K and Cs adlayers on Al(111) have been studied by Auger electron spectroscopy (AES), temperature programmed desorption (TPD), electron energy loss spectroscopy (EELS) and low energy electron diffraction (LEED) to investigate changes of electronic states accompanied by structural transformations of the alkali adlayers. Two types of adlayers with various coverages were prepared for each alkali metal by two methods; as-deposition at 85 K (AD series) and thermally desorbing excess alkali atoms (TD series). For the K AD series, a K-associated loss feature, suddenly appears simultaneously with a two-dimensional condensation at a coverage far below the complete monolayer and the loss energy stays almost constant during growth of the condensed islands. While for the Cs AD series, which shows neither the condensation nor the island growth, such a delayed appearance of loss feature and a constant loss energy are not observed. For both TD series, a drastic change of the EEL spectrum is observed when the desorbing temperature is increased (the coverage is decreased); the nature of an alkali-induced loss peak changes from a surface plasmon to a ns → np like individual excitation, which suggests a "metallic-to-ionic" change. These transitions are associated with thermally induced structural transformations including significant restructuring of the substrate. Although there is a difference in structural transformations between the K and the Cs TD series, both alkali atoms might interact more strongly with Al atoms after the transformations.
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