Angle-resolved electron energy loss spectroscopy of valence-shell and Si 2p pre-edge excitation of SiF4: Bethe surface and absolute generalized oscillator strength measurement

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 keV impact e nergy. The GOS profiles of the pre-edge features are generally consistent w ith the spectral assignments based on the term values of the virtual and Ry dberg states from earlier valence and inner-shell studies. In particular, t he GOS profiles for these low-lying preionization-edge features in the vale nce shell are found to be dominated by a strong maximum at zero momentum tr ansfer, consistent with the proposed assignment of predominantly dipole-all owed Rydberg and mixed valence-Rydberg transitions. In the case of the lowe st-lying preionization-edge 1t(1)-->6a(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 s hell, the GOS profiles for the well resolved, intense sigma (*) resonance a nd three higher-lying Si 2p pre-edge features have been determined and are found to be largely dominated by dipole-allowed (Rydberg) excitations. Diff erences 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 ar e compared with those reported for other cage-like molecules, including CF4 and SF6. (C) 2001 American Institute of Physics.