Relativistic Methods for Chemists (eBook)

Relativistic Methodsfor Chemists 2
Preface 6
Contents 8
1 An Introduction to Relativistic Quantum Chemistry 16
2 Relativistic Effects and the Chemistry of the Heavier Main Group Elements 78
3 Why do we Need Relativistic Computational Methods? 113
4 Two-Component Relativistic Theories 179
5 Relativistic Density Functional Theory 205
6 Relativistic Pseudopotentials 229
7 Four-Component Electronic Structure Methods 292
8 The Effects of Relativity in Materials Science: Core Electron Spectra 363
9 Relativistic Symmetries in the Electronic Structure and Properties of Molecules 384
10 Relativistic String-Based Electron Correlation Methods 417
11 Electronic Structure and Chemistry of the Heaviest Elements 460
12 Relativistic Effects on Magnetic Resonance Parameters and Other Properties of Inorganic Molecules and Metal Complexes 530
Index 608

"CHAPTER 8 THE EFFECTS OF RELATIVITY IN MATERIALS SCIENCE: CORE ELECTRON SPECTRA (p. 351-352)

Abstract: Core electron spectroscopies like X-ray photo-electron spectroscopy, X-ray absorption spectroscopy and electron energy loss spectroscopy are powerful tools to investigate the electronic structure of transition metal, lanthanide and rare earth materials. On the other hand, the interpretation of the spectra is often not straightforward. Relativistic effects and in particular spin-orbit interactions, electron-electron interaction in the valence shell and between core and valence electrons, solid state effects may all play a role in the core electron spectra. Dynamical and non-dynamical electron correlation effects may also be non-negligible. The spectra can be interpreted and predicted using first principles computational methods that take into account both relativity and electron correlation. Furthermore, such approaches enable the interpretation of the complex processes in terms of physical mechanisms. This chapter discusses the effects of relativity on the core spectra of transition metal, lanthanide and actinide materials and a number of much used computational approaches to describe and interpret the spectra.

Keywords: X-ray photo-electron spectra, X-ray absorption spectra, Electron energy loss spectra, Ab initio relativistic quantum chemical methods, Spin-orbit effects, Branching ratios

8.1. INTRODUCTION

In many cases the consequences of relativity in materials manifest themselves clearly in the electronic, magnetic and optical and properties. In this chapter we focus on the interpretation of relativistic effects on spectroscopic properties, in particular on core and deep valence excitation and ionization spectra of transitionmetal, lanthanide and actinidematerials. For excitation from p, d,. . . shells spin-orbit angular momentum coupling is a prominent relativistic effect. Core spectroscopies like X-ray Photo-electron Spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and Electron Energy Loss Spectroscopy (EELS) can nowadays in many cases give very accurate information on the electronic structure of matter. On the other hand, the interpretation of the spectra is often rather complicated, especially in materials that contain open shell ions.

For example, the relative intensities of spin-orbit split peaks in electron energy-loss spectroscopy and near edge X-ray absorption spectroscopy can only in special cases be roughly deduced from the statistical weights of the core levels. In most cases the angular momentum couplings of the open shell valence electrons, the external crystal field and covalent effects strongly affect these ratios [1–3]. A quantitative estimate of these effects on the branching ratios cannot be found without the aid of computational results.

The shifts of core electron binding energies as observed in X-ray photo-electron spectra (XPS) of an ion in a particular chemical environment with respect to a chosen reference can also provide detailed information about the ground state electronic structure of the materials. Especially in the case of XPS the effects of charge transfer (CT) present a complication for the interpretation of the spectra. The creation of a hole in one of the core orbitals of an atom lowers the excitation energy of socalled charge transfer states, in which an electron is transferred from the neighboring atoms to the core-ionized atom. Hence, the hole may become screened through CT, depending on the electron affinity of the ionized atom."