bluuebunny/crossref_metadata_2025 huggingface dataset:
Small-angle light scattering of polymer liquid crystals under shear flow
Authors: Takebe Tomoaki , Hashimoto Takeji , Ernst Benoit , Navard Patrick , Stein Richard S.
Published: July 2002

We presented some experimental results on flow small-angle light scattering (‘‘flow SALS’’) of lyotropic polymer liquid crystals which involved real-time, in situ, and simultaneous measurements of mechanical properties and small-angle light scattering (SALS) of a complex structured fluid. The flow-SALS studies were conducted under steady-state Couette flow and during relaxation process occurring after the cessation of the steady-state shear flow. We found the following universal behavior on the structure and properties for two kinds of the liquid crystal systems, i.e., poly(γ-benzyl L-glutamate) in m-cresol and hydroxypropyl cellulose in water. (i) Upon increasing shear rate γ̇, the ‘‘polydomain’’ structure gradually tends to be transformed to the essentially ‘‘monodomain’’ texture through annihilation of the disclinations. On the top of this general tendency, we found more specific characteristic of the fluid systems with γ̇, (ii) at γ̇<γ̇c, a critical shear rate above which a remarkable shear thinning occurs, the disclinations are more or less uniformly distributed in space, while at γ̇>γ̇c the disclinations are localized into domains and dispersed in the more or less uniform medium with a high degree of orientation, (iii) the relaxation after cessation of the steady state shear depends on its γ̇ before the cessation and is quite different below and above γ̇c, and (iv) the cessation from γ̇>γ̇c involves formation of the ‘‘band texture’’ during the relaxation process of the fluids.
Isotope-dependent rate constants for CS−2 formation in Cs (n
s,n
d)+CS2 collisions
Authors: Carman H. S. , Klots C. E. , Compton R. N.
Published: July 2002

Negative ion formation during collisions between Cs (ns,nd) Rydberg atoms and CS2 molecules has been studied for intermediate values of the effective principal quantum number (n*=10–26). Rate constants for CS−2 formation are found to be different for the two isotopes C32S34S and C32S32S for a narrow range of n* near n*=17, with the rate constant for 12C32S34S− production being up to 4.5 times larger than that for 12C32S32S−.
A nonreptation model for polymer dynamics in the melt and concentrated solutions
Authors: Herman Michael F.
Published: July 2002

A model for the motion of a dense multichain polymer system involving linear chains is presented. The model is based on the assumption that chain motion perpendicular to the direction of the chain backbone dominates the dynamics over motion along the backbone. This model is motivated by the results of recent Monte Carlo computer simulations on multichain systems for chain lengths ranging up to 800 segments. It is found in these simulations that motion perpendicular to the backbone dominates. Since many chains are highly intertwined with any given chain, we assume that the chain motion at an interchain contact point proceeds by sliding along the contour of each entangling chain until the exterior of the ball of density formed by the entangling chain is reached. At this point the contact is broken. This requirement that a chain move by sliding along the random flight contour of another chain at a contact point results in an effective bead friction coefficient for three-dimensional motion which is an increasing function of time. A continuous distribution of contact times is calculated within this model. The chain dynamics are evaluated within a Rouse-like model employing a time-dependent bead friction coefficient and a time-dependent number of chain segments. The number of segments in the model Rouse chain at time t is equated with the number of contacts of duration longer than t. The asymptotic large N dependencies of the center-of-mass diffusion coefficient and the terminal relaxation time are found to be N−2 and N3, respectively. Calculations are performed for chains of various lengths, and the results of the model are qualitatively very similar to those reported from the Monte Carlo simulations.
Jet-cooled spectroscopy of transition metal containing polyatomic molecules: The visible spectrum of yttrium imide
Authors: Simard B. , Balfour W. J. , Vasseur M. , Hackett P. A.
Published: July 2002

Laser vaporization of yttrium metal in a molecular beam apparatus using He/ammonia (14NH3, 15NH3, 14ND3) mixtures as carrier gas has led to the observation of several new vibronic bands in the 500–700 nm region, which are attributed to the yttrium imide, Y–N–H, molecule. The technique should be applicable to other transition metal containing polyatomics of current interest.
Effect of pressure on conformational equilibria of liquid 1-chloropropane and 1-bromopropane
Authors: Katō Minoru , Taniguchi Yoshihiro
Published: July 2002

The effects of pressure on conformational equilibria of liquid 1-chloropropane and 1-bromopropane has been measured up to 2.5 kbar by Raman spectroscopy. Pressure effects on relative integrated intensities of the carbon–halogen stretching vibration of the two conformers show that both gauche conformers increase when compressed. The volume changes of the trans to the gauche are −1.1±0.2 and −1.2±0.2 cm3 /mol for 1-chloro- and 1-bromopropanes at 20 °C, respectively. The volume changes are mainly due to the solvation volumes as shown from calculation of the van der Waals volumes. The solvation volume cannot be accounted for by the quadrupole-induced electrostatic interactions in liquids. The solvation volume arises from local intermolecular interactions.
Highly excited vibration–rotation states of floppy triatomic molecules by a localized representation method: The HCN/HNC molecule
Authors: Mladenović M. , Bačić Z.
Published: July 2002

All rovibrational levels of HCN/HNC up to ∼16 000 cm−1, relative to the HCN minimum, for J=0, 1, 2, have been calculated accurately. All internal degrees of freedom are included in these calculations, performed on the realistic, empirical potential surface by Murrel et al. [J. Mol. Spectrosc. 93, 307 (1982)]. Body-fixed mass-scaled Jacobi coordinates are employed, together with the discrete variable representation of the large amplitude motion (LAM) angular coordinate, and a 2-D distributed Gaussian basis for the radial degrees of freedom. The successive diagonalization–truncation procedure results in a compact matrix representation of the full rovibrational Hamiltonian, allowing accurate and efficient determination of a large number (>350 for J=2, p=0 case) of highly excited LAM rovibrational states of HCN/HNC. This approach is suitable for a broad class of floppy, isomerizing triatomic molecules and van der Waals complexes. In addition to energy levels and wave functions, expectation values of Jacobi coordinates, 〈R〉, 〈r〉, and 〈θ〉, are calculated for most states. The majority of calculated J=1,2 levels lie above the top of the isomerization barrier, and are delocalized to a varying degree over both local minima. Rotation appears to lower the energy threshold for extensive delocalization; for the states with J=1, or 2, it is ∼460–480 cm−1 below that for J=0 states. Moreover, increasing rotational excitation affects significantly the degree of localization of a given state.
The local field distribution in a fluid
Authors: Simon Steven H. , Dobrosavljević Vladimir , Stratt Richard M.
Published: July 2002

The distribution of potentials or fields felt at any given point in a liquid (the local field distribution) ends up being the crucial element in calculating quantities ranging from the inhomogeneous broadening of spectral lines to the rates of irreversible electron transfer. Indeed, the usefulness of this distribution in even its simplest form, the version which assumes a completely uncorrelated environment, has long been appreciated. However, there are a number of difficulties with this version. When the fluid density is low enough to make a neglect of correlations reasonable, the distribution function can still be awkward to calculate numerically. Much more seriously, the omission of correlations among the surrounding atoms is totally unrealistic in a dense liquid. We show here that it is possible to arrive at expressions for the local field distribution that are both accurate under dense liquid conditions and are straightforward to evaluate numerically. The key to this development turns out to be the recognition that the short-ranged and long-ranged contributions to the local field play qualitatively different roles—which can be separated formally using a device we call a closest particle expansion. The qualitative differences between the results for correlated and uncorrelated particles are discussed, as is the appropriateness of the commonly used Gaussian approximation.
Nonadditivity in an analytical intermolecular potential: The water–water interaction
Authors: Saint-Martin H. , Medina-Llanos C. , Ortega-Blake I.
Published: July 2002

We present an analytical potential for intermolecular interactions which includes nonaddivity. We fitted this analytical form to an ab initio energy surface for the water–water interaction and looked into the fidelity of the fitting to two-body interactions, as well as three-body and four-body nonadditives. We tested this potential in a Monte Carlo simulation of liquid water. The results obtained led us to conclude that the model performs very well, giving the best reported fit to the radial distribution functions, the enthalpy per water molecule, and other parameters. The potential appears as an inexpensive, accurate, and flexible potential to be used in numerical simulation studies.
Theoretical studies of the spectroscopy of excess electrons in water clusters
Authors: Barnett R. N. , Landman Uzi , Makov Guy , Nitzan Abraham
Published: July 2002

Variational calculation based on a continuum dielectric model, and numerical simulations based on the RWK2-M water potential and on a pseudopotential for the electron–water interaction, are used to evaluate excitation energies and optical spectra for bound interior states of an excess electron in water clusters and in bulk water. Additionally, optical data for surface states are obtained from numerical simulations. The simulation approach uses adiabatic dynamics based on the quantum-classical time-dependent self-consistent field (TDSCF) approximation and the fast-Fourier transform (FFT) algorithm for solving the Schrödinger equation. Both approaches predict very weak or no cluster size dependence of the excitation spectrum for clusters that support interior solvated electron states. For an electron attached to the cluster in a surface localization mode, bound excited states exist for most nuclear configurations of clusters down to (H2O)−18, and the corresponding excitation energy is strongly shifted to the red relative to that associated with stable internal states in larger clusters. Binding and excitation energies associated with surface states are about half the value of these quantities for interior states. The present variational continuum dielectric theory is in relatively good agreement with the simulation results on the size dependence of the relative stability of interior states. However, it strongly underestimates the vertical excitation energy of the solvated electron. It is suggested that optical spectroscopy of excess electrons in water clusters could serve as a sensitive probe of the transition from surface to interior localization modes as the number of water molecules in the cluster is increased.
The C 3Πg Rydberg and 1 3Πg valence states of O2
Authors: Friedman R. S. , Dalgarno A.
Published: July 2002

Experimental data on the positions and widths of the vibrational energy levels of the C 3Πg Rydberg state of molecular oxygen are interpreted. The energy levels are described as resonances in the elastic scattering of oxygen atoms in the open 1 3Πg valence state caused by a coupling to the closed C 3Πg Rydberg state. The coupled scattering equations are set up with an empirical valence potential energy curve and Rydberg-valence interaction potential and it is shown that the energy level positions and widths can be reproduced by an interaction of 63 meV and a potential energy curve that crosses the Rydberg curve on the attractive part of its outer limb.
Macro–micro relations in dipolar orientational relaxation: An exactly solvable model of dielectric relaxation
Authors: Bagchi B. , Chandra A.
Published: July 2002

The relationship between the microscopic orientational relaxation time (τm) and the collective, macroscopic relaxation time (τM) of a dipolar liquid has been a subject of considerable interest. We examine this relation for the kinetic Ising model of Glauber, used recently by Skinner to describe dielectric relaxation in glassy polymers. In this model, exact solutions for both the same spin and the collective spin time correlation functions are possible. We find that these two correlation functions are entirely different from each other. The self spin correlation function is highly nonexponential, as pointed out by Glauber and by Skinner. But, the collective spin relaxation is single exponential, and the wave vector and frequency dependent dielectric function is of simple Debye form. The reason for this difference is discussed.
The spatial distribution of product desorption in the oxidation of carbon monoxide on platinum (110)(1×2) reconstructed surfaces
Authors: Matsushima Tatsuo
Published: July 2002

The spatial distribution of the desorption of carbon dioxide produced on platinum (110)(1×2) reconstructed surfaces was studied in the wide range of the reactant coverages by means of angle-resolved thermal desorption and low-energy electron diffraction. Heating the surface covered by oxygen atoms and carbon monoxide yields four peaks in the CO2 formation: P1-around 400 K, P2- (300 K), P3- (250 K), and P4-CO2 peaks (170 K). The desorption of each CO2 indicated a sharp angular distribution collimated along the bulk surface normal in the [11̄0] direction. On the other hand, the desorption of P1-and P2-CO2 produced at small CO coverages showed two-directional desorption collimated at either about +23 or −23 degrees off the bulk surface normal in the [001] direction. It indicates that the reactive desorption takes place on declining terraces of the reconstructed plane. The desorption of P3- and P4-CO2, and also of P2-CO2 produced at high initial CO coverages, showed a single peak in the angular distribution curve in the same direction, which was collimated along the bulk surface normal. This suggests that the reconstruction is at least partly lifted during heating procedures, and CO2 is formed on the nonreconstructed as well as the reconstructed plane. It was supported by low-energy electron diffraction observations.
Formation of hydroxyl and water from photoreaction of hydrogen and molecular oxygen coadsorbed on Pt(111)
Authors: Germer Thomas A. , Ho W.
Published: July 2002

The formation of 18OH and H218O from photoreaction of coadsorbed H and 18O2 on Pt(111) is observed and studied using thermal desorption spectroscopy, photoinduced desorption, and electron energy loss spectroscopy. The formation of 18OH is monitored as a function of irradiation time and wavelength λ=330–600 nm. Coadsorption of 18O2 with hydrogen produces negligible changes in the vibrational energies and photodesorption yield of O2. Hydroxyl is formed via photodissociation of O2 and subsequent reaction with hydrogen. Some of the photoproduced hydroxyl further react to form water. The OH formed is also produced thermally during reaction of hydrogen gas with atomic oxygen.
The missing fluorescence of s-trans butadiene
Authors: Zerbetto Francesco , Zgierski Marek Z.
Published: July 2002

Geometry of the S0 and S1 states of s-trans butadiene is optimized and the corresponding force fields calculated using a multiconfiguration self-consistent field (MCSCF) method in the complete active space formulation. Three stable conformations of the S1(2Ag) state are shown to be nonplanar (S2, C2, and C1 symmetry), quasidegenerate and separated by a barrier along torsional coordinates of ca. 1600 cm−1. The planar C2h conformation forms a transition state between the S2 and C2 configurations. It is shown by model calculations that the out-of-plane deformation of the S1 state increases dramatically its rate of internal conversion to the ground state. Thus, although the energy gap for the transition is larger in butadiene than that in octatetraene, the internal conversion S1uS0 is estimated to be 3–5 orders of magnitude faster in butadiene. Fast internal conversion in this molecule explains its lack of fluorescence.
Laser-induced fluorescence spectroscopy of the B̃–X̃ and Ã–X̃ transitions of CaNH2
Authors: Whitham C. J. , Jungen Ch.
Published: July 2002

Laser-induced fluorescence spectra of the B̃ 2B1–X̃ 2A1 and Ã 2B2–X̃ 2A1 transitions of the CaNH2 radical, produced in a pulsed supersonic expansion, have been recorded. The ΔKa = ±1 rotational subbands showed an intensity alternation revealing the molecule to be planar or quasiplanar. The excited Ã and B̃ states exhibit unusually large spin splittings which are shown to arise from the coupling between the electronic orbital motion and the a-axis nuclear rotation. This interaction has been successfully modeled in terms of a-axis rotation–orbital and spin–orbital interactions of an electron localized in a pπ orbital on the calcium atom.
A special method for analyzing anisotropic nuclear magnetic resonance parameters: Acetonitrile in liquid crystals
Authors: Lounila Juhani , Ala-Korpela Mika , Jokisaari Jukka
Published: July 2002

A reliable analysis of the nuclear magnetic resonance (NMR) spectral parameters of partially oriented molecules requires the calculation of the effects of the correlation between the molecular vibration and rotation. However, in many cases the information content of the spectral data is not sufficient for an unambiguous determination of all the adjustable parameters involved in such an analysis. The present paper describes a special method to simplify the analysis significantly, so as to make seemingly underdetermined problems solvable. The method is applicable to the molecules which contain segments composed of one or more light bonds attached to a heavier bond. It is applied to the anisotropic couplings Dij of acetonitrile (CH3CN) oriented in various liquid crystals. The analysis leads to the following rα geometry: ∠HCH=109.22°±0.06°, rCH/rCC =0.751±0.002 and rCN/rCC =0.788±0.005. In addition, detailed information on (1) the indirect coupling anisotropies ΔJCC and 2ΔJCN, (2) the 1H and 13C chemical shift anisotropies, (3) the external torques acting on the CH bonds, and (4) the orientational order parameters of the CH3C segment of the acetonitrile molecule is obtained.
Pump–probe measurements of state-to-state rotational energy transfer rates in N2 (v=1)
Authors: Sitz G. O. , Farrow R. L.
Published: July 2002

We report direct measurements of the state-to-state rotational energy transfer rates for N2 (υ=1) at 298 K. Stimulated Raman pumping of Q-branch (υ=1←0) transitions is used to prepare a selected rotational state of N2 in the υ=1 state. After allowing an appropriate time interval for collisions to occur, 2+2 resonance-enhanced multiphoton ionization is used (through the a 1Πg←X 1Σ+g transition) to detect the relative population of the pumped level and other levels to which rotational energy transfer has occurred. We have performed a series of measurements in which a single even rotational level (Ji=0–14) is excited and the time-dependent level populations are recorded at three or more delay times. This data set is then globally fit to determine the best set of state-to-state rate constants. The fitting procedure does not place any constraints (such as an exponential gap law) on the J or energy dependence of the rates. We compare our measurements and best-fit rates with results predicted from phenomenological rate models and from a semiclassical scattering calculation of Koszykowski et al. [J. Phys. Chem. 91, 41 (1987)]. Excellent agreement is obtained with two of the models and with the scattering calculation. We also test the validity of the energy-corrected sudden (ECS) scaling theory for N2 by using our experimental transfer rates as basis rates (J=L→0), finding that the ECS scaling expressions accurately predict the remaining rates.
Kinetics of molecular tunneling at intermediate temperatures
Authors: Burrows Wes , Strauss Herbert L.
Published: July 2002

The kinetics of NH3D+ rotation in (NH4)2SO4 are followed from about 8 to 60 K. The rotation is between distinct and well-defined positions in the crystal and occurs by tunneling. At low temperatures the data are fit by a phonon assisted mechanism. At higher temperatures (40-60 K), the data are fit by simple quantum transition state theory.
Intramolecular vibrational energy relaxation seen as expansion in phase space. I. Some experimental results for H2O+(X̃ 2B1), C2H+4(X̃ 2B3), and HCN+(B̃ 2∑+)
Authors: Lorquet J. C. , Pavlov-Verevkin V. B.
Published: July 2002

It has been shown by Heller that a nonstationary wave packet resulting from a Franck–Condon transition evolves on the potential energy surface of the final electronic state and propagates through phase space at a rate which can be determined from the autocorrelation function ↓C(t)↓2=↓〈(0)‖(t)〉↓2. Since C(t) can be obtained by Fourier transformation of an optical spectrum S(E), i.e., from an observable quantity, it is possible to derive from an experimental measurement information concerning the density operator of a so-called dynamical statistical ensemble (DSE). This density operator, denoted ρav, represents a statistical mixture of the eigenstates of the system with weights determined by the dynamics of the system. It becomes diagonal after a so-called break time 𝒯B. Its measure, according to a definition due to Stechel, can be interpreted as an effective number of states (denoted 𝒩) that significantly contribute to the dynamics. The break time 𝒯B represents the finite period of time allowed to expand in the phase space and after which no further progress can be made. Therefore, the number 𝒩∞ of phase space cells which are accessed after a very long interval of time (or in practice after the break time) remains limited.

Information on the validity of statistical theories of unimolecular reactions is contained in the fraction ℱ of the available phase space which is eventually explored. In order to assess the representativity of the sampling, it is necessary to account for the selection rule which requires all the states counted in 𝒩∞ to belong to the totally symmetric representation. It is also appropriate to estimate the role played by Fermi resonances and similar vibrational interactions which bring about energy flow into zero-order antisymmetric modes. A method to carry out the necessary partitionings is suggested. The functions 𝒩T and ℛT, and the quantities 𝒯B, 𝒩∞, 𝒩 *, and ℱ have been determined from experimental data in three cases. In each case, the rate ℛT=d𝒩T/dT starts from an initial value of zero, increases up to a maximum which is reached after a time of the order of 10−14 s, and then exhibits an overall decrease upon which oscillations are superimposed. For state X̃ 2B1 of H2O+, 𝒯B≂2.4×10−14 s and ℱ≂0.3. The wave packet never accesses that part of the phase space that corresponds to the excitation of antisymmetric vibrations. For state X̃ 2B3u of C2H+4, 𝒯B≂1.6×10−13 s and ℱ≂5×10−4. This fraction raises to 6×10−3 if measured with respect to the effectively available phase space. When the spectrum consists of a discrete part followed by a dissociation continuum, the method can be extended to study the behavior of the bound part of the wave packet only. This has been applied to state B̃ 2∑+ of HCN+ which is characterized by a very irregular spectrum. This case offers an example of complete occupation of phase space after a break time which is of the order of 2×10−13 s.
New assignment of Fermi resonance spectra
Authors: Kellman Michael E. , Xiao Lin
Published: July 2002

A new quantum number assignment is proposed for bend–stretch vibrational spectra with 2:1 Fermi resonance. Using a classification based on the catastrophe map and polyad phase spheres of the Fermi resonance fitting Hamiltonian, quantum numbers are assigned corresponding to the semiclassical dynamics of each energy level. The method is applied to assign experimental spectra of CH(CF3)3. The separability of the wave functions is discussed in terms of ‘‘nodal coordinates’’ for which the new quantum numbers are constants of the motion.
Self-assembled structure of a semidilute solution of polymer mixtures under shear flow
Authors: Takebe Tomoaki , Fujioka Kiyotoshi , Sawaoka Ryuji
Published: July 2002

The domain structure self-assembled under a steady Couette flow was investigated on a semidilute solution of polymer mixture (polymer A + polymer B + solvent) at a composition near the critical one by use of the in situ light scattering method. This method permits a quantitative analysis of the scattering profile I(qy) in the plane perpendicular to the shear flow as a function of shear rate S at a given quench depth ΔT(0)=Tc(0)−T. Here, qy is the component of the scattering vector q in the plane concerned, Tc(0) the critical temperature at S=0, and T[<Tc(0)] the temperature of the experiment. Effects of shear rate on the self-assembled structure were pronounced, and they were classified into five regimes A to E. At the lowest S (regime A), I(qy) was expressed by a linear combination of the Porod scattering and the Ornstein–Zernike (OZ) scattering, the former being due to the domain structure and the latter to critical composition fluctuations inside the domains. At higher S (regime B), I(qy) was complex. However, with increasing S further (regimes C and D), it was universally represented by the squared Lorentzian form {1+[qy(ξ⊥)d]2}−2, with a shear-rate-dependent length parameter (ξ⊥)d which depends on S−n with n=1/4 to 1/3. This fact indicates the self-assembling of a domain structure which we call oriented random two-phase structure. At the highest S (regime E), I(qy) followed the OZ scattering, thus suggesting the system to change to the shear-induced homogenized state.
Dynamics of molecular liquids: A comparison of different theories with application to wave vector dependent dielectric relaxation and ion solvation
Authors: Wei Dongqing , Patey G. N.
Published: July 2002

Explicit expressions are given for the Fourier–Laplace transform of the van Hove function for fluids of particles interacting through orientation dependent pair potentials. The expressions are obtained from the Kerr approximation together with diffusion models for the self part of the van Hove function and apply at all frequencies and wavelengths. Both spherical (scalar diffusion constants) and nonspherical (tensorial diffusion constants) diffusors are considered and the theory is applied to k-dependent dielectric relaxation and ion solvation dynamics. The required equilibrium structure is obtained using the hypernetted chain (HNC) or reference hypernetted chain (RHNC) theories and Kerr/RHNC(HNC) results are given for fluids of dipolar hard spheres, dipolar hard ellipsoids, and water-like particles. Comparisons are made with earlier work based upon coupling equilibrium theories, such as the mean spherical approximation (MSA) for dipolar hard spheres with a dynamical equation of the Smoluchowski–Vlasov (SV) type. It is shown that for spherical diffusors with the self part of the van Hove function treated at the Fick–Debye level, the SV and Kerr equations are in fact equivalent. However, even for spherical diffusors, the results obtained can differ substantially depending upon the equilibrium theory and/or the molecular model employed. For nonspherical diffusors, the anisotropy of the rotational diffusion tensor can also be an important parameter influencing the k-dependent dielectric relaxation.
Microscopic models for iodine photodissociation quantum yields in dense fluids
Authors: Dardi Peter S. , Dahler John S.
Published: July 2002

Experimental measurements of the photodissociation quantum yield for iodine in a variety of solvents show that the quantum yield becomes less than unity at moderate densities (∼0.01 of liquid densities) and that as liquid densities are reached, the quantum yield drops quite dramatically with increasing density. We examine two models for the iodine photodissociation quantum yields. The first was proposed by Otto, Schroeder, and Troe [J. Chem. Phys. 81, 202 (1984)] to explain the drop in quantum yield at moderate densities. It assumes the formation of van der Waals complexes with the solvent which, when excited can fragment to produce iodine in a distribution of vibrational states in the ground electronic state. We calculate the concentrations of van der Waals complexes with ethane and confirm that there are large enough concentrations to explain the experimental results. The second model is developed to explain the steep drop in quantum yield at high densities. It is based on solvent caging with trapped pairs of iodine atoms recombining to form iodine molecules. Calculations based on this model agree well with experimental results. We discuss the implications that the van der Waals model has on the interpretation of molecular beam experiments involving iodine van der Waals complexes. At high densities both mechanisms (the moderate density mechanism and the caging mechanism) must be superimposed even though it is the caging mechanism which leads to the stronger density dependence. The realization that two distinct pathways exist for returning the excited iodine to its ground state significantly clarifies the interpretation of picosecond experiments examining iodine photochemistry in liquids.
Nature of second hydration shell in Mn+2–aquoion system: Influence on proton relaxivity in nuclear magnetic resonance
Authors: Lata K. Ramani , Sahoo N. , Das T. P.
Published: July 2002

Using the unrestricted Hartree–Fock procedure, we have studied the numbers and locations of water molecules in the second hydration shells of the Mn+2–aquoion system. Two arrangements for the second hydration shell were considered, one involving 8 water molecules located along the body diagonals of a cube with Mn+2 ion at the body center and the other with 12 water molecules located in the directions of the lines joining the Mn+2 ion and the midpoints of the sides of the cube. Both these arrangements of the second hydration shells were considered in the presence of the octahedral arrangement of six water molecules in the first hydration shell. The total energies of the composite clusters were minimized with respect to the metal–oxygen distance for the second shell of water molecules to determine the equilibrium geometries in the two cases. From a consideration of the hydration energies, the eight-molecule configuration was found to be the more likely one for the second hydration shell, the metal–oxygen distance for the second shell being 3.75 Å as compared to 2.19 Å for the first. A physical reason associated with steric effects within the second shell water molecules and between the first and second shell molecules is suggested for the greater stability of the 8-member hydration shell as compared to the 12-member one. Using the calculated geometry, the contribution to the proton relaxivity in aqueous solution from the second hydration shell was determined to be 9.9% of that from the first hydration shell in the dipolar interaction regime, in magnetic fields of 0.25 T (corresponding to proton resonance frequency of about 10 MHz) and above and progressively smaller in importance as one went to lower fields corresponding to the contact interaction regime.
Four-body nonadditivity in liquid water
Authors: Gil-Adalid L. , Ortega-Blake I.
Published: July 2002

Here we present an ab initio study of the many-body expansion for the water–water interaction. Emphasis is made in the difference in convergence of the expansion for the different phases of water. It is shown that for the geometrical arrangements that appear in liquid water, the four-body nonadditivity is larger and the convergence slower than for the other two phases. Hence particular attention should be given for the proper description of the interactions in the aggregates that will arise in the numerical simulations of liquid water.
Quantum simulation of hydrogen migration on Ni(100): The role of fluctuations, recrossing, and multiple jumps
Authors: Haug Kenneth , Metiu Horia
Published: July 2002

We examine the mobility of a hydrogen atom adsorbed on a Ni(100) in a temperature range (200–400 K) where the motion consists of jumps between lattice sites. We view these jumps as isomerization reactions and calculate their rate constants by using the flux–flux correlation function theory. We examine in detail the effect of lattice fluctuations and lattice distortion on the jumping rates and test the accuracy of several short time approximations which provide an extension of the transition state theory to quantum systems. We find that the magnitude of the diffusion coefficient is affected by multiple jumps and that recrossing effects are significant. By comparing the present quantum results to those obtained previously by classical simulations, we find that in this temperature range the quantum effects are small (i.e., at most a factor of 6) and originate mostly from the differences in the magnitudes of the thermodynamic quantities appearing in the rate coefficient expression. Numerical experiments show that it is possible to calculate the transition state rate coefficient in quantum systems by using a free particle approximation to calculate the short time evolution of the flux–flux correlation function. This approximation provides significant computer time savings and will permit calculations for quantum systems with a very large number of degrees of freedom.
The multiconfigurational particle–particle propagator method for directly determining vertical double ionization potentials and double electron affinities
Authors: Graham Richard L. , Yeager Danny L.
Published: July 2002

We present and develop the multiconfigurational particle–particle propagator (MCP2P) method for directly determining vertical double ionization potentials (DIPs) and double electron affinities. The MCP2P method gives highly accurate DIPs. In the first calculations with this method (obtained using a moderate-sized MCP2P complete active space) the average absolute deviation from Δfull configuration interaction within the same basis set for the DIPs to the lowest (in energy) five 1,3S Be2+ states is 0.04 eV.
Dynamical processes in organic glassforming van der Waals liquids
Authors: Meier G. , Gerharz B. , Boese D. , Fischer E. W.
Published: July 2002

This report presents measurements of the frequency dependent dielectric loss ε″(ω) and quasielastic light scattering of thermal density fluctuations of simple organic glassforming van der Waals liquids BCDE (bis-phenol-C-dimethylether) and BKDE (bis-kresol-C-dimethylether) having glass transition temperatures of Tg=240 K and Tg=261 K, respectively. The dielectric measurements of BCDE were carried out in a temperature range between 250 and 316 K, those of BKDE between 270 and 384 K covering a frequency range from 10−1 to ∼109 Hz in each case. Quasielastic light scattering data were taken between the glass transition temperatures and T≂Tg+30 K in a correlation time window of 10−6 s up to 102 s. As main results we have found the following: The relaxation process in the BCDE melt can be described using a Kohlrausch–Williams–Watts (KWW) function with a temperature independent β parameter. Its value was found to be identical either from the light scattering result or from the stretched exponential fit to the Fourier transform of the dielectric loss data.

The temperature dependence of the mean relaxation times of the BCDE melt was fitted with a Williams–Landel–Ferry (WLF) equation, or Vogel–Fulcher–Tammann equation, leading to WLF constants close to the universal ones. The BKDE system, however, behaves differently, where the dynamic response in the bulk amorphous state could only be described using a temperature dependent β parameter. This finding was confirmed by either the light scattering method as well as the dielectric relaxation method. Additionally we were able to measure a second dielectrically active process in BCDE only, arising in the high frequency region with an Arrhenius type of activation energy. In the BKDE melt, however, the superposition of different processes causes a temperature dependence of the β parameter when the data is forcefitted with a single KWW function. To correlate the fast process of BCDE (β process) with a possible molecular model of motion we have performed additional 2H-NMR solid echo measurements on both materials around and well below Tg. Thereby a 180° phenyl ring flip motion in BCDE was found in contrast to BKDE. This assignment of the secondary relaxation process is discussed together with the results from other methods.
Effect of reagent rotation on product energy disposal in the light atom transfer reaction O(3P)+HCl(v=2,J=1,6,9)→OH(v′,N′)+Cl(2P)
Authors: Zhang Rong , van der Zande Wim J. , Bronikowski Michael J. , Zare Richard N.
Published: July 2002

A rovibronic-state-to-rovibronic-state experiment has been performed on the reaction O(3P)+HCl(v=2,J=1,6,9)→OH(v′,N′)+Cl(2P). The O(3P) atoms are produced with a known energy by photolysis of NO2. The HCl(v=2,J) molecules are prepared by IR excitation of thermal HCl using an optical parametric oscillator. All energetically accessible OH rovibrational product levels are probed by laser-induced fluorescence for each prepared HCl rotational level. The OH(v′=0,N′) rotational distribution shows a dip at N′=11, the depth of which decreases with increasing HCl rotational excitation. The available energy of reaction is partitioned so that 40% appears as OH vibration (V′), 32% as OH rotation (R′), and 28% as product translation (T′). This energy partitioning does not change with HCl rotation, in contrast to the general expectation for light atom transfer reactions of approximate conservation of internal angular momentum (R→R′). A substantial vibrational inversion is observed, in agreement with the vibrational adiabaticity (V→V′) expected for such reactions.
Bond-breaking without barriers. II. Vibrationally excited products
Authors: Green William H. , Mahoney Arthur J. , Zheng Qi-Ke , Moore C. Bradley
Published: July 2002

Ketene is photolyzed in a supersonic jet, and the vibrationally excited singlet methylene CH2 (ã 1A1), produced is detected by laser-induced fluorescence. The appearance thresholds and yield curves of individual methylene rovibrational states are obtained by scanning the photolysis laser wavelength. As observed previously by probing the (0,0,0) state at lower photolysis energies, there are no barriers to dissociation and nuclear spin is conserved. Sharp steps are observed just above the energetic threshold in each of these photofragment excitation (PHOFEX) curves. This suggests that the rotational state distributions are given by phase space theory (PST). The quantum yield of the (0,1,0)101 rovibrational state is measured and the quantum yield for (0,1,0) inferred. These values are larger than predicted by PST, and are close to values predicted by variational Rice–Ramsberger–Kassel–Marcus (RRKM) theory and by the separate statistical ensembles (SSE) method. This indicates that near the (0,1,0) energy threshold the (0,0,0) yield is constrained, as by a tight transition state. The appearance of steps spaced by the energies of a free CO rotor in the PHOFEX curves close to the thresholds of each vibrational state probed indicates that the near threshold flux of vibrationally excited products is controlled by a loose ‘‘transition state’’ on a vibrationally adiabatic surface. These observations are consistent with the variational RRKM theory for dissociations without barriers in which each product vibrational state evolves on its own vibrationally adiabatic potential surface and has its own transition state. As the energy increases above the threshold for a vibrational state, its transition state moves in along the reaction coordinate and tightens. Thus total rates increase less rapidly with energy than in PST and vibrational distributions are skewed towards higher levels than in PST.
Structure of adsorbates on alkali halides (theory). I. HBr on LiF(001)
Authors: Polanyi J. C. , Williams R. J. , O’Shea S. F.
Published: July 2002

We report a first computation of the geometry of HBr adsorbed on a LiF(001) substrate. The interaction energy of 200 HBr molecules with periodic boundary conditions was determined with respect to adsorbate–substrate and adsorbate–adsorbate electrostatic interaction (point charges in HBr simulated its dipole and multipoles) as well as a short-range ‘‘core’’ potential (comprising dispersion forces and repulsive overlap). The monolayer heat of adsorption at T=100 K according to this model was 0. 27 eV, which was also the experimental value [E. B. D. Bourdon, et al., J. Chem. Phys. (submitted)]. The major contribution to this energy came from electrostatic binding. The preferred HBr location at all surface coverages (0.25, 0.50, and 1.0 ML) in the temperature range T=70–130 K was with Br over Li+ and H almost eclipsing one of the four neighboring F−. In this configuration, Br–H is tilted down by 23 ° from the crystal plane in an arrangement resembling a Br–H– –F− hydrogen bond. This computed geometry agrees well with recent experimental findings for HBr/LiF(001) using polarized infrared spectroscopy [P. M. Blass et al.; J. Chem. Phys. (submitted)], and is also in qualitative accord with the observed angular distribution of fast H from the photolysis of HBr adsorbed on LiF(001). At the separations dictated by the LiF(001) lattice, the net adsorbate–adsorbate interaction is attractive, leading to a tentative prediction of island formation at lower coverages under equilibrium conditions.
Dense inhomogeneous fluids: Functional perturbation theory, the generalized Langevin equation, and kinetic theory
Authors: Pozhar Liudmila A. , Gubbins Keith E.
Published: July 2002

We present a functional perturbation theory (FPT) to describe the dynamical behavior of dense, inhomogeneous fluid mixtures, and from this show rigorously that the generalized Langevin equations are a first order form of this FPT. These equations lead to linearized kinetic equations for the singlet dynamical distribution function and for the higher distribution functions. These kinetic equations for inhomogeneous fluid mixtures reduce to those of Sung and Dahler [J. Chem. Phys. 80, 3025 (1984)] in the case of homogeneous fluids. Finally, we prove that the kinetic equations derived can be used to derive a ‘‘smoothed density’’ postulate, in which the local transport coefficients for inhomogeneous fluids are equated to those for a homogeneous fluid of the same smoothed density.
Smoluchowski fluctuation theory of dielectric relaxation
Authors: Kim Hyung J. , Friedman Harold L. , Raineri Fernando O.
Published: July 2002

A recently proposed theory for transport coefficients in ionic solutions can be reinterpreted so that it becomes applicable to calculating the dielectric relaxation processes in dipolar fluids and in ionic solutions in dipolar solvents. The resulting theory is the same as the Smoluchowski–Vlasov theory which has been derived in various other ways by Munakata, Calef and Wolynes, and Chandra and Bagchi. Here the theory is applied to calculate the time correlation function for the fluctuating polarization density field in simple dipolar fluid, and in an ionic solution in a dipolar solvent. The results are similar to those derived some time ago by Berne using a ‘‘forced diffusion equation’’ as the theory for calculating the response. Comparison is also made with the results of Chandra and Bagchi using the Smoluchowski–Vaslov theory.
Hyperspherical formulation of the photodissociation of ozone
Authors: Le Quéré F. , Leforestier C.
Published: July 2002

In a preceding paper [J. Chem. Phys. 92, 247 (1990)], we reported a preliminary three-dimensional quantum calculation of the dissociation of the ozone molecule in the Hartley band. We present here a fully converged calculation of the autocorrelation function on the excited 1B2 potential energy surface of Sheppard and Walker. The study has been reformulated in terms of hyperspherical coordinates, in order to make use of the permutation-inversion symmetry of the ozone molecule. The wave-function has been discretized on a three-dimensional hyperspherical grid. The autocorrelation function 〈φ0‖φt〉 has been computed within the time formulation, by means of the Lanczos algorithm. A calculation performed for the total angular momentum value J=0 shows a good agreement with the experimental results of Johnson and Kinsey, except for the overall intensity of the recurrence peaks. The calculated photodissociation spectrum reproduces the same oscillation pattern as observed experimentally.
The silyl anion (SiH−3): Cubic/quartic force field and anharmonic contributions to the fundamental vibrational frequencies
Authors: Shen Mingzuo , Xie Yaoming , Yamaguchi Yukio , Schaefer Henry F.
Published: July 2002

Fundamental vibrational frequencies for the silyl anion have been determined using two distinct vibrational theoretical methods, namely, the standard second-order perturbation theory and Pulay’s variational theory. Several full quartic force fields were determined and used in both vibrational methods. Ab initio quantum mechanical methods used to generate energy derivatives include self-consistent-field (SCF), configuration interaction with single and double excitations (CISD), and coupled cluster with single and double excitations (CCSD), in conjunction with basis sets including double zeta plus polarization (DZP), triple zeta plus double polarization (TZ2P), and TZ2P plus diffuse p functions on the silicon atom [TZ2P+diff(Si)]. SCF energy third derivatives were determined using analytic methods, while SCF fourth derivatives were determined from finite differences of third derivatives. CISD analytic energy gradients were used to generate second, third, and fourth derivatives by finite difference methods. Anharmonic corrections determined from both perturbation and variational theories are found to be quite insensitive to basis set size and electron correlation. The two theoretical methods for the treatment of vibrations have shown small but systematic differences in their prediction of anharmonic corrections. The fundamental vibrational frequencies obtained agree reasonably well with the limited available experimental results.
Lowest energy excited singlet state of isolated c
i
s-hexatriene
Authors: Buma Wybren Jan , Kohler Bryan E. , Song Kyuseok
Published: July 2002

In a previous letter [J. Chem. Phys. 92, 4622 (1990)] we reported the first observation of the 2 1Ag state of cis-hexatriene in a supersonic jet expansion by using resonance enhanced multiphoton ionization spectroscopy. Here, the vibrational analysis of the 1 1Ag→2 1Ag excitation spectrum of cis-hexatriene is presented. The excitation spectrum shows that cis-hexatriene in the 2 1Ag state deviates slightly from planarity; a conclusion which is corroborated by ab initio calculations indicating that the nonplanarity primarily involves the terminal hydrogen atoms. Except for observable intensity in the low frequency modes associated with the small out of plane distortion, the vibronic development of the 1 1Ag→2 1Ag transition in cis-hexatriene is similar to that observed for other polyenes: the 0–0 transition is the most intense feature and the next most intense band is the C■C stretching fundamental. Thus the general features of the electronic structure of the cis-hexatriene 2 1Ag state are analogous to those of other polyenes.