C(2×2) oxygen-induced core-level shifts and surface states of Pd(100)

Surface states induced by the C(2×2) phase of oxygen on a Pd(100) surface were studied by means of surface core-level-shift (SCLS) spectroscopy and photoelectron angular distributions that were obtained by using forward-scattering spectroscopy. This combination of the two powerful techniques proved to be very useful in that surface and bulk states were observed to have two distinct energies as determined by the SCLS spectroscopy applied to the Pd 3${\mathit{d}}_{5/2}$ and 3${\mathit{d}}_{3/2}$ core levels. Angular distributions of the surface and bulk components of the Pd 3${\mathit{d}}_{5/2}$ core level were studied separately. Photoelectron angular distributions were found to be consistent with the forward-scattering mechanism in which the same Pd atom residing in its lattice site gives raise to two distinct 3${\mathit{d}}_{5/2}$ photoemission peaks separated by 0.55 eV. Forward-scattering results obtained along the 〈001〉 and the 〈011〉 directions also suggest that surface states extend four to five layers into the bulk and they can be characterized by a wave function whose amplitude decays in the direction of the inward normal. Coefficients of the atomic orbitals that represent oxygen-induced surface states were mapped experimentally. Structural and mechanistic implications of the long-range nature of the O-Pd interaction are discussed.