Radiative-Auger transitions in soft-x-ray plasma emission

A broad, prominent emission blend is observed between 15 and 22 {} from a plasma produced in a θ-pinch deuterium discharge seeded with neon. The occurrence of this feature is associated with radiative-Auger-transition channels in various intermediate ion species, following the creation of short-lived $1s$-orbit vacancies by the dielectronic capture of free electrons. The calculated blended spectrum from several stages of ionization agrees with the measured contour. A similar feature is detected near 27 {} with oxygen seeding. The measured intensities and shapes indicate that 2¹S→1¹S two-photon spontaneous emission in heliumlike ions is negligible for existing plasma conditions.