bluuebunny/crossref_metadata_2025 huggingface dataset:
A rapidly convergent CI expansion based on several reference configurations, using optimized correlating orbitals
Authors: Taylor Peter R.
Published: February 2003

A method is presented for obtaining CI expansions, comprising all single and double excitations from an arbitrary set of reference configurations, in a rapidly convergent form. This rapid convergence results from the use of mutually nonorthogonal sets of correlating orbitals, and the method thus represents a generalization of the PNO–CI scheme. A computational implementation requires the following steps: generation of a configuration list and symbolic Hamiltonian matrix elements between configurations, optimization of correlating orbitals, transformation of required integrals, the construction of a numerical Hamiltonian matrix from these data, and the extraction of one or more roots from this matrix. Each of these steps is discussed in terms of the algorithm used in the present application. The results of illustrative calculations on the methylene and lithium monoxide molecules are given, together with observed computation times, and it is concluded that the present scheme should prove as useful for CI expansions based on several reference configurations as has the PNO–CI method for the simpler case of a single reference configuration.
Quantum corrections to the thermodynamic properties of adsorbed fluid phases
Authors: Glandt Eduardo D.
Published: February 2003

The first Wigner–Kirkwood correction to the third and fourth virial coefficients for the two-dimensional Lennard-Jones fluid is reported. The quantum effects on the vapor–liquid critical temperature are well described by kTc/ε=(kTc/ε) (classical)−0.15Λ2. The first correction to the Helmholtz free energy (and thus to its derivatives) is calculated using Percus–Yevick radial distribution functions. A Monte Carlo computer simulation is used, at a single temperature kT/ε=1, to confirm the accuracy of the Percus–Yevick results.
The dipolar correlation factor, the electrostatic field, the dipole moment, and the Coulombic interaction energy of water molecules in clathrate hydrates
Authors: Johari G. P.
Published: February 2003

The dipolar correlation factor g, the electrostatic field E, the dipole moment μ, and the Coulombic interaction energy U of water molecules in clathrate hydrates of structure I and structure II have been calculated from the molecular data for their crystal structures in space groups Pm3n and Fd3m, respectively. For dendritic probabilities of orientation of water molecules in fully proton-disordered structures, the values of g are 2.876 and 2.899, of E are 0.2861 and 0.262 Mesu cm−2, of μ are 2.4133 and 2.4135 D, and of U are −7.99 and −7.93 kJ/mole H bond. The dipole moment of H2O is increased by nearly 30% over its vapor phase value. The dielectric permittivity of the two clathrate hydrates calculated from the above values of g and μ are significantly higher than those measured. The values of g, μ, E, and U are somewhat lower than in ice Ih and Ic. The results are discussed in terms of the lack of complete disorder of proton positions in the two structures of clathrate hydrates.
Molecular models of proton pumps
Authors: Nagle J. F. , Mille M.
Published: February 2003

Three theoretical models of proton pumps are discussed at the molecular level. The models contain hydrogen-bonded chains, which are the best understood channels, structurally and kinetically, for proton conduction within membranes. The first model, called the integral injector model, has an active site which involves a pK change. The second model, called the switch model, utilizes a conformational change in a retinal as the active site. The third model, called the active chain model, involves a conformational change in the protein, and the active site is delocalized to include the entire hydrogen-bonded chain. The models presented were chosen in part because of their possible relevance to the bacteriorhodopsin proton pump and comparisons are made to this system, but the basic principles involved have greater generality. In greater part they were chosen to explore the general family of theoretical proton pumps. Each model illustrates general principles that form a basis for generating additional models in the family.
Liquid crystal model for hydrocarbons under shock wave conditions
Authors: Pucci R. , March N. H.
Published: February 2003

To interpret shock wave data on hydrocarbons, Ree has proposed a model in which carbon, having tetrahedral coordination, and molecular hydrogen separate. He then shows that the Hugoniot curves can be ordered according to their hydrogen:carbon ratio and that a quantitative fit of the high pressure data proves possible. In the present work, an alternative proposal is put forward, in which the C–H bonds are not dissociated. Instead, it is suggested that pressure can induce a π-electron transition in which the delocalized behavior parallel to the molecular plane can give way to localized behavior in which the π electrons can participate in intermolecular bonding. In this model, the Hugoniot curves can be ordered according to the π-electron densities. A natural explanation of the discontinuities in the Hugoniot curves is, on the present proposal, a transition from an isotropic liquid phase to a nematic liquid crystal phase. Again, the model leads to an approximate equation of state which can be compared with the available experimental data. The agreement at very high pressures lends support to the model as being useful at a semiquantitative level.
Perturbative examination of avoided crossings
Authors: Ramaswamy Ramakrishna , Marcus R. A.
Published: February 2003

Quantum perturbation theory is used to examine the eigenvalues of a nonseparable Hamiltonian system in the classically regular and irregular regimes. As a function of the perturbation parameter, the eigenvalues obtained by exact (matrix diagonalization) methods undergo an avoided crossing. In the present paper perturbation theory is used as an approximate method to predict the locations of such avoided crossings in energy-parameter space. The sparsity of such avoided crossings in the Hénon–Heiles system is seen to produce regular sequences in the eigenvalues even when the classical motion is predominantly chaotic.
Vibrational–translational relaxation in liquid chloroform
Authors: Takagi K. , Choi P.-K. , Negishi K.
Published: February 2003

Ultrasonic measurements were made in liquid chloroform over the frequency range from 3 MHz to 5 GHz by means of three experimental techniques, pulse-echo overlap, high-resolution Bragg reflection, and Brillouin scattering. The observed velocity dispersion revealed two relaxation processes, one at 650 MHz and the other at 5.1 GHz at 20 °C. They are interpreted in terms of vibrational–translational relaxation. Quantitative analysis of specific heat shows the lowest (261 cm−1) and the second lowest (366 cm−1) fundamental vibrational modes should have a common relaxation time at 50 ps and the group of all above the third mode (667 cm−1) at 290 ps. The present results are combined with recent data obtained by Laubereau et al. with the picosecond spectroscopy technique; a diagram illustrating V–T and V–V energy transfer is presented. A brief comment is given also on V–T and V–V processes in dichloromethane.
Effect of near-resonant energy transfer on vibrational relaxation in liquid dichloromethane–benzene mixtures
Authors: Choi Pak-Kon , Takagi Kenshiro , Negishi Katsuo
Published: February 2003

The velocity and absorption of sound waves are measured in liquid dichloromethane–benzene mixtures in the frequency range from 3 MHz to 6 GHz using three ultrasonic techniques, high-resolution Bragg reflection, Brillouin scattering, and pulse-echo overlap. Vibrational double relaxation is observed in every mixture studied. The higher-frequency relaxation is assigned to the lowest mode of benzene, and the lower-frequency one to all but the lowest mode of both molecules. These modes of both molecules are shown to be coupled with each other by the near-resonant intermolecular vibrational–vibrational energy transfers between the second lowest mode (704 cm−1) of dichloromethane and the modes (707, 675 cm−1) of benzene. The concentration dependence of the relaxation frequency is analyzed with the binary collision theory, and the rates of vibrational–translational energy transfer in collisions between different kinds of molecules are estimated.
Monte Carlo sampling techniques and the evaluation of unimolecular rate constants
Authors: Doll J. D.
Published: February 2003

We discuss the evaluation of unimolecular dissociation constants using Monte Carlo sampling methods. Numerical applications are presented for four atom argon clusters. The energy-dependent rate constants produced by the present procedure agree well with those produced by the dynamical studies of Brady et al. [J. Chem. Phys. 71, 2467 (1979)]. The procedures outlined here are applicable to the problem of evaluating RRKM dissociation rate constants for general interaction potentials.
Differential elastic scattering of Ne*(3s 3P2,0) by Ar, Kr, and Xe: Optical potentials and their orbital interpretation
Authors: Gregor R. W. , Siska P. E.
Published: February 2003

Optical potentials for the title systems have been derived by simultaneous fitting of thermal-energy elastic scattering angular distributions reported here and ionization cross section and quenching rate constant data from other laboratories. The real parts of these potentials are similar to sodium–rare gas van der Waals potentials, with well depths ε=0.126 kcal/mole for Ne*+Ar, 0.192 kcal/mole for Kr, and 0.288 kcal/mole for Xe, with rm=5.0 Å for all three systems. A direct inversion of Ne*+Xe rainbow scattering yields a potential in good accord with the parametric form used in fitting. One-electron model potential calculations also produce curves in good agreement with experiment, and shed light on the effect of orbital mixing (hybridization of the Ne*3s orbital) on the shape of the repulsive branch of the potentials. The derived resonance widths Γ (imaginary parts of the optical potentials) give evidence that a Coulomb or radiative mechanism dominates the thermal-energy quenching by Penning ionization, in contrast to the exchange mechanism thought to be dominant for He*(2 1S,2 3S). The neon 3d orbital, mixed into the excited electron’s orbital by exchange repulsion, appears to play a key role in this mechanistic changeover.
A stochastic approach to the theory of intramicellar kinetics. II. Master equation for reversible reactions
Authors: Hatlee Michael D. , Kozak John J.
Published: February 2003

In this paper we focus on a particular class of intramicellar kinetic processes: reversible reactions of molecularity two taking place in the interior (or perhaps the immediate vicinity) of a micellar assembly. We formulate a stochastic master equation to describe an ensemble of compartmentalized, distributed systems wherein the (only) microscopic chemical event is a photoinduced, reversible, bimolecular reaction. Then, for an initial distribution of reactants assumed to be Poissonian, we calculate and characterize the overall (macroscopic) dynamics displayed by such a system. The resulting kinetic description is compared with the behavior found previously for the simpler case of irreversible, intramicellar kinetic processes. Finally, we introduce and then investigate the notion of an ’’apparent’’ equilibrium constant Q for a reversible reaction taking place in a compartmentalized, distributed system and compare this quantity with the ’’canonical’’ equilibrium constant K obtained for the (same) reversible reaction carried out in bulk, homogeneous solution. The full behavior of Q=Q(K) is explored numerically, and we prove analytically that Q∼K2 in the small K limit and that Q is independent of K in the limit of large K. The experimental consequences of the theory are dicussed in some detail.
Stochastic theory of intramolecular energy transfer in the presence of radiation
Authors: Dougherty Eugene P. , Augustin Stuart D. , Rabitz Herschel
Published: February 2003

A theory for internal energy redistribution in polyatomic molecules perturbed by strong radiation fields is constructed. Use is made of stochastic theory, which assumes that a random phase approximation is valid after appropriate time intervals. This approximation permits the Schrödinger equation to be replaced with a finite-difference master equation for the probabilities of occupying the various quantum levels. The semiclassical theory of the radiation–molecule interaction is employed in this work. A laser line profile is incorporated in the formalism, and this is used to simulate the effects of rotational states and collisions. At each stochastic step the energy changes in the molecules due to radiation are estimated. Thus, energy conservation is explicitly taken into account. Model calculations for SO2, whose transition dipole moment matrix elements and anharmonic force constants have been determined, indicate a complex interplay of anharmonic and radiative coupling. Power density, laser linewidth, and detuning from resonance are observed to play a significant role in energy redistribution.
Diatomic molecules as perturbed Morse oscillators. VI. High-precision eigenfunctions.
Authors: Huffaker J. N.
Published: February 2003

Radial eigenfunctions for vibrating, rotating diatomic molecules are obtained as linear combinations of Morse eigenfunctions by matrix diagonalization in a truncated Morse basis. Results are obtained for CO for v⩽28, J⩽160 from a recently obtained high-precision analytic vibrational potential. Overlap integrals with numerical Cooley–Cashion eigenfunctions are used to evaluate the suitability of alternative matrix techniques and the accuracies of several kinds of truncations.
Energy transfer between electronically excited zinc and calcium atoms
Authors: Coombe R. D. , Wodarczyk F. J. , Horne R. K.
Published: February 2003

A series of experiments have been performed in which zinc atoms carried in a flowing stream of helium are excited to the 4(3P1) state by optical pumping with a XeCl excimer laser. Addition of calcium atoms to the flow results in quenching of the excited zinc atoms. The cross section for this energy transfer process is σ?3×10−16 cm2, corresponding to Zn(4 3PJ) quenching in fewer than 10 collisons with calcium atoms. Subsequent to optical excitation of the zinc atoms, emission was observed from both the 4 1P1 and 5 3S1 excited states of calcium. Integration of the intensities of the calcium emissions indicated that excitation of the singlet and triplet manifolds in calcium occurs with comparable probability, suggesting that spin conservation is not a strong constraint in the energy transfer process. Summation of the intensities indicate that the E–E transfer process accounts for most of the quenching of the excited zinc atoms.
Photolytic spectroscopy of simple molecules. I. The production of 5 2D atoms from Cs2
Authors: Collins C. B. , Anderson J. A. , Popescu D. , Popescu Iovitzu
Published: February 2003

A two-photon technique is reported for the measurement of relative cross sections for the selective photolysis of simple molecules into particular product channels. In a demonstration of this method two independently tunable dye lasers were used to sequentially dissociate molecules of Cs2 for wavelengths in the visible range and then to excite the resulting products to Rydberg states which could be readily ionized for detection. With this system the spectra were determined for the selective photolysis of Cs2 into product atoms in the 5 2D3/2 and 5 2D5/2 states. Two relatively broad absorption bands were found, each of which led to the preferential population of a different fine structure components of the 5 2D products. Analysis of the photolysis spectra indicated that both dissociation and predissociation mechanisms were responsible and led to the identification of a new 1Σu and 3Σu state correlated with the 5 2D atomic states.
Thixotropic behavior of lysozyme solutions
Authors: Giordano R. , Fontana M. P. , Wanderlingh F.
Published: February 2003

It is shown that lysozyme solutions exhibit thixotropic properties down to the very low concentration of 10−4 by weight. The entire range of concentration up to 30% by weight is investigated. It is found that there are three ranges, namely, from 10−4 to 2×10−3; from 3×10−3 to 10−2, and larger than 15%, each of them characterized by different behavior. Thixotropic properties can be explained in terms of the building up of a structure that is both time and shear dependent. It is also shown that a yield point exists. From the experimental results the time constant of the process, the yield point, and the dependence on the shear rate are found.
Monte Carlo study of freely jointed ring polymers. I. Generation of ring polymers by dimerization method
Authors: Chen Yi-der
Published: February 2003

Ring polymers of the freely jointed model are efficiently obtained by joining two linear chains together (dimerization). The rings produced are equivalent to those obtained by the direct Monte Carlo method of Bruns and Naghizadeh, based on the radius of gyration and the various distribution functions. The method can be used to generate any number of ring polymers with any number of bonds and is therefore useful in future studies of ring polymer statistics.
Vibrational frequencies of halogen atoms adsorbed on silver metal surfaces
Authors: Nichols Henry , Hexter Robert M.
Published: February 2003

The vibrational frequencies of halogens adsorbed on Ag(100) are calculated. The frequency of a single adsorbed atom vibration perpendicular to the surface is obtained from AgX (X=F, Cl, Br, I) crystal frequencies. Interaction between dipoles consisting of the effective charge on the adsorbed atom and its induced image charge produces a coverage dependent frequency. The effective charge on the adsorbed atom is assumed to be 1.0e, while the electronic polarizability of the real-plus-image atom system is assumed to be twice the electronic polarizability of X−. At full coverage, the calculated frequencies for the k=0 modes are 437, 241, 150, and 113 cm−1, respectively, for X=F, Cl, Br, I, which compare well with experimental values, and these frequencies are assigned to external vibrational model of halogen atoms adsorbed on silver.
Dynamics of ion channeling at low energies: Nonnormal incidence
Authors: Adams John E. , Doll Jimmie D.
Published: February 2003

Penetration depth profiles and backscattered ion energy analyses have been calculated for a beam of deuterons incident upon the (110) face of a tungsten single crystal at small angles away from the surface normal. We find that while channeling in this energy regime (80 eV) is characterized by relatively large acceptance angles, the energy spectra of the backscattered ions are quite sensitive to changes in the angle of incidence.
Analytic configuration interaction gradient studies of SH4, sulfurane
Authors: Yoshioka Yasunori , Goddard John D. , Schaefer Henry F.
Published: February 2003

The potential energy surface of SH4, the prototype sulfurane, has been examined in light of previous theoretical studies suggesting that there may be two distinct energy minima. The first of these has the four H atoms in roughly octahedral positions, as in the experimentally known SF4 molecule, and belongs to point group C2v. The second isomer is square pyramidal, point group C4v. Molecular electronic structure theory has been applied using basis sets of triple zeta (TZ) and triple zeta plus polarization (TZ+P) quality. Configuration interaction (CI) including in some cases all single and double excitations was carried out and geometries optimized via newly developed analytic CI gradient techniques. At the TZ SCF and TZ CI levels of theory there are both C2v and C4v relative minima, separated by +2.4 and −1.6 kcal, respectively. However, using the more complete TZ+P SCF and TZ+ P CI methods, the C2v minimum disappears, leaving only the C4v minimum. Geometries corresponding to the expected C2v minimum lie about 6 kcal above the square pyramid structure. The C4v mimimum lies 74.6 kcal above separated H2S+H2 at the TZ+ P CI level of theory, and this is reduced to 72.4 kcal when the effects of higher excitations (unlinked clusters) are considered. Harmonic vibrational frequencies have been predicted at the three simpler levels of theory to prove the proposed designations of the various stationary points. For the square pyramidal SH4, the predicted frequencies are quite sensitive to basis set, as might be expected for an extremely flat potential energy surface such as this. The square planar geometry lies only slightly above the square pyramid, which faces a barrier of 2.3 kcal to inversion. The transition state for dissociation via SH4→SH2+H2 was also located at several levels of theory and has an interesting structure. The barrier to dissociation is ∼46 kcal and the activation energy ∼42 kcal, suggesting that SH4 should be a ’’makable’’ molecule.
Effects of periodic boundary conditions on equilibrium properties of computer simulated fluids. I. Theory
Authors: Pratt Lawrence R. , Haan Steven W.
Published: February 2003

An exact formal theory for the effects of periodic boundary conditions on the equilibrium properties of computer simulated classical many-body systems is developed. This is done by observing that use of the usual periodic conditions is equivalent to the study of a certain supermolecular liquid, in which a supermolecule is a polyatomic molecule of infinite extent composed of one of the physical particles in the system plus all its periodic images. For this supermolecular system in the grand ensemble, all the cluster expansion techniques used in the study of real molecular liquids are directly applicable. As expected, particle correlations are translationally uniform, but explicitly anisotropic. When the intermolecular potential energy functions are of short enough range, or cut off, so that the minimum image method is used, evaluation of the cluster integrals is dramatically simplified. In this circumstance, a large and important class of cluster expansion contributions can be summed exactly, and expressed in terms of the correlation functions which result when the system size is allowed to increase without bound. This result yields a simple and useful approximation to the corrections to the particle correlations due to the use of periodic boundary conditions with finite systems. Numerical application of these results are reported in the following paper.
Neutron-diffraction study of Ho2C at 4–296 K
Authors: Atoji Masao
Published: February 2003

By neutron powder diffraction, trigonal Ho2C has been shown to become ferromagnetic below 100 K. The paramagnetic scattering indicates the free Ho3+-ion moment, while the saturation moment in the ferromagnetic phase is 7.16 Bohr magnetons (72% of the free-ion moment), showing a sizeable crystal-field effect. The preferential crystallite orientation induced by the applied magnetic field has shown that the ordered moments are aligned parallel to the [104] axis which corresponds to the [100] axis of the high-temperature cubic modification. The previously proposed HoN-type magnetic ordering is not compatible with our results. The residual, disordered moments exhibit a ferromagnetic short-range order superposing on the ferromagnetic long-range order. Crystallographic-structure data at 296 and 4 K are also presented.
Neutron-diffraction study of cubic ErC0.6 in the temperature range 1.6–296 K
Authors: Atoji Masao
Published: February 2003

Neutron-diffraction measurements have shown that the form of ErC0.6 that has a cubic, NaCl-type structure is paramagnetic above 90 K, exhibiting a free Er3+ moment. Below 90 K, ErC0.6 becomes a ferromagnet with a saturation moment of 2.5 Bohr magnetons (only 28% of the maximum free-ion moment), indicating a large crystal-field effect. By measuring the preferential crystallite orientation induced by the applied magnetic field, the direction of the ferromagnetically ordered moment was found to be parallel to the 〈100〉 axis. A ferromagnetic, short-range ordering coexists with the ferromagnetic long-range ordering at temperatures down to 1.6 K.
Mixtures of hard ions and dipoles against a charged wall: The Ornstein–Zernike equation, some exact results, and the mean spherical approximation
Authors: Blum Lesser , Henderson D.
Published: February 2003

The Ornstein–Zernike equation for a mixture of ions and dipoles near a hard charged wall is obtained. It is shown that the same exact contact and monotonicity theorems, previously derived for the primitive (continuum dielectric) case, also are valid for this model. Rather simple expressions for the contact density, potential difference, capacitance, and distribution functions are obtained in the mean spherical approximation (MSA). These expressions reduce to previously known results in the limits of low and high concentrations of ions. It is found that cooperative alignment of the dipoles near the wall results in an increased potential difference and reduced capacitance of the double layer compared to that calculated when the solvent is represented by a continuum dielectric.
Binding energies and structure of transition metal negative ions
Authors: Feigerle C. S. , Corderman R. R. , Bobashev S. V. , Lineberger W. C.
Published: February 2003

The photoelectron spectra of 11 transition metal negative ions have been obtained in a crossed beam experiment using a sputter type ion source and a fixed frequency ArII laser (488 nm). The electron affinities (E.A.’s) are measured to be E.A.(Ti)=0.080±0.014 eV, E.A.(V)=0.526±0.012 eV, E.A.(Cr)=0.667±0.010 eV, E.A.(Zr)=0.427±0.014 eV, E.A.(Nb) =0.894±0.025 eV, E.A.(Mo)=0.747±0.010 eV, E.A.(Rh)=1.138±0.008 eV, E.A.(Pd)=0.558±0.008 eV, E.A.(Ta)=0.323±0.012 eV, E.A.(W)=0.816±0.008 eV, and E.A.(Ir)=1.566±0.008 eV. The ground states of the negative ions of these elements are determined from analysis of the photoelectron spectra and all are found to be of a dks2 configuration with the exception of Pd−(d10 s). Excited electronic states of Pd−[(4d95s2) 2D5/2] and Ta−[(5d46s2) 3P0] are identified. Calculation of relative intensities of photelectron transitions is used extensively in the analysis of the spectra and is compared with experiment for a number of cases. The spin–orbit separations for Rh−(3F) and Ta−(5D) are measured and compared with values obtained from ratio isoelectronic extrapolation. The accuracy of this extrapolation technique is discussed and an extensive table of extrapolated splittings for transition metal negative ions is given. Plots of the s-electron binding energies of the transition metal negative ions versus the number of d electrons exhibit smooth trends and striking similarities for each of the three transition series.
Vibrational spectra and phase transitions of crystalline bromoform
Authors: Burgos E. , Halac E. , Bonadeo H.
Published: February 2003

We have obtained the Raman and infrared spectra of bromoform crystals at different temperatures in the range −175 to 5 C. Under different experimental conditions, three crystalline phases are grown: α, stable between the melting point and −5 C; β, stable between −5 and −175 C, and γ, which can be trapped by sudden freezing of the liquid to liquid nitrogen temperatures. The γ phase transforms irreversibly into β at temperatures above about −80 C. Possible crystal structures for the three phases are discussed on the basis of the spectroscopic evidence. The results explain apparent contradictions in previously published spectra.
Temperature dependence of the 15N and 1H nuclear magnetic shielding in NH3
Authors: Jameson Cynthia J. , Jameson A. Keith , Cohen Sheila M. , Parker Harriet , Oppusunggu D. , Burrell Patricia M. , Wille S.
Published: February 2003

The temperature and density dependence of the 15N and the 1H nuclear resonance in 15NH3 gas have been observed. The density dependence which is a measure of the effect of intermolecular interactions on the nuclear shielding is linear, with slopes of −0.041±0.002 ppm/amagat for the 15N nucleus and −0.0032±0.0001 ppm/amagat for the 1H nucleus. The shielding in the limit of zero pressure σ0 varies with temperature due to rovibrational motion. This is of special interest for the 15N shielding in NH3 because of the previously reported exceptional temperature dependence of 31P in PH3. It is found that 15N in NH3 is also an exceptional case and, in fact, near 300 K 15N in NH3 and 31P in PH3 have nearly identical values of dσ0/dT(+0.00651±0.00082 ppm/deg for 15N in NH3 from 320 to 380 K). All other cases, involving a variety of nuclei in a wide variety of molecular types, exhibit dσ0/dT<0.
Dynamic light scattering and sedimentation experiments on silica dispersions at finite concentrations
Authors: Kops-Werkhoven M. M. , Fijnaut H. M.
Published: February 2003

We have performed photon correlation spectroscopy measurements on silica particles dispersed in cyclohexane. The diffusion coefficient D of the particles has been measured as a function of their volume fraction φ and could be described by D=D0(1+kDφ), where D0 is the diffusion coefficient at infinite dilution and the constant kD has a value of 1.3±0.2. This value is in agreement with the theoretical values of kD for hard spheres of Burgers, Batchelor, and Felderhof. These results have been compared with time averaged light scattering and sedimentation measurements. The time averaged light scattering experiments confirmed the hard sphere character of the particles earlier found by van Helden. The sedimentation constant has been measured as a function of the volume fraction of the particles resulting in a virial constant ks=6±1, which agrees with the theoretical values for hard spheres.
Laser spectroscopy of defect chemistry in CaF2:Er3+
Authors: Moore D. S. , Wright J. C.
Published: February 2003

Alkaline earth fluoride crystals doped with trivalent lanthanide ions have been studied as model systems for the solid state defect equilibria important in all solids. Problems have arisen in these systems because the behavior does not agree qualitatively with the expected behavior. Most explanations attribute the problems to competing defect equilibria which were previously not considered. In this paper, we have measured the concentrations of all the lanthanide defect sites as a function of both annealing temperature and dopant concentration. Clusters are seen to be an important part of these defect equilibria and could be contributing competing equilibria. At high annealing temperatures, however, the site distribution simplifies so there can be no competing equilibria and yet the anomalous behavior remains. Nonideality effects must be responsible for this behavior.
A photoionization mass spectrometric investigation of CH3CN and CD3CN
Authors: Rider D. M. , Ray G. W. , Darland E. J. , Leroi G. E.
Published: February 2003

A photoionization mass spectrometric investigation of CH3CN and CD3CN from the onset of ionization to ∼20 eV is reported. Photoionization efficiency (PIE) curves of the acetonitrile parent ions and of the fragments C2H2N+, C2HN+, CH2N+, CH+3, CH+2 and their deuterated analogs have been measured. The parent ion curves display isotope-dependent direct ionization as well as autoionization structure. Excitation of one quantum of the doubly degenerate CCN bending mode (ν8) in the ground state of CH3CN+ is observed, indicating the presence of Jahn–Teller coupling. The measured ionization potentials are 12.194±0.005 eV (CH3CN) and 12.235±0.005 eV (CD3CN). Autoionizing Rydberg series converging to the first and second excited electronic states have been observed and assigned. At their thresholds, C2H2N+ and C2D2N+ are produced exclusively from parent ion states which are populated by autoionization. Appearance potentials of the fragment ions for which PIE curves were obtained are reported, and their heats of formation have been calculated where more reliable literature values are not available. The observed fragment ion appearance potentials and relative ion yields indicate that H-atom migrations in the molecular ion are important in the fragmentation mechanisms.
Dissociative attachment from vibrationally and rotationally excited HCl and HF
Authors: Allan M. , Wong S. F.
Published: February 2003

An electron impact mass spectrometer is used to study dissociative attachment in HCl, DCl, and HF at incident energies 0–4 eV. Target molecules in different excited nuclear states are prepared with an iridium oven source. The cross sections for Cl−/HCl, Cl−/DCl, and F−/HF all show an order of magnitude increase with each increase of vibrational quantum (v=0, 1, and 2). In Cl−/HCl the threshold cross section for the v=2 level at 0.1 eV reaches (7.8±4.7)×10−15 cm2. Together, this large cross section and the recent potential-energy-curve calculations of HCl and HCl− show that the conventional theory of dissociative attachment may not be applicable here. A new mechanism for dissociative attachment is suggested.
Electric field effects in the infrared multiphoton dissociation of CF2HCl
Authors: Gozel P. , van den Bergh H.
Published: February 2003

The infrared multiphoton dissociation of chlorodifluoromethane is studied in a dc electric field. The reaction yield is measured as a function of the laser energy fluence, the CF2HCl pressure, and the electric field strength in the range 0⩽E⩽5.4 kV cm−1. At low fluence (F<1J cm−2) and intensity, under essentially collision free conditions, the multiphoton dissociation is strongly enhanced by the electric field. This is tentatively explained by a breakdown in the angular momentum selection rules caused by the applied field.
Temperature dependence of partial molar heat capacities of binary critical mixtures: Study of the system isobutyric acid/water
Authors: Pelger M. , Klein H. , Woermann D.
Published: February 2003

Data of the temperature dependence of partial molar heat capacities of isobutyric acid and water near the critical point are reported. It is found that the singular part of the partial molar heat capacity of isobutyric acid in a critical mixture has a positive sign whereas that of water has a negative sign. A relationship between the critical amplitudes of specific heat capacity, partial molar heat capacities, temperature derivative of specific volume, and temperature derivatives of partial molar volumes is derived and checked using experimental data of the system isobutyric acid/water. The relationship is obeyed within the precision of the measurements.
Hydration and complex formation study on concentrated MCl2 solutions [M=Co(II), Ni(II), Cu(II)] by x-ray diffraction technique
Authors: Magini Mauro
Published: February 2003

Concentrated aqueous chloride solutions containing equal amounts of transition metal ions of almost equal ionic radii (Co2+, Ni2+ and Cu2+) have been examined by the x-ray diffraction technique. CoCl2 and Nicl2 solutions show distribution curves very similar to each other which leads to the conclusion that the same dominant species should occur in both systems. The analysis of the structure functions can be achieved by considering M(H2O)2+6 (M=Co2+, Ni2+) as the dominant species although indication is given in favor of the presence of about 50% of the M(H2O)5Cl+ complex [and the rest M(H2O)2+6] in both solutions. The distribution curve of CuCl2 solution is markedly different from those of cobalt and nickel. Equatorial positions, in the distorted octahedral copper coordination are, on the average, occupied by about 2.8 water molecules (at 1.95 Å) and about 1.2 chloride ions (at 2.25 Å). Occupancy of the axial copper positions is discussed, though no unique conclusion can be obtained with respect to this point from the scattering data.
On correlation functions and the onset of chaotic motion
Authors: Koszykowski M. L. , Noid D. W. , Tabor M. , Marcus R. A.
Published: February 2003

A variety of correlation functions computed over a microcanonical ensemble for the Hénon–Heiles system are investigated. We find the general trend is that of a gradual change to some form of decaying behavior as the motion becomes predominantly chaotic. The decay of a mode energy correlation function indicates a time scale for intramolecular energy redistribution.
A molecular theory for the solid–liquid interface
Authors: Haymet A. D. J. , Oxtoby David W.
Published: February 2003

We present an order parameter theory of the solid–liquid interface which uses structural information about the uniform liquid phase. The order parameters are the coefficients of a Fourier expansion of the nonuniform density in terms of reciprocal lattice vectors characteristic of the uniform solid phase. The theory provides explicit formulas for the interfacial density profile and the surface free energy. Connection is made with recent theories of freezing and of the liquid–vapor interface. Two special cases of particular interest are considered: the flat interface, for which numerical simulations have been attempted, and the spherical ball, which is important in theories of liquid–solid nucleation.
Photoselection in uniaxial liquid crystals: The advantages of using saturating light pulses for the determination of orientational order
Authors: Naqvi K. Razi
Published: February 2003

It is contented that by using a pulsed laser source for excitation and investigating the fluorescence anisotropy in uniaxial liquid crystalss, one can determine all the moments and therefore the orientational order of the molecules. (AIP)
Formation of Fe* in collisions between metastable Ar*(4s 3P2,0) and Fe(CO)5
Authors: Kobovitch John , Krenos John
Published: February 2003

A molecular beam apparatus was used to measure luminescence from Fe* formed by Ar*/Fe(CO)5 dissociative collisions. The bimolecular origin of this process was confirmed. Populations of septet levels of iron relative to quintet populations are much smaller than those obtained in a flow tube and, in addition, population ratios within the 7FJ0 multiplet differ. The small septet level populations observed are attributed to a time bias in the beam measurements. Degeneracy-corrected cross-section ratios [(σJ/gJ)/(σJ−1/gJ−1)] for 7FJ0 levels (J=2−5) are all ∼1, whereas the flow tube values are ∼2 in each case. Our results follow statistical behavior and suggest that for septet levels, intramultiplet electronic relaxation occurs at pressures used in the flow tube measurements. (AIP)
Reply to Comment by Thomas on ‘‘On rainbow scattering in inelastic molecular collisions’’
Authors: Bowman Joel M. , Lee Ki Tung
Published: February 2003

The comments of Thomas1 on the location of rainbows in inelastic molecular scattering of Ref. 2 are discussed and evaluated. It is contended that more insight into the nature of reainbows in rotatinally inelstic collisions is obtained by using the arguments in ref. 2. (AIP)
Near thermal charge transfer between Ar+2 and N2
Authors: Holzscheiter H. M. , Church D. A.
Published: February 2003

The near thermal charge transfer reaction of Ar+2 with N2 has been studied at total pressures below 10−7 Torr using a stored ion technique. Ar+2 ions produced by electron impact double ionization of Ar gas were selectively stored for times the order of seconds in a split-ring Penning-type ion trap. The decay with time of the initial ion sample number in a mixture of Ar and N2 gases was fit to the sum of two exponentials, corresponding to different reaction rates for the 3P and 1D low-lying Ar+2 levels. The observed Ar+2 number decrease is attributed to the double-charge transfer process Ar+2+N2→Ar+N2+2→Ar+N++N+ in accord with recent flow-tube measurements. A rate constant for the metastable Ar+2 (1D) level reaction with a value k(1D)=1.4×10−9 cm3/sec is obtained, using the previously measured rate constant for the Ar+2(3P) state.
Dissipation in an oscillatory reaction mechanism with periodic input
Authors: Termonia Yves , Ross John
Published: February 2003

We investigate dissipation in externally driven systems beyond marginal stability where, in the absence of external effects, the system has oscillations. The reaction mechanism we choose for study is that proposed by Higgins to describe the free oscillations in the phosphofructokinase reaction of the glycolytic pathway. Entrainment of that reaction is realized by imposing a periodic supply of fructose-6-phosphate. The condition of a periodic supply can be met in biological systems, for instance if transport occurs across a membrane with concentration dependent permeability. An essential reverse reaction is added on the input side of the mechanism, which then permits fructose-6-phosphate to interact with its own supply. A convenient definition of efficiency is introduced to measure the effect of a periodic source on the alteration of free energy dissipation. We go beyond a linear analysis in two respects : First, we integrate the kinetic equations numerically and second, we use the equivalent linearization method of Bogoluibov and Mitropolsky, which is not restricted to small amplitude motion. There is a large resonance effect on the dissipation in the fundamental entrainment band (much larger than for submarginal conditions) and smaller effects at higher harmonics and one subharmonic. A large change in efficiency, from positive to negative, occurs in a very narrow range of frequency of applied concentration variation, which provides the possibility of regulation of dissipation. The equivalent linearization method applied to the fundamental entrainment band gives qualitatively correct trends and results.
Ionizing reaction cross sections in the collision of argon atoms in high Rydberg states with various molecules
Authors: Dimicoli I. , Botter R.
Published: February 2003

Measurements of absolute ionizing reaction cross sections in collisions between argon atoms in high Rydberg states (HR) and CCl4, CCl3F, CH3I, SF6, C6F6, CF4, and CO are reported together with their dependence on the HR argon velocity (570–2100 m/sec). The identity of the positively and negatively charged reaction products has been investigated with a coincidence TOF mass spectrometer. A comparison of the present data with results of other bound and free electron experiments and theoretical predictions is given.
Ion-pair formation in the collision of high Rydberg argon atoms with SF6 and C6F6 and negative ion lifetimes
Authors: Dimicoli I. , Botter R.
Published: February 2003

The ion-pair formation in collision between argon atoms in high Rydberg states (HR) with SF6 and C6F6 has been investigated. The absolute cross sections are inversely proportional to the Ar(HR) velocity and increase monotonically with increasing principal quantum number n. Measurements of the negative ion lifetimes against autodetachment are reported and are ≳20 μsec for SF6− and ∼1 and ≳20 μsec for C6F6−. The present data are reviewed in light of the theoretical predictions of ’’free electron’’ models and the results of free electron experiments.
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o studies of structural features not easily amenable to experiment. II. The influence of bond delocalization effects on the molecular structures of some lithium fluoride clusters
Authors: Swepston P. N. , Sellers H. L. , Schäfer Lothar
Published: February 2003

The geometries of linear and cyclic forms of Li2F2 and Li3F3 were determined by unconstrained ab initio force relaxations using the 4-31G basis set on F and 5-21G on Li. Optimized Li–F bond distances depend strongly on their molecular environment. Linear clusters are more stable than bent chains. Planar, cyclic forms are more stable than open chains. Some of the refined structures deviate significantly from the equilibrium geometries calculated for these molecules on the basis of electrostatic bonding models. It is concluded that the ionic bonding concept may be intrinsically deficient in quantitatively rationalizing the molecular structures even of systems the potential energy minima of which are primarily determined by electrostatic interactions.
Approximate kinetic energy relations in density functional theory
Authors: March N. H.
Published: February 2003

Density functional theory applied to N independent particles yields Nμ=∫ρδt/δρ dτ+U, (1) where ρ is the particle density, μ the chemical potential, T=∫tdτ is the kinetic energy, and U the potential energy. The kinetic energy functional of the Thomas–Fermi theory implies ∫ρδt/δp dτ=(1+2/d)T, (2) where d is the dimensionality, 1–3. Equation (2) is tested here by appeal first to models; closed shells in a bare Coulomb field and in a harmonic well. For the one-dimensional oscillator, it is shown that (2) is exact for all N since δt/δρ=3t/ρ+(1/4)ρ″/ρ. (3) For the other models considered, it is demonstrated that the right-hand side of (2) is a useful upper bound to the left-hand side even for small N. This bound property is also confirmed to lowest order in a gradient expansion.
The complete active space SCF (CASSCF) method in a Newton–Raphson formulation with application to the HNO molecule
Authors: Siegbahn Per E. M. , Almlöf Jan , Heiberg Anders , Roos Björn O.
Published: February 2003

The complete active space (CAS) SCF method is presented in detail with special emphasis on computational aspects. The CASSCF wave function is formed from a complete distribution of a number of active electrons in a set of active orbitals, which in general constitute a subset of the total occupied space. In contrast to other MCSCF schemes, a CASSCF calculation involves no selection of individual configurations, and the wave function therefore typically consists of a large number of terms. The largest case treated here includes 10 416 spin and space adapted configurations. To be able to treat such large CI expansions, a density-matrix oriented formalism is used. The Newton–Raphson scheme is applied to calculate the orbital rotations, and the secular problem is solved with recent developments of CI techniques. The applicability of the method is demonstrated in calculations on the HNO molecule in ground and excited states, using a triple-zeta basis and different sizes of the active space. With a reasonable choice of active space, the calculations converge in 6–10 iterations. This is true also for states which are not the lowest state of the symmetry in question. The equilibrium geometry for the ground state is RNO=1.215(1.212) Å, RNH =1.079(1.063) Å, ϑHNO=108.8(108.6) °, the experimental values given in parenthesis for comparison. The best estimates for the transition energies to the lowest 3A″ and 1A″ states are 0.67(0.85) eV and 1.52(1.63) eV, respectively. The results obtained indicate that the choice of active space may be crucial for the convergence properties of CASSCF calculations.
A molecular theory of Stokes–Einstein behavior. II. Rotational diffusion of a Brownian sphere
Authors: Masters A. J. , Madden P. A.
Published: February 2003

The analysis of the translational diffusion coefficient of a Brownian particle, given in the preceding paper, is extended to treat the rotational motion. Agreement with the hydrodynamic value for the frequency dependent friction coefficient is found.
Microwave rotational spectrum, molecular geometry, and intermolecular interaction potential of the hydrogen-bonded dimer OC–HCl
Authors: Soper P. D. , Legon A. C. , Flygare W. H.
Published: February 2003

The microwave rotational spectra of several isotopic species of a weakly-bound dimer (CO,HCl) have been measured. For each isotopic species, values of the rotational, centrifugal distortion, and chlorine nuclear quadrupole coupling constants were obtained. These constants are consistent with a linear equilibrium geometry with the atom order OC–HCl, thus establishing the presence of a hydrogen bond to the carbon atom of the CO molecule.. The potential binding of the HCl and CO molecules is also discussed.
On the time dependence of optical activity
Authors: Harris R. A. , Stodolsky L.
Published: February 2003

A dynamical theory of the low temperature relaxation of optical isomers is presented. A set of Bloch equations descriptive of the relaxation of chirality are derived from ’’first principles.’’ A collision induced stability of optical isomers is found, verifying earlier qualitative predictions. A microscopic description of the damping is found in terms of the S matrix for collisions.