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Photon Absorption (photon + absorption)
Selected AbstractsCa2+ -dependent Regulation of Phototransduction,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 4 2008Ricardo Stephen Photon absorption by rhodopsin triggers the phototransduction signaling pathway that culminates in degradation of cGMP, closure of cGMP-gated ion channels and hyperpolarization of the photoreceptor membrane. This process is accompanied by a decrease in free Ca2+ concentration in the photoreceptor cytosol sensed by Ca2+ -binding proteins that modulate phototransduction and activate the recovery phase to reestablish the photoreceptor dark potential. Guanylate cyclase-activating proteins (GCAPs) belong to the neuronal calcium sensor (NCS) family and are responsible for activating retinal guanylate cyclases (retGCs) at low Ca2+ concentrations triggering synthesis of cGMP and recovery of the dark potential. Here we review recent structural insight into the role of the N-terminal myristoylation in GCAPs and compare it to other NCS family members. We discuss previous studies identifying regions of GCAPs important for retGC1 regulation in the context of the new structural data available for myristoylated GCAP1. In addition, we present a hypothetical model for the Ca2+ -triggered conformational change in GCAPs and retGC1 regulation. Finally, we briefly discuss the involvement of mutant GCAP1 proteins in the etiology of retinal degeneration as well as the importance of other Ca2+ sensors in the modulation of phototransduction. [source] Thermal transfer in SWNTs and peapods under UV-irradiationPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 11 2007P. Puech Abstract Results of UV irradiation experiments in vacuum on SWNTs and peapods, as followed by in-situ Raman spectroscopy with increasing laser power up to 300 mW are reported. UV micro-Raman measurements were used to record spectra free of black-body radiation. Photon absorption was found to induce both the shifting and broadening of the G+ and G, bands. The local temperature increase (up to a temperature gain of ,1000 K) was able to be reliably calculated from the band feature variation, more specifically considering the G+ band. Meanwhile, a specific sensitivity to oxidation of the nanotubes from peapods was revealed, due the fullerene-catalysed dissociation of molecular oxygen traces into active species. In addition, both silica and diamond substrates were used. The different thermal conductivity of the substrates has the effect of changing the laser power levels needed to induce structural transformations. Carrying-out UV irradiation of nanotube-based material while monitoring the induced temperature via in situ Raman spectroscopy therefore appears as a new tool likely to be useful for the nano-engineering of nanotube-based devices. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The Quest for Nanoscale Magnets: The example of [Mn12] Single Molecule MagnetsADVANCED MATERIALS, Issue 43 2009Guillaume Rogez Abstract Recent advances on the organization and characterization of [Mn12] single molecule magnets (SMMs) on a surface or in 3D are reviewed. By using nonconventional techniques such as X-ray magnetic circular dichroism (XMCD) and scanning tunneling microscopy (STM), it is shown that [Mn12]-based SMMs deposited on a surface lose their SMM behavior, even though the molecules seem to be structurally undamaged. A new approach is reported to get high-density information-storage devices, based on the 3D assembling of SMMs in a liquid crystalline phase. The 3D nanostructure exhibits the anisotropic character of the SMMs, thus opening the way to address micrometric volumes by two photon absorption using the pump-probe technique. We present recent developments such as µ-SQUID, magneto-optical Kerr effect (MOKE), or magneto-optical circular dichroism (MOCD), which enable the characterization of SMM nanostructures with exceptional sensitivity. Further, the spin-polarized version of the STM under ultrahigh vacuum is shown to be the key tool for addressing not only single molecule magnets, but also magnetic nano-objects. [source] Monte Carlo model of UV-radiation interaction with TiO2 -coated spheresAICHE JOURNAL, Issue 10 2007Gustavo E. Imoberdorf Abstract Photocatalysis is one of the advanced oxidation techniques that are being studied for the treatment of polluted air and water from different sources. From a kinetic point of view, photocatalytic reaction rates are strongly dependent not only on the reactant and product concentrations, but also on the rate of photon absorption. Unfortunately, the local rate of photon absorption is usually difficult to evaluate because of (i) the inherent complexity of the system and (ii) the lack of data concerning the photocatalyst optical properties. The final objective of this project is focused on the development of a complete model of the radiation field; the bed structure, and the flow pattern to describe the operation of a fixed bed photocatalytic reactor. In this article, the interaction between radiative energy and TiO2 -coated fused-silica sphere beds was studied. The proposed model was built applying the Monte Carlo method, taking into account the complex reflection/refraction/absorption interactions between radiation and the packed bed. To obtain experimental measurements, an ad hoc device was designed and built. This device allows us to validate the proposed radiation model, and to obtain the optical parameters of the composite photocatalyst, i.e., the refractive index and the surface rough index of the fused-silica spheres, as well as the refractive index and the optical thickness of the TiO2 films. © 2007 American Institute of Chemical Engineers AIChE J, 2007 [source] Increasing the efficiency of photodynamic therapy by improved light delivery and oxygen supply using an anticoagulant in a solid tumor modelLASERS IN SURGERY AND MEDICINE, Issue 7 2010Liyong Yang MS Abstract Background and Objective The main factors in photodynamic therapy (PDT) are: photosensitizer retention, photon absorption, and oxygen supply. Each factor has its unique set of problems that poses limitation to the treatment. Both light delivery and oxygen supply are significant bottlenecks in PDT. Vascular closure during PDT reduces oxygen supply to the targeted tissue. On the other hand, with the changes in blood perfusion, the tissue optical properties change, and result in variation in irradiation light transmission. For these reasons, it becomes very important to avoid blood coagulation and vascular closure during PDT. Study Design/Materials and Methods The efficiency of PDT combined with the anticoagulant heparin was studied in a BALB/c mouse model with subcutaneous EMT6 mammary carcinomas. Mice were randomized into three groups: control, PDT-only, and PDT with heparin. The photosensitizer Photofrin® was used in our experiments. Light transmission, blood perfusion, and local production of reactive oxygen species (ROS) were monitored during the treatment. The corresponding histological examinations were performed to determine the thrombosis immediately after irradiation and to evaluate tumor necrosis 48,hours after the treatment. Results The results clearly demonstrated that PDT combined with pre-administered heparin can significantly reduce thrombosis during light irradiation. The blood perfusion, oxygen supply, and light delivery are all improved. Improved tumor responses in the combined therapy, as shown with the histological examination and tumor growth assay, are clearly demonstrated and related to an increased local ROS production. Conclusion Transitory anticoagulation treatment significantly enhances the antitumor effect of PDT. It is mainly due to the improvement of the light delivery and oxygen supply in tumor, and ultimately the amount of ROS produced during PDT. Lasers Surg. Med. 42:671,679, 2010. © 2010 Wiley-Liss, Inc. [source] Formation of hard very high energy gamma-ray spectra of blazars due to internal photon,photon absorptionMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2008Felix A. Aharonian ABSTRACT The energy spectra of TeV gamma-rays from blazars, after being corrected for intergalatic absorption in the extragalactic background light (EBL), appear unusually hard, a fact that poses challenges to the conventional models of particle acceleration in TeV blazars and/or to the EBL models. In this paper, we show that the internal absorption of gamma-rays caused by interactions with dense narrow-band radiation fields in the vicinity of compact gamma-ray production regions can lead to the formation of gamma-ray spectra of an almost arbitrary hardness. This allows significant relaxation of the current tight constraints on particle acceleration and radiation models, although at the expense of enhanced requirements to the available non-thermal energy budget. The latter, however, is not a critical issue, as long as it can be largely compensated by the Doppler boosting, assuming large (>10) Doppler factors of the relativistically moving gamma-ray production regions. The suggested scenario of formation of hard gamma-ray spectra predicts detectable synchrotron radiation of secondary electron,positron pairs which might require a revision of the current ,standard paradigm' of spectral energy distributions of gamma-ray blazars. If the primary gamma-rays are of hadronic origin related to pp or p, interactions, the ,internal gamma-ray absorption' model predicts neutrino fluxes close to the detection threshold of the next generation high-energy neutrino detectors. [source] Photon-assisted tunneling in ac driven double quantum dot spin pumpsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 6 2006R. Sánchez Abstract In this work, we study the effect of an applied ac gate voltage on the spin filtering and pumping properties of a lateral double quantum dot, in the Coulomb blockade and weak coupling regimes, considering not only the effect of the ac potential on the inter-dot tunneling but also on the tunneling through the leads. This last effect accounts for additional photon absorption and emission processes and therefore affects the spin polarization of the pumped current. In particular, we find that the spin down filtering property can be affected depending on the intensity of the ac field. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Coherence control of electron spin currents in semiconductorsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 10 2006Henry M. van Driel Abstract We provide an overview of some of our recent work on the use of one color and two color optical techniques to generate and control electronic spin currents in semiconductors for which a spin,orbit interaction exists. The generation process relies on the quantum interference between different absorption pathways, such as that between single and two photon absorption or those involving different polarization states of a monochromatic beam. For different crystal orientations and/or beam polarizations it is possible to generate a spin current with or without an electric current, and an electrical current with or without a spin current. In our experiments, which are conducted either at 80 K or 295 K, we typically employ nominally 100 fs pulses centered near 1500 and 750 nm. The currents generated are quasi-ballistic and the carriers typically move distances of ,1,10 nm, determined by the momentum relaxation time, which is of the order of 100 fs. The transient characteristics of spin-polarized electrical currents generated in strained GaAs at room temperature by ,100 fs pulses is detected by the emitted THz radiation. Pure spin currents can be detected by taking advantage of the accumulation of up and down spins on opposite sides of tightly focused pump beams. The spin states are detected through differential transmission measurements of tightly focused right and left circularly polarized, near-band-edge probe pulses, delayed by several picoseconds from the pump pulses to allow carrier thermalization to occur. By spatial scanning across the differential spin profiles and determining the amplitude of the response we are able to translate this into nm spatial resolution of spin displacement. Finally, the ability to generate ballistic currents using purely optical techniques allows us to generate transverse Hall-like currents, with transverse charge currents generated from pure spin currents and transverse spin currents generated from pure charge currents. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] High efficiency all-GaAs solar cellPROGRESS IN PHOTOVOLTAICS: RESEARCH & APPLICATIONS, Issue 2 2010Abderrahmane Belghachi Abstract The reduction of surface recombination in GaAs solar cells is known to be a major concern for photovoltaic cells designers. A common technique used to reduce this effect is to cover the GaAs surface with a wide band gap window layer, therefore the creation of a heterojunction. To avoid a heterojunction with its inconveniences; interface surface states, poor photon absorption in addition to the technological exigencies, one can use an all-GaAs solar cell. In this type of structure, a thin highly doped layer is created at the surface known as a front surface field (FSF). The main role of an FSF layer is to reduce the effect of front surface recombination and the enhancement of light-generated free carriers' collection. This is achieved by the drastic reduction of the effective recombination at the emitter upper boundary. In this work, a simple analytical model is used to simulate the influence of the FSF layer on GaAs solar cell parameters; photocurrent, open circuit voltage and energy conversion efficiency. The effects of the FSF layer doping density and its thickness on the cell performance are discussed by using computed results. Copyright © 2010 John Wiley & Sons, Ltd. [source] | ||||||||||