We also observed that the VB-RIXS spectra can directly be compared to the GS DOS for qualitative analysis. Complementary to the LFDFT model where multiplet effects and charge transfer electronic structure are taken into consideration, we show in Fig- ures 4a and 4b qualitative descriptions of the VB-RIXS and HR-XANES spectra by using the total DOS of PuO 2 and its projection onto Pu and O basis functions of different angular momenta (projected DOS, or pDOS). Figure shows that the unoccupied 5f DOS of Pu aligns with the absorption peak of the experimental Pu M 5 -edge HR-XANES spec-trum of PuO 2 in Figure . The VB-RIXS spectrum aligns also closely with the calculated occupied valence band DOS of PuO 2 (Figure ). In the energy region (between -3 and -8 eV), the Pu(f) pDOS is mostly spread below the total DOS, indicating that Pu(f) electron density is delocalized and mixed with O(2p) states. The combination of this analysis with the observation that the VB-RIXS spectrum is spread over approximately 12 eV illustrates that there is 5f electron density, which is not strongly localized on the Pu atoms. The experimental results therefore support the conclusion from the theoretical analysis that part of the Pu 5f density is diffuse and originates from the covalent interactions of Pu with the oxygen ligands. 5f electron configuration of Pu in PuO 2 vs. Pu 3+ (aq) and Pu 4+ (aq): It is illustrative to compare the PuO 2 data with those for the aqueous Pu 3+ and Pu 4+ ions. Figure displays the Pu M 5 -edge HR-XANES spectra of PuO 2 , Pu 3+ and Pu 4+ in aqueous solution [Pu 3+ (aq) and Pu 4+ (aq)]. In Reference 7, the authors proposed that the shoulder observed in the Pu M 5 -edge HR-XANES spectrum, situated on the high energy side of the main peak, arises from a mixture of different Pu ONs in the PuO 2 sample. Specifically, it was argued that the shoulder arises from a contaminant with higher ON, probably Pu(VI), as results of non-stochiometric behavior of PuO 2+x , where x originates from Pu(VI) generated by surface corrosion at atmospheric pressure and in the presence of water. However, we show in the following that the M 5 -edge shoulder instead originates from final state effects resulting from a 4f core-hole, which contributes to the HR-XANES spectral shape. Butorin showed by crystal field multiplet calculations, that the shapes of the Pu M 5 -edge XANES and HR-XANES spectra differ; the former accounts for a core-hole in the 3d states, whereas the latter involves core-holes in both the 3d and 4f states of Pu. Notably, our computed Pu M 5 -edge HR-XANES of PuO 2 uniquely reveals a shoulder on the high-energy side of the most intense absorption feature, along with a distinct resonance peak in the corresponding RIXS map (Figure ). The LFDFT calculation of the Pu M 5 -edge CC-RIXS map and the Pu M 5 -edge HR-XANES obtained from it closely resemble the experimental Pu M 5 -edge HR-XANES. The observed shoulder in the Pu M 5 -edge HR-XANES, as well as in the respective CC-RIXS spectra, do not result from mixtures of Pu oxidation states. Instead, it is caused by a final state effect induced by the interaction between the 4f core-hole and the 5f electrons. Notably, this shoulder is present and even more pronounced in the spectrum of Pu 4+ in aqueous solution (cf. Figure ). Figure also presents the Pu M 5 -edge HR-XANES of Pu 3+ (aq). The Pu M 5 -edge HR-XANES spectrum of Pu 3+ (aq) measured with lower energy resolution is shown in Figure illustrating disappearance of the double peak structure due to increased spectral broadening. We reported previously Pu M 5 -edge HR-XANES spectra of Pu 3+ (aq) and Pu 3+ incorporated in a borosilicate glass. The main absorption peak manifested as two peaks with similar intensities. Pu M 5 -edge RIXS and HR-XANES spectra of Pu 3+ Cl 3 calculated with crystal field multiplet theory also revealed a shoulder on the high energy side of the main absorption resonance. It is well established that the energy position of the rising absorption edge is sensitive to the electron density of Pu. A noticeable shift of about 1.6 ± 0.01 eV of the Pu M 5 -edge HR-XANES PuO 2 spectrum to higher energy, compared to the spectrum of Pu 3+ (aq), reveals a substantial difference in the electronic structure and overall larger electron density on Pu in Pu 3+ (aq). An energy shift of about 1-2 eV corresponds to a change of the oxidation state of Pu or U by one. Note that this energy shift is smaller for the molecular plutonyl ions PuO 2 1+/2+ because of the strong covalency of the Pu-O axial bonds (cf. Figure ). Considering the spectroscopic data, we conclude c Sum of 7s, 5f, 5f', and 6d occupations.