Field effects on the Rydberg product-state distribution from dielectronic recombination

Correlation between double and nonresonant single ionization

We performed simultaneous measurements of electrons and ions produced in Xe photoionization driven by an 800-nm, 100-fs laser pulse. The obtained energy and angular resolved electron spectra allow the identification of the electronic states populated during the ionization. Xe2+ ions appear at the same laser intensity as electrons emerging from a nonresonant 9-photon ionization process of Xe. Similar to optical tunneling ionization, the nonresonant ionization delivers low-energy electrons needed for the formation of Xe2+ by a backscattering process.

Centrifugal electron-ion recombination

Transient electric-field pulses have been used to stimulate electron/ion recombination in a low density plasma in the presence of a static electric field. The measured recombination rates exhibit a strong dependence on the relative orientation of the pulsed and static fields. For weak pulses, the recombination rate is significantly higher for orthogonal as opposed to parallel or antiparallel field configurations. The enhanced recombination rate is attributed to the dynamic stabilization of high-m Rydberg levels that are populated during the pulse. Classical simulations confirm the importance of angular momentum rather than energy transfer.

Dielectronic recombination in plasmas. II. Initial excited states

Ions in a plasma recombine with electrons by both direct and resonant modes. The latter, the dielectronic recombination, can be a dominant process at temperature near T approximately equal to Z2 Ry, for ions with charge Z. The rates are usually given for target ions in their ground states, and contributions from all doubly excited intermediate states and final singly excited states of the recombined ions are summed over. To facilitate applications of the rates in plasma modelling in terms of rate equations, simple rate formulas are often devised. However, at finite temperature, a sizable fraction of ions is initially in an excited state, and after recombination, ions are usually left in singly excited final states. Thus new empirical rate formulas are needed that exhibit an explicit dependence on final as well as initial states of the ions before and after the recombination. We have calculated properly adjusted rates where (a) the target ions are allowed to be in their ground and excited states, and (b) contributions to the individual final states are explicitly separated. Multiple cascades are important in such calculations. For Al3+ ions we show that rates for the initial excited states are much larger than that for the ground state.