We report on the first X-ray absorption fine structure (XAFS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy are well established methods for retrieving structural information about the composition of solid state materials and soft matter. The water window spectral range between 284 eV and 543 eV is of special interest as it contains the K-shell absorption edges of the biological building blocks: carbon (284 eV), nitrogen (410 eV) and oxygen (543 eV). Up until recently only facility scale light sources have been capable of generating coherent water window radiation: synchrotrons with a high degree of spatial coherence and hundreds of femtoseconds pulse durations, and X-ray free electron lasers with a high degree of spatial coherence and femtosecond temporal resolution [1]. High harmonic generation (HHG) [2, 3] offers an attractive alternative approach since it is realizable on a small table-top scale and is capable of generating fully coherent radiation, i.e. femto-to atto-second and possibly even zeptosecond pulse durations. The ability to generate coherent water window radiation from HHG is extremely exciting as it would bring ultra-short time resolution to structural probing with a table top method. HHG is most commonly driven by Ti:sapphire sources at 800 nm with the highest achievable photon energy, the so called cutoff, scaling linearly with the laser intensity and quadratically with the driving wavelength [4]. While the water window range is reachable with such sources via nonphase-matched HHG [5], the contradicting requirements of increasing the cutoff with higher laser intensity while avoiding excessive ionisation, severely limits the achievable flux in the water window. A solution to this dilemma is to use a source with a similar peak intensity and pulse duration, but at much longer emission wavelengths in order to exploit the quadratic wavelength scaling of the HHG cutoff. A drawback of such an approach is the unfavourable single atom response scaling of harmonic yield with λ −9 [6] which can however be mitigated, to a large extent, through high gaspressure phase matching [7]. This concept was demonstrated by reaching a 1.6 keV cutoff when driving with a mid-IR laser system [8]. Despite this cutting-edge result, the 20 Hz repetition rate and stability of the system have thus far proved insufficient for applications, thereby underlining the need for significant improvements of the laser parameters.We find that while high X-ray flux can be achieved through phase-matched HHG driven by kHz or higher repetition rate long-wavelength sources, achieving sufficient intensity and carrier to envelope phase (CEP) stability of the driver laser is an essential key both for producing attosecond pulses and for generating reproducible X-ray spectra from each laser pulse and throughhout an X-ray measurement.Currently at the kHz level and with long wavelength drivers, the lower end of the water window at 300 eV was reached using a Ti:sapphire pumped optical parametric amplifier (OPA) at 1.5 μm [9]...

We report on the first streaking measurement of water-window attosecond pulses generated via high harmonic generation, driven by sub-2-cycle, CEP-stable, 1850 nm laser pulses. Both the central photon energy and the energy bandwidth far exceed what has been demonstrated thus far, warranting the investigation of the attosecond streaking technique for the soft X-ray regime and the limits of the FROGCRAB retrieval algorithm under such conditions. We also discuss the problem of attochirp compensation and issues regarding much lower photo-ionization cross sections compared with the XUV in addition to the fact that several shells of target gases are accessed simultaneously. Based on our investigation, we caution that the vastly different conditions in the soft X-ray regime warrant a diligent examination of the fidelity of the measurement and the retrieval procedure.

Recent developments in attosecond technology led to table-top x-ray spectroscopy in the soft x-ray range, thus uniting the element-and statespecificity of core-level x-ray absorption spectroscopy with the time resolution to follow electronic dynamics in real-time. We describe recent work in attosecond technology and investigations into materials such as Si, SiO 2 , GaN, Al 2 O 3 , Ti, and TiO 2 , enabled by the convergence of these two capabilities. We showcase the state-of-the-art on isolated attosecond soft x-ray pulses for x-ray absorption near-edge spectroscopy to observe the 3d-state dynamics of the semi-metal TiS 2 with attosecond resolution at the Ti L-edge (460 eV). We describe how the element-and state-specificity at the transition metal L-edge of the quantum material allows us to unambiguously identify how and where the optical field influences charge carriers. This precision elucidates that the Ti:3d conduction band states are efficiently photo-doped to a density of 1.9 Â 10 21 cm À3 . The light-field induces coherent motion of intra-band carriers across 38% of the first Brillouin zone. Lastly, we describe the prospects with such unambiguous real-time observation of carrier dynamics in specific bonding or anti-bonding states and speculate that such capability will bring unprecedented opportunities toward an engineered approach for designer materials with pre-defined properties and efficiency. Examples are composites of semiconductors and insulators like Si, Ge, SiO 2 , GaN, BN, and quantum materials like graphene, transition metal dichalcogens, or high-Tc superconductors like NbN or LaBaCuO. Exiting are prospects to scrutinize canonical questions in multi-body physics, such as whether the electrons or lattice trigger phase transitions.

Phase transitions of solids and structural transformations of molecules are canonical examples of important photo-induced processes, whose underlying mechanisms largely elude our comprehension due to our inability to correlate electronic excitation with atomic position in real time. Here, we present a decisive step towards such new methodology based on water-windowcovering (284 eV to 543 eV) attosecond soft X-ray pulses that can simultaneously access electronic and lattice parameters via dispersive X-Ray absorption fine-structure (XAFS) spectroscopy. We validate attoXAFS with an identification of the σ * and π * orbital contributions to the density of states in graphite simultaneously with its lattice's four characteristic bonding distances. This work demonstrates the concept of attoXAFS as a powerful real-time investigative tool which is equally applicable to gas-, liquid-and condensed phase.X-ray absorption fine-structure (XAFS) spectroscopy is a powerful element-specific technique, providing electronic as well as structural, and chemical information with atomic resolution 1-3 . In XAFS, electronic information is extracted from the near-edge XAFS (XANES or NEXAFS), which arises

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