Electronic structure of acetylene on X-ray photoelectron and x-ray absorption spectroscopy

The electronic structure of acetylene adsorbed at room temperature on (Formula presented) its thermal stability, and its reactivity in the presence of a silicon flux have been studied by synchrotron radiation x-ray photoemission of valence bands and core-levels (C (Formula presented) Si (Formula presented)) and x-ray absorption spectroscopy (C (Formula presented) edge). We observe that the electronic occupancy of both adatoms and restatoms is modified by the adsorption process. The alignment of the C-C molecular axis by the chemisorption bond leads to a strong polarization dependence of the C (Formula presented)-edge spectra. The orientation of the C-C molecular axis is then determined to be nearly parallel to the surface plane and the bonding of di-σ type. Annealing sequences of an acetylene-saturated surface (up to a maximum temperature of 955 K) induce strong changes in the electronic structure of both molecule and substrate. There are hints of molecule/molecule reactions involving the in-plane π bond. We observe also the penetration of atomic carbon into the surface silicon layers, which in turn induces changes in the electronic structure of the silicon surface atoms. In the case of an exposure of an acetylene predosed surface to a silicon flux at 300 K, the molecule floats over the growing silicon layer, maintaining a di-σ bonding, as the in-plane π bond does not interact with incoming silicons.