The manuscript describes the preliminary design of a confocal soft X-ray microscope that utilizes microchannel plates (MCPs) as both the condenser and objective. This project funded by the INFN's Vth Committee aims to demonstrate the feasibility of a versatile X-ray microscope based on diffractive metalenses of low cost. The experimental setup includes a customized high-vacuum (HV) chamber with two hexapods manipulators for the precise alignment of the lens. Preliminary data obtained using flat MCPs are also presented. Synchrotron radiation tests have demonstrated the condensing capabilities and dispersive properties of MCPs, highlighting their potential for the manipulation of the radiation beam.

We determine with high accuracy the energy of the inner-shell transition 1s(2)2s(2 1)S(0) -> 1s2s(2)2p(3/2) P-1(1) O-16(K alpha)4+ at 554.372(3) eV (lambda= 22.36480(12) & Aring;) as well as its small shift of 2.2 +/- 1.3 meV (Delta lambda= 0.089(52) m & Aring;) for the O-18 isotope. This transition blends with a K alpha line of O5+ used in astrophysical diagnostics, potentially affecting its reliability. In contrast to our experimental uncertainty of +/- 3 meV, advanced electronic structure predictions for this four-electron system, including quantum electrodynamic (QED) corrections on the order of 100 meV, still scatter by more than +/- 250 meV. Ions generated and stored in an electron beam ion trap were excited at the ELETTRA synchrotron facility with monochromatic soft x rays, with photon energies corrected by an additional spectrometer. Upon resonant excitation of O4+ and their subsequent autoionization, we separate the photoions of each isotope by a time-of-flight measurement. This way, we resolve soft x-ray isotopic shifts of a few meV, obtain very accurate data on essential astrophysical ions, and test calculations down to the level of QED contributions.

This paper describes the design and experimental validation of the OctoIonGuide, a compact setup for high-resolution photoionization studies of low-charge atomic ions with synchrotron radiation. The system integrates an axial ion source, a 40 cm RF octupole ion guide, and a quadrupole mass analyser, providing precise ion transport and mass selection. A proof-of-principle experiment on Xe+ ions in the 20-22 eV range demonstrated the ability to resolve Rydberg resonances converging to the Xe2+. The recorded spectra showed excellent agreement with reference data, confirming the accuracy and reliability of the setup. The novelty of this work lies in combining these components into a single, integrated platform, offering improved ion handling and compatibility with synchrotron and free-electron laser facilities. The OctoIonGuide represents a versatile tool for studying ionization dynamics relevant to laboratory astrophysics and plasma physics.

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.

The effects of hydrogen bonding (HB) on the electronic structure of melamine films are investigated by analyzing the electronic states of hydrogen-bonded aggregates in terms of the molecular orbitals (MOs) of the isolated monomer. This approach provides an orbital description of HB directly connected to the spectroscopic signatures observed in valence-level photoemission spectroscopy (VL-PES) and N 1s near edge X-ray absorption fine structure spectroscopy (NEXAFS). The orbital analysis reveals that the hydrogen bond in melamine is described by the interaction between the triazine nitrogen lone pairs (Nlp) and occupied σ(N-H) orbitals, leading to the formation of bonding and antibonding HB states. The antibonding states acquire additional stabilization through a small admixture of the unoccupied σ*(N-H) orbitals of the monomer. Hydrogen bonding is expected to induce splittings of states with lone-pair and σ(N-H) character. In the valence photoemission spectrum, however, these splittings are too small to be resolved directly. In contrast, clear fingerprints of hydrogen bonding emerge in the N 1s NEXAFS spectrum. The MOs derived from the monomer LUMO+1 and LUMO+2, both characterized by σ*(N-H) character, undergo substantial modifications upon hydrogen-bond formation, including orbital mixing, changes in spatial localization, and a redistribution of spectral intensity over a broad photon-energy range. These effects lead to measurable shifts and intensity changes in the NEXAFS resonances, providing direct spectroscopic evidence of the hydrogen-bond-induced perturbation of the electronic structure and supporting the molecular-orbital description of hydrogen bonding in a complex organic supramolecular network.

We present α-Al2O3 XAS, XES and RIXS measurements across the Al L2/L3 edges at about 79 eV excitation energy. In the emission spectra, we identify two fluorescence peaks, corresponding to electronic transitions into the 2p core hole from mixed states of Al 3s and Al 3d character, both mixed with O 2p orbitals. Even if the XAS spectrum shows more than one resonance, surprisingly only one clear RIXS signal with energy loss equal to 10.7 eV is present in the data. Nevertheless, this allows us to tentatively extract from the measured high-resolution data the linewidths for fluorescence and RIXS transitions, with the latter being almost a factor of two smaller than the former.

Two synchrotron-based studies on 4H-pyran-4-thione, photoelectron spectroscopy and vacuum ultraviolet (VUV) absorption spectra were performed. A highly resolved structure was observed in the photoelectron spectrum (PES), in contrast to an earlier PES study, where little structure was observed. The sequence of ionic states was determined using configuration interaction and coupled cluster methods. The vibrational structure of the lowest three PES bands was analyzed by configuration interaction and density functional calculations, providing a detailed explanation of the observed profiles. Several vibrational bands in the VUV absorption spectrum showed a similar structure to the bands in the PES and were identified as Rydberg states.

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.

Gratings used in laboratory applications such as compensated and non-compensated monochromators have usually a plane geometry and require additional optical elements, mirrors, to focalize the selected spectral component. An active grating, whose curvature radius can be adjusted with a suitable mechanism, can at the same time select a spectral component and focalize it, reducing the number of mirrors in the apparatus. In this paper, we present the mechanical configuration and the optical properties in the extreme ultraviolet of a low-cost active grating. The system has been tested in the 13-50 eV region, showing good optical properties and high efficiency, and the shape of the bent grating has been analyzed using a wavefront sensor. The proposed system could be used both in table-top experiments and in large scale facilities.

The ultraviolet and vacuum ultraviolet (VUV) photo absorption spectra of fulvene were reconsidered by a combination of configuration interaction and density functional methods and extended to the newly acquired VUV photo absorption spectrum of the 6,6-dimethylfulvene derivative, where several Rydberg states have been identified. Singlet states of fulvene were studied using multi-root multi-reference configuration interaction with the H2C unit either coplanar with, or perpendicular to the ring. In contrast to ethylene, the lowest states are coplanar. The vibrational structure of the lowest (1B2) excited states of fulvene is well reproduced by calculated values. The second singlet state of fulvene, previously assigned as 1A1 on the basis of intensity, is incompatible with the calculated planar 1A1 state, which itself is a saddle point. This state shows significant quartic character on bending, and the best interpretation of the observed UV band of fulvene is of a bent form. The UV spectral state with low onset intensity is probably a 1B1 state of CS symmetry. Theoretical Rydberg states were determined for fulvene; the closest fit to the two known Rydberg states is to the 3p and 4p states (1B1). Comparison of the separation of the 2A2 and 2B1 states in the photoelectron spectra of the two compounds, with the threshold photoelectron spectrum of fulvene, shows that the 2B1 state vibrational structure is largely lost for both molecules. A reconsideration of the interaction between the X2A2 and A2B1 ionic states has led to the identification of the 4A2 quartic state of fulvene.

The calibration activities of the COmpact SPectrometer—COSP for the FERMI Free-Electron Laser (FEL) facility at the Elettra Synchrotron (Italy) are presented. COSP is an in-house built grating spectrometer designed to be used during the optimization of the FERMI parameters and to control the relative stability between different FEL harmonics in the multi-harmonic emission mode. The spectrometer is designed to work in single-shot mode at a repetition rate of 50 Hz providing medium resolution in a wide spectral range in order to either measure the separate intensities of the harmonics being mixed in a multi-color experiment or to quantify the amount of possible spurious harmonics. These activities are of key importance in the new class of experiments based on the wave mixing paradigm tested at the seeded FEL FERMI.

Exploiting acceleration gradients that are up to three orders of magnitude higher than those achievable using conventional radiofrequency-based devices, plasma-based devices promise a revolution in particle acceleration, enabling particles to reach high energies over much shorter distances than existing accelerators [...]

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.

We report a study of the electronic and nuclear relaxation dynamics of the photoexcited RNA base uracil in the gas phase using time-resolved core-level photoelectron spectroscopy together with high-level calculations. The dynamics was investigated by trajectory surface hopping calculations, and the core ionization energies were calculated for geometries sampled from these. The molecule was excited by a UV laser and dynamics probed on the oxygen, nitrogen, and carbon sites by core electron spectroscopy. We find that the main de-excitation channel of the initially excited S2(ππ*) state involves internal conversion to the S1(nπ*) state with a time constant of 17 ± 4 fs, while a portion of S2(ππ*) population returns directly to the ground state by internal conversion. We find no evidence that the S1(nπ*) state decays to the ground state; instead, it decays to triplet states with a time constant of 1.6 ± 0.4 ps. Oscillations of the S1(nπ*) state O 1s intensity as a function of time correlate with those of calculated C4═O8 and C5═C6 bond lengths, which undergo a sudden expansion following the initial π → π* excitation. Our calculations support our interpretation of the data and provide detailed insight into the relaxation processes of uracil.

The electronic structure of 2(5H)-thiophenone in the gas phase was investigated by ultraviolet photoelectron spectroscopy and x-ray photoemission spectroscopy (XPS) and near edge x-ray absorption fine structure (NEXAFS) spectroscopy at the C 1s, O 1s, and S 2p edges. All assignments of the experimental results are supported by both ab-initio electron propagator outer-valence Green's function (OVGF) calculations for the valence photoemission bands and density functional theory (DFT) and relativistic time dependent DFT calculations for the core levels XPS and NEXAFS spectra. Overall good agreement between experiment and theory is observed; this is especially true for core electron excitations which has permitted an unambiguous assignment of the observed spectral features in terms of single-particle excitations to virtual molecular orbitals. The assignment of the valence band spectra based on OVGF calculations, although satisfactory, points to the importance of electron correlations effects that partially break the single particle picture of ionization.

In the framework of the ELETTRA 2.0 project, the MOST beamline will replace the present GasPhase and CIPO beamlines. MOST aims to provide high flux in the wide photon energy range 15-2200 eV, spectral resolution better than 5000 in the almost whole spectral range, high spectral purity, full polarization control and transmission almost independent from the input polarization. The optical layout and the performances are here described.

Experimental characterization of the structural, electronic and dynamic properties of dilute systems in aqueous solvents, such as nanoparticles, molecules and proteins, are nowadays an open challenge. X-ray absorption spectroscopy (XAS) is probably one of the most established approaches to this aim as it is element-specific. However, typical dilute systems of interest are often composed of light elements that require extreme-ultraviolet to soft X-ray photons. In this spectral regime, water and other solvents are rather opaque, thus demanding radical reduction of the solvent volume and removal of the liquid to minimize background absorption. Here, we present an experimental endstation designed to operate a liquid flat jet of sub-micrometre thickness in a vacuum environment compatible with extreme ultraviolet/soft XAS measurements in transmission geometry. The apparatus developed can be easily connected to synchrotron and free-electron-laser user-facility beamlines dedicated to XAS experiments. The conditions for stable generation and control of the liquid flat jet are analyzed and discussed. Preliminary soft XAS measurements on some test solutions are shown.

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.

Since third-generation synchrotron radiation facilities are highly brilliant sources, the scattering of a partially coherent beam gives rise to strong interference effects. We present and discuss in this contribution the comparison of simulated and experimental diffraction patterns collected with an optical device made by two identical flat Micro Channel Plates (MCPs) illuminated by synchrotron radiation at different energies. The experimental patterns clearly show the increase of the density of the main peak with respect to those collected with a single flat MCP. Data demonstrate that the use of a device based on two MCPs is an ideal optical system to condense the primary radiation in a narrow intense central peak.