ALUMINUM-HYDRIDE DESORPTION FROM HYDROGEN COVERED ALUMINUM SINGLE-CRYSTAL SURFACES

Formation and desorption of aluminum hydride from hydrogen covered Al(110), Al(100), and Al (111) has been investigated, using thermal desorption spectroscopy. Both desorption of aluminum hydride and molecular hydrogen has been detected with branching ratios depending on surface structure. Production of aluminum hydride is negligible on the rough (110) surface, whereas on the flat (111) plane up to 50% of the adsorbed hydrogen is desorbed in form of aluminum hydride. Furthermore, aluminum hydride formation is strongly enhanced with an increase of the heating rate. Desorption of molecular hydrogen follows a close to zero order reaction, with a desorption energy around 17 +/- 1 kcal/mol. Aluminum hydride desorption can be described best by a fractional order (almost-equal-to 0.5), with a desorption energy of 27 +/- 1 kcal/mol. Angular distribution measurements reveal desorption distribution functions of D(THETA) almost-equal-to cos-THETA - cos3-THETA for aluminum hydride and D(THETA) almost-equal-to cos2-THETA - cos15-THETA for molecular hydrogen, strongly dependent on surface structure.