D−production by multiple charge-transfer collisions in alkaline-earth-metal vapors

${\mathrm{D}}^{-}$ negative-ion equilibrium fractions of deuterium in collisions with Mg, Ca, Sr, and Ba alkaline-earth-metal vapors have been measured over the energy range 1.25-100 keV. For Ca, Sr, and Ba targets, the yield of ${\mathrm{D}}^{-}$ ions is almost completely independent of the identity of the target over the energy range 4-35 keV. At low energies, an abrupt increase in ${\mathrm{D}}^{-}$ production is observed for targets of Ca, Sr, and Ba but not Mg, and occurs at approximately the same energy, 4 keV, in all three cases. Possible mechanisms responsible for the structure in ${\mathrm{D}}^{-}$ production are discussed.