Hydrogen detection by SIMS: Hydrogen on polycrystalline vanadium

The interaction of hydrogen with polycrystalline vanadium has been studied by SIMS and flash-desorption. Two well defined vanadium surfaces with different amounts of residual oxygen were exposed to H₂-doses up to 1000 L. The investigations were carried out under UHV conditions. The experiments result in a better understanding of the vanadium-hydrogen interaction, especially with regard to the influence of residual oxygen. The residual oxygen concentration can be characterized by the O^? emission; it increases with the number of O₂ exposure/ heating cycles. Different groups of secondary ions and flash signals, characteristic of certain chemical environments of hydrogen, could be distinguished. The ratios $\text{VH}{}^{\mathrm{+}}\text{V}{}^{\mathrm{+}}$ and $\text{V}{}_2\text{H}{}^{\mathrm{+}}\text{V}{}^{\mathrm{+}}{}_2$ proved to be typical of the direct V-H bond concentration. This can be derived from their behaviour during H₂ and O₂ exposure, e.g. Bulk dissolved hydrogen is indicated by a dosedepending H₂ flash signal at 900 K. Bulk solution of H₂ is blocked by a monomolecular oxygen layer. An additional H₂ flash signal at 500 K together with increasing values of $\text{VH}{}^{\mathrm{+}}\text{V}{}^{\mathrm{+}}$ and $\text{V}{}_2\text{H}{}^{\mathrm{+}}\text{V}{}^{\mathrm{+}}{}_2$ is observed for relatively oxygen free targets after H₂ exposure. The detection limit for metal-bonded hydrogen was found to be less than 100 ppm of one monolayer.