2. Excitation modes for gas lasers

2.1 Energy levels in molecules

Atomic energy levels were described in the article ; as some gas lasers operate on the vibration-rotation levels of molecules, a brief description of these levels and their population is necessary for understanding population inversions in molecular lasers.

Gaseous molecules have one (diatomic molecules) or more modes of vibration v; within each mode there are levels of vibration; finally, each level of rotation is subdivided into levels of rotation. Transitions between levels take place according to quantum selection rules: Δv = 1 and ΔJ = 0, 1

Translational energy (thermal energy) is virtually continuous.

Vibration energy is equal (to a first...