  A c-axis oriented thin film and a high density sintered pellet of MgB2 have
been studied by x-ray photoemission spectroscopy, and compared to measurements
from MgO and MgF2 single crystals. The as-grown surface has a layer which is
Mg-rich and oxidized, which is effectively removed by a nonaqueous etchant. The
subsurface region of the pellet is Mg-deficient. This nonideal near-surface
region may explain varied scanning tunneling spectroscopy results. The MgB2
core level and Auger signals are similar to measurements from metallic Mg and
transition metal diborides, and the measured valence band is consistent with
the calculated density of states.
