duces the symmetry from O h to C 4v . This results in a drastic change of the symmetry selection rules, as seen in the experimental RIXS spectrum. For example, RIXS through the core-excited state |F 1s -1 6a 1 1g to the symmetry forbidden scattering channels in the ungerade final states |1t -1 2u 6a 1 1g and |5t -1 1u 6a 1 1g become allowed due to the vibronic coupling.