You have to take into account that there are two different main processes that are observed in a UPS spectrum. After excitation of valence electrons by the incident ultraviolet radiation there are:
- The primary (photo-)electrons: the ones that do not undergo inelastic scattering within the material, i.e., they do not lose their energy and hence they do not lose information about the initial state they existed.
The primary electrons produce distinct spectral features, representing in first approximation the Density of States of the material. Thus, one information one can extract from these features is the binding energies of electronic states.
- The secondary (photo-)electrons: these electrons undergo inelastic scattering events within the material as they travel out of it. The majority of the secondary electrons undergo this inelastic collisions, hence they are ejected out of the material with reduced kinetic energy (these electrons lose information about their initial state).
The secondary electrons give a continuous background that is superimposed with the distinct features given by the primary electrons. The onset of the signal stemming from the secondary electrons is used to calculate the work function of the material.
The reason that the kinetic energy is not zero, is that upon electrical contact between sample and analyser (they are at electrical equilibrium at the end), there is a contact potential formed, which is equal to:
wf_sample - wf_analyser, where
wf = work function.
wf_analyser < wf_sample, then this contact potential will accelerate electrons toward the analyser, giving them non-zero kinetic energy.