Fig. 1: Experimental setup and XUV spectroscopy of CO₂ molecules.

a Schematic view of the experimental setup used for the measurement of RABBIT spectrograms in a mixture of randomly aligned CO₂ molecules and argon atoms (MCP: microchannel plate). b Electronic configuration of CO₂ and binding energies of the first four cationic states together with the Dyson orbitals for the \({B}^{2}{\Sigma }_{u}^{+}\) and \({C}^{2}{\Sigma }_{g}^{+}\) cationic states. c XUV-only spectra measured in coincidence with the parent cation \({{{{\rm{CO}}}}}_{2}^{+}\) for the integration angle (0° − 90°) along the polarisation direction of the XUV field. A 200-nm-thick aluminium filter was used to filter out the IR radiation. The vertical lines indicate the expected positions of the photoelectron peaks associated with a cation in the X²Π_g (dashed red lines), A²Π_u (yellow dotted lines), \({B}^{2}{\Sigma }_{u}^{+}\) (green dash-dotted lines), and \({C}^{2}{\Sigma }_{g}^{+}\) (blue dotted lines) state. The shaded blue areas indicate the expected energy ranges for the sidebands associated with an ion in the \({B}^{2}{\Sigma }_{u}^{+}\) state. The binding energies of the first ionic states are taken from refs. ^62,63.