Negative shake-up energy in core ionization

In recent years unusual multipeak structures have been detected in the core electron ionization spectra of certain highly polar aromatic molecules. In this work the ionization spectrum of para-nitroaniline, NO₂C₆H₄NH₂, in the Nls energy region is calculated by ab initio methods taking configuration interaction between the single hole-hole configurations and manifold of two-hole-one-particle excited configurations into account. The splitting of the Nls(NO₂) line observed in the ESCA spectrum of para-nitroaniline is reproduced by the calculation. The calculation shows that in the presence of the core hole the lowest π → π* excitation energy is negative. This negative shake-up energy is responsible for the multi-peak structure of the Nls(NO₂) line. It is shown that the computational results can be simply rationalized by considering the charge distribution of the highest occupied and lowest unoccupied valence orbitals. The close analogy between the multi-peak phenomena in the K-shell ESCA spectra of nitroaniline (and related highly polar aromatic compounds) and molecules chemisorbed on metal surfaces is emphasized.