X-ray Photoelectron Spectroscopy (XPS), is also known as ESCA (Electron Spectroscopy for Chemical Analysis).

XPS

XPS provides quantitative compositional information from the top 10 atomic layers of a sample surface for the elements lithium to uranium. Furthermore, information regarding the chemical states of any elements present can also be obtained.
The sample is irradiated with a beam of monochromatic soft X-rays. Photoelectron emission results from the atoms in the specimen. The kinetic energies of these electrons relates to the atom and orbital from which they originated. The distribution of kinetic energies from a sample is then measured directly by the electron spectrometer.
Atomic orbitals from atoms of the same element in different chemical environments are found to possess slightly different (but measurable) binding energies. These "chemical shifts" arise because of the variations in electrostatic screening experienced by core electrons as the valence and conduction electrons are drawn towards or away from the specific atom. Differences in oxidation state, molecular environment and co-ordination number all provide different chemical shifts.
Photoelectron binding energy shifts are, therefore, the principal source of chemical information. It should be noted that these shifts can be very small and can only be detected using a high performance instrument with suitable software.


Angle Dependent XPS

By varying the sample orientation with respect to the analyser, the variation of surface chemistry with depths in the range 2‑10nm can be probed non-destructively.


Depth Profiling XPS

A locally destructive technique which measures changes in sample composition as a function of depth (up to 1µm below the surface) by removing surface material with a focused ion beam.