Davide Faccialà

We studied high-order harmonic generation (HHG) inside a microfluidic device fed with helium by analyzing the propagation of the fundamental field and the evolution of emerging XUV radiation in a multimode regime. The low-ionization regime investigated here is identified as a particularly straightforward framework for isolating pure mode-beating-induced effects, enabling a direct assessment of the role of modal dynamics in shaping the harmonic generation process. By varying the modal population through the manipulation of the coupling conditions, we demonstrated the possibility of optimizing the intensity and spectral content of the emitted harmonics, achieving an extension of the cutoff energy by more than 30 eV. Furthermore, we modeled the spatial evolution of the XUV field inside the waveguide, revealing the emergence of mode-beating-induced phase-matching channels that give rise to complex harmonic spatial profiles. This work demonstrates the potential of HHG inside waveguides for all-optical, on-site shaping of the generation process via modal-based spatial modulation of the driving field.

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.

Chirality is the property of an object that cannot be superimposed with its mirror image by any translation or rotation. This property plays a crucial role in the biological activities of molecules and in the optical and magnetic properties of materials. This review will show recent advances in the study of ultrafast chiral processes occurring in molecular systems and materials investigated by high-order harmonic generation (HHG) spectroscopy, both from a theoretical and experimental point of view. HHG is a highly nonlinear optical process providing coherent XUV radiation with attosecond duration. The chemical physics community, in its broadest sense, has developed an interest in HHG spectroscopy, given the increasing number of studies of complex molecules, including those of biological relevance, chemical reactions, and characterization of the structure of solids by means of an ultrafast probe. Indeed, HHG is a powerful tool to capture the photoinduced electron dynamics in their natural timescale, also enabling the investigation of transient chirality, which is so far mostly unexplored. The study of chirality on the ultrafast timescale represents an opportunity to bridge the gap between atomic physics, chemical physics, and materials science in their traditional definitions and gain insight into fundamental chiral processes at the electron level.

Attosecond light sources based on high-order harmonic generation (HHG) constitute to date the only table-top solution for producing coherent broadband radiation covering the spectral range from the extreme ultraviolet to the soft x-rays. The so-called emission cutoff can be extended towards higher photon energies by increasing the driving wavelength at the expense of conversion efficiency. An alternative route is to overdrive the process by using higher laser intensities, with the challenging requirement of interacting with higher plasma densities over short propagation distances. Here, we address this challenge by exploiting free-space propagation in a differentially pumped glass chip designed for optimal gas confinement over sub-mm lengths. By driving HHG with multicycle pulses at either 800 nm or 1500 nm, we demonstrate a cutoff extension by a factor of two compared to conventional phase matching approaches and surpassing the present record using multicycle fields. Our three-dimensional propagation simulations, in excellent agreement with the experiment, confirm that gas confinement is crucial since efficient phase matching of cutoff harmonics occurs only for short propagation lengths. Additionally, we show that the high photon energy component is not only temporally confined to the leading edge of the driving pulse, but also spatially confined in the near-field to an off-axis contribution due to reshaping of the driving field along propagation inside the medium. Our findings contribute to the fundamental understanding of HHG across different regimes.

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 CS₂ molecules, where the role of internal conversion (IC) and intersystem crossing (ISC) in determining the predissociation dynamics is controversial. We present XPS spectra from photoexcited CS₂, 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 predissociation 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 predissociation dynamic pathway.

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.

In this study, we investigate the electronic structure of hemithioindigo–hemistilbene (HTI) photoswitches and their functionalized derivatives, HTI-OMe and HTI-SMe, using x-ray photoemission spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. HTI compounds are known for their high quantum yield, thermal bistability, and rapid photoisomerization, making them promising candidates for applications in molecular motors, optical materials, and photocatalysis. Our analysis, supported by first-principles simulations, reveals how the conjugation of heteroatoms within the π-system affects the core-level chemical shifts and ionization intensities in XPS, while NEXAFS probes the influence of substituents on virtual molecular orbitals and energy transitions. In particular, the comparison between different functionalized HTIs allowed us to evaluate the effect of electronic relaxation following core-level photoionization and photo-excitation. These results provide a detailed understanding of the influence of functionalization on the electron distribution of HTI compounds, providing a robust foundation for the study and control of ultrafast charge transfer and photoswitching mechanisms in these molecular systems.

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.

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 (Gabalski, et al 2023 J. Phys. Chem. Lett. 14 7126–7133).

Thermotropic liquid crystals are versatile optical materials that exhibit a state of matter intermediate between liquids and solids. Their properties can change significantly with temperature, pressure, or other external factors, leading to different phases. The transport properties within these materials in different phases are still largely unexplored, and their understanding would enable exciting prospects for innovative technological advancements. High-order harmonic spectroscopy proved to be a powerful spectroscopic tool for investigating the electronic and nuclear dynamics in matter. Here, we report the first experimental observation of high-order harmonic generation in thermotropic liquid crystals in two different phase states, nematic and isotropic. We found the harmonic emission in the nematic phase to be strongly dependent on the relative orientation of the driving field polarization with respect to the liquid crystal alignment. Specifically, the harmonic yield has a maximum when the molecules are aligned perpendicularly to the polarization of the incoming radiation. Our results establish the first step for applying high-order harmonic spectroscopy as a tool for resolving ultrafast electron dynamics in liquid crystals with unprecedented temporal and spatial resolution.