Abstract
Absolute generalized oscillator strengths (GOSs) of valence-shell electronic transitions of and as functions of energy loss (0–150 eV) and momentum transfer (i.e., the Bethe surfaces) have been determined using angle-resolved electron energy-loss spectroscopy at an impact energy of 2.5 keV. The assignments for the prominent low-lying preionization-edge energy-loss features of and were inferred from the term values reported previously and from their characteristic experimental GOS profiles. In particular, the GOS profiles of the low-lying Rydberg transitions (which originated from the nonbonding highest occupied molecular orbitals) below the ionization edge at 11.1, 11.9, 12.7, and 13.7 eV in and at 8.5, 9.6, and 10.6 eV in were determined. These GOS profiles were found to be dominated by a strong maximum at zero-momentum transfer, which is characteristic of dipole interaction. Weak secondary maxima (and minima) were also observed and could be interpreted qualitatively in terms of the spatial overlaps between the initial-state and final-state orbital wave functions.
In addition, the low-lying feature at 7.2 eV in could be attributed predominantly to electronic excitations from the Cl 3p nonbonding (n) orbitals (2, 9e, 9, and 8e) to a C-Cl σ* antibonding orbital (10), according to a single-excitation configuration-interaction (CI) excited-state calculation. The experimental GOS profile of this low-lying feature was found to have a shape that is characteristic of a mixture of dipole-allowed and nondipole interactions, with maxima at momentum transfers of 0 and ∼0.9 a.u., respectively. Furthermore, the CI calculation indicated that some of these n(Cl 3p)→σ*(C-Cl) excitations in , like other Cl-containing freons [ (n=0–3) and (m=1,2)], could also lead to dissociation of the C-Cl bond. © 1996 The American Physical Society.
- Received 25 July 1995
DOI:https://doi.org/10.1103/PhysRevA.53.1476
©1996 American Physical Society