Adsorption and reaction of acetylene and ethylene on the $\mathrm{Si}\left(001\right)2\times 1$ surface

The electronic structures and the thermal reaction of chemisorbed ${\mathrm{C}}_2{\mathrm{H}}_2$ and ${\mathrm{C}}_2{\mathrm{H}}_4$ on the $\mathrm{Si}\left(001\right)2\times 1$ 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 $\left({\pi }_{\mathrm{C}-\mathrm{C}}^{*}\right)$ of ${\mathrm{C}}_2{\mathrm{H}}_2$ and ${\mathrm{C}}_2{\mathrm{H}}_4$ double-σ-bonded on the top of the Si dimer. The thermal evolution of mainly ${\mathrm{C}}_2{\mathrm{H}}_2$ 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 ${\mathrm{C}}_2{\mathrm{H}}_2$ and ${\mathrm{C}}_2{\mathrm{H}}_4,$ for the coverages greater than $\sim 0.25$ monolayer (ML). The ${\pi }_{\mathrm{C}-\mathrm{C}}^{*}$ resonance of those physisorbatelike ${\mathrm{C}}_2{\mathrm{H}}_4$ species in NEXAFS exhibits an unusual polarization dependence indicating adsorption with their molecular planes aligned perpendicular to the surface. The dissociation of ${\mathrm{C}}_2{\mathrm{H}}_2$ chemisorbates is shown to occur at $600-700$ K 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. Most of the Si dangling bonds are passivated by, at least partly, the hydrogen adsorbates at this stage. At $\sim 1000$ K, the desorption of hydrogen occurs, which accompanies the appearance of a broad SiC-like feature in the UP spectra at $\sim 3$ eV below Fermi level.