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The appearance of high mobility electrons at the LaAlO3/SrTiO3 (LAO/STO) interface has raised strong interest in the material science community and a lively debate on the origin of the phenomenon. In particular, in view of
the large band gaps of the two bulk single crystals constituting this heterostructure, the realization of a conducting
system was totally unexpected. A possible explanation is an electronic reconstruction of the interface, realizing
a transfer of electrons from the LaAlO3 surface to SrTiO3 near the interface, thereby avoiding the polarization
catastrophe associated with the alternating polar layers of the LaAlO3 film. The predictions of theoretical models
based on this idea are quite peculiar and need to be verified by specific experiments able to address the electronic
properties of the LAO/STO buried interface. Here, by using x-ray spectroscopy techniques, we show that the
appearance of an electron system is correlated to the removal of the degeneracy of the titanium 3d states, and
doped electrons appear in a band preferentially created by the hybridization between 3dxy states of titanium
and oxygen 2px,y states. This splitting is consistent with an ordering of the Ti 3dxy orbital belonging to the
TiO6 octahedra close to the interface, as theoretically proposed. However, the valence of titanium ions remains
prevalently 4+, therefore other mechanisms should be also considered for the stabilization of the system.
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