arXiv subjects

Alessandro Gessini

Publications and source records attributed to Alessandro Gessini.

5 recordsLinked to original sources

arXiv2026

Spin waves excited by hard x-ray transient gratings

Recent progress in ultrafast x-ray sources helped establish x-rays as an important tool for probing lattice and magnetic dynamics initiated by femtosecond optical pulses. Here, we explore the potential of ultrashort hard x-ray pulses for driving magnetic dynamics. We use a transient grating technique in which a spatially periodic x-ray excitation pattern gives rise to material excitations at a well-defined wave vector, whose dynamics are monitored via diffraction of an optical probe pulse. The excitation of a ferrimagnetic gadolinium bismuth iron garnet film placed in an external tilted magnetic field by x-rays at the Gd L3 edge results in both magnetic and non-magnetic transient gratings whose contributions to the diffracted signal are separated by polarization analysis. We observe the magnetization precession at both longitudinal acoustic and spin wave frequencies. An analysis with the Landau-Lifshitz-Gilbert equation indicates that the magnetization precession is driven by strain resulting from thermal expansion induced by absorbed x-rays. The results establish x-ray transient gratings as a tool for driving coherent phonons and magnons, with the potential of accessing wave vectors across the entire Brillouin zone.

physics.optics↗

arXiv2025

Exploring the Growth Dynamics of Size-selected Carbon Atomic Wires with in situ UV Resonance Raman Spectroscopy

Short carbon atomic wires, the prototypes of the lacking carbon allotrope carbyne, represent the fundamental one-dimensional system and the first stage in carbon nanostructure growth, which still exhibits many open points regarding their growth and stability. We introduce an in situ UV resonance Raman approach for real-time monitoring of the growth of carbon atomic wires during pulsed laser ablation in liquid without perturbing the synthesis environment. We track single-chain species' growth dynamics, achieving size selectivity by exploiting the peculiar optoelectronic properties of carbon wires and the tunability of synchrotron radiation. We systematically explore diverse solvents, finding size- and solvent-dependent production rates linked to the solvent's C/H ratio and carbonization tendency. Carbon atomic wires' growth dynamics reveal a complex interplay between formation and degradation, leading to an equilibrium. Water, lacking in carbon atoms and reduced polyynes' solubility, yields fewer wires with rapid saturation. Organic solvents exhibit enhanced productivity and near-linear growth, attributed to additional carbon from solvent dissociation and low relative polarity. Exploring the dynamics of the saturation regime provides new insights into advancing carbon atomic wires' synthesis via PLAL. Understanding carbon atomic wires' growth dynamics can contribute to optimizing PLAL processes for nanomaterial synthesis.

physics.chem-ph↗

arXiv2021

Hard X-ray Transient Grating Spectroscopy on Bismuth Germanate

Optical-domain Transient Grating (TG) spectroscopy is a versatile background-free four-wave-mixing technique used to probe vibrational, magnetic and electronic degrees of freedom in the time domain. The newly developed coherent X-ray Free Electron Laser sources allow its extension to the X-ray regime. Xrays offer multiple advantages for TG: their large penetration depth allows probing the bulk properties of materials, their element-specificity can address core-excited states, and their short wavelengths create excitation gratings with unprecedented momentum transfer and spatial resolution. We demonstrate for the first time TG excitation in the hard X-ray range at 7.1 keV. In Bismuth Germanate (BGO), the nonresonant TG excitation generates coherent optical phonons detected as a function of time by diffraction of an optical probe pulse. This experiment demonstrates the ability to probe bulk properties of materials and paves the way for ultrafast coherent four-wave-mixing techniques using X-ray probes and involving nanoscale TG spatial periods.

physics.optics↗

arXiv2020

Element- and enantiomer-selective visualization of ibuprofen dimer vibrations

In chemistry, biology and materials science, the ability to access interatomic interactions and their dynamical evolution has become possible with the advent of femtosecond lasers. In particular, the observation of vibrational wave packets via optical (UV-visible-IR) spectroscopies has been a major achievement as it can track the motion of nuclei within the system. However, optical spectroscopies only detect the effect of the interatomic vibrations on the global electronic surfaces. New tuneable, pulsed and polarized sources of short-wavelength radiation, such as X-Ray Free electron lasers, can overcome this limitation, allowing for chemical and, of primary importance in biochemistry, enantiomeric selectivity. This selectivity may be complemented by taking into account the chemical shifts of atoms belonging to different molecular moieties

cond-mat.soft↗

arXiv2017

Rayleigh anomalies and disorder-induced mixing of polarizations at nanoscale in amorphous solids. Testing 1-octyl-3-methylimidazolium chloride glass

Acoustic excitations in topologically disordered media at mesoscale present anomalous features with respect to the Debye's theory. In a three-dimensional medium an acoustic excitation is characterized by its phase velocity, intensity and polarization. The so-called Rayleigh anomalies, which manifest in attenuation and retardation of the acoustic excitations, affect the first two properties. The topological disorder is, however, expected to influence also the third one. Acoustic excitations with a well-defined polarization in the continuum limit present indeed a so-called mixing of polarizations at nanoscale, as attested by experimental observations and Molecular Dynamics simulations. We provide a comprehensive experimental characterization of acoustic dynamics properties of a selected glass, 1-octyl-3-methylimidazolium chloride glass, whose heterogeneous structure at nanoscale is well-assessed. Distinctive features, which can be related to the occurrence of the Rayleigh anomalies and of the mixing of polarizations are observed. We develop, in the framework of the Random Media Theory, an analytical model that allows a quantitative description of all the Rayleigh anomalies and the mixing of polarizations. Contrast between theoretical and experimental features for the selected glass reveals an excellent agreement. The quantitative theoretical approach permits thus to demonstrate how the mixing of polarizations generates distinctive feature in the dynamic structure factor of glasses and to unambiguously identify them. The robustness of the proposed theoretical approach is validated by its ability to describe as well transverse acoustic dynamics.

cond-mat.dis-nn↗