Resonant Auger decay induced by the symmetry-forbidden 1a_1g → 6a_1g transition of the SF₆ molecule

Resonant Auger electron spectroscopic study at the symmetry-forbidden $1a1g→6a1g$ excitation below the S $K$-shell threshold of SF$6$ 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 SF$6$ on the femtosecond (fs) timescale. In particular, for the potential-energy curves of the states S $1s−16a1g$ and S $2p−26a1g$⁠, 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.