6.3.2 Neutral Measurements

All three samples (CVD diamond, BaZrO\( _{3}\), MgO) were tested using neutral particles. As primary particles CO\( ^{+}_{2}\) ions and N\( _{2}^{+}\) ions were used. The CO\( _{2}^{+}\) ions neutralized and dissociated into CO and O neutrals in the neutralizer. This configuration was used for neutral oxygen measurements. The CO molecule was no problem because the TOF allowed resolution of the particle mass. No large amounts of CO were measured, thus the CO fraction was neglected in the further calculations. CO\( ^{+}_{2}\) particles in the energy range of 150eV to 900eV were used corresponding to an energy range of 30eV to 300eV per O\( ^{0}\) atom. N\( ^{+}_{2}\) molecules were used for sputtering measurements since N and N\( _{2}\) do not form negative ions.. The N\( ^{+}_{2}\) ions were neutralized and partly dissociated at the neutralization surface. The neutral N\( _{2}\) and N particles then hit the conversion surface eventually sputtering an oxygen or hydrogen atom (e.g., from water on the surface or out of the conversion surface material itself) away into the TOF. As nitrogen does not have a stable negative charge state all detected negative ions should originate from the CS itself and not from the neutral beam impinging on the CS. N\( ^{+}_{2}\) ions in the energy range of 110eV to 655eV were used again resulting in an energy range of 30eV to 300eV per N\( ^{0}\) atom. For each surface CO\( ^{+}_{2}\) and N\( ^{+}_{2}\) measurements were performed at corresponding mean energies per oxygen or nitrogen atom. By turning the setup out of the view of the beam, background measurements could be performed to estimate the count rate due to residual gas ions.

With all three surfaces a substantial conversion to negative ions was observed even at the lowest energies investigated. Preliminary data are shown in Figure 6.12. A 190eV O\( ^{0}\) beam (made out of a 600eV CO\( ^{+}_{2}\) ion beam) was impinging on the CVD diamond surface. Beside the expected O\( ^{-} \) peak, C\( ^{-} \), H\( ^{-} \), and H\( ^{-}_{2} \) peaks are also visible. In the lower panel a background spectrum is shown with the beam going into the chamber but not hitting the conversion surface. In the background spectrum only C\( ^{-} \), H\( ^{-} \), and H\( ^{-}_{2} \) peaks are visible suggesting that these particles were sputtered from the surface by particles accelerated to the surface (the surface was on minus 19kV during these measurements). The large hydrogen component originated from the hydrogen passivation layer on top of the conversion surface and from absorbed water. The incident 190eV O\( ^{0}\) particles did not produce noticeable sputtering by themselves as verified with a N\( ^{0}\) beam. When using N\( ^{0}\) particles all detected carbon, oxygen and hydrogen were either sputtered from the surface or background particles. Nitrogen was chosen for several reasons to estimate sputtering and background: Its mass is similar to oxygen, it does hot have a stable negative charge state and would therefore not show up in the TOF spectra, it is not contained as component in any of the investigated surfaces, and the CASYMS source could provide a high intensity primary N\( ^{+}_{2}\) beam for the neutralizer.

Figure 6.12: Preliminary TOF spectra obtained using a 190eV O\( ^{0}\) beam impinging on the CVD diamond conversion surface. The converted negative ions were accelerated to 14keV in the extraction lens prior entering the TOF. The time of flight increases with channel number.

March 2001 - Martin Wieser, Physikalisches Institut, University of Berne, Switzerland