Adsorption and reaction of acetylene and ethylene on the Si(001)2x1 surface

The electronic structures and the thermal reaction of chemisorbed C2H2 and C2H4 on the Si(001)2x1 surface have been investigated by carbon K-edge near-edge x-ray absorption fine structure (NEXAFS) and ultraviolet photoemission spectroscopy (UPS) using synchrotron radiation. The bonding and antibonding states due to the interaction of the molecules and the Si surface atoms are identified by detailed polarization-dependent UPS and NEXAFS measurements, respectively. These bonding and antibonding states are shown to originate from the hybridization between the occupied Si dangling bonds and the lowest unoccupied molecular orbitals (pi(C.C)*) Of C2H2 and C2H2 double-sigma-bonded on the top of the: Si dimer. The thermal evolution of mainly C2H2, is investigated in detail for a wide temperature range of 60 - 1500 K from the condensation to the surface alloy formation. The coexistence of the physisorbatelike and the chemisorbed molecular species is observed at 70-90 K for C2H2 and C2H4, for the coverages greater than similar to 0.25 monolayer (ML). The pi(C-C)* resonance of those physisorbatelike C2H4 species in NEXAFS exhibits an unusual polarization dependence indicating adsorption with their molecular plants aligned perpendicular to the surface. The dissociation of C2H2 chemisorbates is shown to occur at 600-700 R as observed by UPS. After the dissociation of molecules, the atomic hydrogen adsorbates are identified by the: monohydridelike surface resonance states in the UP spectra at 800-950 K. 19 lost of the Si dangling bonds are passivated by, at least partly, the hydrogen adsorbates at this stage. AT similar to 1000 K, the desorption of hydrogen occurs, which accompanies the appearance of a broad SiC-like feature in the UP spectra at similar to 3 eV below Fermi level.