Breakdown of Dipole Selection Rules in Electron-Excited 3p-3d Transitions within Localized Transition-Metal Ions at Low Incident Energies

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Metal M 2,3 inner-shell electron-energy-loss spectra in layered transition-metal tri-thiophosphates MPS 3 (M=Mn, Fe, Ni) have been measured at low incident energies in reflection geometry. The spectra reveal the multiplet structures of the final states of localized divalent metal ions. The relative intensities of the peaks are varied systematically as incident energy decreases. Large deviation from the X-ray absorption spectra are found at low incident energies. The variations are interpreted in terms of the incident energy dependence of momentum transfer involved in the inelastic scat-tering process and exchange interaction between incident and target electrons. It is found that dipole selection rules are broken in 3p-3d transitions within the localized 3d transition-metal ions as incident energy approaches the excitation threshold.