The C 1s NEXAFS spectrum of benzene below threshold: Rydberg or valence character of the unoccupied σ-type orbitals

High-resolution C 1s near edge X-ray absorption fine structure (NEXAFS) spectra of benzene and benzene-d6 were measured and compared with the results of state-of-the-art density functional theory (DFT) calculations demonstrating excellent agreement. The spectrum below threshold is dominated by four resonances, each two of them with σ- and π-type final-state orbitals. The π-type resonances have been ascribed in previous work unanimously to antibonding valence orbitals while the character of the σ-type resonances is still the subject of discussion. Based on the present theoretical results and the observed vibrational fine structures both σ-type orbitals are assigned to be valence type with minor admixtures of Rydberg character.