Electron impact study of the energy levels of trans-1,3-butadiene: II. Detailed analysis of valence and Rydberg transitions

In this electron energy loss investigation of the electronic states of trans-1,3-butadiene, high energy resolution (25 meV) and a wide range of incident energies (9.5 to 49 eV) have been employed to locate and assign valence and Rydberg transitions in the typically congested 5--9 eV (2500 to 1400 A) region of the spectrum of a polyatomic molecule. Comparisons of the observed relative differential scattering cross sections (DCS) of various features in the lower energy (5.0 to 6.8 eV) region with the unambiguous assignments of these features from optical absorption and multiphoton ionization spectra allowed certain conclusions to be drawn regarding the typical DCS of Rydberg and valence singlet--singlet transitions. These ''rules'' have been applied to the congested 7--9 eV energy loss region of the butadiene spectrum. A diffuse process near 7.3 eV has been assigned as a forbidden valence transition, probably 1A/sub g/reverse arrow1A/sub g/, on the basis of its relative DCS. The identification of the 7.07 and 8.00 eV transitions as successive members of the same Rydberg series is supported, and both transitions are demonstrated to contain considerable valence component