RATE CONSTANTS FOR CESIUM ION AND ATOM DESORPTION ON IRIDIUM WITH GRAPHITE ISLANDS - PARALLEL PROCESSES STUDIED BY FIELD REVERSAL

The desorption of Cs as ions and neutrals from Ir polycrystalline surfaces is studied by the field reversal kinetics method, which involves molecular beam techniques in an UHV environment. The electric field outside the surface under study is switched rapidly between accelerating and retarding field. The surface temperatures studied lie between 1100 and 1900 K. The shortest ionic desorption time constant measured in this study is 300 ns, and the longest neutral time constant is 0.5 s. Two neutral desorption channels and two ionic channels are observed in the temperature dependence of the desorption rate constants. The results agree with a model where graphite islands with a total coverage of less than 10(-2) exist on the surface. The desorption energy of Cs+ on Ir is 2.10 +/- 0.05 eV, in excellent agreement with previous low-temperature results by Zandberg et al. On graphite, the desorption energy for Cs+ is 2.67 +/- 0.11 eV, which agrees very well with the previous results from our group. The two neutral desorption energies, which both correspond to desorption via the graphite islands, are found to be 3.10 +/- 0.35 and 2.38 +/- 0.10 eV. A barrier of approximately 0.7 eV for transport from the iridium surface to the graphite islands is also found.