Density dependence of dielectronic recombination in selenium

Abstract

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\ifmmode\times\else\texttimes\fi{}${10}^{20}$ electrons/${\mathrm{cm}}^{3}$ 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\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ ${\mathrm{cm}}^{3}$/sec at 1.0 keV, which is to be compared with the zero-density value of 2.8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ ${\mathrm{cm}}^{3}$/sec. In the latter case (F-like to Ne-like), the effective recombination rate coefficient is found to be 1.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ ${\mathrm{cm}}^{3}$/sec, which is substantially reduced from the zero-density result of 3.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ ${\mathrm{cm}}^{3}$/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\r{} 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.