SU5: a calibrated variable-polarization synchrotron radiation beam line in the vacuum-ultraviolet range

SU5 is a high-resolution variable-polarization synchrotron radiation (SR) beam line with which linear and circular dichroism experiments are performed in the vacuum ultraviolet (VUV) range (5-40eV), based on an electromagnetic crossed undulator called the Onduleur Plan/Helicoidal du Lure à Induction Electromagnétique (OPHELIE). To get precise knowledge of the polarization state of the emitted SR and to take into account the polarization transformations induced by reflection on the various optics, we set up an in situ VUV polarimeter to provide a precise and complete polarization analysis of the SR atthe sample location. The overall measured polarization performances were highly satisfactory, with measured linear polarization rates of more than 98% (83%) in the vertical (horizontal) linear polarization mode and an average 92.1% (95.2%) circular polarization rate for the right- (left)-handed circular polarization mode, which, to our knowledge, are the highest reported values in the VUV range. Despite some uneven photon energy efficiency, the OPHELIE crossed undulator behaves as expected in terms of polarization, permitting full control of the emitted polarization by manipulation of the vertical-to-horizontal magnetic field ratio (rho(und)) and the relative longitudinal phase (phi(und)).     

Performance of antireflecting coating-AlGaAs window layer couplingfor terrestrial concentrator GaAs solar cells

In this paper, we present the performance of optical coating systems coupled with AlGaAs window layers over GaAs solar cells. Single, double, and triple antireflecting coatings and window layers with constant and graded aluminum content are considered. Comparison between constant and graded window layers is established. To better represent reality, practical factors such as absorption of materials even for antireflecting coatings and the oxidation at window layer surface due to its high aluminum content are also included in the calculations. The design criteria to determine the optimum thickness of each layer is the achievement of maximum photogenerated current density. For this purpose and to account for terrestrial concentrator GaAs solar cells, the inclusion of direct terrestrial solar spectrum together with the internal spectral response of the device are taken into account. Finally, the best antireflecting coating/AlGaAs window layer couplings for different cases are presented     

Anti‐HIV Small‐Molecule Binding in the Peptide Subpocket of the CXCR4:CVX15 Crystal Structure

The CXC chemokine receptor 4 (CXCR4) is involved in chemotaxis and serves as a coreceptor for T‐tropic HIV‐1 viral entry, thus making this receptor an attractive drug target. Recently, crystal structures of CXCR4 were reported as complexes with the small molecule IT1t and the CVX15 peptide. Follow‐up efforts to model different antagonists into the small molecule CXCR4:IT1t crystal structure did not generate poses consistent with either the X‐ray crystal structure or site‐directed mutagenesis (SDM). Here, we compare the binding pockets of the two CXCR4 crystal structures, revealing differences in helices IV, V, VI, and VII, with major differences for the His203 residue buried in the binding pocket. The small molecule antagonist AMD11070 was docked into both CXCR4 crystal structures. An AMD11070 pose identified from the CXCR4:CVX15 model presented interactions with Asp171, Glu288, Trp94, and Asp97, consistent with published SDM data, thus suggesting it is the bioactive pose. A CXCR4 receptor model was optimized around this pose of AMD11070, and the resulting model correlated HIV‐1 inhibition with MM‐GBSA docking scores for a congeneric AMD11070‐like series. Subsequent NAMFIS NMR results successfully linked the proposed binding pose to an independent experimental structure. These results strongly suggest that not all small molecules will bind to CXCR4 in a similar manner as IT1t. Instead, the CXCR4:CVX15 crystal structure may provide a binding locus for small organic molecules that is more suitable than the secondary IT1t site. This work is expected to provide modeling insights useful for future CXCR4 antagonist and X4‐tropic HIV‐1 based drug design efforts.     

Optimizing TCP and RLC interaction in the UMTS radio access network

TCP, the dominant transport protocol for Internet applications, suffers severe performance degradation due to packet losses when a wireless link is present in the end-to-end path. For this reason, the 3G specification entity, 3GPP, has defined a reliable link layer protocol, RLC, to support packet switched services over UMTS. RLC provides error recovery in the radio access network by means of an ARQ algorithm. Early studies supported the benefit of using a reliable link protocol, while more recent studies outline new problems arising from RLC and TCP interaction and how to overcome them. This article describes the most relevant issues concerning TCP-RLC interaction and evaluates the most practical enhancement approaches, based on optimum parameter configuration at the transport and link layers. We devote special attention to RLC, whose specific configuration decisions are left to operators, and provide specific guidelines for setting its parameters. In addition, we propose two operational changes for enhancing the buffer management strategy, one of the main drawbacks of RLC.     

Weathering of road tar. II

The techniques employed on samples from the Hercies Road Experiment (Part I) were used to determine if chemical changes occurring during the weathering of road tar could be detected and to obtain information on the nature of these changes. The main changes were attributable to evaporation and condensation, and chemical changes due to the action of air and light play a relatively minor part.     

The quantitative effects of crystal habit change on the film surface form and the preferred orientation during the growth of vacuum-condensed films

Our recent systematic observations by reflection electron diffraction showed that a basic feature of film growth of high melting point materials condensed in vacuum on substrates initially at room temperature is that, after the initial thin film of randomly oriented crystals, and the growth of faces and preferred orientation of a particular type at the film surface, a new type of face and preferred orientation is often developed in further strata of deposit growth.Calculations are made showing the effect of a change in crystal habit on the film surface element distribution and the nature of the new preferred orientation which is developed.The {211} habit and orientation in gold films, in further growth after the first-developed {111} habit and orientation, is discussed as an example of the calculations, which indicate a tilt δ of approximately 25° for the {211} orientation when the vapour angle of incidence i is 45°. This agrees with our observations, confirming that the {211} orientation arises from the habit change and not from nucleation by secondary {111} twinning.     

Credit risk in the pool—implications for private capital investments in Brazilian power generation

The new Brazilian Electric Sector Regulation of 2004 introduced two negotiation markets: the regulated pool and the free market. Competition in the pool is enforced via energy auctions, where the winning generator has to sign long-term power purchase agreements simultaneously with all distributors at the bidding-price.

To estimate the appropriate credit risk spread of the pool, we implement a clustering methodology to rank and rate the distributors. The results show an average spread between 5.75% and 8.5%, which corresponds to a credit rating of B− according to the spreads available in Reuters 2004. This estimation is at least 208 basis points higher than the credit rating Ba1/BB+ assigned to the distributors by the National Electric Energy Agency (ANEEL) in the periodic tariff revisions. Distributors with higher risk/spread are located in the South–Southeast, compared to the low risk/spread ones concentrated in the North–Northeast.

We estimate the opportunity cost of capital in real terms in the range of 13–16% to account for the credit risk of the pool. Essential to determine the bidding price at the auctions, this estimation is higher than the 11.26% opportunity cost estimated by ANEEL. The pool's credit risk has to be taken into consideration, especially for compensating new private capital investments in Brazilian power generation.