C(2X2) OXYGEN-INDUCED CORE-LEVEL SHIFTS AND SURFACE-STATES OF PD(100)

Surface states induced by the C(2 X 2) phase of oxygen on a Pd(100) su rface were studied by means of surface core-level-shift (SCLS) spectro scopy and photoelectron angular distributions that were obtained by us ing forward-scattering spectroscopy. This combination of the two power ful techniques proved to be very useful in that surface and bulk state s were observed to have two distinct energies as determined by the SCL S spectroscopy applied to the Pd 3d5/2 and 3d3/2 core levels. Angular distributions of the surface and bulk components of the Pd 3d5/2 core level were studied separately. Photoelectron angular distributions wer e found to be consistent with the forward-scattering mechanism in whic h the same Pd atom residing in its lattice site gives raise to two dis tinct 3d5/2 photoemission peaks separated by 0.55 eV. Forward-scatteri ng results obtained along the [001] and the [011] directions also sugg est that surface states extend four to five layers into the bulk and t hey can be characterized by a wave function whose amplitude decays in the direction of the inward normal. Coefficients of the atomic orbital s that represent oxygen-induced surface states were mapped experimenta lly. Structural and mechanistic implications of the long-range nature of the O-Pd interaction are discussed.