Nonadditivity of convoy- and secondary-electron yields in the forward-electron emission from thin carbon foils under irradiation of fast carbon-cluster ions

We have measured energy spectra of secondary electrons produced by fast-carbon-cluster Cₙ⁺ $(n=1--4)$ bombardment of thin carbon foils (3.2, 7.3, 11.9, and 20.30.3em0exμg∕cm²). For clusters of identical velocity, the convoy-electron yield is enhanced with increasing cluster size n, while the yield of secondary electrons is reduced. The yield of convoy electrons normalized to the number of injected atoms increases proportionally with cluster size n. This proportionality suggests that there is only a weak vicinage effect on the number of primary electrons scattered by the projectile. The vicinage effect observed in low-energy secondary electrons must therefore arise from either transport or transmission through the surface.