Density dependence of dielectronic recombination in selenium

Dielectronic recombination has been found to be the dominant recombination process in the determination of the ionization balance of selenium near the Ne-like sequence under conditions relevant to the exploding-foil EUV laser plasmas. The dielectronic recombination process tends to populate excited levels, and these levels in turn are more susceptible to subsequent excitation and ionization than are the ground-state ions. If one defines an effective recombination rate which includes, in addition to the primary recombination, the subsequent excitation and ionization of the additional excited-state population due to the primary recombination, then this effective recombination rate can be density-sensitive at relatively low electron density. We present results for this effective dielectronic recombination rate at an electron density of 3×{}10²⁰ electrons/cm³ for recombination from Ne-like to Na-like selenium and from F-like to Ne-like selenium. In the former case, the effective recombination rate coefficient is found to be 1.8×{}10^-11 cm³/sec at 1.0 keV, which is to be compared with the zero-density value of 2.8×{}10^-11 cm³/sec. In the latter case (F-like to Ne-like), the effective recombination rate coefficient is found to be 1.3×{}10^-11 cm³/sec, which is substantially reduced from the zero-density result of 3.3×{}10^-11 cm³/sec. We have examined the effects of dielectronic recombination on the laser gain of the dominant Ne-like 3p-3s transitions and have compared our results with those presented by Whitten et al. [Phys. Rev. A 33, 2171 (1986)]. The results obtained for the gain of the J=2 transitions at 206.9 and 209.8 A{} disagree by a factor of 2, and the calculated F-like to Ne-like isoelectronic sequence ratio differs by a factor of 4. These discrepancies are investigated and are found to be due to three factors: inclusion of post-recombination collision effects, inclusion of Rydberg levels, and choice of line shape.