Surface core-level shifts of Ge(100)-2X1

Core-level photoemission spectra of the buckled Ge(001)-2x1 surface taken at various energies and emission angles are analyzed, using the inelastic mean free path (IMFP) to constrain the intensities of the contributions to the 3d signal of the first two surface layers and the bulk. This ansatz successfully represents data from the clean Ge(100) surface at all emission angles and photon energies. The surface core-level shifts of the up-dimer atom, down-dimer atom, and subsurface atom are then found at -442, -183, and +93 meV, respectively. They are in good agreement with the final-state theoretical calculations by Pehlke and Scheffler [Phys. Rev. Lett. 71, 2338 (1993)]. The IMFP is 3.5+/-0.2 Angstrom at 26 eV kinetic energy. Furthermore, we found that hulk and surface atoms have distinct spin-orbit splittings of 594 and 561 meV, respectively.