The longer answer is that the FWHM of the Ag 3d 5/2 peak is a useful indication performance. Energy resolution of an XPS instrument is usually measured as the full width at half-maximum (FWHM) of the Ag 3d 5/2 peak. XPS is a spectroscopic technique and therefore the energy resolution of the instrument is of fundamental importance. The chemical or bonding information of the element is derived from these chemical shifts. 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 environment of an atom alters the binding energy (BE) of a photoelectron which results in a change in the measured kinetic energy (KE). The importance of surfaces in materials science is discussed in greater detail elsewhere 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 surface sensitivity of XPS is determined by the distance that that photoelectron can travel through the material without losing any kinteic energy. The position and intensity of the peaks in an energy spectrum provide the desired chemical state and quantitative information. The kinetic energy of these emitted electrons is characteristic of the element from which the photoelectron originated.

These ejected electrons are known as photoelectrons. 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. 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.

Xps peak with a mg ka source