Sm M4,5 emission spectra of Sm metal and SmB6 with fluorescence-excitation and near-threshold electron-excitation methods.

Abstract

The Sm ${M}_{4}$,5 fluorescent emission and characteristic isochromat spectra of Sm metal and ${\mathrm{SmB}}_{6}$ were measured to obtain information on the valence mixing of ${\mathrm{SmB}}_{6}$. It was found that the fluorescent spectra do not exhibit resonant emission, which had been observed in the electron-excited spectra measured with relatively low electron-beam energy. The fluorescent spectrum of ${\mathrm{SmB}}_{6}$ is quite similar to that of Sm metal and the fluorescent and characteristic isochromat spectra of ${\mathrm{SmB}}_{6}$ show no distinct indication of the spectrum corresponding to the ${\mathrm{Sm}}^{2+}$ ion. The characteristic isochromat spectra at the photon energy of the resonant peak of these materials show that an electron-beam energy beyond 1085 eV is required for the generation of the resonant emission (3${d}_{5/2}^{\mathrm{\ensuremath{-}}1}$4${f}^{n+1}$\ensuremath{\rightarrow}3${d}^{10}$4f $^{n}$), where n is equal to 5 for the ${\mathrm{Sm}}^{3+}$ ion. This is ascribed to the formation of the transient state 3${d}^{9}$4${f}^{n+2}$, which is caused by both the electron excited from the 3d level and the incident electron losing the initial energy. The possibility of a valence change from divalent to trivalent is discussed as the reason no distinct indication of the spectrum corresponding to the ${\mathrm{Sm}}^{2+}$ ion in ${\mathrm{SmB}}_{6}$ is observed.