Electron-impact excitation of low-lying preionization-edge electronic and Rydberg transitions of fluoroform and chloroform: Bethe surfaces and absolute generalized oscillator strengths
- Department of Physics and Department of Chemistry, The University of Waterloo, Waterloo, Ontario, N2L3G1 (CANADA)
Absolute generalized oscillator strengths (GOSs) of valence-shell electronic transitions of CHF{sub 3} and CHCl{sub 3} as functions of energy loss (0{endash}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 CHF{sub 3} and CHCl{sub 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 CHF{sub 3} and at 8.5, 9.6, and 10.6 eV in CHCl{sub 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 CHCl{sub 3} could be attributed predominantly to electronic excitations from the Cl 3{ital p} nonbonding ({ital n}) orbitals (2{ital a}{sub 2}, 9{ital e}, 9{ital a}{sub 1}, and 8{ital e}) to a C-Cl {sigma}{asterisk} antibonding orbital (10{ital a}{sub 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 {approximately}0.9 a.u., respectively. (Abstract Truncated)
- OSTI ID:
- 279015
- Journal Information:
- Physical Review A, Vol. 53, Issue 3; Other Information: PBD: Mar 1996
- Country of Publication:
- United States
- Language:
- English
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