Resumo em Inglês:

When Goldstein's report on the "positive light" (or what is known as "Kanalstrahlen", canal rays) in gas discharge tubes first appeared in 1886, Willy Wien had just finished his thesis at the Helmholtz Institute in Berlin. Eleven years later he performed his first experiments on canal rays and found that they consisted of inert, charged and neutral particles. The charged component in canal rays could be de ected using electric and magnetic fields, enabling Wien to roughly determine their mass-to-charge ratio. Improving vacuum conditions and detection efficiency, Thomson finally resolved the lightest constituents of canal rays: the hydrogen ions H+ and H2+. This marked the beginning of mass spectrometry. The first mass spectrographs were parabola-image instruments being used by Thomson to discover isotopes. Until about 1923, canal rays became the most common ion source. Also Aston used canal rays as an ion source for the first double focussing mass spectrometer. - Wien continued his work on canal rays up to the end of his life (he died in 1928). He investigated their interaction with matter, i.e. the mean free path of canal rays in gases with respect to charge exchange and atomic excitation. His particular interest was addressed to the physics of light emission by canal rays, such as the line spectrum and the splitting of these lines in magnetic and electric fields, the Doppler effect and lifetimes.

Resumo em Inglês:

Mother Nature (the Ultimate Chemist): South America holds some of the richest rain forests in the world where Nature, through the abundance of the essentials of life has produced a rich spectrum of biological molecules. The early inhabitants of these regions, over thousands of years, have, through experimentation, identified plants and animals that contain components that they have found to be beneficial for their health or so toxic that they can be used as a weapon.

Resumo em Inglês:

In the last two decades, several new types of ion sources have been introduced in organic mass spectrometry like bombardment with heavy ions, PDMS, and laser light, MALDI. Hand in hand with the new ionization techniques followed also a renaissance for the 50 year old time-of-flight technique for the mass analyses. In this paper the basics of the ToF technique is described in a tutorial non-theoretical way for the beginner together with some practical hints. The electrostatic mirror, the delayed extraction technique as well as some recent technical developments are also included.

Resumo em Inglês:

Laboratory data is needed on electronically-induced desorption from low-temperature solids: ices, organics, hydrated salts, glasses and certain minerals. Many bodies in the outer solar system are bombarded by relatively intense fluxes of fast ions and electrons as well as solar UV photons. This can cause both changes in their optical reflectance as well as desorption of atoms and molecules from their surfaces. Stimulated desorption produces Na and K 'atmospheres' above the 'rocky' surfaces of the moon and Mercury and H2O, H2 and O2 'atmospheres' about icy outer-solar system bodies. Since theses bodies contain other surface materials, direct detection by spacecraft or remote detection by telescopes of the desorbed atoms and molecules can be used, along with laboratory data, to determine the surface composition and geological processes occurring on distant bodies. This paper describes the relevance of stimulated desorption to the ambient neutrals and plasma in Saturn's magnetosphere, in preparation for CASSINI's arrival, and to the production of atmospheres on the moons of Jupiter being studied by the Galileo spacecraft.

Resumo em Inglês:

The correlative method of unsymmetrized self-consistent field (CUSF) is used to study dynamical characteristics of a strongly anharmonic crystal with body-centered cubic lattice, namely, the inter-atomic and mean square relative displacements. We present the general formulae for crystals with anharmonicity, including the strong one, up to fourth anharmonic terms. Taking into account the second order of the method we calculate correlations in this lattice between the nearest, second, third, fourth and fifth neighbors. The influence of more distant interactions is discussed. The results are compared with those obtained previously for an weakly anharmonic BCC crystal. We use the Schiff potencial for Na and also Lennard-Jones potentials for comparison.

Resumo em Inglês:

We derive a system of equations to describe stationary axisymmetric MHD equilibria characterized by toroidal and poloidal flows as well as plasma anisotropy due to strong magnetic field and eventual auxiliary heating methods. The system consists of a nonlinear partial differential equation for the poloidal magnetic flux function and an algebraic Bernoulli-type equation, which relates plasma density with several surface functions. We analyse the ellipticity of the equation and the plasma density bifurcation as the Alfvén-Mach number is changed.

Resumo em Inglês:

We present a symmetry preserving approach to1/N (N being the number of colors) corrections in the Nambu-Jona-Lasinio model. This is achieved through an explicit 1/N expansion where N is considered as a free parameter. Verification and use of the pertinent Ward identity to the next to leading order in 1/N is made in obtaining the Goldberger-Treiman relation. Although the present results are not new, the approach may be useful for deriving higher (than one loop) corrections to the NJL model in a systematic way.

Resumo em Inglês:

The effect of including a van Hove singularity in the density of state of a renormalized BCS equation in s and d waves and its appropriateness in describing some properties of high-Tc cuprates in the weak-coupling region are studied in two space dimensions. The specific heat and knight shift as a function of temperature exhibit scaling below the critical temperature in d wave. We also study the jump in the specific heat at the critical temperature Tc in s and d waves, which can have values significantly higher than the standard BCS values and which increases with Tc, as experimentally observed in many d-wave high-Tc materials. The experimental results on the specific heat and knight shift of the Y-123 system are compared with the theoretical predictions.

Resumo em Inglês:

By fitting the Bethe formula to experimental data available in the literature, we determined the integrated oscillator strength for the Ar8+ formation by electronic collision on neutral Argon. The results are compared with a previous empirical calculation and with a set of experimentally obtained multiple ionisation data.

Resumo em Inglês:

The phenomenon of macroscopic homogenization is illustrated with a simple example of diffusion. We examine the conditions under which a d-dimensional simple random walk in a symmetric random media converges to a Brownian motion. For d = 1, both the macroscopic homogeneity condition and the diffusion coeficient can be read from an explicit expression for the Green's function. Except for this case, the two available formulas for the effective diffusion matrix kappa do not explicit show how macroscopic homogenization takes place. Using an electrostatic analogy due to Anshelevich, Khanin and Sinai [AKS], we discuss upper and lower bounds on the diffusion coeficient kappa for d >1.

Resumo em Inglês:

On 30th June 1989 high sensitivity-spectral resolution observations of solar radio bursts were carried out in the frequency range of 18 - 23 GHz. The burst observed at 17:46 UT was different from the 60 bursts observed so far in the sense that it exhibited a frequency fine structure superimposed on the ongoing burst in its rising phase, i.e. an additional enhancement of the flux density of the order of 10 SFU, observed only in the 21 and 22 GHz frequency channels, lasting for about 4 s. Interaction of an emerging loop with an adjacent loop accelerated particles in that loop from which the broadband burst was emitted due to the gyrosynchrotron emission. The observed fine structure is interpreted as due to thermal gyro-emission at 6th harmonic of the gyrofrequency originated from a hot kernel with short lifetime located at the top of emerging loop. We derived the hot kernel source parameters, th e temperature as 8 <FONT FACE="Symbol">´</font>10(7) K, the magnetic field as <img src="http:/img/fbpe/bjp/v29n3/510i1.gif" alt="510i1.gif (46 bytes)"> 1250 G and the density as <img src="http:/img/fbpe/bjp/v29n3/510i1.gif" alt="510i1.gif (46 bytes)"> 5 ×10(12) cm-3.

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The properties of coherent X-radiation (CXR), generated in crystals by electrons in the tens of MeV energy range, were investigated theoretically. The analytical expressions for CXR spectral-angular and angular cross sections were obtained with the use of small-angle multiple scattering approximation. Our theoretical results are in good agreement with experiment. The obtained results can be used in designing quasi-monochromatic, tunable and polarized X-radiation sources based on the CXR process.

Resumo em Inglês:

Traveling convection vortices (TCV) are studied using a theoretical ideal MHD model for the equilibrium and perturbed plasma. In this first approach, we do not consider the viscosity and the flow of the plasma column in the equilibrium. The linearized equations are solved using normal mode analysis. The solutions show an m = 1 kink instability which is in good agreement with the experimental data. It is calculated numerically a growth rate of 28 minutes to be compared with experimental data, indicating a quite good result with our model. Clearly, the results indicate the behaviour of the TCV as a kind of kink instability as supported by the agreement between theoretical and experimental values for the growth rate. Also the filamentation currents calculated through the theoretical model agree quite well with the experimental observations by satellites.

Resumo em Inglês:

The quantum mechanical problem posed by the internal motion of three particles subject to Coulomb interactions is variationally solved by means of an Eckart-Gaussian (EG) ansatz that exhibits an exponential behavior with respect to the radial coordinates {r1,r2}, and a harmonic Gaussian-type dependence on the interparticle distance r12, thereby providing explicit correlation. The proposed wavefunction is of the form (e-alpha1r1-beta1r2 + e-beta2r1-alpha2r2) r l12 e<FONT FACE="Symbol">-(g</FONT> 2r1-u0)2, through which ground state energies are calculated for a few two-electron atoms-considering finite nuclear mass effects-and molecular ions corresponding to electronic and mesonic systems. The physical interpretation and advantages of the EG wavefunction are discussed in terms of the relative masses of the particles in the analyzed systems. A useful application of the variational method is presented where the underlying structure of the 3-body wavefunction combines an atomic- and a molecular-like description of the system. The obtained energies agree with the exact results within 10-4 - 10-2 Hartrees.

Resumo em Inglês:

The motion of a particle with charge q and mass m in a magnetic field given by B = kB0 + B1 [icos(omegat) + jsin(omegat)] and an electric field which obeys <FONT FACE="Symbol">Ñ</FONT> ×E = -<FONT FACE="Symbol">¶</FONT>B/<FONT FACE="Symbol">¶</FONT>t is analysed classically and quantum-mechanically. The use of a rotating coordinate system allows the analytical derivation of the particle classical trajectory and its laboratory wavefunction. The motion exhibits two resonances, one at omega = omegac = -qBo/m, the cyclotron frequency, and the other at omega = omegaL = -qBo/2m, the Larmor frequency. For omega at the first resonance frequency, the particle acquires a simple closed trajectory, and the effective hamiltonian can be interpreted as that of a particle in a static magnetic field. In the second case a term corresponding to an effective static electric field remains, and the particle orbit is an open line. The particle wave function and eigenenergies are calculated.

Resumo em Inglês:

We measured the photoluminescence (PL) spectra of a series of Ge n quantum wells as a function of temperature, from 2K to 50K. The PL spectra at 2.1K are dominated by broad emission lines, which can be interpreted as recombination across the indirect gap of the Si/Ge microstructure and are strongly influenced by the interface morphology of each sample. Beyond T <img src="http:/img/fbpe/bjp/v29n3/547i1.gif" alt="547i1.gif (50 bytes)"> 15K, all samples show identical spectra in which the broad structures are replaced by thin, strong lines. We interpret these changes as a quenching of the recpmbination across the gap PL of the microstructure and the appearance of defect-related peaks from the Si substrate.

Resumo em Inglês:

We use the two dimensional extended Hubbard Hamiltonian with the position of the attractive potential as a variable parameter, within a BCS type approach, to study the interplay between the superconductor transition temperature Tc and hole content for high temperature superconductors. This method gives some insight on the range and intensity of the Cooper pair interaction. It suggests why different compounds have different values for their measured coherence lengths, and describes the experimental results of the superconducting phase diagram Tc × n . The calculations may also be used to study the effect of the applied pressure with the assumption that it increases the attractive potential which is accompanied by an increase in the superconductor gap. In this way we obtain a microscopic interpretation for the intrinsic term and a general expansion for Tc in terms of the pressure which reproduces well the experimental measurements on the Bi2Sr2CaCu2O8+y superconductors.

Resumo em Inglês:

Dirac-Fock calculations have been performed using a segmented contraction of Gaussian basis sets entirely generated from the Dirac-Fock Hamiltonian. Both Dirac-Fock-Coulomb and Dirac-Fock-Breit self-consistent field calculations have been performed for Ar (Z=18), Ge (Z=32), Sn (Z=50), Xe (Z=54) and Hg (Z=80). It is found that contractions of large exponent bases of s and p symmetry species have little effect on the total and orbital energies for light atoms (Ar and Ge) and a large effect for intermediate (Sn and Xe) and heavy (Hg) atoms.

Resumo em Inglês:

Charret et al. applied the properties of Grassmann generators to develop a new method to calculate the coefficientes of the high temperature expansion of the grand canonical partition function of self-interacting fermionic models on d-dimensions (d > or = 1). The method explores the anti-commuting nature of fermionic fields and avoids the calculation of the fermionic path integral. We apply this new method to the relativistic free Dirac fermions and recover the known results in the literature without the beta-independent and mu-independent infinities that plague the continuum path integral formulation.

Resumo em Inglês:

The author describes his first attempt in 1958 at the unification of electromagnetic and weak interactions and his prediction in the same paper of the neutral Z0 boson which would be the intermediate quantum exchanged in an eventual electron-neutron weak interaction (as muonic neutrinos were not known at that time).

Resumo em Inglês:

Some renormalization group approaches have been proposed during the last few years which are close in spirit to the Nightingale phenomenological procedure. In essence, by exploiting the finite size scaling hypothesis, the approximate critical behavior of model on infinite lattice is obtained through the exact computation of some thermal quantities of the model on finite clusters. In this work some of these methods are reviewed, namely the mean field renormalization group, the effective field renormalization group and the finite size scaling renormalization group procedures. Although special emphasis is given to the mean field renormalization group (since it has been, up to now, much more used to study a wide variety of different systems) a discussion of their potentialities and interrelations to other methods is also presented.