It is the nature of the outermost layers of a solid material which determines the use and usefulness of almost every material, since it is the outer layers which play the most important roles in chemical reactions, surface protection and all high end materials use. 

X-ray photoelectron spectroscopy (XPS), also known as ESCA (electron spectroscopy for chemical analysis) provides both elemental and chemical state information virtually without restriction on the type of matreial which can be analysed.  The sample is illuminated with x-rays - monochromatic or achromatic Al Kα or Mg Kα - and photoelectrons are emitted from the surface.  The kinetic energy of these emitted electrons is characteristic of the element from which the photoelectron originated.  The position and intensity of the peaks in an energy spectrum provide the desired chemical state and quantitative information.

The chemical state of an atom alters the binding energy (BE) of a photoelectron which results in a change in the measured kinetic energy (KE).  The BE is related to the measured photoelectron KE by the simple equation;  BE = hν - KE where hv is the photon (x-ray) energy.  The chemical or bonding information of the element is derived from these chemical shifts. 

In modern spectrometers the x-rays are energy filtered or monochromatised using a quartz crystal to give x-rays with very little energy spread.  This monochromatic x-ray illumination of the sample enables high energy resolution of chemical shifts as well as detailed study of line pofiles and subtle bonding changes evedent in the valence band. 

Photoelectrons may also be collected from the surface in two dimensions to generate elemental or chemical state images of the surface.