  A new method involving mass spectrometry and modelling is described in this
work, which may highlight the production mechanisms of negative ions on surface
in low pressure plasmas. Positive hydrogen ions from plasma impact a sample
which is biased negatively with respect to the plasma potential. Negative ions
(NI) are produced on the surface through the ionization of sputtered and
backscattered particles and detected according to their energy and mass by a
mass spectrometer placed in front of the sample. The shape of the measured
negative-ion energy distribution function (NIEDF) strongly differs from the
NIEDF of the ions emitted by the sample because of the limited acceptance angle
of the mass spectrometer. The reconstruction method proposed here allows to
compute the distribution function in energy and angle (NIEADF) of the
negative-ions emitted by the sample based on the NIEDF measurements at
different tilt angles of the sample. The reconstruction algorithm does not
depend on the NI surface production mechanism, so it can be applied to any type
of surface and/or NI. The NIEADFs for HOPG (Highly Oriented Pyrolitic Graphite)
and Gadolinium (low work-function metal) are presented and compared with the
SRIM modelling. HOPG and Gd show comparable integrated NI yields, however the
key differences in mechanisms of NI production can be identified. While for Gd
the major process is backscattering of ions with the peak of NIEDF at 36 eV, in
case of HOPG the sputtering contribution due to adsorbed H on the surface is
also important and the NIEDF peak is found at 5 eV.
