Optically forbidden excitations of the3ssubshell in the3dtransition metals by inelastic scattering of fast electrons

Abstract

Electron-energy-loss spectra of 75-keV electrons have been recorded from thin evaporated films of the $3d$ transition metals. Spectra have been measured in the region of the $3s$ subshell excitation ($M_1$ edge) at different momentum transfers. At small $q$, no significant intensity at the $M_1$ threshold is found, but at larger $q(>~1{\AA }^{-1\mathrm{}})$, weak but sharp peaks are observed. These are attributed to the optically forbidden $3s\to 3d$ transitions. Widths of these peaks for the metals titanium, chromium, iron, and nickel and their oxides Ti${\mathrm{O}}_2$, ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$, and NiO are measured and compared with both the expected widths for the unoccupied portions of the $3d$ bands and the intrinsic $M_1$ core level widths.