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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

X-ray free-electron lasers provide unique opportunities for exploring ultrafast dynamics and for imaging the structures of complex systems. Understanding the response of individual atoms to intense X-rays is essential for most free-electron laser applications. First experiments have shown that, for light atoms, the dominant interaction mechanism is ionization by sequential electron ejection, where the highest charge state produced is defined by the last ionic state that can be ionized with one photon. Here, we report an unprecedentedly high degree of ionization of xenon atoms by 1.5 keV free-electron laser pulses to charge states with ionization energies far exceeding the photon energy. Comparing ion charge-state distributions and fluorescence spectra with state-of-the-art calculations, we find that these surprisingly high charge states are created via excitation of transient resonances in highly charged ions, and predict resonance enhanced absorption to be a general phenomenon in the interaction of intense X-rays with systems containing high-Z constituents. Researchers create high ionization states, up to Xe36+, using 1.5 keV free-electron laser pulses. The higher than expected ionization may be due to transient resonance-enhanced absorption and the effect may play an important role in interactions of intense X-rays with high-Z elements and radiation damage.

Laser-induced electron diffraction is an evolving tabletop method, which aims to image ultrafast structural changes in gas-phase polyatomic molecules with sub-{\AA}ngstr\"om spatial and femtosecond temporal resolution. Here, we provide the general foundation for the retrieval of multiple bond lengths from a polyatomic molecule by simultaneously measuring the C-C and C-H bond lengths in aligned acetylene. Our approach takes the method beyond the hitherto achieved imaging of simple diatomic molecules and is based upon the combination of a 160 kHz mid-IR few-cycle laser source with full three-dimensional electron-ion coincidence detection. Our technique provides an accessible and robust route towards imaging ultrafast processes in complex gas phase molecules with atto- to femto-second temporal resolution., Comment: 16 pages, 4 figures