Most large molecules are chiral in their structure: they exist as two enantiomers, which are mirror images of each other. Whereas the rovibronic sublevels of two enantiomers are almost identical (neglecting a minuscular effect of the weak interaction), it turns out that the photoelectric effect is sensitive to the absolute configuration of the ionized enantiomer. Indeed, photoionization of randomly oriented enantiomers by left or right circularly polarized light results in a slightly different electron flux parallel or antiparallel with respect to the photon propagation direction-an effect termed photoelectron circular dichroism (PECD). Our comprehensive study demonstrates that the origin of PECD can be found in the molecular frame electron emission pattern connecting PECD to other fundamental photophysical effects such as the circular dichroism in angular distributions (CDAD). Accordingly, distinct spatial orientations of a chiral molecule enhance the PECD by a factor of about 10.

Atoms and molecules attached to rare gas clusters are ionized by an interatomic autoionization process traditionally termed 'Penning ionization' when the host cluster is resonantly excited. Here we analyze this process in the light of the interatomic Coulombic decay (ICD) mechanism, which usually contains a contribution from charge 1 arXiv:1910.06230v1 [physics.atm-clus] 14 Oct 2019 exchange at short interatomic distance, and one from virtual photon transfer at large interatomic distance. For helium (He) nanodroplets doped with alkali metal atoms (Li, Rb), we show that long-range and short-range contributions to the interatomic autoionization can be clearly distinguished by detecting electrons and ions in coincidence.Surprisingly, ab initio calculations show that even for alkali metal atoms floating in dimples at large distance from the nanodroplet surface, autoionization is largely dominated by charge exchange ICD. Furthermore, the measured electron spectra manifest ultrafast internal relaxation of the droplet into mainly the 1s2s 1 S state and partially into the metastable 1s2s 3 S state.Interatomic decay processes have recently been found to play a crucial role in the interaction of biological matter with energetic radiation. Both free radicals and low-energy electrons produced by ICD processes can induce irreparable damage of the genome (double strand breaks in DNA) causing cancer or cell death. 1-3 Upon electronic excitation, weakly bound systems such as van der Waals or hydrogen bonded complexes and clusters can relax by interatomic autoionization if the excited state energy exceeds their adiabatic ionization energy. In the case of rare gas clusters doped with atomic or molecular impurities, this process has traditionally been termed Penning ionization, 4-10 in analogy to the collisional autoionization occurring in crossed atomic beams involving excited atoms, mostly rare gases prepared in metastable excited states. 11 This process is mainly driven by charge exchange between two interacting atoms or molecules which come so close to one another that their valence orbitals overlap. However, already in the early days of systematic Penning ionization studies, it was realized that the autoionization rate contains a second contribution describing energy transfer in the form of a virtual photon exchange. 12Since the seminal work by L. Cederbaum in 1997, such non-local autoionization processes involving two or more atomic or molecular centers have been formulated in the theoretical framework termed interatomic/intermolecular Coulombic decay (ICD). 13 This approach mainly refers to the autoionization of weakly bound systems that are inner-shell excited by

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Alkali metal dimers attached to the surface of helium nanodroplets are found to be efficiently doubly ionized by electron transfer-mediated decay (ETMD) when photoionizing the helium droplets. This process is evidenced by detecting in coincidence two energetic ions created by Coulomb explosion and one low-kinetic energy electron. The kinetic energy spectra of ions and electrons are reproduced by simple model calculations based on diatomic potential energy curves, and are in agreement with ab initio calculations for the HeNa 2 and HeKRb systems. This work demonstrates that ETMD is an important decay channel in heterogeneous nanosystems exposed to ionizing radiation.

We report Lyman series emission cross sections of neutral hydrogen dissociation fragments after valence (15-34 eV) and inner-shell (533-542 eV) excitation of water vapor with monochromatic synchrotron radiation as functions of the exciting-photon energy. In the valence excitation energy region the thermodynamical limits of the production of the differently excited hydrogen fragments is directly observed and absolute emission cross sections were determined. For resonant innershell excitations, the fluorescing excited hydrogen state is found to be strongly dependent on the molecular or Rydberg-like character of the excitation.

Resonant interatomic Coulombic decay (RICD) in inner-valence excited neon clusters is observed by a combination of vacuumultraviolet (VUV) and UV/visible fluorescence spectroscopy. These ultrafast interatomic electronic processes efficiently quench radiation emission from inner-valence excited clusters. After RICD took place, outer-valence excited clusters relax further by emission of fluorescence. The direct correspondence of the structures observed in the VUV and UV/visible fluorescence signals implies that the final states of the spectator RICD decay by a cascade of radiative decays: First, by the Rydberg-to-Rydberg transitions in the UV/visible spectral range, and then, by the Rydberg-to-valence transition in the VUV range. Our study demonstrates a possibility of detecting interatomic electronic processes by UV/visible fluorescence spectroscopy.

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We report on the status of a users’ end-station, MAC: a Multipurpose station for Atomic, molecular and optical sciences and Coherent diffractive imaging, designed for studies of structure and dynamics of matter in the femtosecond time-domain. MAC is located in the E1 experimental hall on the high harmonic generation (HHG) beamline of the ELI Beamlines facility. The extreme ultraviolet beam from the HHG beamline can be used at the MAC end-station together with a synchronized pump beam (which will cover the NIR/Vis/UV or THz range) for time-resolved experiments on different samples. Sample delivery systems at the MAC end-station include a molecular beam, a source for pure or doped clusters, ultrathin cylindrical or flat liquid jets, and focused beams of substrate-free nanoparticles produced by an electrospray or a gas dynamic virtual nozzle combined with an aerodynamic lens stack. We further present the available detectors: electron/ion time-of-flight and velocity map imaging spectrometers and an X-ray camera, and discuss future upgrades: a magnetic bottle electron spectrometer, production of doped nanodroplets and the planned developments of beam capabilities at the MAC end-station.