We know that metals have electrons that are loosely
bound to the nucleus in the outermost shells. The temperature difference changes the number of free electrons. At room temperature, there are large numbers
of free electrons that are randomly moving inside the metal.
Even though electrons are freely moving inside the metal,
they cannot leave the surface of the metal. Because the free electrons are
attracted by positive nuclei of the metal. These attractive electrons will not
leave the nuclei at room temperature.
If the free electrons want to leave the metallic
surface, that free electrons must cross a potential barrier that is created by
the positive nuclei of the metal. This potential barrier is called as a surface
barrier because these potential barriers prevent free electrons from
leaving the metallic surface.
All kinds of free electrons possess kinetic energy and that
energy is different for different electrons. Electron kinetic energy is not
sufficient to overcome the surface barrier.
Suppose an additional energy is given to those free electrons,
they will have sufficient energy to cross the surface barrier. The liberation
of electrons from any surface of a substance is called electron emission.
The minimum energy needed for an electron to escape from the
metal surface is called the work function of that metal. That work function is
function is different for different metals. Like copper work function energy is
4.65 eV, aluminium work function is 4.28 eV.
Types of electron
emission
These are the four types of electron emission,
1. Thermionic emission
2. Field emission
3. Photo electric emission
4. Secondary emission
Thermionic emission
When a metal is heated to a high temperature, the free
electrons get sufficient energy to leave the metal surface. So that free electrons
are emitted from the metallic surface.
This thermionic emission of electrons is used in
cathode-ray tubes, X-ray tubes and electron microscopes.
Field emission
When a very strong electric field is applied across the metal
surface, the free electrons overcome the surface barrier of the metal.
Field emission scanning electron microscope and field
emission display are using this field emission method.
Photoelectric emission
When electromagnetic radiation of suitable frequency is
incident on the surface of the metal, that energy is transferred from the
radiation to the free electrons.
Photodiodes and photoelectric cells are made with this
photoelectric emission.
Secondary emission
When a beam of fast-moving electrons strikes the surface of
the metal, the kinetic energy of the striking electrons is transferred to the
free electrons on the metal surface.
Image intensifiers and photomultiplier tubes are made up of
this secondary emission method.
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