$C1s$ and $O1s$ photoelectron satellite spectra of CO with symmetry-dependent vibrational excitations

The photoelectron shake-up satellite spectra that accompany the $C1s$ and $O1s$ main lines of carbon monoxide have been studied by a combination of high-resolution x-ray photoelectron spectroscopy and accurate ab initio calculations. The symmetry-adapted cluster-expansion configuration-interaction general-$R$ method satisfactorily reproduces the satellite spectra over a wide energy region, and the quantitative assignments are proposed for the 16 and 12 satellite bands for $C1s$ and $O1s$ spectra, respectively. Satellite peaks above the $π−1π*$ transitions are mainly assigned to the Rydberg excitations accompanying the inner-shell ionization. Many shake-up states, which interact strongly with three-electron processes such as $π−2π*2$ and $n−2π*2$⁠, are calculated in the low-energy region, while the continuous Rydberg excitations are obtained with small intensities in the higher-energy region. The vibrational structures of low-lying shake-up states have been examined for both $C1s$ and $O1s$ ionizations. The vibrational structures appear in the low-lying $C1s$ satellite states, and the symmetry-dependent angular distributions for the satellite emission have enabled the $Σ$ and $Π$ symmetries to be resolved. On the other hand, the potential curves of the low-lying $O1s$ shake-up states are predicted to be weakly bound or repulsive.