Differential Measurement of Electron Ejection after Two-Photon Two-Electron Excitation of Helium

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

XUV pump-XUV probe transient absorption spectroscopy at FELs

The emergence of ultra-intense extreme-ultraviolet (XUV) and X-ray free-electron lasers (FELs) has opened the door for the experimental realization of non-linear XUV and X-ray spectroscopy techniques. Here we demonstrate an experimental setup for an all-XUV transient absorption spectroscopy method for gas-phase targets at the FEL. The setup combines a high spectral resolving power of E/ΔE ≈ 1500 with sub-femtosecond interferometric resolution, and covers a broad XUV photon-energy range between approximately 20 and 110 eV. We demonstrate the feasibility of this setup firstly on a neon target. Here, we intensity- and time-resolve key aspects of non-linear XUV-FEL light-matter interactions, namely the non-resonant ionization dynamics and resonant coupling dynamics of bound states, including XUV-induced Stark shifts of energy levels. Secondly, we show that this setup is capable of tracking the XUV-initiated dissociation dynamics of small molecular targets (oxygen and diiodomethane) with site-specific resolution, by measuring the XUV transient absorption spectrum. In general, benefitting from a single-shot detection capability, we show that the setup and method provides single-shot phase-locked XUV pulse pairs. This lays the foundation to perform, in the future, experiments as a function of the XUV interferometric time delay and the relative phase, which enables advanced coherent non-linear spectroscopy schemes in the XUV and X-ray spectral range.ISSN:1359-6640ISSN:1364-549

Strong-Field Extreme-Ultraviolet Dressing of Atomic Double Excitation

We report on the experimental observation of a strong-field dressing of an autoionizing two-electronstate in helium with intense extreme-ultraviolet laser pulses from a free-electron laser. The asymmetricFano line shape of this transition is spectrally resolved, and we observe modifications of the resonanceasymmetry structure for increasing free-electron-laser pulse energy on the order of few tens of Microjoules.A quantum-mechanical calculation of the time-dependent dipole response of this autoionizing state, drivenby classical extreme-ultraviolet (XUV) electric fields, evidences strong-field-induced energy and phaseshifts of the doubly excited state, which are extracted from the Fano line-shape asymmetry. Theexperimental results obtained at the Free-Electron Laser in Hamburg (FLASH) thus correspond to transientenergy shifts on the order of a few meV, induced by strong XUV fields. These results open up a new way ofperforming nonperturbative XUV nonlinear optics for the light-matter interaction of resonant electronictransitions in atoms at short wavelengths