Reconstructing Images from Projections Using the Maximum-Entropy Method. Numerical Simulations of Low-Aspect Astrotomography

The reconstruction of images from a small number of projections using the maximum-entropy method (MEM) with the Shannon entropy is considered. MEM provides higher-quality image reconstruction for sources with extended components than the Hogbom CLEAN method, which is also used in low-aspect astrotomography. The quality of image reconstruction for sources with mixed structure containing bright, compact features embedded in a comparatively weak, extended base can be further improved using a difference-mapping method, which requires a generalization of MEM for the reconstruction of sign-variable functions.We draw conclusions based on the results of numerical simulations for a number of model radio sources with various morphologies.Comment: 11 pages, 9 figure

Ultrafast X-ray scattering of xenon nanoparticles: imaging transient states of matter

Sem informaçãoFemtosecond x-ray laser flashes with power densities of up to 10(14) W/cm(2) at 13.7 nm wavelength were scattered by single xenon clusters in the gas phase. Similar to light scattering from atmospheric microparticles, the x-ray diffraction patterns carry information about the optical constants of the objects. However, the high flux of the x-ray laser induces severe transient changes of the electronic configuration, resulting in a tenfold increase of absorption in the developing nanoplasma. The modification in opaqueness can be correlated to strong atomic charging of the particle leading to excitation of Xe4+. It is shown that single-shot single-particle scattering on femtosecond time scales yields insight into ultrafast processes in highly excited systems where conventional spectroscopy techniques are inherently blind.Femtosecond x-ray laser flashes with power densities of up to 10(14) W/cm(2) at 13.7 nm wavelength were scattered by single xenon clusters in the gas phase. Similar to light scattering from atmospheric microparticles, the x-ray diffraction patterns carry information about the optical constants of the objects. However, the high flux of the x-ray laser induces severe transient changes of the electronic configuration, resulting in a tenfold increase of absorption in the developing nanoplasma. The modification in opaqueness can be correlated to strong atomic charging of the particle leading to excitation of Xe4+. It is shown that single-shot single-particle scattering on femtosecond time scales yields insight into ultrafast processes in highly excited systems where conventional spectroscopy techniques are inherently blind.108915Sem informaçãoSem informaçãoBMBF [05KS4KT1, 05KS7KT2]HGF Virtuelles Institut [VH-VI-103, VH-VI-302]Sem informaçãoWe would like to thank all staff at FLASH for their outstanding support. Funding is acknowledged from BMBF 05KS4KT1 and 05KS7KT2, as well as HGF Virtuelles Institut VH-VI-103 and VH-VI-302

Monte Carlo simulations of membrane signal transduction events: Effect of receptor blockers on G-protein activation

Cells have evolved elaborate strategies for sensing, responding to, and interacting with their environment. In many systems, interaction of cell surface receptors with extracellular ligand can activate cellular signal transduction pathways leading to G-protein activation and calcium mobilization. In BC 3 H1 smooth muscle-like cells, we find that the speed of calcium mobilization as well as the fraction of cells which mobilize calcium following phenylephrine stimulation is dependent upon receptor occupation. To determine whether receptor inactivation affects calcium mobilization, we use the receptor antagonist prazosin to block a fraction of cell surface receptors prior to phenylephrine stimulation. For cases of equal receptor occupation by agonist, cells with inactivated or blocked receptors show diminished calcium mobilization following phenylephrine stimulation as compared to cells without inactivated receptors. Ligand/receptor binding and two-dimensional diffusion of receptors and G-proteins in the cell membrane are studied using a Monte Carlo model. The model is used to determine if receptor inactivation affects G-protein activation and thus the following signaling events for cases of equal equilibrium receptor occupation by agonist. The model predicts that receptor inactivation by antagonist binding results in lower G-protein activation not only by reducing the number of receptors able to bind agonist but also by restricting the movement of agonist among free receptors. The latter process is important to increasing the access of bound receptors to G-proteins.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43997/1/10439_2006_Article_BF00000009.pd

Time-resolved pump-probe experiments at the LCLS

The first time-resolved x-ray/optical pump-probe experiments at the SLAC Linac Coherent Light Source (LCLS) used a combination of feedback methods and post-analysis binning techniques to synchronize an ultrafast optical laser to the linac-based x-ray laser. Transient molecular nitrogen alignment revival features were resolved in time-dependent x-ray-induced fragmentation spectra. These alignment features were used to find the temporal overlap of the pump and probe pulses. The strong-field dissociation of x-ray generated quasi-bound molecular dications was used to establish the residual timing jitter. This analysis shows that the relative arrival time of the Ti:Sapphire laser and the x-ray pulses had a distribution with a standard deviation of approximately 120 fs. The largest contribution to the jitter noise spectrum was the locking of the laser oscillator to the reference RF of the accelerator, which suggests that simple technical improvements could reduce the jitter to better than 50 fs

Numerical simulation of small angle scattering (SAXS) for large atomic clusters

FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOIn the context of direct imaging of single nanoparticles, we discuss strategies for simulation of the small angle X-ray scattering (SAXS) of nanoparticles, propose a new scalar numerical approach applicable to arbitrary collections of atoms and compare it to the existing alternatives, both scalar and vector. The simulations are also compared with SAXS experimental data obtained recently at the FLASH FEL in Hamburg (lambda(FEL) 32 nm), demonstrating the direct evaluation of size for very large Argon clusters with a single FEL pulse.In the context of direct imaging of single nanoparticles, we discuss strategies for simulation of the small angle X-ray scattering (SAXS) of nanoparticles, propose a new scalar numerical approach applicable to arbitrary collections of atoms and compare it to the existing alternatives, both scalar and vector. The simulations are also compared with SAXS experimental data obtained recently at the FLASH FEL in Hamburg (lambda(FEL) 32 nm), demonstrating the direct evaluation of size for very large Argon clusters with a single FEL pulse.166/167SI2127FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOBundesministerium fur Forschung und Technologie (BMBF) [05 KS4KTC/1]Sem informaçãoARBC is grateful to Prof I. Torriani (UNICAMP) for patient assistance regarding X-ray diffraction theory and SAXS, and for directing his attention to the publications of the Hamburg outstation of EMBL. Financial support by FAPESP (Fundacao de Amparo a Pesquisa do Estado de Sao Paulo, SP, Brazil), the Bundesministerium fur Forschung und Technologie (BMBF) under grant 05 KS4KTC/1 and from the HGF virtuelle Institut VH-VI 103 (Germany) are kindly acknowledged

Clusters in intense FLASH pulses: ultrafast ionization dynamics and electron emission studied with spectroscopic and scattering techniques

International audienceFLASH, the first FEL operating at short wavelength, has paved the way for novel types of experiments in many different scientific disciplines. Key questions for the first experiments with this new type of light source are linked to light – matter interaction and ionisation processes. This paper gives an overview of the ultrafast ionisation dynamics and electron emission of pure and doped rare gas clusters illuminated with intense short-wavelength pulses by summarizing the findings of recent years work at FLASH. Atomic clusters are ideal for investigating the light – matter interaction because their size can be tuned from the molecular to the bulk regime thus allowing to distinguish between intra and interatomic processes. The ionisation processes turned out to be strongly wavelength dependent. Plasma absorption, while dominant at 13 eV becomes insignificant at photon energies above 40 eV. The cluster ionisation and disintegration proceeds in several steps on a time scale from fs to ps. Insight into the involved processes can be obtained with ion and electron spectroscopy. The high intensity of FLASH pulses opens the door for a new imaging approach to study nanoparticles. Scattering patterns of single and few clusters can be recorded in a single shot. Initial results of scattering experiments and their comparison to Miecalculations show that two and three dimensional structural information of gas phase particles can be obtained this way

Clusters in intense FLASH pulses: ultrafast ionization dynamics and electron emission studied with spectroscopic and scattering techniques

Sem informaçãoFLASH, the first FEL operating at short wavelength, has paved the way for novel types of experiments in many different scientific disciplines. Key questions for the first experiments with this new type of light source are linked to light-matter interaction and ionization processes. This paper gives an overview of the ultrafast ionization dynamics and electron emission of pure and doped rare gas clusters illuminated with intense short-wavelength pulses by summarizing the findings of recent years' work at FLASH. Atomic clusters are ideal for investigating the light-matter interaction because their size can be tuned from the molecular to the bulk regime, thus allowing us to distinguish between intra and interatomic processes. The ionization processes turned out to be strongly wavelength dependent. Plasma absorption, while dominant at 13 eV, becomes insignificant at photon energies above 40 eV. The cluster ionization and disintegration proceed in several steps on a time scale from fs to ps. Insight into the involved processes can be obtained with ion and electron spectroscopy. The high intensity of FLASH pulses opens the door for a new imaging approach to study nanoparticles. Scattering patterns of single and few clusters can be recorded in a single shot. Initial results of scattering experiments and their comparison to Mie calculations show that two-and three-dimensional structural information of gas phase particles can be obtained this way.FLASH, the first FEL operating at short wavelength, has paved the way for novel types of experiments in many different scientific disciplines. Key questions for the first experiments with this new type of light source are linked to light-matter interaction and ionization processes. This paper gives an overview of the ultrafast ionization dynamics and electron emission of pure and doped rare gas clusters illuminated with intense short-wavelength pulses by summarizing the findings of recent years' work at FLASH. Atomic clusters are ideal for investigating the light-matter interaction because their size can be tuned from the molecular to the bulk regime, thus allowing us to distinguish between intra and interatomic processes. The ionization processes turned out to be strongly wavelength dependent. Plasma absorption, while dominant at 13 eV, becomes insignificant at photon energies above 40 eV. The cluster ionization and disintegration proceed in several steps on a time scale from fs to ps. Insight into the involved processes can be obtained with ion and electron spectroscopy. The high intensity of FLASH pulses opens the door for a new imaging approach to study nanoparticles. Scattering patterns of single and few clusters can be recorded in a single shot. Initial results of scattering experiments and their comparison to Mie calculations show that two-and three-dimensional structural information of gas phase particles can be obtained this way.4319Sl111Sem informaçãoSem informaçãoBMBF [05KS4KTC1]DFG [BO 3169/2-2]HGF Virtuelles Institut [VHVI-302]Sem informaçãoWe would like to thank the DESY staff for their outstanding support and our colleagues from Universitat Rostock, MPI-PKS, MPG-ASG, MPI-K, MPG-HLL, DESY and Amolf for inspiring and fruitful collaborations. Funding is acknowledged from BMBF grant no 05KS4KTC1, DFG BO 3169/2-2 and HGF Virtuelles Institut VHVI-302

Trace amine-associated receptor 1 activation silences GSK3β signaling of TAAR1 and D2R heteromers

Trace amine-associated receptor 1 (TAAR1) activation by selective endogenous agonists modulates dopaminergic neurotransmission. This results in antipsychotic-like behavior in vivo which might be initiated by an interaction of TAAR1 and dopamine D2L receptor (D2R). Here we analyzed the functional link between TAAR1 and D2R using highly potent and selective TAAR1 agonists, and newly generated tools such as TAAR1 knock-out and TAAR1 overexpressing rats as well as specific anti-rat TAAR1 antibodies. We provide data from co-immunoprecipitation experiments supporting a functional interaction of the two receptors in heterologous cells and in brain tissue. Interaction of TAAR1 with D2R altered the subcellular localization of TAAR1 and increased D2R agonist binding affinity. Using specific beta-arrestin 2 (betaArr2) complementation assays we show that the interaction of TAAR1 with D2R reduced betaArr2 recruitment to D2R. In addition, we report that besides Galphas-protein signaling TAAR1 also signals via betaArr2. In the presence of D2R, cAMP signaling of TAAR1 was reduced while its betaArr2 signaling was enhanced, resulting in reduced GSK3beta activation. These results demonstrate that betaArr2 signaling may be an important pathway for TAAR1 function and that the activation of the TAAR1-D2R complex negatively modulates GSK3beta signaling. Given that patients with schizophrenia or bipolar disorder show increased GSK3beta signaling, such a reduction of GSK3beta signaling triggered by the interaction of D2R with activated TAAR1 further supports TAAR1 as a target for the treatment of psychiatric disorders