Sequential and direct ionic excitation in the strong-field ionization of 1-butene molecules

We study the Strong-Field Ionization (SFI) of the hydrocarbon 1-butene as a function of wavelength using photoion-photoelectron covariance and coincidence spectroscopy. We observe a striking transition in the fragment-associated photoelectron spectra: from a single Above Threshold Ionization (ATI) progression for photon energies less than the cation D0–D1 gap to two ATI progressions for a photon energy greater than this gap. For the first case, electronically excited cations are created by SFI populating the ground cationic state D0, followed by sequential post-ionization excitation. For the second case, direct sub-cycle SFI to the D1 excited cation state contributes significantly. Our experiments access ionization dynamics in a regime where strong-field and resonance-enhanced processes can interplay

Molecular orbital imprint in laser-driven electron recollision

Electrons released by strong-field ionization from atoms and molecules or in solids can be accelerated in the oscillating laser field and driven back to their ion core. The ensuing interaction, phase-locked to the optical cycle, initiates the central processes underlying attosecond science. A common assumption assigns a single, welldefined return direction to the recolliding electron. We study laser-induced electron rescattering associated with two different ionization continua in the same, spatially aligned, polyatomic molecule. We show by experiment and theory that the electron return probability is molecular frame-dependent and carries structural information on the ionized orbital. The returning wave packet structure has to be accounted for in analyzing strong-field spectroscopy experiments that critically depend on the interaction of the laser-driven continuum electron, such as laser-induced electron diffraction

Strong field ionization of small hydrocarbon chains with full 3D momentum analysis

Strong field ionization of small hydrocarbon chains is studied in a kinematic complete experiment using a reaction microscope. By coincidence detection of ions and electrons different ionization continua populated during the ionization process are identified. In addition, photoelectron momentum distributions from laser-aligned molecules allow to characterize the electron wavepackets emerging from different Dyson orbitals

Generation and characterisation of few-pulse attosecond pulse trains at 100 kHz repetition rate

The development of attosecond pump-probe experiments at high repetition rate requires the development of novel attosecond sources maintaining a sufficient number of photons per pulse. We use 7 fs, 800 nm pulses from a non-collinear optical parametric chirped pulse amplification laser system to generate few-pulse attosecond pulse trains (APTs) with a flux of >106 photons per shot in the extreme ultraviolet at a repetition rate of 100 kHz. The pulse trains have been fully characterised by recording frequency-resolved optical gating for complete reconstruction of attosecond bursts (FROG-CRAB) traces with a velocity map imaging spectrometer. For the pulse retrieval from the FROG-CRAB trace a new ensemble retrieval algorithm has been employed that enables the reconstruction of the shape of the APTs in the presence of carrier envelope phase fluctuations of the few-cycle laser system. © 2020 The Author(s). Published by IOP Publishing Ltd

Generation and characterisation of few-pulse attosecond pulse trains at 100 kHz repetition rate

The development of attosecond pump–probe experiments at high repetition rate requires the development of novel attosecond sources maintaining a sufficient number of photons per pulse. We use 7 fs, 800 nm pulses from a non-collinear optical parametric chirped pulse amplification laser system to generate few-pulse attosecond pulse trains (APTs) with a flux of >106 photons per shot in the extreme ultraviolet at a repetition rate of 100 kHz. The pulse trains have been fully characterised by recording frequency-resolved optical gating for complete reconstruction of attosecond bursts (FROG-CRAB) traces with a velocity map imaging spectrometer. For the pulse retrieval from the FROG-CRAB trace a new ensemble retrieval algorithm has been employed that enables the reconstruction of the shape of the APTs in the presence of carrier envelope phase fluctuations of the few-cycle laser system

Inter-kingdom Signaling by the Legionella Quorum Sensing Molecule LAI-1 Modulates Cell Migration through an IQGAP1-Cdc42-ARHGEF9-Dependent Pathway

Small molecule signaling promotes the communication between bacteria as well as between bacteria and eukaryotes. The opportunistic pathogenic bacterium Legionella pneumophila employs LAI-1 (3-hydroxypentadecane-4-one) for bacterial cell-cell communication. LAI-1 is produced and detected by the Lqs (Legionella quorum sensing) system, which regulates a variety of processes including natural competence for DNA uptake and pathogen-host cell interactions. In this study, we analyze the role of LAI-1 in inter-kingdom signaling. L. pneumophila lacking the autoinducer synthase LqsA no longer impeded the migration of infected cells, and the defect was complemented by plasmid-borne lqsA. Synthetic LAI-1 dose-dependently inhibited cell migration, without affecting bacterial uptake or cytotoxicity. The forward migration index but not the velocity of LAI-1-treated cells was reduced, and the cell cytoskeleton appeared destabilized. LAI-1-dependent inhibition of cell migration involved the scaffold protein IQGAP1, the small GTPase Cdc42 as well as the Cdc42-specific guanine nucleotide exchange factor ARHGEF9, but not other modulators of Cdc42, or RhoA, Rac1 or Ran GTPase. Upon treatment with LAI-1, Cdc42 was inactivated and IQGAP1 redistributed to the cell cortex regardless of whether Cdc42 was present or not. Furthermore, LAI-1 reversed the inhibition of cell migration by L. pneumophila, suggesting that the compound and the bacteria antagonistically target host signaling pathway(s). Collectively, the results indicate that the L. pneumophila quorum sensing compound LAI-1 modulates migration of eukaryotic cells through a signaling pathway involving IQGAP1, Cdc42 and ARHGEF9

The tumor suppressor CYLD regulates the p53 DNA damage response

The tumour suppressor CYLD is a deubiquitinase previously shown to inhibit NF-κB, MAP kinase and Wnt signalling. However, the tumour suppressing mechanisms of CYLD remain poorly understood. Here we show that loss of CYLD catalytic activity causes impaired DNA damage-induced p53 stabilization and activation in epithelial cells and sensitizes mice to chemical carcinogen-induced intestinal and skin tumorigenesis. Mechanistically, CYLD interacts with and deubiquitinates p53 facilitating its stabilization in response to genotoxic stress. Ubiquitin chain-restriction analysis provides evidence that CYLD removes K48 ubiquitin chains from p53 indirectly by cleaving K63 linkages, suggesting that p53 is decorated with complex K48/K63 chains. Moreover, CYLD deficiency also diminishes CEP-1/p53-dependent DNA damage-induced germ cell apoptosis in the nematode Caenorhabditis elegans. Collectively, our results identify CYLD as a deubiquitinase facilitating DNA damage-induced p53 activation and suggest that regulation of p53 responses to genotoxic stress contributes to the tumour suppressor function of CYLD

Generation and characterization of isolated attosecond pulses at 100 kHz repetition rate

The generation of coherent light pulses in the extreme ultraviolet (XUV) spectral region with attosecond pulse durations constitutes the foundation of the field of attosecond science. Twenty years after the first demonstration of isolated attosecond pulses, they continue to be a unique tool enabling the observation and control of electron dynamics in atoms, molecules, and solids. It has long been identified that an increase in the repetition rate of attosecond light sources is necessary for many applications in atomic and molecular physics, surface science, and imaging. Although high harmonic generation (HHG) at repetition rates exceeding 100 kHz, showing a continuum in the cutoff region of the XUV spectrum, was already demonstrated in 2013, the number of photons per pulse was insufficient to perform pulse characterization via attosecond streaking, let alone to perform a pump-probe experiment. Here we report on the generation and full characterization of XUV attosecond pulses via HHG driven by near-single-cycle pulses at a repetition rate of 100 kHz. The high number of 106 XUV photons per pulse on target enables attosecond electron streaking experiments through which the XUV pulses are determined to consist of a dominant single attosecond pulse. These results open the door for attosecond pump-probe spectroscopy studies at a repetition rate 1 or 2 orders of magnitude above current implementations

Arp3 controls the podocyte architecture at the kidney filtration barrier

Podocytes, highly specialized epithelial cells, build the outer part of the kidney filtration barrier and withstand high mechanical forces through a complex network of cellular protrusions. Here, we show that Arp2/3-dependent actin polymerization controls actomyosin contractility and focal adhesion maturation of podocyte protrusions and thereby regulates formation, maintenance, and capacity to adapt to mechanical requirements of the filtration barrier. We find that N-WASP-Arp2/3 define the development of complex arborized podocyte protrusions in vitro and in vivo. Loss of dendritic actin networks results in a pronounced activation of the actomyosin cytoskeleton and the generation of over-maturated but less efficient adhesion, leading to detachment of podocytes. Our data provide a model to explain podocyte protrusion morphology and their mechanical stability based on a tripartite relationship between actin polymerization, contractility, and adhesion

Subfemtosekunden-Prozesse in Molekülen untersucht mit Hilfe von Koinzidenzspektroskopie

Studying 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