The adsorption of hydrogen on Cs thin films deposited on Al(111) has been investigated by means of electron energy loss spectroscopy (EELS) and temperature programmed desorption (TPD). When hydrogen is adsorbed at a surface temperature of 85 K, three hydride phases are formed depending on Cs coverage. For Cs coverages below one monolayer (ML), hydrogen adsorption causes formation of a stable surface complex, CsAlH2, in which H atoms are bound to Al with a geminal structure. In the coverage range between 1 and 2 ML, a surface hydride of Cs and the CsAlH2 complex are formed on the surface, while above 2 ML, both the surface hydride and a bulk hydride of Cs are formed. When a hydrogen-adsorbed thick Cs film is heated from 85 K, Cs atoms start to desorb from 270 K, while hydrogen atoms initially located in surface region do not desorb with Cs but penetrate into bulk and finally reach the Al substrate resulting in the formation of the CsAlH2 complex. This complex is quite stable and decomposes above 450 K desorbing H2 and Cs. Hydrogen adsorption on Cs-Al surface alloy phases was also studied and it was found that its adsorption state is essentially the same as that on the Cs submonolayer overlayers, that is, formation of the CsAlH2 complex.
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