  Core level X-ray Photoelectron Spectroscopy (XPS) studies have been carried
out on polycrystalline MgB_2 pellets over the whole binding energy range with a
view to having an idea of the charge state of Magnesium (Mg). We observe 3
distinct peaks in Mg 2p spectra at 49.3 eV (trace), 51.3 eV (major) and 54.0 eV
(trace), corresponding to metallic Mg, MgB_2 and MgCO_3 or, divalent Mg species
respectively. Similar trend has been noticed in Mg 2s spectra. The binding
energy of Mg in MgB_2 is lower than that corresponding to Mg(2+), indicative of
the fact that the charge state of Mg in MgB2 is less than (2+). Lowering of the
formal charge of Mg promotes the sigma to pi electron transfer in Boron (B)
giving rise to holes on the top of the sigma-band which are involved in
coupling with B E_2g phonons for superconductivity. Through this charge
transfer, Mg plays a positive role in hole superconductivity. B 1s spectra
consist of 3 peaks corresponding to MgB_2, boron and B_2O_3. There is also
evidence of MgO due to surface oxidation as seen from O 1s spectra.
