ABSOLUTE DETERMINATION OF THE LAYER-PERPENDICULAR BAND-STRUCTURE OF VSE2 AND TIS2 BY COMBINED VERY-LOW-ENERGY ELECTRON-DIFFRACTION AND PHOTOEMISSION

The layer-perpendicular dispersions E(k(perpendicular to)) of the typi cal layered TMDCs VSe2 and TiS2 were studied by combining determinatio n of the upper unoccupied bands by very-low-energy electron diffractio n (VLEED) with mapping of the lower occupied bands by photoemission (P E). We found that the upper bands of these materials are very complica ted, and are compatible neither with the free-electron, nor with the g round-state approximation. Knowledge of the upper bands allowed us to carry out a PE experiment optimized for the k(perpendicular to)-resolv ed mapping of the lower bands. The PE data were consistently rationali zed, using a map of the PE intensity as a function of the binding ener gy E-i and the photon energy hv. We found that the PE intensity is wel l described by direct, k(perpendicular to)-conserving, transitions, wi th minor shifts of PE peaks being basically a consequence of their bro adening due to finite electron and hole lifetime. Finally the lower ba nds were mapped explicitly, using the PE peaks with minimal shifts and the experimental upper bands. The obtained E(k(perpendicular to)) is very consistent, and shows overall agreement with full-potential LAPW calculations.