DESORPTION-KINETICS OF H-2 FROM AL(100), AL(110) AND AL(111)

Hydrogen desorption from three different aluminium single crystal surfaces [(100), (110), and (111)] has been studied using angle resolved thermal desorption spectroscopy. Adsorption of hydrogen was facilitated using dissociated atomic hydrogen from a hot Knudsen cell. Saturation coverages between 1.3 and 2.4 monolayers have been obtained. The shape of the desorption spectra exhibits a zero order desorption mechanism for all three aluminum surfaces. This behavior can be explained using a model in which desorption is assumed to take place simultaneously from hydrogen lattice gas in equilibrium with a condensed two-dimensional phase. The angular distribution of the desorption probability for associatively desorbing H-2 molecules exhibits a strongly forward focused function. The sharpest distribution is obtained for the (110) surface with D(phi) proportional cos20-phi. This indicates a very high activation barrier for adsorption and desorption.