Paper VT1-ThA4
Temperature Programmed Desorption Measurements of the Binding Energy of Water to Stainless Steel Surfaces

Thursday, October 18, 2007, 3:00 pm, Room 618

The presence of water vapor is the limiting factor in achieving ultra-high vacuum (UHV) in an unbaked stainless steel system. While the "water problem" has been of scientific and technical interest for many decades, fundamental measurements of water interactions with stainless steel systems are not well characterized, including the binding energy and sticking coefficient of water on stainless steel. In addition, outgassing rate measurements of water from stainless steel surfaces are typically hampered by the problem of re-adsorption, leading to inaccurate measurement results. The NIST Pressure and Vacuum group has undertaken a study of the binding energy of water to stainless steel surfaces. A temperature programmed thermal desorption apparatus was constructed for this purpose and will be described in detail. The apparatus uses computer control to linearly heat a stainless steel filament at a rate of 3 °C/s while a quadrupole mass spectrometer detects the thermally desorbed species. The system is designed with a high pumping speed to minimize the problem of re-adsorption/desorption, and has reproducibly shown a water thermal desorption peak at 139 °C. A model for thermal desorption, first employed by Redhead in 1962¹ was used to determine the activation energy, or binding energy of water to stainless steel. This technique has reproducibly yielded a water binding energy between 25.6 and 26.0 kcal/mole. These results may justify lower baking temperatures than are traditionally used for achieving UHV. Future work will focus on the apparent interplay of water desorption and hydrogen desorption observed during UHV system bake-outs.

¹ Redhead, P.A., Vacuum, 12, 203 (1962).