RECENT ADVANCES IN THE PRACTICAL AND ACCURATE CALCULATION OF CORE ANDVALENCE XPS SPECTRA OF POLYMERS - FROM INTERPRETATION TO SIMULATION

Core and valence X-ray Photoelectron Spectroscopies (XPS) are routinel y used to obtain information on the chemical composition, bonding and homogeneity of polymer surfaces. In spite of their apparent conceptual simplicity, Core and Valence Electron Binding Energies (CEBEs and VEB Es) a few electron-volts (eV) or fractions of an eV apart are difficul t to interpret. We present some results obtained with various recent t heoretical. approaches. An emphasis is made on a procedure based on th e Density Functional Theory (DFT) that enables the calculation of CEBE s and VEBEs which are in remarkable agreement with experiment. The met hod has been tested on numerous small (3-6 atoms) to fairly large (15- 25 atoms) molecules, and shows an average absolute deviation with expe riment of only 0.20 eV for CEBEs and 0.30 eV for VEBEs, i.e. compatibl e with the resolution of the best XPS experiments carried out at the m oment. Besides the quality of its predictions, the procedure takes adv antage of the speed and CPU time scaling of DFT as a function of syste m size: it is computationally tractable, even for surprisingly large s ystems such as polymers, and may be an interesting accurate alternativ e to interpret and simulate XPS-probing on real systems, We illustrate the usefullness and pitfalls of this approach in fundamental as well as applied fields such as in the study of Polyacrylonitrile (PAN), Pol ytetrafluoroethylene (PTFE), Polyvinyldifluoride (PVdF) and gamma-Amin opropyltriethoxysilane (gamma-APS, an adhesion promoter). (C) 1997 Els evier Science B.V.