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

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.