Effects of plasma microfields on radiative transitions from atomic levels above the ionization threshold

The effects of plasma electric microfields on linelike optical features arising from atomic levels above the ionization threshold are investigated within the framework of the quasistatic and single-frequency dynamical field theories of spectral-line broadening. The $2p^2{}_{}{}^3{}_{}{}^{}P\to 1s2p{}_{}{}^3{}_{}{}^{}P$ and $2s2p{}_{}{}^3{}_{}{}^{}P\to 1s2s{}_{}{}^3{}_{}{}^{}S$ transitions in helium and heliumlike ions are treated as illustrations. The mixing of the doubly excited levels in the perturbing microfields produces Stark broadening of the emission lines and induces autoionization of the $2p^2{}_{}{}^3{}_{}{}^{}P$ level, which, unlike the $2s2p{}_{}{}^3{}_{}{}^{}P$ level, is metastable against autoionization in the field-free environment. Determination of the complete Stark-broadening profiles in thermal plasmas is complicated by the need to include the effects of both the (quasistatic) ion and the (dynamical) electron fields. Under nonequilibrium conditions, where electric fields from either electron or ion plasma waves can far exceed nearby particle fields, the calculation and interpretation of the line shapes may be simplified and could provide a diagnostic probe of the wave-field properties.