Hi Edouard
Sorry, but for some reason, I cannot see the image. However materials with strong plasmons ususally need a modified form of the Tougaard bacckground – examples are G, Al and Si in tehir elemental forms.
You may be better of using a U3 background type in CasaXPS some one of the values is always calculated for you, else may be worth using REELS to obtains the cross section data for the metallic Na – see for eample DOI: 10.1016/j.elspec.2009.08.005