Core-level spectroscopy of hydrocarbons adsorbed on Si(100)-(2×1): A systematic comparison

Adsorbate and substrate core-level binding energies for adsorption layers of six unsaturated hydrocarbons $({\mathrm{C}}_2{\mathrm{H}}_2,{\mathrm{C}}_2{\mathrm{H}}_4,{\mathrm{C}}_3{\mathrm{H}}_4,{\mathrm{C}}_4{\mathrm{H}}_6,{\mathrm{C}}_6{\mathrm{D}}_6({\mathrm{C}}_6{\mathrm{H}}_6),$ and $1,2-{\mathrm{C}}_2{\mathrm{H}}_2{\mathrm{Cl}}_2)$ on the Si(100)-(2×1) surface have been determined by high-resolution x-ray photoemission spectroscopy; results for the clean and the (2×1)-H covered surface have been obtained for comparison. Remarkable differences in the Si $2p$ and C $1s$ surface core-level shifts for the various adsorbates are found, which range from 327 to -169 meV and from 1220 to -260 meV, respectively. Both initial- and final-state effects are necessary to explain the strong but unsystematic variations of the observed shifts. Additionally, from a comparison of the C $1s$ intensities the absolute saturation coverages are determined to be 0.87, 0.84, 1.15, 0.79, 0.36, and 0.44 molecules per unit cell of the clean surface for ${\mathrm{C}}_2{\mathrm{H}}_2,{\mathrm{C}}_2{\mathrm{H}}_4,{\mathrm{C}}_3{\mathrm{H}}_4,{\mathrm{C}}_4{\mathrm{H}}_6,{\mathrm{C}}_6{\mathrm{D}}_6,$ and $1,2-{\mathrm{C}}_2{\mathrm{H}}_2{\mathrm{Cl}}_2,$ respectively.