Description
XPS
Photoemission is based on the photoelectric effect. An incident photon is absorbed by an atom which uses its energy hν to emit an electron. In a photoemission experiment, this photoelectron is collected by an analyzer able to measure its kinetic energy as well as, in angle-resolved photoelectron spectroscopy (ARPES), its emission angle. In this lecture, we will describe this process and the information contained in photoemission spectra.
One can first make a clear difference between the observation of core levels, atomic-like electrons described by the usual quantum numbers n, l and ml, and the more or less delocalized valence electrons described by their binding energy and their wavevector k.
Core level spectroscopy gives a quite direct access to the local chemical environment of the excited atom. They can also be measured in a pump-probe experiment, where a pulse of UV or visible light prepares the material into an excited state (pump) prior to its analysis by photoemission with a soft X-ray photon (probe). Variation of core level binding energy with the time delay between the pump and the probe allows for a measurement of the excited state lifetime, a key parameter for materials to be used in solar cells or for hydrogen production by water photoelectrolysis.
On valence states, ARPES is a unique tool to give an image of the material band structure. The basis of this technique will also be described.
