[en] Sputtered negative ion spectra from a carbon target under 12 keV, H₂⁺ and D₂⁺ bombardment were investigated at 300 K and elevated temperature. At 300 K the absolute yield of ¹²C^- was measured and the ionization probability determined. The ionization probability is shown to be in good agreement with that predicted from the calculated survival probability of an affinity level in an atom leaving the surface. The yield of ion clusters, ¹²C₂^- and ¹²C₃^- was determined and are consistent with the existence of clusters as continguous atoms on the surface prior to sputtering. A cluster formation process where by sputtered atoms re-combine away from the target surface is shown to be highly improbable. At elevated temperature the yield of ¹²C^- and ¹²C₂^-increased and this is attributed to two mechanisms, i.e. a a changing surface adsorbate coverage and changes in surface topography. Both of these processes can result in a surface work function change and so influence ion yield. (author)