The key energy scales of Gd-based metallofullerene determined by resonant inelastic x-ray scattering spectroscopy

Endohedral metallofullerenes, formed by encaging Gd inside fullerenes like C₈₀, can exhibit enhanced proton relaxitivities compared with other Gd-chelates, making them the promising contrast agents for magnetic resonance imaging (MRI). However, the underlying key energy scales of Gd _x Sc_3-x N@C₈₀ (x  =  1-3) remain unclear. Here, we carry out resonant inelastic x-ray scattering (RIXS) experiments on Gd _x Sc_3-x N@C₈₀ at Gd N _4,5-edges to directly study the electronic structure and spin flip excitations of Gd 4f electrons. Compared with reference Gd₂O₃ and contrast agent Gadodiamide, the features in the RIXS spectra of all metallofullerenes exhibit broader spectral lineshape and noticeable energy shift. Using atomic multiplet calculations, we have estimated the key energy scales such as the inter-site spin exchange field, intra-atomic 4f-4f Coulomb interactions, and spin-orbit coupling. The implications of these parameters to the 4f states of encapsulated Gd atoms are discussed.