The spin-forbidden f-f multiplet excitations are observed for the first time as the strongest features in the electron-energy-loss spectra of Gd, Dy, and Sm metals at low primary-electron energies. The exchange nature of the excitation process is argued, in analogy with that of spin-flip Stoner excitations in d-band ferromagnets. A resonance enhancement observed for primary-electron energies near the 4d-4f core threshold is found to be consistent with the proposed mechanism.
