Abstract
A very simple derivation is given of the ionization probability P− for negative ions sputtered off a metal surface. The result P− ≈ e−(ϕ−A)/cv⊥ shows the experimentally observed dependences on work function ϕ, atomic affinity A, and outwards velocity v⊥. Estimates of c show good agreement with experiment, too. The physical picture behind the process is discussed.
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