Electronic excitation to singlet states of 1,3-C4F6, c-C4F6 and 2-C4F6 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(4)F(6) isomers, hexafluoro-1,3-butadiene (1,3-C(4)F(6)), hexafluorocyclobutene (c-C(4)F(6)), and hexafluoro-2-butyne (2-C(4)F(6)), 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 n = 3 members of the Rydberg series have been assigned converging to the lowest ionization energy limits of the C(4)F(6) isomers and classified according to the magnitude of the quantum defects (δ).