Fig. 1
General working principle of extreme-ultraviolet time-resolved photoelectron spectroscopy. The figure shows a set of neutral and cationic potential energy surfaces in adiabatic (solid) and diabatic representations (dashed). At position 1, a WP is generated in the Franck-Condon region of the first exicted state of the neutral (N-ES) with a UV pump photon (blue arrow). As the excited-state WP moves along the potential surface, the vertical Ip to the cationic groundstate (C-GS) and the first cationic excited state (C-ES), never exceeds the XUV photon energy (violet arrow). During the propagation of the WP, the electronic character changes (depicted as the leading electronic configurations), therefore changing the electronic overlap between neutral and cationic states. At position 2, the transition to the C-GS is Koopmans forbidden (dotted gray arrow) while the transition to the C-ES is allowed (dashed black arrow). Upon relaxation of the WP to the ground-state of the neutral (N-GS), the effect is inverted (position 3)
