O 1s excitation and ionization processes in the CO2 molecule studied via detection of low-energy fluorescence emission

Oxygen 1s excitation and ionization processes in the CO2 molecule have been studied with dispersed and non-dispersed fluorescence spectroscopy as well as with the vacuum ultraviolet (VUV) photon-photoion coincidence technique. The intensity of the neutral O emission line at 845 nm shows particular sensitivity to core-to-Rydberg excitations and core-valence double excitations, while shape resonances are suppressed. In contrast, the partial fluorescence yield in the wavelength window 300-650 nm and the excitation functions of selected O+ and C+ emission lines in the wavelength range 400-500 nm display all of the absorption features. The relative intensity of ionic emission in the visible range increases towards higher photon energies, which is attributed to O 1s shake-off photoionization. VUV photon-photoion coincidence spectra reveal major contributions from the C+ and O+ ions and a minor contribution from C2 +. No conclusive changes in the intensity ratios among the different ions are observed above the O 1s threshold. The line shape of the VUV-O+ coincidence peak in the mass spectrum carries some information on the initial core excitation.