ELECTRONIC-STRUCTURE OF ACETYLENE ON SI(111)-7X7 - X-RAY PHOTOELECTRON AND X-RAY-ABSORPTION SPECTROSCOPY

The electronic structure of acetylene adsorbed at room temperature on Si(111)-7 x 7, its thermal stability, and its reactivity in the presen ce of a silicon flux have been studied by synchrotron radiation x-rap photoemission of valence bands and core-levels (C 1s, Si 2p) and x-ray absorption spectroscopy (C IC edge). We observe that the electronic o ccupancy of both adatoms and restatoms is modified by the adsorption p rocess. The alignment of the C-C molecular axis by the chemisorption b ond leads to a strong polarization dependence of the C K-edge spectra. The orientation of the C-C molecular axis is then determined to be ne arly parallel to the surface plane and the bonding of di-sigma type. A nnealing sequences of an acetylene-saturated surface (up to a maximum temperature of 955 K) induce strong changes in the electronic structur e of both molecule and substrate. There are hints of molecule/molecule reactions involving the in-plane pi bond. We observe also the penetra tion of atomic carbon into the surface silicon layers, which in turn i nduces changes in the electronic structure of the silicon surface atom s. In the case of an exposure of an acetylene predosed surface to a si licon flux at 300 K, the molecule oats over the growing silicon layer, maintaining a di-sigma bonding, as the in-plane pi bond does not inte ract with incoming silicons.