Electronic spectroscopy of 1,3,5,7-cyclooctatetraene by low-energy, variable-angle electron impact

The electron-impact energy loss spectrum of 1,3,5,7-cyclooctatetraene has been measured at electron impact energies of 30, 50, and 75 eV, and scattering angles varying from 5° to 80°. Three transitions with maxima at 3.05, 4.05, and 4.84 eV are identified as singlet → triplet excitations. The significance of the lowest lying of these triplet states in the quenching process of dye laser solutions (in particular rhodamine 6G) is discussed and an exciplex mechanism for triplet quenching is suggested. Singlet→singlet transitions are observed at 4.43, 6.02, and 6.42 eV. These spin-allowed transitions have been observed optically and are assigned as ? 1A1 →1A2, ? 1A1→1E, and ? 1A1→1E excitations. Three new, singlet → singlet transitions are observed at 6.99, 8.41, and 9.05 eV and are tentatively assigned as the ? 1A1→1B2, ? 1A1→1E, and ? 1A1 →1E, π→π* excitations. Several superexcited features between 10 and 15 eV have been observed and are believed to involve excitations to autoionizing Rydberg states.