Optoelectronic applications root in excited electronic states. In semiconductors, there are two types of excited states: many-body states like bound electron-hole states, so-called excitons, and simpler quasi-particle states, typically referred to as quasi-free carriers (QFCs). In general, both types coexist in a dynamic interplay of exciton and QFC populations.
We present a novel, at first glance counter-intuitive approach for accessing exciton and QFC dynamics on ultrafast time scales: the photoemission lineshapes of core levels, i.e. states deep below the frontier orbitals, turn out to be sensitive probes of the electron dynamics occurring in the valence and conduction band. By combining time-resolved photoemission spectroscopy simultaneously probing excited states and core levels with a novel lineshape model, we retrieved how the character of excited states changes from excitonic to QFC-like. Our core finding is that the strength of screening of excited states is encoded in the core-level lineshape. Our approach is therefore generic and applicable to a range of electronic phase transitions like metal-insulator, topological and superconductor phase transitions.