The negative ionization of sputtered carbon atoms

The formation and survival probability of negative carbon ions sputtered fr om a graphite sample was determined. The energy distribution of C- ions was measured experimentally and the energy distribution of all sputtered carbo n atoms was obtained from a computer simulation with the program TRIM.SP. F rom the two distributions, we obtained the probability that a carbon atom w ould capture an electron from the graphite and subsequently survive as it m oves away from the surface. The results show two different situations. At l ow energy (below 25 eV), the sputtered particles are all negatively ionized when the affinity level crosses the graphite Fermi level and afterwards, t he population decay follows a semi-classical rate equation. At higher energ y (above 25 eV), a velocity-dependent fraction reaches the crossing distanc e as neutral atoms. (C) 1999 Elsevier Science B.V. All rights reserved.