Exact partial wave expansion of optical beams with respect to an arbitrary origin

Using an analytical expression for an integral involving Bessel and Legendre functions, we succeed in obtaining the partial wave decomposition of a general optical beam at an arbitrary location relative to the origin. We also showed that solid angle integration will eliminate the radial dependence of the expansion coefficients. The beam shape coefficients obtained are given by an exact expression in terms of single or double integrals. These integrals can be evaluated numerically on a short time scale. We present the results for the case of a linear-polarized Gaussian beam.     

Heavy metal in sediments and bioaccumulation in the bivalve Corbula gibba in a drilling discharge area

The longterm bioavailability of heavy metals in sediments of a Northern Adriatic Sea shelf area affected by drilling mud and cutting discharges was discussed. Levels of Mn, Cu, Cr, Zn and Pb in different geochemical phases of the sediment and in soft tissues of the bivalve Corbula gibba were recorded and the relationships between biological and geochemical metal investigated. Total metal content, acetic acid extractable-, exchangeable-, carbonate-, easily reducible-, moderately reducible-, oxidable- and residual-fractions were determined on sediment samples. Corbula gibba was collected from wet sediments at the same times and sites, and the soft-tissue metal contents were determined. Correlations show that the fractions with greatest bioavailability are the exchangeable and carbonate for Cr and the exchangeable, carbonate, easily and moderately reducible fractions for Zn. Data also show a possible bioavailability of Pb only from the residual fraction, consisting of very resistant matter of more recent anthropogenic origin. Near the platform, total Mn content in sediments and in C. gibba tissues show a strong correlation suggesting that this organism is sensitive to variations of the Mn-oxi-hydroxides superficial film. No relationships were found between biological and sediment-bound Cu, however the discharged muds did not cause Cu enrichment. The metal fraction determined by weak acetic acid extraction at no point seems related to metal levels in Corbula gibba.     

Atomic structure of icosahedral B4C boron carbide from a first principles analysis of NMR spectra

Density functional theory is demonstrated to reproduce the 13C and 11B NMR chemical shifts of icosahedral boron carbides with sufficient accuracy to extract previously unresolved structural information from experimental NMR spectra. B4C can be viewed as an arrangement of 3-atom linear chains and 12-atom icosahedra. According to our results, all the chains have a CBC structure. Most of the icosahedra have a B11C structure with the C atom placed in a polar site, and a few percent have a B (12) structure or a B10C2 structure with the two C atoms placed in two antipodal polar sites.     

The ad hoc return channel: a low-cost solution for Brazilian interactive digital TV

The upcoming terrestrial digital television technology brings a new class of services to traditional TV sets. A set-top box may, for example, access the Internet and send email. The interactive return channel makes these new services possible. This channel allows TV viewers to interact by sending data back to the TV broadcaster. In this article we analyze the viability of a wireless ad hoc network to implement the return channel. For this purpose, an ad hoc network is a low-cost and flexible solution. We evaluate the performance of an ad hoc return channel using the IEEE 802.11 wireless technology for different Brazilian geographical scenarios. The results show that ad hoc networking is a promising solution for the return channel of Brazilian interactive digital TV     

Measurement of multi-muon events in a flash chamber tracking calorimeter at sea level

Summary The rate and angular distribution of multi-muons events in extensive air showers (EAS) are measured at sea level by a high-efficiency flash chamber calorimeter with an angular resolution of about 0.5°. Data are compared with the predictions of the low-energy composition model (LEC) of the primary cosmic-rays energy spectrum in the range (10¹⁴”10¹⁶) eV and agreement has been found. The measured time correlation between multi-muons events is also discussed. No anomaly has been found. Partially supported by Ministero della Pubblica Istruzione. Marcello Conversi died on September 27, 1988. The present paper is dedicated to his memory.     

First-principles calculation of vibrational Raman spectra in large systems: signature of small rings in crystalline SiO2

We present an approach for the efficient calculation of vibrational Raman intensities in periodic systems within density functional theory. The Raman intensities are computed from the second order derivative of the electronic density matrix with respect to a uniform electric field. In contrast to previous approaches, the computational effort required by our method for the evaluation of the intensities is negligible compared to that required for the calculation of vibrational frequencies. As a first application, we study the signature of 3- and 4-membered rings in the Raman spectra of several polymorphs of SiO2, including a zeolite (H-ZSM-18) having 102 atoms per unit cell.     

Caminatal, an aldehyde sesterterpene with a novel carbon skeleton from the Antarctic sponge Suberites caminatus

A new sesterterpene, caminatal 1, containing a bicyclic terpenoid moiety with an aldehyde appendage in combination with an isolated isoprenic aromatic ring leading to a novel carbon skeleton has been obtained from the Antarctic sponge Suberites caminatus. Its structure and relative stereochemistry were established by spectroscopic evidence and biogenetic considerations.     

Large monolithic silica-based macrocellular foams with trimodal pore system

Silica-based materials with hierarchical pore systems at three different length scales (small mesopores-large mesopores-macropores) have been prepared through a nanotectonic approach by using mesoporous nanoparticles as building blocks; the resulting materials present a highly accessible foam-like architecture and can be prepared as large monoliths.     

Accurate first principles prediction of 17O NMR parameters in SiO2: assignment of the zeolite ferrierite spectrum

17O NMR parameters, both the chemical shifts and the quadrupolar parameters, are calculated for SiO2 polymorphs using density functional theory with the generalized gradient-corrected PBE functional. The gauge including projector augmented wave (GIPAW) method (Pickard, C. J.; Mauri, F. Phys. Rev. B2001, 63, 245101) ensures the reproduction of all electron results while using computationally efficient pseudopotentials. The use of plane-waves permits fully converged calculations to be performed on structures containing 144 atoms in the unit cell, without the need to resort to the cluster approximation. The calculated NMR parameters of cristobalite, quartz, coesite, and faujasite are in excellent agreement with experimental data. This demonstrates that density functional theory is able to reproduce with high accuracy the 17O NMR parameters in SiO2 systems. This precision is used to assign the spectrum of the zeolite ferrierite. The data calculated for SiO2 are used to confirm that no simple correlation between the chemical shift and Cq NMR parameters and Si-O-Si angle exists, emphasizing the importance of predictive theories in this field.     

Is it feasible to reconstruct body segment 3-D position and orientation using accelerometric data?

The analysis of the mechanics of the musculo-skeletal system during the execution of a motor task requires the determination of the instantaneous position and orientation of the body segments involved in relation to an inertial system of reference. By using adequately assembled uniaxial accelerometric sensors, an easy-to-manage measurement system can be obtained that estimates the three-dimensional position and orientation (P&O) of a body segment through an appropriate analytical model. However, the extent to which experimental errors, in particular accelerometers (ACs) assembly inaccuracies, affect such estimation has never been systematically investigated. This paper systematically analyzes the sensitivity of analytical models of body segment P&O reconstruction through a six-AC system and a nine-AC system to different sources of experimental error. We simulated and statistically assessed the performance of these models in the case of body segment motions typical of movements under muscular control. The results obtained indicated that the inaccuracy in the orientation of the individual AC's active axes and the offset error in the AC responses were the major sources of P&O estimation errors. In particular, no accurate estimation of position was possible with the analytical models analyzed. Under the motion conditions simulated in this study, no substantial advantages were found in using a nine-AC system rather than a six-AC system. Considering that the magnitudes of the simulated experimental errors were quite low (< or = 0.1 deg: AC's orientation; < or = 10(-4) m: uncertainty of the distance between two ACs; < or = 10(-2) ms(-2): random error; 0.5 x 10(-2) ms(-2): offset error), the results indicate that none of the two ACs systems analyzed is suitable for body segment P&O estimation in routine biomechanical applications.