Inner-shell and valence-shell electronic excitation of SF6, SeF6 and TeF6 by high energy electron impact: An investigation of potential barrier effects

Inner- and valence-shell electron energy loss spectra of gaseous SF₆, SeF₆ and TeF₆ have been measured at high impact energy (2.0–3.7 keV) and zero degree scattering angle. The resulting inner-shell excitation spectra include F 1s, S 2s and 2p in SF₆: F 1s, Se 3s, 3p and 3d in SeF₆ and F 1s, Te 4s, 4p, 4d and 3d in TeF₆. The results for each of these hexafluorides are interpreted, in the framework of the potential barrier model, as excitations to resonances involving a common manifold of virtual valence (inner-well) orbitals and also to Rydberg (outer-well) orbitals. The below-edge shape resonances in corresponding inner-shell excitation spectra of these hexafluorides show very similar variations in intensities in accord with expectations based on electric-dipole selection rules. This is an indication that these below-edge resonances in SeF₆ and TeF₆ are of the same symmetry as those in SF₆ (i.e. a_1g and t_1u). The continuum (above-edge) shape resonances in SeF₆ also show similar spectral behavior to those of SF₆ and can be understood by including Se 4d and S 3d orbitals in the basis for the respective MO schemes. However, in TeF₆ completely different spectral behavior is observed for the continuum resonances. In particular, there is a dramatic series of intense resonances observed above the Te 3d and 4d edges. The TeF₆ spectrum can only be understood by extending the MO basis set to include the Te 4f orbitals which are even lower lying than the Te 5d orbitals. Therefore, these continuum resonances which are also seen in the F 1 s and Te 3p spectra of TeF₆ are assigned to l=3 (f-type) continuum shape resonances due to involvement of the 4f orbitals rather than the l=2 (d-type) continuum resonances observed in SF₆ and SeF₆. This is the first reported observation of such f-type continuum shape resonances and such considerations will likely prove to be important in the understanding of near-edge spectra of heavy atom containing species. The VSEELS spectra which are very similar for the three hexafluorides also show significant continuum shape resonances. A consideration of both the ISEELS and the VSEELS spectra indicates that there is a weakening of the potential barrier in going through the series from SF₆, SeF₆ to TeF₆.