Time-dependent density functional theory study of the X-ray absorption spectroscopy of acetylene, ethylene, and benzene on Si(100)

The carbon Is X-ray absorption spectroscopy (XAS) of acetylene, ethylene, and benzene in gas phase and adsorbed on the Si(100) surface is studied using time-dependent density functional theory (TDDFT) with hybrid exchange correlation functionals. The computed spectra are sensitive to the proportion of Hartree-Fock exchange in the exchange-correlation functional. The fraction of Hartree-Fock exchange is optimized to provide accurate predictions of the 1s -> pi* excitation energies. In gas phase, the spectra of acetylene, ethylene and benzene are dominated by 1s -> pi* excitations with further weaker bands at higher energies below the ionization threshold arising predominantly from Rydberg excitations. In the spectrum for ethane, bands arising from excitation to Rydberg and sigma(C-C)* excitations are observed. For the molecules adsorbed on Si(100), excitations to Rydberg states are less evident. The calculations indicate that acetylene adsorbed on the Si(100) surface has an intense pi* band, which lies 1.5 eV lower than in gas phase, at an energy similar to gas-phase ethylene. Additional bands arising from sigma(Si-C)* and sigma(C-H)* excitations are observed also. The spectrum for ethylene on the surface has a broad feature that is largely due to the sigma(C-C)* excitation, and at low energy a sigma(Si-C)* band is predicted. Benzene is studied in two binding configurations. Both configurations have intense pi* bands arising from the Is orbitals localized on the carbon atoms not bonded to the surface, and the location of this band is sensitive to the adsorption configuration. The results show that TDDFT with a modified hybrid exchange-correlation functional can provide accurate predictions of the XAS of organic molecules on the Si(100) surface and can assist in the elucidation of the structure of adsorbed molecules using polarized XAS.