High-resolution electron energy-loss spectroscopy of ethylene: analysis of the vibrational structure of the triplet ( π to π*) state

Low-energy electron impact energy-loss spectra of the triplet (π to π*) state of ethylene have been obtained using scattering angles in the range 2 to 60 degrees . The high resolution obtained ( approximately 0.025 eV FWHM) has allowed many vibrational levels to be observed which have not been resolved in previous energy-loss or photoabsorption studies. Analysis of the vibrational structure indicates that a long progression of the C-C stretching mode is involved as well as at least three quanta of the CH 2 torsional mode. The results obtained are compared with theoretical predictions about the geometry of the triplet (π to π*) state of ethylene.