Figure 1

(a) Schematic illustration showing some key parameters affecting the MRI relaxivities: the electronic spin relaxation time of metal ions T _1e,2e , the rotational correlation time τ _R and inner sphere water exchange reaction T _1m  + k _ex . (b) From top to bottom: experimental XAS spectra (thin lines) of Gadodiamide, Gd₁Sc₂N@C₈₀, Gd₃N@C₈₀, and Gd₂O₃ compared with simulated XAS spectrum using single Gd³⁺ ion (thick line). Simulation parameters are listed in Table 1 (baseline). (c) Zoom-in pre-edge region of XAS spectra. The simulated spectra with Gd³⁺ and Gd²⁺ ion are denoted by thick red and blue lines, respectively. The blue vertical lines mark the main peak positions and are guides for eyes. (d) Schematic illustration of RIXS process producing the inelastic features with term symbols ⁶P _J , ⁶D _J , and ⁶G _J (in [3.5 eV, 8 eV] energy loss window). (e–g) RIXS maps of (e) Gd₂O₃, (f) Gd₃N@C₈₀, and (g) simulated Gd³⁺ single ion. The experimental RIXS maps in panels (e) and (f) are produced by interpolating the RIXS spectra recorded at 141, 141.5, 142, 142.4, 142.7, 143.1, 143.5, 144.5, and 145.5 eV excitation photon energies. The intensity of elastic peak (zero energy loss) is scaled to 1 with color bars shown on the right. White dashed lines in panels (e) and (f) denote the excitation photon energies used in Figs 2 and 3.