EXCITED ELECTRONIC STATES OF CYCLOHEXENE, 1,4-CYCLOHEXADIENE, NORBORNENE, AND NORBORNADIENE AS STUDIED BY ELECTRON-IMPACT SPECTROSCOPY

The excited electronic states of cyclohexene, 1,4-cyclohexadiene, norbornene (bicyclo[2.2.1]-2-heptene), and norbornadiene (bicyclo[2,2,1]-2,5- heptadiene) have been studied by electron impact at scattering angles from 5° to 80°, and impact energies of 30 and 50 eV. Low-lying features with intensity maxima at 4.24 eV in cyclohexene and 4.10 eV in norbornene are identified as singlet → triplet transitions. Similar features in the spectra of 1,4-cyclohexadiene and norbornadiene extending from 3.4 to 5.4 eV and 2.9 to 4.5 eV, respectively, are believed to result from superposition of two low-lying singlet → triplet transitions in each molecule. In norbornadiene, these features have estimated intensity maxima at 3.4 and 3.9 eV, while in 1,4-cyclohexadiene they appear to be more highly overlapped, yielding a single intensity maximum at 4.29 eV. The singlet → singlet excited state spectra of these molecules are discussed from the point of view of a model in which ethylene units interact via through-bond and through-space effects. In each of these four molecules, transitions to several superexcited states are observed. © 1979 American Institute of Physics.