Description
It has been suggested in the past [1,2] that the attosecond delays measured in the experimental method of "reconstruction of attosecond beating by interference of two-photon transitions" (RABBITT) would be affected by IR-driven dipole transitions within the residual ion, provided that the energy separation of the residual ion states in question were close to a resonance with the IR. Signatures of this "coupling delay" were simulated theoretically in CO$_2$ [1] and in C$_2$H$_2$ [2], but experimental confirmation remained elusive due to the very complex photoelectron spectra of molecular targets possessing sufficiently densely spaced electronic states.
In the present work [3] we leverage the photoelectron-photoion coincidence spectrometry to validate the coupling delay prediction in channels associated with the states $B$ and $C$ of CO$_2$$^+$. While the $C$ state is dissociative, the $B$ state is stable, allowing to distinguish the two channels by measurement of photoelectron spectra in coincicence with the corresponding molecular fragments. We confirm that RABBITT in CO$_2$ becomes effectively a three-leg process, where an additional interference pathway contributes to the standard XUV$+$IR/XUV$-$IR pair. In the high-energy limit we find a link between the coupling delay and purity of the reduced density matrix of the entangled photoelectron-photoion system traced over photoelectron degrees of freedom.
In a further follow-up study [4] we analyze in detail the fragmentation process of CO$_2$$^+$ in the $C$ state and experimentally discover a massive time delay variation of ~800 as in the RABBITT signal over a narrow photoelectron energy range $3-8$ eV. We trace this variation to a competition of IR driven transitions in the continuum and in the residual ion.
[1] Benda & al., Phys. Rev. A 105 (2022) 053101
[2] Delgado & al., Phys. Rev. A 111 (2025) 063107
[3] Makos & al., Nat. Commun. 16 (2025) 8554
[4] Makos & al., arXiv:2604.23441
