Adsorption of 2-butyne on Si(0 0 1) at room temperature: A valence band photoemission study

We present an angle resolved ultraviolet photoemission spectroscopy study of the adsorption of 2-butyne (CH₃-CC-CH₃) on Si(0 0 1)-2 × 1 at room temperature. We recorded valence band photoemission spectra for two azimuthal positions of a vicinal silicon surface, where all the rows formed by the surface silicon dimers are parallel. The photoemission symmetry selection rules allow the determination of the orientation of the molecular orbitals. The photoemission signal of the HOMO is enhanced when the electric field is parallel to the dimer rows. This showed that the π orbital left intact after the cyclo-addition reaction of the molecule with one silicon dimer is parallel to the dimer rows. This indicates that each 2-butyne molecule adsorbs on one silicon dimer. In spite of the size of the system and the vicinity of the orbitals, the angle resolved study points out that no dispersion of the electronic bands occurs. Not all the surface dimers are reacted so some disorder still exists on the surface preventing the formation of Bloch states.