The angle-resolved inner-shell photoionization of R-trifluoromethyloxirane, C3H3F3O, is studied experimentally and theoretically. Thereby, we investigate the photoelectron circular dichroism (PECD) for nearly-symmetric O 1s and F 1s electronic orbitals, which are localized on different molecular sites. The respective dichroic β1 and angular distribution β2 parameters are measured at the photoelectron kinetic energies from 1 to 16 eV by using variably polarized synchrotron radiation and velocity map imaging spectroscopy. The present experimental results are in good agreement with the outcome of ab initio electronic structure calculations. We report a sizable chiral asymmetry β1 of up to about 9% for the K-shell photoionization of oxygen atom. For the individual fluorine atoms, the present calculations predict asymmetries of similar size. However, being averaged over all fluorine atoms, it drops down to about 2%, as also observed in the present experiment. Our study demonstrates a strong emitter-and site-sensitivity of PECD in the one-photon inner-shell ionization of this chiral molecule.

We report on the experimental observation of strong-field dressing of an autoionizing two-electron state in helium with intense extreme-ultraviolet laser pulses from a freeelectron laser. The asymmetric Fano line shape of this transition is spectrally resolved, and we observe modifications of the resonance asymmetry structure for increasing free-electron-laser pulse energy on the order of few tens of µJ. A quantum-mechanical calculation of the time-dependent dipole response of this autoionizing state, driven by classical extreme-ultraviolet (XUV) electric fields, reveals a direct link between strongfield-induced energy and phase shifts of the doubly excited state and the Fano line-shape asymmetry. The experimental results obtained at the Free-Electron Laser in Hamburg (FLASH) thus correspond to transient energy shifts on the order of few meV, induced by strong XUV fields. These results open up a new way of performing non-perturbative XUV nonlinear optics for the light-matter interaction of resonant electronic transitions in atoms at short wavelengths.Quantum mechanics provides a consistent description of the structure and dynamics of atoms, the constituents of our macroscopic world. In particular, it describes how bound excited states in atoms are formed through the Coulomb interaction of the positively charged nucleus and the negatively charged electrons. With the obvious exception of the ground state, such states possess a finite lifetime, with singly excited states decaying through photon emission via the interaction with the radiation field. For two-electron excitations of neutral atoms, the Coulomb interaction between the electrons is much more effective such that at least one electron will eventually be ionized, which typically marks the leading contribution to the decay of the excited state for the case of light atoms. Thus ionization is a fundamentally important and very basic effect that accompanies the physics of multi-electron excitations in atoms [1]. An interesting situation arises if the interaction of such states with the radiation field is significantly increased which nowadays can be achieved by using extreme ultraviolet (XUV) or x-ray light sources. In addition, the properties of these radiation fields can often be well controlled, thus providing a unique toolbox for exploring the dynamics of excited states, e.g., by performing time-resolved investigations with lab-based attosecond high-order harmonic generation (HHG) sources [2,3], or facility-based femtosecond XUV/x-ray freeelectron lasers (FELs) [4,5]. The latter deliver particularly high intensities for XUV/x-ray nonlinear optics [6] with ultrafast time resolution and site-specific core-level access [7], and nowadays even approach the attosecond regime [8].The helium atom consists of two electrons bound to a nucleus, representing the ideal case of a Coulombic three-body system, which serves as a benchmark for developing a theoretical description [1,9,10] and most importantly also for controlling the dynamics of two bound electrons with stron...

We demonstrate time-resolved nonlinear extreme-ultraviolet absorption spectroscopy on multiply charged ions, here applied to the doubly charged neon ion, driven by a phase-locked sequence of two intense free-electron laser pulses. Absorption signatures of resonance lines due to 2p-3d boundbound transitions between the spin-orbit multiplets 3 P0,1,2 and 3 D1,2,3 of the transiently produced doubly charged Ne 2+ ion are revealed, with time-dependent spectral changes over a time-delay range of (2.4 ± 0.3) fs. Furthermore, we observe 10-meV-scale spectral shifts of these resonances owing to the AC Stark effect. We use a time-dependent quantum model to explain the observations by an enhanced coupling of the ionic quantum states with the partially coherent free-electron-laser radiation when the phase-locked pump and probe pulses precisely overlap in time. PACS numbers: .In interaction with matter the oscillating electric field of a laser not only induces transitions between bound electronic states but also affects the states and transitions themselves. It splits [1], shifts [2, 3] and modifies the width [4, 5] and the shape [6-8] of spectral transition lines depending on the amount of detuning out of resonance with the laser frequency and the field strength. Only for sufficiently high field strengths, at which more than one photon can interact with the quantum system on its intrinsic time and energy scale, these phenomena are accessible. Modern ultrafast lasers are effective driver and control tools for nonlinear effects at visible frequencies and have become the "working horses" for nonlinear coherent spectroscopies [9], in time domain and frequency domain, including the quantum control of boundbound electronic transitions (see, e.g., [10] and references therein).Since the advent of short-wavelength free-electron lasers (FELs) [11,12] the field of nonlinear spectroscopy is being extended into the extreme-ultraviolet (XUV) and x-ray spectral ranges [13][14][15][16][17][18][19][20][21][22][23][24]. One advantage of employing x-rays is the ability to access bound-bound electronic transitions associated with the spatially localized innerelectronic shell and the potential to probe site-specific spectroscopic information of a sample. Since experimental studies on the impact of XUV/x-ray nonlinear effects on inner-shell-excited resonances are often ham-pered by the extremely short Auger decay times, yet, such research is rare. Nonetheless, first x-ray nonlinear line-shape modifications of inner-shell transitions have been studied experimentally [16] by employing Augerelectron spectroscopy. By contrast, we here address the valence electrons of the doubly charged neon ion, Ne 2+ , and manipulate-in the absence of any competing ultrafast decay channel-the ground state to excited state transitions between spin-orbit multiplets with intense XUV-FEL radiation. Being sensitive to the atomic/ionic dipole response and associated spectral line-shape modifications, our work demonstrates a direct view on XUV nonlinear effects occurring i...

Short-wavelength free-electron lasers with their ultrashort pulses at high intensities have originated new approaches for tracking molecular dynamics from the vista of specific sites. X-ray pump X-ray probe schemes even allow to address individual atomic constituents with a ‘trigger’-event that preludes the subsequent molecular dynamics while being able to selectively probe the evolving structure with a time-delayed second X-ray pulse. Here, we use a linearly polarized X-ray photon to trigger the photolysis of a prototypical chiral molecule, namely trifluoromethyloxirane (C3H3F3O), at the fluorine K-edge at around 700 eV. The created fluorine-containing fragments are then probed by a second, circularly polarized X-ray pulse of higher photon energy in order to investigate the chemically shifted inner-shell electrons of the ionic mother-fragment for their stereochemical sensitivity. We experimentally demonstrate and theoretically support how two-color X-ray pump X-ray probe experiments with polarization control enable XFELs as tools for chiral recognition.

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The Fano absorption line shape of an autoionizing state encodes information on its internal atomic structure and dynamics. When driven near-resonantly with intense extreme ultraviolet (XUV) electric fields, the absorption profile can be deliberately modified, including observable changes of both the line-shape asymmetry and strength of the resonance, revealing information on the underlying dynamics of the system in response to such external driving. We report on the influence of the XUV pulse parameters at high intensity that can be achieved with a free-electron laser (FEL) with statistically broadened spectra based on self-amplified spontaneous emission (SASE). More specifically, the impact of the FEL pulse duration is studied for the example of the doubly excited 2s2p resonance in helium, where line-shape modifications have been measured with XUV transient absorption spectroscopy in Fraunhofer-type transmission geometry. A computational few-level-model provides insight into the impact of different average pulse durations of the stochastic FEL pulses. These findings are supported by measurements performed at the Free-Electron Laser in Hamburg (FLASH) and provide further insight into XUV strong-coupling dynamics of resonant transitions driven by intense high-frequency FEL sources.

High-intensity ultrashort pulses at extreme ultraviolet (XUV) and x-ray photon energies, delivered by state-of-the-art free-electron lasers (FELs), are revolutionizing the field of ultrafast spectroscopy. For crossing the next frontiers of research, precise, reliable and practical photonic tools for the spectro-temporal characterization of the pulses are becoming steadily more important. Here, we experimentally demonstrate a technique for the direct measurement of the frequency chirp of extreme-ultraviolet free-electron laser pulses based on fundamental nonlinear optics. It is implemented in XUV-only pump-probe transient-absorption geometry and provides in-situ information on the time-energy structure of FEL pulses. Using a rate-equation model for the time-dependent absorbance changes of an ionized neon target, we show how the frequency chirp can be directly extracted and quantified from measured data. Since the method does not rely on an additional external field, we expect a widespread implementation at FELs benefiting multiple science fields by in-situ on-target measurement and optimization of FEL-pulse properties.

We report on numerical results revealing line-shape asymmetry changes of electronic transitions in atoms near-resonantly driven by intense extreme-ultraviolet (XUV) electric fields by monitoring their transient absorption spectrum after transmission through a moderately dense atomic medium. Our numerical model utilizes ultrashort broadband XUV laser pulses varied in their intensity (1014–1015 W/cm2) and detuning nearly out of resonance for a quantitative evaluation of the absorption line-shape asymmetry. It will be shown how transient energy shifts of the bound electronic states can be linked to these asymmetry changes in the case of an ultrashort XUV driving pulse temporally shorter than the lifetime of the resonant excitation, and how the asymmetry can be controlled by the near-resonant detuning of the XUV pulse. In the case of a two-level system, the numerical model is compared to an analytical calculation, which helps to uncover the underlying mechanism for the detuning- and intensity-induced line-shape modification and links it to the generalized Rabi frequency. To further apply the numerical model to recent experimental results of the near-resonant dressing of the 2s2p doubly excited state in helium by an ultrashort XUV free-electron laser pulse we extend the two-level model with an ionization continuum, thereby enabling the description of transmission-type (Fraunhofer-like) transient absorption of a strongly laser-coupled autoionizing state.