Primary-energy dependence of the momentum transfer in reflection inner-shell-electron energy-loss spectra of layered transition-metal dichalcogenides

Abstract

Sulfur ${L}_{2.3}$ and titanium ${L}_{2.3}$ and ${M}_{2.3}$ inner-shell-electron energy-loss spectra in 1T-${\mathrm{TiS}}_{2}$, 1T-${\mathrm{TiSe}}_{2}$, 2H-${\mathrm{TaS}}_{2}$, and 2H-${\mathrm{MoS}}_{2}$ have been measured at various primary energies in the reflection mode. A remarkable primary-energy dependence is found. As the primary energy decreases, the first peak that is assigned to the unoccupied ${t}_{2g}$ band for the 1T compounds band for 2H-${\mathrm{TaS}}_{2}$ increases in intensity relative to the second peak. The peak positions are almost unchanged. They are in agreement with those of band-structure calculations and x-ray absorption spectra. The primary-energy dependence has been discussed mainly in terms of the breakdown of dipole selection rules due to momentum transfer involved in the inelastic scattering process and anisotropy in electronic structures.