Population inversion of rotational levels of diatomic molecules due to adiabatic expansion of a gas

It is shown for the first time that a population inversion of rotational levels of diatomic molecules can be obtained as a result of adiabatic expansion of a gas mixture. The inversion is a consequence of the kinetics of the rotational relaxation of molecules in a monatomic gas. The results of numerical calculations are given for different conditions of flow of HCl+He mixtures and allowance is made for one- and twoquantum collisional transitions. One-quantum radiative transitions from inverted levels j = 13–20 are found to be located in the wavelength range λ= 40–20 μ.