When a gas is stimulated in such a degree, that electrons are separated from the atoms or molecules of the gas it starts to be transformed into a plasma. Opposite to the gas where all particles are neutral, a plasma contains free electrons and positive charged ions. These ions are very reactive, because they have a strong tendency to find replacement for the lost electron. By accepting electrons from other atoms, molecules or ions, they react to new molecular compounds.

Processes to stimulate ionisation can be very high temperature, very strong electric field strength or high voltage electric fields oszillating at high frequency.

The stimulation process not only produces ions but also radicals or stimulated atoms. Stimulated atoms are characterized by valence electrons elevated to a higher energy level. 

Ions, radicals and stimulated atoms remain in this state only for fractions of a second until they either find a reaction partner or recombine or relax to the base state of the atom. Each recombination process and each relaxation process cause the emission of one photon. Recombination usually results inthe emission of a photon in the UV frequency range, relaxation in visible light. This photon emission is responsible für the colour of a low pressure plasma which is characteristic for the individual process gas.

Many chemical processes profit from the high reactivity of active species in a plasma.

"Plasma" is the historic greek designation for "matter" and has been introduced to designate a ionisized gas by Irving Langmuir in 1928.

Many informations about plasma You find in section ⇒ What is Plasma

Low pressure plasma
The Diener electronic low pressure plasma system Bell Jar enables to perfectly observe a plasma process