By use of high-resolution electron-energy-loss spectroscopy, atomic vibrations of the sodium and the lithium chemisorbed Al(111) surfaces have been investigated as a function of coverage. Owing to the screening of the adsorbate-adsorbate interaction by electrons of the Al atoms located between the alkali-metal atoms, the vibrational energy of Na on Al(111) remains almost constant (11.6-12.6 meV) over the coverage range of θ = 1 15- 1 3. The vibrational energy of Li on Al(111) also remains constant (17.6-18.1 meV) from θ = 1 30 to θ = 1, which strongly suggests a substitutional adsorption geometry for this system. Good agreement has been found between a recent ab initio calculation and the present experiment for the vibrational energy of the Al(111)(√3 × √3)R30° -Na surface.
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