Resonant Electron Capture by Ions into Rydberg States of Atoms

Abstract

Resonant mechanisms of electron–ion recombination accompanied by the formation of Rydberg atoms in a plasma containing atomic and molecular ions are investigated. An analytical approach is developed for the description of three-particle electron capture into a Rydberg state as a result of resonant energy transfer from a free electron to the electronic shell of a quasimolecular ion formed during the collision of an atomic ion with a buffer gas atom. An efficient method is proposed to calculate dissociative recombination rates under thermal excitation of all rotational–vibrational (rovibrational) levels of the molecular ion. The dependence of the cross sections and the rate constants of the processes on the principal quantum number is established, and the relative role of these processes is determined in a wide range of temperatures of the electron, T_e, and gas, T, components of the plasma. Conditions are found under which integral contributions of the continuous spectrum of the molecule and of the whole rovibrational quasicontinuum to the total rate of resonant electron capture are dominant. A specific analysis is carried out by an example of Ne + Xe⁺ + e and Ar + Xe⁺ + e heteronuclear systems with significantly different dissociation energies (D₀ = 33 and 171 meV) of the ground electronic term of the RgXe⁺ (Rg = Ne and Ar) ion. It is shown that the capture rate constants essentially depend on the binding energy |ε_n| of the resulting Xe(n) atom, the temperatures T and T_e, and the relationship between D₀ and the thermal energy k_BT.