The electronic structures of the Eu/ Si͑111͒-͑3 ϫ 2͒ and ͑2 ϫ 1͒ surfaces have been investigated by angleresolved photoelectron spectroscopy. On the ͑3 ϫ 2͒ surface, we identify six surface states in the gap and a pocket of the bulk band projection. Among the five surface states observed in the bulk band gap, the dispersions of three of them agree well with those of the surface states of monovalent atom adsorbed Si͑111͒-͑3 ϫ 1͒ surfaces. The dispersions of the two other surface states observed in the band gap agree well with those observed on the Ca/ Si͑111͒-͑3 ϫ 2͒ surface, which has basically the same structure as that of monovalent atom adsorbed Si͑111͒-͑3 ϫ 1͒ surfaces. Taking these results into account, we conclude that the five surface states observed in the band gap originate from the orbitals of Si atoms that form a honeycomb-chain-channel structure. In the case of the ͑2 ϫ 1͒ surface, two semiconducting states are observed in the bulk band gap. The difference in binding energy of these two states at the ⌫ point agrees well with that of the surface states obtained theoretically for a clean Si͑111͒-͑2 ϫ 1͒ surface with a Seiwatz structure, and the dispersion of the upper state shows good agreement with the corresponding theoretical surface state. These observations indicate that the two surface states in the band gap originate from Si atoms that form a Seiwatz chain. The present results support the structures of the Eu/ Si͑111͒-͑3 ϫ 2͒ and ͑2 ϫ 1͒ surfaces proposed in the literature.

We have investigated the electronic structures of the so-called Eu-and Ca-induced Si͑111͒-͑5 ϫ 1͒ surfaces by using angle-resolved photoelectron spectroscopy ͑ARPES͒ and low-energy electron diffraction ͑LEED͒. The LEED patterns of these surfaces indicate that the periodicities of both surfaces are actually ͑5 ϫ 4͒. In the ARPES study, seven surface states were observed on each ͑5 ϫ 4͒ reconstruction. Of these surface states, the dispersions of five of them show good agreement with those of the Eu-and Ca-induced ͑3 ϫ 2͒ honeycombchain-channel ͑HCC͒ surfaces and the dispersions of the two other states agree well with those of the Eu-and Ca-induced ͑2 ϫ 1͒ Seiwatz surfaces along the ͓110͔ direction-i.e., the direction parallel to the adsorbate chain. Taking the dispersion behavior of these surface states into account, we conclude that the interaction between the nearest-neighbor HCC chain and Seiwatz chain is quite small and that the electronic structure of one chain hardly affects the electronic structure of its neighboring chain. We also discuss the atomic structure of the Eu-and Ca-induced Si͑111͒-͑5 ϫ 1͒ reconstructions based on their electronic structures.

The surface structures of the ͑quasi-͒one-dimensional reconstructions induced by the adsorption of Eu on Si͑111͒ have been investigated by low-energy electron diffraction ͑LEED͒ and high-resolution core-level photoelectron spectroscopy. Different phases were observed in LEED depending on the Eu coverage. The lowest coverage phase has a ͑3 ϫ 2͒ periodicity, and the highest coverage phase has a ͑2 ϫ 1͒ one. Of the intermediate phases, the LEED pattern of the so-called ͑5 ϫ 1͒ surface indicates that this surface has actually a ͑5 ϫ 4͒ periodicity. The Eu 4f core-level spectra show that the Eu coverages of the ͑3 ϫ 2͒, ͑5 ϫ 4͒, and ͑2 ϫ 1͒ phases are 1 / 6 monolayer ͑ML͒, 0.3 ML, and 0.5 ML, respectively, and that the valence state of the adsorbate is 2+ in all these three phases. In the Si 2p core-level spectra, three surface components were observed in both the lowest and highest coverage phases. By considering the energy shift and intensity of each surface component, we conclude that the structure of the ͑3 ϫ 2͒ phase is basically the same as that of the honeycomb-chain-channel model, and that the ͑2 ϫ 1͒ phase is formed by -bonded Seiwatz Si chains. Regarding the ͑5 ϫ 4͒ phase, two extra Si 2p surface components were observed together with the three components observed in the two end phases. Taking the energy shifts and intensities of the extra surface components into account, we propose a structural model of the ͑5 ϫ 4͒ phase.