Precision Determination of Adsorption Layers on Stainless Steel Mass Standards by Mass Comparison and Ellipsometry: Part II: Sorption Phenomena in Vacuum

Abstract

Using the two independent measuring techniques, mass comparison and ellipsometry, described in Part I, adsorption layers on stainless steel mass standards and artefacts of nominal value 1 kg have been determined directly and precisely as a function of pressure in the range 5 × 10-3 Pa ⩽ p ⩽ 1 × 105 Pa. For clean, polished surfaces (average peak-to-valley height Rz ⩽ 0,12 μm) reversible sorption isotherms with a coefficient of (0,024 ± 0,005) μg cm-2 were found due to the transition from normal pressure (relative humidity h = 0,03) to the pressure of 0,1 Pa. For uncleaned surfaces this coefficient rises by a factor of about 2,5. At pressures below 0,1 Pa irreversible adsorption was observed with a constant adsorption rate of about 0,012 μg cm-2 d-1, independent of the surface cleanliness. This sorption effect is attributed to continuous condensation of oil particles originating from the turbomolecular pump used.

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