Question

Please explain the step by step concept of XPS ,auger and chemical shift in the graph as simply as possible.

Answer 1

In XPS the photon is absorbed by an atom in a molecule or solid, leading to ionization and the emission of a core (inner shell) electron. By contrast, in UPS the photon interacts with valence levels of the molecule or solid, leading to ionisation by removal of one of these valence electrons.

The kinetic energy distribution of the emitted photoelectrons (i.e. the number of emitted photoelectrons as a function of their kinetic energy) can be measured using any appropriate electron energy analyser and a photoelectron spectrum can thus be recorded.

The process of photoionization can be considered in several ways : one way is to look at the overall process as follows :

A + h?  ? A⁺ + e-

Conservation of energy then requires that :

E(A) + h?  = E(A⁺ ) + E(e-)

Since the electron's energy is present solely as kinetic energy (KE) this can be rearranged to give the following expression for the KE of the photoelectron:

KE =  h? - ( E(A⁺ ) - E(A) )

The final term in brackets, representing the difference in energy between the ionized and neutral atoms, is generally called the binding energy (BE) of the electron - this then leads to the following commonly quoted equation :

KE =  h? - BE

Experimental Details

The basic requirements for a photoemission experiment (XPS or UPS) are:

1. a source of fixed-energy radiation (an x-ray source for XPS or, typically, a He discharge lamp for UPS)
   
2. an electron energy analyser (which can disperse the emitted electrons according to their kinetic energy, and thereby measure the flux of emitted electrons of a particular energy)
   
3. a high vacuum environment (to enable the emitted photoelectrons to be analysed without interference from gas phase collisions)

4. For each and every element, there will be a characteristic binding energy associated with each core atomic orbital i.e. each element will give rise to a characteristic set of peaks in the photoelectron spectrum at kinetic energies determined by the photon energy and the respective binding energies.

   The presence of peaks at particular energies therefore indicates the presence of a specific element in the sample under study - furthermore, the intensity of the peaks is related to the concentration of the element within the sampled region. Thus, the technique provides a quantitative analysis of the surface composition and is sometimes known by the alternative acronym , ESCA (Electron Spectroscopy for Chemical Analysis).

5. the XPS spectrum of Pd metal

   The diagram below shows a real XPS spectrum obtained from a Pd metal sample using Mg K_?radiation

   

   - the main peaks occur at kinetic energies of ca. 330, 690, 720, 910 and 920 eV.

   Since the photon energy of the radiation is always known it is a trivial matter to transform the spectrum so that it is plotted against BE as opposed to KE.

   

   The most intense peak is now seen to occur at a binding energy of ca. 335 eV

   Working downwards from the highest energy levels ......

6. the valence band (4d, 5s) emission occurs at a binding energy of ca. 0 - 8 eV ( measured with respect to the Fermi level, or alternatively at ca. 4 - 12 eV if measured with respect to the vacuum level ).
7. the emission from the 4p and 4s levels gives rise to very weak peaks at 54 eV and 88 eV respectively
8. the most intense peak at ca. 335 eV is due to emission from the 3d levels of the Pd atoms, whilst the 3p and 3s levels give rise to the peaks at ca. 534/561 eV and 673 eV respectively.
9. the remaining peak is not an XPS peak at all ! - it is an Auger peak arising from x-ray induced Auger emission. It occurs at a kinetic energy of ca. 330 eV (in this case it is really meaningless to refer to an associated binding energy).