ESR Observation of theΓ6Excited State ofEr3+in the Dilute AlloyPdEr

We report the first ESR signal of an excited crystalline state (${\ensuremath{\Gamma}}_{6}$) in a metal (PdEr). The intensity of the ${\ensuremath{\Gamma}}_{6}$ isotropic resonance compared to the $\ensuremath{\Gamma}_{8}^{(3)}$ resonances of the ground-state quarter allows the determination of the energy scaling parameter $W$ of the crystalline field. A more precise value is deduced from the splitting of the transitions 1 \ensuremath{\rightarrow} 2 and 3 \ensuremath{\rightarrow} 4 of the $\ensuremath{\Gamma}_{8}^{(3)}$ ground state: $W=\ensuremath{-}0.163\ifmmode\pm\else\textpm\fi{}0.015$ K, in accordance with the work of Praddaude.