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Abstract

We report high-resolution C 1s near-edge x-ray-absorption fine-structure (NEXAFS) spectra of the C₆-ring-containing molecules benzene (C₆H₆), 1,3- and 1,4-cyclohexadiene (C₆H₈), cyclohexene (C₆H₁₀), cyclohexane (C₆H₁₂), styrene (C₈H₈), and ethylbenzene (C₈H₁₀) which allow us to examine the gradual development of delocalization of the corresponding π electron systems. Due to the high experimental resolution, vibrational progressions can be partly resolved in the spectra. The experimental spectra are compared with theoretical NEXAFS spectra obtained from density-functional theory calculations where electronic final-state relaxation is accounted for. The comparison yields very good agreement between theoretical spectra and experimental results. In all cases, the spectra can be described by excitations to π*- and σ*-type final-state orbitals with valence character, while final-state orbitals of Rydberg character make only minor contributions. The lowest C 1s→1 π* excitation energy is found to agree in the (experimental and theoretical) spectra of all molecules except for 1,3-cyclohexadiene (C₆H₈) where an energy smaller by about 0.6 eV is obtained. The theoretical analysis can explain this result by different binding properties of this molecule compared to the others.