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

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.