Absolute generalized oscillator strengths (GOSs) of valence-shell electronic transitions of ${\mathrm{CHF}}_3$ and ${\mathrm{CHCl}}_3$ 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 ${\mathrm{CHF}}_3$ and ${\mathrm{CHCl}}_3$ 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 ${\mathrm{CHF}}_3$ and at 8.5, 9.6, and 10.6 eV in ${\mathrm{CHCl}}_3$ 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 ${\mathrm{CHCl}}_3$ could be attributed predominantly to electronic excitations from the Cl 3p nonbonding (n) orbitals (2${\mathit{a}}_2$, 9e, 9${\mathit{a}}_1$, and 8e) to a C-Cl σ* antibonding orbital (10${\mathit{a}}_1$), 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 ${\mathrm{CHCl}}_3$, like other Cl-containing freons [${\mathrm{CF}}_{\mathit{n}}$${\mathrm{Cl}}_{4\mathrm{-}\mathit{n}}$ (n=0–3) and ${\mathrm{CHF}}_{\mathit{m}}$${\mathrm{Cl}}_{3\mathrm{-}\mathit{m}}$ (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