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...

Non-collinear four-wave mixing (FWM) techniques at near-infrared (NIR), visible, and ultraviolet frequencies have been widely used to map vibrational and electronic couplings, typically in complex molecules. However, correlations between spatially localized inner-valence transitions among different sites of a molecule in the extreme ultraviolet (XUV) spectral range have not been observed yet. As an experimental step towards this goal we perform time-resolved FWM spectroscopy with femtosecond NIR and attosecond XUV pulses. The first two pulses (XUV-NIR) coincide in time and act as coherent excitation fields, while the third pulse (NIR) acts as a probe. As a first application we show how coupling dynamics between odd-and even-parity inner-valence excited states of neon can be revealed using a two-dimensional spectral representation. Experimentally obtained results are found to be in good agreement with ab initio time-dependent R-matrix calculations providing the full description of multi-electron interactions, as well as few-level model simulations. Future applications of this method also include site-specific probing of electronic processes in molecules.

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...

The reconstruction of the full temporal dipole response of a strongly driven time-dependent system from a single absorption spectrum is demonstrated, only requiring that a sufficiently short pulse is employed to initialize the coherent excitation of the system. We apply this finding to the time-domain observation of Rabi cycling between doubly excited atomic states in the few-femtosecond regime. This allows us to pinpoint the breakdown of few-level quantum dynamics at the critical laser intensity near 2 TW=cm 2 in doubly excited helium. The present approach unlocks single-shot real-time-resolved signal reconstruction across timescales down to attoseconds for nonequilibrium states of matter. In contrast to conventional pump-probe schemes, there is no need for scanning time delays in order to access real-time information. The potential future applications of this technique range from testing fundamental quantum dynamics in strong fields to measuring and controlling ultrafast chemical and biological reaction processes when applied to traditional transient-absorption spectroscopy.

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