bluuebunny/crossref_metadata_2025 huggingface dataset:
Flaw length distribution measurement in brittle materials
Authors: Rabinovitch A. , Zlotnikov R. , Bahat D.
Published: July 2002

A technique was developed in an experiment of fractured soda-lime glass which yielded a wide mist zone. This technique enabled us to measure the lengths of a large number (≈ 12 000) of secondary cracks (SC) in this zone. Statistical analysis of these lengths across the mist as a function of distance from fracture origin revealed some unexpected results. (a) The total number of SC remains constant for ≈ 40% of the width of the mist zone and subsequently decreases monotonically down to ≈ 50% of the initial number towards the end of this zone. (b) The length distribution of SC can be interpreted as constituting of two populations: (1) A nucleated SC population which remains unchanged across the whole mist. This distribution consists of two subpopulations: (i) half Gaussian (around 84%), and (ii) decaying exponential (around 16%). (2) A rather small portion of the nucleated population of SC (≈ 9% throughout) which grow monotonically by a factor of up to 40 with distance across the mist. (c) This factor agrees well with our theoretical calculation of the growth mode, and yields a reasonable estimate of the radius of the initial critical flaw (1.85 mm).
Kinetics of the thermally and photoinduced solid state reaction of Ag with As33S67 films
Authors: Wagner T. , Dale G. , Ewen P. J. S. , Owen A. E. , Perina V.
Published: July 2002

The kinetics of the solid state chemical reaction of silver with amorphous As33S67 films in a conventional sandwich structure was measured by optically monitoring the change in thickness of the undoped chalcogenide. For the thermally stimulated process, the kinetics data indicate a two-stage process, with the second stage having a sublinear time dependence. The effect of broadband illumination is to reduce the activation energy by approximately one order of magnitude, but the photoinduced process still has two identifiable stages. In the case of the photostimulated process, rate coefficients and activation energies have been measured and the composition and structure of the reaction products have been determined by Rutherford backscattering spectroscopy (RBS). The RBS spectra obtained at different stages of the photoinduced reaction process show a step-like form for the Ag concentration profile during the course of the reaction and indicated a homogeneous distribution of Ag in the final reaction products, the Ag concentration being 19.5 at. %.
Epitaxial and highly electrical conductive La0.5Sr0.5TiO3 films grown by pulsed laser deposition in vacuum
Authors: Wu Wenbin , Lu Fei , Wong K. H. , Pang Geoffrey , Choy C. L. , Zhang Yuheng
Published: July 2002

The target material with nominal composition of La0.5Sr0.5TiO3 sintered in air is an insulator and not a single-phase compound. By pulsed laser ablation in vacuum at the multiphase La–Sr–Ti–O target, however, highly electrical conductive and epitaxial La0.5Sr0.5TiO3 films have been fabricated on LaAlO3(001) substrates. Structural characterization using three-axis x-ray diffraction (θ–2θ scan, ω-scan rocking curve, and φ scan) reveals that the films have a pseudocubic structure and grow on the substrates with a parallel epitaxial relationship. Atomic force microscopy images show the films have quite smooth surface, for a film 200 nm thick, the roughness Ra is about 0.31 nm over the 1 μm×1 μm area. Resistivity versus temperature measurements indicate that the films are metallic at 2–300 K and have resistivity of 64 μΩ cm at 300 K, which is about one order lower than that of the single-phase La0.5Sr0.5TiO3 bulk materials. After the same deposition procedure, epitaxial La0.5Sr0.5TiO3 films have also been grown on TiN buffered (001) Si substrates.
Crystal field parameters in rare earth compounds: A mixed ‘‘covalo-electrostatic model’’
Authors: Garcia D. , Faucher M.
Published: July 2002

A limited LCAO model was applied to a series of 12 rare earth or rare earth doped compounds and one actinide compound, namely, LiYF4:Nd3+, KY3F10:Eu3+, YOCl:Eu3+, BaFCl:Sm2+, SrFCl:Sm2+, Nd2O3, Nd2O2S, NdAlO3, LaAlO3:Eu3+, LaCl3:Nd3+, YF3:Gd3+, Y2O3:Eu3+, and CsUF6. The procedure and the approximations are detailed. The overlap integrals are calculated and fitted to analytical functions of the distance for nine rare earth–ligand pairs: Nd3+, Eu3+, Tm3+/O2−, F−, and Cl− utilizing literature radial wave functions. For each species, the ionization energies are assigned mean values. The one-electron energies given by the diagonalization of the interaction matrix are transformed into sets of Bkq crystal field parameters. The comparison with literature phenomenological values is made in terms of scale and reliability factors. In most cases the results are satisfactory.
O+2 ions dissociation studied by threshold photoelectron–photoion coincidence method
Authors: Richard-Viard M. , Dutuit O. , Lavollée M. , Govers T. , Guyon P. M. , Durup J.
Published: July 2002

The threshold photoelectron–photoion coincidence method was used to study O+2 dissociation. Previous results were confirmed and a careful analysis of the time of flight peak shapes using a Monte Carlo simulation gave us new results. The lifetime of the B 2Σ−g state was measured to be 70∓25 ns independently of the vibrational quantum number. The III 2∏u state which dissociates towards the O+(2D0)+O(3P) and O+(2P0)+O(3P) limits shows anisotropic distribution of fragment ions. The c 4Σ−u(v=0) state which was previously observed to dissociate into O+(4S0)+O(1D), its adiabatic limit, is seen to predissociate also about 40% towards the ground state limit.
Mass spectrometric study of the thermochemistry of gaseous EuTiO3 and TiO2
Authors: Balducci G. , Gigli G. , Guido M.
Published: July 2002

The gaseous molecule EuTiO3 has been investigated in a high temperature mass spectrometric study of vapors over the europium–titanium–oxygen system. From the enthalpy of reaction: EuTiO3(g)=EuO(g)+TiO2(g) and proper ancillary data, the atomization energy of this molecule has been determined. In addition, from the study of the gaseous exchange reaction: TiO2(g)+Eu(g)=TiO(g)+EuO(g) the dissociation energy of TiO2(g) has been derived and compared with previous results. The dissociation energies proposed are: D○0,at(EuTiO3) =2278±28 kJ mol−1 and D○0,at(TiO2) =1260±12 kJ mol−1.
The atom in a molecule: A density matrix approach
Authors: Li Lemin , Parr Robert G.
Published: July 2002

A method based on first-order density matrices is proposed to define an atom in a molecule, which is in accord with a previously given definition using density functional theory. The promotion energy Ep is expressed in terms of a defined division function α(r). By minimizing Ep with respect to α(r), one can obtain α(r), by which an atom in a molecule is uniquely determined. It is shown that such an ‘‘atom’’ satisfies a local virial theorem, depicted intuitively, Some other properties of the atom in a molecule are also discussed. Comparison is made with a definition due to Bader, and an approximate but simple approach to determine an atom in a molecule is proposed.
Benzene adsorbed on silver: An electron energy loss and surface-enhanced Raman study
Authors: Wolkow Robert A. , Moskovits M.
Published: July 2002

Vibrational electron energy loss (EELS) and surface-enhanced Raman (SERS) spectra of benzene on evaporated silver display marked coverage dependence for ‘‘cold-deposited’’ surfaces prepared at or above 150 K. In contrast, silver deposition at 60 K leads to less complex behavior. On ‘‘warm’’ prepared surfaces, 150–300 K, there exists a low coverage spectrum and a distinctly different high coverage (still submonolayer) spectrum. On cold prepared surfaces, 60 K, only one type of spectrum appears and it closely resembles the high coverage spectrum seen on a warm prepared surface. The dependence on silver preparation temperature, evidence that different sites lead to different spectra, and the expectation that benzene forms substantial π bonds only, implies that this is not a compressional reorientation effect like that observed previously for pyridine on Ag(111). It appears that cold prepared surfaces have very rough features giving rise to only one type of spectrum, while warm prepared surfaces have smooth and rough areas resulting in the low and high coverage spectra, respectively. The low and high coverage spectra may be an indication that the two types of site support unique orientations. Since however, orientation alone does not account for all observed features it may be that the rough silver surface provides a degeneracy lifting mechanism, possibly a field gradient effect like that proposed in relation to SERS.
Surface analysis of polyethylene and hexatriacontane by high resolution electron energy loss spectroscopy
Authors: Pireaux J. J. , Thiry P. A. , Caudano R. , Pfluger P.
Published: July 2002

Surfaces of polyethylene and n-hexatriacontane were for the first time studied by high resolution electron energy loss spectroscopy (HREELS). The vibrational band energies and intensities were recorded and compared to transmission infrared data. The attribution of most of the bands to methyl, methylene, and C–C vibrations is straightforward; but some of the bands remain unassigned. The intensities of the methyl associated electron energy losses are greatly enhanced. This confirms the extreme surface sensitivity of the spectroscopy and is consistent with a C–C skeleton preferentially oriented perpendicular to the surface. Measured cross section data vs electron impact energy (E0) show that while a dipolar-type interaction mechanism is dominating some regions of the cross-section curve, other resonance scattering may contribute significantly for low and high E0. This cross-section curve shows also that the polymer surfaces are to be considered as a ‘‘condensed gas,’’ with weak intermolecular interaction, instead of a real solid state material with collective (phonon) vibrational excitations. No evidence of radiation damage could be found in the vibrational spectra.
Generalization of analytic energy third derivatives for the RHF closed-shell wave function: Derivative energy and integral formalisms and the prediction of vibration–rotation interaction constants
Authors: Gaw Jeffrey F. , Yamaguchi Yukio , Schaefer Henry F. , Handy Nicholas C.
Published: July 2002

The general restricted Hartree–Fock (RHF) closed-shell energy analytic third derivative method is presented together with details concerning its implementation and that of the derivative integral algorithm. The viability of the energy equation and the integral techniques are illustrated by calculations on water, hydrogen cyanide, and formaldehyde with double zeta (DZ), double zeta plus polarization (DZP), triple zeta (TZ), triple zeta plus polarization (TZP), and triple zeta plus two sets of polarization (TZ2P) basis sets. Cubic force constants in reduced normal coordinates are reported for these basis sets. The cubic force constants are then applied in the prediction of the vibration–rotation interaction constants αBr (B=A, B, C) for these molecules.
Measurement of the HCl+(A 2Σ+–X 2Π) electronic transition moment using quasiresonant charge transfer at low energy
Authors: Glenewinkel-Meyer Th. , Müller B. , Ottinger Ch. , Tischer H.
Published: July 2002

HCl+(A) was selectively prepared in the v′=0, 3, 5, and 6 levels using ion impact excitation. The resulting A–X spectra are largely free from band overlap. From intensity measurements on 16 bands, the electronic transition moment function M(R) was derived in the region of R=1.1–1.8 Å. Computer simulations based on this M(R) function were performed both for these and for additional spectra excited by electron impact. In all cases an excellent reproduction of the measured spectra was achieved. The M(R) dependence obtained in this work agrees very well with a large-scale ab initio calculation.
Magnetic properties in terms of localized quantities. XII. Normal- and hypervalent sulfur compounds
Authors: Schindler Michael
Published: July 2002

Magnetic susceptibility and NMR chemical shift tensors are calculated for a number of sulfur containing compounds by means of the IGLO method (IGLO stands for individual gauge for localized molecular orbitals). The molecules investigated are H2S, the normal- and hypervalent sulfur fluorides SF2, SF4, SF3SF, and SF6, the triatomics OCS, SCS, OSO, NSF, NSCl, and SCl2, further NSF3, F2SO2, FSSF, SSF2, the thionyl compounds OSX2, X=CH3, F, Cl, the three-membered ring systems C2H4X, X=S, SO, SO2, and thiophene, C4H4S. With the provision that basis sets including at least two sets of polarization functions for the second row atoms are used, we obtain a rather satisfactory agreement between theory and experiment. Effects of antisymmetry are so large in NSF and NSCl that the anisotropies Δσ of their 33S, 35Cl, and 19F NMR shifts, calculated from the unsymmetric tensors differ from those obtained from the corresponding symmetrized eigenvalue problem by up to 100 ppm. Although a number of the molecules studied cannot be described entirely by localized MOs, MO contributions to magnetic properties provide valuable insight when discussing the orientation of the principal shielding axis systems, and when comparing iso-(valence)electronic molecules. Though iso-valencelectronic with ozone, SO2 shows a much smaller paramagnetic deviation from experimental data than the former, thus indicating that correlation effects, though important, are somewhat less essential for molecules containing second row elements.
The vibronic theory of resonance hyper-Raman scattering
Authors: Chung Y. C. , Ziegler L. D.
Published: July 2002

The intensities of vibrational resonance hyper-Raman transitions are developed in the usual framework of vibronic theory. In analogy to linear Raman scattering, the leading terms of RHR scattering, A, B, and C, are found when the electronic transitions moments, appearing in the transition hyperpolarizability, are expanded to first order in nuclear coordinate dependence. Noncentrosymmetric molecules are predicted to be the strongest nonlinear scatterers and derive activity from the RHR A term. In contrast to the vibronic analysis of linear Raman scattering, both one and two-photon allowed transitions contribute to RHR scattering cross section at the same order of vibronic theory (B term) for centrosymmetric molecules. RHR scattering spectra of gas phase CH3I, NH3, and CS2 illustrate A- and B-term RHR activity.
Vibrational relaxation of the 2ν2 level of NH3
Authors: Dubé P. , Reid J.
Published: July 2002

Lead salt diode lasers are used in conjuction with a Q-switched CO2 laser to measure collisional energy transfer from the 2ν2 level of NH3. The (V–T,R) relaxation rate from the 2ν2 level is found to be approximately an order of magnitude faster than the comparable relaxation rate from the ν2 level. (V–V) transfer from the ν2 to the 2ν2 levels plays an insignificant role in the dynamics of energy transfer in the ν2 mode of NH3.
Electronic spectra of jet-cooled isopropyltropolones in the S1–S region. Proton tunneling in the S1 state
Authors: Sekiya Hiroshi , Takesue Hiroto , Nishimura Yukio , Li Zhi-Hong , Mori Akira , Takeshita Hitoshi
Published: July 2002

Laser fluorescence excitation spectra and dispersed fluorescence spectra of jet-cooled 3-, 4-, and 5-isopropyltropolone in the S1–S0 region have been measured to investigate the effect of unsymmetrical and symmetrical substitution of an isopropyl group on proton tunneling. The 0++ and 0−+ transitions have been identified in the excitation spectra of 3-isopropyltropolone and its −OD derivative. The tunneling doublet splittings in the S1 state have been determined to be 58 and 14 cm−1 for normal and deuterated species, respectively. The increase in tunneling splitting of 3-isopropyltropolone compared with that of tropolone suggests that the substitution of the isopropyl group induces an asymmetry in the double-minimum potential well. In contrast with the excitation spectrum of 3-isopropyltropolone, no 0−− and 0−+ transitions have been detected in 4- and 5-isopropyltropolone. The nonobservation of these transitions is likely to be due to substantial increase in the potential-energy barrier to tunneling in the S1 state.
On the dual role of the Knudsen layer and unsteady, adiabatic expansion in pulse sputtering phenomena
Authors: Kelly Roger
Published: July 2002

When a surface is bombarded with pulses as from a laser and the number of monolayers sputtered per pulse is sufficiently small, the emitted particles fly freely from the target surface. For yields comparable to 0.5 monolayer in 10 ns, a limited number of gas-phase interactions is known to occur, leading to a Knudsen layer. As a result the particles develop moderate forward peaking (∼cos4 θ) and begin to flow (Mach number, M≊1). It is common, however, for yields to exceed 0.5 monolayer in 10 ns. We show that the resulting gas-phase interactions cause the Knudsen layer to evolve into an unsteady, adiabatic expansion which is formally like a gun which fires a finite charge into an infinite, one-dimensional barrel. An explicit solution can be obtained which has the form a=f1(x/t) and u=f2(x/t), where a is the speed of sound and u is the flow velocity. The solution is assumed to be valid up to a distance where the gas density ρ falls to the critical value ρc where free flight sets in. The particles are then characterized by extents of forward peaking much stronger than ∼cos4 θ (e.g., ∼cos25 θ in work by Namiki et al.) and correspondingly high M (cos25 θ is equivalent to an M near 4). In describing these effects, M and γ (the heat-capacity ratio) take the role of the basic parameters, the knowledge of which is essential for understanding what is happening.
Investigation of the free energy functions for electron transfer reactions
Authors: King Gregory , Warshel Arieh
Published: July 2002

The free energy functions for electron transfer reactions in solution are explored using a previously developed microscopic simulation approach that provides a clear definition of these functions and the variable (the reaction coordinate) used as their argument. The issue of whether the curvatures of the two functions (which correspond to states with nonpolar and polar solutes) are different is given special attention. It is found, in contrast to some previous suggestions, that the curvatures of the two functions are quite similar, even when one would expect differences due to dielectric saturation effects, and that Marcus’ approximation (and, in fact, the linear response theory inherent in this approximation) provides a valid description of the solvent’s role in electron transfer reactions over a wide range of conditions. The present study demonstrates that direct simulations of the reactant and product states do not provide the data needed for determination of the free energy functions in high energy regions. On the other hand, a free energy perturbation approach developed in our early studies does allow one to obtain statistically significant data for these regions of high free energy. Inclusion of the solvent electronic polarizabilities in the potential results in a marked lowering of the solvent reorganization energy, but does not change the ratio of the curvatures of the free energy functions for the nonpolar and polar states. Finally, the effect of solvent tunneling is evaluated with the dispersed polaron method and shown to be rather small, and it is argued that the observed deviations from the classical Marcus theory in the inverted region are due to quantum mechanical tunneling associated with the solute vibrations, rather than to differences in the curvatures of the solvent free energy functions.
Higher analytic derivatives. II. The fourth derivative of self-consistent-field energy
Authors: Maslen Paul E. , Jayatilaka Dylan , Colwell Susan M. , Amos Roger D. , Handy Nicholas C.
Published: July 2002

This is the second in a series on the ab initio calculation of the second, third, and fourth derivatives of the energy of a molecule with respect to nuclear coordinates. A knowledge of these derivatives yields, in particular, anharmonic spectroscopic constants. Here we discuss our implementation of the formula for the fourth derivative of the self-consistent-field energy and present full quartic force fields in internal coordinates for H2O and CO2.
NO3 radical studied by laser-induced fluorescence
Authors: Kim Bongsoo , Hunter Philip L. , Johnston H. S.
Published: July 2002

The fluorescence emission spectra of NO3 excited at 14 742, 15 109, 15 882, 16 053, and 16 555 cm−1 are reported. On the basis of fundamentals, overtones, and combination of five vibrational frequencies (368, 753, 1053, 1500, and 2010 cm−1 ) we assign 18 out of 20 observed bands. The fluorescence bands exhibit two different shapes, one shows a sharp spike overlapped with a broadband, and the other shows a broadband only. From the literature we obtain a potential-energy surface that has D3h symmetry with three identical shallow minima, each representative of a local C2v structure and located with threefold symmetry around the central axis. Such a potential-energy function can split degenerate D3h vibrational modes, giving ‘‘pseudorotations,’’ as a structure with one long and two short bonds permutes around the three minima. On the time scale of molecular rotations, vibrational motions average over the three local C2v structures to give D3h structure and rotational spectra. This model qualitatively explains both the five fundamental frequencies observed by fluorescence and the definite D3h properties of high-resolution infrared spectra. We suggest that a molecular theoretical model with fine spatial resolution sees the miniwells and reports C2v as minimum-energy structure, but a model with less fine resolution overlooks the three shallow minima and reports the larger-scale D3h structure.
Temperature dependence of interactions of an ion pair in water: A molecular dynamics study
Authors: Dang Liem X.
Published: July 2002

We present a comparative study of the potentials of mean force (PMF) and the related thermodynamic properties for Na+–Cl− ion pair using a rigid simple point charge (SPC/E) water model at two different temperatures, 298 and 373 K. The PMFs were calculated using the thermodynamic perturbation theory and the long-range interactions were evaluated using an Ewald summation technique. The PMFs for both temperatures display two minima corresponding to the contact and solvent-separated ion pairs; however, they are different in details. At room temperature, the solvent separated region is favored over the contact region. On the other hand, the calculated PMF at higher temperature indicate that the contact ion pair is more favorable. These observations are in general agreement with a recent Raman study of salt solutions of the similar systems. We also carried out a calculation at room temperature using the spherical truncation technique. The calculated PMF using this technique is somewhat more repulsive compared to the calculated PMF using an Ewald summation technique. However, these PMFs have similar characteristics. When comparing the present calculations with those reported in previous works on the same systems, our calculated PMF at room temperature is identical to the results of simulations using a flexible water model. This result suggests that the more stable solvent-separated ion pair at room temperature may not be due to the inclusion of the internal motions of water molecules. It is probably due to the balancing of the intermolecular forces.
An infrared and Raman study of the reorientational motions and phase transition in and structure of CsPF6
Authors: Heyns Anton M. , de Waal Danita
Published: July 2002

CsPF6I is face-centered cubic and the PF6− ions are randomly distributed among the six equivalent orientations in the crystal. The ions librate in equilibrium positions for an average time τ, and also jump from one orientation to another in a time which is much shorter than τ. The six different orientations in CsPF6I are of much lower symmetry than the full molecular one. The full vibrational spectra of CsPF6 are reported over the temperature range of 300–4 K and can be analyzed in Phase I in terms of (Oh,C̄4v) where C̄4v denotes the symmetry of the ion and Oh that of the site symmetry S. The polarized Raman spectra of single crystals of CsPF6I are interpreted in terms of all the different orientations of the PF6− ions and excellent agreement is obtained between predicted and experimental results. At 90 K CsPF6 undergoes a phase transition and a crystal structure of P4/nmm-D4h7 is assigned to Phase II. This transition is of the second order and shows behavior which corresponds to those of critical phenomena. An order parameter ξ is defined and a critical constant β is calculated to be equal to 0.3. In CsPF6II two orientations of the PF6− ions exist, both conforming to 4mm-C4v symmetry and the disorder persisting in this phase causes the splitting of the vibrational bands to be fully resolved below 60 K only. An activation energy of ∼3.0 kJ/mol is calculated for the motions of the PF6− ions between 90 and 60 K from the variation of the linewidths and intensities of vibrational bands.
Infrared profile of van der Waals dimers HCl–RG* (RG*=Ar, Kr, Xe) trapped in rare gas matrices
Authors: Girardet C. , Lakhlifi A. , Laroui B.
Published: July 2002

The infrared profile of the van der Waals dimers HCl–RG* trapped in a ternary matrix containing small molar ratio of hydracids (≤1/1000) and of rare gas dopants (≂1/10) is theoretically investigated. The homogeneous profile of a single dimer is determined, using a conventional cumulant expansion of the coupling Hamiltonian between a renormalized optical system described by the vibration-orientation Hamiltonian of the trapped molecule and a renormalized bath defined by the lattice modes and the molecule and dopant center of mass motions. The influence of the ternary matrix inhomogeneities is also studied within the framework of a stochastic model which describes the interaction between the various species in the solid. A detailed analysis of the homogeneous and inhomogeneous broadenings of the infrared lines of these dimers is done on the basis of relatively accurate interaction potentials, previously determined. The comparison of the infrared profiles with recent FTIR spectroscopic data shows a fair agreement in most cases.
On the convergence of the symmetrized Rayleigh–Schrödinger perturbation theory for molecular interaction energies
Authors: Ćwiok T. , Jeziorski B. , Kol/os W. , Moszynski R. , Szalewicz K.
Published: July 2002

The generalized Heitler–London perturbation theory for molecular interaction energies proposed recently by Tang and Toennies [J. Chem. Phys. 95, 5981 (1991)] is proved to be equivalent to the symmetrized Rayleigh–Schrödinger perturbation expansion. This theory is applied to the interaction of two hydrogen atoms and is shown to reproduce the interaction energy in the region of the van der Waals minimum to within about 1% already in the third-order treatment. It is shown that the convergence radius of this theory is the same as that of the polarization perturbation theory, i.e., is marginally greater than unity at large interatomic distances R. This proximity to unity results in a prohibitively slow high-order convergence of the expansion in the region of the van der Waals minimum. Consequently, for the singlet state at large R a small part of the exchange energy cannot be recovered in practice by a direct term-by-term summation of the series. The perturbation series resulting from the application of the theory to the antisymmetric triplet state of H2 converges to an unphysical energy lying above the energy of the triplet state. At large R the difference between the physical energy and the unphysical limit of the series is very small and can be neglected in practical applications of the theory.
Measurements of the dc electric dipole polarizabilities of the alkali dimer molecules, homonuclear and heteronuclear
Authors: Tarnovsky V. , Bunimovicz M. , Vušković L. , Stumpf B. , Bederson B.
Published: July 2002

The average static (dc) electric dipole polarizabilities of the NaK and KCs heteronuclear alkali dimers have been measured, and those of the homonuclear molecules remeasured using molecular beam deflection in an inhomogeneous electric field. A congruent magnetic field enabled removal of the monomer constituents of the beam. The resulting deflection curves reflect both the permanent electric dipole force (for the heteronuclear dimers only), and the induced (polarizability) dipole force. It is assumed that the molecular constituents of the beam possess modified Maxwell–Boltzmann velocity and rotational state distributions. A Mach number is introduced to describe the sonic beam properties. The data analysis consists of a two-parameter least squares best fit of the experimental deflection curves to computed ones, the parameters being the polarizability and the Mach number. Values of the permanent heteronuclear dipole moments are taken from the literature. The obtained static electric dipole polarizabilities in units of (Å3) are 34±3 for Li2 (948 K), 40±3 for Na2 (676 K), 77±6 for K2 (542 K), 79±6 for Rb2 (527 K), 104±8 for Cs2 (480 K), 51±3 for NaK (612 K), and 89±5 for KCs (488 K). Comparison with recent calculations as well as earlier measurements of the homonuclear species are given. Two empirical rules are proposed for determining polarizabilities and dipole moments of heteronuclear molecules.
On the thermodynamic stability of clathrate hydrate. I
Authors: Tanaka Hideki , Kiyohara Kenji
Published: July 2002

The thermodynamic stability of a clathrate hydrate has been investigated by examining the free energy of formation of clathrate hydrate II encaging propane. The total free energy has been divided into several contributions—the interaction between water and guest propane molecules, the entropic contribution arising from the combinations of cage occupancy, and also the free energy due to intermolecular vibrations. The present method avoids some of the fundamental assumptions in the van der Waals and Platteeuw theory. This enables us to assess separately the factors which have a bearing on the thermodynamic stability of the hydrate. Kinetic stability has also been investigated by calculating molecular dynamics trajectories having initially excited several characteristic vibrational modes. We show, for propane in large cages, that the potential energy surface of the guest molecule in a cage has a single minimum and molecular motions can be approximated accurately to a collection of harmonic oscillators. It is found that the intermolecular vibrational modes for water molecules shift toward the higher frequency regions in the presence of the guest molecules. This shift of the vibrational motions of water molecules gives rise to an increase in the chemical potential of water compared with that evaluated under the fixed lattice sites approximation where only vibrations of the guest molecules are taken into account. The empty clathrate hydrate structures melt into liquid water within a few picoseconds when several of the lowest frequency modes are excited. On the other hand, the guest molecules are found to prevent the clathrate structures from melting even 10 ps after the excitation of the same frequency modes.
A study of 13C-enriched chemical vapor deposited diamond film by means of 13C nuclear magnetic resonance, electron paramagnetic resonance, and dynamic nuclear polarization
Authors: Lock Herman , Wind Robert A. , Maciel Gary E. , Johnson Curtis E.
Published: July 2002

13C nuclear magnetic resonance (NMR), with and without dynamic nuclear polarization (DNP), and electron paramagnetic resonance (EPR) measurements are presented on two 13C-enriched chemical vapor deposited (CVD) diamond films. The samples were prepared by the decomposition in a microwave discharge of a gas mixture of CH4 and H2, with and without O2. 13C NMR spectra, obtained with or without magic-angle spinning (MAS) both at a field of 14 T using direct polarization (DP) and at 3.5 T via cross polarization (CP), are presented. Because the samples contain both nuclear and (unpaired) electron spins, irradiation of the sample with microwaves with a frequency at or near the electron Larmor frequency leads to an enhancement of the nuclear spin polarization via the effect referred to as dynamic nuclear polarization (DNP). This enhancement (at a field of 1.4 T), combined with the effects of isotopic enrichment (14% 13C), makes it possible to measure 13C DNP–CP–MAS spectra within a couple of hours, compared to almost 20 h for the CP–MAS spectra taken at high field without DNP. Data based on DNP–CP–MAS experiments with variable CP-match time or dipolar-dephasing time are shown and discussed in terms of structural models. The cross-polarization signals are rationalized in terms of estimates of the chemical shifts of hydrogenated diamond surfaces; this approach indicates that most of the hydrogens in the polycrystalline sample are at the intergrain boundaries.
Radii of monopositive atomic ions
Authors: Gadre Shridhar R. , Sen K. D.
Published: July 2002

An electrostatic model of ‘‘touching spheres’’ of ionic charges with a common matching electron density in conjunction with the electron density corresponding to the minimum value of the anionic electrostatic potential is used to calculate approximate radii of the monopositive alkali metal ions. The theoretical values derived from the self-interaction corrected local-spin-density functional scheme are found to be in good agreement with the empirical estimates.
Benchmark calculations with correlated molecular wave functions. I. Multireference configuration interaction calculations for the second row diatomic hydrides
Authors: Woon David E. , Dunning Thom H.
Published: July 2002

Multireference configuration interaction calculations (valence electrons only) based on generalized valence bond (GVB) and complete active space (CAS) self-consistent field wave functions are used to compute potential energy functions and spectroscopic constants for the second row diatomic hydrides of aluminum through chlorine. The correlation consistent basis sets of Dunning and co-workers have been used. This suite of sets—standard and augmented sets of double through quintuple zeta quality—provides a systematic means of improving the description of chemical bonding. The regularity of De and re as a function of basis set quality allows extrapolation to an estimated ‘‘complete’’ basis set limit. The error in the CAS+1+2 predictions of De for the five species varies from 0.3 kcal/mol (AlH) to 1.4 kcal/mol (HCl) with a root-mean-square (rms) error of 0.7 kcal/mol. The error in re varies from 0.0008 Å (SH) to 0.0028 Å (SiH) with a rms error of 0.002 Å. Other properties are described with comparable accuracy. GVB+1+2 results are only slightly less accurate (rms errors of 1.3 kcal/mol and 0.003 Å, respectively). The intrinsic errors inherent in the use of GVB+1+2 and CAS+1+2 wave functions, i.e., the errors at the ‘‘complete’’ basis set limit, are found to be much smaller than previously thought. Triple zeta quality sets are found to provide a good compromise between accuracy and cost.
Resonant energy dependence of the hyperpolarizability β: On the possibility of double resonance enhancement
Authors: Moylan Christopher R.
Published: July 2002

The dependence of the first molecular hyperpolarizability on the energy of the transition to the first excited state is examined under the assumptions of the two-level model of Oudar and Chemla. It is found that the resonance enhancement term is not maximized when λmax lies between the fundamental wavelength and its harmonic, contrary to frequency assertion. It is also found that the explicit λmax dependence of β is much smaller than the implicit dependence. These two facts imply that it is indeed desirable for a nonlinear chromophore’s λmax to lie to the red of a laser’s second harmonic, but not because of double resonance enhancement.
On the bending vibration of NH−2: An ab initio study
Authors: Botschwina Peter , Seeger Sigrun , Flügge Jörg
Published: July 2002

Large-scale ab initio calculations predict the origin of the bending vibration of the amide anion at 1449.5 cm−1. This band has a small absolute IR intensity of 2.5 km mol−1, about four times smaller than that of its first overtone which gains intensity through slight Fermi resonance with the symmetric stretching vibration. The present results are in poor agreement with the interpretation of a recent matrix study [S. Suzer and L. Andrews, J. Chem. Phys. 89, 5347 (1988)].
Lattice cluster theory of compressible diblock copolymer melts
Authors: Dudowicz Jacek , Freed Karl F.
Published: July 2002

We generalized to diblock copolymer systems the lattice cluster theory, which includes the influence on thermodynamic properties of monomer structure, local correlations, and compressibility (often called ‘‘equation of state effects’’). The thermodynamic properties are expanded in powers of the inverse lattice coordination number and the van der Waals interaction energies. This cluster expansion for the Helmholtz free energy is represented diagrammatically similar to the case of blends, but diblock copolymers introduce new features into the theory due to the presence of correlations in the region of the junction between two blocks. The theory predicts the composition and molecular weight dependence for both the entropic and enthalpic portions of the effective interaction parameter. The influence of monomer structure on thermodynamic properties is illustrated using model calculations for the order–disorder transition temperature as a function of composition and pressure.
Integral equation theory for associating liquids: Dimer and trimer concentrations for model 1–3 electrolytes
Authors: Wang Jun , Haymet A. D. J.
Published: July 2002

Ion association in a 1–3 electrolyte is studied using an asymmetrically charged soft-sphere model. The ions in this model have identical size, and the aqueous solvent is treated as a structureless dielectric continuum. The fraction of monomers, dimers, and trimers in the electrolyte are predicted by an extension of Wertheim’s associating liquids integral equation theory. Over the range of concentrations from 0.002 to 1.5M, the predictions for the thermodynamic quantities such as the reduced excess energy and the osmotic coefficient, are compared with existing molecular dynamics simulations of a ‘‘hard-core’’ model of 1–3 electrolytes. The influence of charge asymmetry on ion association is examined by comparing the 1–3 predictions with those for 1–1, 1–2, and 2–2 model electrolytes with the same Debye screening length.
Viscosities of a series of gaseous fluorocarbons at 25 °C
Authors: Dunlop Peter J.
Published: July 2002

Viscosities have been measured at 25 °C for methane, ethane, and most of the fluorinated compounds of those species; some viscosities are also reported for several other simple hydrocarbons and fluorocarbons. In the case of the two pairs of configurational isomers of ethane, quite large differences in the viscosities of each species of a pair are apparent.
Comment on ‘‘Non-Rice–Ramsperger–Kassel–Marcus dynamics and the statistics of reaction rates in chaotic systems’’ [J. Chem. Phys. 9
8, 7898 (1993)]
Authors: Wilkie Joshua , Brumer Paul
Published: July 2002

Results from chaos theory on the scarring of wave functions are applied to the problem of analyzing semiclassical reaction rates. Specifically, we show that uniform support of an eigenstate over the energy surface yields a Rice–Ramsperger–Kassel–Marcus (RRKM) estimate of the rate, while scarring of an eigenstate results in corrections to the RRKM rate due to periodic trajectories which recross the transition state. Non-RRKM behavior is also demonstrated for strongly averaged rates where recrossing effects are negligible.
Conductivity in polyacetylene. I. Ab initio calculation of charge localization, bond distances, and reorganization energy in model molecules
Authors: Rodríguez-Monge Lucía , Larsson Sven
Published: July 2002

The electronic structure and geometry of neutral or charged molecules of the type trans-H(CH)nH is calculated using ab initio methods. The reorganization energy λ for adding or subtracting electrons is obtained for the cases with n=10, 11, and 12 using (U)MP2 with a 6-31G basis set. We find contributions to λ not only from C–C bond distance changes but also from C–C–C bond angle changes, whereas the contribution from C–H bond length and C–C–H bond angle changes are unimportant. The change of bond length when electrons are added to or subtracted from a neutral molecule with an even number of carbon atoms is typically 0.05–0.08 Å and occurs over a large part of the molecule, even if the donated charge is concentrated near the positive alkali ion. λ for one-electron reduction or oxidation is large in an even C chain but small in an odd C chain. The introduction of electronic correlation at (at least) the level of second-order Mo/ller–Plesset perturbation theory is important in the calculation of bond lengths and reorganization energies.
The equation of state of polyamorphic germania glass: A two-domain description of the viscoelastic response
Authors: Smith K. H. , Shero E. , Chizmeshya A. , Wolf G. H.
Published: July 2002

We have measured the quasistatic room temperature equation of state of GeO2 glass under hydrostatic conditions to 7.1 GPa. From ambient pressure to 4 GPa the compression displays completely reversible elastic behavior. Above 4 GPa the glass becomes anelastic and exhibits a dramatic increase in the static compressibility. This change in elastic response is concomitant with the onset of the previously reported pressure-induced germanium coordination change. The equation of state data can be quantitatively described by a two-domain model composed of four- and six-coordinated germanium clusters. The model accurately reproduces the previously measured change in the average Ge–O bond length of germania with pressure and rationalizes the different pressure dependent compressional behavior observed in quasistatic and ultrasonic measurements. We further conjecture that the vitreous polyamorphism exhibited by germania glass at high pressures, and the pressure-induced crystal-to-amorphous transition of quartz-isotypic GeO2, both result from similar underlying coordination instabilities in the germania tetrahedral framework.
Femtosecond pump–probe spectroscopy of intermolecular vibrations in molecular dimers
Authors: Tanimura Yoshitaka , Mukamel Shaul
Published: July 2002

The dynamics of intermolecular vibrations in molecular aggregates can be directly monitored by ultrafast spectroscopic techniques. Starting with the third order response function obtained using the path-integral approach, we calculate the pump–probe spectrum of a molecular dimer, taking into account the effects of an intermolecular vibration coupled to the solvent. The various contributions to the signal which represent distinct Liouville space paths are identified and analyzed.
Comment on ‘‘Orientation dependence of 2H nuclear magnetic resonance spin-lattice relaxation in phospholipid and phospholipid:cholesterol systems’’ [J. Chem. Phys. 101, 749 (1994)]
Authors: Nagle J. F.
Published: July 2002

A kinetic inequality that is fundamental to a proposed new method of quantitative analysis of NMR data is shown to be incorrect for the simplest version of the rotational isomeric model. While the claim that the rotational isomeric model is inconsistent with dynamical data was therefore not valid, the constraints introduced by this method should prove useful in refining dynamic models of lipid bilayers.
Computer simulation of regular Liesegang structures
Authors: Büki A. , Kárpáti-Smidróczki É. , Zrínyi M.
Published: July 2002

A heterogeneous chemical pattern formation, the so-called Liesegang phenomenon, was studied by means of computer simulation. A one dimensional algorithm based on Ostwald’s supersaturation theory was developed and used for determining the effect of different initial conditions on the final structure. Four ‘‘reduced’’ parameters have been introduced that unambiguously determine the spacing coefficient of the final structure. The relationship between the initial concentration of the outer electrolyte and the spacing coefficient that was suggested by Matalon and Packter has been found to be valid only for high outer electrolyte concentrations.
Transport coefficients of liquid butane near the boiling point by equilibrium molecular dynamics
Authors: Daivis Peter J. , Evans Denis J.
Published: July 2002

We present very precise results for the linear viscosity, thermal conductivity, and self-diffusion coefficient of the Ryckaert–Bellemans model of liquid butane near the boiling point, calculated from their respective Green–Kubo formulas using equilibrium molecular dynamics simulations. These results are used as a basis for the appraisal of previous calculations of these transport coefficients, which vary considerably. We find excellent agreement between our results and the results of the most precise nonequilibrium molecular dynamics simulations. We directly examine the system-size dependence of these transport coefficients for system sizes between 64 and 864 molecules and find that it is negligible, within experimental errors, for the viscosity and the thermal conductivity. The self-diffusion coefficient increases with increasing system size. The long-time decay of the shear stress, heat flux, and velocity autocorrelation function is also discussed quantitatively.
Tunneling in the H2S+O(3P)→HS+OH reaction: A theoretical study
Authors: Yokoyama Keiichi , Takayanagi Toshiyuki
Published: July 2002

Title reaction has been investigated by a quantum mechanical reactive scattering method. A potential energy surface has been constructed on the basis of ab initio calculations at the MP2(fc)/6-311G(3df,3pd) level of theory. The reaction probabilities have been calculated under an assumption of a collinear atom-diatom collision. It has been found that OH(v=1) is mainly produced in the reaction at room temperature. The rate constants evaluated from the reaction probabilities were 2 orders of magnitude higher than those calculated by the transition-state theory, implying that quantum mechanical tunneling plays an important role in this reaction even at room temperature.
Magnetic interactions between the reduced bacteriopheophytin and quinone electron acceptors in reaction centers of the photosynthetic purple bacterium rhodopseudomonas viridis: An X-band and Q-band electron paramagnetic resonance study
Authors: Van den Brink Johan S. , Gast Peter , Hoff Arnold J.
Published: July 2002

The electron paramagnetic resonance (EPR) signal associated with the photo-accumulated radical anion of the primary electron acceptor (I⋅−, a bacteriopheophytin radical) in reaction centers (RCs) of the photosynthetic purple bacterium rhodopseudomonas viridis shows a characteristic splitting of about 14.0 mT. This splitting has been attributed to an exchange interaction between I⋅− and the reduced complex of the second electron acceptor, a quinone molecule, and a divalent, high-spin (S=2) Fe-ion, [Q⋅−AFe++]. The magnetic structure of the three-spin complex, Fe++ (S=2), Q⋅−A (S=1/2), and I⋅− (S=1/2), is assessed by Q-band (34.8 GHz) and X-band (9.2 GHz) EPR spectroscopy. The EPR spectrum of [I⋅−Q⋅−AFe++] is simulated accurately for the first time, using the magnetic parameters for the quinone-iron complex [W. F. Butler, R. Calvo, D. R. Fredkin, M. Y. Okamura, and G. Feher [Biophys. J. 45, 947 (1984)]. A largely isotropic interaction between I⋅− and Q⋅−A is required (JIQ=−7.5 mT), together with an anisotropic interaction between I⋅− and the Fe++-ion, whose y-component, CIFe,y, is −3.5 mT. The simulations were insensitive to the magnitude of the x,z-components, CIFe,x/z. The experimental magnetic interactions correspond very well with values calculated from the distances in the RC crystal structure. Thus, the interaction between I⋅− and Q⋅−A is largely isotropic (exchange), whereas the interaction between I⋅− and Fe++ has a purely dipolar character. This result is used to determine the principal directions of the magnetic interaction tensors of the Fe++-ion.