Electron energy-loss spectra of coupled electronic states: Effects of Rydberg-valence interactions inO2

Abstract

The concept of the ``generalized transition moment'' is extended beyond the region of applicability of the first Born approximation and is used in the analysis of electron energy-loss spectra for ${\mathrm{O}}_{2},$ presented here, which have been measured at intermediate impact energies. A coupled-channel theoretical treatment of the strongly mixed Rydberg and valence states that dominate the optically allowed spectrum is used to explain the relative intensities of many unusual features occurring in the 7--11.2-eV energy-loss region. For these electronically excited states of ${}^{3}{\ensuremath{\Sigma}}_{u}^{\ensuremath{-}}$ and ${}^{3}{\ensuremath{\Pi}}_{u}$ symmetry, the evolution of the shape of the corresponding electron energy-loss spectrum as the scattering conditions are changed is controlled essentially by one parameter: the ratio of the diabatic generalized transition moments into the Rydberg and valence components of the mixed electronic states. The generalized Rydberg transition moment is found to decrease much faster than the valence moment as the momentum transferred in the collision increases. The results of the coupled-channel analysis also indicate that the more diffuse spectral features are generally asymmetric, those in the 7--9.8-eV region changing slightly in position as the scattering conditions are altered. Thus, the Gaussian--line-shape-based models that have been employed previously in attempts to decompose the diffuse part of the electron energy-loss spectrum for ${\mathrm{O}}_{2},$ into contributions from different electronic states, are unlikely to have given meaningful results. As a consequence of the Rydberg-valence interactions, it is found that some vibrational levels of the mixed states give rise to features in the electron energy-loss spectrum which are anomalously strong at the low impact energies and large scattering angles for which normal optically allowed transitions are expected to decline in relative strength. These ``persistent lines'' are easily confused with those from the optically forbidden transitions which increase in relative intensity under such conditions. Thus, the coupled-channel technique is found to be a valuable aid to the assignment of features in the electron energy-loss spectrum.