Parsec | EPFL Graph Search

Le parsec (), de symbole pc, est une unité de longueur utilisée en astronomie. Il est défini comme valant exactement , soit environ . Le nom parsec est la contraction de « parallaxe-seconde », une expression se rapportant à sa définition historique, désormais obsolète ( figure). Terminologie est emprunté à l'anglais parsec, mot-valise proposé par l'astronome britannique Herbert Hall Turner à partir de par[allax] (en français, « parallaxe ») et de sec[ond] (« seconde »). D'après Frédéric Arenou, le parsec a été utilisé pour la première fois en par l'astronome allemand Hermann Kobold, sous le nom de Sternweite (littéralement « distance stellaire » en allemand). En 1913, l'astronome britannique Frank Watson Dyson propose de nommer l'unité macron et Turner astron. Le terme de macron fut rejeté par Frank Watson Dyson, par crainte d'une confusion avec le terme micron. En , la commission « No

Surface 3D Micro Free Forms: Multifunctional Microstructured Mesoporous alpha-Alumina by in Situ Slip Casting Using Excimer Laser Ablated Polycarbonate Molds

Paul Bowen, Patrik Hoffmann, Sriharitha Rowthu

Ceramic surface microstructuring is a rapidly growing field with a variety of applications in tribology, wetting, biology, and so on. However, there are limitations to large-area microstructuring and fabrication of three-dimensional (3D) micro free forms. Here, we present a route to obtain intricate surface structures through in situ slip casting using polydimethylsiloxane (PDMS) negative molds which are replicated from excimer laser ablated polycarbonate (PC) master molds. PC sheets are ablated with a nanosecond KrF (lambda = 248 nm) excimer laser mask projection system to obtain micron-scale 3D surface features over a large area of up to 3 m(2). Complex surface structures that include 3D free forms such as 3D topography of Switzerland, shallow structures such as diffractive optical elements (60 nm step) and conical micropillars have been obtained. The samples are defect-free produced with thicknesses of up to 10 mm and 120 mm diameter. The drying process of the slip cast alumina slurry takes place as a one-dimensional process, through surface evaporation and water permeation through the PDMS membrane. This allows homogeneous one-dimensional shrinkage during the drying process, independent of the sample's lateral dimensions. A linear mass diffusion model has been proposed to predict and explain the drying process of these ceramic colloidal suspensions. The calculated drying time is linearly proportional to the height of the slurry and the thickness of the negatively structured PDMS and is validated by the experimental results. An experimentally observed optimum Sylgard PDMS thickness range of similar to 400 mu m to 1 mm has achieved the best quality microstructured green compacts. Further, the model predicts that the drying time is independent of the microstructured areas and was validated using experimental observations carried out with microstructured areas of 300 mm(2), 1200 mm(2), and 120 cm(2). Therefore, in principle, the structures can be further replicated in areas up to 3 m(2) with the same drying time for the same slurry height. The surface-structured ceramics display interesting wetting properties, for example, eicosane-coated mesoporous microstructured alumina shows superhydrophobic behavior. Additionally, ceramic bulk samples could be further used as second-generation very hard and low-wear molds for further microfabrication.

Amer Chemical Soc2015

J1721+8842: a gravitationally lensed binary quasar with a proximate damped Lyman-alpha absorber

Hung-Hsu Chan, Frédéric Courbin, Cameron Alexander Campbell Lemon, Martin Raoul Robert Millon, Eric Gérard Guy Paic

High-redshift binary quasars provide key insights into mergers and quasar activity, and are useful tools for probing the spatial kinematics and chemistry of galaxies along the line of sight. However, only three sub-10-kpc binaries have been confirmed above z = 1. Gravitational lensing would provide a way to easily resolve such binaries, study them in higher resolution, and provide more sightlines, though the required alignment with a massive foreground galaxy is rare. Through image deconvolution of StanCam Nordic Optical Telescope (NOT) monitoring data, we reveal two further point sources in the known, z approximate to 2.38, quadruply lensed quasar (quad) J1721+8842. An ALFOSC/NOT long-slit spectrum shows that the brighter of these two sources is a quasar with z = 2.369 +/- 0.007 based on the C III] line, while the C III] redshift of the quad is z = 2.364 +/- 0.003. Lens modelling using point-source positions rules out a single source model, favouring an isothermal lens mass profile with two quasar sources separated by similar to 6.0 kpc (0.73 '') in projection. Given the resolving ability from lensing and current lensed quasar statistics, this discovery suggests a large population of undiscovered, unlensed sub-10-kpc binaries. We also analyse spectra of two images of the quad, showing narrow Ly alpha emission within the trough of a proximate damped Lyman-alpha absorber (PDLA). An apparent mismatch between the continuum and narrow line flux ratios provides a new potential tool for simultaneously studying microlensing and the quasar host galaxy. Signs of the PDLA are also seen in the second source, but a deeper spectrum is still required to confirm this. Thanks to the multiple lines of sight from lensing and two quasar sources, this system offers simultaneous subparsec- and kiloparsec-scale probes of a PDLA.

EDP SCIENCES S A2022

Morphology Effectively Controls Singlet-Triplet Exciton Relaxation and Charge Transport in Organic Semiconductors

Verner Thorsmolle

We present a comparative study of ultrafast photoconversion dynamics in tetracene (Tc) and pentacene (Pc) single crystals and Pc films using optical pump-probe spectroscopy. Photoinduced absorption in Tc and Pc crystals is activated and temperature-independent, respectively, demonstrating dominant singlet-triplet exciton fission. In Pc films (as well as C-60-doped films) this decay channel is suppressed by electron trapping. These results demonstrate the central role of crystallinity and purity in photogeneration processes and will constrain the design of future photovoltaic devices.