Negative Hydrogen Ion Production By Backscattering From Alkali Metal Targets

Measurements have been made of the total backscattered D− and H− yields from Cs, Rb, K, Na and Li surfaces bombarded with D2, D3, H2+ and H3+ in the energy range 0.15 to 4 keV/nucleus. All measurements were made at a background pressure less than 10−9 Torr and the alkali-metal surfaces were evaporated onto a substrate in situ to minimize contamination of surfaces. For each target, the D− and H− yields exhibited maxima (as high as 8% per incident deuteron or proton for Cs); the maxima occurred at incident energies between 300 and 1000 eV/nucleus and always occurred at a lower incident energy for H than for D for a given target. Both the H− and D− yields decreased, at any measured energy, in going from Cs to Li in the order given above. Measurements of the H− yield were also made for H3+ bombarding a W substrate, as a function of the work function of the target, as Li was deposited on the W. The work function of the target showed a minimum as the Li coverage was increased and the H− yield showed a corresponding maximum which was almost two orders of magnitude higher than the H− yield for a thick Li target.