Recombination of ${\mathrm{W}}^{19+}$ ions with electrons: Absolute rate coefficients from a storage-ring experiment and from theoretical calculations

Experimentally measured and theoretically calculated rate coefficients for the recombination of ${\mathrm{W}}^{19+}(\left[\mathrm{Kr}\right]4d^{10}4f^9$) ions with free electrons (forming ${\mathrm{W}}^{18+}$) are presented. At low electron-ion collision energies, the merged-beam rate coefficient is dominated by strong, mutually overlapping, recombination resonances as already found previously for the neighboring charge-state ions ${\mathrm{W}}^{18+}$ and ${\mathrm{W}}^{20+}$. In the temperature range where ${\mathrm{W}}^{19+}$ is expected to form in a collisionally ionized plasma, the experimentally derived recombination rate coefficient deviates by up to a factor of about 20 from the theoretical rate coefficient obtained from the Atomic Data and Analysis Structure database. The present calculations, which employ a Breit–Wigner redistributive partitioning of autoionizing widths for dielectronic recombination via multi-electron resonances, reproduce the experimental findings over the entire temperature range.

Figure 1

Populations of the 2469 levels of the $\left[\mathrm{Kr}\right]4d^{10}4f^9$ ground configuration and the $\left[\mathrm{Kr}\right]4d^{10}4f^{8}5s$ and $\left[\mathrm{Kr}\right]4d^{10}4f^{8}5p$ first excited configurations of ${\mathrm{W}}^{19+}$ as a function of ion storage time. The thick solid line represents the population of the $\left[\mathrm{Kr}\right]4d^{10}4f^9{}^6{H}_{15/2}$ ground level, the dashed, dash-dotted, dash-dot-dotted, and short-dashed lines denote the populations of the long-lived metastable $\left[\mathrm{Kr}\right]4d^{10}4f^9{}^6{F}_{9/2},{}^4{M}_{21/2},{}^6{F}_{3/2}$, and ${}^4{L}_{19/2}$ levels, respectively. The thin solid lines represent the remaining 10 levels from Table 1. The dotted line represents the sum of the populations of the 2554 short-lived levels, which are not listed in Table 1.

Figure 2

Partitioned-damped intermediate coupling rate coefficients for electron-ion recombination of ${\mathrm{W}}^{19+}$, via $\mathrm{\Delta }n=0$ promotions of $4d$ and $4f$, for the lowest 16 initial levels. The ground level curve is highlighted in black—it is the uppermost curve in the inset and at the peak at 200 eV—as is the long-lived ${}^4{M}_{21/2}$ level, in blue.

Figure 3

Comparison of our measured (symbols) and various calculated merged-beam recombination rate coefficients. The short-dashed line (labeled PD) is the result of the fully partitioned calculation including autoionizing and radiative damping. The long-dashed line (labeled RR) is the calculated rate coefficient for radiative recombination. The inset shows the same data on a double logarithmic scale. The full circle (labeled ST) is the rate coefficient from the statistical theory by Dzuba et al. [9].

Figure 4

Experimentally derived (thick solid line) and theoretical rate coefficients for electron-ion recombination of ${\mathrm{W}}^{19+}$ in a plasma. The error bars represent the combined statistical and systematic uncertainty (see text) of the experimentally derived rate coefficient. The short-dashed line (labeled PD) is the result of the present partitioned-and-damped theory. The dash-dotted line is the plasma recombination rate coefficient from the ADAS database [8, 18]. The long-dashed line is the calculated RR plasma rate coefficient. The shaded area indicates the plasma temperature range where ${\mathrm{W}}^{19+}$ is expected to form in a collisionally ionized plasma [19].