The development of UHV techniques spawned the revival of electron spectroscopy in about 1970. The necessity for such UHV conditions for the study of solid surfaces can be judged from the accompanying figure, which gives the "universal" electron escape depth in Å as a function of the electron kinetic energy in the range of interest between about 10 and 1000 eV. The escape depth is only on the order of a few Å. It can be seen that only those electrons ejected (be they x-ray, uv, or electron impact induced) which do not lose energy by inelastic scattering on their passage to the surface will be detected and be able to appear in the kinetic energy spectrum. The implication is that any spectroscopy of a solid surface involving electrons, samples only electrons from a very thin layer of the surface. If one wishes to learn about the properties of the solid, one has to work with atomically clean surfaces, for the investigation of the surface states and of adsorbed molecules requires UHV conditions to prevent interference from adsorbed contaminants.
QUESTION: Why does the escape depth, as a function of energy, yield a roughly universal curve for all metals (Figure 4.1) although the dielectric function diuffers from material to material?