Ab initio spectroscopy is a powerful combination of quantum-based theories and computer simulations, covering a wide range of theoretical and computational methods that incorporate many-body effects and interactions showing up in the excited state. This framework, combining density-functional theory (DFT) with many-body perturbation theory, not only allows for analyzing data obtained by experimental probes, but also for shining light onto the underlying physics. I will demonstrate this with a series of selected materials and excitation processes:

Oxygen K-edge spectra from the wide-gap semiconductor Ga₂O₃ will reveal how signals from atoms located in a particular environment can be selectively enhanced or quenched by adjusting the crystal orientation [1]. These results suggests ELNES, combined with ab initio many-body theory, to be a very powerful technique to characterize complex systems, with sensitivity to individual atomic species and their local environment.

With the example of self-assembled phases of functionalized azo-benzene it will be shown, how excitonic effects in core and valence excitations can be tuned by molecular packing [2], and how this may affect the switching functionality of the molecules.

Finally, I will discuss how transmission electron microscopy can be used for mapping atomic orbitals, exploring its capabilities by a first principles approach [3]. For defected graphene, exhibiting either an isolated vacancy or a substitutional nitrogen atom, different kinds of images are to be expected, depending on the orbital character (see Figure 1).

[1] C. Cocchi, H. Zschiesche, D. Nabok, A. Mogilatenko, M. Albrecht, Z. Galazka, H. Kirmse, C. Draxl, and C. T. Koch, submitted to Phys. Rev. B (2016).

[2] C. Cocchi and C. Draxl, Phys. Rev. B 92, 205105 (2015).

[3] L. Pardini, S. Löffler, G. Biddau, R. Hambach, U. Kaiser, C. Draxl, and P. Schattschneider, Phys. Rev. Lett. (2016); in print.

Figures:

Figure 1: Simulated real-space intensity of the electron's exit wave function after propagation of an incident planewave through a graphene layer in presence of a nitrogen substitutional atom (upper panels) and a vacancy (bottom panels) from three selected energy regions.

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EMC Abstracts - https://emc-proceedings.com/abstract/from-core-and-valence-excitations-to-orbital-mapping-a-theorists-perspective/