Core-level photoemission study of thallium adsorbed on a Si(111)-(7x7) surface: Valence state of thallium and the charge state of surface Si atoms

The coverage-dependent valence state of Tl adsorbed on a Si(111)-(7x7) surface and the coverage dependence of the charge states of surface Si atoms have been investigated by high-resolution core-level photoelectron spectroscopy. Although two different reconstructions were observed in low-energy electron diffraction at different coverages, a (1x1) pattern at a Tl coverage of 1 monolayer (ML) and a (√(3)x√(3)) pattern at a coverage of 1/3 ML, the binding energy of the Tl 5d core-level was the same at Tl coverages up to 1 ML. Taking the valence state on a (1x1) surface reported in the literature into account, we conclude that the valence state of Tl is 1+, and that the 6s2 electrons of Tl are inactive as an inert pair in the Tl-Si bonding on a Si(111) surface at a coverage of 1 ML and below. In the Si 2p core-level spectra, one surface component was observed on the (1x1) surface, and three surface components were observed on the (√(3)x√(3)) surface. The binding energies and intensities of the Si 2p surface components indicate that the charge state of the surface Si atoms on Tl/Si(111)-(1x1) is the same as that of the (√(3)x√(3)) surfaces induced by the other group III metals, but they are different on the Tl/Si(111)-(√(3)x√(3)) surface