2,109 research outputs found
A detailed experimental and theoretical study of the femtosecond A -band photodissociation of C H3I
The real time photodissociation dynamics of C H3I from the A band has been studied experimentally and theoretically. Femtosecond pump-probe experiments in combination with velocity map imaging have been carried out to measure the reaction times (clocking) of the different (nonadiabatic) channels of this photodissociation reaction yielding ground and spin-orbit excited states of the I fragment and vibrationless and vibrationally excited (symmetric stretch and umbrella modes) C H3 fragments. The measured reaction times have been rationalized by means of a wave packet calculation on the available ab initio potential energy surfaces for the system using a reduced dimensionality model. A 40 fs delay time has been found experimentally between the channels yielding vibrationless CH3(ν=0) and I(2P3/2) and I*(2P1/2) that is well reproduced by the calculations. However, the observed reduction in delay time between the I and I* channels when the C H3 fragment appears with one or two quanta of vibrational excitation in the umbrella mode is not well accounted for by the theoretical model. © 2008 American Institute of Physics.This work has been financed by the Spanish MEC through Grant Nos. CTQ2005-08493-C02-01, FIS-2007- 62002, and Consolider program “Science and Applications of Ultrafast Ultraintense Lasers” No. CSD2007-00013.Peer Reviewe
Unique challenges accompany thick-shell CdSe/nCdS (n \u3e 10) nanocrystal synthesis
Thick-shell CdSe/nCdS (n \u3e10) nanocrystals were recently reported that show remarkably suppressed fluorescence intermittency or blinking at the single-particle level as well as slow rates of Auger decay. Unfortunately, whereas CdSe/nCdS nanocrystal synthesis is well-developed up to n \u3c 6 CdS monolayers (MLs), reproducible syntheses for n \u3e 10 MLs are less understood. Known procedures sometimes result in homogeneous CdS nucleation instead of heterogeneous, epitaxial CdS nucleation on CdSe, leading to broad and multimodal particle size distributions. Critically, obtained core/shell sizes are often below those desired. This article describes synthetic conditions specific to thick-shell growth (n\u3e 10 and n\u3e 20 MLs) on both small (sub2 nm) and large (\u3e4.5 nm) CdSe cores. We find added secondary amine and low concentration of CdSe cores and molecular precursors give desired core/shell sizes. Amine-induced, partial etching of CdSe cores results in apparent shell-thicknesses slightly beyond those desired, especially for very-thick shells (n \u3e20 MLs). Thermal ripening and fast precursor injection lead to undesired homogeneous CdS nucleation and incomplete shell growth. Core/shells derived from small CdSe (1.9 nm) have longer PL lifetimes and more pronounced blinking at single-particle level compared with those derived from large CdSe (4.7 nm). We expect our new synthetic approach will lead to a larger throughput of these materials, increasing their availability for fundamental studies and applications
G-band Spectral Synthesis in Solar Magnetic Concentrations
Narrow band imaging in the G-band is commonly used to trace the small
magnetic field concentrations of the Sun, although the mechanism that makes
them bright has remained unclear. We carry out LTE syntheses of the G-band in
an assorted set of semi-empirical model magnetic concentrations. The syntheses
include all CH lines as well as the main atomic lines within the band-pass. The
model atmospheres produce bright G-band spectra having many properties in
common with the observed G-band bright points. In particular, the contrast
referred to the quiet Sun is about twice the contrast in continuum wavelengths.
The agreement with observations does not depend on the specificities of the
model atmosphere, rather it holds from single fluxtubes to MIcro-Structured
Magnetic Atmospheres. However, the agreement requires that the real G-band
bright points are not spatially resolved, even in the best observations. Since
the predicted G-band intensities exceed by far the observed values, we foresee
a notable increase of contrast of the G-band images upon improvement of the
angular resolution. According to the LTE modeling, the G-band spectrum emerges
from the deep photosphere that produces the continuum. Our syntheses also
predict solar magnetic concentrations showing up in continuum images but not in
the G-band . Finally, we have examined the importance of the CH
photo-dissociation in setting the amount of G-band absorption. It turns out to
play a minor role.Comment: To appear in ApJ, 554 n2 Jun 20, 33 pages and 9 figure
Fully Convolutional Networks for Continuous Sign Language Recognition
Continuous sign language recognition (SLR) is a challenging task that
requires learning on both spatial and temporal dimensions of signing frame
sequences. Most recent work accomplishes this by using CNN and RNN hybrid
networks. However, training these networks is generally non-trivial, and most
of them fail in learning unseen sequence patterns, causing an unsatisfactory
performance for online recognition. In this paper, we propose a fully
convolutional network (FCN) for online SLR to concurrently learn spatial and
temporal features from weakly annotated video sequences with only
sentence-level annotations given. A gloss feature enhancement (GFE) module is
introduced in the proposed network to enforce better sequence alignment
learning. The proposed network is end-to-end trainable without any
pre-training. We conduct experiments on two large scale SLR datasets.
Experiments show that our method for continuous SLR is effective and performs
well in online recognition.Comment: Accepted to ECCV202
When the path is never shortest: a reality check on shortest path biocomputation
Shortest path problems are a touchstone for evaluating the computing
performance and functional range of novel computing substrates. Much has been
published in recent years regarding the use of biocomputers to solve minimal
path problems such as route optimisation and labyrinth navigation, but their
outputs are typically difficult to reproduce and somewhat abstract in nature,
suggesting that both experimental design and analysis in the field require
standardising. This chapter details laboratory experimental data which probe
the path finding process in two single-celled protistic model organisms,
Physarum polycephalum and Paramecium caudatum, comprising a shortest path
problem and labyrinth navigation, respectively. The results presented
illustrate several of the key difficulties that are encountered in categorising
biological behaviours in the language of computing, including biological
variability, non-halting operations and adverse reactions to experimental
stimuli. It is concluded that neither organism examined are able to efficiently
or reproducibly solve shortest path problems in the specific experimental
conditions that were tested. Data presented are contextualised with biological
theory and design principles for maximising the usefulness of experimental
biocomputer prototypes.Comment: To appear in: Adamatzky, A (Ed.) Shortest path solvers. From software
to wetware. Springer, 201
Subharmonic bifurcation cascade of pattern oscillations caused by winding number increasing entrainment
Convection structures in binary fluid mixtures are investigated for positive
Soret coupling in the driving regime where solutal and thermal contributions to
the buoyancy forces compete. Bifurcation properties of stable and unstable
stationary square, roll, and crossroll (CR) structures and the oscillatory
competition between rolls and squares are determined numerically as a function
of fluid parameters. A novel type of subharmonic bifurcation cascade (SC) where
the oscillation period grows in integer steps as is found
and elucidated to be an entrainment process.Comment: 7 pages, 4 figure
Perfil Clínico del tratamiento de infección del tracto urinario con antibióticos de amplio espectro en un hospital 2012
Objetivo. Determinar el perfil clínico del tratamiento de la infección del tracto urinario (ITU) con antibióticos de amplio espectro en el Hospital Central de la Fuerza Aérea del Perú, entre los meses de Agosto y Noviembre del 2012. Materiales y métodos Estudio observacional, descriptivo y transversal realizado en el Hospital Central de la Fuerza Aérea de Lima-Perú, entre los meses de Agosto y Noviembre del 2012. Se tomo 71 historias clínicas de pacientes con diagnóstico de ITU que recibieron antibióticos de amplio espectro. Se analizó las variables con frecuencias (cualitativas), medidas de tendencia central y dispersión (cuantitativas). Los datos fueron procesados en el paquete estadístico STATA 11.0. Resultados. La edad promedio fue 67.08 ± 19.21 años, el 61,43% (n=43) fueron varones y el 38,57% (n=27) mujeres. El servicio con mayor prescripción fue el Servicio de Emergencia 44,29% (n=31). La comorbilidad más frecuente fue Diabetes Mellitus (DM) 24,29% (n =17). La clínica más frecuente fue disuria 35,71% (n=25). El hallazgo de laboratorio más frecuente fue leucocitosis 65,71% (n=46). Y se prescribió con mayor frecuencia ceftriaxona 34,29% (n=24). Conclusión. El perfil clínico del tratamiento de ITU con antibióticos de amplio espectro es diverso, asociándose con mayor frecuencia a la presencia de antecedente de DM y disuria
El análisis proteómico mediante 2D-DIGE y MS/MS permite discriminar en un mismo experimento entre ratones silvestres (WT) o deficientes CD38 (CD38ko) y entre ratones afectados o no por la artritis inducida por colágeno
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An atomic scale study of Si-doped AlAs by cross-sectional scanning tunneling microscopy and density functional theory
Silicon (Si) donors in GaAs have been the topic of extensive studies since Si
is the most common and well understood n-type dopant in III-V semiconductor
devices and substrates. The indirect bandgap of AlAs compared to the direct one
of GaAs leads to interesting effects when introducing Si dopants. Here we
present a study of cross-sectional scanning tunneling microscopy (X-STM) and
density functional theory (DFT) calculations to study Si donors in AlAs at the
atomic scale. Based on their crystal symmetry and contrast strengths, we
identify Si donors up to four layers below the (110) surface of AlAs.
Interestingly, their short-range local density of states (LDOS) is very similar
to Si atoms in the (110) surface of GaAs. Additionally we show high-resolution
images of Si donors in all these layers. For empty state imaging, the
experimental and simulated STM images based on DFT show excellent agreement for
Si donor up to two layers below the surface
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