Dielectronic Recombination in Photoionized Gas: The Importance of Fine-structure Core Excitations

At the low electron temperatures existing in photoionized gases with cosmic abundances, dielectronic recombination (DR) proceeds primarily via nl j → nl excitations of core electrons (Δ n =0 DR). At these temperatures, the dominant DR channel often involves 2 p 1/2 →2 p 3/2 fine-structure core excitations, which are not included in LS -coupling calculations or the Burgess formula. Using the heavy-ion storage ring at the Max-Planck-Institut für Kernphysik in Heidelberg, Germany, we have verified experimentally for Fe XVIII that DR proceeding via this channel can be significant in relation to other recombination rates, especially at the low temperatures characteristic of photoionized gases. At temperatures in photoionized gases near where Fe XVIII peaks in fractional abundance, our measured Fe XVIII to Fe XVII Δ n =0 DR rate coefficient is a factor of ~2 larger than predicted by existing theoretical calculations. We provide a fit to our measured rate coefficient for ionization equilibrium models. We have carried out new fully relativistic calculations using intermediate coupling, which include the 2 p 1/2 →2 p 3/2 channel and agree to within ~30% with our measurements. DR via the 2 p 1/2 →2 p 3/2 channel may also have spectroscopic implications, providing unique spectral signatures at soft X-ray wavelengths that could provide good electron temperature diagnostics.