Ultra violet photoemission spectroscopy (UPS) is analogous to XPS but the excitation source is a helium discharge source. 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 (x-ray) energy. The consequence of this lower photon energy is that only the low binding energy valence electrons may be excited using the He source. A further consequence of the low photon energy is UPS is more surface sensitive than XPS and thus very sensitive to surface contamination.
UPS is 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.