bluuebunny/crossref_metadata_2025 huggingface dataset:
Intermolecular forces in van der waals dimers
Authors: Hurst G. J. B. , Fowler P. W. , Stone A. J. , Buckingham A. D.
Published: December 2004

For several van der Waals molecules, the results of ab initio intermolecular perturbation theory are compared with the simple Buckingham‐Fowler model. A selection of dimers is studied, with emphasis on cases where the experimental structure is not reliably predicted by the hard‐sphere electrostatic model. Cases are reported where the electrostatic term dominates, in agreement with the model: HF···HF, H3N···HF, N2O···HF, and H2CO··· HF; cases where the electrostatic term dominates but the molecular shapes are not well described by hard spheres: CO2···H2O and CIF···HF; and cases where there is a delicate balance between the electrostatic and other terms:H2O···HF, H2O···HCCH, CO2···HF, CO2···N, and CH3CN···HCN. It is concluded that, in general, the electrostatic interaction is central to the strength and orientation dependence of the forces between molecules and that the model provides a useful approximation to this interaction, while perturbation theory gives a more complete picture.
Two particle transition density and two particle bond order and their applications to electronic energy shifts caused by two particle interactions via geometric deformations
Authors: Hohlneicher Georg , Gutmann Michael
Published: December 2004

The concept of two particle transition density (TPTD) and two particle bond order (TPBO) is introduced and formally developed in order to extend the considerations of Callis, Scott, and Albrecht [1] to two particle interactions. These interactions may become important if the TPTD (TPBO) for one particle effects is small or the perturbation considered is essentially a two particle perturbation. As an example the geometric deformation of annulene perimeters is studied in detail. This deformation can be interpreted as a two particle perturbation because one particle effects do not contribute considerably. As a result of the perturbation characteristic energy shifts of the four lowest electronic singlet excitations depending on the type of deformation can be observed.
Research in the method of large molecular calculations utilizing transferability of LMO
Authors: Huai Sun , An‐min Tian , Guo‐Sen Yan
Published: December 2004

The localized molecular orbitals of some saturated hydrocarbons and their derivatives have been formed using ab initio method and M. P. [1–2] localization procedure.

Two models, SLMO and ELMO, a set of parameters of LMO Fock matrix elements, and a technique called “Group Effect” are proposed. Based on these, we developed a procedure to simulate the ab initio calculations on large molecules. Some test calculations have been done. The results are compared with those of the ab initio method. In general, absolute errors of orbital energies are about 10−3 a.u., and the relative errors of total energies are about 10−4.

For the original applications, we applied this procedure to some large systems of alkane and their derivatives as well as three Crown‐ether compounds. Satisfactory results are obtained.
Structure and properties of nonclassical polymers. III. Magnetic characteristics at finite temperatures
Authors: Tyutyulkov N. N. , Karabunarliev S. C.
Published: December 2004

Within the Ising model, the two‐dimensional alternant nonclassical polymers display magnetic order at finite temperatures and are characterized by a nonzero critical point Tc.

It is shown that magnetic ordering at finite temperatures occurs when interchain interactions in a quasi‐ dimensional nonclassical altemant polymer are taken into account.
A theoretical study of hydrogen surface coverage over Ni(100)
Authors: Ruette Fernando , Ludeñta Eduardo V. , Hernǎandez Antonio
Published: December 2004

We have used the MINDO/SR molecular orbital method in order to model the migration of hydrogen atoms over a Ni(100) surface. The present calculations indicate the existence of two different states for adsorbed hydrogen, a result which is in agreement with experimental thermal desorption data and LEED. A detailed analysis of the electronic factors involved in this process is presented.
Quantum theory of dielectric relaxation in polar solids
Authors: Georgiev M. , Gochev A. , Diaz‐Gongora A.
Published: December 2004

A novel approach to the impurity dipole relaxation in polar solids is described in which the dipole reorientational sites are formed by an appropriate electron—phonon coupling interaction. The electronic states used for a two‐site formulation are defined as ones pertaining to the impurity and/or the associated intrinsic point defect. The theory takes into account the adiabaticity of the electron transfer in addition to the barrier‐controlled ionic tunneling. It compares favorably with available experimental relaxation‐time data on alkali halides.
Looking at orbitals in the laboratory: The experimental investigation of molecular wavefunctions and binding energies by electron momentum spectroscopy
Authors: Brion C. E.
Published: December 2004

The experimental technique of electron momentum spectroscopy (EMS) (i.e., binary (e, 2e) spectroscopy) is discussed together with typical examples of its applications over the past decade in the area of experimental quantum chemistry. Results interpreted within the framework of the plane wave impulse and the target Hartree—Fock approximations provide direct measurements of, spherically averaged, orbital electron momentum distributions. Results for a variety of atoms and small molecules are compared with calculations using a range of Fourier transformed SCF position space wavefunctions of varying sophistication. Measured momentum distributions (MD) provide a “direct” view of orbitals. In addition to offering a sensitive experimental diagnostic for semiempirical molecular wavefunctions, the MD's provide a chemically significant, additional experimental constraint to the usual variational optimization of wavefunctions. The measured MD's clearly reflect well known characteristics of various chemical and physical properties. It appears that EMS and momentum space chemistry offer the promise of supplementary perspectives and new vistas in quantum chemistry, as suggested by Coulson more than 40 years ago. Binding energy spectra in the inner valence region reveal, in many cases, a major breakdown of the simple MO model for ionization in accord with the predictions of many‐body calculations.

Results are considered for atomic targets, including H and the noble gases. The measured momentum distribution for H2 is also compared with results from Compton scattering. Results for H2 and H are combined to provide a direct experimental assessment of the bond density in H2, which is compared with calculations. The behavior of the outer valence MD''s for small row two and row three hydride molecules such as H2O and H2S, NH3, HF, and HCl are consistent with well known differences in chemical and physical behavior such as ligand‐donor activity and hydrogen bonding. MD measurements for the outermost valence orbitals of HF, H2O and NH3 show significant differences from those calculated using even very high‐quality wavefunctions. Measurements of MD's for outer σg orbitals of small polyatomic molecules such as CO2, COS, CS2, and CF4 show clear evidence of mixed s and p character. It is apparent that EMS is a sensitive probe of details of electronic structure and electron motion in atoms and molecules.
Strong nonadiabatic effects in reactions of photoisomerization
Authors: Smedarchina Z.
Published: December 2004

Reactions of photoisomerization proceeding through a “funnel” are discussed (Fig. 1a, θ ≅ θ*, where θ is the angle of isomerization). Strong nonadiabatic interactions in the region of conical intersection of the multidimensional adiabatic potentials Us and Us0 are supposed to be responsible for the ultrafast nonradiative transition S → K2 S0 (K2 ≅ 1010‐1012 s−1). The K2 dependence on the solvent viscosity (isomerization of t‐stilbene in series ethanol‐‐octanol) and polarity (isomerization of cyanine dyes in polar solvents) was determined to be in a good agreement with experiments.
Final state effects in the photoionization process
Authors: Hohlneicher Georg , Marquardt Bernd
Published: December 2004

Different methods to take into account many body effects in photoionization processes are discussed. Numerical results are shown for some indirect methods, several approximations within the Green's function formalism, and for potential models. The suitability of the Green's function method for the investigation of satellite structure is considered.
Thermal and state‐selected rate constant calculations for O(3p) + H2 → OH + H and isotopic analogs
Authors: Garrett Bruce C. , Truhlar Donald G.
Published: December 2004

We present a new parametrization (based on ab initio calculations) of the bending potentials for the two lowest potential energy surfaces of the reaction O(3P) + H2, and we use it for rate constant calculations by variational transition‐state theory with multidimensional semiclassical tunneling corrections. We present results for the temperature range 250–2400 K for both the rate constants and the intermolecular kinetic isotope effects for the reactions of O(3P) with D2 and HD. In general, the calculated rate constants for the thermal reactions are in excellent agreement with available experiments. We also calculate the enhancement effect for exciting H2 to the first excited vibrational state. The calculations also provide information on which aspects of the potential energy surfaces are important for determining the predicted rate constants.
On the pressure dependence of the muon spin polarization in mixtures of noble gases
Authors: Turner Ralph Eric , Senba Masayoshi , Arseneau Donald J.
Published: December 2004

In muon spin rotation (μSR) experiments, where spin‐polarized positive muons are stopped in condensed matter, three magnetically distinguishable chemical environments can be observed. That is, the Larmor frequencies associated with diamagnetic environments and two types of paramagnetic environments (muonium and radicals) can be resolved. The chemical identities of the latter two are distinct since their Larmor frequencies are distinct, whereas the chemical identities of the possible diamagnetic species are not determined by the Larmor frequency since chemical shifts can not be resolved in μSR experiments. However, two different diamagnetic species have been observed in experiments performed on mixtures of noble gases. Their distinction arises through different thermal rate constants that lead to “fast” and “slow” relaxing components of the diamagnetic signal. The pressure dependencies of the amplitudes associated with these components are related to the stopping dynamics of muons in noble gas targets. A set of coupled rate equations for muon spin dynamics, based upon quantal Boltzmann equations, have been developed to describe this process in single component gases. This theory is now extended to mixtures. In particular, the dynamics of the muon spin is generated by the muonium hyperfine interaction and by time dependent rate constants for the various chemical species that are assumed to be present, namely, muonium and three diamagnetic species. Radicals have not yet been observed in low pressure gases. The coupled quantal rate equations are solved for two models of the stopping dynamics wherein the rates are taken as square box functions of time.
Free energy simulations: Applications to the study of liquid water, hydrophobic interactions and solvent effects on conformational stability
Authors: Beveridge D. L. , Mezei M. , Ravishanker G. , Jayaram B.
Published: December 2004

Free energy simulations using the Metropolis Monte Carlo method and the coupling parameter approach with umbrella sampling are described for several problems of interest in structural biochemistry: the liquid water, the hydrophobic interaction of alkyl and phenyl groups in water, and solvent effects on the conformational stability of the alanine dipeptide and the dimethyl phosphate anion in water. Proximity analysis of the results is employed to identify stabilizing factors. Implications of the result with respect to the structural chemistry of proteins and nucleic acids is considered.
The case of disordered two‐dimensional electron systems
Authors: Srivastava Vipin
Published: December 2004

We investigate the question of the existence or nonexistence of mobility edges that separate the localized and extended states in two‐dimensional disordered electron systems treated within the Anderson model. Evidence is produced to show that the mobility edges exist if the amount of disorder is less than a critical value. The following three different agruments are presented: (1) exact enumeration of self‐avoiding walks that contribute to the renormalized perturbation series of self‐energy whose convergence (divergence) indicates localization (delocalization); (2) “quantum percolation” in a strongly disordered binary alloy; and (3) influence of magnetic field on localized and extended states (the quantum Hall Effect problem).
From macroscopic irreversibility to microscopic reversibility via a nonlinear schrödinger‐type field equation
Authors: Schuch D. , Chung K. ‐M.
Published: December 2004

The aim of our work was to find an unambiguous connection between irreversible macroscopic dynamics and reversible microdynamics that makes it possible to manifest irreversibility on a submacroscopic level without the use of coarse graining arguments. On this level the state of a physical system can be described by a field amplitude Ψ and the time evolution of this system is determined by a field equation for Ψ. For conservative systems, this field equation is formally identical with the linear Schrödinger equation, which can be constructed with the help of the classic Hamiltonian function for the corresponding problem. Regarding irreversible phenomena like damping due to a frictional force, for those dissipative systems no classic Hamiltonian function exists. Therefore the corresponding field equation cannot be obtained in the usual way. Nevertheless, also for dissipative systems it is possible to obtain a field equation in an unambiguous way using only Newton's form of classic mechanics. The result of our method is a nonlinear Schrödinger‐type field equation with a logarithmic nonlinearity. We discuss in more detail the properties of our logarithmic nonlinearity that corresponds to a macroscopic frictional force in a unique way. A figurative interpretation in terms of environment and interaction can be given. From a more formal point of view, the compatibility of our nonlinear operator with principles known from the theory of linear operators is investigated. One of the surprising results is the fact that although our nonlinear Hamiltonian HNL is “Hermitean” in the usual sense, in contrast to the linear theory an operator exp(i · HNL) is not unitary. Furthermore, in our theory the time‐derivative of the mean value of an operator is not only essentially determined by (the mean value of) its commutator with the Hamiltonian. There also occurs an additional term that causes irreversibility of the evolution and is connected with the feature of our theory that (in general) time derivative and construction of the mean value are no longer commuting operations. This fact shows some similarity with coarse graining theories, but in our theory the reason can be traced back unambiguously to an irreversible physical phenomenon.
Etude MNDO des composes conjugues enneagonaux
Authors: Ouamerali Ourida , Gayoso Jose
Published: December 2004

We present MNDO calculations of a series of nine‐membered conjugated compounds. The geometries have been fully optimized. Cyclononatetraenyl and benzocyclononatetraenyl anions, nonapyrrol, and nonathiophene appear to be planar aromatic compounds. Following MNDO, nonafulvene has a planar prefered conformation, but a polyenic structure, while nonafuran and cyclononatetraene are gauche molecules. The Universal Huckel Method (UHM), a simple all valence electron empirical technique which describes σ systems using localized MO, gives, qualitatively, the same results as the MNDO method. The quantum calculation carried out with MNDO and UHM have been used to describe the physicochemical properties of ninememberedring conjugated compounds under consideration. To our knowledge, we present here the first theoretical study dealing with nine‐membered conjugated molecules.
Electronic charge density and mean kinetic energy of lithium orbitals in LiH, LiH+, Li2, Li2+, LiH2+, and Li2H+
Authors: Cardelino B. H. , Eberhardt W. H. , Borkman R. F.
Published: December 2004

The electron density near the lithium nucleus in the species LiH, LiH+, Li2, Li2+, LiH2+, and Li2H+ was analyzed by transforming the SCF molecular orbitals into a sum of atomic contribnutions, for both core and valence orbitals. These “hybrid‐atomic” orbitals were used to compare: electron densities, orbital polarizations, and orbital mean kinetic energies with the corresponding lithium atom quantities. Core‐orbital electron densities at the lithium nucleus were observed to increase by up to 0.5% relative to the lithium atom 1s orbital. Lithium cores also exhibited polarization but, surprisingly, in the direction away from the internuclear region. Similar dramatic changes were seen in the electron densities of the valence orbitals of lithium: The electron density at the nucleus for these orbitals increased two‐fold for homonuclear species and twenty‐fold for heteronuclear triatomic species relative to the electron density at the nucleus in lithium atom. The polarization of the valence orbital electronic charge, in the vicinity of the lithium nucleus, was also away from the internuclear region. The mean “hybrid‐atomic” orbital kinetic energies associated with the lithium atom in the molecules also showed changes relative to the free lithium atom. Such changes, accompanying bond formation, were relatively small for the lithium core orbitals (within 0.2% of the value for lithium atom). The orbital kinetic energies for the lithium valence electrons, however, increased considerably relative to the lithium atom: By a factor of about 2 in homonuclear diatomics, by a factor of 7 in heteronuclear diatomics, and by a factor of 11 in the triatomic species. In summary, the total electronic density (core plus valence) at the lithium nucleus remained remarkably constant for all of the species studied, regardless of the effective charge on lithium. Thus, the drastic changes noted in the individual lithium orbitals occurred in a cooperative fashion so as to preserve a constant total electron density in the vicinity of the lithium nucleus. In all cases, bond formation was accompanied by an increase in the orbital kinetic energy of the lithium valence orbital. We suggest that these two observations represent important and significant features of chemical bonding which have not previously been emphasized.
On the state of the art of quantum chemistry
Authors: Löwdin Per‐Olov
Published: December 2004

A brief general survey of the current state of the art of quantum chemistry is given with some aspects also towards the future. It is emphasized that, if one wants to incorporate such concepts as temperature, entropy, and free energy into quantum chemistry, it is necessary to make a transition from pure quantum mechanics based on wavefunctions to the more general “quantum statistics” based on density matrices and system operators. In addition to the Schrödinger equations, one obtains the Liouville equations, and it is shown that both the time‐dependent and the time‐independent equations may be solved in both cases by using analogous Hilbert‐space methods. Some of the methods for solving the time‐independent eigenvalue problems are reviewed, and the need for giant “number crunchers” in this connection are discussed. It is shown that the resolvent methods combined with the “inner projection” technique for the calculations provide a powerful tool for handling the eigenvalue problems in the future in both the Hamiltonian and Liouvillian formalisms. It is stressed that, by going over to supercomputers, one may gain a factor of 100, and that one may gain another factor of 100 by going over to more powerful theoretical methods; however, for programming reasons, it will take a long time before one can reach the combined efficiency factor 100 ✗ 100.
Density functional calculation of quinone electrode potentials
Authors: Reynolds Christopher A.
Published: December 2004

Here, we have applied density functional methods, in combination with free energy hydration calculations, to calculate two‐electron electrode potentials for quinones and naphthoquinones. While we find that the free‐energy perturbation method, implemented within a molecular dynamics framework, is superior to the PM3—SM3 continuum method for determining free energies of hydration, the computationally less expensive PM3—SM3 method does perform well when there is not an internal hydrogen bond. Generally, all the density functional approaches investigated gave good energetics when applied to this problem, but the Beck '88—Vosko—Wilk—Nusair combination of functionals for the exchange‐correlation energy gave the best results. The density functional results are marginally better than the Møller‐Plesset second‐order perturbation results. Moreover, because the results are obtained using a thermodynamic cycle which involves taking differences in total energies, the results are not too dependent on the quadrature scheme used to calculate the exchange‐correlation energy. By using semiempirically optimized geometries and the PM3—SM3 method for determining free energies of hydration, it has been possible to calculate electrode potentials for a series of large molecules (naphthoquinones) to within about 30 mV of experiment. This result is extremely encouraging and shows that density functional methods offer great promise in the design of redox‐active molecules such as bioreductive anticancer agents. © 1995 John Wiley & Sons, Inc.
Density functional study of the intershell interaction energy in carbon onions
Authors: Diez R. Pis , Iñtiguez M. P.
Published: December 2004

The intershell interaction energy in carbon onions is self‐consistently obtained by using the local density approximation for the π electrons in a spherically averaged pseudopotential. Onions formed by two, three, and four shells with up to 1800 atoms are considered. Our results agree well with recent large‐scale ab initio calculations and with experimental evidence that onions are more stable than are single fullerenes for cluster sizes larger than 100 atoms. The small electronic redistribution that takes place when onions are formed supports the picture of weak van der Waals forces between single fullerenes. © 1995 John Wiley & Sons, Inc.
Proton transfer in small model systems: A density functional study
Authors: Barone Vincenzo , Orlandini Laura , Adamo Carlo
Published: December 2004

The structures, interaction energies, and proton‐transfer features of some representative intermolecular complexes are determined by using a density functional which incorporates gradient corrections and, as recently suggested by Becke, some Hartree–Fock exchange. The results are compared with those obtained by high‐order many‐body perturbation theory and by a number of more conventional density functionals. Hydrogen‐bond strengths and interatomic distances between heavy atoms are well reproduced by several density functionals. However, inclusion of some Hartree–Fock exchange is mandatory to improve XH bond lengths, and, especially, energy barriers governing proton transfer. Use of the new functional significantly improves the agreement with experimental and post‐Hartree–Fock results. This paves the route for a detailed theoretical study of proton‐transfer processes in large, biologically significant systems. © 1995 John Wiley & Sons, Inc.
Comparison of the performance of various gradient‐corrected exchange and correlation functionals in density functional theory: Case studies of CO and N2O molecules
Authors: Chermette H. , Lembarki A. , Gulbinat P. , Weber J.
Published: December 2004

An investigation of the influence of various gradient‐corrected exchange and correlation functionals on the bond lengths and dipole moments of CO and N2O has been carried out using density functional theory. It is shown that whereas some functionals are found to be more sensitive to the basis set quality than are others, the more commonly used gradient‐corrected functionals lead to properties in very good agreement with experiment provided that a sufficiently large basis set is employed. © 1995 John Wiley & Sons, Inc.
Applications of some bounds to density functionals for atoms
Authors: Porras I. , Gálvez F. J.
Published: December 2004

In this article, some applications of lower bounds to density‐dependent functionals in terms of radial expectation values are performed. These diverse applications include accurate upper bounds to the exact kinetic energy and to conjecture a wide set of relationships among radial and momentum expectation values. Some open questions for the improvement of these results are remarked upon. © 1995 John Wiley & Sons, Inc.
Density functional calculations of structures and ionization energies for heavy group V cluster anions
Authors: Kaschner R. , Saalmann U. , Seifert G. , Gausa M.
Published: December 2004

The structure and electronic structure of heavy‐group V cluster anions (Sb, Bi) are calculated with density functional methods within the local spin density approximation (LSDA). The influence of gradient corrections of the exchange and correlation energy is investigated. The calculated vertical and adiabatic ionization energies are in very good agreement with data from photoelectron spectroscopy (PES) for Sb, whereas the relatively large deviations for Bi can be reduced by the consideration of relativistic effects in a scalar‐relativistic manner. Concerning the structures, a strong similarity to the corresponding P clusters was found. In particular, the negatively charged pentamers are planar rings (with similarities to the aromatic [C5H5]− anion) with especially high ionization energies. © 1995 John Wiley & Sons, Inc.
On the electronic structure of transition‐metal oxide cations:DFTCalculations for VO+ and MoO+
Authors: Broclawik Ewa
Published: December 2004

Two transition‐metal oxide diatomic cations, VO+ and MoO+ are considered in this article. Ground‐ and excited‐state properties of the cations are derived from spin‐polarized DF calculations, including spectroscopic constants and metal–oxygen bonding features. A set of ionization potentials are calculated and, for vanadium oxide, compared with photoelectron spectroscopy data and a few available ab initio calculations. All calculated properties are close to experiment, the agreement being much better than for other traditional quantum chemical calculations. Present results together with our earlier findings for neutral molecules provide an excellent confirmation of the good performance of DFT in the case of transition‐metal systems. © 1995 John Wiley & Sons, Inc.
Full ab initio calculation of second‐order Raman spectra of semiconductors
Authors: Windl W. , Karch K. , Pavone P. , Schütt O. , Strauch D.
Published: December 2004

We present the first full ab initio calculation of second‐order Raman spectra in semiconductors based on density functional perturbation theory. The method is applied to the Γ spectra of the elemental semiconductors diamond, silicon, and germanium and reproduces the experimental data excellently. Using first‐principles phonons, we also calculated the corresponding overtone densities of states and Γ Raman spectra employing phenomenological polarizability coefficients. Finally, we analyze the results of the different approaches pointing out the relevance of a full first‐principles derivation. © 1995 John Wiley & Sons, Inc.
Ab initio calculation of structural, lattice dynamical, and thermal properties of cubic silicon carbide
Authors: Karch K. , Pavone P. , Windl W. , Strauch D. , Bechstedt F.
Published: December 2004

We present first‐principles calculations of the structural, lattice dynamical, and thermal properties as well as Raman results for cubic silicon carbide (3C SiC). The plane‐wave pseudopotential approach to density functional theory (DFT) in the local density approximation has been used to calculate the equilibrium properties of 3C SiC, i.e., the ground‐state energy, the band structure, the valence electron density, the lattice constant, the bulk modulus, its pressure derivative, and the ionicity factor of the chemical bonds. The linear‐response theory within DFT has been used to obtain the phonon frequencies, the eigenvectors, and the mean‐square atomic displacements. Furthermore, we calculated the mode Grueisen parameters, the internal‐strain parameter, the elastic constants, the Born effective charge, and the high‐frequency dielectric constant. The specific heat at constant volume and at constant pressure, the thermal expansion coefficient, the temperature dependence of the lattice constant, and that of the isothermal and adiabatic bulk modulus have been derived within the quasi‐harmonic approximation. Finally, the second‐order Raman spectrum of 3C SiC has been calculated using phenomenological polarizability coefficients and ab initio frequencies and eigenvectors. © 1995 John Wiley & Sons, Inc.
A model for the charge capacity of 1T–TiS2 intercalated with Li
Authors: Mendizabal Fernando , Contreras Renato , Aizman Arie
Published: December 2004

The variation of the charge capacity of a double‐layer 1T–TiS2 model system upon increasing intercalation with lithium is examined at a molecular level. The charge capacity is modeled through the global hardness index of density functional theory. The results obtained qualitatively reproduce the experimental trend observed in the voltage–composition variation curve in this system. Comparison with previous theoretical models show that the present approach may give a more complete information about the double‐layer capacitance, since it takes into account the lithium ionicity and the host–guest specific interactions. © 1995 John Wiley & Sons, Inc.
Nonlocal density functional calculations of the jellium metal surface
Authors: Jȩdrzejek C. , Wyżliński A. , Gryko J.
Published: December 2004

The Becke exchange functional is used for calculation of properties of the jellium model using the slab geometry inside a box with the infinite potential barriers at the boundaries. We simplify semianalytical representation of matrix elements. We calculate the surface energies and work functions with self‐consistent electron densities. For all densities (here, we give results in erg/cm2 for rs = 2.07 bohr, in comparison with the LSD approximation (−602)) and the uncorrected Pw GGA‐II (−730), the Becke‐II exchange only (−1212), and the Becke‐II exchange with Perdew86 correlation (−830) [always close to Pw GGA‐I (−814)] give smaller surface energies. The most important factor determining values of surface energies from different GGAS seems to be a form of a correlation potential. We also calculate the effect of finite slab thickness and the vacuum region thickness on the surface energy at the LSD level and indicate its importance in various jellium model calculations. © 1995 John Wiley & Sons, Inc.
Signal‐to‐noise measurement in X‐ray photoelectron spectroscopy
Authors: Koenig M. F. , Grant J. T.
Published: December 2004

A study of signal‐to‐noise (S/N) was made in a Kratos ES300 XPS system on a variety of materials using both monochromatic and conventional X‐ray sources covering a wide range of count rates. Equations relating S/N to total counts (S + B), background (B), and the signal‐to‐background (S/B) ratio were derived and tested for data obtained in a system limited by noise from counting statistics.
Quantitative composition‐depth profiles of thin films
Authors: Frankenthal R. P. , Siconolfi D. J.
Published: December 2004

A technique utilizing a layer by layer analysis has been developed for obtaining the composition‐depth profiles of thin films from Auger electron spectroscopy (AES) combined with ion sputtering. The technique, which has monolayer resolution and gives significantly more accurate results than prior methods, has been used for analyzing the composition of the native oxide on a series of iron‐chromium alloys. The new results differ significantly from those that do not consider layer by layer variations in composition, and they are in good agreement with those obtained some years ago by ion‐scattering spectrometry (ISS). In addition, it was confirmed that no preferential sputtering of Fe or Cr occurs during argon ion sputtering of the alloys.
Effect of hydroxylation upon the ion scattering spectra of oxide surfaces
Authors: Kelso J. F. , Pantano C. G.
Published: December 2004

The low energy sputter peak is an artifact found in many ISS spectra at energies below those predicted for binary elastic backscattering. In this study, a correlation is shown between the intensity of the low energy sputter peak in 3He+ ion scattering spectra and the degree of hydroxylation of the surfaces of silicon and aluminum oxide. The spectra for the clean and hydroxylated surfaces of silica glass (SiO2) were examined. It was found that as the degree of hydroxylation increased, the sputter peak intensity increased and the amount of elastic scattering from surface atoms decreased due to atomic shielding by the chemisorbed hydroxyls. The spectra for materials hydroxylated in bulk (silica gel (SiO2·nH2O) and gibbsite (Al(OH)3)) were also analyzed and these further verified this interpretation of the sputter peak intensity. On the basis of these observations, the low energy sputter peak in ISS has been attributed to the detection of positively sputtered ions such as the hydrogen (H+) and matrix species (e.g., Si+, SiO+, and O+). These were detected in a separate experiment by secondary ion mass spectroscopy (SIMS). It could not be determined whether the dependence of the sputter peak intensity upon the extent of hydroxylation is due solely to the sputter ion yield of hydrogen species or is related, in part, to an enhancement of the secondary ion yields of the oxide matrix species as well.
Effect of oxygen exposure on Auger spectra
Authors: Sherman Robert
Published: December 2004

The effect of residual vacuum contamination on Auger spectra, and subsequently on quantitative analysis, is well known. These effects become especially obvious when measuring the extent of phosphorus grain boundary segregation in the many alloys that require various methods of in situ fracture. To gain insight into the effects of vacuum contamination on the phosphorus Auger signal, we exposed a grain facet to controlled oxygen dosages and monitored the decreasing phosphorus‐to‐iron PPHR as a function of the increasing oxygen‐to‐iron PPHR. This communication reports the results of this experiment and suggests ways to compensate for vacuum contamination.
Electrochemical polymerization of pyrrole on carbon fibres surface
Authors: Lazzaroni R. , Dujardin S. , Riga J. , Verbist J. J.
Published: December 2004

The electrochemical polymerization of pyrrole using carbon fibres as substrates gives strongly adhesive films which improve the fibres' mechanical properties.
Primary‐beam diffraction modulated electron emission (PDMEE) for surface structural analysis
Authors: Valeri S. , Di Bona A. , Gazzadi G. C.
Published: December 2004

Scattering interference of primary exciting electrons leads to a dependence of the Auger emission intensity on the beam incidence angle. For beam energy in the kiloelectron‐volt range, maxima in the Auger yield occur when the primary beam is aligned with low‐index crystal axes, therefore the Auger angular intensity distribution directly reflects the symmetries and the degree of local order. We investigated the potential for this effect, primary‐beam diffraction modulated electron emission (PDMEE) to be used for surface structural characterization, in comparison with the Auger electron/photoelectron diffraction (AED/PD) technique. The angular intensity distributions of both low and high kinetic energy Auger electrons were measured on both GaAs and InP (110) surfaces. We show, by using highly collimated energetic electron beams, that the angular resolution is significantly improved in PDMEE with respect to AED/PD. Unlike the AED/PD case, the angular intensity distribution is independent of the kinetic energy of the Auger electrons, therefore low‐energy, surface‐sensitive electrons also directly reflect crystal symmetries, owing also to the fact that the specific momentum character of the Auger electrons can be neglected. Anisotropy in the intensity angular distribution as large as 70% was measured, comparable to AED/PD anisotropy values. Both a retarding field (low‐energy electron diffraction optics) and a dispersion field cylindrical mirror analyser were used for PDMEE analysis, and instrumental performances are discussed comparatively.
Secondary ion yields depending on the primary oxygen ion energy in cameca IMS‐4f instrument
Authors: Tomita M. , Homma Y. , Saito K.
Published: December 2004

The secondary ion yields of six element in silicon were measured as a function of the primary oxygen ion energy in a Cameca IMS‐4f SIMS instrument. The ion yields of elements with a high ionization potential decrease steeply with primary ion energy owing to the decreasing surface oxygen concentration. The decrease of the ion yield depends on the ionization potential. This relation and the known relative sensitivity factors enable a semiquantitative analysis of impurities in silicon to be carried out without standard samples.

A similar study combined with oxygen flooding shows that the yields are now very high and independent of the primary ion energy. Profile distortions due to segregation effects, however, can occur.

A comparison of the ion yields obtained with and without energy offset under low and high oxygen surface coverage shows that the shape of the arsenic secondary ion energy distribution is drastically changed leading to pronounced differences in ion yields.
Direct observation of a Pb–dithiophosphinate complex on galena mineral surfaces using SIMS
Authors: Brinen J. S. , Nagaraj D. R.
Published: December 2004

Static SIMS measurements of galena mineral surfaces after diisobutyl dithiophosphinate adsorption shows the presence of a 1:1 complex of the reagent and Pb. This assignment is based not only on the observed mass but on the observed isotope ratio.
Cytology and immunocytochemistry of the nucleus of the lateral line lobe in the electric fish Gnathonemus petersii (mormyridae): Evidence suggesting that GABAergic synapses mediate an inhibitory corollary discharge
Authors: Mugnaini Enrico , Maler Leonard
Published: December 2004

Knollenorgans, Low‐threshold electoreceptors found in mormyrid fish, are involved primarily, if not exclusively, in communication. The rhombencephalic nucleus of the lateral line lobe (nLLL) is the target nucleus of knollenorgan afferents. Cells in the nLLL receive a few medium size to large endings with round synaptic vesicles (classified as spoon; large club; small club‐, and rodlet‐shaped endings) with which they form nexus (gap junction) and asymmetrical chemical synapses associated with the round synaptic vesicles. In addition these endings emit thin collaterals which terminate as small boutons on nLLL neurons; these boutons also have round vesicles and make mixed (electrotonic and chemical) synapse. In addition, cells in the nLLL receive synaptic input from numerous small boutons containing pleomorphic vesicles and making symmetric synapses. We have not found any interneurons within nLLL. Our ultrastructural analysis suggests that boutons synapsing on nLLL neurons belong to only two afferent fiber systems and that the wiring diagram of nLLL is extremely simple. We have studied the immunolocalization in nLLL of glutamic acid decarboxylase (GAD), the enzyme essential for the synthesis of GABA that is also a useful marker for this widely distributed inhibitory neurotransmitter. GAD immunoreactivity was confined to the small boutons with pleomorphic vesicles. GAD was also found in a nucleus projecting to the nLLL, here named the sublemniscal nucleus (SL), which probably conveys corollary discharge signals to the nLLL. This GABAergic projection may be responsible for the potent inhibition associated with the electric organ discharge command that has been described in these cells.
Contrasting properties of K+ conductances induced by baclofen and γ‐aminobutyric acid in slices of the guinea pig hippocampus
Authors: Ogata Nobukuni , Inoue Masumi , Matsuo Tadashi
Published: December 2004

Properties of membrane K+ conductances induced by baclofen and γ‐aminobutyric acid (GABA) in the hippocampus were investigated by using guinea‐pig brain slices.

Baclofen hyperpolarized the membrane and decreased the input resistance of pyramidal cells through the activation of a membrane K+ conductance.

GABA caused a biphasic response in pyramidal cells, consisting of hyperpolarizing and depolarizing components. Combined application of picrotoxin and bicuculline eliminated the major part of the depolarizing component of the biphasic response and produced a relatively pure hyperpolarizing response which was also mediated by an increase in K+ conductance.

The K+ conductance change induced by baclofen showed prominent inward rectification. However, the K+ conductance induced by GABA did not show an obvious rectifying property.

The K+ conductance activated by baclofen was strongly antagonized by a low concentration (5 × 10−6 M) of 4‐aminopyridine (4‐AP). In contrast, the K+ conductance activated by GABA was insensitive to 4‐AP even at a high concentration of 10−3 M.

The slow inhibitory postsynaptic potential (slow i.p.s.p.) evoked by stimulation of the mossy fibres was totally suppressed by a low concentration of baclofen (5 × 10−6 M). Whereas GABA (10−3 M) decreased the amplitude of the slow i.p.s.p., the reduction of the amplitude was proportional to the decrease in the amplitude of the electrotonic potentials produced by constant inward current injections.

These results suggest that the hyperpolarizations induced by GABA and baclofen may be generated by K+ conductances of different kinetic and pharmacologic properties.
Serum protein leakage in aged human brain and inhibition of ligand binding at alpha2‐ adrenergic and cholinergic binding sites
Authors: Pappolla Miguel A. , Andorn Anne C.
Published: December 2004

Serum proteins are known to extravasate into the brain parenchyma in senile and presenile dementia (Glenner: Hum. Pathol. 16:433–435, 1986; Wisniewski and Kozlowski: Ann. NY Acad. Sci. 396:119–129, 1982). We have recently demonstrated that human serum Cohn fraction IV (alpha‐globulin enriched) inhibits ligand binding at putative dopamine and serotonin2 receptors labeled by [3H]spiroperidol in human brain (Andorn, Pappolla, Fox, Klemens, and Martello: Proc. Natl. Acad. Sci. USA 83:4572–4575, 1986). We now demonstrate that serum proteins can be identified in the neuropil and in neuronal cell bodies in normal aged brain, that alpha‐globulin‐enriched fractions inhibit ligand binding at alpha2‐adrenergic and muscarinic binding sites in human brain as well, and that serum proteins can be identified within neuronal cytoplasm and axons.
The GABAA receptor: New insights from single‐channel recording
Authors: Mathers D. A.
Published: December 2004

The GABAA receptor of mammalian neurons exists as a macromolecular complex incorporating several interacting components. These include a chloride‐permeable ion channel and a recognition site for GABA. The binding of GABA molecules at the latter site triggers the transient opening of the chloride ion channel, resulting in a flow of charge which inhibits the generation of action potentials in the cell. The precise amount of charge passed during this event is modulated by ligand binding at at least three regulatory sites. These sites act as receptors for barbiturates, benzodiazepines, and certain convulsants. The extracellular patch clamp method has now been used to study the gating of chloride channels by GABA and the modulation of this process by drugs.

Even in the absence of drugs, GABAA channels exhibit gating behavior, indicating the presence of multiple open and closed states and of substate conductance levels. GABAA channels from different preparations show considerable diversity in their detailed gating characteristics.

In the presence of the barbiturate pentobarbital, GABAA channels open in prolonged bursts, consistent with the potentiating effect of this drug on macroscopic GABA responses. In contrast, the convulsant penicillin decreases charge transfer through open GABAA channels by shortening the average duration of the open state. Benzodiazepine receptor ligands exert complex effects on the GABAA channel. A general model of barbiturate and benzodiazepine potentiation of GABAA receptor responses is proposed.
Dopamine receptors and the dopamine hypothesis of schizophrenia
Authors: Seeman Philip
Published: December 2004

The discovery of neuroleptic drugs in 1952 provided a new strategy for seeking a biological basis of schizophrenia. This entailed a search for a primary site of neuroleptic action. The Parkinsonian effects caused by neuroleptics suggested that dopamine transmission may be disrupted by these drugs. In 1963 it was proposed that neuroleptics blocked “monoamine receptors” or impeded the release of monoamine metabolites. The neuroleptic concentration in plasma water or cerebrospinal fluid was of the order of 2 nM for haloperidol in clinical therapy. A systematic research was made between 1963 and 1974 for a primary site of neuroleptic action which would be sensitive to 2 nM haloperidol and stereoselective for (+)‐butaclamol. Direct evidence that neuroleptics selectively blocked dopamine receptors occurred in 1974 with the finding that nanomolar concentrations of these drugs stereoselectively inhibited the binding of [3H]‐dopamine or [3H]‐haloperidol. These binding sites, now termed D2 dopamine receptors (which inhibit adenylate cyclase), are blocked by neuroleptics in direct relation to the antipsychotic potencies of the neuroleptics. No such correlation exists for D1 receptors (which stimulate adenylate cyclase).

Based on the fact that dopamine‐mimetic drugs elicited hallucinations, and that neuroleptics caused rigidity, Van Rossum in 1966 had suggested a hypothesis that dopamine pathways may be overactive in schizophrenia. The D2‐selective blockade by all neuroleptics (except the monoamine‐depleting reserpine) provided strong support for the dopamine hypothesis. Further support now comes from postmortem data and in vivo positron tomographic data, both of which indicate that the density of D2 receptors are elevated in the schizophrenic brain. The postmortem data indicate a bimodal pattern with half the schizophrenics having striatal D2 densities of 14 pmol/g (control is 13 pmol/g) and the other half having 26 pmol/g. Current positron tomographic data indicate D2 densities of 14 pmol/g in control subjects, but values of 34 pmol/g in drug‐naive schizophrenics. Future tests of the dopamine hypothesis of schizophrenia may entail an examination of the amino acid composition and genes for D2 receptors in schizophrenic tissue, an examination of the ability of the D2 receptor to become phosphorylated and to desensitize into the low‐affinity state, and an examination of the interaction of D2 receptors with D1 receptors or other neurotransmitters.