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

We simulate energy-dependent photoionization cross sections of neutral and doped diphenyl polyenes, including alpha,omega-diphenyltetradecaheptaene (D P7) and its sodium-doped species (2Na/DP7), in order to explore the possibi lities of characterizing doping effects by means of photoelectron spectrosc opy using tunable photon sources. The validity of density of state versus f ull cross section calculations for interpreting the ultraviolet photoelectr on spectra is tested. It is found that the doping-induced potentials trap t he photoelectron at low energies and gives rise to an energy dependence of the cross sections that is sharply resonant. The commonly used discrete pho toionization energy at 21.2 eV, "He I," transcends the resonant region for most channels, including the channels pertaining to the charge-induced stat es in the band gap, thereby still motivating an analysis of the outer valen ce levels using density-of-state diagrams at that particular photon energy. [SO163-1829(99)08607-5].