ELECTRON-ENERGY-LOSS SPECTROSCOPY OF CONDENSED BUTADIENE AND CYCLOPENTADIENE - VIBRATIONALLY RESOLVED EXCITATION OF THE LOW-LYING TRIPLET-STATES

Low-energy electron-energy-loss spectra of 1,3-butadiene and 1,3-cyclopentadiene deposited on a thin film of solid argon are measured at a temperature of 15 K. This new method allows us to resolve vibrational structure within the low-lying triplet bands. The vibrational frequencies observed in the first triplet band of butadiene are in agreement with theoretical predictions under the assumption of a metastable planar structure for the lowest triplet state (13B(u)). The lowest observable vibronic band of the second triplet transition of butadiene is located at 4.63 eV. The 0-0 transition to the first triplet state of cyclopentadiene is resolved and located at 2.555 eV. Concerning the second triplet band of cyclopentadiene, we find evidence from a comparison with optical spectra that the onset lies in the energy range from 4.55 to 4.7 eV. This is the first likely observation of T2 for a cis-diene.