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

Sulfur L2.3 and titanium L2.3 and M2.3 inner-shell-electron energy-loss spectra in 1T-TiS₂, 1T-TiSe₂, 2H-TaS₂, and 2H-MoS₂ 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 t2g band for the 1T compounds band for 2H-TaS₂ 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.