  The influence of surface enhanced covalency on the Madelung potential is
experimentally investigated using angle-resolved photoemission for (100), (110)
and (111) SrTiO$_3$ surfaces after annealing in UHV at 630 {\deg}C.
Deconvolution of the core level spectra (O 1s, Sr 3d and Ti 2p) distinguishes
bulk and surface components, which are interpreted in terms of surface enhanced
covalency (SEC). By comparing the experimentally measured binding energies with
theoretical calculations developed in the framework of the Localized-Hole
Point-Ion Model, we quantitatively determine the effective electron occupancy
at bulk and surface Sr and Ti sites. Our results confirm the essentially ionic
character of Sr$-$O bond and the partially covalent character of Ti$-$O bond in
bulk STO. The cation Ti and Sr electron occupation is greater for all the three
surfaces than in the bulk. Surface covalency shifts the Madelung potential at
the surface by {\Delta}EM. {\Delta}EM is a minimum for the (111) surface, and
increases through (100), attaining a maximum for (110). The angle-resolved
valence band spectra and the work function values also confirm this trend. The
results are consistent with d$-$d charge fluctuations dominating at the
surface, whereas metal-ligand charge transfers are more energetically
favourable in the bulk.
