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

Abstract

We report total and differential cross sections for electron-impact excitation of the T (a\ifmmode \tilde{}\else \~{}\fi{} $^{3}$${\mathit{B}}_{1\mathit{u}}$) and ${(}^{1}$${\mathrm{B}}_{1\mathrm{u}}$) 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.