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Physical review letters 129(18), 183204 (2022). doi:10.1103/PhysRevLett.129.183204, We report the measurement of the photoelectron angular distribution of two-photon single-ionization near the $2p^2$ $^1D^e$ double-excitation resonance in helium, benchmarking the fundamental nonlinear interaction of two photons with two correlated electrons. This observation is enabled by the unique combination of intense extreme ultraviolet pulses, delivered at the high-repetition-rate free-electron laser in Hamburg (FLASH), ionizing a jet of cryogenically cooled helium atoms in a reaction microscope. The spectral structure of the intense self-amplified spontaneous emission free-electron laser pulses has been resolved on a single-shot level to allow for post selection of pulses, leading to an enhanced spectral resolution, and introducing a new experimental method. The measured angular distribution is directly compared to state-of-the-art theory based on multichannel quantum defect theory and the streamlined $R$-matrix method. These results and experimental methodology open a promising route for exploring fundamental interactions of few photons with few electrons in general., Published by APS, College Park, Md.

The European physical journal / D 75(7), 205 (2021). doi:10.1140/epjd/s10053-021-00218-0, Ionization of laser-dressed atomic helium is investigated with focus on photoelectron angular distributions stemming from two-color multi-photon excited states. The experiment combines extreme ultraviolet (XUV) with infrared (IR) radiation, while the relative polarization and the temporal delay between the pulses can be varied. By means of an XUV photon energy scan over several electronvolts, we get access to excited states in the dressed atom exhibiting various binding energies, angular momenta, and magnetic quantum numbers. Furthermore, varying the relative polarization is employed as a handle to switch on and off the population of certain states that are only accessible by two-photon excitation. In this way, photoemission can be suppressed for specific XUV photon energies. Additionally, we investigate the dependence of the photoelectron angular distributions on the IR laser intensity. At our higher IR intensities, we start leaving the simple multi-photon ionization regime. The interpretation of the experimental results is supported by numerically solving the time-dependent Schr��dinger equation in a single-active-electron approximation., Published by Springer, Heidelberg

A reaction microscope dedicated to multi-particle coincidence spectroscopy on gas-phase samples is installed at beamline FL26 of the free-electron laser FLASH2 in Hamburg. The main goals of the instrument are to follow the dynamics of atoms, molecules and small clusters on their natural time-scale and to study non-linear light–matter interaction with such systems. To this end, the reaction microscope is combined with an in-line extreme-ultraviolet (XUV) split-delay and focusing optics, which allows time-resolved XUV-XUV pump–probe spectroscopy to be performed.

Applied Sciences 10(8), 2953 (2020). doi:10.3390/app10082953, The reaction microscope (REMI) endstation for atomic and molecular science at the free-electron laser FLASH2 at DESY in Hamburg is presented together with a brief overview of results recently obtained. The REMI allows coincident detection of electrons and ions that emerge from atomic or molecular fragmentation reactions in the focus of the extreme-ultraviolet (XUV) free-electron laser (FEL) beam. A large variety of target species ranging from atoms and molecules to small clusters can be injected with a supersonic gas-jet into the FEL focus. Their ionization and fragmentation dynamics can be studied either under single pulse conditions, or for double pulses as a function of their time delay by means of FEL-pump–FEL-probe schemes and also in combination with a femtosecond infrared (IR) laser. In a recent upgrade, the endstation was further extended by a light source based on high harmonic generation (HHG), which is now available for upcoming FEL/HHG pump–probe experiments., Published by MDPI, Basel

The Jovian plasma Dynamics and Composition analyzer (JDC), which is part of the Particle Environment Package (PEP) on board of the JUICE spacecraft, uses high voltages up to 5kV, during operations. These high voltages are used in the ion-optical part of the instrument, where the strongest electrical fields are encountered. Preventing internal high voltage discharges in this region is a key design driver. For this reason, a design rule of keeping the field strength -2 to 1 mbar. To successfully operate using the Below the Paschen minimum pressure the gas flow in the instrument is a non-collisional molecular flow. The time spent to further reduce pressure inside of the instrument when the instrument is exposed to vacuum (e.g. in space or in a test facility) depends on the internal outgassing source strength, the temperature and the geometric shape of the outgassing path.We determine the time constants of the pressure reduction in molecular flow regime by placing micro pirani pressure sensors inside critical volumes of real instruments.We compare the outgassing performance of the Miniature Ion Precipitation Analyzer (MIPA) on Bepi Colombo with the JDC sensor. Measurements showed too long outgassing time constants for JDC in order to be able to operate the instrument during the very time constraint on-ground test campaign where only approximately 3 days are available to reach a save vacuum of -5 mbar inside of the instrument. We present the implemented solution to improve outgassing performance of JDC and show it is sufficient for the on-ground test campaign.

We present a HHG-based vacuum ultraviolet (VUV) source at the free electron laser FLASH2. The source provides ultrashort pulses from 10 to 40eV, coupled to the REMI end-station (beamline FL26) for VUV-FEL pump-probe experiments.

We present a VUV beamline installed as pump-probe source at the free-electron laser FLASH. The source is based on high-order harmonic generation driven by femtosec-ond near-infrared laser pulses synchronized with the FEL burst mode.

Charge transfer (CT) at avoided crossings of excited ionized states of argon dimers is observed using a two-color pump-probe experiment at the free-electron laser in Hamburg (FLASH). The process is initiated by the absorption of three 27-eV-photons from the pump pulse, which leads to the population of Ar2+*–Ar states. Due to nonadiabatic coupling between these one-site doubly ionized states and two-site doubly ionized states of the type Ar+*–Ar+, CT can take place leading to the population of the latter states. The onset of this process is probed by a delayed infrared (800 nm) laser pulse. The latter ionizes the dimers populating repulsive Ar2+ –Ar+ states, which then undergo a Coulomb explosion. From the delay-dependent yields of the obtained Ar2+ and Ar+ ions, the lifetime of the charge-transfer process is extracted. The obtained experimental value of (531 ± 136) fs agrees well with the theoretical value computed from Landau-Zener probabilities.

Physical review letters 122(7), 073001 (2019). doi:10.1103/PhysRevLett.122.073001, Time delays for atomic photoemission obtained in streaking or reconstruction of attosecond bursts by interference of two-photon transitions experiments originate from a combination of the quantum mechanical Wigner time and the Coulomb-laser coupling. While the former was investigated intensively theoretically as well as experimentally, the latter attracted less interest in experiments and has mostly been subject to calculations. Here, we present a measurement of the Coulomb-laser coupling-induced time shifts in photoionization of neon at 59.4 eV using a terahertz (THz) streaking field (λ=152 μm). Employing a reaction microscope at the THz beamline of the free-electron laser in Hamburg (FLASH), we have measured relative time shifts of up to 70 fs between the emission of 2p photoelectrons (∼38 eV) and low-energetic (, Published by APS, College Park, Md.

Physical review / A 98(3), 033408 (2018). doi:10.1103/PhysRevA.98.033408, A kinematically complete experiment on two-photon double ionization of Ar by free-electron laser radiation with a photon energy of 27.93 eV was performed. The electron energy spectra show that double ionization is dominated by the sequential process. Comparison of the electron angular distributions to our data for single ionization and to theory confirms that even in the sequential process the electrons from both ionization steps are correlated with each other through polarization of the intermediate Ar$^+$ state. Furthermore, a very important role of autoionization in both ionization steps is found., Published by APS, Woodbury, NY

While a lot of papers on RoboCup's robotic 2D soccer simulation have focused on the players' offensive behavior, there are only a few papers that specifically address a team's defense strategy. In this paper, we consider a defense scenario of crucial importance: We focus on situations where one of our players must interfere and disturb an opponent ball leading player in order to scotch the opponent team's attack at an early stage and, even better, to eventually conquer the ball initiating a counter attack. We employ a reinforcement learning methodology that enables our players to autonomously acquire such an aggressive duel behavior, and we have embedded it into our soccer simulation team's defensive strategy. Employing the learned NeuroHassle policy in our competition team, we were able to clearly improve the capabilities of our defense and, thus, to increase the performance of our team as a whole.

Physical review / A 102(6), 062809 (2020). doi:10.1103/PhysRevA.102.062809, Photoelectron emission from excited states of laser-dressed atomic helium is analyzed with respect to laser intensity-dependent excitation energy shifts and angular distributions. In the two-color exteme ultraviolet (XUV)-infrared (IR) measurement, the XUV photon energy is scanned between 20.4 eV and the ionization threshold at 24.6 eV, revealing electric dipole-forbidden transitions for a temporally overlapping IR pulse (≈10$^{12}$Wcm$^{−2}$). The interpretation of the experimental results is supported by numerically solving the time-dependent Schrödinger equation in a single-active-electron approximation., Published by Inst., Woodbury, NY