REELS is an abbreviation for Reflected Electron Energy Loss Spectroscopy
REELS is an analytical technique which is based upon the measurement of the kinetic energy of electrons reflected from a surface.

Mechanism
When high energy electrons impinge on a surface, many are elastically reflected but some suffer an energy loss during the encounter with the surface.

Schematic Diagram Showing the Reflection of Electrons from a Solid Surface

Some of the electrons which have given up energy to the surface will have excited one of the modes of collective oscillation within the electronic structure of the solid. Since these oscillations have characteristic energies, the incident electrons lose discrete quantities of energy to excite the oscillations and there are therefore peaks in the energy loss spectrum. These collective modes of oscillation are known as plasmons and an incident electron which excites one of these oscillations is said to have suffered a plasmon loss. The spectrum below shows the energy loss spectrum from an aluminium bond pad.

Energy Loss Spectrum from an Aluminium Bond Pad

The energy loss spectrum from aluminium is sensitive to the cleanliness of the surface.
Energy losses can also be due to ionisation processes within the solid.

Analytical Capabilities
REELS can be a useful analytical tool when it is necessary to distinguish materials which have similar elemental composition:

• Crystallographic forms of the same material (e.g. graphite, diamond and amorphous carbon)    
• Elements present in different chemical states    
• Organic materials    
• Metals and hydrides

Instrument Requirements
To be useful for REELS imaging measurements it is necessary for an Auger instrument to have the following properties:

• An incident electron beam with very small energy spread (e.g. from a field emission source)    
• An energy analyser capable of resolving the loss peaks from the elastic peak (<0.1% energy resolution)    
• A beam of electrons which has a small spot size at the required energy (e.g. <100 nm at 1 keV)