A peculiar excited electronic state of allene (1,2-propadiene)

The synchrotron photoelectron spectra of allene (C3H4) and perdeuterated allene (C3D4) have been obtained at 40–127 eV photon energies. The study confirms the existence of a peak at 12.7 eV binding energy which was observed by Baltzer et al. [Chem. Phys. 196 (1995) 551]. High-level Complete Active Space Multiconfiguration SCF (CASSCF) and MRSDCI theoretical calculations indicate that the peak at 12.7 eV is most likely a peculiar correlation peak of 2E symmetry which gains intensity as a result of strong vibronic coupling (Jahn–Teller-like) between at least two electronic states. The photon energy dependence of the 12.7 eV peak intensity is reported.