${\mathrm{D}}^{-}$ production by charge transfer of (0.3 - 3)-keV ${\mathrm{D}}^{+}$ in thick alkaline-earth vapor targets: Interaction energies for ${\mathrm{CaH}}^{+}$, CaH, and ${\mathrm{CaH}}^{-}$

Equilibrium charge-state fractions of energetic deuterium ions and atoms emerging from alkaline-earth (magnesium, calcium, strontium, and barium) vapor targets are reported in the range of 0.3- to 3-keV incident ${\mathrm{D}}^{+}$ energy. Deuterium negative-ion production in thick barium and calcium vapor targets equals similar production in cesium vapor (∼34%). Moreover, the maxium ${\mathrm{D}}^{-}$ production in strontium vapor exceeds that for all other known gas or vapor targets and reaches 50% at 500 eV. Theory does not quantitatively describe these results, although ab initio molecular-interaction-energy calculations on the neutral and negative-ion CaH systems lead to the prediction of large ${\mathrm{D}}^{-}$ yields at low energies. The theoretical prediction is based on the lack of a strong coupling between the negativeion and neutral molecular states. This implies that there are small ${\mathrm{D}}^{-}$ electron-detachment cross sections at energies less than 1 keV. The present measurements agree with previous measurements done at higher energies.