Calculated final state effects of the PMDA-ODA polyimide XPS photoemission spectrum

Abstract

X-Ray photoemission spectroscopy has been extensively utilized in probing surfaces and interfaces involving the technologically important class of polymers: the polyimides. We have previously shown that, within the Koopmans' approximation, the calculated core level positions can yield an accurate characterization of the relative positions of the chemically inequivalent atoms observed in the photoemission spectrum (XPS) of the PMDA-ODA polyimide. Whereas the relative positions agreed well with experiment, the absolute magnitudes of the calculated XPS peak positions were tens of electron volts above the measured values. To resolve this discrepancy as well as to examine details involving the relative XPS peak positions, the carbon(C1s), nitrogen(N1s), and oxygen(O1s) ionization energies of separate molecular units, pyromellitic diimide (PMDI) and hydroxyaniline, were calculated taking into account final state effects, allowing for core hole relaxation. This technique is called the ΔSCF method. The present calculations show that core-hole relaxation, treated in the ΔSCF approximation, shifts the calculated values of the photoemission peaks to within one electron volt of the observed peak positions.