Nonmonotonous distribution of population of the $a^3\Sigma^+_g$ triplet state rotational levels in corona discharge in cryogenic helium gas

We observed the spectra within the wavelength range of $910$–$990$ nm of emission of the corona discharge in supercritical helium gas at $6$–$11$ K. This spectral range contains the molecular bands of the $\rm He^*_2$ excimer radiative transitions $(c^3\Sigma^+_g \to a^3\Sigma^+_u)$ between the electron-vibrational-rotational levels of the $c^3\Sigma^+_g$ and the $a^3\Sigma^+_u$ triplet states. We have determined the populations of the rotational levels (quantity of molecules in the given excited state among the excited molecules of the discharge) of the $c^3\Sigma^+_g$ excited state from experimental values of the intensity of the respective electron-vibrational-rotational lines. The calculated population distribution is nonmonotonous. The population of the level with the rotation quantum number $K' = 18$ (the level number) is higher than those of the levels with the other $K'$ values.