Enhancement of the excitation and deexcitation rate coefficients of ions in dense plasmas: The role of autoionizing states

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Abstract
In processes of excitation–deexcitation and of ionization–recombination of ions immersed in dense plasmas, multistep processes through various intermediate states are important. In the present paper we deal with the excitation and deexcitation that have doubly excited, or autoionizing, states as intermediate states. We call these the dielectronic-capture ladderlike (DL) processes. Hydrogenlike 1s⇆2s or 1s⇆2p transitions are considered for the purpose of illustration. The DL excitation rate coefficient is obtained as a function of electron density with an approximation that is based on the picture of dielectronic capture of the 1s ion into the doubly excited states followed by the collisional ladderlike excitation-ionization chain. The DL deexcitation rate coefficient results from autoionization of the doubly excited level ions that are in local thermal equilibrium with respect to the 2l-level populations (l is s or p). Both of the rate coefficients are interpreted as an extrapolation of the excitation cross section due to lowering of the threshold energy of the 1s→2l excitation and are found to obey the principle of detailed balance. Thus, the DL processes may be regarded as enhancements of the direct excitation and deexcitation. A detailed collisional-radiative-model calculation, which includes 60 doubly excited levels and relevant collisional and radiative transitions, is performed, and the above approximations are found to be consistent with the results.