arXiv subjects

Roberto Costantini

Publications and source records attributed to Roberto Costantini.

4 recordsLinked to original sources

arXiv2026

X-ray driven displacive excitation of coherent phonons

Modulating electron-phonon coupling offers a route to control structural displacements and tune material functionality. Valence-to-conduction band transitions, however, provide limited leverage over the driving force. Here, we demonstrate coherent lattice dynamics in trigonal tellurium using free-electron laser pulses tuned to the Te N4,5-edge. Over a broad fluence range, the oscillation amplitude obeys the displacive excitation of coherent phonons framework, extended to core resonance with twice the driving efficiency of a visible pump. Ab initio calculations decompose the force into competing multiband contributions, inaccessible to optical excitation, whose balance shifts as carriers relax. Tunable extreme-ultraviolet and X-ray pulses thus open a regime in which the displacive response is set by band-dependent coupling to the lattice, not by the number and temperature of the photocarriers alone.

cond-mat.mtrl-sci↗

arXiv2022

Atom-Specific Probing of Electron Dynamics in an Atomic Adsorbate by Time-Resolved X-ray Spectroscopy

The electronic excitation occurring on adsorbates at ultrafast time scales from optical lasers that initiate surface chemical reactions is still an open question. Here, we report the ultrafast temporal evolution of X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) of a simple well known adsorbate prototype system, namely carbon (C) atoms adsorbed on a nickel (Ni(100)) surface, following intense laser optical pumping at 400 nm. We observe ultrafast (~100 fs) changes in both XAS and XES showing clear signatures of the formation of a hot electron-hole pair distribution on the adsorbate. This is followed by slower changes on a few ps time scale, shown to be consistent with thermalization of the complete C/Ni system. Density functional theory spectrum simulations support this interpretation.

physics.chem-ph↗

arXiv2021

Ultrafast adsorbate excitation probed with sub-ps resolution XAS

We use a pump-probe scheme to measure the time evolution of the C K-edge X-ray absorption spectrum (XAS) from CO/Ru(0001) after excitation by an ultrashort high-intensity optical laser pulse. Due to the short duration of the X-ray probe pulse and precise control of the pulse delay, the excitation-induced dynamics during the first ps after the pump can be resolved with unprecedented time resolution. By comparing with theoretical (DFT) spectrum calculations we find high excitation of the internal stretch and frustrated rotation modes occurring within 200 fs of laser excitation, as well as thermalization of the system in the ps regime. The ~100 fs initial excitation of these CO vibrational modes is not readily rationalized by traditional theories of nonadiabatic coupling of adsorbates to metal surfaces, e. g. electronic frictions based on first order electron-phonon coupling or transient population of adsorbate resonances. We suggest that coupling of the adsorbate to non-thermalized electron-hole pairs is responsible for the ultrafast initial excitation of the modes.

physics.chem-ph↗

arXiv2020

Keto-enol tautomerization drives the self-assembly of Leucoquinizarin on Au(111)

The self-assembly of Leucoquinizarin molecule on Au(111) surface is shown to be characterized by the molecules mostly in their keto-enolic tautomeric form, with evidences of their temporary switching to other tautomeric forms. This reveals a metastable chemistry of the assembled molecules, to be considered for their possible employment in the formation of more complex hetero-organic interfaces

cond-mat.mtrl-sci↗