bluuebunny/crossref_metadata_2025 huggingface dataset:
Lattice cluster theory for phase behavior of rectangular mesogens
Authors: Li Wenshuo , Freed Karl F.
Published: July 2002

The lattice cluster theory is generalized to study a system of hard, rigid, rectangular mesogens, which may exhibit isotropic liquid and nematic and discotic nematic liquid crystalline phases. A common partition function for all three phases is derived as a functional of the different possible orientational distribution functions. The partition function is expressed in the familiar form of a cluster expansion with a zeroth order uncorrelated mean field portion and corrections originating from correlations between different numbers of mesogens. The entropies for the three phases are evaluated to second order in mesogen density φ by including in the cluster expansion only corrections originating from correlations containing two objects. (A previous paper illustrates the application of the general theory to order φ4 for the packing entropy of the smallest square objects, and work is currently in progress to pursue these technically involved fourth order computations for the present case.) Models with different dimensionality represent the influence of greater orientational freedom and are shown to produce qualitatively different phase diagrams. One model for extra orientational freedom is obtained by considering the single object entropy lost upon ordering, while another model is generated by comparing the d-dimensional lattice model second virial coefficient with that of the continuous space three-dimensional object.
Microscopic treatment of substrate effects on linear optical properties of model Langmuir–Blodgett films
Authors: Munn R. W.
Published: July 2002

An algebraic treatment is developed for the effect of a plane substrate with isotropic relative permittivity on the linear optical response of layers of molecules treated as sets of point submolecules so as to incorporate molecular elongation and orientation. Results are modified from those for free-standing films by planewise sums describing the field at a dipole due to its image in the substrate. Numerical calculations of refractive indices are performed for a monolayer of molecules initially packed in a tetragonal structure normal to the substrate and then tilted to yield monoclinic structures. The head-group polarizability is varied relative to that of the other submolecules, as is the spacing between the layer and the substrate. Substrate effects are largest when a very polarizable head group of a tilted molecule is adjacent and very close to the substrate; in extreme cases the refractive indices diverge. The results support the idea that first-layer effects on optical properties could be dielectric as well as structural in origin.
Linear viscoelasticity of dense colloidal suspensions
Authors: Cichocki B. , Felderhof B. U.
Published: July 2002

In earlier exact calculations of the linear viscoelastic behavior of semidilute suspensions of hard spheres we have shown that hydrodynamic interactions have a significant influence on the frequency dependence of the dynamic shear viscosity. Scaling with the mean relaxation time is not sufficient. Here we reanalyze experimental data of van der Werff et al. on the linear viscoelasticity of dense hard sphere suspensions with hydrodynamic interactions, and determine values of the parameters characterizing the shape of the dispersion curves. We find that the data cannot be explained by scaling the exact result for a semidilute suspension of hard spheres without hydrodynamic interactions with just the mean relaxation time.
Multibody distribution function contributions to the heat capacity for the truncated Lennard-Jones fluid
Authors: Freasier Ben C. , Czezowski Adam , Bearman Richard J.
Published: July 2002

We have performed a series of standard NVT Monte Carlo simulations in which we have calculated the configurational heat capacity, Cv, of a truncated Lennard-Jones model fluid by the fluctuation theorem. As we found in a previous investigation, the heat capacity exhibited two extrema as a function of density on sufficiently low temperature isotherms. During the course of these runs, we have decomposed the heat capacity into the sum of quantities, Cv(2), Cv(3), and Cv(4), which are averages over two, three, and four body distribution functions. We analyze combinations of the three contributions to gain insight into the nature of the extrema. In particular, we find that the sum Cv(2)+Cv(3) has a maximum and a minimum at all temperatures studied. The magnitude and slope of Cv(2)+Cv(3) compared with Cv(4) determines whether or not Cv itself has maxima and minima.
Calculation of rovibronic structures in the lowest nine excited 1Σ+g+1Πg+1Δg states of H2, D2, and T2
Authors: Yu S. , Dressler K.
Published: July 2002

We have calculated the rovibronic structures which arise from the simultaneous radial and angular couplings among the lowest nine excited 1Σ+g, 1Πg, and 1Δg states in H2, D2, and T2. Using ab initio potential curves, adiabatic corrections, and nonadiabatic coupling functions wherever possible, we have solved the coupled vibronic equations for all bound states below the 1s+2l dissociation limit up to J=5 in H2 and D2, and up to J=7 in T2. The good agreement of the bulk of the data with spectroscopic results enables us to eliminate spurious assignments in old spectroscopic work (e.g., all old assignments involving the 4s O 1Σ+g state are spurious), and to make numerous new assignments. We present updated tables of spectroscopic term values for H2 and D2. The absolute energy errors of the ab initio Born–Oppenheimer energies around the minima of the potential curves appear to be at most of the order of the radiative energy corrections (≂0.1 cm−1) in the states 1σg2sσg E and (1σu)2 F, less than 1 cm−1 in the states 3dσ G, 3s H, and 3dπ I, less than 2 cm−1 in the 4s O state, and approximately 4 cm−1 in the 4dσ P state.
Ab initio study of He(1S)+Cl2(X 1Σg,3Πu) potential energy surfaces
Authors: Chal/asiński Grzegorz , Gutowski Maciej , Szcze̢śniak M. M. , Sadlej Joanna , Scheiner Steve
Published: July 2002

The potential energy surface of the ground state He+Cl2(1Σg) is calculated by using the perturbation theory of intermolecular forces and supermolecular Mo/ller–Plesset perturbation theory approach. The potential energy surface of the first excited triplet He+Cl2(3Πu) was evaluated using the supermolecular unrestricted Mo/ller–Plesset perturbation theory approach. In the ground state two stable isomers are found which correspond to the linear He–Cl–Cl structure (a primary minimum, De=45.1 cm−1, Re=4.25 Å) and to the T-shaped structure with He perpendicular to the molecular axis (a secondary minimum, De=40.8 cm−1, Re=3.5 Å). The small difference between these geometries is mainly due to the induction effect which is larger for the linear form. The results obtained for the T-shaped minimum are in good agreement with the excitation spectroscopy experiments which observed only the T-shaped form [Beneventi et al., J. Chem. Phys. 98, 178 (1993)]. In the lowest triplet states correlating with Cl2(3Πu), 3A′ and 3A″, the same two isomers correspond to minima. Now, however, the T-shaped form is lower in energy. The 3A′ and 3A″ states correspond to (De,Re) of (19.9 cm−1, 3.75 Å) and (30.3 cm−1, 3.50 Å), respectively, whereas the linear form is characterized by (19.8 cm−1, 5.0 Å). The binding energy for the T form in the lower 3A″ state is in good agreement with the experimental value of Beneventi et al.
Inclusion of Hartree–Fock exchange in density functional methods. Hyperfine structure of second row atoms and hydrides
Authors: Barone Vincenzo
Published: July 2002

Isotropic and anisotropic hyperfine coupling constants of second row atoms have been studied using extended basis sets with a number of different density functionals. Local functionals give even the wrong sign for some atoms, and the same occurs when the Perdew’s correlation functional is coupled to the Becke exchange one. Much better results are obtained using the Lee–Yang–Parr correlation functional. Introduction of some Hartree–Fock exchange in the functional delivers a further significant improvement, approaching the accuracy of the most refined post-Hartree–Fock computations. Purposely tailored basis sets are also introduced which are small enough to be used in molecular computations, but still give high quality geometries and hyperfine coupling constants.
Photoionization and photodissociation of nitric oxide in the range 9–35 eV
Authors: Erman P. , Karawajczyk A. , Rachlew-Källne E. , Strömholm C.
Published: July 2002

Photoionization and photodissociation of NO has been studied using 9–35 eV synchrotron light and detection of fluorescence as well as of mass selected NO+, N+, O+, and O− ions. Rydberg series converging to all known states in NO+ below 24 eV have been analyzed and classified and several of them are found to be predissociated by various NO+ states as well as by NO ion pair states. The complex structure in the 950–1200 Å excitation region is suggested to originate from interaction between a ‘‘new’’ NO valence state at T0=77 470 cm−1 and high vibrational levels of Rydberg series converging to the NO+ ground state.
Effect of polar additives on charge transport in a molecularly doped polymer: Survey of various additives
Authors: Young Ralph H. , Fitzgerald John J.
Published: July 2002

We have investigated the mobility of holes in a molecularly doped polymer, polystyrene doped with tri-p-tolylamine and small amounts of polar additives. The additives come from several chemical families (including cyano, nitro, and sulfonyl compounds) and range in dipole moment from 0 to 6.6 D. The mobility is dramatically reduced and its dependence on electric field strength is greatly increased by some of them. The size of both effects depends mainly on the concentration and net dipole moment of the additive; structure-specific variations are secondary. The results are compared with two current models. A model in which the additive acts as a ‘‘dipole trap’’ predicts the wrong dependence on the concentration of the additive. Attempts to interpret the effects with the Gaussian disorder model of Bässler and co-workers [H. Bässler, Phys. Status Solidi B 175, 15 (1993)] are only partially successful. Possible reasons are described.
Magnetic modulation of hyperfine quantum beats
Authors: Hemmi Naoki , Cool Terrill A.
Published: July 2002

Weak magnetic fields are observed to produce significant alterations in the temporal evolution of fluorescence emitted from cyanogen following laser excitation of the vibronically allowed 410Ã(1Σ−u )←X̃(1Σ+g) transition near 219 nm. Magnetically induced modulations of molecular quantum beats are found to arise from Zeeman splittings among the hyperfine levels of spin–orbit coupled molecular eigenstates. These magnetic modulations are accurately described by a simple model in which fluorescence interferences that arise from coupling between the Zeeman components of the molecular eigenstates are analogous to the interference of light passing through the equally spaced slits of a transmission diffraction grating. Landé g factors derived from the magnetically induced modulations are in good agreement with the values directly measured from the Zeeman broadened envelopes of quantum beat frequency components.
Rotational spectra and van der Waals potentials of Ne–Ar
Authors: Grabow J.-U. , Pine A. S. , Fraser G. T. , Lovas F. J. , Suenram R. D. , Emilsson T. , Arunan E. , Gutowsky H. S.
Published: July 2002

The high sensitivity and resolution of Fourier-transform microwave spectroscopy using a pulsed jet coaxial to a Fabry–Perot resonator have been exploited to measure pure rotational transitions of several isotopomers of the weakly polar Ne–Ar van der Waals dimer in natural abundance. Transitions of the most abundant isotopomer, 20Ne–40Ar, could be observed with an excellent signal-to-noise ratio with a single polarization pulse. The ground-state rotational constants for this species yield a zero-point separation of R0≂360.7 pm. Simple model van der Waals potentials have been fit to the microwave transitions for the various isotopomers, providing estimates of the equilibrium spacing at the well minimum of Re=348.0(2) pm. More elaborate potentials based on ab initio calculations or on molecular-beam scattering cross sections and thermodynamic and transport properties have also been tested. The induced electric dipole moment is estimated to be μ0=7.3(1.6)×10−33 C m [0.0022(5) D] by comparison of π/2 polarization pulses with a reference molecule (Ar–CO2) whose dipole moment is known from Stark effect splitting measurements. Uncertainties in parentheses are one standard deviation.
Hydrogen on polycrystalline diamond films: Studies of isothermal desorption and atomic deuterium abstraction
Authors: Koleske D. D. , Gates S. M. , Thoms B. D. , Russell J. N. , Butler J. E.
Published: July 2002

Studies of hydrogen isothermal desorption and abstraction from polycrystalline diamond surfaces are presented. The surface H and D coverages (θH and θD) are measured in real time by mass analyzing the recoiled ions generated in a time-of-flight scattering and recoil spectroscopy (TOF-SARS) experiment. For surface temperatures (Ts) from 825 and 920 °C, isothermal H2 desorption is 1st order in θH with a measured activation energy, ET, of 69±6 kcal/mol and a pre-exponential factor, ν, of 1010.5±0.9 s−1. For H2 desorption from diamond, the estimated ΔET based on bond energy calculations is ≊88 kcal/mol, substantially higher than the experimentally measured ET. This difference suggests π-bonding of the surface after H2 desorption is involved. Using a simple bond order argument, the π-bonding contribution is estimated to be ≊21 kcal/mol. The abstraction and replacement of absorbed H by atomic deuterium (Dat) is explained by three first-order reactions. Under a constant Dat flux, the rate of abstraction of adsorbed H by Dat is 1st order in θH, with an ‘‘apparent activation energy’’ (Ea) of 0.8±0.2 kcal/mol and ν=(1.3±0.2)10−3 s−1. The low Ea and 1st order kinetics imply that H is abstracted from the surface by a generalized Eley–Rideal or direct mechanism. Using the relative rate of Dat abstraction of H to Dat adsorption on clean diamond, we estimate an upper limit for the abstraction activation barrier of 16 kcal/mol. Under identical reaction conditions, the rate for Hat abstraction of D is ≊1/3 the rate for Dat abstraction of H. We interpret this isotope effect using momentum transfer arguments.
Experimental determination of dipole moments, hyperfine interactions, and ab initio predictions for PtN
Authors: Jung K. Y. , Steimle T. C. , Dai D. , Balasubramanian K.
Published: July 2002

Platinum nitride, PtN, was identified in the supersonic coexpansion of a laser ablation generated platinum vapor and ammonia by laser induced fluorescence spectroscopy. The intense blue band system was assigned as the (0,0) d 4Π1/2–X 2Π1/2 transition with the determined spectroscopic fine parameters being (in cm−1):T00=18586.3608(28), B″=0.4541(7), (p+2q)″=0.1219(15), B′=0.4164(7), and (p+2q)′=0.2039(8). The Stark shifts and splitting were analyzed to produce ground and excited electronic state values for the permanent electric dipole moment of 1.977(7)D and 1.0(1)D, respectively. The splitting in the field free spectrum for the 195Pt isotopomer was analyzed to produce magnetic hyperfine parameters (in cm−1) h1/2″= 0.0639(30), h1/2′ = 0.1571(36), d′=−0.0979(7), and d″=−0.0034(15). A sophisticated ab initio calculation of the physical properties of the low-lying states was performed. The experimental results are generally consistent with the ab initio predictions that the band system is the (0,0) d 4Π1/2–X 2Π1/2 transition. A discussion of the nature of the electronic states is given.
The simple theory of demixation in the presence of amphiphile
Authors: Moskalenko A. , Dawson K. A.
Published: July 2002

The Blume–Emery–Griffiths three-spin state lattice model is applied to describe three component mixtures of water, polymer, and amphiphile. A simple mean-field theory similar to the Flory–Huggins approach is used to analyze the lattice model. The shape of the demixation curve is derived analytically in the mean-field approximation for small concentrations of amphiphile. We analyze the system in the vicinity of the critical point. In addition we consider mixtures of two polymers in solvent, and discuss the demixation curve for such systems.
Monte Carlo density functional theory of nonuniform polymer melts
Authors: Yethiraj Arun , Woodward Clifford E.
Published: July 2002

A theory for nonuniform polymer melts is presented, which combines density functional theory with Monte Carlo methods. The theory treats the ideal gas functional exactly via a single chain simulation and uses the weighted density approximation for the excess free energy functional. The bulk fluid properties required in the theory are obtained from a generalized Flory equation of state. The predictions of the theory are compared to Monte Carlo simulations for the density profiles of semiflexible polymer melts confined between flat plates. Good agreement between theory and simulation is found for 3mers and 20mers and for several densities and molecular stiffnesses.
Electric field induced second harmonic generation/third harmonic generation measurements on molecules with extended charge transfer: Absorption domain and strong resonance effects
Authors: Puccetti G.
Published: July 2002

An enlarged method of performing electric field induced second harmonic generation (EFISHG) or third harmonic generation (THG) measurements of molecular hyperpolarizabilities in the strongly resonant regime is proposed. The influence of absorption coefficients and anomalous dispersion are discussed in detail and corrections proposed which require no change in the experimental setup. An alternative method of data interpretation is presented and its validity discussed for two examples of molecular structures.
Path-integral treatment of multi-mode vibronic coupling. II. Correlation expansion of class averages
Authors: Krempl Stefan , Winterstetter Manfred , Domcke Wolfgang
Published: July 2002

A path-integral approach to real-time quantum dynamics is presented which is suitable to treat the dynamics of vibronic coupling or spin boson models. In these models the vibrational dynamics is nonseparable as a consequence of the electronic inter-state coupling. The sum over all possible paths in electronic-state space generated by the usual Trotter procedure is expressed in terms of single-mode averages over classes of paths and statistical mode correlations. The averages for classes of a given length can be calculated iteratively from averages over shorter paths. This expansion is formally exact and finite for a finite number of modes. Usually only a limited number of terms has to be evaluated in order to obtain converged results. The scaling of the computational effort with respect to the number of time steps and the number of modes is given by a low-order power law, depending on the chosen class structure and the order of the expansion. The usual time-dependent wave-packet propagation and the full path enumeration, which exhibit an exponential scaling behavior with respect to either the number of modes or the number of time steps, can be considered as opposite limiting cases of the correlation expansion (CE) of the path integral. The convergence of the CE is tested by application to a two-state four-mode model representing S1-S2 vibronic coupling in pyrazine, for which exact references (time-dependent correlation functions) are available. The potential of the CE approximation for the treatment of multi-mode problems is demonstrated by application to an extended 24-mode vibronic-coupling model. This model is suitable to provide a microscopic description of ultrafast optical dephasing processes in large molecules.
Molecular dynamics simulation of the wave vector-dependent static dielectric properties of methanol–water mixtures
Authors: Skaf Munir S. , Ladanyi Branka M.
Published: July 2002

The static wave vector-dependent dielectric properties of methanol–water mixtures at room temperature are investigated through molecular dynamics simulation. We report results for the transverse and longitudinal components of the permittivity tensors, as well as the short- and long-ranged portions of the Kirkwood g factors at small wave vectors k, calculated from the mean squared dipole densities for mixtures of different compositions. In addition, we have calculated dipolar symmetry projections h110(r) and h112(r) of the pair distribution functions for each mixture and determined the asymptotic behavior of the second projection at large distance r. We find that h110(r) and h112(r) as well as their contributions from different molecular pairs are strongly composition dependent, but that the resulting Kirkwood correlation factor is nearly independent of composition. We obtained the dielectric tensor ε(k) over a wide range of k using Fourier–Hankel transforms of h110(r) and h112(r). Our results indicate that this real space route is an attractive alternative approach to the static, k-dependent dielectric properties of polar liquids. The k-dependent transverse components of the permittivities obtained this way are in good agreement with the values calculated from the mean squared dipole densities. The longitudinal permittivities, on the other hand, show a less satisfactory agreement at small wave vectors due to inaccuracies in the numerical transforms.
The glass transition of charged polymer colloids
Authors: Härtl W. , Versmold H. , Zhang-Heider X.
Published: July 2002

Dynamic and static light scattering is applied to concentrated suspensions of a new species of charged colloidal suspensions with a low refractive index. The polymer colloids which were obtained by polymerizing perfluorobutylacrylate could be index matched in a water/glycerol mixture. At volume fractions Φ greater than 0.22 a glass transition can be observed, which is manifested by the nondecaying components of the intermediate scattering functions. The intermediate scattering functions for a supercooled liquid sample and the predictions of the mode-coupling theory for the β relaxation are compared over a large Q range. By using an exponent parameter λ=0.76 a good agreement between theory and experiment can be obtained.
Vibrational spectroscopy of the chlorobenzene cation using zero kinetic energy photoelectron spectroscopy
Authors: Wright Timothy G. , Panov Sergey I. , Miller Terry A.
Published: July 2002

The one-color (1+1) resonance-enhanced multiphoton ionization (REMPI) spectrum of the S1(1B2) state of chlorobenzene has been recorded and is similar to that obtained by other workers. Zero-kinetic-energy (ZEKE) photoelectron spectroscopy was then used to probe the vibrational levels in the ground electronic state of the cation (I+0) using a two-color photoionization scheme via the S1 electronic state. By using different intermediate vibrational levels in the S1 state, different vibrations could be accessed in the ion. Vibrational symmetry selection rules for the I+0←S1 ionization appear to hold well. Exciting through different S1 vibrational levels has revealed the probable mixing of the S1 normal coordinates in I+0. A previously-identified Fermi resonance in the S1 state is also confirmed by the ZEKE spectra. The adiabatic ionization energy is measured as 73 170±5 cm−1.
Electronic spectroscopy of jet-cooled iron monocarbide. The 3Δi←3Δi transition near 493 nm
Authors: Balfour Walter J. , Cao Jianying , Prasad C. V. V. , Qian Charles X. W.
Published: July 2002

We report the first gas phase spectroscopic study of iron monocarbide. FeC molecules were generated in a laser vaporization molecular beam source and detected by laser induced fluorescence. Twenty-six vibronic bands have been recorded in the 430–500 nm region. Rotational analyses have been carried out for 22 of the bands. There are three lower states involved, two with Ω″=2 and one with Ω″=3. Based on our experimental observations and the ab initio calculations on RuC [Shim et al. J. Phys. Chem. 91, 3171 (1987)] the observed bands are interpreted as 3Δi←3Δi and 1Δ←1Δ transitions. The ground state electronic configuration is assigned to 1δ39σ1. Evidence for three excited electronic states (one 3Δi state, one 1Δ state and another state with Ω′=3) and for perturbations in several of their vibrational levels is presented.
Pulsed-nozzle Fourier-transform microwave investigation of the large-amplitude motions in HBr–CO2
Authors: Rice J. K. , Lovas F. J. , Fraser G. T. , Suenram R. D.
Published: July 2002

Microwave spectra of H79Br–CO2 and H81Br–CO2 and their D and 18O isotopomers have been measured using a pulsed-nozzle Fourier-transform microwave spectrometer. The spectra are consistent with a T-shaped Br–CO2 geometry, as concluded previously by Zeng et al. [Y. P. Zeng, S. W. Sharpe, S. K. Shin, C. Wittig, and R. A. Beaudet, J. Chem. Phys. 97, 5392 (1992)] from an investigation of the rotationally resolved infrared spectrum of the asymmetric C=O stretching vibration of the complex. Only b-type Ka=1←0 transitions are observed, with the symmetry-allowed a-type ΔKa=0 transitions being too weak to be detected. The absence of a strong a-type spectrum implies that the HBr axis is nearly parallel to the b-inertial axis of the complex, which itself is parallel to the C∞ axis of the CO2. The Ka=1←0 energy level spacing is approximately 1.2 GHz larger than that predicted from the infrared rotational constants due to an additional contribution to the splitting arising from the hindered-rotation tunneling of the HBr through a Cs or C2v transition state. Because the Bose–Einstein statistics of the spin-zero oxygen nuclei allow only symmetric tunneling states for Ka even and antisymmetric tunneling states for Ka odd, no doubling of the lines is observed. No evidence was obtained for this tunneling motion in the infrared spectrum of Zeng et al., since the tunneling state selection rules are symmetric↔symmetric and antisymmetric↔antisymmetric for the band studied. A dynamical modeling of the 79Br and 81Br nuclear quadrupole coupling constants gives an equilibrium ∠CBrH angle of ∼103° and an HBr zero-point bending amplitude of ∼24°. The implication of this study on the interpretation of experiments on the photoinitiated reaction of H atoms with CO2 using an HBr–CO2 precursor are discussed.
Predissociation mechanism and spin-rotation constant of the HCO B̃ 2A′ state
Authors: Lee Shih-Huang , Chen I-Chia
Published: July 2002

Formyl radicals produced from photolysis of acetaldehyde at 310 nm were supersonically cooled and detected via the B̃–X̃ transition using the laser-induced fluorescence (LIF) technique. Spectra at 0.16 cm−1 resolution and fluorescence lifetimes of HCO B̃(0,0,0), (0,0,1), and (0,1,0) levels were measured. The observed lifetimes decrease rapidly with variation of the rotational quantum number Ka from 0 to 2 but slowly with the rotational quantum number N from 0 to 8. Experimental data indicate that the B̃ state is coupled to a predissociating state via an a-type Coriolis interaction to account for the rotationally dependent lifetime for the low vibrational levels of the HCO B̃ state. Correction of the fluorescence quantum yield for individual rotational states is necessary to obtain accurate ground state populations by LIF when using the B̃–X̃ transitions. The intensity distribution of the two spin states observed implies a negative value of the spin-rotation parameter for the B̃ state, in contrast with the ground electronic state.
Density-functional calculation of core-electron binding energies of C, N, O, and F
Authors: Chong Delano P.
Published: July 2002

The unrestricted generalized transition-state model using B88/P86 functional with Dunning’s cc-pV5Z basis set, found to be an excellent method of calculating core-electron binding energies (CEBEs), was further applied to many more molecules, some of which contain atoms from the third period. Estimation of relativistic corrections has also been refined. The average absolute deviation of over 50 calculated CEBEs from experiment is 0.30 eV before inclusion of approximate relativistic corrections (Crel), and 0.23 eV after adding Crel. Those molecules with observed CEBEs served to confirm our procedure, whereas the other cases provided our prediction of CEBEs.
Observation of an indirect pathway in the femtosecond study of alkyl nitrite photodissociation in the S1 state
Authors: Mestdagh J. M. , Berdah M. , Dimicoli I. , Mons M. , Meynadier P. , d’Oliveira P. , Piuzzi F. , Visticot J. P. , Jouvet C. , Lardeux-Dedonder C. , Martrenchard-Barra S. , Soep B. , Solgadi D.
Published: July 2002

The present work has addressed the question of the dissociation time of four alkyl nitrites upon photoexcitation in the S1 state (methyl nitrite, n- , and t-butyl nitrites and i-amyl nitrite). The time resolved Laser Induced Fluorescence technique has been used in the femtosecond regime under bulk conditions. The photodissociation has been initiated at 351 nm by 150 fs pump pulses, and has been probed using a two-photon process at 467 nm by 200 fs pulses. The LIF signal has qualitatively the same shape for the four nitrites: it passes through a transient peak before reaching a plateau. The two-photon process that induces the detected fluorescence is nonresonant for detection of the dissociation product NO through the A←X transition. Conversely, the two-photon process is resonant or quasiresonant for detection of the excited nitrite molecule in the S1 level before it dissociates. This leads to an enhanced detection efficiency of the non-dissociated excited molecule versus that of the NO fragment. A simple kinetic model has been developed to account for this detection scheme. It shows that the transient peak is observable only if the lifetime of the excited molecule is comparable or larger than the temporal width of the lasers. In that case, the model allows the determination of an effective lifetime of the excited molecule from the measured LIF signal. Lifetimes have been found in the range of 125 fs for the four nitrites investigated. This has allowed us to answer a controversy on the dissociation mechanism of methyl nitrite and to confirm that an indirect pathway exists in the photodissociation of this molecule.
A charge exchange model of sequential fragmentation applied to dissociative double ionization of hydrazoic acid and halogen azides
Authors: Hsieh S. , Eland J. H. D.
Published: July 2002

A model of charge separation reactions including charge exchange between fragments is developed to include sequential reactions. The model uses known molecular structures, assumes an exponential distribution of intermediate lifetimes, and requires a mean lifetime as the only disposable parameter. The general predictions and quantitative calculations of relative intensities, peak widths, and peak shapes are shown to agree with photoelectron–photoion–photoion coincidence spectra of HN3, ClN3, and IN3 and other small molecules studied previously.
Potential inversion via variational generalized inverse
Authors: Zhang Dong H. , Light John C.
Published: July 2002

The determination of potential energy surfaces (PES) from values calculated ab initio at a set of points or from spectral data (vibration–rotation energy level information and rotation constants) are important and often difficult problems. The former is a ‘‘potential interpolation’’ problem, the latter a ‘‘potential inversion’’ problem. These are indeterminate problems in which the known data is insufficient to yield a unique solution. We present here a new constrained variational approach to the direct solution of these problems. The constraints are the known exact values of the potential or the exact perturbation corrections desired. The variational functional is chosen to provide control of the magnitude and smoothness of the correction function or potential. The method is very simple, very fast computationally, and yields exact solutions to the perturbation or interpolation equations in a single application. The potential inversion is completed by iteration to converge the perturbation solutions for a given set of assigned levels, and then by repeating with additional levels assigned in sequence to the data set to yield a physically acceptable PES very quickly. This procedure yields a smooth PES from which the energy levels in the dataset are calculated exactly. Information on rotational constants may also be used. Both interpolation and inversion procedures are applied to the the two dimensional (R,θ) PES for ArOH(A 2Σ+). A combined application of these two procedures is also presented in the paper, where we first interpolate a PES from ab initio points and then correct the ab initio fitted surfaces using spectral data.
Solvation dynamics in hard-sphere solvents
Authors: Evans Glenn T.
Published: July 2002

The time correlation function for the solvation energy fluctuation is calculated in context of a hard-sphere kinetic theory. The theory has three ingredients: a frequency measuring the thermal fluctuations in the potential, a frequency measuring the rate of passage of a solvent–solvent pair through the interaction region, and a memory function representing the longer time dynamics associated with particle recollisions. The calculated time correlation function is determined wholly in terms of hard-sphere properties, and when compared with a recent molecular dynamics simulation, the agreement is surprisingly good. The presence of fully correlated recollisions is made known by the appearance of a plateau in the time correlation function.
Ab initio studies of CN adsorbed on Ni(111)
Authors: Yang Hong , Caves Thomas C. , Whitten Jerry L.
Published: July 2002

The adsorption of cyanide (CN) on Ni(111) is treated using an ab initio embedding theory. The Ni(111) surface is modeled as a three-layer, 28-atom cluster with the Ni atoms fixed at bulk lattice sites. The present calculations show that CN is able to bind to the surface either via the carbon, or nitrogen, or in a side-on geometry with very small differences in total energy (≊0.1 eV). Adsorption energies at threefold, bridge and atop sites are comparable, with the fcc threefold site more favorable over other adsorption sites by ≊0.1 eV. At the fcc threefold site, adsorption energies and C–N stretching frequencies are 5.0 eV and 2150 cm−1 for the η1-cyanide-N, 4.9 eV and 1970 cm−1 for the η1-cyanide-C, and 4.9 eV and 1840 cm−1 for the η2-cyanide-C,N, respectively. Dipole moment calculations show that the bonding of CN to the Ni surface is largely ionic, while η2-cyanide-C,N has more covalent character. Calculated energy barriers in going from η1-cyanide-C to η2-cyanide-C,N, and from η2-cyanide-C,N to η1-cyanide-N are around 0.1 eV. Thus although CN is strongly bound to the surface (at ≊5 eV), within an energy range of ≊0.2 eV, the molecule is free to rotate to other geometries. During this rotation there are large changes in the dipole moment.
Electronic transitions of cobalt carbide, CoC, near 750 nm: A good example of case (bβS) hyperfine coupling
Authors: Barnes M. , Merer A. J. , Metha G. F.
Published: July 2002

The laser induced fluorescence spectrum of jet-cooled CoC near 750 nm has been measured at high resolution following the reaction of laser-ablated cobalt atoms with methane. The X2Σ+ ground state of CoC is an unusually good example of Hund’s case (bβS) coupling. Since Co has a nuclear spin I=7/2, each rotational level is split by the Fermi contact interaction into G=3 and G=4 components, where G=I+S; the splitting for N=0 is more than 0.5 cm−1. The X2Σ+ state begins to uncouple toward case (bβJ) with increasing rotation. Transitions to various 2Π excited states occur in the region 13 000–14 500 cm−1; the most prominent of these (for which high resolution spectra have been recorded) lie at 13 079 cm−1 (2Π3/2) and 13 343 cm−1 (2Π1/2). The (bβS) coupling in the ground state produces some unexpected hyperfine intensity patterns, which have been studied in detail. A very low-lying 2Δi state, whose Ω=5/2 and Ω=3/2 components lie at 221 and 1173 cm−1, has been identified. Laser excitation of the 2Π3/2–2Δ5/2 transition has been observed by monitoring the strong 2Π3/2–X2Σ+ emission, which has allowed the 2Δ5/2 state to be characterized at high resolution. A total of 879 rotational-hyperfine transitions between the various 2Π1/2, 2Π3/2, 2Δ5/2, and 2Σ+ states have been assigned and fitted. Matrix elements for a 2Σ+ state in case (bβS) coupling are listed.
On the time decay of the photoinduced condensation in supersaturated vapors
Authors: Baranovskii S. D. , Dettmer R. , Hensel F. , Uchtmann H.
Published: July 2002

Calculations are carried out to solve the thus far puzzling problem of the long-time trends in the photoinduced condensation of supersaturated vapors observed in thermal diffusion cloud chambers. These trends are the long-time delay of the condensation after switching the light on and the long-time decay after terminating the light. The calculations show that the supersaturation level which governs the condensation process cannot be affected significantly by irradiation under conventional experimental circumstances, and hence its change cannot be the cause of the long-time transients. These trends are attributed to the slow diffusion-limited growth of liquid droplets before they achieve the observable size of 10−3–10−2 cm. The predictions are in good agreement with experimental data.
Structure of the CCCN and CCCCH radicals: Isotopic substitution and ab initio theory
Authors: McCarthy M. C. , Gottlieb C. A. , Thaddeus P. , Horn M. , Botschwina P.
Published: July 2002

The millimeter-wave rotational spectra of the 13C isotopic species of the CCCCH and CCCN radicals and CCC15N were measured and the rotational, centrifugal distortion, and spin-rotation constants determined, as previously done for the normal isotopic species [Gottlieb et al., Astrophys. J. 275, 916 (1983)]. Substitution (rs) structures were determined for both radicals. For CCCN, an equilibrium structure derived by converting the experimental rotational constants to equilibrium constants using vibration–rotation coupling constants calculated ab initio was compared with a large-scale coupled cluster RCCSD(T) calculation. The calculated vibration–rotation coupling constants and vibrational frequencies should aid future investigations of vibrationally excited CCCN. Less extensive RCCSD(T) calculations are reported here for CCCCH. The equilibrium geometries, excitation energies (Te), and dipole moments of the A2Π excited electronic state in CCCN and CCCCH were also calculated. We estimate that Te=2400±50 cm−1 in CCCN, but in CCCCH the excitation energy is very small (Te=100±50 cm−1). Owing to a large Fermi contact interaction at the terminal carbon, hyperfine structure was resolved in 13CCCCH. Measurements of the fundamental N=0→1 rotational transition of CCCCH with a Fourier transform spectrometer described in the accompanying paper by Chen et al., yielded precise values of the Fermi contact and dipole–dipole hyperfine coupling constants in all four 13C species. The Fermi contact interaction is approximately two times larger in CCCN, allowing a preliminary estimation of hyperfine coupling constant bF in 13CCCN and C13CCN from the millimeter-wave rotational spectra.
The 2345 multimode resonance in acetylene: A bifurcation analysis
Authors: Rose John P. , Kellman Michael E.
Published: July 2002

This paper reports on a classical phase space bifurcation analysis of the 2345 Fermi resonance of acetylene. The 2345 Fermi resonance is a multimode nonlinear, resonance coupling that is important to the vibrational dynamics and energy flow of highly excited acetylene. The bifurcation analysis is performed on an integrable Hamiltonian that represents a planar five-mode model of acetylene in which the ν2, ν3, ν4, and ν5 vibrational modes are nonlinearly coupled through the 2345 Fermi resonance. The phase space structures of the 2345 Fermi resonance are shown to be analogous to but more complicated than phase space structures of the two-mode, 1:1 and 2:1 Fermi resonance. The results are presented in terms of bifurcation diagrams and molecular catastrophe maps. The bifurcation analysis of this multidimensional system with a complicated multimode resonance is a step beyond the simple integrable, resonantly coupled two-mode systems that are now well understood. Analysis of this integrable system also represents a necessary step toward using a multiresonance, i.e., ‘‘chaotic’’ model to decipher the vibrational spectra of highly excited acetylene, based on knowledge of the anharmonic modes born from bifurcations of the low-energy normal modes.
Anharmonic oscillator representation of nonlinear optical susceptibilities of a charged soliton, a neutral soliton, and a polaron in conjugated polymers
Authors: Takahashi Akira , Mukamel Shaul
Published: July 2002

The signatures of a charged soliton, a neutral soliton, and a polaron in the resonant and off-resonant optical susceptibilities of conjugated polymers are calculated using the Pariser–Parr–Pople (PPP) model which includes Coulomb interactions. The optical response is obtained by solving equations of motion for the reduced single-electron density matrix, derived using the time dependent Hartree–Fock (TDHF) approximation. The density matrix clearly shows the electronic structures induced by the external field. The roles of charge density, spin density, bond order, and spin bond order waves and how they contribute to the optical response are analyzed. Both charged and neutral solitons show one absorption peak inside the gap of the half-filled case; the frequency of the neutral soliton peak is about 0.7 eV higher than that of a charged soliton. A polaron shows two absorption peaks inside the gap.
Interaction-induced contributions to polarizability anisotropy relaxation in polar liquids
Authors: Ladanyi Branka M. , Liang Ying Q.
Published: July 2002

We use molecular dynamics simulation to investigate polarizability anistropy relaxation in two polar liquids, methanol and acetonitrile, which have similar dielectric constants at room temperature, but are very different at the molecular level. Interaction-induced contribution to the polarizability is included using first-order perturbation theory and separated into a component which projects along the sum of molecular polarizability anistropies and relaxes through collective reorientation and a ‘‘collision induced’’ component which relaxes through other mechanisms involving mainly translational motion. We find that interaction-induced effects on the polarizability anisotropy time correlation are important on all relevant time scales, especially for the more polarizable acetonitrile. In methanol, even though most of the molecular polarizability is along the CO bond, we find that the OH bond dynamics make a substantial direct contribution to polarizability anisotropy relaxation. We compare our results to the experimentally determined nuclear portion of the optical Kerr effect response and discuss their implications for the use of this response in solvation dynamics theories. We find that the short-time optical Kerr response of acetonitrile is dominated by collision-induced polarizability dynamics, while librational orientational dynamics is the main contributor for methanol.
Core electron chemical shifts in conjugated molecules and polymers
Authors: Gel’mukhanov Faris , Ågren Hans
Published: July 2002

Modern experiments and quantum calculations have revealed interesting trends in core electron spectra of oligomers and polymers. In particular, the core electron chemical shifts show salient features that are structurally dependent, but which have not been completely understood. From π-electron theory we derive a model for core electron binding energy shifts in conjugated molecules and polymers. The alternant behavior of the site dependent shifts in the linear and polycyclic hydrocarbons are predicted by the model. The alternations show end-to-bulk amplification, respectively, damping for the cyclic and the linear polyenes. The polycyclic hydrocarbons show the strongest alternation effects for the bulk atoms, while for the polyenes the alternation is strongest for the terminal atoms. The analysis is based on renormalized perturbation theory and on the analytic solutions of the Hückel equations. The theory and the proposed model can be applied also for investigations of the influence of impurities on physical properties of polymers.
Decomposition reactions as general Poisson processes: Theory and an experimental example
Authors: Rydén Tobias , Wernersson Mikael
Published: July 2002

The classical theory of decomposition reaction kinetics depends on a ‘‘large scale’’ assumption. In this paper we show how this assumption can be replaced by the assumption that the nucleation process is a space–time Poisson process. This framework is unifying in the sense that it includes many earlier formulas as special cases, and it naturally takes boundary effects into account. We consider the conversion of a sphere in detail, and fit the parameters of this model to gypsum decomposition experimental data. The so obtained model shows, for this particular reaction, that the boundary effects decrease with temperature.
Calculation and interpretation of total energies in electron propagator theory
Authors: Ortiz J. V.
Published: July 2002

Ground state total energies and one-electron density matrices can be calculated from contour integrals over the electron propagator. Ionization energies and corresponding Feynman–Dyson amplitudes are related simply to ground state properties. Total energy formulas derived from electron propagator theory are transparent generalizations of Hartree–Fock expressions. Computationally useful methods for evaluating integrals over the Coulson contour are derived. An approximate integration scheme is introduced and compared to exact results. Several decouplings of the electron propagator that have been employed frequently for electron binding energies are used to calculate size-extensive total energies. These methods do not yield satisfactory correlation energies, but they provide a reasonable account of bending potentials for water, ammonia, and methane. Total energy contributions derived from propagator poles and residues are calculated as a function of bond angle distortions. These results are compared with simple molecular orbital notions that seek to explain the instability of D∞h water, D3h ammonia, and D4h methane with respect to the equilibrium geometries of these molecules.
On the nature of the intramicellar and intermicellar forces governing the liquid crystalline phase behavior of aqueous solutions of disklike micelles
Authors: Boden N. , Harding R. , Gelbart W. M. , Ohara P. , Jolley K. W. , Heerdegen A. P. , Parbhu A. N.
Published: July 2002

The liquid crystalline phase diagrams for aqueous solutions of the homologous series of surfactants CF3(CF2)nCO−2Cs+ (n=5–8) have been determined. They all exhibit the classical isotropic (I)-to-discotic nematic (ND) and ND-to-smectic lamellar (L) sequences of transitions with increasing concentration, as previously established for the n=6 system [N. Boden, S. A. Corne, and K. W. Jolley, J. Phys. Chem. 91, 4092 (1987)]. The effect of increasing n is to displace the transitions to higher temperatures. The behavior of all of the surfactant systems can be represented on a universal phase diagram. Both the I-to-ND and the ND-to-L transitions at corresponding concentrations are found to occur when the axial ratio of the disklike micelles attains a singular value in each case, irrespective of the value of n. The form of the experimental phase diagrams can be qualitatively understood in terms of a simple ‘‘zeroth’’ order theory which uses the results of Onsager’s theory [L. Onsager, Ann. NY Acad. Sci. 51, 627 (1949)] applied to disks to find the critical axial ratios in the coexisting isotropic and nematic phases and, separately, a dilute solution, self-assembly theory of disklike micelles [W. E. McMullen, A. Ben-Shaul, and W. M. Gelbart, J. Colloid Interface Sci. 98, 523 (1984)] to determine the temperatures at which these axial ratios are achieved at each concentration. The same treatment is also shown to account for the experimental phase behavior of mixed-chain-length systems.