1,720,971 research outputs found
Kohärente Beugungsabbildung von Elektronendynamiken im Attosekundenbereich
No full textWith the emerging availability of intense attosecond pulses and pulse trains in the X-ray and extreme ultraviolet (XUV) regime at free-electron lasers, and high harmonic generation sources, fast electronic dynamics in single nanostructures are becoming accessible for imaging techniques. This work addresses the detection of ultrafast electronic dynamics down to the attosecond domain in single nanoparticles using coherent light scattering. In particular, scattering images of helium nanodroplets, which serve as a simple model system, are produced using XUV light.
In order to provide the necessary particularly high intensities, an intense XUV light source based on high harmonic generation has been developed in a collaboration of several research groups at the Max Born Institute. Besides the optimization of the generation process itself, the geometry of the associated beamline significantly determines the achievable intensities. To use the maximum intensities also for imaging helium droplets, a new experimental setup was designed. It includes focusing optics specially developed for this experiment and several tools for characterizing the pulses interacting with the helium droplets.
Both the development of the intensity-optimized XUV light source and the experimental setup have made it possible to detect a change in the scattering response of helium droplets induced by a near-infrared (NIR) laser pulse on the femto- to even attosecond time scale. In agreement with a previous measurement, a reduced scattering signal was now routinely observed during simultaneous illumination of the droplets with NIR and XUV pulses. Moreover, a change in the amount of scattered light during simultaneous illumination was observed when the time delay of the two pulses was varied in the attosecond range. In fact, an oscillation of the scattered signal with a period of about half an NIR laser pulse cycle was measured and reproduced several times.
By modeling the experiment through the dipole response of a single electron in a helium atom, the observed change can be attributed to phenomena known from atomic physics, namely the AC Stark shift and light-induced states. In particular, the observed oscillation per NIR half cycle is attributed to quantum path interference occurring in light-induced states.
The experiment in this work is one of the first to detect attosecond electron dynamics in individual nanoparticles by coherent light scattering. This opens up new possibilities for the temporally and spatially resolved study of electronic processes in condensed matter on the nanoscale.Mit der zunehmenden Verfügbarkeit intensiver Attosekundenpulse und -pulszüge im Röntgen- und extrem ultraviolettem (XUV) Bereich an Freie-Elektronen-Laser und mit Hilfe von der Erzeugung hoher Harmonischer wird die schnelle elektronische Dynamik in einzelnen Nanostrukturen für bildgebende Verfahren zugänglich. Diese Arbeit befasst sich mit dem Nachweis ultraschneller elektronischer Dynamiken bis in den Attosekundenbereich in einzelnen Nanopartikeln mittels kohärenter Lichtstreuung. Im speziellen werden Streubilder von Heliumnanotröpfchen, welche als einfaches Modellsystem dienen, mit XUV Licht erzeugt.
Damit die notwendigen besonders hohen Intensitäten zur Verfügung stehen, wurde in einer Zusammenarbeit mehrerer Forschungsgruppen am Max-Born-Institut eine intensive XUV-Lichtquelle auf Basis von der Erzeugung von hohen Harmonischen entwickelt. Neben der Optimierung des Erzeugungs-Prozesses selbst, bestimmt die Geometrie der zugehörigen Strahlführung maßgeblich die erreichbaren Intensitäten. Um auch bei der Abbildung von Heliumtröpfchen von maximalen Intensitäten profitieren zu können, wurde zudem ein neuer Versuchsaufbau konzipiert. Dieser umfasst unter anderem eine speziell für dieses Experiment entwickelte Fokussieroptik und mehrere Werkzeuge zur Charakterisierung der mit den Heliumtröpfchen wechselwirkenden Pulse.
Sowohl die Entwicklung der intensitätsoptimierten XUV-Lichtquelle als auch des Versuchsaufbaus haben es ermöglicht, eine durch einen nahinfraroten (NIR) Laserpuls induzierte Änderung der Streureaktion von Heliumtröpfchen auf der Femto- bis sogar Attosekunden-Zeitskala nachzuweisen. Es konnte in Übereinstimmung mit einer früheren Messung nun routinemäßig ein reduziertes Streusignal bei gleichzeitiger Ausleuchtung der Tröpfchen mit NIR- und XUV-Pulsen beobachtet werden. Darüber hinaus wurde insbesondere eine Änderung der Menge an gestreutem Licht während der gleichzeitigen Ausleuchtung auch dann beobachtet, wenn die Zeitverzögerung der beiden Pulse im Attosekundenbereich variiert wurde. Tatsächlich wurde eine Oszillation des Streusignals mit einer Periode von etwa einem halben NIR-Laserpulszyklus gemessen und mehrfach reproduziert.
Durch die Modellierung des Experiments über die Dipolantwort eines einzelnen Elektrons in einem Heliumatom kann die beobachtete Änderung auf aus der Atomphysik bekannte Phänomene zurückgeführt werden, nämlich die AC-Stark-Verschiebung und lichtinduzierte Zustände. Insbesondere wird die beobachtete Oszillation pro NIR-Halbzyklus Quantenpfadinterferenzen, die in lichtinduzierten Zuständen auftreten, zugeschrieben.
Das Experiment dieser Arbeit ist eines der Ersten, das eine Elektronendynamik im Attosekundenbereich in einzelnen Nanopartikeln durch kohärente Lichtstreuung nachweist. Damit öffnen sich neue Möglichkeiten zur zeitlich und räumlich aufgelösten Untersuchung elektronischer Prozesse in kondensierter Materie auf der Nanoskala
Angle-Resolved Observation of X-Ray Second Harmonic Generation in Diamond
Full text linkThis thesis reports angularly-resolved observation of X-ray second harmonic generation (XSHG) in diamond at several phase-matching geometries. The XSHG signal was produced by ultra-short, highly intense X-ray pulses with a photon energy of 9.831 keV generated by a free-electron laser. In some geometries for high pulse energies more than 10 second harmonic photons per pulse were generated.
Different phase-matched geometries were used for XSHG to investigate the angular dependence of the efficiency of the process. Furthermore, for each phase-matching condition, the quadratic dependence for second harmonic generation at each geometry was verified and the crystal rocking curves were measured. The results for the angular dependence of the efficiency are in good agreement with the theory as shown by simulations (done by Priyanka Chakraborti).
Improved interpretations of the data are enabled by a new analysis algorithm (developed within the framework of this thesis) that interprets the detected photon signal using a deeper understanding of the used 2D detector. Further an extensive analysis on established interpretation methods is presented.
Adviser: Matthias Fuch
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Full text linkWe 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
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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