Techniques for the Detection of Photodesorbed Negative Ions

Abstract

The negative ion channel in stimulated desorption has been the subject of very few studies. The existing experimental data, consisting entirely of electron-stimulated desorption (ESD) experiments, have been summarized in a recent review by KNOTEK [1]. Systems studied have included H⁻ from H/W [2], O⁻ from CO/W [3], O⁻ from O/W [4,5], O⁻ from O/Mo [5], and O⁻ from Mo(100) [6] (the other substrates being polycrystalline). For the surfaces investigated above, the ion yield for negatives was one to two orders of magnitude smaller than that for positives. In some cases, both positive and negative ions could be assigned to the same chemical state on the surface through coverage dependence of the signal, for example [2]. There is also a report of ESD of H⁻ from potassium-dosed polycrystalline Mo in an investigation relating to the conversion of proton beams to negative ions at surfaces [7]. The associated application is in the production of intense neutral beams for fusion experiments. It is also noteworthy that negative ions could have deleterious effects on the performance of forthcoming synchrotron radiation sources utilizing positron beams because of trapping and scattering effects.

Work supported by the U.S. Department of Energy, BES-Materials Sciences, under Contract No. W-31-109-ENG-38.

On leave from SPAS-Commissariat a l’Energie Atomique, CEN Saclay, 91191, Gif-s-Yvette, France.

Work supported by NSF Grant No. DMR-8405-173.