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Abstract:
The core‐hole excitation spectrum of benzene below 12.0 eV is calculated within a symmetry‐adapted first‐order configuration interaction approach based on the CNDO/S equivalent‐core approximation. One‐electron singlet excitation energies of the neutral molecule are shown to approximate the corresponding "singlet"‐coupled doublet energies in the delocalized core‐hole species. Intensities in accord with experiment are obtained within the sudden approximation by projecting the C2ν reduced‐symmetry equivalent‐core solutions out of the first‐order delocalized description. The total core‐hole excitation intensity below 12.0 eV is found to be largely attributable to three one‐electron π*←π excitations of singlet‐coupled doublet character which are split into the observed number of features, with correspondingly observed strengths, by first‐order configuration interaction with energetically degenerate excitations in the "triplet"‐coupled doublet manifold—"spin‐symmetry breaking configurations." The observed core‐hole excitation satellite at 10.7 eV and the asymmetry on the high binding energy side of the well‐resolved satellite at 8.3 eV are interpreted in terms of two excitations of 1E2g (π*←π) neutral molecule parentage. The assignment of these higher‐energy features as 1E2g(π*←π) in origin provides further support for the numerous theoretical predictions and recent conclusions based on spectroscopic investigations for states of 1E2g symmetry at approximately 8.0 and 10.0 eV in neutral benzene.
