Desorption kinetics at atmospheric pressure:: alkali interactions with rhodium and steel surfaces

The kinetics for alkali ion emission from rhodium and stainless steel surfaces are studied with field reversal (FR) technique under atmospheric conditions. Rapid electric FR outside the surface disturbs the surface population of alkali metal atoms by retarding or accelerating the desorbing ions, and the flux of ions from the surface is monitored to determine the rate constants for desorption. The rhodium surface is oxidized at surface temperatures below 1373 K, and the oxides decompose at higher temperatures. This phase transition is clearly observed as a change in the desorption kinetics for alkali ions, with a higher alkali stability on the oxidized surface compared to the metallic surface. The kinetics for Na+, K+ and Cs+ desorption from Rh in air are characterized, and rate constants in the range 10(-2)-10(3) s(-1) are obtained at surface temperatures of 990-1710 K. Rate constants for Nat and K+ desorption from stainless steel are determined in the temperature range 920-1450 K. The heterogeneous character of the Rh and steel surfaces is reflected in the observed desorption kinetics that show effects of combined desorption and diffusion steps on the surfaces. The results confirm that the FR technique can be used as an in situ probe, capable of monitoring phase transitions and kinetics on hot surfaces at elevated pressure. (C) 2000 Elsevier Science B.V. All rights reserved.