In Photoemission Spectroscopy, PES, a sample is excited with photons of a well-known energy. The photons are absorbed by the sample, causing an emission of electrons. The kinetic energy, EK, of the emitted electrons is measured by the electron spectrometer. The binding energy, EB of the electrons being
EB = hν - EK
A PES measurement thereby yields a spectrum of the number of emitted electrons as a function of energy (binding or kinetic).
XPS (X-Ray Photoemission Spectroscopy):
For XPS, X-Rays are used as ionizing radiation. All elements have their characteristics set of core electron binding energies. As the core electron orbitals have almost an atomic character with regards to molecules and solids, XPS offers a possibility of performing qualitative analysis of sample. The detailed position of the core levels also depend on the chemical state of the probed atom (chemical shift). Being surface sensitive, XPS can be used for studying the chemical status of elements on the surface.
It is possible to use PES for quantitative analysis as the surface concentration of an element is proportional to the measured peak intensity of the corresponding core level. However, the intensity also depends on other factors, such as the differential cross section of a particular orbital, instrumental broadening and polarization.
UPS (Ultraviolet Photoemission Spectroscopy):
For UPS, Ultraviolet Radiation is used as ionizing radiation. Being less energetic, ultraviolet radiation is highly surface sensitive. Ultraviolet photons are produced using a gas discharge lamp, filled with helium. As lower energy photons are used, most core level photoemissions are not accessible using UPS, so spectral acquisition is limited to the valence band region.