Simulation study of angle-resolved photoemission spectra and intramolecular energy-band dispersion of a poly(tetrafluoroethylene) oligomer film

Theoretical simulations of the angle-resolved ultraviolet photoemission spectra (ARUPS) for the oligomer of poly(tetrafluoroethylene) [(CF2)(n); PTFE] were performed using the independent-atomic-center approximation combined with ab initio molecular orbital calculations. Previously observed normal-emission spectra for the end-on oriented sample (with long-chain axis perpendicular to the surface) showed the incident photon-energy (hnu) dependence due to the intramolecular energy-band dispersion along the one-dimensional chain, and the present simulations successfully reproduced this hnu dependence of the observed spectra. We employed the experimentally observed helical structure for PTFE oligomers for the simulations. We also calculated the density of states (DOS) for the planar zigzag structure, and examined the changes in the electronic structure due to the difference in the molecular structure by comparing the DOS for the helical and planar zigzag structures. Only a small change in the DOS was found between these structures, showing little change of the electronic structure between these conformations. We also evaluated the inner potential V-0, which is the parameter defining the energy origin of the free-electron-like final state, and checked the validity of the value of -10 eV estimated in our previous study using the experimentally observed hnu dependence of the peak intensity. The estimation of V-0 was performed by pursuing the best agreement between the energy-band dispersion [E=E(k)] relation along the chain direction obtained from the simulated spectra and the experimentally deduced one. An excellent agreement in the topmost band was achieved when the assumed inner potential V-0 was set at about zero. This value of V-0 is much different from the value of V-0=-10 eV in the previous study, suggesting the invalidity of the previous assumption at the estimation of V-0 from the peak intensity variation with hnu. Using the presently obtained V-0, we could derive more reliable E=E(k) dispersion relation from the observed ARUPS spectra. The comparison of this newly derived relation gave good agreement with theoretically calculated E=E(k) relations, in contrast to the poor agreement for the previous results with V-0=-10 eV. (C) 2004 American Institute of Physics.