XPS and bonding: when and why can relaxation effects be ignored

The arguments behind the use of the term 'chemical Shift' in X-ray photoele ctron (ESCA) spectroscopy are explored. The factors that relate to initial state (chemical) vs final state (relaxation) contributions are described al ong with relationships to applied cases, with a particular emphasis on oxid es. It is demonstrated that some major classes of oxides, such as silicates , are excellent examples of systems for which the measured binding energy s hifts are directly related to differences in initial state chemistry. (C) 1 999 Elsevier Science B.V. All rights reserved.