Achieving few-femtosecond resolution for a pump-probe experiment is crucial to measuring the fastest electron dynamics. As almost all molecules resonantly absorb radiation in the UV and/or visible, generating ultrashort pump pulses in this part of the optical spectrum has huge potential for probing excited state electronic dynamics while not ionising the molecule or material. However, traditional UV-Vis pump pulses cannot achieve few-fs durations and usually operate at fixed wavelengths. Almost every material and even air begin to be absorbent and highly dispersive in this region, making the generation, transport, and characterisation of ultrashort UV pulses extremely challenging. Technology to produce few-fs widely tuneable UV pulses was recently realised by the generation of resonant dispersive wave (RDW) emission in gas-filled hollow capillary fibres (HCF) [1]. RDW is a revolutionary new way to pump photochemical systems that overcome many limitations of traditional pumping regimes.

High-harmonic generation (HHG) in solids driven by femtosecond lasers is a promising method for the compact production of coherent extreme ultraviolet (XUV) radiation but so far has been limited to photon energies below 40 eV. Here, we report the highest ever recorded photon energy for a harmonic in a solid sample, reaching 50 eV (31st harmonic) in 100-µm-thick MgO, using a 780 nm, 30 fs driving pulse. This is achieved through optimization of the spectrometer and detection efficiency, as well as an increase in emission efficiency enabled by a larger excitation area and the use of a multi-cycle pulse. We observe that the harmonic cutoff exhibits nontrivial behavior as a function of laser field strength, suggesting that an extension to our existing understanding of the generation process may be needed. This work demonstrates further the potential for compact XUV sources beyond 50 eV based on solid-state media.

The detailed understanding of electronic coherence in quantum systems requires measurements on the attosecond timescale. Attosecond x-ray pulses enable the study of electronic coherence in core-excited molecular systems. Here we report on the coherent motion of electrons in the 1,1-difluoroethylene ion following ionization of the K shell of the two nonequivalent carbon sites with a subfemtosecond x-ray pulse. Using the angular streaking technique to track the Auger-Meitner decay, we observe temporal modulations of the emission, indicating the electronic coherence of the core-excited ionic states, and extract a 6.5±0.8 fs average lifetime of the core-level vacancies. A quantum-mechanical model is employed to interpret the measurement, and we find the observed temporal modulations are independent of charge density oscillations. This work opens a new regime of coherent electronic motion, beyond charge migration, where electronic coherence manifests in the nonlocal quantum correlation between atomic sites while charge density oscillation is absent. Our results broaden the landscape of electronic coherence in molecular systems. Published by the American Physical Society 2025

Combinatorial post-translational modifications (PTMs) of proteins, such as histones, govern cell differentiation and organismal development, and are widely thought to play a key role in aging, development of cancers, neurodevelopmental disorders, neurodegenerative and other diseases. Nonetheless, current understanding of the precise biological function of different modification patterns is limited by the difficulty of mass spectrometry to identify and quantify different combinatorial isomers in their mixtures as they naturally occur. This profound difficulty is a result of the fundamental incompleteness of the information contained in a one-dimensional mass spectrum featuring the mass-to-charge ratios and relative abundances of the individual peptide fragments. Here we demonstrate that the fragment-fragment correlations revealed by the recently developed two-dimensional partial covariance mass spectrometry (2D-PC-MS) allow one to solve the combinatorial PTM puzzles that cannot be tackled by the standard mass spectrometry as a matter of principle. We introduce the concept of 2D-PC-MS marker ion correlations and show that they can provide the missing PTM location information that enables identification of co-fragmentated combinatorially modified isomers in their mixtures. We demonstrate experimentally the use of the marker ion correlations to fully analyze and resolve mixtures of doubly acetylated histone H4 peptides, a problem that was previously branded “mathematically impossible”. Our accompanying comprehensive in silico study reveals that the marker ion correlations can be used to unambiguously identify five times more combinatorially modified tryptic peptides and three times more combinatorially modified Glu-C peptides of human histones than is possible using the standard MS/MS.

Pump-probe experiments with sub-femtosecond resolution are the key to understanding electronic dynamics in quantum systems. Here we demonstrate the generation and control of sub-femtosecond pulse pairs from a two-colour X-ray free-electron laser (XFEL). By measuring the delay between the two pulses with an angular streaking diagnostic, we characterise the group velocity of the XFEL and demonstrate control of the pulse delay down to 270 as. We demonstrate the application of this technique to a pump-probe measurement in core-excited para-aminophenol. These results demonstrate the ability to perform pump-probe experiments with sub-femtosecond resolution and atomic site specificity.

Femtosecond-fast and nanometre-size pulses of electrons are emerging as unique probes for ultrafast dynamics at the nanoscale. Presently, such pulses are achievable only in highly sophisticated ultrafast electron microscopes or equally complex setups involving few-cycle-pulsed lasers with stable carrier-envelope phase (CEP) and nanotip probes. Here, we show that the generation of femtosecond pulses of nanoscale tunnelling electrons can be achieved in any ultrafast optical laboratory, using any (deep-UV to mid-IR) femtosecond laser in combination with photosensitive asymmetric nanogap (PAN) diodes fabricated via easy-to-scale adhesion lithography. The dominant mechanism producing tunnelling electrons in PANs is strong-field emission, which is easily achievable without CEP locking or external bias voltage. We employ PANs to demonstrate ultrafast nanoscopy of metal-halide perovskite quantum dots immobilised inside a 10-nm Al/Au nanogap and to characterise laser pulses across the entire optical region (266-6700 nm). Short electron pulses in PANs open the way towards scalable on-chip femtosecond electron measurements and novel design approaches for integrated ultrafast sensing nanodevices.

Achieving few-femtosecond resolution for a pump-probe experiment is crucial to measuring the fastest electron dynamics and for creating superpositions of valence states in quantum systems. However, traditional UV-Vis pump pulses cannot achieve few-fs durations and usually operate at fixed wavelengths. Here, we present, to our knowledge, an unprecedented temporal resolution and pump tuneability for UV-Vis-pumped soft X-ray transient absorption spectroscopy. We have combined few-fs deep-UV to visible tuneable pump pulses from resonant dispersive wave emission in hollow capillary fiber with attosecond soft X-ray probe pulses from high harmonic generation. We achieve sub-5-fs time resolution, sub-fs interferometric stability, and continuous tuneability of the pump pulses from 230 to 700 nm. We demonstrate that the pump can initiate an ultrafast photochemical reaction and that the dynamics at different atomic sites can be resolved simultaneously. These capabilities will allow studies of the fastest electronic dynamics in a large range of photochemical, photobiological and photovoltaic reactions.

Novel ultrafast x-ray sources based on high harmonic generation and at x-ray free electron lasers are opening up new opportunities to resolve complex ultrafast processes in condensed phase systems with exceptional temporal resolution and atomic site specificity. In this perspective, we present techniques for resolving charge localization, transfer, and separation processes in organic semiconductors and organic photovoltaic devices with time-resolved soft x-ray spectroscopy. We review recent results in ultrafast soft x-ray spectroscopy of these systems and discuss routes to overcome the technical challenges in performing time-resolved x-ray experiments on photosensitive materials with poor thermal conductivity and low pump intensity thresholds for nonlinear effects.

Free-electron lasers have demonstrated their capability of generating intense attosecond X-ray pulses, which are the key to studying electron dynamics at their natural time scale and in specifically targeted electronic states, but come at the expanse of complicated generation schemes and stochastic pulse shapes. Here, we demonstrate a novel and simple operation concept based on the manipulation of the electron-bunch chirp-dispersion and working with the full 4.5 MHz repetition rate at the European XFEL in Germany. With a high-fidelity single-shot temporal characterisation, we detect X-ray pulses with durations of down to 200 attoseconds and peak powers reaching into the terawatt regime at 1 keV photon energy. As a direct application, we present simultaneous measurements of nonlinear X-ray-matter interaction via time-resolved electron spectroscopy. Using the derived temporal pulse information and restricting the durations to a regime where individual X-ray pulses are shorter than the single-core-hole life time in neon atoms, we reveal an otherwise hidden peak-intensity dependence in the nonlinear dynamics of double-core-hole formation. Our results open the field of attosecond science to the investigation of electronic processes not only in the ground state but also in systems driven far off their equilibrium. They shed light on highly transient intermediate steps in complex electronic dynamics and thus promise to help build the conceptual bridge between fundamental physical processes and chemical photo-reactions.

We investigate here the influence of pulse duration on high harmonic spectra in a wide band gap dielectric, MgO. Employing a Ti:S pulse compressed in a hollow core fiber, we can effectively tune the output pulse duration from 30 fs to 4.5 fs. By systematically varying both intensity and pulse duration, we explore macroscopic scaling laws governing harmonic generation in solids. We examine the cut-off energy, divergence, and emission wavelength. Notably, we show in the measured harmonics spectra differences in the highest energy reached, the shape of the harmonics and their center emitted energy. Pulse duration significantly impacts the emission process, thereby modulating the spectro-spatial characteristics of the generated harmonics, even at constant intensity levels.

Ultrashort pulses can excite or ionize molecules and populate coherent electronic wave packets, inducing complex dynamics. In this Letter, we simulate the coupled electron-nuclear dynamics upon ionization to different electronic wave packets of (deuterated) benzene and fluoro-benzene molecules, quantum mechanically and in full dimensionality. In fluoro-benzene, the calculations unravel both interstate and intrastate quantum interferences that leave clear signatures of attochemistry and charge-directed dynamics in the shape of the autocorrelation function. The latter are in agreement with experimental high-harmonic spectroscopy measurements of benzenes and fluoro-benzene.

The coupling of electronic and nuclear motion in polyatomic molecules is at the heart of attochemistry. The molecular properties, transient structures, and reaction mechanism of these many-body quantum objects are defined on the level of electrons and ions by molecular wave functions and their coherent superposition, respectively. In the present contribution, we monitor nonadiabatic quantum wave packet dynamics during molecular charge motion by reconstructing both the oscillatory charge density distribution and the characteristic time-dependent nuclear configuration coordinate from time-resolved Auger electron spectroscopic data recorded in previous studies on glycine molecules [Schwickert et al. Sci. Adv. 2022, 8, eabn6848]. The electronic and nuclear motion on the femtosecond time scale was induced and probed in kinematically complete soft X-ray experiments at the FLASH free-electron laser facility. The detailed analysis of amplitude, instantaneous phase, and instantaneous frequency of the propagating many-body wave packet during its lifecycle provides unprecedented insight into dynamical processes beyond the Born-Oppenheimer approximation. We are confident that the refined experimental data evaluation helps to develop new theoretical tools to describe time-dependent molecular wave functions in complicated but ubiquitous non-Born-Oppenheimer photochemical conditions.

X-ray free-electron lasers are sources of coherent, high-intensity X-rays with numerous applications in ultra-fast measurements and dynamic structural imaging. Due to the stochastic nature of the self-amplified spontaneous emission process and the difficulty in controlling injection of electrons, output pulses exhibit significant noise and limited temporal coherence. Standard measurement techniques used for characterizing two-coloured X-ray pulses are challenging, as they are either invasive or diagnostically expensive. In this work, we employ machine learning methods such as neural networks and decision trees to predict the central photon energies of pairs of attosecond fundamental and second harmonic pulses using parameters that are easily recorded at the high-repetition rate of a single shot. Using real experimental data, we apply a detailed feature analysis on the input parameters while optimizing the training time of the machine learning methods. Our predictive models are able to make predictions of central photon energy for one of the pulses without measuring the other pulse, thereby leveraging the use of the spectrometer without having to extend its detection window. We anticipate applications in X-ray spectroscopy using XFELs, such as in time-resolved X-ray absorption and photoemission spectroscopy, where improved measurement of input spectra will lead to better experimental outcomes.

We present a SWIR wavelength dependence study of high harmonics from MgO and α -Al 2 O 3 . We report robust enhancement for harmonic energies corresponding to the widest bandgap accompanied by gradual changes to the harmonic anisotropy.

In molecular systems, the ultrafast motion of electrons initiates the process of chemical change. Tracking this electronic motion across molecules requires coupling attosecond time resolution to atomic-scale spatial sensitivity. In this work, we employ a pair of attosecond x-ray pulses from an x-ray free-electron laser to follow electron motion resulting from the sudden removal of an electron from a prototypical aromatic system, para-aminophenol. X-ray absorption enables tracking this motion with atomic-site specificity. Our measurements are compared with state-of-the-art computational modeling, reproducing the observed response across multiple timescales. Sub-femtosecond dynamics are assigned to states undergoing non-radiative decay, while few-femtosecond oscillatory motion is associated with electronic wavepacket motion in stable cation states, that will eventually couple to nuclear motion. Our work provides insight on the ultrafast charge motion preceding and initiating chemical transformations in moderately complex systems, and provides a powerful benchmark for computational models of ultrafast charge motion in matter.

The photoelectric effect is not truly instantaneous, but exhibits attosecond delays that can reveal complex molecular dynamics. Sub-femtosecond duration light pulses provide the requisite tools to resolve the dynamics of photoionization. Accordingly, the past decade has produced a large volume of work on photoionization delays following single photon absorption of an extreme ultraviolet (XUV) photon. However, the measurement of time-resolved core-level photoionization remained out of reach. The required x-ray photon energies needed for core-level photoionization were not available with attosecond tabletop sources. We have now measured the x-ray photoemission delay of core-level electrons, and here report unexpectedly large delays, ranging up to 700 attoseconds in NO near the oxygen K-shell threshold. These measurements exploit attosecond soft x-ray pulses from a free-electron laser (XFEL) to scan across the entire region near the K-shell threshold. Furthermore, we find the delay spectrum is richly modulated, suggesting several contributions including transient trapping of the photoelectron due to shape resonances, collisions with the Auger-Meitner electron that is emitted in the rapid non-radiative relaxation of the molecule, and multi-electron scattering effects. The results demonstrate how x-ray attosecond experiments, supported by comprehensive theoretical modelling, can unravel the complex correlated dynamics of core-level photoionization.

Solid High Harmonic Generation (HHG) has seen a recent surge in interest, as a promising path for bright, compact, and tuneable XUV sources as well as a spectroscopic tool to understand ultrafast electron dynamics in solids. The high electron density renders solids a good candidate for HHG [1]. However, to optimise the efficiency for the HHG process, a thorough understanding of the electron motion between the conduction and valence bands is key, yet remains inconclusive [2]. Probing the electron potential to have access to the microscopic properties of the crystal is the first step to achieve this understanding [3]. It has been previously demonstrated that the spectral properties of HHG are strongly related to the crystal symmetry and geometry [4].

We present spatially- and spectrally-resolved measurements of high harmonic generation in MgO and sapphire from 30 fs pulses at 780 nm. We show ring-like features that are distinctly different to gas-phase HHG. The harmonic yield shows harmonic independent 4-fold rotational symmetry from MgO and harmonic dependent 6-fold symmetry in sapphire.

Combinatorial post-translational modifications (PTMs),such asthose forming the so-called "histone code", have beenlinked to cell differentiation, embryonic development, cellular reprogramming,aging, cancers, neurodegenerative disorders, etc.Nevertheless, a reliable mass spectral analysis of the combinatorialisomers represents a considerable challenge. The difficulty stemsfrom the incompleteness of information that could be generated bythe standard MS to differentiate cofragmented isomeric sequences intheir naturally occurring mixtures based on the fragment mass-to-chargeratio and relative abundance information only. Here we show that fragment-fragmentcorrelations revealed by two-dimensional partial covariance mass spectrometry(2D-PC-MS) allow one to solve the combinatorial PTM puzzles that cannotbe tackled by the standard MS as a matter of principle. We introduce2D-PC-MS marker ion correlation approach and demonstrate experimentallythat it can provide the missing information enabling identificationof cofragmentated combinatorially modified isomers. Our insilico study shows that the marker ion correlations can beused to unambiguously identify 5 times more cofragmented combinatoriallyacetylated tryptic peptides and 3 times more combinatorially modifiedGlu-C peptides of human histones than is possible using standard MSmethods.

Ultrashort pulses can excite or ionize molecules and populate coherent electronic wavepackets, inducing complex dynamics. In this work, we simulate the coupled electron-nuclear dynamics upon ionization to different electronic wavepackets of (deuterated) benzene and fluoro-benzene molecules, quantum mechanically and in full dimensionality. In fluoro-benzene, the calculations unravel both inter-state and intra-state quantum interferences that leave clear signatures of attochemistry and charge-directed reactivity in the shape of the autocorrelation function. The latter are in agreement with experimental high harmonic spectroscopy measurements of benzenes and fluoro-benzene.