Fig. 4: Three-path RABBIT measurement and effect of the coupling between the ionic states on the entangled photoelectron-photoion wave function.

Schematic representation of the two-colour photoionisation process on the CO₂ molecule described by the two states \({B}^{2}{\Sigma }_{u}^{+}\) and \({C}^{2}{\Sigma }_{g}^{+}\). The absorption of an XUV photon of the harmonic 2q ± 1 determines a state described by an ion in the \({B}^{2}{\Sigma }_{u}^{+}\) state (Φ_B) and a photoelectron with energy e_2q±1 = (2q ± 1)ℏω_IR − I_p(B) (path 1 for 2q + 1 and path 2 for 2q − 1, respectively). The subsequent interaction with the IR leads to the emission (absorption) of an IR photon for the path 1 (path 2), resulting in the final state Ψ_B2q. The absorption of a photon of the harmonic 2q + 1 can also lead to the emission of a photoelectron e_C = (2q + 1)ℏω_IR − I_p(C) ≈ e_2q and an ion initially in the \({C}^{2}{\Sigma }_{g}^{+}\)-state (Φ_C) (path 3). The interaction with the IR field determines an ionic coupling that leads to the transition from the \({C}^{2}{\Sigma }_{g}^{+}\) to the \({B}^{2}{\Sigma }_{u}^{+}\)-state, thus reaching the same final state as the other two paths.