Elena Babici

Ultrafast photoexcitation can drive solids far from equilibrium by dynamically renormalizing their electronic structure, yet direct time-domain access to bulk band-structure reshaping with orbital and momentum selectivity has remained limited. Here, we show in photoexcited graphite that femtosecond carbon K -edge resonant inelastic x-ray scattering (RIXS), used in the time domain, provides a momentum- and orbital-selective spectroscopic fingerprint of transient valence-band reshaping. Momentum- and orbital-selective measurements along the K → M contour disentangle π and σ contributions and reveal a structured redistribution of spectral weight emerging within ∼70 femtoseconds and evolving on picosecond timescales. Nonequilibrium first-principles RIXS calculations performed within a frozen-lattice approximation show that the earliest response is dominated by electronic correlations and transient screening. At longer delays, the dynamics become strongly momentum dependent: Near K , the signal reflects the decay of the electronic contribution to the renormalized band structure under a hot-phonon bottleneck, whereas at the M -point van Hove singularity, it reveals a characteristic spectroscopic signature consistent with electron-phonon–driven band red shift and lifetime broadening induced by strongly coupled optical phonons. A three-temperature model constrained by the orbital-resolved tr-RIXS data captures the ensuing energy cascade through a hot-phonon reservoir dominated by the E 2 g and A 1 ′ modes. These results establish time-domain RIXS as a complementary probe of transient band-structure renormalization and mode-selective energy flow in quantum materials.

Time-resolved resonant inelastic X-ray scattering (tr-RIXS) is a powerful technique for probing quasiparticle interactions in quantum materials under nonequilibrium conditions. Here, we implement tr-RIXS at the carbon K-edge to investigate the ultrafast dynamics of core excitons coupled to vibrational modes in graphite. Using femtosecond X-ray pulses from a free-electron laser, we monitor the temporal evolution of vibronically dressed excitons and their interaction with symmetry-selective optical phonons. By tuning the incident photon energy across the 1s → σ^* resonance and analyzing the integrated inelastic sideband intensity, we reveal a detuning-controlled crossover between two complementary dynamical regimes. Phenomenological modeling and first-principles calculations reproduce both the magnitude and detuning dependence of the spectral-weight changes. In this work, enabled by the unique capabilities of X-ray free-electron lasers, we demonstrate how tr-RIXS can access coupled electronic and lattice dynamics with elemental and symmetry specificity, opening new routes to control vibronic interactions in light-element and low-dimensional quantum materials.