The 3d surface core-level shifts of the clean Ge(100)(2×1) surface have been investigated at 300 K by photoemission. A previously unobserved (to our knowledge) shifted component at about +0.19 eV relative to the bulk emission has been clearly identified and associated with dimer down-atom emission. The binding energy difference of 0.72 eV between dimer down-atom and dimer up-atom 3d core levels, consistent with a charge transfer similar to that occurring in the Si(100)-(2×1) surface, supports the so-called ionic model of the Ge(100)-(2×1) buckled-dimer reconstruction
