Filming vibronic core excitons in graphite with femtosecond inelastic X-ray scattering

When light strikes a material, the response unfolds on ultrafast timescales. Electrons react almost immediately, while the atomic lattice begins to vibrate shortly afterwards. This rapid interplay between electronic motion and atomic vibrations governs how materials absorb energy, dissipate heat, and ultimately determines their physical properties. Yet observing these processes as they happen has long remained a major experimental challenge.

In a study published in Nature Communications, a team led by Elettra Sincrotrone Trieste has now captured this ultrafast interaction in graphite using time-resolved resonant inelastic X-ray scattering (tr-RIXS) at the MagneDyn beamline of the FERMI free-electron laser. The experiment provides a direct, time-domain view of how electronic excitations known as core excitons evolve and interact with lattice vibrations immediately after photoexcitation.

Core excitons are short-lived bound states created when an X-ray photon excites a deeply bound electron, in this case from the carbon 1s level. In graphite, these excitons are strongly coupled to specific vibrational modes of the lattice. This coupling gives rise to vibronically dressed excitons, hybrid states that carry detailed information about the interaction between electrons and atomic motion at the most fundamental level.

To observe these processes in real time, the researchers combined an ultrafast optical pump pulse with femtosecond X-ray probe pulses delivered by a free-electron laser. By tuning the X-ray energy across the carbon 1s → σ* resonance and monitoring the resulting inelastic scattering signal, the experiment tracked how energy is redistributed between electronic and vibrational degrees of freedom following excitation. The concept of time-resolved RIXS applied to the detection of vibronic excitations in graphite is explained in Figure 1.

A key finding of the study is the identification of a detuning-dependent crossover between distinct dynamical regimes. By adjusting the X-ray excitation energy, the researchers were able to selectively emphasize different pathways through which energy flows from excited electrons into lattice vibrations. This reveals a new level of control and selectivity in probing ultrafast electron–phonon interactions.

Figure 1: (a) In resonant inelastic X-ray scattering (RIXS), a femtosecond X-ray pulse creates a short-lived core exciton that couples to lattice vibrations before emitting a scattered photon with reduced energy. (b) This coupling produces characteristic phonon sidebands in the RIXS spectrum. (c) Schematic pump–probe scheme: an ultrafast optical pulse excites the electronic system, followed by a delayed X-ray probe that tracks the ensuing dynamics. (d) Pumped RIXS spectra compared with unpumped spectra show a transient modification of the phonon sideband at resonant excitation, revealing ultrafast electron–phonon dynamics.

The experimental observations are supported by phenomenological modeling and first-principles calculations, which reproduce both the magnitude of the measured effects and their dependence on excitation conditions. Together, experiment and theory demonstrate that tr-RIXS offers a uniquely element-specific and symmetry-sensitive window into ultrafast dynamics, allowing electronic and lattice contributions to be disentangled on their natural timescales.

A second pathway stems from the acoustic response of the material itself. When a laser pulse is absorbed in a thin subsurface layer, it produces a longitudinal acoustic pulse that propagates both into the bulk and toward the surface. At an ideally flat interface, the upward-traveling pulse would simply reflect with reversed strain. Real surfaces, however, are never perfectly smooth; their inherent roughness scatters part of this reflected pulse into both bulk and surface acoustic modes, offering another route for generating the unexpectedly short-wavelength SAWs observed in the experiment.

Beyond graphite, this work highlights the broader potential of time-resolved RIXS to investigate vibronic interactions in a wide range of light-element and low-dimensional quantum materials. The approach opens new opportunities to explore how energy flows at the atomic scale, an essential step toward understanding and ultimately controlling material functionality on ultrafast timescales.

Marco Malvestuto^1,2, Beatrice Volpato³, Elena Babici³, Richa Bhardwaj¹, Antonio Caretta¹, Simone Laterza¹, Fulvio Parmigiani^1,3, Michele Manfredda¹, Alberto Simoncig¹, Marco Zangrando^1,2, Alexander Demidovich¹, Peter Susnjar¹, Enrico Massimiliano Allaria¹, Alexander Darius Brynes^1,4, David Garzella¹, Luca Giannessi¹, Primož Rebernik¹, Filippo Sottocorona¹ and Dino Novko^5,6
1 Elettra Sincrotrone Trieste S.C.p.A., Trieste, Italy
² CNR – Istituto Officina dei Materiali (IOM), Trieste, Italy
³ Department of Physics, University of Trieste, Trieste, Italy
⁴ Science and Technology Facilities Council (STFC), Daresbury Laboratory, Warrington, United Kingdom
⁵ Centre for Advanced Laser Techniques, Institute of Physics, Zagreb, Croatia
⁶ Donostia International Physics Center (DIPC), Donostia–San Sebastián, Spain

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