# Photoelectron  spectroscopy

In the method of photoelectron spectroscopy electrons are excited from the core levels or occupied valence band states and are analysed by their kinetic energy giving a replica of the electronic structure of solids. Analysis of the core levels (x-ray photoelectron spectroscopy, XPS) gives information about the chemical states of elements that might be influenced by the corresponding atomic coordination. In the case of the angle-resolved photoelectron spectroscopy (ARPES) the electrons emitted from the valence band are additionally analysed by their emission angle (θ) and detected by the 2D detector giving photoemission intensity map for the particular k-direction in the Brillouin zone, which are used for the analysis of the electronic structure of the studied objects.

In many cases experimentally obtained angle-resolved photoemission spectroscopy (ARPES) data can be directly compared with the respective results of the band-structure calculations. For example, in case of graphene and graphene/metal interfaces (if quasiparticle excitations, like phonons or low-energy plasmons, in the close vicinity to the Fermi level are not considered) one can perform a direct comparison of the experimental and DFT calculated energy dispersions for the graphene-related π and σ bands (electron correlations in graphene are very small due to the delocalased nature of the respective electrons). In the general case, because of a large mismatch between graphene and metal lattice constants, a supercell approach is used to model graphene-metal interfaces. The latter brings in an inconvenience: The folding of the bands into the smaller supercell Brillouin zone gives rise to complicated band structures. Thus, in order to make such comparison straightforward, the band unfolding procedure has to be applied to the graphene/metal long-range structures. Such procedure can be performed with the BandUP code that allows to unfold the band structure of the system on the $(1\times 1)$ primitive unit cells of the respective symmetry (graphene or metallic substrate).