Helium nanodroplets provide a unique and versatile platform for investigating strong-field-driven nanoplasma dynamics. In this work, we present large-scale, GPU-accelerated particle-in-cell simulations using PIConGPU to study the interaction of pure helium nanodroplets containing up to 10^6 atoms with intense near-infrared femtosecond laser pulses, and compare the results with single-shot velocity-map electron imaging and ion measurements. The simulations describe the plasma evolution from the first ionization events to collective electron motion, nanoplasma formation, and early expansion. We show that the calculated electron and ion observables reproduce the main features of the measured spectra in systems with similar cluster sizes and laser intensities. Our results demonstrate that PIConGPU captures the essential physics of nanoplasma formation previously addressed mainly with molecular-dynamics or TDDFT approaches, while remaining computationally efficient and applicable to much larger systems. This establishes PIConGPU as a powerful and scalable tool for connecting nanoplasma theory with experimentally accessible observables.

Attosecond photoelectron interferometry, based on the measurement of photoelectron spectra generated by a two-colour field, provides access to the photoionisation dynamics of quantum systems. In general, due to the entanglement between the wave function of the emitted photoelectron and that of the parent ion, the dynamics driven by the infra-red field in the photoion can affect the properties of the photoemitted electronic wave packet, when the measurement protocol corresponds to the projection of the total time-dependent wave function onto a specific final state of the bipartite system. This is particularly relevant for molecules, due to their rich internal electronic and vibrational energy structure. Here we show how the polarisation of the ion influences the photoionisation dynamics by introducing an additional time delay in the photoelectrons emitted from CO2 molecules. The delay stems from the entanglement between the photoion and the photoelectron created in the photoionisation process.

We present coincidence measurements of two-photon double ionization (TPDI) of argon driven by femtosecond pulses tunable around 26.5eV photon energy, which are obtained from a high-harmonic generation source. The measured photoelectron spectra are interpreted with regard to three TPDI mechanisms. Theoretical predictions are obtained by an approximate model for direct TPDI and atomic structure calculations, which are implemented into a Monte Carlo simulation. The prevailing mechanism involves the excitation and prompt photoionization of an autoionizing resonance in neutral argon. We provide evidence for pronounced electron-electron interaction in this ultrafast ionization process. Furthermore, we show that the dominant TPDI mechanism can be altered by slight tuning of the photon energy. The present work paves the way for scrutinizing and controlling nonlinear photoionization in the extreme ultraviolet using table-top sources.

Laser-driven dynamics in polyatomic molecules poses a complex many-body problem. Understanding intense light-matter interaction is crucial for steering intramolecular quantum dynamical processes. Here, we record time-resolved x-ray diffraction images of C60 molecules during and after their interaction with intense near-infrared fields, giving direct access to structural changes of the molecules and their fragmentation in real time. Tuning the intensity of the excitation pulses, we uncover a transition from a weak-field regime of excited but stable molecules to a high-field regime dominated by Coulomb explosion. In the transition region, the molecules expand by up to 50% of their initial size within just 140 fs, with major fragmentation only setting in afterward. This work demonstrates that x-ray diffractive imaging is capable of retrieving time-resolved structural information of large molecules reshaped by intense laser fields. Laser-driven fragmentation is a first step toward observing molecular processes modified by laser fields of increasing intensity.

We formulate a general hybrid quantum-classical technique to describe the interaction of diatomic molecules with XUV pulses. We demonstrate the accuracy of our model in the context of the interaction of the O_2 molecule with an XUV pulse with photon energy ranging from 20 eV to 42 eV. We account for the electronic structure and electron ionization quantum mechanically employing accurate molecular continuum wavefunctions. We account for the motion of the nuclei using classical equations of motion. However, the force of the nuclei is computed by obtaining accurate potential-energy curves of O_2 up to O_2^2+, relevant to the 20 eV-42 eV photon-energy range, using advanced quantum-chemistry techniques. We find the dissociation limits of these states and the resulting atomic fragments and employ the Velocity Verlet algorithm to compute the velocities of these fragments. We incorporate both electron ionization and nuclear motion in a stochastic Monte-Carlo simulation and identify the ionization and dissociation pathways when O_2 interacts with an XUV pulse. Focusing on the O^+ + O^+ dissociation pathway, we obtain the kinetic-energy release distributions of the atomic fragments and find very good agreement with experimental results. Also, we explain the main features of the KER in terms of ionization sequences consisting of two sequential single-photon absorptions resulting in different O^+ and O^2+ electronic state configurations involved in the two transitions.

The dynamics of an x-ray ionized two-component core-shell nanosystem is experimentally probed using doped helium (He) nanodroplets. First, a soft x-ray pump pulse selectively inner-shell ionizes the core cluster of heavy rare-gas atoms, causing electron migration from the He shell to the highly-charged core. This leads to a strong damping of the core cluster’s spatial expansion. Thus, He droplets act as a tamper that reduces the radiation damage of embedded nanostructures, a property that could be exploited for improving x-ray coherent diffraction images. A near-infrared probe pulse, igniting and then driving the formation of a He nanoplasma, is used to obtain time-resolved information about the tamper effect. As evidenced by molecular dynamics simulations, the tamper effect is manifested by the time constant of the falling edge of the pump-probe signal curve.

Abstract For photo-dissociation of a single hydrogen molecule ( $$H_2$$ H 2 ) with combined XUV and IR laser pulses, we demonstrate optical control of the emission direction of the photoelectron with respect to the outgoing neutral fragment (the H-atom). Depending on the relative delay between the two laser fields, adjustable with sub-femtosecond time resolution, the photoelectron is emitted into the same hemisphere as the H-atom or opposite. This emission asymmetry is a result of entanglement of the two-electron final-state involving the spatially separated bound and emitted electron.

Photoionization can initiate structural reorganization of molecular matter and drive formation of new chemical bonds. Here, we used time-resolved extreme ultraviolet (EUV) pump - EUV probe Coulomb explosion imaging of carbon dioxide dimer ion (CO2)(2)(+) dynamics, that combined with ab initio molecular dynamics simulations, revealed unexpected asymmetric structural rearrangement. We show that ionization by the pump pulse induces rearrangement from the slipped-parallel (C-2h) geometry of the neutral CO2 dimer towards a T-shaped (C-2v) structure on the similar to 100 fs timescale, although the most stable slipped-parallel (C-2h) structure of the ionic dimer. Moreover, we find that excited states of the ionized CO2 dimer can exhibit formation of a CO3 moiety in the C2O42+ complex that can persist even after a suitably time-delayed second photoionization in a metastable C2O42+ dication. Our results suggest that charge asymmetry plays an important role in the ionization-induced dynamics in such dimers that are present in CO2 rich environments

We observe and modify a molecular vibrational wave packet in an electronically excited state of the neutral hydrogen molecule. In an extreme-ultraviolet (XUV) time-domain absorption spectroscopy experiment, we launch a vibrational wave packet in the D 1ju 3pn state of H2 and track its time evolution via the coherent dipole response. The reconstructed time-dependent dipole from experimentally measured XUV absorption spectra provides access to the revival of the vibrational wave packet, which we control via an intense near-infrared (NIR) pulse. Tuning the intensity of the NIR pulse, we observe the revival of the wave packet to be significantly modified, which is supported by the results of a multilevel simulation as well as an analytical model based on state-specific phase shifts. The NIR field is applied only 7 fs after the creation of the wave packet but influences its evolution up to at least its first revival at 270 fs. This experimental approach for nonlocal-in-time laser modification of quantum dynamics in a pump-control scheme enabled by molecular self-probing is generally applicable to a large range of molecules and materials as it only requires the observation of absorption spectra.

High temporal resolution is essential for ultra-fast pump-probe experiments. Arrival time jitter and drift measurements, as well as their control, become critical especially when combining XUV or X-ray free-electron lasers (FELs) with optical lasers due to the large scale of such facilities and their distinct pulse generation processes. This paper presents the application of a laser pulse arrival time monitor that actively corrects the arrival time of an optical laser relative to the FEL's main optical clock. Combined with post-analysis single pulse jitter correction this new approach improves the temporal resolution for pump-probe experiments significantly. Benchmark measurements on photo-ionization of xenon atoms performed at FLASH beamline FL26, demonstrate a sub-50 fs FWHM overall temporal resolution.

Following up on a previous paper on two-color photoionization of Ar(3p) [Bharti et al., Phys. Rev. A 103 (2021) 022834], we present measurements and calculations for a modified three-sideband (3-SB) version of the "reconstruction of attosecond beating by interference of two-photon transitions" (RABBITT) configuration applied to He(1s). The 3-SB RABBITT approach allows us to explore interference effects between pathways involving different orders of transitions within the continuum. The relative differences in the retrieved oscillation phases of the three sidebands provide insights into the continuum-continuum transitions. The ground state of helium has zero orbital angular momentum, which simplifies the analysis of oscillation phases and their angle-dependence within the three sidebands. We find qualitative agreement between our experimental results and the theoretical predictions for many cases but also observe some significant quantitative discrepancies.

The relaxation processes in atomic xenon following core ionization of the 4d and 4p subshells by extreme ultraviolet (XUV) pulses from a free-electron laser (FLASH) are investigated using ion time-of-flight spectroscopy. We compare the dynamics following ionization and Auger-Meitner decays at 90-eV photon energy, i.e., near the giant resonance, with those at 160 eV, near the Cooper minimum, where cross sections for photoionization of the 4d and 4p subshells are similar. Final states with charges higher than 4 show signatures of sequential absorption of two XUV photons, followed by subsequent Auger-Meitner decay. The averaged lifetimes of some important excited states are measured in a two-color XUV-pump-near-infrared-probe experiment. A transient enhancement in the ion yield of Xe5+ with an average lifetime of (49 +/- 3) fs is obtained, attributed to transient intermediate states following the decay of 4d double-core-hole states.

We have measured triple differential momentum distributions for electron detachment in collisions of 30- and 300-keV anions (C− and Si−) with atoms and molecules (He, Ar, and N2). The entire angular range of the ejected electrons was covered. The binary encounter ring, well known from positive ion-atom collisions, was observed in most cases. In a quasifree-electron scattering model this structure represents elastic scattering of the active electron from the target. However, especially for the slowest projectiles, this model fails to reproduce the angular distributions. In this regime it is probably necessary to use quasimolecular models to describe the collision. Indications for a projectile cusp are observed, implying that excitation-induced dipole moments in the ion projectile may be important. Published by the American Physical Society 2024

Isotopic substitution in molecular systems can affect fundamental molecular properties including the energy position and spacing of electronic, vibrational and rotational levels, thus modifying the dynamics associated to their coherent superposition. In extreme ultraviolet spectroscopy, the photoelectron leaving the molecule after the absorption of a single photon can trigger an ultrafast nuclear motion in the cation, which can lead, eventually, to molecular fragmentation. This dynamics depends on the mass of the constituents of the cation, thus showing, in general, a significant isotopic dependence. In time-resolved attosecond photoelectron interferometry, the absorption of the extreme ultraviolet photon is accompanied by the exchange of an additional quantum of energy (typically in the infrared spectral range) with the photoelectron-photoion system, offering the opportunity to investigate in time the influence of isotopic substitution on the characteristics of the photoionisation dynamics. Here we show that attosecond photoelectron interferometry is sensitive to isotopic substitution by investigating the two-color photoionisation spectra measured in a mixture of methane (CH$_4$) and deuteromethane (CD$_4$). The isotopic dependence manifests itself in the modification of the amplitude and contrast of the oscillations of the photoelectron peaks generated in the two-color field with the two isotopologues. The observed effects are interpreted considering the differences in the time evolution of the nuclear autocorrelation functions in the two molecules.

The electronic and nuclear dynamics inside molecules are essential for chemical reactions, where different pathways typically unfold on ultrafast timescales. Extreme ultraviolet (XUV) light pulses generated by free-electron lasers (FELs) allow atomic-site and electronic-state selectivity, triggering specific molecular dynamics while providing femtosecond resolution. Yet, time-resolved experiments are either blind to neutral fragments or limited by the spectral bandwidth of FEL pulses. Here, we combine a broadband XUV probe pulse from high-order harmonic generation with an FEL pump pulse to observe dissociation pathways leading to fragments in different quantum states. We temporally resolve the dissociation of a specific O2+ state into two competing channels by measuring the resonances of ionic and neutral fragments. This scheme can be applied to investigate convoluted dynamics in larger molecules relevant to diverse science fields.

Ultrafast H2+ and H3+ formation from ethanol is studied using pump-probe spectroscopy with an extreme ultraviolet (XUV) free-electron laser. The first pulse creates a dication, triggering H2 roaming that leads to H2+ and H3+ formation, which is disruptively probed by a second pulse. At photon energies of 28 and 32 eV, the ratio of H2+ to H3+ increases with time delay, while it is flat at a photon energy of 70 eV. The delay-dependent effect is ascribed to a competition between electron and proton transfer. High-level quantum chemistry calculations show a flat potential energy surface for H2 formation, indicating that the intermediate state may have a long lifetime. The ab initio molecular dynamics simulation confirms that, in addition to the direct emission, a small portion of H2 undergoes a roaming mechanism that leads to two competing pathways: electron transfer from H2 to C2H4O2+ and proton transfer from C2H4O2+ to H2.

Upon ionization, water forms a highly acidic radical cation H 2 O +· that undergoes ultrafast proton transfer (PT)—a pivotal step in water radiation chemistry, initiating the production of reactive H 3 O + , OH ⋅ radicals, and a (hydrated) electron. Until recently, the time scales, mechanisms, and state-dependent reactivity of ultrafast PT could not be directly traced. Here, we investigate PT in water dimers using time-resolved ion coincidence spectroscopy applying a free-electron laser. An extreme ultraviolet (XUV) pump photon initiates PT, and only dimers that have undergone PT at the instance of the ionizing XUV probe photon result in distinct H 3 O + + OH + pairs. By tracking the delay-dependent yield and kinetic energy release of these ion pairs, we measure a PT time of (55 ± 20) femtoseconds and image the geometrical rearrangement of the dimer cations during and after PT. Our direct measurement shows good agreement with nonadiabatic dynamics simulations for the initial PT and allows us to benchmark nonadiabatic theory.

We report a joint experimental and theoretical study of a three-sideband (3-SB) modification of the reconstruc-tion of attosecond beating by interference of two-photon transitions (RABBIT) setup. The 3-SB RABBIT scheme makes it possible to investigate phases resulting from interference between transitions of different orders in the continuum. Furthermore, the strength of this method is its ability to focus on the atomic phases only, independent of a chirp in the harmonics, by comparing the RABBIT phases extracted from specific SB groups formed by two adjacent harmonics. We verify earlier predictions that the phases and the corresponding time delays in the three SBs extracted from angle-integrated measurements become similar with increasing photoelectron energy. A variation in the angle dependence of the RABBIT phases in the three SBs results from the distinct Wigner and continuum-continuum coupling phases associated with the individual angular-momentum channels. A qualitative explanation of this dependence is attempted by invoking a propensity rule. Comparison between the experimental data and predictions from an R matrix (close-coupling) with time dependence calculation shows qualitative agreement in most of the observed trends.

In extreme ultraviolet spectroscopy, the photoionization process occurring in a molecule due to the absorption of a single photon can trigger an ultrafast nuclear motion in the cation. Taking advantage of attosecond photoelectron interferometry, where the absorption of the extreme ultraviolet photon is accompanied by the exchange of an additional infrared quantum of light, one can investigate the influence of nuclear dynamics by monitoring the characteristics of the photoelectron spectra generated by the two-color field. Here, we show that attosecond photoelectron interferometry is sensitive to the nuclear response by measuring the two-color photoionization spectra in a mixture of methane (CH4) and deuteromethane (CD4). The effect of the different nuclear evolution in the two isotopologues manifests itself in the modification of the amplitude and contrast of the oscillations of the photoelectron peaks. Our work indicates that nuclear dynamics can affect the coherence properties of the electronic wave packet emitted by photoionization on a time scale as short as a few femtoseconds.

Velocity map imaging (VMI) is a powerful technique to deduce the kinetic energy of ions or electrons that are produced from a large volume in space with good resolution. The size of the acceptance volume is determined by the spherical aberrations of the ion optical system. Here we present an analytical derivation for velocity map imaging with no spherical aberrations. We will discuss a particular example for the implementation of the technique that allows using the reaction microscope recently installed in the cryogenic storage ring (CSR) in a VMI mode. SIMION simulations confirm that a beam of electrons produced almost over the entire volume of the source region, with a width of 8 cm, can be focused to a spot of 0.1 mm on the detector. The use of the same formalism for position imaging, as well as in a mixed mode where position imaging is in one axis and velocity map imaging is in a different axis, is also discussed.