5SCl thus obtained, the ionization energy (IE) is (8.994±0.007) eV. Based on GAUSSIAN-2 calculations, the observed ionization of C2H5SCl near the threshold region is likely to form doublet C2H5SCl+ from singlet C2H5SCl; the calculated IE 8.978 eV agrees well with the experimental value. A vibrational frequency of doublet C2H5SCl+, was found to be (557±60) cm−1, which agrees satisfactorily with a theoretical value of 560.9 cm−1. © 1999 American Institute of Physics.

Notes: The accuracy of the time-dependent self-consistent-field (TDSCF) approach assuming partial factorization of the total wave packet is tested against an exact treatment, when applied to calculate asymptotic properties. The test is carried out in the framework of a three-dimensional simulation of the Ar–HCl UV photodissociation dynamics. All the partially-separable TDSCF ansatzs possible for this problem are investigated. The quality of the TDSCF results is found to be strongly dependent on the specific partially-separable ansatzs applied. In general, the TDSCF predictions are in very good (even quantitative) agreement with the exact ones for magnitudes associated with direct photodissociation dynamics, and are qualitative in the case of indirect photodissociation. The deviation of the TDSCF results from the exact dynamics is interpreted in terms of an error operator defined as the difference between the exact and the TDSCF Hamiltonians. The analysis of this operator also explains the different accuracy of the partially-separable ansatzs investigated. Based on this analysis, a simple procedure is suggested to estimate the relative average quality of the different TDSCF ansatzs. © 1999 American Institute of Physics.

Notes: In the framework of spherical geometry for jellium and local spin density approximation, we have obtained the equilibrium rs values, r¯s(N,ζ), of neutral and singly ionized "generic" N-electron clusters for their various spin polarizations, ζ. Our results reveal that r¯s(N,ζ) as a function of ζ behaves differently depending on whether N corresponds to a closed-shell or an open-shell cluster. That is, for a closed-shell one, r¯s(N,ζ) is an increasing function of ζ over the whole range 0≤ζ≤1, and for an open-shell one, it has a decreasing part corresponding to the range 0〈ζ≤ζ0, where ζ0 is a polarization that the cluster assumes in a configuration consistent with Hund's first rule. In the context of the stabilized spin-polarized jellium model, our calculations based on these equilibrium rs values, r¯s(N,ζ), show that instead of the maximum spin compensation (MSC) rule, Hund's first rule governs the minimum-energy configuration. We therefore conclude that the increasing behavior of the equilibrium rs values over the whole range of ζ is a necessary condition for obtaining the MSC rule for the minimum-energy configuration; and the only way to end up with an increasing behavior over the whole range of ζ is to break the spherical geometry of the jellium background. This is the reason why the results based on simple jellium with spheroidal or ellipsoidal geometries show up MSC rule. © 1999 American Institute of Physics.

Notes: Two possible routes are considered to arrive at a one-particle reduced density matrix formulation of electronic structure theory. In the first scheme, an extended Fock matrix H is defined that has twice the dimension of the one-particle basis set. The corresponding Green's function, defined as the upper left block of (ω1−H)−1, yields the exact one-particle density matrix and energy. The poles of the Green's function are precisely the ionization potentials and electron affinities of the extended Koopmans theorem. In the second scheme, a generalized Fock equation [F(ρ),ρ]=X is derived that is satisfied by the exact non-idempotent one-particle density matrix. The antisymmetric matrix X on the right-hand side is obtained from the irreducible part of the two-particle reduced density matrix, while F is the usual Fock matrix defined using the correlated one-matrix. The generalized Fock equation is a necessary condition but does not determine ρ uniquely. Alternatively, the one-matrix can be obtained from the irreducible part of the two-matrix directly, using a sum rule. The analysis leads to some additional desiderata and separability properties that may be imposed on traditional wave function based approaches. Possibilities for practical computational schemes are addressed briefly. © 1999 American Institute of Physics.

Notes: The first high resolution infrared spectrum of the ionic complex N2⋅⋅H+⋅⋅N2 and its deuterated derivative is reported. The spectra were obtained in direct absorption in a supersonic slit nozzle plasma. The observed rovibrational transitions were assigned to the ν3 antisymmetric NN stretching vibration and the spectrum is consistent with a linear centrosymmetric equilibrium structure. The band origin is found at 2352.2364(6) cm−1 and the ground state rotational constant is determined as B″=0.081 809(14) cm−1. The assignment is supported by ab initio calculations including electron correlation effects. The best estimate for the equilibrium structure is Re (NN)=1.095 Å and re (N⋅⋅H)=1.277 Å. The transition moment of the ν3 band of N2⋅⋅H+⋅⋅N2 is predicted to be 0.21 D, an order of magnitude larger than for the NN stretching vibration of HN2+. The equilibrium dissociation energy De for fragmentation into N2 and HN2+ is calculated to be ∼5900 cm−1. © 1999 American Institute of Physics.

Notes: We use the stochastic Schrödinger equation approach to examine an experiment performed by Scherer, Jonas, and Fleming [J. Chem. Phys. 99, 153 (1993)]. They have excited I2 molecules dissolved in n-hexane with a strong, ultrashort pump pulse. This creates moving wave packets on both the excited and the ground electronic states. The coherent motion of these packets is studied by measuring the absorption of a weak probe pulse, as a function of the delay time between the pulses. We show how the oscillations in the signal are connected to different excitation mechanisms and to the interference between various packets created by the lasers. Our purpose is to clarify the dynamics of the packets and to show that the stochastic Schrödinger equation approach can be used to study their decoherence. We did not intend to try to develop a realistic model for the I2 dynamics and its interaction with the solvent and do not perform a detailed comparison with the experiment. Nevertheless, the calculations reproduce, semiquantitatively, the observations. © 1999 American Institute of Physics.

Notes: It is shown that exactly the same results of the memory function theory of diffusion-influenced reactions, presented by Yang, Lee, and Shin, can be derived from the nonequilibrium reduced distribution function (RDF) theory. Instead of the usual dynamic superposition approximation (SA), which has been widely used to truncate the hierarchy of RDF evolution equations, we introduce another type of truncation approximation. The new approximation provides simple analytic solutions that are in better agreement with the computer simulation and the known exact results than those obtained with the SA. © 1999 American Institute of Physics.

Notes: The steep non-Arrhenius temperature dependence at low temperatures of the shear viscosity of water and its backwards-sounding increased fluidity under pressure for temperatures below 33 °C are two of the anomalies of this liquid that have been known for a very long time. The purpose of the present paper is to show how these two important characteristics of water emerge quantitatively from an explicit two-state outer-neighbor mixture model that we have used to explain many other properties of this substance. It will be shown here that both of these viscosity anomalies are directly related to the steep variations with temperature and pressure of the fractional compositions of ice-Ih-type bonding and ice-II-type bonding in the two-state mixture. This compositional dependence has already been obtained in earlier work from the variations of the density and the isothermal compressibility of water with temperature. The viscosity analysis presented here thus helps to unify further all the properties of this liquid under a single, very simple structural characteristic. © 1999 American Institute of Physics.

Notes: An analysis of the electronic structure of the high-spin 3d7 Co(II) ion in the approximately octahedral Co(II)(acac)2(H2O)2 complex is presented in terms of crystal fields of descending symmetry from octahedral to orthorhombic. The energies and wave functions resulting from the interplay of these fields with the spin–orbit coupling are used to obtain zero-field splittings, magnetic moments, magnetic susceptibilities, and g values for the complex. The calculated temperature dependence of the susceptibility is compared to the reported dependence for Co(II)(acac)2(H2O)2, yielding bounds on the strength of the tetragonal component of the crystal field. The calculated anisotropy in the susceptibility is used in an analysis of our observed pseudocontact NMR shifts for methyl and methine protons in the complex. A procedure is outlined for using a crystal field analysis to compute pseudocontact contributions to proton chemical shifts starting from g values extracted from ESR spectra. The relationship between molecular structure and crystal-field splittings is also explored via a series of ab initio electronic structure calculations for the M(II)(acac)2(H2O)2 complexes with M=Mn, Co, Ni, and Zn. © 1999 American Institute of Physics.

Notes: The free energy and dynamics of the dissociation reactions of the [Na+(Cl−)2] ion complex in mesoscopic water clusters are examined. The free energy surface shows the existence of stable single and double solvent-separated complex species formed from ionization of the stable double-contact ion complex. The reaction occurs on the cluster surface for mesoscale clusters composed of tens of water molecules. Passage between stable species is an activated process but barrier crossing has a large diffusive component so that dynamical corrections to transition state theory are large. The structure of the decay of the time-dependent rate constant reflects the diffusive character of the recrossing dynamics so that a plateau is not established on a 10 ps time scale in contrast to ionization dynamics in bulk fluids. © 1999 American Institute of Physics.

Notes: We employ classical Molecular Dynamics simulations to study the cooperative dynamics of two low molecular weight glass-formers, propylene carbonate and salol. The length scales of dynamic heterogeneities are estimated. After appropriate scaling both glass-formers display a similar temperature dependence of this length scale. Local structural properties like density, regularity, and potential energy are correlated with different local dynamical observables. We find that the dynamic heterogeneities are most strongly related to the local potential energy. To obtain an optimum correlation the local dynamics has to be characterized by the residence time. © 1999 American Institute of Physics.

Notes: The theoretical model described in part I is applied to calculate the vibrational energies and transition moments for low-lying levels of O3 trapped in rare gas matrices. Results are given for molecules trapped in distorted face-centered-cubic (fcc) and hexagonal-closed-packed (hcp) lattice structures. New harmonic and anharmonic constants are determined that lead to matrix dependent calculated energy levels. Changes are significant for harmonic and third-order anharmonic ones. Moreover the symmetry of the potential in which the ozone oxygen nuclei move is shown to be altered. Calculated energy levels compare well with observed ones and allow predictions of unobserved ones. The 2ν3→ν3 fluorescence observed in different rare gas matrices is confirmed. Transition moments hardly differ from one matrix to the other for 2ν3→ν3 and ν1+ν3 transitions although for the latter, it is one order-of-magnitude higher in a double than in a single substitutional site. © 1999 American Institute of Physics.

Notes: Reported here is a detailed study of the vibrational relaxation of C2H2 and C2HD upon scattering from LiF(001). While direct (specular) scattering shows no vibrational quenching for either molecule, the residence times associated with trapping–desorption are long enough to give significant quenching. By studying this quenching as a function of surface temperature, thereby varying the residence times, we show that vibrational relaxation proceeds via multiple steps. The differences between C2H2 and C2HD quenching can be understood in terms of the presence of a Fermi resonance in the former, which enhances the relaxation rates associated with the early steps. A simple kinetic model is used to fit the data, confirming that vibrational quenching proceeds via a cascading mechanism, which is likely to be quite general for polyatomic molecules. © 1999 American Institute of Physics.

Notes: We present an analysis of the thermodynamic properties of chain molecules formed from Yukawa segments using the statistical associating fluid theory with interactions of variable range (SAFT-VR) and the high-temperature expansion of the mean-spherical solution (MSA-HTE) to the Ornstein–Zernike equation for a simple Yukawa fluid. The SAFT-VR expressions derived previously for this system allow the MSA-HTE equation of state to be reformulated in terms of first-order perturbation quantities, thus improving its accuracy. Furthermore, the MSA-HTE solution provides a full theoretical derivation of the perturbation theory used in SAFT-VR, together with a completely analytical equation of state for chain molecules composed of segments which interact via the Yukawa potential. © 1999 American Institute of Physics.

Notes: A self-avoiding, self-interacting polymer chain is studied both on a lattice and in the continuum using a Born–Green–Yvon integral equation approach. Equivalent theoretical approximations are made in both cases, allowing for an unambiguous comparison between the lattice and continuum models. The theory preserves the universal scaling behavior for polymer chain dimensions in the high-temperature limit and, with a lowering of temperature, predicts a universal collapse transition behavior for both lattice and continuum chains. Implications for the modeling of polymer solutions are discussed. © 1999 American Institute of Physics.