bluuebunny/crossref_metadata_2025 huggingface dataset:
Chemical relaxation of H bonds in formic acid vapor studied by resonant photoacoustic spectroscopy
Authors: Winkler A. , Mehl J. B. , Hess P.
Published: July 2002

The chemical relaxation of hydrogen bonds in dimeric formic acid (methanoic acid) was studied in the gas phase by means of precise measurement of the speed of sound and the sound absorption as functions of the pressure and the temperature by exciting standing acoustic waves. The first radial acoustic resonance of a cylindrical cavity was excited by a modulated CO2 laser beam. Resonance profiles were measured and recorded by a computer-controlled system. Due to the high information content of this method, a consistent set of thermodynamic and kinetic parameters can be obtained. The equilibrium constants of the dimer/monomer and the cis-/trans-monomer equilibrium, the dissociation rate constant of (HCOOH)2 and the mean relaxation time of the vibrational states of the monomer–dimer mixture were determined by fitting a detailed theoretical model of the resonator to the measured values for the resonance frequency and the resonance broadening for total pressures in the range from 0.3 to 50 mbar and temperatures from 290 to 325 K. We obtained 159±2 J/(mol K) for the entropy and 61.8±0.5 kJ/mol for the enthalpy of dissociation at 300 K. We inferred a value for the enthalpy of isomerization of 10±5 kJ/mol. For the first time the pressure dependence of the dissociation rate constant was determined. It was found that the unimolecular decay is in the second-order regime at these low pressures as expected. The mean collision efficiency for the dissociation process relative to the dimer was obtained for HCOOH, He, and Ar to be 0.5±0.2, 0.05±0.02, and 0.08±0.02, respectively, independent of the temperature. We measured an average (pτ) value of 10±2 ns bar for the relaxation of the vibrational degrees of freedom. The activation energy of the dissociation of dimeric formic acid was determined to be 33±1 kJ/mol.
Bonding between CO and the MgO(001) surface: A modified picture
Authors: Nygren Martin A. , Pettersson Lars G. M. , Barandiarán Zoila , Seijo Luis
Published: July 2002

The interaction energy and equilibrium distance for adsorption of CO on a perfect MgO(001) surface has been calculated, using a cluster approach for representing the surface. The cluster size has been varied from one single Mg2+ ion up to Mg14O5. Ab initio model potentials (AIMP) were used to embed the explicitly described cluster while the rest of the crystal was taken to be point charges. The AIMP model potentials are shown to be a necessary and reliable improvement, compared with using only point charges as a model for the surrounding crystal. It is found that the electrostatic attraction and Pauli repulsion almost cancel each other and the small binding energy obtained is ascribed to dispersive forces. The interaction energy is calculated to be 0.07–0.09 eV (depending on which approach to basis set superposition errors corrections is taken); this is rather low in comparison with the experimental value of 0.3–0.4 eV.
The reorientational behavior of nematic and smectic liquid crystals in a magnetic field
Authors: Magnuson Matthew L. , Fung B. M.
Published: July 2002

A new method which uses carbon-13 NMR with rapidly decelerated sample spinning to study the reorientation of a nematic and a smectic liquid crystal is presented. When a macroscopically ordered liquid crystal is subjected to a new torque of sufficient magnitude, the direction of the macroscopic ordering changes in response to the new torque. In our experiments, the liquid crystal is spun rapidly at an angle slightly less than the magic angle, so that its director aligns along the spinning axis. The rapid spinning is suddenly stopped, and the liquid crystal molecules reorient so that the director would align along the magnetic field. The carbon-13 chemical shift, which is very sensitive to the angle that the molecular segment forms with respect to the magnetic field, is observed as a function of time following the stopping of the spinner. By fitting the chemical shift data as a function of time to a physically meaningful equation, a time constant can be calculated for various molecular segments of the liquid crystal molecule. This time constant provides information about the macroscopic reorientation of the liquid crystal from the alignment caused by the spinning torque to the alignment caused by the magnetic field. A room temperature nematic, 4-n-pentyl-4′-cyanobiphenyl (5CB), and a room temperature smectic, 4-n-octyl-4′-cyanobiphenyl (8CB), are studied by this technique. The reorientation time constant of 8CB is about 300 times larger than that of 5CB, and the reorientation time constant of the aromatic core is slightly smaller than the reorientation time constant of the aliphatic chain for both compounds. These results are among the first experimental data in the field of studying liquid reorientation on the level of the molecular segment, and a possible explanation for these results is presented.
Adiabatic rotation of effective spin. II. Spin-rotational relaxation
Authors: Serebrennikov Yu. A. , Steiner U. E.
Published: July 2002

The theory of electron spin-rotational (SR) relaxation in systems with an effective spin Seff=1/2 is formulated in terms of the adiabatic rotation of effective spin (ARES) approach. It is shown that SR relaxation results from the orientational random walk of the axes of the intramolecular electric field potential (ligand field) to which a spin-bearing atomic center is exposed. The validity of the stochastic treatment presented here is not limited by the Redfield conditions. The general expression obtained for the time constant of electron spin relaxation in liquid phase reproduces the well-known result of Hubbard–Atkins–Kivelson theory if it is specialized to the case of systems with weak spin–orbit coupling.
Guided-ion beam measurements of X++NO (X=Ar, N2) reactions
Authors: Graul Susan T. , Williams Skip , Dressler Rainer A. , Salter Richard H. , Murad Edmond
Published: July 2002

Cross section and product ion time-of-flight measurements are presented for the X++NO (X=Ar, N2) collision systems over a collision energy range of 0.1–20 eV (c.m.). The experiments are carried out in an octopole guided-ion beam apparatus. Charge transfer is the main channel observed and dissociative charge-transfer products are observed for collision energies equal to or greater than their respective thermodynamic thresholds. A weak channel leading to ArN+ and/or ArO+ is observed in the X=Ar system. The charge-transfer cross sections of the two collision systems are similar in magnitude and have approximately an E−1/2 dependence at near-thermal collision energies. The near-thermal cross sections are significantly less than the Langevin–Gioumousis–Stevens predictions for a reaction mediated by ion–induced dipole interactions and complex formation. The time-of-flight distributions of the Ar+, N+2+NO charge-transfer products are characteristic of near-resonant charge-transfer processes. At all collision energies studied, the charge-transfer reaction is dominated by a direct mechanism, but below 1 eV, the formation of a long-lived intermediate complex is also significant. The results are discussed in relation to statistical models and to constraints imposed by requirements of energy resonance and Franck–Condon overlap in charge-transfer reaction dynamics.
Analytic solution of the restricted primitive model of electrolyte solutions with double sticky interaction between the oppositely charged ions
Authors: Pizio Orest A.
Published: July 2002

A model for the description of the association phenomena and chemical reactions in the ionic fluids is considered. It generalizes the sticky electrolyte model and provides simultaneous formation of species with different bond length. The interaction between the oppositely charged ions is characterized by the presence of two narrow and deep attractive square wells inside a hard-core region. It determines the double sticky electrolyte model (DSEM). Application of the simple interpolation scheme by Zhou and Stell and the approximation by Rasaiah and Zhu for the cavity function is investigated for DSEM in order to provide the mass action limiting law. Analytic solution for the DSEM is presented.
The preparation of polymer melt samples for computer simulation studies
Authors: Brown David , Clarke Julian H. R. , Okuda Motoi , Yamazaki Takao
Published: July 2002

We have previously proposed a method for preparing dense amorphous polymer samples which was designed to circumvent the need for long periods of relaxation to equilibrium [J. I. McKechnie, D. Brown, and J. H. R. Clarke, Macromolecules 25, 1562 (1992)]. In the current article, we examine in more detail the application of the method to the preparation of polymer melts using precise data from massively parallel simulations. We expose deficiencies in the original method and introduce a modification which improves the equilibration. The limitations of the overall procedure are discussed in detail.
Conformational analysis of double-tethered chain molecules at an impenetrable interface: A Monte Carlo study
Authors: Jones Ronald L. , Spontak Richard J.
Published: July 2002

Due to tremendous interest in surface modification, numerous theoretical and simulation efforts have sought to elucidate the conformational properties of chain molecules grafted, or tethered, at one end to an impenetrable interface. Few studies have, however, addressed the analogous properties of chains in which both ends are anchored to an interface and behave as ‘‘sticky feet’’ in the presence of a good solvent. In this work, on-lattice Monte Carlo simulations have been performed to examine the characteristics of double-tethered chains in the limit of low interfacial coverage. Phantom and self-avoiding chains possessing up to 200 monomers are considered in 2 D random-flight simulations, as are corresponding non- and single-tethered chains to facilitate property comparison. While single-tethered chains adopt a more extended conformation than their nontethered analogs, segmental density distributions reveal that the existence of a second tethered site exerts little influence on monomers near the chain origin, but promotes overall coil localization near the interface. Scaling relationships for chain dimensions such as the root-mean-square (rms) end-to-end distance and coil radius are derived here in terms of both monomer and tether number.
NO electronic desorption processes from step sites on Pt(112): A comparison between photo- and electron-stimulated desorption
Authors: Heiz U. , Xu J. , Gadzuk J. W. , Yates J. T.
Published: July 2002

Using Fourier-transform infrared reflectance absorption spectroscopy as a surface analytical method, the total photo- and electron-stimulated desorption cross sections for specific chemisorbed NO species on the steps of Pt(112) have been measured. These species-specific cross sections for photo- and electron stimulated desorption are compared with each other. An unexpected result was obtained for photo-stimulated desorption, using photons with an energy of 3.5 eV; bridged-NO desorbs with a higher cross section (1.1×10−22 cm2) than terminal-NO (6.1×10−23 cm2). For electron stimulated desorption, using electrons of an energy of 275 eV, the opposite is observed. Terminal-NO desorbs with a higher cross section (2.3×10−18 cm2) than bridged-NO, with a cross section of 8.7×10−19 cm2. The photo-stimulated process is discussed within the ‘‘hot electron’’ model, and it is postulated that the higher expected quenching rate of the NO−(a) species produced from the more strongly-bound bridged-NO is more than compensated by propagation of the short-lived intermediate NO−(a) toward the metal surface. This leads to a closer approach to the surface of NO−(a) produced from bridged-NO than from terminal-NO, giving a higher probability of photodesorption of bridged-NO compared to terminal-NO. For the bridged-NO species, this is due to a steeper slope of the NO−(a) potential energy surface at the Franck–Condon excitation point compared to the slope for the terminal-NO which is initially located further from the surface. Electron stimulated desorption yields the expected relationship between the total desorption cross section for bridged- and terminal-NO, with the more strongly-bound bridged-NO having the smaller cross section. Here it is postulated that bridged- and terminal-NO connect to highly-excited NO states which do not differ so much in their dynamical interaction with the surface.
Toeplitz matrices within DVR formulation: Application to quantum scattering problems
Authors: Eisenberg Eli , Ron Shlomo , Baer Michael
Published: July 2002

It is shown that the matrix obtained from the infinite order discrete variable representation (DVR) of a scattering problem has the structure of a Toeplitz matrix. The resulting properties can be used to reduce the associated infinite system of algebraic equations to a finite (and relatively small) one. An example is worked out to show the efficiency of the combined Toeplitz DVR approach.
Coherent anti-Stokes Raman spectroscopy of shock-compressed liquid nitrogen/argon mixtures
Authors: Moore D. S. , Schmidt S. C. , Shaw M. S.
Published: July 2002

Single-pulse multiplex coherent anti-Stokes Raman scattering (CARS) was used to obtain vibrational spectra of 20%/80% liquid nitrogen/argon mixtures, shock compressed to several high-pressure/high-temperature states. A semiclassical model for CARS spectra was used to extract best fit vibrational frequencies, peak Raman susceptibilities, and Raman linewidths from the data. Up to a maximum single shock pressure of 17.1 GPa, the N2 vibrational frequency was found to increase monotonically with pressure. The vibrational frequencies measured in both the singly and doubly shocked N2/Ar mixtures correspond within experimental error to those for pure nitrogen at equivalent pressures and temperatures, implying that the influence of the interaction potential on the N2 vibrational frequency for the N2/Ar collision is not significantly different from that of a N2/N2 collision at these conditions. The transition intensity and linewidth data suggest that thermal equilibrium of the vibrational levels is attained in less than 10 ns at these shock pressures. Vibrational temperatures obtained were used to improve the potential function of argon used to calculate equation-of-state pressures and temperatures. The measured linewidths suggest that the nitrogen vibrational dephasing time decreased to about 1 ps at the highest pressure shock state.
Photoassociative spectroscopy of 1g, 0+u, and 0−g states of Na2
Authors: Ratliff L. P. , Wagshul M. E. , Lett P. D. , Rolston S. L. , Phillips W. D.
Published: July 2002

We use photoassociation of ultracold Na to study transitions from free atoms to bound molecules. We obtain rovibrational spectra of the 1g, the 0+u, and, for the first time, the 0−g ‘‘purely long-range’’ state of Na2, all of which dissociate to Na(3 2S1/2)+Na(3 2P3/2).
Low-energy electron attachment to excited nitric oxide
Authors: Kuo C. T. , Hardwick J. L. , Moseley J. T.
Published: July 2002

Dissociative attachment of low energy electrons to the A2Σ+ state of nitric oxide has been measured quantitatively, and the cross section for this process has been determined. A magnetically collimated electron gun has been used as a controlled source of low energy electrons, and the mass analyzed detection system has been calibrated using the known dissociative attachment cross section of CCl4. The cross section for the reaction NO*(A2Σ+)+e−(0.5 eV)→N(4S)+O−(2P) is measured to be (2.0±0.5)×10−15 cm2 and is found to be independent of rotational quantum number within the statistical uncertainty.
An electrostatic model for physisorption at ionic solids: Application to CO adsorption on NaCl(100)
Authors: Jug Karl , Albert Katrin , Gerwens Heiko
Published: July 2002

An electrostatic model for physisorption at ionic solids is developed. It is based on the asymptotic density model recently introduced for the efficient calculation of electrostatic potentials. The shape of the interacting systems is generated by appropriate isodensity surfaces. An optimization procedure for orientational effects is described. The model is applied to the dimers of CO2 and H2O. Finally, the CO adsorption on NaCl(100) is simulated by NaCl clusters of varying surface size. The influence of the position and site of the adsorbed molecule, the size of the surface, and the optimization of the cluster on orientation and adsorption energy is presented and analyzed. It is found that the adsorption energy is primarily dependent on the distance between the nearest atoms of cluster and adsorbant, which is determined by the shape of the isodensity surface of the cluster and, to a lesser extent, by long range attractions.
Three-dimensional nuclear dynamics on conically intersecting potential energy surfaces of O+3 (2A1–2B2)
Authors: Müller H. , Köppel H. , Cederbaum L. S.
Published: July 2002

Three-dimensional calculations of the nuclear dynamics of the ozone cation have been performed involving the vibronically coupled 2A1–2B2 electronic states and using the results of large-scale ab initio computations of Schmelz et al. [Chem. Phys. Lett. 183, 209 (1991)]. Anharmonic (diabatic) potential surfaces are employed and the vibronic coupling term is taken to be a linear function of the asymmetric stretch coordinate. These calculations are compared to the first and second bands of the experimental photoelectron spectrum. Most features of its peculiar shape can be correctly reproduced and interpreted in this way. The band maximum for zero temperature is assigned to the fourth peak and the first peak of the experimental spectrum is interpreted as a hot band (vibronic temperature ≊275 K). In addition the properties of a quadratic model Hamiltonian are investigated and compared to the afore mentioned calculations. It is found that the model Hamiltonian is suitable to reproduce the full three-dimensional computations. As a by-product of this work the vertical ionization potentials (IP) of the interacting states are determined to be IP(2A1)=12.78 eV and IP(2B2)=13.02 eV.
Molecular dynamics implementation of the Gibbs ensemble calculation
Authors: Palmer Bruce J. , Lo Chaomei
Published: July 2002

A molecular dynamics version of the Gibbs ensemble calculation is proposed. This calculation is based on an extended Hamiltonian formalism that treats the temperature, volume, and the coupling of a single particle to the rest of the system as continuous dynamical degrees of freedom with their own equations of motion. Calculations on the truncated and shifted Lennard-Jones fluid are presented and compared to the Gibbs ensemble Monte Carlo results of Smit. Quantitative agreement is found between the molecular dynamics and Monte Carlo calculations.
Surface waves on polymer solutions: Complete capillary wave–elastic wave crossover
Authors: Cao B. H. , Kim M. W. , Cummins H. Z.
Published: July 2002

We study the surface waves of polyethyleneoxide (PEO) in water by surface heterodyne light scattering. The complete capillary wave–elastic wave crossover is observed as the polymer concentration is increased. At high polymer concentrations, the peak frequencies of the experimental power spectra increase as the polymer concentration is increased, while the corresponding frequency widths decrease as the polymer concentration is increased. This unusual behavior of peak frequency and frequency width results from the strong viscoelasticity of the bulk which gives the additional restoring force of the surface waves at high polymer concentrations, as predicted by Harden et al.’s [J. Chem. Phys. 94, 5208 (1991)] theoretical calculation. Furthermore, the adsorbed monolayer of 1000 K PEO appears to have no effect on the surface waves.
Structure and vibrations of the phenol-ammonia cluster
Authors: Schiefke A. , Deusen C. , Jacoby C. , Gerhards M. , Schmitt M. , Kleinermanns K. , Hering P.
Published: July 2002

The phenol-ammonia 1:1 complex has been investigated by mass resolved hole burning spectroscopy and ab initio methods at the HF/6-31G(d,p) and HF/6-31++G(d,p) levels of theory. By means of spectral hole burning four bands in the region of intermolecular vibrations could be assigned to the 1:1 complex. The ab initio computed cluster structure and its normal vibrations are reported and compared to the experimental results. Anharmonic calculations were carried out for the ammonia torsion. The results are compared to structurally related complexes.
Lower bound for the nonrelativistic ground state energy of the lithium atom
Authors: King Frederick W.
Published: July 2002

A Temple lower bound for the nonrelativistic ground state energy of the lithium atom is determined using large scale Hylleraas-type wave functions. Weinstein lower bounds for the three lowest nonrelativistic energy levels are also reported. All the component expectation values necessary to construct the lower bounds are given. Detailed information is given to assess the convergence of the calculations.
Study of the low-lying states of Ge2 and Ge−2 using negative ion zero electron kinetic energy spectroscopy
Authors: Arnold Caroline C. , Xu Cangshan , Burton Gordon R. , Neumark Daniel M.
Published: July 2002

The low-lying states of Ge2 and Ge−2 are probed using negative ion zero electron kinetic energy (ZEKE) spectroscopy. The ZEKE spectrum of Ge−2 yields an electron affinity of 2.035±0.001 eV for Ge2, as well as term energies and vibrational frequencies for the low-lying states of Ge−2 and Ge2. Specifically, we observe transitions originating from the anion 2Πu(3/2) ground state and 3Σ+g excited state (Te=279±10 cm−1) to several triplet and singlet states of Ge2. Term values and vibrational frequencies are determined for the Ge23Σ+g ground state, the low-lying 3Πu excited state (Te=337 cm−1 for the 2u spin–orbit component), and the somewhat higher lying 1Δg, 3Σ+g, and 1Πu states. We also determine the zero-field splitting for the X0+g and 1g components of the 3Σ+g state and the splittings between the 2u, 1u, and 0±u spin–orbit components of the 3Πu state. Detailed comparisons are made with Si2 and Si−2.
Molecular relaxation and excess entropy in liquids and their connection to the structure of glass
Authors: Murthy S. S. N. , Paikaray A. , Arya N.
Published: July 2002

We have examined the nature of glass formation in three fragile glass forming liquids, viz., o-terphenyl, phenylsalicylate, and α-phenyl–o-cresol using heat capacity and dielectric relaxation (106–10−3 Hz) measurements. Dielectric relaxation rates and excess entropies of the equilibrium liquid are determined. The temperature dependence of these quantities suggest that the underlying glass transition process (of which the transition at Tg is a kinetic reflection), is possibly nonideal. And also we have studied the structure of the glass of one of the above three liquids, viz., o-terphenyl using dipolar solutes by dielectric spectroscopy and the results are compared to that of the dipolar solutes in their own matrix. Our results indicate that the glasses are statistically uniform materials and all the solute molecules contribute to the secondary relaxation. The role of molecular size and shape in this context is discussed.
Length scales for fragile glass-forming liquids
Authors: Mountain Raymond D.
Published: July 2002

Molecular dynamics simulation results are used to demonstrate the existence of a growing length in supercooled, fragile glass-forming liquids. This length is the longest wavelength, propagating shear wave the fluid can support. Explicit results are reported for an equimolar soft-sphere mixture. A possible connection between this length and the size of locally rigid clusters is discussed.
Linking structure and vibrational mode coupling using high-resolution infrared spectroscopy: A comparison of gauche and trans 1-chloro-2-fluoroethane
Authors: Miller C. Cameron , Stone Stephen C. , Philips Laura A.
Published: July 2002

The high-resolution infrared spectrum of 1-chloro-2-fluoroethane in a molecular beam was collected over the 2975–2994 cm−1 spectral region. The spectral region of 2975–2981 cm−1 contains a symmetric C–H stretching vibrational band of the gauche conformer containing the 35Cl isotope. The spectral region of 2985–2994 cm−1 contains three vibrational bands of the trans conformer. Two of the three bands are assigned as an antisymmetric C–H stretch of each of the two different chlorine isotopes. The third band is assigned as a symmetric C–H stretch of the 35Cl isotope. The gauche conformer of 1-chloro-2-fluoroethane showed doublet patterns similar to those previously observed in 1,2-difluoroethane. The model for 1,2-difluoroethane is further refined in the present work. These refinements suggest that the coupling dark state in 1,2-difluoroethane is composed of 1 quantum C–H bend, 1 quantum C–C stretch, and 12 quanta of torsion. For 1-chloro-2-fluoroethane the dark state could not be identified due to a small data set. The trans conformer of 1-chloro-2-fluoroethane showed no evidence of mode coupling in the three vibrational bands. Including 2-fluoroethanol in this series of molecules, the extent of vibrational mode coupling did not correlate with the density of states available for coupling. Therefore, density of states alone is insufficient to explain the observed trend. A correlation was observed between the degree of intramolecular interaction and vibrational mode coupling.
Thermodynamic and stochastic theory of nonequilibrium systems: A Lagrangian approach to fluctuations and relation to excess work
Authors: Suárez Alberto , Ross John , Peng Bo , Hunt Katharine L. C. , Hunt Paul M.
Published: July 2002

The dynamics of fluctuations in systems approaching a nonequilibrium steady state, with or without detailed balance, are investigated by means of a Lagrangian function, which is derived from the generator of time displacement (Hamiltonian) of the mesoscopic evolution equation. In the thermodynamic limit, the stationary probability distribution for the fluctuating variables is expressed in terms of the action of this stochastic Lagrangian along the fluctuational trajectory, the most probable path of infinite duration for the generation of a particular fluctuation away from the steady state. The fluctuational trajectory is related by a gaugelike transformation to the deterministic trajectory, which is the most probable path for the relaxation of the macroscopic system to the steady state. This framework is applied to the analysis of one-variable chemical reactions modeled by a constant step master equation, and to two-variable systems in the linearized region around the steady state, where the fluctuations are described by a linear Fokker–Planck equation. In these examples, the thermodynamic significance of the action along the fluctuational trajectory is established by relating the irreversible (odd under time inversion) part of the Lagrangian and the time derivative of a deterministic excess work.
Dipole binding: An experimental test for small cluster structure calculations
Authors: Desfrançois C. , Abdoul-Carime H. , Khelifa N. , Schermann J. P. , Brenner V. , Millie P.
Published: July 2002

An experimental method for the discrimination between nearly degenerate isomers of size selected clusters of closed-shell polar molecules is presented. It is based upon electron attachment properties of dipole fields since a minimum value (≊2.5 D) of molecular dipoles is required for electron binding. When neutral clusters are created in different configurations only those with large enough resulting dipole moments bind electrons and give birth to stable anions. These dipole-bound anions have the geometries of their neutral parents which are here calculated within the framework of the exchange perturbation theory as developed by Claverie. Anions are created by Rydberg electron transfer to cold neutral clusters containing up to six molecules of acetonitrile, water, ammonia, or methanol. Structure and dipole moment calculations account well for the ‘‘magic’’ numbers observed in mass spectra distributions. Detailed comparisons between experimental data and calculated geometries lead to informations on the structure of several isomers of each studied neutral cluster.
Free jet infrared spectroscopy of SiF4-rare gas complexes
Authors: Urban R.-D. , Jörissen L. G. , Matsumoto Y. , Takami M.
Published: July 2002

The rotation-vibration spectra of 28SiF4-Ar and 28SiF4-84Kr complexes have been studied in the 10 μm region. The triply degenerate ν3 (F2) band of SiF4 reduces to nearly degenerate parallel (A) and perpendicular (E) band of the complex with the C3v structure. The two bands coupled by an A−E Coriolis interaction have been analyzed simultaneously. No splitting due to internal motion has been observed. Band shifts by the formation of the complex are discussed in conjunction with the magnitudes of A−E splitting.
A theoretical study of bridged vs atop interactions of Pt2 with CO
Authors: Roszak S. , Balasubramanian K.
Published: July 2002

Potential energy curves for the low-lying electronic states of the Pt2CO complex are studied using the Mo/ller–Plesset second-order perturbation theory (MP2) and the complete active space multiconfiguration self-consistent field method (CASSCF). Multireference singles and doubles configuration interaction (MRSDCI) computations that included up to two million configurations were also made. The results for Pt2CO are compared with experimental results for chemisorption of CO on a Pt surface. The atop and bridged bondings of CO on the Pt-surface are modeled using potential energy curves for the ground state linear and bridged Pt2CO structures. It is shown that the atop interaction proceeds without a barrier while the bridge interaction has to surmount a barrier, even though the bridge bonding leads to a more stable equilibrium complex. The calculated vibrational frequencies at the MP2 level for Pt2CO and Pt3CO are compared with the experimentally determined values for different chemisorptive sites. The differences between the atop and bridged chemical bonds are discussed using the Mulliken population analysis. The spin–orbit effect is studied utilizing a relativistic configuration interaction (RCI) approach.
The B(1/2 2P3/2)→X(1/2 2Σ+) transition in XeBr
Authors: Clevenger Jason O. , Tellinghuisen Joel
Published: July 2002

The B(1/2 2P3/2)→X(1/2 2Σ+) transition in XeBr is recorded at high resolution, using a CCD array detector to record spectra from Tesla discharge sources containing isotopically pure 136Xe with 81Br2 or 79Br2. The high signal/noise capabilities of the detector permit the measurement of discrete vibrational structure in this system, which has normally been treated as a purely bound–free transition. The assignments comprise 119 υ′–υ″ bands for 136Xe81Br and 86 for 136Xe79Br, spanning υ′=0–33 and υ″=0–16. The van der Waals ground state is analyzed through fits to the customary polynomials in (υ+1/2) and to near-dissociation expansions. Franck–Condon calculations are used to locate the X-state potential on the internuclear axis relative to the B state, which is modeled as a Rittner potential. The following fundamental spectroscopic constants (units cm−1, for 136Xe81Br) are obtained from the analysis: Te′=35 863.2, ωe′=135.72, ωexe′=0.32, ωe″=25.7, ωexe″=0.62. The ground state has a dissociation energy 𝒟e″=254±2 cm−1 and supports 24 bound vibrational levels.
A density functional theory study of frequency-dependent polarizabilities and Van der Waals dispersion coefficients for polyatomic molecules
Authors: van Gisbergen S. J. A. , Snijders J. G. , Baerends E. J.
Published: July 2002

A method for calculating frequency-dependent polarizabilities and Van der Waals dispersion coefficients, which scales favorably with the number of electrons, has been implemented in the Amsterdam Density Functional package. Time-dependent Density Functional Theory is used within the Adiabatic Local Density Approximation (ALDA). Contrary to earlier studies with this approximation, our implementation applies to arbitrary closed-shell molecular systems. Our results for the isotropic part of the Van der Waals dispersion energy are of comparable quality as those obtained in TDCHF calculations. The ALDA results for the relative anisotropy of the dipole dispersion energy compare favorably to TDCHF and MBPT results. Two semi-empirical ways to calculate the dispersion energy anisotropy are evaluated. Large bases which include diffuse functions are necessary for a good description of the frequency-dependent properties considered here.
Free jet expansions of binary atomic mixtures: A method of moments solution of the Boltzmann equation
Authors: Mazely Troy L. , Roehrig Gillian H. , Smith Mark A.
Published: July 2002

The method of moments is used to obtain solutions to the Boltzmann equation describing flow and thermal relaxation of binary atomic mixtures undergoing adiabatic free jet expansion. Differential moment equations are solved within the framework of an anisotropic Maxwellian distribution function. Numerical integration of these equations yields the temporal dependence of the four moments of the distribution function for each flow component; density, hydrodynamic velocity, parallel temperature, and perpendicular temperature. Results are presented for mixtures of the rare gases which yield new insight into the driving forces behind the phenomena of velocity and temperature slip. The model can easily be extended to ternary and higher atomic mixtures.
TiNb, ZrNb, ZrV, and HfV molecules: ESR, ground states, and isovalency
Authors: Van Zee R. J. , Li S. , Weltner W.
Published: July 2002

Electron-spin-resonance (ESR) spectroscopy of these molecules in argon matrices at 2 K reveals that each has a 4∑ ground electronic state, the same as TiV, also containing 9 valence (s+d) electrons. TiTa and ZrTa were not observed. An analogy can be drawn with the 15-valence-electron Brewer–Engel molecules VNi, VPd, VPt, NbNi, which also have 4∑ ground states and a ...σ1dδ2 configuration, where σ is an sdσ hybrid.
Effect of chain length and surface density on looped polymers grafted to an impenetrable surface
Authors: Jones Ronald L. , Spontak Richard J.
Published: July 2002

Chemical modification of inorganic surfaces through the use of grafted polymers constitutes an important means by which to tailor surface properties. While numerous theoretical and simulation efforts have addressed dense, single-grafted polymer layers (i.e., brushes), few have sought to examine layers comprised of double-tethered macromolecules (i.e., loops). In this work, bond-fluctuation (BF) simulations have been performed in the presence of an impenetrable surface and good solvent to ascertain the effect of surface anchor density (σ) and chain length (N) on the segmental density distribution and layer thickness of grafted polymer loops. At low σ, the density distribution for loops is accurately described by self-consistent field (SCF) theories for brushes of half-chain length, whereas the parabolic form of the SCF distributions is replaced by a blocklike profile, indicative of density saturation, at high σ. For N≳20, the σ signaling the onset of chain impingement is found to obey the same scaling relationship as that of brushes, namely, N−6/5. Median segment density distributions for grafted loops at low σ are comparable to SCF tail-end distributions of half-length brushes but again, due to density saturation, deviate from SCF predictions at high σ.
Ab initio model potential study of the optical absorption spectrum of Mn2+-doped CaF2
Authors: Pascual José Luis , Seijo Luis , Barandiarán Zoila
Published: July 2002

An ab initio calculation of the optical absorption spectrum of Mn2+-doped CaF2 is performed in which electrostatic and quantum embedding effects originated by a relaxed and polarized CaF2 lattice on the ligand field ground and excited states of a MnF86− cluster have been considered. The theoretical spectrum is calculated by means of the complete active space self-consistent-field (CASSCF) and average coupled pair funtional (ACPF) methods, correlating up to 23 electrons. An excellent overall agreement with the experiments is found and a detailed analysis of the results is presented. The initial assignment of the 4A1g(4G) and 4Eg(4G) states, lately reversed, is supported. The wrong assignment of the 4T1g(4P) state is shown to be responsible for a recently proposed change of the value of the crystal field splitting paramenter initially accepted, 10Dq=4250 cm−1, which is in turn supported here. Also, new assignments for the absorptions to the 4T1g(4F) and 4T2g(4F) excited states are suggested.
Maxwell-displacement-current across phospholipid monolayers due to phase transition
Authors: Iwamoto Mitsumasa , Kubota Tohru , Zhong-can Ou-Yang
Published: July 2002

With Maxwell-displacement-current-measuring method, the phase transitions of phospholipid monolayers of L-α-dilaurylphosphatidylethanolamine (DLPE) and L-α-dimyristoyl- phosphatidylethanolamine (DMPE) on a water surface were detected. Some fine features, such as the first order phase transition from gaseous to the polar ordering phase and the polar ordering of lipid molecules in the solid phase coexisting with fluid, were revealed for the first time.
Unambiguous assignment of the van der Waals modes of benzene–Ar by analysis of the rotationally resolved UV-spectra and comparison with multidimensional calculations
Authors: Riedle E. , van der Avoird A.
Published: July 2002

We present calculations of the van der Waals (vdW) states of electronically excited benzene-Ar in which the coupling to the vibronic angular momentum of the excited 61 state of benzene is explicitly included. It is predicted, in particular, that the vibrational angular momentum of the degenerate (E1) vdW bending fundamental leads to parallel Coriolis coupling between two substates and consequently to a perturbed rotational structure of the corresponding parallel band in the UV spectrum. A detailed analysis of these bands in the UV spectra of C6H6-Ar and C6D6-Ar is given and found to agree with the theory. This yields not only a set of accurate rotational and Coriolis coupling constants, but also an unambiguous assignment of all observed vdW transitions in the UV spectra of these complexes. The vibrational frequencies, the changes of rotational constants upon vdW excitation, and the intensities calculated with some of the available model potentials, and the isotopic shifts in these quantities are in good agreement with experiment. It is noteworthy that the Franck-Condon principle, which holds for the vibronic intensities in normal molecules, does not simply apply to the intermolecular vibrations in vdW complexes. The libration of the vibronic transition dipole moment of the monomer gives an additional source of intensity to the (non-totally symmetric) bending modes. This allows the hitherto doubted observation of the transition to the vdW bending fundamental.
Quantum-mechanical study of the I2–Ne vibrational predissociation dynamics: A three-dimensional time-dependent self-consistent-field approach
Authors: García-Vela A.
Published: July 2002

A three-dimensional time-dependent self-consistent-field (TDSCF) approach is proposed to study the vibrational predissociation of the I2(B)–Ne van der Waals (vdW) complex. Jacobian coordinates are used within the assumption of zero-total angular momentum. In the method the total wave function is factorized such that the bending mode of the system is explicitly separated, while the coupling between the remaining two degrees of freedom is treated exactly. The decay dynamics of several resonances corresponding to different initial vibrational states of I2 is investigated through long-time wave packet propagations. Calculated resonance lifetimes are compared to experimental data and found to be in quantitative agreement with them. The results show that predissociation of the complex is mainly governed by the coupling between the I2 and the vdW stretching vibrations, whereas the bending mode has a rather weak effect on the dynamics. The good quality of the TDSCF description of this long-time dynamical process is due to the adaptation of the decoupling approximations applied in the method to the physical situation of the system. The validity of the approach is discussed in the light of the results.
Equation of state for fluids of hard heteronuclear diatomic and symmetric triatomic molecules
Authors: Maeso M. J. , Solana J. R.
Published: July 2002

A model previously developed for the equation of state of linear homonuclear fused hard sphere fluids is generalized to fluids with heteronuclear molecules. The model only requires two parameters, which can be determined from the geometrical characteristics of the molecules, for which analytical expressions are derived. Results for fluids with heteronuclear hard diatomic and symmetric triatomic molecules agree with simulation data within their accuracy for almost all the fluids considered.
On the electronic structure of the palladium monoxide and the methane adsorption: Density functional calculations
Authors: Broclawik Ewa , Yamauchi Ryo , Endou Akira , Kubo Momoji , Miyamoto Akira
Published: July 2002

Electronic structure of the palladium monoxide and its interaction with a methane molecule has been investigated by means of density functional theory. The two triplets, 3Π and 3Σ−, lie very close in energy, with the indication at the 3Π ground state of the oxide. A methane molecule interacts with the open shell PdO and forms two stable adsorption complexes: in collinear on palladium and bridging conformations. The scission of the C–H bond in adsorbed methane requires moderate activation energy of 24.5 kcal/mol and the dissociation product is very stable, however, the singlet–triplet crossing occurs at the transition state.
Equation of state for a Yukawa fluid in the mean spherical approximation
Authors: Herrera J. H. , Ruiz-Estrada H. , Blum L.
Published: July 2002

The analytical solution of the mean spherical approximation (MSA) for a fluid of hard spheres and a Yukawa potential is obtained by solving a quartic equation for an inverse length parameter Γ. Simple expressions for the thermodynamics and correlation functions in terms of Γ exist in the literature. In this paper we obtain a simple form of the equation of state via the energy. This is compared with the result of Henderson et al. [D. Henderson, L. Blum, and J. P. Noworyta, J. Chem. Phys. 102, 4973 (1995)].
Transition from order to chaos in molecular wave functions and spectra
Authors: Arranz F. J. , Borondo F. , Benito R. M.
Published: July 2002

In this Communication we describe how the transition from regularity to classical chaos in molecular Hamiltonian systems shows up at the quantum level in the structure of the corresponding wave functions and spectra. By changing the value of ℏ we show how the scars result from combinations of regular wave functions.
Infrared spectroscopic studies of structure and phase transitions in a Langmuir monolayer of H(CH2)23COOH
Authors: Li Mengyang , Rice Stuart A.
Published: July 2002

We report the results of linked theoretical and experimental studies of external infrared reflectivity by a Langmuir monolayer. The new feature in our analysis is the inclusion of the influence of the factor group splitting, arising from the presence of two molecules in the unit cell of a close packed two-dimensional monolayer lattice (CS phase), on the polarized reflection spectra of the monolayer. Our experimental study of the polarized reflection spectra of water supported Langmuir monolayers of tetracosanoic acid [H(CH2)23COOH] provide the first measurements of the factor group splittings of the CH2 symmetric and antisymmetric stretch vibrations in the CS phase of a monolayer. The existence of a nonzero factor group splitting is direct evidence for the presence of herringbonelike hydrocarbon chain packing in a two-dimensional crystalline phase of the monolayer. We also report values for the collective tilt angle in the tetracosanoic acid monolayer, as a function of surface area per molecule. The collective tilt angles for the unstrained I phase and the U phase of the tetracosanoic acid monolayer found in this work agree with those obtained from grazing incidence x-ray diffraction studies.
Study of the solid–liquid transition for Ar55 using the J-walking Monte Carlo method
Authors: López Gustavo E.
Published: July 2002

We present here a study of the solid–liquid transition for Ar55 using classical Monte Carlo techniques. Calculations of the heat capacity as a function of temperature show the expected solid–liquid coexistence region which is identified by anomalies in the heat capacity as a function of temperature. The problem of quasiergodicity observed in this transition is considerable reduced by the implementation of J-walking Monte Carlo. Calculations of the distribution of energies clearly demonstrate the proper sampling of configurational space achieved using the J-walking Monte Carlo algorithm.
Monte Carlo simulation and reference hypernetted chain equation results for structural, thermodynamic, and dielectric properties of polar heteronuclear diatomic fluids
Authors: Lombardero M. , Martín C. , Lomba E. , Lado F.
Published: July 2002

We study fluids of heteronuclear two-center Lennard-Jones molecules with embedded point dipoles using both numerical simulation and integral equation theory. Extensive Monte Carlo simulations are performed for the structural, thermodynamic, and dielectric properties of two models of such fluids, with unusually long simulation runs to assure convergence of the dielectric constant. The results are used to test a generalization of a reference hypernetted chain approximation (RHNC-VM) used previously for nonpolar heteronuclear diatomics. Very good agreement is found between the two methods. We conclude that the RHNC-VM integral equation is a reliable method for studying both polar and nonpolar fluids of diatomic molecules.