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Physical review / X 11(3), 031001 (1-9) (2021). doi:10.1103/PhysRevX.11.031001, In this work, we use an extreme-ultraviolet (XUV) free-electron laser (FEL) to resonantly excite the I: 4$d_{5/2}–σ^∗$ transition of a gas-phase di-iodomethane (CH$_2$I$_2$) target. This site-specific excitation generates a 4$d$ core hole located at an iodine site, which leaves the molecule in a well-defined excited state. We subsequently measure the time-dependent absorption change of the molecule with the FEL probe spectrum centered on the same I: 4$_d$ resonance. Using ab initio calculations of absorption spectra of a transient isomerization pathway observed in earlier studies, our time-resolved measurements allow us to assign the timescales of the previously reported direct and indirect dissociation pathways. The presented method is thus sensitive to excited-state molecular geometries in a time-resolved manner, following a core-resonant site-specific trigger., Published by APS, College Park, Md.

We present a technique for the direct measurement of the frequency chirp of high-frequency free-electron-laser pulses based on fundamental nonlinear optics. It is implemented in transient absorption geometry.

Measuring bound-state quantum dynamics, excited and driven by strong fields, is achievable by time-resolved absorption spectroscopy. Here, a vacuum beamline for spectroscopy in the attosecond temporal and extreme ultraviolet (XUV) spectral range is presented, which is a tool for observing and controlling nonequilibrium electron dynamics. In particular, we introduce a technique to record an XUV absorption signal and the corresponding reference simultaneously, which greatly improves the signal quality. The apparatus is based on a common beam path design for XUV and near-infrared (NIR) laser light in a vacuum. This ensures minimal spatiotemporal fluctuations between the strong NIR laser and the XUV excitation and reference beams, while the grazing incidence optics enable broadband spectral coverage. The apparatus combines high spectral and temporal resolution together with an increase in sensitivity to weak absorption signatures by an order of magnitude. This opens up new possibilities for studying strong-field-driven electron dynamics in bound systems on their natural attosecond time scale.

Physical review letters 123(10), 103001 (2019). doi:10.1103/PhysRevLett.123.103001, 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 $2p-3d$ bound–bound transitions between the spin-orbit multiplets $^3P_{0,1,2}$ and $^3D_{1,2,3}$ 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., Published by APS, College Park, Md.

Watching as helium goes topsy-turvy Theorists have long pondered the underpinnings of the Fano resonance, a spectral feature that resembles adjacent rightside-up and upside-down peaks. An especially well-studied instance of this feature appears in the electronic spectrum of helium as a transient state undergoes delayed ionization. Two studies have now traced the dynamics of this state in real time. Gruson et al. used photoelectron spectroscopy to extract the amplitude and phase of the electron wave packet after inducing its interference with reference wave packets tuned into resonance at variable delays. Kaldun et al. used extreme ultraviolet absorption spectroscopy to probe the transient state while variably forcing ionization with a strong near-infrared field. Science , this issue pp. 734 and 738

We create a tunable temporal gate by strong-field depletion of a previously excited two-electron system. This allows us to temporally resolve the emergence of a Fano resonance - a long-standing quest in attosecond science.

Firstly, we demonstrate time-resolved four-wave-mixing spectroscopy on inner-valence transitions in neon using high-harmonic generated (HHG) pulses. Secondly, we present the design of a two-dimensional spectroscopy setup for extreme ultraviolet (XUV) and soft-X-ray pulses.