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There are myths in the literature concerning the quantities of water adsorbed on technical surfaces in vacuum. This talk will survey what is known about water on surfaces of materials used in UHV systems (oxides of Fe, Cr, Al, Cu, stainless steel, etc.), based on careful studies of water adsorbed on well-characterized surfaces. We take a surface science perspective, and introduce temperature programmed desorption (TPD) and how it provides information about binding energy and coverage. Surface electron spectroscopy and vibrational spectroscopy measurements demonstrate that a monolayer of molecular water is seldom stable on planar surfaces under uhv conditions at 300K. Desorption and outgassing of molecular water from surfaces under high or ultrahigh vacuum conditions are almost invariably a consequence of recombination (e.g., H+OH → H2O). Any molecular water present on technical surfaces at 300K in uhv is at the fractional monolayer level, at very special binding sites. Recent studies of water adsorption at water vapor pressures >1 Torr demonstrate that the saturation coverage is generally less than 1 monolayer even at such high pressures. Of course, pores and high area powders can complicate the issue but the principles are the same as for planar surfaces.
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