Surface core-level shifts on Nb(001)

High-resolution 3d5/2 core-level spectra of the clean Nb(001) surface have been obtained at 130 K with 240- and 250-eV synchrotron radiation. The most remarkable result is the positive sign (0.49±{}0.01 eV) of the surface core-level shift (the positive sign means that the surface signal is shifted to higher binding energies relative to the bulk one), which is in stark contradiction to the earlier theoretical predictions, but conforms the results from the most recent ab initio calculations. Using a nonlinear curve-fitting program, we have also tentatively obtained the subsurface emission shifted by 0.13±{}0.01 eV, a value that is comparable with those for the Ta(001) and W(001) surfaces.