Three Dimensionality and Orbital Characters of the Fermi Surface in (Tl,Rb)_{y}Fe_{2-x}Se_{2}

We report a comprehensive angle-resolved photoemission spectroscopy study of the tridimensional electronic bands in the recently discovered Fe selenide superconductor (Tl,Rb)_{y}Fe_{2-x}Se_{2} (T_{c}=32 K). We determined the orbital characters and the k_{z} dependence of the low energy electronic structure by tuning the polarization and the energy of the incident photons. We observed a small 3D electron Fermi surface pocket near the Brillouin zone center and a 2D like electron Fermi surface pocket near the zone boundary. The photon energy dependence, the polarization analysis and the local-density approximation calculations suggest a significant contribution from the Se 4p_{z} and Fe 3d_{xy} orbitals to the small electron pocket. We argue that the emergence of Se 4p_{z} states might be the cause of the different magnetic properties between Fe chalcogenides and Fe pnictides.