Low-Energy Mutual Neutralization Studies for Early Universe Hydrogen Chemistry

Low-energy interactions between light ions, as they occur in low density plasmas, are ideally studied under merged-beam conditions. This was the motivation for building the dual-source setup in operation at UCL, Louvain-la-Neuve, since the early eighties. Although initially developed for the study of charge exchange~[1], mutual neutralization and transfer ionization, this machine has produced a host of total cross section measurements for a wide variety of associative ionization and other reactive processes involving charged reactants, from H$^+$ to CO$^+$, in collision with H$^-$, D$^-$, C$^-$ and O$^-$~[2]. A recent paper by Glover et al.~[3] has revived the interest for mutual neutralization studies, by stressing the need of the astrophysical community for a precise determination of the low-energy cross section of the H$^+$/H$^-$ reaction. The mutual neutralization acts as a sink for negative ions which otherwise dominate the primordial formation of H$_2$ by associative detachment with ground state H. Absolute measurements in the range 5 meV to 5 eV are needed to rule out earlier experimental work~[4] contradicting the most recent theoretical predictions~[5]. Our setup is currently modified to incorporate coincident imaging techniques, giving access to differential cross sections besides the branching among accessible neutral channels. Mutual neutralization reactions of H$^-$ with H$_2^+$ and H$_3^+$ will also be investigated, for the role they play in laboratory plasmas~[6].\\ [4pt] [1] S. Sz\"ucs, M. Karemera, M. Terao, and F. Brouillard, \emph{J. Phys. B} \textbf{17}, 1613 (1983).\\ [0pt] [2] E. A. Naji et al., \emph{J. Phys. B} \textbf{31}, 4887 (1998), A. Le Padellec et al., \emph{J. Chem. Phys.}, \textbf{124}, 154304 (2006) and references therein.\\ [0pt] [3] S. C. Glover, D. W. Savin, and A.-K. Jappsen , \emph{Astrophys. J.} \textbf{640}, 553 (2006). \\ [0pt] [4] J. Moseley, W. Aberth, and J. R. Peterson, \emph{Phys. Rev. Lett.} \textbf{24}, 435 (1970).\\ [0pt] [5] M. Stenrup, {\AA}. Larson, and N. Elander, \emph{Phys. Rev. A} \textbf{79}, 012713 (2009).\\ [0pt] [6] M. J. J. Eerden et al., \emph{Phys. Rev. A} \textbf{51}, 3362 (1995).