Thermionic Emission

There are free electrons in metals which can be removed from the metal’s surface because they are not bound to any particular atom but are free to move about the crystal lattice. There are typically as many free electrons as there are atoms within metals. It is these free electrons which allow current to flow in metals whenever there is a potential difference across it. The amount of energy needed to liberate these electrons from the metal is called its work function (φ).

There is a common analogy between this removal of electrons and the evaporation of molecules from a liquid. Liquid molecules can become vapour molecules if they acquire sufficient energy to escape the liquid’s surface. There is a variance in the energies of electrons in a metal in a similar fashion to the variance in the energies of molecules in a liquid, due to collisions with other particles throughout the structure. If a liquid molecule with sufficient energy makes its way to the surface, it evaporates. At temperatures commonly encountered on Earth, the electrons in metals don’t have enough energy for emission in a similar way. Heating the metal to a high temperature causes the electrons and the atoms to have a greater kinetic energy such that more electrons can escape. This is called thermionic emission. This process is important in producing X-rays and was used in old television sets (cathode ray tubes). The current of these tubes is dependent on the temperature at emission.

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