Detection of metastable He₂* molecules in helium plasma

Metastable He₂* molecules, ubiquitous in helium systems, consist of a metastable He* (2³S₁) and a ground-state He atoms. Their detection through the optical transition a³Σ⁺_u (ν = 0) - e³Π_g(ν = 0) has been performed in He plasma (P=8-267 mbar) with a tunable cw laser at 465 nm, based on the sum frequency generation of 923 and 937 nm in the PP-KTP crystal. The laser has been characterized for its wavelength tuning and efficiency of nonlinear conversion. Absorption rates provide direct access to He₂* number densities, based on quantitative spectroscopic data obtained for the probed rotational branches. Studies mainly involve 3He₂* but its isotopologues are also studied in isotopic mixtures and pure ⁴He gas. He₂* absorption spectra yield rotational temperatures close to the gas temperatures. In addition to the spin-statistics-driven line intensities alternation, an alternation of 3He₂* line shapes occurs indicative of the underlying structure of the rotational states. Time evolution of absorption signals has been recorded in discharge afterglow providing quantitative information about dynamics and relative abundances of the metastable species. The He₂* density varies for strong laser excitation of He* at 1083 nm, as formation rate from the 2³P state is 100-times higher. Simultaneous absorption measurements on atoms and molecules during the metastability exchange optical pumping (MEOP) experiments has been performed to study the potential contribution of He₂* to the laser-enhanced ³He nuclear relaxation observed in MEOP. Preliminary results has shown expected increase in molecular density that is so far mild in comparison with the spectacular enhancement of ³He nuclear relaxation rate. (author)