2-Butyne on Si(0 0 1) at room temperature: An XPS and NEXAFS study

We present here a study of the adsorption of 2-butyne (CH ₃-CC-CH₃) on the Si(0 0 1)-2 × 1 silicon surface at room temperature using synchrotron radiation photoemission spectroscopy (XPS) and X-ray absorption spectroscopy (NEXAFS). In particular, the Si2p and C1s core levels were followed by real-time photoemission (measuring while dosing). The intensity of the Si2p surface state component gradually diminishes with an increasing exposure to the gas. The C1s photoemission line is decomposed into two main components with their vibrational series, attributed to the two inner carbons (bonded to silicon) and the two methyl carbons (protruding into the vacuum), respectively. C1s real-time XPS indicates that the chemical bonding of the molecule does not change from low coverage to saturation coverage. NEXAFS spectroscopy performed at the C K-edge using linearly polarized radiation reveals the presence of a π*(CC) molecular orbital parallel to the surface, resulting from the opening of the triple CC bond and the formation of two σ(Si-C) bonds. The attachment of the molecule via C-H bond scission (conserving the CC bond) is excluded. The clear-cut observation of a CC bond, combined to our preceding angle-resolved UV photoemission spectroscopy (ARUPS) work [Bournel et al., Surf. Sci. 601 (2007) 3750] favors the on-dimer adsorption model at saturation (∼3 L).