DIELECTRONIC RECOMBINATION IN PHOTOIONIZED GAS - THE IMPORTANCE OF FINE-STRUCTURE CORE EXCITATIONS

At the low electron temperatures existing in photoionized gases with c osmic abundances, dielectronic recombination (DR) proceeds primarily v ia nl(j) --> nl(j)' excitations of core electrons (Delta n = 0 DR). At these temperatures, the dominant DR channel often involves 2p(1/2) -- > 2p(3/2) fine-structure core excitations, which are not included in L S-coupling calculations or the Burgess formula. Using the heavy-ion st orage ring at the Max-Planck-Institut fur Kernphysik in Heidelberg, Ge rmany, we have verified experimentally for Fe XVIII that DR proceeding via this channel can be significant in relation to other recombinatio n rates, especially at the low temperatures characteristic of photoion ized gases. At temperatures in photoionized gases near where Fe XVIII peaks in fractional abundance, our measured Fe XVIII to Fe XVII Delta n = 0 DR rate coefficient is a factor of similar to 2 larger than pred icted by existing theoretical calculations. We provide a fit to our me asured rate coefficient for ionization equilibrium models. We have car ried out new fully relativistic calculations using intermediate coupli ng, which include the 2p(1/2) --> 2p(3/2) channel and agree to within similar to 30% with our measurements. DR via the 2p(1/2) --> 2p(1/2) c hannel may also have spectroscopic implications, providing unique spec tral signatures at soft X-ray wavelengths that could provide good elec tron temperature diagnostics.