Re-analysis of the K-shell spectrum of benzene

The carbon K-shell spectrum of gaseous benzene has been recorded by inner-shell electron energy loss spectroscopy (ISEELS) under electric-dipole conditions (2 keV, small angle) with a resolution of 0.17 eV. Ab initio configuration interaction calculations performed in the frame of the equivalent core model have helped in the assignment of the experimental spectrum. They emphasize the importance of the multi-configurational approach and the role of the Rydberg orbitals in the assignment of some features. The band located at 289.09 eV is assigned to Rydberg excitations and a 3pi* transition mixed with doubly excited configurations, and not to an anti-bonding (C-H) transition as suggested previously. Because of a large 3s/3d character of the (C-H)* orbital, the corresponding peak appears above the ionization threshold. The 293.8 eV band contains in-plane resonances and very likely also several doubly excited states with a 3pi* contribution.