The emitted characteristic fluorescence by the samples was detected by a Johann type X-ray emission spectrometer, where the sample, cylindrically bent analyzer crystals and a single diode VITUS silicon drift detector (KETEK, Germany) are placed on a Rowland cycle with 1 m diameter equal to the bending radius of the crystals. To enable experiments at tender, 3 to 4 keV, X-ray energies, a glove box encapsulating the spectrometer and continuously flushed with helium was employed. For the Pu M 5 -edge HR-XANES experiments, 1 Si(220) analyzer crystals was positioned at a Bragg angle of 75.2°(3351 eV, M α1 ) and covered with masks only for Pu 4+ (aq). This was done to decrease the diameter of the crystal to 50 mm, leading to an increase in experimental energy resolution. For Pu M 3 -edge HR-XANES and M 5 -edge VB-RIXS, two Pu 4+ /three Pu 3+ Ge(311) crystals were aligned at Bragg angles of 74.3°( 3775 eV) for Pu M 5 -edge VB-RIXS and 74.8°(3765 eV, M γ1 ) for M 3 -edge HR-XANES. Before HR-XANES spectra were recorded, the respective emission line (M α1 or M γ1 ) was scanned using a fixed excitation energy above the edge (3800 eV for Pu M 5 -edge or 4600 eV for Pu M 3 -edge). Then the emission energy of the spectrometer was fixed at the maximum intensity of the respective emission line. Pu M 5 -edge/M 3 -edge HR-XANES spectra were recorded with a step size of 0.1/0.3 eV across the most intense absorption peak (white line, WL) (for details cf. Table in SI). At least two scans were recorded per sample and averaged. For the Pu M 5 -edge VB-RIXS measured for PuO 2 and Pu 4+ (aq) , the excitation energy was fixed at the maximum of the Pu M 5 -edge HR-XANES and the emission energy was scanned from 3760 eV to 3779.2 eV with a step size of 0.28 eV using the x-ray emission spectrometer and an integration time of 48 sec/energy step.