Plasma causes stimulation of atoms so that there valence electrons are elevated to a higher energy level or even are able to exit from the nuclear shell. In the later case a positive ion and a free electron result from the stimulation, As stimulated electrons try to return to their basic state and positive ions try to catch free electrons in any stationary plasma allways also inverse processes are active, i.e. recombination to the basic state. As during the process of recombination the atom returns to lower energy the resulting energy is emitted in form of electromagnetic radiation. The energy quantum emitted in case of recombination of an ion and an electron is very high and the high energy radiation resulting is invisible UV. The radiation caused by an electron returning from a stimulated level to the basic level usually is emitted in form of visible radiation. As the band gap between the basic level and elevated levels within a nuclear shell is an energy amount typical for any atom, also the frequency of the electromagnetic radiation emitted is characteristic for any specific atom. This results in a colour typical for any specific process gas used to generate a plasma.
Typical colours are for example:
CF4: blue SF6: milky blueSiF4: light blueSiCl4: light blue Cl2: milky green CCl4:light green H2: rose O2: milky yellow N2: red to yellow Br2: red He: red to violett Ne: pure red Ar: dark red
The colour of the plasma not only characterizes the chemical nature of the process gas but also its density and the degree of contamination.