Negative ions formed at grid surfaces in a flux of excited Cs atoms and ions

Abstract

In a thermionic energy converter with a carbon layer on the colder electrode, the so-called collector, a thermal plasma with special properties is formed. The flux of Cs from such a converter contains highly excited Cs atoms, even doubly excited states Cs 2★ and negative excited ions (Cs ★) − [J. Lundin and L. Holmlid, Adv. Mass Spectrom., 11A (1989) 204; J. Phys. Chem., 95 (1991) 1029]. In the present experiments, a shielded grid is mounted in this flux and held at a negative potential. Negative ions are formed at the grid and observed by quadrupole mass spectrometry. Ions H − 2, CO − 2, O −, OH −, C − 2 and Mo − are observed as negative peaks in the mass spectra, at current levels up to 0.1 nA. Ion energy analysis is formed by varying the voltage of the collector at the exit of the quadrupole mass spectrometer. This analysis shows that the ions are formed at the negative grid, and experiments with a lens instead of the grid give no negative ions. The grid must have a very low work function to be able to give such large ion currents. It is suggested that the grid surface is covered by a layer of excited states.