𝜔 0 , 𝛤 are the line center frequency and the collisional relaxation rate (given in s -1 ). The wave number and the most probable value of the absorber speed are 𝑘 = 𝜔 0 𝑐 ⁄ , 𝑣 𝑎0 = √2𝑘 𝐵 𝑇 𝑚 𝑎 ⁄ . T is the temperature, kB the Boltzmann constant and ma the molecular mass of the absorber.