Electron impact study of the energy levels of trans-1,3-butadiene

The electron impact energy loss spectrum of trans-1,3-butadiene has been investigated at incident energies from 7.5 to 34 eV and scattering angles from 0 to 700. Energy resolutions of 45 to 75 meV were used. The energy loss spectrum at 33 eV and 00 was found to correspond almost exactly to the optical absorption spectrum measured by McDiarmid. The only minor difference was a weak band which appeared in the energy loss spectrum at 7.80 eV. An investigation of the relative intensities of the three diffuse vibronic bands of the intense N→V1 transition centered near 5.92 eV showed that the relative intensities of these bands were not constant below incident energies of 15 eV and that whereas the low energy 5.74 eV vibronic band was enhanced at 00 scattering angle, the high energy 6.08 eV band was enhanced at 700. These results are consistent with the presence of two forbidden transitions in the 5.7 to 6.2 eV energy loss region: a symmetry forbidden transition near 5.80 eV and a singlet--triplet transition near 6.10 eV: in excellent agreement with some, but not all, of the recent theoretical calculations on 1,3-butadiene. Detailed studies of the higher energy region above 6.5 eV energy loss did not reveal the presence of any underlying valence transitions although the results are not conclusive because of the intense Rydberg structure in this region. The Rydberg transitions at 7.08 and 8.0 eV were found to have differential scattering cross sections which were very different from those of the nearby transitions