Charge-transfer excitations in lanthanum compounds measured by resonant inelastic x-ray scattering at the $M_5$ edge

We present resonant inelastic x-ray scattering spectra from La metal ${\mathrm{La}}_2{\mathrm{CuO}}_4$ and ${\mathrm{LaF}}_3.$ Monochromatic photon excitation with energy across the La ${3d}_{5/2}$ ${(M}_5)$ threshold was used and the valence scattering was monitored at selected incoming energies. The spectra vs the outgoing photon energy show the elastic peak, the $5p-{3d}_{5/2}$ decay peak and, in between, a feature due to charge transfer excitation. This brings an electron from the ligand to the lanthanum $4f$ states (in the case of ${\mathrm{La}}_2{\mathrm{CuO}}_4$ and ${\mathrm{LaF}}_3)$ and results from a charge fluctuation from ${4f}^0$ to ${4f}^1$ occupation in the case of La metal. The energy transferred to the solid when this process occurs is remarkably different in the three cases and correlates with the difference in the electronegativity with the lanthanum neighbors. The present approach allows us to reveal features that are completely hidden in the x-ray absorption spectra.