Rami Arieli: "The Laser Adventure" Chapter 6, Ion Gas Lasers, page 1
6.1.4 Argon Ion (Ar+) Gas Laser

The Argon laser was invented in 1964 by William Bridges at Hughes.

Argon ion laser contains a tube filled with Argon gas which transforms into plasma in an excited state.
(Plasma is a state of matter in which the electrons are separated from the atoms and molecules, which means that it contains free electrons and ions).

A schematic diagram of the energy levels of the Argon laser is shown in figure 6.4.

 
Figure 6.4: Energy Level Diagram of Ion Argon Laser.

The two main laser transitions are at visible wavelengths:

Blue 0.488 [mm]
green 0.5145 [mm],
but the Argon ion laser emits also in the UV spectrum:
0.3511 [mm]
0.3638 [mm].