Atomic-site-specific nonoptical excitation spectroscopy of chlorofluoromethanes CF4−nCln (n=0–4): Absolute generalized oscillator strength measurement of below-edge electronic excitations of C 1s and Cl 2p shells

Abstract

The absolute transition probabilities, more commonly known as absolute generalized oscillator strengths (GOSs), have been determined for selected electronic excitations from the C 1s or Cl 2p shell to the lowest-lying σ* orbitals for chlorofluoromethanes CF4−nCln (n=1–4) using angle-resolved electron energy loss spectroscopy at 2.5 keV impact energy. The GOS profiles of these inner-shell transitions were found to have shapes characteristic of predominant dipole-allowed transitions. Furthermore, the proportions of dipole-allowed components of these GOS profiles appeared to increase with successive chlorination for these molecules. In particular, the GOS profile of the C 1s→σC–Cl* [lowest unoccupied molecular orbital (LUMO)] transition of CF3Cl was found to contain relatively more intense higher momentum-transfer (K) components than those of the corresponding features in CF2Cl2 and CFCl3 (and CCl4). The GOS profiles for the Cl 2p3/2,1/2→σC–Cl* transitions of CF3Cl were found to contain a well-defined secondary maximum at K2∼1.2 a.u., characteristic of predominant nondipole interaction, in addition to the primary dipole-dominated peak at zero momentum transfer. Within the present statistics, our GOS measurements have not, however, revealed any discernible difference between the Cl 2p3/2→11a1 and Cl 2p1/2→11a1 features that could be attributed to spin effects. Furthermore, the dipole-dominated GOS profile was found to become more concentrated in the lower momentum transfer region for the Cl 2p3/2→σC–Cl* transition in CCl4 than that in CFCl3. The present work illustrates the possibilities of atomic-site-specific investigation of the excited states by means of GOS measurements of electronic excitations from localized inner-shell initial states.