A theoretical and experimental model system investigation of core ionisation phenomena for polymers
The photoemission of electrons from molecules upon their irradiation by X-rays forms the basis of E.S.C.A. (XPS) spectroscopy. The electrons remaining in the molecule experience an effective increase in nuclear charge accompanying photoionisation, and undergo a "relaxation" process. The energy associated with this (the relaxation energy) affects not only the intensity and shape of the experimentally determined peak, but also its position (or binding energy) to a significant extent and gives rise to accompanying lower kinetic energy, satellite structure. By means of well-established quantum mechanical methods, it is possible to calculate theoretically the binding energies and relaxation energies for core electron photoionisation, and the transition energies and intensities of accompanying shake-up satellites. A series of C,H,N,0, containing molecules has been investigated encompassing a wide range of functionalities of interest to the polymer chemist. Trends in binding energies as a function of electronic environment are established and their dependence upon changes in relaxation effects is noted. The manner in which weak interactions of the ground state are enhanced on going to the core hole state manifold is demonstrated in a determination of the classical or non-classical nature of carbocations of current interest and in a study of hydrogen bonding in 1,3 dicarbonyl systems. Shifts in binding energy are found to be highly characteristic of a given structural type and in the case of the latter this is also manifest in the accompanying satellite structure. Finally, the importance of such structure is emphasised in studies relating to the isomeric hydrocarbons and benzoquinones, where the factors determining shifts in binding energy are so short range in nature as to render them negligible.