Figure illustrates the Pu M 3 -edge HR-XANES spectra (3p → 6d) of PuO 2 , Pu 4+ (aq) and Pu 3+ (aq). The spectra of PuO 2 and Pu 4+ (aq) exhibit pre-edge features arising from electric quadrupole 3p → 5f excitations, which may increase in intensity from possible mixing of Pu 5f with 6d states in non centrosymmetric coordinations of the Pu 4+ (aq) structure. This pre-edge feature is likely hidden below the WL in the Pu M 3 -edge HR-XANES spectrum of Pu 3+ (aq). There is some controversy related to how many water molecules do constitute the solvation sphere of the Pu 4+ ion in aqueous solution It is assumed that either 8 or 9 water molecules can be coordinated to Pu. However, our calculations of Pu 4+ (aq) coordinated with 9 vs. 8 water molecules revealed no important differences in the 5f occupations, and the populations follow the same trends with increasing fractions of HF exchange (see Table and). The presence of rather pronounced pre-edge features suggests that the local atomic environment of Pu 4+ is distorted. The N 5 (4f 5/2 ) core-hole lifetime broadening in the final state of the Pu M 3edge excitation process is 5.2 eV, corresponding to a core-hole lifetime of 1.2 femtosec-onds. The timescale of molecular vibrations in a solution ranges from femtoseconds to picoseconds. This suggests that the spectrum represents a distribution of molecular geometries at a specific moment. Note that the main absorption peak is slightly broader for PuO 2 as it is expected for a solid state material. The PuO 2 and Pu 4+ (aq) occupation display small differences for both 5f and 6d electron density on Pu (non-diffuse n(5f) and n(6d) in Table ) as suggested also by the lack of an energy shift between the corresponding Pu M 3 -edge HR-XANES spectra. There is a substantial energy shift of 2.53 ±0.15 eV to lower energy of the Pu M 3 -edge HR-XANES spectrum of Pu 3+ (aq) considering the first inflection point of the spectra (5.4 eV shift of the WL energy positions). Interestingly, the 6d electron density on Pu is higher for PuO 2 and Pu 4+ (aq) compared to Pu 3+ (aq), which clearly suggests that the Pu M 3 -edge HR-XANES is much influenced also by variations of the 5f electron density localized on Pu. This is additional indirect evidence that the 5f electron configurations are different for Pu 3+ and PuO 2 , with higher non-diffuse 5f electron density on Pu 3+ (cf. Table ). The differences in Pu-O bond lengths are illustrated by the energy shifts of the post-edge feature. Additionaly, we performed quantitative analyses depicted in Figure , where the Pu M 5 -edge VB-RIXS spectra of PuO 2 and Pu 4+ (aq) were modeled with Voigt profiles using the XES NEO software. The intense elastic peak around 0 eV was fit with a double Lorentzian function, whereas peaks in the VB energy region were modeled mainly with Voigt functions with prevalent Gaussian contributions. Note that within the elastic line there are also some 5f→5f excitations which cannot be separately de-convoluted because of the broadening of the peak. Direct comparison between the VB-RIXS spectra of PuO 2 and Pu 4+ (aq) provides consistent evidence for larger 5f orbital covalency in PuO 2 as the amplitudes of the fitted VB spectra for PuO 2 are overall larger than those for Pu 4+ (aq). The elastic scattering peak has much higher intensity for the Pu 4+ (aq) spectrum and strongly overlaps with the valence band emission. Note that the probability for valence-band emission is small and it is experimentally very challenging to measure these spectra with improved energy resolution, which would lead to a better separation between the elastic peak and the VB-RIXS features (Table and Table in SI). In principle, it is possible to detect the localized Pu 5f electrons by measuring 5f electron transitions within the valence band. The shoulder at about -2.5 eV in the VB-RIXS spectrum of PuO 2 in Figure is partially probing this localized 5f electron density on Pu. However, higher experimental resolution would be necessary for better energy separation to allow more detailed analyses of such spectral features. The experimental Pu M 5 -edge VB-RIXS data support the theoretical results because the modeled total area of the VB-RIXS peaks is larger for PuO 2 (488 ±27 arbitrary units, cf. Table ) as compared to Pu 4+ (aq) (110 ±10 arbitrary units, cf. Tables ) following the trend of the diffused(delocalized)/locaalized theoretical 5f occupation.