The key instrumentation necessary for photoemission consists of a radiation source, an electron energy analyzer, and a means of preparing and maintaining a clean sample surface in ultrahigh vacuum. Ancillary sample characterization capability, both in situ and ex situ, is also important, especially for non-stoichiometric samples like high-temperature superconductors. We discuss each of these in turn, emphasizing some of the important attributes of each. We do not go into enough depth to allow a reader to construct the instrumentation. We emphasize the crucial characteristics of resolution and signal-to-noise ratio, and how they are limited by instrumentation.
Work functions may be as high as 5 eV, so light sources for photoemission must emit photons of energies higher than this. Since O2 and N2 absorb photons starting just above 6 eV, the entire optical path must be evacuated to a pressure below about 10–5 Torr in order to ensure significant transmission over light paths of a meter or more. For this reason, the spectral range above about 6 eV is called the vacuum ultraviolet region, Its name changes at around 50–100 eV to the soft x-ray region, for which the optical path must also be in vacuum.
Radiation sources used in the earlier valence-band studies (Berglund and Spicer, 1964b) often were hydrogen discharges. These gave a weak continuum from the H2 molecule and a strong emission line, the Lyman-α line at 10.2 eV, from the H atom (Samson 1967).