Multiple excitations and charge transfer in the ESCA N1s (NO₂) spectrum of paranitroaniline. A theoretical and experimental study

The Nls (NO₂) ESCA spectrum of paranitroaniline (PNA) is analyzed theoretically and experimentally. New monochromatized high-resolution spectra in the vapor (gas) and condensed phases are presented. In the condensed phase spectrum two distinct peaks are discerned, whereas the gas phase spectrum shows a drastically different feature in which the two peaks are reduced to an asymmetric band. The gas phase spectrum has been interpreted based on large-scale MCSCF wave functions calculated using a complete configuration expansion in the π valence orbital space. Emphasis is placed on the identification of features due to "shake" effects. Multiple excitations in the charge transfer excited states, as well as configuration near-degeneracy effects in the NO₂ group, are found to be crucial for this identification. Our results are different from an earlier many-body, Green's function, treatment of PNA. We find that the charge transfer shake states lie higher in energy (shakeup) and not lower (negative shakeup).