Carbon K-shell electron energy loss spectra of 1- and 2-butenes, t r a n s-1,3-butadiene, and perfluoro-2-butene. Carbon–carbon bond lengths from continuum shape resonances

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Abstract
Electron energy loss spectra of 1‐butene, cis‐2‐butene, trans‐2‐butene, trans‐1,3‐butadiene, and perfluoro‐2‐butene in the region of carbon K‐shell (C 1s) excitation and ionization have been recorded under dipole‐dominated inelastic electron scattering conditions. The features observed below the C 1s I.P. in the spectra of the butenes and butadiene are assigned to promotions of C 1s electrons to unoccupied valence (π*) and Rydberg orbitals while broad features observed above the edge are assigned to σ(C–C) and σ(C–C) shape resonances. These spectra, along with carbon K‐shell spectra of other hydrocarbons, are used to demonstrate that there is a quantitative relationship between carbon–carbon bond lengths and the location of σ shape resonances relative to the C 1s ionization threshold (I.P.). The C 1s spectrum of perfluoro‐2‐butene demonstrates dramatic potential barrier effects, namely suppression of Rydberg transitions and strong enhancement of σ(C–C) and σ(C–F) shape resonances in the region of the C 1s ionization threshold.