Electron-impact excitation of the T and V states of ethylene: An ab initio study

We report total and differential cross sections for electron-impact excitation of the T (a 3B1u) and (1B1u) states of ethylene from threshold to 20 eV. The calculations were carried out using the complex Kohn variational method. For collision energies above the threshold for excitation of the V state, we carried out three-state close-coupling calculations using accurate multiconfiguration target states. An averaged natural orbital technique was used to obtain a compact representation of the V-state wave function that properly reflects the mixed valence-Rydberg character of this state. The total and small-angle differential cross sections for exciting the optically allowed V state are sensitive to the inclusion of high partial-wave contributions, which were computed using a simple perturbative scheme. In the energy range between the T and V states, where only two electronic states are open, we carried out dressed two-state calculations using optical potential techniques to incorporate the effect of closed channels. We found significant d-wave resonance behavior in the T-state excitation cross section near 5 eV. At higher energies, the T-state differential cross sections are backward peaked, while the V state cross sections show strong forward peaking that is typical of an optically allowed transition