Nonoptical excited state spectroscopy of CF3Cl, CF2Cl2, and CFCl3: Bethe surfaces, and absolute transition probability measurement of preionization-edge valence and Rydberg transitions by angle-resolved electron energy loss spectroscopy

Abstract

Absolute transition probabilities or generalized oscillator strengths (GOSs) of valence-shell electronic transitions of CF3Cl, CF2Cl2, and CFCl3 as functions of energy loss and momentum transfer (or Bethe surfaces) have been determined using angle-resolved electron energy loss spectroscopy (EELS) at an impact energy of 2.5 keV. Low-lying electronic excitation features in the energy loss region of 6.8–8.1 eV are observed. Using the results of single-excitation configuration interaction excited-state calculations, we show that these excitation features can be attributed predominantly as electronic transitions from the Cl 3p nonbonding (n) orbitals to a C–Cl σ* antibonding orbital (i.e., n→σ* transitions), some of which may lead to dissociation of the C–Cl bond. Moreover, the absolute GOS profiles of these low-lying excitation features have been determined for the first time. In particular, the GOS profile of the n→σ* feature of CF3Cl at 7.7 eV has been found to have a shape characteristic of a quadrupole transition. On the other hand, the GOS profiles of analogous excitation features for CF2Cl2 and CFCl3 are found to have more complicated structures. The combined profiles of the GOSs of these n→σ* excitation features in the CF4−mClm (m=1–4) series indicate increased dipole component with the number of chlorine atoms. The possible mechanisms and significance of this trend in the GOSs of these n→σ* transitions have been discussed. Furthermore, the GOS profiles of low-lying preionization-edge Rydberg transitions (that originate from the Cl 3p nonbonding orbitals) are also determined, and found to contain not only strong maxima at zero momentum transfer, which are characteristic of predominant dipole-allowed interactions, but also weak secondary maxima (and minima). The nature of these secondary extrema in the GOS profiles is discussed by considering the spatial overlaps of the initial-state and final-state orbital wave functions. Finally, we provide new tentative assignments for other valence-shell energy loss features of CF3Cl, CF2Cl2, and CFCl3 using the ionization potentials and term values reported in the literature.