Vibrational characterization of different benzene phases on flat and vicinal Si(100) surfaces

Based on high-resolution electron energy loss spectroscopy and temperature-programmable desorption, benzene chemisorption on vicinal and nominally flat Si(100) surfaces has been studied for various adsorption, annealing, and site blocking treatments. Three different chemisorbed benzene (C6H6 and C6D6) phases with distinct thermal desorption characteristics and different vibrational spectra have been separated and characterized on both substrates. All three phases are identified as 1,4-cyclohexadiene-like structures with butterfly geometry. Whereas the dominant phase is di-sigma bonded to the two Si atoms of a single Si-Si dimer, the benzene orientation (double bond orientation) in the other phases is rotated. Di-sigma bonding to Si atoms of adjacent Si-Si dimer for the latter cases is most likely. Coverage and temperature dependent conversions between the different phases have been addressed by vibrational spectroscopy.