The negative ionization of sputtered carbon atoms

The formation and survival probability of negative carbon ions sputtered from a graphite sample was determined. The energy distribution of C ^- ions was measured experimentally and the energy distribution of all sputtered carbon atoms was obtained from a computer simulation with the program TRIM.SP. From the two distributions, we obtained the probability that a carbon atom would capture an electron from the graphite and subsequently survive as it moves away from the surface. The results show two different situations. At low energy (below 25 eV), the sputtered particles are all negatively ionized when the affinity level crosses the graphite Fermi level and afterwards, the population decay follows a semi-classical rate equation. At higher energy (above 25 eV), a velocity-dependent fraction reaches the crossing distance as neutral atoms.