Density functional theory study of the near edge X-ray absorption fine structure and infrared spectroscopy of acetylene and benzene on group IV semiconductor surfaces

The near edge X-ray absorption fine structure and infrared spectroscopy of acetylene and benzene adsorbed on C(1 0 0)-2 × 1, Si(1 0 0)-2 × 1 and Ge(1 0 0)-2 × 1 surfaces is studied with density functional theory calculations. Time dependent density functional theory calculations of the near edge X-ray absorption fine structure with a modified exchange-correlation functional agree well with experiment, and show that the spectral features arise from excitation to π∗, σ CH ∗ and σ XC ∗ orbitals, where X represents C, Si or Ge. The σ XC ∗ excitation energies are dependent on the surface, and for acetylene, the location of the π∗ band also varies with the surface. Calculations of the vibrational modes show the CH stretching frequencies for carbon atoms bonded directly to the surface vary significantly between the three surfaces, while those for carbon atoms not bonded to the surface do not change significantly.