To account for the different Pu concentrations of the different samples, the VB-RIXS spectra were normalized applying the following procedure: The excitation energy was set to 3.85 keV. This energy was chosen because the Pu M 5 -edge HR-XANES spectra have rather flat intensity and no strong shape resonances in this energy region. Over the course of 120 seconds, the emitted photons were measured. This number was then used for the normalization of the VB-RIXS spectra.