Electron-impact excitation of low-lying preionization-edgen→σ* and Rydberg transitions of CHF2Cl and CHFCl2: Absolute generalized oscillator strength measurement

Abstract

Angle-resolved electron energy loss spectroscopy has been used to determine the absolute generalized oscillator strengths (GOSs) of valence-shell electronic transitions of difluorochloromethane (CHF2Cl) and dichlorofluoromethane (CHFCl2) as functions of energy loss and momentum transfer at an impact energy of 2.5 keV. Absolute GOS profiles of the prominent low-lying preionization-edge energy loss features of CHF2Cl and CHFCl2 were determined and found to be consistent with the previous assignments of the underlying transitions made by VUV photoabsorption spectroscopy. In particular, the lowest-lying features at 8.0 eV in CHF2Cl and at 7.5 eV in CHFCl2 have been attributed predominantly to electronic excitations from the Cl 3p nonbonding (n) orbitals to the C–Cl σ* antibonding orbital, in good accord with single-excitation configuration interaction (CI) excited-state calculations. The corresponding GOS profiles of these n(Cl 3p)→σ*(C–Cl) (HOMO→LUMO) transitions revealed an interesting trend of increased dipole character with increasing Cl content, i.e., from an essentially quadrupole-dominated profile, characterized by a maximum at K2∼0.9 a.u., in CHF2Cl to a mixed dipole-quadrupole profile in CHFCl2 and CHCl3. The CI calculations further showed that some of the underlying n(Cl 3p)→σ*(C–Cl) transitions in CHF2Cl, CHFCl2, and CHCl3, like the other chlorofluorocarbons: CF3Cl, CF2Cl2, CFCl3, and CCl4, could lead to dissociation of the C–Cl bond. In addition, the GOS profiles of the remaining low-lying preionization-edge features at 9.8 and 11.2 eV in CHF2Cl and at 9.4, 10.7, and 11.6 eV in CHFCl2 were also determined. These features have been previously assigned as Rydberg transitions originated from the nonbonding HOMOs. In particular, these experimental GOS profiles were found to be dominated by a strong maximum at K=0, which is indicative of strong dipole interactions. The weak secondary maxima observed at K2∼2.8–3.5 a.u. could be interpreted qualitatively in terms of the spatial overlaps between the initial-state and final-state orbital wave functions. Together with our earlier work on CHF3 and CHCl3, the present work on the remaining members of the CHFmCl3−m (m=0–3) series, CHF2Cl and CHFCl2, provides further evidence for the empirical trends on the preionization-edge structures observed in the CFnCl4−n (n=0–4) series.