RATE CONSTANTS FOR THERMAL CESIUM ION DESORPTION ON CLEAN AND GRAPHITE-COVERED IRIDIUM STUDIED BY LASER-INDUCED DESORPTION

Desorption of cesium ions from clean and graphite-covered polycrystalline iridium was studied at surface temperatures of 600-910 K. Cs was adsorbed on the surface and the population decay due to thermal desorption was followed to obtain desorption rate constants. The surface population of Cs at different times was probed by laser-induced desorption, which gave a signal of laser-induced desorbed ions proportional to the surface concentration. With this method ionic rate constants in the range 10(-5)-10(-1) s-1 were obtained. The desorption energy of Cs+ on Ir was found to be 2.07 +/- 0.16 eV, in agreement with previous experimental results. On graphite-covered Ir three parallel desorption modes were found, all with a desorption energy of 1.8 eV, but with frequency factors of 7 x 10(8), 9 x 10(9) and 2 x 10(11) s-1, respectively. The highest frequency factor corresponds to desorption from a state with a desorption energy of 1.8 eV on the graphite layer. The two lower factors are due to Cs at graphite grain bounderies, and between the graphite layer and the Ir surface. Diffusion of these populations to the state on top of the graphite layer, followed by desorption, is concluded to be the mechanism behind the observed results.