Resonances in the K-Shell Spectra of Fluorinated Organic Molecules

The inner shell electron energy loss spectra of a series of fluoroalkanes (CnF2n+2, n=1–6), fluoroethenes (C2 HnF4–n, n=0–4) and fluorobenzenes (C6 HnF6–n, n=0–6) are presented. C ls→ σ* (C-F) transitions are identified in each spectrum 3 to 5 eV below the ionization threshold(s), consistent with the correlation between positions of σ* resonances and molecular bond lengths. The analogies between spectral signatures of molecular shape resonances and atomic giant resonances are discussed briefly. The growth of potential barrier effects with the development of a fluorine cage around a core excited atom are demonstrated with the absolute cross-sections for C ls→ σ* (C-F) in the spectra of, C6 F6, C2F4, C2F6and CF4, derived by sum rule normalization from the energy loss spectra.