Aspects on Direct and Inverse Photoemission Studies of Layered Transition Metal Dichalcogenides

Angle resolved photoemission spectroscopy (ARPES) and angle resolved inverse photoemission spectroscopy (ARIPES) have been established as powerful experimental tools to probe the momentum resolved electronic structure of solids and their surfaces. ARIPES in the vacuum ultra-violet (VUV) energy range is a technique complementary to the now widely used angle resolved photoelectron spectroscopy (ARPES) in the study of occupied states of single crystals, since it can probe the unoccupied momentum resolved electronic structure in an energy regime not accessible to ordinary photoemission, i.e. between the Fermi level and the vacuum level. It is based on the fact that electrons penetrating into a solid surface cause emission of photons in a momentum conserving interband transition. For recent reviews in this field, see e.g. [1–6] and references given there. In a classic angle resolved photoemission experiment photons with energy hv (typically 5–40 eV) and polarization characterized by the vector potential A hit a single crystal surface under a defined angle of incidence, Figure 1. The kinetic energy distribution of the emitted photoelectrons is measured and varies as a function of the emission angles In the time reversed process of inverse photoemission electrons with a given low kinetic energy hv 30 eV incident under the angles cause emission of a characteristic photon spectrum. Both types of spectra contain a large amount of microscopic information on the momentum resolved electronic structure of the crystal and its surface. Photoemission as well as inverse photoemission have already been very successfully, but in most cases separately, applied in a wide field of studies of clean surfaces, adsorbates and interfaces of metals and semiconductors.