Satellites to Δn = 1 transitions between high-lying levels of multiply ionized atoms

In a θ pinch discharge satellites to Δn=1 transitions between high-lying levels are observed for the ions Si ix, Si x, and Si xi, but not for Si xii. They are identified as Δn=1 transitions between the corresponding levels of doubly excited systems. At high densities, the series of Rydberg levels above their respective thermal limit are collisionally coupled to their ionization limit: The intensity ratio of a transition to that of its satellite thus offers the unique possibility of measuring the ratio of the population density in the ground energy level of the next ionization stage to that in the lowest excited levels of this ion