Resonant Auger electron spectroscopic study at the symmetry-forbidden 1a1g6a1g excitation below the S K-shell threshold of SF6 is reported. Partial electron yield and resonant KLL Auger spectra have been measured by using monochromatized undulator synchrotron radiation. By changing the photon energy in small steps, a so-called 2D map is produced. In this map, the dipole-forbidden transition exhibits spectral features (e.g., an S-shaped dispersion relation), which are well known and understood for dipole-allowed transitions. We validate by a theory that for the case of dipole-forbidden transitions, these spectral features can be analyzed in the same way as previously established for the dipole-allowed ones. This approach grants information on the nuclear dynamics in the K-shell core-excited states of SF6 on the femtosecond (fs) timescale. In particular, for the potential-energy curves of the states S 1s16a1g and S 2p26a1g, the slopes at the equilibrium distance of the ground state are derived. Symmetry breaking as a result of ultrafast vibronic coupling is revealed by the population of the electronically forbidden excited state.

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