Rami Arieli: "The Laser Adventure" Chapter 6, Carbon Dioxide Lasers, page 2
Lasing transitions in CO2 laser
Lasing transitions in CO2 laser occur when the molecule is going from higher energy level of the asymmetric mode into one of the other two, as can be seen in figure 6.6.
  1. The transition to the symmetric stretching mode correspond to the wavelength of 10.6 [mm].
  2. The transition to the bending mode correspond to the wavelength of 9.6 [mm].
 
Figure 6.6: Energy Level Diagram of CO2 Laser

Each of the vibrational energy level is subdivided into many rotational levels. Transitions can occur between vibrational energy levels with different rotational levels, so there are many lasing lines around the main vibrational transitions.