Electron-impact excitation of low-lying preionization-edge electronic and Rydberg transitions of fluoroform and chloroform: Bethe surfaces and absolute generalized oscillator strengths.

Abstract

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 \ensuremath{\sigma}* 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 \ensuremath{\sim}0.9 a.u., respectively. Furthermore, the CI calculation indicated that some of these n(Cl 3p)\ensuremath{\rightarrow}\ensuremath{\sigma}*(C-Cl) excitations in ${\mathrm{CHCl}}_{3}$, like other Cl-containing freons [${\mathrm{CF}}_{\mathit{n}}$${\mathrm{Cl}}_{4\mathrm{\ensuremath{-}}\mathit{n}}$ (n=0--3) and ${\mathrm{CHF}}_{\mathit{m}}$${\mathrm{Cl}}_{3\mathrm{\ensuremath{-}}\mathit{m}}$ (m=1,2)], could also lead to dissociation of the C-Cl bond. \textcopyright{} 1996 The American Physical Society.