The transition from the adsorbed state to a surface alloy in the Sn/Ni(1 1 1) system

Abstract

Conventional X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), and high resolution photoelectron spectroscopy with synchrotron radiation (SRPES) were used in order to study the overlayer to surface alloy transition in the Sn/Ni(1 1 1) system, as a function of temperature and initial Sn coverage. The TPD of CO adsorbed at 150 K, shows complete blocking of CO adsorption on the perfect surface alloy (0.33 ML Sn), whereas for less than 0.33 ML Sn, CO adsorbs on the alloy-free regions with the same local saturation coverage as on the clean Ni(1 1 1) surface. For RT deposition of 0.1 to 2.6 ML Sn/Ni(1 1 1), only a new sharp c(4 × 2) LEED structure was observed at exactly 0.25 ML, whereas Sn in excess of 1 ML yielded a c(2 × 2) LEED structure, when flashed at 700 K, and the sharpest (√3 × √3) R30° alloy structure, when annealed at 1000 K. A series of high resolution Sn4d SRPE spectra with 50 eV photon energy taken upon short heating of 1.2 ML Sn/Ni(1 1 1) at increasing temperatures, showed the gradual transition from the overlayer (Sn4d 5/2 BE 24.00 eV) to the alloyed (Sn4d 5/2 BE 23.73 eV) state of Sn. The combination of LEED and SRPES results suggests that the overlayer-to-alloy transition occurs via Sn adsorption at the threefold hollow Ni(1 1 1) sites. A theoretical estimation of the Sn4d 5/2 chemical shift using data from ab initio electronic structure calculations yielded a value of −0.36 eV, in reasonable agreement with the experiment.