Adsorption of ${\mathrm{C}}_2{\mathrm{H}}_2$ and ${\mathrm{C}}_2{\mathrm{H}}_4$ on Si(001): Core-level photoemission

The debating issue of Si $2p$ surface-core-level shifts and C $1s$ line shape for the Si(001) surface with unsaturated hydrocarbon molecules $({\mathrm{C}}_2{\mathrm{H}}_2$ and ${\mathrm{C}}_2{\mathrm{H}}_4)$ adsorbed is reexamined carefully with high-resolution photoemission. The present data unambiguously show the Si $2p$ components due to the Si-molecule bonding on the surface with binding-energy shifts from the bulk Si $2p$ of $0.24\pm 0.02$ and $0.18\pm 0.01\mathrm{eV}$ for the ${\mathrm{C}}_2{\mathrm{H}}_2$ and ${\mathrm{C}}_2{\mathrm{H}}_4$ adsorbates, respectively. This result is consistent with the simple picture of the charge transfer from Si to the adsorbed hydrocarbon molecules, but is in contrast to some of the recent experimental reports and a recent theoretical expectation of almost no surface-core-level shifts for both adsorbate systems. The C $1s$ core levels are found to consist of two components for both ${\mathrm{C}}_2{\mathrm{H}}_2$ and ${\mathrm{C}}_2{\mathrm{H}}_4$ adsorbates with binding-energy shifts of $0.37\pm 0.02$ and $0.43\pm 0.02\mathrm{eV},$ respectively. The binding-energy shifts and the relative intensities of these components are invariant over the whole coverage range up to saturation. This supports the interpretation of the higher-binding-energy and smaller-intensity components as the C-H vibrational satellite.