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The journal of physical chemistry / A 125, 10138���10143 (2021). doi:10.1021/acs.jpca.1c06033, We performed a time-resolved spectroscopy experixment on the dissociation of oxygen molecules after the interactionwith intense extreme-ultraviolet (XUV) light from the free-electronlaser in Hamburg at Deutsches Elektronen-Synchrotron. Using anXUV-pump/XUV-probe transient-absorption geometry with asplit-and-delay unit, we observe the onset of electronic transitionsin the O$^{2+}$ cation near 50 eV photon energy, marking the end ofthe progression from a molecule to two isolated atoms. We observetwo different time scales of 290 �� 53 and 180 �� 76 fs for theemergence of different ionic transitions, indicating differentdissociation pathways taken by the departing oxygen atoms.With regard to the emerging opportunities of tuning the centralfrequencies of pump and probe pulses and of increasing the probe���pulse bandwidth, future pump���probe transient-absorptionexperiments are expected to provide a detailed view of the coupled nuclear and electronic dynamics during molecular dissociation, Published by Soc., Washington, DC

Nature Communications 12(1), 643 (1-7) (2021). doi:10.1038/s41467-020-20846-1, 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 anionized 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., Published by Nature Publishing Group UK, [London]

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.

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., Faraday Discussions, 228, ISSN:1359-6640, ISSN:1364-5498

Journal of physics / B 53(23), 234002 (1-8) (2020). doi:10.1088/1361-6455/abbe2d, 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., Published by IOP Publ., Bristol

The femtosecond-timescale formation of a Fano resonance in doubly excited helium has been recently observed, enabling a time-domain view into two-electron correlation dynamics. Measuring the absorption spectral line shape, the buildup of the resonance is revealed by imposing a temporal gate on the dipole response using strong-field ionization. Here, we apply this approach to the Rydberg series of the doubly excited states in helium. This reveals the characteristic times of emergence of isolated two-electron resonances from the continuous single-ionization background absorption, as well as their time-dependent line-shape asymmetry and the time it takes to separate individual spectral absorption lines within the series. Furthermore, we time resolve the dynamics of the excited wave packet by reconstructing its dipole response in the time domain and thereby characterize the ionization gate to close as fast as only 1.2 fs. These results represent an approach to resolve on the femtosecond and attosecond timescale the strong-field-ionization dynamics of excited coherent wave packets.

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.

Accessing attosecond (as) dynamics directly in the time domain has been achieved by several pioneering experiments over the course of the last decade. Extreme ultraviolet (XUV) group delays and, later, ionization time delays on the order of a few attoseconds have been extracted by photoemission or high-harmonic spectroscopy. Here, we present and benchmark an approach based on attosecond transient absorption spectroscopy to quantify deliberately induced delays by employing resonant photoexcitation of three XUV transitions with a precision of less than 5 as. While here we quantify the sensitivity to these delays via a chirp on the attosecond pulse by using thin-foil metallic filters, the method enables future studies of attosecond delays probed through resonant excitations.

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 free-electron 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 $\mu$J. 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 strong-field-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., Comment: 16 pages, 3 figures

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

The laser-field-modified dipole response of the first ionization threshold of helium is studied by means of attosecond transient absorption spectroscopy. We resolve light-induced time-dependent structures in the photoabsorption spectrum both below and above the ionization threshold. By comparing the measured results to a quantum-dynamical model, we isolate the contributions of the unbound electron to these structures. They originate from light-induced couplings of near-threshold bound and continuum states and light-induced energy shifts of the free electron. The ponderomotive energy, at low laser intensities, is identified as a good approximation for the perturbed continuum response.

Two-electron dynamics of an excited model atom interacting with moderately strong laser fields is analyzed in the time domain. We solve the time-dependent Schrödinger equation (TDSE) for two electrons confined to the same one-dimensional configuration space, accounting also for the electron-electron interaction. The computational method allows direct access to the time-dependent population of the relevant atomic states during and right after the interaction with a near-infrared (NIR) laser pulse. We compare the ionization dynamics of singly excited states and doubly excited states. We find that doubly-excited initial states exhibit enhanced double ionization yield, with non-trivial dynamics including contributions from direct and sequential processes, while the electrons leave the atom either back-to-back or in the same direction.

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 $2\text{ }\text{ }\mathrm{TW}/{\mathrm{cm}}^{2}$ 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.

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 $2\text{ }\text{ }\mathrm{TW}/{\mathrm{cm}}^{2}$ 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.

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.

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., 5 pages, 3 figures

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.

We introduce and demonstrate an in situ method for characterizing intense few-cycle laser pulses with transient absorption spectroscopy. Thereby, we obtain crucial pulse properties precisely where the physics of ultrafast quantum dynamics experiments takes place.

We report on the experimental observation of the buildup of the 2s2p Fano resonance in helium in the time domain, which has been under theoretical investigation for more than a decade. The emergence of the absorption line is temporally resolved by interrupting the natural decay of the excited state via saturated strong-field ionization at a variable time delay. We compare the experimental data with full ab-initio simulations to validate the time-gating by strong-field ionization and thereby confirm the recently developed theory for the formation of Fano line-profiles.