Coadsorption of cesium and hydrogen on Al(111): induction of another two distinct adsorption states of hydrogen and its suppressive effect on aluminum hydride desorption

Recently, we found out that aluminum hydrides desorb from hydrogen-adsorbed Al(111) surfaces. In this work, coadsorption states of cesium and hydrogen on Al(111) and the effects of cesium on the desorption of aluminum hydrides have been studied by means of temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS) and work-function changes. The desorption of aluminum hydrides is profoundly suppressed by Cs. The suppressive effect of a Cs atom extends to a much larger area than that expected from the area geometrically covered by the Cs atom. Three types of hydrogen binding sites are formed in the presence of preadsorbed Cs adatoms; that is, Cs sites, Cs-influenced Al sites in the vicinity of Cs atoms and Cs-effect-free Al sites. The hydrogen atoms at the Cs sites form a CsAlH₂ surface complex. This complex is so stable that it decomposes above 450 K resulting in the simultaneous desorption of H₂ and Cs leaving Al. The hydrogen atoms in the Cs-influenced area around the CsAlH₂ complex are considered to be embedded into subsurface sites. This hydrogen species does not form an aluminum hydride and desorbs as H₂. Appearance of these two types of hydrogen located inside the Cs-influenced area is the origin of the suppression of the aluminum-hydride desorption.