We report a joint experimental and theoretical study using a combination of polarization-controlled freeelectron-laser (FEL) and near-infrared (NIR) pulses in a synchronized two-color photoionization scheme. Excited He+ ions, created by extreme ultraviolet (XUV) circularly polarized radiation from the XUV-FEL FERMI in the oriented 3p (m= +1) state, are exposed to circularly polarized 784-nm NIR radiation with peak intensities from 1012 to 1013 W/cm2. The angular distribution of the ejected electrons exhibit a strong dichroism depending on the NIR intensity. While the corotating case is defined by a single path, for the counterrotating case, there are two dominant pathways whose relative strength and phase difference are determined.

The Wigner delay is defined as the energy derivative of the scattering phase of a particle in a given potential, unveiling the time taken (or gained) due to the interaction. The characterisation of this delay plays a central role in attosecond science, where the time resolution allows to gain information on the time interval required for a photoelectron to be emitted into the continuum after the absorption of a single photon. Attosecond interferometric techniques, based on two-colour (extreme ultraviolet and near-infrared) photoionisation schemes, cannot provide a direct measurement of the Wigner delay, because the low-frequency photon contributes with an additional delay, which is imprinted on the outgoing photoelectron. The isolation of the Wigner delay is usually achieved by appealing to the asymptotic approximation, which assumes that the two-photon delay is separable into a Wigner and a near-infrared-induced phase and provides a universal analytical expression for the latter. In this study, we introduce a self-referencing approach based on the implementation of non-consecutive extreme ultraviolet harmonics, in order to test the validity of the asymptotic approximation. We demonstrate its breakdown by observing a deviation of a few tens of milliradians (corresponding to a few attoseconds) between its predictions and the experimentally measured phases of the sideband oscillations generated in our scheme, in agreement with full-dimensional simulations.

Time-resolved X-ray photoelectron spectroscopy (XPS) is used to track the photodissociation dynamics of 2-iodothiophene following 262 nm excitation. The transient XPS features include both direct ionization of the initially populated excited states and pronounced satellite peaks arising from shake-down processes. While the direct ionization signals exhibit only minimal energy shifts during C-I bond cleavage, the shake-down transitions undergo a substantial, 5 eV, shift over the reaction coordinate. By correlating these shifts with simulated C-I bond lengths, a direct structural mapping is established that reveals the exceptional sensitivity of shake-down channels to molecular geometry. These results demonstrate that shake-down transitions provide a new and powerful probe of ultrafast structural dynamics.

In seeded free-electron lasers (FELs), harmonic up-conversion of an external seed laser can be used to generate coherent emission at short wavelengths. The harmonic generation process is sensitive to the electron beam slice energy spread, which limits the highest harmonic conversion factor. We present here an experimental study of the impact of the slice energy spread on the FEL performance in two different harmonic generation schemes: high-gain harmonic generation and echo-enabled harmonic generation (EEHG). The slice energy spread is controlled by means of a laser heater installed in the low energy region of the linear accelerator that drives the FEL. The measurements of the FEL pulse energy as a function of the electron beam slice energy spread were performed at different harmonics. Our results confirm the robustness of EEHG to the slice energy spread and open the way toward fully coherent FEL pulses in the soft x-ray domain.

Ultrafast time-resolved photoelectron spectra are reported for the vacuum-ultraviolet (VUV) photoionization of acetylene following excitation to the Ã1Au state via UV absorption at 200 nm. The excitation energy lies above the lowest dissociation threshold to C2H X̃2Σ+ + H, as well as above the threshold for adiabatic dissociation of the Ã1Au state to form C2H (Ã2Π) + H. The time-dependent mass spectra and photoelectron spectra provide insight into the intramolecular decay processes of the Ã1Au state. In addition, photoelectron spectra of the Ã1Au state with VUV light access both the X̃2Πu and Ã2Σg+ states of the ion, as well as the predicted, but previously unobserved, 1 2Πg state, which corresponds to a two-hole, one-particle configuration that lies in close proximity to the Ã2Σg+ state. The 1 2Πg state is split into 2A2 + 2B2 and 2Ag + 2Bg states in the cis and trans configurations, respectively. Electronic structure calculations, along with trajectory calculations, reproduce the principal features of the experimental data and confirm the assignment of the 1 2Πg state.

Time-resolved resonant inelastic X-ray scattering (tr-RIXS) is a powerful technique for probing quasiparticle interactions in quantum materials under nonequilibrium conditions. Here, we implement tr-RIXS at the carbon K-edge to investigate the ultrafast dynamics of core excitons coupled to vibrational modes in graphite. Using femtosecond X-ray pulses from a free-electron laser, we monitor the temporal evolution of vibronically dressed excitons and their interaction with symmetry-selective optical phonons. By tuning the incident photon energy across the 1s → σ* resonance and analyzing the integrated inelastic sideband intensity, we reveal a detuning-controlled crossover between two complementary dynamical regimes. Phenomenological modeling and first-principles calculations reproduce both the magnitude and detuning dependence of the spectral-weight changes. In this work, enabled by the unique capabilities of X-ray free-electron lasers, we demonstrate how tr-RIXS can access coupled electronic and lattice dynamics with elemental and symmetry specificity, opening new routes to control vibronic interactions in light-element and low-dimensional quantum materials. Core excitons are strongly localised excitonic states impacting x-ray absorption and resonant inelastic scattering (RIXS) spectra. Here, the authors demonstrate an application of free electron laser-driven ultrafast RIXS spectroscopy to study previously unclear aspects of core exciton-phonon interactions in graphite.

Tracking the multifarious ultrafast electronic and structural changes occurring in a molecule during a photochemical transformation is a challenging endeavor that benefits from recent experimental and computational progress in time-resolved techniques. Measurements of valence electronic states, which provide a global picture of the bonding structure of the molecule, and core electronic states, which provide insight into the local environment, traditionally require different approaches and are often studied separately. Here, we demonstrate that X-ray pulses from a seeded free-electron laser (FEL) enable the measurement of high-resolution, time-resolved X-ray photoelectron spectra (XPS) that capture weak satellite states resulting from shake-down processes in a valence-excited molecule. This approach effectively combines the advantages of both valence- and core-state investigations. We applied this method to investigate photoexcited CS_2 molecules, where the role of internal conversion (IC) and intersystem crossing (ISC) in determining the pre-dissociation dynamics is controversial. We present XPS spectra from photoexcited CS_2, obtained at the FERMI FEL. High-resolution measurements, compared to the corresponding spectra obtained from accurate multireference quantum chemical calculations, reveal that shake-down satellite channels are highly sensitive to both valence electronic and geometric changes. Previous studies of the pre-dissociation dynamics have led to uncertain assignments of the branching between singlet and triplet excited states. We derive a propensity rule that demonstrates the spin-selectivity of the shake-downs. This selectivity allows us to unequivocally assign contributions from the bright and dark singlet excited states, with populations tracked along the pre-dissociation dynamic pathway.

The metrology of attosecond pulse trains is based on a cross-correlation technique between a comb of extreme ultraviolet harmonics generated by the high-order harmonic generation process and a synchronised infrared field. The approach, usually referred to as reconstruction of attosecond beating by interference of two-photon transitions (RABBIT), allows one to recover the relative phase between the comb of consecutive odd harmonics, thus providing access to the attosecond temporal structure of the radiation. Seeded free-electron lasers have recently demonstrated the generation of combs consisting of even and odd harmonics of the seeding radiation. In this scheme, each harmonic is generated by an independent undulator (or set thereof), providing an additional degree of freedom in selecting the specific harmonics that make up the extreme ultraviolet comb. Here, we present results on the generation and temporal characterisation of a comb consisting of non-consecutive harmonics. The single-shot correlation analysis of the photoelectron spectra and the reordering of the single-shot data using an attosecond timing tool allow the reconstruction of the group delay dispersion of the harmonic comb and the temporal reconstruction of the attosecond pulse train.

The light-induced ultrafast switching between molecular isomers norbornadiene and quadricyclane can reversibly store and release a substantial amount of chemical energy. Prior work observed signatures of ultrafast molecular dynamics in both isomers upon ultraviolet excitation but could not follow the electronic relaxation all the way back to the ground state experimentally. Here we study the electronic relaxation of quadricyclane after exciting in the ultraviolet (201 nanometres) using time-resolved gas-phase extreme ultraviolet photoelectron spectroscopy combined with non-adiabatic molecular dynamics simulations. We identify two competing pathways by which electronically excited quadricyclane molecules relax to the electronic ground state. The fast pathway (<100 femtoseconds) is distinguished by effective coupling to valence electronic states, while the slow pathway involves initial motions across Rydberg states and takes several hundred femtoseconds. Both pathways facilitate interconversion between the two isomers, albeit on different timescales, and we predict that the branching ratio of norbornadiene/quadricyclane products immediately after returning to the electronic ground state is approximately 3:2.

Wave packet interferometry with vacuum ultraviolet light has been used to probe a complex region of the electronic spectrum of molecular nitrogen, N2. Wave packets of Rydberg and valence states were excited by using double pulses of vacuum ultraviolet (VUV), free-electron-laser (FEL) light. These wave packets were composed of contributions from multiple electronic states with a moderate principal quantum number (n ∼ 4–9) and a range of vibrational and rotational quantum numbers. The phase relationship of the two FEL pulses varied in time, but as demonstrated previously, a shot-by-shot analysis allows the spectra to be sorted according to the phase between the two pulses. The wave packets were probed by angle-resolved photoionization using an infrared pulse with a variable delay after the pair of excitation pulses. The photoelectron branching fractions and angular distributions display oscillations that depend on both the time delays and the relative phases of the VUV pulses. The combination of frequency, time delay, and phase selection provides significant control over the ionization process and ultimately improves the ability to analyze and assign complex molecular spectra.

Ultrashort pulses in the deep-UV region around 200 nm have important applications in laser spectroscopy and Free-Electron laser technology [1,2]. Such pulses can be obtained using Ti:Sapphire laser followed by a fourth harmonic generation (FHG). In this case due to phase matching limitations of the available nonlinear crystals the fourth harmonic (FH) can only be generated in a three-step setup starting with second harmonic generation (SHG), followed by third harmonic generation (THG) realized by sum frequency mixing (SF) of SH and fundamental pulses, and then a final FHG stage based on sum frequency mixing of third harmonic (TH) and fundamental pulses.

Tailored light-matter interactions in the strong coupling regime enable the manipulation and control of quantum systems with up to unit efficiency, with applications ranging from quantum information to photochemistry. While strong light-matter interactions are readily induced at the valence electron level using long-wavelength radiation, comparable phenomena have been only recently observed with short wavelengths, accessing highly-excited multi-electron and inner-shell electron states. However, the quantum control of strong-field processes at short wavelengths has not been possible, so far, due to the lack of pulse shaping technologies in the extreme ultraviolet (XUV) and X-ray domain. Here, exploiting pulse shaping of the seeded free-electron laser (FEL) FERMI, we demonstrate the strong-field quantum control of ultrafast Rabi dynamics in helium atoms with high fidelity. Our approach unravels a strong dressing of the ionization continuum, otherwise elusive to experimental observables. The latter is exploited to achieve control of the total ionization rate, with prospective applications in many XUV and soft X-ray experiments. Leveraging recent advances in intense few-femtosecond to attosecond XUV to soft X-ray light sources, our results open an avenue to the efficient manipulation and selective control of core electron processes and electron correlation phenomena in real time.

We have generated elliptically polarized femtosecond pulses from a seeded free-electron laser (FEL) in the spectral range 700-800 eV and conducted demonstrative experiments based on magnetic circular dichroism, at the Fe L3 edge. This was achieved by making use of the radiation produced at the third harmonic of the fundamental FEL wavelength lambda. A planar radiator emits linearly polarized harmonics along the undulator axis, while a helical undulator produces off-axis harmonic radiation with circular polarization. However, off-axis circularly polarized light is characterized by a rather low flux and, as a result, it is unsuitable for most experiments that rely on circular dichroism. Here, we demonstrate that on-axis radiation with a substantial degree of circular polarization at lambda/3 can be generated using a variable-polarization (APPLE-type) radiator, finely tuned in a hybrid-polarization mode. The experiments, which demonstrate the potential of the adopted configuration for magnetic dichroism studies, were carried out at the FERMI FEL facility in Trieste (Italy). These results pave the way to experiments at the 2p-3d resonances of the transition metals most relevant for magnetic studies (Fe, Co, Ni) and make it possible to envisage investigations, e.g., spectroscopic studies with different core-hole lifetimes, covering both the M (50-70 eV) and L (600-900 eV) edges, at the same seeded FEL source.

The photodissociation dynamics of UV excited CS2 are investigated using time-resolved Auger-Meitner (AM) spectroscopy. AM decay is initiated by inner-shell ionisation with a femtosecond duration x-ray (179.9 eV) probe generated by the FERMI free electron laser. The time-delayed x-ray probe removes an electron from the S(2p) orbital leading to secondary emission of a high energy electron through AM decay. We monitor the electron kinetic energy of the AM emission as a function of pump-probe delay and observe time-dependent changes in the spectrum that correlate with the formation of bound, excited-state CS2 molecules at early times, and CS + S fragments on the picosecond timescale. The results are analysed based on a simplified kinetic scheme that provides a time constant for dissociation of approximately 1.2 ps, in agreement with previous time-resolved x-ray photoelectron spectroscopy measurements.

The circular dichroism (CD) of photoelectrons generated by near-infrared (NIR) laser pulses using multiphoton ionization of excited He+ ions in the 3p(m= +1) state is investigated. The ions were prepared by circularly polarized extreme ultraviolet (XUV) pulses. For circularly polarized NIR pulses co- and counter-rotating relative to the polarization of the XUV pulse, a complex variation of the CD is observed as a result of intensity- and polarization-dependent Freeman resonances, with and without additional dichroic AC-Stark shifts. The experimental results are compared with numerical solutions of the time-dependent Schrödinger equation to identify and interpret the pronounced variation of the experimentally observed CD.

We report a two-dimensional pump-control-probe spectroscopic study of the dynamics of singly excited He Rydberg-state wave packets with a seeded extreme ultraviolet (XUV) free-electron laser (FEL) source. A pair of coherent XUV pulses, defined by their coarse time separation and relative phase, created and manipulated the wave packets. The He atoms were postionized by infrared (IR) pulses, and the ion yield was measured as a function of XUV phase and IR arrival time. We tagged and sorted the relative phase of the XUV pulse pair on a single-shot basis by fitting each FEL spectrum with a suitable function that accounts for nonidealities of the XUV pulse pairs, associated with the seeding process; more generally, the fit returns the time-dependent electric field of the FEL spectra. The experimental two-dimensional maps of ion yields, measured as a function of IR (probe) delay and of XUV (pump-control) phase, were compared with the solution of the first-order time-dependent Schr & ouml;dinger equation for this field. Despite the fact that the experimental conditions imply strong excitation, beyond the approximations of first-order perturbation theory, the simulated map satisfactorily reproduces the experimental one for temporally well-separated pulses. We show that by selecting data at appropriate values of pump-control phase, we enhance or suppress the amplitude of chosen wave-packet components consisting of two or more Rydberg states. When the temporal overlap of the pulse pair cannot be neglected, the phase reconstruction is underdetermined, and we provide a simplified comparison between data and simulations.

Ultrashort deep ultraviolet (DUV) pulses serve as indispensable tools for investigating molecular dynamics on the femtosecond scale. Nonlinear frequency upconversion of near-infrared (NIR) light sources in a sequence of nonlinear crystals is a common method for their generation. However, preserving the temporal duration of the starting source encounters challenges owing to phase-matching bandwidth limitations within the harmonic generation process. Here we propose an approach for circumventing this limitation and demonstrate it for the case of generation of the third harmonic of 800 nm pulses in a two-stage scheme (second harmonic generation succeeded by sum-frequency mixing of the fundamental and second harmonic pulses). Expanding the bandwidth of the DUV pulse involves the utilization for the last mixing process of two nonlinear crystals, detuned to convert opposite sides of the spectrum. The implementation of this approach yields 20 µJ, 263 nm DUV pulses as short as 19 fs after compression. The setup is very compact and extremely stable due to the common-path scheme, which makes it very interesting for a variety of advanced ultrafast spectroscopy applications.

We present a compact, intrinsically stable common path interferometer for the seeding of free-electron lasers (FELs). The interferometer can handle the required ultraviolet seed wavelengths and features an excellent phase stability of 10 mrad at 265 nm. By seeding the FEL FERMI, we demonstrate the generation of extreme ultraviolet (XUV) pulse pairs with tunable delay and a delay stability of 6 as at 52.5 nm. Prospective applications are Fourier transform spectroscopy, nonlinear spectroscopy and coherent control experiments in the XUV and x-ray domain.

Generation of high energy few-fs pulses in the ultraviolet (UV) still represents challenges due to compression and phase control difficulties in this spectral range. Presented here is a pulse compression approach utilizing cross-phase modulation within a thin solid-state medium induced by a strong, spatially and temporally controllable near-infrared (NIR) pulse acting on a weaker, 400 nm UV pulse. Through this method, four-fold compression is attained within a single fused silica plate, resulting in a 13 fs UV pulse with preserved beam quality. With some further technical adjustments, this method’s applicability could be extended to deep or even vacuum UV, where direct compression is difficult.

A new compact common-path scheme for Fourth-Harmonic Generation (FHG) of femtosecond laser pulses is proposed here. Guidelines for selection of non-linear crystals and birefringent crystals used for delay pre-compensation are derived based on approximation of non-saturated sum-frequency mixing of Gaussian pulses. We experimentally demonstrate the scheme and study its performance for a different combinations of BBO nonlinear crystals for the case of generation of sub-200 nm deep ultraviolet pulses by FHG of pulses delivered by a Ti:Sapphire ultrafast laser. Pulses with duration between 130 fs and 370 fs and pulse energy of 19 to 85 mu J were generated. It is shown, that in addition to its compactness and intrinsic stability, the scheme allows a superior spectrum and pulse quality.