Fig. 2

Quantitative analysis of the RXES spectra. The spectra are fitted using three components corresponding to the Yb³⁺ (blue filling) and Yb²⁺ (green) emission lines and the weak quadrupole transitions E2 (white) in the pre-edge region, respectively²¹. The line shape of the Yb³⁺ and Yb²⁺ lines is not symmetric and has been determined from the experimental data by looking at the difference spectrum between high and low T data (see Supplementary Note 1). The Yb valence is then determined from the ratio of the Yb²⁺ and Yb³⁺ integrated intensities knowing that the Yb³⁺ emission line is reduced to 13% of its maximum intensity when the incident photon energy is set to the Yb²⁺ resonance of the L₃ absorption edge (see ref. ²¹ and Supplementary Note 2). The same lineshapes are used to fit the Yb²⁺ and Yb³⁺ components of the RXES spectra for all samples and at all temperatures