Attosecond Time Delay in Bound–Bound Transitions Probed by Fano Resonances

Abstract This letter presents a method for probing the attosecond time delay between two radiatively resonant transitions from Fano structures, which arise from interference between the extreme ultraviolet free induction decay (XFID) emission and high-order harmonics. The ellipticity dependence of the Ne⁺ XFID yield confirms that the ionic excited-state populations originate from inelastic recollision between tunneling electrons and parent ions. Subsequent extraction of relative phases from Fano structures enables the determination of the time delay (∼ 22 as) between the two decay pathways. This work provides an experimental approach to probe the attosecond time delay between different XFID channels and contributes to a deeper understanding of the tunneling-plus-rescattering model in strong laser fields.