Angle-resolved electron energy loss spectroscopy of valence-shell and Si 2p pre-edge excitation of SiF_4: Bethe surface and absolute generalized oscillator strength measurement

摘要

Absolute generalized oscillator strengths (GOSs) of discrete transitions in the preionization-edge region of the valence and Si 2p inner shells of SiF 4 have been determined as functions of energy loss and momentum transfer by using angle-resolved electron energy loss spectroscopy at 2.5 ke V impact energy. The GOS profiles of the pre-edge features are generally consistent with the spectral assignments based on the term values of the virtual and Rydberg states from earlier valence and inner-shell studies. In particular, the GOS profiles for these low-lying preionization-edge features in the valence shell are found to be dominated by a strong maximum at zero momentum transfer, consistent with the proposed assignment of predominantly dipole-allowed Rydberg and mixed valence-Rydberg transitions. In the case of the lowest-lying preionization-edge 1 t_1 —> 6 a_1 feature, which is formally dipole-forbidden, the present work shows that such a shape for the GOS profile is, however, not exclusive to just dipole-allowed transitions. In the Si 2p shell, the GOS profiles for the well resolved, intense #sigma#~*resonance and three higher-lying Si 2p pre-edge features have been determined and are found to be largely dominated by dipole-allowed (Rydberg) excitations. Differences in and between the GOS profiles for the valence-shell and Si 2p pre-edge features in SiF4 are identified. No discernible secondary extrema can be found in any of these GOS profiles. The present GOS results for SiF4 are compared with those reported for other cage-like molecules, including CF_4 andSF_6.