H−andD−production by backscattering from alkali-metal targets

Abstract

Measurements have been made of the total backscattered ${\mathrm{D}}^{-}$ and ${\mathrm{H}}^{-}$ yields from thick, clean targets of Cs, Rb, K, Na, and Li, bombarded with ${\mathrm{H}}_2^{+}$, ${\mathrm{H}}_3^{+}$, ${\mathrm{D}}_2^{+}$, ${\mathrm{D}}_3^{+}$ with incident energies from 0.15 to 4.0 keV/nucleus. All of the measurements were made at background pressures less than ${10}^{-9\mathrm{}}$ Torr, and the alkali-metal targets were evaporated onto a cold substrate ($T\sim 77$ K) in situ to assure thick, uncontaminated targets. For each target, the ${\mathrm{H}}^{-}$ and ${\mathrm{D}}^{-}$ yields exhibited maxima (as high as 0.08 per incident proton and deuteron) at incident energies between 0.3 and 1.4 keV/nucleus. For both hydrogen and deuterium incident at any energy, the negative-ion yield decreases in going from Cs to Li in the order given above. Also, a definite isotope effect was observed for every target used, with the ${\mathrm{H}}^{-}$ yield peaking at a lower incident energy than the ${\mathrm{D}}^{-}$ yield and in most cases, the maximum ${\mathrm{H}}^{-}$ yield was higher than the maximum ${\mathrm{D}}^{-}$ yield. Measurements of the ${\mathrm{D}}^{-}$ yield as a function of Cs coverage were also made for ${\mathrm{D}}_3^{+}$ bombarding a Ni substrate. The ${\mathrm{D}}^{-}$ yield maximized at or near the coverage at which the surface work function reached a minimum.