The recent discovery of superconductivity in ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$ has drawn significant attention in the field. A key open question regards the evolution of the electronic structure with respect to hole doping. Here we exploit x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) to probe the doping-dependent electronic structure of ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$. Upon doping, a high-energy feature in Ni $L_3$-edge XAS develops in addition to the main absorption peak, while XAS at the O $K$-, Nd $M_3$- and Nd $M_5$-edge exhibits a much weaker response. This implies that doped holes are mainly introduced into Ni $3d$ states. By comparing our data to atomic multiplet calculations including $D_{4h}$ crystal field, the doping-induced feature in Ni $L_3$-edge XAS is consistent with a $d^8$ spin-singlet state in which doped holes reside in the $3d_{x^2-y^2}$ orbitals. This is further supported by the softening of RIXS orbital excitations due to doping, corroborating with the Fermi level shift associated with increasing holes in the Ni $3d_{x^2-y^2}$ orbital.

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• Electronic structure
• Impurities in superconductors
• Superconducting gap
• Superconducting phase transition
• Cuprates
• Thin films
• Methods in superconductivity
• Resonant inelastic x-ray scattering
• X-ray absorption spectroscopy