Probing La-based nickelates with Ni 1$s$ core-level photoelectron spectroscopy

We present a comparative Ni core level photoemission study of La${}_3$Ni${}_2$O${}_7$, Nd${}_3$Ni${}_2$O${}_7$, and LaNiO${}_3$ using both the Ni $2p$ and the Ni $1s$. We address the challenges in analyzing the widely investigated Ni $2p$ spectra arising from the substantial overlap in energy of the Ni $2p$ with the La $3d$. We show that on the other hand the deep Ni $1s$ core level does provide a clean view on the intrinsic electronic excitations and we highlight its potential to resolve detailed differences in the electronic structure within the strongly correlated Ruddlesden-Popper series La${}_{n+1}$Ni${}_n$O${}_{3n+1}$.