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Description
The negative ion formation in the cesium sputter ion sources occurs on the surface of a cathode containing the ionized material. The cathode is covered by a thin layer of cesium (Cs), which lowers the work function of the surface enhancing the negative ion formation. Vogel [1] recently introduced a hypothesis that the negative ion current can be enhanced by exposing the cathode to a laser beam resonantly exciting neutral cesium atoms to 7p electronic states, which acts as a catalyst for negative ion production via so-called ion pair production. We have recently demonstrated that the photo-assisted production of negative ions can be provoked by lasers at various wavelengths with the photon energy exceeding a certain threshold, which questioned the resonant ion pair production hypothesis [2]. Furthermore, the laser-assisted production of negative ions of oxygen (O-) as well as aluminium (Al-) was observed with the off-resonance diode lasers [3]. This observation opens the door for practical applications of photo-assisted negative ion production for all negative ion species, not just those with their electron affinity level in resonance with the excited states of neutral Cs. In this paper we present new results for Cl-, Br- and I- ions, obtained with a higher power, better focused and controlled laser setup. Finally, we explain the laser enhancement with a qualitative model.
