  This work focuses on the production of negative-ions on graphite and diamond
surfaces bombarded by positive ions in a low pressure (2 Pa) low power (20 W)
capacitively coupled deuterium plasma. A sample is placed opposite a mass
spectrometer and negatively biased so that surface produced negative ions can
be self-extracted from the plasma and measured by the mass spectrometer. The
ratio between negative-ion counts at mass spectrometer and positive ion current
at sample surface defines a relative negative-ion yield. Changes in
negative-ion production yields versus positive ion energy in the range 10-60 eV
are analysed. While the negative-ion production yield is decreasing for diamond
surfaces when increasing the positive ion impact energy, it is strongly
increasing for graphite. This increase is attributed to the onset of the
sputtering mechanisms between 20 and 40 eV which creates negative ions at
rather low energy that are efficiently collected by the mass spectrometer. The
same mechanism occurs for diamond but is mitigated by a strong decrease of the
ionization probability due to defect creation and loss of diamond electronic
properties.
