Question

With SPM techniques, we can often identify the surface composition of atoms using the cross-section or size of the atom itself. In XPS, we can also perform such analysis, although typically with greater sensitivity and through a completely different mechanism. Why is it possible to identify individual elements with XPS based on binding energies? (Hint: why are X-rays used to perform photoelectron spectroscopy, and not a lower energy photon source?)

Answer 1

The SPM techniques are based upon scanning a probe just above a surface while monitoring some interaction (like quantum tunneling STM and atomic forces in AFM) between the probe and the surface. The interaction between the sharp probe and surface provides 3D topographic image of surface at the atomic scale. However, these details are limited upto few atomic distance only.

X-ray Photoelectron Spectroscopy (XPS) works on the principle of photoelectric effect where X-ray photon are sent into the material. The photon knocks out the elctron from the material by giving its energy to elctrons. These emitted electrons which are called "photoelcectrons" carry all the necessary information about material. Besides identifying elements in the specimen, the spectrum of photoelctrons can also tell how much of each element is in the sample.

The energy conservation demands

where K.E. is kinetic energy of ejected photoelctron and B.E. is binding energy which depends on the sample material . From KE.energy and BE energy, an energy spectrum of intensity (number of photoelectrons versus time) versus binding energy (the energy the electrons had before they left the atom) can be obtained. Each prominent energy peak on the spectrum corresponds to a specific element.