Atomic photoionization in a changing plasma environment

We present a detailed theoretical study on the photoionization of one- and two-electron atoms subject to an external changing plasma environment based on the Debye-H\"uckel model. Specifically, by examining the migration of the bound excited states into the continuum in the presence of plasma, our investigation has led to resonancelike photoionization structures immediately above the ionization thresholds for H, H${}^{$-${}}$, and He atoms. For He, unlike the usual Fano-Beutler resonances due to the configuration mixing between the doubly excited components and the single ionization channel with different ${$\ell${}}_{1}{$\ell${}}_{2}$ angular momenta combinations in the final-state wave function, these plasma-induced resonancelike structures result from the mixing of the quasibound and continuum components of the same ${$\ell${}}_{1}{$\ell${}}_{2}$ combination in the final-state function. With a number of specific examples, we show that the general features of these spectra could be linked directly to the overlap at small $r$ between the effective wave functions of the outgoing ionized electron and the atomic electron in its initial states.