HIGH-RESOLUTION ELECTRON ENERGY-LOSS SPECTROSCOPY OF ETHYLENE - ANALYSIS OF THE VIBRATIONAL STRUCTURE OF THE TRIPLET (PI-]PI-STAR) 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 CH2 torsional mode. The results obtained are compared with theoretical predictions about the geometry of the triplet (π to π*) state of ethylene.