Production of negative ions at graphite (HOPG) surface in H$_{2 }$ and D$_{2}$ plasmas

Negative ion formation by dissociative attachment on molecules in low pressure plasmas has been largely studied, whereas, negative ion formation on surfaces has been few investigated. In this work we show that under positive ion bombardment, a huge number of negative ions are produced on a graphite surface placed in low pressure H$_{2}$/D$_{2}$ plasma. Our goal is to identify the negative ion production mechanisms. In this aim, the graphite sample (HOPG) is put in a helicon reactor, in front of a Hidden EQP mass spectrometer. The sample was exposed to single positive ion dominated H$_{2}$/D$_{2}$ plasma (H$_{3}^{+}$/H$_{2}^{+}$, D$_{3}^{+}$/D$_{2}^{+})$ at low pressures (0.2-1 Pa). The sample is biased negatively with respect to plasma and negative ion energy distribution functions are recorded and analysed. Two surface production mechanisms have been identified: sputtering of adsorbed hydrogen atom as negative ion and backscattering of a positive ion as a negative ion.