Ionization potentials of ethylene, allene and butatriene by a green function method

The vertical valence ionization potentials of ethylene, allene and butatriene are computed by a many-body Green function method. For ethylene it is shown that diffuse d-type polarization functions must be added to the basis to describe accurately the correlation energy changes which arise in the diffuse part of the charge cloud. This is of particular importance for the π-ionization of all three molecules considered here. For butatriene, whose photoelectron spectrum contains one band more than expected from the number of orbitals, it is shown that one of the first three bands must be due to a vibrational phenomenon so that two of the bands form one composite band. There is some indication that both the second and the third band arise from the ionization leading to the first ²B_3u state.