Enhancement of the Excitation and Deexcitation Rate Coefficients of Ions in Dense Plasmas : The Role of Autoionizing States

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In processes of excitation-deexcitation and 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 DL processes. Hydrogen-like ls⟷2ℓ (ℓ=s or p) 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 ls ion into the doubly excited states followed by the collisional ladder-like excitation-ionization chain. The DL deexcitation rate coefficient results from autoionization of the doubly-excited-level ions that are in LTE with respect to the 2ℓ-level populations. Both the rate coefficients are interpreted as an extrapolation of the excitation cross section due to lowering of the threshold energy of the ls⟶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. 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.
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