Theoretical aspects of relaxation phenomena accompanying core and valence ionization in some organic systems

Abstract

Irradiation of molecules by X-rays leads to photo- emission of electons, this forming the basis of XPS or ESCA spectroscopy. The electrons remaining In the molecule experience an effective Increase In nuclear charge accompanying photolonization, and undergo a "relaxation" process. The energy associated with this (the relaxation energy) affects not only the intensity and shape of the experimentally measured peak, but also its position (or binding energy) to a significant extent. By means of well-established quantum mechanical methods. It Is possible to calculate theoretically both binding energies and relaxation energies for core-electron photoionizatlon. Specifically, ab initio LCAO MO SCF calculations within the Hartree-Fock formalism have been performed on a wide variety of organic systems. The relaxation energy as a function of increasing chain- length In a series of linear and bent alkanes has been Investigated, and found to be responsible for the experimentally observed shifts In core binding energies. The valence (2s) Ionized species have also been Investigated, with particular emphasis on line-widths. A study of the simple carbocations has been made, a particular point of Interest being the classical or non-classical descriptions of the bonding in specific cases. To Investigate the effect of extra-molecular contributions to relaxation energies, a series of hydrogen-bonded dimers has been studied as a simple prototype system. Intermolecular contributions to relaxation energies are found to be of the same sign, irrespective of the sign for the shift in core binding energy. The core-like, valence (2s) hole-state species are found to have parallel trends In shifts. A method has been developed for partitioning the total relaxation energy Into contributions from each occupied orbital in the molecule of interest. This procedure Is shown to be both mathematically rigorous, and In accord with chemical Intuition, and Is applied to several series of related molecules. From studies of this nature, a clearer understanding of the relationship between chemical bonding, and the reorganization processes accompanying core-ionization is possible. Finally, as an extension of the Investigation into extra- molecular relaxation energies, and a further example of the use of relaxation energy partitioning, some simple prototype systems for the adsorption of small molecules onto metal surfaces are considered. In the case of CO on Ni, It Is found that a bent conformation Is needed to Interpret the available XPS data.