Ultrafast hybrid exciton dynamics in lithium fluoride

We explored the sub-femtosecond dynamics in lithium fluoride using transient reflection spectroscopy. This technique probes ultrafast dynamics initiated by a few-femtosecond infrared pump pulse with attosecond pulses in the 25−50 eV energy range, providing direct access to non-equilibrium charge dynamics in the deep conduction band. Under strong-field excitation, this energy region is characterized by a remarkably rich ultrafast response arising from the interplay between virtual charges and an excitonic state, which is hidden in static measurements but becomes strongly visible in pump-probe experiments. A combined theoretical effort based on time-dependent density functional theory and ab initio many-body perturbation theory enables an unambiguous identification of the observed dynamics as originating from a hybrid excitonic state deep in the conduction band of lithium fluoride. This work reports the first observation and characterization of these elusive quasiparticles, demonstrating that attosecond spectroscopy is uniquely sensitive to weakly visible excitons and highlighting the intrinsic limitations of static investigations of equilibrium properties of this widebandgap insulator.