1,721,077 research outputs found
Time-resolved photoemission in one- and two-electron atoms
No full textDie Möglichkeit der direkten Beobachtung der Elektronenbewegung in Atomen, Molekülen und Festkörpern auf ihrer natürlichen Zeitskala ist von großem Interesse für die Quantenphysik. Für solche Echtzeituntersuchungen werden ultrakurze Referenzen benötigt die im letzten Jahrzehnt mit der Entwicklung von wohlkontrollierten und ultrakurzen Laserpulsen, mit einer Dauer von weniger als hundert Attosekunden, zugänglich wurden. Aus diesen experimentellen Fortschritten entstand das Forschungsgebiet der Attosekundenphysik. Eine zentrale Frage, die sich bei der zeitaufgelösten Beobachtung von Photoemission stellt, ist, ob Einsteins photoelektrischer Effekt unverzüglich vonstatten geht, also das Elektron ohne Zeitverzögerung direkt nach der Absorption des Photons emittiert wird, oder ob das elektronische Wellenpaket eine gewisse Zeit braucht um sich zu bilden. Solche Verzögerungen im Photoemissionsprozess können experimentell mittels sogenannter pump-prope Aufbauten wie "attosecond streaking" gemessen werden.Wir untersuchen atomare Photoionisationsprozesse für deren präzise Beschreibung die zeitabhängige Schrödingergleichung für das Atom in externen elektromagnetischen Feldern gelöst werden muss. Ab initio ist dies nur für Atome mit ein oder zwei Elektronen numerisch möglich. Unser Ziel ist letztendlich nicht nur eine genaue Simulation von Attosekundenexperimenten, sondern vielmehr die korrekte Deutung der erhaltenen Ergebnisse, da die Messapparatur selbst den zu untersuchenden Prozess verändern kann. Für Streuprozesse kann eine quantenmechanische Zeitverzögerung mittels der Eisenbud-Wigner-Smith (EWS) Zeitverschiebung angegeben werden, die der Gruppenverzögerung eines Wellenpakets entspricht. Wir zeigen wie dieses Konzept auch für atomare Photoionisation angewandt werden kann, wobei hier das langreichweitige Coulombpotential besonders behandelt werden muss. Die Coulombkraft induziert bei der Messung mittels "Streaking" eine scheinbare Zeitverschiebung (Coulomb-laser-coupling), für die wir eine genaue Erklärung anbieten. Wir untersuchen weiters, wie sich die Wechselwirkung eines leicht polarisierbaren Anfangs- oder Endzustandes mit dem untersuchenden Laserfeld auf die gemessene Verzögerung auswirkt.Letztlich zeigen wir, dass die quantenmechanische Zeitverschiebung mittels "Streaking" gemessen werden kann, solange alle Verzerrungseffekte korrekt berücksichtigt werden.Schließlich untersuchen wir wie sich die vieldiskutierten Elektronenkorrelationen auswirken, indem wir Zeitverzögerungen in Helium, dem einfachsten Mehrelektronenatom, analysieren und finden einen Beitrag, der durch die Verschränkung des ionisierten Elektrons mit dem zurückbleibenden Ion stammt. Weiterhin wenden wir die Konzepte der Zeitverzögerung auf die korrelierte Emission von zwei Elektronen an, indem wir sequentielle Zweielektronendoppelionisation von Helium untersuchen. Wir finden, dass die Zeitordnung im Doppelionisationsprozess tatsächlich sowohl in der EWS Zeitverzögerung, als auch in Streakingsimulationen sichtbar wird.The direct observation of electronic dynamics in atoms, molecules, and solids on its natural time scale is of great interest in quantum physics. For such observations, an ultrafast probe is needed which became available in the last decade with the advent of very well controlled ultrashort laser pulses with durations of only a few tens of attoseconds, creating the new research field of attosecond science. A fundamental question posed by the observation of photoemission in real time is whether Einstein's photoelectric effect happens instantaneously, i.e. if the electron is ejected without delay upon absorption of the photon, or if there is a finite time the free electronic wavepacket takes to be formed. Experimentally, such photoemission time delays are accessible by pump-probe schemes such as the "attosecond streaking" technique. We theoretically study photoionization in atoms for which the time-dependent Schrödinger equation in the external electromagnetic fields has to be solved. A full ab initio numerical solution is only possible for one- and two electron systems. Our aim is not only the accurate simulation of attosecond experiments but the correct interpretation of the observables for which intrinsic properties and measurement-induced effects have to be disentangled. The intrinsic quantum mechanical time delay for scattering is given by the Eisenbud-Wigner-Smith (EWS) time delay (corresponding to the group delay of a wavepacket). We illustrate how this concept can be applied to atomic photoionization where the long-ranged Coulomb potential requires special care. In streaking setups, the Coulomb force induces an apparent time shift (Coulomb-laser-coupling) for which we offer an accurate description.Another measurement-induced effect stems from the interaction of polarizable initial or final states with the probing streaking field. We demonstrate that the quantum mechanical time delay becomes accessible by attosecond streaking when the distortion effects are correctly accounted for.Furthermore, we investigate the much-discussed role of electron-electron correlation in time-resolved processes, studying time shifts in helium, the simplest multi-electron atom, and find a contribution due to dynamical correlation effects. Moreover, we apply the concepts of time delay to the correlated emission of two electrons by studying sequential two-photon double ionization of helium. We find that, indeed, the time ordering of the double ionization process becomes visible in the EWS time delay as well as in streaking simulations.<br /
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Operans fysik : Övertoner i ljud och ljus
No full textFör otränade öron och ögon är det inte lätt att se vad operasång och attofysik har med varandra att göra. I det här bidraget visar dock Johan Mauritsson och Hedvig Jalhed att det finns påtagliga paralleller. Artikeln är en frukt av ett populärvetenskapligt projekt där artikelförfattarna använder just dessa beröringspunkter för att sprida såväl visdom som skönhet för både öron och ögon
Temporal Aspects of High-Intensity Laser-Matter Interactions
No full textThe availability of short-pulse, high-intensity lasers has opened doors to new areas in atomic and plasma physics. The short pulses (a few femtoseconds), that are available today, enable unprecedented temporal measurements, while the high peak power accessible (several terawatts) allows physicists to study the interaction between light and relativistic plasmas. The aim of this work was twofold: i) to generate and characterize extreme ultraviolet (XUV) harmonic emission from the interaction between a gas and an intense laser pulse, and ii) to study phenomena occurring in the interaction between laser-produced plasmas and laser pulses with intensities exceeding 1018 W/cm2. The first part was aimed at improving our understanding of high-order harmonic emission in general and its time–frequency structure in particular. Spectrally, this harmonic emission consists of a comb of frequencies where each tooth in the “comb” corresponds to a pulse with a duration of the order of the laser pulse driving the generation process (typically a few tens of femtoseconds). Temporally, the superposition of several of these harmonics may form a train of attosecond pulses. In this work different methods of characterizing these XUV pulses have been studied, both experimentally and theoretically. Based on the technique of so-called sideband generation, which is simply the cross-correlation signal between XUV pulses and ultrashort infrared pulses (~ 10 fs) in a photoelectron spectrum, time–frequency characterization of individual harmonics has been demonstrated. Using a similar technique, trains of pulses of attosecond duration have also been observed. In the second part, the interaction between plasmas and laser pulses at relativistic intensities has been studied. Fascinating phenomena are observed, such as relativistic channelling and the acceleration of electrons to energies of several MeV over distances as short as 1 mm. The work focused mainly on the influence of the laser pulse duration in the laser–plasma interaction
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