Mechanisms responsible for chemical shifts of core-level binding energies and their relationship to chemical bonding

A comprehensive review of different mechanisms which contribute to the chem ical shifts of core-level binding energies, BEs, is made. A principle focus is on showing how the mechanisms can be used to relate the BE shifts to fe atures of the chemical bonding and chemical interactions in the studied sys tem. Several initial state mechanisms are identified; while some are well. known, the importance of others has been only recognized fairly recently. A theoretical framework is presented which places the analysis and interpret ation of these BE shifts on a firm foundation. A rigorous definition and di stinction of initial and final state effects is presented. This definition is applied to show that initial state effects are often the dominant factor s for the chemical BE shifts. It is also shown that, in many cases, theoret ical approaches involving the use of constrained variations can permit a cl ear and definitive separation of the contributions of the different mechani sms. Several representative applications to the analysis and interpretation of core-level BE shifts are described which show how the theoretical metho ds of analysis can be used to identify the mechanisms important for the BE shifts. Often more than one mechanism makes an important contribution to th e shifts and it is common that the contributions will be canceling. When al l of the relevant mechanisms are taken into account in the analysis of the BE shifts, these shifts do provide valuable information about the chemical bonding and electronic structure of the materials being studied. The mechan isms presented and the theoretical frameworks described provide a unified v iew of BE chemical shifts. (C) 1999 Elsevier Science B.V. All rights reserv ed.