Resonances in the silicon L core level cross sections of fluoromethylsilane compounds characterized by photoabsorption spectroscopy and MS-Xα calculations

Abstract

High-resolution Si 2p and 2s photoabsorption spectra of the five fluoromethylsilane compounds, Si(CH 3) x F 4- x ; x =0–4, obtained in the gas phase using monochromatized synchrotron radiation are presented. Transition state MS-Xα calculations for both the pre- and post-edge regions of the Si 2p and 2s edges of all five molecules are also reported. The theoretical results and trends observed in the experimental spectra of the molecular series facilitate the identification and assignment of numerous spectral features. In the Si 2p pre-edge region, absorptions are assigned as transitions to antibonding or Rydberg orbitals. Combined intensities of transitions to Rydberg orbitals are found to be comparable to the intensities of transitions to antibonding orbitals. For these molecules the Si 2s pre-edge regions of the spectra exhibit less structure, but assignments analogous to those made for the Si 2p pre-edge are made. Three continuum resonances are observed above the Si 2p ionization threshold in the photoabsorption spectra of all five molecules, one near threshold, another ≈ 5 eV above threshold and a high energy peak ≈ 20 eV above threshold. Low energy peaks are shape resonances attributed to transitions into SiC antibonding orbitals lying slightly above threshold. The medium energy peaks are assigned as shape resonances resulting from electronic transitions to quasibound states in the continuum supported by the electronegative fluorine ligands. An atomic-like delayed onset phenomenon is used to explain the higher energy peaks.