ARPES experiments give information about the electronic band structure of a crystalline bulk material. However, they are not not obtained in a direct way: only the component parallel to the surface of the electronic momentum is conserved;  the perpendicular component may be obtained, or using some theoretical information, or performing complicated experimental procedures involving more than one surface. ARPES spectra contain information both on electronic bulk and surface states (if they exist), and normally is not easy to separate the two contributions. Theoretical electronic structure calculations may help to do this job. They must be performed for both, the surface and the bulk material,  and the results would be related together in order to identify surface and bulk features. In this talk I shall present results using this theoretical approach in order to understand ARUPS spectra. All electronic structure calculations were performed using the Wien2k package for the following systems:

 (a) Atomic N on the Cu(001) surface. The ARUPS spectra on clean Cu(001) and N-implanted and annealed Cu(001)-N surfaces, taken at the same azimuthal angle, shows a metallic to semiconductor change in character under N incorporation, in correspondence with the two crystalline limits: bulk Cu and crystalline Cu₃N. Slabs of clean and N-adsorbed Cu(001) surfaces are used to characterize these systems and to interpret the ARUPS spectra.  Perpendicular N-relaxation  is also evaluated  on the  hollow site, and  N 1s  core level shifts  are compared with  XPS  measurements.

(b) Transition metal chalcogenides layer materials, like a-MoTe₂. These compounds are layered materials formed by sandwiches of three atomic planes (chalcogen-metal-chalcogen, covalently bonded) separated by a van der Waals gap from the following sandwich. Band structure calculations confirm this compound is an indirect semiconductor with the valence band maximum (VBM) at the G point, as well as several other features of the ARUPS spectra. Slabs calculations, considering different number of sandwiches, let us identify some surface related states. For the limit case of only one sandwich of three layer atoms (the equivalent to graphene in graphite), the VBM changes noticeably from the G to the K point (as it is found in graphite and graphene). The ARUPS spectra and theoretical calculations of TM chalcogenides shows the energy of this two points very close, with a bulk character for the highest occupied state at the G point, and a surface related state at the highest occupied  K point.