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Nature physics 18(4), 423 - 428 (2022). doi:10.1038/s41567-022-01507-0, Following structural dynamics in real time is a fundamental goal towards a better understanding of chemical reactions. Recording snapshots of individual molecules with ultrashort exposure times is a key ingredient towards this goal, as atoms move on femtosecond (10$< {−15}$ s) timescales. For condensed-phase samples, ultrafast, atomically resolved structure determination has been demonstrated using X-ray and electron diffraction. Pioneering experiments have also started addressing gaseous samples. However, they face the problem of low target densities, low scattering cross sections and random spatial orientation of the molecules. Therefore, obtaining images of entire, isolated molecules capturing all constituents, including hydrogen atoms, remains challenging. Here we demonstrate that intense femtosecond pulses from an X-ray free-electron laser trigger rapid and complete Coulomb explosions of 2-iodopyridine and 2-iodopyrazine molecules. We obtain intriguingly clear momentum images depicting ten or eleven atoms, including all the hydrogens, and thus overcome a so-far impregnable barrier for complete Coulomb explosion imaging—its limitation on molecules consisting of three to five atoms. In combination with state-of-the-art multi-coincidence techniques and elaborate theoretical modelling, this allows tracing ultrafast hydrogen emission and obtaining information on the result of intramolecular electron rearrangement. Our work represents an important step towards imaging femtosecond chemistry via Coulomb explosion., Published by Nature Publishing Group, Basingstoke

Have you ever wondered how plants make use of sunlight to do photosynthesis? What happens inside your eye such that your brain can perceive your smartphone screen or your coffee cup? It turns out this is eventually triggered by tiny rearrangements of individual atoms. Our aim is to develop an imaging method sensitive enough to watch such processes in single molecules on the atomic level.

Journal of the American Society for Mass Spectrometry 32(3), 670 - 684 (2021). doi:10.1021/jasms.0c00390, Near-edge X-ray absorption mass spectrometry (NEXAMS) is an action-spectroscopy technique of growing interest for investigations into the spatial and electronic structure of biomolecules. It has been used successfully to give insights into different aspects of the photodissociation of peptides and to probe the conformation of proteins. It is a current question whether the fragmentation pathways are sensitive toward effects of conformational isomerism, tautomerism, and intramolecular interactions in gas-phase peptides. To address this issue, we studied the cationic fragments of cryogenically cooled gas-phase leucine enkephalin ([LeuEnk+H]$^+$) and methionine enkephalin ([MetEnk+H]$^+$) produced upon soft X-ray photon absorption at the carbon, nitrogen, and oxygen K-edges. The interpretation of the experimental ion yield spectra was supported by density-functional theory and restricted-open-shell configuration interaction with singles (DFT/ROCIS) calculations. The analysis revealed several effects that could not be rationalized based on the peptide’s amino acid sequences alone. Clear differences between the partial ion yields measured for both peptides upon C 1s → π*(C═C) excitations in the aromatic amino acid side chains give evidence for a sulfur–aromatic interaction between the methionine and phenylalanine side chain of [MetEnk+H]$^+$. Furthermore, a peak associated with N 1s → π*(C═N) transitions, linked to a tautomeric keto-to-enol conversion of peptide bonds, was only present in the photon energy resolved ion yield spectra of [MetEnk+H]$^+$., Published by ACS Publications, Washington, DC

Nonlinear x-ray spectroscopy The extension of nonlinear optics to the x-ray spectral domain is a promising direction in the development of x-ray spectroscopy. Although theoretical concepts of nonlinear x-ray spectroscopy were developed decades ago, scientists still struggle to implement them because of the elusive nature of nonlinear effects. Eichmann et al. now present atomic momentum spectroscopy (AMS), which is based on the detection of the scattered atom after momentum transfer from x-ray photons (see the Perspective by Pfeifer). The authors show how AMS can observe stimulated x-ray Raman scattering signals at the neon K edge on a single-atom level and distinguish them from other competing processes. These results pave the way for future nonlinear x-ray spectroscopy methods for the study of x-ray–matter interactions. Science , this issue p. 1630 ; see also p. 1568

Photoinduced isomerization reactions lie at the heart of many chemical processes in nature. The mechanisms of such reactions are determined by a delicate interplay of coupled electronic and nuclear dynamics occurring on the femtosecond scale, followed by the slower redistribution of energy into different vibrational degrees of freedom. Here we apply time-resolved photoelectron spectroscopy with a seeded extreme ultraviolet free-electron laser to trace the ultrafast ring opening of gas-phase thiophenone molecules following ultraviolet photoexcitation. When combined with ab initio electronic structure and molecular dynamics calculations of the excited- and ground-state molecules, the results provide insights into both the electronic and nuclear dynamics of this fundamental class of reactions. The initial ring opening and non-adiabatic coupling to the electronic ground state are shown to be driven by ballistic S–C bond extension and to be complete within 350 fs. Theory and experiment also enable visualization of the rich ground-state dynamics that involve the formation of, and interconversion between, ring-opened isomers and the cyclic structure, as well as fragmentation over much longer timescales.

The idea of using ultrashort X-ray pulses to obtain images of single proteins frozen in time has fascinated and inspired many. It was one of the arguments for building X-ray free-electron lasers. According to theory1, the extremely intense pulses provide sufficient signal to dispense with using crystals as an amplifier, and the ultrashort pulse duration permits capturing the diffraction data before the sample inevitably explodes2. This was first demonstrated on biological samples a decade ago on the giant mimivirus3. Since then a large collaboration4has been pushing the limit of the smallest sample that can be imaged5,6. The ability to capture snapshots on the timescale of atomic vibrations, while keeping the sample at room temperature, may allow probing the entire conformational phase space of macromolecules. Here we show the first observation of an X-ray diffraction pattern from a single protein, that ofEscherichia coliGroEL which at 14 nm in diameter7is the smallest biological sample ever imaged by X-rays, and demonstrate that the concept of diffraction before destruction extends to single proteins. From the pattern, it is possible to determine the approximate orientation of the protein. Our experiment demonstrates the feasibility of ultrafast imaging of single proteins, opening the way to single-molecule time-resolved studies on the femtosecond timescale.

Physical chemistry, chemical physics 24, 23096 - 23105 (2022). doi:10.1039/D2CP02252D special issue: "Ions, electrons, coincidences and dynamics: Festschrift for John H.D. Eland", We investigated the dissociation of dications and trications of three polycyclic aromatic hydrocarbons (PAHs), fluorene, phenanthrene, and pyrene. PAHs are a family of molecules ubiquitous in space and involved in much of the chemistry of the interstellar medium. Ions are formed by ionization with 30.3 nm extreme ultraviolet (XUV) photons, and their velocity map images are recorded using a PImMS multi-mass imaging sensor. Application of recoil-frame covariance analysis allows the total kinetic energy release (TKER) associated with multiple fragmentation channels to be determined to high precision. Experimental measurements are supported by Born-Oppenheimer molecular dynamics (BOMD) molecular dynamics simulations., Published by RSC Publ., Cambridge

Journal of physics / B 55(1), 014001 (2022). doi:10.1088/1361-6455/ac489d, The ultraviolet (UV)-induced dissociation and photofragmentation of gas-phase CH$_2$BrI molecules induced by intense femtosecond extreme ultraviolet (XUV) pulses at three different photon energies are studied by multi-mass ion imaging. Using a UV-pump���XUV-probe scheme, charge transfer between highly charged iodine ions and neutral CH$_2$Br radicals produced by C���I bond cleavage is investigated. In earlier charge-transfer studies, the center of mass of the molecules was located along the axis of the bond cleaved by the pump pulse. In the present case of CH$_2$BrI, this is not the case, thus inducing a rotation of the fragment. We discuss the influence of the rotation on the charge transfer process using a classical over-the-barrier model. Our modeling suggests that, despite the fact that the dissociation is slower due to the rotational excitation, the critical interatomic distance for charge transfer is reached faster. Furthermore, we suggest that charge transfer during molecular fragmentation may be modulated in a complex way., Published by IOP Publ., Bristol

Here, we report on the nonlinear ionization of argon atoms in the short wavelength regime using ultraintense x rays from the European XFEL. After sequential multiphoton ionization, high charge states are obtained. For photon energies that are insufficient to directly ionize a $1s$ electron, a different mechanism is required to obtain ionization to Ar$^{17+}$. We propose this occurs through a two-color process where the second harmonic of the FEL pulse resonantly excites the system via a $1s \rightarrow 2p$ transition followed by ionization by the fundamental FEL pulse, which is a type of x-ray resonance-enhanced multiphoton ionization (REMPI). This resonant phenomenon occurs not only for Ar$^{16+}$, but through multiple lower charge states, where multiple ionization competes with decay lifetimes, making x-ray REMPI distinctive from conventional REMPI. With the aid of state-of-the-art theoretical calculations, we explain the effects of x-ray REMPI on the relevant ion yields and spectral profile., Comment: 8 pages, 5 figures, one supplementary material (not included)

Journal of physics / B 55(14), 144001 - (2021). doi:10.1088/1361-6455/ac6b6b, We demonstrate the application of event-driven Timepix3-based detectors in combination with a double-sided velocity-map-imaging spectrometer to record the full 3D momentum of charged particles at the free-electron-laser facility FLASH. We measured the XUV induced fragmentation of $\text{N}_2$ using 250 kHz FLASH bursts with sub-pixel spatial resolution and up to 1.7 ns temporal resolution for photoelectrons. To further demonstrate the capabilities of this camera at even higher repetition rates we measured single-shot images of He($1s$) photoelectrons for bursts with a repetition rate of 1 MHz. Overall, with the Timepix3 camera we overcome limitations of standard-camera technology for advanced-imaging experiments with requirements on high event-rates and high spatio-temporal resolution., Published by IOP Publ., Bristol

XUV multiphoton ionization of molecules is commonly used in free-electron laser experiments to study charge transfer dynamics. However, molecular dissociation and electron dynamics, such as multiple photon absorption, Auger decay, and charge transfer, often happen on competing time scales, and the contributions of individual processes can be difficult to unravel. We experimentally investigate the Coulomb explosion dynamics of methyl iodide upon core–hole ionization of the shallow inner-shell of iodine (4d) and classically simulate the fragmentation by phenomenologically introducing ionization dynamics and charge transfer. Under our experimental conditions with medium fluence and relatively long XUV pulses (∼75 fs), we find that fast Auger decay prior to charge transfer significantly contributes to the charging mechanism, leading to a yield enhancement of higher carbon charge states upon molecular dissociation. Furthermore, we argue for the existence of another charging mechanism for the weak fragmentation channels leading to triply charged carbon atoms. This study shows that classical simulations can be a useful tool to guide the quantum mechanical description of the femtosecond dynamics upon multiphoton absorption in molecular systems.

Physical review / A 103(2), 022832 (2021). doi:10.1103/PhysRevA.103.022832, We report angle-resolved measurements on photoelectrons emitted upon near-threshold two-photon double ionization (TPDI) of Kr irradiated by free-electron laser (FEL) pulses. These photoelectron angular distributions (PADs) are compared with the results of semirelativistic R-matrix calculations. As reported by Augustin et al. [Phys. Rev. A 98, 033408 (2018)], it is found that the presence of autoionizing resonances within the bandwidth of the exciting FEL pulse strongly influences the PADs. In contrast to measurements on lower-Z targets such as Ne and Ar, the larger spin-orbit interaction, inherent in 4p-subshell hole states of Kr, permits us to resolve and study PADs associated with some of the fine-structure components of the Kr$^+$ and Kr$^{2+}$ ions., Published by Inst., Woodbury, NY

We report on a multiparticle coincidence experiment performed at the European X-ray Free-Electron Laser at the Small Quantum Systems instrument using a COLTRIMS reaction microscope. By measuring two ions and two electrons in coincidence, we investigate double core-hole generation in O_{2} molecules in the gas phase. Single-site and two-site double core holes have been identified and their molecular-frame electron angular distributions have been obtained for a breakup of the oxygen molecule into two doubly charged ions. The measured distributions are compared to results of calculations performed within the frozen- and relaxed-core Hartree-Fock approximations.

Journal of physics / B 53(22), 224001 (2020). doi:10.1088/1361-6455/abb1fd, The predissociation dynamics of the 6s (B$^2$E) Rydberg state of gas-phase CH$_3$I were investigated by time-resolved Coulomb-explosion imaging using extreme ultraviolet (XUV) free-electron laser pulses. Inner-shell ionization at the iodine 4d edge was utilized to provide a site-specific probe of the ensuing dynamics. The combination of a velocity-map imaging (VMI) spectrometer coupled with the pixel imaging mass spectrometry (PImMS) camera permitted three-dimensional ionic fragment momenta to be recorded simultaneously for a wide range of iodine charge states. In accord with previous studies, initial excitation at 201.2 nm results in internal conversion and subsequent dissociation on the lower-lying A-state surface on a picosecond time scale. Examination of the time-dependent yield of low kinetic energy iodine fragments yields mechanistic insights into the predissociation and subsequent charge transfer following multiple ionization of the iodine products. The effect of charge transfer was observed through differing delay-dependencies of the various iodine charge states, from which critical internuclear distances for charge transfer could be inferred and compared to a classical over-the-barrier model. Time-dependent photofragment angular anisotropy parameters were extracted from the central slice of the Newton sphere, without Abel inversion, and highlight the effect of rotation of the parent molecule before dissociation, as observed in previous works. Our results demonstrate the ability to perform three-dimensional ion imaging at high event rates and showcase the potential benefits of this approach, particularly in relation to further time-resolved studies at free-electron laser facilities., Published by IOP Publ., Bristol

Optica 9(4), 429 - (2022). doi:10.1364/OPTICA.454920, Ultrafast X-ray spectroscopies require flexible X-ray properties together with high temporal and spectral resolution. Here, we demonstrate simultaneous sub-20 fs and sub-eV resolutions for pump/probe experiments, without the need for additional photon arrival-time monitors., Published by OSA, Washington, DC

The application of a novel fast optical-imaging camera, TimepixCam, to molecular photoionization experiments using the velocity-map imaging technique at a free-electron laser is described. TimepixCam is a 256 × 256 pixel CMOS camera that is able to detect and time-stamp ion hits with 20 ns timing resolution, thus making it possible to record ion momentum images for all fragment ions simultaneously and avoiding the need to gate the detector on a single fragment. This allows the recording of significantly more data within a given amount of beam time and is particularly useful for pump–probe experiments, where drifts, for example, in the timing and pulse energy of the free-electron laser, severely limit the comparability of pump–probe scans for different fragments taken consecutively. In principle, this also allows ion–ion covariance or coincidence techniques to be applied to determine angular correlations between fragments.

Journal of optics 24(2), 025502 (2022). doi:10.1088/2040-8986/ac4318, We present the results of an experiment investigating the generation of high-order harmonics by a femtosecond near-infrared (NIR) laser pulse in the presence of an extreme ultraviolet (XUV) field provided by a free-electron laser (FEL), a process referred to as XUV-assisted high-order harmonic generation (HHG). Our experimental findings show that the XUV field can lead to a small enhancement in the harmonic yield when the XUV and NIR pulses overlap in time, while a strong decrease of the HHG yield and a red shift of the HHG spectrum is observed when the XUV precedes the NIR pulse. The latter observations are in qualitative agreement with model calculations that consider the effect of a decreased number of neutral emitters but are at odds with the predicted effect of the correspondingly increased ionization fraction on the phase matching. Our study demonstrates the technical feasibility of XUV-assisted HHG experiments at FELs, which may provide new avenues to investigate correlation-driven electron dynamics as well as novel ways to study and control propagation effects and phase matching in HHG., Published by IOP Publ., Bristol

We present the first pump/probe measurements performed at the SQS instrument of the European XFEL. We obtain the first characterization of the femtosecond temporal resolution and show the feasibility for the investigation of ultrafast dynamics.

Physical review / X 10(4), 041056 (2020). doi:10.1103/PhysRevX.10.041056, The nature of transient electronic states created by photoabsorption critically determines the dynamics of the subsequently evolving system. Here, we investigate K-shell photoionized atomic neon by absorbing a second photon within the Auger-decay lifetime of 2.4 fs using the European XFEL, a unique high-repetition-rate, wavelength-tunable x-ray free-electron laser. By high-resolution electron spectroscopy, we map out the transient Rydberg resonances unraveling the details of the subsequent decay of the hollow atom. So far, ultra-short-lived electronic transients, which are often inaccessible by experiments, were mainly inferred from theory but are now addressed by nonlinear x-ray absorption. The successful characterization of these resonances with femtosecond lifetimes provides the basis for a novel class of site-specific, nonlinear, and time-resolved studies with strong impact for a wide range of topics in physics and chemistry., Published by APS, College Park, Md.

31st International Conference on Photonic, Electronic, and Atomic Collisions, ICPEAC XXXI, Caen Normandy, France, 23 Jan 2019 - 30 Jan 2019; Journal of physics / Conference Series 1412, 112009 (2020). doi:10.1088/1742-6596/1412/11/112009, (X-ray) free-electron lasers are employed to site specifically interrogate atomic fragments during ultra-fast photolysis of chiral molecules via time-resolved photoelectron circular dichroism., Published by IOP Publ., Bristol

Physical review research 2(2), 023053 (2020). doi:10.1103/PhysRevResearch.2.023053, We investigate the frustrated absorption phenomenon of atomic systems driven by x-ray pulses of extremely high intensity. When an atom is exposed to intense x-ray pulses generated by x-ray free-electron lasers (XFELs), it undergoes complex ionization dynamics characterized by sequential multiphoton multiple ionization. Counterintuitively, as the pulse duration becomes shorter so that the intensity increases, the ionization becomes suppressed because of hollow-atom formation and the reduction of cross section. This is called frustrated absorption. However, as we report here, the paradigm of frustrated absorption can break down at extremely high intensity. By using a state-of-the-art theoretical tool, we examine the pulse-duration dependence of x-ray multiphoton ionization dynamics of heavy atoms, revealing that the reduced ionization for shorter pulses is due to the suppression of Auger decays, rather than the frustration of photoabsorption. Moreover, we predict a situation where ionization is, in fact, enhanced as the pulse duration is decreased and explain the mechanism why this happens. The present results demonstrate that the breakdown of frustrated absorption will emerge at the highest fluence currently available at XFEL facilities and will play an important role when terawatt-attosecond x-ray pulses come into realization., Published by APS, College Park, MD

A simple model for sequential multiphoton ionization by ultraintense x rays is presented. The derived scaling of the ion yield with pulse energy quantitatively reproduces the experimental data, which shows that the ion yield increases according to the ``power-law'' behavior typical of multiphoton ionization, followed by saturation at high pulse energies. The calculated average time interval between ionizations for producing ions at a certain charge state is found to be proportional to the pulse duration and independent of all other x-ray pulse parameters. This agrees with previous studies where the kinetic energy of fragment ions with a given charge state produced by the intense x-ray ionization of molecules was found to be independent of the pulse energy, but to increase with a smaller pulse duration due to the smaller time interval between ionizations.

We report experimental results on the diffractive imaging of three-dimensionally aligned 2,5-diiodothiophene molecules. The molecules were aligned by chirped near-infrared laser pulses, and their structure was probed at a photon energy of 9.5 keV (λ ≈ 130 pm) provided by the Linac Coherent Light Source. Diffracted photons were recorded on the Cornell-SLAC pixel array detector, and a two-dimensional diffraction pattern of the equilibrium structure of 2,5-diiodothiophene was recorded. The retrieved distance between the two iodine atoms agrees with the quantum-chemically calculated molecular structure to be within 5%. The experimental approach allows for the imaging of intrinsic molecular dynamics in the molecular frame, albeit this requires more experimental data, which should be readily available at upcoming high-repetition-rate facilities.

Physical review / X Expanding access 10(2), 021052 (2020). doi:10.1103/PhysRevX.10.021052, A central motivation for the development of x-ray free-electron lasers has been the prospect of time-resolved single-molecule imaging with atomic resolution. Here, we show that x-ray photoelectron diffraction—where a photoelectron emitted after x-ray absorption illuminates the molecular structure from within—can be used to image the increase of the internuclear distance during the x-ray-induced fragmentation of an O$_2$ molecule. By measuring the molecular-frame photoelectron emission patterns for a two-photon sequential $K$-shell ionization in coincidence with the fragment ions, and by sorting the data as a function of the measured kinetic energy release, we can resolve the elongation of the molecular bond by approximately 1.2 a.u. within the duration of the x-ray pulse. The experiment paves the road toward time-resolved pump-probe photoelectron diffraction imaging at high-repetition-rate x-ray free-electron lasers., Published by APS, College Park, Md.

We present a comprehensive experimental and theoretical study on superfluorescence in the extreme ultraviolet wavelength regime. Focusing a free-electron laser pulse in a cell filled with Xe gas, the medium is quasi-instantaneously population inverted by $4d$-shell ionization on the giant resonance followed by Auger decay. On the timescale of $\ensuremath{\sim}10\text{ }\text{ }\mathrm{ps}$ to $\ensuremath{\sim}100\text{ }\text{ }\mathrm{ps}$ (depending on parameters) a macroscopic polarization builds up in the medium, resulting in superfluorescent emission of several Xe lines in the forward direction. As the number of emitters in the system is increased by either raising the pressure or the pump-pulse energy, the emission yield grows exponentially over four orders of magnitude and reaches saturation. With increasing yield, we observe line broadening, a manifestation of superfluorescence in the spectral domain. Our novel theoretical approach, based on a full quantum treatment of the atomic system and the irradiated field, shows quantitative agreement with the experiment and supports our interpretation.

Background and purpose: Aside from the enhancement of physical dose deposited by antiprotons annihilating in tissue-like material compared to protons of the same range a further increase of biological effective dose has been demonstrated. This enhancement can be expressed in an increase of the relative biological effectiveness (RBE) of antiprotons near the end of range. We have performed the first-ever direct measurement of the RBE of antiprotons both at rest and in flight.Materials and methods: Experimental data were generated on the RBE of an antiproton beam entering a tissue-like target consisting of V79 cells embedded in gelatin with an energy providing a range of approximately 10 cm.Results: The RBE in the entrance channel (the "plateau") is only slightly above the value for a comparable proton beam, and remains low until the proximal edge of the spread-out Bragg peak (SOBP). A steep increase of RBE is seen starting from the onset of the SOBP.Conclusions: This paper reports the final results of the experiment AD-4/ACE at CERN on the first-ever direct measurement of RBE of antiprotons and constitutes the first step toward developing treatment plans. (C) 2016 Elsevier Ireland Ltd. All rights reserved.

The photodissociation dynamics of CH

JoVE 140, e57055 (2018). doi:10.3791/57055, This protocol describes key steps in performing and analyzing femtosecond pump-probe experiments that combine a femtosecond optical laser with a free-electron laser. This includes methods to establish the spatial and temporal overlap between the optical and free-electron laser pulses during the experiment, as well as important aspects of the data analysis, such as corrections for arrival time jitter, which are necessary to obtain high-quality pump-probe data sets with the best possible temporal resolution. These methods are demonstrated for an exemplary experiment performed at the FLASH (Free-electron LASer Hamburg) free-electron laser in order to study ultrafast photochemistry in gas-phase molecules by means of velocity map ion imaging. However, most of the strategies are also applicable to similar pump-probe experiments using other targets or other experimental techniques., Published by JoVE, [S.l.]

Physical chemistry, chemical physics 20, 20205 - 20216 (2018). doi:10.1039/C8CP00936H, A photofragmentation study of gas-phase indole $(C_8H_7N)$ upon single-photon ionization at a photon energy of 420 eV is presented. Indole was primarily inner-shell ionized at its nitrogen and carbon 1s orbitals. Electrons and ions were measured in coincidence by means of velocity map imaging. The angular relationship between ionic fragments is discussed along with the possibility to use the angle-resolved coincidence detection to perform experiments on molecules that are strongly oriented in their recoil-frame. The coincident measurement of electrons and ions revealed fragmentation-pathway-dependent electron spectra, linking the structural fragmentation dynamics to different electronic excitations. Evidence for photoelectron-impact self-ionization was observed., Published by RSC Publ., Cambridge

International audience; Due to its element and site specificity, inner-shell photoelectron spectroscopy is a widely used technique to probe the chemical structure of matter. Here, we show that time-resolved inner-shell photoelectron spectroscopy can be employed to observe ultrafast chemical reactions and the electronic response to the nuclear motion with high sensitivity. The ultraviolet dissociation of iodomethane (CH3I) is investigated by ionization above the iodine 4d edge, using time-resolved inner-shell photoelectron and photoion spectroscopy. The dynamics observed in the photoelectron spectra appear earlier and are faster than those seen in the iodine fragments. The experimental results are interpreted using crystal-field and spin-orbit configuration interaction calculations, and demonstrate that time-resolved inner-shell photoelectron spectroscopy is a powerful tool to directly track ultrafast structural and electronic transformations in gas-phase molecules.

Interatomic Coulombic Decay (ICD) is a relaxation phenomenon, which takes place in weakly bound atomic and molecular systems, typically within a few to hundreds of femtoseconds depending on the system and the particular decay mechanism. The creation of ICD-active states requires the production of highly excited systems, usually populated by innershell ionization or excitation. To this end, XUV and X-ray radiation from synchrotrons was conventionally applied for the majority of experiments due to the desired state-selective ionization of certain sub-shells. The advent of Free-Electron Lasers (FELs) has enabled an entirely new class of experiments, which finally allow to trace ICD directly in the time domain due to the femtosecond pulse duration. Within this paper, the first time-resolved ICD measurement using an XUV-pump–XUV-probe scheme will be discussed in detail. The experiment was performed on neon dimers and ICD was triggered by removing a 2s electron from one of the neon atoms using a 58 eV pulse from the FEL in Hamburg (FLASH). The onset of ICD was probed with a delayed copy of the trigger pulse that further ionized one of the two Ne+ ions emerging after ICD. Thus, the delay-dependent yield of coincident Ne+ + Ne2+ ion pairs contains the lifetime of the 2s-innershell vacancy decaying via ICD. The result of 150 fs ± 50 fs is in good agreement with theory but only for those calculations that explicitly take nuclear motion into account.