Ultrafast RIXS of orbital-resolved valence-band reshaping in photoexcited graphite.

Abstract

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 <i>K</i>-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 [Formula: see text] contour disentangle [Formula: see text] and [Formula: see text] 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 [Formula: see text], the signal reflects the decay of the electronic contribution to the renormalized band structure under a hot-phonon bottleneck, whereas at the [Formula: see text]-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 [Formula: see text] and [Formula: see text] modes. These results establish time-domain RIXS as a complementary probe of transient band-structure renormalization and mode-selective energy flow in quantum materials.