bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermodynamic properties of model molecules with hexagonal symmetry from statistical mechanical theory
Authors: Friedrich Axel , Lustig Rolf
Published: July 2002

The thermodynamic behavior of planar hexagonal model molecules interacting through a 6- and 7-center Lennard-Jones potential is considered. The influence of molecular size on various thermodynamic properties is elucidated. It is shown that the second virial coefficients obey a microscopic scaling, which causes serious ambiguities for devising effective potential models for real substances. By use of a perturbation theory the vapor–liquid coexistence curves and the critical points are established. Some regularities of distinct properties on the pVT surface are found. The reduced coexisting curves can be used to obtain unambiguous potential parameters for real substances with hexagonal molecular scaffold by using only two experimental points. Thus, all results are rationalized by empirical correlations for easy use. For benzene as an example this procedure results in a model that reproduces vapor pressures and dew densities over two orders of magnitude. Bubble densities are given within a few percent and second virial coefficients are predicted within the experimental scatter throughout the measured temperature range.
Potential energy surfaces for the Ta++C2 reaction
Authors: Roszak S. , Balasubramanian K.
Published: July 2002

Relativistic complete-active space self-consistent field and multireference configuration interaction computations including spin–orbit coupling are made on the TaC2+ cation, the most abundant of metal carbides observed in direct laser vaporization and Knudsen effusion–mass spectrometric experiments. The suggested mechanism of TaC2+ formation via recombination reactions between Ta+ and C2 is consistent with the calculated potential energy curves for the low-lying electronic states. Two nearly degenerate unsymmetrical linear (Ta–C–C+) and symmetrical C2v(C–Ta–C+) isomers suggest that the products can include a mixture of both isomers.
Monte Carlo study of a semiflexible liquid crystal model: The smectic phase
Authors: Mazars M. , Levesque D. , Weis J. J.
Published: July 2002

The mechanical stability of two different smectic arrangements formed by molecules having a rigid core and a semiflexible tail has been investigated by Monte Carlo simulation as a function of temperature, pressure, and chain length.
Intramolecular energy transfer rates for vinyl bromide and deuterium-substituted vinyl bromides from power spectrum line splittings
Authors: Pan Ran , Raff Lionel M.
Published: July 2002

Continuous frequency modulated (CFM) line splittings are used to determine the energy transfer rate coefficients for the local C–Br and C=C vibrational modes in vinyl bromide and the C–H stretching modes in doubly deuterium-substituted vinyl bromides. The global potential developed by Abrash et al. is employed in all calculations. Energy transfer rate coefficients are extracted from the fine structure spacing of the numerically computed power spectrum of the bond coordinates. The consistency of the averaged individual rate coefficients is evaluated by comparison with results obtained from local mode energy decay curves. It is found that the total intramolecular vibrational relaxation (IVR) rate coefficients for all modes investigated are large relative to the unimolecular decomposition rate. However, previous studies show that IVR is not globally rapid so statistical behavior of the unimolecular reaction is not expected. It is shown that near overlapping resonances in the power spectrum make it difficult to accurately extract CFM line splittings. This limitation effectively precludes the use of power spectra to investigate IVR rates for some modes. For the specific case of vinyl bromide, it is demonstrated that the C–Br and C=C stretching modes have sufficiently isolated bands that IVR rates out of these modes can be determined from the line splittings. However, the superposition of the three C–H stretching fundamentals makes it essentially impossible to investigate these modes in vinyl bromide. For the case of doubly deuterium-substituted vinyl bromides, the C–H stretching fundamental is well isolated so that IVR relaxation rates can be easily obtained from the power spectrum line splittings. The consistency of the IVR rate coefficients obtained from line splittings is investigated by calculation of these coefficients from the envelopes of bond energy decay curves. The differences between the two results varies from 15% for the C=C stretch to 43% for one of the C–H stretching modes. The average deviation is 30% which is in accord with the accuracy of the method (±25%) previously estimated by Agrawal et al. The effect of initial local excitation energy on the line splittings and associated rate coefficients is investigated for the C–Br stretching mode. The results show that the line splitting and rate coefficients are nearly independent of excitation energy below 0.8 eV. Above this energy, both the line splittings and the IVR rate coefficients increase rapidly. This is interpreted as being due to increased intermode coupling at higher energies produced by the greater vibrational anharmonicity. It is concluded that CFM line splittings can be effectively used as a probe of energy transfer rates in six-atom molecules provided the modes under examination have reasonably isolated bands in the power spectrum.
Luminescence spectra of matrix isolated N2 at high pressure and low temperature
Authors: Semling M. , Jordan M. , Syassen K. , Jodl H. J. , Signorini G. F.
Published: July 2002

N2 molecules with a concentration of ca. 1% were isolated in Xe, Kr, and Ar. X-ray stimulated luminescence A ∑u+2→X ∑g+1) was investigated as a function of pressure (<30 GPa), temperature (300 K and 77 K), and matrix material. Observed UV transitions were interpreted as rovibronic transitions of the impurity molecule along with phonon sidebands as lattice excitations. Spectra were analyzed due to band maxima of vibron progressions, bandwidth, and bandshape, Franck–Condon profiles, and molecule constants, each as a function of pressure. The main result was a shift of band maxima with pressure to lower energies at a rate of 100 cm−1/GPa in Xe, 50 cm−1/GPa in Kr, and 10 cm−1/GPa in Ar. We modeled and explained these pressure-induced shifts of electronic bands by two mechanisms: a dielectric effect and a volume effect.
Experimental and theoretical studies of the monolayer structure of OCS adsorbed on NaCl(001): Coexistence of orientationally inequivalent phases
Authors: Picaud S. , Girardet C. , Glebov A. , Toennies J. P. , Dohrmann J. , Weiss H.
Published: July 2002

The structure of a physisorbed carbonyl sulfide (OCS) monolayer on a well-defined NaCl single crystal surface has been studied by helium atom scattering (HAS), low energy electron diffraction (LEED) and polarized Fourier-transform infrared spectroscopy, in the temperature range 30–100 K. The monolayer growth proceeds via large islands and both HAS and LEED indicate a (2×1) commensurate structure with two molecules per unit cell. The binding energy is estimated to be about 0.26 eV from desorption experiments. By contrast, the occurrence of three intense and three weaker absorption peaks in the infrared spectrum of the asymmetric stretching vibration ν3 is difficult to reconcile with the conclusions drawn from HAS and LEED. Therefore semi-empirical potential calculations are performed to determine the monolayer structure. Energy minimization rules out a high molecular density for the layer, and shows the existence of two isoenergetical (2×1) and (2×2) phases with respectively two and four molecules per unit cell, which differ only by the orientations of the molecules. Because of their similar geometry, these two structures cannot be discriminated by HAS and LEED, but the mixing of these two phases with different vibrational couplings between the admolecules provides a reasonable interpretation of the main features of the infrared spectrum.
The mobilities of ions and cluster ions drifting in polar gases
Authors: de Gouw Joost A. , Krishnamurthy M. , Leone Stephen R.
Published: July 2002

The mobility of ions drifting in polar gases is explored both theoretically and experimentally. New experimental results are presented for (i) NO+ ions drifting in H2O (the reduced zero-field mobility K0(0) is 0.66±0.07 cm2 V−1 s−1), (ii) H3O+(H2O)3 ions drifting in H2O (K0(0)=0.43±0.06 cm2 V−1 s−1), and (iii) NO+(CH3COCH3)n ions (n=2,3) drifting in CH3COCH3 (K0(0)=0.041 ±0.010 cm2 V−1 s−1 for n=2 and K0(0)=0.050±0.015 cm2 V−1 s−1 for n=3). A number of theoretical models for ion mobilities in polar gases are described. The models are compared with the available experimental data and a reasonable agreement is obtained. For larger cluster ions the measured mobilities are considerably smaller than the calculated values. Some possible reasons for the discrepancies are discussed, including momentum transfer outside the capture cross section, dipole–dipole interactions, ligand exchange, inelastic collisions, and the validity of Blanc’s law.
Photochemistry of dimethylcadmium on compound semiconductor surfaces
Authors: Lasky P. J. , Lu P. H. , Khan K. A. , Slater D. A. , Osgood, Jr. R. M.
Published: July 2002

We present dynamical studies of the surface photochemistry of dimethylcadmium (DMCd) adsorbed on GaAs(110) and CdTe(110) surfaces. This important precursor for chemical vapor deposition is studied using time-of-flight (TOF) spectroscopy in conjunction with temperature programmed desorption (TPD). For the case of greater than one monolayer coverages, on these surfaces, both photofragmentation and photodesorption are observed following irradiation. The relative probabilities of these two processes are found to be strongly dependent on adsorbate coverage and incident photon energy. Direct photoabsorption by the adsorbed molecule is found to dominate the photofragmentation process, with the substrate strongly reducing the probability of photofragmentation at low coverages due to the operation of substrate mediated channels for excited state relaxation. The CH3 fragments observed following photodissociation are found to have kinetic energies that are invariant with the energy of the exciting photon (193 and 248 nm), an effect attributed to an efficient curve crossing between excited states in this molecule. The molecular desorption process is found to exhibit an unusual coverage dependence which is linked to the probability of deposition of a significant amount of vibrational excitation in the overlayer via substrate mediated de-excitation processes. Following the adsorption of one monolayer on the GaAs substrate, the photochemistry observed is significantly different due to the thermal dissociation of DMCd to form adsorbed methylgallium moieties. In this case, multiple features are observed in the TOF spectrum with kinetic energies significantly different to those observed for the gas phase or physisorbed molecule. Correlation of the TOF spectral features with thermal desorption data allows these TOF features to be assigned to specific adsorbed intermediates.
The thermal diffusion factor of the van der Waals binary mixture
Authors: Castillo Rolando , Orozco Jorge
Published: July 2002

The explicit dependence of the thermal diffusion factor with respect to composition and interaction parameters for the van der Waals binary mixture is obtained in the framework of the mean-field kinetic variational theory and in the scheme of Scott and van Konynenburg. Here, we present a numerical study where the global behavior of the thermal diffusion factor is described in terms of molecular masses, sizes, and interaction parameters, along the phase diagram of this model mixture. This numerical study allows us to understand what molecular parameters modify the sign of the thermal diffusion factor. In addition, a comparison is made between the thermal diffusion factor coming from the van der Waals mixture and from the hard-sphere mixture.
Electronic structure of CHClBrI
Authors: Novak Igor , Choon Ng Siu , Jin Shengxi , Potts Anthony W.
Published: July 2002

He I and He II photoelectron spectra (ultraviolet photoemission spectroscopy) of one of the smallest chiral molecules, CHClBrI, were recorded and analyzed on the basis of empirical arguments and density functional theory calculations. We had previously suggested that investigations into the possible phenomenon of circular dichroism in gas phase photoionization be made. This work reports a necessary study of the electronic structure of one of the prime candidates for such studies, the CHClBrI molecule. Its molecular and electronic structure and orbital interactions are discussed in the light of these experimental and theoretical results.
Site-specific fragmentation following Si:2p core-level photoionization of F3SiCH2CH2Si(CH3)3 condensed on a Au surface
Authors: Nagaoka Shin-ichi , Mase Kazuhiko , Nagasono Mitsuru , Tanaka Shin-ichiro , Urisu Tsuneo , Ohshita Joji
Published: July 2002

We used photoelectron spectroscopy and the energy-selected-photoelectron photoion coincidence method to study site-specific fragmentation following Si:2p photoionization of 1-trifluorosilyl-2-trimethylsilylethane [F3SiCH2CH2Si(CH3)3, FSMSE] condensed on a Au surface. The photoelectron spectrum of FSMSE has two peaks for 2p-electron emission: One for the Si atom bonded to three methyl groups (Si [Me]) and one for the Si atom bonded to three F atoms (Si [F]). H+ and F+ ions are predominantly desorbed coincidentally with the Si[Me]:2p and Si[F]:2p electrons.
Response to “Comment on ‘Long time behavior of the angular velocity autocorrelation function’ ” [J. Chem. Phys. 107, 291 (1997)]
Authors: Masters A. J.
Published: July 2002

Felderhof and Cichocki [J. Chem. Phys. 107, 291 (1997), preceding paper] claim that tagged particle reorientation should play no role in theories of the angular velocity autocorrelation function (AVACF) as it is a nonlinear effect. In this reply we show that reorientation does indeed contribute to the AVACF to linear order and that the conclusions of our previous paper hold true.
Resonant two-photon ionization spectroscopy of LiCu
Authors: Russon Larry M. , Rothschopf Gretchen K. , Morse Michael D.
Published: July 2002

Jet-cooled LiCu has been investigated using resonant two-photon ionization spectroscopy. A long vibrational progression was observed and identified as the [20.5] Σ+1←XΣ+1 band system. Ten bands of the system were rotationally resolved for Li637Cu, giving bond lengths of r0=2.2618(3) Å for the X1Σ+ state and re′=2.74(4) Å for the [20.5] Σ+1 state. The fitted spectroscopic parameters of the [20.5] Σ+1 state were used to obtain a RKR estimate of the [20.5] Σ+1 potential energy curve. The unusual shape of this curve is thought to derive from avoided crossings between the Li+Cu− ion pair state and covalent states, with the Li+Cu− ion pair state ultimately correlating to the ground electronic state of the molecule.
Three-dimensional analytical infinite order sudden quantum theory for triatomic indirect photodissociation processes
Authors: Grinberg Horacio , Freed Karl F. , Williams Carl J.
Published: July 2002

Our previously developed analytical infinite order sudden (IOS) quantum theory of triatomic photodissociation is extended to describe indirect photodissociation processes through a real or virtual intermediate state. The theory uses the IOS approximation for the dynamics in the final dissociative channels and an Airy function approximation for the continuum states. These approximations enable us to evaluate the multi-dimensional non-separable transition amplitudes analytically (as one-dimensional quadratures), despite the different natural coordinates for the initial bound, the intermediate resonant, and the final dissociative states. The fragment internal energy distributions are described as a function of the initial and final quantum states and the photon excitation energy. The theory readily permits the evaluation of rotational distributions for high values of the total angular momentum J in the initial bound molecular state, a feature that would be very difficult with close-coupled methods. In paper II we apply the theory to describe the photofragment yield spectrum of NOCl in the region of the T1(13A″)←S0(11A′) transition.
Spontaneous emission in lipid-water system
Authors: Cho Minhaeng
Published: July 2002

A theoretical description of the quantum electromagnetic field in the lipid-water system is presented, and the spontaneous emission rate of an excited molecule bound in the lipid layer is calculated. By using the reflectivity data of liquid water, the emission rates as a function of the wavelength and the index of refraction of hydrocarbon are calculated. Depending on the alignment of the transition dipole, the spontaneous emission rate is found to be modified significantly. When the alignment of the transition dipole is perpendicular to the lipid-water interfaces, the emission rate is more strongly suppressed by the presence of the boundaries, in comparison to the parallel case. It is suggested that one could experimentally measure this alignment dependence of the spontaneous emission process in the lipid-water system, and some possible implications to the antenna system in the photosynthesis are briefly discussed.
Disordered and ordered C28 solids
Authors: Kim Jeongnim , Galli Giulia , Wilkins John W. , Canning Andrew
Published: July 2002

Using tight-binding molecular dynamics, we have performed computer experiments to mimic the gas phase growth of a disordered solid composed of C28 fullerenes. The growth has been simulated by repeated low energy collisions of molecules coming from random directions. The resulting solid is composed of undamaged C28 cages, with most fullerenes being three- and four-fold coordinated, similar to C atoms in amorphous materials. The system contains a high percentage of distorted sp2 C sites and only a small proportion of sp3 sites. These results help clarify the structure of disordered films obtained experimentally by small fullerene deposition on surfaces. Furthermore, we have compared the properties of the disordered C28 solid (a-C28) with those of ordered C28 solids. We have found that the energy of a-C28 is close to that of hyperdiamond (0.1 eV/atom higher) and differs by a few meV from that of other ordered structures, such as 2D-hypergraphite, hexagonal and clathrate solids. This indicates that in condensed phases C28 molecules can act as carbon superatoms, while showing more bonding flexibility than C atoms; in particular the capability of acting as six-fold coordinated building blocks of hexagonal solids, which are as stable as a-C28.
Deuteron NMR studies of guest motion in alkanoic acid urea inclusion compounds
Authors: Vold Robert L. , Hoatson Gina L. , Subramanian Raju
Published: July 2002

The orientation dependence of deuteron relaxation times of Zeeman (T1Z) and quadrupole (T1Q) order have been measured in the temperature range −30 to +45 °C for polycrystalline urea inclusion compounds containing alkyl deuterated octanoic acid-d18, dodecanoic acid-d23, and hexadecanoic acid-d31. These data were fitted to a model, based on Redfield relaxation theory, which incorporates rapid rotation about the long molecular axis and slower, limited angle libration of individual CD bonds about a perpendicular axis. The activation energies for rotational diffusion about the long axis are 17.4±0.8 kJ/mol for octanoic acid-d18, 14.4±0.6 kJ/mol for dodecanoic acid-d23, and 22.5±0.4 kJ/mol for hexadecanoic acid-d31 guests. These values are larger than those for alkane guests with the same number of carbon atoms, and they increase more rapidly with increasing chain length. Quadrupole coupling constants, determined at room temperature from measurements on large single crystals, are consistent with a small gauche population near the methyl end of the alkanoic acid chains. Analytic models which relate the measured quadrupole coupling constants and relaxation times to conformational probabilities and dynamics require the assumption that rotations about individual C–C bonds are uncorrelated. Newtonian molecular dynamics simulations reveal that this incorrect assumption introduces significant error.
State-specific desorption in condensed Si(CH3)2Cl2 following resonant excitation at the Cl 2p edge
Authors: Chen J. M. , Lu K. T. , Liu R. G. , Lay J. W. , Liu Y. C. , Chuang T. J.
Published: July 2002

Photon-stimulated ion desorption (PSID) from solid Si(CH3)2Cl2 following Cl 2p core-level excitation has been investigated along with the resonant photoemission spectra using synchrotron radiation. The Cl L23-edge total-electron yield spectrum of solid Si(CH3)2Cl2 mainly exhibits the Cl 2p→15a1*, Cl 2p→10b1* and shape resonance peaks. A significant dissimilarity between the Cl L23-edge total-electron yield spectrum and Cl+ PSID spectrum of solid Si(CH3)2Cl2 is observed. The desorption of atomic Cl+ ions is detected predominantly at the Cl 2p→15a1* excitation, while scarcely any Cl+ ions are observed following the Cl 2p→10b1* excitation and Cl 2p→shape resonance excitation. This indicates that the character of bound terminating orbital has a significant influence on the desorption processes. The enhanced Cl+ desorption yield at the Cl 2p→15a1* resonance is attributed to the the fast desorption process originating from the localization of the spectator electron in the strong antibonding orbital.
Comparison of three Brownian-dynamics algorithms for calculating rate constants of diffusion-influenced reactions
Authors: Zhou Huan-Xiang
Published: July 2002

A new algorithm for calculating the rate constants of diffusion-influenced reactions from Brownian-dynamics simulations is introduced and compared with two previous algorithms. It is based on the mean residence time of the pair of reactant molecules in the reactive region after the molecules are started from that region. Of the previous algorithms, one is based on the capture probability of one reactant molecule started on a spherical surface enclosing the other reactant molecule [Northrup et al., J. Chem. Phys. 80, 1517 (1984)]; the other is based on the survival probability of the pair of reactant molecules started in the reactive region [Zhou, J. Phys. Chem. 94, 8794 (1990)]. In the implementation of the residence-time based algorithm, analogy can be drawn between diffusion-influenced bimolecular reactions and diffusive energy-barrier crossing processes. When the reactive region is small, the pair of reactant molecules will be near the reactive region even after many multiples of the mean residence time have elapsed. Hence the residence time in the reactive region will not be significantly affected by the presence of an interaction potential U if the potential is smooth around the reactive region. This rationalizes an earlier analytic result k=k0〈exp(−U/kBT)〉, where k and k0 are the rate constants in the presence and absence of the potential. The three simulation algorithms are applied to the binding of a pointlike ligand to an immobile sphere with a reactive patch in the presence and absence of a Coulomb potential. The survival-probability based algorithm is always the most accurate and efficient one.
Towards a quantum-chemical description of crystalline insulators: A Wannier-function-based Hartree-Fock study of Li2O and Na2O
Authors: Shukla Alok , Dolg Michael , Fulde Peter , Stoll Hermann
Published: July 2002

A recently proposed approach for performing electronic-structure calculations on crystalline insulators in terms of localized orthogonal orbitals is applied to the oxides of lithium and sodium, Li2O and Na2O. Cohesive energies, lattice constants and bulk moduli of the aforementioned systems are determined at the Hartree-Fock level, and the corresponding values are shown to be in excellent agreement with the values obtained by a traditional Bloch-orbital-based Hartree-Fock approach. The present Wannier-function-based approach is expected to be advantageous in the treatment of electron-correlation effects in an infinite solid by conventional quantum-chemical methods.
Optical and x-ray studies of thin mixed crystals of NaCl-typed RbI and CsCl-typed CsI
Authors: Hirai T. , Ichimura N. , Hashimoto S. , Nakayama T. , Yoshimura Y. , Iwasaki H.
Published: July 2002

Thin Rb1−xCsxI mixed crystals have been grown from the melt in a very small gap formed in between a pair of quartz plates and reflection spectra were measured in the uv-light region at low temperature. Continuous spectral changes of the exciton bands with the composition x suggest strongly the formation of extended solid solution both in the RbI-rich (0.0⩽x⩽0.4) and CsI-rich (0.7⩽x⩽1.0) regions. The formation was confirmed by x-ray diffraction, and the compositional change of the lattice constant was determined. These results are in contrast to the eutectic type phase relation with only a limited solid solubility observed in the bulk state of the RbI–CsI binary system.
Exact distribution function for discrete time correlated random walks in one dimension
Authors: Hanneken J. W. , Franceschetti D. R.
Published: July 2002

A discrete time correlated random walk in one dimension is investigated. Combinatorial arguments are used to calculate the exact probability distribution PN(L), the probability that the correlated random walker arrives at a distance L steps to the right of its starting point after exactly N steps. PN(L) is calculated using arbitrary initial conditions which permit the influence of end effects and boundary conditions to be calculated and several special cases are considered in detail. PN(L) with arbitrary initial conditions is calculated both with and without a bias for motion in one direction yielding a useful model for the combined diffusion and drift of charged particles undergoing a correlated random walk in an applied field. The relation of the correlated random walk to the Ising model is also discussed.
Measurements of the infrared spectra and vapor pressure of the system carbon dioxide–acetylene at cryogenic temperatures
Authors: Gough T. E. , Rowat T. E.
Published: July 2002

The system carbon dioxide–acetylene at cryogenic temperatures has been reinvestigated. Measurements have been made of the vapor pressure as functions of composition and temperature, and of infrared spectra of cryofilms at 90 K as a function of composition. It was concluded that, under suitable conditions, carbon dioxide and acetylene can co-crystallize to form cubic crystals of the stoichiometry CO2⋅C2H2, and that this phase is metastable with respect to the two pure phases. This metastability was confirmed by following the decomposition of CO2⋅C2H2 at 90 K as a function of time, and the fractional decomposition was found to vary sigmoidally with time. The kinetics of the decomposition are roughly described by a model in which the rate increases as product nuclei grow and decreases as the reactant is depleted (Avrami model).
A primitive model of a charged hard ellipsoidal fluid
Authors: Rickayzen G.
Published: July 2002

A method is developed for the theoretical investigation of the structure of fluids comprised of hard nonspherical molecules carrying electric charge. The development is based upon the mean spherical approximation for the direct correlation function of such fluids. Since the equations based upon this approximation cannot in general be solved analytically an approximate ansatz for the direct correlation function containing a small number of free parameters is introduced. The free parameters are then determined from a standard variational principle. The ansatz itself is chosen to produce the known results in the strong coupling (large charge) limit and in the special case of hard spheres. In order to ensure that the solution in the limit of strong coupling has a tractable analytic form a new model is proposed for the charge distribution on an individual molecule. The method is applied in detail to a model of charged hard ellipsoids which is a generalization of the primitive model for ionic fluids and is found to be practical; it reduces to the primitive model in the special case of charged hard spheres. It is shown that the approximation preserves the conservation of charge. Properties investigated include the direct correlation function itself, the electrostatic energy of the fluid, the pair distribution function and the electrical potential surrounding an individual molecule. Results for these quantities are obtained for a range of densities and charge covering four orders of magnitude and for molecules with elongations (ratios of lengths of axes) from 0.5 (oblate) to 10 (prolate). The direct correlation function itself is given in an analytic form which can be used together with an appropriate density functional to investigate the structure of the electrical double layer formed by the fluid at a solid boundary.
Resonant two-photon ionization spectra of van der Waals complexes p, m, o-C6H4F2⋯NH3(ND3)
Authors: Hu Yihua , Yang Xin , Yang Shihe
Published: July 2002

We have studied the resonant two-photon ionization (R2PI) spectra of three van der Waals (vdW) complexes p, m, o-C6H4F2⋯NH3(ND3) through the S1←S0 transition with mass selectivity. The stretching frequencies of the three vdW complexes were found to be quite close (86 cm−1). From the photodissociation mechanism and the relationship between anharmonicity of the stretching vibration and the dissociation energy, we estimated the dissociation energies of all the three vdW complexes in the S0 and S1 states. A quantum ab initio calculation on p-C6H4F2⋯NH3 at the MP2/6-31G** level gave the following geometry: The N atom of NH3 is located on the symmetry axis (Z-axis) and 3.53 Å above the benzene ring; the C3 axis of NH3 is at an angle of 52.5° with the Z-axis of p-C6H4F2 with one of the hydrogen atoms pointing towards the benzene ring; the rotation of NH3 around the Z-axis is nearly free. The calculated bond dissociation energies and the expectation of internal rotation are consistent with our experimental results.
Disappearances of energy minima and loss of order in polydisperse colloidal systems
Authors: Lacks Daniel J. , Wienhoff Jeffrey R.
Published: July 2002

Simulations are carried out to determine how polydispersity (δ) affects the potential energy surface of a colloidal system. Increases in polydispersity are shown to cause the disappearance of potential energy minima, such that energy minima corresponding to homogeneous highly ordered structures exist only for δ<0.10, and energy minima corresponding to homogeneous partially ordered structures exist only for δ<0.15. These results show that colloidal systems cannot form homogeneous crystals, even as metastable phases, above certain values of polydispersity.
Solvation dynamics of fluoroprobe in diethylether
Authors: Brown Ross , Middelhoek Rob , Glasbeek Max
Published: July 2002

We present classical molecular dynamics simulations of the solvation dynamics of a strongly polar dye molecule in a weakly polar solvent. Solvation times and amplitudes are compatible with recent measurements. Several differences in the solvation dynamics relative to the widely studied case of strong polar solvents are exhibited: lesser importance of the inertial component of the Stokes shift, relaxation times close to individual solvent reorientational times, instead of the longitudinal relaxation time of the solvent, large amplitudes of translational and orientational relaxation over about 10 ps and the presence of a long-lived solvation shell in the excited state.
Coexistence curve and turbidity of aqueous solutions of oligooxyethylene alkyl ether with the addition of urea near the critical point
Authors: Shimofure Shinsuke , Kubota Kenji , Kita Rio , Dobashi Toshiaki
Published: July 2002

Coexistence curves and turbidity of aqueous solutions of tetraoxyethylene mono-n-octyl ether C8E4 without and with the addition of urea were examined focusing on the critical behavior. The critical exponent β, which characterizes the shape of coexistence curve, for aqueous C8E4 solution was determined as 0.329±0.003 in agreement with 3D Ising model. Turbidity measurement for aqueous C8E4 solution also yielded Ising values γ=1.26±0.05 and ν=0.63±0.02 for the critical exponents of the isothermal compressibility and correlation length, respectively. Addition of urea up to 7 M concentration did not modify the critical exponents, although the critical temperature increased by about 20 °C with the addition of urea. Coexistence curves of oxyethylene mono-isobutyl ether C4E1 with the addition of 7 M urea also yielded β=0.326±0.002 in agreement with 3D Ising model. These are in contrast to our recent results that the critical exponents for heptaoxyethylene mono-n-tetradecyl ether C14E7 in an aqueous salt solution were modified to Fisher’s renormalized Ising values by the addition of salt [J. Chem. Phys. 109, 711 (1998)]. The difference in the critical behavior for the nonionic surfactant solutions might be attributed to the solution structure, structure of micellar aggregation of surfactant.
Auger decay of the C 1s−12π* resonance in carbon monoxide: Vibrationally and angularly resolved spectra
Authors: Kukk E. , Bozek J. D. , Cheng W.-T. , Fink R. F. , Wills A. A. , Berrah N.
Published: July 2002

Auger electron spectra from the decay of the ν=0, 1, and 2 levels of the C 1s−12π* state were measured with sufficiently high electron and photon energy resolution to completely resolve the vibrational structure of the final electronic states. The results are compared with ab initio calculations with emphasis on the analysis of the spectator Auger transitions. The anisotropy parameters extracted from angle-resolved spectra show variations within the vibrational envelopes of the participator Auger decay and exhibit a complex oscillating behavior for the spectator Auger transitions. In addition, two-dimensional (2D) imaging of the electron emission across the C 1s−12π* resonance was performed. The ratio of resonant and nonresonant ionization processes is estimated from the 2D map. Strong vibrational lifetime interference effects apparent in the 2D imaging are discussed.
Response to “Comment on ‘Molecular simulation and continuum mechanics study of simple fluids in nonisothermal planar Couette flows’ ” [J. Chem. Phys. 111, 10730 (1999)]
Authors: Khare Rajesh , de Pablo Juan , Yethiraj Arun
Published: July 2002

This response clarifies some of the issues raised in the comment by Todd et al. It is emphasized that at the typical shear rates currently used in nonequilibrium molecular-dynamics simulations, fluids undergo significant viscous heating. While artificial thermostats can be used to keep the temperature constant, we have shown that actual confined fluids at high shear rates are unable to dissipate heat at the rate required to avoid viscous heating. This raises questions as to the relevance of thermostating an entire confined fluid subject to a high shear rate; such a situation is simply not physically realizable. In our work we argue that removing heat through the walls of a confined system provides a better representation of reality.
Collisional effects on angular momentum orientation in acetylene X̃ 1Σg+ (ν2″=1,j″). II. Disorientation by rotationally elastic and multiple inelastic collisions
Authors: Rudert Armin D. , Martin José , Gao Wen-Bin , Zacharias Helmut , Halpern Joshua B.
Published: July 2002

This is the second of two papers about collisionally induced changes in molecular orientation. In the first paper [A. D. Rudert, J. Martin, W. B. Gao, J. B. Halpern, and H. Zacharias, J. Chem. Phys. 111, 9549 (1999)] the orientation was measured in the prepared state and in other states populated by collisional transfer from the initially excited one. It was shown that a significant amount of the initial orientation is retained in collisions, even for large changes in the rotational quantum number. In this paper the decay of the orientation due to elastic and multiple inelastic collisions is investigated. The measurements clearly show that for acetylene self-collisions the orientation decay [〈koritotal〉=7.6±1.0 (μs Torr)−1] is much slower than the depopulation of the prepared rotational state [〈ktot〉=25±1.8 (μs Torr)−1]. By using a set of master equations, rate constants are derived which describe the effects of both rotationally elastic and multiple inelastic collisions. From this model rate constants for orientation decay due to rotationally elastic collisions, korielastic, can be derived. These rate constants decrease from korielastic=10.7 (μs Torr)−1 for j″=1 to korielastic=3.8 (μs Torr)−1 for j″=15. The rate constants for orientation decay are found to be equal to previously measured rate constants for the alignment decay. A model describing the collisionally induced change of the direction of the molecular angular momentum vector is presented which reconciles both alignment and orientation decay measurements. It is shown that mj-changing, rotationally elastic collisions completely destroy any orientation or alignment and probably occur perpendicular to the plane of molecular rotation. This is in contrast to rotationally inelastic collisions which occur primarily in the plane of rotation.
Equilibrium states of inhomogeneous mean-field quantum spin systems with random sites
Authors: Blobel A. , Messer J.
Published: July 2002

For inhomogeneous mean-field quantum spin systems with random sites, we prove that the weak*-limit points of the sequence of n-particle Gibbs states, on a suitable C*-algebra, are superpositions of inhomogeneous product states, which are given by solutions of a generalized Curie–Weiss equation. Under certain conditions there is only one weak*-limit point. As applications, the BCS model and a spin system with cosine interaction are studied.
Strong approximation of time evolution operators for a finite system of oscillators with nonlinear coupling
Authors: Kröger Helmut
Published: July 2002

We consider a system in three space dimensions consisting of a finite number of oscillators with a nonlinear interaction. Using projectors on N-particle subspaces of the Fock space, we show that the time evolution operator is strongly approximatable by exponentials of self-adjoint finite-rank operators (finite-dimensional Hermitian matrices), which can easily be calculated in the corresponding eigenrepresentation.
Dual physical interpretation of the energy tensor: Nonexistence of the Kerr–Newman example
Authors: Ferdon D. W. , Safko J. L.
Published: July 2002

Several recent papers have dealt with the possibility of interpreting the Kerr–Newman metric as a viscous fluid as well as its usual interpretation of a rotating, charged black hole. In this paper we show that there is no possible viscous fluid with the Kerr–Newman metric.
The dynamical structure of gravitational theories with GL(4,R) connections
Authors: Antonowicz Marek , Szczyrba Wiktor
Published: July 2002

We investigate here GL (4,R)-gauge theories of gravity based on variational principles. The components of tetrad fields eμ(α), the components of metrics g(α)(β), and the components of connections Γλ(α)(β) are taken as the gravitational potentials. Matter potentials are the components of GL (4,R)-tensor fields φΣ. We derive the conservation laws for a general theory, that is, the Belinfante–Rosenfeld and Bianchi identities, and find minimal systems of independent variational equations. The natural GL (4,R)-covariant Hamiltonian formulation of the theory induces a GL (3,R)-covariant Hamiltonian formulation related to a chosen slicing of space-time. The Hamiltonian field equations corresponding to this formulation describe the dynamics of the system. We determine 20 symplectic constraints, 20 gauge transformations, and 20 gauge variables generic for a general gravitational Lagrangian. As an example, we consider the Gl (4,R)–Einstein theory in vacuum as well as in the presence of a vector field and find the complete canonical formulation in both cases.
The representation of Lie groups by bundle maps
Authors: Sen R. N.
Published: July 2002

This paper deals with representations of connected Lie groups by bundle maps of fiber bundles. It is pointed out that a large class of such representations can be obtained from the bundle structure theroem, and explicit constructions are given, first on principal bundles and then on associated bundles. Examples are provided to show that, for bundle representations, the theorem of full reducibility breaks down even for compact Lie groups. Finally, a general construction is given for obtaining representations of a Lie group on an arbitrary principal bundle. However, it is not known whether this exhausts all possibilities.
Spontaneous compactification and Ricci-flat manifolds with torsion
Authors: McInnes Brett
Published: July 2002

The Freund–Rubin mechanism is based on the equation Rik=λgik (where λ>0), which, via Myers’ theorem, implies ‘‘spontaneous’’ compactification. The difficulties connected with the cosmological constant in this approach can be resolved if torsion is introduced and λ is set equal to zero, but then compactification ‘‘by hand’’ is necessary since the equation Rik =0 can be satisfied both on compact and on noncompact manifolds. In this paper we discuss the global geometry of Ricci-flat manifolds with torsion, and suggest ways of restoring the ‘‘spontaneity’’ of the compactification.
Dual mass, ℋ-spaces, self-dual gauge connections, and nonlinear gravitons with topological origin
Authors: Magnon Anne
Published: July 2002

An analogy between source-free, asymptotically Taub–NUT magnetic monopole solutions to Einstein’s equation and self-(anti-self-) dual gauge connections is displayed, which finds its origin in the first Chern class of these space-times. A definition of asymptotic graviton modes is proposed that suggests that a subclass of Penrose’s nonlinear gravitons or Newman’s ℋ-spaces could emerge from nontrivial space-time topologies.
Non-Hamiltonian symmetries of a Boussinesq equation
Authors: Ten Eikelder H. M. M. , Broer L. J. F.
Published: July 2002

For a class of Hamiltonian systems there exist infinite series of non-Hamiltonian symmetries. Some properties of these series are illustrated using a Boussinesq equation. It is shown that the recursion operators generated by these non-Hamiltonian symmetries are powers of the original recursion operator. A class of recursion formulas for the constants of the motion (not for the corresponding symmetries!) is given.
A simple Grassmannian path integral representation of the Dirac propagator
Authors: Rumpf Helmut
Published: July 2002

Starting from the affinely parametrized supersymmetric Dirac particle model a Grassmannian path integral expression for the propagator of the Dirac equation, minimally coupled to an external electromagnetic field, is derived. A purely ‘‘bosonic’’ path integral representation of the propagator of the iterated minimally coupled Dirac equation is also obtained. It appears that a Nicolai mapping exists, even in a formal sense, only in the case of a constant external field.
Coordinate-independent formulation of the Langevin equation
Authors: Batrouni G. G. , Kawai H. , Rossi Pietro
Published: July 2002

A diffusion process on a compact Riemannian manifold is considered, and a coordinate-invariant Fokker–Planck equation is formulated. A covariant form of the Langevin equation is also derived, and the formalism is applied to the stochastic quantization of lattice gauge theories.
An application of filtering theory to parameter identification in quantum mechanics
Authors: Beumee J. G. B. , Rabitz H.
Published: July 2002

A method for inverting observations on quantum mechanical systems to obtain estimates of unknown parameters residing in the Hamiltonian is presented. The quantal system is represented in matrix form with respect to a chosen basis, and it is assumed that the associated expansion coefficients are truncated to a finite dimension. The uncontrollable laboratory noise will be modeled by means of an inhomogeneous white noise process so that the experimental observations are represented as stochastic variables satisfying a stochastic differential equation. It will be assumed that measurements obtained from an experiment are now equivalent to a realization of these stochastic variables. It is known from filtering theory that the minimum variance estimate of the unknown parameters in the quantal model is now given by the expectation of the unknowns conditional on this realization. This estimator can be calculated analytically from the associated a posteriori probability density if the original quantal system does not contain any random elements. This probability density for the unknown matrix elements is calculated, and it is demonstrated that for a full Hamiltonian matrix the asymptotic variance of the parameter estimator decreases as a third power in time and a fourth power in the initial conditions. Some differences with the minimum least-square method are mentioned, and a few issues of numerical implementation are discussed.
On a Weyl-type theorem for higher-order Lagrangians
Authors: Castagnino M. , Domenech G. , Noriega R. J. , Schifini C. G.
Published: July 2002

Weyl’s theorem is extended making use of the theory of concomitants to obtain a Lagrangian density for the massless bosonic fields without dimensional constants. It turns out to be quadratic in the gravitational field and encompasses all the theories that usually appear in the literature. It is shown that the gauge invariance of the Lagrangian follows from the invariance of the field equations.
On head-wave amplitudes
Authors: Drijkoningen G. G. , Chapman C. H. , Thompson C. J.
Published: July 2002

The head-wave contribution to a reflection is investigated by two different methods and it is shown that the new result presented by Lerche and Hill [J. Math. Phys. 26, 1420 (1985)] for the head-wave amplitude is in error due to the use of an inappropriate mathematical method.
Maxwell’s equations for a transverse field
Authors: Gordon William B.
Published: July 2002

Hodge–deRham theory is applied to Maxwell’s equations for a transverse electromagnetic wave with a given wave-front surface S. It is shown that the E and B fields, considered as tangent fields to S, are harmonic in the sense of Hodge theory. If S is a spheroid it is known that the space of harmonic fields on S has dimension zero, and hence transverse fields with spheroidal wave fronts do not exist. The same result holds, but for a different reason, if S is a noncircular cylinder or a surface of revolution, and it is conjectured that smooth, singularity-free, transverse solutions to Maxwell’s equations exist only if S is a plane or a circular cylinder.
On hyper-relativistic quantum systems
Authors: Maduemezia Awele
Published: July 2002

A hyper-relativistic system is defined as one whose equation of motion is form invariant under coordinate transformations induced by a semisimple group whose algebra is contractible to the algebra of the Poincaré group. Such a system lies, categorically, in the domain between the special theory of relativity and the general theory, for whereas the former requires covariance under transformations between inertial systems, the latter imposes covariance with respect to arbitrary continuous transformations. In this paper, a new interpretation of a particular fiber-bundle structure constructed on the timelike homogeneous space M=SO(4,2)/SO(4,1) is presented, and Minkowski space-time is realized as a subspace of the standard fiber of the tangent bundle over this hyperquadric. Through the process of group contraction, coupled with the commutation of the momentum vector fields with the principal bundle of linear frames with which the tangent bundle is associated, a hierarchy of ‘‘Heisenberg commutation relations,’’ parametrized by the point spectrum of the center of the contracted algebra, is obtained. The classical Newtonian gravitational potential field enters as the fifth coordinate of an extended space-time manifold.
One loop effective measure for sigma models coupled with gravity
Authors: Guadagnini E.
Published: July 2002

The one loop effective measure for a two-dimensional generalized sigma model with torsion in the presence of an arbitrary gravitational background is computed. The consequences of the result on the relation between the conformal anomalies and the divergences of the model are considered.