Symmetric resonant charge transfer in ${\mathrm{H}}^{+}$+H and ${\mathrm{He}}^{2+}$+He collisions at extremely low energies

Symmetric resonant charge transfer (RCT) in ${\mathrm{H}}^{+}$+H and ${\mathrm{He}}^{2+}$+He collisions has been investigated in the ultralow-collision-energy range ${10}^{\mathrm{-}3}$–${10}^{\mathrm{-}7}$ eV. The RCT cross section has been computed by partial-wave expansion in the molecular-orbital expansion method within the fully quantum-mechanical formalism. The calculated RCT cross sections become constant below E≃${10}^{\mathrm{-}6}$ eV, corresponding to a scattering length of ∼6$a_0$ and ∼2$a_0$ for ${\mathrm{H}}_2$${\mathrm{}}^{+}$ and ${\mathrm{He}}_2$${\mathrm{}}^{2+}$, respectively.