The Electronic States of the Pentatetraene Radical Cation

HeIα excited photoelectron spectra of pentatetraene and the inferred ionization energies are reported. The first band has a characteristic Franck ‐ Condon envelope similar to the first photoelectron bands of allene and butatriene. The four bands found below 16 eV ionization energy have been assigned to the X̃ 2 E, Ã 2 E, B̃ 2 E and C̃ 2 B 2 states of the radical cation of pentatetraene by comparison with STO ‐ 3G and SPINDO calculations on the cumulene series. The correlation scheme includes the 2 s shell ionization energies of ethylene, allene and butatriene. The π‐orbital ionization trends of the cumulenes are discussed in the framework of localized orbitals calculated with the STO ‐ 3G basis set.