Colour of the plasma

The colour of a plasma is generated by emissions from energetically excited atoms, ions or molecules during their relaxation to lower-energy states. Since the energy levels in any gas have different energy transitions, every process gas exhibits different characteristic emissions, and accordingly different characteristic colours. The typical colours of some gases frequently used in plasma processes are as follows: CF4: blue
SF6: pale blue
SiF4: light blue
SiCl4: light blue
Cl2: pale green
CCl4: pale green
H2: pink
O2: pale yellow
N2: red to yellow
Br2: reddish
He: red to purple
Ne: brick red
Ar: dark red The colour of the plasma not only indicates which process gas has been used, but also the quality of the process gas, i.e. whether it is free of contaminations.

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