What are F-centres in a crystal?

Adsorption is an exothermic process. Heat is releases whenever molecules are adsorbed on the surface.
So, rate of adsorption decreases whenever temperature is increases. This due to the fact that, when temperature is increases, the kinetic energy of adsorbed molecules gets increased and they overcome the electrostatic force of attraction by the adsorbent surface.

It has been observed that if a crystal of NaCl is heated in sodium vapour, it acquires a yellow colour.
This yellow colour is due to the formation of a non-stoichiometric compound of sodium chloride in which there is a slight excess of sodium ions.
What happens in this case is that some sodium metal gets doped into sodium chloride crystal which, due to the crystal energy, gets ionised into Na+and e–. This electron occupies a site that would otherwise be filled by a chloride ion,
The extra electron in thus shared between all the six Na+ions which implies that this electron is not localised at the vacant Cl–site. On the other hand, this electron is similar to the delocalised p electrons present in molecules containing conjugate double bonds.
Light is absorbed when this delocalised electron makes an easy transition from its ground state to an excited state. As a result, the non – stoichiometric form of sodium chlorideappears coloured. Because of this, the sites occupied by the extra electrons are known ascolour centres.These are also calledF-centres. This name comes from the German wordFarbemeaningcolour.