Formation of negative ions by charge transfer: He/sup -/ to Cl/sup -/ Page: 3 of 13
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or from papers by Heineaeier and Hvelplund. Binding energies of
negative ions have been summarized by Hotop and Lineberger.lO
Experimenters measure equilibrium charge-state fractions
(equilibrium yields, Fi“^ or optimum conversion efficiency
(niopt). The latter is dependent on the geometry of the ex-
perimental arrangement, and is a lower bound to the foraer.ll
Since data are sparse for formation of negative ions other than
H- and D- by charge transfer, both are presented here. The
reader is reminded that can be lower than F.“ by an
Several systems considered here have more than 3 states, irt
which case charge-state fractions as a function of target
thickness can exhibit ccrptex behavior. An example is
helium,12 for which a minimum of 4 states must be considered:
He*. He°(ls2)lS. He°(ls2s)3S and He-; other states,
e.g.» He° (ls2s)*S or the P states must sometimes also be
considered. The He- fraction exhibits an optimum fraction,
Fopt, at a target thickness less than that for equilibrium
(see discussion below).
He-, Ne-, Ar-
Oonnally and Thoemingl3 showed in 1967 that He- is
produced from He* by a two-step process in cesium vapor, in
which He triplet metastable atoms ilsZs)3S are produced in the
first collision and He- in the second; Jorgensen et al.l* had
previously noted the role of the oetastable He atom in He-
formation. The process is
He* ♦ Cs -He° (ls2s)3S ♦ Cs*
Heo (1s2s)3s ♦ Cs -He- (lsZsZp)4P ♦ Cs*.
This two-step process is necessary because He- is a quartet
state, requiring all three electron spins to be aligned.
Schlachter et al.15 made a detailed study of this process,
using a 4-state model to demonstrate the role of the He0
triplet metastable state in He- formation. Charge-state
fractions for 25-keV He* in cesium vapor are shown in Fig. la;
the He- fraction is seen to reach a maximum at a target
thickness of less than 1 x 1015 cm-2. Singlet and triplet
metastable atom fractions are shown in Fig. lb, which were
obtained from the data in Fig. la by use of a 4-state-conponent
model; the triplet metastable fraction also has a maximum at less
than 1 x 1015 cm-2. Helium negative ions are created by
electron attachment to triplet metastable atoms.
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Schlachter, A.S. Formation of negative ions by charge transfer: He/sup -/ to Cl/sup -/, article, December 1, 1983; [Berkeley,] California. (digital.library.unt.edu/ark:/67531/metadc1074466/m1/3/: accessed November 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.