A Time-Domain Generalization of Extended Koopmans’ Theorem

We introduce a time-domain generalization of the extended Koopmans' theorem within the framework of the multiconfigurational time-dependent Hartree-Fock (MCTDHF) theory. This formulation naturally yields well-defined time-dependent hole states formed by removing one electron from the multielectron system, enabling the instantaneous construction of reduced density matrices for the photofragments during MCTDHF simulations with negligible computational overhead. Leveraging this foundation, we derive the equation of motion for the time-dependent Dyson orbitals and develop a systematic approach to extract hole-resolved observables directly from the time-dependent \textit{ab initio} wavefunctions, such as channel-resolved photoelectron momentum distributions. The proposed method is universally applicable to both projection-based and flux-based schemes, offering a powerful tool for disentangling correlated electron-hole dynamics in ultrafast multichannel ionization processes.