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Molecular contamination due to outgassing of organic materials in vacuum can be a source of significant degradation for space equipment, especially for sensitive optical instruments. Thus, on-ground contamination measurements in vacuum are essential to ensure the flight hardware cleanliness and for making in-flight performance predictions. A suitable detection technique shall work in-situ and in real-time, i.e. it shall be possible to measure the real-time build-up of contamination layers of a few nanometers and less on surfaces in vacuum in the presence of an outgassing source. In this paper we address a new experimental setup for controlled contamination and high-sensitivity spectroscopic analysis of the contaminants on representative material for typical space optics. The central method of choice for the detection of organic contaminants incorporated in this setup is laser-induced fluorescence (LIF), which provides the demanded distinguishability of the applied contamination species. Different excitation wavelengths in the UV spectral range are used. LIF measurements are accompanied by in operando verification of amount and species of the contaminants by a thermoelectric quartz crystal microbalance (TQCM) and a mass spectrometer (MS). This parameter study aims for exploring the laser parameter dependent detection limits and the temperature-dependent condensation process with respect to various substrate-contaminant combinations, in order to reveal strategies to prevent contamination for space applications.

A new implementation for XAS measurements based on zone plates is presented. With a two-color scheme, several fluorescence yields can be probed simultaneously., X-ray absorption spectroscopy (XAS) is a powerful element-specific technique that allows the study of structural and chemical properties of matter. Often an indirect method is used to access the X-ray absorption (XA). This work demonstrates a new XAS implementation that is based on off-axis transmission Fresnel zone plates to obtain the XA spectrum of La0.6Sr0.4MnO3 by analysis of three emission lines simultaneously at the detector, namely the O 2p–1s, Mn 3s–2p and Mn 3d–2p transitions. This scheme allows the simultaneous measurement of an integrated total fluorescence yield and the partial fluorescence yields (PFY) of the Mn 3s–2p and Mn 3d–2p transitions when scanning the Mn L-edge. In addition to this, the reduction in O fluorescence provides another measure for absorption often referred to as the inverse partial fluorescence yield (IPFY). Among these different methods to measure XA, the Mn 3s PFY and IPFY deviate the least from the true XA spectra due to the negligible influence of selection rules on the decay channel. Other advantages of this new scheme are the potential to strongly increase the efficiency and throughput compared with similar measurements using conventional gratings and to increase the signal-to-noise of the XA spectra as compared with a photodiode. The ability to record undistorted bulk XA spectra at high flux is crucial for future in situ spectroscopy experiments on complex materials.

The unique properties of free-electron lasers (FELs) allow for the systematic study and precise characterization of electronic and structural chemical and biochemical reactions in space and time—from minutes to femtosecond time resolution. Rationalizing the ultra-fast behaviour in the chemical reactions of unimolecular reactions (reactions of one type of molecule), allows entirely new types of opto-electronic devices (such as solar cells or organic light emitting diodes) to be constructed. In bimolecular reactions (with two types of molecules), FELs allow for entirely new types of electronic and structural characterization schemes.

Understanding and controlling the relaxation process of optically excited charge carriers in solids with strong correlations is of great interest in the quest for new strategies to exploit solar energy. Usually, optically excited electrons in a solid thermalize rapidly on a femtosecond to picosecond timescale due to interactions with other electrons and phonons. New mechanisms to slow down thermalization will thus be of great significance for efficient light energy conversion, e.g., in photovoltaic devices. Ultrafast optical pump–probe experiments in the manganite Pr$_{0.65}$Ca$_{0.35}$MnO$_{3}$, a photovoltaic, thermoelectric, and electrocatalytic material with strong polaronic correlations, reveal an ultraslow recombination dynamics on a nanosecond-time scale. The nature of long living excitations is further elucidated by photovoltaic measurements, showing the presence of photodiffusion of excited electron–hole polaron pairs. Theoretical considerations suggest that the excited charge carriers are trapped in a hot polaron state. Escape from this state is possible via a slow dipole-forbidden recombination process or via rare thermal fluctuations toward a conical intersection followed by a radiation-less decay. The strong correlation between the excited polaron and the octahedral dynamics of its environment appears to be substantial for stabilizing the hot polaron.

Scientific reports 7(1), 8849(2017). doi:10.1038/s41598-017-09052-0, We have implemented and successfully tested an off-axis transmission Fresnel zone plate as spectral analyzer for resonant inelastic X-ray scattering (RIXS). The imaging capabilities of zone plates allow for advanced two-dimensional (2D) mapping applications. By varying the photon energy along a line focus on the sample, we were able to simultaneously record the emission spectra over a range of excitation energies. Moreover, by scanning a line focus across the sample in one dimension, we efficiently recorded RIXS spectra spatially resolved in 2D, increasing the throughput by two orders of magnitude. The presented scheme opens up a variety of novel measurements and efficient, ultra-fast time resolved investigations at X-ray Free-Electron Laser sources., Published by Macmillan, London

X-ray spectroscopy is a powerful tool to study the local charge distribution of chemical systems. Together with the liquid jet it becomes possible to probe chemical systems in their natural environment, the liquid phase. In this work, we present X-ray absorption (XA), X-ray emission (XE) and resonant inelastic X-ray scattering (RIXS) data of pure water and various salt solutions and show the possibilities these methods offer to elucidate the nature of ion-water interaction.

Femtosecond optical pump−probe spectrosco- py has been employed for studying the directly linked electron donor−acceptor system pyrene-N,N-dimethylaniline (PyDMA) in solid state. This DMA-pyrene derivative discussed is being applied as a molecular diode system switching on an ultrafast time scale. Our ultrafast solid-state studies reveal a complex photochemistry of this molecular crystal system. Strong couplings of the optically induced charge-transfer state with the radical ion pair state allow a femtosecond transition of the latter. One could see on the highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) description that a pure optical transition switches the system from a conducting to a blocked system because the molecular orbitals (MOs) of DMA moiety lie in a node plane of the LUMO. Within 800 fs the system relaxes back to the ground state and/or forms a radical ion pair, which is the surprising result of our study; when the system was probed further, the system underwent vibrational cooling and enhanced population inversion of the radical ion pair.

SPIE Optical Engineering + Applications, San Diego, California; Proceedings of SPIE 8849, 8809I (2013). doi:10.1117/12.2023992, Published by SPIE, Bellingham, Wash.

Microscopy and microanalysis 24(S2), 182 - 183 (2018). doi:10.1017/S1431927618013260, Published by Cambridge University Press, New York, NY