arXiv subjects

Gianluca Geloni

Publications and source records attributed to Gianluca Geloni.

At least 19 recordsLinked to original sources

arXiv2026

Generation of high-fluence and high-intensity hard x-ray attosecond pulses at European XFEL

By combining hard x-ray attosecond pulses from the European XFEL with total-reflection focusing x-ray optics, we generated nanofocused hard x-ray attosecond pulses with intensities and fluences comparable to the highest values attained in the hard x-ray regime. A peak intensity on the order of 10$^{20}$ W/cm$^2$ is confirmed through the observation of saturation in amplified spontaneous emission from copper atoms. These x-ray pulses enable new scientific opportunities, including the exploration of higher-order nonlinear light--matter interactions, damage-free structure determination, and coherent control of atoms and molecules.

physics.optics↗

arXiv2026

Photoinduced enhancement of chemical shift sensitivity to local vibrations

The advent of novel free-electron laser sources enabling time-resolved x-ray photoelectron spectroscopy (tr-XPS) provides a unique opportunity to monitor local chemical environments in real time by measuring sub-eV shifts in core-electron binding energies. These shifts reflect the interplay between electronic excitation and nuclear motion, an interplay that remains largely unexplored. In our combined theoretical and experimental study of fluoropyridine (C$_5$H$_4$FN), we investigate this link by monitoring the evolving chemical environment at the N and F atomic sites as the photoexcited $S_1$ state relaxes to the ground state via a conical intersection. We find that the F site responds primarily to vibrational relaxation, showing minimal sensitivity to the electronic excited state. In contrast, excitation to $S_1$ induces a measurable energy shift at the N site and significantly enhances its sensitivity to local vibrations within the ring. This behavior arises from a photoinduced redistribution of charge, which also increases the Coulomb interaction between the 1s electron at the N atom and the atomic partial charge at an adjacent C atom. This insight opens new avenues for exploring ultrafast dynamics and conical intersection pathways in more complex systems, from photostable DNA bases to light-harvesting materials.

physics.chem-ph↗

arXiv2026

Attosecond soft X-ray pulses generated by chirp-dispersed manipulation in an XFEL reveal nonlinear core-electron dynamics in neon

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.

physics.optics↗

arXiv2026

Spatiotemporal shaping of attosecond X-rays with time-dependent orbital angular momentum

Attosecond X-ray pulses are indispensable for exploring ultrafast phenomena in matter on Angstrom and attosecond scales. Here we propose a new method to realize spatiotemporal shaping of attosecond X-rays through temporal control of the orbital angular momentum mode content using an X-ray free-electron laser. The method exploits transverse-mode-dependent frequency pulling in a deliberately detuned second stage, together with slippage between the seed and the amplified radiation. Three-dimensional simulations show a double-spike waveform in which the two spikes carry different dominant topological charges. The spike separation is tunable and can reach several hundred attoseconds. This provides a source-level route to spatiotemporally structured attosecond X-rays with controllable temporal structure and topological mode content.

physics.acc-ph↗

arXiv2025

Unraveling real-time chemical shifts in the ultrafast regime

Traditional x-ray photoelectron spectroscopy (XPS) relies upon a direct mapping between the photoelectron binding energies and the local chemical environment, which is well-characterized by an electrostatic partial charges model for systems in equilibrium. However, the extension of this technique to out-of-equilibrium systems has been hampered by the lack of x-ray sources capable of accessing multiple atomic sites with high spectral and temporal resolution, as well as the lack of simple theoretical procedures to interpret the observed signals. In this work we employ multi-site XPS with a narrowband femtosecond x-ray probe to unravel different ultrafast dissociation processes of a polyatomic molecule, fluoromethane (CH$_{3}$F). We show that XPS can follow the cleavage of both the C-F and C-H bonds in real time, despite these channels lying close in binding energy. Additionally, we apply the partial charges model to describe these dynamics, and verify this extension with both advanced ab-initio calculations and experimental data. These results enable the application of this technique to out-of-equilibrium systems of higher complexity, by correlating real-time information from multiple atomic sites and interpreting the measurements through a viable theoretical modelling.

physics.chem-ph↗

arXiv2025

Generation of high-power attosecond x-ray FEL pulses carrying orbital angular momentum

X-ray beams carrying orbital angular momentum (OAM), are emerging as a powerful tool to probe matter. Recently, a method called self-seeded FEL with OAM (SSOAM) has been proposed to generate high-power x-ray OAM pulses, which places the traditional optical elements in the linear regime of the FEL amplification process before saturation to reduce the thermal load of the optical element. In this work, we propose to utilize the SSOAM scheme to produce attosecond x-ray vortices with high intensity. Numerical simulations demonstrate the x-ray OAM pulses with peak powers of more than one hundred gigawatts and a pulse duration of the order of hundred attoseconds can be achieved using the proposed method.

physics.acc-ph↗

arXiv2024

Spatial coherence of synchrotron radiation degraded by grating monochromators

In this work, we present a physical and theoretical description of the significant degradation of X-ray spatial coherence properties due to the use of grating monochromators at synchrotron facilities. We present a comprehensive study of the effects of spatial coherence degradation as a function of grating parameters under different focusing and propagation conditions. The focus is on the effect of spatial coherence degradation caused by the grating alone, starting from a fully coherent incident photon beam generated by a filament source. The mathematical description and theoretical analysis of the spatial coherence degradation is presented in the framework of statistical optics.

physics.optics↗

arXiv2023

X-ray Free-Electron Laser accelerator to study on Quantum Electro-Dynamics

X-ray free-electron lasers (XFELs) utilize high-density and high-energy electron bunches which are well-suited to produce Compton back-scattering radiation. Here we study study interaction of such electron bunches during head-on collision with retroreflected X-ray pulses, emitted by an XFEL. Such collisions allow one to conduct experiments on electron-positron pair production, both through trident and gamma-gamma colliding processes. We discuss cost-effective setups to study such processes, taking advantage of the existing conventional as well as proposed X-ray FEL infrastructure. We estimate parameters of the proposed experiments and compare them with other projects under construction.

physics.acc-ph↗

arXiv2022

Self-Seeded Free-Electron Lasers with Orbital Angular Momentum

X-ray beams carrying orbital angular momentum (OAM) are an emerging tool for probing matter. Optical elements, such as spiral phase plates and zone plates, have been widely used to generate OAM light. However, these optics are challenging to use at x-ray free-electron lasers (XFELs) due to the high impinging intensities. Here, we propose a self-seeded FEL method to produce intense x-ray vortices. Unlike passive filtering after amplification, an optical element will be used to introduce the helical phase in the linear regime, significantly reducing the thermal load on the optical element. The generated OAM pulse is then used as a seed and significantly amplified. Theoretical analysis and numerical simulations demonstrate that the power of the OAM seed pulse can be amplified by more than two orders of magnitude, reaching peak powers of several tens of gigawatts. The proposed method paves the way for high-power and high-repetition-rate OAM pulses of XFEL light.

physics.acc-ph↗

arXiv2022

Effect of Aberration of Light in X-ray Free Electron Lasers

We discuss the physics of a microbunched electron beam kicked by the dipole field of a corrector magnet by describing the kinematics of coherent undulator radiation after the kick. Particle tracking shows that the electron beam direction changes after the kick, while the orientation of the microbunching wavefront stays unvaried. Therefore, electrons motion and wavefront normal have different directions. Coherent radiation emission in a downstream undulator is expected to be dramatically suppressed as soon as the kick angle becomes larger than the divergence of the output radiation. In fact, according to conventional treatments, coherent radiation is emitted along the normal to the microbunching wavefront. Here we show that kinematics predicts a surprising effect. Namely, a description of coherent undulator radiation in the laboratory frame yields the radical notion that, due light aberration, strong coherent radiation is produced along the direction of the kick. We hold a recent FEL study made at the LCLS as a direct experimental evidence that coherent undulator radiation can be kicked by an angle of about five times the rms radiation divergence without suppression. We put forward our kinematical description of this experiment.

physics.acc-ph↗

arXiv2022

Modulated Electron Bunch with Amplitude Front Tilt in an Undulator

In a previous paper we discussed the physics of a microbunched electron beam kicked by the dipole field of a corrector magnet by describing the kinematics of coherent undulator radiation after the kick. We demonstrated that the effect of aberration of light supplies the basis for understanding phenomena like the deflection of coherent undulator radiation by a dipole magnet. We illustrated this fact by examining the operation of an XFEL under the steady state assumption, that is a harmonic time dependence. We argued that in this particular case the microbunch front tilt has no objective meaning; in other words, there is no experiment that can discriminate whether an electron beam is endowed with a microbunch front tilt of not. In this paper we extend our considerations to time-dependent phenomena related with a finite electron bunch duration, or SASE mode of operation. We focus our attention on the spatiotemporal distortions of an X-ray pulse. Spatiotemporal coupling arises naturally in coherent undulator radiation behind the kick, because the deflection process involves the introduction of a tilt of the bunch profile. This tilt of the bunch profile leads to radiation pulse front tilt, which is equivalent to angular dispersion of the output radiation. We remark that our exact results can potentially be useful to developers of new generation XFEL codes for cross-checking their results.

physics.acc-ph↗

arXiv2022

Misconception Regarding Conventional Coupling of Fields and Particles in XFEL Codes

Maxwell theory is usually treated in the lab frame under the standard time order (light-signal clock synchronization). Particle tracking in the lab frame usually treats time as an independent variable. Then, the evolution of electron beams is treated according to the absolute time convention (non-standard clock synchronization). This point has never received attention in the accelerator community. There are two ways of coupling fields and particles. The first, Lorentz's way, consists in `translating' Maxwell's electrodynamics to the absolute time world-picture. The second, Einstein's way, consists in `translating' particle tracking results to the electromagnetic world-picture. Conventional particle tracking shows that the electron beam direction changes after a transverse kick, while the orientation of the microbunching fronts stays unvaried. We show that under Einstein's time order, in the ultrarelativistic asymptote the orientation of the planes of simultaneity is always perpendicular to the electron beam velocity. This allows for the production of coherent undulator radiation from a modulated electron beam in the kicked direction without suppression. We hold a recent FEL study at the LCLS as experimental evidence of microbunching wavefront readjusting after a large kick. In a previous paper we quantitatively described this result invoking the aberration of light, corresponding to Lorentz's way of coupling fields and particles. Here we give details of the Lorentz's approach used in that paper. We also demonstrate that an `inverse translation' of particle tracking results to the standard time order resolves puzzles related with the strong qualitative disagreement between simulations and experiments. Previous simulation results in ultrarelativistic electron beam physics should be reexamined in the light of the difference between particle tracking and electromagnetic world-pictures.

physics.acc-ph↗

arXiv2022

Evidence of Wigner Rotation Phenomena in the Beam Splitting Experiment at the LCLS

A result from particle tracking states that, after a microbunched electron beam is kicked, its trajectory changes while the orientation of the microbunching wavefront remains as before. Experiments at the LCLS showed that radiation in the kicked direction is produced practically without suppression. This could be explained if the orientation of the microbunching wavefront is readjusted along the kicked direction. In previous papers we showed that when the evolution of the electron beam modulation is treated according to relativistic kinematics, the orientation of the microbunching wavefront in the ultrarelativistic asymptotic is always perpendicular to the electron beam velocity. There we refrained from using advanced theoretical concepts to explain or analyze the wavefront rotation. For example, we only hinted to the relation of this phenomenon with the concept of Wigner rotation. This more abstract view of wavefront rotation underlines its elementary nature. The Wigner rotation is known as a fundamental effect in elementary particle physics. The composition of non collinear boosts does not result in a simple boost but, rather, in a Lorentz transformation involving a boost and a rotation, the Wigner rotation. Here we show that during the LCLS experiments, a Wigner rotation was actually directly recorded for the first time with a ultrarelativistic, macroscopic object: an ultrarelativistic electron bunch in an XFEL modulated at nm-scale of the size of about 10 microns. Here we point out the role of Wigner rotation in the analysis and interpretation of experiments with ultrarelativistic, microbunched electron beams in FELs. After the beam splitting experiment at the LCLS it became clear that, in the ultrarelativistic asymptotic, the projection of the microbunching wave vector onto the beam velocity is a Lorentz invariant, similar to the helicity in particle physics.

physics.acc-ph↗

arXiv2019

Superradiant Undulator Radiation for Selective THz Control Experiments at XFELs

The generation of frequency-tunable, narrow-bandwidth and carrier-envelope-phase stable THz pulses with fields in the MV/cm regime that can be appropriately timed to the femtosecond X-ray pulses from free-electron-lasers is of highest scientific interest. It will enable to follow the electronic and structural dynamics stimulated by (non)linear selective excitations of matter on few femtosecond time and Ångstrom length scales. In this article, a scheme based on superradiant undulator radiation generated just after the XFEL is proposed. The concept utilizes cutting edge superconducting undulator technology and provides THz pulses in a frequency range between 3 and 30 THz with exceptional THz pulse energies. Relevant aspects for realization and operation are discussed point by point on the example of the European XFEL.

physics.acc-ph↗

arXiv2018

ROSA: Reconstruction of Spectrogram Autocorrelation for Self-Amplified Spontaneous Emission Free-Electron Lasers

In recent years X-ray Free Electron Lasers (XFELs) proved to be unmatched sources of ultrashort pulses of spatially coherent quasimonochromatic X-ray radiation. Diagnostics of XFEL emission properties, in particular pulse duration, spectrum and temporal profile is extremely important in order to analyze the experimental results. In this paper we propose a cost-effective method to examine these properties of Self-Amplified Spontaneous Emission (SASE) pulses. It only requires an ensemble of measured SASE spectra and provides the temporal autocorrelation of the ensemble-averaged Wigner distribution of SASE FEL pulses.

physics.optics↗

arXiv2018

Effects of energy spread on Brightness and Coherence of undulator sources

The (spectral) brightness for partially transverse coherent sources as Synchrotron Radiation (SR) and Free-Electron Laser (FEL) sources can be defined as the maximum of the Wigner distribution. Then, the brightness includes information on both coherence and wavefront characteristics of the radiation field. For undulator sources, it is customary to approximate the single-electron electric field at resonance with a Gaussian beam, leading to great simplifications. Attempts to account for the modified spatial and angular profile of the undulator radiation in the presence of detuning due to energy spread currently build on the simplified brightness expression derived under the assumption of Gaussian beams. The influence of energy spread on undulator radiation properties is becoming important in view of diffraction-limited rings with ultralow emittance coming on-line. Here we discuss the effects of energy spread on the brightness of undulator radiation at resonance, as well as relevant relations with coherence properties.

physics.acc-ph↗

arXiv2017

Relativity and Accelerator Engineering

From a geometrical viewpoint, according to the theory of relativity, space and time constitute a four-dimensional continuum with pseudo-Euclidean structure. This has recently begun to be a practically important statement in accelerator physics. An X-ray Free Electron Laser (XFEL) is in fact the best, exciting example of an engineering system where improvements in accelerator technology makes it possible to develop ultrarelativistic macroscopic objects with an internal fine structure, and the theory of relativity plays an essential role in their description. An ultrarelativistic electron bunch modulated at nanometer-scale in XFELs has indeed a macroscopic finite-size of order of 10 $μ$m. Its internal, collective structure is characterized in terms of a wave number vector. Here we will show that a four-dimensional geometrical approach, unusual in accelerator physics, is needed to solve problems involving the emission of radiation from an ultrarelativistic modulated electron beam accelerating along a curved trajectory. We will see that relativistic kinematics enters XFEL physics in a most fundamental way through the so-called Wigner rotation of the modulation wave number vector, which is closely associated to the relativity of simultaneity. If not taken into account, relativistic kinematics effects would lead to a strong qualitative disagreement between theory and experiments. In this paper, several examples of relativistic kinematics effects, which are important for current and future XFEL operation, are studied. The theory of relativity is applied by providing details of the clock synchronization procedure within the laboratory frame. This approach, exploited here but unusual in literature, is rather "practical", and should be acceptable to accelerator physicists.

physics.class-ph↗

arXiv2017

On Radiation Emission from a Microbunched Beam with Wavefront Tilt and Its Experimental Observation

In this paper we compare experimental observations and theory of radiation emission from a microbunched beam with microbunching wavefront tilt with respect to the direction of motion. The theory refers to the work [1], which predicts, in this case, exponential suppression of coherent radiation along the kicked direction. The observations refer to a recent experiment performed at the LCLS [2,3], where a microbunched beam was kicked by a bend and sent to a radiator undulator. The experiment resulted in the emission of strong coherent radiation that had its maximum along the kicked direction of motion, when the undulator parameter was detuned to a value larger than the nominal one. We first analyze the theory in detail, and we confirm the correctness of its derivation according to the conventional theory of radiation emission from charged particles. Subsequently, we look for possible peculiarities in the experiment, which may not be modeled by the theory. We show that only spurious effects are not accounted for. We conclude that the experiment defies explanation in terms of the conventional theory of radiation emission.

physics.acc-ph↗