Photoionization cross sections of charge-induced gap states: A theoretical study of doped diphenylpolyenes

We simulate energy-dependent photoionization cross sections of neutral and doped diphenyl polyenes, including α,ω-diphenyltetradecaheptaene (DP₇) and its sodium-doped species (2Na/DP₇), in order to explore the possibilities of characterizing doping effects by means of photoelectron spectroscopy using tunable photon sources. The validity of density of state versus full cross section calculations for interpreting the ultraviolet photoelectron spectra is tested. It is found that the doping-induced potentials trap the photoelectron at low energies and gives rise to an energy dependence of the cross sections that is sharply resonant. The commonly used discrete photoionization energy at 21.2 eV, ``He I,'' transcends the resonant region for most channels, including the channels pertaining to the charge-induced states in the band gap, thereby still motivating an analysis of the outer valence levels using density-of-state diagrams at that particular photon energy.