H−production on a graphite surface in a hydrogen plasma

We measure H− negative ions by means of a mass spectrometer in a helicon plasma reactor. The H2 plasma operates at a low injected RF power (50–300 W), in a capacitive regime, under low pressure conditions (between 0.4 and 1 Pa). A highly oriented pyrolytic graphite (HOPG) graphite sample centred in the expanding chamber and facing the mass spectrometer nozzle placed 40 mm away is negatively biased. Negative ions formed on the graphite surface upon positive ion bombardment are detected according to their energy by the mass spectrometer. We obtain the H− ion distribution function (IDF) showing two main features: first, a high energy tail attributed to negative ions created via two-electron capture following and impact on the HOPG sample and, second, a main peak which can be attributed to negative ions created on the surface by the sputtering of adsorbed hydrogen and/or two-electron capture. Finally, we show that negative ion production is proportional to the positive ion flux and strongly depends on the positive ion energy.