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 (root 3x root 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 6s(2) 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 (root 3x root 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 (root 3x root 3) surfaces induced by the other group III metals, but they are different on the Tl/Si(111)-(root 3x root 3) surface.