Here χ is the angle between the incoming photon polarization e and the wave vector of the emitted photon k defined as cos χ = e • k . According to our simulations the dominant contribution to the RIXS cross section is given by one-electron transitions. In this case the RIXS process is simply the core -excitation F 1s → ψ i followed by the emission transition ψ f → F 1s. In this case the final state |ψ -1 f ψ 1 i with the hole in MO ψ f and electron on the unoccupied MO ψ i allows only one core-excited state |F1s -1 ψ 1 i . This means that the interference between electronic intermediate states is absent. However, the interference between intermediate vibrational states is very important. This approximation allows to get from Eq. ( ) the following expression for the RIXS cross-section σ(ω , ω, χ) = 1 9