Repositório RCAAP

It is presented a theoretical study on electron -CO collisions in the (5-20) eV energy range. Specifically, elastic differential cross sections (DCS) are calculated using the configuration interaction (CI) method to describe the target. Static and exchange contributions to the interaction potential are obtained exactly from a CI wave function with single and double excitations. The DCS are obtained using the Schwinger variational iterative method (SVIM). It is observed that the discrepancies between Hartree-Fock and CI results are more significant for low energies and small angles.

Accurate potential energy curves, transition moments, spectroscopic constants, radiative transition probabilities, and lifetimes for the states X²sigma+ and A²pi of BeF, MgF and CaF are reported using high level calculations. Diagonal transitions dominate for the A²<FONT FACE=Symbol>P ®</FONT> X²sigma+ system. Radiative lifetimes for nu' = 0 are predicted to be 6.81 ns, 7.16 ns and 19.48 ns, respectively for BeF, MgF and CaF. The result calculated for the CaF molecule are in excellent agreement with the experimental result equal to 21.9±4.0 ns.

The core-to-sigma* excited state is repulsive for the bond elongation; on the other hand, the core ionized state and the core-to-pi* state are bound and the core-to-Rydberg states are almost parallel to the potential energy curve of the core ionized state. Thus, the core-electron excitation to the unoccupied sigma* orbital can be mixed with the one-electron Rydberg or continuum orbital, as dependent on the bond distance, and even with the unoccupied pi* orbital in some cases. Within the framework of one-electron picture, we show sigma* orbitals mixed with Rydberg character in the 1s excitation of O2 and CH3F, and Rydberg orbitals mixed with valence character in the 1s excitation of CH4, CO2, and N2O.

We study the charge transport by polarons in conjugated polymers in the presence of impurities. The effects on the polaron motion due to the symmetry of the chain is considered. The polaron dynamics in doped conjugated polymers is numerically studied using the Su-Schrieffer-Heeger (SSH) model combined with the Pariser-Parr-Pople (PPP) model modified to include an external electric field and the Brazovskii-Kirova (BK) symmetry breaking interaction. The time dependent Hartree-Fock approximation is used. The electric field is used to accelerate the polaron in a polymer chain. Polaron trapped by the impurity and polaron-impurity collisions are considered. When the polaron-structure is around the impurity site a charge oscillation can be observed. Nevertheless, when there are collisions between the polaron and the impurity, the polaron-structure can pass, be trapped or be reflected. These effects are determined by the strength of the radical parameter and by the electric field intensity. The effects on the polaron are analyzed and an effective potential is determined. Therefore, the effective potential determines the polaron behavior for each case, as the polaron pass, or it is trapped or reflected by an impurity.

Sequences of increasing size atom centered basis sets of Gaussian-type functions for the ground state of the CS molecule are generated with the molecular improved generator coordinate Hartree-Fock (HF) method. At the HF level, total and orbital energies and electric dipole moment and, at the second-order Mphiller-Plesset (MP2) level, correlation and dissociation energies and electric dipole moment were calculated and compared with the results obtained with other Gaussian basis sets reported in the literature. Considering our largest basis set, the HF energy is in error by 56.2 µhartree and the second order correlation energy corresponds to ~ 80% of an estimate of the limiting value. At the MP2 level, the dipole moment and the dissociation energy computed with our largest basis set are in good agreement with the corresponding experimental values. The CS molecule is considered a prototype for systems containing atoms from different rows of the periodic table.

This article reviews previously employed methods to study several valence electronic transitions, optically forbidden or not, enhancing intensity through vibronic coupling. Electronic transition dipole moments were calculated using several ab initio methods including electron correlation. In this method the square of the electronic transition dipole moments are directly calculated along the normal coordinates of vibration and then expanded with a polynomial function. Afterwards, analytical vibrational integration using harmonic wave functions, of the square of the transition moments function, allows us to obtain partial (i.e. for each vibrational mode) and total optical oscillator strengths (OOS), for the vibronic transition of interest. We illustrate the accuracy of the method through valence transitions of benzene (C6H6), formaldehyde (H2CO), acetone (C3H6O) and formic acid (HCOOH).

The number of citations of a scientific publication or of an individual scientist has become an important factor of quality assessment in science. We report a study of the statistical distribution of the citation index of both scientific publications and scientists. We give numerical evidence that Tsallis (power law) statistics explains the entire distribution over eight orders of magnitude (10-4 to 10(4)). Also, we draw Zipf plots in order to analyze the statistical distribution of the citation index of Brazilian and international physicists and chemists. The relatively small group of Brazilian scientists seems more adequate to explain the dynamics of the citation index. In this case, we find that the distribution of the citation index can also be explained by a gradually truncated power law with similar parameters. We finally discuss possible mechanisms behind the citation index of scientists and scientific publications.

We used a set of properties of the interactions among the spinal neurons in order to develop a computer model for a spinal reflex circuit. The model equations take into account the synaptic characteristics of the somatodendritic membrane of neurons in a morphofunctional unity of the spinal reflex activity. This model is based on the idea that the responses of spinal alpha-motoneurons to a sensorial stimulation can be modulated by the serial activation of a motor command chain. We developed a Fortran program for simulating a physiological situation. The results are discussed in terms of available experimental data for the motoneuron firing rate.

It is a well-known result that the scalar field spectrum is composed of two chiral particles (Floreanini-Jackiw particles) of opposite chiralities. Also, that a Siegel particle spectrum is formed by a nonmover field (a Hull's noton) and a FJ particle. We show that the spectrum of the chiral boson on a circle has a particle that is not present in its currently well-known spectrum.

A detailed study of the photoluminescence properties in undoped and Te-doped AlGaAsSb alloys lattice matched to InP is presented. Photoluminescent temperature and excitation intensity dependences are employed to discuss the origin of the dominant transition and the influence of the presence of Al and Te on the fluctuation of the electrostatic potential in the epitaxial layers. The behavior of PL dominant transitions is associated with the quasi-donor-acceptor-pair (QDAP) model at a low temperature interval. The temperature dependence of photoluminescence intensities showed characteristics similar to those observed for amorphous semiconductors and disordered superlattices. The presence of two V elements, or the presence of Sb in the ternary and quaternary (III)-III-V-V alloys, causes a reduction in photoluminescence intensity temperature dependence, when compared to (III)-III-III-V alloys.

We have fabricated tin oxide (SnO2) nanorods on palladium-coated substrates by carrying out the thermal evaporation of solid Sn powders. We have employed X-ray diffraction, scanning electron microscope, transmission electron microscope and photoluminescence (PL) spectroscopy to characterize the synthesized products. The obtained 1D nanomaterials with a jagged surface were SnO2 with rutile structure. PL spectra exhibited visible light emission.

In order to evaluate an atomic clock, it is important to determine the main frequency shifts caused by external fields, device imperfections, etc. Scanning the frequency of the main oscillator, the Ramsey fringe is obtained and used to determine the main shifts. The gravitational shift (10-17), second order Doppler (1.65×10-13), Black-Body radiation shift (2.9×10-14), quadratic Zeeman effect (5×10-13), Rabi Pulling (1.3×10-13) and cavity pulling (1.3×10-13) have been evaluated. The short term stability, (6.6±0,2)×10-13, was obtained.

In this work we report efficiency measurements on light-emitting diodes with electrochemically synthesized sulfonated polyaniline as hole transport layer. The anode used in our devices is fluorine-doped tin oxide, the blocking layer is electrochemically synthesized poly(9,9-dioctyl-1,4-fluorenylenevinylene) and the electron transporting material and emitter is tris-(8-hydroxyquinoline) aluminum. Sulfonated polyaniline based devices presented efficiency of 0.79 cd/A.

The conservation laws associated with a previously studied metric nonsymmetric theory of gravitation are established and their physical significance is discussed. Conservation laws are obtained for the generalized energy and momentum. Some applications of the results are made for a spherically symmetric fluid with a view to future study of stelar equilibrium.

Perturbations around black holes have been an intriguing topic in the last few decades. They are particularly important today, since they relate to the gravitational wave observations which may provide the unique fingerprint of black holes' existence. Besides the astrophysical interest, theoretically perturbations around black holes can be used as testing grounds to examine the proposed AdS/CFT and dS/CFT correspondence.

In this work we investigate the evolution of a Universe consisted of a scalar field, a dark matter field and non-interacting baryonic matter and radiation. The scalar field, which plays the role of dark energy, is non-minimally coupled to space-time curvature, and drives the Universe to a present accelerated expansion. The non-relativistic dark matter field interacts directly with the dark energy and has a pressure which follows from a thermodynamic theory. We show that this model can reproduce the expected behavior of the density parameters, deceleration parameter and luminosity distance.

An analysis of the solutions for the field equations of generalized scalar-tensor theories of gravitation is performed through the study of the geometry of the phase space and the stability of the solutions, with special interest in the Brans-Dicke model. Particularly, we believe to be possible to find suitable forms of the Brans-Dicke parameter w and potential V of the scalar field, using the dynamical systems approach, in such a way that they can be fitted in the present observed scenario of the Universe.

We derive, in the large scale limit, analytical solutions for a dark energy model described by the Born-Infeld field plus a perfect fluid, both for homogeneous background and first order perturbations. These analytical results are compared with numerical solutions in a model with radiation, pressureless matter and the field. We investigate the non-adiabatic perturbed pressure associated with this attractor and whether it corresponds to isocurvature contribution.

It is very common to find numerical studies of dark energy and dark matter. Among these, one can find the interesting proposal of unifying dark matter and dark energy with the use of a single component with an 'exotic' equation of state. However, there is a certain shortage of analyses involving exact analytic models following this proposal. Therefore, here are presented examples of simple exact cosmological models which can reproduce some of the desired properties of an unified dark matter/energy fluid.

The method of Morales and Ramis determines whether a given Hamiltonian system is non-integrable. We apply this method to Friedmann Robertson Walker models with a self-interacting scalar field and cosmological constant. It is shown that, with the exception of a set of measure zero, these models are non-integrable. Our results complement those of Helmi and Vucetich who used the Painlevé property to find integrable models within this class.