1,721,058 research outputs found
Direct Imaging of Transient Fano Resonances in N2 Using Time-, Energy-, and Angular-Resolved Photoelectron Spectroscopy
Get PDFAutoionizing Rydberg states of molecular N2 are studied using time-, energy-, and angular-resolved photoelectron spectroscopy. A femtosecond extreme ultraviolet pulse with a photon energy of 17.5 eV excites the resonance and a subsequent IR pulse ionizes the molecule before the autoionization takes place. The angular-resolved photoelectron spectra depend on pump-probe time delay and allow for the distinguishing of two electronic states contributing to the resonance. The lifetime of one of the contributions is determined to be 14±1 fs, while the lifetime of the other appears to be significantly shorter than the time resolution of the experiment. These observations suggest that the Rydberg states in this energy region are influenced by the effect of interference stabilization and merge into a complex resonance
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
Shape Resonances as a Probe of an Evolving Nuclear and Electronic Structure in Molecules
Get PDFShape resonances are a ubiquitous phenomenon in electron–molecule scattering, in which the impinging electron is resonantly captured in a pseudo-bound state that is supported by the molecular potential. To study the electron scattering dynamics, we use time- and angle- resolved photoelectron spectroscopy here. With this technique, the transient evolution of the photoelectron angular distributions (PADs) from the ionization of an excited-state species can be measured. In the PADs, the electron–molecular-ion scattering dynamics are contained because the photoelectron necessarily interacts with the potential of the parent molecule as it escapes. The aim of this thesis is to investigate to what extent molecular dynamics, which are triggered by a pump laser pulse, are reflected in the PADs of the photoelectron spectra generated by an ionizing probe pulse, and how these effects can be rationalized in a photoelectron-scattering picture.
Three experimental studies are covered in this thesis: In the first experiment, CF3I molecules are impulsively aligned in space by a short near-infrared pulse, which creates a rotational wave packet. During the revival of the rotational wave packet, PADs are measured for different molecular-axes distributions by photoionization with an ultrashort XUV pulse generated through high-order harmonic generation (HHG). Comparing the PADs thus obtained to the results of quantum-scattering calculations carried out with the ePolyScat suite of programs, we show that the alignment-dependent change in the PADs can be largely explained by two prominent shape resonances that contribute to the PADs in a distinctly different way geometrically.
In the second experiment, we investigate the laser-assisted photoelectron recollisions that occur in strong-field ionization of atoms and molecules. We show how the differential scattering cross sections (DCSs) for the electron–molecular-ion collision process can be extracted from the resulting photoelectron spectrum. Then, we apply this approach to the investigation of the excited-state dynamics of I2 molecules that are prepared in the A or B state, leading to photodissociation and the creation of a vibrational wave packet, respectively. Again, by comparing to calculations carried out with ePolyScat, we conclude that the observed modulations in the DCSs of the rescattered electrons can be very well explained by considering two prominent shape resonances involved, the l=6 resonance of the diatomic molecular ion and the l=3 resonance of the free iodine atomic ion.
In the third study, the time-resolved core-shell photoionization of dissociating halomethane molecules, namely CH3I and CH2ICl, is investigated employing ultrashort soft x-ray pulses provided by the free-electron laser FLASH in Hamburg, which are able to ionize the 4d shell of iodine close to the well-known “giant” photoionization resonance (again related to the l=3 shape resonance). We find that the dissociation clearly manifests as a shift of the 4d core-level binding energy, and that the time scale and temporal onset of this effect is distinctly different from that of the photoion measurements, which are commonly exploited to quantify the dissociation dynamics
Fundamental Carrier-Envelope Phase Noise Limitations during Pulse Formation and Detection
Get PDFThe difference between the positions of the maximum peak of the carrier wave of a laser pulse and the maximum
of its intensity envelope is termed carrier-envelope phase (CEP). In the last decades, the control and stabilization
of this parameter has greatly improved, which enables many applications in research fields that rely on
CEP-stable pulses such as attosecond science and optical frequency metrology. Further progress in these fields depends
strongly on minimizing the CEP noise that restricts stabilization performance. While the CEP of most high
repetition-rate low-energy laser oscillators has been stabilized to a remarkable precision, some types of oscillators
show extensive noise that inhibits precise stabilization. The CEP stabilization performance of low repetition-rate
high peak-power amplified laser systems also remains limited by noise, which is believed to stem mainly from the
CEP detection process.
In this thesis, the origins of the CEP noise within four oscillators as well as the noise induced by the measurement
of the CEP of amplified pulses are investigated. In the first part, the properties of the CEP noise of one
Ti:sapphire oscillator and three different fiber oscillators are extracted by analyzing the unstabilized CEP traces by
means of time-resolved correlation analysis of carrier-envelope amplitude and phase noise as well as by methods
that reveal the underlying statistical noise properties. In the second part, investigations into the origin of CEP noise
induced by the measurement of the CEP of amplified pulses are conducted by comparing several different CEP
detection designs that are based on f -2 f interferometry. These detection setups differ in the employed sources of
spectral broadening as well as frequency doubling media, both necessary steps to measure the CEP. The results
in both parts of this thesis show that white quantum noise dominates most CEP measurements. In one particular
fiber oscillator, the strong white noise is found to be a result of a correlating mechanism within the employed
SESAM. During amplifier CEP detection, the CEP noise is found to be originating only to a marginal degree from
the number of photons that are detected during the measurement, which excludes shot noise as a limiting source.
Instead, the analysis reveals that the origin of the observed strong white noise can be interpreted as a loss of coherence
during detection. This type of coherence is termed here intra-pulse coherence and describes the phase
transfer within f -2 f interferometry. Its degradation is a result of amplitude-to-phase coupling during the spectral
broadening process that leads to pulse-to-pulse fluctuations of the phases at the edges of the extended spectrum.
Numerical simulations support the concept of intra-pulse coherence degradation and show that the degradation is
substantially stronger during plasma-driven spectral broadening as compared to self-phase modulation-dominated
spectral broadening. This difference in degradation also explains the much stronger CEP noise typically observed
in amplified systems as compared to oscillators, as the former typically rely on filamentation-based and hence
plasma-dominated spectral broadening for CEP detection. The concept of intra-pulse coherence constitutes a
novel measure to assess the suitability of a spectral broadening mechanism for application in active as well as in
passive CEP stabilization schemes and provides new strategies to reduce the impact of the CEP detection on the
overall stabilization performance of most lasers.Diese Arbeit beschäftigt sich mit der Identifizierung und Minimierung fundamentaler Rauschquellen, die zu einer
Limitierung des erreichbaren Carrier-Envelope Phasen (CEP) Jitters führen. Die Carrier-Envelope Phase beschreibt
die Differenz zwischen dem Maximum der Trägerwelle und dem Scheitelpunkt der Intensitätseinhüllenden. In den
letzten Jahrzehnten hat sich die Kontrolle und Stabilisierung der CEP deutlich verbessert, was zu einem schnellen
Fortschritt in Forschungsfeldern geführt hat, bei denen CEP-stabile Pulse notwendig sind. Diese Forschungsfelder
umfassen die Attosekundenforschung und optische Frequenzmetrologie. Weitere Entwicklungen in diesen Feldern
hängt stark von der Minimierung von CEP Rauschen ab, welches die CEP Stabilisierung stark beeinträchtigt.
Obwohl die CEP der Pulse der meisten Laseroszillatoren mit hohen Repetitionsraten äußerst genau stabilisiert
werden kann, existieren einige Laseroszillatoren bei denen starke Rauschquellen eine Stabilisierung verhindern
oder stark einschränken. Des Weiteren zeigen vor Allem verstärkte System mit niedrigen Repetitionsraten und
hohen Spitzenleistungen eine Beschränkung der CEP Stabilisierung aufgrund von Rauschen, dass vermutlich zum
großen Teil durch den Detektionsprozess entsteht. In dieser Arbeit ist der Ursprung von CEP Rauschen in vier unterschiedlichen
Laseroszillatoren sowie während der Detektion der CEP von verstärkten Systemen untersucht worden.
Im ersten Teil wurden die Eigenschaften des CEP Rauschens eines Ti:Saphir-basierten Oszillators und drei
verschiedener Faserlaser analysiert. Hierzu wurde das Rauschen unter anderem mittels zeitaufgelöster Korrelationsanalyse
von Carrier-Envelope Amplituden- und Phasenrauschen sowie mittels Methoden, die die statistischen
Eigenschaften des Rauschens offenlegen, analysiert. Im zweiten Teil der Arbeit wurde das Rauschen untersucht,
welches durch den Messprozess der CEP von verstärkten Pulsen mittels f -2 f Interferometrie entsteht. Experimentell
wurden hierzu vier unterschiedliche Detektionsanordnungen verwendet, die sich durch die Nutzung unterschiedlicher
nichtlinearer Prozesse zum Erzeugen der spektralen Verbreiterung sowie zur Erzeugung der zweiten
Harmonischen unterscheiden. Die Ergebnisse in beiden Teilen der Arbeit zeigen dominierendes weißes Quantenrauschen
in den meisten CEP Messungen. In einem bestimmten Faserlaser, in dem besonders starkes weißes
Rauschen vorlag, konnte der Ursprung einerWechselwirkung innerhalb des verwendeten halbleiterbasierten sättigbaren
Absorbers zugeordnet werden. Bei der Detektion der CEP bei verstärkten Systemen wurde hingegen gezeigt,
dass niedrige Photonenzahlen und damit Schrotrauschen nur zum kleinen Teil für die starken weißen Rauschanteile
verantwortlich gemacht werden kann. Stattdessen kann die Ursache des starken Rauschens einem Verlust von Kohärenz
zugeordnet werden. Diese Art von Kohärenz ist hier mit intra-Puls Kohärenz bezeichnet und beschreibt
den Phasentransfer innerhalb der Detektion mittels f -2 f Interferometrie. Der Verlust von intra-Puls Kohärenz ist
eine Folge von Amplituden-zu-Phasen Koppelung während der spektralen Verbreiterung. Von Puls zu Puls führt
dies zu Fluktuationen der Phase an beiden Rändern der erzeugten spektralen Verbreiterung. Numerische Simulationen
unterstützen das Konzept der intra-Puls Kohärenz und zeigen auf, dass die Degradation bedeutend stärker
bei plasmadominierten Prozessen ausfällt als im Vergleich zu spektraler Verbreiterung mittels Selbstphasenmodulation.
Dieser unterschiedlich starke Verlust der intra-Puls Kohärenz erklärt das deutlich höhere Rauschniveau in
verstärkten Systemen im Vergleich zu Oszillatoren, da verstärkte Systeme plasmadominierte Prozesse zur spektralen
Verbreiterung nutzen. Das Konzept der intra-Puls Kohärenz stellt ein neues Maß zur Einschätzung einer
Methode zur spektralen Verbreiterung für eine bestimmte Anwendung dar, die sowohl in aktiven sowie passiven
CEP Stabilisierungen von Lasern eine Rolle spielt. Es ermöglicht somit neue Strategien, um den Einfluss der
Detektion auf die CEP Stabilisierung der meisten Laser zu senken
Subfemtosekunden-Prozesse in Molekülen untersucht mit Hilfe von Koinzidenzspektroskopie
No full textStudying dynamics in molecules occurring on the few-femtosecond to subfemtosecond
timescale is a formidable challenge, due to the wealth of phenomena
exhibited by molecular systems. Complex manifolds of electronic states
featuring electron-nuclear and electron-electron correlations complicate the
interpretation of experimental data. In order to improve the situation, it is
desirable to perform experiments where as much information as possible is
obtained about the processes under scrutiny. When ionization is involved,
this amounts to the detection of the full momentum vectors of all charged
particles created in a single event, i.e. electrons and ions, the latter of which
may dissociate into smaller fragments. This can be accomplished in coincidence
experiments using a reaction-microscope detector. In this thesis, a reaction
microscope is employed with the aim of studying attosecond dynamics taking
place in molecules.
First, the polyatomic molecule 1,3-butadiene is investigated using intense
femtosecond laser pulses. According to the well-known three-step model, an
electron is released from the molecule via strong-field ionization and may
subsequently return to and rescatter from its parent ion, all of which happens
within a single laser cycle. A common phenomenon in the response of molecules
to strong fields is ionization to multiple final electronic states of the cation.
Here, the coincidence capabilities of the reaction microscope are exploited to
demonstrate directly for the first time that the resulting multiple electron
continua display differences in their rescattering behaviour. Using aligned
molecules, it is furthermore shown that the probability for the electron to
return to the core is dependent on the orientation of the molecule with respect
to the laser polarization direction, since the returning electron wave packet
retains structural information on the shape of its initial bound state.
The other goal of the thesis is to take the step from experiments relying on
the sub-cycle dynamics occurring in a femtosecond laser pulse to attosecond
pump-probe coincidence spectroscopy. To this end, a beamline combining
a reaction microscope with a two-colour, attosecond-stable interferometric
setup based on high-harmonic generation is presented. The setup is designed
to operate at a repetition rate of 100 kHz, which is an order of magnitude
larger than other setups currently combining attosecond spectroscopy with
coincidence detection and which affords shorter acquisition times for coincidence
experiments. First test results, in particular the first full characterization of
attosecond pulse trains driven by sub-8 fs pulses at a repetition rate of 100 kHz,
permit an optimistic perspective that, in the near future, the beamline will be
capable of providing attosecond and few-cycle femtosecond pulses for pumpprobe
experiments on molecular targets, promising to uncover novel insights
into the complex attosecond dynamics of polyatomic molecules.Die direkte Beobachtung ultraschneller dynamischer Prozesse in Molekülen,
welche auf einer Zeitskala von weniger als einer Femtosekunde ablaufen können,
stellt aufgrund der großen Komplexität solcher molekularen Systeme eine große
Herausforderung dar. Daher ist es wünschenswert, experimentell möglichst
viele Informationen über die zu untersuchenden Prozesse zugänglich zu machen.
Durch Koinzidenzmessungen mit einem Reaktionsmikroskop ist es möglich,
in Ionisationsexperimenten die vollständigen Impulsvektoren aller geladenen
Teilchen (Elektronen und positiv geladene Ionen), die in einem einzelnen Ionisationsereignis
entstehen, zu bestimmen. In der vorliegenden Arbeit wird ein
solches Reaktionsmikroskop mit dem Ziel, Attosekunden-Prozesse in Molekülen
zu untersuchen, eingesetzt.
Zunächst wird der Einfluss intensiver Femtosekunden-Laserpulse auf das
mehratomige Molekül 1,3-Butadien betrachtet. Die Vorgänge, welche in
Atomen und Molekülen im starken elektrischen Feld solcher Pulse ablaufen,
können durch ein gängiges Dreischrittmodell beschrieben werden, wobei die
drei Schritte innerhalb einer einzigen Oszillationsperiode des elektrischen Feldes
stattfinden: Ein Elektron wird durch Starkfeldionisation freigesetzt und dann
kurze Zeit später durch das Laserfeld zu seinem Mutterion zurückbeschleunigt,
an welchem es schließlich rückstreuen kann. Es ist weiterhin bekannt, dass
in Molekülen mehrere elektronische Zustände des Ions durch Starkfeldionisation
besetzt werden können. Mit Hilfe von Koinzidenzmessungen wird in der
vorliegenden Arbeit nun erstmals direkt gezeigt, dass die mehreren daraus
resultierenden Elektronenkontinua ein unterschiedliches Rückstreuverhalten
aufweisen. Des Weiteren wird mit Hilfe ausgerichteter Moleküle demonstriert,
dass in 1,3-Butadien die Rückkehrwahrscheinlichkeit des Elektrons von der
Molekülorientierung abhängt, da das zurückkommende Elektronenwellenpaket
die Struktur des ursprünglichen gebundenen Zustands des Elektrons teilweise
beibehält.
Ein weiteres Ziel dieser Arbeit ist es, Attosekunden-Pump-Probe- und
Koinzidenzspektroskopie miteinander zu verbinden. Daher wird ein neuer
experimenteller Aufbau vorgestellt, der ein Reaktionsmikroskop und ein attosekundenstabiles
Zweifarben-Interferometer, basierend auf der Erzeugung
hoher Harmonischer, kombiniert. Die Repetitionsrate von 100 kHz ist um
eine Größenordnung höher als in vergleichbaren Aufbauten, die derzeit in
Verwendung sind, was eine kürzere Messdauer für Koinzidenzexperimente
ermöglicht. Ergebnisse erster Testmessungen mit dieser Apparatur werden
vorgestellt, insbesondere die erste vollständige Charakterisierung von kurzen,
durch sub-8 fs-Pulse und bei 100 kHz erzeugten Attosekunden-Pulszügen. Diese
Ergebnisse zeichnen ein positives Bild im Hinblick auf die Möglichkeit künftiger
Koinzidenzexperimente an Molekülen mit Hilfe des präsentierten Attosekunden-
Pump-Probe-Aufbaus, welche tiefe Einblicke in die komplexen Vorgänge solcher
Systeme ermöglichen könnten
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