Core Level-Resolved Attosecond Electron Dynamics in GaAsSb

Strong near-infrared laser fields drive ultrafast electron motion in solids on attosecond timescales, giving rise to the dynamical Franz-Keldysh effect (DFKE). However, it is not fully understood how the measured sub-cycle response depends on the specific initial states of the transitions probed. Here, we simultaneously track the field-driven dynamics from multiple atomic core levels over a broad photon-energy range in monocrystalline GaAsSb. Even though the underlying electron dy- namics are independent of the probing initial state, a comparison between experiment and DFKE simulations reveals an effective phase difference of about 70 attoseconds between the responses probed via As and Sb core levels. Our simulations show that this phase difference arises from interference of the distinct signals from spin-orbit-split core levels. Each state produces a char- acteristic fishbone-like pattern in the transient absorption spectrum, and closely spaced states generate observable interference between their contributions, leading to an observed phase shift that is dependent on the spin-orbit-split energy. These results extend the DFKE framework to a state-resolved regime and provide a basis for understanding how spin–orbit splitting of a probe core level can influence the measured absolute phase of attosecond electron dynamics.