Spectroscopic methods:

These methods make use of the principle that when a beam of electrons or high-energy radiation (usually X-rays) are bombarded at the surface, the surface electrons from the upper layer of are ejected. These electrons are called secondary electrons. Since the energy of these electrons is very low, these are assumed to be produced from only a few layers of the surface. Electrons of this energy (10-30 eV) cannot reach the surface from underlying layers. In spectroscopic methods, surfaces are analyzed by capturing secondary electrons of different energies, which are characteristic of their environment. Based on spectroscopic theory some of the techniques are discussed below.

X-Ray Photoelectron Spectroscopy (XPS):

X-ray photoelectron spectroscopy (XPS) was developed in mid 1960 by K. Siegbohn and his colleagues. Later on, K. Siegbohn was awarded Nobel Prize for physics in 1981 for the work on XPS. This technique provides a wide range of information regarding atomic composition, oxidation states, and chemical structures. Because of its versatility it is also called Electron Spectroscopy for Chemical Analysis (ESCA). Basic principle of this technique is based on phenomenon called photoelectric effect, rationalized by Albert Einstein in 1905. When photons of known energy (usually X-rays) knock the surface, an electron from K-shell is knocked out; kinetic energy of this electron is measured in the spectroscopy. The spectrum is given as the binding energy as function of electron counting rate. Binding is one unique character of different elements. Binding energy of an atom can be calculated from the following equation.

Binding Energy (eV), B.E. = h – K.E – W

where, h = Incident energy, K.E = Kinetic Energy of the ejected electron and W = Work function.

In the XPS spectrum, the innermost orbital appears a at higher binding energy than the outer orbital. Binding energies of 1s orbitals increase with atomic number.