State-Resolved Mutual Neutralization of ${\mathrm{Mg}}^{+}$ and ${\mathrm{D}}^{-}$

We present experimental final-state distributions for Mg atoms formed in ${\mathrm{Mg}}^{+}+{\mathrm{D}}^{-}$ mutual neutralization reactions at center-of-mass collision energies of $59\pm 12\mathrm{meV}$ by using the merged-beams method. Comparisons with available full-quantum results reveal large discrepancies and a previously underestimated total rate coefficient by up to a factor of 2 in the 0–1 eV ($<{10}^4\mathrm{K}$) regime. Asymptotic model calculations are shown to describe the process much better and we recommend applying this method to more complex iron group systems; data that is of urgent need in stellar spectral modeling.

Figure 1

$\mathrm{Mg}/\mathrm{D}$ particle separation distributions, $r$, at $E_{\mathrm{CM}}=59\mathrm{meV}$ ($E_R=32\mathrm{meV}/u$) in 0.05 mm bins and normalized to unit area. Left panel: experimental data (black histogram) compared to Newtonian Monte Carlo (NMC) model data (black curve). The latter is obtained by fitting the intensities of the final $\mathrm{Mg}(3snl{}^{2\mathrm{S}+1}\mathrm{L})$ states (pink curves) to the experimental curve. The inset plot is zoomed in on the weak $3p$ ${{}^1\mathrm{P}}^{\mathrm{o}}$ channel at 42 mm. Right panel: comparison of the experimental data to synthetic NMC distributions based on the theoretical branching fractions listed in Table 1. The red and blue curves show models for neutralization of ${\mathrm{Mg}}^{+}$ with ${\mathrm{H}}^{-}$ based on the FQ and LZ-LCAO branching fractions in Table 1, respectively. The green curve corresponds to neutralization with ${\mathrm{D}}^{-}$, as in the present experiment, based on LZ-LCAO results. Compensation for missed MN events near the edge of the detector are included in the model results shown in the right panel (see the text for details).

Figure 2

The DESIREE cryogenic merged-beams double electrostatic ion-beam storage ring facility at Stockholm University [46, 47]. The main quantity of interest to the present experiment is the distance, $r$, between the neutral products as they reach the microchannel plate (MCP) detector following a mutual neutralization event inside one or several, biased drift tubes in the merging section. Further details are given in the text and additional definitions of the various technical components can be found in Refs. [14, 15].