Negative-ion surface production in hydrogen plasmas: modeling of negative-ion energy distribution functions and comparison with experiments

A negatively biased graphite sample (highly oriented pyrolitic graphite) was placed in a H2 low-pressure plasma. The negative ions were produced on the graphite surface upon positive-ion bombardment. Surface-produced H− negative-ion distribution functions (NIDFs) were measured by means of an energy-resolved mass spectrometer. The shapes of the recorded NIDFs depend not only on the surface production mechanisms but also on the negative-ion trajectories in the plasma and their collection probability by the mass spectrometer. In order to gain an insight into the surface production mechanisms, NIDFs were computed using Stopping and Range of Ions in Matter simulations and calculations of the ion transmission function between the sample and the mass spectrometer detector. The calculations were based on 3D modeling of the sheath potential in the extraction region and 3D modeling of ion transport inside the mass spectrometer. The excellent agreement between experiments and computations led to a better understanding of the experimental NIDFs. The method developed in this work to study H-surface production on graphite can be generalized to any other negative ions and/or surface material.