Spectroscopic methods:

Auger Electron Spectroscopy (AES)

Unlike ESCA Auger (pronounced as OJ) Electron Spectroscopy is based on a two step process. In AES, the source is electron beam unlike x-rays in XPS. Electrons of energy 3-20 keV are incident upon a conducting sample. These electrons cause core electrons from the atoms contained in the sample to be knocked out, giving photoelectrons and atoms with a core hole. The atom then relaxes when electrons from higher level drop into the core hole, some energy is released in this transition, this released energy ejects another electron from the next higher level, which is called auger electron. The kinetic energy of auger electron is the difference between the energy released in relaxation of excited ion and energy required to remove the second electron from its orbit. The energy of auger electron is characteristic of the element that emits it and can be used to identify the element.

Auger electron spectroscopy is a popular technique for determining the composition of the top few layers of the surface. Except Hydrogen and Helium it is sensitive for all other elements. But it is more sensitive to lighter elements than heavy elements.

Auger electron emission is described as KLL, LMM, and MNN etc. For example KLL process involves initial removal of a K electron followed by a transition of an electron from L or higher levels to K. The energy released in this process ejects second electron from L Shell, which is called auger electron. This process occurs almost simultaneously. Auger electron emission is purely a surface phenomenon. Since auger electrons are very weak, only electrons from the surface of a few atomic layers can reach the detector.

Instrumentation:

It is somewhat similar to XPS except for the electron source. A common electron gun is the source of electrons. Its basic mechanism involves the application of a high voltage of about 50keV applied to “V” Wehnelt cylinder which produces electrons. These produced electrons are converged to a spot by positive potential. This spot is used to scan the surface.

Low Energy Electron Diffraction(LEED)

This technique is especially used in crystallography. If a crystal is cut along a certain plane, then the atoms near the surface may be disturbed from their equilibrium positions. Such changes can be explored with LEED. The surface crystallography structures can be determined by bombarding the surface with low energy electrons (10-200eV) and diffracted electrons are observed as spots on a phosphorescent screen. The relative position of the spots on the screen shows the surface crystallographic structure.

The diffracted spots move as the energy of the incident electron changes and the intensity of the spots as a function of incident electron energy reveals much about surface reconstruction. In order to determine the true structure, much computer analysis is to be done.

Applications:

This instrument is basically used for crystallographic surface analysis.