The effect of 2 x 1 reconstruction upon the core-electron binding energies of the outermost (100) layers was determined by using high-resolution photo-emission data. A previously unobserved 0.19eV crystal-field splitting was resolved for the up-atoms of the asymmetrical surface dimers, whose average core-level shift was -0.4eV. The signal from the down-atoms was clearly identified and had a shift of 0.22eV. These findings indicated a charge flow of about 0.05e from the sub-surface to the surface layers, with a substantially larger difference of about 0.34e between the up-atoms and down-atoms in the dimer.

G.K.Wertheim, D.M.Riffe, J.E.Rowe, P.H.Citrin: Physical Review Letters, 1991, 67[1], 120-3