Electron-Ion Recombination of Low-Charged Heavy Atomic Ions at the Cryogenic Storage Ring

Abstract

For plasma modeling, e.g., in astrophysical environments, reliable data on the recombination of atomic ions with free electrons are required. Especially low-charged heavy ions, which have a high mass-to-charge ratio m/q, and which are found, e.g., in kilonovae, can have a complex electronic structure and therefore the recombination of these ions is difficult to treat theoretically. So far, no experimental data are available on the recombination of low-charged heavy ions. Due to technical limitations, previous electron-ion recombination experiments in storage rings were restricted to ions with a low mass-to-charge ratio m/q ≤ 15. Only in extremely challenging single-pass experiments, recombination of an ion with a higher m/q = 24 was measured. In this work, first experiments on electron-ion recombination of atomic ions were performed at the Cryogenic Storage Ring CSR at the Max-Planck-Institut für Kernphysik in Heidelberg. In two experiments, absolute merged-beams rate coefficients for electron-ion recombination were measured for Ne²⁺ and Xe³⁺ ions. Recombination features from radiative and dielectronic recombination have been observed. The first experiment, performed with Ne²⁺ ions (m/q = 10), demonstrates the general feasibility of electron-ion recombination experiments with atomic ions at CSR. The second experiment, performed with Xe³⁺ ions, which have a high mass-to-charge ratio of m/q = 43, proves the feasibility of recombination experiments with low-charged heavy ions at CSR. The experiments show that recombination of previously not accessible low-charged heavy atomic ions can be studied in the cryogenic environment of CSR.