Population inversion in a magnetized hydrogen plasma expansion as a consequence of the molecular mutual neutralization process.

A weakly magnetized expanding hydrogen plasma, created by a cascaded arc, was investigated using optical emission spectroscopy. The emission of the expanding plasma is dominated by H{α} emission in the first part of the plasma expansion, after which a sharp transition to a blue afterglow is observed. The position of this sharp transition along the expansion axis depends on the magnetic field strength. The blue afterglow emission is associated with population inversion of the electronically excited atomic hydrogen states n=4-6 with respect to n=3. By comparing the measured densities with the densities using an atomic collisional radiative model, we conclude that atomic recombination processes cannot account for the large population densities observed. Therefore, molecular processes must be important for the formation of excited states and for the occurrence of population inversion. This is further corroborated at the transition from red to blue, where a hollow profile of the excited states n=4-6 in the radial direction is observed. This hollow profile is explained by the molecular mutual neutralization process of H₂+ with H⁻, which has a maximum production for excited atomic hydrogen 1-2 cm outside the plasma center.