An ion-impact study of singlet-triplet transitions in ethylene and alkyl-substituted ethylenes

The T <- N (π → π*) transitions in ethylene, all of its methyl derivatives, and 1-butene were observed by means of He+-impact energy-loss spectroscopy. With a resolution of better than 0.1 eV, a vibrational structure was just resolved in the T <- N excitation spectrum of ethylene. The T <- N transition energy was measured for the alkylethylenes with a lower resolution. The results are in good agreement with the electron-impact data, indicating that the vibro-rotational excitation due to the impulsive momentum transfer is negligible in the present method. The T <- N excitation cross-sections were found to decrease by half upon the unit substitution of alkyls. The relative invariance of the T <- N transition energy and the decrease in the T <- N cross-sections upon alkyl substitution may be attributable to the decrease in the intra- and intermolecular Coulomb and exchange interactions between π- and π*-orbitals. (author)