Abstract
The recontamination and long-term stability of materials suitable for mass standards was studied with x-ray photoemission spectroscopy after low-pressure plasma cleaning using hydrogen and oxygen. The cleaned samples were stored in air or argon at ambient pressure and under UHV conditions and the surface chemical state was monitored continuously. We found the recontamination exhibits initially self-limited growth. In the long term over three years, we found no indication for further accumulation of contaminants. From the uncertainties of our observations we provide an upper limit for the long-term contamination which is the lowest of all the results published up to now. The stability of PtIr and Au upon different cleaning processes and storage conditions is described in detail.
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