Ultraviolet photoemission spectroscopy (UPS) is analogous to XPS but the excitation source is a helium discharge lamp. UPS can provide details relating to the electronic structure of the valence bands in atoms, solids and molecules. This technique can also be used to determine the work function of a material. Depending on the operating conditions of the source the photon energy can be optimised for He I = 21.22eV or He II = 40.8eV which is significantly lower energy than Al or Mg Kα used in XPS. As with XPS the BE is related to the measured photoelectron KE by the simple equation; BE = hν – KE where hv is the photon (ultraviolet radiation) energy. The consequence of using this lower photon energy is that only the low binding energy, valence electrons are excited using the UV source. A further consequence of the low photon energy is that UPS is more surface sensitive than XPS and thus very sensitive to surface contamination. UPS is often combined with gentle gas cluster ion source sample cleaning. Using large, low energy ion clusters ensures that only surface contamination is removed by sputter cleaning. It is vital that the etching does not change the chemistry of the material being studied. UPS is also very useful as a technique to determine the work function of the material being analysed and is finding increasing application in characterisation of organic and inorganic photovoltaics, organic LEDs. Information content from UPS experiments: - Investigation of valence band and highest molecular orbitals. - Work function measurements of materials. Ultraviolet photoemission spectroscopy can be applied to various fields including, but not limited to: materials science, surface science, semiconductor & electronic materials and solar energy. UPS provides information on the valence electrons, those involved in the bonding of the atoms within the material. The insight into a material’s electronic band structure is crucial in understanding its electrical and optical properties. This information aids in the development of new materials for electronic devices, energy storage, and optoelectronic applications.
