In this case the laser active atoms form a gas. The first example found experimentally was the He-Ne laser in which a gas mixture of helium and neon atoms is used (fig. 1.1). The laser transitions take place in Ne especially at A = 0.6328 ym, A = 1.15 μm and A = 3.39 ym. The pumping of the Ne atoms is particularly interesting. In the gas mixture which typically contains 1.0 mm Hg of He and 0.1 mm Hg of Ne a dc or ac discharge takes place. By it electrons of sufficiently high energy are liberated which can excite the He atoms by collisions. The electrons of the He atoms recombine by means of a cascade and preferably accumulate in the long living metastable states

Fig. 1.1. Example of an experimental set-up of the He-Ne laser. The gas discharge tube of the laser is denoted by L. At the endfaces the mirrors are mounted under the Brewster angle. The laser is mounted in the way indicated in order to avoid vibrations.

2³ S and 2¹ S (fig. 1.2). Because these long living levels practically coincide with the 2S and 3S levels of Ne, by means of collisions the excited He atoms can transfer their energy to the Ne atoms which are thereby brought into excited states. These states serve as initial states for the laser transition or even for a cascade of laser transitions. A further important class of gas lasers is represented by ion lasers. Laser active transitions occur in ions of the gases of He or Ar. Ionization and excitation is achieved by electron impact. The wave-length of the emitted light lies in the ultraviolet.

Here the excited state of an electron in a molecule is generated by a chemical process. An example is provided by the reaction between flu

Fig. 1.2. The energy level scheme of helium and neon. The energy of the excited helium atom is transferred by means of collisions to neon