Multiplet effects in X-ray spectroscopy

This review gives an overview of the presence of multiplet effects in X-ray spectroscopy, with an emphasis on X-ray absorption studies on 3d transition metal ions in inorganic oxides and coordination compounds. The first part of the review discusses the basics of multiplet theory and respectively, atomic multiplets, crystal field effects and charge transfer effects are explained. The consequences of 3d-spin-orbit coupling and of 3d systems in symmetries lower than cubic are discussed. The second part of the paper gives a short overview of all X-ray spectroscopies, where the focus is on the multiplet aspects of those spectroscopies and on the various configurations that play a role in combined spectroscopies such as resonant photoemission, resonant X-ray emission and coincidence spectroscopy. The review is concluded with a section that gives an overview of the use of multiplet theory for 3d coordination compounds. Some new developments are sketched, such as the determination of differential orbital covalence and the inclusion of π-(back)bonding.