4f7-4f7transitions in Gd, oxidized Gd, and epitaxial Gd silicide

4f-4f transitions in the rare earths are investigated in a detailed study of the 4${\mathit{f}}^7$-4${\mathit{f}}^7$ excitations of Gd observed by electron-energy-loss spectroscopy in reflection mode. Consistent quasiatomic losses are observed in polycrystalline Gd metal, oxidized Gd, and an epitaxially ordered Gd silicide phase. Sharp 4${\mathit{f}}^{7\mathrm{-}}$4${\mathit{f}}^7$ transitions, whose positions are independent of environment, are superposed on a broad loss background which changes markedly from system to system. Systematic variations in the intensity of multiplet components with electron primary energy and angle of scattering are presented, and attempts are made to distinguish contributions which arise from large-angle electron-exchange collisions without elastic scattering and those which involve both large-angle elastic scattering and inelastic exchange scattering. Theoretical estimates of spin-flip cross sections are consistent with the importance of high-momentum-transfer processes.