bluuebunny/crossref_metadata_2025 huggingface dataset:
A theoretical study of x-ray photoelectron spectra of model molecules for polymethylmethacrylate
Authors: Naves de Brito A. , Correia N. , Svensson S. , Ågren H.
Published: July 2002

We explore the usefulness of the delta self-consistent-field (ΔSCF) approximation in connection with high-resolution x-ray photoelectron spectra for component and structural analysis of organic compounds. Results for core electron binding energy shifts for model molecules of the polymethylmethacrylate polymer are presented. A previously devised method for proper self-consistent-field solutions for core hole states in molecules is evaluated. The results indicate that chemical shifts can be obtained within a few tenths of an eV. A discussion is presented on the inherent errors in the ΔSCF approximation, the proper corrections for zero-point vibrational energies, and the role of relaxation of core orbitals.
Tricritical points in H2O–oil–amphiphile mixtures
Authors: Kahlweit M. , Strey R. , Aratono M. , Busse G. , Jen J. , Schubert K. V.
Published: July 2002

Quaternary mixtures of water (A), an oil (B), a nonionic amphiphile (C), and an appropriately chosen fourth component offer an opportunity for searching for tricritical points (tcp) at atmospheric pressure. It is shown that for reaching a tcp, one has to couple an A–B–C mixture that shows the phase sequence 2_→3→2̄ with rising temperature, with a second ternary mixture that shows a 2_→2̄ transition, the bar denoting in which of the two phases the amphiphile is mainly dissolved. With weakly structured solutions, that is, with short-chain amphiphiles as (C) this can be done by either adding an oil with a lower carbon number, or by adding a nonaqueous polar protic solvent such as formamide. With strongly structured solutions, that is, with long-chain amphiphiles, one has to add a short-chain amphiphile for destroying the structure as a prerequisite for reaching a tcp. Insofar, our earlier presumption that with long-chain amphiphiles, a tcp may also be reached, either by increasing their amphiphilicity or by lowering the carbon number of the oil, does not seem to apply. Experience shows that in A–B–C′ mixtures with sufficiently short-chain amphiphiles as C′ that separate into three phases: the amphiphile-rich middle phase always wets the A/B interface. If a short-chain amphiphile is added to an A–B–C mixture with a nonwetting middle phase one will, therefore, inevitably find a nonwetting→wetting transition as one approaches a tcp.
The interaction of rotation and local mode tunneling in the overtone spectra of symmetrical hydrides
Authors: Lehmann Kevin K.
Published: July 2002

In the ‘‘local mode limit’’ where the tunneling time for vibrational energy exchange is long compared to the classical rotational period, one expects that the effective rotational Hamiltonian will reflect the reduced symmetry of the local mode state. Hamiltonians in the local mode basis are given for interaction of rotation and local mode tunneling for molecules of the XH2, XH3, and XH4 type. Transformation of these Hamiltonians to a symmetrized basis (which diagonalizes the vibrational problem), produces rotational couplings between the vibrational states. Relations between the spectroscopic constants are derived that are less restrictive than those given earlier by Halonen and Robiette, but reduce to them when the assumptions of their model are met. The present algebraic procedure can be easily extended to include higher order terms. The effect of these couplings is to reduce the size of the pure vibrational splittings. This is due to the fact that in the rovibrational problem, in general, one must reorient the angular momentum vector in the body frame as well as transfer the vibrational action between bonds. This increases the length of the tunneling path and thus decreases the rate of vibrational energy transfer. Model calculations show that a simple semiclassical picture can rationalize the observed trends.
Suppression of vibron state formation in Arx(N2)1−x mixed crystals
Authors: De Kinder Jan , Bouwen August , Goovaerts Etienne , Schoemaker Dirk
Published: July 2002

The dephasing of the k≂0 states of the vibrons in Arx(N2)1−x crystals was studied by picosecond time-resolved coherent anti-Stokes Raman scattering as a function of temperature (0<T<40 K) and Ar concentration (0<x<0.15). Homogeneous (exponential) and inhomogeneous (Gaussian) contributions to the observed dephasing signal were separated. In the α phase, an increase of the inhomogeneous linewidth was obtained starting from small concentrations of Ar atoms. For larger Ar concentrations, a decrease of the exponential dephasing time T2 and a temperature dependence of the increasingly faster Gaussian decay is observed. Substitution of N2 molecules by Ar atoms increases the librational amplitude. Through anharmonic terms in the crystal Hamiltonian the vibron state formation is reduced and as a result, the T2 time decreases. This suppression of vibron state formation, which counteracts the reduction of the inhomogeneous broadening, is responsible for the increasing and temperature dependent inhomogeneous linewidth. The Ag−Tg factor group splitting increases almost linearly with Ar concentration. Surprisingly, no inhomogeneous line broadening could be resolved for β-Arx(N2)1−x crystals and the observed dephasing time T2 remains long compared to the α phase, even for high Ar concentrations.
On the derivation of the Debye theory of dielectric relaxation from the Langevin equation in the presence of the driving field. II. Inclusion of inertial effects for rotation in three dimensions
Authors: Coffey W. T.
Published: July 2002

It is shown how the inertial response in the presence of a driving field of an assembly of molecules free to rotate in space (with dipole–dipole coupling effects ignored) may be obtained by direct averaging of the Langevin equation. The procedure is illustrated by considering the linear dielectric response of an assembly of needle like rotators choosing as variables in the Langevin equation the Hermite polynomials in the angular velocities and the associated Legendre functions of order l. This leads directly, using the statistical properties of the white noise driving torques, to the differential–difference equations which govern the dielectric response. These are converted into an infinite set of linear simultaneous first order differential equations which are solved by successively limiting the size of the transition matrix to 2×2, 3×3, etc. The continued fraction solution previously obtained by Sack is recovered using this procedure. The 2×2 approximation to the solution, effectively corresponding to the Rocard equation, is discussed in relation to recent far infrared absorption measurements.
Rotational tunneling of methane on MgO surfaces: A neutron scattering study
Authors: Larese J. Z. , Hastings J. M. , Passell L. , Smith D. , Richter D.
Published: July 2002

High-resolution inelastic neutron scattering was used to investigate the rotational tunneling of methane molecules in a ■×■R45° commensurate, square-lattice–solid monolayer adsorbed on MgO (100) surfaces. Good matches to the observed transition energies were obtained using potential functions with C2v symmetry, suggesting that the preferred orientation of the molecule is the dipod-down configuration with two opposite edges of the H-atom tetrahedron parallel to the surface plane.
Fourier transform far infrared spectroscopy of a C4 bending mode
Authors: Withey P. A. , Shen L. N. , Graham W. R. M.
Published: July 2002

A study of the Fourier transform far infrared spectrum of the C4 radical produced by trapping the products of the vacuum ultraviolet photolysis of 1,3-butadiene and acetylene in Ar at 10 K has resulted in the identification for the first time of a bending vibration at 172.4 cm−1. The assignment is confirmed by 13C isotopic data; is consistent with the earlier identification of the antisymmetric C=C stretching mode at 1543.4 cm−1; and is in agreement with the predictions of ab initio calculations for linear C4.
Solid state proton spin relaxation in ethylbenzenes: Methyl reorientation barriers and molecular structure
Authors: Beckmann Peter A. , Happersett Laura , Herzog Antonia V. , Tong William M.
Published: July 2002

We have investigated the dynamics of the ethyl groups and their constituent methyl groups in polycrystalline ethylbenzene (EB), 1,2-diethylbenzene (1,2-DEB), 1,3-DEB, and 1,4-DEB using the solid state proton spin relaxation (SSPSR) technique. The temperature and Larmor frequency dependence of the Zeeman spin-lattice relaxation rate is reported and interpreted in terms of the molecular dynamics. We determine that only the methyl groups are reorienting on the nuclear magnetic resonance time scale. The observed barrier of about 12 kJ/mol for methyl group reorientation in the solid samples of EB, 1,2-DEB, and 1,3-DEB is consistent with that of the isolated molecule, implying that in the solid state, intermolecular electrostatic interactions play a minor role in determining the barrier. The lower barrier of 9.3±0.2 kJ/mol for the more symmetric 1,4-DEB suggests that the crystal structure is such that the minimum in the anisotropic part of the intramolecular potential is raised by the intermolecular interactions leading to a 3 kJ/mol decrease in the total barrier. We are able to conclude that the methyl group is well away from the plane of the benzene ring (most likely orthogonal to it) in all four molecules, and that in 1,2-DEB, the two ethyl groups are in the anticonfiguration. Our SSPSR results are compared with the results obtained by microwave spectroscopy and supersonic molecular jet laser spectroscopy, both of which determine molecular geometry better than SSPSR, but neither of which can determine ground electronic state barriers for these molecules.
Perturbations in the multiphoton ionization spectrum of the F 1Δ state of HCl
Authors: Xie Yongjin , Reilly Peter T. A. , Chilukuri Santaram , Gordon Robert J.
Published: July 2002

The 2+1 REMPI spectra of the F(0–0), F(1–0), E(0–0), and V(11–0) transitions of HCl were measured in a time-of-flight molecular beam machine. Both room temperature HCl, and rotationally and vibrationally excited HCl obtained by photodissociating vinyl chloride at 193 nm, were used. Several anomalies in the F state were observed. These include a decrease in intensity of the P, Q, and R branches between J′=3 and J′=10 for the 0–0 transition, a falloff in intensity for J′>5 for the P, Q, and R branches of the 1–0 transition, and an enhanced intensity loss of the Q(9) line of the 0–0 transition. Accurate values of the lambda-doublet splitting constants for these transitions were also obtained. The intensity loss in the 0–0 transition is due to an interaction with some unobserved bound state, which could also explain the 1–0 perturbation. A candidate for the perturbing state is the e 3Σ+(1). The isolated perturbation of the Q(9), 0–0 line could be caused by an interaction with the g(0+) state.
Theoretical study of the quenching of N2(A 3Σ+u) by hydrogen atoms
Authors: Sperlein Robert F. , Golde Michael F.
Published: July 2002

A b initio calculations on the interactions of N2(A 3Σ+u) and N2(X 1Σ+g) with H(1 2S) atoms in a T-shape (C2v) geometry have been performed. The 2B2 state, resulting from the interaction of N2(A) with H, is strongly attractive, in contrast to the repulsive interaction of N2(X) with H in this geometry. When the C2v symmetry is relaxed, coupling of these two states in the vicinity of the ‘‘crossing region’’ is observed. The efficient quenching of N2(A) in collisions with H atoms is discussed in terms of the calculated properties of N2H.
Quantum study of the redistribution of flux during inelastic collisions
Authors: Alexander Millard H.
Published: July 2002

A new method is presented for the study of the mechanism of inelastic atomic and molecular collisions. This involves the determination of the current density associated with, separately, the incoming and outgoing scattering wave functions in either an asymptotic (diabatic) or locally adiabatic basis. This yields a picture of how the incoming flux, initially associated with a given internal state, redistributes itself as a function of the interparticle separation both as the particles approach, and, subsequently, as the particles recede. It is shown that the separation into incoming and outgoing flux, which is valid asymptotically, continues to be valid as the collision partners approach, without mixing of the contributions from the incoming and outgoing waves. A simple extension of our linear-reference-potential, log-derivative propagation technique can be used to compute the redistribution of the initial flux. It is argued that analysis in a fully adiabatic basis, which corresponds to the local eigenvectors of the collision system, provides the most meaningful physical insight. A simple stabilization correction can be introduced, which prevents adiabatically closed channels from numerically contaminating the determination of flux redistribution among the locally open channels. Application is made to a pedagogical two-state problem, to a multistate collision system involving four different electronic potential curves, and to a second multistate collision system involving a closed-channel resonance.
Coverage-dependent desorption measurements for CO/Ag(011)
Authors: Peterson L. D. , Kevan S. D.
Published: July 2002

We report time-resolved electron energy loss spectroscopy results for the thermal desorption of CO physisorbed onto Ag(011). The measurements were performed isothermally and effectively at constant coverage. We observe that the logarithm of the measured pseudo-first-order desorption rate constant increases approximately linearly with coverage over the temperature and coverage ranges accessible to our measurements. The results are analyzed in terms of a quasiequilibrium model in which the desorption rate constant is expressed analytically as a virial expansion. Using this analysis, we determine that at low coverage the molecules adsorb with their axes parallel to the surface plane and are oriented with respect to the crystalline axes. We find that long-range interactions extending out to 7–8 neighbor shells must be included to fit our results. The magnitude of these long-range interactions is reasonably well described by the gas phase intermolecular potential if screening by substrate image charges is included.
Attenuation length measurements in cadmium arachidate Langmuir–Blodgett films
Authors: Sastry Murali , Ganguly P. , Badrinarayanan S. , Mandale A. B. , Sainkar S. R. , Paranjape D. V. , Patil K. R. , Chaudhary S. K.
Published: July 2002

Attenuation lengths in cadmium arachidate [(C19H39COO)2Cd] multilayer Langmuir–Blodgett films deposited on thick lead arachidate [(C19H39COO)2Pb] films have been determined in this communication. A discrete layer model appropriate to the ordered structure of these built-up films is proposed and applied for the first time to the measurements of the x-ray photoemission intensity variation with electron takeoff angle and attenuation lengths for 950–1350 eV electrons are evaluated. These lengths agree with earlier attenuation length measurements in Langmuir–Blodgett films and are higher than in most inorganic materials.
Sedimentation, diffusion, and Taylor dispersion of a flexible fluctuating macromolecule: The Debye–Bueche model revisited
Authors: Frankel I. , Mancini Francesco , Brenner Howard
Published: July 2002

The role of hydrodynamic flexibility stemming from a lack of configurational rigidity in the sedimentary and dispersive transport of macromolecules in dilute polymer solutions is analyzed within the framework of generalized Taylor dispersion theory. A macromolecular chain is modeled as a thermally fluctuating porous Brownian spongelike sphere which—in contrast with the classical investigations of Debye–Bueche and Brinkman for the rigid porous sphere case—is allowed to undergo thermal fluctuations in size (assumed governed by a Hookean elastic potential). Our results show an increase of up to about 20% in the average translational mobility of the flexible (size-fluctuating) sphere above that of the comparable rigid Debye–Bueche/Brinkman sphere (of equilibrium preaveraged radius). Coupling between mobility variations (arising from instantaneous fluctuations in sphere radius) and diffusive sampling of such sphere radii in size space, respectively, gives rise to a ‘‘Taylor’’ dispersion mechanism, which enhances the diffusivity of the macromolecule above and beyond its purely molecular value. Indeed, the dispersivity (dyadic) of the sedimenting sphere is shown to be anisotropic, possessing a value different from its (mean) molecular diffusivity in the direction of net sedimentation. Both sedimentation and dispersion effects associated with size fluctuations are shown to be maximized at finite, intermediate values of the nondimensional sponge stiffness parameter S, rather than being monotonically decreasing functions of S. The relative importance of both effects increase with decreasing permeability of the sphere. With an increasing number (N≫1) of monomer units composing the chain, the isotropic (molecular) portion of the dispersivity dyadic decreases like N−1/2, whereas the anisotropic Taylor-dispersion portion, which is exclusively associated with the size fluctuations, increases like N3. The latter contribution, which is negligible for ordinary gravitational settling (owing to the smallness of the relevant Langevin parameter), may, however, become significant for ultracentrifugation of polymer solutions composed of long (N>105) macromolecular chains.
Quadrupolar relaxation—an approach to rotational diffusion
Authors: Kontaxis G. , Sterk H. , Kalcher J.
Published: July 2002

The 7Li relaxational behavior of phenyl-lithium in isotropic solution has been investigated. The multiexponential quadrupolar relaxation dynamic facilitates the determination of the rotational diffusion constants together with the quadrupolar coupling constant, despite the zero average of the quadrupolar splitting on account of molecular tumbling. The rotational diffusion constants obtained are in very satisfactory agreement with those derived from our phenyl-lithium 13C relaxation studies.
IF(A,B) electronic excitation in F+I2F induced by the surface reaction F(ad)+I2(ad)
Authors: Wagemann K. , Chen X. R. , Wanner J.
Published: July 2002

This paper reports on a dynamical study of the reaction of an effusive F atom beam with iodine molecules condensed on a temperature-controlled LiF(001) surface. The observation of IF ground state products by laser-induced fluorescence and of IF(A,B) excited species by visible chemiluminescence can be explained by means of a reaction scheme consisting of two channels. In the first channel IF (X 1Σ+) ground state molecules are formed on the surface and desorb into the gas phase above a threshold temperature of about 200 K, where they are probed by laser-induced fluorescence. The vibrational population ratio [N′(v=0)/N′(v=1)] shows equilibrium of the desorbing molecules with the surface. The population distribution of the IF(v=0) rotational manifold can be well-characterized by a rotational temperature which follows the surface temperature. Surface equilibration of products implies that, unlike in the gas phase reaction F+I2, the products have no memory of their exoergic formation, indicating a Langmuir–Hinshelwood mechanism. In a second reaction channel I2F, as the intermediate in the reaction of fluorine atoms with adsorbed iodine molecules, stabilizes at the surface and desorbs into the gas phase above a surface temperature of 300 K. There it reacts with F atoms according to the chemiluminescence process F+I2F→IF*+IF. This gas phase reaction is identified by the exoergicity limit of the IF (B 3Π) state vibrational population distribution, obtained from analysis of the B→X chemiluminescence. In comparison with Kahler and Lee’s molecular beam synthesis of I2F from I2 and F2 with subsequent secondary encounter with the product F atom, the above reaction scheme represents an alternative induction of the F+I2F reaction. It can explain visible IF chemiluminescence from fluorine–iodine systems under experimental conditions in which F atoms are present and I2 wall adsorption is possible. In this work, evidence is given that I2F is a common precursor in the two reaction channels. The overall branching ratio strongly favors the formation of IF ground state molecules in these experiments.
Doppler profiles and fine-structure branching ratios of O(3P
j) from photodissociation of carbon dioxide at 157 nm
Authors: Matsumi Yutaka , Shafer Neil , Tonokura Kenichi , Kawasaki Masahiro , Huang Yu-Lin , Gordon Robert J.
Published: July 2002

The O(2p 3Pj) (j=2, 1, and 0) fragments produced in the 157 nm photodissociation of CO2 were detected by resonance-enhanced multiphoton ionization in a molecular beam. The Doppler profiles and fine-structure branching ratios were measured for the oxygen-atom photofragment in the 3Pj states. The Doppler profiles were analyzed to give an anisotropy parameter of β=2.0±0.2 and an internal energy equivalent to 3.9±0.3 vibrational quanta of CO. The fine-structure populations were found to be 0.70±0.05, 0.16±0.03, and 0.14±0.03 (with error bars of ±σ) for j=2, 1, and 0, respectively. A mechanism is proposed in which complex on the 1B2 surface undergoes intersystem crossing to the 3B2 surface. A phase-space model with a constraint on the impact parameter is shown to be consistent with the observed energy release. The nonstatistical fine-structure population could be caused by long-range interactions on the triplet surface. In a bulb experiment, O(3P) was produced by quenching of O(1D). The fine-structure populations of the resulting O(3Pj) were 0.64±0.04, 0.25±0.04, and 0.11±0.04. This state distribution is consistent with a long-lived complex which decays to give statistical products.
Conformational statistics of semiflexible polymers with excluded volume
Authors: Kholodenko A. L.
Published: July 2002

The renormalization group treatment (including the crossover effects) of conformational properties of semiflexible polymers with excluded volume is developed in close analogy with that known for the fully flexible case. Obtained results represent further extension of recently discovered connection between Dirac’s fermions and semiflexible polymers [Ann. Phys. (N.Y.) 202, 186 (1990)].
Phase equilibria and critical lines in model water/salt mixtures
Authors: Harvey Allan H.
Published: July 2002

The mean spherical approximation solution for the thermodynamics of a mixture of equal-sized dipolar hard spheres and charged hard spheres is used to calculate phase equilibria and critical lines. The binary system is characterized by the dimensionless ratio of the dipolar strength to the strength of the charges. At high values of this ratio, the critical curve is interrupted by a liquid–liquid equilibrium and the phase diagram (under the usual classification scheme) is type III. As the ratio is lowered, the critical curve becomes continuous; this is type I or II behavior. The continuous critical line is maintained until the critical temperature of the ionic component exceeds that of the dipolar component by a factor of approximately 3.56; such a lengthy unbroken critical line is unusual in real fluid systems but is observed in many mixtures of water with strong electrolytes. At sufficiently low values of the dimensionless ratio, the critical line is interrupted very near the critical point of the dipolar component; these systems exhibit type IV or V behavior. The simple model used here reproduces the qualitative features of the phase diagrams of real water/salt systems. Insights from the model suggest at least partial explanations for the unusually long unbroken critical lines observed in many of these systems.
A simple model for polarizabilities of organic polymers
Authors: Kavanaugh Theresa C. , Silbey Robert J.
Published: July 2002

The hyperpolarizabilities of organic polymers are modeled using coupled Morse oscillators. Analytic solutions for the polarizability and the hyperpolarizabilities are obtained for both small and large chain-length polymers, and compared to computational results. The conjugation length of the polarizabilities is discussed.
High resolution Fourier-transform infrared spectroscopy of CHCl2F in supersonic jets: Analysis of ν3, ν7, and ν8
Authors: Snels Marcel , Quack Martin
Published: July 2002

The spectrum of CHCl2F (fluorodichloromethane, CFC 21) has been measured in the region of the ν3, ν7, and ν8 fundamentals (700–1300 cm−1, including the atmospheric window) by high resolution interferometric Fourier-transform spectroscopy [0.004 cm−1 bandwidth apodized, full width at half-maximum (FWHM)] in supersonic jet expansions with a rotational temperature of ∼70 K and at room temperature in static cells. The supersonic jet spectra allowed an analysis of the rotational fine structure to be started successfully and carried on to the room temperature spectra. Effective Hamiltonian constants have been obtained with accurate band centers for the two most abundant isotopes CH35Cl2F(CH35Cl37ClF), ν̃8=807.1749(805.0492) cm−1, ν̃3=1079.4276(1079.3893) cm−1, and ν̃7=1239.1940(1238.6884) cm−1. Excited state rotational constants and improved ground state rotational constants including quartic centrifugal distortion in the Watson Hamiltonian are reported as well as hot bands and further bands in anharmonic resonance with the fundamentals. The results are important for and discussed in relation to simulations of atmospheric absorption, line coincidences with CO2 lasers for sub-Doppler spectroscopy and infrared (IR) multiphoton excitation and anharmonic intramolecular dynamics.
Dipolar and conductivity relaxations in LiCl–propylene glycol systems
Authors: Pathmanathan K. , Johari G. P.
Published: July 2002

The effect of LiCl on the dielectric properties of propylene glycol has been studied both in their glassy and liquid states over a wide frequency range. The data are analyzed in terms of both the complex permittivity ε* and complex electrical modulus M* formalisms. The dielectric and modulus relaxation rates follow a Vogel–Fulcher–Tamman equation, and the dc conductivity follows a non-Arrhenius relation. The data are expressed in terms of a single relaxation time for ionic or protonic conduction process, and an asymmetric distribution of relaxation times for the dipolar process. The two processes which are separate in pure propylene glycol become merged at high LiCl concentrations with an increase in the distribution of relaxation times, which is attributed to ion-pair formation and/or partial breaking of the H-bonded structure of the solvent. The electrical properties of these solutions are adequately described in terms of a formalism for M* that does not assume a nonexponential decay of electric field due to ionic diffusion, and it is maintained that ionic and/or protonic conduction in viscous liquids is Maxwellian in character.
Observation of autoionizing Rydberg series converging to SO+3(2E′,2A′1)
Authors: Norwood K. , Ng C. Y.
Published: July 2002

The ionization energy of SO3 is determined to be 12.828±0.010 eV from photoionization mass spectrometric measurements of SO3. Two autoionization Rydberg series converging to the SO3(2E′) state (17.901 eV) are identified. Three window resonances observed in the region 610–690 Å are assigned as members of an autoionizing Rydberg series converging to the SO+3(2A1) state (20.603 eV).
Fluctuations and phase diagrams for ternary mixtures. II
Authors: Teubner Max
Published: July 2002

Further results on phase diagrams of ternary mixtures and fluctuations in coexisting phases are presented. For two coexisting phases the ratio of the fluctuations parallel to the binodal can be read from the phase diagram. For three coexisting phases all the nine independent fluctuations in the three phases can be obtained from the phase diagram up to a constant factor. A new geometric constraint on three-phase triangles is obtained which is also valid for pure substances and binary mixtures.
Optical response of liquid mercury
Authors: Tuminello P. S. , Arakawa E. T. , Inagaki T. , Parks J. E.
Published: July 2002

In many of the results of previous investigations, systematic differences have been observed in the directly measured reflectances of liquid Hg and those calculated from the optical constants determined by ellipsometry. We have performed a comprehensive set of experiments on liquid Hg at room temperature in order to resolve the problem of whether these discrepancies are real. A summary of some of our results have been published [Phys. Rev. B 40, 11994 (1989)]. Here we present a more detailed account of the experimental details and some new results. The spectral range of these experiments was confined to the visible spectral region. Normal-incidence reflectances of liquid Hg under vacuum and in contact with dielectric overlayers were measured. Reflectances of polarized light were measured at a photon angle of incidence of 70° for liquid Hg under vacuum. The optical constants of liquid Hg in contact with various dielectric overlayers were determined by ellipsometry and also by measuring reflectances as a function of angle of photon incidence at a MgF2–Hg interface. The results of the direct reflectance measurements were consistent with the optical properties determined by ellipsometry. No evidence was found for transition layers on the surface of liquid Hg.
Quantum scattering studies of inelastic collisions of NH(A 3Π) with helium: Fine-structure and Λ-doublet propensities
Authors: Alexander Millard H. , Dagdigian Paul J. , Lemoine Didier
Published: July 2002

The results of full close-coupled calculations of state-to-state cross sections for rotationally inelastic collisions of NH in its A 3Π electronic state with helium, based on the recently calculated ab initio potential energy surfaces of Jonas and Staemmler [Z. Phys. D 14, 143 (1989)], are presented. The calculated Λ-doublet resolved cross sections have been compared with predictions based on formal analyses of the scattering equations both in the Hund’s case (a) and (b) limits. For transitions involving low J levels, a strong propensity toward conservation of the e/f label was found, as expected in the case (a) limit. For higher J, the cross sections connecting related pairs of Λ-doublet levels were found to be unequal, reflecting a quantum mechanical interference between the two potential energy surfaces arising from the interaction of a molecule in a Λ>0 state with a perturber. For transitions connected by even l terms in the expansion of the potentials, a simple analysis, based on the relative strengths of the l=2 coupling matrix elements of the electrostatic potential, was found capable of explaining the relative ordering of the cross sections in most cases. A similar success for predicting transitions coupled by odd l terms in the potential was not found; this reflects the fact that the l=3 terms are relatively small for the NH(A 3Π)–He interaction. The calculated cross sections for large J also exhibit a propensity for conservation of the fine-structure label, as expected in the case (b) limit. As an indication of the reliability of the calculated interaction potential and our treatment of the collision dynamics, appropriately summed calculated cross sections reproduce well the experimental rates for transitions from selected f levels into all e levels, as measured by Stuhl and co-workers.
Integral equation theory for the adsorption of chain fluids in slitlike pores
Authors: Yethiraj Arun , Hall Carol K.
Published: July 2002

The adsorption of hard chains in slitlike pores is studied via an integral equation theory. The theory uses the growing adsorbent model in conjunction with the polymer reference interaction site model (polymer-RISM) theory to predict the wall–fluid correlation functions; wall–fluid and fluid–fluid correlations are treated using the Percus–Yevick closure relations. The theory is compared to Monte Carlo simulation data for the density profiles of 4-mers and 8-mers in slitlike pores; the theory is fairly accurate, though it tends to overestimate the density at the wall at low densities and underestimate the density at the wall at high densities. The theory also underestimates the amplitude of the oscillations in the density profile. The theory is very accurate, however, for the adsorption isotherm of 4-mers and 8-mers.
Lattice models for the MSXα description of transition metal impurities in ionic crystals
Authors: Till S. J. , Howard S. T. , Huke J. P. , Parsons I. W.
Published: July 2002

The development of a near MSXα description of transition metal impurities in polar solids is described. Non-muffin-tin corrected MTXα calculations are performed for clusters comprising the impurity together with one or two shells of adjacent lattice ions stabilized by potentials calculated from lattice models of increasing sophistication. The method is illustrated by its application to Cr3+:MgO and Cu+:NaX (X=Cl,F). The calculations are much faster than Hartree–Fock (+configuration interaction) techniques and the method offers an attractive route for computing the potential surfaces and wave functions which determine the impurity related properties of the doped host. The representation of the confining lattice potential is shown to be of paramount importance. Clusters comprising the impurity plus the two nearest shells of lattice ions embedded in a lattice of adjacent Hartree–Fock–Slater ions and remote point charges are shown to give good quality potential surfaces, but only when lattice–cluster exchange effects are included.
Infrared signatures for isomerization and melting in inhomogeneous van der Waals clusters
Authors: Kmetic Mary Ann , LeRoy Robert J.
Published: July 2002

Molecular dynamics simulations are combined with a model for predicting the perturbed infrared spectrum of SF6 in rare gas clusters and used to generate synthetic spectra for a range of cluster isomers and effective temperatures. It is shown that the perturbed infrared spectrum of a vibrationally degenerate chromophore solvated in a cluster can be an incisive tool for determining the number, arrangement, and dynamical state of the solvent particles surrounding the chromophore. The simulations described herein also draw attention to a kind of melting behavior in which the time scale of the ‘‘measurement’’ directly affects the perception of whether the system is liquidlike or solidlike.
Thermophysical properties of gaseous refrigerants from speed-of-sound measurements. III. Results for 1,1-dichloro-2,2,2-trifluoroethane (CHCl2-CF3) and 1,2-dichloro-1,2,2-trifluoroethane (CHClF-CClF2)
Authors: Goodwin A. R. H. , Moldover M. R.
Published: July 2002

The speed of sound in gaseous 1,1-dichloro-2,2,2-trifluoroethane (CHCl2CF3, commonly known as R123) has been measured at temperatures T  between 260 and 335 K at pressures p from 1.6 to 77 kPa. Perfect-gas heat capacities and second and third acoustic virial coefficients have been calculated from the results. The second and third acoustic virial coefficients are used to estimate the density virial coefficients B(T ) and C(T ) and an effective square-well potential. The acoustic estimates of B(T ) and C(T ) are consistent with B(T ) and C(T ) deduced from high-quality (p,Vm, T ) results. The combined values of B(T ) and C(T ) cover a reduced temperature range of 0.57 to 0.99. We have also measured the speed of sound in 1,2-dichloro-1,2,2-trifluoroethane (CHClF-CClF2, commonly known as R123a). This isomer of R123 is a significant impurity in R123 as manufactured and used. We provided estimates of the vibrational relaxation time for R123.
Chemical equilibrium in simple fluids: Solvent density dependence of the dimerization equilibrium of 2-methyl-2-nitrosopropane in argon and xenon
Authors: Kimura Y. , Yoshimura Y.
Published: July 2002

We have studied the dimerization equilibrium of 2-methyl-2-nitrosopropane (MNP) at 60 and 35 °C in argon and xenon up to densities about twice the critical density of the solvent. With an increase in the solvent density, the dimerization equilibrium constant at 60 °C decreases in the low-density region, whereas it increases in the high-density region. The inversion of the density dependence occurs around ρr≂0.8 in argon and ∼1.4 in xenon, where ρr means the density reduced by the critical density of the solvent fluid. The equilibrium constant in xenon at 35 °C increases with increasing solvent density in the low-density region (ρr<∼0.5), while it decreases in the medium-density region (0.5<ρr<1.5). The equilibrium constant in argon at 35 °C has a similar density dependence to that at 60 °C. The internal energy change for the dimerization shows a large density dependence in the low-density region of xenon, while it changes little with density in argon. The theoretical calculation by the Percus–Yevick (PY) approximation for a simple reaction model gives a poor result for the density dependence of the equilibrium constant, although the temperature dependence of the equilibrium constant is reproduced qualitatively. The calculation for a more realistic model including the molecular anisotropy of MNP suggests that the density dependence at the low-density limit is sensitive to the interaction model.
Calculation of the exchange integral for short range intramolecular electronic energy transfer in bichromophoric molecules
Authors: Levy Sharona-Tal , Speiser Shammai
Published: July 2002

A novel calculation of the exchange integral used for describing short range electronic energy transfer (EET) is presented. The calculation is applied to describe intramolecular EET in specially designed bichromophoric molecules. The results indicate that the simple Dexter exponential distance dependence, which is usually assumed to account for short range exchange interaction is valid in some limited cases. However, a more elaborate approach, such as presented here, is required in order to fully describe the molecular geometry effects on short range intramolecular EET.
Beyond the time-dependent Hartree grid approximation for curve-crossing problems
Authors: Campos-Martinez Jose , Waldeck Janet R. , Coalson Rob D.
Published: July 2002

A new ‘‘configuration-interaction’’ method is presented which extends the single-surface time-dependent Hartree grid (TDHG) approximation into a formally exact algorithm for obtaining multidimensional quantum wave-packet dynamics on nonradiatively coupled electronic potential surfaces. As a numerical example, photofragmentation cross sections are computed for a two-degree-of-freedom model of direct dissociation. For systems prepared in vibrationally excited states of the ground electronic potential the TDHG approximation fails due to ‘‘direct correlation’’ effects, while our method provides accurate results.
An experimental study of collision broadening of rotational spectral lines of 13CH3 13C15N in the ground and nν8, n=1 and 2 vibrationally excited states at microwave frequencies
Authors: Hajsaleh J. , Al-Share M. , Johri M. , Johri G. K. , Roberts J. A.
Published: July 2002

A double modulation microwave spectrometer was used to determine the linewidth parameter for some rotational components in the nν8 vibrations of methyl cyanide under 13C and 15N substitution. The linewidth parameters for self-broadening of the ΔJ=2←1 rotational components for the ground, ν8, and 2ν8 vibrations were determined over a pressure range of 1–13 mTorr and at a temperature of 300 K. An experimental method is presented to correct for modulation broadening when high derivatives are used to extract the absorption profile information from the signals. The eighth derivative profile was explored to determine if the spectral line shape remains Lorentzian over a range of modulation levels from 10% of Δν to more than 100% of Δν. These tests showed that the ratio of inner to next inner peak separations, designated in this paper as Δ, of the eighth derivative was the same as that for an assumed Lorentzian line shape. Thus, line shapes were assumed to be Lorentzian for theoretical analysis of the derivative profiles and comparisons made between experiment and theory on that basis. Dipole moments for vibrationally excited energy levels for the ν8 and 2ν8 vibrations were calculated from the linewidth parameter data after all corrections had been made for modulation, Doppler, and other nonpressure broadening effects.
Reorientational motion of a cross-link junction in a poly(dimethylsiloxane) network measured by time-resolved fluorescence depolarization
Authors: Stein Alan D. , Hoffman D. A. , Frank C. W. , Fayer M. D.
Published: July 2002

The reorientational dynamics of a cross-link junction in poly(dimethylsiloxane) networks, measured by the fluorescence anisotropy decay of a chromophore tagged to the cross-link, have been investigated over a range of temperatures from Tg+75 to Tg+150. The probe chromophore, 1-dimethylamino-5-sulfonylnaphthalene amide (dansyl amide), is pendant to a trifunctional silane that acts as a cross-linking molecule. In cyclohexanol, the fluorescence anisotropy decay is in agreement with Debye–Stokes–Einstein hydrodynamic theory (rotational diffusion) demonstrating that the cross-linker can be used as a probe of orientational relaxation. The fluorescence anisotropy decays at a rapid rate in an end-linked poly(dimethyl siloxane) network reflecting fast reorientational motion of the cross-link junction. This reorientation appears diffusive and has a temperature dependence in accord with the Williams–Landel–Ferry equation. A model is proposed that suggests that reorientation and translational motion of the cross-link occur simultaneously and are both coupled to fluctuations of the polymer chain ends.
On calculations of resonance parameters from stabilization graphs
Authors: Macías A. , Riera A.
Published: July 2002

The performance of the simplest methods that have been proposed to calculate resonance widths using the stabilization method is compared. For the model problem considered here, our method yields accurate results with a minimal computational effort.
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o study of the electronic structure of polymeric quasi-one-dimensional superlattices of furan and thiophene
Authors: Bakhshi A. K.
Published: July 2002

A b initio band structure results of polyfuran (PFU) and polythiophene (PTP) obtained with a double zeta basis set are reported. The electronic density of states (DOS) of the various quasi-one-dimensional copolymers (superlattices) of the type (AmBn)x (A=furan; B=thiophene) have been calculated numerically within the ab initio self-consistent field (SCF) tight binding approximation. These copolymers on the basis of the band positions of PFU and PTP are found to belong to the class of type I superlattices. The trends in their electronic structure and conduction properties as a function of (i) composition (m/n); (ii) block sizes m and n; and (iii) arrangement of the blocks in the copolymer chain are discussed.
A phase-space theory and Monte Carlo sampling method for studying nonadiabatic unimolecular reactions
Authors: Marks Alison J. , Thompson Donald L.
Published: July 2002

A statistical method for calculating nonadiabatic unimolecular rate constants is developed. The rate constant is considered as a weighted flux through a ‘‘transition-state,’’ represented by the locus of intersection of two diabatic potential energy surfaces. Phase-space points close to the transition-state contribute to the rate constant, with a weight given by a transition probability (e.g., the Landau–Zener transition probability). The resulting expressions are approximated using a Monte Carlo technique. The statistical model is general and applicable to large polyatomic systems. Application is made to N2O predissociation, and the results are compared with those of classical trajectory surface-hopping calculations. A comparison is also made to the phase-space theory developed by Zahr et al. [G. E. Zahr, R. K. Preston, and W. H. Miller, J. Chem. Phys. 62, 1127 (1975)]. The Monte Carlo calculations are in good agreement with the trajectory results.
Water molecules in different crystal surroundings: Vibrational O–H frequencies from a
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Authors: Ojamäe Lars , Hermansson Kersti
Published: July 2002

A b initio quantum-mechanical calculations of anharmonic frequencies for the water O–H vibrations have been performed for a series of crystalline hydrates. In each case, the potential-energy curve for the uncoupled water O–H stretch was derived at the Mo/ller–Plesset MP2 level. Nearest neighbors of the water molecule were explicitly included in the supermolecule and the rest of the surroundings were mimicked by point charges to reproduce the crystal field out to infinity. The time-independent Schrödinger equation for the motion of the proton in this potential well was solved variationally and the frequency was obtained from the energy difference between the 0 and 1 eigenstates. Computed frequencies can be directly compared with existing infrared data for isotope-isolated water molecules in these hydrates. The compounds selected (LiClO4⋅3H2O, LiHCOO⋅H2O, LiOH⋅H2O ) exhibit experimental O–H frequency shifts in a wide range, from −150 down to −930 cm−1. Good agreement is found between experimental and theoretical frequencies (experimental values in parentheses): 3596 (3556) for LiClO4⋅3H2O, 3129 (3112) and 3488 (3390) for LiHCOO⋅H2O, and 2817 (2775) cm−1 for LiOH⋅H2O. Correlation curves of typical H-bond parameters such as ν(O–H) vs R(H⋅⋅⋅O), re (O–H) and νOH/νOD have been computed and compared with experiment. The vibrational intensities are also discussed.
Fermi resonance in ammonia adsorbed on silica surfaces
Authors: Zerda T. W. , Brodka A. , Hopkins B. J.
Published: July 2002

Fermi resonance in ammonia adsorbed on silica of derivatized surfaces is investigated using Raman spectroscopy. From experimental band shapes the frequencies of hypothetical uncoupled modes and the Fermi coupling parameter are calculated and discussed in terms of surface coverage. It is shown that the Fermi resonance is strongly perturbed by interactions between ammonia and the silica surface. Surface hydroxyl groups and strained cyclic rings are identified as adsorption sites.
A coincidence counting study of polyatomic ion induced sputtering
Authors: Park M. A. , Cox B. D. , Schweikert E. A.
Published: July 2002

Coincidence counting methods were used to examine the desorption of secondary ions from a CsI surface via keV atomic and polyatomic projectile impacts. A correlation between the emission of I− and CsI−2 secondary ions was attributed to the common chemical origin of the ions. The degree to which I− and CsI−2 were correlated was observed to change as a function of the kinetic energy and complexity of the primary ion as well as the yield of I−. This is attributed to a change in the relative importance of competing ion formation processes as a function of the energy in the desorption site.
Fluorescence decay and depolarization in membranes
Authors: Toptygin D. , Svobodova J. , Konopasek I. , Brand L.
Published: July 2002

In optically discontinuous and/or anisotropic environments such as lipid membranes, the radiative decay rate is dependent on the orientation of the emission dipole. A theory of fluorescence decay and depolarization in membranes has been developed that takes the orientational dependence into account. The theory predicts nonexponential total emission intensity decay and emission anisotropy decay different than the one in optically isotropic systems. 1,6-diphenyl-1,3,5-hexatriene fluorescence in L-α-dipalmitoyl-phosphatidylcholine large unilamellar vesicles has been analyzed in terms of this theory. It has been shown that the orientational dependence of the radiative rate is sufficient to account for the complexity of the intensity decay in membranes. The analysis also allowed the determination of the refractive index, orientational order, and rotational dynamics in the membrane.