Resonant inelastic x-ray scattering spectra were measured for a series of Ce solid solutions (Ce-Th and Ce-Sc) across the $\gamma \mathrm{\text{−}}\alpha$ phase transition. They reveal a well-defined feature associated with the $4f^2$ configuration when the incident energy is tuned to the Ce $L_3$ preedge region. This component is normally hidden in x-ray absorption spectra because of lifetime broadening. The $f^1/f^2$ ratio estimated by resonant inelastic x-ray scattering presents a sharp drop across the $\gamma \mathrm{\text{−}}\alpha$ transition and hysteresis as a function of temperature that closely resemble the magnetization loop. These measurements confirm recent dynamical mean-field theory calculations that unexpectedly predict significant double occupancy of $f$ orbitals in the ground state.

• Received 19 December 2003

DOI:https://doi.org/10.1103/PhysRevLett.93.067402