Unsupervised real-world knowledge extraction via disentangled variational autoencoders for photon diagnostics

We present real-world data processing on measured electron time-of-flight data via neural networks. Specifically, the use of disentangled variational autoencoders on data from a diagnostic instrument for online wavelength monitoring at the free electron laser FLASH in Hamburg. Without a-priori knowledge the network is able to find representations of single-shot FEL spectra, which have a low signal-to-noise ratio. This reveals, in a directly human-interpretable way, crucial information about the photon properties. The central photon energy and the intensity as well as very detector-specific features are identified. The network is also capable of data cleaning, i.e. denoising, as well as the removal of artefacts. In the reconstruction, this allows for identification of signatures with very low intensity which are hardly recognisable in the raw data. In this particular case, the network enhances the quality of the diagnostic analysis at FLASH. However, this unsupervised method also has the potential to improve the analysis of other similar types of spectroscopy data

Electronic Quantum Coherence in Glycine Molecules Probed with Ultrashort X-ray Pulses in Real Time

Structural changes in nature and technology are driven by charge carrier motion. A process such as charge-directed reactivity that can be operational in radiobiology is more efficient, if energy transfer and charge motion proceeds along well-defined quantum mechanical pathways keeping the coherence and minimizing dissipation. The open question is: do long-lived electronic quantum coherences exist in complex molecules? Here, we use x-rays to create and monitor electronic wave packets in the amino acid glycine. The outgoing photoelectron wave leaves behind a positive charge formed by a superposition of quantum mechanical eigenstates. Delayed x-ray pulses track the induced electronic coherence through the photoelectron emission from the sequential double photoionization processes. The observed sinusoidal modulation of the detected electron yield as a function of time clearly demonstrates that electronic quantum coherence is preserved for at least 25 femtoseconds in this molecule of biological relevance. The surviving coherence is detected via the dominant sequential double ionization channel, which is found to exhibit a phase shift as a function of the photoelectron energy. The experimental results agree with advanced ab-initio simulations.Comment: 54 pages, 11 figure

Optical-optical double resonance experiments on Potassium-Argon and determination of the adiabatic potentials as well as the spin-orbit function in the B²Σ - A²Π - system

In der vorliegenden Arbeit wird über die laserspektroskopische Untersuchung des van der Waals-Moleküls Kalium-Argon berichtet. Die KAr-Moleküle wurden in einer Überschall-Molekülstrahlapparatur erzeugt, in der ein Gemisch aus Kaliumdampf und Argon-Gas durch eine schmale Düse in ein Vakuum expandiert. Im Überschallstrahl wurden die KAr-Moleküle durch einen kontinuierlich durchstimmbaren Laser angeregt und die induzierte Fluoreszenz gemessen. Auf diese Weise wurden mit der Doppelresonanzmethode Experimente im Wellenzahlbereich zwischen 13048 cm-1 und 13073 cm-1 durchgeführt. Außerdem wurde das Absorptionsspektrum von KAr von 13032 cm-1 bis 13049 cm-1 rotationsaufgelöst aufgenommen, wodurch nun zusammen mit den Ergebnissen früherer Untersuchungen ein lückenloses Anregungsspektrum von 12866 cm-1 bis 13080 cm-1 vorliegt. Mit Hilfe der Doppelresonanzmessungen konnten erstmalig die drei untersten Schwingungsniveaus v = 0,1,2 des angeregten elektronischen Zustands B2Σ identifiziert werden. Durch Auswertung von 243 Absorptionslinien und 65 Doppelresonanzsignalen konnten die Molekülparameter dieser Niveaus ermittelt werden. Weiterhin konnten drei lokale Störungen zwischen Schwingungsniveaus der Zustände B2Σ und A2Π3/2 beobachtet werden. Im Fall der Störung B2Σ,v = 1 - A2Π3/2,v = 14 gelang eine quantitative Analyse. Darin wurden Molekülkonstanten des Niveaus A2Π3/2,v = 14 und der Störparameter B14,1 berechnet. Für die ersten angeregten Zustände B2Σ und A2Π wurden in einem quantenmechanischen Verfahren interatomare Wechselwirkungspotentiale berechnet. Dazu wurden die Parameter geeigneter Potentialfunktionen durch numerisches Lösen der Schrödingergleichung an die experimentell bestimmten Schwingungsniveaus angepaßt. Für den B2Σ-Zustand, dessen Potential hier aus den nun vorliegenden Daten zu den Schwingungsniveaus v = 0...6 erstmals ermittelt wurde, geschah dies zunächst in einer Einzelanpassung. Darüberhinaus wurde unter Hinzunahme der experimentellen Daten zu den Schwingungsniveaus v = 6...11 der Zustände A2Π1/2 und A2Π3/2 eine globale Berechnung unter Verwendung einer adiabatischen Näherung vorgenommen, die die Wechselwirkung der Zustände A2Π1/2 und B2Σ durch den Spin-Bahn-Operator berücksichtigt. Mit dieser Anpassung gelang neben der Berechnung der adiabatischen Potentiale für B2Σ, A2Π1/2 und A2Π3/2 auch eine Bestimmung der Spin-Bahn-Funktion A(R). Für den A2Π1/2-Zustand konnte im Bereich der Störung ein gegenüber früheren Ergebnissen verbessertes Potential ermittelt werden. Die Resultate für die Gleichgewichtsparameter der bestimmten adiabatischen Potentiale sind: A2Π1/2 A2 Π3/2 B2Σ Re in Å 3.30(2) 3.31(2) 6,99(2) De in cm-1 424(3) 438(3) 26.06(4) </center

A non-invasive online photoionization spectrometer for FLASH2

The stochastic nature of the self-amplified spontaneous emission (SASE) process of free-electron lasers (FELs) effects pulse-to-pulse fluctuations of the radiation properties, such as the photon energy, which are determinative for processes of photon-matter interactions. Hence, SASE FEL sources pose a great challenge for scientific investigations, since experimenters need to obtain precise real-time feedback of these properties for each individual photon bunch for interpretation of the experimental data. Furthermore, any device developed to deliver the according information should not significantly interfere with or degrade the FEL beam. Regarding the spectral properties, a device for online monitoring of FEL wavelengths has been developed for FLASH2, which is based on photoionization of gaseous targets and the measurements of the corresponding electron and ion time-of-flight spectra. This paper presents experimental studies and cross-calibration measurements demonstrating the viability of this online photoionization spectrometer

Towards Tip Vortex Measurements on Rotors in the High Pressure Wind Tunnel Göttingen (HDG) of the DLR

The wake of a helicopter rotor features vortices originating from the blade tips. The vortices are relevant for the rotor noise and rotor performance, hence, the analysis of their propagation, growth and decay is of particular interest. It is widely believed that the vortex Reynolds number, defined as the ratio of the vortex circulation to the fluid viscosity, is an important factor in this process. Being related to turbulence and instability mechanisms, the Reynolds number effect on the vortex evolution is difficult to capture in numerical simulations. Therefore, experiments with a variation of the Reynolds number are required. Atmospheric subscale test facilities cannot reproduce the Reynolds numbers encountered on full-size helicopter rotors due to size limitations, whereas field measurements on free-flying helicopters are affected by the engine exhaust flow, wind gusts, blockage of the fuselage, tail rotor, etc. Furthermore, neither approach enables a variation of the Reynolds number independently of the rotational speed and the rotor dimensions. Therefore, we chose DLR’s High Pressure Wind Tunnel Göttingen (HDG) for measurements with a varying Reynolds number. The current work discusses the experimental methodology and sample results from a pretest

Non-invasive online wavelength measurements at FLASH2 and present benchmark

At FLASH2, the free-electron laser radiation wavelength is routinely measured by an online spectrometer based on photoionization of gas targets. Photoelectrons are detected with time-of-flight spectrometers and the wavelength is determined by means of well known binding energies of the target species. The wavelength measurement is non-invasive and transparent with respect to running user experiments due to the low gas pressure applied. Sophisticated controls for setting the OPIS operation parameters have been created and integrated into the distributed object-oriented control system at FLASH2. Raw and processed data can be stored on request in the FLASH data acquisition system for later correlation with data from user experiments or re-analysis. In this paper, the commissioning of the instrument at FLASH2 and the challenges of space charge effects on wavelength determination are reported. Furthermore, strategies for fast data reduction and online data processing are presented