Observation of population inversion in He I due to a double electron capture process

The first observation of inverted population density due to a double electron capture process has been performed for He plasma in contact with H2 molecular gas in the TPD-II device. Recently, the process He2+ + H2 → He∗ (1s3l) + 2H+ has been found to have a very large cross-section (10−15 cm2 or more) in collisions at low energies (below 1 eV) due to an orbiting effect and accidental resonance. To see the population density of He I levels, a 2 m vacuum UV monochromator was used and resonance spectra (np1P → 1s1S, n = 2,3) were measured. In contrast to the quick disappearance of the transition 2p1P → 1s1S of He I, the transition 3p1P → 1s1S has exhibited a noticeable increase of one order of magnitude associated with the injection of H2 gas. As a result, population inversion between 2p1P and 3p1P was concluded to have occurred.