The adsorption of Cs on Al(111) has been studied by electron energy loss spectroscopy (EELS) and low energy electron diffraction (LEED). Two series of Cs overlayers with various coverages were prepared by two methods; as-deposition at 85 K (AD series) and thermally desorbing excess Cs (TD series). We find an abrupt change of EEL spectra for the TD series at a coverage of approximately 0.5; the nature of a Cs-induced loss peak changes from a surface plasmon to a Cs 6s → 6p like excitation, which is indicative of a metallic-to-ionic change. This transition is associated with a simultaneous structural transformation. Both EELS and LEED for the TD series are different from those for the AD series at the coverages between ca. 0.5 and 0.2. Irreversible structural and electronic transformations from the AD to the TD series also occur at these coverages by heating above 200 K without desorbing Cs. This indicates that the transformation is induced by a thermally activated process.
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