X-Ray Photo-emission Spectrometry XPS

 X-Ray Photo-emission Spectrometry (XPS): 

Principle: When a primary X-ray beam of precisely known energy impinges on sample 

atoms, inner shell electrons are ejected and the energy of the ejected electrons  is measured. 

The difference in the energy of the impinging  X-ray and the ejected  electrons gives the 

binding energy (Eb) of the electron to the atom. Since,  this binding energy of the emitted 

electron depends on the energy of the electronic orbit and the element it can be used to 

identify the element involved. Further, the chemical form or environment of the atom affects 

the binding energy to a considerable extent to give rise to some chemical shift, which can be 

used to identify the valence state of the atom and its exact chemical form. This technique is 

mostly referred as Electron Spectroscopy for Chemical Analysis (ESCA).  

 

An associated process with this method is that when the electron is ejected from the inner 

orbital a vacancy is left with. Hence, another electron from the outer orbits may fall to fill the 

vacancy and by doing so emits X-ray fluorescence. The energy of this X-ray fluorescence is 

sometimes transferred to a second electron to make it to be ejected. This second electron thus 

emitted is termed as  Auger electron  and the method  Auger Spectroscopy  (after French 

Physicist  Pierre Auger).  Its applications are more or less similar to ESCA and both the 

methods are used in conjunction since in both cases the energies involved are similar.   

 

Instrumentation:  It consists of a radiation source for primary X-rays, monochromator, the 

energy analyzer (to resolve the electrons generated from the samples by energy) and detector 

to measure the intensity of the resolved electrons. The analysis is done in high vacuum. 

Applications : It is mainly used for surface analysis, especially in the qualitative identification 

of the elements in a sample. Based on the chemical shifts, the chemical environment around 

the atoms can also be estimated. This measurement is useful in determining the valence states 

of the atoms present in various moieties in a sample. Quantitative measurements can be made 

by determining the intensity of the ESCA lines of each element. 

 

Disadvantages : High vacuum is necessary for the system to avoid the low energy electrons to 

be collided with other impurities, which may result in low sensitivity. It is not possible to 

detect the impurities at the ppm or ppb levels. The whole instrumentation is highly 

complicated.