Electronic Excitation to Singlet States of 1,3‑C<sub>4</sub>F<sub>6</sub>, c‑C<sub>4</sub>F<sub>6</sub> and 2‑C<sub>4</sub>F<sub>6</sub> by Electron Impact - Electron Energy-Loss Spectroscopy and ab Initio Calculations

We report on the first measurements of the electron impact electronic excitation cross sections for C<sub>4</sub>F<sub>6</sub> isomers, hexafluoro-1,3-butadiene (1,3-C<sub>4</sub>F<sub>6</sub>), hexafluorocyclobutene (c-C<sub>4</sub>F<sub>6</sub>), and hexafluoro-2-butyne (2-C<sub>4</sub>F<sub>6</sub>), measured at 100 eV, 3° scattering angle, while sweeping the energy loss over the range 2.0–15.0 eV. Under these experimental conditions, optically allowed transitions are favored. The electronic state spectroscopy has been investigated and the assignments supported by quantum chemical calculations. The <i>n</i> = 3 members of the Rydberg series have been assigned converging to the lowest ionization energy limits of the C<sub>4</sub>F<sub>6</sub> isomers and classified according to the magnitude of the quantum defects (δ).