Emission spectra of HeAr2+ and HeKr2+ heterotrimer ions produced in a helium flowing afterglow Original Research Article

Emission spectra resulting from clustering reactions of He+ with a heavier rare gas Rg(Rg=Ne, Ar, or Kr) have been studied in a helium flowing afterglow at various stagnation pressures of Rg. At low stagnation pressures, emission spectra of HeRg+ heterodimer ions were found for Rg=Ne, Ar, and Kr due to radiative association and three-body clustering reactions. At high stagnation pressures, new continuous bands were found for Rg=Ar and Kr at longer-wavelength region of the heterodimer bands. They were attributed to bound-free transitions of HeRg2+ heterotrimer ions. The HeRg2+ bands consisted of two components: the first continuum degraded to the red from near the HeRg+(B 2Σ+–X 2Σ+) transition, and the second continuum, a roughly Gaussian feature at longer wavelengths. The first and second components were ascribed to the B 1/2–X 1/2 and B 1/2–A2 1/2 transitions of HeRg2+, respectively. The emission intensity of the second continuum increased more rapidly than that of the first one with increasing the stagnation pressure of Rg or a foreign gas. It was explained by the fact that the second continuum arises dominantly from low vibrationally excited levels formed by collisional relaxation of the upper vibrational levels of HeRg2+(B 1/2). The geometries of HeRg2+ in the upper B 1/2 state and the lower X 1/2 and A2 1/2 states were attributed to Rg⋯He+Rg and Rg⋯HeRg+, respectively, and the bound-free character derives from attractive Rg⋯He+ and repulsive Rg⋯He bonds in Rg⋯(HeRg)+.