bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermodynamic stability of phases and transition kinetics under adiabatic conditions
Authors: Umantsev A.
Published: July 2002

A study of equilibrium states of a thermodynamic system whose evolution is governed not only by the temperature, but also by the ordering field is carried out. It is found that an adiabatically insulated system may have a new type of nonuniform state of equilibrium which is inhomogeneous in temperature. The comparison is made of the stability conditions in isothermal and adiabatic systems. The steady motion of an interface boundary during a first-order phase transition is investigated. It is shown, that depending upon the values of the diffusion coefficients, different regimes can exist. For small thermal diffusivity, the temperature of the final phase after the exothermal transition can be above the equilibrium point. The kinetic problem is reformulated to a dynamical system, and a numerical procedure to solve the latter is presented. Numerical results are discussed in comparison with analytic ones.
Influence of bulk diffusion on the adsorption of hard spheres on a flat surface
Authors: Senger B. , Schaaf P. , Voegel J. C. , Johner A. , Schmitt A. , Talbot J.
Published: July 2002

Irreversible adsorption of hard spheres onto a solid surface is analyzed by using a generalization of the ‘‘random sequential adsorption’’ (RSA) model: ‘‘diffusion random sequential adsorption’’ (DRSA). In addition to the irreversible nature of the adsorption process and exclusion effects of the adsorbed configurations of hard spheres, the new model also considers the interactions between adsorbed particles and particles from the bulk, diffusing toward the surface. It is shown, in particular, that this affects the structure of adsorbed configurations for coverages different from the jamming limit coverage θ∞. Surprisingly, θ∞ appears to be identical for configurations generated by RSA and DRSA algorithms. Moreover, the structures of the configurations, as characterized by the radial distribution function g(r), are also identical at the jamming limit, whereas they differ for lower coverages. The coupling between the bulk diffusion process and the ‘‘adsorption process’’ is also analyzed as a function of the coverage. New approximate boundary conditions at the interface for the adsorption–diffusion equations are derived and the adsorption rate is calculated for a particular example.
Small-angle neutron scattering study on weakly charged temperature sensitive polymer gels
Authors: Shibayama Mitsuhiro , Tanaka Toyoichi , Han Charles C.
Published: July 2002

The static structure factor for poly(N-isopropylacrylamide-co-acrylic acid) (NIPA/AAc) gels was investigated in terms of small-angle neutron scattering (SANS). The NIPA/AAc gels underwent a discrete volume phase transition at 50.8 °C from swollen to shrunken states as temperature increased. The static structure factors for NIPA/AAc gels were well described by a Lorentz-type scattered intensity function at temperatures below 34 °C which was near the so-called Θ temperature of NIPA in D2O. At higher temperatures, the static structure factor had a distinct scattering maximum although the gel was still in the highly swollen state. The scattering maximum was located around q=0.02 Å and was temperature and concentration dependent, where q is the magnitude of the scattering vector. The appearance of the peak indicates the strong concentration fluctuations that created polymer rich and poor regions in the system. The contrast in the concentrations appeared to result from the competing interactions in the network, i.e., the electrostatic repulsive interaction between AAc segments on the network and the hydrophobic attractive interaction among NIPA segments. The static structure factors were analyzed quantitatively with the theory of Borue and Erukhimovich for polyelectrolyte solutions in a poor solvent. It is found that the concentration fluctuations lead to a microphase separation between polymer rich and poor domains before the system undergoes the volume-phase transition.
A simple model for dense phases of two-dimensional hard rods and its application to mono- and bidisperse alkanes physisorbed on graphite
Authors: Hentschke R. , Askadskaya L. , Rabe J. P.
Published: July 2002

Based on a simple mean field model, we study the positional ordering in dense systems of mono- and bidisperse parallel hard rods in two dimensions. We obtain phase diagrams exhibiting nematic, columnar, and crystalline order as a function of the packing density, the lengths, and the mixing ratio of the particles. Even though our theory neglects the details of the molecular interactions, it proves to be applicable to physisorption of large anisotropic molecules on smooth surfaces. This is demonstrated on the basis of scanning tunneling microscopy experiments on dense monolayers of highly ordered alkane chains physisorbed on graphite. Crystalline as well as columnar orderings are observed as a function of molecular weight for monodisperse adsorbates and, in addition, in terms of the mixing ratio for bidisperse adsorbates, in accord with the predictions of the model.
Free energy perturbation calculations on multiple mutation bases
Authors: Yoneda Shigetaka , Umeyama Hideaki
Published: July 2002

A new method of using multiple potential functions (mutation bases) in free energy perturbation calculations is proposed to make more effective mutation pathways. The method, which is a generalization of the conventional coupling parameter approach, allows pathways of more freedom and of smoothly curved shapes to be produced. Molecular dynamics simulations were carried out by the method to derive relative free energy on a model system made of 64 four-atom molecules in the periodic boundary condition. Effectiveness of five mutation pathways was compared using the resulting free energy values and the final ratios of trans conformations. For confidence, 15 free energy perturbation calculations were carried out for each shape and length of the mutation pathways, and means and standard errors were calculated. Results showed that one of the pathways, which is impossible without the multiple mutation bases, is more effective than the down-scaled pathway proposed by Mark et al. [J. Chem. Phys. 94, 3808 (1991)], and is much more effective than the simple mutation of the conventional coupling parameter approach.
Chain length and density dependence of the chemical potential of lattice polymers
Authors: Szleifer Igal , Panagiotopoulos Athanassios Z.
Published: July 2002

The chemical potential of lattice polymers is calculated in two alternative ways: by the Widom insertion method using the Rosenbluth and Rosenbluth sampling technique and by the modified Widom method, based on the insertion of one segment to an existing polymer chain. In the first part of this paper we present a detailed derivation of the modified Widom technique for lattice systems. We then proceed to calculate the chemical potential for chains of up to 50-mers in monomeric and polymeric solvents. We observe marked odd-even effects on the chemical potential. The density dependence of the chemical potential is found to vary with chain length. For most temperatures and densities studied the chemical potential of chain molecules in a fixed environment becomes linear in chain length for molecules longer than 10–20 segments. The results are compared to the classical lattice theories, which are found to be best at high densities, as expected based on previous investigations.
Random sequential adsorption on a line: Mean-field theory of diffusional relaxation
Authors: Privman Vladimir , Barma Mustansir
Published: July 2002

We develop a new fast-diffusion approximation for the kinetics of deposition of extended objects on a linear substrate, accompanied by diffusional relaxation. This new approximation plays the role of the mean-field theory for such processes and is valid over a significantly larger range then an earlier variant, which was based on a mapping to chemical reactions. In particular, continuum-limit off-lattice deposition is described naturally within our approximation. The criteria for the applicability of the mean-field theory are derived. While deposition of dimers, and marginally, trimers, is affected by fluctuations, we find that the k-mer deposition kinetics is asymptotically mean-field-like for all k=4,5,...,+∞, where the limit k→∞, when properly defined, describes deposition-diffusion kinetics in the continuum.
Microwave and millimeter-wave spectra, electric dipole moment, and internal rotation effects of methyl hydroperoxide
Authors: Tyblewski M. , Ha T.-K. , Meyer R. , Bauder A. , Blom C. E.
Published: July 2002

The microwave spectrum of methyl hydroperoxide which has been investigated in the range of 12–307 GHz is dominated by the internal rotation of the OH group. The molecule exists in a gauche equilibrium configuration. Pure rotational transitions as well as much stronger torsional–rotational transitions have been assigned. A model of the overall rotation for a molecule with two strongly coupled tunneling states has been used for the analysis of all assigned transitions. Quantum-mechanical tunneling between the two equivalent gauche minima splits the ground torsional state by 448.7603(1) GHz. Additional small splittings of the microwave transitions have been attributed to the internal rotation of the CH3 group with a threefold barrier of 1120(10) cm−1. The permanent electric dipole components have been estimated from Stark splittings. Ab initio calculations of the molecular structure and of the potential functions for internal rotations of the OH and CH3 groups complement the experimental study.
Raman scattering evidence of a ferroelastic distortion at 105 K in K2ZnCl4
Authors: Hedoux A. , Noiret I. , Guinet Y. , Odou G. , Lefebvre J.
Published: July 2002

Raman scattering measurements on K2ZnCl4 have been performed from 20 up to 250 K in the lattice mode frequency range and in the Zn–Cl stretching mode region. The splitting of certain modes in the low-frequency region and a change in the slope of the frequency vs temperature plot at 105 K show the existence of a phase transition at this temperature. Characteristic phenomena are also observed for the phase transition near 150 K.
Two-photon dissociation of HD+ with one intermediate resonance in the presence of two different laser fields
Authors: Datta Banani , Bhattacharyya S. S.
Published: July 2002

The time-dependent dissociation probabilities, line shapes of dissociation rate, and photofragment kinetic-energy distribution have been investigated for resonant two-photon dissociation of HD+ from the 1sσg(v=0, J=0) state in the presence of two radiation fields of different frequencies ω1 and ω2. Simultaneous dissociation in two different electronic states at two different final energies occur through the absorption of either ω1+ω2 or 2ω2 photons. Only one of the two fields causes a near-resonant coupling either with (v=6, J=1) through absorption of a photon of frequency ω1 or with the (v=14, J=1) state through absorption of a photon of frequency ω2. Dissociation probabilities have been calculated as functions of the resonant and off-resonant field amplitudes and times. The dissociation probability at fixed times show interesting variation with the field amplitudes. Conditions for time-dependent interference oscillations in dissociation probabilities are discussed and these oscillations demonstrated. Intensity-dependent line shapes for dissociation rate are obtained for various other combinations of fields. Long-time kinetic-energy distribution of photofragments is similar to the energy spectrum of photoelectrons obtained from atomic autoionization. The photofragment spectra for two energies in two final electronic continuum states are compared for different values of the field amplitudes and detunings.
Stability of van der Waals compounds and investigation of the intermolecular potential in helium–xenon mixtures
Authors: Barrat Jean-Louis , Vos Willem L.
Published: July 2002

The stability of the stoichiometric solid structures Xe(He)2 and Xe(He)13 is investigated up to 30 GPa pressure at room temperature, T=300 K. The Gibbs free energy of these so-called van der Waals compounds is computed using simple semianalytical methods that are in good agreement with molecular dynamics computer simulations. The interatomic interactions are described by empirical pair potentials adjusted to fit high pressure experimental data. The Xe(He)2 phase is found to be stable with respect to phase separation into pure components, while Xe(He)13 is unstable. These results agree with simple packing considerations, but are rather sensitive to the precise form of the helium–xenon interaction. A brief comment is made about the occurrence of a Ne(He)2 compound, that was recently found by Jean-Louis et al.
Dipole moment differences of nonpolar dyes in polymeric matrices: Stark effect and photochemical hole burning. I
Authors: Altmann R. B. , Renge I. , Kador L. , Haarer D.
Published: July 2002

The difference of the induced dipole moments Δμind between the ground and excited state of (centro-)symmetric dye molecules (tetraphenylporphin and tetrapropylporphycene) embedded in different nonpolar and polar polymers is investigated. The technique of photochemical hole burning combined with Stark effect measurements was used to determine the values as a function of the burning frequency. For almost all systems an increase of Δμind from higher to lower optical frequencies was found. A plot of the <Δμind≳ values measured at the absorption band maximum vs the low-temperature values of the dielectric constant ε shows a clear correlation between the dipole moment difference and the low-temperature ε values over the whole investigated range which is accessible with commercially available polymers. In order to evaluate the data the dielectric constant ε of each sample was determined in a series of separate experiments as a function of frequency and temperature.
Departures from the soft collision model for Dicke narrowing: Raman measurements in the Q branch of D2
Authors: Forsman J. W. , Sinclair P. M. , May A. D. , Duggan P. , Drummond J. R.
Published: July 2002

With high quality spectral data, we have observed departures from the soft collision model for translational motion, in the transition region from Doppler broadening to Dicke narrowing. The departures are in agreement with theoretical calculations based on the Boltzmann equation. The implications of the results concerning the dynamics of fluids are discussed. In addition we show that the mass diffusion constant describes the translational diffusion of the optical coherence and we give precise measurements of the broadening coefficients of the Q(0) to Q(6) lines.
Collision-induced dissociation of Fen+ (n=2–19) with Xe: Bond energies, geometric structures, and dissociation pathways
Authors: Lian Li , Su C.-X. , Armentrout P. B.
Published: July 2002

The kinetic energy dependence of the collision-induced dissociation (CID) of Fe+n (n=11–19) with Xe is studied by using a guided ion beam mass spectrometer. As previously reported for smaller iron cluster ions, these iron cluster ions dissociate exclusively by evaporation, the sequential loss of Fe atoms. Bond energies for the larger iron cluster ions D0(Fe+n−1–Fe), n=11–19, as well as updated values for the smaller cluster ions, n=2–10, are reported. These energies are derived after explicit consideration of the effect of secondary collisions between Fe+n and Xe, and of the lifetimes of the energized iron cluster ions. Bond energies for neutral iron clusters are derived by combining these ionic bond energies with literature ionization energies for Fen. Except for the dimers, we find that the bond energies of ionic and neutral iron clusters are very similar and change nonmonotonically as a function of cluster size, with local maxima at n=6, 7, 13, 15, and 19; and local minima at n=8, 14, and 18. Evidence for the existence of weakly bound isomers of iron cluster ions is also observed.
Non-Lorentzian depolarized Raman line shapes in n-pentanol
Authors: Di Fabrizio E. , Nardone M. , Nucara A. , Gallo P. , Ruocco G.
Published: July 2002

High resolution depolarized Raman spectra of n-pentanol in the −30 to 60° C temperature range are reported. The spectra appear as a continuously decreasing line shape centered at zero frequency. A quantitative analysis of this line shape shows a high frequency exponential tail, which can safely be assigned to intermolecular collision phenomena, and a low frequency narrow contribution. The line shapes of the low frequency contribution are seen to deviate from a single Lorentzian, the effect being more pronounced as the temperature is increased. The data have been analyzed assuming the non-Lorentzian line shape as due either to a distribution of Debye relaxation times or to a single non-Debye relaxation mechanism. The analysis in terms of a distribution of relaxation times suggests a broadening of the distribution as the temperature increases, while the ‘‘average’’ relaxation time shows an Ahrenius behavior which compares well with the one observed in the ultrasonic attenuation measurements. Similar results have been obtained considering the nonexponential relaxation mechanisms. Absolute intensities are also evaluated with respect to the Brillouin doublet and discussed in terms of the possible microscopic scattering mechanisms.
Infrared spectral evidence of N≡C–C≡C–N≡C: Photoisomerization of N≡C–C≡C–C≡N in an argon matrix
Authors: Smith Alice M. , Schallmoser Günter , Thoma Anton , Bondybey Vladimir E.
Published: July 2002

Ultraviolet (UV) laser photolysis of dicyanoacetylene, N≡C–C≡C–C≡N, isolated in an argon matrix at 16 K produces the iso-nitrile, N≡C–C≡C–N≡C, in sufficient concentration for direct Fourier-transform-infrared measurements of all five stretching vibrational fundamentals. The assignments for ν1 (2287.1 cm−1), ν2 (2203.6 cm−1), ν3 (2044.8 cm−1), ν4 (1202.3 cm−1), and ν5 (610.1 cm−1) are supported by a normal coordinate analysis using 20 vibrational frequencies from 7 isotopomers and by ab initio results of Botschwina et al. The di-isonitrile C≡N–C≡C–N≡C is also produced in small amounts and its infrared-active asymmetric stretches assigned as ν4 (2114.9 cm−1) and ν5 (1287.5 cm−1). As isotopic vibrational frequencies for N≡C–C≡C–C≡N were measured for the first time, the disputed assignments for the three totally symmetric stretching vibrations were re-examined through a normal coordinate analysis. Laser-induced fluorescence measured by a Fourier-transform-spectrometer during photolysis reveals the 1Σg+←3Σu+ electronic transition in dicyanoacetylene at T00=25 607 cm−1 and four vibronic transitions involving two totally symmetric stretching states of the ground electronic state.
Site-specific branching ratios for H-atom production from primary haloalkanes photolyzed at 193, 222, and 248 nm
Authors: Brum Jeffrey L. , Deshmukh Subhash , Wang Zhongrui , Koplitz Brent
Published: July 2002

Selectively deuterated compounds are used to investigate the ‘‘site-specific’’ nature of H-atom production resulting from the photolysis of primary haloalkanes. The molecules investigated are 1-iodopropane, 1-bromopropane, iodoethane, bromoethane, and chloroethane, with photolysis being initiated at 193, 222, and 248 nm. Hydrogen and deuterium isotopes are systematically used to label chemically distinct carbon atoms within a given molecule. H- and D-atom Doppler profiles are generated via two-photon (121.6+364.7 nm) ionization resonant with Lyman-α, and the relative H/D ratios are used to quantify the probability for hydrogen production from each carbon site. In general, photolysis of an intermediate, presumably the alkyl radical, is implicated as being a key step in the overall process. When using 248 nm radiation, the photolysis process is dominated by C–H (or C–D) bond cleavage at the β carbon position regardless of the system investigated. In contrast, results using 193 nm excitation display no obvious preference for bond cleavage at a particular carbon site, again regardless of the system. The data produced by 222 nm photolysis show a qualitative tendency for C–H (or C–D) bond cleavage at the β carbon, but the quantitative results are not as self-consistent as either the 248 nm or the 193 nm data. Also, the results of ab initio calculations on the ground and excited states of the ethyl radical are used to help interpret the observed site specificity.
Temperature dependence of the splay and bend viscosities of PAA
Authors: Devanand K.
Published: July 2002

Quasielastic laser light scattering is used to study the splay and bend modes of p-azoxyanisole (PAA) as a function of temperature. At values of the scattering wave vector, where more than one mode is present, extraction of single mode parameters is complicated by the need to fit the data to sums of exponentials. At the relatively large scattering angles at which the modes are isolated, the scattered signals are usually weak. By using the 647.1 nm line of a Krypton Ion laser, significant improvement in signal-to-noise ratios are achieved and by experimentally isolating a single mode, more reliable fits are obtained. The two viscosities, ηsplay and ηbend, are obtained from the decay rates of the director fluctuations using the K1 and K3 data from Freedricksz transition experiments performed at Cambridge [J. Phys. 40, 1513 (1985)]. An empirical approach is used to describe the temperature dependence of the viscosities. The nature of the coupling of the order parameter to the viscosities is believed to be responsible for the behavior of the two viscosities.
Free energy, entropy, and internal energy of hydrophobic interactions: Computer simulations
Authors: Smith David E. , Haymet A. D. J.
Published: July 2002

Hydrophobic interactions are investigated by molecular dynamics computer simulations of the free energy, entropy, and internal energy of association of two methane molecules in water. Recently we reported a computer simulation calculation of the entropy of association of methane in water [J. Am. Chem. Soc. 114, 5875 (1992)], where entropy drives nonpolar solutes together at short distances. Here this method is compared with two other general methods for the calculation of the entropic contribution to the free energy. The calculated thermodynamic quantities for methane association are in good agreement with available experimental measurements. Solute contact configurations are found to be of greater importance than solvent-separated configurations, in conflict with earlier theoretical and simulation studies of similar systems. In some cases, this conflict may be understood in terms of differences in the assumed, model intermolecular potential energies.
Vibrational state specificity and selectivity in the reactions N+OH→NO(v)+H and N+NO(v)→N2+O
Authors: Smith Ian W. M. , Tuckett Richard P. , Whitham Christopher J.
Published: July 2002

The vibrational state distribution of NO formed in the radical–radical reaction N+OH→NO(v′≤9)+H; ΔH00=−204.0 kJ mol−1 has been determined using a pulse-and-probe technique. OH radicals were generated by pulsed laser photolysis of H2O2 at 266 nm in a flow of N2 which had been passed through a microwave discharge to produce N atoms. The vibrational distribution of NO, measured by laser-induced fluorescence (LIF) spectroscopy, is similar to that predicted by phase-space theory and corresponds to an average yield of ca. 31% of the energy available to the reaction products. Experiments in which LIF signals were observed as the pulse–probe time delay was varied showed that populations within different vibrational levels (v=0–8) displayed similar kinetics, consistent with rapid removal of NO(v) by reaction with N atoms, N+NO(v)→N2+O, at rates which exhibit a mild dependence on v.
The water–methanol complexes. I. A matrix isolation study and an ab initio calculation on the 1-1 species
Authors: Bakkas N. , Bouteiller Y. , Loutellier A. , Perchard J. P. , Racine S.
Published: July 2002

The infrared spectrum of the CH3OH:H2O complex isolated in a nitrogen matrix is here reported. The complex is identified through observation of all three internal modes of the water moiety and 10 out of 12 vibrations of the methanol subunit. Several conformers of the CH3OH...OH2 are evidenced through ir photochemical and thermal conversion process, but none corresponds to the reverse CH3HO...HOH structure, with water playing the role of the proton donor. Ab initio calculations on both structures have been performed to obtain their equilibrium geometries and vibrational spectra. They allow to account for the ir spectral changes (frequencies as well as intensities) of the two submolecules interacting through hydrogen bonding within the complex.
Intermediate ordering in a liquid supported monolayer: A molecular dynamics study
Authors: Gao Jun , Rice Stuart A.
Published: July 2002

We report the results of molecular dynamics simulations of a water supported monolayer of F(CF2)6(CH2)6OC6H4COOH. A grazing incidence x-ray diffraction study of this system showed, unexpectedly, the absence of any strongly ordered condensed phase. We find that the monolayer has a partially ordered structure with a range of order intermediate between that typically found in monolayers of, e.g., arachidic acid, and that expected for a two-dimensional liquid. The calculated width of the first order diffraction peak is about ten times larger than for arachidic acid, yet is ten times smaller than expected for a two-dimensional liquid. We argue that monolayers with intermediate order represent a different class of systems that those heretofore studied, and that these systems may have some unusual properties.
The near ultraviolet photodissociation dynamics of azomethane
Authors: North Simon W. , Longfellow Cheryl A. , Lee Yuan T.
Published: July 2002

The photodissociation of azomethane following absorption of a single 351 nm photon was studied using the method of molecular beam photofragment translational spectroscopy. The dissociation was observed to proceed via cleavage of both C–N bonds to yield N2 and two methyl radicals. The measured time-of-flight spectra show evidence that the two methyl radicals possess unequal velocities in the azomethane center of mass suggesting that the dissociation is not symmetric. The angles between the asymptotic center-of-mass velocities for all three fragments are strongly correlated, implying that the methyldiazenyl radical (CH3N2) intermediate decomposes within a fraction of its rotational period. We conclude, therefore, that the dissociation is concerted, not stepwise as was inferred from recent time-resolved experiments. The overall translational energy distributions for all the photofragments in the azomethane center of mass reveal that an average of 60% of the total available energy appears as translation. A possible mechanism, consistent with the experimental findings, will be proposed and discussed.
Late stage dynamics of phase separation processes of binary mixtures containing surfactants
Authors: Kawakatsu Toshihiro , Kawasaki Kyozi , Furusaka Michihiro , Okabayashi Hirofumi , Kanaya Toshiji
Published: July 2002

Late stage dynamics of phase separation processes of immiscible binary mixtures containing surfactants (amphiphilic molecules) is investigated by computer simulations on the hybrid model proposed by the present authors [T. Kawakatsu and K. Kawasaki, Physica A 167, 690 (1990)]. With use of this hybrid model, one can investigate large scale phenomena while retaining the intramolecular structures of surfactant molecules. Simulations are performed both for irregular bicontinuous and micellar domain formation processes taking the thermal fluctuation effects into account. In the very late stage, the coarsening of the domain structures is considerably slowed down both for bicontinuous and micellar domain formation processes due to the low interfacial tension of the surfactant–adsorbed interfaces. Scaled scattering structure functions are also calculated, which possess the characteristic features of the experimentally observed scattering functions of microemulsions and polymer blends containing amphiphilic block copolymers. We found that the calculated structure function for the system containing surfactants is different from that for the simple binary mixture without surfactant. Also reported are the important effects of thermal fluctuations on micellar domain structures, where the domain formation is considerably accelerated by the thermal fluctuation.
On the time evolution of spin operators under the influence of a zero field splitting Hamiltonian
Authors: Baram Asher
Published: July 2002

It is shown that closed form expressions may be evaluated, for the time evolution of spin operators under the influence of a zero field splitting Hamiltonian, for all spin systems with S≤3. Explicit expression are given for all S≤2.
The elasticity of ideal polymer chains
Authors: Altenberger Andrzej R. , Rosa Epaminondas , Dahler John S.
Published: July 2002

The systems considered here are composed of noninteracting ideal chains, formed from many identical beads, each of which is bound to its two nearest neighbors with an arbitrary potential of interaction. The thermodynamic properties of these systems are examined for several bond potentials. Comparisons then are made between the properties of the model systems and the experimentally observed behaviors of rubbery materials. Connections also are established between the thermodynamic and elastic properties of these model systems and those given by the traditional theories of polymeric elasticity.
Measurement of the X 2Σ+–A 2Π splitting in CsO via photoelectron spectroscopy of CsO−
Authors: Sarkas H. W. , Hendricks J. H. , Arnold S. T. , Slager V. L. , Bowen K. H.
Published: July 2002

We present the photoelectron spectrum of CsO−, recorded using 2.540 eV photons. This spectrum provides a direct measurement of the X 2Σ+–A 2Π energy splitting in CsO, which is found to be 0.135±0.025 eV. This work also establishes that the ground state of CsO− is 1Σ+. In addition, the adiabatic electron affinity of CsO is found to be 0.273±0.012 eV, while the D0 value for the X 1Σ+ state of CsO− (with respect to Cs+O−) is found to be 1.84±0.15 eV. Molecular parameter estimates for CsO− are also extracted from the spectrum.
Spectroturbidimetry theory for determining orientation distributions of spheroidal particles in the Rayleigh–Debye–Gans and Rayleigh scattering regimes
Authors: Buehler Christopher S. , Caruthers James M. , Franses Elias I.
Published: July 2002

The general direct and inverse problems of turbidity of spheroids which are monodisperse in size and shape and fall in the Rayleigh–Debye–Gans (RDG) and Rayleigh scattering regimes are solved analytically. The theory applies to single and independent scattering from nonabsorbing optically isotropic particles. From turbidity measurements at one incident direction, one wavelength, and three incident polarizations, it is shown that three functions or moments of the orientation distribution functions can be obtained for both regimes. For RDG particles, use of turbidity spectra (at multiple wavelengths) can recover more information, i.e., more moments. The complete orientation distribution function, with no empirical fits, can in principle be obtained from turbidity spectra at many incident directions for the RDG regime, but not for the Rayleigh regime. The approach used is more general, and should provide potentially much more orientation information than measurements of conservative linear dichroism.
Rise profile of the thermal lens signal: Contribution of the temperature lens and the population lens
Authors: Terazima Masahide , Hirota Norobu
Published: July 2002

Time profiles of the rise parts of variations of a probe beam light densities under the thermal lens (TL) experimental condition are measured with nanosecond pulsed laser excitation. The rise profile in benzene can be explained well by the probe beam expansion only by a refractive index lens due to the density fluctuation, which is induced by heat from the radiationless transition. In water, the signal is fitted well by the sum of two contributions of the refractive index change; i.e., due to the density (the thermal lens) and temperature (the temperature lens) variations. The coefficient of the temperature variation (∂n/∂T)ρ is evaluated from the relative signal intensity. The contribution of the population lens is clearly observed in the rise curve of the ‘‘TL signal’’ after the photoexcitation of C60 in benzene.
Lattice dynamics of halogenated anthracene derivatives under pressure
Authors: Della Valle Raffaelle Guido , Brillante Aldo
Published: July 2002

Lattice dynamics calculations have been performed for β-9,10-dichloroanthracene and 9,10-dibromoanthracene under pressure. The potential has been fitted to the ambient pressure crystallographic structure and to lattice phonon frequencies of both compounds. The pressure dependence of the structures and frequencies has been calculated. A pressure induced phase transition which resembles the experimental transition is predicted for the chloro derivative at ≊3 GPa.
Competition between atomic shell and electronic shell structures in aluminum clusters
Authors: Baguenard B. , Pellarin M. , Lermé J. , Vialle J. L. , Broyer M.
Published: July 2002

Under usual experimental conditions, aluminum clusters have specific geometric arrangement. By heating the nozzle, we obtain melted aluminum clusters, and a new periodicity appears in mass spectra, which corresponds to electronic shells up to 1800 electrons.
Thermal decomposition of H3O− produced in reaction of OH− with H2CO
Authors: Viggiano A. A. , Miller Thomas M. , Miller Amy E. Stevens , Morris Robert A. , Paulson John F. , Brown Eileen R. , Sutton Emmett A.
Published: July 2002

The ion–molecule reaction OH−+H2CO→H3O−+CO has been studied at 300 K with isotopic labeling of reactants. The H3O− product is only observed in small abundance because the ion dissociates into OH−+H2 upon multiple collisions in a helium buffer gas. Without isotopic labeling, the pseudo-first-order kinetics plots for the reactions of OH− with H2CO and OD−+D2CO were found to be curved as a result of the regeneration of OH− or OD− reactant. A scavenger technique was used to remove the H3O− (or D3O−) produced prior to dissociation, to reveal the true first-order attenuation of OH− (or OD−) in reaction with H2CO (or D2CO). The rate constant for the OH−+H2CO reaction is 7.6×10−10 cm3 s−1, and for OD−+D2CO is 5.7×10−10 cm3 s−1. For the isotopically mixed cases OH−+D2CO and OD−+H2CO, the rate constants are equal to 1.3×10−9 cm3 s−1, about twice as large as those for the reactions involving only a single hydrogen isotope, indicating that isotopic exchange is an important process. The rate constants for the thermal dissociation of H3O− and D3O− in helium were found to be 1.6×10−12 and 1.1×10−12 cm3 s−1, respectively, within a factor of 2. The results are discussed in terms of other thermal dissociation reactions of ions.
Reactions of low work function metals Na, Al, and Ca on α,ω-diphenyltetradecaheptaene. Implications for metal/polymer interfaces
Authors: Dannetun P. , Lögdlund M. , Fredriksson C. , Lazzaroni R. , Fauquet C. , Stafström S. , Spangler C. W. , Bredás J. L. , Salaneck W. R.
Published: July 2002

The interactions between different low work function metals aluminium, calcium and sodium, and α,ω-diphenyltetradecaheptaene, a model molecule for certain conjugated polymers, have been investigated using both x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. The spectra are interpreted with the help of the results of quantum chemical calculations performed within the local spin density (LSD) approximation methodology. The metals are found to interact with the conjugated system in very different ways. Aluminium forms a covalent bond, which strongly modifies the π-electronic structure of the conjugated molecule, while both the sodium and the calcium atoms act as doping agents, inducing new states in the otherwise forbidden bandgap. These new gap states can be viewed as a soliton–antisoliton pair for the Na/DP7 and a bipolaronic-like defect for Ca/DP7.
The detection of fast neutral fragments following the photodissociation of krypton cluster ions
Authors: Smith J. A. , Winkel J. F. , Jones A. B. , Stace A. J. , Whitaker B. J.
Published: July 2002

Mass and kinetic energy resolved krypton cluster ions, Kr+n, have been photodissociated in the entrance to a time-of-flight (TOF) device of variable length. The subsequent deflection of all ions allowed for time resolved measurements to be undertaken on the neutral photofragments. Following the absorption of a photon (hν=2.33 eV), all cluster ions up to Kr+25 were found to eject one or, possibly, two neutral atoms with relatively high kinetic energies. An analysis of the laser polarization dependence of this event showed that the atoms are ejected on a time scale which is short compared with the rotational period of a cluster (10–100 ps). Remaining internal energy within the cluster ions is dissipated through the further loss of neutral atoms, but with low kinetic energies. The latter process is found to be isotropic with respect to the angle of polarization of the laser radiation. Kinetic energy releases calculated from the TOF spectra exhibit a gradual decline as a function of cluster size out as far as Kr+13 and, thereafter, maintain a constant value. This pattern of behavior is significantly different from that observed previously [Smith et al., J. Chem. Phys. 97, 397 (1992)] for argon cluster ions, Ar+n. A careful analysis of the kinetic energy data for Kr+3 photodissociation reveals that, in at least one of the reaction paths, the Kr+ fragment can only be formed in the ground spin–orbit state. This observation implies that photofragmentation proceeds via a 1(1/2)g←1(1/2)u transition. The implications of this result for the analogous Ar+3 photofragmentation are discussed.
The effect of solvation on the conformation of freely jointed repulsive trimers
Authors: Grayce Christopher J. , de Pablo Juan J.
Published: July 2002

The effect of solvation on the conformation of a dense one-component fluid composed of freely jointed repulsive trimers is calculated theoretically and compared to results from computer simulation. All monomers in the fluid interact with one another via the purely repulsive, shifted–truncated Lennard-Jones potential. We are able to confirm an earlier suggestion that when the conformation of nonpolar flexible molecules is calculated with a site–site solvation potential and the reference interaction site model (RISM) theory, better results are obtained with the recently derived Percus–Yevick (PY) style solvation potential than the hypernetted-chain (HNC) style or Gaussian-fluctuation solvation potential. Interestingly, although RISM-based theory predicts well the general shape of the probability distribution of trimer conformations, it cannot recover a small feature observed in the simulations and expected on simple physical grounds. At high density, there are three preferred conformations of the trimer, corresponding to the three ways in which a flexible trimer can be fitted into a dense fluid with locally hexagonally close-packed (hcp) symmetry. RISM-based theories predict a preference for only two conformations of the trimer. To predict the preference for all three conformations, information about three-site correlations in the fluid must be incorporated into the theory. We present a theory that includes this information and is thereby capable of reproducing the results of our simulations.
Valence bond calculations of the potential energy surface for CH4→CH3+H
Authors: Wu Wei , McWeeny Roy
Published: July 2002

A valence bond study of the potential energy surface for methane CH4→CH3+H is performed at the 6–31G level using (i) a valence bond self-constituent field (VB-SCF) method; (ii) a valence bond configuration interaction (VBCI) method; and (iii) an antisymmetrized product of strong-orthogonal geminals (APSG) method (also in VB form). The calculations show that, although the energies are somewhat inferior (on an absolute scale) to those obtained in very large CI calculations, the VB reduced potential energy surfaces behave better, in the intermediate range 2–3 Å, than those obtained using (i) the Mo/ller–Plesset fourth-order perturbation (MP4) approximation, (ii) configuration interaction with all singles and doubles (CISD), and (iii) coupled clusters with all singles and doubles (CCSD). The results are in very good agreement with those obtained from multi-reference configuration interaction (MR-CISD) calculations. The lower absolute energies obtained in the very extensive CI calculations indicate a better description of electron correlation, both in the molecule and in its dissociation products, but evidently they do not imply a better overall description of the PE surface. The remarkable fact is that a single VB structure, with carefully optimized orbitals, provides an excellent description of the whole dissociation process.
Far-infrared reflection–absorption spectroscopy of thin polyethylene oxide films
Authors: Da Costa V. M. , Fiske T. G. , Coleman L. B.
Published: July 2002

We report the temperature dependent far-infrared spectrum of ultra-thin films of polyethylene oxide (PEO). Using the orientational specificity of infrared and far-infrared reflection–absorption spectroscopy and in-situ recrystallization, we find that during spin coating the PEO helices are initially in the plane of the film, but on crystallization reorient to be normal to the substrate. A splitting of the C–O torsional mode near 109 cm−1 is identified as arising from a distortion of the normal helical structure of PEO. Comparison with transmission spectra of cast films demonstrates the value of far-infrared reflection–absorption spectroscopy (FIRRAS) in the study of crystalline polymers in the far infrared.
Semiclassical propagation for multidimensional systems by an initial value method
Authors: Kay Kenneth G.
Published: July 2002

A semiclassical initial value technique for wave function propagation described by Herman and Kluk [Chem. Phys. 91, 27 (1984)] is tested for systems with two degrees of freedom. It is found that chaotic trajectories cause a serious deterioration in the accuracy and convergence of the technique. A simple procedure is developed to alleviate these difficulties, allowing one to propagate wave functions of a moderately chaotic system for relatively long times with good accuracy. This method is also applied to a very strongly chaotic system, the x2y2 or ‘‘quadric oscillator’’ model. The resulting energy spectra, obtained from the autocorrelation function of the wave function, are observed to be in good agreement with the corresponding quantal spectra. In addition, the density of states spectra, computed from the trace of the semiclassical propagator, are found to determine many individual energy levels of this system successfully.
Small-angle neutron scattering: A critical study of the contrast approximation
Authors: Fries P. H. , Kunz W. , Calmettes P. , Turq P.
Published: July 2002

A recent hypernetted chain study has yielded molecular correlations functions of a nonaqueous electrolyte solution. In the present paper these functions are taken to calculate small-angle neutron scattering intensities of this solution in the framework of the so-called ‘‘contrast-approximation’’ commonly used for the interpretation of this kind of experiments. A comparison of this result to a rigorous calculation of the total scattering cross section reveals serious inconsistencies, even at very small scattering angles. Therefore the question is raised as to whether the contrast approximation can give a realistic description of small-angle scattering data.
The effect of shear-flow on critical correlations in colloidal systems: Microstructure, turbidity, and dichroism
Authors: Dhont Jan K. G. , Verduin Henk
Published: July 2002

On the basis of the Smoluchowski equation, ‘‘the Liouville equation for colloidal systems,’’ shear flow distortion effects on the structure factor for temperatures and densities close to the spinodal, c.q. the critical point, are considered. The results are valid in the classical region where the equilibrium structure factor attains the Ornstein–Zernike form. From the structure factor distortion we derive scaling relations for the shear-rate dependent turbidity and flow dichroism for various flow geometries. The dimensionless group which expresses the effect of the shear flow is essentially λ=Pe0(γ̇)/(ξRV)4, with Pe0(γ̇)=γ̇R2V/2D0 (γ̇ is the shear rate, RV the range of the pair-interaction potential, ξ−1 the correlation length, D0 is the Stokes–Einstein diffusion coefficient). As a consequence, very small shear rates (small values of Pe0) have very large effects close to the spinodal, c.q. the critical point, since then ξRV is a small number. Flow dichroism can be many decades larger than for systems far into the stable region of the phase diagram. Relaxation of turbidity and flow dichroism as a shear flow is turned off is also considered. The temperature, density, shear rate, and time dependence of the relaxation is described by scaling functions depending on the two dimensionless groups λ and γ̇t (t is the time lapse after switching off the shear flow).
Response to ‘‘Comment on ‘Mean force potential for the calcium–chloride ion pair in water’ ’’ [J. Chem. Phys. 102, 3483 (1995)]
Authors: Guàrdia E. , Padró J. A.
Published: July 2002

Discrepancies between the mean force potential for the Ca2+–Cl− ion pair in water obtained by Dang and Smith and in our paper [Guàrdia et al., J. Chem. Phys. 99, 4229 (1993)] are attributed to the different interatomic potentials used for its calculation. Despite their differences both interaction models reproduce the experimental structural data now available.
Nonadiabaticity and intramolecular electronic energy transfer in the photodissociation of 1-bromo-3-iodopropane at 222 nm
Authors: Stevens Jonathan E. , Kitchen David C. , Waschewsky Gabriela C. G. , Butler Laurie J.
Published: July 2002

The photodissociation of 1-bromo-3-iodopropane (1,3-C3H6BrI) at 222 nm is studied with crossed laser-molecular beam experiments. Irradiation at this wavelength excites an n(Br)→σ*(C–Br) transition which promotes the molecule to an approximately diabatic excited state potential energy surface which is dissociative in the carbon–bromine bond. This surface intersects an approximately diabatic surface of n(I)→σ*(C–I) character at extended C–Br distances; this surface is dissociative in the carbon–iodine bond. Crossings from the surface initially accessed to the intersecting surface correspond to intramolecular excitation transfer from the carbon–bromine to the carbon–iodine bond. The incidence of such transfer and hence of carbon–iodine bond fission depends upon the strength of the off-diagonal potential coupling of the two diabatic states. These experiments test the dependence of the coupling and consequent energy transfer upon the separation distance of the C–Br and C–I chromophores. The data show C–Br fission dominates C–I fission by a ratio of 4:1 and determine the center-of-mass translational energy distributions and angular distributions of these processes. The measured anisotropy parameters are β(C–Br)=1.6±0.4 and β(C–I)=0±0.2. A third photofission process, IBr elimination, also contributes to the observed signal. The results of the study of C–Br and C–I fission are compared to previous studies on similar molecules to understand how the branching depends on the relative positioning of the C–Br and C–I chormophores.