Ultraviolet photoelectron spectroscopy (UPS) is a method of probing the occupied DOS of the near surface of a material. In this technique, UV photons of energy hv are created in a continuous discharge source by applying a high voltage to a gas to cause breakdown. The photons are then targeted onto the sample under investigation. These photons will liberate electrons with sufficient energy, from the sample and into the vacuum where their kinetic energies may be analysed to gain an idea of their energy origin in the density of states.
As the photon may penetrate several nanometres into the surface, some bulk states as well as surface electronic states are probed via this technique. As such, the escape depth of the electron involved is also important.
In our UPS arrangement, we use photons generated by the He I alpha transition which have a characteristic energy of 21.22 eV (lambda = 58.43 nm) This energy is close to that of the 23S metastable He state state used in metastable de-excitation spectroscopy (MDS). For this reason, UPS acts as a very useful complementary technique to MDS.
