Variation of the Pb and Bi 5dbinding energies and branching ratios for thin Pb and Bi overlayers on Ni(110)

Thin Pb and Bi overlayers on Ni(110) have been studied by use of variable-energy photoelectron spectroscopy. The overlayers are characterized by use of the Pb and Bi 5d core-level intensities and binding-energy shifts. The Pb 5d cross section (σ) and 5$d_{5/2}$:5$d_{3/2}$ branching-ratio (BR) dependence as a function of photon energy are quite sensitive to the thickness of the overlayer: as the thickness increases, the cross-section maximum shifts to lower photon energy. This shift is reflected by the point at which the BR crosses the statistical 1.5 value: 57.5 eV for 0.5 monolayer to 42.5 eV for bulk Pb. We postulate that this behavior is due to the sensitive nature of the outgoing f-wave potential, as shown recently by theoretical calculations of heavy-metal d cross sections.