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Electronic transitions involving Zn-3d and O-2s bands are detected.Ab-initio band structure calculations performed at the DFT-LDA level help tointerpret the observed transitions. The calculations are also extended to theGW approximation in order to determine the electronic bandgap. Finally, theplasmon frequency is found to be ~!

The experimental characterization of surfaces is often obtained through spectro scopic techniques like photoemission inverse photoemission electron energy loss spectroscopy and optical techniques as Reflectance Anisotropy and Differential Reflectivity etc It is hence very important to describe accurately the electronic excitations with highly reliable and efficient ab-initio approaches In the last two decades Density Functional Theory has proven to be a very powerful tool for elec tronic ground state properties but shows significant deviations from the expen ments when excited states are involved The use of many body Green’s functions theory, with DFT calculations as zero order approximation is now the state of the art to obtam quasi particle excitation energies and optical spectra both in bulk and at surfaces In this paper we will present the current status of this theoretical and computational approach showing results for several semiconductor surfaces