RESUMO
The origin and assignment of the complex main and satellite X-ray photoelectron spectroscopy (XPS) features of the cations in ionic compounds have been the subject of extensive theoretical studies using different methods. There is agreement that within a molecular orbital model, one needs to take into account different types of configurations. Specifically, those where a core electron is removed, but no other configuration changes are made and those where in addition to ionization, there are also shake or charge-transfer changes to the ionic configuration. However, there are strong disagreements about the assignment of XPS features to these configurations. The present work is directed toward resolving the origin of main and satellite features for the Ni 2p XPS of NiO based on ab initio molecular orbital wave functions (WFs) for a cluster model of NiO. A major problem in earlier ab initio XPS studies of ionic compounds has been the use of a common set of orbitals that was not able to properly describe all the ionic configurations that contribute to the full XPS spectra. This is resolved in the present work by using orbitals that are optimized for averages of the occupations of the different configurations that contribute to the XPS. The approach of using state-averaged (SA) orbitals is validated through comparisons between different averages and through use of higher order excitations in the WFs for the ionic states. It represents a major extension of our earlier work on the main and satellite features of the Fe 2p XPS of Fe2O3 and proves the reliability and the generality of the assignments of the character and origin of the different features of the XPS obtained with orbitals optimized for SAs. These molecular orbital methods permit the characterization of the ionic states in terms of the importance of shake excitations and of the coupling of ionization of 2p1/2 and 2p3/2 spin-orbit split sub shells. The work lays the foundation for definitive assignments of the character of main and satellite XPS features and points to their origin in the electronic structure of the material.
RESUMO
Over the past three decades, the widespread utility and applicability of X-ray photoelectron spectroscopy (XPS) in research and applications has made it the most popular and widely used method of surface analysis. Associated with this increased use has been an increase in the number of new or inexperienced users which has led to erroneous uses and misapplications of the method. This article is the first in a series of guides assembled by a committee of experienced XPS practitioners that are intended to assist inexperienced users by providing information about good practices in the use of XPS. This first guide outlines steps appropriate for determining whether XPS is capable of obtaining the desired information, identifies issues relevant to planning, conducting and reporting an XPS measurement, and identifies sources of practical information for conducting XPS measurements. Many of the topics and questions addressed in this article also apply to other surface-analysis techniques.