X-ray photoelectron spectroscopy (XPS), also known as ESCA (electron spectroscopy for chemical analysis) is a surface analysis technique which provides both elemental and chemical state information virtually without restriction on the type of material which can be analysed. It is a relatively simple technique where the sample is illuminated with X-rays which have enough energy to eject an electron from the atom. These ejected electrons are known as photoelectrons. 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 surface sensitivity of XPS is determined by the distance that that photoelectron can travel through the material without losing any kinteic energy. These elastiaclly scattered photoelectrons contribute to the photoelectron peak, whilst photoelectrons that have been inelastically scattered, losing some kinetic energy before leaving the material, will contribute to the spectral background. The importance of surfaces in materials science is discussed in greater detail elsewhere
The chemical environment 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 profiles and subtle bonding changes evident in the valence band. There is growing interest in using higher energy X-ray excitation sources relative to the commonly used Al Kα (1486.6 eV) such as Ag Lα ( 2984.2 eV), Cr Kα (5414.9 eV) or Ga (9252 eV). One significant advantage of using higher energy photons is the ability to excite photoelectrons from higher binding energy core-levels. It is also the case that, with increased kinetic energy of core level electrons, the information depth also increases allowing greater sampling depth relative to conventional Al Kα. This is demonstrated by the increase in the information depth for Si 2p excited by Al Kα and Ag Lα which is 7 nm and 13 nm respectively .
Photoelectrons may also be collected from the surface in two dimensions to generate elemental or chemical state images of the surface.
 M. P. Seah and W. A. Dench, Surf. Interface Anal. 1, 2 (1979)