A metal can be considered as the collection of a conglomeration of crystal with various shapes and sizes. Each crystal consists of a nucleus and orbits surrounding nucleus. The nucleus can be considered as the positive charged portion and in the orbits, electrons are revolving. Since electrons have negative charge, we can consider orbits with negatively charged electrons revolving with a velocity of light. The valence electrons, ie, the electrons in the outermost orbits decide the chemical behavior of an atom. When we brought similar atoms close to each other, the electrons in the metal try to move from one atom to another. In a random way, the valence electrons with high potential energy will move very freely from atom to atom. These electrons which can move freely in an atom are called as “free electrons”. When the valence electrons reach the surface of metal, it encounters a potential energy barrier; the kinetic energy of such electrons will get reduced to zero and is turned back into the body of the metal.
If the energy is greater than zero, it emits from the metal surface. The “work function” of the metal can be defined as this minimum amount of energy required at absolute temperature to make some electrons to escape from the metal.
The electron emission can be classified as,
1. Thermionic Emission
2. Secondary Emission
3. Photoelectric Emission
4. High Field Emission
From the name itself, the thermionic emission deals with the effect of heating. We know that when a metal is heated, its temperature increases and the kinetic energy of some of the electrons in the metal may increase beyond the fermilevel so as to surmount the potential energy barrier of the surface. These electrons can escape from the metal and yields to a type of emission called ‘Thermionic Emission’. Thermionic emitters are of two types,
1. Directly heated Emitter
2. Indirectly Heated Emitter (Oxide Coated Emitter)
Directly heated Emitters are,
1. Tungsten Emitter
2. Thoriated Tungsten Emitter
When a moving particle strikes a solid with higher velocity, major portion of its kinetic energy will get transferred to one of the electrons and enables the escape of electrons through the potential barrier at the surface of the solid yields to a process of electron emission called as secondary emission. The electrons thus liberated are called as the secondary electrons, the high velocity particles strikes the solid to cause the secondary emission and are called as primary particles. Such electron emission is desirable in devices like electron multiplier tubes, dynatrons, television camera tubes etc. and which is undesirable in most of other devices. The secondary emission ratio can be defined as the number of secondary electrons emitted per primary particle. When the kinetic energy of a primary particle is large, it will energize and leads to liberate more than one electron on the target surface.
The secondary emission ratio depends on,
1. Target Material and Surface Condition.
2. Energy of primary particle.
3. Type of primary particle.
4. Angle of incidence of the target surface.