Combined Experimental and Ab Initio Multireference Configuration Interaction Study of the Resonant Inelastic X-ray Scattering Spectrum of CO₂

The fundamental problem of the symmetry breaking in the resonant inelastic X-ray scattering (RIXS) of the CO2 gas molecule is studied. The measurements were performed under catalytically relevant conditions within an in-house constructed reaction cell. The experimental RIXS plane is constructed from a sequence of resonances, covering the near-edge X-ray absorption fine structure (NEXAFS) spectrum up to 539 eV. The spectra show significant sensitivity with respect to the excitation frequency. The NEXAFS absorption spectrum, as well as the corresponding RIXS spectra, is interpreted with the aid of multireference configuration interaction (MRCI) theory. In this framework, the configuration interaction space spans the space of the intermediate and final states with single and single–double excitations. The dynamic character of the RIXS spectra is investigated by considering the electronic–nuclear vibrational coupling with the bending and antisymmetric stretching vibrations in the important intermediate excited states. In addition, the vibronic coupling mechanism involving the Renner–Teller effect and the core–hole localization pseudo-Jahn–Teller effect of the intermediate states is fully considered. The physical origin of the observed spectral features is discussed qualitatively and quantitatively in terms of individual core-to-valence excitations and valence-to-core decays, respectively. The computational protocol presented here, based on multireference wave function ab initio theory, serves as an important reference for future theoretical and experimental applications of RIXS spectroscopy.